Post iron-doping of activated nitrogen-doped carbon spheres as a high-activity oxygen reduction electrocatalyst

Iron-nitrogen-doped carbon materials (Fe-N-C) have emerged as the most promising nonprecious alternative catalysts for commercial Pt/C toward oxygen reduction reaction (ORR). However, the general synthesis of Fe-N-C catalysts involves complex pyrolysis reactions, which not only obtains the difficulty in optimizing pore structures, but also leads to low porosity and specific surface area due to lacking of efficient pore regulation. Herein, we report an efficient post Fe-doping strategy to synthesize high-performance Fe-N-C catalysts using activated N-doped carbon spheres (ANCSs) with improved porosity as the supports. Large amount of micropores in the ANCSs generated by CO2 activation are advantageous for enhanced exposure of N-functional groups which can efficiently complex more Fe ions during post Fe-doping, enabling the facile formation and increasing the accessibility of high-density of Fe-N-x active sites onto the pore surfaces. The resulting Fe-N-C catalysts demonstrated well-defined hierarchical micro-/mesoporosity, large surface area and optimized active sites. Ascribed to the synergy of the above factors, they showed very high ORR activity with a four-electron dominant pathway superior and comparable to that of 20 wt% Pt/C in alkaline and acidic electrolyte, respectively, also presenting excellent stability and methanol tolerance. The present research paves a new avenue for preparing various metal-N-C catalysts used in heterogeneous catalysis, batteries and super-capacitors.