Graphene-encapsulated Co9S8 nanoparticles decorated N, S co-doped carbon nanotubes: An efficient bifunctional oxygen electrocatalyst for rechargeable zinc-air battery.

An inexpensive and efficient bifunctional electrocatalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), is central to the rechargeable zinc-air battery. Herein, a novel nanohybrid, N and S co-doped carbon nanotubes decorated with porous graphene layers encapsulated Co9S8 nanoparticles, was fabricated by one-step heat treatment process. The N, S doped species are major activity sites for the ORR, while Co9S8 nanoparticles are mainly responsible for the OER. Compared with commercial 20wt% Pt/C and Ir/C electrocatalysts, this nanohybrid exhibits comparable ORR half-wave potential (0.831 V vs. RHE) and OER potential (1.591 V at 10 mA·cm-2), better long-term stability in an alkaline medium, as well as a narrower potential gap (0.76 V) between ORR and OER. Furthermore, as air electrode of the rechargeable zinc-air battery, it delivers low charge-discharge voltage gap (0.65 V at 5 mA·cm-2), high open circuit potential (1.539 V), good specific capacity of 805 mA h gZn-1 (at 5 mA·cm-2) and excellent cycling stability (48 h), superior to the commercial Pt/C+Ir/C catalyst, demonstrating promising applications for renewable energy conversion devices.