In Situ Atomic Force Microscopy (AFM) Study of Oxygen Reduction Reaction on a Gold Electrode Surface in a Dimethyl Sulfoxide (DMSO)-Based Electrolyte Solution

In the present study, the morphological changes on a gold electrode during the oxygen reduction (ORR) and oxygen evolution reaction (OER) processes in a dimethyl sulfoxide (DMSO)-based electrolyte solution were investigated using an electrochemical atomic force microscope (EC-AFM) with the help of vibrational spectroscopy measurements. The growth of the ORR products on the electrode surface, which was mainly assigned to lithium peroxide (Li2O2), was directly confirmed by the EC-AFM. It was found that the water concentration in the solution significantly affects the morphology of the ORR products. The growth of anisotropic Li2O2 particles on the gold electrode surface has been confirmed to be an electrochemical process. No evidence was found to support the disproportionation growth mechanism. These ORR products were fully decomposed at a potential as high as 4.4 V (vs Li+/Li) in the subsequent OER process, more positive than that determined by a surface-enhanced Raman spectroscopy (SERS) measurement. Combined with infrared absorption spectroscopy and SERS measurements, we propose that the oxidation decomposition of the Li2O2 deposits first occurs at its interface with the gold electrode surface, while that of the remaining particles takes place at a higher overpotential. On the contrary, the ORR deposits could be fully decomposed at a potential as low as 3.6 V when tetrathiafulvalene (TTF) was included in the solution. We confirmed by EC-AFM that the electrochemically generated TTF+ can mediate the decomposition of the Li2O2 at a lower potential through a homogeneous oxidation mechanism.