Identification of true active sites in N-doped carbon-supported Fe₂P nanoparticles toward oxygen reduction reaction

Transition metal-based nanoparticles (NPs) are emerging as potential alternatives to platinum for catalyzing the oxygen reduction reaction (ORR) in zinc-air batteries (ZAB). However, the simultaneous coexistence of single-atom moieties in the preparation of NPs is inevitable, and the structural complexity of catalysts poses a great challenge to identifying the true active site. Herein, by employing in situ and ex situ XAS analysis, we demonstrate the coexistence of single-atom moieties and iron phosphide NPs in the N, P co-doped porous carbon (in short, Fe-N₄-Fe₂P NPs/NPC), and identify that ORR predominantly proceeds via the atomic-dispersed Fe-N₄ sites, while the presence of Fe₂P NPs exerts an inhibitory effect by decreasing the site utilization and impeding mass transfer of reactants. The single-atom catalyst Fe-N₄/NPC displays a half-wave potential of 0.873 V, surpassing both Fe-N₄-Fe₂P NPs/NPC (0.858 V) and commercial Pt/C (0.842 V) in alkaline condition. In addition, the ZAB based on Fe-N₄/NPC achieves a peak power density of 140.3 mW cm^-2, outperforming that of Pt/C-based ZAB (91.8 mW cm^-2) and exhibits excellent long-term stability. This study provides insight into the identification of true active sites of supported ORR catalysts and offers an approach for developing highly efficient, nonprecious metal-based catalysts for high-energy-density metal-air batteries.