▎ 摘　　要

Graphene is considered as one of the most promising electrode materials for supercapacitors. Despite many advantages of graphene-based electrodes, there remains a major limitation, which is low value of specific capacitance. This limitation is observed in graphene-based electrodes' due to their low quantum capacitance. The present article explored the variations of quantum capacitance of graphene sheets through their functionalization with monovalent and divalent functional groups. The findings of this report demonstrate that functionalization of graphene may improve the gravimetric quantum capacitance compared to pristine graphene. The monovalent functional groups show symmetric behavior in negative and positive applied voltages. They have a significant effect on the quantum capacitance, especially at low voltages. In contrast, the divalent functional groups show asymmetric behavior in negative and positive applied voltages. They have an additive effect on the quantum capacitance, especially at higher voltages and negative bias. However, both kinds of functional groups enhance the quantum capacitance compared to pristine graphene. (C) 2015 Elsevier Ltd. All rights reserved.