Complex Iridate Solid Solutions for Catalyzing Oxygen Evolution Reaction: Comparison of Elemental Leaching and Stability Numbers

As predicted by the Hume-Rothery rules, forming solid solutions of rutile IrO₂ with other metal oxides that have different crystal structures is thermodynamically challenging. Consequently, achieving high solubility of foreign elements in Ir-based solid-solution oxides has been significantly limited. We demonstrate that hexagonal-perovskite BaIrO₃ can serve as a flexible matrix oxide capable of incorporating a wide spectrum of (post)transition-metal cations with different electronic structures, ranging from d⁰ to d¹⁰ configurations. Among 12 cation solutes, Ta⁵⁺, Nb⁵⁺, and Zr⁴⁺ are found to be stable without substantial leaching during the oxygen evolution reaction (OER) under acidic condition. Acceptor-type trivalent cations, including Sc³⁺, In³⁺, and Fe³⁺, are identified to leach out gradually from the particle surface while enhancing the OER catalytic activity. Both X-ray absorption spectroscopy and ab initio molecular dynamics simulations consistently show that the robust face-sharing [IrO₆] octahedral framework of the solid solutions remains unperturbed unless electrochemical leaching rapidly occurs. As a result, notably high S-numbers, on the order of 10⁶, are achievable at pH = 1. Although our work focuses on single-element incorporation, it is suggested that the solid-solution methodology is an effective strategy for developing stable, long-lasting OER catalysts with further reduced Ir usage for acidic water oxidation.