Nonhalogenated Surface-Active Ionic Liquid as an Electrolyte for Supercapacitors

Abstract

We report a nonhalogenated surface-active ionic liquid (SAIL) that consists of the surface-active anion 2-ethylhexyl sulfate and the tetraoctylammonium cation ([N8,8,8,8]­[EHS]). We explored the thermal and electrochemical properties, i.e., degradation, melting and crystallization temperatures, ionic conductivity, and electrochemical potential window of neat SAIL and its binary mixture with acetonitrile. This SAIL was tested as an electrolyte in a multiwalled carbon nanotube (MWCNT)-based supercapacitor at various temperatures from 298 to 373 K. In addition, we also tested the binary mixture of SAIL with acetonitrile as an electrolyte at lower temperatures (253–298 K). The electrochemical performance of SAIL and the SAIL/acetonitrile binary mixture as a function of temperature was compared with that of a standard electrolyte, an aqueous solution of 6 M KOH, in the same MWCNT-based supercapacitor. The solution resistance (R s), charge transfer resistance (R ct), and equivalent series resistance (ESR) decreased with an increase in temperature for all SAIL-based electrolytes. We found that the supercapacitor cell with SAIL as an electrolyte has a high specific capacitance (C elec in F g–1), a high energy density (E in Wh kg–1), and a high power density (in W kg–1) compared to those for the binary mixture of SAIL with acetonitrile and for the 6 M KOH aqueous electrolytes, particularly at elevated temperatures. For the SAIL/MWCNT-based supercapacitor, C elec increased from 75 F g–1 at 298 K to 169 F g–1 at 373 K, whereas the energy density increased from 42 Wh kg–1 (at 298 K) to 94 Wh kg–1 (at 373 K) and the power density increased from 75 kW kg–1 (at 298 K) to 169 kW kg–1 (at 373 K) at a scan rate of 2 mV s–1 (potential window = 4 V). This study reveals that SAIL can potentially be used as an electrolyte for high-temperature electrochemical applications for energy storage devices.

Non-halogenated Imidazolium and Phosphonium-based Surface-Active Ionic Liquids as Electrolytes for Supercapacitors

“…(2) and (3) were plotted against each other and generally referred as the Ragone plot. Figure 12 shows 12 All other solid data points have been taken from the literature for a comparison; blue 57 , wine 58 , royal blue 60 , red 61 , black 62 , pink 63 , green 64 , olive 65 , orange 66 , violet 67 , dark yellow 68 and magenta 69 . V that is much wider than ECWs of aqueous and organic solvent-based electrolytes.…”

“…16,19,25 Therefore, SAILs can be used as electrolytes or components of electrolytes in supercapacitors and batteries to operate under a wider range of temperatures. 12,16 A more thorough investigation of SAILs may increase the traditional understanding of ILs in electrochemistry. There is very little known about the SAILs and, to the best of our knowledge, few studies performed on this new class of ionic liquids, which comprise a surface-active anion like 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate (AOT) 16 and 2-ethylhexyl sulfate 12 as electrolytes or as components of electrolytes in supercapacitors.…”

“…[31][32][33][34] The electrochemical performance of studied here SAILs as electrolytes is also compared with that of the previously reported SAIL, [N8, 8,8,8][EHS], and its binary 50 wt% mixture with acetonitrile and with 6 M KOH(aq) as electrolytes in MWCNT-based supercapacitors in the same temperature range. 12 We showed that using SAILs as electrolytes, one could achieve improved specific capacitance and a high energy and power density, 5 particularly, at elevated temperatures up to 373 K and with a good cycling stability of the S1) are provided in the Electronic Supplementary Information (ESI).…”

“…The order of the specific capacitance and energy densities calculated from CV curves obtained at a scan rate of 2 mV s -S15 in the ESI showsCV curves obtained at 2 mV s -1 for MWCNTs/[C6C1Im][EHS] and MWCNTs/[P4,4,4,4][EHS] capacitors at different temperatures.) The data for tests on a MWCNTs/[N8,8,8,8][EHS] capacitorwas taken from our previous study 12. The energy density and the power density values at different scan rates for MWCNTs/SAILs supercapacitors are also shown in FiguresS16 and S17in the ESI.…”

Non-halogenated Imidazolium and Phosphonium-based Surface-Active Ionic Liquids as Electrolytes for Supercapacitors

“…Few studies on ammonium and phosphonium cationbased ILs with common anions like bis(trifluoromethanesulfonyl)imide, [NTf2] -, or bis(fluorosulfonyl)imide, [FSI] -, have shown a promising performance of these halogenated RTILs as electrolytes with a low viscosity and a three-four fold larger electrochemical potential window (ECW) as compared to ECWs of molecular organic and standard KOH(aq) and H2SO4(aq) based electrolytes. 12 However, due to high costs and the presence of halides, the use of halogenated RTILs is rather limited as electrolytes in supercapacitors. [13][14][15] Halogen containing anions are prone to hydrolysis giving rise to halide anions.…”