Engineering the Electronic Structure of Active Centers in Metalloporphyrins to Boost Oxygen Reduction Reaction Activity

Inspired by the ligand-promoted high reactivity of the heme active center in cytochrome c oxidase (CcO) toward oxygen reduction reaction (ORR), this work proposes a new strategy for improving the efficiency of metalloporphyrin-catalyzed ORR. We carried out a comparative study between two graphene/cobalt porphyrin complexes, the rGO/TMPPCo formed by noncovalent pi-pi stacking, and the rGO-Ci/TMPPCo with cobalt porphyrins (TMPPCo) axially coordinated with the 1-carboimidazole (Ci) ligands on reduced graphene oxide (rGO) surface. The involvement of the axial ligand was found to greatly reduce the ORR overpotential by 120 mV in acidic medium, while hydrodynamic tests revealed an unaltered reaction mechanism. This positive effect of axial ligand has been rationalized by both electronic "push effect" and steric "trans effect". The results of this work highlight the importance of a rationally engineered electronic structure of active sites in heterogeneous catalysis, providing insights for the fabrication of highly-efficient ORR electrocatalysts.