Deep Eutectic Solvent-based Ionic Liquid Electrolytes for Electrical Double-layer Capacitors

Through hydrogen‐bonding between choline chloride (CC) and hydrogen bond donors, the binary systems, liquidized at their eutectic point (deep eutectic solvents, DESs), can be considered as organic salts in the liquid state (a kind of ionic liquids). Such cheap and biocompatible ionic liquids were developed for electrolytes of electrical double‐layer capacitors (EDLCs). This work employs glycerol (G), malonic acid (M), and urea (U) to form DESs (GCC, UCC, and MCC). The stable potential window of these DESs is about 2 V while only GCC shows a nearly rectangular CV curve at room temperature at 1 mV/s. The ionic conductivity of GCC is equal to ca. 1.3 and 10.0 mS/cm at 25 and 75 °C, respectively. γ‐Butyrolactone (GBL), working as a good solvent to disperse GCC, significantly reduces the viscosity of GCC, increases the ionic conductivity, and enhances the performances of GCC‐based EDLCs. This GCC‐GBL system shows the promising applicability in EDLCs at 10 mV/s at 25 °C. In this work, viscosity of DESs is found to dominate their performance in EDLCs while the mesoporous structure of reduced graphene oxide (RGO) further reduces the loss of electrolyte‐accessible area at higher scan rates, emphasizing the important influence of material pore structure on the capacitive performances. The Arrhenius plots of log(κ) against 1/T for (1) GCC + 0.5 M LiClO4, (2) GCC/GBL (= 4), and (3) GCC/GBL (= 4) + 1 M LiClO4 reveal that the high conductivity and lower viscosity of the GCC/GBL system reveal a synergistic effect between glycerol and GBL on the dissociation of choline chloride (i.e., GBL effectively dilutes choline cations and glycerol‐complexing chloride anions) which provides the ionic conductivity in this gel‐like electrolyte.