Origin of Surface Amorphization and Catalytic Stability of Ca2-xIrO4 Nanocrystals 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 Ca2-xIrO4 nanocrystals during acidic OER in the H2SO4 electrolyte, while there is no obvious surface amorphous state in the HClO4 electrolyte, using scanning transmission electron microscopy imaging. The X-ray absorption near-edge structure (XANES) spectra reveal that a few CaSO4 molecules are adsorbed on the Ca2-xIrO4 surface in the H2SO4 electrolyte, but the Ca coordination environments of the Ca2-xIrO4 surface are almost unchanged in the HClO4 electrolyte. Density functional theory calculations suggest that the Ca2IrO4 surface with leached Ca atoms is responsible for the excellent acidic OER activity, and the strong binding strengths of SO42- and CaSO4 on the surface of Ca2-xIrO4 induce surface amorphization. Chronopotentiometric measurements indicate the critical role of acid anions for the long-term catalytic stability of Ca2-xIrO4 nanocrystals in representative acidic electrolytes. Our results demonstrate the formation mechanism of surface amorphization on Ca2-xIrO4 nanocrystal electrocatalysts and provide insights into the influence of different electrolytes on catalytic stability for highly active acidic OER nanocatalysts.