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We report, to our knowledge for the first time, a combined experimental and density functional theory (DFT) investigation into the activity and stability of cobalt, molybdenum, and copper arsenides as catalysts for the hydrogen evolution reaction (HER). We find CoAs and MoAs to be the most active arsenide materials. We discuss the trends between calculated surface vacancy formation energies and catalyst stability. Using a simple thermodynamic model of HER activity, we find consistent trends between hydrogen binding free energy and the experimentally observed activity.

The present work was undertaken to elucidate the facet-dependent activity of Ag for the electrochemical reduction of CO 2 to CO. To this end, CO 2 reduction was investigated over Ag thin films with (111), (100), and (110) orientations prepared via epitaxial growth on single-crystal Si wafers with the same crystallographic orientations. This preparation technique yielded larger area electrodes than can be achieved using single-crystals, which enabled the electrocatalytic activity of the corresponding Ag surfaces to be quantified in the Tafel regime. The Ag(110) thin films exhibited higher CO evolution activity compared to the Ag(111) and Ag(100) thin films, consistent with previous single-crystal studies. Density functional theory calculations suggest that CO 2 reduction to CO is strongly facet-dependent, and that steps are more active than highly coordinated terraces. This is the result of both a higher binding energy of the key intermediate COOH as well as an enhanced double-layer electric field stabilization over undercoordinated surface atoms located at step edge defects. As a consequence, step edge defects likely dominate the CO 2 reduction activity observed over the Ag(111) and Ag(100) thin films. The higher activity observed over the Ag(110) thin film is then related to the larger density of undercoordinated sites compared to the Ag(111) and Ag(100) thin films. Our conclusion that undercoordinated sites dominate the CO 2 reduction activity observed over close-packed surfaces highlights the need to consider the contribution of such defects in studies of single-crystal electrodes.

We demonstrate that applying atomic layer deposition-derived molybdenum disulfide (MoS2) catalytic coatings on copper gallium diselenide (CGSe) thin film absorbers can lead to efficient wide band gap photocathodes for photoelectrochemical hydrogen production. We have prepared a device that is free of precious metals, employing a CGSe absorber and a cadmium sulfide (CdS) buffer layer, a titanium dioxide (TiO2) interfacial layer, and a MoS2 catalytic layer. The resulting MoS2/TiO2/CdS/CGSe photocathode exhibits a photocurrent onset of +0.53 V vs RHE and a saturation photocurrent density of -10 mA cm-2, with stable operation for >5 h in acidic electrolyte. Spectroscopic investigations of this device architecture indicate that overlayer degradation occurs inhomogeneously, ultimately exposing the underlying CGSe absorber.

Objective evaluation of the performance of electrocatalysts for CO2 reduction has been complicated by a lack of standardized methods for measuring and reporting activity data. In this perspective, we advocate that standardizing these practices can aid in advancing research efforts toward the development of efficient and selective CO2 reduction electrocatalysts. Using information taken from experimental studies, we identify variables that influence the measured activity of CO2 reduction electrocatalysts and propose procedures to account for these variables in order to improve the accuracy and reproducibility of reported data. We recommend that catalysts be measured under conditions which do not introduce artifacts from impurities, from either the electrolyte or counter electrode, and advocate the acquisition of data measured in the absence of mass transport effects. Furthermore, measured rates of electrochemical reactions should be normalized to both the geometric electrode area as well as the electrochemically active surface area to facilitate the comparison of reported catalysts with those previously known. We demonstrate that, when these factors are accounted for, the CO2 reduction activities of Ag and Cu measured in different laboratories exhibit little difference. Adoption of the recommendations presented in this perspective would greatly facilitate the identification of superior catalysts for CO2 reduction arising solely from changes in their composition and pretreatment.

© The Author(s) 2016. Hydrogen production via electrochemical water splitting is a promising approach for storing solar energy. For this technology to be economically competitive, it is critical to develop water splitting systems with high solar-to-hydrogen (STH) efficiencies. Here we report a photovoltaic-electrolysis system with the highest STH efficiency for any water splitting technology to date, to the best of our knowledge. Our system consists of two polymer electrolyte membrane electrolysers in series with one InGaP/GaAs/GaInNAsSb triple-junction solar cell, which produces a large-enough voltage to drive both electrolysers with no additional energy input. The solar concentration is adjusted such that the maximum power point of the photovoltaic is well matched to the operating capacity of the electrolysers to optimize the system efficiency. The system achieves a 48-h average STH efficiency of 30%. These results demonstrate the potential of photovoltaic-electrolysis systems for cost-effective solar energy storage.

Electrocatalytic nitrate reduction reaction (NO 3 RR) presents an innovative approach for sustainable NH 3 production. However, selective NH 3 production is hindered by the multiple intermediates involved in the NO 3 RR process and the competitive hydrogen evolution reaction. Hence, the development of highly efficient NO 3 RR catalysts is paramount. Herein, we report highly efficient bimetallic catalysts derived from hydroxy double salt (HDS). Under NO 3 RR conditions, Cu 1 Co 1 -HDS undergoes in situ reconstruction, forming nanocomposites of homogeneously distributed metallic Cu 0 and Co(OH) 2 . Reconstruction-induced Cu 0 rapidly converts NO 3 - to NO 2 - , which is further hydrogenated to NH 3 by Co(OH) 2 . Homogeneously mixed Cu and Co species maximize this synergistic effect, achieving outstanding NO 3 RR performance including the highest NH 3 yield rate (4.625 mmol h -1 cm -2 ) reported for powder-type NO 3 RR catalysts. Integration of Cu 1 Co 1 -HDS with a commercial Si solar cell attained 4.53% solar-to-ammonia efficiency from industrial wastewater-level concentrations of NO 3 - (2000 ppm), demonstrating practical application potential for solar-driven NH 3 production. This study provides a strategy for enhancing the NH 3 yield rate by optimizing the compositions and distributions of Cu and Co.

Anion exchange membrane water electrolysis (AEMWE) is a promising technology to produce hydrogen from low-cost, renewable power sources. Recently, the efficiency and durability of AEMWE have improved significantly due to advances in the anion exchange polymers and catalysts. To achieve performances and lifetimes competitive with proton exchange membrane or liquid alkaline electrolyzers, however, improvements in the integration of materials into the membrane electrode assembly (MEA) are needed. In particular, the integration of the oxygen evolution reaction (OER) catalyst, ionomer, and transport layer in the anode catalyst layer has significant impacts on catalyst utilization and voltage losses due to the transport of gases, hydroxide ions, and electrons within the anode. This study investigates the effects of the properties of the OER catalyst and the catalyst layer morphology on performance. Using cross-sectional electron microscopy and in-plane conductivity measurements for four PGM-free catalysts, we determine the catalyst layer thickness, uniformity, and electronic conductivity and further use a transmission line model to relate these properties to the catalyst layer resistance and utilization. We find that increased loading is beneficial for catalysts with high electronic conductivity and uniform catalyst layers, resulting in up to 55% increase in current density at 2 V due to decreased kinetic and catalyst layer resistance losses, while for catalysts with lower conductivity and/or less uniform catalyst layers, there is minimal impact. This work provides important insights into the role of catalyst layer properties beyond intrinsic catalyst activity in AEMWE performance., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Computationally predicting the performance of catalysts under reaction conditions is a challenging task due to the complexity of catalytic surfaces and their evolution in situ, different reaction paths, and the presence of solid-liquid interfaces in the case of electrochemistry. We demonstrate here how relatively simple machine learning models can be found that enable prediction of experimentally observed onset potentials. Inputs to our model are comprised of data from the oxygen reduction reaction on non-precious transition-metal antimony oxide nanoparticulate catalysts with a combination of experimental conditions and computationally affordable bulk atomic and electronic structural descriptors from density functional theory simulations. From human-interpretable genetic programming models, we identify key experimental descriptors and key supplemental bulk electronic and atomic structural descriptors that govern trends in onset potentials for these oxides and deduce how these descriptors should be tuned to increase onset potentials. We finally validate these machine learning predictions by experimentally confirming that scandium as a dopant in nickel antimony oxide leads to a desired onset potential increase. Macroscopic experimental factors are found to be crucially important descriptors to be considered for models of catalytic performance, highlighting the important role machine learning can play here even in the presence of small datasets., (© 2024 Wiley-VCH GmbH.)

The reconstruction of Cu catalysts during electrochemical reduction of CO 2 is a widely known but poorly understood phenomenon. Herein, we examine the structural evolution of Cu nanocubes under CO 2 reduction reaction and its relevant reaction conditions using identical location transmission electron microscopy, cyclic voltammetry, in situ X-ray absorption fine structure spectroscopy and ab initio molecular dynamics simulation. Our results suggest that Cu catalysts reconstruct via a hitherto unexplored yet critical pathway - alkali cation-induced cathodic corrosion, when the electrode potential is more negative than an onset value (e.g., -0.4 V RHE when using 0.1 M KHCO 3 ). Having alkali cations in the electrolyte is critical for such a process. Consequently, Cu catalysts will inevitably undergo surface reconstructions during a typical process of CO 2 reduction reaction, resulting in dynamic catalyst morphologies. While having these reconstructions does not necessarily preclude stable electrocatalytic reactions, they will indeed prohibit long-term selectivity and activity enhancement by controlling the morphology of Cu pre-catalysts. Alternatively, by operating Cu catalysts at less negative potentials in the CO electrochemical reduction, we show that Cu nanocubes can provide a much more stable selectivity advantage over spherical Cu nanoparticles., (© 2024. The Author(s).)

Metal-organic frameworks (MOFs) offer an interesting opportunity for catalysis, particularly for metal-nitrogen-carbon (M-N-C) motifs by providing an organized porous structural pattern and well-defined active sites for the oxygen reduction reaction (ORR), a key need for hydrogen fuel cells and related sustainable energy technologies. In this work, we leverage electrochemical testing with computational models to study the electronic and structural properties in the MOF systems and their relationship to ORR activity and stability based on dual transitional metal centers. The MOFs consist of two M1 metals with amine nodes coordinated to a single M2 metal with a phthalocyanine linker, where M1/M2 = Co, Ni, or Cu. Co-based metal centers, in particular Ni-Co, demonstrate the highest overall activity of all nine tested MOFs. Computationally, we identify the dominance of Co sites, relative higher importance of the M2 site, and the role of layer M1 interactions on the ORR activity. Selectivity measurements indicate that M1 sites of MOFs, particularly Co, exhibit the lowest (<4%), and Ni demonstrates the highest (>46%) two-electron selectivity, in good agreement with computational studies. Direct in situ stability characterization, measuring dissolved metal ions, and calculations, using an alkaline stability metric, confirm that Co is the most stable metal in the MOF, while Cu exhibits notable instability at the M1. Overall, this study reveals how atomistic coupling of electronic and structural properties affects the ORR performance of dual site MOF catalysts and opens new avenues for the tunable design and future development of these systems for practical electrochemical applications.

Ammonia (NH 3 ) is a key commodity chemical for the agricultural, textile and pharmaceutical industries, but its production via the Haber-Bosch process is carbon-intensive and centralized. Alternatively, an electrochemical method could enable decentralized, ambient NH 3 production that can be paired with renewable energy. The first verified electrochemical method for NH 3 synthesis was a process mediated by lithium (Li) in organic electrolytes. So far, however, elements other than Li remain unexplored in this process for potential benefits in efficiency, reaction rates, device design, abundance and stability. In our demonstration of a Li-free system, we found that calcium can mediate the reduction of nitrogen for NH 3 synthesis. We verified the calcium-mediated process using a rigorous protocol and achieved an NH 3 Faradaic efficiency of 40 ± 2% using calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip) 4 ] 2 ) as the electrolyte. Our results offer the possibility of using abundant materials for the electrochemical production of NH 3 , a critical chemical precursor and promising energy vector., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Renewable energy-driven bipolar membrane water electrolyzers (BPMWEs) are a promising technology for sustainable production of hydrogen from seawater and other impure water sources. Here, we present a protocol for assembling BPMWEs and operating them in a range of water feedstocks, including ultra-pure deionized water and seawater. We describe steps for membrane electrode assembly preparation, electrolyzer assembly, and electrochemical evaluation. For complete details on the use and execution of this protocol, please refer to Marin et al. (2023). 1 ., Competing Interests: Declaration of interests The authors have patents submitted and issued (US patent # 11,268,200) related to the content of this manuscript., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

In situ techniques are essential to understanding the behavior of electrocatalysts under operating conditions. When employed, in situ synchrotron grazing-incidence X-ray diffraction (GI-XRD) can provide time-resolved structural information of materials formed at the electrode surface. In situ cells, however, often require epoxy resins to secure electrodes, do not enable electrolyte flow, or exhibit limited chemical compatibility, hindering the study of non-aqueous electrochemical systems. Here, a versatile electrochemical cell for air-free in situ synchrotron GI-XRD during non-aqueous Li-mediated electrochemical N 2 reduction (Li-N 2 R) has been designed. This cell not only fulfills the stringent material requirements necessary to study this system but is also readily extendable to other electrochemical systems. Under conditions relevant to non-aqueous Li-N 2 R, the formation of Li metal, LiOH and Li 2 O as well as a peak consistent with the α-phase of Li 3 N was observed, thus demonstrating the functionality of this cell toward developing a mechanistic understanding of complicated electrochemical systems., (open access.)

Improving electrocatalyst stability is critical for the development of electrocatalytic devices. Herein, we utilize an on-line electrochemical flow cell coupled with an inductively coupled plasma-mass spectrometer (ICP-MS) to characterize the impact of composition and reactant gas on the multielement dissolution of Mn(-Cr)-Sb-O electrocatalysts. Compared to Mn 2 O 3 and Cr 2 O 3 oxides, the antimonate framework stabilizes Mn at OER potentials and Cr at both ORR and OER potentials. Furthermore, dissolution of Mn and Cr from Mn(-Cr) -Sb-O is driven by the ORR reaction rate, with minimal dissolution under N 2 . We observe preferential dissolution of Cr totaling 13% over 10 min at 0.3, 0.6, and 0.9 V vs RHE, with only 1.5% loss of Mn, indicating an enrichment of Mn at the surface of the particles. Despite this asymmetric dissolution, operando X-ray absorption spectroscopy (XAS) showed no measurable changes in the Mn K-edge at comparable potentials. This could suggest that modification to the Mn oxidation state and/or phase in the surface layer is too small or that the layer is too thin to be measured with the bulk XAS measurement. Lastly, on-line ICP-MS was used to assess the effects of applied potential, scan rate, and current on Mn-Cr-Sb-O during cyclic voltammetry and accelerated stress tests. With this deeper understanding of the interplay between oxygen reduction and dissolution, testing procedures were identified to maximize both activity and stability. This work highlights the use of multimodal in situ characterization techniques in tandem to build a more complete model of stability and develop protocols for optimizing catalyst performance.

Ni and nitrogen-doped carbons are selective catalysts for CO 2 reduction to CO (CO 2 R), but the hypothesized NiN x active sites are challenging to probe with traditional characterization methods. Here, we synthesize 61 Ni-enriched model catalysts, termed 61 NiPACN, in order to apply 61 Ni Mössbauer spectroscopy using synchrotron radiation ( 61 Ni-SR-MS) to characterize the structure of these atomically dispersed NiN x sites. First, we demonstrate that the CO 2 R results and standard characterization techniques (SEM, PXRD, XPS, XANES, EXAFS) point to the existence of dispersed Ni active sites. Then, 61 Ni-SR-MS reveal significant internal magnetic fields of ∼5.4 T, which is characteristic of paramagnetic, high-spin Ni 2+ , in the 61 NiPACN samples. Finally, theoretical calculations for a variety of Ni-N x moieties confirm that high-spin Ni 2+ is stable in non-planar, tetrahedrally distorted geometries, which results in calculated isotropic hyperfine coupling that is consistent with 61 Ni-SR-MS measurements.

GaInP 2 has shown promise as the wide bandgap top junction in tandem absorber photoelectrochemical (PEC) water splitting devices. Among previously reported dual-junction PEC devices with a GaInP 2 top cell, those with the highest performance incorporate an AlInP 2 window layer (WL) to reduce surface recombination and a thin GaInP 2 capping layer (CL) to protect the WL from corrosion in electrolytes. However, the stability of these III-V systems is limited, and durability continues to be a major challenge broadly in the field of PEC water splitting. This work provides a systematic investigation into the durability of GaInP 2 systems, examining the impacts of the window layer and capping layer among single junction pn-GaInP 2 photocathodes coated with an MoS 2 catalytic and protective layer. The photocathode with both a CL and WL demonstrates the highest PEC performance and longest lifetime, producing a significant current for >125 h. In situ optical imaging and post-test characterization illustrate the progression of macroscopic degradation and chemical state. The surface architecture combining an MoS 2 catalyst, CL, and WL can be translated to dual-junction PEC devices with GaInP 2 or other III-V top junctions to enable more efficient and stable PEC systems.

The development of inexpensive and abundant catalysts with high activity, selectivity, and stability for the oxygen reduction reaction (ORR) is imperative for the widespread implementation of fuel cell devices. Herein, we present a combined theoretical-experimental approach to discover and design first-row transition metal antimonates as excellent electrocatalytic materials for the ORR. Theoretically, we identify first-row transition metal antimonates─MSb 2 O 6 , where M = Mn, Fe, Co, and Ni─as nonprecious metal catalysts with good oxygen binding energetics, conductivity, thermodynamic phase stability, and aqueous stability. Among the considered antimonates, MnSb 2 O 6 shows the highest theoretical ORR activity based on the 4e - ORR kinetic volcano. Experimentally, nanoparticulate transition metal antimonate catalysts are found to have a minimum of a 2.5-fold enhancement in intrinsic mass activity (on transition metal mass basis) relative to the corresponding transition metal oxide at 0.7 V vs RHE in 0.1 M KOH. MnSb 2 O 6 is the most active catalyst under these conditions, with a 3.5-fold enhancement on a per Mn mass activity basis and 25-fold enhancement on a surface area basis over its antimony-free counterpart. Electrocatalytic and material stability are demonstrated over a 5 h chronopotentiometry experiment in the stability window identified by theoretical Pourbaix analysis. This study further highlights the stable and electrically conductive antimonate structure as a framework to tune the activity and selectivity of nonprecious metal oxide active sites for ORR catalysis.

Platinum is an important material with applications in oxygen and hydrogen electrocatalysis. To better understand how its activity can be modulated through electrolyte effects in the double layer microenvironment, herein we investigate the effects of different acid anions on platinum for the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution/oxidation reaction (HER/HOR) in pH 1 electrolytes. Experimentally, we see the ORR activity trend of HClO 4  > HNO 3  > H 2 SO 4 , and the OER activity trend of HClO 4 [Formula: see text] HNO 3 ∼ H 2 SO 4 . HER/HOR performance is similar across all three electrolytes. Notably, we demonstrate that ORR performance can be improved 4-fold in nitric acid compared to in sulfuric acid. Assessing the potential-dependent role of relative anion competitive adsorption with density functional theory, we calculate unfavorable adsorption on Pt(111) for all the anions at HER/HOR conditions while under ORR/OER conditions [Formula: see text] binds the weakest followed by [Formula: see text] and [Formula: see text]. Our combined experimental-theoretical work highlights the importance of understanding the role of anions across a large potential range and reveals nitrate-like electrolyte microenvironments as interesting possible sulfonate alternatives to mitigate the catalyst poisoning effects of polymer membranes/ionomers in electrochemical systems. These findings help inform rational design approaches to further enhance catalyst activity via microenvironment engineering., (© 2022. The Author(s).)

CO 2 emissions can be transformed into high-added-value commodities through CO 2 electrocatalysis; however, efficient low-cost electrocatalysts are needed for global scale-up. Inspired by other emerging technologies, the authors report the development of a gas diffusion electrode containing highly dispersed Ag sites in a low-cost Zn matrix. This catalyst shows unprecedented Ag mass activity for CO production: -614 mA cm -2 at 0.17 mg of Ag. Subsequent electrolyte engineering demonstrates that halide anions can further improve stability and activity of the Zn-Ag catalyst, outperforming pure Ag and Au. Membrane electrode assemblies are constructed and coupled to a microbial process that converts the CO to acetate and ethanol. Combined, these concepts present pathways to design catalysts and systems for CO 2 conversion toward sought-after products., (© 2021 Wiley-VCH GmbH.)

Tuning bimetallic effects is a promising strategy to guide catalytic properties. However, the nature of these effects can be difficult to assess and compare due to the convolution with other factors such as the catalyst surface structure and morphology and differences in testing environments. Here, we investigate the impact of atomic-scale bimetallic effects on the electrochemical CO 2 reduction performance of Cu-based catalysts by leveraging a systematic approach that unifies protocols for materials synthesis and testing and enables accurate comparisons of intrinsic catalytic activity and selectivity. We used the same physical vapor deposition method to epitaxially grow Cu(100) films decorated with a small amount of noble or base metal atoms and a combination of experimental characterization and first-principles calculations to evaluate their physicochemical and catalytic properties. The results indicate that the metal atoms segregate to under-coordinated Cu sites during physical vapor deposition, suppressing CO reduction to oxygenates and hydrocarbons and promoting competing pathways to CO, formate, and hydrogen. Leveraging these insights, we rationalize bimetallic design principles to improve catalytic selectivity for CO 2 reduction to CO, formate, oxygenates, or hydrocarbons. Our study provides one of the most extensive studies on Cu bimetallics for CO 2 reduction, establishing a systematic approach that is broadly applicable to research in catalyst discovery.

Bridging polymer design with catalyst surface science is a promising direction for tuning and optimizing electrochemical reactors that could impact long-term goals in energy and sustainability. Particularly, the interaction between inorganic catalyst surfaces and organic-based ionomers provides an avenue to both steer reaction selectivity and promote activity. Here, we studied the role of imidazolium-based ionomers for electrocatalytic CO 2 reduction to CO (CO 2 R) on Ag surfaces and found that they produce no effect on CO 2 R activity yet strongly promote the competing hydrogen evolution reaction (HER). By examining the dependence of HER and CO 2 R rates on concentrations of CO 2 and HCO 3 - , we developed a kinetic model that attributes HER promotion to intrinsic promotion of HCO 3 - reduction by imidazolium ionomers. We also show that varying the ionomer structure by changing substituents on the imidazolium ring modulates the HER promotion. This ionomer-structure dependence was analyzed via Taft steric parameters and density functional theory calculations, which suggest that steric bulk from functionalities on the imidazolium ring reduces access of the ionomer to both HCO 3 - and the Ag surface, thus limiting the promotional effect. Our results help develop design rules for ionomer-catalyst interactions in CO 2 R and motivate further work into precisely uncovering the interplay between primary and secondary coordination in determining electrocatalytic behavior.

Understanding the differences between reactions driven by elevated temperature or electric potential remains challenging, largely due to materials incompatibilities between thermal catalytic and electrocatalytic environments. We show that Ni, N-doped carbon (NiPACN), an electrocatalyst for the reduction of CO 2 to CO (CO 2 R), can also selectively catalyze thermal CO 2 to CO via the reverse water gas shift (RWGS) representing a direct analogy between catalytic phenomena across the two reaction environments. Advanced characterization techniques reveal that NiPACN likely facilitates RWGS on dispersed Ni sites in agreement with CO 2 R active site studies. Finally, we construct a generalized reaction driving-force that includes temperature and potential and suggest that NiPACN could facilitate faster kinetics in CO 2 R relative to RWGS due to lower intrinsic barriers. This report motivates further studies that quantitatively link catalytic phenomena across disparate reaction environments., (© 2021 Wiley-VCH GmbH.)

The design and fabrication of lattice-strained platinum catalysts achieved by removing a soluble core from a platinum shell synthesized via atomic layer deposition, is reported. The remarkable catalytic performance for the oxygen reduction reaction (ORR), measured in both half-cell and full-cell configurations, is attributed to the observed lattice strain. By further optimizing the nanoparticle geometry and ionomer/carbon interactions, mass activity close to 0.8 A mg Pt -1 @0.9 V iR-free is achievable in the membrane electrode assembly. Nevertheless, active catalysts with high ORR activity do not necessarily lead to high performance in the high-current-density (HCD) region. More attention shall be directed toward HCD performance for enabling high-power-density hydrogen fuel cells., (© 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

In situ characterization of electrochemical systems can provide deep insights into the structure of electrodes under applied potential. Grazing-incidence X-ray diffraction (GIXRD) is a particularly valuable tool owing to its ability to characterize the near-surface structure of electrodes through a layer of electrolyte, which is of paramount importance in surface-mediated processes such as catalysis and adsorption. Corrections for the refraction that occurs as an X-ray passes through an interface have been derived for a vacuum-material interface. In this work, a more general form of the refraction correction was developed which can be applied to buried interfaces, including liquid-solid interfaces. The correction is largest at incidence angles near the critical angle for the interface and decreases at angles larger and smaller than the critical angle. Effective optical constants are also introduced which can be used to calculate the critical angle for total external reflection at the interface. This correction is applied to GIXRD measurements of an aqueous electrolyte-Pd interface, demonstrating that the correction allows for the comparison of GIXRD measurements at multiple incidence angles. This work improves quantitative analysis of d-spacing values from GIXRD measurements of liquid-solid systems, facilitating the connection between electrochemical behavior and structure under in situ conditions.

Alloying is a powerful tool that can improve the electrocatalytic performance and viability of diverse electrochemical renewable energy technologies. Herein, we enhance the activity of Pd-based electrocatalysts via Ag-Pd alloying while simultaneously lowering precious metal content in a broad-range compositional study focusing on highly comparable Ag-Pd thin films synthesized systematically via electron-beam physical vapor co-deposition. Cyclic voltammetry in 0.1 M KOH shows enhancements across a wide range of alloys; even slight alloying with Ag (e.g. Ag 0.1 Pd 0.9 ) leads to intrinsic activity enhancements up to 5-fold at 0.9 V vs. RHE compared to pure Pd. Based on density functional theory and x-ray absorption, we hypothesize that these enhancements arise mainly from ligand effects that optimize adsorbate-metal binding energies with enhanced Ag-Pd hybridization. This work shows the versatility of coupled experimental-theoretical methods in designing materials with specific and tunable properties and aids the development of highly active electrocatalysts with decreased precious-metal content.

The electrochemical CO 2 reduction reaction (CO 2 RR) using Cu-based catalysts holds great potential for producing valuable multi-carbon products from renewable energy. However, the chemical and structural state of Cu catalyst surfaces during the CO 2 RR remains a matter of debate. Here, we show the structural evolution of the near-surface region of polycrystalline Cu electrodes under in situ conditions through a combination of grazing incidence X-ray absorption spectroscopy (GIXAS) and X-ray diffraction (GIXRD). The in situ GIXAS reveals that the surface oxide layer is fully reduced to metallic Cu before the onset potential for CO 2 RR, and the catalyst maintains the metallic state across the potentials relevant to the CO 2 RR. We also find a preferential surface reconstruction of the polycrystalline Cu surface toward (100) facets in the presence of CO 2 . Quantitative analysis of the reconstruction profiles reveals that the degree of reconstruction increases with increasingly negative applied potentials, and it persists when the applied potential returns to more positive values. These findings show that the surface of Cu electrocatalysts is dynamic during the CO 2 RR, and emphasize the importance of in situ characterization to understand the surface structure and its role in electrocatalysis.

Catalytic systems whose properties can be systematically tuned via changes in synthesis conditions are highly desirable for the next-generation catalyst design and optimization. Herein, we present a two-dimensional (2D) conductive metal-organic framework consisting of M-N 4 units (M = Ni, Cu) and a hexaaminobenzene (HAB) linker as a catalyst for the oxygen reduction reaction. By varying synthetic conditions, we prepared two Ni-HAB catalysts with different crystallinities, resulting in catalytic systems with different electric conductivities, electrochemical activity, and stability. We show that crystallinity has a positive impact on conductivity and demonstrate that this improved crystallinity/conductivity improves the catalytic performance of our model system. Additionally, density functional theory simulations were performed to probe the origin of M-HAB's catalytic activity, and they suggest that M-HAB's organic linker acts as the active site with the role of the metal being to modulate the linker sites' binding strength.

The recirculation of gases in a sealed reactor system is a broadly useful method in catalytic and electrocatalytic studies. It is especially relevant when a reactant gas reacts slowly with respect to residence time in a catalytic reaction zone and when mass transport control through the reaction zone is necessary. This need is well illustrated in the field of electrocatalytic N 2 reduction, where the need for recirculation of 15 N 2 has recently become more apparent. Herein, we describe the design, fabrication, use, and specifications of a lubricant-free, readily constructed recirculating pump fabricated entirely from glass and inert polymer (poly(ether ether ketone) (PEEK), poly(tetrafluoroethylene) (PTFE)) components. Using these glass and polymer components ensures chemical compatibility between the piston pump and a wide range of chemical environments, including strongly acidic and organic electrolytes often employed in studies of electrocatalytic N 2 reduction. The lubricant-free nature of the pump and the presence of components made exclusively of glass and PEEK/PTFE mitigate contamination concerns associated with recirculating gases saturated with corrosive or reactive vapors for extended periods. The gas recirculating glass pump achieved a flow rate of >500 mL min -1 N 2 against atmospheric pressure at 15 W peak power input and >100 mL min -1 N 2 against a differential pressure of +6 in. H 2 O (∼15 mbar) at 10 W peak power input., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Electrochemical CO reduction can serve as a sequential step in the transformation of CO 2 into multicarbon fuels and chemicals. In this study, we provide insights on how to steer selectivity for CO reduction almost exclusively toward a single multicarbon oxygenate by carefully controlling the catalyst composition and its surrounding reaction conditions. Under alkaline reaction conditions, we demonstrate that planar CuAg electrodes can reduce CO to acetaldehyde with over 50% Faradaic efficiency and over 90% selectivity on a carbon basis at a modest electrode potential of -0.536 V vs. the reversible hydrogen electrode. The Faradaic efficiency to acetaldehyde was further enhanced to 70% by increasing the roughness factor of the CuAg electrode. Density functional theory calculations indicate that Ag ad-atoms on Cu weaken the binding energy of the reduced acetaldehyde intermediate and inhibit its further reduction to ethanol, demonstrating that the improved selectivity to acetaldehyde is due to the electronic effect from Ag incorporation. These findings will aid in the design of catalysts that are able to guide complex reaction networks and achieve high selectivity for the desired product., Competing Interests: The authors declare no competing interest.

The electrochemical oxygen evolution reaction (OER) is the half-cell reaction for many clean-energy production technologies, including water electrolyzers and metal-air batteries. However, its sluggish kinetics hinders the performance of those technologies, impeding them from broader implementation. Recently, we reported the use of zirconium phosphate (ZrP) as a support for transition metal catalysts for the oxygen evolution reaction (OER). These catalysts achieve promising overpotentials with high mass activities. Herein, we synthesize ZrP structures with controlled morphology: hexagonal platelets, rods, cubes, and spheres, and subsequently modify them with Co(ii) and Ni(ii) cations to assess their electrochemcial OER behavior. Through inductively coupled plasma mass-spectrometry measurements, the maximum ion exchange capacity is found to vary based on the morphology of the ZrP structure and cation selection. Trends in geometric current density and mass activity as a function of cation selection are discussed. We find that the loading and coverage of cobalt and nickel species on the ZrP supports are key factors that control OER performance.

Ni,N-doped carbon catalysts have shown promising catalytic performance for CO 2 electroreduction (CO 2 R) to CO; this activity has often been attributed to the presence of nitrogen-coordinated, single Ni atom active sites. However, experimentally confirming Ni-N bonding and correlating CO 2 reduction (CO 2 R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile-derived Ni,N-doped carbon electrocatalysts (Ni-PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO 2 R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X-ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square-planar geometry that strongly resembles the active sites of molecular metal-porphyrin catalysts., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Silicon has shown promise for use as a small band gap (1.1 eV) absorber material in photoelectrochemical (PEC) water splitting. However, the limited stability of silicon in acidic electrolyte requires the use of protection strategies coupled with catalysts. Herein, spin coating is used as a versatile method to directly coat silicon photoanodes with an IrO x oxygen evolution reaction (OER) catalyst, reducing the processing complexity compared to conventional fabrication schemes. Biphasic strontium chloride/iridium oxide (SrCl 2 :IrO x ) catalysts are also developed, and both catalysts form photoactive junctions with silicon and demonstrate high photoanode activity. The iridium oxide photoanode displays a photocurrent onset at 1.06 V vs reversible hydrogen electrode (RHE), while the SrCl 2 :IrO x photoanode onsets earlier at 0.96 V vs RHE. The differing potentials are consistent with the observed photovoltages of 0.43 and 0.53 V for the IrO x and SrCl 2 :IrO x , respectively. By measuring the oxidation of a reversible redox couple, Fe(CN) 6 3-/4- , we compare the charge carrier extraction of the devices and show that the addition of SrCl 2 to the IrO x catalyst improves the silicon-electrolyte interface compared to pure IrO x . However, the durability of the strontium-containing photoanode remains a challenge, with its photocurrent density decreasing by 90% over 4 h. The IrO x photoanode, on the other hand, maintained a stable photocurrent density over this timescale. Characterization of the as-prepared and post-tested material structure via Auger electron spectroscopy identifies catalyst film cracking and delamination as the primary failure modes. We propose that improvements to catalyst adhesion should further the viability of spin coating as a technique for photoanode preparation.

Electrochemical CO[Formula: see text] reduction is a potential route to the sustainable production of valuable fuels and chemicals. Here, we perform CO[Formula: see text] reduction experiments on Gold at neutral to acidic pH values to elucidate the long-standing controversy surrounding the rate-limiting step. We find the CO production rate to be invariant with pH on a Standard Hydrogen Electrode scale and conclude that it is limited by the CO[Formula: see text] adsorption step. We present a new multi-scale modeling scheme that integrates ab initio reaction kinetics with mass transport simulations, explicitly considering the charged electric double layer. The model reproduces the experimental CO polarization curve and reveals the rate-limiting step to be *COOH to *CO at low overpotentials, CO[Formula: see text] adsorption at intermediate ones, and CO[Formula: see text] mass transport at high overpotentials. Finally, we show the Tafel slope to arise from the electrostatic interaction between the dipole of *CO[Formula: see text] and the interfacial field. This work highlights the importance of surface charging for electrochemical kinetics and mass transport.

Conversion of CO 2 into valuable molecules is a field of intensive investigation with the aim of developing scalable technologies for making fuels using renewable energy sources. While electrochemical reduction into CO and formate are approaching industrial maturity, a current challenge is obtaining more reduced products like methanol. However, literature on the matter is scarce, and even more for the use of molecular catalysts. Here, we demonstrate that cobalt phthalocyanine, a well-known catalyst for the electrochemical conversion of CO 2 to CO, can also catalyze the reaction from CO 2 or CO to methanol in aqueous electrolytes at ambient conditions of temperature and pressure. The studies identify formaldehyde as a key intermediate and an unexpected pH effect on selectivity. This paves the way for establishing a sequential process where CO 2 is first converted to CO which is subsequently used as a reactant to produce methanol. Under ideal conditions, the reaction shows a global Faradaic efficiency of 19.5 % and chemical selectivity of 7.5 %., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We demonstrate the translation of a low-cost, non-precious metal cobalt phosphide (CoP) catalyst from 1 cm 2 lab-scale experiments to a commercial-scale 86 cm 2 polymer electrolyte membrane (PEM) electrolyser. A two-step bulk synthesis was adopted to produce CoP on a high-surface-area carbon support that was readily integrated into an industrial PEM electrolyser fabrication process. The performance of the CoP was compared head to head with a platinum-based PEM under the same operating conditions (400 psi, 50 °C). CoP was found to be active and stable, operating at 1.86 A cm -2 for >1,700 h of continuous hydrogen production while providing substantial material cost savings relative to platinum. This work illustrates a potential pathway for non-precious hydrogen evolution catalysts developed in past decades to translate to commercial applications.

Multimetallic Ir-based systems offer significant opportunities for enhanced oxygen evolution electrocatalysis by modifying the electronic and geometric properties of the active catalyst. Herein, a systematic investigation of bimetallic Ir-based thin films was performed to identify activity and stability trends across material systems for the oxygen evolution reaction (OER) in acidic media. Electron beam evaporation was used to co-deposit metallic films of Ir, IrSn 2 , IrCr, IrTi, and IrNi. The electrocatalytic activity of the electrochemically oxidized alloys was found to increase in the following order: IrTi < IrSn 2 < Ir ∼ IrNi < IrCr. The IrCr system demonstrates two times the catalytic activity of Ir at 1.65 V versus RHE. Density functional theory calculations suggest that this enhancement is due to Cr active sites that have improved oxygen binding energetics compared to those of pure Ir oxide. This work identifies IrCr as a promising new catalyst system that facilitates reduced precious metal loadings for acid-based OER catalysis.

With promising activity and stability for the oxygen reduction reaction (ORR), transition metal nitrides are an interesting class of non-platinum group catalysts for polymer electrolyte membrane fuel cells. Here, we report an active thin-film nickel nitride catalyst synthesized through a reactive sputtering method. In rotating disk electrode testing in a 0.1 M HClO 4 electrolyte, the crystalline nickel nitride film achieved high activity and selectivity to four-electron ORR. It also exhibited good stability during 10 and 40 h chronoamperometry measurements in acid and alkaline electrolyte, respectively. A combined experiment-theory approach, with detailed ex situ materials characterization and density functional theory calculations, provides insight into the structure of the catalyst and its surface during catalysis. Design strategies for activity and stability improvement through alloying and nanostructuring are discussed.

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO 2 reduction (CO 2 R). There are variety of factors that impact CO 2 R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO 2 R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO 2 R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO 2 R.

The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative 1-4 to the energy-intensive Haber-Bosch process, which dominates industrial ammonia production. However, there are considerable scientific and technical challenges 5,6 facing the electrochemical alternative, and most experimental studies reported so far have achieved only low selectivities and conversions. The amount of ammonia produced is usually so small that it cannot be firmly attributed to electrochemical nitrogen fixation 7-9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes 9 , or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream 10 , in the atmosphere or even in the catalyst itself. Although these sources of experimental artefacts are beginning to be recognized and managed 11,12 , concerted efforts to develop effective electrochemical nitrogen reduction processes would benefit from benchmarking protocols for the reaction and from a standardized set of control experiments designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15 N 2 that enables us to reliably detect and quantify the electrochemical reduction of nitrogen to ammonia. We demonstrate experimentally the importance of various sources of contamination, and show how to remove labile nitrogen-containing compounds from the nitrogen gas as well as how to perform quantitative isotope measurements with cycling of 15 N 2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aqueous media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran 13 . The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochemical reduction of nitrogen to ammonia.

Electrocatalytic transformation of carbon dioxide (CO 2 ) and water into chemical feedstocks offers the potential to reduce carbon emissions by shifting the chemical industry away from fossil fuel dependence. We provide a technoeconomic and carbon emission analysis of possible products, offering targets that would need to be met for economically compelling industrial implementation to be achieved. We also provide a comparison of the projected costs and CO 2 emissions across electrocatalytic, biocatalytic, and fossil fuel-derived production of chemical feedstocks. We find that for electrosynthesis to become competitive with fossil fuel-derived feedstocks, electrical-to-chemical conversion efficiencies need to reach at least 60%, and renewable electricity prices need to fall below 4 cents per kilowatt-hour. We discuss the possibility of combining electro- and biocatalytic processes, using sequential upgrading of CO 2 as a representative case. We describe the technical challenges and economic barriers to marketable electrosynthesized chemicals., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The rational improvement of current and developing electrochemical technologies requires atomistic understanding of electrode-electrolyte interfaces. However, examining these interfaces under operando conditions, where performance is typically evaluated and benchmarked, remains challenging, as it necessitates incorporating an operando probe during full electrochemical operation. In this study, we describe a custom electrochemical flow cell that enables near-surface-sensitive operando investigation of planar thin-film catalysts at significant hydrogen evolution reaction (HER) rates (in excess of -100 mA cm-2) using grazing incidence X-ray methods. Grazing-incidence X-ray spectroscopy and diffraction were implemented on the same sample under identical HER conditions, demonstrating how the combined measurements track changing redox chemistry and structure of Cu thin-film catalyst surfaces as a function of electrochemical conditions. The coupling of these methods with improved mass transport and hydrodynamic control establishes a new paradigm for operando measurement design, enabling unique insights into the key fundamental processes occurring at the catalyst-electrolyte interface.