Electropolymerizable isocyanate-based electrolytic additive to mitigate diffusion-controlled self-discharge for highly stable and capacitive activated carbon supercapacitors

To mitigate the self-discharge of supercapacitors (SCs), numerous researches have reported about the effective use of cell resistance, although this might provoke a deterioration of charge/discharge performances. In this work, isocyanate-based electrolytic additive, 2-isocyanatoethylmethacrylate (ICEMA), is polymerized by two methods, in-situ electropolymerization (EP) and radical-polymerization (RP), on activated carbon (AC) electrode to mitigate the diffusion-controlled self-discharge, which is a major contribution to the self-discharge herein. Although the radical-polymerized PICEMA reveals favorable suppression of self-discharge, only confined in bulk electrolyte and meso-region, this exhibits severe increases in all impedance parameters and deteriorated charge/discharge capabilities. The electropolymerized PICEMA, however, meets the bifunctional perspective, revealing aptly retained capacitance and suppressed self-discharge behavior throughout micro- and meso-regions. ICEMA is effectively electropolymerized on positive AC electrode at + 0.9 ~ 1.2 V (vs. AC), which is confirmed by the electrochemical impedance spectroscopy, electrochemical quartz microbalance, and X-ray photoelectron spectroscopy. The functionality of isocyanate group to suppress self-discharge is also confirmed by comparing methoxy group of ethylene glycol methyl ether methacrylate (EGMEMA).