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Acidic Oxygen Evolution Reaction Activity–Stability Relationships in Ru-Based Pyrochlores

Hubert, McKenzie A and Patel, Anjli M and Gallo, Alessandro and Liu, Yunzhi and Valle, Eduardo and Ben-Naim, Micha and Sanchez, Joel and Sokaras, Dimosthenis and Sinclair, Robert and Nørskov, Jens K and King, Laurie A and Bajdich, Michal and Jaramillo, Thomas F (2020) Acidic Oxygen Evolution Reaction Activity–Stability Relationships in Ru-Based Pyrochlores. ACS Catalysis. pp. 12182-12196. ISSN 2155-5435

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Abstract

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.

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