Unequal electrode capacitance in a symmetric supercapacitor results in an unequal voltage distribution on the two electrodes and the supercapacitor usually cannot utilize the full electrochemical window of electrolyte. In this study, the voltage distribution on the two electrodes of graphene supercapacitor is optimized by an adjustment of electrode mass ratio from 1 to 1.5 and an enlarged energy density of 118 Wh kg(-1) is achieved at operating voltage 4.7 V using an ionic liquid electrolyte 1-methyl-1-propylpiperidinium bis (trifluoromethyl sulfonyl) imide (MPPp-TFSI). (c) 2017 Elsevier Ltd. All rights reserved.