Optimizing the eg occupancy of magnesium cobalt spinel oxides via Fe substitution to promote oxygen evolution reaction

Cobalt-based spinel oxides are a promising class of electro-catalysts for oxygen evolution reaction (OER). Herein, a series of Mg(Co1−xFex)2O4 (x = 0.0–1.0) oxides are designed as OER catalysts. MgCo2O4 is selected to fix Co3+ in the octahedral position, which is active site for the OER. We elucidate the volcanic relationship between the OER activity and Fe substitution concentration in Mg(Co1−xFex)2O4, which shows an optimal activity at 20 at% Fe substitution. Superconducting Quantum Design (SQUID) technique reveals that Fe substitution tunes the eg orbital electronic configuration of Co3+ from low spin to high spin. The optimized eg occupancy of 0.91 is obtained in MgCo1.6Fe0.4O4. Density functional theory calculations verify that the improved OER performance stems from the enhanced Co-O covalence after Fe substitution, which promotes the transfer of electrons from oxygen, thus accelerates the catalytic oxygen evolution reaction. The formation energy of O* decrease as well by Fe substitution, and the conversion of OH* into O* is identified to be the rate-determining step. This work complements the optimizing electronic configuration of cobalt cations for OER and provides favorable principles for the rational design of OER electro-catalysts.