Graphene with KI-modified pore structure and its electrochemical capacitors application

The use of electrochemical capacitors is greatly limited by their poor volumetric energy density, and the key for improving it is to develop porous yet compact carbon materials. In recent years, the densification of porous carbons by capillary forces during drying has been used as a main method to balance the density and porosity of porous carbons for high volumetric performance. But there are still deficiencies in fine tuning the pore structure, which limits the compatibility of porous carbons with high-voltage ionic liquids. We report a KI-assisted method to prepare graphene-based porous carbons with high density and high capacitance. During the synthesis of the carbons, graphene oxide was dispersed in KI solutions with concentrations of 12.5-37.5 mmol/mL followed by hydrothermal treatment at 180 °C for 6 h. The obtained hydrogels were dried in 0.01 MPa of air at 60 °C for 72 h. By this procedure, KI was loaded into a matrix of compact graphene and formed crystals to suppress over shrinkage of pores during drying to produce densification and pseudo-capacitance. Electrochemical characterization of the resulting materials indicated that the ion-accessible surface area and pseudo-capacitance of the porous carbons were increased. The KI/graphene composite with an optimum concentration of KI of 25 mmol/mL achieved both a high electrode density of 0.96 g cm-3 and a high volumetric capacitance of 115 F cm-3 (at 1 A g-1) in an ionic liquid electrolyte (1-alkyl-3-methylimidazolium tetrafluoroborate). A symmetric cell assembled using this material had a high volumetric energy density of 19.6 Wh L-1.