Defect Engineered Ni Doped MnCO2O4 Spinel Acts as Effective Bifunctional Electrocatalyst for Rechargeable Zinc-Air Batteries

Herein, an effective strategy using defect engineering promoted by Ni atom doping on MnCo2O4 is proposed for bifunctional catalytic activities of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). A series of NixMn1-xCo2O4 with x=0, 0.1, 0.2, and 0.3 is hydrothermally synthesized. Tuning the electronic structure by Ni-doped MnCo2O4 results in modulating the valence states by dominating Mn4+ / Co3+ with the increase of oxygen vacancies. At optimized Ni content, Ni0.2Mn0.8Co2O4 reveals activity toward ORR with a current density of -1.02 mA cm(-2) at 0.95 V (vs. RHE), a high electron transfer number of 3.536, and the highest half-wave potential of 0.855 V, which is comparable to Pt/C. For OER, Ni0.2Mn0.8Co2O4 demonstrates the lowest overpotential of 503 mV to achieve a current density of 10 mA cm(-2). Additionally, rechargeable zinc-air batteries (RZABs) with Ni0.2Mn0.8Co2O4 catalyst demonstrate a capacity of 808.6 mA h g(-1) at 1.0 mA cm(-2) with power densities of 459 mW cm(-2) at 675 mA cm(-2). The improved binding strength (Mn-O/Co-O bond) and the enhanced stability by Ni atom doping in MnCo2O4 for ORR/OER is proved by ex-situ FT-EXAFS. This work highlights defect engineering on Ni-doped MnCo2O4 used as bifunctional ORR/OER catalysts for high-performance RZABs.