Implementation of a choline bis(trifluoromethylsulfonyl)imide aqueous electrolyte for low temperature EDLCs enabled by a cosolvent

We report a carbon/carbon capacitor based on micro/mesoporous carbon electrodes with cost-effective and eco-friendly aqueous choline bis(trifluoromethylsulfonyl)imide (ChTFSI) electrolyte with a cosolvent enabling low-temperature operation down to −30 °C. For this purpose, a MgO-templated hierarchical carbon (MP98B) with an average mesopore diameter of 3.5 nm was prepared by pyrolysis of magnesium citrate hydrate at 900 °C. To reach lower temperatures, the melting point and viscosity of the aqueous electrolyte were reduced by mixing water (W) with an organic solvent (methanol, M, or isopropanol, I) of high dielectric constant and low viscosity. 5 mol kg −1 (5 m) ChTFSI in an optimized volume fraction of cosolvent, M 0.75 W 0.25, and I 0.75 W 0.25 , showed the highest conductivity; the higher conductivity in M 0.75 W 0.25 (22.8 and 3.1 mS cm −1 at 20 and −30 °C, respectively) than in I 0.75 W 0.25 (8.5 and 0.5 mS cm −1 at 20 and −30 °C, respectively) is attributed to the lower viscosity of the M 0.75 W 0.25 solution. The electrochemical stability window (ESW) of 5 m ChTFSI in M 0.75 W 0.25 and I 0.75 W 0.25 (1.6 V) on an MP98B electrode was determined by applying the S-method. Meanwhile, by adjusting the mass ratio of the two electrodes, a MP98B/MP98B capacitor using the 5 m electrolyte in M 0.75 W 0.25 could operate with a good life span up to 1.6 V while exhibiting a better charge propagation, greater specific capacitance, and higher specific energy than in I 0.75 W 0.25 .