Graphene-Encapsulated Co₉ S₈ Nanoparticles on N,S-Codoped Carbon Nanotubes: An Efficient Bifunctional Oxygen Electrocatalyst

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 nanohybrid, in which N,S-codoped carbon nanotubes were decorated with Co₉ S₈ nanoparticles encapsulated in porous graphene layers, was fabricated by a one-step heat-treatment process. The N,S dopant species were the major active sites for the ORR, and Co₉ S₈ nanoparticles were mainly responsible for the OER. Compared with commercial 20 wt % Pt/C and Ir/C electrocatalysts, this nanohybrid exhibited a comparable ORR half-wave potential (0.831 V vs. reversible hydrogen electrode) and OER potential (1.591 V at 10 mA cm^-2 ), better long-term stability in an alkaline medium, and a narrower potential gap (0.76 V) between ORR and OER. Furthermore, as air electrode of the rechargeable zinc-air battery, it delivered a low charge-discharge voltage gap (0.65 V at 5 mA cm^-2 ), high open-circuit potential (1.539 V), good specific capacity (805 mA h g $\frac{-1}{\mathrm{Zn}}$ at 5 mA cm^-2 ), and excellent cycling stability (48 h), superior to those of commercial Pt/C and Ir/C catalysts, and thus showed promise for applications in renewable energy conversion devices.