Synergistic spin-ligand effects on the oxygen reduction activity of the FePPc electrocatalyst

Abstract

As a typical single-atom catalyst (SAC), iron(II) polyphthalocyanine (FePPc) with an Fe–N₄ structure was experimentally found to have excellent ORR activity. Spin-related effects have been found to significantly influence the ORR activity of graphene-based TM–N–C SACs, yet they are rarely discussed in TMPPc catalysts, which severely limits the development of this material family for fuel cell applications. In this work, we employed density functional theory (DFT) calculations to investigate the spin-dependent ORR activity of FePPc and X–FePPc (X is the axial ligand). A potential-dependent spin crossover effect was observed in some ORR intermediates, which potentially affects the path of spin state evolution during the ORR. Polarization current simulation shows that the axial ligand significantly promotes ORR activity due to the inhibited partitioning of Fe(II)^IS and enhanced activity of Fe(III)^HS. Scaling relationship analysis shows that both the d-band center and magnetic moment can describe the G_*OH of X–FePPc, independent of the spin state, but their applicability depends on changes in the coordination number. According to volcano analysis, we also predict that stabilizing the IS state can further optimize the ORR activity of FePPc*X. Our work reveals synergistic spin-ligand effects on the ORR activity of FePPc catalysts, which could further benefit the design of spin-related electrocatalysts for green energy technologies.