Versatile electrochemical manufacturing of mixed metal sulfide/N-doped rGO composites as bifunctional catalysts for high power rechargeable Zn-air batteries

The development of rechargeable zinc-air batteries requires air cathodes capable of performing both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with high performance and an extended operational lifespan. Here, we present a cost-effective and versatile electrochemical method for the direct assembly of such electrocatalysts, consisting of nitrogen-doped reduced graphene oxide (NrGO) and mixed transition metal sulfides (NiCoMnSx or NCMS). To this end, we use a small electric bias to electro-deposit both NrGO and NCMS directly on conductive graphene foam, resulting in a perfect porous network and two interpenetrated paths for the easy transport of electrons and ions. The NCMS/NrGO composite shows one of the highest limiting currents reported so far for a non-noble metal catalyst. Additionally, it exhibits outstanding bifunctional performance for the ORR/OER, superior to both mixed transition metal compounds and noble metals from previous reports. Thus, it serves as a highly efficient air cathode for practical zinc-air batteries featuring high power densities (124 mW cm-2) and long catalyst durability (1560 cycles, around 260 h). We attribute the excellent performance to the synergistic effect between hetero-structured metallic sites and nitrogen dopants. Our approach can be used for preparing efficient zinc-air cathodes on conductive 3D carbon substrates with arbitrary shapes and good performance. Electrodeposition was used for direct synthesis of nitrogen-dopged reduced graphene oxide (NrGO) and mixed transition metal sulfides (NCMS). The resulting NCMS/NrGO composite exhibits excellent OER/ORR peroformance as Zn-air battery cathode.