Superior bifunctional oxygen electrocatalysts based on Co2MnO4 with mixed site occupancy, Mn-rich surfaces and twin defects

Co/Mn-based spinel oxides are promising electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), however, it is challenging to achieve high activities for ORR and OER simultaneously in one composition. Here, we report Co2MnO4 (C2M) with Mn-rich surfaces and abundant twin defects synthesized by a simple coprecipitation and annealing route, which exhibits excellent ORR activity (half-wave potential E1/2 = 0.830 V), long ORR durability (91% current retained after 12 h-chronoamperometry test at 0.45 V vs. RHE) and a low ΔE = 0.767 V between E1/2 and E10 mA/cm2 in alkaline electrolyte, which enables its excellent performances in rechargeable Zn-air batteries showing a high peak energy density of 136 mW/cm2 and a large specific capacity of 809.4 mAh/gZn. Combining comprehensive experimental investigations and DFT simulations, we reveal that the Mn-rich surfaces benefit bridge adsorption of O2 to promote the 4-electron reaction pathway in ORR, and the mixed site occupancy in the crystal structure induces Mn/Co-O orbital hybridizations to form an impurity energy level in the band gap to facilitate charge transfer. Furthermore, the bifunctional performance and the ORR durability can be further enhanced by acid treatment to show a lower ΔE = 0.750 V and to retain 98.5% ORR current in 12 h. This work provides not only a promising bifunctional electrocatalyst but also a new perspective of exploring simple synthesis method suitable for mass-producible electrocatalysts with highly active surfaces.