Density Functional Theory Calculation for Quantum Capacitance of Graphene Oxide as Supercapacitor Electrodes

Graphene oxide has become an attractive candidate electrode material for supercapacitors thanks to its higher specific capacitance than that of graphene. Quantum capacitance makes relative contributions to the specific capacitance, which is considered as the major limitation of graphene electrodes, while the quantum capacitance of graphene oxide is rarely concerned. This study explores the quantum capacitance of graphene oxide which bears epoxy and hydroxyl groups on its basal plane by employing density functional theory (DFT) calculations. Results demonstrate the total density of states near the Fermi level is significantly enhanced by introducing oxygen-containing groups, which is beneficial for the improvement of the quantum capacitance. Moreover, the quantum capacitances of these two oxygen-containing group concentrations are compared, which reveals more epoxy and hydroxyl groups result in higher quantum capacitance. Notably, hydroxyl concentration has considerable effect on the capacitive behaviors. Supercapacitors are considered as one of the most promising energy storage devices due to their high-power density, fast charging and discharging behavior, and long life cycle. Considerable efforts have been made to design and synthesize novel electrode materials for supercapacitors with relatively high power and energy densities. Graphene, a two-dimensional singleatom-layer carbon material with a high specific surface area as high as 2630 m/g and good electrical conductivity induced by covalently bonded sp carbon atoms, is regarded as one of the most attractive candidate electrode materials of supercapacitors. However, compared with other energy storage devices, graphene supercapacitors exhibit low specific capacitance. Graphene oxide is a single-layer graphite oxide in which hydroxyl and epoxy groups are accommodated in large quantities on its basal plane while comparatively smaller amounts of carbonyl and carboxyl groups are accommodated on its edge. 14] Graphene oxide can be regarded as an intermediate of graphene preparation by the approach of oxidation–exfoliation– reduction of graphite, which is one of the most widely applied method for mass production of graphene owing to its low cost, high yield, and simple synthesis procedure. Moreover, graphene oxide possesses higher capacitance than reduced graphene in symmetrical two-electrode cell with KOH aqueous electrolyte, and this enhanced capacitance is attributed to the good wettability of aqueous electrolytes and the pseudocapacitance effect when oxygen-containing groups are introduced into graphene oxide. However, although aqueous electrolytes can induce good capacitive performance, the intrinsic deficiency of water-based electrolytes in lower operating voltage may limit the overall energy density (E = 1 2 CV). Because water has a thermodynamic decomposition voltage of 1.229 V at 25 C, the tests for the capacitive behaviors of supercapacitors with water-based electrolytes are generally operated at about 1 V (i.e., from -0.2~0.8 V or -0.3~0.7 V). By contrast, ionic liquids allow for high operating voltages and good thermal stability and nonvolatility , which imply that they can also be used for supercapacitors. Hence, the question on whether graphene oxide shows better performance in ionic liquid electrolyte than graphene must be addressed. On the basis of experimental and theoretical studies, the total interfacial capacitance (CT) of graphene-based electrodes can be depicted by the following equation under the condition of no pseudo-capacitance effect: 1