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 m2 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.