Factors Affecting the Rate of Electrocatalytic O2 Reduction By Iron Porphyrin Complexes

Electrochemical O₂ reduction reaction (ORR) catalyzed by iron porphyrin complexes are investigated using surface enhanced resonance Raman spectroscopy coupled to rotating disc electrochemistry. The data suggests that the cleavage of O-O bond is the rate determining step (rds). Further the competitive hydrolysis of this rate determining Fe^III-OOH species results in 2e/2H⁺ reduction of O₂. Additional functionalities are included in the design of these iron porphyrin complexes to enhance the O-O cleavage rate and thus increase the rate of ORR. While inclusion of residues that allow facile PCET to the active site helps deter the 2e/2H⁺ pathway, introduction of the push effect of a thiolate axial ligand or the pull effect of protonated residues in the distal side is found to enhance the rate of ORR by 100-1000 times relative to an unfunctionalized porphyrin.