The data presented in this work show the preparation of an electrochemical double layer capacitor with improved capacitance employing an ionic liquid based composite gel polymer electrolyte containing nanostructured SiO2. The composite gel electrolyte has good self-standing characteristics, conductivity, electrochemical window and high thermal stability. The use of this electrolyte promotes an increase of about 72% in the device capacitance compared to a capacitor prepared with the gel electrolyte without SiO2 at ambient temperature. The cell is very stable when cycled at a high current density (1.0 A g(-1)) since it loses only 9% of its initial capacitance after 2000 cycles, with values of energy and power density of 32.2 Wh kg(-1) and 0.9 kW kg(-1), respectively. Additionally, the SiO2-based electrolyte allows the capacitor to be used at high temperature (100 degrees C) with a superior performance without ionic liquid leakage. Therefore, the use of such composite gel electrolyte makes possible the construction of miniaturized, safe and completely solid state devices. (C) 2015 Published by Elsevier Ltd.