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(Co _1−xFe _x) ₂O ₄ (x = 0.0–1.0) oxides are designed as OER catalysts. MgCo ₂O ₄ is selected to fix Co ³⁺ 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(Co _1−xFe _x) ₂O ₄, which shows an optimal activity at 20 at% Fe substitution. Superconducting Quantum Design (SQUID) technique reveals that Fe substitution tunes the e _g orbital electronic configuration of Co ³⁺ from low spin to high spin. The optimized e _g occupancy of 0.91 is obtained in MgCo _1.6Fe _0.4O ₄ . 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.