Strengthening oxygen reduction activity based on the cooperation of pyridinic-N and graphitic-N for atomically dispersed Fe sites

Abstract

The effects of different N-types coordinated with single atomic sites (SASs) on the oxygen reduction reaction (ORR) has not been deeply explored. Herein, we first predicted by density functional theory (DFT) calculations that pyridinic-N and graphitic-N on Fe SAS can cooperatively enhance ORR activity. Then, Fe SASs containing both pyridinic-N and graphitic-N were synthesised by a dual-nitrogen strategy. The Fe SASs supported on a 3D N-doped carbon nanotubes/graphene network structure (Fe–N–C/GC) with more pyridinic-N and graphitic-N sites were prepared by a dual nitrogen strategy. The highly active Fe–N₄ sites and 3D structure enabled the outstanding ORR performance of Fe–N–C/GC with a half-wave potential of 0.85 V and a limiting current density of 6.05 mA cm⁻², which is comparable to the performance of Pt/C in alkaline media. Furthermore, the assembled H₂/O₂ fuel cell with Fe–N–C/GC can deliver a peak power density of 406 mW cm⁻² and an open-circuit voltage of 0.90 V, exceeding those of previously reported Fe-related catalysts. DFT calculations demonstrated that the anti-bonding d_z2, d_xz, and d_yz orbitals of the Fe atom had higher filling degrees, which reduced the energy barrier for the formation of oxygen intermediates and promoted ORR activity. This was ascribed to the synergistic effect between pyridinic-N and graphitic-N sites.