Polaronic Competition Triggers the H$_2$O$_2$ Evolution on Perovskite Oxides during Water Oxidation

Perovskite oxides, particularly calcium-based perovskites (CaMO$_3$), are promising catalysts for the oxygen evolution reaction (OER) due to their high efficiency and economic feasibility. However, a comprehensive mechanistic understanding that elucidates the relationship between catalytic selectivity and mechanistic pathways has yet to be achieved. In this study, we employ density functional theory (DFT) to investigate the mechanism of the OER in a series of Ca-based perovskites. Our findings indicate that early transition metals in CaMO$_3$ favor a conventional OER pathway, characterized by a four-step concerted proton−electron transfer process leading to O$_2$ formation from H$_2$O. In contrast, perovskites containing Mn, Fe, and Co exhibit two proton−electron transfer steps, favoring the selective formation of hydrogen peroxide (H$_2$O$_2$) over O$_2$. This shift in selectivity is attributed to polaronic effects, which strengthen the metal-oxo bonding, enabling a transition from a conventional OER to H$_2$O$_2$ evolution. Furthermore, we reveal that the thermodynamic stability of these perovskites in aqueous environments is significantly influenced by pH, where acidic conditions destabilize the perovskite structure. These insights suggest that modulating polaronic effects and maintaining high pH environments are key to optimizing both the stability and catalytic activity of perovskite oxides in OER applications.