Lattice-Relaxation-Enabled Design of Ru–Ir Oxides for Acidic Oxygen Evolution Reaction: Insights from Pair Distribution Function and Operando Multiedge XAS Analyses

Designing oxygen evolution catalysts that are both active and durable under acidic conditions requires balancing the high activity of Ru oxides with the exceptional stability but limited abundance of Ir oxides. Here, we demonstrate that an optimized Ru_0.875Ir_0.125O<i>_x</i>-300 (annealing temperature (AT)) catalyst achieves an overpotential of 208 mV at 10 mA cm^-2 and sustains stable operation for over 100 h, surpassing commercial IrO₂ and RuO₂ benchmarks. Despite extensive efforts, the structural and electronic evolution underlying the local-to-long-range ordering in mixed Ru-Ir oxides remains elusive because conventional X-ray diffraction (XRD) fails to resolve short-range structural motifs. Here, we combine atomic pair distribution function (PDF) analysis with operando multiedge X-ray absorption spectroscopy (XAS), including extended X-ray absorption fine structure (EXAFS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) at the Ir L₃ and Ru K edges, and surface-sensitive O K-edge spectroscopy, to track atomic and electronic transformations across composition and annealing temperature. Unlike conventional XRD or single-edge XAS, this combined approach enables direct correlation between short-/medium-range structural ordering and element-specific electronic responses under operando conditions. PDF analysis identifies the onset of rutile-like ordering near 300 °C (AT) in Ru-rich oxides, accompanied by cooperative Ir-O contraction and Ru-O expansion. Under anodic bias, Ir-O bonds contract further and accumulate 5d holes. In contrast, Ru-O bonds remain nearly invariant in the crystalline 300 °C (AT) phase but strongly shorten in the low-symmetry 250 °C (AT) state, reflecting the transition from a flexible to a rigid lattice. The combined fingerprints reveal that mixed-octahedra relaxation, Ir-O shortening coupled with moderated Ru-O response within a rutile-like framework, governs the activity-stability tradeoff, as revealed by operando EXAFS measurement. These findings establish lattice relaxation during the low-symmetry-to-rutile structural evolution as a mechanistic basis and design principle for durable, low-Ir acidic OER catalysts. This activity-stability balance is further supported by in a single-cell proton exchange membrane water electrolyzer, confirming relevance under membrane-electrode assembly (MEA) conditions.