Insights into Acidic Oxygen Evolution Reaction over Ru-Pyrochlore Oxide Electrocatalysts: Role of Lanthanide Cations in Dictating Stability

Abstract

Oxygen evolution reaction (OER) is a key anodic process in electrochemical upgrading and energy storage. However, its sluggish kinetics require high overpotentials to achieve practical electrochemical rates over the state-of-the-art IrO2 electrocatalyst. More cost-effective RuO2 exhibits higher activity than IrO2 but is limited by long-term stability. Incorporating Ru into ordered mixed metal oxide frameworks has shown promise for mitigating instability; however, the design principles needed to stabilize Ru cations in these oxides remain poorly understood. Herein, we investigate factors that govern Ru cation stability in lanthanide-based Ru-pyrochlores (Ln2Ru2O7, Ln = Pr, Nd, Sm, Eu, and Gd) under acidic OER conditions. We probe how A-site Ln identity influences metal–oxygen (M–O, M = Ln, Ru) bonding and link these structural changes to differences in electrochemical reactivity and stability. By integrating structural characterization (X-ray diffraction, angle-resolved X-ray photoelectron spectroscopy), electrochemical measurements (cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy), and time-resolved cation dissolution analyses, we show how A-site Ln identity influences the intrinsic metal–oxygen bonding in the pyrochlore lattice, dictating the oxide's susceptibility to dissolution under oxidative acidic conditions, consequently impacting electrochemical stability. Across the Ln2Ru2O7 series, decreasing Ln cation size correlates with enhanced electrochemical stability of Ru-pyrochlores. This manifests as two regimes during constant current studies: an initial period of significant cation dissolution during which electrochemical activity is maintained, followed by attenuated dissolution and the onset of electrochemical instability. The duration of the stable regime extends systematically as the A-site cation size decreases. In more stable pyrochlores, preferential Ln dissolution leads to formation of a Ru-enriched near-surface region, the extent of which inversely correlates with subsequent deactivation rates. These findings establish that Ru cation stability in pyrochlores is governed by A-site Ln identity, which controls the kinetics of cation dissolution, the extent of surface restructuring toward Ru-enriched states, and ultimately, long-term electrochemical durability, providing a compositional lever for designing acid-stable Ru-based OER electrocatalysts.