Imidazolium-based ILs are among the most extensively studied ILs and electrolytes in SC because of their low viscosity and high conductivity, especially when paired with a range of carbon electrodes, as seen in a recent study (Ortega et al., 2020). Where imidazolium ILs [(EMIM)(Tf₂N)] and ([(EMIM)(BF₄)] were paired with different carbon electrode materials such as; activated carbon (AC), mesoporous carbon (MES), multi-walled carbon nanotubes (MWCNT), and reduced graphene oxide (RGO) in SC.

The study aimed to obtain insights on; compatibility of the ions with the electrodes, factors affecting the charge accumulation and, its influences on the SC performance. Interestingly, although the ILs investigated contain the same cation, their properties were entirely different. For instance, (EMIM)(Tf₂N) is hydrophobic, while (EMIM)(BF₄) is hydrophilic. In addition, (EMIM)(BF₄) showed higher ionic conductivity (14 mS cm⁻¹) despite its higher viscosity (42 cP). The observations reference the role of the paired anion with imidazolium plays in the physical properties. Figure 3A,B shows; the specific capacitance, energy density and, power density constructed with (EMIM)(Tf₂N) and (EMIM)(BF₄ ) with the selected electrode material.

We can conclude from Figure 3 that imidazolium-based ILs make better SC with electrode materials containing a high volume of mesopores. The increased power density was achieved with open and accessible electrode material surfaces like mesoporous carbon (Lian Y.-M. et al., 2019) and multi-walled carbon nanotubes (Zhang et al., 2018). These observations are generally valid for a wide range of SC with IL electrolytes. In carbon-based electrode materials with high pore volumes and many micropores (like activated and mesoporous carbons), the interactions between the imidazolium cations and the desorption of co-ions present in the pores govern the specific capacitance and resistance of the electrochemical double-layer capacitance. A summary of the performance of different SC containing neat imidazolium ILs electrolyte is showed in Table 2 .

The performance (specific capacity and energy density) of the SC in Table 2 emphasizes that matching the pore size of the electrode with the ionic size of the ILs electrolyte is not the only essential criteria in selecting carbon electrode materials for SC. Other interactions of the ILs with electrode materials, including ion-ion interaction within the ILs, functional groups on the carbon electrode surface, are among other considerations, as will be seen in subsequent sections of this work. A feature observation that qualitatively defines a SC as an electrochemical double layer supercapacitor is the near box-like shape of the cyclic voltammetry response, which also depicts the influence of ion diffusion limitation on limiting current density. Ion diffusion limitations are often defined by electrical conductivity and prevalent electrode mesopore (Hu et al., 2019). The key is that the pore structure of the electrode creates an easy path for the electrolyte ions to access the electrode surface, as shown by Tamailarasan et al. (Tamailarasan and Ramaprabhu, 2012).

Ion diffusion limitations can also be solely a result of the IL property, the physicochemical properties of two imidazolium containing tris(pentafluoroethyl)trifluorophosphate (FAP) and tetracyanoborate {[B(CN)₄]⁻} anions were evaluated, and their performance as the electrolyte in SC was studied (Seki et al., 2012) (Table 2). Due to the larger size of the (FAP)⁻ anion, the viscosity of (FAP)^- anion-based IL (43 mPa.s) was higher in comparison with [B(CN)₄]⁻ anion-based counterpart (15 mPa.s). At the same time, the ionic conductivity of (EMIM)(FAP) was almost three times lower (6.9 mS cm⁻¹) than (EMIM) [B(CN)₄] (19.0 mS cm⁻¹) at 30°C. In other words, the nature of the anions playing a significant role in determining the properties of ILs and the SC performance.

In addition to single/isolated charge-discharge response, an important performance metric for SC is capacity retention and cycling stability (Kurig et al., 2012). In investigations of the chemical reversibility of IL SC systems with (EMIM)⁺ cation and different anions such as (BF₄)⁻, (Tf₂N)⁻, (FSI)⁻, (FPA)⁻, [B(CN)₄]⁻ and (SCN)⁻ (Kurig et al., 2011). The oxidation of the (SCN)⁻ to thiocyanogen caused deviation from the ideal capacitive behaviour. Cycling stability in SC can also be linked to the wettability of the electrolyte on the electrode pore walls due to the presence of functional groups on the pore walls (Thangavel et al., 2018).

Generally, we observed from these earlier studies that the relatively good performance of imidazolium cation based ILs as SC electrolytes might be related to small ionic size, appreciable electrochemical stability (depending on the paired anion), low viscosity, and high conductivity.

To further boost the energy density of SC, research efforts have been dedicated to redox-active ILs. An additional charge storage mechanism (pseudocapacitance) is offered from the redox species inherent in the ILs rather than applying pseudocapacitive solid electrode materials. SC assembled with redox-active ILs further blur the lines between batteries and SC, given the galvanostatic charge-discharge response of these SC begins to look more “battery-like”. Fic et al. (2019) took advantage of pseudo halide anion property and excellent transport properties of selenocyanate (SeCN) anion based ILs to develop a redox-active IL for SC. These ILs proved promising candidates as they were previously extensively studied in dye-sensitized solar cells (Wang et al., 2004).

An (EMIM)(SeCN) was tested as a SC electrolyte adopting an activated carbon electrode (Fic et al., 2019). Due to low viscosity (17 mPs) and relatively high conductivity (25 mScm⁻¹), (EMIM)(SeCN) showed promising capacitance (Table 3), especially at higher current density (42 Fg⁻¹ at 2.5 Ag⁻¹). The appreciable performance of the (SeCN)⁻ anion based ILs might be related to their reasonable electrochemical stability, low viscosity, small ionic volumes, and, therefore, high ionic conductivity. Other previous research efforts applied; Cu (II) metal-containing ILs (Sun et al., 2010), bromide ion species (Br₂/Br₃ ⁻) (Yamazaki et al., 2012), and hydroquinone (Sathyamoorthi et al., 2015).

The higher capacitance value of copper containing ILs is associated with the pseudocapacitance contribution from Cu (II)/Cu redox reaction (Table 3). Imidazolium cation can coordinate with the metals through its acidic proton on the C₂ carbon atom and form the Cu (II) complex of the ILs (Blue et al., 2006). It is worth noting that thermal stability and the effect of halide content on transport properties of ILs with the metal complexes are often unreported. It is known that even a minimal concentration of halide impurities can detrimentally affect the viscosity of ILs (Seddon et al., 2000). Given these concerns and SC performance targets, the optimal position and polarity of the redox-active component/molecule in the IL also need to be evaluated.

Bi-redox ILs with Anthraquinone, on anion [(AQ-PFS)⁻] and 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) on imidazolium cation [(MIM-TEMPO)⁺] have also been demonstrated as electrolyte for SC (Mourad et al., 2017). The bi-redox IL introduced had to be dissolved in (BMIM)(Tf₂N) (because of high viscosity) to enable testing at room temperature. In contrast, neat bi-redox ILs electrolytes were tested at elevated temperatures (60°C) because of the significant decrease in viscosity with increasing temperature.

Applying only (BMIM)(Tf₂N) electrolyte, the double-layer charge storage mechanism is seen without any Faradaic reactions. In the case of bi-redox ILs dissolved in (BMIM)(Tf₂N), the redox-active ions, (AQ-PFS)⁻ and (MIM-TEMPO)⁺ are electro adsorbed on the surface of the carbon electrode and also undergo Faradaic reactions. The introduction of the reducible moiety on the anion and oxidizable species on the cation helped generate bi-anions and bi-cations at the anode and cathode. From the above discussion, we can conclude that the introduction of the redox active materials can improve the performance of the SC. However, it can detrimentally affect the transport properties and electrochemical stability of the ILs. The summary of the performance of the redox-active ILs is summarized in Table 3.

The unique feature of the protic ILs (PILs) is the presence of an acidic and liable proton (hydrogen) on the cationic core, which can involve coordinating with metals (Menne et al., 2014). However, this exchangeable hydrogen on the cationic core results in low thermal stability of PILs. Due to the relatively easy synthesis procedure and non-requirement of purification steps, protic ILs receive much attention in various applications such as; electrolyte for lithium-ion rechargeable batteries (Menne et al., 2013; Vogl et al., 2014), extraction processes (Wang et al., 2020; Rodrigues et al., 2018; Janssen et al., 2016).

In a recent study (Mayrand-Provencher and Rochefort, 2009), PILs were prepared from aromatic heterocyclic compounds (base) containing two nitrogen atoms and trifluoroacetic acid. The effect of multiple proton exchange sites and the impact of the acid strength (pKa) on the physicochemical properties of these PILs and the capacitance of SC containing RuO₂ electrodes were evaluated. At the same time, specific conductivities recorded varied from 0.71 to 9.07 mScm⁻¹ at 27°C. In all the PILs studied, conductivity improved with the amount of acid used, and the highest value was obtained in a base and acid ratio of 1:2. This increase in conductivity is related to the decrease in the viscosity obtained at that ratio.

As expected with PILs, the pH (potential of hydrogen) of the resulting SC electrolyte has a significant influence on the specific capacitance value, as demonstrated by Demarconnay and co-workers (Demarconnay et al., 2013). PILs such as pyrrolidinium nitrate [(Pyr)(NO₃)] and triethylammonium bis(trifluoromethylsufonyl)imide [(N_222H)(Tf₂N)] with high conductivity, varying pH, and water content were employed as electrolytes with carbon-based electrodes. Varying pH of the electrolyte from 7 to 11 increased the specific capacitance from 121 to 208 Fg⁻¹ for (Pyr)(NO₃). The higher capacitance value at pH11 was attributed to the pseudocapacitive contribution arisen due to the increased mobility of PILs proton by the presence of multiple amine sites on the electrode material. The water content in the electrolyte detrimentally affected the operating voltage and specific capacitance value. Considering the difficulty in drying the PILs, care must be taken when selecting the anions of the PILs. The PILs containing hydrophobic anions tend to reduce the water content in the PILs and can contribute to enhanced performance.

In summary, PILs demonstrated so far in the literature are promising candidates for carbon electrode SCs because of the pseudo-faradaic reaction between the oxygen-containing functional groups on the activated carbon and the proton on the PILs. The low operating temperature some PILs offer (Montes-Morán et al., 2004) might be valuable in various applications like aerospace.

Like PILs, sulfonium-based ILs are also relatively easy to synthesize for application as SC electrolytes, as shown by Anouti et al. (2012). They synthesized trimethylsulphonium bis(trifluoromethylsufonyl)imide (Me₃S) (Tf₂N) by the simple alkylation of dimethylsulfide with corresponding alkyl halides, followed by a metathesis reaction of the resulting halide salt with Li (Tf₂N). Although the (Me₃S)(Tf₂N) was obtained as solid at room temperature (T_m = 45.5°C), the IL reported high conductivity (20.42 mS cm⁻¹) at 80°C, which is significantly higher than the ammonium-based ILs used in SC applications. (Me₃S) (Tf₂N) also showed very low viscosity (3 mPas) at similar experimental conditions. The smaller ionic size and high fluidity at 80°C promoted the high conductivity in (Me₃S) (Tf₂N) ILs, typical for the ILs containing sulfonium cation. When neat (Me₃S) (Tf₂N) was applied as SC electrolyte (Maton et al., 2013), a potential window of 5.3V (vs. Ag/Ag⁺) was reported at 50°C with a capacitance value of 130 Fg^-1 and 140 Fg⁻¹ at 50°C and 80°C, respectively. These values are almost double that of (Pyr _1,4) (Tf₂N) at similar conditions with the same activated carbon electrodes.

The observed results with (Me₃S) (Tf₂N) reinforce the influence of high conductivity, lower viscosity, and small size of the (Me₃S)⁺(0.6 nm) cation compared with the (Pyr_1,4)⁺(1.1 nm) cation. The higher energy and power densities recorded for the sulfonium ILs make them a promising electrolyte class for SCs. However, the thermal stability of sulfonium-based ILs is still a concern at elevated temperatures (Zhang et al., 2009; Coadou et al., 2016).

Sampaio and co-workers also (Sampaio et al., 2019) conducted molecular dynamics studies to evaluate the electrical, dynamical and structural properties of three sulfonium based ILs diethylmethylsulfonium bis(trifluromethylsulfonyl)imide [(S₂₂₁)(Tf₂N)], triethylsulfonium bis(trifluromethylsulfonyl)imide [(S₂₂₂)(Tf₂N)] and diethylpropylsulfonium bis(trifluromethylsulfonyl)imide[(S₂₂₃)(Tf₂N)] at atomic level. It was observed that both viscosity and ionic conductivity of these ILs varied according to cation size. However, the differences in the properties had a negligible effect on the performance of the SC, especially on the specific capacitance value, which is contradictory to the earlier reports (Li S. et al., 2016) and our previously highlighted relationships. We suspect the near uniformity of their SC-specific capacitance is due to microporous activated carbon as the electrode material, which has a smaller pore size than the ionic size of the ions in the ILs. Although the easy synthesis and favourable transport properties of sulphonium-based ILs make them suitable candidates for SC electrolytes, design protocols to improve their thermal stability are necessary for their use in high-temperature supercapacitors.

Most ILs used as electrolytes for SCs are based on aromatic or non-aromatic five/six -membered rings (imidazolium, pyrrolidinium or piperidinium). ILs with seven-membered ring (Azepanium) are also applicable and were synthesized, with their physicochemical and electrolyte performance in SC was compared with the established (Pyr_1,4)(Tf₂N) electrolyte in a previous study (Pohlmann et al., 2015). The presented azepanium based ILs (N-butyl-N-methylazepanium bis(trifluoromethanesulfonyl)imide [(Azp_1,4)(Tf₂N)] and N-methyl-N-hexylazepanium bis(trifluoromethanesulfonyl)imide [(Azp_1,6)(Tf₂N)] displayed higher viscosity and lower conductivity compared to their pyrrolidinium analogues. This observation is possibly due to the larger ring size of the azepanium cation. However, the azepanium based ILs showed comparable thermal stability (>350°C) as pyrrolidinium ILs. The determination of the maximum operating voltage showed that the positive potential limit was not affected by the ILs structure. On the other hand, the negative potential limit was influenced by the size of the cation. The inferior performance of the azepanium based ILs as an electrolyte compared with the pyrrolidinium analogue was related to their high viscosity and lower conductivity. Nevertheless, considering the lower cost of azepanium ILs, modifications to reduce their viscosity and improve the ionic conductivity can transform them into electrolytes of choice in SCs.

The performance of ILs based electrolytes in SC can be improved by the cautious design of ILs, including introducing functional groups to conventional ILs. Modifying the IL cations by grafting the alkyl side chain with heteroatoms (Oxygen, Nitrogen, Sulphur, etc.) can dramatically improve the ionic transport properties. The heteroatom on the alkyl chain can alter the electronic distribution of the cationic core, which will improve ionic transport properties and enhance electrochemical performance. Rennie et al. (2013), tethered oxygen atom on the alkyl side chain (ether functionalization) of the imidazolium cation [1-(2-ethoxyethyl)-2,3-dimethylimidazolium bis(trifluromethylsulfonyl)imide], (MMIM ₂₀₂) [(Tf₂N) and phosphonium cation (2-methoxyethyl)trimethylphosphonium bis(trifluromethylsulfonyl)imide, [(P _222,201) (Tf₂N))], which significantly improved the specific capacitance and reduced the resistance of a SC containing mesoporous carbon electrode. The presence of the ether group reduced the viscosity of the IL. A similar trend was observed for ILs containing other cationic cores under investigation for viscosity and thermal stability. Electrochemical impedance spectroscopy results of ILs showed significant deviation from the ideal behaviour due to the inhomogeneity on the electric double layer formed, which is typically prominent in viscous ILs with low conductivity. The improved performance of the ether group-containing ILs might be related to the presence of an electronegative oxygen atom, which might create a remote electronegative region on the cationic core and facilitated the creation of a denser double layer on the electrode surface and resulted in the displacement of a large amount of charge on the electrode.

A comparative study of electrochemical performance of ILs based on two weakly coordinating anions aluminum hexafluroisopropoxilate {[Al(hfip)₄]⁻} and (Tf₂N)⁻ was also performed in a SC (Roznyatovskaya et al., 2015). The [Al(hfip)₄]^– anion showed less electrode interaction and improved transport properties compared to (Tf₂N)⁻ anion, when both were evaluated with carbon electrodes possessing micro to macropores—to study the effect of pore size-ion size matching (sieving effect) on capacitance behaviour. Both areal and specific capacitance values with cells combining electrodes with different pore sizes and ILs are shown in Figure 4. The sieving effect is clear in the case of electrodes with micropores. ILs with larger cation [(N₁₄₄₄)⁺] and anions {[Al(hfip)₄] ⁻} displayed lower capacitance value. The larger size of the {[Al(hfip)₄]^–} anion (1.166 nm) is not able to pass through the microporous carbon electrode with an average pore size of 0.86 nm - resulting in lower capacitance than {Tf₂N}⁻ anion. Conversely, there was no significant difference in the capacitance for ILs with mesoporous carbon electrodes with larger pores as expected.

ILs with pyrrolidinium cations having simple alkyl group [(Pyr _1,3)⁺, (Pyr _1,4)⁺] and ether groups [(Pyr _{1, 201})⁺] paired with dicyanamide (DCA) anion have been reported as electrolytes in SCs with an activated carbon-based composite electrode (Wolff et al., 2015). It was previously observed that dicyanamide anion based ILs are typically electrochemically less stable than their (Tf₂N)⁻ analogues (Zarrougui et al., 2018). Nevertheless, their high conductivity makes them a good candidate for SC electrolytes. Table 4 shows the SC performance of some reported Pyrrolidinium and Piperidinium ILs.

Both pyrrolidinium and piperidinium cationic cores have also been investigated for SC electrolytes in combination with tricyanomethanide {[C(CN)₃]^–} and Tetracyanoborate [B(CN)₄]^– anions (Martins et al., 2017; Martins et al., 2018). It was observed that the ILs with pyrrolidinium and tricyanomethanide combinations showed similar viscosity at 25°C, while (Pip _1,4)[C(CN)₃] and (Pip _1,4)[B(CN)₄] had significantly higher viscosities due to relatively larger cation size. Consequently, pyrrolidinium-based ILs show better performance as SC electrolytes than the piperidinium counterpart (Table 3), which showed a significant iR drop during discharge. The effect of pyrrolidinium ion size on resulting higher viscosity electrolytes like (Pyr _1,4)(FAP) was also demonstrated (Sillars et al., 2012) on the capacitance of SC applying carbon electrodes. It was observed that the higher viscosity leads to the weakening of the interaction between the electrode and electrolyte ions. From the above discussion, it is clear that the viscosity of ILs playing a significant role in determining the supercapacitor performance, which must be considered while designing new ILs electrolytes for SC.

More ILs have been recently developed and studied in electrochemical energy storage applications like SC. An example is ILs derived from dimethylformamide (DMF) type cation, and tetrafluoroborate anion, (EDMF)(BF₄) prepared and studied as a potential high-capacitance electrolyte for SC by Chen and co-workers (Chen et al., 2019). The moderate size and quasi-linear nature of the (EDMF)⁺ cation increased ion mobility and helped the formation of a compact electric double layer resulting in better performance of the SC with (DMF)(BF₄) electrolyte.

Most ILs applied as electrolytes in SC contain fluorine in the anions. A recent study (Khan and Shah, 2020)s synthesized an IL with a non-fluorinated Furoate anion, in combination with tetrabutylphosphonium furoate [(P₄₄₄₄)(FuA)], and tested it as an electrolyte for a SCs. The advantage with some of these developed ‘non-conventional’ ILs is that high ionic conductivities and operating potential windows in SC applications can be reported, comparable to the most applied high performing ILs like (EMIM)(BF₄). In addition, their synthesis can be simplified. However, there are still setbacks with the newly developed ILs in thermal stability. For example; the Furoate anion based ILs like previously mentioned (P₄₄₄₄)(FuA), are thermally less stable (<250 °C) in comparison to their fluorinated analogues, which is not surprising because carboxyl groups can easily undergo decarboxylation at elevated temperatures (Clough et al., 2013; Cao and Mu, 2014). Nevertheless, this report opened the possibility of developing high performing ILs electrolytes by avoiding fluorinated anions.

The mixing of two neat ILs created a new electrolyte that behaved differently than the constituent ILs as seen in the study highlighted in Figure 5 (Van Aken et al., 2015). The use of such IL mixtures can potentially result in electrolytes with balanced charge storage, high potential window, and high energy density (Van Aken et al., 2015).

Another study (Lin R. et al., 2011) explored IL mixtures by mixing two ILs with the same anion bis(fluorosulfonyl)imide [(FSI]⁻], with similar cations [(Pyr _1,3)⁺ and (Pip _1,3)⁺, which prevented ordered arrangement of the ions in the crystal lattice to avoid crystallization. The result was a new IL with a broader liquid range compared to the constituent ILs. Mixing of the two ILs extended the liquid range of the newly formed mixture to as low as −50°C, resulting in the capability to operate a SC with extended operating temperature from −50 to 100°C.

As suggested so far, the electrochemical performance of these non-conventional ILs is defined by their molecular structure. Therefore, the structure–performance influence is always worthy of investigation. Mousavi et al. (2016) reported the effect of ILs structure on the physical properties, electrochemical stability, capacitance, and specific energy when applied as electrolytes in SC using carbon electrodes with uniform and highly interconnected mesopores. ILs containing different anions (tetrafluoroborate, trifluoromethanesulfonate, trifluoromethanesulfonimide and most used cations (imidazolium, ammonium, pyridinium, piperidinium, and pyrrolidinium) were selected for the study seen in Figure 6.

The electrochemical stability measurements of the ILs showed that anions have little effect on the cathodic stability of these ILs. On the other hand, the cation has a significant influence on the anodic limit of the ILs. Quaternary ammonium ILs showed the highest anodic stability compared to their aromatic counterparts. Given cation structure substantially impacts the cathodic stability of the ILs, non-aromatic quaternary ammonium cationic cores displayed better stability than their aromatic analogues. The reason is that cations with lower LUMO (lowest unoccupied molecular orbit) energy levels are prone to reduction. The investigators observed that alkyl spacer lengths on the cations had no significant influence on the electrochemical stability of the ILs. Cation size also had a considerable effect on the viscosity and conductivity of the electrolyte and the capacitance of assembled SC. In the study (Mousavi et al., 2016), imidazolium and pyridinium-based ILs provided the highest cell capacitance, and ammonium-based ILs offer potential windows much larger than imidazolium and pyridinium ILs. Although alkyl chain lengths on the imidazolium cation did not influence their electrochemical potential window, it significantly influenced the specific capacitance of SCs with imidazolium ILs (Figure 6).

The increase in the alkyl chain length on the imidazolium cation (larger cation) decreased the gravimetric specific capacitance due to the decrease in the concentration of the counterions in the electrode pores due to the sieving effect of the electrolyte. In addition, the increased ionic size also reduces the accessible surface area. (EMIM)(Tf₂N) displayed the highest capacitance among the imidazolium ILs studied (Mousavi et al., 2016). This study clearly shows that the size of the ILs ions should be considered when selecting electrode materials for ILs based supercapacitors.