Hierarchical porous carbon nanofibers with facilely accessible iron-based active sites for efficient oxygen reduction in alkaline and acidic media

Reasonable design of hierarchically porous structures with mass transport maximization and highly accessible active sites still remains a great challenge. Herein, we successfully fabricated Fe3O4 nanoparticles embedded in the nitrogen-doped hierarchical porous carbon nanofibers as efficient and durable electrocatalysts for oxygen reduction reaction (ORR), which was synthesized by a stepwise electrospinning-pyrolysis strategy. The unique hierarchically porous carbon nanofibers could not only improve the dispersion and exposure of metal active sites, but also maximize the mass transport, thereby enhancing the accessibility of active sites and chemical reaction rate with efficient transport channels for reactants and products. Remarkably, the optimized sample (Fe3O4@Fe–N/HPCNF) displayed outstanding ORR activity with an onset potential of 0.99 V and a half-wave potential of 0.88 V in alkaline media, even comparable to that of commercial Pt/C. Additionally, Fe3O4@Fe–N/HPCNF catalyst also revealed excellent ORR performance with a half-wave potential of 0.75 V in acidic solution. More importantly, the optimal power density of Fe3O4@Fe–N/HPCNF assembled Zn-air battery can achieve 116 mW cm−2. The excellent electrocatalytic performance is benefited from the synergistic coupling between unique hierarchically porous carbon nanofibers and highly accessible Fe-based active sites.