N,S-Doped hollow carbon nanosheet-encapsulated Co9S8 nanoparticles as a highly efficient bifunctional electrocatalyst for rechargeable zinc–air batteries

Abstract

The development of bifunctional electrocatalysts based on non-noble metals for the oxygen reduction/evolution reactions (ORR/OER) that have rationally designed structures and inexpensive components is of practical significance for the commercialization of rechargeable zinc–air batteries. Here, we report the rational synthesis of Co₉S₈ nanoparticles embedded in N,S co-doped hollow carbon nanosheets (Co₉S₈/NSC) as highly efficient oxygen electrocatalysts. The catalyst is formed when a Co-based zeolitic imidazolate framework (ZIF-67), grown on a Zn-based ZIF (ZIF-8) template, is partially vulcanized following thioacetamide (TAA) and thermal treatment. The resulting catalyst, Co₉S₈/NSC-3, shows satisfactory bifunctional electrocatalytic activity in 0.1 M KOH, in which the half-wave potential (E_1/2) for the ORR is 0.82 V and the overpotential for the OER at 10 mA cm⁻² is just 350 mV. Furthermore, as the air electrode material in a practical demonstration of a rechargeable liquid zinc–air battery, Co₉S₈/NSC-3 exhibits promising battery performance with a high specific capacity of 804 mA h g⁻¹ and a pleasing charge/discharge cyclability of over 140 h at 10 mA cm⁻². The satisfactory activity of Co₉S₈/NSC-3 can be attributed to the synergistic effect of the Co₉S₈ nanoparticles with the N,S-doped hollow carbon nanosheet structure, resulting in an effective electrochemically active surface with fully exposed active sites that give fast catalytic kinetics.