Effect of the Axial Halogen Ligand on the Oxygen Reduction Reaction Performance of Transition Metal–Nitrogen–Carbon Catalysts

The high cost of a cathode oxygen reduction reaction (ORR) catalyst is the key technical bottleneck of developing the proton-exchange membrane fuel cell. The performance of the commonly studied ORR materials is hindered by unfavorable scaling relationships for binding energies of reaction intermediates. In this work, the density functional theory calculations are applied to explore the binding behaviors of ORR intermediates and catalytic mechanism on axially halogen-coordinated M–N–C catalysts (MN4Xn, M = Fe, Co, Ni; X = F, Cl, Br, I; n = 0–2). The results show that all MN4 can serve stably to avoid aggregation and dissolution. The favorable scaling relationship for ORR intermediates and matchless theoretical highest ORR activity at volcano peak unravel intrinsically why the M–N–C SACs hold a state-of-the-art place for ORR performance. The CoN4Cl and CoN4Br stand out with the ultralow ORR overpotential values of 0.25 and 0.26 V, respectively. Finally, a new intrinsic descriptor φ is proposed to appropriately describe the ORR activity of the non-axial-coordinated and axial-coordinated M–N–C catalysts. These findings reveal the intrinsic advantages of M–N–C SACs for ORR catalysis and rationally provide theoretical insights into designing advanced SAC materials.