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 (A 2 Ru 2 O 7 , A = Y, Nd, Gd, Bi) as OER catalysts and demonstrate improved activity and stability of catalytic Ru sites relative to RuO 2 . Furthermore, we combine complementary experimental and theoretical analysis to understand how the A-site element impacts activity and stability under acidic OER conditions. Amongst the A 2 Ru 2 O 7 studied herein, we find that a longer Ru-O bond and a weaker interaction of the Ru 4d and O 2p orbitals compared to RuO 2 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 (mol O 2 evolved /mol Ru dissolved ) as a comparative metric, the A 2 Ru 2 O 7 materials studied in this work show substantially greater stability than a standard RuO 2 and commensurate stability to some Ir mixed metal oxides. The insights described herein provide a path to further enhance Ru catalyst activity and durability, ultimately improving the efficiency of water electrolyzers.

Zirconium phosphate (ZrP), an inorganic layered nanomaterial, is currently being investigated as a catalyst support for transition metal-based electrocatalysts for the oxygen evolution reaction (OER). Two metal-modified ZrP catalyst systems were synthesized: metal-intercalated ZrP and metal-adsorbed ZrP, each involving Fe(II), Fe(III), Co(II), and Ni(II) cations. Fourier transform infrared spectroscopy, X-ray powder diffraction, thermogravimetric analysis, and X-ray photoelectron spectroscopy were used to characterize the composite materials and confirm the incorporation of the metal cations either between the layers or on the surface of ZrP. Both types of metal-modified systems were examined for their catalytic activity for the OER in 0.1 M KOH solution. All metal-modified ZrP systems were active for the OER. Trends in activity are discussed as a function of the molar ratio in relation to the two types of catalyst systems, resulting in overpotentials for metal-adsorbed ZrP catalysts that were less than, or equal to, their metal-intercalated counterparts.

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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 (SrCl2: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 SrCl2: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 SrCl2: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 SrCl2 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.

The morphology of ZrP supports affects the loading and coverage of Co and Ni species, explaining their different OER performances.

Improved electrochemical oxygen evolution catalysis is crucial for many clean-energy production technologies. Recently, transition-metal-modified zirconium phosphate (ZrP) catalysts were studied for the oxygen evolution reaction (OER) in alkaline media. These studies suggest that the OER occurs preferentially on the surface of the layered ZrP nanoparticles rather than the interlayer gallery. Herein, ZrP nanoparticles are exfoliated with tetrabutyl­ammonium hydroxide (TBA+OH–) to further expose surface sites which are subsequently modified with Co and Ni cations by an ion-exchange reaction. Because of the greater surface accessibility of the exfoliated ZrP support, higher loadings of catalyst material were achieved along with improved site access for catalysis. These new composite materials have improved geometric area normalized overpotentials than metal-adsorbed ZrP nanoparticles without exfoliation. Specifically, Co-modified and Ni-modified exfoliated ZrP show a reduction in overpotential at a current density of 10 mA/cm2 by 41 and 181 mV, respectively.

Reducing precious metal content and improving the efficiency of proton exchange membrane water electrolyzers is critical for producing renewable hydrogen cost-effectively. Mixed metal iridium oxide catalysts (AIr x O y , A = nonprecious metal) have demonstrated superior oxygen evolution reaction (OER) activity relative to IrO2 catalysts while utilizing less Ir. However, improved stability is required if these materials are to be implemented commercially. In this work, we use a combination of ex situ and in situ characterization techniques to study physical and electronic properties of Y2Ir2O7 as it evolves during OER in acidic electrolyte. We identify and quantify dissolution of Y and Ir, finding that this material exhibits similar stability to other reported mixed metal Ir oxides (104–105 molO2 evolved/molIr dissolved) and appears to become more stable over time. We find that the catalyst surface becomes enriched with Ir after electrochemical testing. We further monitored the Ir oxidation state in situ using high-energy resolution fluorescence detected X-ray absorption spectroscopy. Our results suggest that the Ir oxidation state is dynamic: an IrO x surface forms that is more oxidized than the bulk pyrochlore material but subsequently dissolves. Such detailed characterization of material properties can be used to develop design principles for improving catalyst stability.