Structural evolution during annealing of thermally reduced graphene nanosheets for application in supercapacitors

The surface functional groups of thermally reduced graphene nanosheets (TRG) prepared by vacuum promoted thermal expansion of graphene oxide are tailored by progressive carbonization. The residual carbon ratios after annealing at various temperatures from 250 to 1000 °C increase progressively from 44.3 to 84.8%. The oxygen containing functional groups are intensively removed at higher annealing temperature. The thermally stable phenols, ethers, and carbonyls become the major components in 1000 °C annealed TRG. Though the starting material G250 owns a high specific capacitance of 170.5 F/g, the value decreases to only 47.5 F/g when the annealing temperature is increased to 1000 °C. The oxygen containing functional groups can enhance the capacitance performance of TRGs by introducing abundant pseudocapacitance active sites through reversible Faradic redox reactions. The correlation between the structural evolution and electrochemical performance of TRGs provides new insight for designing graphene based electrodes with controllable properties for advanced supercapacitors.