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

Abstract

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 FeIII-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.