Linear acrylate monomers with different polyethylene oxide chains were studied as electrolyte additives to widen the operating potentials of conventional supercapacitors (SCs). For the SC cells composed of symmetrical activated carbon (AC), electropolymerization occurred on the negative AC electrode at the cell potentials above 2.4 V. The polymeric layer suppressed further electrochemical reactions of electrolyte components above the reduction potential of the reference electrolyte. Electropolymerization was induced by various conditions of cyclic voltammetry on both AC and glassy carbon electrodes and the formation of a passive polymeric layer was confirmed by Fourier transform infrared spectroscopy, scanning electron microscopy, electrochemical impedance spectroscopy, and Brunauer-Emmett-Teller analysis. The electrochemical performances of SCs depended on monomer content and size. SCs containing 0.5 wt.% MMA showed better cycling stability and less gas evolution when exposed to a potential higher than the potential window of the electrolyte, even above 4.0 V. (C) 2017 Elsevier Ltd. All rights reserved.