Hollow Co₃ O₄ 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 Co₃ O₄ nanospheres embedded in nitrogen-doped carbon nanowall arrays on flexible carbon cloth (NC-Co₃ O₄ /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-Co₃ O₄ /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.