Electrochemical Investigations of the Suitability of 1-Propyl-2,3-dimethylidazolium bis(trifluoromethylsulfonyl)imide as the Electrolyte for Application in Micro–Mesoporous Carbon-Electrode-Based Supercapacitors and Other Electrochemical Systems

The electrochemical properties of the hydrophobic room-temperature ionic liquid 1-propyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide (PMMIm(TFSI)) were investigated, for the first time, using an electrochemical double-layer capacitor-mimicking cell containing two identical-sized micro–mesoporous molybdenum carbide-derived carbon electrodes (MMP-C(Mo2C)), by applying cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. Surprisingly, despite the substitution of the slightly acidic hydrogen atom with a methyl group at the carbon atom located between two nitrogen atoms in the imidazolium cation, the EIS and CV measurements demonstrated that PMMIm(TFSI) began to decompose electrochemically at the same cell potential (ΔE) as 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm(BF4)), specifically at ΔE = 2.75 V. However, the CV and EIS data indicated that PMMIm(TFSI) decomposed with a significantly lower intensity than EMIm(BF4). Therefore, we believe that the use of PMMIm(TFSI) as the electrolyte will enable the construction of safer supercapacitors that can tolerate short periods of over-polarization up to ΔE = 4.0 V. However, when the ΔE ≤ 3.2 V was applied, EMIm(BF4) offered higher maximum power compared to PMMIm(TFSI). We found that the calculated maximum gravimetric power precisely describes the maximum ΔE applicable for a supercapacitor candidate.