Axial ligand effect on the stability of Fe–N–C electrocatalysts for acidic oxygen reduction reaction

Iron and nitrogen co-doped carbons (Fe–N–C) have comparable activity to Pt-based catalysts for oxygen reduction reaction (ORR), but with much poorer durability in acidic electrolytes. Recently, regulating the coordination environment of Fe center (in-plane or axially) to boost the ORR activity of Fe–N–C has attracted many interests, and the axial OH ligand is even regarded as a necessary part of a highly-active structure. However, the influence of these regulations on the stability is still not clear. Herein, we performed kinetic and thermodynamic calculations based on density functional theory with explicit consideration of electrode potential to study the OH axial ligand effect on the stability of Fe–N–C electrocatalysts. We found that although the OH ligand can enhance the ORR onset potential to some extent, it substantially increases the H2O2 selectivity, pushing ORR diverted to the 2e- + 2e-pathway. In the latter 2e-process (H2O2 reduction), harmful hydroxyl radicals could be produced upon H2O2 dissociation. Therefore, from the perspective of catalysts’ stability, OH ligand coordination on the metal center is not a good way to develop stable ORR catalysts.