Structural supercapacitors are energy storage devices that can function as structural materials. We synthesized composite solid polymer electrolytes (CSPEs) from 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIm][OTF]), poly (ethylene glycol) monomethyl ether acrylate (PEGA) and functionalized silica filler. Two types of fumed silica were used: one had an unmodified surface, and the other an organically modified surface. The CSPEs were prepared by adding ionic liquids (IL) to the PEGA and the ratios between PEGA and IL were 7: 3 and 5: 5, respectively. The functionalized silica was synthesized by the sol-gel method under acidic conditions using methacryloxypropyl trimethoxysilane (MAPTMS), whereas the effects of silica filler on the electrochemical and thermal properties of CSPEs were investigated using electrochemical impedance spectroscopy, cyclic voltammetry, and differential scanning calorimetry. The ionic conductivity of CSPEs based on PEGA/[OTF]_SiO2 at various concentrations of [EMIm][OTF] was 5.7×10−4 and 4.8×10−4S/cm, and their specific capacitance was 10.0 and 9.5 F/g, respectively. With the addition of silica filler, the ionic conductivity and specific capacitance of the synthesized CSPEs were lower than those of the neat CSPEs.