The origin of high electrochemical stability of iridium oxides for oxygen evolution

Abstract

Understanding the dissolution mechanism of oxygen evolution reaction (OER) catalysts is essential for designing efficient and stable electrolyzers. Iridium oxide (IrO₂), the most stable single oxide OER catalyst, represents an ideal subject for investigating and decoding the secrets of electrochemical stability under high anodic potentials. Using constant-potential DFT calculations, we reveal that the exceptional stability of IrO₂ originates from a highly activated surface reconstruction step. Compared with Ru reconstruction on RuO₂(110), which occurs readily (<1 eV) via a water oxidation induced mechanism, Ir reconstruction cannot be facilitated with concerted water oxidation, imposing a significantly higher barrier (>2 eV). We further illustrate that the distinct Ir reconstruction kinetics stems from the inherent stability of Ir⁴⁺ in the rutile phase – a consequence of relativistic effects, which makes the oxidation of Ir⁴⁺ unfavorable even at highly anodic potentials. Instead, the formation of a high-energy surface-adsorbed IrO₄ precursor is required before further oxidation can occur to form soluble IrO₃ or IrO₂(OH) species. Our findings suggest that stabilizing the relative stability of rutile Ru⁴⁺ with respect to that of higher oxidation states could be a working strategy for the design of stable Ru-based OER catalysts.