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

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 (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.

High-Efficiency of Bi-Functional-Based Perovskite Nanocomposite for Oxygen Evolution and Oxygen Reduction Reaction: An Overview

“…Among these nanocubes (Figure 3b) and nanocrystals (Figure 3c), pc-LSMO nanocrystals could exhibit better ORR activity (21.4 A g −1 ) with remarkable cyclic stability under alkaline conditions [49]. Ruthenium-based pyrochlores (A2Ru2O7, A=Y, Nd, Bi) composite has shown the distribution of nanoparticles, which is analyzed by STEM-EDX analysis and the systematic theoretical study of OER activities [50]. Perovskite-structured calcium and strontium-doped rare-earth cobaltate powders were synthesized by various processes.…”

Novel engineering of ruthenium‐based electrocatalysts for acidic water oxidation: A mini review

“…Generally, as shown in Figure 7(A), pyrochlore oxides can be denoted as A 2 B 2 O 6 O′ or [A 2 O′][B 2 O 6 ], which consists of a network of corner‐sharing BO 6 octahedra with interstitial sites being occupied by O′ and A atoms (A 2 O′) 109 . The Ru‐based pyrochlore oxides, in which the B sites are dominated by Ru atoms have also drawn a great deal of notice because of the lowered usage of Ru, while maintaining the excellent catalytic activity and stability toward OER 49,50,52–54,110–113 . For examples, Kim et al 49 used a sol–gel method to synthesize Y 2 Ru 2 O 7−δ, in which all diffraction of Y 2 Ru 2 O 7−δ was consistent with cubic phase ( Fd3m ) pyrochlore‐type Y 2 Ru 2 O 7 (YRO) without any unpurified phase.…”

Mixed Metal-Antimony Oxide Nanocomposites: Low pH Water Oxidation Electrocatalysts with Outstanding Durability at Ambient and Elevated Temperatures

“…[Mn+Sb]Ox catalyst is of the same order of magnitude as that reported for the ruthenium pyrochlore catalysts elsewhere. [48]…”