Hollow Co₃O₄ Nanosphere Embedded in Carbon Arrays for Stable and Flexible Solid-State Zinc–Air Batteries

Highly active and durable air cathodes to catalyze both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are urgently required for rechargeable metal–air batteries. In this work, an efficient bifunctional oxygen catalyst comprising hollow Co₃O₄ nanospheres embedded in nitrogen-doped carbon nanowall arrays on flexible carbon cloth (NC-Co₃O₄/CC) is reported. The hierarchical structure is facilely derived from a metal–organic framework precursor. A carbon onion coating constrains the Kirkendall effect to promote the conversion of the Co nanoparticles into irregular hollow oxide nanospheres with a fine scale nanograin structure, which enables promising catalytic properties toward both OER and ORR. The integrated NC-Co₃O₄/CC can be used as an additive-free air cathode for flexible all-solid-state zinc–air batteries, which present high open circuit potential (1.44 V), high capacity (387.2 mAh g⁻¹, based on the total mass of Zn and catalysts), excellent cycling stability and mechanical flexibility, significantly outperforming Pt- and Ir-based zinc–air batteries.