Symposium EN20 : Deposition, Transformation and Reaction at Functional Interfaces for Electrochemical Energy Systems

We have been studying electrode and electrolyte materials for Li-ion batteries. In the past 15 years, we have also studied the materials for Na-ion batteries [1,2]. As we described in the papers, yearly paper numbers of Na batteries have increased drastically in 2010s. Indeed, research and development of high-performance positive/negative electrode materials for Na-ion are conducted actively. Because of larger ionic size of sodium than lithium, much wider variety of materials chemistry attracts researchers’ interests as post Li-ion battery.
From 2013, we began to expand our research target to K-ion batteries, which is potentially expected to demonstrate higher-power and higher-voltage than Li-ion batteries, in spite of heavier and larger potassium atoms [3]. We study a variet of potassium insertion materials based on our experience of lithium and sodium ones, and their electrochemistry is examined and compared in corresponding non-aqueous electrolytes. We will talk about and introduce our recent progress on alkali-metal insertion materials for Na-ion and K-ion batteries and will discuss similarity and difference compared with Li-ion ones.
References:
[1] N. Yabuuchi, S. Komaba, et al., Chem. Rev., 114, 11636 (2014).
[2] K. Kubota, S. Komaba et al., J. Electrochem. Soc., 162, A2538 (2015).
[3] X. Bie, S. Komaba et al, J. Mater. Chem. A, 5, 4325 (2017).

Lithium-ion batteries are ubiquitous in everyday life, and are transforming mobility through electric vehicles, and electricity grid through the storage of intermittent renewables. Metrics such as energy density, lifetime and safety are controlled by phenomena that span enormous length scales, ranging from sub-Angrostroms to centimeters and beyond. Despite the significant progress over the past three decades, we still lack a complete understanding of how each length scale connects to one another, and most importantly, controls the behavior of the device. One grand challenge for materials used in lithium-ion batteries, like other so-called hierarchical materials, is to bridge the enormous span in length scales through integration of theory, advanced characterization, and data analytics. In this talk, I will provide an overview of our group’s recent activities on addressing this challenge through (1) a bottom-up approach, that is, understanding the fundamental nature of battery operation at the ion/electron, particle and agglomerate length scales, and (2) a top-down approach, that is, analyzing massive set of battery cycling data to discover new battery management protocols. This approach of merging predictive and data-driven design of lithium-ion batteries has already contributed to breakthroughs in several electrode materials.

Lithium metal is most attractive anode material for high-energy-density Li-ion battery. However, the strong reducing nature of lithium metal results in poor stability and low coulombic efficiency in lithium batteries. Despite decades of research efforts to stabilize lithium metal anode, the development of lithium battery is still greatly impeded by the lack of knowledge about lithium-stable materials chemistry. So far, only a few materials are known to be stable against Li metal.
We will present our recent work, which provides novel strategies of using nitrides to stabilize lithium metal anode. We uncovered many lithium-stable materials out of chemistry across the periodic table, by using first-principles calculations based on large materials database. We found that most oxides, sulfides, and halides, which were commonly studied as protection materials, are reduced by lithium metal due to the reduction of metal cations. On the contrary, nitride anion chemistry exhibits unique stability against Li metal, which is either thermodynamically intrinsic or a result of stable passivation. Many nitrides materials, which were rarely investigated as lithium metal protection materials before, can be promising candidate lithium metal protection materials to achieve long-term stability.
Our work established essential guidelines for selecting, designing, and discovering materials for lithium metal protection. In addition, we also proposed multiple novel strategies of using nitride materials and high nitrogen doping to form stable solid-electrolyte-interphase for lithium metal anode. Our computation results provide novel guidance and strategies for stabilizing lithium metal anode and would contribute to the development of high-energy-density rechargeable lithium batteries.

Nowadays, substantial works have been devoted to electrochemical energy storage and conversion due to the increasing global energy demand. The individual nanowire/sheet electrochemical devices can establish an intrinsic relationship between the electrochemical performance and electronic transport. Meanwhile, it is a fantastic platform to tune the electrochemical behavior (including electrochemical energy storage and electrocatalytic) with external fields, which is meaningful on understanding the reaction mechanism and further developing new optimize strategy.¹

In the electrochemical energy storage field, graphene has been widely used to enhance the device-performance due to its high conductivity and large surface area. However, it is still unclear how graphene influences the electrochemical performance and reaction mechanisms of electrode materials. To detect this mechanism in depth, we designed a single-nanowire electrochemical probe to explore the intrinsic mechanisms of the electrochemical reactions in situ.² It is demonstrated that the high capacity is resulted from increasing MnO₂ NWs intercalation capacitance, and the pores in the graphene provide channels for fast ion diffusion without degrading the rate of electron transport. Such devices are further used to detect the ion transport in Li/Na ion based energy storage devices.³ Our results show that the ions would choose the shortest pathway along the radial direction to intercalate into the nanowire structure.

In the electrocatalytic field, we designed an in-situ testing platform with individual Ni-graphene nanosheet based OER devices.⁴ We demonstrated that the oxygen acts as a barrier to reduce the concentration of OH⁻ ions at catalyst surface, slowing down the charge transfer process and OER kinetics. By removing oxygen in the electrolyte, a significant decrease in Tafel slope of over 20% and an early onset potential of 1.344 V vs. RHE are achieved. Afterwards, the individual electrocatalytic devices were applied to understand how does the external electric field tune the HER behavior. Increasing the back gate voltage from 0 to 5 V, the overpotential of MoS₂ nanosheet decreases from 240 to 38 mV.⁵ Such strategy is further extended to VSe₂ nanosheet.⁶ Our results indicate that HER performance improves along with the increasing of the negative back gate voltage. Besides, charge transfer resistance and high-frequency time constant drop dramatically, which demonstrates a much faster charge transfer process.

Reference
1. Mai L, Yan M, Zhao Y. Nature 2017, 546(7659): 469.
2. Hu P, Yan M, Wang X, Han C, He L, Wei X, et al. Nano letters 2016, 16(3): 1523-1529.
3. Xu X, Yan M, Tian X, Yang C, Shi M, Wei Q, et al. Nano letters 2015, 15(6): 3879-3884.
4. Wang P, Yan M, Meng J, Jiang G, Qu L, Pan X, et al.Nature Communications 2017, 8.
5. Wang J, Yan M, Zhao K, Liao X, Wang P, Pan X, et al. Advanced Materials 2017, 29(7).
6. Yan M, Pan X, Wang P, Chen F, He L, Jiang G, et al. Nano Letters 17(7): 4109.

Transforming transportation and the electricity grid with high performance, low cost energy storage requires development of beyond Li-ion technology and innovations in electrodes and electrolytes, alike. Proposed technologies such as multivalent systems (e.g., Mg²⁺, Ca²⁺ and Zn²⁺) have attracted increased interest. While these systems are inherently quite different, many of them suffer from lack of suitable electrolytes. To address the need for novel and optimized electrolytes, an automatic high-throughput computational infrastructure has been constructed and coupled with experimental analysis. Here we present our multi-scale modelling approach for understanding the the solvation structures, the stability, the conductivity and other relevant properties relevant for multivalent energy storage. We uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potential, e.g., at the metal anode. We elucidate systematic correlations between molecular level interactions and composite electrolyte properties, such as electrochemical stability, solvation structure, and dynamics. We find that multivalent electrolytes are highly prone to ion pair formation, even at modest concentrations, for a wide range of solvents with different dielectric constants, which have implications for dynamics as well as charge transfer. In particular, at Mg metal potentials, the ion pair undergoes partial reduction at the Mg cation center (Mg²⁺→Mg⁺), which competes with the charge transfer mechanism and can activate the anion to render it susceptible to decomposition. Specifically, TFSI⁻ exhibits a significant bond weakening while paired with the transient, partially reduced Mg⁺. This instability is contrasted to the behavior in Zn electrolytes, where the origin of anodic stability for a range of nonaqueous zinc electrolytes is traced to the solvent. Finally, we apply our insights to the design of novel salts and demonstrate realized synthesis and electrochemical validation.

Commercial rechargeable lithium-ion batteries (LIBs) use LiCoO₂ (LCO) as the cathode, which shows relatively low energy capacity (~140 mAh/g) and suffers poor cycle life performance. To develop next-generation LIBs with high capacity and long cycle life, surface modifications on cathodes have been applied. We start with thin layers of Al₂O₃ synthesized from sol-gel method to coat on the LCO surfaces and to test the optimal conditions, namely thickness and annealing temperature, for LIB performances in both half-cell and full cell configurations. The best performance (longest cycle life with high retention rate) is found to be 0.5 wt% coated Al₂O₃ annealed at 600°C. After identifying the best thickness for surface coating, we are moving to test new types of Li⁺ conducting surface coating, Li₂SrSiO₄, as predicted by density functional theory on LCO-based (LiNi_xMn_yCo_1-x-yO₂) cathodes for high-density LIBs with long cycle life.

Lithium cobalt oxide (LiCoO₂, LCO) is one of the most extensively studied/used cathode materials for rechargeable lithium ion batteries. Well-defined, single crystalline LiCoO₂ thin films and multilayered structures are highly desirable for fundamental investigations of the charge/discharge processes. In our work, epitaxial LiCoO₂ thin films with different orientations and strain states were grown by pulsed laser deposition (PLD). Direct lithium contact was made under a scanning transmission electron microscope (STEM) using an in situ Nanofactory holder. The structural and chemical evolution of the constructed LCO thin film electrode upon lithium contact were studied by STEM, EELS, EDX, and NBD to better understand the reaction products, lithium diffusion pathways, and intercalation induced phase changes, in particular occurring at the interfaces and antiphase boundaries.

Room temperature Sodium ion batteries (NIB) may offer key benefits over other commercial battery technologies such as lithium-ion batteries and lead acid batteries particularly in cost and safety. Sodium is the fourth most abundant element on the planet and is found in sea water. Therefore not only is it a low cost material but it is not geographically limited in terms of supply, unlike for lithium. Safety benefits include the low toxicity of sodium compared to lead acid batteries and the potential for safe transport of cells at zero state of charge and volts. NIB technology however is still in its infancy and despite recent advances, significant knowledge gaps in terms of cell design, life-time and performance still exist. Here we discuss a novel sodium ion cell chemistry based upon Sn-containing nickelate sodium layered oxide materials and a hard carbon. The energy density and the life-time is significantly affected by the balance of the anode to the cathode, and the voltage ranges of the cell. The influence on cell design is discussed, and the performance characteristics and failure mechanisms presented. In particular the failure mechanisms relating to the structural changes of the cathode during cycling and the relationship to the charging profiles and cycle life.

The research on room temperature sodium ion batteries (NIB) started alongside lithium ion batteries (LIB). Due to the higher energy and power density compared with NIB, LIB gained the higher industrial interest. With the increasing amount of portable devices and stationary energy storage technologies to support renewable energy generators, the demand for Li is increasing¹. Na is a much more abundant material, it is not geographically limited and offers an alternative to Li ion technology, particularly if volumetric energy density is not the main driving factor for its application. The chemistry of NIB and LIB are similar and are based upon a Li (or Na) containing inorganic transition metal cathode and a carbon anode. However, Na cannot be used with a graphite anode as Na ions become trapped within the graphite layers. Hard carbon (HC) materials do not show this issue and therefor are used in NIB. Furthermore, HC offers a potentially sustainable anode material as many other materials such as coconut shells and banana peels can be used^2,3. In HC the mixed sp² and sp³ hybridisation leads to a cross-linking between the layers and a lack of long range ordering in the c direction. This decreases the average resistivity of the HC particles compared to graphite as the resistivity in graphite differs between parallel and perpendicular orientation to the c direction [4]. Although electronic conductivity is essential, the influence of the ionic transport of carrier ions within the materials and composite electrode is important as well.
Publications concentrate on ionic transport mechanisms and characteristics within the solid state phase of the active material. The investigation of these characteristics in composite electrodes is widely neglected in the literature. Within this work electrochemical techniques are used to determine the ionic and electronic properties of HC composite electrodes. Galvanostatic intermittent titration technique, electrochemical impedance spectroscopy and cycling were used to characterise different HC electrode compositions to investigate the effects of additives on the overall half-cell performance. Hence, electronic and ionic conductive additives were used to design various HC composite electrodes. The electrodes were used to analyse apparent diffusion coefficients, surface layer formation, and ohmic resistance depending on state of charge and health. The influence of electrode parameters such as tortuosity, porosity and volume changes have been investigated to identify the limitations of HC. The outcome of these experiments will be used to modify electrode parameters to improve the cycle life and cycling performance required by the desired applications.
[1] J. Barker, et. al, Electrochem. Solid-State Chem., 6, A1 (2003)
[2] Y. Li, et. al, Adv. Energy Mater., 1 (2016)
[3] D.A. Stevens, J.R. Dahn, J. Electrochem. Soc., 1271 (2000)
[4] H.O. Pierson, p. 61, Handbook of carbon, graphite, diamond, and fullerenes, William Andrew (1993)

Currently, sulfur encapsulation in high surface area, nanoporous conducting carbon is the most widely studied approach to improve the cycling stability of Li-S batteries. However, the relatively large amount of high surface area carbon results in two fundamental problems with this approach. First, a large amount of electrolyte volume to sulfur (E/S) ratio (typically > 20 ml_E/g_s) is needed to fully wet the porous sulfur cathode. Second, the large amount use of high surface area carbon greatly decreases the overall energy density in the system, especially for volumetric energy density, and makes it difficult to compete with other battery technologies.
Here, we report a soft gel encapsulation approach for rechargeable Li−S cell under lean electrolyte conditions. The polymer gel immobilizes the electrolyte and confines polysulfide within the ion conducting phase, enabling smooth charge transfer on the interface under a very lean electrolyte condition of E/S of 3.3 mL_E/g_S and good cycle life. The cell failure mechanism study using solid NMR indicates that the passivation of the cathode which needs further investigation (Nano Lett. 17, 3061, 2017). To solve the critical passivation issue of cathode especially under lean electrolyte condition, we proposed a new approach that does not depend on the conventional sulfur encapsulation with high surface area carbon was proposed to reduce electrolyte absorption. The new approach generates a large spherical porous agglomerated particles with self-sustaining structures to avoid cathode passivation, leading ~100% sulfur utilization with good cycling (Nature Energy 2, 813, 2017).
In addition, the dissolution of polysulfide would cause the sulfur redistribution more or less during cycling in the conventional solvating electrolyte system, which is considered to harmful for the long-term cycling of Li-S batteries. Our recent research on non-solvating electrolyte suggests that it would be alternative new strategy for sulfur batteries to avoid active material redistribution enabling stable cycling.

Lithium-sulfur (Li-S) batteries have been considered as a promising candidate for next-generation energy storage devices, owing to the extremely high theoretical specific capacity and energy density of sulfur. However, the shuttle effect of lithium polysulfide hampered its commercial application. Trapping of polysulfides requires appropriate molecular functionalities built into a suitable nanostructure in the sulfur cathode. In this work, we present a scalable, room temperature, one-step approach to fabricate gelatin-encapsulated hollow sulfur nanospheres (GEHS) for sulfur cathode, allowing excellent control over electrode design from nanoscale to macroscale. The typical scanning electron microscopy (SEM) images of the gelatin-encapsulated hollow S nanospheres have been carried out. These SEM images show the particle size is highly monodispersed and particles are in the range of 170–220 nm. The stability was also checked for the different batteries by cycling at different rates. The GEHS cathode, cycling at 0.1, 0.2, 0.5, and 1.0 C rates, displayed reversible capacities of about 989, 893.7, 844.4 and 832 mA h g^-1, respectively. When the rate was switched back to 0.1C, the electrode nearly regained its original capacity of about 967 mA h g^-1. The rate capability of GEHS cathode is clearly superior to that of normal cathode, indicating that the GEHS cathode material is highly robust.

Acknowledgements
Financial support from the National Natural Science Foundation of China (No. 51672020) is gratefully appreciated.

Lithium metal batteries and sodium metal batteries (LMBs and NMBs) are an emerging alternative to conventional lithium ion batteries (LIBs) due to their greatly improved energy. Despite the heavy focus on specific energy (energy per weight) in scientific literature, it is the energy density (energy per volume) that is the primary consideration for most portable, stationary and even automotive applications. Researchers are pursuing LMB and NMB cathodes such as S and Se, which have the potential to provide 2-5 times capacity of LIBs. Selenium possesses similar chemical and electrochemical properties to sulfur, but orders of magnitude higher electrical conductivity, giving it a performance edge. Our approach here is fundamentally different from that of previous studies. Rather than seeking to create a nanostructured high – surface area electrode based on selenium and a nano – carbon, we achieve the opposite by creating a low surface area monolithic electrode-grade film. On a per volume basis, a dense electrode is twice as energetic as the same electrode when it is fabricated into a micro or a nano powder. Because our dense low surface area Se-carbon electrodes remain nanostructured on the inside, the kinetics and cyclability remain very good. For Li storage, the cathode delivered reversible capacity of 1028 mAh cm^-3 (578 mAh g^-1) and 82% retention over 300 cycles. The electrodes yield superb volumetric energy densities, being 1727 Wh L^-1 for Li-Se and 980 Wh L^-1 for Na-Se normalized by total composite volume. Such an approach also brings the selenium system closer to commercial electrode formulations, where the surface areas are purposely kept relatively low as to minimize parasitic surface reactions with the electrolyte.

Lithium-ion batteries are promising battery technologies to provide high energy and high power for applications such as electric vehicles or electrical grids. Recent studies have observed that lithium-ion battery electrode materials (e.g. TiO₂) containing intentional structural defects exhibit enhanced electrochemical charge storage capacity. In this work we investigate the irradiation effect on structure and electrochemical response of TiO₂ nanotubes through proton ion irradiations, because irradiation is known to produce an excess of defects in a material. In addition, we invetigated heavy ion irradiation on TiO₂ single crystals to elucidate the effects if irradiating species and crystallographic orientation on defect production and microstructure evolution. We have observed defect generation upon irradiation in both nanostructured and single crystal TiO₂ samples and changes in electrochemical response in nanostructured TiO₂ anode.

While there have been recent advances in solid ion conductors exhibiting conductivities comparable to liquid electrolytes, how to best capitalize on these materials discoveries to enable new energy storage technology is currently not known. Of particular interest is the integration solid-state electrolyte to allow for the safe and stable use of metallic Li anodes. Because Li offers a ~4X increase in volumetric energy density compared to state-of-the-art graphite anodes, significant gains in cell energy density are possible.
The solid electrolyte based on garnet-type oxide, of nominal composition Li₇La₃Zr₂O₁₂ (LLZO), simultaneously exhibits fast-ion conductivity and stability against metallic Li. Typically, LLZO is studied in the polycrystalline form and made using conventional ceramic processing techniques such as solid-state powder synthesis, calcination, and densification. These process introduce defects such as pores, grain boundaries, impurities, surface contamination, etc.. This paper discusses the major outcomes of a systematic study to characterize the effect of each microstructural defect and its impact on the maximum tolerable current density at and above which Li metal propagates through LLZO.
Aspects such as grain size, grain boundary orientation, mechanical properties, surface chemistry, and external variables such as cell temperature and stack pressure will be described. EIS, DC, SEM, Raman, XPS, acoustic emission, and indentation analysis was used as an integral part of the systematic study. Altogether, the results of this study define a path towards the stable plating of Li through LLZO in the mA/cm² range. These findings could facilitate the development of viable, high energy density solid-state batteries.

All-solid-state rechargeable batteries embody the promise for high energy density, low cost, and improved stability. The success of all-solid-state rechargeable batteries is largely impeded by high resistance at electrode-electrolyte interfaces. Among all the causes for high interfacial resistance, Li deficiency has been proposed as one of the major culprit. Yet the experimental evidence is elusive due to the challenges associated with probing Li distribution. In this contribution, three-dimensional ⁷Li Magnetic Resonance Imaging (MRI) is employed to examine the homogeneity of Li distribution in a superionic conductor Li₁₀GeP₂S₁₂ within symmetric Li/Li₁₀GeP₂S₁₂/Li battery cells. ⁷Li MRI and derived Li histograms reveal depletion of Li from the electrode-electrolyte interface and increased heterogeneity of Li distribution in the bulk upon electrochemical cycling. The degree of Li loss at electrode-electrolyte interfaces, instead of solid-electrolyte-interphase formation, is determined as the dominant cause for high interfacial resistance. Significant Li loss at interfaces is mitigated via a facile modification with a Polyethylene oxide (PEO)/LiTFSI thin film. The Li/PEO-coated Li₁₀GeP₂S₁₂/Li shows excellent long-term cycling stability with minimized Li loss. This study demonstrates a powerful tool for non-invasively monitoring Li distribution at the interfaces and in the bulk of all-solid-state batteries and a convenient strategy for improving interfacial stability.

Two basic needs for the development of high-performance solid-state batteries: the discovery of fast Li-ion conductors to use as solid electrolytes and the mitigation of large interfacial resistance. Both theoretical and experimental efforts have dedicated to the discovery of lithium fast ion conductors with ionic conductivity > 1 mS cm^–1. In situ monitoring of the synthesis process will make the discovery process more efficient compared with traditional trial-and-error approach. In situ X-ray/Neutron Diffraction are often employed. In this contribution, we demonstrate in situ high-temperature NMR for following the synthesis process of solid electrolytes. The advantages of NMR over diffraction techniques are: 1) it unveils local structural disorder and defects that are not accessible by diffraction; 2) it is capable of determining ion dynamics and phase evolution at the same time. This is critical to the establishment of real time structure-property correlation and facilitate real time screening of candidates for fast ion conductors.
The heterogeneity of Li distribution in Li solid electrolytes results in both interfacial resistance and metallic Li microstructure formation. The ability to non-destructively probe Li distribution in solid electrolytes is important but challenging. 2D ⁷Li MRI was employed to monitor the distribution of Li ion concentration in a symmetric Li/c-LLZO/Li battery cell. Li deficiency layers (LDLs) near electrolyte-electrode interfaces upon electrochemical cycling were observed. The LDLs at the interfaces were associated with large interfacial impedance, which gradually increased with electrochemical cycling, and resulted in the malfunction of battery cells. In this work, we demonstrate the capabilities of ⁷Li NMR/MRI for capturing the salient facts of solid-state batteries which are inaccessible by other techniques.

Cryo-electron microscopy (cryo-EM) received the 2017 Nobel Prize in Chemistry for its ability to elucidate the nanostructure of biomolecules in their native state, revolutionizing the field of structural biology. Here, we pioneer an approach to utilize this powerful technique to enable new discoveries for batteries¹ (Y. Li, Y. Cui, et al. Science 2017, DOI: 10.1126/science.aam6014) and show that cryo-EM can potentially have a similar impact in materials science.

Whereas conventional transmission electron microscopy (TEM) studies are unable to preserve the native state of chemically-reactive and beam-sensitive battery materials (e.g. Li metal) after operation, such materials remain pristine at cryogenic conditions. It is then possible to atomically resolve individual Li metal atoms and their interface with the solid electrolyte interphase (SEI). We observe that dendrites in carbonate-based electrolytes grow along the <111> (preferred), <110>, or <211> directions as faceted, single-crystalline nanowires. These growth directions can change at kinks with no observable crystallographic defect. Furthermore, we reveal distinct SEI nanostructures formed in different electrolytes that explain why certain additives lead to better performance. With cryo-EM, we open up exciting new opportunities for scientific discovery, which will be critical for providing fundamental insight to battery materials design.


1. Yuzhang Li*, Yanbin Li*, A. Pei, K. Yan, Y. Sun, C-L Wu, L-M, Joubert, R. Chin, A.L. Koh, Y. Yu, J. Perrino, B. Butz, S. Chu, Y. Cui. “Atomic structure of sensitive battery materials and interfaces revealed by cryo-electron microscopy,” Science (2017) DOI: 10.1126/science.aam6014

*Denotes equal contribution

Lanthanum strontium cobalt ferrite (LSCF) cathodes in solid oxide fuel cells can experience rapid degradation under high operating temperatures. Coarsening of the microstructure is one of the mechanisms that lead to such degradation because of the loss of reactive surface area. Thus, insight into the dynamics of coarsening in morphologically complex LSCF microstructures is required to improve overall performance of SOFC. Serial sectioning with FIB-SEM of both as-fired and experimentally annealed microstructures was carried out to reconstruct the three-dimensional microstructures. Using the reconstructed as-fired LSCF microstructure as the initial condition, coarsening by surface diffusion is simulated with a phase-field model based on the conserved Cahn-Hilliard equation with a concentration dependent mobility term. We find that the characteristic length scale, S_v^-1, obeys the temporal power law, ~t^1/4. This is expected for microstructural systems coarsening via surface diffusion, thereby verifying the phase-field simulation code. The morphological evolution of the as-fired LSCF microstructure is examined statistically with interfacial shape distribution (ISD) and locally with isosurfaces colored with local curvatures. We find that interfaces of the microstructure are mostly saddle-shaped, which is consistent with morphology of bicontinuous structures. As coarsening proceeds, high-curvature interfaces disappear and the overall curvature distribution becomes more symmetric with respect to the average mean curvature of the microstructure. Lastly, we have calculated tortuosities of both the LSCF and pore phases. The tortuosity values are nearly constant during the coarsening process; therefore, the electrode performance is not significantly affected by the tortuosity change. The simulation indicates that, while the morphology of the structure undergoes significant change during coarsening, the connectivity of the bicontinuous microstructure remains relatively unchanged. Therefore, we hypothesize that the performance degradation is due to the reactive surface area change and reactivity reduction due to strontium segregation.

Surface cation segregation and phase separation, of strontium in particular, has been suggested to be the key reason behind the chemical instability of perovskite oxide surfaces and the corresponding performance degradation of solid oxide electrochemical cell electrodes. However, there is no well-established solution for effectively suppressing Sr-related surface instabilities. Here, we control the degree of Sr-excess at the surface of SrTi_0.5Fe_0.5O_3-δ thin films, a model mixed conducting perovskite O₂-electrode, through lattice strain, which significantly improves the electrode surface reactivity. Combined theoretical and experimental analyses show that Sr cations are intrinsically under a compressive state in the SrTi_0.5Fe_0.5O_3-δ lattice and that the Sr-O bonds are weakened by the local pressure around the Sr cation, which is the key origin of surface Sr enrichment. Based on these findings, we successfully demonstrate that when a large-sized isovalent dopant is added, Sr-excess can be remarkably alleviated, improving the chemical stability of the resulting perovskite O₂-electrodes. Furthermore, in-situ characterizations of lattice strain and surface composition of STF thin films will be discussed for clarifying the behavior of strontium cations at various temperatures.

The electroreduction of CO2 provides a promising means of converting surplus electricity and a waste emission into a sustainable fuel. In principle, it could be conducted in two steps: (i) CO2 reduction to CO; (ii) CO reduction to a more reduced, energy rich product, such as ethanol. Although several catalysts exist for the first step, only Cu-based materials show high activity and Faradaic selectivity for CO reduction. In particular, nano-structured, oxide-derived Cu shows exceptionally high CO reduction activity and selectivity at potentials as positive as -0.3 V in 0.1 M KOH.¹
I will present a series of investigations where we elucidate the origin of the superior performance of oxide derived Cu. Using temperature programmed desorption of CO, we found the CO reduction activity is correlated to the presence of a site with particularly strong binding to CO.² On this catalyst, acetaldehyde is an intermediate in the formation of ethanol.³ I will compare the electrochemical response of nano-structured oxide derived Cu to polycrystalline Cu and vacuum-prepared single crystal Cu.⁴
Finally, I will also show that dynamic time-dependent effects can enhance CO reduction on Cu nanoparticles.⁵ In particular, the transient presence of molecular O₂ can selectively accelerate methane production. Density functional theory calculations suggest that adsorbed O can lead to more favourable scaling relations between the reaction intermediates.
On the basis of our results, I will propose avenues for future improvements to this important reaction.
1 Li, C. W., Ciston, J. & Kanan, M. W. Nature 508, 504, (2014).
2 Verdaguer-Casadevall, A., Li, C. W., Johansson, T. P., Scott, S. B., McKeown, J. T., Kumar, M., Stephens, I. E. L., Kanan, M. W. & Chorkendorff, I. J. Am. Chem. Soc. 137, 9808, (2015).
3 Bertheussen, E., Verdaguer-Casadevall, A., Ravasio, D., Montoya, J. H., Trimarco, D. B., Roy, C., Meier, S., Wendland, J., Nørskov, J. K., Stephens, I. E. L. & Chorkendorff, I. Angew. Chem.-Int. Edit. 55, 1450, (2016).
4 Bertheussen, E., Hogg, T. V., Abghoui, Y., Engstfeld, A. K., Chorkendorff, I. & Stephens, I. E. L. submitted.
5 Scott, S. B., Trimarco, D. B., Bodin, A., Bagger, A., Mazzanti, N., Sørensen, J. E., Pedersen, T., Hansen, O., Stephens, I. E. L., Vesborg, P. C. K., Rossmeisl, J. & Chorkendorff, I. submitted.

Electrochemical methods represent one possible strategy for directing clean sources of electricity towards the conversion of carbon dioxide to fuels and feedstocks. Such methods enable the use of electrical potential for making and breaking chemical bonds, often replacing pressure and temperature, and allow for small scale, modular conversion of point sources of carbon dioxide. We are interested in catalysts which can selectively convert carbon dioxide to syngas, which is a key industrial feedstock than can be used to produce diverse hydrocarbons, alcohols, acids, and esters. In our work, we have developed metal phthalocyanine catalysts immobilized on porous carbon supports with high turnover frequencies for reducing carbon dioxide. Our supported catalyst has enabled mechanistic studies that provide a detailed understanding of the molecular-level picture of how metal phthalocyanines interact with carbon dioxide and the proton/electron transfers involved in the rate limiting step. Our results highlight the interplay of kinetics and transport in the widely observed behavior of metal phthalocyanines, as well as strategies for mitigating transport limitations.

Overconsumption of fossil energy has increased the CO₂ concentration in the atmosphere, leading to severe climatic problems. Toward establishing a sustainable energy system, it is desirable to develop carbon-neutral fuels, for which a promising approach is electrochemical conversion of CO₂ to useful products powered by electricity generated from renewable energy sources. However, due to slow kinetics and diverging reaction of electrochemical CO₂ reduction, high-performance catalysts are required. In this work, we have designed three copper (Cu)-complexes and one of them (CuPc) have exhibited high efficiency and selectivity for electrochemical reduction of CO₂ to methane. Using in situ X-ray absorption spectroscopy we find CuPc undergoes reversible structural and oxidation state changes to form nanometer size Cu metallic clusters, while the other two decompose irreversibly. With the help from density functional theory, the small but stable Cu clusters generated in situ during reaction is identified as the reaction active site of electrochemical CO₂ reduction and is responsible for CuPc’s high efficiency and selectivity. The insights learned from this study provide new ways to control the catalytic site at molecule level for the development of high electrocatalytic nano-materials.

Interfaces in electrochemical devices are critical, they take part in controlling charge transfer rates, can affect reactivty and stability and ultimately, device performance. Li ion batteries depend on the Solid Electrolyte Interphase (SEI) to control battery stability and charge, discharge rates. Electrochemical catalyst reactivity is dependent on adsorption at the interface as well as the interface at the support. The interfaces of these materials can be tailored to optimize wanted reactions and decrease unwanted reactions. Additives in Li ion batteries have allowed operation at voltages that would otherwise cause electrode reactions with the electrolyte leading to capacity fading. Addition of plasmonic materials to catalysts can generate near and far fields that could enhance catalytic rates and lower activation barriers. In situ and operando characterization techniques are aiding in the tailoring of these interfaces. Advances in interface manipulation within the energy storage and Catalysis areas at the Army Research Lab will be discussed

Growing concerns of the economical, societal and environmental impacts of the human age (i.e. Anthropocene) have led to finding innovative carbon management solutions to mitigate its negative effects and to move on to a more sustainable era. As a part of the effort, electrochemically converting carbon dioxide to value-added products has gained significant interest. However, developments in the field have been slow, mainly from the difficulties associated with finding efficient catalysts for multicarbon (C₂-C₃) formation. Not only it is difficult to selectively form multicarbon products from CO₂, but the overpotentials required to enable their formation is significantly high limiting the efficiency. Recently, we discovered a Cu-based electrocatalyst that could selectively reduce CO₂ to C₂-C₃ products at significantly reduced overpotentials, compared to what have been mostly reported so far. This active catalyst was made from using copper nanoparticle ensembles as precursors and inducing their structural transformation during CO₂ electrolysis. In-situ structural change led to the formation of cuboidal particles which facilitated the formation of ethylene, ethanol, and n-propanol as the major C₂-C₃ products. The onset of these products occurred as low as -0.5 volts (vs. RHE, reversible hydrogen electrode) and C₂-C₃ faradaic efficiency (FE) > 50% was acquired at only -0.75 volts. This catalytic structure also remains active for longer periods demonstrated by stable performance over 10 hours. Investigating the structural evolution in the relevant electrochemical environments has led to identifying unique traits of the process that shed light into the dynamic processes involved for materials in electrochemical conditions. Furthermore, structural characteristics responsible for selective C₂-C₃ formation have been studied to correlate with electrokinetic studies identifying CO dimerization as a rate determining step for C₂ products and a separate pathway for C₃.

The conversion of carbon dioxide (CO₂) to carbon monoxide (CO) on nanostructured silver or gold based electrocatalysts is an attractive process for a sustainable carbon cycle.^1-2 However, industrial applications in massive scale of those metal catalysts can be limited due to their expensive cost. Zinc is a non-noble metal with low price and abundant reserves. Remarkably, bulk Zn metal was historically reported to catalyze for CO₂-to-CO conversion, thus, Zn can be a promising metal to replace precious metals in the carbon dioxide reduction (CO₂RR). In this study, a porous nanostructured Zn-based electrocatalyst was synthesized by reducing from its oxide (RE-Zn) to facilitate the activity for CO₂RR. We discovered that the activation environment using saturated CO₂ gas in electrolyte in pretreatment step plays a significant role in the fabrication of Zn-based catalyst to activate the high selection for CO production in CO₂RR later. Meanwhile, using Ar gas bubbling in pretreatment environment instead CO₂ gas can lead to less CO product of the Zn-based electrocatalyst. A Faradaic efficiency reached to 78.5% is achieved on the RE-Zn activated in CO₂-bubbled KHCO₃ electrolyte, which is about 10% higher than that of RE-Zn activated in Ar-bubbled electrolyte. Moreover, the CO₂-pretreated catalyst in KCl electrolyte is highly effective in improving the selective CO production with a Faradaic efficiency of 95.3%. By high-resolution X-ray photoelectron spectroscopy studies of the interfacial surfaces on the high performing Zn electrodes, the higher amount of oxidized zinc states has been found. Thus, the active sites on RE-Zn for electrochemical CO₂RR might be induced by the oxidized zinc states.³

References
1. Kim, H.; Jeon, H. S.; Jee, M. S.; Nursanto, E. B.; Singh, J. P.; Chae, K.; Hwang, Y. J.; Min, B. K., Contributors to Enhanced CO2 Electroreduction Activity and Stability in a Nanostructured Au Electrocatalyst. ChemSusChem 2016, 9 (16), 2097-2102. (DOI: 10.1002/cssc.201600228)
2. Kim, C.; Jeon, H. S.; Eom, T.; Jee, M. S.; Kim, H.; Friend, C. M.; Min, B. K.; Hwang, Y. J., Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles. Journal of the American Chemical Society 2015, 137 (43), 13844-13850. (DOI: 10.1021/jacs.5b06568)
3. Nguyen, D. L. T.; Jee, M. S.; Won, D. H.; Jung, H.; Oh, H.-S.; Min, B. K.; Hwang, Y. J., Selective CO₂ Reduction on Zinc Electrocatalyst: The Effect of Zinc Oxidation State Induced by Pretreatment Environment. ACS Sustainable Chemistry & Engineering Just Accepted Manuscript. (DOI: 10.1021/acssuschemeng.7b02460)

To utilize intermittent renewable sources, much attention has been devoted to proton exchange membrane (PEM) electrolyzer as a promising grid-scale energy storage solution. It can convert electric energy into chemical energy in the form of H₂ which is a clean energy carrier with zero emission. Ir is the only element that has both high electrocatalytic activity towards the oxygen evolution reaction (OER) and good stability at highly corrosive environment of the anode. However, Ir is even scarcer than Pt on the earth, which hinders its development and commercialization. Although a couple of fancy Ir-Ni or Ir-Cu nanoparticles were reported for Ir minimization recently, they could not be applied to the PEM electrolyzer due to severe leaching of the secondary metal.^[1] Leached 3d metal ions contaminate PEM, which has deteriorating effect on ion conductivity and cell performance.
Although the control of Ir oxide nanostructure has been difficult, we have concentrated on tuning Ir oxide property itself for better OER performance considering its potential application. Herein, we report one-dimensional structure of ultrathin IrO₂ nanoneedles with enhanced OER electrocatalytic activity and durability. ^[2] More specifically, IrO₂ nanoneedles have a diameter of 2 nm consisting of 6~8 atomic layers and a length of approximately 30 nm. The nanoneedles were synthesized through scalable molten salt method on a gram scale. At higher temperatures than the melting point of the salt, the liquid salt served as a solvent and an oxygen donor. Aspect ratios of the particles were controlled by the amount of used shaping agent. Ultrathin structure gave sufficient electrochemical surface area as well as BET specific surface area.
Prepared one-dimensional ultrathin IrO₂ nanoneedles exhibited enhanced OER performance. The higher the aspect ratio, the better the OER activity was. Thanks to one-dimensional structure, electrical conductivity and stability increased, which made them overcome the inverse relation between activity and stability of OER electrocatalysts. Typically, higher activity causes poorer durability in the OER electrocatalysts. Our IrO₂ nanoneedles could enhance both activity and durability in the OER by the unique shape. When the nanoneedles were introduced into PEM water electrolyzer, they showed better efficiency and durability compared to the unshaped IrO₂ particles case. We believe this finding would stimuli the development of Ir based OER electrocatalysts and PEM water electrolyzer.

References
[1] J. Lim et al., Chem. Commun. 2016, 52, 5641-5644.
[2] J. Lim et al., Adv. Funct. Mater. 2017, 1704796.
l., Adv. Funct. Mater. 2017, 1704796.

Sustainable development of high efficiency, cost-effective, and durable catalysts for hydrogen production from hydrogen evolution reaction (HER) using non-precious metal is urgent to tackle the global demand for renewable and clean alternative energy to help resolve the global warming issue. Up until now, an enormous amount of research on two-dimensional transition metal dichalcogenides, cubic pyrite-phase, and transition metal phosphides have been conducted as viable alternatives to platinum catalyst. In the past decade, considerable efforts have been dedicated to maximize the number of exposed active sites, facile charge transport, and optimize the free energy hydrogen adsorption, which are key factors that primarily contribute to the HER activity through anion or cation substitution. However, it is a great challenge to comply with all of the above-mentioned factors through only anion or cation substitutions, which results in inferior efficiency compared to the state-of-the-art Pt catalyst.
Motivated by this challenge, we reasoned that by anion-cation double substitutions in the cubic pyrite-phase, a synergistic effect may occur to satisfy all of the above-mentioned factors, which may enhance the overall performance of the HER activity. Here, we present the simultaneous incorporation of vanadium and phosphorus into the CoS₂ moiety for preparing three-dimensional (3D) mesoporous cubic pyrite-metal Co_1-xV_xSP. We demonstrated that the higher catalytic activity of CoS₂ after V incorporation can be primarily attributed to abundance active sites, whereas P substitution is responsible for improving HER kinetics and intrinsic catalyst. Interestingly, due to the synergistic effect of P-V double substitution, the 3D Co_1-xV_xSP shows superior electrocatalysts toward the HER with a very small overpotential of 55 mV at 10 mA cm^-2, a small Tafel slope of 50 mVdec^-1, and a high turnover frequency of 0.45 H₂ s^-1 at 10 mA cm^-2, which is very close to commercial 20% Pt/C. Density functional theory (DFT) calculation reveals that the superior catalytic activity of the 3D Co_1-xV_xSP is contributed by the reduced the kinetic energy barrier of rate-determining HER step as well as the promotion of the desorption H2 gas process. Thus, the new Co_1-xV_xSP catalyst exhibits exceptional HER activity, which outperforms the current state-of-the-art catalysts, is one of the most promising candidates for effective non-precious metal electrocatalysts.

It is still a great challenge to prepare efficient, cost-effective and stable electrocatalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) for many energy technologies, like water splitting, fuel cell and metal-air batteries¹. The earth-abundant first-row transition-metal-based catalysts have been developed for Oxygen revolution reaction. And among numerous low-costed oxides investigated, cobalt oxides are promising for both OER and ORR^1-2. Herein, we introduced a simple strategy to synthesis the N-doped mesoporous nanofibers with an optimal non-noble metal dispersion by electrospinning. The mesoporous structure can help to increase the reaction area for catalytic reaction, while the metal elements can help tune the catalytic activity of these material. Up to now, the best multimetal combination we demonstrated is Fe:Co:Mo (with a particular ratio ), which shows a relative long life, a positive potential of 0.94V and a half-wave potential of 0.804V under the alkaline media, close to that of commercial Pt/C.

Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (No. 21671096), the Shenzhen Key Laboratory Project (No. ZDSYS201603311013489), the Natural Science Foundation of Shenzhen (No. JCYJ20170412153139454, JCYJ20150331101823677), and the Innovative Entrepreneurship Training Program of Southern University of Science and Technology (2016S14, 2017X07).

References
1. Maiyalagan, T.; Jarvis, K. A.; Therese, S.; Ferreira, P. J.; Manthiram, A., Spinel-type lithium cobalt oxide as a bifunctional electrocatalyst for the oxygen evolution and oxygen reduction reactions. Nature Communications 2014, 5.
2. Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; García-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L.; García de Arquer, F. P.; Dinh, C. T.; Fan, F.; Yuan, M.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P.; Li, Y.; De Luna, P.; Janmohamed, A.; Xin, H. L.; Yang, H.; Vojvodic, A.; Sargent, E. H., Homogeneously dispersed multimetal oxygen-evolving catalysts. Science 2016, 352 (6283), 333-337.

The growing consumption of fossil fuels and related environmental pollution is prompting huge research on a clean, safe and sustainable alternative energy source. Among various solutions, electro-driven water splitting is believed to be one of the promising and appealing strategies to achieve the conversion of electric energy into chemical energy [1]. Because of the high cost, scarcity and unsatisfied stability of the noble metal (Pt, Pb, Ru etc), transition metal based catalysts, such as metal alloy, metal carbides/nitride, metal chalcogenides, metal phosphate, metal oxides, hydroxides attracted lots of attention [2]; however, hydroxyl salts, another promising alternatives received much less attention till now. In most cases, hydroxyl salts were used as a kind of precursor or intermediate product while people underestimate the function of hydroxyl ions within the electrocatalytic system utilized.
Here we synthesized a series of hydroxyl salts, including metal phosphate hydroxides, metal carbonate hydroxides and metal sulfate hydroxides and metal nitrate hydroxides. It is discovered that the differences of constitution and morphology have a great influence on the performance of the basic salt. Among these four kinds of hydroxyl salts, metal phosphate hydroxides and metal carbonate hydroxides have a good bifunctional electrocatalytic activity for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) after proper modification in alkane solution. Then taking metal phosphate hydroxides {Co₂(PO₄)OH} as an example, we discussed the effects of hydroxyl and phosphate on the OER performance. The overpotential of Co₃(PO₄)₂, Co(OH)₂ and Co₂(PO₄)OH are 275 mV, 220 mV and 275 mV at 10 mA/cm², respectively, while the corresponding Tafel slope is 138 mv/decade, 98 mv/decade and 78 mv/decade. In this case, we can conclude that hydroxyl improves the Tafel slope while phosphate have a negative impact on both overpotential and Tafel slope. All these findings may strengthen the understanding of hydroxyl salt and explore other kinds of hydroxyl salt as highly active electrocatalysts towards efficient overall water splitting.
References
M. Fang, G. Dong, R. Wei, J. C. Ho, Adv. Energy Mater. 2017, 1700559.
M. S. Faber, S. Jin, Energy Environ. Sci. 2014, 7, 3519.

This work aims to expand our understanding of processes occurring at Gallium Phosphide (GaP)/electrolyte interfaces with adsorbed sensitizers. On native p-GaP interfaces, cationic dyes readily adsorb and inject holes upon photoexcitation. However, the sensitization appears to be mediated by a surface degradation process. Accordingly, we argue that a passivation layer is needed for long term sensitization of GaP. We will discuss possible organic passivating layers that will effectively lower the decomposition rate of the surface and provide points of attachment for increased sensitizer loading. Monolayers consisting of polar functional groups will be described for dye adsorption through electrostatic interactions. Data on planar p-GaP (100) and (111A) surfaces will be first discussed to identify rate limiting aspects. However, prospects for sensitization of macroporous p-GaP will also be discussed. Data from linear sweep voltammetry, X-ray photoelectron spectroscopy, and spectra response measurements will be presented.

Recently, as the concerns on the global warming and the depletion of fossil fuels, the most immediate scientific challenge is developing clean and renewable energy. Hydrogen has emerged as one of the promising alternatives to carbon-based fuels because hydrogen is eco-friendly energy.
There are lots of methods to produce hydrogen. One easy way to produce extremely pure hydrogen is water electrolysis, through hydrogen evolution reaction (HER).
Currently, noble metals such as platinum are generally regarded as the state-of-the-art electrocatalysts for improving the kinetics and efficiency of HER. However, their prohibitive price and scarcity limit their application industrially. It is therefore highly desirable to discover highly active and stable HER electrocatalysts composed of inexpensive and earth abundant elements.
In the last few years, boron based materials have been proposed as low-cost alternative electrocatalysts for HER. Recently, binary molybdenum borides (MoB, Mo₂B, MoB₂ and Mo₂B₄), iron boride (FeB₂) and amorphous cobalt boride (Co₂B) were reported to have excellent catalytic properties for the HER.
In this work, we have synthesized binary tungsten borides. The synthesized materials were characterized by X-ray power diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray analysis (EDX) and scanning electron microscope (SEM). Linear sweep voltammetry (LSV) was performed to determine the electrochemical properties by using a three electrodes system, and cyclic voltammetry (CV) was conducted in order to measure stability.

Electrocatalytic conversion of CO₂ into long-chain hydrocarbon represents important research directions in adding value to CO₂-based chemicals and realizing its practical application. Long chain hydrocarbons may change current fossil fuel-based industry in that those chemicals have similar energy density with gasoline, high compatibility with current infrastructure. However, most of the electrocatalysts produce C1, C2, and C3 chemicals, and methods for producing long chain hydrocarbons are not available thus far. Cu electrode is a good candidate for producing long chain hydrocarbon because several ketones and aldehyde compounds involved in aldol condensation have been proposed as reaction intermediates for the electrochemical CO₂ reduction. Although Cu electrode is the only reported catalyst that can generate such key intermediate species, it preferentially produces C₂H₄, C₂H₅OH like C2 chemicals as its final product, rather than inducing C2 dimerization. It is assumed that the currently used Cu electrode is not suitable for holding the enol tautomer of aldehyde or ketones due to its moderate affinity for oxygen. Utilization of Cu binary alloy can be one effective strategy to overcome such limitations by incorporating high oxygen affinity elements into the structure. Indeed, with Cu alloy metal catalyst, superior efficiency and new products that were not expected in a pure metal electrode have been reported recently. However, we realized that the underlying principles of their outstanding performance have not been fully addressed. In particular, possible phase segregation with concurrent composition changes, which has been widely observed in the field of metallurgy, has not been considered at all. Moreover, surface-exposed metals can easily form oxide species, which is another pivotal factor that determines overall catalytic properties. Here, to clarify the alloying effect which determines the reduction of CO₂ to long-chain hydrocarbons, the current understanding of the structural-property relationships of various alloy systems was discussed. In order to expect the alloying effect from the alloy structure, we proposed a possible microstructure and naturally occurring surface oxide species from the viewpoint of the thermodynamic stability of the alloy structure and element mixing. Inspired from the thermodynamic database, we developed a new Cu binary alloy catalyst. Remarkably, by mixing copper with high oxygen affinity metal, we observed value-added hydrocarbon products which have never been reported in Cu based system. In this manner, we expect the basic principle of material science can guide us to develop new binary alloy catalysts to further improve CO₂ conversion and, ultimately, achieve long chain hydrocarbon production.

Over the years, it is highly demanded for new energy technologies, which can provide clean and environmentally friendly solutions, to alleviate the impact of greenhouse-gases on the global environment. To make more efficient use of energy, electrochemical energy systems offer promising approaches and play key roles in energy sustainability. In a typical electrochemical process, the interfacial properties between the electrodes and electrolytes, such as the structures and morphologies, have great influence on the device performance. In order to well control the process and understand the growth mechanism of different morphologies, in this study a numerical model is utilized to investigate the formation and evolution of thin films during electrochemical deposition (ECD). The alterable parameters affecting the nucleation and growth process of thin films, such as the current, temperature, additive and pH, are taken into consideration in the model. The influence of these parameters on the formation and evolution of characteristic morphologies during ECD are also demonstrated in the simulations. Depending on the growth factors, such as the growth rate, substrate interfacial energy, etc., different types of the microstructures have been reconstructed and characterized with different growth mechanisms. According to the calculation results, a featured surface with flattened morphology is generally driven by the surface diffusion, while the factor of the deposition kinetics is inclined to roughen the surface of thin films. The ultimate aim of this research is attempted to establish a theoretical model of electrochemical deposition process to enhance the development of electrochemical energy applications.

Over the last decades, scientists have endeavored to develop nanoscopic machines and envisioned that these tiny machines could be exploited in biomedical applications and novel material fabrication. Recently, visible-/near-infrared light-driven nanomotors based on a single silicon nanowire by employing the photoelectrochemical redox reaction has been successfully demonstrated.¹ However, the conversion efficiency from light into mechanical energy is still unsatisfactory due to the inefficient photoelectrochemical reaction of redox couples, such as hydroquinone/benzoquinone, at the electrodes. To improve the energy conversion efficiency, we systematically examined a series redox couples by experimental study and numerical simulation. Their performance can be assessed by the proposed figure of merit, which involves parameters such as the overpotential and the kinetic constants of corresponding electrode reactions. This result provides the roadmap for the selection of high efficient redox couple system, which moves a step forward to efficiently harness the light energy for nanoscale mechanical motion and opens up new opportunities for the realization of many novel functions such as biocompatible nanorobots and controllable self-assembly.

1. Wang, J. et al. A Silicon Nanowire as a Spectrally Tunable Light-Driven Nanomotor. Adv. Mater., 1701451, doi:10.1002/adma.201701451.

Novel current collector consisting of C-coated Ni nanowires is loaded with Si nanoparticles as Li-ion battery anode. Ni wires are synthesized by heating Ni(Ac)₂ with glycerol at 400^oC for 40 min. Ni nanowires are produced by oxalic acid etching of Ni wires followed by H₂ reduction. Carbon coating on Ni nanowires is performed at relatively low temperature (350^oC) with acetylene to improve electrical conductivity. Commercial Si nanoparticles mixed with polyacrylic acid binder are coated onto Ni nanowires attached on Ni foam (Si/Ni NWF) as the anode of Li-ion battery. The Si/Ni NWF anode can be cycled 750 times at C/2 with capacity ~ 300 mAh g^-1 without extra carbon additive since the electrical conductivity is improved by the intimate contact between Si nanoparticles and C coated Ni nanowires. Superior stability is attributed to the void space accommodating volume expansion of Si nanoparticles.

In heterogeneous catalysis, metal-support interactions may improve catalytic performance. Pt nanoparticles dispersed on CeO₂, for example, are more active for CO oxidation than those dispersed on non-reducible metal oxides. The enhancement arises from CeO₂’s ability to donate oxygen locally along the perimeter of the metal-support interface. The ease with which oxygen may be removed from the CeO₂ lattice has been shown theoretically to depend on the interfacial atomic structure [1]. At present, though, there is little experimental data on the atomic structures that comprise the metal-support interface. Additionally, despite the fact that the metal deposition method creates the metal-support interface, the effects of different deposition methods on the interfacial structure and catalytic performance have gone largely uninvestigated.

Two deposition methods are of interest to this work: impregnation and photodeposition. Impregnation is the conventional metal deposition method, and serves as a benchmark. Photodeposition, on the other hand, is a method in which metal ions deposit as nanoparticles due to direct reduction onto the support by electrons photoexcited within the support. Photodeposition has been demonstrated to decorate nanostructured CeO₂ cubes with well-dispersed ~2 nm Pt nanoparticles [2]. In that work, however, the interfacial structures and catalytic activity were not assessed nor compared to impregnated catalysts.

The present study investigates the effect of the metal deposition method on the interfacial structure and catalytic activity of Pt/CeO₂ catalysts. Nanostructured CeO₂ cubes will be synthesized and loaded with 2 wt. % Pt via photodeposition and impregnation. The catalysts’ activity for CO oxidation will be assessed with a quartz tube microreactor coupled to a gas chromatography system. Pt particle size distributions will be determined with probe-corrected scanning transmission electron microscopy (TEM). Interfacial structures will be visualized with aberration-corrected TEM (AC-TEM). The results will be analyzed with the aim of establishing atomic-level structure-function relationships between the metal-support interface and catalytic activity. Understanding these relationships will facilitate the engineering of highly active Pt/CeO₂ catalysts in the energy and environmental remediation reactions where they are indispensably used.

[1] Vayssilov et al; Nature Materials 10, 310–315 (2011)
[2] Vincent et al; Microscopy and Microanalysis 23(S1), 966-967 (2017)
[3] We gratefully acknowledge the support of NSF grant CBET-1604971 and ASU’s John M. Cowley Center for High Resolution Electron Microscopy.

We are fabricating all-thin-film solid oxide fuel cell (SOFC) structures in order to probe fundamental transport properties of SOFC electrolytes in an out-of-plane measurement geometry using epitaxial films with ideal model interfaces. Epitaxial multilayer thin-film structures consisting of a bottom electrode (SuRuO₃ (SRO) or Ba_0.93La_0.07SnO₃ (BLSO)) and an electrolyte Sm_0.2Ce_0.8O_2-δ (SDC20) (600 nm - 1000 nm) on SrTiO₃ (100) single crystal substrates were fabricated using pulsed laser deposition. The heteroepitaxy of the multi-layer structures was confirmed by X-ray diffraction and high-resolution scanning transmission electron microscopy, and atomically sharp and structurally coherent interfaces were observed in the electrolyte/electrode bilayers. Electrochemical impedance spectroscopy (EIS) measurements of the devices at temperature range of 623 K to 823 K in air reveal electrochemical properties of SDC20 which are quantitatively consistent with known bulk transport properties of SDC20. This work demonstrates the utility of out-of-plane all-thin-film heteroepitaxial electrochemical devices as a model platform for directly investigating intrinsic transport properties of electrochemical materials in single-crystal-like heterostructures with well-defined sharp interfaces. We are also fabricating various combinatorial libraries where the composition of the cathode or the electrolyte layer is continuously varied across the library chips. Systematic measurements of microdot arrays on the libraries allow identification of optimum compositions as well as delineation of the rate determining step in the reaction. We will also discuss our progress in fabrication of epitaxial Li solid state battery structures. This work is carried out in collaboration with Yangang Liang, Xiaohang Zhang, Chris J. Kucharczyk, Huairuo Zhang, Leonid A. Bendersky, and Sossina M. Haile, and is funded by DOE, NSF, and NIST.

Ni-based materials as highly efficient and low-cost electrocatalysts for hydrogen as well as oxygen evolution reactions (HER & OER) have been developed to produce high-purity hydrogen fuel. Bifunctional catalysts with earth abundant elements often show the quite large applied overpotential originated from slow kinetics on cathode for HER and anode for OER for the overall water splitting reaction in alkaline solutions. Herein, we report that crystalline Ni₃S₂ thin films have been synthesized using low-temperature atomic layer deposition (ALD) at 250^oC without any post annealing. A new ALD chemistry was demonstrated by using bis(1-dimethylamino-2-methyl-2-butoxy) nickel (II) (Ni(dmamb)₂) and H₂S as precursors. Homogeneous and conformal depositions of Ni₃S₂ films were achieved on 4-inch wafer. Owing to their high conductivity and stability, crystalline Ni₃S₂ films were found to be highly efficient bifunctional electrocatalyst for both HER and OER in alkaline solutions. The performance of planar ALD Ni₃S₂ films is among the best performance for overall water splitting up to date. As-deposited Ni₃S₂ displays the efficient catalytic activity and stability in HER even on non-conductive substrate, e.g., SiO₂. We obtained the low overpotential of 300 mV and high turnover frequency for HER. On the other hand, our ALD Ni₃S₂ is also capable to serve as high efficient and stable electrocatalyst for OER with overpotential of 400 mV. The full cell two-electrode electrolyzers were constructed using Ni₃S₂ films as both cathode and anode. And it is able to maintain stable potential of 2.0 V at current density of 10 mA/cm² for 100 hours.

Transition metal dichalcogenides are being studied due to their unique optoelectronic properties, which can be utilized in the hydrogen evolution reaction (HER). In particular, metallic molybdenum disulfide (MoS₂) nanosheets have been studied for HER due to their higher reactivity for HER, earth abundance, low-cost, and non-toxicity, which makes MoS₂ a candidate to replace platinum for HER. In this study, we modify the fundamental electronic properties of metallic (1T phase) MoS₂ nanosheets through covalent chemical functionalization, and thereby directly influence the HER kinetics, surface energetics, and stability. We also explore the degradation mechanism for HER of the unfunctionalized 1T MoS₂ nanosheets. Chemically-exfoliated, metallic (1T) MoS₂ nanosheets are functionalized with organic phenyl rings containing electron donating or withdrawing groups. We find that MoS₂ functionalized with the most electron donating functional group (p-(CH₃CH₂)₂NPh-MoS₂) is the most efficient catalyst for HER in this series, with initial activity similar to the pristine metallic phase of MoS₂. The p-(CH₃CH₂)₂NPh-MoS₂ is more stable than unfunctionalized metallic MoS₂ and outperforms unfunctionalized metallic MoS₂ for continuous H₂ evolution within 10 min under the same conditions. With regards to the entire studied series, the overpotential and Tafel slope for catalytic HER are both directly correlated with the electron donating strength (Hammett parameter) of the pendant group on the phenyl ring. The results are consistent with a mechanism involving ground-state electron donation or withdrawal to/from the MoS₂ nanosheets, which modifies the electron transfer kinetics and catalytic activity of the MoS₂ sheet. We show that the functional groups preserve the metallic feature of the MoS₂ films, inhibiting conversion to the thermodynamically stable semiconducting state (2H) when annealed at 150 °C for 24 h in a nitrogen atmosphere. We propose that this protection is critical to maintaining the catalytically active state of 1T MoS₂ nanosheets. To test this hypothesis, we measure via X-ray photoelectron spectroscopy the chemical environment of the MoS₂ electrode after HER to determine how the p-(CH₃CH₂)₂NPh-MoS₂ electrode changes compared to the unprotected MoS₂ electrode.

We describe the spontaneous formation of a composite chalcogenide materials that consist of two-dimensional (2-D) materials dispersed in bulk and their unusual charge transport properties for application in hydrogen evolution reactions (HERs). When MoS₂ as a representative 2-D material is deposited on transition metals such as Cu in a controlled manner, the sulfidation reactions also occur with the metal. This process results in remarkably unique structures, i.e., bulk layered heterojunctions (BLHJs) of Cu-Mo-S that contain MoS₂ flakes inside, which are uniformly dispersed in the Cu₂S matrix. The resulting structures were expected to induce asymmetric charge transfer via layered frameworks and tested as electrocatalysts for HERs. Upon suitable thermal treatments, the BLHJ surfaces exhibited the efficient HER performance of approximately 10 mA/cm² at a potential of as low as -0.1 V versus a reversible hydrogen electrode (RHE). The Tafel slope was approximately 30 to 40 mV/dec. The present strategy was further generalized by demonstrating the formation of BLHJs on other transition metals such as Ni. The resulting BLHJs of Ni-Mo-S also showed the remarkable HER performance and the stable operation over 10 days without using Pt counter electrodes by eliminating any possible issues on the Pt contamination.
Ref. H. Shin, et. al., “Bulk Layered Heterojunction as an Efficient Electrocatalyst for Hydrogen Evolution”, Science Advances, 3, E1602215 (2017)

The direct methanol fuel cells (DMFCs) has attracted considerable attention as one of the most promising green power sources during the last few decades. The selectivity and sensitivity of a support material on the anodic catalyst are very essential and can highly improve the performance of the DMFCs. However, Platinum is considered to be the most important catalyst that can be used for the electro-oxidation of fuel alcohol such as ethanol and methanol. In present study, one of the Pt-based tri-metallic catalysts has been developed. The catalyst comprising NiCu bimetallic alloy nanoparticles decorated with ultra-low Pt as shell onto bimetallic alloys. The overall nanocomposite material was supported on reduced graphene oxide (rGO). The reduced graphene oxide is considered to be a promising candidate to support the catalyst in DMFCs. The synthesis has been performed by using wet-chemical method. A series of Pt-NiCu/rGO nanocomposites were synthesized with different compositions and conditions to obtain the optimal conditioned material. Afterwards, morphological and structural characterizations of Pt-NiCu/rGO composites were performed by means of scanning electron microscopy (SEM), element mapping, Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD) analysis (XRD), thermogravimetric analysis (TGA), and X-ray Photoelectron Spectroscopy (XPS). All the physical characterizations revealed the successful formation of as-desired material. Moreover, the catalytic performance of this anode material was studied by cyclic voltametry (CV) and chronoamperometery (CA) for the whole series of Pt-NiCu/rGO catalysts. The electrochemical results showed good performance for the electro-oxidation of methanol at anodic end of DMFCs. The present study opened up a broad avenue for developing lower cost Pt-based catalysts with better performance in the field of DMFCs research.
Keywords: nanocomposite; we-chemical method; catalysts; anodes; direct methanol fuel cells
Acknowledgements
This work was made possible by NPRP grant # 9 – 219-2-105 from the Qatar National Research Fund (A Member of The Qatar Foundation). The finding achieved herein is solely the responsibility of the authors.

Many reactions central to energy conversion processes such as the O₂ reduction reaction (ORR), the O₂ evolution reaction, and the CO₂ reduction reaction involve the transfer of both protons and electrons. We have developed electrochemical platforms that allow for enhanced physiochemical control at the electrode-electrolyte interface and that enable us to quantitatively modulate the kinetics of proton and electron transfer to electrocatalysts. The platforms consist of a molecular electrocatalyst appended to a self-assembled monolayer (SAM) via azide-alkyne click chemistry, which is subsequently covered by a proton-permeable membrane. By changing the length of the alkyl SAM, the kinetics of electron transfer to the catalyst can be controlled. Altering the permeability of the membrane through the use of proton carriers allows us to tune the kinetics of proton transfer to the catalyst.
Enhancing selectivity is a grand challenge for ORR catalysts, which must exclusively produce water in fuel cells. CO₂ reduction catalysts must also be selective and should not produce undesirable side products such as H₂. We demonstrate that the selectivity of a Cu-based ORR catalyst can be significantly improved by controlling proton and electron transfer rates. Under normal conditions, this non-precious metal ORR catalyst produces ~10% deleterious H₂O₂ side product. However, with properly regulated proton and electron transfer rates, the catalyst produces exclusively water. We also discuss how modulating proton and electron transfer rates in the membrane-modified electrochemical platform affects the selectivity of molecular CO₂ reduction catalysts such as metal porphyrins.

The efficiencies of electrochemical energy conversion devices such as electrolyzers and fuel cells depend on the efficacy of the electrocatalytic processes. Ab initio techniques have been successful in providing insights in these electrocatalytic mechanisms, even motivating computational design of electrocatalysts. There remain, however, many open questions, thus motivating a call for fundamental studies of the adsorption processes on well-defined model surfaces. In this talk, I will present our recent results on a combined theory-experiment approach towards the understanding of iridium oxide catalysts for oxygen evolution reaction (OER), using well-defined surfaces grown using molecular-beam epitaxy and ab initio techniques. I will first discuss our study of SrIrO₃ showing the challenge of the conventional potential-limiting-step approach in predicting the electrocatalytic activity in this system. In the second part, I will focus on IrO₂ and how cyclic voltammetry curves on single-crystal can be used to understand adsorption energetics and benchmark the accuracy of ab initio approaches in electrocatalysis.

The design of efficient and stable oxide catalysts for the oxygen evolution reaction (OER) is crucial for the development of a number of electrochemical energy conversion devices, such as electrolyzers and metal−air batteries^1-3. SrIrO₃ has recently been reported to be highly active for the OER in acid², where the active site was proposed to be IrO_x, a structural oxide created from the dissolution of Sr²⁺ in SrIrO₃. In an effort to better understand the nature of these active sites, we examine the structural evolution of model SrIrO₃ films grown on DyScO₃ (110) substrates using a combination of electrochemical measurements, synchrotron X-ray scattering, and X-ray spectroscopic studies, with an objective to identify how the Sr²⁺ dissolution activates the OER catalysts in acid. We find that SrIrO₃ evolves into a new structure at the solid-liquid interface with a thickness of ~6 pseudo-cubic unit cells. A combination of X-ray spectroscopy and theoretical calculations show that these interfacial structure were formed from a coupled diffusional exchange of Sr²⁺ and lattice oxygen (O^2-), resulting in a reconstructed Sr-Ir-O framework that protects the bulk oxide underneath from further dissolution. Our results illustrate the critical role of a coupled cation-anion exchange on the structural evolution of oxides, offering a new paradigm for achieving high activity and stability and providing a pathway toward future catalyst design for electrochemical energy devices.

References:
1. W. T. Hong, M. Risch, K. A. Stoerzinger, A. Grimaud, J. Suntivich, Y. Shao-Horn, Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis. En. Environ. Sci. 8 (2015) 1404-1427. DOI: 10.1039/c4ee03869j.
2. L. C. Seitz, C. F. Dickens, K. Nishio, Y. Hikita, J. Montoya, A. Doyle, C. Kirk, A. Vojvodic, H. Y. Hwang, J. K. Norskov, T. F. Jaramillo, A highly active and stable IrO_x/SrIrO₃ catalyst for the oxygen evolution reaction. Science 353 (2016), 1011-1014. DOI: 10.1126/science.aaf5050.
3. T. Binninger, R. Mohamed, K. Waltar, E. Fabbri, P. Levecque, R. Kotz, T. J. Schmidt, Thermodynamic explanation of the universal correlation between oxygen evolution activity and corrosion of oxide catalysts. Sci. Rep. 5, (2015) 12167-12174; DOI: 10.1038/srep12167.

Conversion of electrical energy into hydrogen gas via water electrolysis is a strong candidate as an efficient and environmentally-friendly energy storage mechanism for the compensation of the intermittent nature of leading renewable energy sources, such as solar and wind [1,2]. The key to improving the overall efficiency of water electrolysis lies in the development of high-performance catalysts for the oxygen evolution reaction (OER), the rate-limiting half-reaction of water electrolysis [2]. In particular, OER catalysts with high stability in acid are desired due to their compatibility with polymer electrolyte membrane (PEM) electrolyzers, which are more advantageous than the conventional electrolyzers in base due to operability at higher current density and lower gas crossover rate [3]. However, only a handful of materials, such as RuO₂ and IrO₂, are stable under acidic and strongly oxidizing potential conditions [2-4]. A recent breakthrough in OER catalysts is the development of SrIrO₃ epitaxial films, currently the most active OER catalyst stable in acid [5]. Interestingly, this catalyst shows enhancement in catalytic activity over operating time via loss of Sr transforming into IrO_x/SrIrO₃, the mechanism behind which is yet to be understood. This strongly motivates the study of the relationship between catalytic activity and the structural variables of the initially synthesized films.
We synthesized SrIrO₃ epitaxial films on SrTiO₃ (001) substrates under various growth conditions using pulsed layer deposition [6]. We thoroughly characterized their structural properties and compared with their OER catalytic performance in acid. Surprisingly, we discovered that the catalytic activity is enhanced linearly with the film in-plane resistivity, surface cation stoichiometry, and film mosaicity. The detailed trend and the relationship between these variables will be discussed in the presentation. The identification of strong correlations between structural variables and catalytic activity not only provides guidelines to controllably synthesize catalysts and reveals deeper insight to the development of SrIrO₃ catalyst, but also presents an effective approach to enhance catalytic activity of other electrocatalysts in general.

[1] Turner, J. A., Science 305, 972 (2004).
[2] Fabbri, E., et al., Catal. Sci. Technol. 4, 3800 (2014).
[3] Carmo, M., et al., Int. J. Hydrog. Energy 38, 4901 (2013).
[4] McCrory, C. C., et al. J. Am. Chem. Soc. 135, 16977 (2013).
[5] Seitz, L., et al., Science 353, 1011 (2016).
[6] Nishio, K., et al., APL Mater. 4, 036102 (2016).

Lanthanum strontium transition-metal oxide perovskites (La_xSr_1-xMO₃) are among the most promising materials for low temperature solid oxide fuel cell (SOFC) cathodes [1]. However, segregation of strontium to the surface of these perovskites has been observed to occur during synthesis, and at exposure elevated temperatures [2-4]. Surface strontium segregation affects the oxygen exchange at the perovskite-gas interface, which can alter the activity for oxygen electrocatalysis [5,6].
To deepen our understanding of the fundamental causes of strontium segregation and its impact on SOFC performance, we use aberration corrected transmission electron microscopy (TEM) to examine the chemical, crystallographic, and morphological changes that occur at the surface of lanthanum strontium cobaltite (La_0.7Sr_0.3CoO₃) nanocubes upon exposure to oxidizing and reducing atmospheres at elevated temperatures.
We synthesize La_0.7Sr_0.3CoO₃ nanocubes for TEM analysis using a molten salt synthesis method [7], with a mixture of sodium nitrate and sodium nitrite used as the solvent. Identical location ex-situ TEM, and in-situ TEM allow us to image individual nanocubes as they are exposed to different atmospheres. The high spatial resolution of TEM allows us to characterize and compare the changes occurring at the perovskite-gas interfaces for different La_0.7Sr_0.3CoO₃ crystal facets, and in the bulk of the La_0.7Sr_0.3CoO₃ nanocubes. Crystallographic and morphological changes that occur in the La_0.7Sr_0.3CoO₃ nanocubes are explored using lattice resolution imaging in bright-field TEM and in annular dark-field (HAADF) scanning TEM (STEM), whilst chemical changes are explored with hyperspectral mapping using electron energy-loss spectroscopy (EELS).
Our results will provide important feedback to help guide the design of more durable perovskite cathodes for SOFCs.

References:
[1] Gao, Z., et al. Energy Environ. Sci. 9, 1602-1644 (2016)
[2] Crumlin, E. J. et al. Energy Environ. Sci. 5, 6081 (2012).
[3] Chen, Y. et al. Chem. Mater. 27, 5436–5450 (2015).
[4] Wang, H. & Barnett, S.A. ECS Transactions 78, 905-913, (2017).
[5] Feng, Z. et al. J. Phys. Chem. Lett. 5, 1027–1034 (2014).
[6] Rupp et al. J. Mater. Chem. A. 3, 22759-22769, (2015).
[7] Yang, J. et al. J. Alloys & Compounds 508, 301–308 (2010).

Acknowledgements: We gratefully acknowledge the support of NSF grant DMR-1308085 and ASU’s John M. Cowley Center for High Resolution Electron Microscopy.

The oxygen evolution reaction (OER) is a kinetically sluggish and particularly efficiency-limiting reaction with four-electron redox process, which often requires a high overpotential to break O–H bond and form O–O bond. Some state-of-the-art precious-metal electrocatalysts and their oxides (IrO₂ and RuO₂) endowed with high OER activity are seriously restricted by their scarcity, high cost, and inferior stability leading to low market penetration of these catalysts. Transition metal based nanocomposites are regarded as cheap and stable OER electrocatalysts for mass production of oxygen. In particular, NiFe (oxy)hydroxide-based catalysts are among the best-performing nonprecious OER catalysts in alkaline media likely attributed to a strong interaction upon incorporating Fe into NiOOH. It's widely believed that the performance of one-component OER catalyst may be further boosted through being combined with another active component for OER benefiting from enhanced electron transfer efficiency and intense synergetic effects within the strong coupled interface of different components, which becomes one of the most important research targets in the field of OER electrocatalyst.
Herein, by choosing self-supported Ni-Co-P two-dimensional (2D) nanosheets fabricated on the commercial carbon cloth (NiCoP/CC) through a facile electro-deposition route integrated with succeeding in-situ phosphorization process, we present a novel architecture comprising NiCoP/CC followed by electro-depositing NiFe-LDH nanosheets (NiFe-LDH/NiCoP/CC) to form an open three-level hierarchy via the interaction of p-n heterojunction at the semiconductor-semiconductor interface of NiFe-LDH/NiCoP/CC hierarchical nanosheet structure, where NiCoP and NiFe-LDH manifest as p- and n-type semiconductor respectively. Based on the Tauc plots and Mott-Schottky plots, the positions of the conduction band (CB) and the valence band (VB) for NiFe-LDH/NiCoP/CC have been determined, which demonstrates the p-n heterojunction catalyst can be a great boost for the OER through smoothing and promoting electron transfer among NiFe-LDH, NiCoP, and CC. The performance test can further confirm that the catalyst with p-n heterojunction effect deliver extremely low overpotentials (η) of 216, 244, and 255 mV at current densities (j) of 10, 100, and 300 mA cm^-2 respectively; a small Tafel slope of 35.7 mV dec^-1, and great long-term durability in 1 M KOH electrolyte. Impressively, the j at 1.485 V (vs RHE) of NiFe-LDH/NiCoP/CC catalyst is ~10 and ~30 times as those of NiCoP/CC and NiFe-LDH/CC rather than a simple addition, to which the strong p-n heterojunction synergic effect is a key contribution. This proof-of-concept strategy of utilization of p-n heterojunction synergic effect enables the exploration of more efficient and economic OER electrocatalysts and exploits a promising avenue for functional nanocatalysts for use in clean energy technologies.

As one of the cleanest sources of energy, hydrogen is abundant on earth but always found as part of a compound, such as water. The electrolysis of water is considered as a clean mean for large scale hydrogen gas production. However, this large-scale production is still hindered by the high cost and scarcity of noble metal catalysts such as Pt. Recently, non-noble metal materials have emerged as highly active electrocatalysts for the hydrogen evolution reaction (HER) to produce hydrogen gas. Among all the non-noble metal catalysts, our recent research found that MoB₂ [1] exhibits high activity and chemical stability. In addition, density functional theory (DFT) calculations show that several surfaces of MoB₂ are active and the optimum evolution of H₂ occurs on the graphene-like B-terminated {001} surface. Geyer et al. [2] reported that FeB₂ is also highly active for overall water splitting in basic solution. However, TiB₂ [3] is not as active as MoB₂ and FeB₂ for HER reaction. To examine the distinct activities of metal diboride as HER catalysts and how the metals could affect the graphene-like boron layer, DFT was applied to investigate the H-surface adsorption process on MB₂ (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W). Our results indicate that the H-surface binding energy decreases (becomes more negative) as the electronegativity of the metal increases. Therefore, the electron transfer between metal and boron is one of the key parameters to control the HER activity of MB₂. In addition, VB₂ behaves similarly to MoB₂, thus it is predicted to be a highly active HER catalyst candidate. We have also probed the activity of MgB₂ and AlB₂, both are found to be poor HER catalysts. Hence, the type of chemical bonding (covalent, ionic, metallic) in these compounds also plays an essential role on their catalytic activity.

References
[1] P. R. Jothi, Y. Zhang, J. P. Scheifers, H. Park, B. P. T. Fokwa, Sustainable Energy Fuels 2017, 1, 1928.
[2] H. Li, P. Wen, Q. Li, C. Dun, J. Xing, C. Lu, S. Adhikari, L. Jiang, D. L. Carroll, S. M. Geyer, Adv. Energy Mater. 2017, 7, 1700513.
[3] C. S. Lim, Z. Sofer, V. Mazánek, M. Pumera, Nanoscale 2015, 7, 12527.

Understanding and controlling reactions at electrode-electrolyte interfaces remains a major challenging in electrochemical energy storage and conversion, due to the complexity of these systems (e.g., for both the solids and electrolytes) and significant structural and chemical changes that can take place as a function of applied potentials. I will present recent work in which we seek to isolate and understand the role of interfacial reactivity in these systems through in-situ, real-time, observations of electrochemically driven reactions at well-defined model electrode-electrolyte interfaces using X-ray reflectivity. I will discuss two distinct types of electrochemical energy systems: 1) lithium ion battery chemistries in which energy is stored upon lithium ion incorporation into electrodes (e.g., insertion reactions in Li_xMn₂O₄ and conversion reactions in NiO) using well-defined thin-film and multilayer electrode structures; and 2) the electrochemical reduction of CO₂ at bismuth electrolyte interfaces in ionic liquid based electrolytes solutions where we observe dramatic changes in the electrode structure prior to onset of CO₂ reduction. These observations provide new insights into the complex reaction pathways of these materials through in operando observations.

Acknowledgment: This work was supported as part of the Center for Electrochemical Energy Science (CEES) and the Fluid Interface Reactivity Structure and Transport Center (FIRST), which are Energy Frontier Research Centers funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The work was done in collaboration with J. Medina-Ramos, T. Fister, S. S. Lee (ANL), X. Chen, G. Evmenenko, M. Bedzyk (Northwestern), D. Lutterman R. Sacci (ORNL), A. van Duin (Penn State), M. Neurock (U. Minnesota) and J. Rosenthal (U. Delaware).

Internal reforming of hydrocarbons in solid oxide fuel cells (SOFCs) can enhance electrochemical conversion efficiencies by eliminating the need for external fuel reforming [1]. Long-term stability of SOFCs may be limited by carbon deposition from the fuels onto the anode catalyst, causing deactivation of the ceramic-metal composite structure. Ni/CeO₂-based catalysts may inhibit carbon deposition through a process involving lattice oxygen exchange. Natural gas consists primarily of methane (CH₄) but also contains several percent of higher hydrocarbons such as ethane (C₂H₆) and propane (C₃H₈), which have been shown to deposit carbon more easily than CH₄ [2]. We are interested in understanding how CeO₂ can affect carbon deposition at the gas-solid interface by observing structural and chemical changes at the nanoscale. In situ environmental transmission electron microscopy (ETEM) provides the ability to observe atomic-level structural changes under simulated reaction conditions [3]. In situ electron energy-loss spectroscopy (EELS) has allowed dynamic changes in the local oxidation state of CeO₂ to be determined during reduction [4]. A fundamental study of the local structural and chemical changes occurring at Ni/CeO₂ interfaces under reaction conditions will elucidate the mechanisms that enable Ni/CeO₂-based catalysts to inhibit carbon deposition from light hydrocarbons.

In situ ETEM techniques were employed to investigate the atomic-level three-phase interactions occurring at the metal-support interface during carbon deposition from C₂H₆ and C₂H₄ over a model Ni/CeO₂ catalyst. Structural and chemical interfacial changes occurring during species-dependent carbon deposition were determined using atomic-level imaging and EELS. During exposure to C₂H₆, no carbon deposition occurred, and localized reduction zones formed at the Ni/CeO₂ interface through a Mars van Krevelen carbon oxidation mechanism. In contrast, less pronounced reduction zones formed during C₂H₄ exposure, and carbon deposition occurred on Ni surfaces. Rapid dehydrogenation and subsequent graphite formation occurred on Ni surfaces during C₂H₄ exposure, whereas the metal-support interface catalyzed the oxidative dehydrogenation of C₂H₆ and oxidized the resulting carbonaceous species during C₂H₆ exposure. These experiments demonstrate that the ability of the interfacial sites on Ni/CeO₂ to inhibit carbon deposition during reforming is strongly influenced by thermodynamic and kinetic considerations which may show significant variation among different hydrocarbon species [5].
[1] Gür, T.M., et al, Progress in Energy and Combustion Science 54 (2016), p. 1-64.
[2] Bierschenk, D.M., et al, Fuel Cells 10 (2010), p. 1129-1134.
[3] Tao F. and Crozier P.A., Chemical Reviews 116 (2016) p. 3487-3539.
[4] Sharma R., et al, Philosophical Magazine 84 (2004) p. 2731-2747.
[5] We gratefully acknowledge support of NSF grant DMR-1308085 and ASU’s John M. Cowley Center for High Resolution Electron Microscopy.

Nickel-based electrodes and catalysts are often utilized in high-temperature electrochemical CO₂ reduction due to their high performance and low cost. However, nickel is also an excellent catalyst for destructive carbon deposition, which can be mitigated by the use of ceria. In this work, we elucidate the inhibition mechanism during electrochemical CO₂ reduction on dense thin-film model-electrodes consisting of samarium-doped ceria, nickel, and yttria-stabilized zirconia. The results obtained via operando x-ray photoelectron spectroscopy show hat ceria-based electrodes require higher onset overpotentials for carbon deposition and have a high surface coverage of carbonate species. Our density functional theory calculations reveal the crucial role of the surface carbonates as energetic traps that inhibit carbon formation and show that this is most effective with non-stoichiometric CeO_2-δ(100) surfaces. This destabilization of carbon leads to a thin amorphous carbon layer instead of the destructive carbon nanotubes that grow on nickel without ceria present.

I will give an overview of x-ray experiments on electrocatalysts, based on our own work and on an analysis of published results. I will compare in-situ and ex-situ experiments on both model systems and on operando studies. The focus will be on x-ray absorption and resonant inelastic x-ray scattering experiments. Some recent time-resolved photo-electrocatalytic results will be discussed. I will compare x-ray spectroscopy with lab sources, synchrotrons and free-electron lasers.

For many decades, XPS (or ESCA) was the well-accepted, standard method for non-destructive chemical analysis of solid surfaces. To fulfill this task, existing ESCA tools combine reliable quantitative chemical analysis with comfortable sample handling concepts, integrated into fully automated compact designs. Over the last several years it has been possible to develop XPS systems that can work far beyond the standard conditions of high or ultrahigh vacuum. Near Ambient Pressure (NAP) XPS has become a fast growing field in research, inspiring many scientists to transfer the method to completely new fields of application. Thus, by crossing the pressure gap, new insights in complicated materials systems have become possible using either synchrotron radiation or laboratory X-ray monochromators as excitation sources under NAP conditions. Based on this experience, SPECS Surface Nano Analysis GmbH has developed an EnviroESCA as a tool for reproducible chemical high throughput surface analysis under any environmental condition.

In this presentation the physical effect Environmental Charge Compensation is introduced. Together with the concept of measuring samples in residual gas pressure of up to 40mbar this allows for fast measurements on basically every solid, liquid or even gaseous sample, that can be conductive or insulating. Results will be presented from insulating polymers, ceramics and zeolites. The drying process of materials like water soaked superabsorbent polymers and hydrogels are analyzed regarding their differences in wet and dry state. Also measurements on water and aqueous solutions, for instance NaCl, Fe₃SO₄ and Ag nanoparticles in water are presented, to demonstrate, that a chemical surface analysis of liquids is possible. Finally XPS data from biological samples, like hair, artificial biofilms and biofilms with bacteria under different pressures are shown, demonstrating the differences between NAP and UHV analysis.

Photovoltaic cells are increasingly taking advantage of heterostructure deposition technology. A similar trend is observed in the development of photoelectrodes for solar water splitting in electrochemical cells (PEC). One complification in the PEC is that the photoelectrodes are exposed to humidity or liquid aqueous eectrolytes, which, in a polarizing electrical field, can cause corrosion of the electrode material or passivation of important electronic states at the surface. For the optimization of PEC electrodes it is therefore necessary to build up complex heterostructures where the light absorption is optimized independent from the optimization of the electrocatalytic electrode surface and potential buffer layers between absorber and current collector.
We have addressed this analytical problem by employing synchrotron based photoemission spectroscopy in the resonant mode. This allows us to decompose the x-ray spectra into element specifiic and orbital specific components for the electrode surface, the bulk and and underlying buffer layer. Moreover, we have done this study under photoelectrochemical conditions using an ambient pressure spectroscopy instrument.
We find that the oxygen 2p specific component in the valence band spectra scales analog with the DC current in dark and in lght conditions, highlighting the role of the O2p holes in the water oxidation processes. More over we find that the interface between absorber and current collector forms an electronic structure with electronic states that prevent light induced photoelectrons from passing with ease to the metal oxide current collector. Engineering a buffer layer between current collector and absorber seems to "eliminate" these states with the effect that the photocurrent is significantly increased.

The search for a suitable platinum group metal-free (PGM-free) catalyst to drive the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells has intensified with the recent arrival of commercial fuel cell electric vehicles. New classes of PGM-free catalysts are constantly emerging, with the latest efforts focused on atomically dispersed transition metal catalysts derived from metal organic frameworks (MOFs). A critical step in the synthesis of these ORR catalysts is the high temperature heat treatment (up to 1100^oC) required to convert the Fe, Ni, or Co-doped MOFs into graphitic carbons containing the proposed individual metal-nitrogen active sites. We have utilized low-voltage, aberration-corrected scanning transmission electron microscopy (ac-STEM) coupled with electron energy loss spectroscopy (EELS) to establish a clear link between the dispersion of the single metal atoms and catalyst performance as measured by rotating disk electrode (RDE). In this work, we provide additional insight into the interaction between the metal-nitrogen sites and the graphitic carbon support through the application of in situ electron microscopy. By utilizing advanced micro-electro-mechanical system (MEMS)-based heating devices, it is now possible to reproduce these high temperature heat treatments within the electron microscope to directly observe the transformation of the metal-doped MOFs. In situ and ex situ experiments conducted in tandem with RDE and fuel cell measurements will be employed to elucidate the role of annealing temperature, nitrogen-doping, and metal loading on the formation of graphene-embedded metal-nitrogen sites and their subsequent impact on catalyst activity. This research is sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.

The performance of electrochemical devices such as fuel cells or electrolyzers is often dictated by material properties at interfaces. Both fuel cells and electrolyzers contain interfaces between Nafion, a proton conducting polymer film, and catalysts, and electrolytes. It is widely known that the structure and functional properties of Nafion change significantly with water content. To investigate the relevant interfaces between the Nafion, catalyst layers and gas diffusion layers, it is essential to conduct 3D characterization in the near-operando state. Using techniques developed to study biological materials, focused ion beam (FIB) tomography of hydrated fuel cell/electrolyzer components was conducted at cryogenic temperatures. A custom transfer arm was designed and built to exchange samples between a cryo-fixation station and a Quorum Technologies PP3005 cryo-stage in a FIB-SEM. The resulting 3D-reconstructions and their implications for improving the performance of electrochemical devices will be discussed.

Recently, an inexpensive “wet form” of ALD based on self-terminated electrodeposition reactions was uncovered that enables controlled formation of ultrathin films of Pt, Ir and iron group metals and alloys thereof. Common to all these systems is the role of reaction intermediates, namely adsorbed H or OH^-, in the quenching of metal deposition reactions. Further details on the mechanisms of self-terminated deposition reactions will be discussed. Likewise, the utility and relevance of the process to the synthesis of nanoparticles and thin films, and the study of bimetallic electrocatalysts will be detailed.

Template-assisted electrodeposited Fe-Ni-Co nanowires having an Invar-like composition (e.g. Fe-rich, with £ 5 wt % Co) are of interest for their low thermal expansion property, its ability to magnetically align nanowires, and to serve as a catalyst. The morphology of the nanowires is controlled by the deposition conditions (pulse vs dc), and with subsequent etching under the constraint when there is dissimilar structure or composition. Solid wires, tubular regions and novel porous structures were observed and controlled by the role of the hydrogen side reaction during deposition. Post treatment of nanowires via chemical etching, can also be used to shape the nanowires. Two different approaches were taken, 1) creating dissimilar crystalline structure within the nanowire by the choice of the deposition operating conditions followed by a select etch in structure and 2) introducing a sacrificial element, such as Cu, to selectively etch it in a passivating environment for Fe-Ni-Co. Unique thinned nanowires and nanotips were designed, and a kinetic Monte Carlo model was used to explain the role of the composition on the moment the wire breaks. In addition, Au decorated, Fe-Ni-Co nanowires were fabricated and the change in an individual wire electrical conductance determined in order to reflect the surface conditions.

Molybdenum based materials have recently emerged as highly active electrocatalysts for the hydrogen evolution reaction (HER) [1, 2]. Molybdenum borides in contrary have not been extensively studied for their HER activity at the nanoscale, however they were recently shown to be already efficient HER catalysts in the bulk (microscale) [3]. Recently, we reported the first nanoscale molybdenum boride, MoB₂ nanospheres, which outperformed the microscale sample greatly [4]. This prompted us to search for a general method for the synthesis of these materials at the nanoscale. Indeed, we have discovered a simple, one-step, low temperature solid state reaction capable of producing such nanocrystalline molybdenum borides. The structural and morphological characterizations were studied using XRD, XPS and electron microscopy. The obtained molybdenum borides nanostructures were tested for their electroactivity for HER in acidic conditions. We found a boron- and structural dependency of these nanoborides for the HER. Also, the obtained HER activity values represent a significant improvement from those observed for the already highly active bulk borides,[3] thus opening a new avenue for inexpensive nanoscale metal boride nanomaterials s highly efficient HER catalysts.

References:
1. H. Vrubel and X. L. Hu, Angew Chem Int. Edit. 55 (2012) 12703-12706.
2. P. Xiao, M. A. Sk, L. Thia, X. M. Ge, R. J. Lim, J. Y. Wang, K. H. Lim and X. Wang, Energy Environ. Sci. 7 (2014) 2624-2629.
3. H. Park, A. Encinas, J. P. Scheifers, Y. Zhang and B. P. T. Fokwa, Angew. Chem. Int. Ed., 2017, 56, 1-5.
4. P. R. Jothi, Y. Zhang, J. P. Scheifers, H. Park, B. P. T. Fokwa, Sustainable Energy Fuels, 2017, 1, 1928-1934.

At present, there is an increasing interest in the development of new highly efficient and low cost catalytic systems in the field of global energy issues.
Layered double hydroxides (LDHs) are lamellar compounds whose structure consists of positively charged brucite-like layers with interlayer anions balancing the positive charge. LDHs containing transition metals have been widely investigated since they are very promising materials for a large number of possible applications due to their versatility, tunable properties, wide range of compositions and low cost.
Recently, these materials are attracting much interest in the area of electrochemistry for applications such as batteries, supercapacitors, sensors, fuel cells and as efficient electrocatalysts for oxygen evolution reaction (OER). For all these applications a fundamental property is that the active material must be well adherent to the conductive support, thus guarantying the formation of a mechanically stable coating.
Our group has proposed and optimized an electrochemical approach, based on the electrochemical generation of hydroxides by cathodic reduction of nitrate ions, in order to obtain LDH films on any kind of conductive supports of any shape and dimension, including porous substrates and transparent or flexible electrodes. The features of the deposited LDH (amount, thickness and composition) and the adherence of the film to the electrode surface depend on the applied potential, the composition of the electrolytic solution and on the surface morphology of the electrode itself (1).
This contribution is aimed to investigate the performance of LDH films containing Cobalt or Nickel as bivalent cation and Iron and Alluminium as the trivalent one for two important energy applications, i.e. as supercapacitors and as electrocatalysts for OER. In particular our results show that LDHs containing Co as the bivalent metal can be considered good candidate for supercapacitors development, especially when the trivalent metal is Fe: actually for Co/Fe LDH the redox process significantly involves also the inner surface of the LDH, giving a peculiar electrochemical behaviour, where the capacitance has both Faradaic and charge separation origin. This guarantees a wider voltage window than the one displayed by the Al based LDH, making that material even more suitable for supercapacitor development.
As to OER electrocatalysts are concerned the best performances in terms of onset potential, current density at a fixed potential and TOF were obtained for iron-based LDHs both containing Ni or Co as the bivalent metal, our results proving that the presence of iron is crucial to significantly enhance the OER performances of LDHs (2).

REFERENCES
(1) E. Scavetta, A. Mignani, D. Prandstraller, D. Tonelli, Chem.Mater. 19, 2007, 4523–4529.
(2) Y. Vlamidis, E. Scavetta, M. Gazzano, D. Tonelli, Electrochimica Acta, 188, 2016, 653-660

Field-effect gating with solid dielectrics is the basis for modern electronics. Electrolyte gating, however, offers far higher polarizations. Indeed, electrolyte gating has been a breakthrough to electrically induce numerous phase transitions in solids [1,2,3]. These experiments are all done by dripping mm-size drops of the electrolytes onto the active sample. Compared to integrated circuit technology this approach seems “stone-age” to us. These drops are open to the environment, and allow only for limited purity and reproducibility.
Heterostructure electronic circuits have, up to now, been comprised of solid materials only. We have opened this materials space to also include liquids. We demonstrate integrated liquid capacitors and integrated liquid field effect devices which are of equal quality or even outperform standard, bulk devices. This work will accelerate discoveries based on electrolyte gating by providing a new platform, and opens a new area to exploit liquid/solid interfaces in integrated functional devices with technological promise.

[1]Yamada Y. et al. Electrically Induced Ferromagnetism at Room Temperature in Cobalt-Doped Titanium Dioxide. Science 332, 1065-1067 (2011).
[2]Nakano, N. et al. Collective bulk carrier delocalization driven by electrostatic surface charge accumulation. Nature 487, 459-462 (2012).
[3]Jeong, J. et al. Suppression of Metal-Insulator Transition in VO2 by Electric Field- Induced Oxygen Vacancy Formation. Science 339, 1402-1405 (2013).

Freestanding and robust thin-film electrodes are the basis for future renewable energy technology owing to their unique merits: i) additive-free features enable facile fabrication route and long lifetime; ii) can be easily recycled; iii) significant reduce the weight and volume of the energy devices, especially for batteries and supercapacitors; iv) can be integrated into wearable electronic systems. However, previous efforts fail to make significant progress in freestanding metal compound thin-films due to a lack of fundamental understanding and technical breakthrough in freestanding metal compound thin-films. In this work, transformative thin-film nanomanufacturing technology has been developed to fabricate freestanding metal compound thin-films using a facile and scalable electrochemical route. Both experimental and computational studies indicate that a supreme electrochemical performance can be achieved by tuning the microstructure and composition of thin-films. Outstanding energy storage and hydrogen generation properties are therefore obtained by directly using these advanced thin-films as electrodes.

Composite liquid-impregnated surfaces (LIS), whereby a thin stable film of liquid is held inside a textured solid by capillary forces, have been garnering widespread attention recently with applications in anti-fouling and anti-scaling. Corrosion and hydrogen embrittlement are broad problems in several industries, and developing surfaces that resist corrosion has been an area of great interest since the last several decades. Superhydrophobic surfaces that combine hydrophobic coatings along with surface texture have been shown to improve corrosion resistance by creating voids filled with air that minimize the contact area between the corrosive liquid and the solid surface. However, these air voids can incorporate corrosive liquids over time, and any mechanical faults such as cracks can compromise the coating and provide pathways for corrosion. In this work, we systematically study electrochemical activity and anti-corrosion properties of textured surfaces impregnated with a liquid. Since corrosion resistance depends on the area and physico-chemical properties of the material exposed to the corrosive medium, we optimize the design of liquid-impregnated surfaces based on the surface tension, viscosity and chemistry of the impregnating liquid and its spreading coefficient on the solid. We perform all corrosion experiments in a standard three-electrode cell using iron, which readily corrodes in a 3.5% sodium chloride solution, and we impregnate using Krytox, silicone oil and ionic liquids. In order to obtain textured iron surfaces, we sputter-coat thin films (~500 nm) of iron on silicon wafers textured using photolithography, and impregnate them with lubricants. We show that the corrosion rate on LIS is greatly reduced, and offers an over hundred-fold improvement in corrosion protection. Furthermore, we show that the spreading characteristics of the liquid is significant in ensuring corrosion protection: a spreading liquid that covers both inside the texture as well as the top of the texture provides a two-fold improvement in corrosion protection as compared to a non-spreading lubricant that does not cover texture tops. We also show that an increase in viscosity of the liquid scales with greater corrosion protection. We finally provide broader insights into designing liquid-impregnated electrodes specifically for electrochemical applications using ionic liquids, and study the electrochemical interactions at the three-phase catalyst-liquid-liquid interface.

Traditionally, platinum has been considered as the best catalyst for the oxygen reduction reaction (ORR), taking into account its high activity via the four electrons transfer mechanism leading to water as the final product. However, platinum is scarce, its performance degrades in time and it is very expensive. In this regard, carbon materials constitute a good option due to their easy-to-design porous properties, good electronic conductivity and high resistance against corrosion, in addition to their low-cost and wide availability. In this sense, many efforts have been made in developing low cost and highly efficient carbon based-electrocatalysts. Among all the carbon materials reported, Iron(or Cobalt)-Nitrogen-carbon-based catalysts present great potential for the application in fuel cells due to their remarkable electrocatalytic performance and excellent stability. However, the majority of the methodologies reported so far are complex, expensive and time-consuming or they use hazardous substances. Herein, a simple, template-free and cost-effective synthesis procedure for the fabrication of iron-nitrogen-mesoporous carbons is presented. The procedure is based on the straightforward carbonization of a non-alkali organic salt, i.e. citrate salt of calcium. The in-situ formed CaO nanoparticles act as templates, giving rise upon their removal to carbon materials with a porosity made up almost exclusively of mesopores. The subsequent additional heat-treatment step in the presence of urea and ammonium sulfate iron allows the synthesis of Fe-N-doped mesoporous carbons. The final amounts of iron and nitrogen on the sample were optimized to obtain an effective electrocatalysts for the ORR. The combination of all these chemical and structural properties guarantees a large number of highly active and fully accessible catalytic sites and rapid mass-transfer kinetics. When used as electrocatalysts for the ORR in basic media, the Fe-N-doped carbon material predominantly catalyzes the four electron process, with superior onset potential and kinetic current density to that of commercial platinum. In addition, the developed catalysts show a higher stability than commercial Pt/C and excellent electrocatalytic selectivity against methanol crossover. Importantly, it has been demonstrated that both N-sites and Fe-N coordination sites are contributing to the catalytic activity. In acid media, the synthesized materials were tested also, showing a comparable electrocatalyst performance to that of commercial platinum and a better durability.

The global energy crisis coupling with the fast consumption of fossil fuels and the associated environmental issues, has stimulated extensive interest in searching for clean, efficient and sustainable energy storage and conversion systems. Producing oxygen through an oxygen evolution reaction (OER) process can be promising when effective catalysis of water oxidation into oxygen molecules could be achieved. Therefore, developing an efficient OER electro catalyst is vital to the new generation of electrochemical storage and conversion devices such as electrolyzers and metal-air batteries. There are still, however, several challenges that should be solved so that the electrochemical water oxidation process can be economically attractive. One of them is associated with the high overpotential and thereby energy loss at the electrode, where OER occur with a four-electron transfer route, according to the following overall reaction:
2H₂O(l) → O₂(g) + 4H⁺ + 4e^- (1)
Oxygen reduction reaction (ORR), the reverse reaction of OER, also plays an important role in the electrochemical energy storage and conversion field, including the fuel cells and the metal-air batteries. Advanced ABO₃ perovskite oxides with improved catalytic properties have been extensively developed and modified as bifunctional catalysts because of their high versatility in composition, crystalline and electronic structure. In this presentation, Iridum doped (La_0.8Sr_0.2)_1-xMn_1-xIr_xO₃ (x=0, 0.05) and Cobalt doped (La_0.8Sr_0.2)_1-xMn_1-xCo_xO₃ (x=0, 0.05, 0.10) nanoparticles with particle size of 50 nm were succesfully synthesized by the polymer-assisted chemcial solution method as superior bifuctional oxygen catalysts in alkaline solution. The ORR onset potential of the Iridium doped sample is around 0.92 V vs. RHE, which is only 50 mV negative shift relative to the state-of-the-art Pt/C catalyst, whilst the OER onset potential is around 1.55 V, which is comparable to that of the state-of-the-art IrO₂ catalyst. The Cobalt doped samples show enhanced OER activity and reduced total potential, but inferior ORR efficiency.

All-solid-state thin film Li-ion batteries have been studied because they can provide power to microelectronic devices. To develop high performance microbatteries, it is important to develop high quality thin film cathodes with high energy density and long cycle life at low cost. However, cathode materials for a Li-ion battery, which are currently being used commercially, do not yet satisfy the power source requirements for the microelectronic device applications. One effective way to solve this problem is to increase the energy density by using electrochemical cells containing high-voltage cathode materials. In this respect, LiNi_0.5Mn_1.5O₄ (LNMO) is a promising candidate due to its high reaction voltage near 4.7 V vs. Li, structural stability, and the relatively low materials cost.
Various coating techniques (electrostatic spray deposition, sol-gel, pulsed laser deposition, etc.) have been studied to realize high performance LNMO as a thin film. However, such methods are difficult to apply to a large-scale industrial process. Sputtering technique has several advantages over other methods, such as higher deposition rates, process stability and reliability, and the production of high quality thin films on large substrates. In the fabrication of the oxide thin films, the substrate plays an important role in thin film growth. Several substrate materials (alumina, stainless steel, glassy carbon, etc.) have been used to apply LNMO thin films to high performance and various applications. In particular, stainless steel (SUS) substrate is commonly used in Li-ion battery research due to their high electrical conductivity, electrochemical stability and low cost. However, during the thin film annealing process at high temperature, the metal atoms (Fe, Cr, Mn, etc) in the SUS substrate and the metal ions in the oxide thin film diffuse through the cathode/SUS interface. The diffusion of metal ions may modify the chemical composition and crystal structure of the oxide thin film, thereby affecting the electrochemical characteristics of the cathode.
In this work, we have investigated electrochemical properties of two types of interlayers (platinum and In-Sn-O) in order to prevent interdiffusion between SUS substrate and LNMO thin film. Platinum (Pt) is one of the widely used current-collectors in lithium ion batteries and In-Sn-O (ITO) is applied to electronic devices because of its excellent electronic conductivity among oxide materials. We fabricated LNMO film and interlayers using magnetron sputter deposition on SUS substrate and then annealed at 500 ~ 700 °C. As the annealing temperature was increased, Pt reacted with SUS substrate. On the other hand, with the suppression of this reaction by adding ITO between them, the electrochemical properties of LNMO were significantly improved.

The operation of functional electrochemical interfaces often depends on the organization of the active components. Studies of these interfaces regularly rely on fabrication methods that allow for precise control over structural parameters in order to explicate true structure-function relationships. Examples like photolithography, electron-beam lithography, reactive ion etching, and metal evaporation are generally cost prohibitive for scalable approaches to making these materials. Thus, there is an opportunity to identify processes that translate the strategies derived from research prototypes to new methods of low-cost surface fabrication. Here, we describe recent efforts to fabricate hierarchically structured noble metal interfaces via the direct photolithography of cuprous oxide thin films using photoelectrodeposition. Galvanic replacement reactions (GRR) can be used to modify the surface by exchanging Cu for metals like Au or Ag. We will outline experiments to show how GRR can be controlled on Cu₂O electrodes and the resultant surface chemistry of these modifications. Further we explore how patterned Cu₂O can be used as a sacrificial intermediate for GRR-driven metallization. This provides the ability to engineer functional electrochemical surfaces in an inexpensive, scalable, and generalizable way.

A catalyst operates by stabilizing reaction intermediates through surface adsorption. In the oxygen evolution reaction (OER), an electrochemical reaction that limits the water splitting’s efficiency, the OER catalyst stabilizes the intermediates by electrochemically stabilizing oxygen-containing intermediates during the proton and electron transfer events. This presentation will discuss our measurements of the proton and electron transfer rates during the intermediate stabilizations, specifically the de-protonation of OH* to O*. While these measurements are common for homogeneous catalysts, similar experiments have not been reported on heterogeneous surfaces, in particular oxides. Our experiment uses rate-dependent cyclic voltammetry (CV) to probe these rate constants on single-orientation RuO₂(110). By examining a series of electrolytes, we further identify how the rate constant varies with pH to determine whether the proton and electron transfers are coupled or decoupled. Our results offer insights into the surface electron transfer kinetics on heterogeneous surfaces and provide directions to the future design of superior OER catalysts.

Catalysis at surfaces and interfaces where there exists bi- or multi-component cooperation has been identified as crucial for many processes related to energy and environmental applications. Here we show such cooperation-enhanced catalysis can be synthetically controlled at nanoscale and singe-atom scale over well-defined nanocrystals. The first example is M-Pt (M=Co, Ni) core-shell nanocrystals within which desirable/undesirable interfaces between non-precious metal M core and precious metal Pt shell are identified by density functional theory (DFT) calculations and are practically balanced by nanocrystal synthesis. The optimized core-shell nanocrystals exhibit favorable interfacial interaction at nanometer scale through properly coupled electronic and strain effects, leading to an enhanced electrocatalytic efficiency toward oxygen reduction reaction (ORR) in acid. In second example, we take advantage of single-atom Co's synergy with various oxide support for electrochemical oxygen evolution reaction (OER). By choose the proper oxide support nanocrystals, single atom Co conversion and stabilization at high valence OER active site can be controlled through its interaction with supporting materials. The relevant theoretical calculation and structural characterization will be discussed.

The conventional synthesis of hydrogen peroxide(H₂O₂) confronts intense energy cost, tedious separation procedures and high cost, which is not competitive to the traditional oxidants. Hence, the direct synthesis of H₂O₂ from hydrogen and oxygen has attracted intense interest, in which the appropriate catalyst is the determining factor. Most research agreed that the determining reaction in direct synthesis of H₂O₂ will be the absorbed hydrogen atoms reacting with absorbed oxygen molecules as the following:

Previous works on transition metal such as Pd, Au and related alloy have proved that a high H₂O₂ selectivity can only achieved through low rate O-O bond cleavage meantime high reaction rate of adding hydrogen atoms to absorbed oxygen molecules [1, 2]. Moreover, the absorption energy of H₂O₂ also should not be too negative to ensure desorption of formed H₂O₂ for higher productivity and selectivity[3]. Comparing to wide applications in many organic reactions, Ru and Ni as important catalysts have been rarely studied in the synthesis of H₂O₂.
In this work, we propose the mechanism of RuNi alloy as a novel catalyst in direct synthesis of H₂O₂ through density functional theory (DFT) calculations. Hydrogen is highly possible to break on this surface with lowest energy barrier on top of Ni site. The catalyst shows different behaviours with or without oxygen coverage. Partially oxygen coverage can significantly decrease the energy barrier for oxygen absorption and the possibility of oxygen cleavage, which both benefits the further reaction of absorbed oxygen with dissociative hydrogen atoms to form H₂O₂. Moreover, the dissociation of formed H₂O₂ on the surface are also supressed. The absorption energy of formed H₂O₂ on the surface is reasonable to prevent over-binding of H₂O₂ and affect the productivity. In this alloy, Ru is relatively unstable and more active to react with absorbed molecules. Ni can play a role as balancing the binding energy to prevent over electron transfer to achieve high selectivity of H₂O₂. Therefore, this novel alloy RuNi shows high potential for direct synthesis of H₂O₂.

Reference
[1] Q. Liu, J.C. Bauer, R.E. Schaak, J.H. Lunsford, Angewandte Chemie, 120, 6317-6320 (2008).
[2] N.M. Wilson, D.W. Flaherty, J Am Chem Soc, 138, 574-586 (2016).
[3] A. Plauck, E.E. Stangland, J.A. Dumesic, M. Mavrikakis, Proc Natl Acad Sci U S A, 113, E1973-1982 (2016).

Oxide-derived (OD) Cu catalysts have high selectivity towards the formation of multi-carbon products (C₂/C₃) for aqueous electrochemical CO₂ reduction (CO₂R). It has been proposed that a large fraction of the initial oxide can be surprisingly resistant to reduction and that these residual oxides play a crucial role in promoting the formation of C₂/C₃ products. We investigate the stability of residual oxides by synthesizing ¹⁸O enriched OD Cu catalysts and testing them for CO₂R. These catalysts maintain a high selectivity towards C₂/C₃ products (~60%) for up to 5h in 0.1 M KHCO3at -1.0 V vs RHE. However, secondary ion mass spectrometry measurements show that only a small fraction (< 1%) of the original ¹⁸O content remains, showing that residual oxides are not present in significant amounts during CO₂R. Furthermore, we show that OD Cu can reoxidize rapidly, which could compromise the accuracy of ex-situ methods for determining the true oxygen content.

Solid acid fuel cells (SAFCs) based on the electrolyte CsH2PO4 have shown promising power densities at ~250C, a technologically attractive operating temperature. While SAFC development efforts have resulted in impressive performance gains in the last decade, electrocatalysis mechanisms in these devices remain unknown. Here, the hydrogen oxidation kinetics on Pt, Pd and Pt-Pd bimetallic thin film electrodes on proton-conductive CsH2PO4 have been evaluated. In particular, two types of cell geometries were studied. In the first case, a thin (10nm) film of Pt was embedded within CsH2PO4, which was in turn sandwiched between ‘standard’ composite electrodes. This cell configuration was employed to study hydrogen transport through Pt and across the Pt|CsH2PO4 interface. In the second case Pt, Pd, and Pt-Pd bilayer thin film electrodes with varying thicknesses were sputtered on both sides of CsH2PO4 electrolyte discs. These cells were used for studies of hydrogen electro-oxidation across the gas | metal | CsH2PO4 structure. Symmetric cells of both types were characterized by AC impedance spectroscopy under humidified H2 at ~248C. The bilayer films were additionally studied by ex-situ low energy ion scattering and scanning transmission electron microscopy. The results revealed: (1) 10 nm Pt films present negligible bulk resistance to proton transport; (2) the rate-limiting step for hydrogen electro-oxidation on Pt thin films occurs at the internal Pt | CsH2PO4 interface rather than the Pt | gas interface; (3) Pd is substantially more active than Pt for hydrogen electro-reduction; and (4) Pt and Pd undergo extensive intermixing at 250C. From these observations, it is concluded that the dramatic decrease in hydrogen electrooxidation resistance that occurs when Pd is deposited on Pt (on CsH2PO4) is a result of Pd diffusion to the metal | electrolyte interface, at which Pd serves to catalyze the rate-limiting step.

Carbon nanotubes (CNTs) have been interested in various area such as electronics, photonics, energy devices, and other applications. However, it has been a long-standing challenge to establish a straightforward process for the production of uniform CNTs with desired structures and properties. Here, we have fabricated hybrid carbon films which are composed of carbon nanotube arrays grown on reduced graphene oxide films. The processes enabled the formation of diameter, wall-number, and atomic structure controlled carbon nanotube arrays. This method enabled to fabricate the Fe-porphyrinic CNTs, which are outstanding oxygen reduction catalysts via the efficient 4-electron oxygen reduction process. The cyclic and the rotating disk electrode (RDE) voltammograms confirmed the outstanding oxygen reduction reaction (ORR) properties of the Fe-porphyrinic CNT. The fabrication processes of the efficient, bio-mimetic, rigid, electron-conducting carbon nanotube catalysts can have a significant impact on the wide deployment of the current PEMFC technology.