Oxygen Functionalization Reshapes the Electrochemical Interface of Single-Layer Graphene via Defect Evolution and Ion Structuring

Chemical modification, though vital for tailoring graphene-based supercapacitor electrodes, leaves the molecular-scale origins of its impact on the electrochemical stability and interfacial charge storage kinetics unresolved. Here, we uncover how oxygen functionalization modulates the structural integrity and ion dynamics of single-layer graphene (SLG) in an ionic liquid electrolyte using ex situ Raman spectroscopy, in situ sum-frequency generation spectroscopy, and electrochemical impedance spectroscopy, coupled with numerical modeling. We find that oxygen functionalization is electrochemically unstable, leading to defect formation during cyclic voltammetry. An interfacial ion organization is observed: at negative potential, cations adopt a near-planar orientation; at positive potential, the anion adsorption is enhanced. This behavior contrasts with pristine SLG, where cations exhibit a potential-dependent twist. These interfacial modifications lead to ionic relaxation yet increase the overall capacitance, highlighting a fundamental trade-off between charge storage capacity and long-term stability in chemically modified graphene electrodes.