Spectroscopic Investigation for Oxygen Reduction and Evolution Reactions with Tetrathiafulvalene as a Redox Mediator in Li-O₂ Battery

To develop a lithium-oxygen (Li-O₂) battery with a high specific energy, the electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have been systematically investigated on gold and porous carbon electrodes in a DMSO-based electrolyte solution containing tetrathiafulvalene (TTF) as a redox mediator by using in situ UV-vis absorption spectroscopy and surface-enhanced Raman vibrational spectroscopy (SERS) in combination with ex situ infrared and Raman spectroscopies. Our results demonstrate that TTF definitely reduces the overpotential for the OER process and restrains the decomposition of the carbon electrode and solvent during the OER, while the functionality of the redox mediator can change with the electrode materials and morphologies. No free TTF⁺ was observed in solution during a round-trip galvanostatic ORR/OER cycle on the gold electrode surface. The electrochemically generated TTF⁺ was mainly consumed by the oxidative decomposition of lithium superoxide (LiO₂) in solution but not lithium peroxide (Li₂O₂) on the electrode surface formed during the ORR, different from that previously proposed. On the other hand, Li₂O₂ and Li₂CO₃ were observed during the ORR on the porous carbon electrode surface both in the TTF-free and TTF-containing solutions. The TTF⁺ can mediate the oxidation of the Li₂O₂ during the OER. Furthermore, the accumulation of free TTF⁺ was observed in solution during a round-trip galvanostatic ORR/OER cycle on the porous carbon electrode. The amount of the excess TTF⁺ in solution well corresponds to that of the byproduct of Li₂CO₃ formed on the carbon electrode surface. TTF is unlikely to work as an ideal redox mediator at the porous carbon electrode as expected. A better understanding for the mechanism and surface reactions for the ORR/OER cycle on these electrodes has been achieved by the present study.