The effect of an amorphous SiO2 nano-filler on ionic conductivity and crystallinity of poly(ethylene oxide) (PEO)-1-ethyl-3-methylimidazolium hydrogensulfate (EMIHSO4) electrolyte has been investigated. After addition of the filler, the ionic conductivity of the electrolyte reached 2.15 mS/cm at room temperature, a more than 2-fold increase over the electrolyte without filler. Structural and thermal analyses indicated that incorporation of SiO2 into PEO-EMIHSO4 improved and retained the amorphous phase by impeding PEO crystallization. The SiO2 filler was also found to be effective in facilitating ionic conduction via promoting the dissociation of EMIHSO4. Solid electrochemical double layer capacitor (EDLC) devices, leveraging the PEO-EMIHSO4 electrolyte with SiO2 filler, showed a fast capacitive response at 1 V/s and stable cycle life. (C) 2013 Elsevier Ltd. All rights reserved.