Crystal size-mediated solute incorporation and solid-solution stability in RuO2 during acidic oxygen evolution

Abstract

Understanding the miscibility of foreign cations in pristine oxides is a fundamental step toward synthesizing multicomponent single-phase oxide crystals with unprecedented functionalities. In rutile RuO₂, we find that many solute cations, which typically exhibit limited miscibility below a few percent in the bulk, can substitute Ru at levels exceeding 20 at% when the crystal size is reduced below 10 nm. More importantly, this size-dependent enhancement in solubility is not restricted to a few specific elements but appears as a general trend across 20 distinct foreign cations. These results suggest that the phase equilibria of RuO₂ are remarkably altered under the high Laplacian pressure induced by nanoscale crystal dimensions. The successful synthesis of solid-solution RuO₂ nanocrystals smaller than 10 nm, with 20 different compositions, enables quantitative and systematic evaluation of their stability numbers (S-numbers) under identical electrochemical conditions as anodic electrocatalysts for acidic water oxidation. In addition to identifying significant RuO₂-based solid solutions with Ta, Ir, Nb, Sb, and Mn among many proposed candidates for high-durability nanoscale electrocatalysts, the findings in our study demonstrate that the extent of chemical modification in RuO₂ is strongly dependent on crystal size.