The effects of SiO2 and TiO2 nanofillers on a poly(ethylene oxide) (PEO)-1-ethyl-3-methylimidazolium hydrogensulfate (EMIHSO4) electrolyte were studied and compared with respect to ionic conductivity, crystallinity, and dielectric properties over a temperature range from -10 degrees C to 80 degrees C. X-ray diffraction and differential scanning calorimetry were used to study the impact of fillers on the structure of the polymer electrolytes. Using an electrochemical capacitor model, impedance (complex capacitance) and dielectric analyses were performed to understand the ionic conduction process with and without fillers in both semi-crystalline and amorphous states. Despite their different nanostructures, both SiO2 and TiO2 promoted an amorphous structure in PEO-EMIHSO4 and increased the ionic conductivity by a factor of two. While in the amorphous phase, the dielectric constant characteristic of the fillers (TiO2 in this case) contributed to increased conductivity and cell capacitance. Combining complex capacitance and dielectric analyses is an effective approach to study solid electrochemical capacitors and to identify and explain the impact of different fillers on ionic conduction of polymer electrolytes. (C) 2014 Elsevier Ltd. All rights reserved.