Operando Identification of Dual Active Sites in Ca₂IrO₄ Nanocrystals with Yttrium Substitutions Boosting Acidic Oxygen Evolution Reaction

Non-noble metal substitutions of the iridium sites in iridium-based complex oxides while boosting acidic oxygen evolution reaction (OER) is a promising method for developing efficient electrocatalysts, but this remains a huge challenge. Herein, we report yttrium substitution of iridium in Ca2IrO4 (Ca2YxIr1–xO4) nanocrystals for the first time, which is demonstrated by detailed structural characterizations using X-ray absorption spectra, X-ray diffraction patterns, and elemental mapping images. The synthesized Ca2Y0.2Ir0.8O4 catalyst requires a low overpotential of only 213 mV, achieving an acidic OER current density of 10 mA cm–2, which represents an approximately 203.7-fold improvement in iridium mass activity and 204.4-fold improvement in turnover frequency in comparison to that of IrO2 at 1.5 V vs RHE, respectively. Systematic characterizations of electronic structures reveal the synergistic effects between high-valence iridium sites and increased lattice oxygen concentration induced by Y3+ substitutions, which greatly enhance the intrinsic OER activity of Ca2Y0.2Ir0.8O4. Operando X-ray absorption spectra and Raman spectra reveal the iridium and yttrium dual active sites during acidic OER. Our results provide a method for designing dual active sites in iridium-based complex oxides for highly active acidic OER electrocatalysts.