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Summary We report an active zinc-copper (Zn-Cu) bimetallic electrocatalyst for CO2 reduction to CO, prepared by a facile galvanic procedure. Under moderate overpotentials, Zn-Cu catalysts that are Zn rich exhibit intrinsic activity for CO formation superior to that of pure Zn, Cu, and Ag, the last of which is the state-of-the-art catalyst in CO2 electrolyzers. Combinatorial experiments involving catalysts prepared by physical vapor deposition reveal trends across the Zn-Cu system, corroborating the high CO selectivity unrivaled by other alloys and intermetallics. Physical and electrochemical characterization and first principles theory reveal that the origin of this synergy in intrinsic activity is an electronic effect from bimetallic Zn-Cu sites that stabilizes the carboxyl intermediate during CO2 reduction to CO. Furthermore, by integrating Zn-Cu into gas-diffusion electrodes, we demonstrate that bimetallic effects lead to improved electrocatalytic performance at industrially relevant currents. These insights provide catalyst design principles that can guide future development of efficient and earth-abundant CO-producing electrocatalysts.

Selective and stable production of C2 oxygenates from syngas is enabled by a phosphate-passivated mordenite catalyst used in tandem with a commercial Cu/ZnO/AlMgOx catalyst. Unmodified mordenite used in this arrangement is accompanied by substantial hydrocarbon formation and carbon deposition. Therefore, phosphate groups are used to remove or significantly reduce Bronsted acidity in mordenite by capping aluminum sites. Such groups are stable to calcination and catalyst regeneration. Trimethylphosphite is used as a phosphate precursor because it is able to enter the 12 membered-ring pores of mordenite where hydrocarbon formation occurs, but not enter the 8 membered-ring side-pockets, where methanol carbonylation occurs selectively. The catalyst was characterized using X-ray fluorescence, X-ray diffraction, and thermal gravimetric analysis. Optionally, a third bed of Cu/ZnO/AlMgO x catalyst is able to hydrodeoxygenate acetic acid to ethanol. The C2 oxygenate selectivity is over 96% and among the best reported in a single reactor with a syngas feed.

Advanced manufacturing (AM) represents an appealing approach for creating novel electrochemical systems for chemicals synthesis. In this work, we demonstrate AM for rapid development and testing for improved performance for the carbon dioxide reduction reaction across an evolution of vapor-fed reactor designs. In our final design, we observe activation- and mixed-control regimes for a variety of operating conditions via inlet CO2 flow rate and electrochemical potential. Furthermore, we define a dimensionless number (Da) to identify mass transport regimes by exploring the impact of hypothesized underlying mass transport mechanisms, including consumption of CO2via OH−, increased local temperatures, and partial penetration of electrolyte into gas diffusion layer. The accelerated pace of reactor design and development led to high geometric current densities (500 mA cm−2), heightened selectivity (85.5% FE C2+ products), and increased carbon dioxide conversion (16.6%) and cathodic energy efficiency (49.6% CO2R). Using AM vapor-fed reactors, we attain high ethylene (3.67%) and record ethanol (3.66%) yields compared to the literature. This work underscores the promise of AM for accelerating reactor design, understanding of governing phenomena, and improving the performance of catalytic systems.

This work demonstrates that ultrathin (4 nm) tungsten oxide (WO3) coatings on copper gallium selenide (CuGa3Se5) photocathodes have the potential for long-term solar hydrogen evolution. With a combination of a robust 1.84 eV CuGa3Se5 absorber layer, a WO3 protective coating, and a Pt catalyst, we obtain a new durability milestone for any non-silicon photoelectrochemical hydrogen-producing device by passing 21 490 C cm−2 of charge across six weeks of continuously-illuminated chronoamperometric testing under applied bias.

The electrochemical CO2 reduction reaction (CO2RR) 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 CO2RR 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 CO2RR, and the catalyst maintains the metallic state across the potentials relevant to the CO2RR. We also find a preferential surface reconstruction of the polycrystalline Cu surface toward (100) facets in the presence of CO2. 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 CO2RR, and emphasize the importance of in situ characterization to understand the surface structure and its role in electrocatalysis.

Rigorous in situ studies of electrocatalysts are required to enable the design of higher performing materials. Nonplatinum group metals for oxygen reduction reaction (ORR) catalysis containing ligh...

While photoelectrochemical (PEC) solar-to-hydrogen efficiencies have greatly improved over the past few decades, advances in PEC durability have lagged behind. Corrosion of semiconductor photoabsorbers in the aqueous conditions needed for water splitting is a major challenge that limits device stability. In addition, a precious-metal catalyst is often required to efficiently promote water splitting. Herein, we demonstrate unassisted water splitting using a nonprecious metal molybdenum disulfide nanomaterial catalytic protection layer paired with a GaInAsP/GaAs tandem device. This device was able to achieve stable unassisted water splitting for nearly 12 h, while a sibling sample with a PtRu catalyst was only stable for 2 h, highlighting the advantage of the nonprecious metal catalyst. In situ optical imaging illustrates the progression of macroscopic degradation that causes device failure. In addition, this work compares unassisted water splitting devices across the field in terms of the efficiency and stability, illustrating the need for improved stability.

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 N2 reduction, where the need for recirculation of 15N2 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 N2 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 N2 against atmospheric pressure at 15 W peak power input and >100 mL min-1 N2 against a differential pressure of +6 in. H2O (∼15 mbar) at 10 W peak power input.

Silver-based bimetallic catalysts for the oxygen reduction reaction (ORR) are promising for a wide variety of renewable energy technologies, including alkaline fuel cells and metal-air batteries. T...

Nitrates from agricultural runoff and industrial waste streams are a notorious waste product and hazardous pollutant. Traditional electrochemical water remediation approaches aim to solve this prob...

Ni,N-doped carbon catalysts have shown promising catalytic performance for CO2 electroreduction (CO2 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 CO2 reduction (CO2 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 CO2 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.

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 IrOₓ oxygen evolution reaction (OER) catalyst, reducing the processing complexity compared to conventional fabrication schemes. Biphasic strontium chloride/iridium oxide (SrCl₂:IrOₓ) catalysts are also developed, and both catalysts form photoactive junctions with silicon and demonstrate highphotoanode activity. The iridium oxide photoanode displays a photocurrent onset at 1.06 V vs reversible hydrogen electrode (RHE), while the SrCl₂:IrOₓ 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 IrOₓ and SrCl₂:IrOₓ, 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 SrCl₂ to the IrOx catalyst improves the silicon−electrolyte interface compared to pure IrOₓ. However, the durability of the strontium-containing photoanode remains a challenge, with its photocurrent density decreasing by 90% over 4 h. The IrOₓ 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.

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.

Utilizing electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path towards closing the carbon cycle in modern technology. To this day, however, the exact reaction mechanisms towards the plethora of single and multi-carbon products on Cu electrodes are still disputed. This uncertainty even extends to the rate-limiting step of the respective reactions. Since multi-carbon products do not show a dependence on the electrolyte pH in neutral and alkaline media, CO dimerization on the Cu surface has been proposed as the rate-limiting step. However, other elementary steps would lead to the same pH dependence, namely the proton-electron transfer to *CO followed by subsequent coupling or the protonation of the *OCCO dimer. The pH dependence of methane production on the other hand suggests that the rate limiting step is located beyond the first proton-electron transfer to *CO. In order to conclusively identify the rate limiting steps in CO reduction, we analyzed the mechanisms on the basis of constant potential DFT calculations, CO reduction experiments on Cu at varying pH values (3 - 13) and fundamental rate theory. We find that, even in acidic media, the reaction rate towards multi-carbon products is nearly unchanged on an SHE potential scale, which indicates that its rate limiting step does not involve a proton donor. Hence, we deduce that the rate limiting step can indeed only consist of the coupling of two CO molecules on the surface, both in acidic and alkaline conditions. For methane, on the other hand, the rate-limiting step changes with the electrolyte pH from the first protonation step in acidic/neutral conditions to a later step in alkaline conditions. Finally, based on an in-depth kinetic analysis, we conclude that the pathway towards CH4 involving a surface combination of *CO and *H is unlikely, since it is unable to reproduce the measured current densities and Tafel slopes.

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 promising electrocatalytic materials for the ORR. Theoretically, we identify first-row transition metal antimonates – MSb2O6, where M = Mn, Fe, Co, and Ni – as non-precious metal catalysts with promising oxygen binding energetics, conductivity, thermodynamic phase stability and aqueous stability. Among the considered antimonates, MnSb2O6 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. MnSb2O6 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 Pourbaix analysis. This study further highlights the stable and electrically conductive antimonate structure as a promising framework to tune the activity and selectivity of non-precious metal oxide active sites for ORR catalysis.

Climate change has necessitated the development of technologies that are able to efficiently capture, utilize, and store CO2, creating new opportunities to utilize CO2 as a feedstock to produce fuels and chemicals that are vital to our society. In Nature, Sargent, Peters, Agapie, et al. control microenvironments around Cu electrocatalysts to achieve an impressive selectivity of 72% for CO2 conversion to ethylene, stimulating our imagination of what industrial CO2 electrolyzers may look like in the near future.

We demonstrate the translation of a low-cost, non-precious metal cobalt phosphide (CoP) catalyst from 1 cm2 lab-scale experiments to a commercial-scale 86 cm2 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.

Using renewable electricity to convert CO/CO2 into liquid products is touted as a sustainable process to produce fuels and chemicals, yet requires further advances in electrocatalyst understanding, development and device integration. The roughness factor of an electrode has generally been used to increase total rates of production, although rarely as a means to improve selectivity. Here we demonstrate that increasing the roughness factor of Cu electrodes is an effective design principle to direct the selectivity of CO reduction towards multicarbon oxygenates at low overpotentials and concurrently suppressing hydrocarbon and hydrogen production. The nanostructured Cu electrodes are capable of achieving almost full selectivity towards multicarbon oxygenates at an electrode potential of only –0.23 V versus the reversible hydrogen electrode. The successful implementation of this catalytic system has enabled an excellent CO reduction performance and elucidated viable pathways to improve the energy efficiency towards liquid fuels in high-power conversion electrolysers. The roughness factor of an electrode has been generally used to increase total rates of production, though rarely as a means to improve selectivity. Now, Jaramillo, Hahn and co-workers direct the selectivity of CO reduction to multicarbon oxygenates at low overpotentials by increasing the roughness factor of nanostructured Cu electrodes.

CO2 reduction by combined electro- and bio-catalytic reactions is a promising technology platform for sustainable production of chemicals from CO2 and electricity. While heterogeneous electrocatalysts can reduce CO2 to a variety of organic compounds at relatively high reaction rates, these catalysts have limitations achieving high selectivity for any single product beyond CO. Conversely, microbial CO2 reduction pathways proceed at high selectivity; however, the rates at bio-cathodes using direct electron supply via electricity are commonly limiting. Here we demonstrate the use of non-precious metal cathodes that produce hydrogen in situ to support microbial CO2 reduction to C1 and C2 compounds. CoP, MoS2 and NiMo cathodes perform durable hydrogen evolution under biologically relevant conditions, and the integrated system achieves coulombic efficiencies close to 100% without accumulating hydrogen. Moreover, the one-reactor hybrid platform is successfully used for efficient acetate production from electricity and CO2 by microbes previously reported to be inactive in bioelectrochemical systems. The reduction of CO2 by electro- and biocatalysis offers a promising route to the sustainable production of chemicals and fuels. Here, the integration of methanogenic or homoacetogenic microbes with earth-abundant electrodes allows robust, rapid, and selective CO2 reduction with coulombic efficiencies of up to 100%.

The present work was undertaken to elucidate the facet-dependent activity of Ag for the electrochemical reduction of CO2 to CO. To this end, CO2 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 CO2 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 stabilizati...

In2O3 has recently emerged as a promising catalyst for methanol synthesis from CO2. In this work, we present the promotional effect of Pd on this catalyst and investigate structure–performance rela...

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.

Photoelectrochemical water splitting is a promising route for sustainable hydrogen production. Herein, we demonstrate a photoelectrode motif that enables a nanostructured large-surface area electrocatalyst without requiring a nanostructured semiconductor surface with the goal of promoting electrocatalysis while minimizing surface recombination. We compare the photoelectrochemical H2 evolution activity of two silicon photocathode nanostructuring strategies: (1) direct nanostructuring of the silicon surface and (2) incorporation of nanostructured zinc oxide to increase the electrocatalyst surface area on planar silicon. We observed that silicon photocathodes that utilized nanostructured ZnO supports outperformed nanostructured silicon electrodes by ∼50 mV at open circuit under 1 sun illumination and demonstrated comparable electrocatalytic activity.

Gallium indium phosphide (GaInP2) is a semiconductor with promising optical and electronic properties to serve as the large bandgap, top junction in a dual absorber tandem solar water splitting device. Poor intrinsic catalytic ability and surface corrosion in aqueous electrolyte remain key obstacles. Significant progress has been made developing thin-film protection layers and active catalysts for photoelectrochemical devices, but combining these into a catalytic protection layer that can provide long-term stability without sacrificing performance has proven difficult due, in large part, to challenges in developing active and stable interfaces. In this work, we demonstrate that a nanoscale molybdenum disulfide (MoS2) film functions both as an effective protection layer and excellent hydrogen evolution catalyst for GaInP2 photocathodes, with only a ∼10% loss in initial light-limited current density after 100 h, and a photocurrent onset potential better than that of the same state-of-the-art device with a platinum–ruthenium catalyst. Using transient photoreflectance spectroscopy, we probed the carrier dynamics of these photocathodes and show that the MoS2 coated device exhibits improved electron transfer at the surface interface compared to the PtRu catalyzed device. These MoS2 protected devices are among the most active and stable single-absorber photocathodes for solar water splitting to date and offer a promising pathway towards generating hydrogen with high efficiency and significant longevity.

Molybdenum phosphide (MoP) catalyzes the hydrogenation of CO, CO2 , and their mixtures to methanol, and it is investigated as a high-activity catalyst that overcomes deactivation issues (e.g., formate poisoning) faced by conventional transition metal catalysts. MoP as a new catalyst for hydrogenating CO2 to methanol is particularly appealing for the use of CO2 as chemical feedstock. Herein, we use a colloidal synthesis technique that connects the presence of MoP to the formation of methanol from CO2 , regardless of the support being used. By conducting a systematic support study, we see that zirconia (ZrO2 ) has the striking ability to shift the selectivity towards methanol by increasing the rate of methanol conversion by two orders of magnitude compared to other supports, at a CO2 conversion of 1.4% and methanol selectivity of 55.4%. In situ X-ray Absorption Spectroscopy (XAS) and in situ X-ray Diffraction (XRD) indicate that under reaction conditions the catalyst is pure MoP in a partially crystalline phase. Results from Diffuse Reflectance Infrared Fourier Transform Spectroscopy coupled with Temperature Programmed Surface Reaction (DRIFTSTPSR) point towards a highly reactive monodentate formate intermediate stabilized by the strong interaction of MoP and ZrO2 . This study definitively shows that the presence of a MoP phase leads to methanol formation from CO2 , regardless of support and that the formate intermediate on MoP governs methanol formation rate. : © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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 mgPt-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.

Accurately quantifying applied potential is important to ensuring the comparability, accuracy, and precision of electrochemical studies. Reference electrodes (REs) enable knowledge/determination of the applied potential at electrodes in electrochemical systems. Ultimately, the choice of RE will depend on the particular requirements of a given electrochemical system, however, we note it is imperative to ensure the accuracy of the RE potential and its proper translation to a standardized scale. In this work, we highlight that while there are many commercially available REs, these must be experimentally calibrated to a reliable and practical standard potential scale, for instance the reversible hydrogen electrode (RHE) scale for aqueous systems. With representative data, we provide streamlined instructions on how to calibrate any RE to the RHE scale. We also provide guidance to mitigate and/or avoid possible electrolyte contamination issues arising from REs. Moreover, we offer a step-by-step guide on how to build a practical RHE RE, which may be a suitable and desirable option in certain applications. Our work emphasizes the need for the continuous adoption of standardized reference potential scales and demonstrates the versatility of the RHE scale, particularly in aqueous electrochemistry.

Ru-based oxygen evolution reaction (OER) catalysts show significant promise for efficient water electrolysis, but rapid degradation poses a major challenge for commercial applications. In this work, we explore several Ru-based pyrochlores (A2Ru2O7, A = Y, Nd, Gd, Bi) as OER catalysts and demonstrate improved activity and stability of catalytic Ru sites relative to RuO2. Furthermore, we combine complementary experimental and theoretical analysis to understand how the A-site element impacts activity and stability under acidic OER conditions. Among the A2Ru2O7 studied herein, we find that a longer Ru-O bond and a weaker interaction of the Ru 4d and O 2p orbitals compared with RuO2 results in enhanced initial activity. We observe that the OER activity of the catalysts changes over time and is accompanied by both A-site and Ru dissolution at different relative rates depending on the identity of the A-site. Pourbaix diagrams constructed using density functional theory (DFT) calculations reveal a driving force for this experimentally observed dissolution, indicating that all compositions studied herein are thermodynamically unstable in acidic OER conditions. Theoretical activity predictions show consistent trends between A-site cation leaching and OER activity. These trends coupled with Bader charge analysis suggest that dissolution exposes highly oxidized Ru sites that exhibit enhanced activity. Overall, using the stability number (molO2 evolved/molRu dissolved) as a comparative metric, the A2Ru2O7 materials studied in this work show substantially greater stability than a standard RuO2 and commensurate stability to some Ir mixed metal oxides. The insights described herein provide a pathway to enhanced Ru catalyst activity and durability, ultimately improving the efficiency of water electrolyzers.

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. Ag0.1Pd0.9) 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., Electrocatalyst development is key to improving the performance and viability of many electrochemical energy technologies. Here, the authors design Ag-Pd alloys with specifically tuned electronic structures to have enhanced oxygen reduction electrocatalysis and decreased precious metal content.