Origin of the Oxygen Reduction Activity on Boron-Doped Fe–N–C Catalysts for Zinc–Air Battery Applications

Fe–N–C catalysts are considered promising alternatives to platinum-based oxygen reduction electrocatalysts in fuel cells and metal–air batteries. The oxygen reduction reaction (ORR) activity of the Fe–N–C catalysts was further improved by doping with electropositive boron, which outperformed the benchmark Pt/C catalysts. The boron-doped Fe–N–C catalyst was synthesized by two-step pyrolysis of a polyaniline (using FeCl3 in boric acid) precursor, which exhibits superior ORR activity (Eonset = 1.01 and E1/2 = 0.88 V) with a turnover frequency of 3.86 Fe-sites–1 s–1 at 0.8 V. The mechanistic investigation of oxygen and H2O2 reduction reaction on an Fe–N&B/C catalyst using kinetic analysis demonstrates that the ORR proceeds via a pseudo-4-electron pathway, wherein the boron sites catalyze the reduction of a H2O2 intermediate. Furthermore, the poisoning experiments confirm the role of boron. Indeed, the stability of the Fe–N&B/C catalyst has improved due to the improved H2O2 reduction kinetics on boron sites. Theoretical analysis supports the O2 adsorption on boron sites with a relatively smaller activation barrier than the Fe sites. The Fe–N&B/C catalyst exhibits a superior power density (193 mW cm–2) and specific capacity (932 mA h gZn–1) in a zinc–air liquid electrolyte battery. This analysis proves that the Fe–N&B/C materials could be the potential electrocatalytic materials for ORR while placing boron appropriately to induce the synergistic effect.