Co₃O₄/MnO₂/Hierarchically Porous Carbon as Superior Bifunctional Electrodes for Liquid and All-Solid-State Rechargeable Zinc–Air Batteries

The design of efficient, durable, and affordable catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is very indispensable in liquid-type and flexible all-solid-state zinc-air batteries. Herein, we present a high-performance bifunctional catalyst with cobalt and manganese oxides supported on porous carbon (Co₃O₄/MnO₂/PQ-7). The optimized Co₃O₄/MnO₂/PQ-7 exhibited a comparable ORR performance with commercial Pt/C and a more superior OER performance than all of the other prepared catalysts, including commercial Pt/C. When applied to practical aqueous (6.0 M KOH) zinc-air batteries, the Co₃O₄/MnO₂/porous carbon hybrid catalysts exhibited exceptional performance, such as a maximum discharge peak power density as high as 257 mW cm^-2 and the most stable charge-discharge durability over 50 h with negligible deactivation to date. More importantly, a series of flexible all-solid-state zinc-air batteries can be fabricated by the Co₃O₄/MnO₂/porous carbon with a layer-by-layer method. The optimal catalyst (Co₃O₄/MnO₂/PQ-7) exhibited an excellent peak power density of 45 mW cm^-2. The discharge potentials almost remained unchanged for 6 h at 5 mA cm^-2 and possessed a long cycle life (2.5 h@5 mA cm^-2). These results make the optimized Co₃O₄/MnO₂/PQ-7 a promising cathode candidate for both liquid-type and flexible all-solid-state zinc-air batteries.