Symposium S.EN11—Materials, Modeling and Technoeconomic Impacts for Large-Scale Hydrogen and Energy Applications

Hydrogen could be an ideal energy carrier for a sustainable energy supply system because it combusts cleanly without producing CO₂ or any other pollutants, and this chemical energy is easier to store than electricity. However, the current industrial production of hydrogen (most commonly by steam reforming of natural gas) is energy-intensive and produces CO₂ and other pollutants, making the overall system far from being sustainable.

Alternative environment-friendly H₂ production methods include photoelectrolysis or electrolysis of water using a renewable energy source. Photovoltaic cell−electrochemical cell (PV−EC) is such an approach to solar energy-driven water splitting, which combines PV for power generation and EC for electrochemical H₂ production from water. To realize a practical PV−EC system, both PV and EC components should be highly efficient, durable, and cost-effective. The electrochemical cell has received renewed interest recently to improve its performance as well as cost. The electrocatalytic water splitting consists of two electrochemical reactions; hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic or alkaline media. Regardless of the system types and media, sluggish OER kinetics limits the efficiency of overall water splitting reaction owing to its more complex reaction mechanism and higher overpotential (0.2 V) relative to HER (0.05 V). To maximize the efficiency of a PV−EC system, reducing the overpotential of OER by developing a highly efficient oxygen evolution catalyst (OEC) is indispensable. Although noble-metal based OECs including IrO_x and RuO_x exhibit great OER catalytic activity in acidic media, they are not only expensive but often susceptible to corrosion and deactivation in alkaline media without proper modification.

Alternatively, many first-row transition metal oxides, hydroxides, and present great OER activity and stability in alkaline conditions in addition to the advantages of being made of earth-abundant elements and the low price compared to precious metals. Many recent studies have demonstrated great OER performance of binary and ternary mixed oxyhydroxides of Ni, Fe, and Co, synthesized by various methods. However, in order to apply these catalysts to a practical industrial scale, it is required to develop an advanced synthesis method capable of producing multicomponent electrocatalysts with a simpler synthesis step under mild conditions at ambient pressure and temperature preferably with an all-solution-based process.

In the present work, we present an ingenious synthesis method of “precipitating metal nitrate deposition (PMND)” to prepare amorphous phases of unary or binary transition metal oxyhydroxide (TMOH) films containing Ni, Fe, and Co. This technique can easily control the composition of the metals in the catalyst and thus is suitable to study its effects on the catalytic activity of the electrocatalysts. By examining a series of unary and binary TMOH catalysts of 30 different ratios of elements on fluorine-doped tin oxide (FTO) substrate, it is shown that their OER activity is represented by a volcano plot as a function of a single experimental descriptor, i.e., the fraction of hydroxide in the surface oxygen species. We also demonstrate the versatility of the PMND method by preparing the catalysts on various substrates of FTO, nickel foam, nickel mesh, and carbon felt. The optimized NiFe (2:8) electrocatalyst on nickel foam exhibits great OER activity in an alkaline medium superior to conventional noble metal oxides and other reported electrocatalysts of similar composition. Finally, we fabricate a PV−EC device by connecting our optimized EC with a commercial crystalline c-Si PV module, which records a solar-to-hydrogen conversion efficiency (STH) of 9.84 %.

[Y. K. Kim et al. ACS Catalysis 2019, 9 (10), 9650-9662.]

Today the technology around generating and storing efficient and sustainable energy is rapidly evolving and hydrogen technologies offer versatile options. This presentation will provide an overview of the U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office’s early-stage R&D activities in hydrogen production and storage and fuel cell technologies within the Office of Energy Efficiency and Renewable Energy. The presentation will focus on their relevance to the evolving energy storage needs of a modernized grid, and discuss R&D needs and challenges. Specific examples of advanced materials research relevant to address energy storage challenges will be provided. The presentation will also cover DOE’s H2@Scale initiative which will enable innovations to generate cost-competitive hydrogen as an energy carrier, coupling renewables, as well as nuclear, fossil fuels, and the grid, to enhance the economics of both baseload power plants and intermittent solar and wind, enhancing resiliency and avoiding curtailment.

Hydrogen fuel cell technology is gaining momentum throughout the transportation industry for use in a variety of applications - from on road light duty to heavy duty. This presentation will highlight Toyota's image for hydrogen fuel cells in our electrification strategy and our need for a robust supply of renewable low or zero CO2 hydrogen.

Reduction in the cost of polymer electrolyte membrane water electrolyser (PEMWE) technology is urgently required for the realisation of commercially viable green hydrogen production. However, at present up to two thirds of the manufacturing cost of a PEMWE stack is associated with the current collector components, with platinum-coated titanium a common material of choice. This presentation will describe in situ measurements of the local potential at the current collectors during PEMWE cell operation, demonstrating for the first time that the corrosion potential of the anode current collector is completely decoupled from the potential of the anode electrode due to the low ionic conductivity of the deionised water phase. This new observation opens up the possibility of using cheaper materials such as carbon and carbon-coated stainless steel for anode current collector components, which could lead to a dramatic decrease in the cost of the technology.

Clean hydrogen from water splitting driven by electrocatalysis provides an alternative to fossil fuels. However, the oxygen evolution reaction (OER) is the ratedetermining step of water splitting. Therefore, there is an urgent need to develop low-cost and highly efficient OER electrocatalysts. Low-cost carbon-based materials with a high surface area that expose more active centers are the ideal candidates for OER. Challenges to reduce OER overpotentials and to increase stability still limit the practical applications of carbon-based materials. However, substantial overpotentials above thermodynamic requirements limit their efficiency and stability in OER-related energy conversion and storage technologies. Here, we embedded CrN crystals into graphene and in situ electrochemically oxidized them to construct graphene materials with encapsulated Cr⁶⁺ions (Cr⁶⁺@G). These Cr⁶⁺@G materials exhibit the lowest OER overpotential of 197 mV at 10 mA cm⁻² and excellent stability over 200 h at a high current density of about 120 mA cm⁻² in an alkaline electrolyte. Spectroscopic and computational studies confirm a stable ion coordination environment significantly benefiting the downshift of the graphene Fermi level via hybridization of C p orbitals with d orbitals of Cr⁶⁺ ions that enhances the OER activity and stability.
References
1. Yingfang Yao, Zhe Xu, Feng Cheng, Wenchao Li, Peixin Cui, Guangzhou Xu, Sheng Xu, Peng Wang, Guodong Sheng, Yadong Yan, Zhentao Yu,a Shicheng Yan, Zhaoxu Chen, Zhigang Zou, Unlocking the potential of graphene for water oxidation using an orbital hybridization strategy. Energy Environ. Sci., 2018, 11, 407 - 416

Electric energy storage is an important part to establish natural energy-originate electric power like solar cells and wind power generations as a user-on-demand power supply system. Hydrogen storage is a suitable method for a long-term and a large amount of energy storage while the rechargeable battery is good for fast demand response. We proposed a new control concept of the user-on-demand electric supply system taking both advantages of hydrogen storage and rechargeable battery [1]. For the system, not only the rechargeable battery is used as a fast response device but also hydrogen storage is for a large amount of energy storage device, with DC bus voltage as the signal of the power flow balance. In such kind of system, hydrogen is stored by polymer electrolyte electrochemical cell (PEEC) and is consumed by polymer electrolyte fuel cell (PEFC). Thus, the system operating conditions influence the PEEC and PEFC properties. In this report, some of the possible system conditions influencing the PEEC properties are mainly discussed.
The evaluated PEEC was the conventional one, that is, the anodic catalyst is IrOx and the cathodic catalyst is Pt. Step-like electric power changes by the voltage and by the current for a PEEC were performed. The current by the step-like voltage change showed a large overshoot for voltage increase and undershoot for voltage decrease were observed. In contrast, the voltage by the step-like current change showed a relatively smooth increase and decrease without overshoot nor undershoot. This is explained by the EC as a capacitor, and the power change should be controlled by the current.
The performance of water electrolysis is improved with increasing temperature due to the overall electric resistance decreasing and the overvoltage of water splitting decreasing. The hydrogen leakage to the oxygen evolution reaction (OER) side is also well-known properties for PEEC. The leakage was also observed to be increased with cell temperature. This means that the cell temperature must be considered because IrOx water oxidized catalyst may be reduced. This leakage may also affect device reliability, but the details are still obscured.
From the impurity analysis of the supplied water for a PEEC after the current-voltage (I-V) relationship degraded (electric resistivity increased), the component metal ions of PEEC were detected. After replacing the water with pure one, the I-V characteristic was recovered. This probably shows that the dissolved metal ions decrease the ion transferability in the proton exchange membrane. The result shows that especially for the long-term water electrolysis operation requires water purification.

[1] D. Yamashita et al., Int. J. Hydrogen Energy 44 (2019) 27542.

Adsorption of molecules in functionalized and high surface area metal-organic frameworks (MOFs) is of emergent technological importance in a multitude of areas ranging from chemical separations to energy storage. We have been studying the properties of MOFs for storage and separations of industrially important small molecules such as hydrogen, oxygen, carbon dioxide, noble gases, and short chain organics. Besides the geometrical and porosity control available in MOF chemistry, the properties of the frameworks can be tweaked to elevate electrostatic interactions by exposing open metal cation sites or functionalizing ligands. Here, we discuss the information accessible from neutron scattering experiments on hydrogen ansorbed in a selection of nominally rigid MOFs with some surprising subtle flexibility that greatly enhances metal-hydrogen interactions.

Hydrogen is an ideal renewable fuel source due to its superior energy density and absence of harmful emissions. A significant obstacle for a hydrogen-based energy economy is efficient storage post-production, during transportation and within the target technology (e.g. vehicles). Two primary methods are employed to achieve these goals: physical and chemical storage. Physical storage (e.g. high surface area frameworks) is limited by inadequate volumetric capacities near ambient temperatures and pressures. Chemical storage materials exhibit high decomposition temperatures with poor kinetics and cyclability. The synthesis of novel molecules/frameworks or improvement of existing technology is critical for advancing hydrogen storage toward meeting DOE’s technical targets. As such, high-resolution synchrotron x-ray characterization has become increasingly necessary for deriving complex structure-property relationships of developing storage materials. This presentation will highlight recently developed in situ x-ray scattering and spectroscopy measurements performed on leading candidate chemical (borohydrides) and physical (MOFs, COFs) hydrogen storage materials. Approaches for improving decomposition and cyclability of borohydrides include ALD coating, nanoconfinement, eutectic mixtures. Temperature-resolved phase transformations recorded with XRD and SAXS demonstrated structural mechanisms leading to hydrogen release at low temperatures; however in most cases reversibility remained a challenge. The results of these investigations revealed structural limitations of existing materials as well as opportunities for future improvement.

The transition to a sustainable hydrogen economy requires liquid organic hydrogen carriers (LOHCs) to enable efficient, high density storage and transportation of hydrogen-based energy. LOHCs, such as alcohols, require an effective and economical catalyst to promote the chemical reactions which store and release hydrogen from the molecule.¹ In recent years, metal-organic frameworks (MOFs) have emerged as an exciting class of materials with applications in catalysis due to unsurpassed tailorability of pore size and local chemical environment and high thermal and chemical robustness required for industrial-scale processes. The MOF-74 framework (M2(dobdc), where M is a divalent metal ion and dobdc = 2,5-dioxido-1,4-benzenedicarboxylate) is a particularly versatile structure which can incorporate many different open metal sites and has been shown to perform a variety of reactions, including hydrogenolysis of aryl ethers², dehydrogenation of ammonia borane³, and methanol-catalyzed water dissociation⁴. In this work, we investigate a series of mono- and bimetallic MOF-74 catalysts with activity towards the methanol dehydrogenation reaction. Reactions were carried out in a packed bed reactor with methanol vapor and nitrogen carrier gas. While Mg-MOF-74 was found to be inactive for this reaction, other M-MOF-74 (M= Zn, Co, Ni, bimetallic Ni-Mg) catalysts turnover the reaction at pressures of 1 bar with temperatures above 200 °C. At 300 °C, the Ni_0.2Mg_0.8 composition achieved the greatest hydrogen productivity of the sample series at 20.3 mL H₂/min. This catalyst demonstrated stability under the reaction conditions, maintaining the MOF-74 structure and high activity for 6 hours. In contrast, the Ni-MOF-74 catalyst had diminished hydrogen productivity and decomposed under the reaction conditions, suggesting a promotional effect was achieved by maintaining the MOF-74 framework. Ex situ X-ray absorption spectroscopy and density functional theory were used to probe the stability and properties of the open metal sites thought to play a role in the observed catalytic activity. These investigations into methanol demonstrate the catalytic activity of a variety of MOF-74 catalysts towards alcohol dehydrogenation and provide important mechanistic understanding of the factors that affect the catalytic activity and selectivity of MOFs for hydrogen generation from hydrogen carriers.

References
1. Trincado, M., et al. Energy Environ. Sci. 2014, 7, 2464-2503
2. Stavila, V., et al. ACS Catal. 2016, 6, 55-59
3. Srinivas, G., et al. Int. J. Hydrog. Energy. 2012, 37, 3633-3638
4. Fuentes-Fernandez, E.M.A., et al. Appl. Sci. 2018, 8, 270-280

Among various materials for hydrogen storage, alloys and intermetallics forming hydrides are one of the most important classes due to their high volume density, reversibility and safety. Within the class of conventional metals and alloys for hydrogen storage, the body centered cubic (bcc) alloys based on early 3d transition elements (Ti, V, Cr...) represent one of the most promising class due to their high hydrogen storage capacity up to 2 Hydrogen per metal atoms (H/M). These alloys show two plateaus at low and ambient pressure in the Pressure-Composition-Isotherms. But, for practical applications only the second plateau is available reducing the reversible quantity of hydrogen that could be stored. Consequently, new metallurgical concepts and materials are stringently required to develop more efficient multifunctional hydrides.
Recently, a new paradigm of alloying strategy has emerged based on the original concept of multi-principal-element alloys (MPEAs), initially proposed to develop materials with enhanced mechanical properties. The principle is laid on the mixing of elements close to the equimolar proportion for systems up to five and more containing elements. This mixing may lead to the formation of simple single-phased solid solutions (body centered cubic-bcc, face centered cubic-fcc and hexagonal close packed-hcp). The formation and the stability of such phases are still under consideration and seem to be based on several chemical and physical quantities such as, configurational entropy, mixing enthalpy, atomic misfit, valence electron concentration. Among MPEAs, alloys with at least five principal elements with atomic concentrations in the range 5 to 35 % are called high entropy alloys (HEAs). Most of reports concerning these alloys describe their structure, microstructure and mechanical properties, whereas functional properties such as, hydrogen sorption, are only scarcely investigated.
We present here the study of hydrogen absorption properties of MPEAs based on refractory metals. The TiVZrNbX (X = Mg, Al and Ta) alloys have been synthesized by classical metallurgical high temperature methods or mechano-synthesis by ball milling under protective atmosphere. To produce directly metal hydrides we have employed the reactive ball milling under hydrogen gas starting from the pure metal powders.
The properties of TiVZrNbX (X = Mg, Al and Ta), TiZrNbHfTa and related hydrides have been studied by a large set of experimental techniques: laboratory X-ray diffraction, electron microscopy, in situ synchrotron X-Rays or neutron diffraction, pressure-composition-isotherm, thermal desorption spectroscopy and differential scanning calorimetry. All the TiVZrNbX alloys are single-phase bcc and undergo a one-step reaction with hydrogen at room temperature. The single-phase TiZrNbHfTa alloy also crystallizes in a bcc phase but undergoes a two-stage hydrogen absorption reaction to a fcc dihydride phase with an intermediate tetragonal monohydride at high temperature. We suggest that the lattice distortion, δ, as defined for MPEAs, might play an important role: larger δ would favors a single-step reaction with hydrogen (bcc → bct hydride with large hydrogen content), whereas small δ would favor a two-steps phase transition (bcc → bct → fcc), as also encountered for conventional bcc alloys. The most promising alloy is the TiVZrNb composition which, despite a fading of the capacity for the first cycles, shows a stable reversible capacity around 2 wt% for further cycling.
In the light of scarce literature on the subject, hydrogen adsorption in MPEAs/HEAs is an original research topic that might open new routes for the design of promising materials for hydrogen storage.

Several approaches for designing and synthesizing colloidal covalent organic frameworks (COFs) are discussed, along with their unique application as hydrogen storage and delivery media. Using novel catalysts and conditions, the particle size of 3D COFs can be tuned between 50 and 600 nm with surface areas > 500 m²/g. Several functionalization strategies are discussed that help promote long-term colloid stability. When purified and resuspended in a bulky ionic liquid that is size excluded from entering the COF pore, the material behaves as a ‘porous liquid’, dramatically improving gas uptake in the liquid. Using temperature programmed desorption, we investigate how the frozen liquid matrix can be used to trap gas in the COF pores and effectively tune gas desorption temperatures. Finally, a technique for growing a solid-state COF monolith from densely packed COF colloids is presented, along with its characterization.

Since the depletion of fossil fuel is inevitable, exploring an alternative green energy carrier is of an urgent demand. Hydrogen fuel produced from renewable resources such as photovoltaic cells coupled with electrolyzers for water splitting can offer a very attractive route to address the aforementioned issue. The major bottleneck for deploying such technology is to find a practical electrolcatalyst that can efficiently oxidize water. Many research efforts have been made towards finding new electrocatalytic materials to enhance the oxygen evolution reaction (OER). However, precious metals, such as Ru and Ir, containing oxides are still the best-known electrocatalysts for OER. This entails finding a new technique to improve the electrocatalytic activity of cost-effective metal oxides. In this study, we attempt a different approach to promote more active sites for OER using transition metal oxides (i.e., Co₃O₄, NiO, and CuO) and tailoring their heterointerfaces with CeO₂. The metal-oxide/metal-oxide (MO/MO) heterointerfaced nanoparticles were prepared by co-synthesizing the two MOs using wet chemistry. X-ray diffraction spectrums and scanning tunneling microscope images confirm the formation of heterointerfaces between CeO₂ and the other MOs under study. That is, low magnification TEM images show that the synthesized particles compose of nano-sized crystallites of Co₃O₄ embedded on CeO₂ matrix. Electrochemical performance CeO₂/MOs are significantly improved compared to pure CeO₂ or any other MOs alone. Among all the CeO₂/MO structures, CeO₂/Co₃O₄ shows the highest cathodic shift (~ 0.65 V) and also it shows the lowest Tafel slop (~ 59 mV/dec). Mott-Schottky analysis illustrates that the presence of MOs interface with CeO₂ has formed an n-p junction where CeO₂ has an n-type characteristic while the other MOs have a p-type characteristic. This property can be very beneficial for the separation of optically excited electron-hole pairs due to the internally induced electric field. Therefore, exploring the potential use of our MO/MO structures as photoelectrodes was conducted using chronoamperometry under a chopped light source at 1.5 V vs. RHE. These results showed that CeO₂/Co₃O₄ electrode has the highest photo-response as compared to the other CeO₂/MOs or any single MO under this study. Additional investigations using density functional theory (DFT) of pure CeO₂, pure Co₃O₄, and hetrerointerfaced CeO₂/Co₃O₄ were performed. It was confirmed using DFT calculations that CeO₂/Co₃O₄ heterointerface has the lowest Gibbs free energy and lowest adsorption energy for OH^* intermediate compared to pure CeO₂ and Co₃O₄. Additional characterization techniques on these MO/MO structures were conducted in this study as well.

Borohydride compounds have been to have great potential for as hydrogen storage materials. Solid state density functional theory calculations were performed to identify key intermediates and gain insight into the reaction pathways involved in the release and uptake of hydrogen from the thermal decomposition of Mg(BH₄)₂. Even tough improved theoretical methods for calculating thermodynamic properties of complex B_xH_y borohydrides have been developed, we find that there are still limitations to understending mechanistic details of these complex reactions, and approaches that focus on NMR and other spectroscopic characterisations are necesery to complement the thermdynamic data and provide mehcanistic insight into these pathways. We will report both on molecular and solid state DFT calculations, and the gains from empolyong both approaches in these studies.

Developing viable solutions for the efficient production and storage of hydrogen requires understanding chemical process occurring at solid-gas, solid-liquid, and solid-solid interfaces in advanced materials. Probing behavior of these interfaces under operating conditions presents significant challenges; however, predictive modeling offers an opportunity for providing key insights into interface chemistry, particularly when operating in tandem with high-fidelity experimental characterization techniques. Within the DOE Hydrogen Materials—Advanced Research Consortium (HyMARC) and the HydroGEN Advanced Water Splitting Materials Consortium, we are using multiscale models to understand properties of reactive interfaces for the production, storage, and delivery of hydrogen. I will provide an overview of our materials modeling strategy within these consortia, ranging from first-principles calculations of interface chemistry to continuum methods for microstructure-level properties. I will then review some of our recent activities for simulating thermodynamic and kinetic properties of hydrogen-related materials. Specific examples will be given of how these computational models have helped to elucidate mechanisms of interface chemical reactions, the formation of new phases, and the impact of solid-state interfaces on key reaction pathways. I will also show how simulations have been combined with experimental probes to improve models and obtain new understanding of materials interfaces under operating conditions. Finally, I will discuss how this understanding is being used to guide new strategies for improving materials functionality for storage, generation, and delivery of hydrogen. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Two-dimensional (2D) materials consisting of a single or a few layers of atoms have superior performance compared to conventional materials or their bulk counterparts in a variety of applications, because of their unique properties, including their flexibility, high specific surface area, and quasi-2D electron confinement. Recently, we have revealed that the hydrogen boride (HB or borophane) sheets with an empirical formula of H₁B₁ can be formed by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB₂) with an average yield of 42.3% at room temperature [1], as a new member of 2D sheets and boron-based nanomaterials [2]. The sheets feature an sp²-bonded boron planar structure without any long range order. A hexagonal boron network with bridge hydrogens is suggested as the possible local structure, where the absence of long range order was ascribed to the presence of three different anisotropic domains originating from the 2-fold symmetry of the hydrogen positions against the 6-fold symmetry of the boron networks. Our recent analysis with soft x-ray absorption and emission spectroscopy at the B K-shell also supports this view and show the semimetallicity of HB sheets [3]. We have then found several intriguing properties of HB sheets for the applications of hydrogen (H₂) release (as muh as 8 wt%) by UV irradiation at room temperature under mild ambient conditions [4], and the solid-acid catalyst [5] that convert C₂H₅OH to C₂H₄ and water. In the presentation, synthesis, characterization, and application of HB sheets will be introduced.

Acknowledgement
This work was done with Mr. H. Nishino, Prof. T. Fujita, Dr. N. T. Cuong, Dr. S. Tominaka, Prof. M. Miyauchi, Prof. S. Iimura, Dr. A. Hirata, Dr. N. Umezawa, Prof. S. Okada, Prof. E. Nishibori, Mr. A. Fujino, Mr. R. Ishibiki, Mr. T. Goto, Dr. S. Ito, Dr. Tateishi, Prof. Niibe, Prof. J. N. Kondo, Dr. T. Fujitani, Prof. I. Matsuda, Prof. J. Nakamura, and Prof. H. Hosono.

[1] H. Nishino, T. Fujita, N. T. Cuong, S. Tominaka, T. Kondo*, et al., J. Am. Chem. Soc. 139, 13761 (2017).
[2] T. Kondo*, Sci. Technol. Adv. Mater. 18, 780 (2017).
[3] I. Tateishi, N. T. Cuong, C. A. S. Moura, T. Kondo, et al. Phys. Rev. Mate. 3, 024004 (2019).
[4] R. Kawamura, N. Cuong, T. Fujita, T. Kondo*, M. Miyauchi*, et al., Nature Communications, 410, 4880 (2019).
[5] A. Fujino, S. Ito, T. Goto, R. Ishibiki, T. Kondo*, et al., ACS Omega, 4, 14100 (2019).

Despite the high theoretical ~15wt% hydrogen content of magnesium borohydride, reusable dehydrogenation/hydrogenation cycling of pure Mg(BH₄)₂ requires economically infeasible high temperatures and pressures in excess of 300°C and 350 bar. However, many properties of Mg(BH₄)₂, such as the melting point, are readily altered through the use of additive compounds, including metal borohydrides, metal hydrides, and organics (ethers/glymes). In the case of organic additives, both the melting point and the hydrogen evolution temperature have been shown to decrease. The zeroth order description of this behavior is that organic molecules (e.g. THF) act as ligands, attach to Mg and effectively change the cation size in the Mg(BH₄)₂ salt which disrupts the solid structure. In this work we investigated a previously unreported family of non-metallic borohydrides as Mg(BH₄)₂ additives. Of specific interest were tetramethylammonium and tetra-n-butylammonium borohydrides. These non-metallic borohydrides have low melting points (<300°C), and much larger cations than Mg²⁺, similar to the zeroth order picture of Mg²⁺:THF complexes. In-situ diffraction, temperature programmed desorption, mass loss, and heat flow were used to establish a full picture of the behavior of these additive systems. With regards to phase mapping, we will report the solid solubility of the additives in Mg(BH₄)₂, the presence of line compounds predicted in literature, and the initially nucleated phases during quench. We also report the phase behavior of quenched dehydrogenated samples as well as the reversibility of re-hydrogenated material. This work demonstrates a novel additive family with high cyclability and low melting points for lowering reaction and phase change temperatures of Mg(BH₄)₂.

Complex metal hydrides have attracted great attention in the hydrogen storage community due to its high gravimetric capacity (14.9 and 8.8 wt% H₂ for Mg(BH₄)₂ and LiNH₂) and relatively small reaction enthalpy (<= 50-100 kJ /mol H₂). However, sluggish kinetics and the formation of stable intermediate compounds result in poor reversibility, high hydrogenation pressure and dehydrogenation temperature, hampering its application for onboard hydrogen storage.
In this presentation, we propose a new concept to destabilized and bypass the formation of unwanted intermediates. In Mg(BH₄)₂ system, using first-principles calculations we explored the interplay between intermediate morphology and reaction pathways. Our results verify that the effective reaction energy landscape strongly depends on the morphological features and associated chemical environment, offering a successfully explanation of the formation of intermediates, especially Mg(B₃H₈)₂, which was observed in nuclear magnetic resonance (NMR) measurements, but predicted too unstable to form in conventional bulk simulations. In addition, our joint experimental-theoretical work in the LiNH₂ system revealed that the high interface energy suppressed the formation of Li₂NH intermediate in nanoscale. Our understanding introduces the possibility of tuning solid-state hydrogen-storage materials by tailoring morphology and internal microstructure, representing a new paradigm for engineering materials that could meet established performance targets.

Fuel cell technologies (FC) make hydrogen (H₂)a promising fuel to produce electrical energy due to their great efficiency and minimal pollution emission. However, one problem is to optimize pure H₂ production. On account of 96% global H₂ production is performed by steam reforming (SR) (76% from natural gas), H₂ purification process must be improved in SR plants to achieve a short-term hydrogen economy.
In order to avoid a cooling process from SR gas in a purification step, materials with high thermal stability and selective oxidation and chemisorption capability of CO must be developed. This work summarizes Li₂MnO₃ behavior as a selective CO oxidizing-captor to purify H₂ from SR gas. Li₂MnO₃ was synthetized by the solid-state method and characterized by XRD, N₂ adsorption-desorption and SEM. To evaluate CO chemisorption process in Li₂MnO₃, dynamic thermal analyses were performed using different gas flow compositions (CO, CO-O₂, CO₂, CO₂-O₂ and N₂). These experiments showed that CO chemisorption is only produced in oxygen absence. Furthermore, CO₂ chemisorption was not evidenced at any CO₂ partial pressure.
Based on these results, isothermal experiments were performed between 550 and 700 °C into a CO atmosphere (N₂ balanced). The isothermal products were identified as Li₂CO₃ and MnO by XRD. It must be pointed out that at temperatures lower than 625 °C LiMnO₂ was identified as well. These results demonstrated that CO is chemisorbed as Li₂CO₃. It may be produced by a surface CO oxidation step (into CO₂) that involves the evolution from Li₂MnO₃ (Mn⁴⁺) into LiMnO₂ (Mn³⁺) or even into MnO (Mn²⁺).
An interesting ~2% weight loss was observed at the beginning of all the isotherms (before weight gain started). In order to elucidate the process associated to this, several thermogravimetric and catalytic experiments were carried out. Their solid and gas products were identified by XRD and mass spectrometry, respectively. The catalytic experiments showed CO₂ production at the same temperature range where Li₂MnO₃ weight loss take place. Moreover, after this ~2% weight loss occurs and before weight gain process would perform, the material composition was analyzed by XRD and ATR-FTIR, where Li₂MnO₃ was the only crystalline phase identified in additionto carbonates.
Based on these results, itmay be pointed out that an oxidation step is performed at the material surface to oxidize CO into CO₂, where some part of it is chemisorbed as Li₂CO₃, while the rest isreleased from the material’s surface. In addition, lithium diffusion from bulk to surface in Li₂MnO₃ may be triggered by oxygen vacancies formation (at surface) due to CO oxidation. This could explain wherefore chemisorption process was not performed in CO₂ atmosphere or in oxygen presence.
To further analyze Li₂MnO₃, dynamical and isothermal experiments were performed varying the CO-CO₂ partial pressures. These results demonstrated that if CO₂ is involved into the gas flow composition, the CO oxidation-capture process is displaced to higher temperatures and the total weight gain decreases as consequence of less superficial oxygen vacancies formation. Moreover, kinetic constants were calculated using the Jander-Zhang diffusion model, which can be related to ΔH^≠ by the Eyring equation. Results showed that, in addition of CO₂, kinetics diffusion become slower than those obtained into a CO atmosphere, although the ΔH^≠ values decreased.
Finally, Li₂MnO₃ was dynamical tested on H₂ and H₂-CO atmospheres (Ar balanced). These experiments showed a preference oxidation of CO over H₂. Nevertheless, ~10% of hydrogen was oxidized into water while CO was chemisorbed.

With depleting fossil fuel reserves and increasing environmental problems, novel strategies have been developed based on renewable energies. A major source of renewable energy is water, which can produce hydrogen energy in large amounts. Electrochemical water splitting is an excellent way to produce hydrogen as an energy source. For generation of hydrogen at a specific rate, under ideal conditions, 1.23 V must be supplied to the water electrolyzers to allow hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in a feasible manner. In practice, OER is a complex reaction with slow kinetics and low overpotential, hence, precious metal catalysts such as iridium and ruthenium oxides (IrO₂, RuO₂) are used to reduce the overpotential for OER¹. However, their high cost and low natural abundance limit their extensive commercialization. One of the plausible approach can be the reduction of metal loading with the use of conductive support. For metal catalysts, the reduction in size of the catalysts (formation of nanoparticles (nps)) can help in increasing active surface area. Further, for conducting support, doped graphene can be used as a support because it can tune the electronic structure of catalysts to improve specific activity. In the current research, we have studied the electrochemical activity of IrO₂ nps catalyst supported on heteroatom-doped reduced graphene oxide (rGO) as the carbon support. Along with alteration of the electronic structure of the IrO₂ nps, heteroatom-doping of graphene² also promotes ion diffusion at the electrode-electrolyte interface, improving the overall performance of the catalyst.

Graphene oxide (GO) was prepared from synthetic graphite (Sigma Aldrich) by modified Hummers’ method³. For boron doping, boric anhydride (B₂O₃, BA) was used as the precursor. Briefly, BA and GO were mixed together and ultrasonicated in water for 1 hour. The mixture was freeze dried and the obtained powder was pyrolysed at 1000 ^oC for 60 min in N₂ atmosphere. The pyrolysed sample was washed with boiling water and ethanol to yield boron-doped rGO (B-rGO). As the second step, IrO₂ nps were decorated onto B-rGO by hydrothermal synthesis at 150 ^oC for 4 h using H₂IrCl₆ as the Ir precursor. The synthesized materials were characterized for its chemical composition, morphology and electrocatalytic activity using various analytical techniques such as X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV) and linear sweep voltammetry (LSV).

XPS analysis of the synthesized catalyst, IrO₂-B-rGO revealed the presence of all constituent elements at their respective binding energies. The XPS spectra were deconvoluted to obtain the types of constituent C and B. Ir 4f peaks for IrO₂-B-rGO were obtained at lower binding energies than the unsupported IrO₂ powder confirming the change in electronic states of Ir due to boron doping. Further, EDX analysis indicated B to be present in ~2.0 wt% and Ir in 3.3, 7.1 and 19.6 wt%. TEM results exhibited the uniform dispersion of IrO₂ over the wrinkled B-rGO sheets with an average particle diameter of 1.5 nm. Electrochemical analysis in 0.5 M H₂SO₄ with Ir loading of ~140 µg cm^-2 on glassy carbon (GC) electrode showed that the onset potential was 1.41 V vs RHE for IrO₂-B-rGO which was ~90 mV lower than that of IrO₂ decorated on undoped rGO (IrO₂-rGO). Despite the low Ir content, higher current density was obtained for B-doped catalyst as compared to the IrO₂-rGO. Increase in current density for B-doped catalyst is attributed to charge redistribution in graphene lattice which alters the electronic states for Ir and provides additional active centres for catalysis.

1. C. C. L. McCrory et al., J. Am. Chem. Soc. 135, 16977−16987 (2013).
2. J. Zhang et al., ACS Catal. 5, 7244−7253 (2015).
3. S. Abdolhosseinzadeh et al., Sci. Rep. 5, 10160 (2005).

We report an investigation of potential kinetic limitations to the rate of hydrogenation of magnesium diboride. The metals Pd, Fe and Ti, known to be proficient at H-H bond dissociation, were introduced into MgB₂ by ball milling. Pd and Fe are directly introduced as crystalline metals, whereas Ti metal was introduced via the reaction between TiF₃ and MgB₂ to form Ti metal and MgF₂. XRD, FTIR, XAS and TEM data show that the additives persist as metals in the MgB₂ solid, free from significant oxidation of the additive itself (i.e. TiO₂, Fe₂O₃, PdO) as well oxidation of the MgB₂ material (i.e. no MgO, B₂O₃). The Pd in the MgB₂ material consists of two morphological forms: Pd particles of a size ~ 2 – 25 nm diameter with a d-spacing essentially the same as bulk Pd, as well as a highly dispersed Pd component within the MgB₂ matrix. The Fe additive decorates the MgB₂ particles as small particles with sizes ranging from ~ 11 – 34 nm diameter with comparatively little atomic-scale dispersal of the Fe additive. TiF₃ reacts with MgB₂ to form Ti metal and MgF₂, with the Ti and the F smeared out within the MgB₂ base solid. Sieverts-acquired MgB₂ hydrogenation rates for the Pd, Fe and Ti modified MgB₂ are higher than for commercial MgB₂, but the improvement is very modest, about a factor of two at most. H-D exchange studies were conducted to confirm H-H bond breaking in these materials. The data shows that H-H bond breaking is slowest for the bulk MgB₂ material, but much faster for the Fe, Pd and Ti modified samples for which H-H bond breaking reaches equilibrium in about 20 minutes. This work shows that H-H bond dissociation is not limiting the rate of hydrogenation of MgB₂ because extensive H-H bond breaking is already occurring after only 20 minutes whereas the initial hydrogenation to ~0.8 weight percent hydrogen takes about 100 hours. The results also show that surface diffusion cannot be limiting the MgB₂ hydrogenation rate because surface diffusion, a requirement for HD generation, is occurring very rapidly. We speculate that it is the intrinsic stability of the B-B extended hexagonal ring structure in MgB₂ that hinders the hydrogenation of this material. This supposition was supported by B K-edge TFY XAS measurements of the materials, which showed spectroscopically that the B-B ring was intact in these systems throughout most of the material.

H₂ storage constitutes a barrier in decarbonizing the energy economy and expanding H₂ utilization into large scale. Design of novel nanostructured materials is currently a high research priority toward a permanent solution to this challenging problem, and development of safe, dense, solid state media to substitute cryogenic storage. In this work, we present in-situ grown hybrid nanostructures based on graphene and CNTs supporting metalorganic framework (MOF) and metallic (e.g. Mg) clusters and nanocrystallites exhibiting controlled microstructure, porosity, and interface characteristics. The resulting nanohybrids exhibit increased storage capacity, while the stability of the active sites is preserved through the presence and controlled coating by the graphitic layers.

Increasing global population and advancement in energy-dependent devices have caused increased use of energy in consumer and industrial appliances, electronic devices, and automobiles creating an urgent need for clean and renewable energy sources. Electrochemical water-splitting is one of the greenest ways to generate clean and high-performance fuel. Water-splitting generates hydrogen and oxygen gases. The generated hydrogen gas can be used as fuel whereas evolved oxygen gas can be used in metal-air batteries or released in the atmosphere as a clean gas. The electrocatalytic properties of most of the materials for water splitting depend upon several factors such as morphology, phase purity, defects, etc. We have synthesized metal-organic framework (MOF) derived cobalt oxide and cobalt sulfide using a facile method for their application in water-splitting as electrocatalysts. 2-methyl imidazole and cobalt nitrate were used for the synthesis of MOF-derived cobalt oxide and MOF-derived cobalt sulfide electrodes. The electrode with MOF-derived cobalt oxide was synthesized via a solvothermal process, while the electrode with MOF-derived cobalt sulfide was prepared through sulfurization using a hydrothermal process. The structural and electrochemical properties of these films were studied in detail. The electrocatalytic activities of the MOF-derived cobalt oxide and sulfide were studied in 1M KOH solution for oxygen evolution and hydrogen evolution reactions. MOF-derived cobalt oxide showed overpotential of 375 mV and 224 mV to achieve a current density of 10 mA/cm² for oxygen and hydrogen evolution processes, respectively. A significant improvement in electrocatalytic properties was observed with the electrode after sulfurization producing MOF-derived cobalt sulfide. MOF-derived cobalt sulfide displayed overpotential of 278 mV and 220 mV at 10 mA/cm² for oxygen and hydrogen evolution processes, respectively. Our results suggest that a facile method of sulfurization of the MOF-derived compound is a way to achieve high electrocatalytic activities for oxygen and hydrogen evolution reactions in the water-splitting process.

Single atom catalysis (SAC) represents a promising design for the next generation of robust catalysts. These systems, which are commonly single metal atoms stabilized on carbonaceous materials, maximize catalytic performance while simultaneously minimizing the use of expensive and scarce platinum-group metals (PGM). While it is accepted that the molecular environment of a SAC is of crucial consequence to the chemical activity, the exact catalytic sites and chemical pathways responsible have been a matter of some controversy. For industrially practical non-PGM SAC, optimizing the defect environment stabilizing the single metal atom is necessary for enhanced performance. We will present work using a first principles approach for materials discovery to investigate three less studied earth abundant transition metals (V, Mo, Ta) stabilized on single vacancy and pyridinic N-doped defect moieties on graphene substrates to understand the role of electronic transfer, spin state and steric effects on SAC. Our results of all investigated metals demonstrate single atom stability on graphene substrates with both defect moieties. By modeling carbon monoxide oxidation, a common benchmark reaction, facilitated by these defect stabilized SAC with climbing image-nudged elastic band (CI-NEB) calculations, we find that all activation energies for transition metals stabilized on the pyridinic N-doped graphene surface defect are ~0.8 eV or lower. These low activation energy barriers suggest that the CO oxidation reaction would proceed at room temperature with pyridinic N-dopants present in the defect environment. The CI-NEB calculations further illustrate that SAC of Ta and V on pyridinic N doped graphene significantly decrease the activation energy of CO oxidation by 27% and 44%, respectively. Bader charge analysis reveals that the electronic charge transfer is surprisingly similar across the two defect moieties investigated for all four earth-abundant transition metals. However, the pyridinic N-dopant introduces an additional magnetic moment of 0.53 µB on average. Plotting the density of states of CI-NEB transition states shows spin destabilization of O2 peaks near the Fermi level, suggesting that these imparted magnetic moments could be destabilizing the O2 bond and hence lowering activation energies for CO oxidation in some cases. We will present results suggesting that magnetic moment could play a significant role in enhancing catalytic performance of SAC on nitrogen doped graphene substrates, as well as more traditionally understood mechanisms such as charge transfer and steric effects. Computational design of materials is used to define processing conditions for three-dimensional porous graphene structures. Fabrication and characterization of macroscopic graphene structures with high surface area and architectures to facilitate mass transport will also be presented.

Use of regenerative fuel cells (RFC) requires bifunctionality in oxygen electrocatalysis of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) to decrease weight and cost for efficient energy conversion. Many RFC based technologies have used noble metals such as Pt and its alloys (Pt/Ir or Pt/Ru) due to their efficient catalytic activity, selectivity and stability in harsh environments. However, their susceptibility to fast degradation during operation, as a result of catalyst agglomeration and poisoning, has driven research to find alternatives [1]. Non-precious transition metal oxides (TMO) including Fe₃O₄, MnO_x and Co₃O₄ have attracted significant attention as a potential candidates [2].
We utilized a composite structure where TiO₂, ZrO₂, or CeO₂ are deposited onto conductive 3D carbon structure such as graphene oxide (GO), metal-organic frameworks (MOFs) or a combination of activated carbon (AC) with GO/MOFs. The use of carbon structures is to leverage their high surface area and excellent electronic conductivity. However, GO contains oxygen based functional groups on the edges and wrinkles that provides an anchor for nanoparticle binding while its basal plane stays relatively non-reactive. To incur a catalytically active sites between the metal oxides and carbon, we functionalize the basal surface of carbon using phosphoric, hydrobromic and/or oxalic acids. After such treatment, various types of carbon structures were hydrothermally reacted with metal precursors (Ce(NO₃)₃ or ZrOCl₂) or nanoparticles (P25). For MOF structures, due their susceptibility towards degradation after the common step of pyrolysis, a thin film of metal oxides are used. However, to adhere the metal oxides properly onto the MOF, we leverage acid treatment similar to our treatment of GO.


The hydroxylated CeO₂/GO hybrids showed the best ORR and OER performance in both alkaline (0.1 M KOH) and acidic media (0.5 M H₂SO₄), in terms of onset/half-wave potential, electron transfer number, and current density (electrochemical performances) when comparing to the performance of Pt/C (for ORR) and IrO₂ (for OER). From a series of material/experimental analyses, a strong tethering of metal oxides upon the basal plane of GO prohibits restacking, and that the particle-carbon interfaces (as oppose to the particle or GO itself) dictates the performance and reaction route, as indicated in density functional theory calculations. In addition, a hybrid catalyst where TiO₂ nanodots are uniformly anchored on phosphorylated MOF by atomic layer deposition (ALD) showed an even better ORR and OER performance in 0.1 M KOH when compared the aforementioned CeO₂/GO hybrid. Materials characterization emphasizes a strong adhesion of metal oxides upon MOF structures, thus providing ample surface interactions for favorable reaction route is important. In addition, an activation of catalytic sites can be realized by a proper engineering of interfaces in each hybrid systems. Finally, we present a facile route of improving operational durability of TMO/carbon hybrids both in alkaline and acidic media: interfacing with an activated carbon after a proper acid treatment.

This project was funded by NASA Advanced STEM Training and Research (ASTAR) Fellowship.

References:

[1] G. Wu and P. Zelenay, Acc. Chem. Res., 46, 1878, 2013.
[2] T. Islamoglu, A. Atilgan, S. Moon, G. Peterson, J. DeCoste, M. Hall, J. Hupp, and O. Farha, Chem. Mater., 29, 2672-2675, 2017.

Metal nanoparticles with high index facets and controllable shape have demonstrated excellent catalytic performance because of their unique surface chemistry. Generally, the selection of proper ligand plays an important role for the shape-control of nanoparticles. Here, different kinds of alkylphosphines with high temperature solution reduction method are adopted to synthesize metal nanostructures. With increasing chain length of alkylphosphines, different morphologies of copper nanoparticles were synthesized in a hydrophobic system, such as nanocubes, tetrahedron nanoparticles, nanowires, and nanosheets. All these copper nanostructures demonstrated excellent catalytic performance and selectivity to carbon dioxides reduction reaction. Such simple strategy is also extended to the design of other metal nanoparticles for their potential applications in catalysis, sensor, and electronics.

Complex metal oxides such as metal tungstates, niobates, cuprates, and vanadates, have long been considered as promising absorber materials for photoelectrochemical (PEC) cells. In particular, the recent demonstration of self-passivated bismuth (Bi)-containing FeWO₄ thin films with suitable photovoltage for coupling to a photocathode, suggests significant potential for FeWO₄-based photoanodes. In this study, we explore the growth of FeWO₄ thin films with the laser plasma control afforded by pulsed laser deposition (PLD). In PLD, thin film growth is mediated by a laser-generated plasma that exhibits rich chemistry and spatiotemporal phenomena. The chemical diversity, variety of gas backgrounds, and shockwave characteristics of the laser plume are highly conducive to kinetic control of materials synthesis. The laser-induced plasma allows wide tunability of plasma parameters. This includes control of the strong gradients of density and temperature that cause deviations from local thermodynamic equilibrium. For all these reasons, synthesis of Bi-containing FeWO₄ by PLD may allow tuning of conductivity, exploration of additional co-doping with other metals, and control of defect concentrations. Using planar and cylindrical Langmuir probes, we have measured the ion density, kinetic energy distribution, and electron temperature in Fe-, W-, and O-rich plumes produced by KrF excimer laser ablation of solid targets that are suitable for growth of FeWO₄. These plasma measurements during PLD conditions show that the plasma density can be adjusted over several orders of magnitude in the 10¹⁸-10²⁰ m^-3 range, consistent with a correspondingly broad range of deposition rates. Changes in target composition have significant impact in the kinetic energy distribution of ions in the plume. Oxygen-rich targets ablated with a typical 1.4 J/cm² laser fluence and 3.0 mm² spot area, lead to plasmas with wide ion kinetic energy distributions with high fractions of ions in the 80-90 eV range. Similar irradiation conditions for W-rich targets result in plumes dominated by kinetic energies below 5 eV. Thin films grown with changes in laser plasma conditions between these extremes show measurable variation in stoichiometry. We will discuss x-ray photoelectron results on these films that allow correlating changes in W oxidation state with plasma plume composition and kinetic energy. Thin film crystal quality, as measured by x-ray diffraction, will also be discussed in relationship to film stoichiometry and laser plasma characteristics during growth.

The incorporation of energy harvesting systems into architectural elements of buildings is gaining attention as a new possible renewable energy technology for the production of electricity. Luminescent solar concentrators (LSC) represent a promising and cost-effective complement to existing semiconductor photovoltaic (PV) technologies that could be employed as semitransparent windows to contribute to the building’s energy consumption. Typically, an LSC consist of an optical waveguide that collect sunlight from a large-area window and concentrates the emission on the smaller-area edges. The emission is obtained from luminescent materials such as organic dyes, rare earth ions or semiconductor quantum dots in the LSC that absorbs the sunlight and, by a stokes shift, emit photons at wavelengths that are better suitable for absorption by the PV devices coupled at the edges of the LSC, reducing the costs of photovoltaic power generation.
Organic dyes have been the most commonly used luminophores for LSC applications mainly due to their high quantum yield and in some cases its low degradation rate, however, organic dyes no only tend to absorb and convert only a relatively small region of the solar spectrum, but also, they present a large spectral overlap between the absorption and emission regions, which increases the reabsorption losses and prevents the viability of fabrication of large-scale efficient devices. On the other hand, colloidal semiconductor nanoparticles or quantum dots (QDs) exhibit very attractive optical properties and have attracted a lot of attention as potential luminophores for LSC applications. Colloidal QDs are promising candidates since their properties mainly arises from the materials they are made, their size and their surface passivation. Also, QDs can be engineered to manipulate their energy bandgap and the stokes shift values to suppress effectively the reabsorption losses.
Here, we report the fabrication of one-pot synthesized and self-contained photoluminescent films based on silicon quantum dots (SiQDs) and its employment on luminescent solar concentrators (SiQDLSC). Silicon quantum dots were synthesized from reduction of (3-aminopropyl) triethoxysilane (APTES) by sodium ascorbate. Besides the reduction of APTES into silicon quantum dots, a silicate matrix host is obtained, which is further employed as the support for the photoluminescent film in the fabrication of the LSC. The size of the synthetized SiQDs were measured by dynamic light scattering and electron microscopy techniques obtaining an average size of 3 nm. Both SiQDs and SiQDLSCs transmittance, absorbance and photoluminescent properties were characterized. The synthesized colloidal SiQDs exhibit an abrupt increase of optical absorption below 425nm extending to the ultraviolet region while emitting photons in a broad band extending from 450 to 600 nm with maximum intensity at 525 nm. SiQDLSC were fabricated in two sets of different dimensions: 1) 50x50x3mm and 2) 25x25x1mm. Set 1 shown a transmittance above 85% while set 2 exhibited above 90% transmittance over the visible region of the electromagnetic spectrum. Both sets shown the absorbance characteristics of the SiQDs decreasing the transmittance (increasing the absorbance) below 425 nm. Photoluminescent properties of the SiQDLSC were measured at the edges of the devices, obtaining broad bands of emission that were slightly blue shifted to around 500nm due to the expected oxidation of the SiQDs surfaces. However, the fabricated SiQDLSC preserve the desired stokes shift that is necessary to prevent reabsorption losses. These results demonstrate a feasible and very attractive method of fabrication of SiQD-based luminescent solar concentrators that presents good transmittance as well as the photoluminescent stokes shift effects desired for their incorporation as PV windows.

Hydrogen (H2) is one of the mostly used gases in industry. The term hydrogen economy dates back to the early 1970s in the context of energy storage and electrical energy ¹. Since, photoelectrochemical cells (PEC) were studied in recurring episodes for solar H2 production ². MRS Meetings are having PEC Symposia since 2009 ³. Since 2015, H2 mobility with commercial available fuel cell cars has become a reality ⁴, which requires a wide network of H2 fuel infrastructure ⁵. Most H2 for industry use today is of fossil origin, but H2 can be produced sustainable by electrolysis. The Swedish consortium for Artificial Photosynthesis is working since 1994 ⁶. In Switzerland, PEChouse started in 2007 as an umbrella activity for the development of PEC technology for solar H2. The Joint Center for Artificial Photosynthesis (JCAP) in California is a large scale research program with a substantial hydrogen economy component ⁷. More clusters, centers and consortia are spreading ⁸. Since about 7 years, there is in Europe a large consortium ⁹ preparing for a large scale R&D initiative, aiming at renewable, artificial fuels and chemicals, in which hydrogen production plays a central role (SUNRISE) ¹⁰.
SUNRISE aims at meeting the goals of the Paris Agreement by decarbonization of the atmosphere. By providing large scale production of green H2, and converting the CO₂ with H2 to solar fuels and base chemicals, SUNRISE wants to reshape the European energy landscape towards a circular and decentralized energy system. Production, storage and conversion of hydrogen are thus key technologies, which need to be further developed and integrated in existing infrastructure and made compatible with the built environment and the natural landscape.
The SUNRISE Consortium encompasses a diverse assembly of disciplines including economy, Life Cycle Assessment, social sciences, engineering, and the basic sciences in physics, chemistry and biology. By a concerted effort, SUNRISE aims to bring disruptive technology from a low readiness level to commercial maturity within a 10-year period.

1. D.P. Gregory et al.: The Hydrogen Economy, in Electrochemistry of Cleaner Environments, (Springer, Boston MA, 1972).
2. A. Braun et al.: J Mater Res 25, 1 (2010).
3. A. Braun et al.. J Mater Res 31, 1545 (2016).
4. Q.L. Chen, A. Braun. MRS Energy & Sustainability 4 (2017).
5. A. Braun: Von der Nordsee bis Venedig: Mit Wasserstoff und Brennstoffzelle Europa “erfahren“. KD Publishing, 2019, ISBN 978-1790981984.
6. A. Magnuson, S. Styring: Aust J Chem 65, 564 (2012).
8. M. Capezzali et al.: PEChouse: Photoelectrochemical watersplitting for solar production of hydrogen (Swiss Federal Office of Energy, 2007).
7. N. Lewis: Joint Center for Artificial Photosynthesis. Abstr Pap of ACS 241, 1 (2011).
8. V. Artero et al. Artif Photosyn 79, 193 (2016).
9. A. Thapper et al. Green (Germany) 3, 43 (2013).
10. A. Abbott: Europe's next euro1-billion science projects: six teams make it to final round. Nature 566, 164 (2019).

As more nations make long-term commitments to clean infrastructure, it is critical that we better understand technologies that act at the interface of traditionally decoupled systems. Hydrogen (H2) will likely lie at the interface of energy storage, transportation, and industry. Here, we use ex-ante techno-economic analysis to characterize pathways for H2 delivery. We evaluate the impact of different process system parameters on the levelized H2 delivery cost ($/kg H2), and identify barriers and opportunities for achieving industrial competitiveness. Three carrier market applications are studied: transmission, which we define as long-distance delivery from a large H2 source to an end use; distribution, which we define as short-distance delivery from a city-gate to fueling stations within the city; and transmission-distribution, which we define as long distance delivery from a large H2 source all the way to fueling stations. For each of these market applications, we evaluate the energy intensity and other key system level performance metrics for high pressure (350 and 500 bar) truck transportation, cryogenic liquid H2 truck transportation, and carrier-based truck transportation. Ancillary equipment and operation conditions, including systems at the H2 source and refueling station which are unique to adsorption systems are included in the analysis.

For a base transmission application, where 50 Mg H2/day is delivered 100 km from a gas terminal to an end point application, the delivery cost is approximately $1.6/kg H2 and $1.8/kg H2 for 500 bar and 350 bar trucks, respectively. Liquid H2 transport is less competitive at this short distance, and costs $3.1/kg H2. A truck packed with metal organic framework (MOF) Ni2(m-dobdc) and modeled at 100 bar and 200K has a delivery cost of $7.7/kg H2. Transmission costs for the MOF-H2 system can reach as low as $1.8/kg H2 (on par with 350 bar trucks) in a scenario where driverless trucks are employed, a highly durable material is developed (15,000 cycles), material costs are $10/kg MOF, and where bed porosity and pellet porosity are substantially lower (or rather increasing H2 uptake without increasing upstream cooling requirements).

For the distribution application, the delivery cost is dominated by the CAPEX and OPEX of the refueling stations. For a base scenario where 50 Mg H2/day is delivered 1 km from a city-gate gas terminal to distributed fueling stations, the delivery cost is between $3.4/kg H2 and $4.6/kg H2 for 500 bar and 350 bar trucks, respectively, and $5.3/kg H2 for liquid H2 transport. Liquid trucks for distribution perform better in larger markets (40% lower costs for a 120 Mg/day market). The MOF-H2 systems studied performs poorly in this market application (above $20/kg H2) as 10x more fueling stations and trucks are required to meet the same daily H2 demand as compared to the liquid or high pressure systems.

These results demonstrate capabilities of the hydrogen delivery models developed at the Lawrence Berkeley National Laboratory for the Department of Energy HyMARC program. Any path to substantial greenhouse gas emissions (GHG) reduction will include market transformations in the transportation sector, as it remains the single largest primary source of GHGs in the United States and the third largest primary source globally. Advances in carrier materials such as liquid organic hydrogen carriers (LOHC) and MOFs, which exhibit a wide range of tunability for hydrogen storage, could radically change the way hydrogen is stored and transported globally to meet fuel demand in zero direct emission vehicles. In this research work, we present a framework for characterizing and benchmarking the performance of emerging carrier systems.

Formic acid (FA) is a potential material for transporting hydrogen with a higher volumetric density (53 g H₂/L) than compressed hydrogen gas (ca. 40 g H₂/L at 700 bar). Hydrogen can be released from FA in a catalytic process which is exergonic (ΔG = -33 kJ/mol) and can therefore be used to generate hydrogen pressure without a mechanical compressor. An equimolar quantity of carbon dioxide is also generated which needs to be separated from the hydrogen. Hydrogen can also be generated from formate salts (FS) in a reaction with water with close to zero free energy change. Gaseous separation is not required in this case since the product is a bicarbonate salt. In aqueous solution, faster kinetics are found for a mixture of FA and FS compared to either end member. We have studied reaction rates, the degree of conversion, and product distribution for FA, FS and their mixtures as a function of composition, pH, catalyst, and temperature. The results will provide valuable inputs technoeconomic modelling to assess the suitability of these compounds as hydrogen carriers.

In spite of the rapid development of hydrogen fuel cell (HFC) technologies, the unstable nature of hydrogen fuel presents complications for the versatile application of this energy production method. Alternatively, direct ethanol fuel cells (DEFC) are a competitive method of clean energy production due to their non-toxic, abundant, and high energy density fuel. A common barrier for modern implementations of these technologies is their dependence on high-cost precious metal catalysts. Furthermore, the search for alternative catalyst materials is limited by a poor understanding of the relationship between reactivity and structural defects. The promising performance of silver as part of heterogeneous catalytic nanoparticles, where under-coordinated sites are prevalent, motivates on investigation of ethanol on Ag(111) defects. In this study we utilize temperature programmed desorption (TPD) experiments and plane-wave density-functional theory (DFT) to investigate fundamental electronic properties and adsorption mechanisms of ethanol on realistic metal surfaces in search for alternative lower cost catalytic materials. TPD experiments determine an analyte adsorption energy by increasing the temperature of the substrate in a high vacuum environment, showing higher adsorption energy of ethanol to defect sites on Ag(111) compared to smooth surfaces. In order to understand the atomic-scale origin of the increased reactivity of under-coordinated surface atoms we used the JDFTx software to calculate theoretical ethanol adsorption. Adsorption on Ag(111) terrace sites as well as on structural defects such as kinks, vacancies, and step edges was modeled using theory with and without van der Waals interactions to investigate the bonding character at each site. The trends in theoretical adsorption energy, geometry of the adsorbate, and bonding character at different defect sites complements and deepens our understanding of the experimental TPD spectra. Therefore, fundamental insights obtained from this study provide a pathway for development of commercially viable, effective catalysts for ethanol oxidation reactions.

The fluid catalytic cracking process with zeolite catalysts is used to produce a large fraction of the world’s gasoline. Although the process has been known for many years, the field remains very active as interest in the mechanism and applications involving new chemical feedstocks such as shale gas pose new challenges. In-situ electron microscopy offers new insights into the structural evolution of zeolites during chemical processes such as growth or steaming, which is used to convert the material to the catalytically active phase. We report the synthesis and characterization of a model FCC material during the different stages of growth using ex-situ scanning and transmission electron microscopy. Based on different growth conditions, zeolite-Y, which is the precursor to the active catalytic phase, defective zeolite phases, which are catalytically inactive, or other phases can be favored. Finally, we report the behavior of the different zeolites under intermediate temperature steaming in situ in an environmental scanning electron microscope. The results are promising towards method development for in-situ observation of zeolite structure evolution in both scanning and transmission electron microscopy for fundamental understanding of how nucleation and growth occur in an isolated, model system and how the structure evolves under high temperatures and pressures.

Energetic nanomaterials (ENMs) find applications in solid-state propellants and explosives. Yet, the large heat release in the Al nanoparticle (NP)-based first-generation ENMs were offset by hindered detonation rates due to the fuel-oxidizer diffusion lengths and rates being compromised by excessive oxide shell formations and NP aggregations.1-3 Efforts have been made tune their energetic behaviors by tailoring their interfacial structures that can control the oxide shell formation while leading to excessive internal stresses within the metallic cores. Yet, there exists weak fundamental understanding and considerable challenges in the rational design and synthesis of such nanostructured architectures. To this end, carbon (C) coatings on Al NPs facilitate safe handing while promoting enhanced activities due to the added advantages of the coating itself oxidizing into gaseous products (CO2, CO etc.) without any residual ash formation, while allowing the C shell to retard NP aggregations. But, the challenge remains in the facile yet, chemically clean colloidal synthesis of these encapsulated NPs without contaminating and/or oxidizing the metal cores. Here, we address this challenge through rational design and structure-property characterizations of graphitic shell coated Al NPs (< 20 nm sizes) dispersed in pyrolyzed C matrices via laser ablation synthesis in solution (LASiS) to preserve high surface areas and interfacial properties of Al NPs.4 Such nanostructures allow tailored design of interfacial structures that can either lead to strain energy manifestation or, rate-controlled release of solid propellants under high pressure/temperature to prevent oxide shell-mediated surface passivation. Energetic activities of the C/Al composite NPs were tested via Laser-induced Air Shock from Energetic Materials (LASEM) technique at the US Army Research Laboratory, Aberdeen Proving Ground, MD. We demonstrate that synthesis parameters such as organic solvents, laser flux and ablation times can be tuned to provide superior control on NP sizes/aggregation, composition, crystallinity, metastable structures and, in turn, their energetic behavior with the aid of the C shell nanostructures and matrices. The study unveils synthesis-structure-property relations in LASiS-based manufacturing of composite ENMs capsuled in graphitic shells that are safe to handle and undergo kinetically controlled energy release under desired conditions. Such Al/C-based composite ENMs can be immediately employed for high energy density munitions in defense applications.
References:
(1) Mukherjee, D.; Wang, M.; Khomami, B.: AIChE Journal 2012, 58, 3341-3353.
(2) Mukherjee, D.; Rai, A.; Zachariah, M. R.: Journal of Aerosol Science 2006, 37, 677-695.
(3) Park, K.; Lee, D.; Rai, A.; Mukherjee, D.; Zachariah, M. R.: Journal of Physical Chemistry B 2005, 109, 7290-7299.
(4) Davari, S. A.; Gottfried, J. L.; Liu, C.; Ribeiro, E. L.; Duscher, G.; Mukherjee, D.: Applied Surface Science 2019, 473, 156-163.

Freshwater is critical to humankind’s survival, as well as to global health and economic development. Providing clean water in a safe, reliable and affordable manner is a considerable challenge as demand increases with a growing population. Although 71% of the Earth’s surface is covered by water, most of it is saltwater and therefore non-potable. Therefore, large-scale and efficient deployments of seawater desalination are greatly needed. Traditional desalination technologies such as reverse osmosis (RO) and multi-stage flash (MSF) are limited by efficiency and cost. Solar vapor generation, on the other hand, is an emerging and promising technology that takes advantage of clean and abundant energy from the sun by converting the solar energy to thermal energy.
More recently, efforts have been made to reduce heat loss[1][2], enhance the broadband absorption of sunlight[3], and design efficient water supply channels. Meanwhile, various materials such as carbon-based materials[4], plasmonic nanoparticles[5], polymer-based materials and metal oxides have been explored in an attempt to further increase the efficiency of solar water evaporation.
However, there are only few studies regarding the radiation loss coming from the broadband solar absorbers, which emit thermal radiation to the environment like a blackbody in the infrared region. Compared with broadband absorbers, the selective solar absorbers with strong solar absorptance and low infrared emittance can efficiently absorb the solar spectrum while largely suppressing the heat loss, resulting in the enhancement of solar evaporation efficiency. Nevertheless, many existing selective absorbers involve either expensive materials or complicated fabrication protocols, which limit the potential for scalable applications. Therefore, developing solar steam generation devices that takes into consideration light absorption, heat loss, and feasible scalability is challenging but also desirable.
In this study, we report on selective solar absorber with uniform self-assembled nanowires fabricated by an electrochemical deposition process. We demonstrate that the solar-thermal performance can be further enhanced by applying selective solar absorbers, compared with graphite broadband absorbers. Our results provide a feasible path for selective solar absorbers with cost-effective and scalable fabrication processes that may be beneficial for various solar-based applications.
[1] G. Ni et al., “Steam generation under one sun enabled by a floating structure with thermal concentration,” Nat. Energy, vol. 1, no. 9, pp. 1–8, 2016.
[2] D. Hou et al., “Hydrophobic nanostructured wood membrane for thermally efficient distillation,” Sci. Adv., vol. 5, no. 8, p. eaaw3203, 2019.
[3] L. Zhou, S. Zhuang, C. He, Y. Tan, Z. Wang, and J. Zhu, “Self-assembled spectrum selective plasmonic absorbers with tunable bandwidth for solar energy conversion,” Nano Energy, vol. 32, no. November 2016, pp. 195–200, 2017.
[4] Y. Li et al., “Graphene oxide-based evaporator with one-dimensional water transport enabling high-efficiency solar desalination,” Nano Energy, vol. 41, no. September, pp. 201–209, 2017.
[5] L. Zhou et al., “3D self-assembly of aluminium nanoparticles for plasmon-enhanced solar desalination,” Nat. Photonics, vol. 10, no. 6, pp. 393–398, 2016.

Barium titanate (BTO) is a perovskite material used in energy storage applications because of its high dielectric constant [1,2]. Wada et al. discovered that the size of BTO nanoparticles strongly affects their dielectric constant [2]. Particles with diameters above 300nm exhibited a dielectric constant of 4000, but a sharp increase in dielectric constant to over 15,000 was observed at a diameter of 70 nm [2]. These intriguing, yet highly contested results have motivated us to develop a process that further investigates the relationship between BTO nanoparticle size and their dielectric constant. With this goal in mind, we have developed novel methods of (1) functionalizing BTO nanoparticles using ball milling and (2) integrating functionalized BTO nanoparticles into a polymer matrix using injection molding. These processes enable us to create a solid colloid from which the dielectric constant of the nanoparticles can be extracted. Here we present our methods of nanoparticle functionalization and nanocomposite fabrication. We also introduce the results of our investigation, which focused on characterizing and measuring the dielectric constants of nanocomposites containing BTO nanoparticles with sizes ranging from 50 to 500 nm. We will also discuss our method of extracting the dielectric constant of the BTO nanoparticles in the polymer matrix and present the effect of nanoparticle volume loading on the dielectric constant of the nanocomposite.

Sandia National Laboratories is a multimission 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.

[1] Park et al. Journal of Korean Physical Society, Vol. 49 (2006): S680-S683.
[2] Wada et al. Japanese Journal of Applied Physics 42.Part 1, No. 9B (2003): 6188-195.

Today the technology around generating and storing efficient and sustainable energy is rapidly evolving and hydrogen technologies offer versatile options. This presentation will provide an overview of the U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office’s early-stage R&D activities in hydrogen production and storage and fuel cell technologies within the Office of Energy Efficiency and Renewable Energy. The presentation will focus on their relevance to the evolving energy storage needs of a modernized grid, and discuss R&D needs and challenges. Specific examples of advanced materials research relevant to address energy storage challenges will be provided. The presentation will also cover DOE’s H2@Scale initiative which will enable innovations to generate cost-competitive hydrogen as an energy carrier, coupling renewables, as well as nuclear, fossil fuels, and the grid, to enhance the economics of both baseload power plants and intermittent solar and wind, enhancing resiliency and avoiding curtailment.

Developing viable solutions for the efficient production and storage of hydrogen requires understanding chemical process occurring at solid-gas, solid-liquid, and solid-solid interfaces in advanced materials. Probing behavior of these interfaces under operating conditions presents significant challenges; however, predictive modeling offers an opportunity for providing key insights into interface chemistry, particularly when operating in tandem with high-fidelity experimental characterization techniques. Within the DOE Hydrogen Materials—Advanced Research Consortium (HyMARC) and the HydroGEN Advanced Water Splitting Materials Consortium, we are using multiscale models to understand properties of reactive interfaces for the production, storage, and delivery of hydrogen. I will provide an overview of our materials modeling strategy within these consortia, ranging from first-principles calculations of interface chemistry to continuum methods for microstructure-level properties. I will then review some of our recent activities for simulating thermodynamic and kinetic properties of hydrogen-related materials. Specific examples will be given of how these computational models have helped to elucidate mechanisms of interface chemical reactions, the formation of new phases, and the impact of solid-state interfaces on key reaction pathways. I will also show how simulations have been combined with experimental probes to improve models and obtain new understanding of materials interfaces under operating conditions. Finally, I will discuss how this understanding is being used to guide new strategies for improving materials functionality for storage, generation, and delivery of hydrogen. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

As more nations make long-term commitments to clean infrastructure, it is critical that we better understand technologies that act at the interface of traditionally decoupled systems. Hydrogen (H2) will likely lie at the interface of energy storage, transportation, and industry. Here, we use ex-ante techno-economic analysis to characterize pathways for H2 delivery. We evaluate the impact of different process system parameters on the levelized H2 delivery cost ($/kg H2), and identify barriers and opportunities for achieving industrial competitiveness. Three carrier market applications are studied: transmission, which we define as long-distance delivery from a large H2 source to an end use; distribution, which we define as short-distance delivery from a city-gate to fueling stations within the city; and transmission-distribution, which we define as long distance delivery from a large H2 source all the way to fueling stations. For each of these market applications, we evaluate the energy intensity and other key system level performance metrics for high pressure (350 and 500 bar) truck transportation, cryogenic liquid H2 truck transportation, and carrier-based truck transportation. Ancillary equipment and operation conditions, including systems at the H2 source and refueling station which are unique to adsorption systems are included in the analysis.

For a base transmission application, where 50 Mg H2/day is delivered 100 km from a gas terminal to an end point application, the delivery cost is approximately $1.6/kg H2 and $1.8/kg H2 for 500 bar and 350 bar trucks, respectively. Liquid H2 transport is less competitive at this short distance, and costs $3.1/kg H2. A truck packed with metal organic framework (MOF) Ni2(m-dobdc) and modeled at 100 bar and 200K has a delivery cost of $7.7/kg H2. Transmission costs for the MOF-H2 system can reach as low as $1.8/kg H2 (on par with 350 bar trucks) in a scenario where driverless trucks are employed, a highly durable material is developed (15,000 cycles), material costs are $10/kg MOF, and where bed porosity and pellet porosity are substantially lower (or rather increasing H2 uptake without increasing upstream cooling requirements).

For the distribution application, the delivery cost is dominated by the CAPEX and OPEX of the refueling stations. For a base scenario where 50 Mg H2/day is delivered 1 km from a city-gate gas terminal to distributed fueling stations, the delivery cost is between $3.4/kg H2 and $4.6/kg H2 for 500 bar and 350 bar trucks, respectively, and $5.3/kg H2 for liquid H2 transport. Liquid trucks for distribution perform better in larger markets (40% lower costs for a 120 Mg/day market). The MOF-H2 systems studied performs poorly in this market application (above $20/kg H2) as 10x more fueling stations and trucks are required to meet the same daily H2 demand as compared to the liquid or high pressure systems.

These results demonstrate capabilities of the hydrogen delivery models developed at the Lawrence Berkeley National Laboratory for the Department of Energy HyMARC program. Any path to substantial greenhouse gas emissions (GHG) reduction will include market transformations in the transportation sector, as it remains the single largest primary source of GHGs in the United States and the third largest primary source globally. Advances in carrier materials such as liquid organic hydrogen carriers (LOHC) and MOFs, which exhibit a wide range of tunability for hydrogen storage, could radically change the way hydrogen is stored and transported globally to meet fuel demand in zero direct emission vehicles. In this research work, we present a framework for characterizing and benchmarking the performance of emerging carrier systems.

Metal nanoparticles with high index facets and controllable shape have demonstrated excellent catalytic performance because of their unique surface chemistry. Generally, the selection of proper ligand plays an important role for the shape-control of nanoparticles. Here, different kinds of alkylphosphines with high temperature solution reduction method are adopted to synthesize metal nanostructures. With increasing chain length of alkylphosphines, different morphologies of copper nanoparticles were synthesized in a hydrophobic system, such as nanocubes, tetrahedron nanoparticles, nanowires, and nanosheets. All these copper nanostructures demonstrated excellent catalytic performance and selectivity to carbon dioxides reduction reaction. Such simple strategy is also extended to the design of other metal nanoparticles for their potential applications in catalysis, sensor, and electronics.

Hydrogen fuel cell technology is gaining momentum throughout the transportation industry for use in a variety of applications - from on road light duty to heavy duty. This presentation will highlight Toyota's image for hydrogen fuel cells in our electrification strategy and our need for a robust supply of renewable low or zero CO2 hydrogen.

Reduction in the cost of polymer electrolyte membrane water electrolyser (PEMWE) technology is urgently required for the realisation of commercially viable green hydrogen production. However, at present up to two thirds of the manufacturing cost of a PEMWE stack is associated with the current collector components, with platinum-coated titanium a common material of choice. This presentation will describe in situ measurements of the local potential at the current collectors during PEMWE cell operation, demonstrating for the first time that the corrosion potential of the anode current collector is completely decoupled from the potential of the anode electrode due to the low ionic conductivity of the deionised water phase. This new observation opens up the possibility of using cheaper materials such as carbon and carbon-coated stainless steel for anode current collector components, which could lead to a dramatic decrease in the cost of the technology.

Among various materials for hydrogen storage, alloys and intermetallics forming hydrides are one of the most important classes due to their high volume density, reversibility and safety. Within the class of conventional metals and alloys for hydrogen storage, the body centered cubic (bcc) alloys based on early 3d transition elements (Ti, V, Cr...) represent one of the most promising class due to their high hydrogen storage capacity up to 2 Hydrogen per metal atoms (H/M). These alloys show two plateaus at low and ambient pressure in the Pressure-Composition-Isotherms. But, for practical applications only the second plateau is available reducing the reversible quantity of hydrogen that could be stored. Consequently, new metallurgical concepts and materials are stringently required to develop more efficient multifunctional hydrides.
Recently, a new paradigm of alloying strategy has emerged based on the original concept of multi-principal-element alloys (MPEAs), initially proposed to develop materials with enhanced mechanical properties. The principle is laid on the mixing of elements close to the equimolar proportion for systems up to five and more containing elements. This mixing may lead to the formation of simple single-phased solid solutions (body centered cubic-bcc, face centered cubic-fcc and hexagonal close packed-hcp). The formation and the stability of such phases are still under consideration and seem to be based on several chemical and physical quantities such as, configurational entropy, mixing enthalpy, atomic misfit, valence electron concentration. Among MPEAs, alloys with at least five principal elements with atomic concentrations in the range 5 to 35 % are called high entropy alloys (HEAs). Most of reports concerning these alloys describe their structure, microstructure and mechanical properties, whereas functional properties such as, hydrogen sorption, are only scarcely investigated.
We present here the study of hydrogen absorption properties of MPEAs based on refractory metals. The TiVZrNbX (X = Mg, Al and Ta) alloys have been synthesized by classical metallurgical high temperature methods or mechano-synthesis by ball milling under protective atmosphere. To produce directly metal hydrides we have employed the reactive ball milling under hydrogen gas starting from the pure metal powders.
The properties of TiVZrNbX (X = Mg, Al and Ta), TiZrNbHfTa and related hydrides have been studied by a large set of experimental techniques: laboratory X-ray diffraction, electron microscopy, in situ synchrotron X-Rays or neutron diffraction, pressure-composition-isotherm, thermal desorption spectroscopy and differential scanning calorimetry. All the TiVZrNbX alloys are single-phase bcc and undergo a one-step reaction with hydrogen at room temperature. The single-phase TiZrNbHfTa alloy also crystallizes in a bcc phase but undergoes a two-stage hydrogen absorption reaction to a fcc dihydride phase with an intermediate tetragonal monohydride at high temperature. We suggest that the lattice distortion, δ, as defined for MPEAs, might play an important role: larger δ would favors a single-step reaction with hydrogen (bcc → bct hydride with large hydrogen content), whereas small δ would favor a two-steps phase transition (bcc → bct → fcc), as also encountered for conventional bcc alloys. The most promising alloy is the TiVZrNb composition which, despite a fading of the capacity for the first cycles, shows a stable reversible capacity around 2 wt% for further cycling.
In the light of scarce literature on the subject, hydrogen adsorption in MPEAs/HEAs is an original research topic that might open new routes for the design of promising materials for hydrogen storage.

Adsorption of molecules in functionalized and high surface area metal-organic frameworks (MOFs) is of emergent technological importance in a multitude of areas ranging from chemical separations to energy storage. We have been studying the properties of MOFs for storage and separations of industrially important small molecules such as hydrogen, oxygen, carbon dioxide, noble gases, and short chain organics. Besides the geometrical and porosity control available in MOF chemistry, the properties of the frameworks can be tweaked to elevate electrostatic interactions by exposing open metal cation sites or functionalizing ligands. Here, we discuss the information accessible from neutron scattering experiments on hydrogen ansorbed in a selection of nominally rigid MOFs with some surprising subtle flexibility that greatly enhances metal-hydrogen interactions.