Graphene-Encapsulated Co9S8 Nanoparticles on N,S-Codoped Carbon Nanotubes: An Efficient Bifunctional Oxygen Electrocatalyst

Graphical Bifunctional electrocatalyst : A nanocomposite with graphene-encapsulated Co 9 S 8 nanoparticles loaded on N,S-codoped carbon nanotubes is fabricated as a highly efficient and stable bifunctional electrocatalyst for the oxygen reduction and evolution reactions. It can be used as the air electrode in the rechargeable Zn–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 nanohybrid, in which N,S-codoped carbon nanotubes were decorated with Co 9 S 8 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 Co 9 S 8 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 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.