Understanding the evolution of catalytically active multi-metal sites in a bifunctional high-entropy alloy electrocatalyst for zinc-air battery application

Zinc-air batteries are known for high theoretical energy density and environmental friendliness. The successful commercial utilization of rechargeable zinc-air batteries is limited by unstable electrochemical interfaces and sluggish kinetics with poor round-trip efficiency. In this study, we report a nanocrystalline high entropy alloy (HEA) comprising Cu-Co-Mn-Ni-Fe (CCMNF) prepared by casting-cum-cryomilling method. This multi-component HEA embodies multiple catalytically active sites with diverse functionalities, thus enhancing the electrochemical redox reactions, e.g., oxygen reduction (ORR) and oxygen evolution reaction (OER). The bifunctional electrocatalytic performance of this HEA is comparable to that of standard catalysts, RuO2 and Pt/C, as evidenced by low overpotential requirements towards OER and ORR. The HEA was tested for use in the air electrode catalyst in the zinc-air battery, where it performed stable oxygen electrocatalysis that was durable over 1045 charging-discharging cycles for similar to 90 hours of continuous operation. The microstructural analysis of HEA at different time scales (0, 24, 87 h) during the zinc-air battery operation suggested a dynamic participation of multiple metal active sites on the catalyst surface. Detailed studies revealed that despite leaching in harsh alkaline operation conditions, the synergistic electronic interactions between the component metal sites sustained good electrocatalytic performance and promoted oxygen electrocatalysis through the modification of electronic and chemical properties. Rechargeable zinc-air battery operations in an aqueous alkaline electrolyte were studied using a novel FCC crystalline high-entropy alloy CCMNF (Co, Cu, Mn, Ni, Fe) as a bifunctional electrocatalyst at the air electrode.