Symposium EN14—Advanced Materials for Hydrogen and Fuel Cell Technologies

Oxygen reduction reaction is a key reaction for proton exchange membrane fuel cells (PEMFCs), which is the perspective of renewable energy for many practical applications. Platinum is the most efficient element for oxygen reduction reaction on the cathode side. However, Pt-based cathode catalysts are undergoing severe degradation due to the high potential changes. Therefore, we introduce a small amount of Au into Pt, form bimetallic PtAu alloy thin film catalyst by magnetron co-sputtering technique, resulting in a significant stability improvement in oxygen reduction reaction. Moreover, the PtAu alloy thin film catalysts show similar electrochemical activity with the identical Pt-loading, indicates its applicability for industrial PEMFCs. The stability of thin film catalysts is studied in half-cell rotating disk electrode (RDE). The morphology, structure and composition of thin film layer are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).

Silicon carbide (SiC) thin films are of interest for hydrogen permeation barriers in fusion reactors. Stainless steels such as 316L are sought for nuclear applications (for example, fission reactor and tritium process components) because of wide availability and low cost, however they are susceptible to tritium uptake that may affect tritium accountancy and release to the cooling system and environment. We examine SiC coated 316L by magnetron sputtering, followed by deuterium permeation testing at elevated temperature and pressure. Permeation was measured in a concentration-driven gas-phase experimental setup. The Arrhenius permeation behavior was derived by varying temperature and pressure gradients in a stepwise fashion. As seen in other studies, more than an order of magnitude reduction in deuterium permeation is found for thin SiC coatings compared with uncoated 316L. The emphasis here is to characterize the SiC coating and 316L substrate interface to elucidate any effects of microstructure on permeation resistance.

High-temperature endothermic reduction of redox-active metal oxides, which can exchange oxygen between the solid and the gas phase at achievable conditions (where what is achievable remains an open question for the field), converts thermal energy to stored chemical energy. A subsequent re-oxidation step either recovers the stored energy as heat or utilizes it to drive other chemical reactions. If the re-oxidation step restores the material to its original state so that the two steps can repeat indefinitely, the sequence of reactions constitutes a thermochemical cycle. Two related processes (reversibly re-oxidizing with oxygen or affecting bond breaking by re-oxidizing with CO₂ and/or water) are somewhat analogous; the production of energized carriers (H₂ and/or CO) is significantly more constrained by thermodynamics and hence subject to greater challenges to objectives for high efficiency and low cost. Nonetheless, both chemistries have been of increasing interest due to their potential for high utilization of the sunlight in high direct normal irradiance (DNI) regions and economic competitiveness for clean energy, and most recently as a role in achieving negative emissions and closing the carbon cycle.

This presentation will discuss new revelations in the thermodynamics of the redox-active metal oxide and the operating conditions of a cycle. We will show recent progress towards developing high-performing materials and cycles for thermochemical energy storage and CO₂/H₂O splitting.

Extracting hydrogen fuel from water splitting will require identifying and optimizing materials for the oxygen evolution reaction, as it is a major bottleneck in water splitting reactions. Photoelectrochemical cells have been at the forefront of technologies with the potential to realize a hydrogen economy at scale, and transition metal oxides in particular have garnered significant interest as photoelectrodes. Bismuth vanadate (BiVO₄) is one such oxide with numerous advantageous optoelectronic properties that make it amenable as a photoanode for oxygen evolution. However, progress towards achieving the optimal photocurrent densities and performance of BiVO₄ as a photoanode has not been systematic, thus necessitating the need to mechanistically understand its material properties, especially at the surface/interface.

Here, we present a combined experimental and computational study aimed at an atomic-level understanding of the BiVO₄ (010) surface with water. Our previous work [1] demonstrated that the surface composition plays a critical role in the photoelectrochemical performance and identified that the relevant surface is actually Bi-rich, not a 1:1 Bi:V ratio as previously thought. Building on this understanding, we performed first-principles calculations with Quantum ESPRESSO (https://www.quantum-espresso.org/) and Qbox (http://qboxcode.org/), and used resonant PES and XPS to elucidate the microscopic interactions between the BiVO₄ surface and water while varying the surface termination. In this presentation, we will compare and contrast the changes in surface energetics between different surface terminations of the BiVO₄ (010) surface upon immersion in water and discuss ways in which this could impact photoelectrochemical performance.


[1] D. Lee, W. Wang, C, Zhou, X. Tong, M. Liu, G. Galli, K.-S. Choi. “The impact of surface composition on the interfacial energetics and photoelectrochemical properties of BiVO₄ .” Nature Energy. 6, 287 (2021).

This work was supported in part by NSF CHE-1764399 and resources at CFN BNL though the DOE, DE-SC0012704.

Aluminum is a promising energy carrier due to its abundance, low cost, and ability to react with water to produce hydrogen gas at a high volumetric energy density (up to 36.3 MJ/L Al). Ordinarily, aluminum’s passivating oxide layer that rapidly forms upon exposure to oxygen prevents its reaction with water; however, in recent years, numerous promising approaches to disrupting this oxide layer have been developed that utilize a variety of room-temperature-liquid metal alloys containing some combination of gallium, indium, and tin, for example. In this work, an activating alloy composed of eutectic gallium and indium (eGa_0.8In_0.2) was studied due to its ability to activate bulk aluminum particles at added mass ratios below 5%. Prior research had shown that surface-treating bulk aluminum pellets (>5 mm in diameter) with this alloy results in complete penetration of the liquid metal along the aluminum grain boundary network, enabling near-unity theoretical hydrogen yield fractions in subsequent reactions with water. Despite the efficacy of this approach for producing hydrogen, however, numerous questions remain about the nature of the underlying reaction mechanism, in particular what happens to the eGaIn throughout the reaction.

In this work, experimental techniques were developed to carry out the aluminum-water reaction at arbitrarily slow reaction rates via controlled exposure to water vapor at room temperature. By starting and stopping the reaction at various stages in the absence of liquid water, standard SEM-EDS and XRD techniques could be used to study the reaction mechanism in significantly greater detail than before. The results of these experiments show that the aluminum-water reaction occurs in two primary stages: (1) rapid disintegration of the bulk aluminum along its grain boundaries via a fractal-like exfoliation process, and (2) the subsequent reaction of water at unoxidized sites on the freshly exposed aluminum grain surfaces. The phase of the activating eGaIn was also tracked and imaged at various stages of the reaction for the first time, showing ultimately that during the bulk disintegration process, reactions between gallium and ambient water and oxygen lead to the dealloying of the initial eGaIn, leaving behind micron-scale solid indium spheres. Given the high cost of the eGaIn relative to the input aluminum, their recovery is important for many practical applications. The operational implications of these results are discussed, and methods for recovering the activating metals are proposed.

Developing efficient earth-abundant electrocatalysts to replace high-cost Pt for alkaline hydrogen evolution reaction (HER) is crucial for sustainable hydrogen production. However, an additional sluggish water dissociation step associated with alkaline HER poses a major bottleneck for real applications. Recently, developing heterostructures catalysts with an electronegative atom (P, N, O, and S) acting as a base to trap the H* has emerged as a paramount strategy to boost HER activity. However, the alkaline HER activity of most of the heterostructure-based catalysts cannot surpass the commercial Pt, due to inadequate H* adsorption energy (ΔG_H*) and high kinetic barrier for water dissociation, mainly determined by the electronic structure and localized charge-density at the interface-region.

Herein, density functional theory (DFT) predictions reveal that the electronic structure and localized charge density at the heterointerface of NiP₂−FeP₂ heterostructure can be significantly modulated upon coupling with metallic Cu, resulting in optimized proton adsorption energy and reduced barrier for water dissociation, synergistically boosting alkaline HER. Motivated by theoretical predictions, we developed a facile strategy to fabricate interface-rich NiP₂−FeP₂ coupled with Cu nanowires (Cu_NW) grown on Cu foam (NiP₂− FeP₂/Cu_NW/Cu_f). Benefiting from the superior intrinsic activity, conductivity, and copious active sites, the obtained catalyst exhibited exceptional alkaline HER activity requiring a low overpotential of 23.6 mV at −10 mA/cm², surpassing the state-of-the-art Pt. Additionally, a full electrolyzer required a cell voltage of 1.42/1.4 V at 10 mA/cm² in alkaline water/seawater with promising stability. This work highlights the importance of optimizing HER kinetics and offers a novel strategy for designing advanced electrocatalysts for various catalytic reactions (CO₂RR, ORR, OER, NRR, and so on).

Designing an efficient oxygen evolution reaction (OER) electrocatalysts based on single-atom catalysts is a highly promising option for cost-effective alkaline water electrolyzers. The main bottleneck in the practical water electrolysis system is the sluggish oxygen evolution reaction (OER) due to multiple proton/electron-coupled steps. Current technologies involve noble metal (Ir, Ru) or earth-abundant hydroxides/oxides as anodes; however, the scarcity and semiconducting nature, respectively, of these materials limit their activity. Unfortunately, earth-abundant metallic alloys have not been employed for OER. Recent work showed that single-atom catalysts (SACs) possess peculiar electronic features and near-unity atom economy, yet poor intrinsic activity and stability hinder their practical applications.

Overall, the instability of the OOH* intermediate and the high energy barrier for the rate-determining step (RDS) (O* to OOH*) on pure bimetallic alloy surfaces represent serious challenges and must be overcome for replacing Ir/Ru as anode materials based on an atomic level of understanding.

In this work, we introduce a simple approach to stabilize OOH* intermediates on a surface oxygen-rich CoFe₂/G alloy with homogeneously anchored Ru single atom (0.1 at.% of Ru) sites to boost the intrinsic activity via a facile micelle incorporated sol-gel and carbothermal reduction method. State-of-the-art characterizations including XANES, EXAFS, AC-TEM, and XPS supported by theoretical calculations confirmed the anchoring of isolated Ru atoms into the surface oxygen-rich CoFe₂ alloy. Remarkably, the resulting Ru_SACoFe₂/G delivered excellent OER activity with an overpotential as low as 180 mV at a current density of 10 mA cm⁻² and a reasonable Tafel slope of 51 mV dec^-1 in a 1 M KOH aqueous solution. These results surpassed reported literature and state-of-the-art Ru-based electrocatalysts (RuO₂ and 5% Ru/C). Density functional theory (DFT) calculations revealed that the pre-adsorbed oxygens on Ru_SACoFe₂ (110) efficiently stabilized the OOH* intermediates and the isolated Ru sites further dramatically reduced the Gibbs free energy for the RDS (O* to OOH*), thereby accelerating the OER process in alkaline medium. To meet the industrial standards of alkaline water electrolysis, we constructed a Ni₄Mo (-) // Ru_SACoFe₂/G (+) alkaline electrolyzer demanding a very low cell voltage of 1.48 V at a current density of 10 mA cm⁻², which was much lower than those values for a commercial Pt/C-RuO₂ couple (1.65 V at 10 mA cm⁻²).

Among numerous semiconductors that can be used for water photosplitting, Si is highly interesting because it absorbs light in the visible range and its electronic structure is suitable to drive water photooxidation [1]. However, Si corrodes strongly in KOH and it is highly reflective.
The concept, presented, here, consists of successively depositing a protective layer and a catalyst onto the Si surface to extend the lifetime and the efficiency of the photoelectrode. To further improve the performances, various surface structuring methods are used to enhance the light absorption, enlarge the active area, and improve the charge collection. Various electrochemical structuring methods have been used to fabricated three different surface geometries: macropores (SiMP) [2], nanospikes (SiNS) [3], and micropillars (SiPillars) [4]. Those different morphologies show a large active area and a low reflectivity. Atomic Layer Deposition (ALD) is a well-adapted method to coat such tortuous surfaces. Thus, it has been applied to grow thin protective oxide layers (TiO₂ or Fe₂O₃) onto Si and SiMP, SiNS and SiPillars. To enhance the photocurrent and to lower the overvoltage corresponding to water photooxidation, metallic Ni has been deposited onto TiO₂-covered Si through a two-step process: (i) conformal ALD of NiO from Ni(CpEt)2 and O3 and (ii) the reduction to Ni by annealing under H₂ atmosphere. Ultra-low loads of IrO₂ have been deposited on Fe₂O₃-covered Si and ultra-thin TiO₂ capping allows for stabilizing the catalyst in an alkaline medium.
A detailed electrochemical study has been performed to assess the stability of the composite electrodes and to measure the water photooxidation performances. The TiO₂ dissolution at open circuit potential under a low illumination mechanism is identified. It proceeds through the insertion of H into the TiO₂ [5].
The efficiency of such multilayered and structured photoelectrodes is drastically improved using significantly fewer catalysts and the photoanodes exhibit long-term stability. The SiNSs covered by Fe₂O₃ and IrO₂ show, indeed, a photocurrent improved by a factor. It demonstrates that the appropriate combination of materials on the surface exhibiting the optimized geometry leads to high-performance photoanodes.

[1] K. Sun, S. Shen, Y. Liang, P. E. Burrows, S. S. Mao, D. Wang, Chem. Rev. 114, 8662 (2014).
[2] L. Santinacci, M. W. Diouf, M. K. S. Barr, B. Fabre, L. Joanny, F. Gouttefangeas, G. Loget, ACS Appl. Mater. Interfaces, 8, 24810 (2016).
[3] G. Loget, A. Vacher, B. Fabre, F. Gouttefangeas, L. Joanny, V. Dorcet, Mater. Chem. Frontiers, 1, 1881 (2017).
[4] F. J. Harding, S. Surdo, B. Delalat, C. Cozzi, R. Elnathan, S. Gronthos, N. H. Voelcker, G. Barillaro, ACS Appl. Mater. Interfaces, 8, 29197 (2016)
[5] M. E. Dufond, J.-N. Chazalviel, L. Santinacci, J. Electrochem. Soc. 168, 031509 (2021)

Herein, we report on the defect engineering of BiPO₄ nanorods (NRs) via a facile room-temperature template-free co-precipitation method, followed by hydrogen treatment. The hydrogen treatment temperature determined the type of induced defects in the fabricated BiPO₄ NRs and consequently their photocatalytic performance. Upon varying the annealing temperature, the x-ray diffraction (XRD) analysis showed phase transformation and x-ray photoelectron spectroscopy (XPS) analysis revealed variation in the oxygen vacancy content. At moderate treatment temperatures (200-300^oC), shallow defects were predominant, which extended the optical activity of the material to the visible region and increased the photocurrent 3 times when compared to that of bare BiPO₄ NRs. However, treatment at higher temperatures completely altered the crystalline structure, destructed the morphology of the BiPO₄ NRs, and severely affected the photoelectrochemical performance.

Within the current context of energy crisis and climate changes, the search for versatile, clean and sustainable energy sources is vital to reduce our dependency on fossil fuels. As such, photoelectrochemical (PEC) water-splitting arises as an ideal route for the obtention of hydrogen: by using sunlight as power input, it is possible to obtain a high-energy, carbon-free fuel.
In this work, we propose the elaboration of functionally graded photoanodes based on titanium oxides through a simple and fast soft chemistry route. The resulting photoanode presents an external mixed-phase TiO₂ layer that transitions to titanium suboxides (including Magnéli phases) allowing prompt charge separation and transport of the photogenerated electrons towards the metallic Ti support for current collection.
Two main factors enable the superior performance of the photoanode: an increased light absorption (due to the synergy between anatase and rutile domains in intimate contact at the outmost layer, and to Ti³⁺ species present at the oxygen deficient subjacent layers) and a more effective charge separation and transport (facilitated by the alignment of the bandgaps within the specific regions and by the increased conductivity of the titanium suboxides). [1, 2]
Such unique features are achieved by using chelating agents to direct the formation of the different TiO₂ phases, and subsequently enable the pore generation and localized reduction of TiO₂ during the calcination step. We explored variety of experimental conditions in order to optimize our process and to determine the influence of each parameter on the resulting material.
The obtained photocurrent densities (ranging from 0.25 to 0.80 mA cm^-2 at0 and 1.2V vs. Ag/AgCl respectively, in 0.5 mol L^-1 Na₂SO₄) are comparable or superior to doped, sensitized and/or nanostructured anatase photoanodes [3, 4], while not requiring the use of toxic components and while undergoing a much simpler, cheaper and environmentally benign productive process. As such, we envision a great potential for the development and up-scaling of adaptable, cheap and robust devices for hydrogen generation through water-splitting.

[1] G. Li, L. Chen, M. E. Graham, and K. A. Gray, Journal of Molecular Catalysis A: Chemical, vol. 275, no. 1–2, 2007.
[2] S. Jayashree and M. Ashokkumar, Catalysts, vol. 8, no. 12, p. 601, 2018.
[3] H. Eidsvåg, S. Bentouba, P. Vajeeston, S. Yohi, and D. Velauthapillai, Molecules, vol. 26, no. 6, 2021.
[4] K. Arifin, R. M. Yunus, L. J. Minggu, and M. B. Kassim, International Journal of Hydrogen Energy, vol. 46, no. 7, 2021.

Electrochemical water splitting is one of the most efficient techniques to produce hydrogen, which provides clean and sustainable energy. Developing an efficient catalyst for hydrogen evolution reaction (HER) based on 2D molybdenum disulfide is a challenge because of restacking of MoS₂ layers that leads to insufficient access to active sites. In this study, the drawbacks of 1T MoS₂ catalyst were overcome by employing carbon nano-onion (CNO). The unique CNO-MoS₂ heterostructure formed by growing few-layered 2D 1T MoS₂ on the CNO core facilitated smooth and barrier-free charge-transfer through the heterojunction formation. This heterostructure not only mitigates restacking but also maintains the structural stability, providing a long-term high performance catalyst. This CNO-MoS₂ exhibited an excellent HER performance with an overpotential of 53 mV vs. RHE and Tafel slope of 40.8 mV dec^-1 for over 25 h, showing it one of the best HER catalysts next to platinum. This study highlights a new highly stable highly performing non-noble metal catalyst for electrochemical water splitting applications.

Gas evolution at electrochemical electrodes is important for many industrial applications. Perhaps the largest activities where electrochemical bubble generation is critical are the chlor-alkali (chlorine gas production) and Hall-Héroult (electrochemical aluminum smelting) processes. Significant effort goes towards mitigating the “anode effect” of the Hall-Héroult process, where carbon dioxide gas inactivates the carbon anode, causing catastrophic increases in overpotential. Similar inefficiencies and problems are caused during water electrolysis when bubbles adhere to or flood electrodes. The limiting effects of bubbles no doubt limits the adoption of traditional, low-temperature electrochemical methods for hydrogen generation from renewable electricity, leaving natural gas reforming as the dominant form of hydrogen production in the US currently. In this way, both understanding and mitigating the negative impacts that bubbles have on electrode performance offers a unique path to enable low-temperature alkaline water electrolyzers to become a commercially viable route of hydrogen production from renewable electricity.

A key disadvantage for alkaline electrolyzers is that while their porous electrodes can increase the active area of the catalyst, the nature of the catalyst material, like nickel-foams, causes the evolved bubbles to stick, inactivating the catalyst surface and causing significant ohmic and activation losses. The extent to which these bubbles can restrict electrochemical performance is significant. For example, even for the well-established chlor-alkali process, the attachment of bubbles to electrode surfaces accounts for approximately 20% of the overpotential at industrially relevant current densities.

These problems motivate this work as we investigate the way that bubble inefficiencies manifest themselves for different types of electrode configurations for hydrogen-evolving electrodes. While previous research has characterized the impacts of electrochemical bubbles, approaches to mitigate bubbles are limited and typically focus on novel cell design, rather than the design of the electrode itself. Here, we will focus on how different architectures of the electrodes affect the performance of the electrode while systematically studying the differences that arise from a bubble perspective. In general, many approaches for improving the performance of non-gas evolving electrodes are currently used in gas-evolving systems as well. However, we will show that the inherent tradeoffs between traditional methods for improving electrode performance, and those that should be adopted for gas-evolving electrodes are distinct. As a result, we provide a novel framework for designing hydrogen-evolving electrochemical electrodes. Furthermore, implementations of the novel electrode motifs based on this new design perspective are also envisioned to improve the performance of water electrolyzer systems.

To provide affordable H₂ with high purity for energy applications, new strategies as H₂-selective membranes gained in interest. This technology presents interesting benefits as it is cost-effective and have a reasonable environmental impact. In addition, a large variety of materials is available, thus, the composition of the membrane can be adapted to the working temperature. This study focuses on the fabrication of dense membranes able to operate at the high temperatures required in industrial processes (400-800°C). The specificity of the present approach is to fabricate new nanocomposite devices that combine the stability of a ceramic matrix, titanium nitride, with the catalytic effect of palladium nanoparticles used as nanofillers. Both TiN films and Pd nanoparticles are deposited onto a hetero-porous alumina tubular membranes by Atomic Layer Deposition (ALD). The composite membrane is optimized to reach the highest H₂-selectivity and permeation (flow), by finely tuning the thickness and the chemical composition of the TiN/Pd coating. ALD is an original technique in this field, but it is well adapted because it allows for coating nanostructured substrates with an excellent control of the thickness and composition.
The fabrication of the membrane follows two steps. First, the TiN deposit must fill the alumina porosity. The coating forms a dense layer on the nanoporosity at the inner part and penetrates into the macroporosity of the outer part, covering the alumina grains. Afterwards, the Pd nanoparticles are deposited. The selectivity and the permeation of the TiN-Pd membrane are assessed using H₂ and N₂. The increase of the temperature up to 400°C allows for achieving an encouraging dihydrogen ideal selectivity and permeance of 35 and 258 GPU (Gas Per Unit), respectively. Moreover, these values increase during the measurement: the selectivity and the permeance rises up to 109 and 7018 GPU, respectively, after 4.5 h at 400°C. These performances are promising as a similar pure Pd membrane presents an ideal selectivity of 23 and a permeance of 991 GPU at 188°C [1]. The investigation of the thermal evolution and the stability of the present membrane is then required. Complementary results will therefore be presented. In another hand, a Pd-based membranes issue is their poisoning in presence of CO and sulfides gases that are present in industrial processes. The ALD of both TiN and Pd can be optimized to encapsulate the Pd nanoparticles within the TiN matrix. This could be a future route to take up the poisoning obstacle and to improve the performances.

[1] Weber et al., Hydrogen selective palladium-alumina composite membranes prepared by Atomic Layer Deposition, J. Membr. Sci. 2020, 596, 117701.

Nucleation and growth mechanism studies of functional oxide films are essential to gain an understanding and thereby control their morphologies. Towards this, we study the effect of solution pH and deposition potential on the nucleation and growth (NG) of the technologically important material Cu₂O as a thin film on the FTO substrates. The Cu₂O films electrodeposited at a low potential value of -20 mV (vs. Hg/HgO) at pH 9 exhibit a slow layer-by-layer NG mode where the 2-D instantaneous nucleation (IN-2D) is identified as the primary contributor. The NG mechanism, however, switches to 3-D diffusion-controlled instantaneous and progressive (IN-3D_diff & PN-3D_diff) mode at the moderate potential value of -100 mV. Further raising the potential to -180 mV results in a complex process involving multiple contributions from IN-3D_diff & PN-3D_diff which would result in an island type morphology. In contrast, the films deposited at pH 12 show a minimal contribution from the 2-D type NG mechanism even at low potential values. At a low potential of -170 mV, the major contribution comes from the diffusion-controlled 3-D type progressive (PN-3D_diff) NG process. An additional contribution; the charge transfer controlled instantaneous (PN-3D_ct) NG process runs in parallel with previous processes when films are subjected to a moderate potential of -210 mV. At high potentials, however, the situation becomes similar to pH 9 having multiple contributions from both charge transfer and diffusion controlled IN and PN processes. The modeling of experimental data reveals that electrodeposition in a low pH, as well as low potential, is preferable to obtain a layer-by-layer deposition of Cu₂O thin films whereas a 3-D island type NG occurs at high potentials. These results pave the pathway to tailor the morphology of other technologically important metal oxides in a similar fashion.

Niobium oxides exist in a plethora of metastable, stoichiometric, nonstoichiometric, and mixed phases, rendering the Nb-O systems very complicated and hard to study. These structures significantly differ in their catalytic activity, electrical conductivity, and photoresponse. Herein, we demonstrate the ability to selectively fabricate pure T-Nb₂O₅ via the addition of small amount of Zr as a phase stabilizer. Moreover, we were able to tune the photoactivity of the material via hydrogen annealing. The photoactivity and stability of the fabricated black Zr-doped Nb₂O₅ nanotubes were correlated with the nature of induced defects upon hydrogen annealing and Zr doping. The H₂-treated nanotubes showed extraordinary and remarkable stability and photoactivity upon their use for solar water splitting. This was accompanied by a noticeable reduction in the bandgap energy from 3.23 eV to 2.5 eV, which is mainly correlated with the introduced oxygen vacancies within the lattice with a remarkable conductivity. Most importantly, the black-defective nanotubes exhibited a photocatalytic activity that is ~ 65 times that of the air-annealed counterparts. The optimized photoanodes attained a hydrogen production rate of ~ 496 µmol h^-1 cm^-2 in 1 M KOH, revealing increased charge carriers transport and separation. The Mott-Schottky and valence band XPS analyses confirmed the increased charge carriers’ concentration and the appropriate band positions of the fabricated black nanotubes relative to the redox potentials of the water.

Designing next-generation advanced electrode materials by engineering their structural and compositional features can provide a feasible strategy to enhance the electrochemical performance of energy conversion devices. In this study, the rational pathway to design and fabricate nanotube arrays of titanium manganese phosphide via etching of titanium-manganese alloy followed by plasma phosphidation in PH₃ environment is presented and discussed. The structural and elemental analyses of the air-annealed electrodes before plasma treatment confirmed the presence of different binary oxides; TiO₂, MnO, and Mn₂O₃. However, the XPS fitting showed the presence of Ti³⁺ and higher ratio of MnO when annealed in hydrogen atmosphere. The presence of composite oxides resulted in a band gap reduction, which increased the light harvesting capability of the material. This synergetic effect resulted also in a shift in the open-circuit voltage (V_OC) and almost 10-fold increase in the photocurrent density compared to the performance of the nanotubes annealed in air. Mott-Schottky analysis showed a four-orders of magnitude enhancement in the carrier density for the electrodes annealed in Hydrogen and treated in PH₃-plasma compared to those annealed in O₂ or air, ascribed to the creation of Ti³⁺ defects and phosphidation. Our study thus paves the way to a new approach for creating high-performance hybrid electrodes for PEC water splitting.

Green and efficient energy technologies are a key point where nanoscience could make a difference in the paradigm shift from fossil fuels to renewable sources. One attractive possibility is the utilization of solar energy to obtain electricity or chemical fuel based on the ability of semiconductor nanomaterials to function as photocatalysts promoting various oxidation and reduction reactions under sunlight. We report on a novel class of Bi-based photocatalysts for hydrogen production via water splitting. A screening DFT investigation was performed on the Bi₂(MO₄)₃ (M = Cr, Mo, and W) systems. The Bi₂(CrO₄)₃ system exhibits the smallest band gap energy, the highest dielectric constant, and the highest absorption in visible region among the other counterpart materials. Consequently, Bi₂(CrO₄)₃ nanoparticles were synthesized via a simple one-pot method at room temperature and characterized by XRD, XPS, DRS, FE-SEM, HR-TEM, and Raman spectroscopy. The as-prepared yellow Bi₂(CrO₄)₃ nanoparticles exhibited a direct band gap energy of 2.45 eV. The photoactivity of the as-prepared Bi₂(CrO₄)₃ nanoparticles was tested toward the photocatalytic hydrogen production, where reasonable rates of 522.44, 174.15, and 88.24 μmol/g/h were achieved under UV, AM 1.5, and visible irradiations, respectively in the absence of any hole scavengers. Those rates are higher than those reported for Bi-based photocatalysts.

We demonstrate, for the first time, the synthesis of highly ordered niobium oxynitride microcones as an attractive class of materials for visible-light-driven water splitting. As revealed by the ultraviolet photoelectron spectroscopy (UPS), photoelectrochemical and transient photocurrent measurements, the microcones showed enhanced performance (~1000% compared to mesoporous niobium oxide) as photoanodes for water splitting with remarkable stability and visible light activity.

Replacing fossil fuels and natural gas with alternative fuels like hydrogen is an important step towards the goal of reaching a carbon neutral economy. As an important intermediate step towards utilizing pure hydrogen, blending hydrogen in an existing natural gas network is a practical choice for reducing carbon emissions in the near future. A computational fluid dynamic (CFD) model is developed to quantify frictional losses and energy efficiency of transport of methane/hydrogen blends across straight pipe sections. The CFD model developed in the present study is first validated by verifying that the results obtained in the present study (for pressure drops, numerical friction number (f_N), and energy specific toll (EST) agree well with those obtained in an earlier study when identical boundary conditions and model parameters such as Redlich-Kwong equation of state, k-ε turbulence model and pipe wall roughness, are invoked. Furthermore, additional CFD models are developed to assess the effects of other factors such as 1) hydrogen concentration, 2) pipe surface roughness of common pipe materials, and 3) pipe diameter on the dynamics of blended gas flows. The principal conclusions from the present study are as follows: (i) High hydrogen concentration in the gas blends (i.e., greater than 25% and 50%) require higher energy for transporting the blended gases in the pipelines. (ii) Polyethylene (PE) or PE coated pipes, with smoother walls, are 33-40% more energy efficient in transporting gas blends than uncoated stainless steel pipes, which are 17-31% more energy efficient than the uncoated cast iron pipes. (iii) Due to relatively fewer gas interactions with wall surfaces which result in frictional losses, than in the case of smaller diameter pipes, it requires lesser energy to transport blended gases in larger diameter pipes. The CFD model and the simulation results provide valuable operational guidelines for transitioning natural gas networks to transport greener blends of methane and hydrogen.

One of the remaining challenges for the development of efficient photoelectrochemical water splitting systems is related to the photoanode, which requires a high-performance light absorber with an excellent catalyst to drive oxygen evolution reaction that needs high potential [1]. The metal-insulator-semiconductor (MIS) architecture employing Si as the photoanode is an attractive system due to the wide light absorption range of Si (E_g 1.1 eV) and high OER catalytic activity of metal [2]. For the latter, metal nanoparticles can be dispersed on the semiconductor surface via electrodeposition in Si MIS photoanodes [3]. Furthermore, surrounding the metal nanoparticles and the uncoated Si with a higher-work-function material leads to significant improvement of photovoltage due to the pinch-off effect [3]. Up to this point, the surrounding layer is generally the oxidized phase of the base metal, such as its oxide or oxyhydroxide, and the fabrication techniques used to introduce the surrounding layer are limited to a few choices, i.e., electrodeposition, photodeposition, or annealing methods [4]. Copper(I) thiocyanate (CuSCN) is a coordination polymer which can be prepared by a simple chemical deposition through conversion of Cu [5]. Because Cu and CuSCN have work functions of 4.5-4.9 [6] and 5.4 eV [7], respectively, the Cu/CuSCN junction on n-Si is expected to develop the pinch-off region and increase the performance. In this talk, we aim to highlight two key developments of Cu-based MIS Si photoanodes prepared by facile methods. Firstly, we employed electrodeposition to coat 30-nm Cu nanoparticles on n-Si, resulting in a photoanode with a low OER onset potential of 1.37 V vs reversible hydrogen electrode (RHE). Secondly, we added CuSCN on the n-Si/SiO_x/Cu photoanode via a chemical deposition in an aqueous bath containing ammonium thiocyanate. The presence of CuSCN on n-Si/SiO_x/Cu was confirmed by X-ray photoelectron spectroscopy and scanning electron microscopy. The optimal n-Si/SiO_x/Cu/CuSCN with a short CuSCN deposition time showed a 100-mV improvement in the onset potential relative to the unmodified n-Si/SiO_x/Cu. Based on open-circuit-potential measurements, the CuSCN-modified photoanode exhibited a high photovoltage of 460 mV. We believe that the findings can provide a new strategy for further surface modification of PEC devices to achieve low-cost solar harvesting technology.

[1] Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110 (11), 6446–6473.
[2] Aroonratsameruang, P.; Pattanasattayavong, P.; Dorcet, V.; Mériadec, C.; Ababou-Girard, S.; Fryars, S.; Loget, G. Structure–Property Relationships in Redox-Derivatized Metal–Insulator–Semiconductor (MIS) Photoanodes. J. Phys. Chem. C 2020, 124 (47), 25907–25916.
[3] Laskowski, F. A. L.; Oener, S. Z.; Nellist, M. R.; Gordon, A. M.; Bain, D. C.; Fehrs, J. L.; Boettcher, S. W. Nanoscale Semiconductor/Catalyst Interfaces in Photoelectrochemistry. Nat. Mater. 2020, 19 (1), 69–76.
[4] Lee, S. A.; Choi, S.; Kim, C.; Yang, J. W.; Kim, S. Y.; Jang, H. W. Si-Based Water Oxidation Photoanodes Conjugated with Earth-Abundant Transition Metal-Based Catalysts. ACS Mater. Lett. 2020, 2 (1), 107–126.
[5] Xu, J.; Xue, D. Fabrication of Upended Taper-Shaped Cuprous Thiocyanate Arrays on a Copper Surface at Room Temperature. J. Phys. Chem. B 2006, 110 (23), 11232–11236.
[6] Gartland, P. O.; Berge, S.; Slagsvold, B. J. Photoelectric Work Function of a Copper Single Crystal for the (100), (110), (111), and (112) Faces. Phys. Rev. Lett. 1972, 28 (12), 738–739.
[7] Worakajit, P.; Hamada, F.; Sahu, D.; Kidkhunthod, P.; Sudyoadsuk, T.; Promarak, V.; Harding, D. J.; Packwood, D. M.; Saeki, A.; Pattanasattayavong, P. Elucidating the Coordination of Diethyl Sulfide Molecules in Copper(I) Thiocyanate (CuSCN) Thin Films and Improving Hole Transport by Antisolvent Treatment. Adv. Funct. Mater. 2020, 30 (36), 2002355.

U as interesting hydrogen storage medium forms with hydrogen the hydride UH₃, which has two allotropic cubic phases, known as α- and β-UH₃. While the transient phase α-UH₃ (bcc lattice of U filled with H) can be stabilized by alloying of Zr, other alloyed hydrides appear as nanocrystalline β-UH₃ phase (more complicated cubic with two different U positions), the grain size of which can be as small as 2-3 nm. Conventional XRD analysis, based on several very wide diffraction peaks, does not give in such case sufficient information e.g. about possible segregation of alloying elements. That is why methods like total scattering with PDF analysis have been applied [1].
Another method applied is Transmission Electron Microscopy, which allows structural characterization at the atomic level. The TEM study became even more important in the thin film research, in which it proved feasible to synthesize the hydride films by reactive sputter deposition of metals in H₂ containing Ar working gas [2,3]. Such synthesis requires typically low temperatures, which allow H to be embedded into the deposited material. Also this synthesis yields very small grain size, which is often accompanied by a pronounced texture. The information from glancing angle XRD is also in such situation limited.
Here we present results of High-Resolution TEM study both on the bulk alloyed UH₃ and on various U-H films. While crushed fine powder can be used as TEM sample in the former case, the films require the focused ion beam lift-out technique, which allows to obtain cross-sectional thin lamellae giving information on film thickness, individual interfaces to the substrate or possible capping layer, as well as on U phases in the main layer. In particular, despite of nanometer grain size it allows to distinguish well the β-UH₃ type phase from the UH₂ phase, which does not exist as a bulk phase but can be prepared in the thin film form [4]. The study helped to exclude any possible segregation of Mo as alloying element and proved that spurious UO₂ remains on a very low level. The analysis is based on structural analyses using STEM and HR-TEM with subsequent Fourier analysis. The TEM micrographs were compared with simulated images using the JEMS software.
The work was supported by the Czech Science Foundation under the grant No. 21-09766S.

[1] L. Havela et al., MRS Advances 1 (2016) 2987.
[2] L. Havela et al., Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146904.
[3] E. Tereshina-Chitrova et al., Materials Chemistry and Physics 260 (2021) 124069.
[4] L. Havela et al., Inorg. Chem. 57 (2018) 14727.

Solid Oxide Fuel Cells are devices that use electrochemical reactions to convert chemical energy from fuel to electricity. Solid Oxide Fuel Cells typically have three different layers, an anode, electrolyte, a cathode layer. In comparison with coal power plants, a Solid Oxide Fuel Cell, produces a higher electrical conversion efficiency. In the last couple of years, the electrical efficiency has increased from 40 to 60%. Solid Oxide Fuel Cells are modular in nature making the power output even greater. When combined with another form of energy the efficiency increases even more. However, Solid Oxide Fuel Cells typically operate at high temperatures (800-1000 degrees Celsius) which hinders the usability. At this high operating temperature, the materials that can be used are also limited. When lowering the operating temperature, the ohmic resistance increases. There are two ways to prevent this increase is to either to change the material used for the electrolyte layer or to make the electrolyte layer into a thin film. Typical Solid Oxide Fuel Cells use Yttrium Stabilized Zirconium as the electrolyte layer. In this research, the electrolyte layer will be a thin film of various concentrations, 1-10%, of the Yttrium Stabilized Zirconium. This layer is produced from a fine dimple grain structure allowing high flow of oxygen mobility. This mobility increases ionic conductivity and decreases the ohmic resistance. The goal of this research is to optimize the experimental film deposition parameters that will lead to minimum surface resistivity. These thin films are produced are produced by the Pulsed Laser Deposition method. This method of production has shown to produce a specific thin film thickness with repetition. With this method there are numerous parameters that can be adjusted. The pressure, frequency, laser energy, etc. The Yttrium Stabilized Zirconium is deposited onto Polished Zirconium, Sapphire, and Silicon. The differences of substrates were used to compare the different characteristics of Yttrium Stabilized Zirconium layer on different materials. An ellipsometer will be used to ensure the thickness of the Yttrium Stabilized Zirconium layer. These thin films will be characterized through electrical measurements such as 4-point probe resistivity measurements as well as Scanning Electron Microscopy, for the structural characterization.

Separation membranes allow purification of hydrogen from gas mixtures produced by various methods, such as the reformation of natural gas, coal gasification, or the gasification of municipal waste. Hydrogen separated in this way may be used in fuel cells to generate electricity or may be fed directly to the natural gas grid to temporarly storage. It is likely that the current network of natural gas will soon be transformed into a “gas” grid where hydrogen would be an essential ingredient. All these require the use of efficient separation membranes, which would make the easy availability of hydrogen possible as is currently the case for natural gas. Thus, the separation membranes would have a much wider scope than normally anticipated and therefore there would be a need for separation membranes that are more efficient and in particular of low cost.

The current study concentrates on proton conducting oxides that can operate at temperatures higher than 450 °C and applicable to the steam reformation of natural gas including the water-gas shift reactions. Proton conducting oxides used for this purpose are quite attractive due to their high hydrogen selectivity and superior tolerance to possible toxic gases (e.g. CO, H₂S, etc.) that may be present in the gas mixtures.

The current study adopts a membrane design methodology based on combinatorial material science. This approach makes use of a magnetron sputtering system whereby a material library of thin-film membranes is produced in a single experiment. The library is then screened by four-probe resistivity and electrochemical impedance measurements so as to identify the compositions that are highly reactive with hydrogen. A map of reactive index prepared in this way is used to determine candidates for hydrogen separation. The membranes are then fabricated in the form of discs and tested for hydrogen permeability. In the current study, BaZr_0.80Y_0.20O₃-SrCe_0.95Yb_0.05O₃ heterostructures are investigated using the above methodology.

This work was supported by TUBITAK (The Scientific and Technological Research Council of Turkey) with project Number 119M065, which the authors gratefully acknowledge.

The rising need for renewable energy storage and conversion serves as the cause to explore suitable approaches to produce hydrogen. Among the different methods, electrochemical water splitting can produce hydrogen at low cost and high efficiency with non-noble metal catalysts, that can be used to accelerate the reaction. Until now, Pt/C and IrO₂ are found to be the best-known catalyst with very low overpotentials for hydrogen and oxygen evolution reaction, respectively. Yet, these are very costly and impractical for large scale industrial use. Hence, design and fabrication of low cost and highly efficient electrode is of utmost importance to produce hydrogen and oxygen with low overpotential thereby increasing the efficiency of overall water splitting. Transition metal oxides, nitrides and sulfides have been widely explored as catalysts for both HER and OER. Transition metal nitrides are found to have good electronic conductivity and superior corrosion resistance. In particular, Mo₂N exhibits high electrochemical stability and good activity towards HER in which the presence of nitrogen in the metal lattice increases proton adsorption. From the volcano plot which shows the activity of HER as a function of M-H bond strength it can be inferred that a nitride material combining Nickel which binds H weakly, along with molybdenum which binds H strongly, will have an activity as that of Pt. Further, VN has been reported to have good performance towards oxygen evolution reaction owing to its high density of states and good chemical stability. In this work, we introduce and study bimetallic nitrides (MoVN) on 3D porous Nickel foam for catalyzing water splitting reactions. Compared to electrodes with planar surfaces, the Nickel foam offers the advantage of a much larger reactive surface per unit of volume and weight, having the potential to increase the generation rate of H₂ and O₂ of the electrolysis cell. Further, the structural characterizations such as XRD, SEM is performed for the mirror samples. The performance of the electrode is tested in N₂/O₂ saturated 1M KOH solution. Hence, we report the synthesis of novel MoVN on Nickel foam using RF Magnetron co-sputtering as an efficient, bifunctional, binder free electrode for overall water splitting.
References
[1] H. Sun, X. Xu, Y. Song, W. Zhou, and Z. Shao, “Designing High-Valence Metal Sites for Electrochemical Water Splitting,” Adv. Funct. Mater., vol. 31, no. 16, pp. 1–44, 2021, doi: 10.1002/adfm.202009779.
[2] B. Cao, G. M. Veith, J. C. Neuefeind, R. R. Adzic, and P. G. Khalifah, “Mixed close-packed cobalt molybdenum nitrides as non-noble metal electrocatalysts for the hydrogen evolution reaction,” J. Am. Chem. Soc., vol. 135, no. 51, pp. 19186–19192, 2013, doi: 10.1021/ja4081056.
[3] W. F. Chen, J. T. Muckerman, and E. Fujita, “Recent developments in transition metal carbides and nitrides as hydrogen evolution electrocatalysts,” Chem. Commun., vol. 49, no. 79, pp. 8896–8909, 2013, doi: 10.1039/c3cc44076a.
[4] R. Adalati, A. Kumar, Y. Kumar, and R. Chandra, “A High-Performing Asymmetric Supercapacitor of Molybdenum Nitride and Vanadium Nitride Thin Films as Binder-Free Electrode Grown through Reactive Sputtering,” Energy Technol., vol. 8, no. 10, 2020, doi: 10.1002/ente.202000466.
[5] B. Wei et al., “Bimetallic vanadium-molybdenum nitrides using magnetron co-sputtering as alkaline hydrogen evolution catalyst,” Electrochemistry Communications, vol. 93. pp. 166–170, 2018, doi: 10.1016/j.elecom.2018.07.012.

Hydrogen has a very important place for green-energy applications. In this context, thermochemical methods based on solar energy come to the fore in hydrogen production. It is possible to produce hydrogen without the need for purification via the two-step thermochemical water splitting (TWS) method. TWS uses metal oxides as redox materials allowing to produce of pure hydrogen at lower temperatures as compared to thermolysis. Therefore, the thermodynamics and kinetics of redox reactions are one of the important factors that determine hydrogen production efficiency and influenced by the structural properties of active materials used in these reactions. For this purpose, perovskite-oxides draw attention to be able to use in TWS reactions due to providing higher structural stability with allowing compositional diversity.

We focused on the structural properties in terms of morphology and surface area, investigating the compositional dependence of La_1-xSr_xMn_0.6Al_0.4O₃ (x=0.2, 0.4 and 0.6) perovskite oxides to provide higher hydrogen production by TWS.

In this study, La_0.6Sr_0.4Mn_0.6Al_0.4O₃ (LSMA6464), La_0.5Sr_0.5Mn_0.6Al_0.4O₃ (LSMA5564), and La_0.4Sr_0.6Mn_0.6Fe_0.4O₃ (LSMA4664) were synthesized by Pechini method. As a precursor, La(NO₃)₂●6H₂O, Sr(NO₃)₂, Mn(NO₃)₂●4H₂O, and Al(NO₃)₃●9H₂Owere used. After pre-calcination at 250 °C for 2 hrs, calcination was carried out at 1300 °C for 6 hrs to obtain desired crystal structure. The crystal structure of perovskite-oxide materials was analyzed by an X-Ray Diffractometer (XRD, Bruker D8 Advance) using Cu-Kα radiation. It was determined that synthesized LSMA6464, LSMA5564, and LSMA4664 perovskite oxides have a cubic structure. The morphology of LSMA6464, LSMA5564, and LSMA4664 was observed by scanning electron microscopy (SEM) and average particle size, as ca. 0.1 μm. BET analysis results confirmed that the increased A-site dopant, Sr, ratio decreased the surface area as 35 m²/g, 28 m²/g and 25 m²/g for LSMA6464, LSMA5564, and LSMA4664, respectively. Correlated with the structural effect of Sr, two-step water splitting results show that increases in the amount of Sr dopant, causes less hydrogen production for LSMA6464, LSMA5564, and LSMA4664 perovskite oxides as 257 μmol/g, 170 μmol/g and 115 μmol/g, respectively.

ACKNOWLEDGMENTS: This work was supported by TÜBITAK (The Scientific and Technological Research Council of Turkey) (Project Number 119M420), which the authors gratefully acknowledge.

Platinum (Pt) group metals (PGMs) remain the best catalyst choice to speed up the sluggish oxygen reduction reactions (ORR) at the cathode of proton-exchange membrane fuel cells (PEMFCs). However, the requirement of a large amount of PGM catalysts has prevented the widespread adoption of PEMFCs. To address this challenge, it is central to achieve low PGM loading PEMFCs with high performance. However, using low PGM loading inevitably compromises the performance at the high current density region. To overcome this dilemma, we demonstrate an ultralow Pt loading and high-performance membrane electrode assembly (MEA) using ultrathin platinum-cobalt nanowires (PtCoNWs) as the cathode catalysts. The PtCoNWs showed an exceptional high electrochemically active surface area (ECSA) and exhibited a record-high mass activity (MA), far surpassing the Department of Energy (DOE) 2020 beginning of life (BOL) target (0.44 A/mg_PGM). In addition, PtCoNWs displayed an ultrahigh total Pt utilization, surpassing all state-of-the-art Pt-alloy catalyst performance in MEA. In-situ XAS studies suggest that the high atomic ordering core of the PtCoNWs contributes to the high ORR activity and stability of PtCoNWs in PEMFCs.

Since the start of fuel cell research, electrocatalysts based on Pt have shown remarkable ORR performances. However, the high cost and low abundance of Pt limits their practical applications in the vehicle industry. The combination of Pt with first-row transition metals lowers the cost and enhances the electrocatalyst activity in the cathode compartment where the Oxygen reduction reaction (ORR) occurs. Additionally, when using an alkaline medium, the ORR is enhanced, metal oxides are more stable and non-noble metal could be used. In this study, we synthesized first-row transition metal catalysts (M= Ni,Co and Cu)—with the Rotating disk slurry electrodeposition (RoDSE) technique—supported on Vulcan XC-72R. We were able to electrodeposit Ni and Co on Vulcan XC-72R using an electrochemical potential of -0.75V vs. RHE and -0.80V vs. RHE to electrodeposit Cu on Vulcan XC-72R applying the RoDSE methodology in 0.1M KClO₄. These M/Vulcan XC-72R catalysts were modified with a Pt precursor via spontaneous galvanic displacement (SGD) and we obtained Pt-M/Vulcan XC-72R to catalyze the ORR in an alkaline medium. First, we tested the M/Vulcan XC-72R for the oxygen evolution reaction (OER) and Ni/Vulcan XC-72R provided the lowest overpotential with 450 mV vs. RHE at 10 mA/cm²_disk in 0.1M KOH. We also performed oxygen polarization to characterize the electrochemical activity towards oxygen reduction in an alkaline medium. Our results indicated that under controlled temperature (25.0°C), a mass loading of 100 mg/cm² on a glassy carbon rotating electrode and at 1,600 rpm the electrocatalysts revealed good performance. The PtNi-Vulcan XC-72R, PtCo-Vulcan XC-72R and PtCu-Vulcan XC-72R catalysts obtained half-wave potentials (E_1/2) of 0.88 V, 0.88 V and 0.82 vs. RHE, respectively, under an oxygen atmosphere in 0.1M KOH, compared with commercial Pt/Vulcan XC-72R E_1/2 = 0.83 V. The particle sizes of the catalysts were obtained using the X-ray diffraction (XRD) diffractograms of the nanoparticles and the Scherrer equation analysis, suggesting sizes between 2-10 nm. Upcoming studies will focus on evaluating the durability of the catalysts under electrochemical cycling and the electronic properties of the samples using X-ray Absorption Spectroscopy (XAS) techniques to determine their atomic states and coordination environments. This work was supported by the National Science Foundation NSFPREM: Center for Interfacial Electrochemistry of Energy Materials (CiE2M) grant number DMR-1827622.

Infiltrated nickel nanocatalysts have been shown to improve the performance of NiYSZ fuel cell anodes, especially at low temperatures, by decreasing feature size and increasing triple phase boundary (TPB) density. However, estimating infiltrated TPB density by microstructural characterization and relating deposited nanocatalyst morphology to performance improvements is difficult because of the complexity of the deposited nanoparticle and scaffold microstructure. In this study, performance improvements in Ni-infiltrated NiYSZ anode symmetric cells are quantified by electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis, and the microstructure characterized by focused ion beam/scanning electron microscopy (FIB/SEM) analysis and cross-sectional SEM. A relationship between infiltrated microstructure and performance improvements is discussed.

Efficient and durable electrocatalysts are instrumental to enabling next-generation fuel cell technologies. At present, costly precious metals are used as state-of-the-art catalysts. In this report, cost-effective tantalum-based alternatives are synthesized via a green and scalable laser pyrolysis method as bifunctional catalysts for direct peroxide-peroxide fuel cells. By varying the laser power, reactor parameters, and ammonia flow rate, five Ta/N/O nanomaterials are prepared containing Ta₂O₅, Ta₄N₅, Ta₃N₅, and TaN in tunable ratios. Electrochemical studies in neutral and alkaline conditions demonstrate that Ta₄N₅ is the active component for H₂O₂ oxidation and reduction. Kinetic isotope effect (KIE) studies show that protons are involved at or before the rate-determining step (RDS). Long-term stability studies indicate that Ta₃N₅ grants Ta/N/O nanomaterials their enhanced longevity during electrocatalytic operations. Taken together, Ta/N/O can act as active and robust electrocatalysts for H₂O₂ reduction and oxidation. Laser pyrolysis is envisioned to produce refractory metal nanomaterials with boosted corrosion resistance for energy catalysis.

Ammonia borane NH3BH3 (AB), a remarkable hydrogen storage material carrying 19.6 wt% of hydrogen, owns attractive properties [1]. It (in thermolytic conditions) has been understandably much investigated within the past two decades leading to destabilization strategies in order to decrease the onset dehydrogenation temperature (<100°C). Nanosizing AB is an attractive approach to make it suitable for solid-state hydrogen storage. The destabilization of the borane (i.e. modified thermal stability and reactivity) has been anticipated via changed atomic charges of the NH3BH3 molecule, perturbed intermolecular N−H H−B network and lowered activation energy. The nanoconfinement of AB has then shown new pathway to dehydrogenation properties. With this knowledge, it was demonstrated that different porous structures so called scaffolds can be used to improve dehydrogenation properties of AB, such as silica (ex. SBA-15) [2], MOF (ex. Mg-MOF-74) [2] or polymers (ex. PMMA) [2]. In regards to efficiency, sustainability and disruption, AB has been nanosized without scaffold. Inspired from the nanosized AB using cetyltrimethylammonium bromide (CTAB) as surfactant [3], and obtained via an anti-precipitation method in solution, we used amine-borane adducts to produce nanosized AB particles (60 to 200nm). Adducts are the keys for shaping. Different adducts R-NH2-BH3 with R a carbonaceous group have been successfully synthesized and characterized. We have produced adducts such as tetradecylamineborane (C16H33NH2BH3AB) and biphenylamineborane (C12H11NH2BH3). The self-assembling properties of adducts have allowed nanostructuring of AB in solution; spherical, lamellar and polyhedral shapes have been observed. We accordingly synthesized nanoparticles of AB without a scaffold, which opens new perspectives for elaborating from these nanostructured new C-doped B-N-based materials. Recent computational works support that such materials would be the most attractive solutions for reversible H2 storage at ambient conditions [4]. However, there is to our knowledge a lack of experimental evidence yet. The present project aims at confirming the potential of high reversible H2 storage in the field of B-based ceramics.
[1] Demirci, U.B., Ammonia borane, a material with exceptional properties for chemical hydrogen storage. International Journal of Hydrogen Energy, 2017. 42(15): p. 9978-10013.
[2] Turani-i-belloto, K., et al, Nanosized ammonia borane for solid-state hydrogen storage: Outcomes, limitations, challenges and opportunities, international journal of hydrogen energy, 2021, 46: p.7351-7370
[3] Valero-Pedraza, M.-J., et al, ammoniaborane nanospheres for hydrogen storage, ACS Appl. Nano Mater., 2019, 2, 1129-1138
[4] Lale, A., et al., Boron nitride for hydrogen storage. ChemPlusChem, 2018, 83: p. 893-903.

Solid hydrogen storage is a technological obstacle for the hydrogen economy broadening. Various materials and methods have been studied under the scope of the industrial purpose such as complex metal hydrides made by the Sievert’s method. Lanthanum-Nickel hydride, LaNi5 had been chosen for its high volumetric (117kgH2/m3) and gravimetric (1,5wt.%) hydrogen capacity. Notwithstanding, this technique requires expensive high purity materials (99,99%) and the process is performed at high temperature (300K) and high pressure (2bar). To overcome these drawbacks, we decided to go through the hydroxide-plasma path. It consists in synthesising by soft chemistry the hydroxide of the selected metal and reducing it under hydrogen plasma. For this study we chose Nickel as the matrix carrier. Three soft chemistry protocols were chosen : (i) coprecipitation and water washing, (ii) polyol and water washing (iii) polyol and ethanol washing. By coprecipitation, the obtained nanopowder is of a controlled dense structure and composition, while by polyol the composition is sensitive to ambient CO2 pollution and washing. Polyol-made nanoparticles have a non-denses lamellar structure sensitive to washing : water can enter between the layers and leads to delamination. It leads to low mechanical strength. For the hydrogen plasma treatment, the main goal was to study the kinetic of the reduction and the composition evolution of the plasma by mass spectrometry. For both nano-nickel hydroxide made by coprecipitation and made by polyol and washed with water, the mechanism followed was : Ni(OH)2>NiO>Ni (solid solution). Under plasma, the phase change lasted 1-2h. But for the nano-nickel hydroxide made by polyol and washed by ethanol, the mechanism was rather different : Ni(OH)2>Ni2H>Ni (solid solution). This shows that nano-nickel hydroxide can be reduced under plasma up to the unstable nickel-hydrogen solid solution. Further work around plasma parameters must be done to overcome this last step of hydration.
Acknowledgements :
This research was partly supported by Segula Technologies and the French National Research Agency. The authors would like to thank Mr Haddad (Segula technologies) for the fruitful discussions.

Violent reaction of Uranium metal hydrogen can be detrimental for mechanical integrity of devices containing this actinide element. The product of such reaction, UH₃, appears usually as a very fine powder, which burns spontaneously if exposed to air. This was a prohibitive feature in considerations of U for hydrogen storage in stationary applications. (Automotive applications were never considered due to the high mass of U). On the other hand, there are some pros. U absorbs H at very low pressures, working essentially as a getter, and all H is released above T = 450 ^oC. Eventually U started to be used as tritium storage medium in prospective nuclear fusion devices.
Our aim is to explore modifications of U hydrides by small substitutions of U by transition metals, which turned out possible using U-T alloys (T = Mo, Zr, V, Nb...) as precursors. It turned out that surface reactivity can be suppressed, H₂ pressure needed for hydrogenation increases and such hydrides adopt the form of brittle but monolithic pieces stable in air, while maintaining the H sorption capacity. It allowed a comfortable study of basic characterictics (magnetic, electrical resistivity, heat capacity), the results of which can be confronted with ab-initio calculations. The effort revealed a bit surprizing fact that UH₃ is not simply the U metal expanded by 60% due to H absorption, but the polar character of U-H bonds is more important for the bulk properties than the overlap of 5f wave functions between nearest neighbours. UH₃ is ferromagnetic below T ≈ 165 K, and one of striking features observed is the enhancement of the Curie temperature for any kind of dopant up to the concentration of 15 at.% [1].
For spectroscopic studies by XPS, UPS, BIS, XMCD, and XAS, we developed the technique of synthesis of U-H films by reactive sputter deposition. The films exhibit a similar properties as bulk U hydrides, but their composition and microstructure can be varied in a broader range, not being constrained by equilibrium thermodynamics. In a certain regime a di-hydride with CaF₂ structure type, not known before and analogous to rare earth dihydrides, could be obtained [2]. The sputter deposition technology opens a possibility for manufacturing sandwich devices, combining ferromagnetism of U hydrides with paramagnetism (and superconductivity) of U-T alloys on a nanoscopic scale, which can give, due to stron spin-orbit coupling, functionalities relevant in spintronics. The spectroscopies applied [3] confirm the effect of polar bonds, based on a charge transfer towards H and strong 6d-1s hybridization, which leaves behind the 5f states, responsible for magnetism. The reduction of the 5f-6d hybridization allows for pronounced magnetic properies of all hydrides studied (UH₂, α- and β-UH₃).
This work was supported by the Czech Science Foundation under the grant No. 21-09766S.
[1] O. Koloskova et al., J. Alloys Comp. 856 (2021) 157406.
[2] L. Havela et al., Inorg. Chem. 57 (2018) 14727.
[3] L. Havela et al., J. Electron Spectroscopy and Related Phenomena 239 (2020) 146904.

Regulating hydrogen transport in structural metals is one of the key challenges for hydrogen related technologies. Here, we will discuss two new technologies that aim to regulate hydrogen ingress and hydrogen desorption. The former is a novel hydrogen barrier coating with multilayer composite structures of metal and oxide. This structure provides (i) enhanced resistance to delamination, (ii) formation of extended space-charge zones at oxide/metal interface that improves H-permeation barrier performance, and (iii) strong micro-cracking resistance and the self-healing potential to retain hydrogen barrier performance even after mechanical damaging. The latter method aims for recovering steels from hydrogen embrittlement at room temperature by accelerated electrochemical desorption of hydrogen. We confirmed that the method can effectively help several commercial steels to recover from the hydrogen damage, by comparing the mechanical behavior of H-precharged specimens after uncontrolled desorption and controlled electrochemical desorption experiments. Several case studies in different steels will be introduced in this presentation to discuss the influence of trapping sites on the effectiveness of this recovery approach.

Two major obstacles to the broader use of hydrogen energy technologies include (1) cost and reliability of hydrogen storage and delivery infrastructure; and (2) cost and complexity of evaluating materials in hydrogen environments. In particular, the ferrous alloys known to have high resistance to hydrogen embrittlement, e.g., high Ni-containing austenitic stainless steels, are relatively expensive. The current study aims at designing Mn-alloyed ferrite-austenite steels as economical materials for hydrogen service, as an alternative to high Ni stainless steels. The alloy design was guided by a literature review on the effects of alloying on austenite stacking fault energy (SFE), which is established to be correlated with the mechanical stability of austenite and hydrogen embrittlement resistance. The composition of a high Mn duplex steel was designed considering SFE and equilibrium volume fraction of austenite in duplex microstructures, obtained through thermodynamic calculations. Moreover, various thermomechanical processing routes are being explored to obtain desirable combinations of austenite stability, volume fraction, and deformation mechanisms and to enhance fracture toughness of the high Mn duplex steel in hydrogen at given or higher strength levels. In addition to reducing the cost of alloys, the present study also focuses on develop and validate a cheaper and easily accessible method to test alloys in hydrogen environments. Electrochemical hydrogen charging methods are being developed in parallel utilizing a 255 duplex stainless steel. The electrochemical charging method is intended to be a surrogate for testing in gaseous hydrogen at a pressure of approximately 103 MPa. The hydrogen embrittlement characteristics of the 255 duplex stainless steel were evaluated as a function of the hydrogen environment (electrochemical variables compared to testing in high-pressure gaseous hydrogen) and the geometry of circumferentially notched tensile specimens.

Hydrogen technologies are a very promising option among the green energy sources. However, safety precaution have to be taking care while using, handling and storing it and sensors are a key issue in the present technological development. All existing sensors are known to have specific issues and gas discrimination and ambient factors influences are among the most frequent issues.
Surface Acoustic Wave (SAW) sensing is one of the development directions using acoustic waves propagating over a piezoelectric surface and are basing their functionality on the mechanical properties change of an ‘active layer’ material, which the wave is passing through in the presence of the detected gas. Nanostructure or nanostructured materials present a series of enhanced properties compared with the bulk materials, due to their high surface-to-volume ration, but their specific parameters need to be controled for optimizing sensor performances. For SAW sensor fabrication we use ZnO material as a versatile wide bang-gap semiconductor, which is having a good absorption of hydrogen isotopes and various other gases. We use Pulsed Laser Deposition (PLD) - Vapour-Liquid-Solid (VLS) techniques to grow ZnO single crystal [0001] nanowires on sensor surface as an active leyer.
By controlling experimental ZnO growing parameters we are able to control VLS grow elementary processes and respectively nanostructure morphology, optimizing the sensor sensibility. By monitoring sensor response to various propagation frequencies we are able to distinguish several (known) gases presence. Some correlations between experimental parameters and sensor response are presented as well as sensor performances on hydrogen isotopes and few other gases detection.

The urgent need for clean energy coupled with the exceptional promise of hydrogen (H) as a clean fuel is driving development of new metals resistant to hydrogen embrittlement. Experiments on new fcc high entropy alloys (HEAs) present a paradox, absorbing more H than Ni or austenitic 304 stainless steel (SS304) but more-resistant to embrittlement. Here, a new theory of embrittlement in fcc metals is presented based on the role of H in driving an intrinsic ductile-to-brittle transition at a crack tip. The theory quantitatively predicts a hydrogen concentration at which a transition to embrittlement occurs, and good agreement with experiments is found for the alloys SS304, SS316L, CoCrNi, CoNiV, CoCrFeNi and CoCrFeMnNi. The theory rationalizes why CoNiV is the most-resistant alloy and why SS316L is more resistant than the HEAs CoCrFeNi and CoCrFeMnNi. The theory opens a path for computationally-guided discovery of new embrittlement-resistant alloys.

The development of hydrogen-resistant steels depends directly on the understanding of the role of alloy composition and microstructure on mechanical behavior. In this work, we describe the progress toward developing elevated strength, cost-effective alloys through the employment of nano-level strengthening features including short range order (SRO). The intent is to develop microstructures that are sufficiently strong, compared to the current generation stainless steels, that they could be used in hydrogen handling applications with reduced section sizes, and thus reduced cost. The alloy development effort derives from features of the 21-6-9 (Nitronic 40) alloy which has been the focus of multiple hydrogen effects studies over the years. The appeal comes from the relatively high strength, depending on the processing, and the use of Mn in place of some level of Ni. A series of developmental alloys in which the Ni/Mn ration is varied across a wide range are also analyzed. From a microstructural standpoint, the Ni/Mn balance can affect several critical materials parameters including alloy phase stability during deformation, stacking fault energy and the tendency for slip-induced twinning. This work covers the connection between the compositional and microstructural differences in these alloys and the associated mechanical behavior. In particular, the study includes the evolution of microstructure during deformation in both hydrogen charged and uncharged specimens to understand the dislocation-strengthening particle/SRO. Interactions, the tendency to transform from cross-slip to planar slip based, and the tendency for twinning, all of which influence performance in hydrogen atmospheres.

Renewable energy from wind turbine or photovoltaic cells will be abundant in future. We have a need for a clean, efficient system that can act as both energy conversion and storage device. A reversible solid oxide cell (RSOC) is such kind of a device that can convert electricity to fuel during the electrolysis mode by conversion of steam to hydrogen and using that hydrogen later as fuel to generate electricity during the fuel cell mode. Hydrogen is always a fuel of interest due to environmental considerations, increasing oil price and limited supply of fossil fuel. However, when operating in the SOEC mode, pressure builds up in the electrolyte-oxygen electrode interface due to the accumulation of molecular oxygen in the pores near the interface. As a result, the oxygen electrode particularly, suffers delamination in the SOEC mode causing serious performance degradation and failure of the system. To mitigate delamination of the oxygen electrode, the Ruddlesden-Popper structure based mixed ionic electronic conductors (MIEC) in the rare earth nickelate family are an attractive option. However, rare earth nickelates have the tendency to react with the YSZ electrolyte and form undesirable phases. To prevent this, a barrier layer is used between the oxygen electrode and electrolyte to prevent any kind of chemical interaction between the oxygen electrode and the electrolyte. In this work, various rare earth doped ceria have been evaluated as barrier layers and their effect on electrochemical polarization has been systematically studied. Symmetrical cells with various doped ceria barrier layers and nickelate-doped ceria composite electrode have been screen printed and EIS data has been obtained at varying temperatures and oxygen partial pressures. DRT analysis of the impedance spectra has provided insights into the reaction mechanism, and identify the rate limiting steps.

Chromium poisoning remains a major obstacle to wide-spread solid oxide fuel cell (SOFC) employment. A mitigation strategy, electrochemical cleaning, has been proposed in which the Cr deposition reaction is reversed by applying a mild electrolytic current. The method has been shown to reverse chromium deposition and performance loss on cells with (La_0.8Sr_0.2)_0.9MnO_3-δ (LSM) air electrodes. Here we demonstrate the applicability of electrochemical cleaning using more Cr-tolerant electrode materials: La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and Nd₂NiO_4+δ (NNO). Three cells with LSM, LSCF, and NNO cathodes were exposed to accelerated poisoning conditions followed by electrochemical cleaning, i.e., mild electrolytic bias and increased fuel humidity. Performance recovery due to electrochemical cleaning is recorded using current-voltage and EIS measurements. Three additional cells were exposed to the same poisoning condition, but not cleaning for post-test Cr content comparison. Cell cross-sections were analyzed using energy dispersive x-ray spectroscopy (EDS). Cells that were poisoned and cleaned compared to their respective baseline poisoned cell demonstrate a decrease in overall Cr content. Whereas, the location of Cr deposition during poisoning is different between LSM and LSCF or LNO air electrodes. Due to the mixed ionic/electronic conductivity of LSCF and NNO electrodes, the effect of Cr deposition on cell performance is less pronounced compared to LSM. Therefore, when using more Cr-tolerant air electrode materials, electrochemical cleaning is effective in Cr removal and performance recovery and is required at a lower frequency compared to cells with primarily electronically conductive air electrodes.

The catalytic and transport properties of mixed ionic-electronic conducting (MIEC) electrodes for solid oxide fuel cells (SOCs) are well documented and utilised, yet poorly understood. In the current study, a novel kinetic framework for the electrochemical behaviour of hydrogen at the MIEC-gas interface will be discussed for three parallel treatments: as an ideal gas, as an adsorbate on the electrode surface and as a charge carrier dissolved in the oxide lattice. This model gives a physically meaningful reason for the enhancement in electrochemical activity of a MIEC electrode as the steam pressure is increased in both fuel cell and electrolysis modes, and is ubiquitous for any electrochemical system where the electric double layer is described as a Heaviside step function.

The process of charge transfer at the MIEC electrode/gas interface comprises of ambipolar exchange of ions and electronic species. The result of such a process causes charge separation and an associated dipole moment at the electrode surface. By applying an overpotential η to the MIEC electrode, an electrostatic surface potential shift away from equilibrium may be established where an effective double layer is formed between the electrode surface and the adsorbed species. Although no net charge transfer occurs, this surface potential shift modifies the surface chemistry and is the driving force for the ambipolar exchange of ions and electronic species.

Density Functional Theory (DFT) calculations were used to study the electrostatic potential at the ceria [111] surface, where we were able to calculate the dipole moment and adsorption energy as a function of hydroxyl coverage. The theory of the electrostatic potential at the MIEC-gas interface was then applied to predict the surface electrochemical properties as a function of local overpotential. [1]

Finally, we extend this model to show the effects of non-equilibrium thermodynamics on the bulk defect concentration and adsorbate induced electrostatic surface potential in MIEC electrodes. This methodology is used to model the current-overpotential characteristics of water electrolysis mechanism at 2 phase and 3 phase boundaries present in cermet electrodes.The mechanistic understanding gained from this model is widely applicable to a range of MIEC systems and provides a basis upon which the operating conditions can be tailored.

[1] Williams, N.J., Seymour, I.D., Leah, R.T., Mukerjee, S., Selby, M. and Skinner, S.J., 2021. Theory of the electrostatic surface potential and intrinsic dipole moments at the mixed ionic electronic conductor (MIEC)–gas interface. doi.org/10.1039/D1CP01639C

Mesoporous N- and Fe,N-carbons exhibit high and stable activities for oxygen and sulfur reduction that are comparable to or surpass those of standard Pt-activated-carbon electrocatalysts. Favorable properties include high nitrogen contents (>15 atom%), high fractions of N moieties at surface sites, 3-nm mesopores to promote diffusion, and electron conductivity to surface N environments where the reduction reactions occur. The types, quantities, and distributions of N-heteroatom environments, especially those at surface sites, are shown to strongly influence macroscopic reduction activities. N- and Fe,N-mesoporous carbons synthesized using difference mesopore templates (e.g., salt versus silica) are explored to understand the atomic level differences that correlate with increased reduction activity, and how they can be optimized. Compared to 20 wt% Pt supported on activated carbon, the salt-templated mesoporous Fe,N exhibits the highest reduction activity, exceeding even that of the commercial Pt-carbon catalyst.
While inclusion of N-heteroatoms improves carbon-based electrocatalyst reduction activity,¹ the atomic-level origins of such properties have remained elusive. Nevertheless, two-dimensional ¹³C-¹⁵N NMR spectra resolve signals from four distinct types of N-heteroatom environments: pyrrolic, graphitic, edge/isolated pyridinic, and pyrazinic/pyridinic moieties, the quantities of which vary by porogen and account for different O₂ reduction activities.^2,3 Chemical shift assignments are corroborated by DFT, with points corresponding to different structural motifs overlaying the spectrum. Importantly, ¹⁵N-¹H NMR spectra enable surface N species to be selectively distinguished from interior moieties via interactions with adsorbed water. These analyses establish that certain types of N-carbon moieties are more important to electrocatalytic performance than others. The incorporation of non-precious transition metals (e.g., Fe) significantly increases electrocatalytic activity, which have been challenging to explain.^4,5 Nevertheless, ⁵⁷Fe Mössbauer spectroscopy and solid-state ¹⁵N NMR and spin-lattice relaxation-time analyses resolve signals from ¹⁵N species that are proximate to paramagnetic Fe-heteroatoms, from which ¹⁵N-Fe distances are estimated. XRD, XPS, XRF, Raman spectroscopy, STEM, and EXAFS are used to characterize the materials and corroborate the hypotheses. Understanding the roles of Fe and N-carbon moieties in electrocatalytic reduction yields new design criteria for syntheses of high performance non-precious-metal electrocatalysts with diverse fuel cell and battery applications.
(1) Gong, et al., Science 2009, 323, 760.
(2) Becwar, et al., submitted.
(3) Wei, et al., J. Phys. Chem. C 2021, in press.
(4) Kim, et al., ACS Appl. Mater. Interfaces 2018, 10, 25337.
(5) Al-Zoubi, et al., J. Am. Chem. Soc. 2020, 142, 5477.

The U.S. Department of Energy’s (DOE) Hydrogen and Fuel Cell Technologies Office (HFTO) funds research, development, and demonstration (RD&D) activities to enable decarbonization across sectors. HFTO’s R&DD portfolio includes hydrogen production, infrastructure, storage, fuel cells, and a wide range of end-uses for hydrogen, including industrial processes (e.g. steelmaking, data centers), energy storage, hydrogen blending with natural gas, and transportation. The Office’s materials R&D activities are conducted largely through national laboratory consortia within the DOE’s Energy Materials Network. These consortia include: H-Mat, focused on materials compatibly with hydrogen; HyMARC, focused on materials-based storage of hydrogen; HydroGEN, focused on advanced water splitting materials; and ElectroCat, focused on platinum group metal (PGM)-free catalysts for fuel cells. The current presentation will provide an overview of the HFTO RD&D portfolio and EMN consortia, including recent accomplishments and future priorities.

Quasi-static tensile properties are the base for any component design. Often, wall thickness are set based on such data. A cost effective test method to acquire the required data for high pressure hydrogen applications is tensile testing using tubular specimen, where the hydrogen pressure is applied to the inner cavity of the specimen. Several structural steels were tested with this method and this study compares the results of tubular specimens with those of conventional specimens tested in a high pressure hydrogen autoclave. Commonalities and differences will be presented and discussed by means of FEM analysis and hydrogen failure modes.

Exposure of rubber materials to diffusive gases at high pressure and subsequent decompression may lead to cavitation and cracking depending on the exposure conditions. This phenomenon has been studied for different gas polymer systems in the past, but the interest in hydrogen as an alternative energy carrier and the application of rubber materials in Fuel Cell Vehicles and equipment for storage and transportation of hydrogen makes it important to study the compatibility of these materials with hydrogen. The relatively high pressures in use, combined with the safety measures required for the usage of hydrogen itself pose serious challenges in terms of material and design. In particular, the safety risk associated with hydrogen leakage makes specific demands on the rubber materials used as seals in terms of durability.
Decompression failure can be addressed at two different scales. At the macroscopic scale of the component, the main challenge is to capture the statistics of cavity fields (known to depend on the mechanical loading, decompression conditions and material properties), the gradients and the full diffuso-mechanical couplings, in order to model the residual mechanical and permeation properties. The question of a Representative Volume Element over which to develop modelling is a key one.
At the cavity scale, the existence of full diffuso-mechanical couplings is a key issue too: the expansion and deflation of the cavity depends on the balance between the external hydrostatic pressure and the internal pressure (which depends itself on the volume of the cavity and the gas content inside it, possibly varied by the gas flux at the cavity wall) but also on the intrinsic properties of the rubber at large strains at the cavity wall, and on the global gas content field. The type of mechanisms responsible for cavitation is another issue, for soft matters in general, regardless of the specific decompression failure context.
The experimental works reported here aim at better understanding of the mechanisms of cavity growth, some possible interaction effects at variable range, with a free surface or between close cavities. The work was based on a very recent 3D time-resolved quantification of the kinetics and morphology of the damage, from an in-situ X-ray tomography experiment initially developed on a laboratory source (spatial resolution 16µm, temporal 100s) and extended under a synchrotron environment on the Anatomix beamline at SOLEIL synchrotron (France).
The gain in spatial (3µm) and temporal (4s) resolution made it possible to access the very first stages of growth, to properly quantify anisotropy and to detect residual damage. These different elements showed the existence of a first rapid spherical growth regime which then evolved into an anisotropic regime which reflected an underlying mechanism of cracking. The correlation with the intrinsic fracture properties of the rubber are considered more in-depth.
Then, the evolution of this damage during successive pressure cycles was investigated, raising the question of non-trivial cumulated effects.
Results mainly concern a series of unfilled EPDM, with low variable degrees of cross-link density. However, this microstructural parameter alone did not appear discriminating.

As the energy sector decarbonizes, hydrogen is being considered as a medium to store and convey renewable energy, for example in the natural gas networks. Therefore, the interactions of hydrogen with structural materials in existing energy infrastructure is needed. Medium density polyethylene (MDPE) pipelines are commonly used in distribution gas networks to deliver natural gas from the transmission system to the end user. The open literature contains limited information on the effects of hydrogen (either pure or blended into natural gas) on these commonly used materials. In this research, the morphological changes, transport and mechanical properties associated with exposure to hydrogen were evaluated. Test specimens of PE2708 medium density polyethylene pipe were saturated with hydrogen at pressures ranging from 0.7 to 3.4 MPa for up to 72 hours, prior to evaluating morphological changes in the polymer, equilibrium hydrogen content, hydrogen diffusivity, nano-indentation properties, and burst behavior. In addition, fatigue life tests were conducted in-situ in the presence of low-pressure gaseous hydrogen. The material shows a decrease in the degree of crystallinity by up to 6% using X-ray diffraction and differential scanning calorimetry after exposure. The reduction in the diffusion coefficient was observed using thermal desorption spectroscopy, and nano indentation testing revealed up to a 30% decrease in hardness and modulus immediately following hydrogen exposure. Burst pressures test results were not significantly reduced. The fatigue life, however, was greater in hydrogen than in air. The implication of these property changes on the structural behavior of MDPE pipelines in hydrogen service are discussed.

The U.S. Department of Energy Hydrogen and Fuel Cell Technologies Office launched the H2@Scale program to improve the durability and reliability of materials for hydrogen infrastructure to facilitate widespread utilization of hydrogen. Pacific Northwest National Laboratory leads a multi-lab effort to understand the mechanisms of hydrogen-polymer interactions with the goal of developing polymers with improved resistance to hydrogen degradation. Model nitrile butadiene and ethylene propylene diene rubber compounds were formulated collaboratively with Kyushu University to investigate how each constituent responds to high-pressure hydrogen exposure in such complex systems. Advanced imaging techniques offer unique advantages to visually study possible changes in polymer morphology associated with hydrogen effects at different length scales. X-ray computed tomography research revealed formation of more voids in plasticized systems after static exposure to 90 MPa hydrogen than unplasticized. Surface damage and plasticizer migration at the submicron level were captured by helium ion microscope. Transmission electron microscopy combined with energy dispersive spectroscopy highlighted hydrogen-induced voids formation around accelerator particles along with sulfur coalescing.

Elastomeric rubber materials serve a vital role as gas barriers in the hydrogen storage and transport infrastructure. With applications including O-rings and hose-liners, these components are exposed to pressurized hydrogen at a range of temperatures, cycling rates, and pressure extremes. High-pressure hydrogen exposure often rapidly leads to cavitation, a hydrogen-induced failure mode following several (de)pressurization cycles. This type of failure is readily visible, occurring as a material rupture during decompression and is due to the oversaturated gas traversing the polymer matrix towards the outside of the rubber. Computational modeling in the Hydrogen Materials Compatibility Program (H-Mat), co-led by Sandia National Laboratories and Pacific Northwest National Laboratory, employs multi-scale materials simulation efforts to build a predictive understanding of cavitation damage. We aim to motivate material formulations that are less sensitive to hydrogen-induced failure. Here, we will discuss the use of atomistic molecular dynamics simulations to predictively assess compositional variations and resulting performance metrics of the commonly used elastomer, ethylene propylene diene monomer (EPDM). We perform systematic studies as a function of increasing crosslink density, which show indications of improved bulk material response to the depressurization of gas at higher crosslink density. However, this performance improvement comes with a tradeoff; microscopic analysis indicates an increase in free volume at higher crosslink density, a necessity for gas localization and the initiation of cavitation. We will also discuss the mobility of hydrogen through the polymer matrix leading up to gas localization. Our diffusion results reveal that the hydrogen molecules can be divided into two classes, one with high mobility and one with low mobility in which the hydrogen gas diffusion is impeded by atomic structural features of the polymer.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. SAND2021-7350 A

Development of robust, cost effective, reliable and safe hydrogen applications requires a new generation of low cost, high strength austenitic steels. In this talk we will review recent collaborative efforts to achieve this goal by intimately coupling experiments with modeling and simulation to establish how the intrinsic chemical composition, in concert with hydrogen, governs microstructural evolution and ultimately leads to fracture initiation and failure. Classical metrics of embrittlement (e.g. stacking fault energy and others) have proven to be insufficient on their own to explain failure and thus guide the design of optimum alloy composition. Several commercial and novel austenitic steels have been characterized via highresolution electron microscopy to reveal ubiquitous short range ordering (SRO) behavior, where SRO is defined as any local deviation from a random solid solution. Direct feedback between these experimental observations and our constitutive modeling framework suggests that such SRO domains significantly modify the mobility of nearby dislocations, whileatomistic simulations reveal that not only should such domains be expected but that the SRO domains may also impact the distribution of hydrogen within the lattice. In addition to these observations, we will present preliminary observations for our novel low-nickel austenitic alloys, which have been developed to suppress other known features associated with hydrogen embrittlement (phase transformation, twinning, etc) so as toexplicitly explore the relationship between the observed SRO domains, slip localization and ultimately the failure modes of austenitic steels in the presence (and absence) of hydrogen.

Safe and efficient hydrogen storage and distribution are key challenges to realizing hydrogen as an alternative energy carrier. To this end, cryogenic liquid and cryo-compressed gaseous hydrogen are considered high energy density alternatives to ambient temperature gaseous hydrogen. However, these alternatives have significant material demands: extreme temperature (20 K) and pressure (700 bar) as well as the insidious effects of hydrogen. Austenitic stainless steels are widely used for cryogenic pressure vessels owing to relatively high ductility even at 4 K. However, the influence of hydrogen on mechanical properties at cryogenic temperatures has rarely been studied. In this research, the tensile properties of 304L and Nitronic 50 (XM-19) stainless steel alloys with internal hydrogen were evaluated at 20 K, 77 K, and 113 K. Test specimens were saturated with internal hydrogen through thermal precharging at 573 K in 138 MPa H₂. In both 304L and Nitronic 50, reduced temperature increased strength properties and reduced elongation. The presence of hydrogen increased strength in both alloys but reduced ductility, as measured by reduction of area. Magnetic evaluation of the uniformly strained region of the 304L test specimens suggest that hydrogen mitigates the strain-induced transformation to α’-martensite in 304L, whereas the Nitronic 50 alloy remains stable with regard to transformation to α’-martensite. The effects of temperature and hydrogen were also explored in gas tungsten arc-welded 304L.

While Fe-Ni-Cr austenitic stainless steels exhibit relatively good resistance to hydrogen embrittlement, they still suffer from significant degradation of ductility, fatigue and fracture properties in gaseous hydrogen environments. Experimental studies in the literature suggest that hydrogen reduces stacking fault energy in austenitic stainless steels. This phenomenon causes a large separation of partial dislocations and lower propensity for cross-slip. Whereas lower stacking fault energy does not correlate well with loss of ductility in the absence of hydrogen, when hydrogen is present lower stacking fault energy trends toward greater loss of ductility. Calculations of stacking fault energy are challenging for austenitic stainless steels. One main issue is that in alloys, stacking fault energy is not a single value but rather varies depending on local composition. Herein, we first report an Fe-Ni-Cr-H quaternary interatomic potential and then use this potential to perform time-averaged molecular dynamics simulations to calculate stacking fault energies for tens of thousands of realizations of local compositions for selected stainless steels alloys with and without internal hydrogen. From statistical analyses, our results suggest that hydrogen reduces stacking fault energy, which likely impacts deformation mechanisms of Fe-Ni-Cr austenitic stainless steels when exposed to hydrogen environments. We then perform validation MD simulation tests to show that hydrogen indeed statistically increases the stacking fault widths due to statistically reduced stacking fault energies.
Acknowledgement- SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Cottrell atmosphere formation occurs when solute atoms segregate to dislocations and is one of the most basic processes by which solutes alter mechanical properties of materials.
Hydrogen Cottrell atmospheres can contribute to hydrogen embrittlement. Using molecular dynamics simulations we study the formation of hydrogen Cottrell atmospheres around edge dislocations in aluminum. By pinning dislocations to one position and using coarse-graining techniques, we can resolve the time-dependent changes in the hydrogen concentration field. Comparing to previous Cottrell atmosphere formation and thermodynamics theories, we observe a surprising reduction in peak atmosphere concentration at the dislocation core. This was found to arise from a repulsive, concentration-dependent H-H interaction at the dislocation core that is absent in the bulk far away from dislocation. Since the essential physics of Cottrell atmosphere formation is accurately represented in molecular dynamics, our results provide important inputs to improve Cottrell atmosphere theories. Based on our findings, we propose next steps to study the interaction between dislocations and their associated hydrogen Cottrell atmosphere.

Hydrogen embrittlement, a process whereby metals suffer the loss of ductility in hydrogen environments, limits the choice of austenitic stainless steels that can be used for reliable hydrogen applications. Although several possible mechanisms for hydrogen embrittlement have been put forward, there is still no clear consensus on the precise ways that hydrogen affects microstructural evolution and leads to failure. In this work, we consider how alloy composition and hydrogen affect the presence or absence of short range order (SRO) in austenitic steels, and how these changes to SRO may ultimately affect microstructure evolution. We present a computational assessment of alloy chemistry, SRO, and hydrogen effects using first-principles calculation and cluster expansion-Monte Carlo (CE-MC) simulation in Fe-Cr-Ni binary and ternary alloys. The degree of SRO present in typical binary and ternary austenitic steels of varying Fe-Ni-Cr composition is estimated from the Warren- Cowley short range order parameters. The composition and temperature dependence of SRO agrees with experimental and other computational works. We then demonstrate the effects of interstitial H on SRO in these alloys using a multicomponent multisublattice CE-MC method. The model results indicate that introducing H modifies the SRO preference of typical Fe-Ni-Cr alloys. In particular, we observe a strong tendency to form Cr and H rich domains. It is broadly presupposed that SRO may cause localized slip since planar glide is frequently reported in alloys known to exhibit SRO. As embrittlement is often accompanied by a transition from homogeneous to localized slip, the interaction between chemistry, SRO, and hydrogen may play a role in the slip localization that takes place during embrittlement of austenitic alloys.

We utilized molecular dynamics to study the hydrogen permeation in metallic materials such as pipeline steels, stainless steel, welding zones, and amorphous coatings. First, the nano/micro- structures and element distribution were obtained through experimental characterization. Then, the corresponding molecular dynamics models were established to represent the actual material structures within a small scale (i.e., 100 nm-1 um). Then, we investigated the effects of crystal defects, crystal structures, grain boundaries, interfaces, and surface roughness on hydrogen permeation. The correlation between pressure, temperature, and hydrogen permeation was also studied. The current study aims to understand the metallic materials compatibility used in pipelines and storage vessels. Meanwhile, the current investigation helps establish a method to accelerate hydrogen permeation to simulate the long-time service in hydrogen transporting and storage for mechanical properties tests. Finally, we would like to seek the possible approach to design materials with specific nano- and micro-structures for hydrogen barrier purposes considering different hydrogen pressure.

The mechanisms of hydrogen-assisted fracture in stainless steels are superficially described in the literature. Here, we attempt to produce causal and quantitative evidence of hydrogen-affected deformation and fracture. We analyze oligocrystalline microstructures to isolate deformation mechanisms in physical samples and create computationally equivalent microstructures to simulate the experimentally-observed responses.

This talk focuses on the development of a framework to simulate stress-strain responses of these microstructures, consisting of a tool to assign a best-fit crystal orientation on a regular 2D grid, an application to simulate grain growth in a 3D volume with boundary conditions imposed by a 2D domain, and a crystal plasticity model to simulate mechanical response.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

The interaction between transition metal carbides and hydrogen is an important topic for both hydrogen trapping and hydrogen storage. There are reports that hydrogen can be trapped in carbide precipitates in steel, increasing their tolerance to hydrogen-induced delayed fracture. This trapping could result from hydrogen interaction with the surface or in the bulk. There is also evidence that hydrogen can be stored in transition metal carbides, notably TiC_0.6. This raises important questions regarding hydrogen interaction and storage in transition metal carbides and its dependence on chemistry. In this talk, we examine how hydrogen binds to carbide precipitate interfaces and in the bulk using a combination of density functional theory and elastic models as a function of chemistry, both the transition element and carbon concentration. Our results reveal that metal-rich transition metal group IVB carbides are superior at trapping and storing hydrogen in the bulk while trapping at coherent interfaces is a function of both lattice mismatch and elastic constants of the carbide precipitates.

After decades of research and success stories, an elusive question remains: have the truly “optimal” hydrogen storage alloys escaped discovery? The traditional approach to novel materials discovery has typically relied on researchers’ significant domain expertise and trial-and-error experimentation in order to narrow down the vast space of possible materials. A recent paradigm involves the use of machine/statistical learning (ML) techniques to form surrogate models that screen materials’ performance properties with many orders of magnitude greater efficiency and significantly cut the time and material costs of experimental search and validation. Focusing on materials-based hydrogen storage, we demonstrate that explainable machine learning techniques can be used to elucidate simple, first-order design rules for predicting metal hydride thermodynamics. Trained purely on experimental data, the model does not rely directly on crystal structure and permits predictions on materials that otherwise lack well defined, computationally tractable unit cell representations. These developments are critical in order to apply these models to screen a novel chemical space of 672 high entropy alloys (HEAs), where we discover that we can finely tune the thermodynamic stability across many orders of magnitude within this set of materials. Afterwards we identify and experimentally validate two novel HEA compositions whose hydrides were significantly destabilized (up to 70x increase in equilibrium pressure) compared to the benchmark HEA hydride, thereby releasing hydrogen at much milder conditions. Exploring an even more comprehensive space of nearly 21,000 proposed HEA compositions, we are now using these ML models to optimize the competing objectives of thermodynamic stability, maximal volumetric/gravimetric capacity, and minimal cost (i.e., finding Pareto optimal materials). Our workflow takes seconds to predict the 138 Pareto optimal candidates, and a handful are currently undergoing laboratory validation within our international collaboration group. Assuming 1 month per HEA is required for synthesis and characterization, random sequential selection within this search space would consume an expected 17 years of experimental time before measuring the first Pareto material by chance.

The successful deployment of hydrogen carriers requires careful consideration of chemical properties, engineering factors, and economic realities. The carrier effort in PNNL focuses on identifying and addressing the research needs of potential use cases that will benefit from large scale stable energy storage. The properties of the carriers must be matched to the needs of the use cases, which cover a wide range of scales, storage duration, and delivery rates. Hydrogen storage for stationary back-up or load-levelling applications poses different challenges from uses where the carrier must be transported over large distances between storage and release. We have determined the requirements of a range of representative use cases and used these to calculate operational performance parameters along with the thermodynamic, kinetic, and physiochemical properties of ideal carriers. We will discuss an example of a hydrogen storage system based on using ethanol to power a neighborhood microgrid. The lessons learned from the experimental research and associated engineering analysis are leading us to improved carrier systems for this and other applications.

The current work applies a framework of thermally activated dislocation motion to obtain fundamental information about the effect of hydrogen on the mechanisms of cyclic deformation in strain hardened 316L stainless steel, a common alloy used for hydrogen service. As-received and hydrogen-precharged specimens were deformed in plastic strain controlled low cycle fatigue tests at amplitudes ranging from 0.3% to 0.6% plastic strain, and a series of plastic strain rate change experiments were performed periodically at the peak tension stress. These tests represent constant temperature and dislocation structure experiments from which changes in operational activation area could be determined as the microstructure evolved. Specifically, operational activation areas were determined from the difference in stress associated with the logarithmic difference in plastic strain rate and then plotted as a function of the cumulative plastic strain. Both material conditions experienced a rapid increase in operational activation area during the first few cycles at all plastic strain amplitudes, followed by a region of steady state that coincides with the reduced rate of softening observed throughout most of the fatigue lifetime. The values of operational activation area were larger in the as-received than in the hydrogen-precharged condition, ranging from 100 to 150 b² and from 60 to 90 b², respectively. After the specimens had been cycled to one-half of the expected lifetime, additional tests were conducted at various plastic strain values around stable hysteresis loops to reveal information about the character of deformation and obstacles to dislocation activity in this stainless steel. Plots of the inverse of the normalized operational activation area as a function of the flow stress, also known as Haasen plots, were used as a first step in inferring the dominant deformation mechanism. The results indicated the presence of an athermal stress associated with the interactions between dislocations and the dominant glide obstacles for both material conditions, and the magnitude of this stress is reduced in presence of hydrogen. Although the operational activation areas are weakly dependent on the plastic strain amplitude for as-received and hydrogen-precharged specimens, the nature of the dominant obstacle to dislocation motion tends to be slightly more thermal than forest dislocations in the high amplitude regime. Values of operational activation area at this stage of fatigue life ranged from 100 to 300 b² and from 70 to 200 b² in the as-received and hydrogen-precharged conditions, respectively. Transmission electron microscopy was used to directly observe the differences in dislocation arrangements with and without hydrogen at half-life.

The H₂ capacities of a diverse set of 918,734 metal-organic frameworks (MOFs) sourced from 19 databases is predicted via machine learning (ML). Using only 7 structural features as input, ML identifies more than 8,000 MOFs with the potential to exceed the capacities of state-of-the-art materials. The identified MOFs are predominantly hypothetical compounds having low densities (<0.31 g/cm�³) in combination with high surface areas (>5,300 m²/g), void fractions (~0.90), and pore volumes (>3.3 cm³/g�). The relative importance of the input features are characterized, and dependencies on the ML algorithm and training set size are quantified. The most important features for predicting H₂ uptake are pore volume (for gravimetric capacity) and void fraction (for volumetric capacity). The ML models are available on the web ( https://sorbent-ml.hymarc.org ), allowing for rapid and accurate predictions of the hydrogen capacities of MOFs from limited structural data; the simplest models require only a single crystallographic feature. DOI: 10.1016/j.patter.2021.100291

Metal hydrides continue to be of interest for materials-based hydrogen storage. However, most research has focused on hydrides that are too stable, i.e. their enthalpy of H₂ desorption is too high for the residual heat of a PEM fuel cell to drive the release. In contrast, metastable hydrides, for which H₂ release is exothermic, have received less attention. Compounds such as alane (AlH₃) are so thermodynamically unstable that direct rehydrogenation following hydrogen release is thought to be impossible.¹ Multistep chemistry involving Lewis acid-base intermediates (e.g. alane-ether complexes) can be used but is impractical for many applications, including on-board vehicular storage. This is unfortunate because AlH₃ and another metastable hydride, LiAlH₄, have attractive gravimetric capacities (10.1 wt% and 8.0 wt%, respectively). Here we show that these hydrides can be thermodynamically stabilized by nanoconfinement using two types of porous hosts: nitrogen-functionalized porous carbon (LiAlH₄@NCMK-3) and bipyridine-functionalized Covalent Triazine Framework (AlH₃@CTF-bipyridine). LiAlH₄@NCMK-3 desorbs 100% of its hydrogen by 240 °C and >80% can be regenerated at <100 MPa H₂, bypassing the Li₃AlH₆ intermediate of bulk.² Even more surprisingly, AlH₃@CTF-bipyridine is reversible at 60 °C under 700 bar hydrogen, 36 times lower than the experimentally measured H₂ plateau pressure of bulk aluminum. DFT calculations and EPR spectroscopy support a radical-based mechanism via AlH₃ binding to bipyridine, resulting in single-electron transfer to form AlH₂(AlH₃)_n clusters.³ Together, our results reopen the case in favor of these high-capacity metastable hydrides for H₂ storage. They also suggest potential for using them in batteries, supercapacitors, and heterogeneous catalysis.

1. J. Graetz, J. Wegrzyn, J. J. Reilly, J. Am. Chem. Soc. 2008, 130, 17790.
2. Y. Cho, S. Li, J. L Snider, M. A. T. Marple, N. A. Strange, J. D. Sugar, F. El Gabaly, A. Schneemann, S. Kang, M. Kang, H. Park, J. Park, L. F. Wan, H. E. Mason, M. D. Allendorf, B. C. Wood, E. S. Cho, V. Stavila ACS Nano, 2021 doi.org/10.1021/acsnano.1c02079.
3. V. Stavila, S. Li, C. Dun, M. A. T. Marple, H. E. Mason, J. L. Snider¹, J. E. Reynolds III, F. El Gabaly, J. D. Sugar, C. D. Spataru, X. Zhou, B. Dizdar, E. H. Majzoub, R. Chatterjee, J. Yano, H. Schlomberg, B. V. Lotsch, J. J. Urban, B. C. Wood, M. D. Allendorf, submitted.

The absorption and desorption of hydrogen by hydride forming metals is well known to exhibit hysteresis, in which the partial pressure of hydrogen absorption is greater than desorption. We will demonstrate, by combined theoretical and experimental work, that a continuous barrier to interface motion is the origin of hysteresis. Hysteresis and elastic energies determined from high precision pressure composition isotherm and in-situ X-ray diffraction measurements on palladium hydride are found to be inconsistent with previous theories. Our experiments show that in bulk samples the hysteresis is present even in a partially transformed system, thus a stress-induced jump in volume fraction of hydride cannot be responsible for the hysteresis. Furthermore, we characterize the absorption and desorption kinetics stepwise and find that the equilibration time is constant within the two-phase region of the palladium-hydrogen phase diagram, indicating that there exists a persistent energy barrier.
Acknowledgement: Support from the EFree Center, an EFRC funded by the U.S. DOE, OBES Award DE-SC0001057

Hydrogen is a promising technology for long-term energy storage and realization of zero CO₂ emission. However, utilizing hydrogen as a fuel presents practical challenges due to its low volumetric energy density and therefore requires development of advanced storage medium. Complex magnesium borohydride, Mg(BH₄)₂, possesses one of the highest gravimetric (14.8 wt.%) and volumetric (112 g/L) hydrogen storage density. Yet practical challenges remain due to kinetic limitations to realize full reversibility for de- and re-hydrogenations. Early theoretical work based on density functional theory predicts hydrogen desorption temperature between 20 and 75°C via reaction Mg(BH₄)₂ –> MgB₂ + 4H₂, however, in experiments, hydrogen release have only been observed above 200°C. It is speculated that this discrepancy is originated from the formation of various B_xH_y intermediates that act as a thermodynamic sink or a kinetic barrier during hydrogen desorption. In this work, we use first-principles methods to explore initial dehydrogenation, from BH₄ to B₃H_x, in bulk γ-Mg(BH₄)₂ and to identify the preferred reaction pathways for the formation of key B₂H_x and B₃H_x intermediates that limit the overall kinetics of BH₄ to B₃H_x transformation. Our results not only provide valuable thermodynamic and kinetic information regarding each of the elemental reaction steps during BH₄ to B₃H_x conversion, but also shed light on the long-standing contradiction between previous theoretical prediction and experimental observation about the existence of B₃H₈ intermediate.

This work was supported by the Hydrogen Materials Advanced Research Consortium (HyMARC), established as part of the Energy Materials Network by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

More efficient, compact storage of hydrogen represents a critical need for fuel cell vehicles and long-duration energy storage. Metal hydrides are promising materials to replace high-pressure tanks, offering high volumetric capacities at relatively low hydrogen pressures. However, these materials often suffer from kinetic limitations that incur undesirable energy cost under operation. Within the DOE Hydrogen Materials—Advanced Research Consortium (HyMARC), we are using multiscale models to model kinetic processes in hydrogen storage materials and identify key limitations for targeted engineering. This talk will present an overview of the materials modeling strategy within HyMARC, which incorporates and integrates atomistic simulations of complex interface chemistry, effective-medium approaches for transport and mechanical stresses, and continuum methods for microstructure evolution. Our aim is to go beyond thermodynamic models to understand harder-to-simulate kinetic processes under nonequilibrium cycling conditions. We will show how modeling coupled with high-fidelity experimental characterization has been employed to elucidate chemical reaction pathways at interfaces and to understand the formation and evolution of new solid phases. We will also present recent progress towards elucidating the interaction of hydrogen with native oxide films, which form on metal surfaces and play a key role in determining the activation temperatures and pressures for many metal hydrides. To this end, multiscale simulations and experiments are being combined with machine learning and graph-theoretic algorithms to simulate hydrogen transport through these polycrystalline and amorphous surface films. The talk will also review our recent efforts to improve kinetic predictions of hydrogen storage materials by incorporating additional physics into the simulations, which better approximates realistic operating conditions. Finally, we will discuss how modeling-derived understanding is being used within HyMARC to guide new improvement strategies, including nanoconfinement and compositional modification. Examples will be drawn from multiple materials systems, including complex light metal hydrides for vehicles and TiFe alloys for stationary energy storage.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

The sluggish kinetics of oxygen electrocatalysis and the resulting high overpotentials necessary to achieve useful current densities limit the performance and efficiency of proton exchange membrane fuel cells (PEMFCs).(1) The best catalysts for the oxygen reduction reactions are based on platinum, leading to limitations in the cost effective implementation of this technology.(2,3) The development of alternative catalysts, with comparable or higher activity and durability to the platinum and derived from earth-abundant materials has thus been an active research area for decades.

Incredible progress has been made over the past decade in increasing both the ORR activity and durability of PGM-free PEMFC cathode catalysts.(4) The class of catalysts demonstrating the highest ORR activities are those typically denoted as “Fe-N-C” and synthesized by heat treating iron salts and zinc-based zeolitic imidazolate frameworks (ZIFs) and/or phenanthroline, as carbon and nitrogen sources, or by heat treating iron-substituted ZIFs. For this class of PGM-free materials, it has been determined that variables such as the metal and carbon-nitrogen macrocycle content, as well as the temperature and atmosphere in which the composites are heat treated are important in determining the activity and activity stability of the resulting catalysts.(5,6) Changing these variables and testing their effect on the resulting catalyst properties is a time-consuming process and only a limited portion of the composite composition and temperature space have been explored for this broad class of materials.

This presentation will describe the development and application of high-throughput methodology to accelerate exploration of the effects of composition and synthesis parameters on the activity of iron-carbon-nitrogen acidic electrolyte ORR electrocatalysts. The structural characterization of the materials using X-ray absorption spectroscopy (XAS) and correlation of the atomic structure with ORR activity will be described, as will the high-throughput testing and optimization of the electrode composition using a 25-electrode array fuel cell hardware.(7)

References
1. H. Yang, X. Han, A.I. Douka, L. Huang, L. Gong, C. Xia, H.S. Park, and B.Y. Xia, Adv. Func. Mater., 31 (2021) 2007602.
2. B. Pivovar, Nature Catalysis, 2 (2019) 562.
3. S. Thompson and D. Papageorgopoulos, Nature Catalysis, 2 (2019) 558.
4. L. Osmieri, J. Park, D.A. Cullen, P. Zelenay, D.J. Myers, and K.C. Neyerlin, Curr. Opin. Electrochem., 25 (2021) 100627.
5. E. Proietti, F. Jaouen, M. Lefevre, N. Larouche, J. Tian, J. Herranz, and J.-P. Dodelet, Nature Comm. 2 (2011) 1.
6. A. Zitolo, V. Goellner, V. Armel, M.-T. Sougrati, T. Mineva, L. Stievano, E. Fonda, and F. Jaouen, Nature Materials, 14 (2015) 937.
7. J. Park and D.J. Myers, J. Power Sources, 480 (2020) 228801.

This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). This work utilized the resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated by Argonne National Laboratory for DOE Office and was authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.

The essence of the electrocatalytic reactions is a heterogeneous reaction process that occurs on the surface of a solid-phase catalyst, including the adsorption, diffusion and dissociation of reactants, electron transfer and the absorption and desorption of the intermediate/reaction products. The surface physical and chemical properties of the catalyst such as geometric morphology, atomic arrangement, element composition, hydrophilicity, etc. are closely related to the adsorption behavior of active species and play a critical role in determining the catalytic activity, selectivity and stability. Moreover, the surface structure of the catalyst is also affected by the chemical environment in which it is located, and then offer significant insights in catalyst design strategies.
Based on the relationship among design, surface structure, environment and activity in catalysis, here we focus on the cathodic oxygen reduction reaction in fuel cells and takes the design and synthesis of platinum-based nanocatalysts as the research object. Starting from the surface design and engineering of nanostructured catalysts, our work studies the effect of 1. surface diffusion behavior under different reaction environments, 2. surface atomic arrangement, 3. exposed crystal planes, and 4. surface charge and the corresponding structure-activity relationship. It aims to combine the influence of the chemical reaction environment on the surface structure of the catalyst and the influence of the surface structure of the catalyst on the catalytic performance to pave the way for the development of highly-efficient and stable platinum-based oxygen reduction catalysts.

[1]. Y. Ma, A. N. Kuhn, W. Gao, T. Al-Zoubi, H. Du, X. Pan, H. Yang*, Nano Energy 79, 105465 (2021).
[2.] Y. Ma, F. Li, X. Ren, W. Chen, C. Li, P. Tao, C. Song, W. Shang, R. Huang*, B. Lv*, H. Zhu*, T. Deng, J. Wu*, ACS Applied Materials & Interfaces 10, 15322 (2018).
[3.] Y. Ma, W. Gao, H. Shan, W. Chen, W. Shang, P. Tao, C. Song, C. Addiego, T. Deng, X. Pan*, J. Wu*, Advanced Materials 29, 1703460 (2017).

The high cost and difficult task to eliminate platinum (Pt) catalyst in Proton Exchange Membrane Fuel Cells (PEM-FCs) are some of the main obstacles that thwart the wide adoption of fuel cells. For this reason, Anion Exchange Membrane Fuel Cell (AEM-FC) has been suggested as an alternative to the PEM-FC technology in which non-Pt metals may be employed. However, the common slow kinetics at the anode experienced during Hydrogen Oxidation Reaction (HOR) has been one of the challenges preventing the achievement of high-power density in the Pt-free AEM-FC. Here, our objective is to improve the efficiency of HOR catalysts by depositing various ratios of CeO_x/Pd catalysts onto carbon support using the Controlled Surface Reactions (CSR) process. It is expected that a homogenous distribution of CeO_x preferentially attached to Pd nanoparticles (NPs) can produce highly active CeO_x-Pd/C catalysts for HOR. In the present study, we offer a comprehensive characterization approach for the synthesized highly active catalyst and correlate the obtained structural/compositional parameters to its performance. The characterization of the catalysts was carried out using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), X-ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HR-TEM), Scanning Transmission Electron Microscopy (STEM) - Energy Dispersive Spectroscopy (EDS), Electron Energy Loss Spectroscopy (EELS), and X-ray Photoelectron Spectroscopy (XPS) to confirm the bulk composition, phases present, morphology, elemental mapping, local oxidation state and surface chemical states, respectively. The HRTEM micrographs indicated that Pd NPs were uniformly distributed on the carbon support with only some minor agglomeration. Additionally, the achieved high interfacial contact between CeO_x and Pd acquired on single Pd NPs was, for the first time, segmented and calculated using High-resolution STEM-EDS maps and Image J processing program by measuring the overlap intensities between Pd and Ce in the NPs. The intimate contact between CeO_x and Pd and the increase in their interfacial contact area with the addition of CeO_x in these NPs was observed. This interfacial contact area is higher than other previously reported for Pd-CeO_x catalysts system synthesized by other methods, suggesting that the CSR method can provide a more selective deposition of CeO_x on Pd. EELS was used to determine that the primary oxidation state of our oxide was in the form of CeO₂ and Ce⁴⁺ was predominantly present in the CeO_x. However, the presence of Ce³⁺ cannot be ruled out, especially near the surface of the NPs. The study also found that the already mentioned interfacial contact area was directly correlated to the electrochemical performance reflected on the HOR activity of the CeO_x-Pd/C catalysts. Additionally, to track the morphological, compositional, and structural changes at the nanoscale we used postmortem Identical location transmission electron microscopy (IL-TEM), to reveal the degradation mechanisms governing this new CeO_x-Pd/C catalyst material following Accelerated Stress Tests in a three-electrode electrochemical cell. The preliminary results have shown that CeO_x can act as a promising anchoring mechanism that prevents the mobility, agglomeration, and detachment of Pd at a considerably high degree when compared to samples without CeO_x.

Alkaline fuel cells are a versatile, low-emission alternative energy source for use in transportation, as well as for portable and stationary power applications. These systems use chemical energy from the electrochemical conversion of hydrogen and oxygen gas to produce electrical energy, with only water and heat as by-products. They operate through two interdependent redox reactions within the cell consisting of oxidation of the hydrogen fuel at the anode and reduction of the oxygen at the cathode. One of the most significant barriers that fuel cells face is the sluggish nature of the oxygen reduction and its dependence on platinum, a rare and costly metal, as a catalyst for the reaction. Through the development and optimization of structures and materials, the amount of catalyst can be decreased without sacrificing performance and enabling an increase in the commercial viability of the device.

An alternative cathode material is palladium, which possesses similar chemical properties to platinum but is available in a greater abundance. Additionally, palladium has exhibited improved electrochemical properties in alkaline fuel cells. It has been shown that the activity of palladium-based catalysts can be further increased when alloyed with ancillary metals such as nickel. The incorporation of nickel, a more abundant transition metal, improves the properties of the catalyst and decreases the amount of palladium needed while increasing its utilization. This creates a more cost-effective solution to preparing catalysts. Furthermore, the use of nanoparticles as the basis for the cathode catalyst allows for a high surface area for the oxygen reduction reaction to take place. Nanoparticles allow for a more efficient use of the catalyst materials and, consequently, decreases the amount of metal required and the overall cost of the catalyst.

This study assesses the viability of palladium nickel nanocatalysts as an alternative cathode material for alkaline-based fuel cells. These nanoparticles are synthesized using a water-in-oil microemulsion method and characterized using a series of electron microscopy and electrochemical techniques. By adjusting the ratio of metal salt precursors, a series of nanocatalysts were prepared with various compositions of palladium and nickel. The analysis of these nanocatalysts included assessing the relationship between their composition, structure, and function to determine an optimal ratio of the palladium and nickel within these materials for increasing their catalytic performance. Initial tests demonstrate a promising activity of these nanocatalysts, although further studies will be needed to assess the durability and to better understand the stability of these catalysts.

The functionality of materials used for energy applications is critically determined by the physical properties of small active regions such as surfaces, interfaces, and grain boundaries. Although small, these regions in materials if well utilized can provide sustainable solutions to alleviate the energy crisis of today’s world. In particular, the capability to manipulate disorder in materials, whether due to zero-dimensional defects (such as vacancies, dopants) or three-dimensional defects (such as pores, cracks, etc.), can bring innovation in designing sustainable technologies. Among various tools, computational materials science plays a significant role in understanding and manipulating disorder in energy materials. Hence, in this talk, I will provide a brief overview of our recent work on understanding and manipulating disorder in oxides such as TiO₂ and Yttria stabilized ZrO₂ for enhancing their performance. I will mainly focus on modeling of realistic materials with structural defects and disorder for next-generation solid oxide fuel cells electrolytes and sustainable heterogeneous catalysis. [1]

[1] Madrid, J. C. M., Ghuman, K. K. Disorder in Energy Materials and Strategies to Model it. Advances in Physics: X., 6:1 (2021).

The growing demand for clean and efficient energy sources has been a direct result of the serious effect on the climate and the environment associated with high emissions of pollutants that are mainly produced from fossil fuel-based energy generators. Currently, a green technology like Proton Exchange Membrane Fuel Cell (PEMFC) can produce zero-emissions energy power by primarily using the electrochemical reactions of hydrogen and oxygen to generate electrical energy. Additionally, due to their high activity, Platinum (Pt)/Platinum alloy nanoparticles distributed on a carbon (C) catalyst support are considered the most efficient catalyst materials use in this technology. However, the main drawbacks from this technology are (i) the scarcity of Pt, making it very expensive for widespread fuel cell application and commercialization, as well as the fact that (ii) carbon support can be prone to corrosion and degradation when operating even at the most typical PEMFC conditions. This carbon corrosion can induce the loss of Pt nanoparticles (NPs) active surface area when Pt detaches from the carbon support or agglomerates into larger Pt NPs, which can lead to premature performance losses in a PEMFC. In this work, we propose a novel approach to prevent carbon corrosion by selectively depositing a corrosion-resistant layer (approximately 1.5 nm thickness) of titanium nitride (TiN) only on the surface of the carbon support while the Pt NPs catalyst centers are left uncoated and accessible for reagents reactions (oxygen O and H⁺ protons). TiN was used as the coating material because of its attractive properties, such as its electronic conductivity, which is enough to enable the free movement of electrons from the current collector toward the catalytic centers. TiN would also act as an anchoring mechanism of Pt NPs, preventing their mobility on the carbon support and subsequent agglomeration. The deposition of TiN layers is carried out at 150°C with a Hollow-Cathode Plasma-Assisted Atomic Layer Deposition (HCPA-ALD) while using Tetrakis(dimethylamino)titanium (IV) (TDMAT) as the metal precursor in Ar/N₂ plasma, from which nitrogen was used as co-reactant. Before HCPA-ALD, to prevent TiN from depositing on top of the Pt NPs, the surface of Pt NPs is selectively coated with a thin film of oleylamine (OAm), a polymer that would only absorb onto Pt NPs, which is later removed from the catalyst and TiN system (after the HCPA-ALD process is finished), by heat treatment at 185°C. Here we will mainly focus on the TiN deposition process and the subsequent Transmission Electron Microscopy (TEM) characterization performed on these nanoparticles, which allows us to prove the successful formation of homogenous coatings and architecture characteristics of these nanoparticles after oleylamine application and TiN deposition. Electrochemical characterization, including Cyclic Voltammetry (CV) and Oxygen Reduction Reaction (ORR), were performed in a Rotating Disc Electrode (RDE) to further confirm the activity and effectiveness of our designed catalyst. Our preliminary results have shown that highly conformal TiN can be successfully grown onto Pt/C nanoparticles (as powders) by using HCPA-ALD with a custom-made agitator mechanism. The second phase of this research will be focused on inspecting the degradation mechanisms this novel catalyst system experiences after potential cycling in a Membrane Electrode Assembly (MEA).

316L porous stainless steel (PSS) is a very suitable support material for particularly thin film Pd-based membranes due to its high thermal, oxidation resistance and having a thermal expansion coefficient similar to Pd. However, commercially available PSS have often very large surface pores that affect the final thickness of the membrane that could be deposited on it. The size of pores at the commercial PSS surface can be as large as 30 µm or more. That’s why the membrane thickness deposited on the PSS can reach several tens of microns in order to create a porous-free membrane. Several studies focus on applying an interlayer between PSS and thin film membrane so as to reduce the size of pores at the PSS surface and thus reduce the thickness of the membrane. However, it is often not easy to control the size of surface pores and keep their size under a threshold. In addition, a non-homogeneous application of the interlayer to be formed on the PSS surface can cause formation of large pores that might be overlooked. The deposition of a thin film membrane on such an interlayer typically causes the formation of pin-holes at the thin film membranes. The presence of pin-holes in the membrane results in a reduction in hydrogen selectivity, even though fully dense metallic membranes principally offer infinite hydrogen selectivity. That’s why modification of PSS surface has to be employed in a controlled manner and should be reproducible.

The current study concentrates on surface modification of PSS as a support material in order to enable thin film (<1µm) deposition on PSS. In this respect, 316L foil was used as a model system to determine the application parameters for the formation of a nanoporous Ni surface layer on the PSS. 316L foils were initially electroplated by Ni-Cu alloys using plating solutions with different Ni:Cu ratios. Subsequently, the Ni-Cu surface layers on 316L foils were subjected to an electrochemical dealloying process in order to dissolve Cu back into the plating solution and remain nano-porous Ni surface layer. In the study, different electroplating solutions were employed to determine the suitable alloy composition that yielded the desired pore size (<100nm) in the Ni surface layer formed after the de-alloying process.

This work was supported by TUBITAK (The Scientific and Technological Research Council of Turkey) with project Number 119M636, which the authors gratefully acknowledge.

The unique physical properties of 2D van der Waals materials have attracted great attention to fulfill the demands of future nanoelectronics for sustainable developments and have recently attracted significant attention for addressing the current worldwide challenges of environmental pollution and energy shortage. The ultrahigh surface area in combination with and extraordinary physiochemical, electronic and optical properties offer the application in hydrogen and fuel cell technologies. The huge number of active sides of 2D van der Waals materials as well as their suitable band gap offer their application as photocatalyst. The lacking control of large-scale material synthesis corresponding to specific requirements in commercial material formation processes is still challenging, which motivated us to develop a unique synthetic approach to layered 2D materials MS₂ (M= Mo^IV, W^IV, Ti^IV, Sn^IV), MS (M=Sn^II, Ge^II). A uniform synthesis route of molecular building blocks for controlled formation of stable precursor classes [M(SEtN(Me)EtS)_x] (M = Mo^IV, W^IV, Ti^IV, x = 2; M = Ge^II, Sn^II, x = 1) was developed. Following a simple synthetic protocol, the reaction of tridentate SNS donor ligand with suitable metal compounds resulted in (air)stable molecular precursors, which enabled the targeted formation of homogeneous crystalline 2D MoS₂, WS₂, TiS₂, SnS₂, SnS by thermal decomposition experiments as well as in wet chemical syntheses via microwave assisted decomposition resulting in SnS and SnS₂ particles.
These molecular building blocks provide an extraordinary synthetic approach to materials for hydrogen economy.

Fuel cells are one of the most advantageous solutions for clean energy applications due to their near-zero-emission, high efficiency, low maintenance cost, and high energy density. However, the development of highly efficient and reliable versions of the current Polymer Electrolyte Membrane Fuel Cells (PEMFCs), mature enough to result in an increased application in the transportation and energy generation sectors, have been subjected to considerable scientific and industrial level effort. In general, PEMFCs produce electricity through the electrochemical reaction of hydrogen and oxygen (or air) taking place in the Membrane Electrode Assembly (MEA) by converting the chemical energy of the hydrogen oxidation and oxygen reduction into electrical energy with water as a byproduct. However, high performances (at scale) in oxygen reduction reaction (ORR) catalysts are a challenge that must be tackled to allow an increase in the energy generation of this technology. For this reason, tunability of the materials at the core of this technology is necessary to meet the demands of not only a specific application but also operating conditions. Here, we study commercially available ORR catalysts based on platinum nanoparticles (Pt-NPs) supported on nitrogen-functionalized carbon (Engineered Carbon Supports^TM, ECSTM) produced by Pajarito Powder, LLC at scale. The ORR performance of three MEAs made with different Pt/ECS catalysts, by IRD Fuel Cells, LLC was studied under low relative humidity conditions. At low Pt loading, 0.1 mg_Pt cm^-2, they displayed competitive performance, achieving current densities between 1.8 and 1.3 Acm^-2 at 0.6 V. A comprehensive study of the physicochemical properties and structure of these Pt/ECS catalysts and their Pt-free equivalent ECSTM carbon support materials was also performed by using several characterization techniques, such: X-ray diffraction, Raman spectroscopy, and Transmission Electron Microscopy (TEM) to investigate structural and morphological properties, X-ray Photoelectron Spectroscopy (XPS) to determine the surface composition and chemical properties, Thermogravimetric Analysis (TGA) to evaluate thermal oxidation properties, and nitrogen sorption (Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH)) to determine the surface area and porosity, of these catalyst materials. This detailed study allowed us to conclude that the best performing catalyst in the conditions used in this work has the highest surface area and the highest concentration of surface dopants (nitrogen and oxygen). However, it is worth mentioning that catalysts supported on low surface area carbon with a high graphitic degree, exhibited competitive performance that can be used in applications where corrosion-resistant carbon is needed. We can also conclude that based on the highly competitive MEA performance for the three Pt/ECS catalysts these carbon supports, developed by Pajarito Powder LLC with engineered morphology and composition through the VariPore^TM synthetic technology, can result in tunable properties suitable for a targeted application. These commercially available materials can be adapted through modification of the versatile synthesis platform to tune the physicochemical properties of carbons for other applications and conditions.

There is a considerable interest in lowering the operating temperature of solid oxide fuel cells. In this respect, (La,Sr)CoO3-(La,Sr)2CoO4 dual phase oxides have attracted much attention as cathode materials due to their enhanced oxygen reduction reaction kinetics[1,2]. Moreover there is much interest to develop these cathodes in amorphous state which not only leads to significantly improved cathode performance, but also are associated with a strong resistance to Sr segregation making them as a suitable choice for IT-SOFCs[3]. In the current study so as to further improve the stability, a third oxide was introduced into the system. Over 20 cathodes compositions, all based on (La,Sr)CoO3-(La,Sr)2CoO4- (Gd,Ce)O2 ternary system, were sputter deposited in combinatorial geometry. A target operating temperature of 550^oC was selected and the full cell performances were measured in each cell so as to identify the best cathode composition. Scenarios how to best keep the amorphous structure at elevated temperatures and with prolonged use are discussed.

[1] ZÇ Torunoglu, D Sari, O Demircan, YE Kalay, T Ozturk, Y Kuru One pot synthesis of (La, Sr) CoO3/(La, Sr) 2CoO4 for IT-SOFCs cathodes, International Journal of Hydrogen Energy 43 (40), 18642-18649
[2] D Sari, F Piskin, ZC Torunoglu, B Yasar, YE Kalay, T Ozturk Combinatorial development of nanocrystalline/amorphous (La, Sr) CoO3-(La, Sr) 2CoO4 composite cathodes for IT-SOFCs, Solid State Ionics 326, 124-130
[3] D Sari, B Yasar, F Piskin, YE Kalay, T Ozturk Segregation Resistant Nanocrystalline/Amorphous (La, Sr) CoO3-(La, Sr) 2CoO4 Composite Cathodes for IT-SOFCs Journal of The Electrochemical Society 166 (15), F1157

SOFC technology currently suffers from material interaction, insulating phase formation, and thermal stresses as it operates at high temperatures, which results in rapid degradation, low reliability, and high cost for commercialization. One of the approaches to reduce the operating temperature of SOFCs is the use of thin-film electrolytes to reduce the ohmic losses experienced at a reduced temperature while offering high thermal stability. That been said, the nanoscale thin films are generally polycrystalline and contain a high density of Grain Boundaries (GBs), which can substantially affect the ion diffusion in electrolytes. Experimentally, the atomistic structure of GBs can be analyzed and observe via high-resolution scanning STEM, but unfortunately, it does not provide sufficient information about the atomic-level structure of these boundaries. Molecular Dynamics (MD) simulations have been used for studying Grain Boundaries’ atomic structure and oxygen-ions transport across YSZ electrolyte over the last decades. However, high symmetry GBs have been commonly targeted even though most boundaries found in polycrystalline samples are a mix of twisted and tilted denominations- commonly named “mixed” GBs. A mixed grain boundary (GB) was modeled at the atomic level using MD in this work. The structure was built from a TEM image of an 8YSZ thin film sample using the amorphization and recrystallization (A&R) technique. Oxygen-ion self-diffusion was studied for a range of temperatures between 700 K and 2300 K and Y₂O₃ concentration between 4-14 mol%. It was found that Y₂O₃ does not segregate toward the GB core for one of the grain orientations. On the contrary, Y³⁺ prefers to accumulate in this particular grain after the A&R process. As a result, it allows a better distribution of oxygen vacancies inside the grain interior what leads to obtain a higher oxygen-ion self-diffusion for this region.

Performance of intermediate-temperature fuel cells and electrolysis cells is often limited by the high polarization resistance and low electrocatalytic activity of the positive electrode (i.e., positrode). Mixed ionic-electronic conducting electrode materials have been developed to improve device performance and, recently, triple ionic-electronic conducting oxides (TIEC) have provided a pathway for further improvement. Transport of each charge carrier in these materials depends on defect concentrations, which are moderated by cation concentrations and by operating conditions. Thus, it is important to understand mixed-species transport in single-phase TIEC materials, including discovery and design principles of TIECs, and high-priority areas for further TIEC materials development.¹
Here, we describe a combinatorial experimental approach to investigate a family of TIEC materials, Ba(Co,Fe,Zr,Y)O₃ (BCFZY), which is of interest for electrocatalytic applications in intermediate-temperature fuel cells and electrolyzers. Layered, half fuel-cell devices were deposited by pulsed laser deposition. An electrolyte layer (BaZr_0.8Y_0.2O_3-δ) was topped with compositionally graded arrays of BCFZY thin film microelectrodes to investigate the effects of chemical composition on performance. An automated benchtop instrument was developed to measure spatially resolved impedance of these films at temperatures up to 500°C under localized dry or humidified gases (air or nitrogen).² Performance of the BCFZY thin films was evaluated as a function of temperature and gas atmosphere.³ Distribution of relaxation times analysis was applied to disentangle individual fuel cell processes with distinct time constants in the frequency-dependent impedance spectra. The dependence of the polarization resistance of each of these processes on chemical composition, temperature, and gas atmosphere are reported.
The BCFZY impedance spectra show that a low-frequency process dominates under humid N₂, while the limiting process under dry air depends on the chemical composition of the electrode material. Total polarization resistance is low in Co-rich materials under both gas atmospheres. Low total polarization resistance is also observed in Y-rich materials under the humid N₂ atmosphere, suggesting a substantial proton-related impedance contribution at low frequencies in BCFZY. These results demonstrate a high-throughput experimental approach to studying triple-conducting materials as positive electrodes for intermediate-temperature electrolyzers and fuel cells.

Funding provided in part by the Office of Energy Efficiency and Renewable Energy (EERE) Hydrogen and Fuel Cell Technologies Office (HFTO), as a part of HydroGEN Energy Materials Network (EMN) consortium.

¹ M.C. Papac, V. Stevanović, A. Zakutayev, and R. O’Hayre, Nat. Mater. 20, 301 (2021).
² M.C. Papac, K.R. Talley, R. O’Hayre, and A. Zakutayev, Rev. Sci. Instrum. 92, 065105 (2021).
³ M.C. Papac, J. Huang, A. Zakutayev, and R. O’Hayre, (in preparation).

It was previously found that the Smith acidity scale is a powerful descriptor for predicting how binary oxide additives influence the oxygen surface exchange kinetics of the mixed ionic-electronic conductor Pr_0.1Ce_0.9O_2-x (PCO10). By infiltrating various binary oxides, ranging from strongly basic (Li₂O) to acidic (SiO₂) onto the surface of PCO10, basic additives reportedly result in surface electron accumulation leading to enhanced oxygen exchange while acidic materials yield surface electron depletion, deactivating the surface. [1,2] These findings are extended by examining the impact of the relative acidity of additives on the polarization resistance of screen printed porous PCO10 electrodes applied symmetrically to YSZ single-crystal wafers by Electrochemical Impedance Spectroscopy (EIS). Chromia, an acidic oxide, is a common poison for cathodes that originates from the metal interconnects used in SOFC stacks. To validate the detrimental effects of chromia, each layer of PCO was infiltrated with a Cr concentration of 230 ppm. This value was selected based on the results of Menzler et al. on a SOFC stack after 3000 h of operation, who found a Cr-deposition amount of 0.8 μg/cm² on a cell cathode of similar morphology. [3] For comparison, the effect of an equivalent mass of lithia (1270 ppm), a basic oxide, was also examined. The area-specific resistance (ASR, Ωcm²) increased over four-fold upon infiltration with of chromia, from approximately 124 to 532 Ωcm² when measured at 450 ^°C and in 0.1 atm O₂, while the ASR of the cell infiltrated with the lithia was lower than the pure cell. The semicircles in the Nyquist plot, are distorted as a result of several overlapping processes. Here, the distribution of relaxation times (DRT) method enables a clearer differentiation between electrochemical processes, e.g. low frequencies processes (LF) such as gas diffusion, surface oxygen exchange and high frequency (HF) processes such as charge transfer processes across electrolyte–cathode interfaces. [4,5] At 450 °C, the HF region remains largely unchanged by surface infiltration, while in the LF there are significant differences between samples. Upon chrome infiltration a new LF frequency peak forms, while after Li infiltration the peaks in the LF region largely disappear. By using a systematic temperature study under varying oxygen concentrations, the different peaks in the DRT will be attributed to specific electrochemical processes, allowing for more precise understanding of how the binary oxides influence the cathode reactivity.

[1] C. Nicollet, C. Toparli, G.F. Harrington, T. Defferriere, B. Yildiz, H.L. Tuller, Acidity of surface-infiltrated binary oxides as a sensitive descriptor of oxygen exchange kinetics in mixed conducting oxides, Nat. Catal. (2020). https://doi.org/10.1038/s41929-020-00520-x.
[2] D.W. Smith, An acidity scale for binary oxides, J. Chem. Educ. 64 (1987) 480–481. https://doi.org/10.1021/ed064p480.
[3] N.H. Menzler, F. Tietz, M. Bram, lzaak C. Vinke, L.G.J. (Bert) de Haart, Degradation Phenomena in SOFCs with metallic interconnects, in: P. Singh, N.P. Bansal (Eds.), Adv. Solid Oxide Fuel Cells IV, The American Ceramic Society, 2008: pp. 93–104.
[4] T.H. Wan, M. Saccoccio, C. Chen, F. Ciucci, Influence of the Discretization Methods on the Distribution of Relaxation Times Deconvolution: Implementing Radial Basis Functions with DRTtools, Electrochim. Acta. 184 (2015) 483–499. https://doi.org/10.1016/j.electacta.2015.09.097.
[5] Y. Niu, Y. Zhou, W. Lv, Y. Chen, Y. Zhang, W. Zhang, Z. Luo, N. Kane, Y. Ding, L. Soule, Y. Liu, W. He, M. Liu, Enhancing Oxygen Reduction Activity and Cr Tolerance of Solid Oxide Fuel Cell Cathodes by a Multiphase Catalyst Coating, Adv. Funct. Mater. 2100034 (2021) 1–11. https://doi.org/10.1002/adfm.202100034.

Understanding hydrogen transport is vital to a multitude of application spaces, including energy storage and corrosion mitigation. However, knowledge of the physical diffusion pathways of hydrogen is limited, with a serious gap existing with regards to how hydrogen transport is affected by the host material’s underlying atomic structure and defects. For the case of polycrystalline materials, answering this question is non-trivial, as diffusion can occur through many structurally unique environments: (1) bulk regions (2) grain boundaries, (3) surfaces, etc. In this work we have chosen to study hydrogen diffusion through titania due to its importance in corrosion mitigation and energy storage. In particular, we aim to understand diffusion through titania grain boundaries, approximated by using amorphous phases due to similarities in their short-range atomic order. Density functional theory was used to generate these amorphous configurations via ab initio molecular dynamics. A spectrum of hydrogen binding energies was then calculated using a multitude of oxygen sites with varying coordination number. Classical molecular dynamics were then performed on larger amorphous systems using a machine learning force field. A graph-based order parameter was then used to characterize the classically derived amorphous phase space which was correlated with oxygen coordination number. Our results qualitatively indicate that hydrogen diffusivity can be tailored to either inhibit or induce diffusion based on the specific local oxygen environments present in the amorphous phase.

MXenes possess attractive hydrophilicity, high conductivity, and durability for application in electrocatalysis. Tailored Ti₃C₂ has been actively employed as active support candidates in Pt-loaded catalytic systems to drive the hydrogen evolution reaction (HER). It is noted that the effect of diverse surface functional groups, which may play an important role to optimize the interaction with loaded Pt nanoparticles, has not been deeply investigated to date. 3D wrinkled Ti₃C₂(OH)_x and Ti₃C₂O_x were prepared after the alkali-induced process. 3D MXene with high concentration of -F groups (Ti₃C₂F_x) were prepared by mixing with HF. The incorporation of different functional groups was confirmed by FT-IR spectroscopy. Collected XRD patterns of the MXene powders revealed that NaOH-based and subsequent treatments did not induce the formation of distinct phases. It can be clearly observed that all MXene have same structure even after subsequent treatments from evidenced by SEM and TEM images. Moreover, there is no substantial change in defect ratio. Specific electrochemical properties were obtained due to different termination groups on the surface of Ti₃C₂. In this work we direct our attention to the effect of these surface terminations on the resulting interactions with Pt to improve the electrocatalytic activity of Pt-loaded Ti₃C₂ materials toward HER.

Numerous studies on oxide electrocatalysts for an efficient oxygen evolution reaction have been conducted to compare their catalytic performance and suggest new compositions. However, two significant constraints have been overlooked: One is the difference in electrical conductivity between catalysts and the other is the strong crystallographic surface orientation dependence of the catalysis in a crystal. Consequently, unless a comprehensive comparison of the oxygen-evolution catalytic activity between samples is made on a crystallographically identical surface with sufficient electron conduction, misleading interpretations on the catalytic performance and mechanism may be unavoidable. To overcome these limitations, we utilize both metallic (001) LaNiO₃ epitaxial thin films together with metal dopants and semiconducting (001) LaCoO₃ epitaxial thin films supported with a conductive interlayer. We identify that Fe, Cr, and Al are beneficial to enhance the catalysis in LaNiO₃ although their perovskite counterparts, LaFeO₃, LaCrO₃, and LaAlO₃, with a large bandgap, are inactive. Furthermore, semiconducting LaCoO₃ is found to have more than one order higher activity than metallic LaNiO₃, in contrast to previous reports. Showing the importance of facilitating electron conduction, our work highlights the impact of the near-Fermi-level d-orbital states on the oxygen-evolution catalysis performance in perovskite oxides.

Oxide semiconductors have been widely studied as photoanodes for solar water splitting to generate clean hydrogen as they are in principle inexpensive and stable [1]. To improve their solar-to-hydrogen efficiency, integration of photoanodes with suitable cocatalysts is an attractive strategy to lower reaction barriers thus boosting the photoelectrochemical water splitting ability [2-4]. Very recently, atomically-dispersed catalysts have gained significant attention for integration with semiconductors for solar-to-fuel conversion, due to their high atom utilization, superior catalytic activity, tunable physicochemical properties, and clear active sites [5, 6]. For example, atomically-dispersed iridium catalysts with superior oxygen evolution catalytic performance, have been demonstrated working efficiently on oxide semiconductors for solar water splitting [7, 8]. As the interest in atomically-dispersed catalysts on photoanodes is just rising since 2010, although the phenomenological advantages of pairing atomically-dispersed catalysts with semiconductors are now widely confirmed, the precise role of such cocatalysts, i.e. acting as a catalyst or playing a non-catalytic role on photoanodes, which is essential for future efficient photoanode design, are far from understood.

Haematite (α-Fe₂O₃) has long been targeted as a promising oxide semiconductor for solar water splitting in the photoelectrochemical (PEC) configuration due to its natural abundance, narrow bandgap to capture visible light, and excellent chemical stability. With the advances in improving the fundamental deficiencies of haematite over the past few decades, haematite is currently the leading performer among oxide photoanodes. Also, haematite serves as a prototype oxide photoanode in many ways. Therefore, in this research, atomically-dispersed iridium catalyst with excellent oxygen evolution catalytic activity was firstly synthesized followed by loading on hematite photoanodes. The photoelectrochemical measurements have shown a remarkable cathodic shift of the turn-on voltage and a significant increase in photocurrent. The detailed mechanism through which the atomically-dispersed iridium catalyst enhanced the photoelectrochemical performance of hematite was thoroughly investigated by in-situ transient absorption spectroscopy, in-situ Raman spectroscopy, and in-situ extended X-ray absorption fine structure analysis along with density-functional theory calculations. By doing this, we hope to unravel the mechanisms involved in the higher activity of atomically-dispersed iridium on hematite for the first time, and hence, to provide a guide to employ such cocatalysts for integrated solar-to-fuel conversion.

References
[1] Sivula, K., van de Krol, R. Nat Rev Mater 1, 15010 (2016).
[2] Montoya, J., Seitz, L., Chakthranont, P. et al. Nature Mater 16, 70–81 (2017).
[3] Nellist, M. R., Laskowski, F. A. L., Lin, F., Mills, T. J. & Boettcher, S. W. Acc. Chem. Res. 49, 733-740 (2016).
[4] Kuang, Y., Yamada, T. & Domen, K. Joule 1, 290-305 (2017).
[5] Yang, X.-F., Wang, A. Q., Zhang, T. et al. Acc. Chem. Res. 46, 1740-1748, (2013).
[6] Zhang, B. Sun, L. Chem Soc Rev 48, 2216-2264, (2019).
[7]Cui, C., Heggen, M., Zabka, WD. et al. Nat Commun 8, 1341 (2017).
[8] Li, W., Sheehan, S. W., Wang, D. W. et al. Angew.Chem. 54, 11428-11432, (2015).
[9] Kay, A., Cesar, I. & Grätzel, M. J. Am. Chem. Soc.128, 15714–15721 (2006).

Dealloyed 3D nanoporous materials with bicontinuous open porous structure, large specific surface area, high electric conductivity, and superior catalytic activity are ideal monolithic electrodes for electrochemical devices. Molybdenum-based intermetallic compounds have been considered as promising low-cost alternatives to noble platinum for the electrocatalytic hydrogen production, but they have rarely been developed into nanoporous structures due to the challenges in controlling the composition and chemical order during low-temperature chemical dealloying. Here, we report the direct fabrication of nanoporous Mo-based intermetallic compounds of Co₇Mo₆ and Fe₇Mo₆ by a liquid metal dealloying method at elevated temperatures. An unexpected derivation of the characteristic length of dealloyed nanoporous structures from the conventional linear trend in “ligament diameter versus inverse homologous temperature (1/T_H = T_m/T_dealloy)” is discovered, suggesting a unique intermetallic effect on the pore formation. Assisted by molecular dynamics simulation, this intermetallic effect is explained by the suppressed surface diffusivity of the ordered μ phases that retards the thermal coarsening. As a result, the nanoporous Co₇Mo₆ with a fine structure and correspondingly large surface area delivers excellent activity toward the electrochemical hydrogen production, together with high durability. This study sheds light on a previously unexplored intermetallic effect in dealloying and would promote the development of advanced nanoporous electrocatalysts for energy applications.

Recently, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a widely used as an active component for water-related electrochemical catalysis and photocatalysis due to its decent electrical conductivity, high optical transparency, and good mechanical flexibility. Nonetheless, its hygroscopic nature, intrinsic inhomogeneity, and poor catalytic activity have limited its broad impact on electrochemical catalyst research. In this research we developed the PEDOT:PSS-platinum nanocomposite electrode by electroplating platinum nanoparticles within the crystalline PEDOT:PSS nanofibrillar matrix and demonstrated its electrochemical catalytic function for hydrogen evolution reaction. We systematically investigated optical, structural, electrical, and electrochemical properties of crystalline PEDOT:PSS-platinum nanoparticle composites. The resultant nanocomposite electrode showed the uniform impregnation of platinum nanoparticles into the whole volume of PEDOT:PSS films, leading to the volumetric electrochemical catalyst with high aqueous stability, high reactant permeability, large electrochemically active surface area (20 m²/g_pt) and enhanced catalytic activity for HER (overpotential of 54.7 mV at 10 mA/cm²) compared to that of Pt nanoparticle-tethered electrode without PEDOT:PSS.

In recent years A₂B₇-type RE-Mg-Ni hydrogen storage alloys have been investigated to replace the conventional AB₅-type negative electrodes due to the higher storage capacity (~400 mAh/g vs a theoretical capacity of 372 mAh/g for AB₅). Due to the high Mg vapour pressure, the exact composition of these A₂B₇ materials is challenging to control during production. At the same time, Mg is easily volatilized into ultra-fine powder particles, thus becoming a safety hazard. As a result, new production techniques have been developed. However, these are either expensive or complicated to reproduce [1]. Thus another option is to produce Mg free compositions. By replacing the Mg element with Y, a similar capacity (392.9 mAh/g) is found for LaY₂Ni₁₀Mn_0.5 [2]. Controlled studies into how Y is affecting the electrochemical capacity, stability and phase composition have been performed by Liu et al. [3] and Zhao et al. [4]. However, the effects on the gas storage properties are not well understood. In the current study, the effects of replacing La with Y on the thermodynamic and kinetic properties of La_6-xY_xNi_19.5MnAl_0.5 (x = 3, 4, 5) are presented.
1. Yan, H.; Xiong, W.; Wang, L.; Li, B.; Li, J.; Zhao, X. Investigations on AB3-, A2B7- and A5B19-type La Y Ni system hydrogen storage alloys. Int. J. Hydrog. Energy, 2017, Vol 42, 2257-2264. doi:10.1016/j.ijhydene.2016.09.049
2. Xiong, W.; Yan, H.; Wang, L.; Verbetsky, V.; Zhao, X.; Mitrokhin, S.; Li, B.; Li, J.; Wang, Y. Characteristics of A2B7-type La Y Ni-based hydrogen storage alloys modified by partially substituting Ni with Mn. Int. J. Hydrog. Energy, 2017, Vol 42, 10131-10141. doi:10.1016/j.ijhydene.2017.01.080
3. Liu, Y.; Yuan, H.; Guo, M.; Jiang, L. Effect of Y element on cyclic stability of A2B7 -type La–Y–Ni-based hydrogen storage alloy. Int. J. Hydrog. Energy, 2019, Vol 44, 22064-22073. doi:10.1016/j.ijhydene.2019.06.081
4. Zhao, L.; Luo, Y.; Deng, A.; Jiang, W. Hydrogen Storage and Electrochemical Properties of the Mg-free A2B7-type La1-xYxNi3.25Mn0.15Al0.1 Alloys with Superlattice Structure. Chemical Journal of Chinese Universities, 2018, Vol 39, 1993-2002. doi:10.7503/cjcu20180169

High-throughput experimentation (HTE) generates large, comprehensive datasets with reasonable efforts. In combination with efficient electrocatalytic screening, the corresponding datasets produced by HTE have a huge potential to generate improved understanding of the governing principles that drive electrocatalytic activity in compositionally complex alloys. The challenge is to identify viable combinations of elements and their mixing ratios out of millions of possible combinations and discover new electrocatalysts with advanced properties.

In this contribution, the electrochemical activity data of > 40 sputtered composition spread materials libraries are analyzed using statistical methods and machine learning. The dataset spans > 15000 different chemical compositions distributed in > 15 compositionally complex material systems from combinations of > 25 chemical elements. Electrochemical characterization was performed using a scanning droplet cell. The electrochemical dataset contains > 20 000 linear sweep voltammograms measured by the scanning droplet cell and covers relevant potential ranges for the hydrogen evolution reaction, the oxygen reduction reaction and the oxygen evolution reaction.

The results of statistical analysis and preliminary predictive machine learning models highlight the value of large experimental datasets generated by HTE for the data-driven development of high-performance catalysts.

Now a days agriculture waste are very challenging as most of the residues burn, which deteriorate the environment directly. The conversion of agricultural waste into energy has twin benefits for society - utilization of waste and sustainable energy production. In this regard, catalytic steam reforming (SR) of bio-oil produces hydrogen, which is a clean and green source of energy having heating value 2.5 times to the petroleum products. In recent years, perovskite oxides have drawn tremendous attention for green hydrogen production due to their excellent catalytic properties. Reported study suggests that the presence of oxygen vacancies of A atoms and ability to break carbon-carbon and carbon-hydrogen bonds of B-site atom make it suitable for steam reforming processes. Here, we report a series of LaNi_xCo_1-xO₃ perovskite catalysts for the SR of a mixture of bio-oil model oxygenates to produce green hydrogen. The effect of variation of reaction temperature, Ni and Co species composition, steam to carbon molar ratio (SCMR), and weight hourly space-time (WHST) on gaseous products yield and bio-oil conversion were evaluated using fixed catalytic bed reactor unit. The LaNi_0.5Co_0.5O₃ exhibited the best catalytic activity (83% hydrogen yield and 95% conversion) towards hydrogen production among all other combinations of LaNi_xCo_1-xO₃ catalysts. The catalyst deactivation study revealed that the deactivation was primarily due to two types of coke (amorphous and carbon nanotubes) deposition validate by the thermogravimetric analysis.

Designing a catalyst with high efficiency toward oxygen evolution reactions (OERs) through sea water splitting is a promising factor in recent times for hydrogen production from renewable sources which can consequently contribute towards energy sustainability. Seawater electrolysis possess a challenge of suppressing chlorine evolution reaction (CER), which leads to the undesirable formation of chlorine gas, while enhancing OER. In the current investigation, Nickel Telluride based electrocatalyst was synthesized through electrodeposition process directly on gold-coated glass substrate. When employed as an OER catalyst, the nickel telluride electrode exhibits a current density of 10 mA/ cm² at an OER overpotential of 180 mV with superior stability in a highly alkaline electrolyte which replicates original unfiltered seawater. More importantly, it was observed that nickel telluride did not lead to generation of Cl₂ through CER in simulated seawater electrolyte. To comprehend the admirable OER activity of the Nickel Telluride electrode, a Tafel plot was derived from the LSV curves. The Tafel slope was found to be 56 mV dec⁻¹, which surpass the revolutionary RuO₂ catalyst at 114.4 mV dec⁻¹, indicating superior kinetic performance of nickel telluride. The long term OER stability of the nickel telluride electrocatalyst in artificial seawater was confirmed through surface-sensitive analytical approaches such as XPS. The stability was reestablished through high resolution surface imaging techniques such as SEM and TEM, which confirms that the catalyst surface did not undergo any deterioration, or compositional change. The oxygen and hydrogen gas evolution for Nickel Telluride catalyst was collected over an electrode surface area of 0.5 cm² at a working potential of 0.6 V over a duration of 2 hours. The ratio between the hydrogen and oxygen gas evolved was 1.82:1 which is very close to the 2:1 ratio for electrocatalytic full water electrolysis. Nickel telluride was hence identified as an ideal catalyst to promote OER through seawater electrolysis with low overpotential and high stability while suppressing generation of toxic chlorine gas.

Photoelectrochemical water splitting (PEC) is a compelling approach to convert solar energy to chemical energy by liberating hydrogen, making it a possible solution to the worldwide energy crisis by eliminating all carbon dioxide emissions. In practice, no single electrode material can meet all PEC water splitting requirements. The major challenges for PEC are electron-hole pair recombination, stability, and reduced absorption of visible light. Transition Metal dichalcogenides (TMD) based heterostructures are being envisioned as novel electrode materials because of their exceptional electrical, chemical, mechanical, and thermal stability which include high electron mobility, highly exposed active sites, layer dependent bandgap, and higher ionic conductivity, etc. TMDs have higher electro conductivity resulting in significant enhancement of the electrochemical performance in PEC. Further, it also exhibits desirable electrochemical behavior through its oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) as a photoelectrode for water splitting. However, due to poor absorbance and significant reflection at the grain boundaries, the light-harvesting ability of bulk or nanosized TMD materials is often limited. The charge carriers produced by light illumination will also reach the surface faster than bulk materials in the semiconductor, because of the lesser transport distance.
In the present work, heterostructures consisting of Tungsten disulfide (WS₂) and cadmium sulfide (CdS) were synthesized using a hydrothermal method and Successive Ionic Layer Adsorption Reaction (SILAR) deposition method. The presence of these nanostructures was confirmed by XRD, Raman, UV-Visible spectroscopy, SEM, TEM, and PL spectroscopy. The morphological study clearly indicates a homogeneous distribution of CdS nanospheres on WS₂ nanostructures, indicating close contact between them which helps in efficient charge separation and migration. Further, the continuous nature of the heterostructures of the two materials is not limited to any visible discrete entities. The porosity of the pristine WS₂ nanostructures allows CdS nanospheres to deposit inside the pores along with the entire thickness of the nanosphere, and hence, the highly homogeneous nature of the heterostructures is obtained as indicated by the physical characterization. Due to this unique structure, the obtained heterostructures showed excellent performance as photoanode. The CdS@WS₂ photoanode demonstrated a photocurrent density of 0.15 mA cm⁻² at 1.23 V vs. RHE, which is over 20 times higher than that of the pure pristine materials. Further, CdS@WS₂ heterostructures, as compared to pure materials, have demonstrated longer emission-decay-life times and dramatically quenched the emission of fluorescence compared to pristine materials. In CdS@WS₂ heterostructures, the absorption edge of the WS₂ nanostructures also exhibits a considerable increase in light absorption in the total visible spectrum. This results in enhanced charge separation with proper band alignment leading to improvements in electrochemical performance of CdS@WS₂ photoelectrodes. Further, the low onset potential further eliminates the need for voltage bias in practical applications.


References:

[1] Peng, W.; Li, Y.; Zhang, F.; Zhang, G.; Fan, X. Roles of Two-Dimensional Transition Metal Dichalcogenides as Cocatalysts in Photocatalytic Hydrogen Evolution and Environmental Remediation. Ind. Eng. Chem. Res. 2017, 56, 4611-4626 https://doi.org/10.1021/acs.iecr.7b00371.

[2] Faraji, M.; Yousefi, M.; Yousefzadeh, S.; Zirak, M.; Naseri, N.; Hwa, T. Two-Dimensional Materials in Semiconductor Photoelectrocatalytic Systems for Water Splitting. Energy. Environ. Sci. 2019, 12, 59-95. https://doi.org 10.1039/c8ee00886h.

Hydrogen (H₂) is often seen as a cleaner alternative to conventional fossil fuels with zero carbon emission. Its usage, however, is limited by its lower explosion limit (4%) and high permeability through many materials that make storage and handling of H₂ a challenging task. Hence, the development of low-cost, fast, and sensitive H₂ detection technologies is of paramount importance to the development of the future hydrogen-based economy. The United States Department of Energy (US-DOE) has set a target for the cost of H₂ sensors for fuel-cell-powered vehicles to be <$15 including the cost of electronics. Paper is an inexpensive, flexible, porous, adaptable, environmentally-friendly, and degradable material that has huge potential for the development of sustainable electronics. We report an ultra-low-cost paper-based sensor for room-temperature detection of hydrogen (H₂) using ultrathin palladium nanowires (PdNWs) and prepared by a simple drop-casting method. To gain insight into the sensing mechanism of ultrathin PdNWs, we compare the performance of polycrystalline PdNWs to that of single-crystalline PdNWs. The polycrystalline PdNWs showed a response (fractional resistance change) of 4.3% to 1% H₂ with response and recovery times of 4.9 s and 10.6 s, respectively. Similarly, the single-crystalline PdNWs showed an 8% response to 1% H₂ with slightly slower response and recovery times of 9.3 s and 13.0 s, respectively. The polycrystalline PdNWs show excellent selectivity to H₂ with a limit of detection of 10 ppm of H₂ in air and good stability under repeated exposure to H₂ and after extended storage in air. We attribute the fast response and recovery times of the ultrathin polycrystalline PdNW-based sensors to synergistic effects of their ultrathin (< 5 nm) diameter, strain-coupled grain boundaries, and the porous paper substrate that facilitates rapid transport to exposed PdNWs. This paper-based sensor, using only ~100 μg Pd per device on a substrate of negligible cost, is one of the fastest chemiresistive H₂ sensors and could be orders of magnitude less expensive than the current state-of-the-art H₂ sensing solutions.

There is a strong requirement to establish a hydrogen energy system with the aim of realizing the sustainable society. Highly efficient solid oxide fuel cells (SOFCs) are capable of supplying both heat and electricity without precious catalytic metals, utilizing various fuels such as natural gas, ammonia, and biogas. SOFCs are expected to operate at low temperatures. Reduction of the operation temperature by couple of hundred degrees from conventional >800°C would expand options of surrounding materials and structures, that contributes to improved performances and cost-effective development of the systems. Thus, it has been of importance to improve conductivity of cathode materials to advance the low-temperature operation efficiency. Meanwhile, lanthanum nickelates La_n+1Ni_nO_3n+1 (n=1, 2, and 3) with layered perovskite structures, such as La₂NiO₄, have been developed and their mixed oxygen-ion/electron conductivity has been researched as an expected property for the cathode application in SOFCs [1–3]. Although, the electronic transport properties of the higher order (n≥2) phases still require further study to understand its correlation with their structures. It is also of significance to find an effect of impurity doping on modification of the structure and electronic properties. In this study, La₄Ni₃O₁₀ (n=3) thin films were epitaxially developed on single crystal substrates, and the effect of tetravalent doping on their structure and properties was investigated.
The pure and (Hf⁴⁺, Sn⁴⁺) co-doped lanthanum nickelate thin films were grown on single crystal NdGaO₃ (110) substrates by pulsed laser deposition (PLD) method using a KrF excimer laser (λ=248 nm) and sintered targets of pure and co-doped La₄Ni₃O₁₀ (0.625 at% for each dopant). The fluence and repetition of the laser beam were E~1.0 J/cm² and f=3 Hz, respectively. Deposition ambience was 10 Pa O₂-flow (base pressure ~3×10^–6 Pa), and the substrate temperature was kept at 700°C. The grown lanthanum nickelate thin films were subsequently heat treated in atmospheric O₂-flow at 950°C for 10 hours. The epitaxy of (Hf⁴⁺, Sn⁴⁺) co-doped La₄Ni₃O₁₀ (001) thin films on NdGaO₃ (110) substrates were obtained after the deposition, that XRD analyses using monochromated CuKα1-ray demonstrated {001} diffraction peaks in 2θ/ω-scan and planar four-fold symmetry in φ-scan detecting {208} diffractions. The co-doped epitaxial La₄Ni₃O₁₀ (001) thin film revealed resistivity of ρ=8.9×10^–2 Ωcm at room-temperature, measured by four-probe DC method. Further development of La_n+1Ni_nO_3n+1 crystal phases other than the case of n=3 was available by modification of the O₂ pressure and deposition rate during the thin film growth. Further detailed structural analyses, physical properties including electric conductivity at raised temperature, and comparative study with pure films will also be presented.
[1] S. Yoo et al., RSC Advances, 2 (2012) 4648–4655.
[2] R. Sayers et al., Solid State Ionics, 192 (2011) 531–534.
[3] Y. Gong et al., JOM, 71 (2019) 3848–3858.

Solar water splitting converting the solar energy into green hydrogen fuel is one significant milestone on the road to a sustainable energy future and has driven many researches in the past years [1]. III-V/Si heterostructure combining the good optical properties of III-V semiconductors with the low cost of Si substrates is considered as an attractive solution for efficient water splitting reactions [2-4].
Here, we first show that surface and interface energies play a central role during the epitaxy of III-V semiconductors on Si. We then demonstrate that the presence of 2D vertical singularities (antiphase boundaries) drastically modify the electronic properties of the heterostructure, yielding very promising photo-electro-chemical (PEC) device performances. The strong absorption of light by the bulk materials is the first requirement for the proper operation of the PEC. Next, it is also essential to efficiently extract and collect the carriers. Therefore, the atomistic studies based on the density functional theory have been dedicated to three different aspects: i) the optoelectronic properties of bulk materials, ii) the study of surface stability to understand III-V/Si heteroepitaxy, and iii) the study of interfaces and 2D vertical singularities used to collect charge carriers.

References:
[1] A. J. Bard and M. A. Fox, “Artificial photosynthesis: solar splitting of water to hydrogen and oxygen”, Accounts of Chemical Research, 28(3), 1995.
[2] I. Lucci et al., “A Stress-Free and Textured GaP Template on Silicon for Solar Water Splitting”, Advanced Functional Materials, 28(30):1801585, 2018.
[3] M. Alqahtani et al., “Photoelectrochemical water oxidation of GaP1− xSbx with a direct band gap of 1.65 eV for full spectrum solar energy harvesting”, Sustainable Energy & Fuels 3, 2019.
[4] I. Lucci et al., “Universal description of III-V/Si epitaxial growth processes” Physical Review Materials 2 (6), 060401, 2018

Direct solar-driven water splitting into hydrogen (H₂) and oxygen (O₂) has emerged as a leading approach to reduce our dependence on fossil fuels as hydrogen can be converted into electrical energy using a fuel cell or transformed into chemical feedstocks. However, the identification of ideal light harvesting semiconductors that meet performance and cost requirements for industrialization still remains a main obstacle. Organic photoelectrochemical cells (OPECs) in which pi-conjugated organic semiconductors (OSs) are coupled with co-catalysts have recently attracted great attention as alternative photoelectrodes for solar water reduction (yielding hydrogen) and oxidation (yielding oxygen), considering unique features of OSs such as precisely tunable optoelectrical properties and solution-processability at low temperature. Nevertheless, the conversion efficiency and stability of OPECs (both photocathode and photoanode) for the solar water splitting have remained particularly poor. Herein, we present high-performance and robust organic photoelectrochemical cells by employing a bulk heterojunction (BHJ) blend of semiconducting polymers as a photoactive layer. Our in-depth study using sacrificial agents for photoreduction and photooxidation unveils critical parameters that significantly affect the performance and operational stability of OPECs: (i) rational selection of semiconducting polymer donor and acceptor to generate free charges efficiently and ensure chemical stability upon illumination, (ii) large surface roughness of interlayers to improve interfacial adhesion, and (iii) mitigation of charge accumulation at the interfaces. By leveraging these insights, our optimized polymer BHJ photocathode and photoanode where the polymer BHJ layers are coupled with hydrogen and oxygen evolution catalysts, respectively, show outstanding performance and unprecedented robustness compared to previous OPECs, demonstrating a new benchmark of OPECs for solar fuel production. In particular, the polymer BHJ photocathode and photoanode exhibit initial 1 Sun photocurrent density values up to 8.7 mA cm⁻² at 0 V vs RHE and 2.3 mA cm⁻² at 1.23 V vs RHE for solar water reduction and oxidation with 100% Faradaic efficiency, respectively, which can rival the performance of their inorganic counterparts. More importantly, further engineering of charge transport layer to establish stability of polymer BHJ photocathodes in wide range of pH leads to the first demonstration of an organic tandem photoelectrochemical cell for complete solar water splitting in the same electrolyte in the absence of a membrane, which is a considerable advance in the field of OPECs. Consequently, our work coupled with the facile solution processability of the semiconducting polymers establishes the use of polymer BHJs as a promising path towards efficient, scalable, and economical photoelectrochemical cells for overall solar water splitting.

References
Han-Hee Cho et al. Nat. Catal. 2021, 4, 431–438
Liang Yao et al. J. Am. Chem. Soc. 2020, 142, 17, 7795–7802

Two dimensional materials, such as the transition metal dichalcogenides (TMDCs) are a good candidate for water splitting catalysts [1,2], as they often have larger band gaps than their bulk counterparts. However, this had to be balanced by the thin layers having a small absorption cross section and difficulties in mounting on a suitable substrate. PdSe₂ is being suggested as a potential water splitting candidate [3]. However its bulk band gap is too small for water splitting [4]. We propose to use this structure as a surface coating to a second TMDC with a larger band gap such as MoS₂ and use this as an example of how such heterostructures could function.
Using density functional theory, implemented in the Vienna Ab-initio Simulation Package, we have investigated the surfaces of TMDC monolayers MoS₂ and PdSe₂, and a Hetero-bilayer of the two, for their potential application as photocatalytic water splitters. The different functional groups involved in the Hydrogen and Oxygen evolution reactions have been added to the monolayers and the hetero-bilayer to determine their energetics. In addition to this, we have looked at how stable these materials are, to both adsorptions and substitutions, in both air and water environments.
References:
[1] Qing Tang and De En Jiang. ACS Catalysis, 6(8):4953–4961, Aug 2016.
[2] B. Amin, et al . Phys. Rev. B, 92:075439, Aug 2015.
[3] C. Long, et al . ACS Appl. Energy Mater., 2, 1, 513-520, 2019.
[4] G. Zhang, et al . Appl. Phys. Lett., 114, 253102, June 2019.

For conducting localized photoelectrochemical analysis of photoanode, several in-situ electrochemical techniques have emerged. Scanning electrochemical microscopy (SECM) is one of the methods by which localized properties of the surface are analyzed with the help of a microelectrode probe. We present here a highly resolved photoelectrochemical property of α-Fe₂O₃ surface through a new and more straightforward approach in SECM set-up. Optical microscope objective is used to illuminate a localized surface from the bottom of the sample and a micro-electrode probe scan the sample from the top. The surface generation and tip collection mode of SECM is used to detect locally products of photoelectrochemical reaction at the sample. Recorded faradaic current is directly correlated to the oxygen evolution quantitatively on the surface of photoanode as the influence of light in terms of oxygen flux. Both qualitative and quantitative information of oxygen evolution will open new gateways in the field of photoelectrochemistry to understand the influence of dopants and hole scavengers by an easy and conventional approach.

Surface modifications and substrate effect growth of photoanode for heterojunction ZnO/In₂S₃ are achieved in this paper. 2D nanolayers of In₂S₃ and 3D ZnO have been synthesized using a two-step process physical vapor deposition (PVD) for ZnO, followed by chemical vapor deposition (CVD) for 2D layers of In₂S₃ for PEC application. Enhanced photocurrent density in heterojunction ZnO/In₂S₃ is reported due to uniformity in film growth, increased surface area, and efficient electron-hole separation at the heterojunction interface. A photocurrent density of 2.4 mAcm^-2 is produced for ZnO/In₂S₃ heterojunction, which is 2.5 and 1.4 times higher than pristine ZnO and In₂S₃ at 1.23V (vs. RHE). Transient photocurrent density plots at different wavelengths confirm that the enhancement in photoelectrical response of the interface at lower energy (2.4 eV) compared to ZnO (3.1 eV) is due to absorption in the In₂S₃ layers with energy 2.3 eV. The present approach can be extended to other heterojunctions with 2D materials having different optical and electronic properties.

Significant progress has been reported in solar water splitting, with solar-to-hydrogen (STH) efficiency as high as 30% already demonstrated. However, two major challenges remain. First, these high-efficiency devices (> 15%) have only been achieved using expensive and non-scalable III-V semiconductors. Low-cost metal-oxide based devices, mainly using BiVO₄ as the absorber, have only achieved STH efficiency of < 10%. Due to stability limitations, many of these metal-oxide based devices are operated in near-neutral pH electrolytes, which poses an additional mass transport challenge. Second, the majority of the demonstrated devices are still at the laboratory scale. Reports on large-area devices are limited, and they typically show much lower efficiencies. This is best illustrated in a recent review:^[1] even when III-V semiconductor-based devices are considered, there is no report of devices with a semiconductor absorber area larger than 10 cm² and STH efficiency > 10%. In this talk, we will discuss the scale-up of our photoelectrochemical water splitting devices based on a complex metal oxide absorber. Factors other than the semiconductor itself are found to be responsible for a total voltage loss of > 500 mV and therefore limit the overall performance of the large-area device.^[2] We use a combination of multiphase multiphysics simulations (2-D and 3-D finite element analysis models) and validation experiments (e.g., in-situ pH fluorescence, particle image velocimetry, shadowgraphy) to quantify and breakdown the different loss mechanisms (substrate and electrolyte Ohmic loss, concentration overpotentials, product crossover, bubble-related losses) while scaling-up photoelectrochemical water splitting devices.^[3-6] Based on these insights, electrochemical engineering strategies to overcome the scale-up related losses are offered.

References
1. J. H. Kim et al., Chem. Soc. Rev. 48, 2019, 1908
2. I. Y. Ahmet et al., Sust. Energy Fuels 3, 2019, 2366
3. F. F. Abdi et al. Sust. Energy Fuels 4, 2020, 2734
4. K. Obata et al. Energy Environ. Sci. 13, 2020, 5104
5. K. Obata et al. Cell Rep. Phys. Sci. 2, 2021, 100358
6. K. Obata & F. F. Abdi, Sust. Energy Fuels, 2021. DOI: 10.1039/D1SE00679G

Tungsten oxide (WO₃) is a technologically relevant earth abundant material known for its unique semiconducting properties. It has been extensively explored as a photoanode material for photoelectrochemical water splitting due to its broad spectrum light absorption (band gap ~2.7 eV) and strong oxidative properties. While the WO₃ photoanodes can potentially yield an efficiency close to 5%, under Air Mass 1.5 (1 Sun) illumination, the reported values are significantly lower. The problems are primarily caused by low film thickness causing insufficient light absorption, low surface area or inefficient charge separation/transfer or a combination of these. An array structure consisting of microns long nanotubes can address these issues; however, efforts in developing such an ideal structure, especially using scalable economical processes such as anodic oxidation, have been very challenging. We have overcome these challenges and discovered the synthesis conditions required for WO₃ nanotube array architecture formation via anodic oxidation. Pristine WO₃ nanotubes were amorphous, but the material transformed into monoclinic WO₃ with heat treatment at 500 °C in oxygen environment. The photocurrent of our champion device was 2 mA.cm^-2 with incident photon to current conversion efficiency (IPCE) about 70 % in the ultraviolet region of the solar spectrum under 1 Sun illumination. In this presentation, we will discuss the influence of fabrication conditions on the nanotube formation and exceptional semiconducting properties of the material in relation with photoelectrochemical water splitting.

High entropy alloys (HEA) comprising four or more constituent elements, are an interesting new class of materials with tunable chemical composition providing the basis for catalytic or electrocatalytic activity towards many reactions. The high electrocatalytic activity of HEA is related to an abundance of active sites, resulting from the statistical distribution of the elements on the surface. With noble metal prices on the rise, noble-metal-free HEA have the potential to decrease the cost for electrocatalysts in various energy conversions technologies.

The challenge involved with both, the experimental and computational exploration of HEA, lies in the enormous number of possible combinations of chemical elements and their mixing ratios, leading to a huge option space for the development of new catalysts. With millions of possible element combinations and roughly 10⁴ possible mixing ratios for each material system, fundamentally new research strategies are required.

In this contribution, a high-throughput experimentation strategy is reported, enabling rapid synthesis and screening of possible HEA catalysts materials for water splitting and fuel cells. Combinatorial co-deposition using magnetron sputtering is applied to cover substantial parts of the total composition space of quinary HEA material systems in materials libraries. Electrochemical high-throughput screening is applied to map the electrocatalytic activity over the chemical compositions.

It is demonstrated that within a HEA material system, the electrochemical activity can be increased significantly (e.g. by a factor of 10 as demonstrated in this contribution) by adjusting the mixing ratios of individual elements. The large electrochemical datasets generated by this approach enable the use of artificial intelligence methods (e.g. supervised and unsupervised) for the optimization of chemical composition and synthesis parameters with an enormous potential for understanding the driving forces of electrocatalytic activity and predicting advanced materials.

Photoelectrochemical (PEC) approaches for the processing of solar fuels and materials are interesting, provided they can be efficiently, stably, scalably, and sustainably implemented. One way to increase the economic and sustainable competitiveness is the utilization of concentrated irradiation [1]. This pathway also allows for the thermal integration of the photoabsorbing and electrocatalytically active components, benefiting the performance as a whole [2]. Scaling of such a thermally integrated photo-electrochemical system keeps the interesting thermal dynamics while also indicating that co-generation of hydrogen and heat increase overall system efficiency [3].
I will discuss material challenges for PEC devices in terms of efficiency and longevity at the laboratory scale, specifically focusing on the unique challenges related to the utilization of concentrated radiation and thermal integration. Additionally, the transition from the laboratory scale (at ~100W power) to the demonstration scale (at ~10kW power) requires tackling alternative challenges related to different approaches for component integration and the design of a complete system, including the incorporation of fitting auxiliary components. I will discuss these integrational challenges at the larger scale, while also discussing requirements for the complete solar processing plant design, both informed by our recent large-scale outdoor and on-sun demonstration activities.


References
[1] M. Dumortier, S. Tembhurne, S. Haussener, Energy Environ. Sci., 8:3614–3628, 2015.
[2] S. Tembhurne, F. Nandjou, S. Haussener, Nature Energy, 10.1038/s41560-019-0373-7, 2019.
[3] I. Holmes-Gentle, S. Tembhurne, C. Suter, S. Haussener, Int. Jou. of Hydrogen Energy, 10.1016/j.ijhydene.2020.12.151, 2020.

Bismuth vanadate (BiVO₄) is a promising photoanode material; however, its efficiency significantly changes depending on the atomic ratio of Bi/V, and there is no suitable method for synthesizing large-area photoanodes. In this study, an efficient BiVO₄ photoanode was fabricated via sputtering, by manipulating the molar ratio of Bi/V with V solution annealing. V solution annealing not only adjusted the atomic ratio of Bi/V but also increased the number of O vacancies, thereby improving the charge-separation and charge-transport efficiencies. Consequently, the photocurrent density of the sputtered photoanode with V solution annealing (BVO-V) was 1.86 mA/cm², which is 23 times higher than that of the sputtered photoanode annealed under air conditions (BVO-A, 81.0 µA/cm²). Furthermore, microcone-patterned fluorine-doped SnO₂ was fabricated to increase the active area and reduce the high reflectance, owing to the dense deposition because of the sputtering. Thus, the photocurrent density of the MC-BVO was 3.11 mA/cm², which is approximately 67% higher than that of BVO-V (1.86 mA/cm²).

Hematite is considered as a promising candidate for photoanodes for water photo-oxidation for direct conversion of solar energy to hydrogen in photoelectrochemical (PEC) water splitting. The water photo-oxidation reaction on hematite surface attracts a lot of scientific interest. Different studies highlight the importance of surface states in the oxygen evolution reaction (OER) mechanism and attribute them various roles in the charge transfer and recombination processes that affect both the photocurrent and the photovoltage. In this work we report a study on the surface states by using time dependent cathodic discharge measurements (CDM) of preconditioned hematite photoanodes that were polarized under illumination and potential sufficient to drive oxygen evolution. We show that upon turning the light off, some of the charged (oxidized) surface states discharge spontaneously while others remain meta-stable in their charged state. When the potential is swept cathodically, in a suitable range and rate, double-peak cathodic discharge wave is observed. The magnitude and the shape of the discharge wave depends on the elapsed time between turning the light off and the cathodic sweep. Specifically, we show that the relative heights of the observed cathodic discharge peaks reverse after sufficient time interval. This observation indicates that the cathodic discharge mechanism involves parallel pathways with different relaxation times rather than a single pathway with consequent serial steps, suggesting parallel reaction pathways for the OER on the surface of hematite photoanodes. This conclusion is corroborated by a microkinetic model that is analytically solved for serial and parallel reaction mechanisms. Furthermore, the model predicts the evolution of the two reaction pathways as a function of time and applied potential.

Photoelectrochemical cells (PEC) have drawn much attention due to their eco-friendly capability of light-to-hydrogen energy conversion. Silicon, a promising candidate for photoelectrode due to its low bandgap and high efficiency in photo water-oxidation, shows low stability during the alkaline-based photoelectrochemical reaction, which spurred the research on materials and deposition methods for thin passivation layer on silicon. Herein, we report on the formation of an efficient passivation layer on silicon photoanode using solution-processed nickel oxide in combination with light irradiation pretreatment. While the thickness of nickel oxide film and the condition for light irradiation pretreatment are systematically varied, the resultant passivation layer is characterized optically, morphologically, and electrochemically. The light irradiation pre-treatment used an excimer lamp ultra violet with a wavelength of 172 nm and was carried out under varying temperature conditions for 1 h at N₂ atmosphere. Thickness of film fabricated through light irradiation pre-treatment are about 50% thinner than that of conventional annealed films and film has a flat roughness. The nickel oxide film fabricated with light irradiation pre-treatment showed higher PEC efficiency with narrow band gap (3.41 eV), higher saturation current (36 mA/cm²), and more left shifted overpotential (-0.3 V from on-set of conventional thermal annealing processing film) than those prepared by the conventional thermal annealing process.

Hematite’s intrinsic properties qualifies it to be one of the most effective photoanodes for photoelectrochemical water splitting (PEC). On the other hand, there are several limiting factors for hematite photoanodes that prohibit them from reaching their maximum theoretical efficiencies. One of the most important negatively affecting factors is the low conductivity in hematite, which results in fast recombination rates and hence the loss of the carriers required to perform the PEC process, which eventually results in low hydrogen production rates. Adding dopants to hematite was proven to be one of the best treatments for the low conductivity issue, since donor dopants can generate sufficient electrons to enhance the conductivity and the overall PEC performance. However, in many cases the desired enhancement in conductivity cannot be attained due to compensators for the donors other than electrons such as iron vacancies and other negatively charged defects. In this research, we investigate the effect of substitutional and interstitial defects resulting from the addition of 1% dopants in hematite to determine the optimal set of dopants that can result in a 1:1 dopant-to-electron ratio while maintaining accessible thermodynamic conditions for hematite sample preparation. The investigated dopants include the whole 3d transition metals, in addition to Zr, Nb and Mo from the 4d transition metals and Hf, Ta and W from the 5d transition metals. The formation energies of the dopant defects are calculated using density function theory simulations with Hubbard U corrections and the results are used to plot Kröger Vink diagrams (KVD) which allow us to determine the donors which results in 1% electrons at reasonably accessible oxygen partial pressures. Furthermore, the diagrams are utilized to highlight the dominant dopant defects, which compensate for the electrons and hence provide an insight about their dominant oxidation state in hematite. A further utility of our computations is investigating the possibility of finding a co-doping scenario that can enhance both electrons and holes simultaneously. Our results suggest that 1% doping with W, Ta, and Nb can achieve a 1:1 electron-to-dopant ratio at the atmospheric pressure. Similarly, 1% Ti, Hf, Zr, and Mo can achieve the same effect but at an oxygen partial pressure two orders of magnitude less that the atmospheric pressure. We also propose a W-Zn doped hematite as an optimal dopant strategy to balance enhancing both electrons and holes at a reasonable oxygen partial pressure. We believe that our computational results can narrow down the search space for performing experiments to test the optimal compositions for doped hematite with the goal of enhancing the hydrogen production rates from PEC.

Hematite (α-Fe₂O₃) is considered as a leading photoanode candidate, primarily due to its bandgap applicability to the solar spectrum. Its photocurrent is estimated to potentially reach 12.6 mA cm^-2, assuming every photon with above band gap energy is absorbed and produces hole-electron pair that contributes to the water splitting reaction. Since hematite’s spatial collection efficiency is commonly perceived as the main cause preventing its photocurrent from reaching this value, extensive efforts were made to overcome this challenge by nanostructuring or light trapping routes. However, the record photocurrent reported for hematite photoanodes reaches about 50% of the theoretical limit. This suggests that optical transitions that do not produce mobile charge carriers contribute to light absorption but not to photocurrent generation, and hence fundamentally limits hematite’s performance. The photogeneration yield spectrum, defined as the wavelength-dependent ratio between the absorption that ultimately contributes to the photocurrent and the overall absorption, is elemental fundamental material property that provides a more realistic estimation of the maximum photoconversion efficiency of hematite photoanodes. A general method, that applies to thin and thick films, to extract the photogeneration yield spectrum and the spatial collection efficiency from optical and photoelectrochemical external quantum efficiency (EQE) measurements using minimal a priory assumptions regarding the charge carrier collection efficiency will be presented in this talk. Analyzing a 30 nm thick Sn-doped hematite film shows that the photogeneration yield drops from ~60% at short wavelengths (300-400 nm) to ~20-30% at long wavelengths (~550-600 nm), indicating that ~50% of the photons in the solar spectrum between 300 and 600 nm are lost for non-contributing absorption. This explains the difficulty in reaching high photocurrent in hematite photoanodes, even with optimal nanostructured morphologies and ultrathin films with high charge carrier separation and collection efficiency.

Hydrogen has unique advantages as an energy carrier, with a high energy density and abilities for long term storage and conversion between electricity and chemical bonds. Although hydrogen currently has a significant role in transportation and agriculture, its use in energy consumption overall has been limited, particularly in the case of electrochemical water splitting. With decreasing electricity prices, electrolysis cost reductions can be achieved and allow for an opportunity for greater use.(1) While load-following renewable power sources can reduce feedstock cost, further cost reductions can be achieved by reducing the platinum group metal (PGM) content.(2) Efforts are needed to understand and mitigate electrolyzer degradation, particularly when accounting for lower PGM loadings and intermittent operation.

In this presentation, studies related to anode catalyst durability will be discussed and include the impact of individual stressors, observed degradation mechanisms, and the development of catalyst-specific accelerated stress tests. First, whether through simulated load-following profiles or accelerated cycling, significant performance losses were found as a result of cycling between open circuit and operating potentials.(3) Losses were further aggravated by using a thinner anode catalyst layer or reducing loading, an increase in cycling frequency, and an increase in cell operating voltage. Performance losses primarily appeared through kinetics and were accompanied by anode catalyst dissolution, migration, catalyst layer changes, and interfacial tearing. Second, how components are integrated into membrane electrode assemblies had a significant impact on catalyst layer properties and electrolyzer durability.(4, 5) Properties were modified by varying ink and spray variables during the deposition of the anode catalyst layer and several trends were found with regards to how catalyst-ionomer integration and catalyst layer uniformity affected performance non-idealities and how losses grew during extended operation.

Finally, membrane thickness and ohmic losses were deconvoluted from catalyst layer durability experiments, allowing for a transition from potential-driven parametric observations to current-driven stress tests for advanced materials and coating processes.(5, 6) Through the screening of available catalysts, increased performance losses were found with materials that incorporated less stable elements or contained sub-stoichiometric oxides, particularly when accounting for PGM thrifting and intermittent or start-stop operation. Perspectives on anode catalyst development thrusts and future needs will be also discussed. Specifically, increasing utilization beyond the catalyst layer/membrane interface is needed to better understand the role of catalyst advancements in mitigating electrolyzer performance losses during operation.

[1] B. Pivovar, N. Rustagi and S. Satyapal, The Electrochemical Society Interface, 27, 47 (2018).

[2] K. Ayers, N. Danilovic, R. Ouimet, M. Carmo, B. Pivovar and M. Bornstein, Annual Review of Chemical and Biomolecular Engineering, 10, 219 (2019).

[3] S. M. Alia, S. Stariha and R. L. Borup, J. Electrochem. Soc., 166, F1164 (2019).

[4] S.M. Alia, K.S. Reeves, J.S. Baxter, D.A. Cullen, J. Electrochem. Soc., 167, 144512 (2020).

[5] Electron microscopy was performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

[6] S.M. Alia, H2@Scale: Experimental Characterization of Durability of Advanced ElectrolyzerConcepts in Dynamic Loading, https://www.hydrogen.energy.gov/pdfs/review19/ta022_alia_2019_o.pdf, 2019.

To improve the performance of electrochemical devices (like fuel cells, electrolyzers, batteries), it is critical to deeply understand very thin ionomeric materials. Ionomers in thin films experience multiple interfaces to which they interact very differently. It is thus of high interest to see how a specific property changes across the thickness of a sub-micron thick ionomer film, and how far the interfacial effects propagate deep down inside it. Techniques, like neutron reflectometry and X-ray computed tomography can offer information about mass or density distribution of water and ionomer across ionomer films. However, ion conduction and mechanical properties, the two most critical performance parameters of ionomers, are still reported as an average value for an entire sample. Understanding the need, we have developed an innovative, simple, and every day-usable strategy using fluorescence confocal microscopy to explore depth-specific ion conduction and stiffness behavior within ionomer bulk membranes and thin films. Ionomer samples were stained with suitable functional dyes first, and then placed under a confocal microscope. A number of xy-plane images were taken and z-stacked to obtain the distribution of relevant properties across the material. The technique revealed a unique distribution of ion conduction and stiffness environment across a range of fluorocarbon and hydrocarbon-based ionomer thin films from substrate to air interfaces and identified the critical parameters governing such distributed behavior.

One of the most promising ways of meeting the high demand for clean and renewable energy is the synthesis of green hydrogen via electrocatalytic water splitting. Nevertheless, green hydrogen production is currently challenged in industry-scale deployment mainly due to high material and stack costs. The present research aims to address this topic by minimizing the amount of iridium by optimization of the electrode design. Here, an Ir-decorated boron-doped Si nanoparticulate electrocatalyst is being developed and integrated into a scalable proton exchange membrane water electrolyzer by coating it to a porous titanium transport layer in an argon atmosphere.
The B-doped silicon powder was synthesized by thermal decomposition of the gaseous precursors SiH₄ and B₂H₆, while the B/Si - ratio was chosen to be 4%. Laser-generated Ir nanoparticles were subsequently deposited onto the Si@B support in an aqueous suspension.
A homogeneous catalyst distribution on the substrate was noticed by SEM, while TEM analysis reveals a high degree of crystallinity within the particles. Furthermore, it was found that electrostatically-stabilized iridium nanoparticles (~8 nm), gained from surfactant-free laser synthesis with gram-scale productivities and quantitatively were successfully adsorbed to conductive boron-doped silicon nanoparticles (100 – 300 nm) from kg-scale gas-phase synthesis under sufficiently optimized pH condition.
The activity of an Ir_10wt%@Si(B) catalyst for oxygen evolution reaction as well as the prepared anode layer on the Ti-substrate (Ir_10wt%@Si(B) +Ti-Layer) showed comparable activity in the range from 0.4 V to 1.3 V which was competitive with the activity of a pure Ir-black catalyst indicating a high Ir-accessibility and catalyst layer conductivity of the anode layer.
For validation of the catalyst’s activity, a PEM electrolyzer test stack based on hydraulic compression of single cells will be developed. The stack will be suitable for simultaneous operation of 5 single test cells with 25 cm² active area each. The test system will be automated for an unattended long-term operation, during which conditions like temperature and pressure can be controlled and monitored.

Anion exchange membrane (AEM) water electrolyzers, with economically feasible and resource-rich transition metal (TMs)-based catalysts, are promising for large-scale hydrogen production. However, compared to noble-metal catalysts, the low activity and stability of these TMs catalysts are one of the main challenges.^[1] Recently, TMs chalcogenides, like sulfides/polysulfide, have been widely used as catalysts for oxygen evolution reaction (OER) and exhibit remarkably high activity due to higher conductivity and more exposed active sites than TMs oxides.^[3-4] However, the electrochemical behavior, microstructural and phase composition changes of these chalcogenides in the OER processes need further investigation.
In this work, a scalable electrochemical method was used to activate an S-rich Nickel polysulfide nanocube microstructure prepared by a one-step hydrothermal method to obtain a Nickel-rich polysulfide/(oxy)hydroxide heterostructure. The activated Ni polysulfides show higher activity (370 mV@50 mA cm^-2) than that of Ni/NiO (>460 mV) and Ni(OH)₂ (>470 mV), and stability (> 60 h) for the oxygen evolution reaction compared to commercial Ni/NiO and synthesized Ni(oxy)hydroxides. Moreover, the results of single-cell tests show that cells based on Ni polysulfides (1450 mA cm^-2) have 650 mA cm^-2 higher current density than cells based on Ni/NiO (800 mA cm^-2) in polarization performance at 2.0 V, and nearly no degradation after 80 h@1000 mA cm^-2. This work provides a strategy for better utilization of TMs-based polysulfides by in-situ electrochemical approach, and their actual stability under high current density.
[1] Buttler, A.; Spliethoff, H., Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review. Renewable and Sustainable Energy Reviews 2018, 82, 2440-2454.
[2] Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A comprehensive review on PEM water electrolysis. International Journal of Hydrogen Energy 2013, 38 (12), 4901-4934.
[3] Zhou, W.; Wu, X.-J.; Cao, X.; Huang, X.; Tan, C.; Tian, J.; Liu, H.; Wang, J.; Zhang, H., Ni₃S₂ nanorods/Ni foam composite electrode with low overpotential for electrocatalytic oxygen evolution. Energy & Environmental Science 2013, 6 (10).
[4] Mabayoje, O.; Shoola, A.; Wygant, B. R.; Mullins, C. B., The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as “Pre-catalysts”. ACS Energy Letters 2016, 1 (1), 195-201.

The proton-conducting solid oxide electrolysis cells (p-SOEC) is an emerging and attractive technology for hydrogen production via water electrolysis at intermediate temperatures (400-600^oC). Compared to its counterpart, oxygen-ion conducting SOEC (o-SOEC) that normally operated at >750^oC, reduced operating temperatures can significantly improve the cell/stack durability, minimize stack sealing problems, enable the use of less expensive materials (e.g., ferritic stainless steels for interconnect), and improve response to rapid start-up and repeat thermal cycling needs. Furthermore, p-SOEC can overcome the problems that the o-SOEC encounter, including the mixture of hydrogen and steam, severe delamination of electrodes at high current densities, and partial oxidation of the Ni-based electrode. While these remarkable merits exist, there are still tremendous research efforts needed to address challenges related to electrolyte and electrode materials in p-SOEC. They include poor sinterability, unproven chemical stability and electronic leakage of electrolyte, as well as limited availability of oxygen electrode materials. It raises concerns about whether the technology could move forward for scale up and commercialization. At Idaho National Laboratory, the extensive effort is placed on these topics by both independent research and close collaboration with the universities and industry. Significant advancement is achieved by integrating the materials and microstructure R&D into cell fabrication and manufacturing, implying a prosperous future of p-SOEC.

High-temperature electrolysis by proton-conducting solid oxide electrolysis cells (p-SOECs) is a highly efficient technology to produce hydrogen at intermediate temperatures (400~600^oC). The robust and stable performance is very critical to evaluate the technical feasibility of this ceramic electrolyzer in the realistic operation, particularly high steam condition. The praseodymium cobaltite-based perovskite PrNi_xCo_1-xO_3-δ (PNC) has been discovered to show triple conduction behavior which is utilized as steam electrode for water splitting reaction. The excellent initial performance and nature of high chemical stability make this electrode with great potential to be a benchmarking material for p-SOECs. To systematically evaluate the electrode properties on more aspects, a series of composition optimization study have been carried out to understand structure-property relationship, chemical/performance stability in high steam condition, and interfacial stability. In addition, the origin of the triple conduction has also been investigated to give more guidance on tuning conductivity and material composition.

The two-step metal oxide water-splitting cycle is one of theost viable approach for Solar Thermochemical Hydrogen (STCH) production. Challenges exists in finding suitable oxides that can satisfy thermodynamics of the STCH redox cycle under viable range of temperatures and partial pressures. Recently, novel quaternary oxides Sr_1-xCe_xMnO₃ (SCM) and Ce_xSr_2-xMnO₄ (CSM) (where x from 0.05 to 0.25) are demonstrated to exhibit promising STCH performance. however, a defect model establishing how changing Ce/Sr composition can be used to tune the STCH redox behavior of SCM/CSM systems is still missing. In this study, we investigate the atomic, electronic and defect structure of these strongly correlated oxides using first principles supercell calculations employing different levels of theory (DFT+U, SCAN+U, Hybrid-DFT). We find that O vacancy formation energy is highly sensitive to the choice of level of theory, and strongly depend on the fraction of Mn⁴⁺/Mn³⁺ in the system, ratio of which changes with Ce concentration (x) as well as with the extent of reduction or oxygen off-stoichiometry. Here, we present a defect model that incorporatea dependence of Ce/Sr composition and oxygen off-stoichiometry on the O vacancy formation energy, followed by the thermodynamic modeling to quantitatively predict the redox behavior of these complex quaternary oxides. This study help develop new general approaches for modeling defects in complex oxides beyond the dilute limit and at high temperatures, which is still a critical scientific challenge for the entire computational community.

As the world pursues the goal of lowering carbon emissions to meet the challenges of global climate change, the utilization of hydrogen in transportation, energy storage, and commodity chemical production is becoming increasingly widespread . Water-splitting using renewable energy sources such as electricity from PV or wind, or heat from concentrating solar, is a green method of hydrogen production. However, widespread adoption of these renewable, carbon-free technologies is hindered by materials challenges, low efficiencies, and high costs.

In this talk, a novel route to hydrogen production via a hybrid thermo- electro-chemical route utilizing a liquid metal solution (LMS) as the water splitting medium will be introduced. In the first step of this process, steam is thermochemically reduced by the LMS producing hydrogen and a metal oxide (Eq. 1), while in the second step, the metal oxide is electrochemically reduced back to metal and oxygen is evolved (Eq. 2):

MM’ (l) + H₂O (g) → M (l) + M’O_x (s) + H₂ (g) (Eq. 1)
M’O_x (l) + M’ (l) + e^- → MM’ (l) + O₂ (g) (Eq. 2)

In this reaction, MM’ is an alloy consisting of two metals, one that is inert to steam (M) and remains as solution and one which is redox-active (M’) and is oxidized during the water-splitting process. The reaction can be driven using heat and electricity from concentrating solar or nuclear power. Alternatively, the electrical input may be provided by wind or PV. The LMS process has the potential to produce hydrogen via water splitting at temperatures much lower than current CSP-driven thermochemical processes and at applied potentials lower than conventional high-temperature electrolysis.

A number of possible binary LMS compositions were identified using machine learning. Several candidates were downselected and tested for hydrogen production in a custom-built flow reactor. The metal alloys were characterized pre- and post-reaction to analyze changes microstructure, grain size, and metal oxide particle growth. The results of these analyses will be presented.

Solar thermochemical hydrogen (STCH) production is a promising route to produce fuels from sunlight via high-temperature water splitting. However, efficient and technologically viable implementations of this process only allow a narrow window of thermodynamic boundary conditions that can be used to cycle the system, thus limiting the design space for suitable metal oxide redox mediators. An oxygen defect redox mechanism can contribute a favorable reduction entropy to expand this window, and computational evaluation of materials with high oxygen defect entropies could play a pivotal role in guiding the discovery and design of suitable oxides. This study employs first-principles calculations to investigate the redox mediating defect mechanism of the STCH candidate material, hercynite (FeAl₂O₄). We compare the results from density functional theory (DFT) with beyond-DFT approaches, including hybrid functionals and the random phase approximation, which are among the most advanced methods currently feasible for supercell defect calculations. Using the predicted total energies, we perform thermodynamic modeling of FeAl₂O₄ reduction and oxidation via free energy minimization that incorporates ideal gas, configurational, and vibrational entropy contributions evaluated within the quasi-harmonic approximation. Special attention is devoted to understanding interactions among co-existing defects, such as the association of pairs and complexes of O vacancies and cation antisite defects, as well as the effect of mutually compensating defect charges. Our results corroborate the notion that the details of defect interactions can be decisive for the viability of hydrogen production within the desirable STCH process window.

Increasing interest in sustainable energy sources and climate change has encouraged research in the production of H2 from H2O. One of the significant methods being explored for the generation of H2 is the thermochemical splitting of H2O and by using oxide materials.
Perovskite oxides have been suggested as promising substances for solar thermochemical separation of H2O and have been widely studied recently. We have carried out a systematic experimental production study of doped Ca and Co-doped perovskites within the compositions of La1- CaxMn1-yCoyO3(Ca=0.2-0.8, Co=0.2-0.8).
In the study, LCMx perovskite oxides were synthesised by a solution-based method (Pechini, 19673). For this purpose, nitrated salts of the elements La, Ca, Mn and Co, suitable proportions for the prescribed composition, were used. Deionized water and citric acid in appropriate proportions were added to the structure. The solution was stirred by heating at 120 ° C with ethylene glycol addition. After gelation, the system was dried, and the obtained powders were ground.
Powders were calcined at three different temperatures. The phase analysis of the powders were performed by using X-ray diffraction (Rigaku SmartLab X) using monochromated Cu Ka radiation at 40 kV and 200 mA. The structures are refined using the Maud programme to identify their crystal structure. BET surface area was evaluated by N2 adsorption at 77 K in a constant volume adsorption apparatus (Quantachrome Autosorb 1C-MS).The Powders obtained were examined morphologically using SEM (JEOL JSM-7600F). The effect of the composition and applied temperatures on the surface area and particle size was investigated.
The studies started by determining the synthesis parameters for each LCMx oxide composition. For this purpose LCMC4664 composition was selected and synthesized under four different conditions (1. citric acid addition, 2.citric acid addition and ph ~8, 3. Citric acid and ethylene glycol addition, 4. Citric acid and ethylene glycol addition and ph ~8. All systems are calcined at 900 ° C-1300 °C. The X-ray diffraction results show that the main diffraction peaks are characteristic of the typical cubic perovskite structure. However, with the addition, the structure turns into an orthorhombic structure. The change in lattice parameters were also obtained with Rietveld refinement of the sample. the Bragg peaks of LCMC perovskites doped on both the A and B sites were noticeably shifted due to the distinct ionic radii of the introduced metal ions
LCMC4664 compositions synthesized with different parameters and also characterized by SEM, EDS and BET to compare their grain structure, morphology, chemical composition and surface area.Within all synthesis parameters TM: CA: EG 1: 1,5: 1,5 pH = 8,00 gives the lowest surface area. Among the synthesis conditions, the highest surface area is achieved was for synthesis condition for TM: Ca 1: 1.5 pH≤1.00 (0.66m2 / g).
The thermochemical H2O splitting performances of the samples were evaluated in a laboratory-scale reactor (and the O2 and H2 produced during the thermochemical process were monitored using a mass spectrometer (MS). We compared the thermochemical performances of the A-site doped and B site doped perovskites. The extents of oxygen vacancies generated by the prepared samples in high-temperature thermal reduction processes were investigated by measuring O2 production, which in turn will determine the maximum solar fuel production.
In the second step, H2O splitting tests were conducted and H2 production was immediately detected after the aspiration of H2O vapour into the system. The thermochemical performance of LCMC systems are discussed with respect to their BET surface area, which is directly related to the solid-gas reaction during the thermochemical catalytic process.
Acknowledgements
This work was supported by TÜBITAK (The Scientific and Technological Research Council of Turkey) (Project Number 119M420), which the authors gratefully acknowledge.

Commercialized membrane electrolyzers use acidic proton exchange membranes (PEMs). These systems offer high performance but require the use of expensive precious-metal catalysts such as IrO₂ and Pt that are nominally stable under the locally acidic conditions of the ionomer. Here I will discuss our efforts to study and develop alternative electrolysis platforms.

First, I will discuss alkaline exchange membrane (AEM) electrolyzers that, in principle, offer the performance of commercialized proton-exchange-membrane electrolyzers with the ability to use earth-abundant catalysts and inexpensive bipolar plate materials. I will present progress in building high-performance AEM electrolyzers and studying fundamental aspects of their operation and degradation. To date, our best systems operate at 1 A cm^-2 in pure water feed at < 1.9 V at a moderate temperature of 69 °C. These devices, however, degrade rapidly (~ 1 mV/h) compared to PEM electrolyzers. I will discuss our work assessing chemical changes to the anode and cathode catalyst and ionomer that is correlated with this performance loss, as well as strategies to mitigate degradation modes.

Second, I will introduce the use of bipolar membranes (BPMs) in electrolysis-type devices. BPMs consist of an AEM and PEM laminated together. Under the appropriate bias, they conduct ionic current by dissociating water into protons and hydroxide at the AEM/CEM junction. Commercially, BPMs are used in electrodialysis, but generally are used at low current densities below 100 mA cm^-2 to avoid large losses in driving water dissociation. I will present our fundamental studies on how to accelerate water dissociation in BPMs (Science, 2020) and how that has enabled BPMs operating at > 3 A cm^-2 and with improved efficiency. BPMs limit crossover and enable operation of a cathode and anode in different pH conditions and are thus seeing substantial interest for CO₂ electrolysis, advanced electrodialysis systems, and water electrolysis; applications that I will highlight.

Ultrasmall Co9S8 nanoparticles are introduced on the basal plane of MoS2 to fabricate a covalent 0D–2D heterostructure that enhances the hydrogen evolution reaction (HER) activity of electrochemical water splitting. In the heterostructure, separate phases of Co9S8 and MoS2 are formed, but they are connected by Co–S–Mo type covalent bonds. The charge redistribution from Co to Mo occurring at the interface enhances the electron-doped characteristics of MoS2 to generate electron-rich Mo atoms. Besides, reductive annealing during the synthesis forms S defects that activates adjacent Mo atoms for further enhanced HER activity as elucidated by the density functional theory (DFT) calculation. Eventually, the covalent Co9S8–MoS2 heterostructure shows amplified HER activity as well as stability in all pH electrolytes. The synergistic effect is pronounced when the heterostructure is coupled with a porous Ni foam (NF) support to form Co9S8–MoS2/NF that displays superior performance to those of the state-of-the-art non-noble metal electrocatalysts, and even outperforms a commercial Pt/C catalyst in a practically meaningful, high current density region in alkaline (>170 mA cm−2) and neutral (>60 mA cm−2) media. The high HER performance and stability of Co9S8–MoS2 heterostructure make it a promising pH universal alternative to expensive Pt-based electrocatalysts for practical water electrolyzer.

As an efficient and sustainable alternative to fossil fuels Hydrogen gas (H₂) has emerged to be of great importance for the future of energy generation and storage. However, H₂ production from the electrocatalytic hydrogen evolution reaction (HER) still remains a challenge. Although platinum and its alloys have been the benchmark electrocatalysts for HER, the scarcity, substandard stability and expensiveness limit their usage making it crucial to develop economical electrocatalysts with elevated activity and stability. In this quest, nanoporous metals, built of 3D scaffolds of bi-continuous ligament-pore structure, have shown growing inclination because of their large surface area to volume ratio and enhanced catalytic properties. This work focusses on Nanoporous Gold (NPG) as a promising candidate with its noble nature, high conductivity and large surface area. Instead of common precursors constituting various alloying metals and high Au concentration, a metastable supersaturated solid solution of AuFe₂ was selected – with cheap and abundant Fe and minimum possible Au concentration according to the parting limit. Long and homogeneous melt-spun ribbons were obtained by rapid solidification of the arc-melted precursor. The as-quenched ribbon was dealloyed chemically in 1 M HNO₃ and 1 M HCl at 70 °C for different durations. The structural and compositional investigation was accomplished using XRD, SEM and EDS techniques. The as-dealloyed samples possessed nanoporosity both on surface and cross-section with high Au content. One of these samples was tested as an electrocatalyst for HER in 0.5 M H₂SO₄. Low onset potential of -4 mV; low overpotential of -0.38 V at a current density of -5 mA/cm²; small Tafel slope of 47 mV/dec; and high exchange current density of 0.12 mA/cm² have been obtained. Thus, the NPG sample demonstrates excellent electrocatalytic activity considering that an overall low-cost, fast and simple synthetic route combined with the cost-effective precursor were employed for the fabrication.

Global energy demand is projected to increase by 50% by 2050. Existing fossil fuels can meet this energy demand only until the end of this century. Fossil fuels are also a major source of greenhouse gas emissions. Therefore, it is one of the biggest challenges to find a sustainable, clean, and efficient energy source. Ongoing research efforts have led to the invention of promising energy storage technologies like alkaline water splitting, metal-air batteries, and fuel cells.¹ The oxygen evolution reaction (OER) is the critical anodic reaction in the aforementioned devices. OER is a four-electron process with sluggish reaction kinetics, thus the overall efficiency of these systems is low. RuO₂ and IrO₂ catalysts are considered the benchmark for OER in both acidic and alkaline media.^2,3 However these noble metal-based catalysts are costly and suffer from disadvantages such as instability (oxidized to RuO₄ and IrO₃) under high anodic potential and dissolution into the electrolyte solution.^4,5 Transition metal-based catalysts are cheap and abundant. Fe, Co, Ni, Mo, and W transition metal-based catalysts have been well explored in the past decade for OER and show promising results.^6-7 However, copper oxide-based catalysts are less explored for OER due to their poor conductivity, inappropriate crystal structure, and narrow bandgap.⁸ Doping is an effective strategy to improve the conductivity and thus the electrocatalytic activity of the material. For example, doping sulfur into Cu₂O has been reported to significantly lower the onset potential for OER.⁹ In this work, we have doped Se into CuO nanoarrays for OER application. We have observed that upon doping Se into CuO nanoarray structures, the overpotential for OER can be lowered significantly. Comprehensive characterization and electrochemical performance of the Se doped CuO nanoarray for OER will be presented.
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Related symposia
Symposium EN14-Advanced Materials for Hydrogen and Fuel Cell Technologies

High-performance water electrolysis is an important device for green hydrogen production. Polymer electrolyte electrochemical cell (PEEC) is considered a suitable device for renewable energy storage due to the toughness of the fluctuated energy sources.
We focused on PEEC as a device to convert electricity into hydrogen in this report. The evaluated PEEC has an electrode area of 12.5 cm² with IrO₂ as the cathode catalyst and Pt as the anode catalyst. The operating temperature of this device was varied from 30 to 70°C in order to evaluate the current-voltage characteristics. The results were compared with a chemical reaction-based device model [1,2].
The threshold voltage and the slope of the polarization curve of the current-voltage characteristics were changed with the operating temperature. The threshold voltage decreased and the slope of the polarization curve increased with increasing operating temperature. There were clarified that the activation energy of the electrochemical reaction mainly affects to change of the threshold voltage, and the resistance of the solid polymer membrane mainly affects the change of the slope of the polarization curve from the device model analysis. The other chemical parameters did not significantly affect the PEEC current-voltage temperature dependence. The results show that the reduction of activation energy of the electrochemical reaction and the suppression of ohmic resistance of the solid polymer membrane were important parameters to improve the performance of PEEC.
The PEEC performances were evaluated quantitatively by the current-voltage temperature dependence using the device model. The results of these quantitative evaluations give guide the material designs for the high-performance PEEC.

[1] B. Lee et al., Int. J. Electrochem. Science, 8 (2013) 235.
[2] V. Liso et al., Mdpi. J. Energies, 11 (2018) 3273.