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

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 (E1/2) 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.