Pod-like structured Co/CoO _x nitrogen-doped carbon fibers as efficient oxygen reduction reaction electrocatalysts for Zn-air battery

In this report, novel pod-like nitrogen-doped carbon fibers encapsulated with cobalt/cobalt oxide nanoparticles (Co/CoO _x -N-C) were prepared by a facile method. Co/CoO _x -N-C can be readily obtained from the carbonization of electrospun ZIF-67@PAN fiber. Both effects of ZIF-67 crystal size and pyrolysis temperature on the ORR were investigated. The optimal Co/CoO _x -N-C-l, which has an average ZIF-67 size of 370 nm and a carbonization temperature of 800 °C, presents features of 1D hierarchical porosity pod-like structure, dispersion of nanoparticles, high content of N dopant, and high degree of graphitization, resulting in an excellent ORR performance. Co/CoO _x -N-C-l presents an onset potential of −0.07 V (vs Ag/AgCl), a half-wave potential of −0.144 V, and a diffusion limited current density at 0.6 V of 4.9 mA/cm ² . The relative current of Co/CoO _x -N-C-l (90.1%) is much higher than that of the 20 wt% Pt/C (79.8%) after a 36,000 s of continuous operation. In addition, Co/CoO _x -N-C-l was further applied as a cathode catalyst of the Zn-air battery. It exhibited an outstanding discharge specific capacity (610 mAh/g) and a comparable discharge voltage to that of 20 wt% Pt/C and RuO ₂ . This study may provide a novel strategy to generate pod-like structured nanocomposites for energy devices.