In situ space-confined growth of Co 3 O 4 nanoparticles inside N-doped hollow porous carbon nanospheres as bifunctional oxygen electrocatalysts for high-performance rechargeable zinc-air batteries.

Developing high-performance and low-cost bifunctional oxygen electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is of great significance for accelerating the commercialization of rechargeable zinc-air batteries (RZABs). Herein, in situ grown Co 3 O 4 nanoparticle-embedded N-doped hollow porous carbon nanospheres (Co 3 O 4 @N-HPCNs) are synthesized via template-assisted pyrolysis as efficient bifunctional ORR/OER electrocatalysts. The N-HPCNs efficiently seize and confine Co 3 O 4 nanoparticles to enhance electronic conductivity and structural stability, while the hollow porous architecture offers adequate mass diffusion pathways to improve the accessibility of reactants and electrolytes on active sites. Therefore, the as-obtained Co 3 O 4 -10%@N-HPCNs display outstanding activity and stability for the ORR and the OER, even outperforming commercial Pt/C and Ru/C catalysts. Liquid RZABs assembled with Co 3 O 4 -10%@N-HPCN cathodes exhibit a large specific capacity of 768.3 mA h g -1 Zn , a high peak power density of 145.6 mW cm -2 and a long-term cycling stability for over 1000 h, demonstrating much-enhanced battery performance in comparison with that of Pt/C + Ru/C based RZABs. Also, flexible quasi-solid-state RZABs assembled with Co 3 O 4 -10%@N-HPCN cathodes exhibit a considerable power density of 132.0 mW cm -2 and a stable charge-discharge voltage for a long period even upon bending. This work provides a new approach for the development of catalysts with high activity, long-term stability and low cost.