Interfacial Evolution on Co-based Oxygen Evolution Reaction Electrocatalysts Probed by Using <i>In Situ</i> Surface-Enhanced Raman Spectroscopy

Abstract

Disclosing the roles of reactive sites at catalytic interfaces is of paramount importance for understanding the reaction mechanism. However, due to the difficulties in the detection of reaction intermediates in the complex heterophase reaction system, disentangling the highly convolved roles of different surface atoms remains challenging. Herein, we used CoO_<i>x</i> as a model catalyst to study the synergy of Co_Td²⁺ and Co_Oh³⁺ active sites in the electrocatalytic oxygen evolution reaction (OER). The formation and evolution of reaction intermediates on the catalyst surface during the OER process were investigated by <i>in situ</i> surface-enhanced Raman spectroscopy (SERS). According to the SERS results in ion-substitution experiments, Co_Oh³⁺ is the catalytic site for the conversion of OH^- to O-O^- intermediate species (1140-1180 cm^-1). CoOOH (503 cm^-1) and CoO₂ (560 cm^-1) active centers generated during the OER, at the original Co_Td²⁺ sites of CoO_<i>x</i>, eventually serve as the O₂ release sites (conversion of O-O^- intermediate to O₂). The mechanism was further confirmed on Co²⁺-Co³⁺ layered double hydroxides (LDHs), where an optimal ratio of 1:1.2 (Co²⁺/Co³⁺) is required to balance O-O^- generation and O₂ release. This work highlights the synergistic role of metal atoms at different valence statuses in water oxidation and sheds light on surface component engineering for the rational design of high-performance heterogeneous catalysts.