1-Ethyl-3-methylimidazolium hydrogensulfate (EMIHSO4), an ionic liquid (IL), was investigated as both liquid and solid electrolyte for electrochemical capacitors (ECs). The high resistivity of EMIHSO4 could be overcome by developing a polymer-EMIHSO4 electrolyte. This polymer electrolyte, preferable for environmental and safety reasons, exhibited better performance than liquid EMIHSO4. ECs fabricated using this polymer electrolyte demonstrated a cell voltage window of 0-1.5 V and very stable cycle life. Good high-rate response up to 1 V/s and a time constant of 1 s were observed through cyclic voltammetry and impedance analyses, respectively. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.005202esl] All rights reserved.