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

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.