Origin of Surface Amorphization and Catalytic Stability of Ca_{2- x}IrO₄Nanocrystals for Acidic Oxygen Evolution: Critical Roles of Acid Anions

Iridium-based perovskite oxides and complex oxides are being developed for efficient acidic oxygen evolution reaction (OER) electrocatalysts; however, the origin of their surface layer amorphization has remained poorly understood and the role of the surface amorphous layer for electrochemical OER performance is not clear. Here, we observe surface amorphization of Ca_2-xIrO₄nanocrystals during acidic OER in the H₂SO₄electrolyte, while there is no obvious surface amorphous state in the HClO₄electrolyte, using scanning transmission electron microscopy imaging. The X-ray absorption near-edge structure (XANES) spectra reveal that a few CaSO₄molecules are adsorbed on the Ca_2-xIrO₄surface in the H₂SO₄electrolyte, but the Ca coordination environments of the Ca_2-xIrO₄surface are almost unchanged in the HClO₄electrolyte. Density functional theory calculations suggest that the Ca₂IrO₄surface with leached Ca atoms is responsible for the excellent acidic OER activity, and the strong binding strengths of SO₄^2-and CaSO₄on the surface of Ca_2-xIrO₄induce surface amorphization. Chronopotentiometric measurements indicate the critical role of acid anions for the long-term catalytic stability of Ca_2-xIrO₄nanocrystals in representative acidic electrolytes. Our results demonstrate the formation mechanism of surface amorphization on Ca_2-xIrO₄nanocrystal electrocatalysts and provide insights into the influence of different electrolytes on catalytic stability for highly active acidic OER nanocatalysts.