Fully exposed copper single-atom sites on mesoporous N/S-codoped graphene for efficient zinc-air battery

Transition metal single-atom catalysts are recently emerging as the most potential candidates for the oxygen reduction reaction due to their remarkable activity and durability, yet subjected to limited availability proportion of active sites to reactant in practice due to the lack of favorable morphology. Herein, the ultrathin mesoporous N/S-codoped graphene is fabricated by a potassium thiocyanate (KSCN) assisted pyrolysis of metal-organic framework (ZIF-8) as the robust substrate for anchoring copper single-atom sites. The KSCN-assisted pyrolysis can not only create N and S coordination atoms but also regulate the fully exposed morphology. The graphene-supported copper single-atom catalysts (g-Cu-SACs) show typical mesoporous structure with larger surface area of 1612 m2 g–1. Spectral and microscopy results reveal highly isolated Cu single-atom sites on mesoporous graphene with unsymmetrical Cu-S1N3 structure. Theoretical analysis demonstrated that the asymmetric coordination environment effectively enhanced O2 adsorption and facilitated the ORR performance. The g-Cu-SACs show excellent catalytic activity in alkaline oxygen reduction reaction with a half-wave potential of 0.920 VRHE and good cycling stability, which are superior to that of three-dimensional copper single-atom catalysts (Cu-SACs, supported by N/S-codoped carbon framework directly derived from ZIF-8). Furthermore, the zinc-air battery assembled with g-Cu-SACs also exhibits high specific capacity and remarkable cycling lifetime. This work provides a novel strategy to construct highly exposed single-atom catalysts for energy-related applications.