Effect of pH-induced chemical modification of hydrothermally reduced graphene oxide on supercapacitor performance

Three kinds of reduced graphene oxides are prepared by hydrothermal reduction under different pH conditions and their pseudocapacitive performances are evaluated using full-cell supercapacitor devices. The pH values are found to have great influence on the performance of the supercapacitors, achieving the highest specific capacitance value reported for hydrothermal reduced graphene oxide supercapacitors. Acidic and neutral media yield reduced graphene oxides with more oxygen-functional groups and lower surface areas but with broader pore size distributions than those in basic medium. The graphene produced in the basic solution (nitrogen-doped graphene) presents mainly electrochemical double layer (ECDL) behavior with specific capacitance of 185 F g^-1, while the graphene produced under neutral or acidic conditions show both ECDL and pseudocapacitive behavior with specific capacitance of 225 F g^-1 (acidic) and 230 F g^-1 (neutral), respectively, at a constant current density of 1 A g^-1. The influence of pH on cycling performance and electrochemical impedance of the supercapacitive devices is also presented.