Microwave-assisted synthesis of carbon nanotubes threaded core-shell CoP_x/Co-N_x-C@CNT and its performance as an efficient bifunctional oxygen catalyst for the rechargeable zinc-air battery

The development of bifunctional cathode catalysts with advantages such as high activity, low cost, and high durability is highly desirable and challenging for zinc-air batteries (ZABs) in energy applications. In this work, a novel type of bifunctional catalyst (CoP_x/Co-N_x-C@CNT) simultaneously effective for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) was prepared via a simplified all-in-one phosphorization-carbonization synthesis, which is distinctively different from the previously reported carbonization and postphosphorization route. With the integration of efficient catalytic factors such as Co-N_x-C–coordinated moieties, cobalt phosphide, graphitic carbon shells, as well as a supportive and conductive matrix, the as-prepared CoP_x/Co-N_x-C@CNT exhibits high catalytic activity for ORR and OER in 0.1 M KOH, with an output positive onset potential of 0.92 V vs reversible hydrogen electrode (RHE) for ORR and a low potential of 1.69 V vs RHE at 10 mA cm⁻² for OER. Remarkably, the assembled liquid and all-solid-state ZAB based on the as-prepared CoP_x/Co-N_x-C@CNT showed superior cycling performance than the results of its Pt/C-based counterparts and some recently reported cobalt phosphide–based ZABs. The good flexibility and reversibility of the all-solid-state ZAB also verified its feasibility in applications such as wearable electronics.