In Situ Mechanistic Investigation of O2 Reduction by Iron Porphyrin Electrocatalysts Using Surface-Enhanced Resonance Raman Spectroscopy Coupled to Rotating Disk Electrode (SERRS-RDE) Setup

Over the past few decades, researchers have used heme/porphyrin-based electrocatalysts that are known to catalyze various electrochemical transformations such as O-2 reduction, water splitting, CO2 reduction, etc. Detailed understanding of the mechanism of the catalysts is essential for their development. However, it is limited because of the lack of direct spectroscopic evidence of the intermediates formed on the electrodes during catalytic turnover. Recently, surface-enhanced resonance Raman spectroscopy coupled to rotating disk electrochemistry (SERRS-RDE) has been developed in our laboratory that can be used to study O-2 reduction by iron porphyrin electrocatalysts under physiological conditions. While reaction intermediates are commonly trapped and characterized in single turnover experiments, SERRS-RDE experiments, on the other hand, probe the system while it is under steady-state conditions. A combination of oxidation state, coordination number, and spin state marker bands, along with the metal ligand vibrations, allowed unprecedented direct identification of O-2-derived intermediates formed in situ on the electrode surface during electrocatalysis. This approach, which combines dynamic electrochemistry with resonance Raman spectroscopy, has now been used to understand the role of different axial ligands and distal superstructures in the mechanism of electrochemical O-2 reduction.