Structural Changes of Spinel MCo₂O₄ (M = Mn, Fe, Co, Ni, and Zn) Electrocatalysts during the Oxygen Evolution Reaction Investigated by In Situ X-ray Absorption Spectroscopy

A comparison of the electrochemical and physicochemical behavior of cobalt-based oxides with spinel structure MCo2O4 (M = Mn, Fe, Co, Ni, and Zn) was conducted to investigate the effect of the oxidation state and cation distribution in the spinel on the electrocatalytic activity of the oxygen evolution reaction (OER) in an alkaline solution. Various spinel MCo2O4 electrocatalysts were synthesized by a facile microwave-assisted synthesis and low-temperature annealing. The overpotential of these MCo2O4 electrocatalysts for the OER is comparable to the reported overpotentials of catalysts based on cobalt oxides. From the findings, the catalytic activity of OER decreases in the order of ZnCo2O4 > NiCo2O4 > FeCo2O4 > Co3O4 > MnCo2O4. It was revealed that the active sites are controlled by the balance of M3+/M2+ cation distribution in octahedral and tetrahedral sites and by the bond strength between M and oxygen atoms at the catalyst surface from the direct combination of in situ X-ray absorption fine structure (XAFS) spectroscopy with the electrochemical experiments to track the oxidation state and the structural changes of electrocatalysts before and after the exposure to the OER conditions. This study provides insights into the effects of cation distributions on the OER activity and demonstrates a promising method for determining the fundamental mechanism of cation-substituted cobalt oxides for OER.