Bubble-like Fe-encapsulated N,S-codoped carbon nanofibers as efficient bifunctional oxygen electrocatalysts for robust Zn-air batteries

Efficient, robust and cost-effective bifunctional oxygen electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are of vital importance to the widespread utilization of Zn-air batteries. Here we report the fabrication of a bubble-like N,S-codoped porous carbon nanofibers with encapsulated fine Fe/Fe5C2 nanocrystals (∼ 10 nm) (FeNSCs) by a facile one-pot pyrolysis strategy. The novel FeNSC nanostructures with high Fe content (37.3 wt.%), and synergetic N and S doping demonstrate remarkable ORR and OER catalytic activities in alkaline condition. Particularly for ORR, the optimal FeNSC catalyst exhibits superior performance in terms of current density and durability in both alkaline and acidic media. Moreover, as catalysts on the air electrodes of Zn-air batteries, the optimal FeNSCs show a high peak power density of 59.6 mW/cm2 and extraordinary discharge-charge cycling performance for 200 h with negligible voltage gap change of only 8% at current density of 20 mA/cm, surpassing its noble metal counterpart (i.e. Pt). The impressive battery stability can be attributed to favorable electron transfer resulting from appropriate graphitization of the bubble-like carbon nanofibers and thorough protection of Fe/Fe5C2 nanoparticles by carbon wrapping to prevent oxidation, agglomeration and dissolution of Fe nanoparticles during battery cycling. The present FeNSC catalyst, which is highly active, robust yet affordable, shows promising prospects in large-scale applications, such as metal-air batteries and fuel cells.