Origin of the Overpotential for the Oxygen Evolution Reaction on a Well-Defined Graphene Electrode Probed by in Situ Sum Frequency Generation Vibrational Spectroscopy

Abstract

To develop an efficient material for the cathode of the lithium-oxygen (Li-O₂) secondary battery, the oxygen reduction and evolution reactions (ORR and OER) on a well-defined graphene electrode have been investigated in a typical organic solvent, dimethyl sulfoxide (DMSO). The adsorption and desorption behaviors of the solvents on the graphene electrode surface were evaluated by an intrinsically surface-selective vibrational spectroscopy of sum frequency generation (SFG) during the ORR and OER. After the initial ORR depositing lithium peroxide (Li₂O₂) on the graphene electrode surface in a LiClO₄/DMSO solution, the SFG spectroscopy revealed that the subsequent OER oxidizing the Li₂O₂ preferentially proceeds at the interface between the Li₂O₂ and graphene rather than that between the Li₂O₂ and bulk solution. Therefore, the OER tends to reduce the electric conductivity between the Li₂O₂ and graphene by decreasing their contact area before a large part of the deposited Li₂O₂ was oxidized, which elucidates the origin of the high overpotential for the OER.