Supercapacitors are electrical energy storage device, which provided low cost, high power density, high cycle-life, large capacitance, high power delivery, fast charge-discharge rate, and mechanical robustness. Supercapacitors are used in a wide range of applications such as the hybrid electric vehicle, pulse power, memory protection, and medical equipment. The supercapacitors store electrical charge in an electric double layer at the interface between a high-surface-area carbon electrode and electrolyte. We produced structural supercapacitor with mechanical robustness made from carbon fiber/epoxy prepreg and composite solid polymer electrolyte. We prepared composite solid polymer electrolytes composed of poly (ethylene glycol) monomethyl ether acrylate, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, and inorganic fillers. The effects of characteristics of inorganic fillers on the thermal and electrochemical properties of composite solid polymer electrolyte were investigated.