Ionic Liquids as Electrolytes for Electrochemical Double-Layer Capacitors: Structures that Optimize Specific Energy

Abstract

Key parameters that influence the specific energy of electrochemical double-layer capacitors (EDLCs) are the double-layer capacitance and the operating potential of the cell. The operating potential of the cell is generally limited by the electrochemical window of the electrolyte solution, that is, the range of applied voltages within which the electrolyte or solvent is not reduced or oxidized. Ionic liquids are of interest as electrolytes for EDLCs because they offer relatively wide potential windows. Here, we provide a systematic study of the influence of the physical properties of ionic liquid electrolytes on the electrochemical stability and electrochemical performance (double-layer capacitance, specific energy) of EDLCs that employ a mesoporous carbon model electrode with uniform, highly interconnected mesopores (3DOm carbon). Several ionic liquids with structurally diverse anions (tetrafluoroborate, trifluoromethanesulfonate, trifluoromethanesulfonimide) and cations (imidazolium, ammonium, pyridinium, piperidinium, and pyrrolidinium) were investigated. We show that the cation size has a significant effect on the electrolyte viscosity and conductivity, as well as the capacitance of EDLCs. Imidazolium- and pyridinium-based ionic liquids provide the highest cell capacitance, and ammonium-based ionic liquids offer potential windows much larger than imidazolium and pyridinium ionic liquids. Increasing the chain length of the alkyl substituents in 1-alkyl-3-methylimidazolium trifluoromethanesulfonimide does not widen the potential window of the ionic liquid. We identified the ionic liquids that maximize the specific energies of EDLCs through the combined effects of their potential windows and the double-layer capacitance. The highest specific energies are obtained with ionic liquid electrolytes that possess moderate electrochemical stability, small ionic volumes, low viscosity, and hence high conductivity, the best performing ionic liquid tested being 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

Supercapacitive Properties of Micropore‐ and Mesopore‐Rich Activated Carbon in Ionic‐Liquid Electrolytes with Various Constituent Ions

“…[TFSI] were clearly highert han those measured in [EMI]BF 4 and [EMI] [OTf]. [20] In line with this trend, Ta shimae tal. reported that their AC electrodes, regardless of the conducting agent (acetylene black or Ketjenblack)u sed, showed highers pecific capacitances in [EMI][TFSI] compared to those in [EMI]BF 4 IL in the range of 0-50 8C.…”

“…As for the IL anion effects, with the [EMI][TFSI],[EMI]BF 4 ,and [EMI][FSI] as electrolyte the capacitances were 93, 108, and1 20 Fg À1 ,r espectively,a t0 .5 Ag À1 .A t 10 Ag À1 ,t he capacitances decreased to 10, 17, and 60 Fg À1 , correspondingt o1 1%,1 6%,a nd 50 %r etention, respectively, compared to those measured at 0.5 Ag À1 .T he capacitance reductionw ith increasing appliedc urrent density can be attributed to the kinetic limitation of ion adsorption/desorptiona nd migration in the AC pores. [15,17,31,32] TFSI À can withstand a higher anodic potential, [20] resulting in al arger V cell .H owever, its large size (see Table S2) occupies more space on the AC surface and thus decreases the charged ensity that can be stored. Moreover FSI À decomposed at al ower potential than BF 4 À and TFSI À .…”

“…reported that TFSI − outperformed BF 4 − and the trifluoromethanesulfonate anion (OTf − ) because of its better compatibility with the carbon surface. The potential window and electrode capacitance measured in 1‐ethyl‐3‐methylimidazolium [EMI][TFSI] were clearly higher than those measured in [EMI]BF 4 and [EMI][OTf] . In line with this trend, Tashima et al.…”

Ionic Liquids as Environmentally Benign Electrolytes for High‐Performance Supercapacitors

“…Hence, these different features of ILs based on different cations have a significant influence on the P s as well as the ED of SCs . It was also observed that ammonia‐based ILs display a larger ESW as compared to pyridinium‐ and imidazolium‐based ILs, the two latter offering higher cell capacitance, however . Nevertheless, pyrrolidinium, imidazolium, as well as pyridinium cations have been plagued with toxicity issues affecting the greenness of such ILs as against organic electrolytes used for the same purpose.…”

Ionic liquids containing tricyanomethanide anions: physicochemical characterisation and performance as electrochemical double-layer capacitor electrolytes

“…[11][12][13][14] Ionic liquids (ILs) are a class of material that has been extensively studied and are considered as an alternative to organic solvent-based electrolytes in energy storage devices, not only due to their intrinsic ionic conductivity but also due to their high chemical and thermal stability, non-flammability, negligible vapour pressure, high electrochemical stability and they can be considered to be less hazardous than organic solvent-based electrolytes. [15][16][17][18] ILs are salts that normally contain an asymmetric cation and an anion with delocalised charge, which results in weak ionic interactions. Conventionally, salts that display a melting temperature lower than 100 1C are considered to be ILs.…”