Revisiting Charge Storage Mechanism of Reduced Graphene Oxide in Zinc Ion Hybrid Capacitor beyond the Contribution of Oxygen-Containing Groups

Recently, developing matchable cathode materials of Zn ion hybrid capacitor still remains difficult owing to insufficient understanding of the charge storage behavior. However, most previous efforts are devoted to explain the effect of oxygen-containing groups without paying attention to graphitic structure. Herein, the charge storage capability and electrochemical kinetics of reduce graphene oxide (rGO) nanosheets are optimized as a function of their surface properties. Beyond the contribution of oxygen-containing groups, an extra contribution from the reversible adsorption/desorption of H⁺ on carbon atom of rGO sheets is confirmed. Electrochemical analysis and density functional theory calculations reveal that H⁺ induces disruption of π cloud in aromatic domain, accompanied by C sp²-sp³ re-hybridization and the distortion/restoration of graphitic structure. The optimal electrochemical performance with a specific capacitance of 245 F g^-1 at 0.5 A g^-1 with 53% retention at 20 A g^-1 is achieved for rGO thermally treated at 200 °C. As a proof-of-concept application, the 3D printed rGO electrode delivers a high areal capacitance of 1011 mF cm^-2 and an energy density of 266 μWh cm^-2. The study is believed to broaden the horizons of proton adsorption chemistry and shed light on the design of novel electrode materials.