N,S dual-doped carbon nanosheet networks with hierarchical porosity derived from biomass of Allium cepa as efficient catalysts for oxygen reduction and Zn-air batteries

Efficient metal-free electrocatalysts for oxygen reduction reactions (ORR) have been actively pursued in recent years to promote the application of fuel cells and metal-air batteries. In this work, hierarchically porous nitrogen and sulfur dual-doped carbon nanosheet networks (N,S-HPC) were facilely synthesized by pyrolysis of Allium cepa chips impregnated with thiourea and KOH. The as-prepared N,S-HPC featured high content of N and S dopants (5.32 at.% for N and 2.23 at.% for S, respectively), abundant catalytically active sites, unique hierarchically porous architecture, large surface area (1859 m(2) g(-1)) and exhibited remarkable electrocatalytic activity toward ORR with positive onset/half-wave potential and large limiting diffusion current. In addition, N,S-HPC showed much better long-time stability and resistance to methanol crossover than Pt/C did. When used as the cathodic catalysts of Zn-air battery, N,S-HPC outperformed Pt/C in terms of the open-circuit potential, discharge current density, peak power density, specific capacity and rate performance, showing promise as an alternative to Pt/C for application in fuel cells and metal-air batteries.