Exploring Cu-Doped Co₃O₄ Bifunctional Oxygen Electrocatalysts for Aqueous Zn–Air Batteries

Abstract

The efficiency of oxygen electrocatalysis is a key factor in diverse energy domain applications, including the performance of metal-air batteries, such as aqueous Zinc (Zn)-air batteries. We demonstrate here that the doping of cobalt oxide with optimal amounts of copper (abbreviated as Cu-doped Co₃O₄) results in a stable and efficient bifunctional electrocatalyst for oxygen reduction (ORR) and evolution (OER) reactions in aqueous Zn-air batteries. At high Cu-doping concentrations (≥5%), phase segregation occurs with the simultaneous presence of Co₃O₄ and copper oxide (CuO). At Cu-doping concentrations ≤5%, the Cu ion resides in the octahedral (O_h) site of Co₃O₄, as revealed by X-ray diffraction (XRD)/Raman spectroscopy investigations and molecular dynamics (MD) calculations. The residence of Cu@O_h sites leads to an increased concentration of surface Co³⁺-ions (at catalytically active planes) and oxygen vacancies, which is beneficial for the OER. Temperature-dependent magnetization measurements reveal favorable d-orbital configuration (high e_g occupancy ≈ 1) and a low → high spin-state transition of the Co³⁺-ions, which are beneficial for the ORR in the alkaline medium. The influence of Cu-doping on the ORR activity of Co₃O₄ is additionally accounted in DFT calculations via interactions between solvent water molecules and oxygen vacancies. The application of the bifunctional Cu-doped (≤5%) Co₃O₄ electrocatalyst resulted in an aqueous Zn-air battery with promising power density (=84 mW/cm²), stable cyclability (over 210 cycles), and low charge/discharge overpotential (=0.92 V).