Abstract
Supercapacitors, also known as electrochemical capacitors (ECs), have drawn increasing attention as promising energy storage devices due to their dramatic features of high power capability, long cycle stability and excellent reliability. However, ECs still suffer from limited applications as a result of their poor energy capability. With unique two-dimensional (2D) stucture, high theoretical surface area and superior electrical conductivity, graphene is an attractive candidate as electrode materials for supercapacitors among carbon-based materials. Nevertheless, the intrinsic capacitance of graphene should be further improved indeed through efficient modifications owing to the aggregative property of graphene. In order to resolve the insuffient capacitance problem, nitrogrn-doped graphene (NDG) was applied as electrode material for supercapacitors, and has been further introduced into an organic electrolyte system to effectively improve the energy density of ECs. In this study, the nitrgen-doped graphene (NDG) with high surface area and porous microstructure was successfully synthesized through a rapid thermal annealing process combining with the concepts of molecular functionalization, thermal expansion-exfoliation, and covalent transformation. Melamine was utilized as the nitrogen source precursor herein. The corresponding characterizations and electrochemical performances of the product are systematically investigatied and further compared with pure graphene fabricated through chemical reduction (labeled as CRG) and thermal reduction (labeled as TRG). The NDG prepared by reducing the melamine/graphene oxide mixture shows high specific surface area (712.48 m2 g-1) and mesoporous structure revealed by the tests of N2 adsorption-desorption isotherms. X-ray photoelectron spectoroscopy (XPS) study indicates the successful nitrogen-doping reaction with a content of 3.17 at.% and the N-doped configurations mainly consist of pyridinic-N (44.6 at.%) and graphitic-N (30.2 at.%) functional groups. Electrochemical measurements demonstrate that the NDG exhibits better capacitive performance than that of the prepared CRG and TRG both in aqueous electrolyte and organic electrolyte. The NDG electrode material shows specific capacitance of 145.0 F g-1 and 126.6 F g-1 in 0.5M Na2SO4 electrolyte and 1M TEABF4/PC electrolyte at the scan rate of 10 mV s-1, respectively. The enhancement of capacitive behavior could attribute to the presence of N-containing functional groups.The material also exhibits good long-term cycle stability after 1500 cycles (4.8% decrease of the initial capacitance in aqueous electrolyte and 7.4% in organic electrolyte). Most importantly, in comparison with the voltage range of 1V in aqueous solution, the operating potential range of this study could reach to 3.2V in organic electrolyte, which can deliver higher energy density than water-based electrolytes. Moreover, the synthesized NDG can deliver remarkable energy density of 62.2 Wh kg-1 at the power density of 1.14 kW kg-1 in the voltage range up to 3.2V by the test of two-electrode symmetric system. In total, the results of this work have demonstrated that the great potential of NDG as an ideal electrode material for supercapacitor application.