Superior stability of a bifunctional oxygen electrode for primary, rechargeable and flexible Zn–air batteries

Abstract

Central to commercializing metal–air batteries is the development of highly efficient and stable catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, a composite catalyst with a unique interpenetrating network (denoted as NiCo₂O₄@MnO₂-CNTs-3) was synthesized and exhibited better bifunctional activity (ΔE = 0.87 V) and durability than both Pt/C and Ir/C catalysts. The improved performance arises from three factors: (i) MnO₂ promotes the ORR while NiCo₂O₄ facilitates the OER; (ii) carbon nanotubes improve the electronic conductivity; and (iii) the highly porous structure enables the adsorption–desorption of O₂ and enhances the structural stability. As a result, the primary and rechargeable Zn–air battery affords a high power density and specific capacity (722 mA h g⁻¹), an outstanding discharge stability (255 mW cm⁻² after 1000 cycles) and a high cycling stability (over 2280 cycles). Electron microscopy and electrochemical analysis revealed that the degradation of the rechargeable Zn–air battery performance resulted from the damage of the air electrode and the hydrogen evolution reaction on the zinc electrode. A flexible Zn–air battery employing a solid-state electrolyte showed an exciting stability (540 cycles) and high power density (85.9 mW cm⁻²), suggesting that the anion exchange membrane effectively prevents the migration of Zn²⁺ ions and the deposition of carbonates.