Supercapacitors utilising ionic liquids

Author’s Accepted Manuscript Supercapacitors Utilising Ionic Liquids Ali Eftekhari

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S2405-8297(17)30146-0 http://dx.doi.org/10.1016/j.ensm.2017.06.009 ENSM169

To appear in: Energy Storage Materials Received date: 19 April 2017 Revised date: 16 June 2017 Accepted date: 17 June 2017 Cite this article as: Ali Eftekhari, Supercapacitors Utilising Ionic Liquids, Energy Storage Materials, http://dx.doi.org/10.1016/j.ensm.2017.06.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Supercapacitors Utilising Ionic Liquids Ali Eftekhari a,b a

The Engineering Research Institute, Ulster University, Newtownabbey BT37 OQB, United

Kingdom b

School of Chemistry and Chemical Engineering, Queen's University Belfast, Stranmillis Road,

Belfast BT9 5AG, United Kingdom

Abstract Ionic liquids (ILs) can provide a broad range of opportunities for fabricating high-energy supercapacitors owing to their wide stable potential windows, flexibility in design, and ionic properties. Although their applicability had not been fully understood due to an impression that ILs are simply alternative electrolytes in the electrochemical systems, only a fraction of research works is currently focused on pure IL electrolytes, as the emerging attentions are towards new possibilities introduced by ILs. This review provides an overview of different roles of ILs in the development of new supercapacitors and attempts to link these works for a better understanding of the IL potentials and challenges. While manipulating the IL electrolytes can pave the path for the fabrication of practical supercapacitors, gel polymer electrolytes might have a better fortune in the commercial development of flexible devices. In addition to different roles of ILs as the electrolyte components, they can also modify the electrode material to enhance the supercapacitor performance. Keywords: Ionic liquids; Polymerised ionic liquids; Supercapacitor; Pseudocapacitance; Gel polymer electrolyte; Porous carbon

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1. Introduction

Ionic liquids (ILs) have huge potentials in the realm of electrochemistry due to the mobility and flexibility of ions. Although ILs had been initially considered as alternative solvents or electrolytes, the applications of ILs in electrochemical systems is way beyond simple solvents/electrolytes. At least in electrochemistry, ILs, similar to molten salts, are indeed a mobile matrix of ions rather than a solvent. A wide range of potential applications of ILs in lithium-ion batteries and new possibilities for designing novel types of electrolytes have been recently discussed [1].

In electrochemical systems, pure IL electrolytes have been successfully utilised for the electrodeposition of metals, which is totally impossible in conventional electrolytes [2-4]. Such electrodeposition processes were previously possible only in molten salts, and ILs are somehow molten salts with low melting points for this purpose. Nevertheless, ILs are substantially different from molten salts, due to fundamental differences in the structural arrangement of large asymmetrical organic ions in comparison with straightforward inorganic ions. However, they both serve the same purpose for the electrodeposition of metals. In fact, this electrodeposition is conducted in a solvent-free cell [5]; whether the electrolyte medium is a molten salt (at high temperature) or an IL (at room temperature). In a similar fashion, electrochemical systems utilising IL electrolyte are sometimes called solvent-free [6-8]. The terminology importance is because of the fact that ILs should not be treated as a replacement of conventional electrolytes as liquid media (contrary to the name, they are not usually ideal liquids) unless a mobile matrix of ions is required. In a sense, ILs can be considered as an alternative to solid electrolytes.

ILs are good candidates for supercapacitors, particularly those working based on the double layer charging, for two reasons: the primary task of the electrolyte is to provide charge species at the electrode/electrolyte interface instead of diffusion of specific electroactive species, and wide stable potential window of ILs guarantees high energy densities even greater than those of organic electrolytes.

In addition to the practical applications, ILs as solvent-free media are straightforward systems for fundamental studies to understand the nature of electrode/electrolyte interface in supercapacitors [9]. Note that the complexity of ILs is due to the novelty of these media in comparison with the conventional electrolytes. As a matter of fact, the emerging interest in ILs was due to the practical potentials, and unfortunately, there are only a limited number of fundamental studies (mostly Page 2 of 72

theoretical) focusing on the electrode/electrolyte interface of supercapacitors in ILs [10-23].

Not only ILs have been successfully employed as the electrolytes of current supercapacitors to widen the potential window, but also they have been utilised in the classical electrolytic capacitors [24]. However, only a fraction of research on this topic is devoted to the pure electrolytes, because the applications of ILs are way beyond this. Although the price of ILs has been dramatically dropped during the last years due to the growing demand and commercial productions, the cost stills remains an issue for the practical developments. This is not due to the synthesis cost but purification. Owing to the highly charged nature of the composing ions, ILs are prone to contamination. When utilising the ILs as pure electrolytes, purity is of utmost importance. Nevertheless, other applications of ILs as will be reviewed here need lesser amounts of the ILs while the purity might not be critical. On the other hand, current works are somehow limited to the common ILs, which are indeed suitable choices for standalone electrolytes.

The common ILs in supercapacitors are based on cations of 1-ethyl-3-methylimidazolium (EMI), 1butyl-3-methylimidazolium

(BMI),

N-Propyl-N-methylpyrrolidinium

(PYR13),

1-Butyl-1-

methylpyrrolidinium (PYR14), Tetraethylammonium (Et4N), etc. On the anion side, the common ions are chloride (Cl), bromide (Br), tetrafluoroborate (BF4), hexafluorophosphate (PF6), bis(fluorosulfonyl)imide (FSI), bis(trifluoromethylsulfonyl)imide (TFSI), etc. Figure 1 illustrates typical IL molecular structures. The popularity of these IL electrolytes in the realm of electrochemistry is due to their acceptable viscosity and ionic conductivity at room temperature. However, for other applications, there should be more suitable choices, which are not popular yet. In general, the aim of this review is not to support a specific application of IL among others. Instead, it is attempted to link all the potential applications to consider new possibilities.

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Figure 1. The molecular structure of typical ILs based on common anions and cations as employed in the electrochemical systems. (a) EMI-BF4, (b) BMI-PF6, and (c) Et4N-TFSI.

2. Fundamentals 2.1. Terminology

The terminology of supercapacitors is not straightforward. The term supercapacitor was used for a commercial prototype of double layer capacitors in the 1970s, but Conway used the same term for capacitors based on pseudocapacitive redox systems [25]. This perfectly makes sense, as the prefix "super" was simply used to describe the high specific capacitance of the corresponding capacitors as compared with the classical capacitors. Two other terms are pseudocapacitors (i.e., obviously based on pseudocapacitance) and ultracapacitor (covering the double layer capacitors). A simple literature search reveals that supercapacitor is the most popular name, which is commonly employed for both types of the capacitors. Although the prefix "ultra" is equivalent to "super," the term ultracapacitor has never been used for pseudocapacitive capacitors.

The term supercapacitor is somehow identical to 'electrochemical capacitor', which covers both Page 4 of 72

types of the capacitors mentioned above; though, some believe it does not cover double layer capacitors since the latter is not based on an electrochemical reaction. Nevertheless, double layer capacitors are based on the conventional electrochemical cells, and the corresponding double layer charging has been investigated by electrochemists for over a century.

The terminology of ILs is also confusing, as the threshold for the melting point is an arbitrary value of 100 °C. The reason that this value is not the room temperature (e.g., 25 °C) is because of the capability of ILs for forming supercooled liquids. This means that ILs with melting points over 25 °C can still be liquid at the room temperature [26]. The basis of IL liquidity is glass transition point rather than melting point.

The critical point is that ILs have been considered as solvents, and thus, liquidity is of utmost importance. However, in many applications, as will be reviewed here, ionicity of these organic salts are the point of interest rather than liquidity. On the other hand, many highly viscous ILs, which are not appropriate solvents, might have potential applications in the electrochemical systems where the IL is employed within solid matrixes of electrodes or electrolytes. Therefore, the most appropriate term is ionic organic salts, because many solid salts can be used as well as ILs in various applications where ionicity is the key feature. On the other hand, the definition of ILs also covers chloroaluminates, which are among the few possible inorganic ILs. However, when referring to ILs in the modern literature, organic ILs are always aimed.

2.2. Electrochemical Window

Since the energy stored by supercapacitors is proportional to the square of operating voltage, the benefit of a wide potential window is more than specific capacitance. Hence, there is a demand for widening the electrochemical potential window, which is limited by the instability of electrolytes at extreme anodic or cathodic potentials. This has limited the applicability of aqueous supercapacitors, which are quite cheap with no complication in the cell design. The stable potential window in aqueous electrolytes is theoretically 1.23 V (plus overpotentials) between the redox potentials associated with hydrogen and oxygen evolution. However, owing to the underpotential processes, the practical stable potential window of aqueous supercapacitors is usually about 1 V. A possible solution is to utilise non-aqueous electrolytes, which are relatively stable over a wide potential window. In addition to the cost, the cell design is more difficult since moisture should be utterly Page 5 of 72

avoided. However, these drawbacks are worth of the potential extended range, as the operating voltage of the supercapacitors can be tripled in the non-aqueous electrolyte, resulting in one order of magnitude increase in the energy density.

Despite the electrochemical stability of organic electrolytes such as acetonitrile (AN), they suffer from practical issues such as flammability. Another choice is ILs, which are strongly stable in significantly wide potentials windows, as they are not subject to decomposition at relatively extreme anodic or cathodic potentials. Many of the common ILs can have a stable potential window as wide as 6 V. Since the electrochemical instability is because of the ion reactions at the electrode surface, the wideness of potential window is directly dependent on both anion and cation. Figure 2 shows how changing one ion can significantly alter the potential window for similar ILs.

Figure 2. Electrochemical stability of typical ILs on a Pt working electrode. The reference electrode is Ag/AgCl and the scan rate 1 mV s–1. Reproduced with permission from Ref.[28]. Copyright 2004, Elsevier.

The stable electrochemical window is the potential range in which the electrolyte is not oxidised or reduced at high or low potentials, respectively. The reason behind the wide stable windows of ILs is that the electrolyte is composed of individual ions, which do not participate in any considerable electrochemical reaction over a wide range of potential. Hence, the IL purity plays a particular role in the stable potential window, and this is the reason that high purity of IL is of utmost importance. For instance, a small amount of moisture can significantly reduce the width of the stable potential window.

It should be taken into account that the electrochemical stability of ILs reported in the literature refers to the conventional conditions employing inert electrodes such as Pt. Figure 3 compares the Page 6 of 72

stability of potential window of some ILs at Pt and carbon electrodes. In the practical applications, the chemical and structural irregularities cause electrochemical interaction of the IL ions at the electrode surface, and thus, the stable potential window is much narrower for carbon electrodes [2730]. As summarised in Table 1, the practical potential windows of IL supercapacitors are in the range of 3–4 V (see also [31-33]). In well-designed systems, it is also possible to achieve wider potential windows. For example, a supercapacitor based on atomically thin graphene in an IL have been reported to have an operating voltage in the range of 4–10 V [34]. Protic ILs usually show narrower potential window [35-37].

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Figure 3. Electrochemical behavior of Pt and carbon electrodes in typical ILs. Reproduced with permission from Ref.[29]. Copyright 2016, Elsevier.

Electrochemical studies of the supercapacitor materials are usually as half-cell in which the counter electrode does not directly contribute to the cell response (at least ideally assumed). In a real supercapacitor cell, two half-cells are coupled together to deliver the capacitive response. In Page 8 of 72

asymmetric cells or even symmetric cells in which the potential of zero voltage is not in the middle, there is a potential imbalance resulting in a shift in the actual potential of one electrode [38-39]. This may lead to the electrolyte decomposition even in the stable potential window. Simon and Gogotsi have elaborated the individual roles of half-cells in the supercapacitors [40]. There are a few methods for balancing the charge across the half-cells [41-42]. A subtle approach is to utilise two different ILs for the half-cells [43].

Another important issue for a stable potential window in practical applications, which is usually ignored, is the cell stability. The electrolyte can be stable over a wide range of potential at the electrode material but not the current collector (i.e., usually aluminium) [44-45]. Similarly, the stability of current collector depends on the choice of IL. On the other hand, tiny impurities may be harmless for the supercapacitor process but can have an enormous impact on the current collector corrosion.

In general, ILs provide a stable potential window normally much wider than that of conventional organic electrolytes [46]. Figure 4 compares conventional electrolytes of supercapacitors: aqueous KOH, LiPF6 salt in an organic solvent, and an IL. As can be seen, the capacitive behaviours are similar delivering similar values for the specific capacitance, but the substantial differences in the stable window result in significant differences in the energy densities; where that of the EMI-BF4 IL electrolyte is one order of magnitude is higher than that of the aqueous KOH.

Figure 4. Comparison of the capacitive behavior of a graphene electrode in aqueous, organic, and IL media. Reproduced with permission from Ref.[46]. Copyright 2013, Royal Society of Chemistry.

A major problem of the energy storage devices is the severe safety risks beyond the stable Page 9 of 72

electrochemical window due to the electrolyte decomposition resulting in the formation of harmful products such as explosive gasses. Romann et al. have recently pointed out a safety feature of IL electrolytes when experiencing extreme potentials [47]. By applying a potential higher than the stable window in tetracyanoborate ILs such as EMI-TCB (E≥2.4 V vs. Ag/AgCl), the IL is electropolymerized to form a blocking layer while doping the graphene electrode to become an insulator. The resulting dielectric polycyanoborani polymer blocks any further electrochemical reaction, which might be harmful to the electrochemical cell.

3. Supercapacitors 3.1. Double Layer-Based Supercapacitors

Supercapacitors in which the capacitive behaviour is merely the result of double layer charging at the electrode/electrolyte interface are usually based on carbonaceous materials due to their high specific surface area. It should be taken into account that the electrochemically accessible area is not equal to the physical surface area, and the material architecture should be specifically designed by considering how the electroactive species can access the electrode surface. Because of the wide potential window, the energy density of IL-based ultracapacitors can be as high as that of Ni/MH batteries, with values greater than 100 Wh kg–1 (more precisely specific energy) [48-49].

The double layer charging in ILs as the solvent-free media is different from that in the conventional media in which the charged ions are distributed by the solvent molecules. Re-arrangement of ions at the double layer in an ion-rich medium is accompanied by strong interactions. On the other hand, solvated ions are replaced by bare ions, which are usually asymmetrical both geometrically and electrically. In the ILs, ion pairing or aggregation are somehow similar to solvation in the conventional solvents, surrounding the bare ions [50-56]. Strong self-interaction of IL ions is indeed the main obstacle in using ILs as standalone electrolytes. To address this issue, dilution of the IL electrolyte is beneficial to keep the IL ions separated. NMR studies showed acetonitrile could improve the ion mobility of IL supercapacitors more than five times [12].

When the surface area is increased in a 3D structure of carbonaceous materials, deviation from the ideal capacitive behavior (as can be characterized by the rectangular shape in CV) is inevitable for two reasons: the nature of double layer is more complicated within a non-uniform 3D structure resulting in charge distribution irregularities, and a significant increase in the functional groups Page 10 of 72

adsorbed, particularly on irregular edges. Nevertheless, the electrochemical behaviour of carbon even with an extremely high surface area of 3290 m2 g–1 is still ideally capacitive with no noticeable deviation (Figure 5).

Figure 5. (a) Cyclic voltammograms and (b) charge/discharge profiles of a carbon electrode. Reproduced with permission from Ref.[252]. Copyright 2013, American Chemical Society.

Despite an incredible flexibility in design, only a few typical ILs have been used in the electrochemical systems. The situation is worse for the supercapacitors in which a very limited range of ILs has been employed so far. Most works employ imidazolium or pyrrolidinium cations while the anions are conventional organic or inorganic ones. The potential window of imidazolium ILs is slightly wider than that of pyrrolidinium ILs, accompanied by a more stable temperaturedependent conductivity [57]. There is an emerging interest to employ new ILs [58-59]. Many of these ILs still have a similar structure, such as azepanium, which has been used for supercapacitor application [59]. Nonetheless, current research works are far behind the opportunity of designing ILs for specific systems.

3.1.1. Carbon Physical Structure

In conventional supercapacitors, the material a porosity directly controls the capacitive behaviour due to a higher surface area, but this is more complicated in IL electrolytes due to the ionically interactive nature of the medium. The electrochemical accessibility should be considered in three scales: macroporosity, mesoporosity, and microporosity.

Macroporosity is formed by the porosity between the carbon nanoparticles or other nano-objects. Owing to the high viscosity of ILs, there is a concern that the electrolyte might be distributed within the macroporosity as compared with the conventional electrolytes. However, the experimental results have suggested that thicker films can deliver similar specific capacitances indicating that the IL electrolyte is fairly available within the electrode macroporosity [60].

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Mesoporous carbons are among the best candidates for the fabrication of supercapacitors due to good electrochemical accessibility [30,61-64], but mesoporosity does not sufficiently provide a high surface area to meet the energy storage capacities demanded by the market. Therefore, it is beneficial to add microporosity within the mesopores. However, this makes the architecture of carbon nanomaterials for supercapacitors complicated. The tricky part is how the micropores are accessible by the electroactive species, and thus, considerable attention has been paid to fundamentally study the possibility of insertion of the IL ions into various types of carbon pores [65]. However, the interaction of IL ions within such narrow pores is not necessarily straightforward, because of ion-wall interactions strengthened by the asymmetry of IL ions and interaction with co-ions [66]. As a result, the capacitive performance has a non-linear dependency on the pore size [61,67]. Merlet et al. computationally suggested that the ion charge within the micropores plays a more significant role than the pore size [21].

The electrolyte cannot be soaked within small micropores, and the electrochemical accessibility is facilitated through a diffusion process similar to the solid-state diffusion in lithium-ion batteries. Redondo et al. compared the capacitive behaviour of a series of microporous carbons with various pore sizes in an IL electrolyte [68] and showed that the specific capacitance is not simply proportional to neither the pore size nor the specific surface area. However, an interesting feature in their report was that the rate capability was noticeably poorer for smaller micropores suggesting the rate-determining role of the diffusion within the mesopores. Computational calculations revealed that the diffusion coefficient within the micropores dramatically increases by the IL ion charge due to the intrinsic ion packing of ILs [69].

It has been theoretically predicted that carbon with appropriate microporosity can have a specific capacity in the range of 340–350 F g–1 originated from the double layer charging only [70]. In practice, a specific capacitance of over 200 F g–1 is rarely achievable, which is due to the complexity of ionic interactions. At high potentials, ions can intercalate into the graphite interlayers; this can increase the charge-storage resistance [71]. Nevertheless, this does not guarantee that microporous carbon can generally be used by the IL. Several research works attempted to increase the pore size to make them fully accessible by the IL ions [72-73]. Beguin et al. clarified that there is no general optimum pore size in ILs without considering the size of electroactive ions [74]. Computational studies of carbon nanotubes with various diameters also suggested the nonlinear dependency of the specific capacitance to the tube diameter [75]. On the other hand, the micropore size should be chosen depending on the IL electrolyte, as the sizes of IL Page 12 of 72

ions and micropores are comparables [76]. As mentioned, the ionic interaction of ILs and the electrochemical accessibility cannot be just judged by geometrical measures.

The electrochemical accessibility of inner parts of pores is not merely controlled by matching the size of diffusing ions with the pore diameter because the ions actively interact with the inner walls. On the other hand, ionophilicity/ionophobicity of the pore plays a particular role for internal accessibility of micropores. When the pore is ionophilic, the initial interaction of ions with the pore blocks further diffusion of ions. ILs provides an excellent opportunity for accessing micropores in the absence of ion solvation, and also because of ion mobility in reaching the micropores. The specific capacitance of a carbon aerogel in an IL (EMI-TFSI) electrolyte has been reported to be 100 times higher than that using a conventional polymer electrolyte [77].

3.1.2. Chemical and Structural Irregularities

It should be taken into account that the accessible surface area is not the only factor controlling the double layer capacitance, but the shape of the double layer formed on the electrode surface (altered by the localised charges and irregularities). In porous carbon, there are ruffled edges and angles, which can result in non-uniform charge distribution [78]. This influence is more significant in ILs, as the entire medium is made of charged ions with direct interactions with the localised charges of the electrode surface. Computational studies revealed that the double layer pattern and the capacitance are highly dependent on the electrode morphological structure [79].

Ordered mesoporous carbon provides a good opportunity for the diffusion of the electroactive species through a well-ordered structure, but the chemical composition and the degree of graphitization play a crucial role in the electrochemical behaviour and the double layer formation [80]. This influence is more dominant in ILs where the ionic medium is entirely interactive. Hall and his coworkers showed that the nature of ILs has an enormous impact on the capacitive behaviour of a well-defined mesoporous carbon [81]. They emphasised that the presence of an ether bond can significantly improve the capacitive behaviour. Figure 6 compares the specific capacitance and impedance spectra of similar ultracapacitors in common ILs. High-frequency impedance spectra show that the mechanisms, particularly charge transfer, are significantly different in addition to the value of specific capacitance.

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Figure 6. (a) Impedance spectra of a carbon electrode in various ILs, and (b) specific capacitance as a function of discharge current. Reproduced with permission from Ref.[81]. Copyright 2013, American Chemical Society.

Graphene and similar carbonaceous materials are typically covered with various functional groups. In supercapacitors, such functional groups contribute to the overall capacitance by pseudocapacitance as a result of the side faradaic processes [82]. However, this is not necessarily the case in ILs. Molecular dynamics (MD) simulations suggested that functional groups, particularly hydroxyl groups, prevent the IL ions to interact directly with the electrode surface [83]. When the IL anion is repulsed by the functional groups, creates a dense layer by further interaction with its native cation. In general, while functional groups are advantageous for the supercapacitors in conventional electrolytes, they can be harmful in ILs.

Experimental data also proved the harmful effect of oxygen functional groups on the capacitive Page 14 of 72

behaviour in ILs [63]. Nevertheless, this is not a general behaviour for all functional groups, as the presence of chlorine-based functional groups has a protective effect to achieve a better cyclability. The influence of oxygen functional groups is less at slow scan rates, indicating that the IL ions can interact at the electrode/electrolyte interface through a rate-determining process (i.e., probably the chemical interaction with the functional groups).

The real mechanism of the interaction of IL ions with carbon has not been fully understood yet. There are also controversial experimental data reported in the literature, probably because of different types of carbon and functional groups, which are labelled similarly. For instance, a porous graphene nanosheet heavily functionalized by oxygen groups has been reported to deliver a specific capacitance of 330 F g–1 in an IL [84].

It is well known that altered sp2 hybrid of carbon on curved graphene is electrochemically more reactive than flat hexagonal arrangement [85]. Molecular dynamic simulations revealed that curvature of carbon nanotubes in an IL-based supercapacitor results in the contribution of the socalled quantum capacitance in addition to the double layer capacitance [86]. The importance of functional groups or chemically active surface has shifted the attentions from graphene to graphene oxide (or its reduced form) for the preparation of IL-based supercapacitors [87]. Even functional groups are now considered as a part of the material structure in classification, and these materials have shown promising electrochemical performance [63,83-84,88-89]. Graphene oxide is intrinsically covered by various functional groups; while electronically active π electrons are engaged in the chemical bonds resulting in poor conductivity. Reduced graphene oxide is something between graphene and graphite oxide; the majority of functional groups have been removed resulting in an improved electrical conductivity, but still, it is far from pure carbon. The interesting point about graphene oxide or its reduced form is that they can have a wider stable potential window even in aqueous solutions [90]. Therefore, they are promising candidates for high voltage performance in ILs [91].

In a similar fashion, adding metal oxides can improve the interaction of carbon electrode with the IL electrolyte, and thus, enhancing the supercapacitor performance [92]. Furthermore, the existing functional groups can be bonded with the IL for the fabrication of G/IL nanocomposites, which have a better integrity with the electrolyte, and then, better supercapacitor performance [93]. This job can also be done by an organic promoter mediating the interaction of the IL ions with the Page 15 of 72

carbon atoms [94].

Although ILs are electrochemically stable in the absence of any destructive chemical interaction of the IL ions at the electrode/electrolyte interface, structural changes upon cycling is still an important issue, which should be carefully considered. It has been reported that the capacitance retention of a sponge graphene in an IL is 90% after 10,000 cycles, but the same electrode retains 98% of its capacitance in H2SO4 [95].

In addition to the surface functional groups, doping the carbon increases its electrochemical activity to participate in faradaic reactions along with the capacitive behaviour. High specific capacitance over a wide potential window of ILs can result in extremely high energy and power densities. It has been reported that a nitrogen-doped graphene oxide can deliver a specific capacitance of 764 F g–1 over almost 5 V [96]. This capacitive performance provides specific power and energy of 6,525 W kg–1 and 245 Wh kg–1. Although the electrolyte can be stable (not subject to decomposition) over a wide potential window, the capacitive behaviour naturally becomes less defined at extreme potentials [96-97].

Diamond-coated silicon nanowires have recently attracted considerable attention as a promising candidate for the supercapacitors in ILs, because of excellent cyclability [98-100] and thermal stability [27,37]. Quite interestingly, this supercapacitor can perform over one million charge/discharge cycles in ILs, while still in the mode of capacitance retention [101]. Etching of silicon does not allow acceptable performance of the silicon-based supercapacitor in an aqueous solution, but silicon displays a good electrochemical stability in the IL media [102].

3.2. Pseudocapacitive Supercapacitors

Pseudocapacitor refers to a type of supercapacitor in which a significant contribution to the electrochemical response is provided by the so-called pseudocapacitance. As the name suggests, pseudocapacitance is not a real capacitance as a result of the double layer charging; instead, multiple redox systems display an electrochemical behaviour resembling the capacitive behaviour. The key difference is: while the characteristic feature of a double layer supercapacitor is to store energy by charge separation precisely similar to the classic electrical capacitors, pseudocapacitors store energy by storing charge exactly similar to the battery systems. Page 16 of 72

The mechanism of pseudocapacitive behaviour is completely different from that of double layer charging, though the electrochemical responses are similar. Figure 7 demonstrates the charge/discharge of a FeOOH pseudocapacitor in an IL electrolyte. Since the IL cation is intercalated into the lattice, the size and geometry of the organic cation should match the open channels of the lattice. In comparison with the intercalation of simple inorganic cations, the IL organic cations are larger with an asymmetrical charge distribution. The latter dictates that the solid-state diffusion is highly dependent on the orientations.

Figure 7. The schematic diagram of the proposed charge storage mechanism of the FeOOH electrode in the EMI-TFSI IL. Reproduced with permission from Ref.[216]. Copyright 2016, Royal Society of Chemistry.

RuO2 was the first example of pseudocapacitors and still the best material delivering a specific capacitance of over 800 F g–1, though suffering from the severe disadvantage of cost. This pseudocapacitor can well perform in ILs too [103], but the advantages of IL cannot compensate the cost issue of ruthenium oxide. The first possible alternatives to RuO2 are similar transition metal oxides.

MnO2 is low-cost and one of the most common electroactive material due to its versatility in the electrochemical behaviour, which can make it a potential battery or supercapacitor material [104105]. Following its popularity, the pseudocapacitive behaviour of manganese oxide has also been studied in ILs [106-109]. Although such pseudocapacitors can operate in a potential window of about 3 V, the specific capacitance is noticeably low, ca. 100 F g–1. Several factors are responsible for the weak performance of pseudocapacitors in IL. Studies of the pseudocapacitive behaviour of MnO2 in IL by various techniques such as X-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and electrochemical quartz crystal microbalance revealed that the large cations of IL electrolytes only adsorb on the electrode surface and do not penetrate into the manganese oxide Page 17 of 72

lattice structure [110-112].

Although ILs are good ionic conductors, they do not provide ionic conductivity for the electroactive films, due to poor wettability. ILs are not widely dispersed within the solid porosity as conventional electrolytes do. For similar reasons, insertion/extraction redox is weaker. The conductivity of these metal oxides is always an issue, and the wettability of electrode materials in ILs controls the charge transfer resistance too. All these together make the conductivity a serious challenge for the IL-based pseudocapacitors. Many of these pseudocapacitors suffer from a huge iR-drop [113-115]. This is the reason that gold modification can significantly improve the pseudocapacitive behaviour in ILs [106,116].

Another approach is to geometrically design the electroactive material nanostructure to match the interacting ions [117]. This can lead to pseudocapacitance of MnO2 with a specific capacitance of 147 F g–1 and a specific energy of 163 Wh kg–1 [117]. Modifying MnO2 by Au and adding an organic solvent to the IL (reducing its viscosity) can enhance the specific capacitance of the supercapacitor to reach 523 F g–1 over a 3-V potential window [116].

Conducting polymers are also promising candidates for supercapacitors [118], as they are fairly conductive, flexible, easy to produce and cast, and cheap. To achieve acceptable pseudocapacitive performance, it is beneficial to increase the surface area, which is not a problem, as nanostructured conducting polymers can be readily synthesised and are indeed very common in the electrochemical systems [119]. However, the main drawbacks of conductive polymers are huge volume and structural changes upon cycling, which are more severe for the nanostructured polymers. Fortunately, a nano-architecture can provide enough space for the intrinsic volume expansion of conducting polymers.

Conducting polymers can be synthesised in ILs, and usually, display well-defined redox systems and pseudocapacitive behaviour in ILs [120-123]. There is also an opportunity to add the IL ions to the polymer backbone during the electropolymerization [124]. Quite interestingly, conductive polymers have good stability in ILs [5]. For instance, poly(3,4-ethylenedioxythiophene) (PEDOT) pseudocapacitor has been reported to have a 80% capacitance retention after 400,000 cycles [125]. The interesting feature is that flexible pseudocapacitors can be feasibly constructed by conductive polymers in ILs [126-129].

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Hybrid composite of conductive polymers, graphene, and IL can be a good structure for the supercapacitors by gaining high surface area, good ionic compatibility, and faradaic redox process. These supercapacitors can have a high specific capacitance (e.g., 662 F g–1) and also acceptable cyclability [130].

4. Liquid Electrolytes 4.1. IL Electrolytes

A unique feature of ILs, which is frequently repeated in the literature, is the possibility of having almost an unlimited number of ILs. In practice, only a few ILs are used for various applications, particularly in the electrochemical systems. Although there are numerous choices of anions and cations, it is not easy and cost-effective to produce them. On the other hand, most of the ILs are liquid only by name; as high viscosity makes them unsuitable for most applications.

It is well known that supercapacitors are strongly dependent on the anions and cations employed, according to their charges, sizes, and shapes. However, the role of ions in the IL-based supercapacitors is much more complicated due to their direct effects on the physical properties of the electrolyte [131]. Thus, the structure of the double layer can be significantly different [132]. The specific capacitance of supercapacitor is significantly changed by the choice of anion [133-135] or cation [136-137]. For instance, anions in an aqueous electrolyte can affect the double layer structure, but the influence on the electrolyte conductivity or viscosity is negligible. Figure 8c shows how changing the anion can significantly modify the conductivity and viscosity of the electrolyte. The same effect is observable for the specific capacitance and the charge transfer resistance of various ILs (Figure 8a and 8b. This vividly suggests that the choice of IL is as important as the choice of electrode material in designing the IL-based supercapacitors.

Figure 8. (a) Cyclic voltammograms and (b) impedance spectra of a carbon electrode in various ILs (different anions for the same cation). (c) The differences in electrical conductivity and viscosity of the corresponding ILs. Reproduced with permission from Ref.[133]. Copyright 2014, Wiley-VCH.

Even when considering the influence of the ion size, the size and charge distribution of the IL Page 19 of 72

anions are much different in comparison with those of solvated anions in conventional electrolytes [74,138-139]. Therefore, the capacitive performance heavily depends on the choice of both anion and cation [140].

Although limited to a few works, there are some reports on utilising novel ILs for the fabrication of various types of supercapacitors [141-143]. Sato et al. synthesised ILs based on a new aliphatic quaternary ammonium cation containing a methoxyethyl group attached to the nitrogen atom, which showed better double layer capacitor performance as compared with counterpart ILs [28]. Devarajan et al. synthesised an oxygen containing spiro ammonium salt, whose electrolyte in acetonitrile showed a wider stable window for a double layer capacitor [58]. However, these are typical ILs introduced in addition to the popular IL electrolyte rather than tailored ILs by considering the supercapacitor mechanism.

4.2. Manipulating the IL Electrolyte

Despite the noticeable advantages of IL electrolytes, there are always rooms for modifications. The simplest way is to mix two ILs to have the same cations but having two types of anions with different sizes and geometries. Lian et al. investigated the capacitive behaviour of an onion-like carbon in two similar ILs and their mixture both experimentally and computationally [144]. They concluded that the specific capacitance is higher in the mixture as compared with the individual IL electrolytes, though, the optimum ratio should be dependent on the ions. Figure 9 compares the distribution of ions at the electrode surface in two pure IL electrolyte and their mixture. Having smaller anions results in the formation a thinner double layer, which is beneficial for a higher specific capacitance. However, the surface area of the electrode which is in direct contact with the ions is not necessarily higher for small anions. In the case of mixed IL electrolytes, the presence of large anions among the small anions disturbs the uniformity of the charge distribution and can result in a thin double layer but with a maximised area of contact at the electrode/electrolyte interface.

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Figure 9. Distributions of cations (red line for EMI) and anions (green line for BF 4 and blue line for TFSI) in EDLs of pure and mixed RTILs near a positive surface with ψ − PZC = 1.5V: (a) x = 0, (b) x = 0.25, and (c) x = 1. Inserts are schematics of the EDL structures. Reproduced with permission from Ref.[144]. Copyright 2016, American Chemical Society.

Eutectic mixtures of protic ILs are also among possible choices for electrolytes of supercapacitors because they can have lower viscosity and are usable in a wide range of temperatures (e.g., from – 50 to 80 °C) [145-148]. The eutectic mixtures are normally made of the same anion while the IL cations are different. A common example is FSI anion with pyrrolidinium and piperidinium cations, which have the same molecular formula. The key point is the difference in the molecular structure of cations avoiding the formation of an ordered lattice.

Another approach is to mix ILs with common organic solvents. While they still have a wide stable potential window for supercapacitors [36,131,137,149-154], the viscosity is reduced, and conductivity is increased. This can improve the conductivity by over one order of magnitude [149]. Obviously, this improvement comes at the cost of sacrificing the wide stable potential window of Page 21 of 72

IL, but still wide enough for the practical performance. It should be taken into account that this is not a general approach, and the mixture should be specifically designed depending on the electroactive material and electrolyte. In some cases, pure ILs can provide a better capacitive behaviour in comparison with alternative IL/solvent mixture [152,155].

A similar phenomenon is observed when adding ionic salts to ILs, as the corresponding ions can improve the compatibility of carbon and IL at the electrode/electrolyte interface [156]. As a matter of fact, introducing various types of ions (e.g., transition metal cations) can alter the ionic arrangement within the IL to improve the capacitive performance [157]. Adding carbon nanotubes to IL can create a nanofluid, which can improve the 5 V performance of supercapacitors [158]. The role of carbon nanotubes is not clear yet, but it is believed that they might serve as electrical shortcuts facilitating the charge transfer. This is similar to the role of carbon nanotubes at diffusion layer facilitating the electrodeposition [104-105]. It has also been reported that some carbon nanostructures undergo irreversible reactions in pure IL electrolytes [159]. Even in a mixture of IL and acetonitrile, excess contraction of IL can reduce the specific capacitance, and a dilute solution of an IL can be the optimum condition for the capacitive performance.

Since the electrical double layer quickly forms on the electrode surface and remains electrochemically inert, it can be a subtle idea to choose electrochemically active species. Following the double layer charging, these electroactive species can participate in the Faradaic reactions to contribute to the overall capacitance [160-162]. The most common example of these systems is employing hydroquinone in aqueous electrolytes. This approach has also been employed in IL electrolytes too [139,162]. The first example of redox-active IL electrolytes was using a Cu(II)containing EMI-BF4 electrolyte in a double layer supercapacitor [162]. In this case, the specific capacitance was doubled in the presence of Cu2+ in the IL electrolyte due to the reversible redox system of Cu/Cu2+ as confirmed by the electrodeposition/electrodissolution of Cu at/from the electrode surface.

Xie et al. used the redox system of ferrocene in an imidazolium IL [139]. Figure 10 illustrates how the ferrocene redox system contributes to the supercapacitor storage capacity. The ferrocenyl anions are the charged species attaching at the electrode surface. Upon the double layer charging, which is the fast reaction, the ferrocene redox system is indeed similar to a battery material delivering a flat plateau as result of its straightforward Faradaic reaction. The interesting point is that the redoxPage 22 of 72

active ions can be chosen in accordance with the IL structure to optimise the electrolyte properties such as viscosity and ionic conductivity.

Figure 10. (a) Galvanostatic charge–discharge profiles for the RILSC with 80 wt.% of [EMI][FcNTf] in acetonitrile at 25 °C and a 2 mA current. (b) The charging potential profile of the positive electrode showing the processes during charge. Reproduced with permission from Ref.[139]. Copyright 2016, Elsevier.

4.3. Ionic Liquid Crystal Electrolytes

In discussing the role of ILs in lithium-ion batteries, it has been recently described that ionic liquid crystals (ILCs) might be superior electrolytes in electrochemical systems [1]. While the ionic conductivity is high enough, the ordered crystal structure provides straightforward channels for the diffusion of counterions. Sasi et al. have recently examined an ILC for potential applications in supercapacitors [163]. Figure 11 displays the structure of an imidazolium-based ILC at different concentrations forming various structure. The ionic conductivity is directly dependent on the crystal structure formed by the ILC concentration. In the optimum concentration (corresponding to Figure 11b and 16c), the highest ionic conductivity in the range of 20–30 mS cm–1 can be obtained. The supercapacitor performance was comparable with those of the IL electrolytes (Table 1). Nevertheless, the ILC structure provides an exceptional flexibility for designing the diffusion of electroactive species. This idea is still in infancy, and profound investigations are required to develop this possibility.

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Figure 11. Concentration dependency of an midazolium-based ionic liquid crystal formed in acetonitrile. AFM images of (a) Nematic batonnet phase (concentration: 0.1 M, conductivity: 3-6 mS cm–1), (b) columnar phase (concentration: 0.3 M, conductivity: 19.4 mS cm–1). PLM images of (c) smectic A phase (concentration: 0.4 M, conductivity: 33.10 mS cm –1), and (d) smectic C phase (concentration: 0.6 M, conductivity: <40 mS cm–1). Reproduced with permission from Ref.[163]. Copyright 2016, American Chemical Society.

4.4. ILs As Electrolyte Salts

Contrary to the molecular solvents, ILs are not just the medium in which the electroactive species transport, but the IL ions are also electroactive species too. In fact, ILs can be used as ionic salts similar to the conventional salts in electrolytes. The specific capacitance of a CuO/RuO 2 pseudocapacitor in 1 M IL aqueous solution has been reported to be over 400 F g–1 [164].

Wang et al. reported that utilising an IL salt in the aqueous electrolyte of a Li-O2 battery result in the complexation of the IL organic ions with the Li ions [165]. These Li-based complexions facilitate the reversible Li/Li+ redox system at the metallic Li anode/electrolyte interface, and consequently, the battery cyclability is improved. This is indeed ascribed to the versatility of the organic ions introduced into the electrolyte by the IL to subtly surround the Li ions, which are Page 24 of 72

indeed the point of interest. Chen and Hempelmann employed a concentrated aqueous electrolyte of BMI-Cl in a flow battery and showed that the stable electrochemical window could be as wide as 3 V even in an aqueous electrolyte made of an IL [166]. This introduced a new set of opportunities for utilising aqueous IL electrolytes. Quite recently, Hu et al. proposed that protic ILs can be used as electrolyte salts in aqueous solutions [167]. The electrochemical behaviours of a polyoxometalate-carbon supercapacitor in aqueous BMI-HSO4 and H2SO4 electrolytes were very similar, but the substantial difference was reported for the cyclability of this electrode in these electrolytes. While the supercapacitor lost over 70% of its capacitance after 2,500 cycles, the capacitance retention was about 90% in the aqueous IL electrolyte.

In an aqueous electrolyte, the comparison of the ILs should be made based on the corresponding hydrated ions. Smaller ions cause stronger polarisation, and thus, attracting the surrounding water molecules. This results in quite large hydrated ions, which is accompanied by weaker supercapacitor performance [168]. On the other hand, ions with longer alkyl chains (i.e., larger ionic radii) generate smaller hydrated ions, which are more suitable for the supercapacitor applications [113]. This is similar to the small sizes of solvated Na+ and K+ as compared with Li+, which have made sodium-ion and potassium-ion batteries [169] practical alternatives to the lithiumion batteries. However, note that Li-ions are still advantageous in the solid-state diffusion, which is absent in the double layer supercapacitors.

This is a new application of ILs or more precisely ionic organic salts (since liquidity is not a factor here) for the preparation of ionic electrolytes for the electrochemical energy storage. In this case, the IL cost is not a major issue, as a small amount is required and the purity is not of vital importance. Albeit, there will be no advantage regarding the electrochemical window. In comparison with the inorganic salts, there can be an unlimited number of ionic organic salts for designing new supercapacitor electrolytes.

5. Solid and Semi-Solid Electrolytes 5.1. Ionic Polymer Electrolytes

ILs are normally highly viscous, and thus, far from ideal liquid electrolytes. On the other hand, there is a growing demand for non-liquid electrolytes due to safety issues of possible cell leakage. ILs can be polymerised to form ionic polymers, which have the ionic mobility of ILs to some Page 25 of 72

extent, while they are not liquid. The term ionic polymer is ambiguous, as it was historically referred to ionomers, which are partially ionic. The family of poly(IL)s had a long history when the corresponding ionic organic salts were subject to radical polymerization in the 1970s. Back in time, they were called polyelectrolytes with no reference to the IL monomer, as the monomers were indeed a class of organic salts with no attention to the liquidity. In the recent literature, ionic polymers made of IL are called polymerised IL or poly(IL)s. Both are abbreviated as PIL, which is also the commonly accepted abbreviation of protic IL too. However, neither is scientifically accurate, as the monomer is not necessarily an IL, and more importantly, in a vast range of applications of polymerised ILs, there is no requirement for the liquidity of the monomer to be highlighted in its name. In any case, there is no unified terminology in this context.

In addition to the possible application of polymerised IL as a polymer electrolyte as will be reviewed in the following Section, they can be the functionalizing agents in the electroactive material composite. For instance, polymerised IL can fairly functionalize graphene (as IL can be polymerised by grafting from graphene). The result is a significant improvement in the capacitive performance [170]. In fact, the compatibility of the polymerised IL and the IL electrolyte facilitate the corresponding interactions at the electrode/electrolyte interface.

5.2. Gel Electrolytes

In addition to the polymerization of ILs, it is also possible to immobilise ILs on a matrix to avoid the liquid leakage problem while still having the ionic mobility. Obviously, this reduces the ion mobility and consequently ionic conductivity [171]. Figure 12 illustrates how an IL is cross-linked within a polymer matrix. As a result, the ionic conductivity is reduced, but the electrochemical stability is enhanced.

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Figure 12. (a) The cross-linked structure between poly-4-vinylphenol (c-P4VPh) and 1-ethyl-3methyl

-TFSI). (b) The dependency of the ionic

conductivity to the IL/polymer ratio. (c) The stable electrochemical windows of the EMI-TFSI and a series of the IL/polymer electrolytes. Reproduced with permission from Ref.[171]. Copyright 2016, Royal Society of Chemistry.

A film electrolyte made of 70% IL and 30% epoxy resin has a conductivity of about 1 mS cm –1 [172]. Gel polymer electrolytes refer to a polymeric matrix entrapping a liquid electrolyte therein. Although gel polymers display solid-like behaviour, they suffer from poor thermal and mechanical stability. The typical viscosity disadvantage of IL is indeed an advantage here, as can contribute to the mechanical integrity of the gel polymer. This is beneficial for better bending capability of flexible supercapacitors and fabrication of yarn supercapacitors [173]. On the other hand, tunable hydrophobicity/hydrophilicity of ILs can assist in designing a more stable gel polymer interaction.

Since the seminal works of Fuller and his coworkers on gel polymer electrolytes based on ILs [174Page 27 of 72

175], numerous works have been devoted to the application of IL-based gel polymer electrolytes in various electrochemical systems, particularly lithium-ion batteries [176-181]. It has become a likely choice in designing flexible solid-state supercapacitors [181-185]. The presence of ILs within the polymer matrix improves the thermal stability and increases the stable electrochemical window, which is normally around 3.5–4.0 [186]. Several IL-based gel polymer electrolytes have been tested for supercapacitors based on various polymeric matrix such as ppoly(acrylonitrile) [187-189], poly vinylidene fluoride-co-hexafluoropropylene [188-189], polyvinylidene fluoride/polyvinyl acetate [190], poly(ethylene oxide) [132,188-189], poly(vinylalcohol) [189], poly(methylmethacrylate) [189], and poly(tetrafluoroethylene) [36], chitosan [191], etc.

Since the ionic conductivity, which controls the electrochemical performance, is defined by the mobility of the IL ions, the choice of IL plays a crucial role in the materials properties and supercapacitor performance of gel polymer electrolytes. Figure 13 compares similar gel polymer electrolytes prepared by blending a polymeric ionic liquid, poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide

(PIL-TFSI),

with

four

similar

ILs,

1-butyl-1-

methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14-TFSI) (labeled as IL-b-PE1), 1butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14-FSI) (labeled as IL-b-PE2), 1-(2hydroxy ethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HEMI-TFSI) (labeled as IL-b-PE3), and 1-Butyl-1-methylpyrrolidinium dicyanamide, (PYR14-DCA) (labeled as IL-b-PE4). In this comparison, the ILs are sharing the same cation or anion. The importance of the polymer and IL compatibility [192] can be seen in the case of PYR14-DCA where the DCA anion. While the conformational flexibility of the fluorinated structure of TFSI and FSI provides a good opportunity for perfect blending with the ionic polymer, the rigid structure of DCA anion results in the nontransparent membrane (Figure 13a). It is just a matter of compatibility with the polymer matrix, as DCA is indeed a good choice of IL anion for the supercapacitor applications [6].

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Figure 13. (a) Chemical structure of PILTFSI and different ILs (PYR14TFSI, PYR14FSI, PYR14DCA and HEMi-TFSI) and the respective photo for each membrane. The mass ratio of ILs to PILTFSI for IL-b-PE1, IL-b-PE2 and IL-b-PE3 is 60:40 whereas for IL-b-PE4 is 30:70. (b) Arrhenius plots of ionic conductivities for different free standing IL-b-PE membranes at different temperatures from 30 to 120 °C. (c) The stable electrochemical windows at 25 °C and 10 mV s –1. (d) Charge/discharge profiles recorded at 2 mA cm–2 Reproduced with permission from Ref.[193].

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Copyright 2016, Elsevier.

The key point is the substantial differences in the ionic conductivity (Figure 13b) and the stable electrochemical window (Figure 13c) of these similar gel polymer electrolytes. Consequently, the supercapacitor performance is highly dependent on the choice of IL in the gel polymer electrode (Figure 13d). Not only the specific capacitance but also the iR drop observed at the beginning of the discharge profiles (obviously as a result of lower ionic conductivity) are directly controlled by the IL utilised in the gel polymer electrolyte. Similar behaviour can be seen for the charge transfer resistance at the electrode/electrolyte interface too [193]. These strong dependencies clearly indicate that the results reported in the literature should not be simply generalised for the gel polymer electrolytes, as the electrochemical performance can be highly tuned by choice of IL and polymer.

In portable electronic devices, leakage of the liquid electrolytes is now a major challenge and safety issue. Thus, practical demand for solid electrolytes is rapidly increasing, though it has been limited by the poor performance of solid electrolytes (such as ionomer-based ionic polymers; e.g., Nafion [194-197]). IL-based solid electrolytes have been successfully employed for the preparation of allsolid-state micro-supercapacitors [198-199], which are flexible. Although IL-based gel polymer electrolytes have good stability over a large potential window, ca. 4.4 V [200], the specific capacitance is still lower than the liquid electrolytes due to lesser direct interface with the internal porosity.

A common problem of the solid electrolytes is weak interaction with the electroactive material at the electrode/electrolyte interface resulting in a high charge transfer resistance. However, the flexibility of polymer gels can reduce the mismatch between the electrode and electrolyte. This influence is more significant for the 3D porous electrode materials, which have a rough interface with the solid electrolyte [201].

Polymer electrolytes can be prepared in ILs, which are also called ion gels [202-204]. This class of polymer electrolytes is indeed polymer-in-salt electrolytes [205]. The electrochemical performance of such ion gels is not straightforward, as highly conductive ion gels do not necessarily deliver the best performance since the compatibility between IL and polymer matrix is more important [193]. Adding ionic salts to the polymer gel can significantly improve the electrochemical performance, as the pseudocapacitance contribution is increased [206]. Page 30 of 72

Most of the common ILs (at least in the electrochemical applications) are based on large organic cations and small inorganic anions. Since the IL properties are mainly dependent on the organic cation, less attention is usually paid to the inorganic anion. Since the anion plays a crucial role in the ionic conductivity of the polymer matrix, the impact of the inorganic anion can be enormous. For example, Liew et al. compared the performance of similar double layer supercapacitors utilising PVA-based gel electrolytes in the presence of a series of BMI ILs [207]. Although the capacitive behaviours of the chloride and bromide systems were identical, the specific capacitance of the iodide-based supercapacitor was almost three times higher. Similar differences were also observed in the charge transfer resistances of these systems. The cycling capabilities of these systems were also substantially different.

Ketabi et al. compared a series of protic ILs for the fabrication of proton conducting polymer electrolytes, which were tested for both double layer and pseudocapacitive supercapacitors [208]. For similar ILs with hydrogen sulphate anions, the specific capacitance could be almost doubled or tripled for the carbon and RuO2 electrodes, respectively. This indicates the importance of the choice of ions in forming the electrode/electrolyte interface, particularly when an electrochemical redox is involved.

In gel polymer electrolytes, the IL viscosity is not an issue as the ultimate intention is to immobilise the liquid throughout the polymer matrix. Therefore, highly viscose ILs, which are not suitable as electrolytes, can also be employed for this purpose. Despite the immobility of the IL, the ionic mobility should be high enough to provide strong electrochemical performance. However, the rate capability, and consequently, the power density, of supercapacitors based on gel polymer electrolytes is lower than those employing liquid electrolytes. Hence, a key focus is on the improvement of the ionic mobility within the IL-based gel polymer electrolytes.

A practical approach is to utilise plastic crystalline succinonitrile as a solid solvent/plasticiser [209211]. Succinonitrile has a high dielectric constant of 55 at room temperature because of its polar nature, and thus, can well dissociate the ionic salts to provide a high ionic conductivity. In comparison with the rigid crystals, a plastic crystal has a short range rotational disorder and long range transitional order. Therefore, a greater plasticity and enhanced diffusivities can be expected [212]. As a result, succinonitrile/Li salt plastic crystal electrolytes exhibit a high ionic conductivity (ca. 10–3 S cm–1) but suffer from poor mechanical stability because of plastic and liquid-like behaviour. Immobilising the corresponding plastic crystal within a polymer matrix can enhance Page 31 of 72

their mechanical properties during the electrochemical performance [213-214].

Hashmi et al. designed a gel polymer electrolyte based on a poly(vinylidene fluoride-cohexafluoropropylene) (PVdF-HFP) matrix in which 75% succinonitrile and an IL instead of the common Li salt was incorporated [213]. Owing to the non-ionic nature of succinonitrile, the electrolyte composition should be optimised to achieve strong electrochemical performance. Pandey et al. synthesized a gel polymer electrolyte composed of PVdF-HFP/Sn/BMI-BF4, which showed a high ionic conductivity over a broad range of temperature from 5 x 10–4 S cm–1 at –30 °C to 1.5 x 10–2 S cm–1 at 80 °C, which is at the level of the pure IL electrolyte [215]. A double layer supercapacitor made of activated carbon and this gel polymer electrode could deliver a specific capacitance of 165 F g–1 at 1.5 A g–1 and retain 80% of this value after 10,000 cycles.

Forming an ionogel based on ILs does not necessary require a polymer matrix. A gel electrolyte can be simply prepared by mixing IL and fumed silica. Shen et al. assembled an all-solid-state flexible supercapacitor utilising polyaniline-derived carbon nanorods as the cathode and FeOOH as the anode while the electrolyte was an ionogel of EMI-TSFI [216]. The flexible supercapacitor had an energy density of 1.44 mWh cm–3 (considering the volume of the whole cell) at 200 °C with an excellent bending capability at high temperature without losing its original performance.

Inorganic materials can be introduced to the polymer matrix to keep the skeleton firm. This may reduce the gel polymer electrolyte flexibility, but improves the capacitive behaviour. Since the diffusion pathways are kept open by the inorganic pillars, the electrochemical behaviour is more well-defined while the specific capacitance is increased [217].

5.3. Poly(Ionic Liquid) Nanocomposites

Application of the polymerised ILs is not limited to the electrolytes of supercapacitors, as they can also be utilised as a part of the electrode material. A key obstacle in using ILs as electrolytes is their poor wettability, which is essential for the efficient formation of electrode/electrolyte interface for the supercapacitor performance [40,218]. The presence of ions in the PILs can assist in direct interaction with the electrolyte [219]. For the case of reduced graphene oxide, the polymer nanocomposite can deliver higher specific capacitance over 220 F g–1 [220].

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Many supercapacitors are based on the idea of merging ultracapacitors and pseudocapacitors by compositing metal oxides and carbon to gain the redox contribution of the former and high surface area of the latter. This increases the surface accessibility of the redox material too in favour of better rate capability. This approach is also beneficial for the IL-based supercapacitors. Mediating the formation of MnO2 on graphene fabricate a supercapacitor with a specific capacitance of 411 F g–1 with 85% capacitance retention over 10,000 cycles [221]. This performance is somewhat between those of carbon-based ultracapacitors and MnO2-based pseudocapacitor in terms of specific capacitance and cyclability.

A major drawback of 2D nanomaterials such as graphene for supercapacitor applications is their strong tendency for agglomeration, and thus, the final electrode does not have a high surface area. A possible approach is to utilise spacers, which are small nanoparticles which separate the graphene sheets. Among possible choices, poly(IL)s can provide a better interfacial interaction of the electrode and a similar IL electrolyte [219]. As a result, the corresponding electrode in a solid-state assembly could achieve a specific capacitance of 185 F g–1 over a wide potential range of 3.5 V [219].

5.4. Graphene Oxide Membranes

The layered structure of graphitic materials is well suitable for the design of membranes [222]. The excellent potential of graphene for being functionalized with various groups provides an opportunity to control the chemical selectivity of these membranes, and thus, graphene membranes have been recently employed in various electrochemical systems. During the production of graphene, its surface is functionalized by various groups, which reduce the membrane conductivity. This is the reason that the graphene oxide is reduced to improve the conductivity. This reduction is not normally completed, and thus, the term 'reduced graphene oxide' is scientifically more accurate. This reduction process increased the graphene conductivity, which is which for the fabrication of electrode materials. However, when designing a graphene-based membrane, the ionic functionality is more important than electrical conductivity [223-224]. Such functional groups can chemically bond with ionic materials to fabricate ionic membranes. In this direction, ILs are excellent candidates due to the versatility of IL ions, which can be well trapped within the graphene layers. IL-based graphene membranes have been used for the fabrication of various supercapacitors [225226]. Owing to the flexibility of graphene membranes, they can serve as solid electrolytes in flexible supercapacitors. On the other hand, the thermal stability of graphene and the trapped ILs Page 33 of 72

can make these supercapacitors suitable for performance under extreme temperatures such as 200 °C [226]. However, the primary pitfall of graphene membranes is the instability of functional groups. Whilst the excess amount of the functional groups significantly reduce the material conductivity to be turned into an insulator, their removal facilitate the release of the ILs trapped within the matrix in the absence of chemical linkage.

6. Other Applications 6.1. ILs As the Electrode Modifiers

Similar to polymerised IL, ILs can also be used as a part of the electroactive material in supercapacitors. Obviously, they are not in liquid form, but the ions are somehow adsorbed within the matrix of an electroactive material. The presence of such mobile ions can facilitate the process of ion exchange in supercapacitors. Functionalized nanomaterials with ILs can improve the electrode wettability owing to the hydrophilicity of outer ions [227-232]. Cobalt hydroxide, which is a promising candidate for pseudocapacitors, showed an excellent pseudocapacitive performance with a specific capacitance of 859 F g–1 when modified by an IL [233]. Functionalizingions provide a better opportunity for faster diffusion and easier adsorption of counterions within the Co(OH)2 structure.

Figure 14 compares the electrochemical behaviour of Co(OH)2/IL electrode in a conventional alkaline medium. It is evident that IL has improved the redox system of Co(OH)2. An interesting feature is that both redox couples have shifted towards less positive potentials, indicating a noticeable reduction in the overpotentials. This is ascribed to a lower charge transfer resistance at the electrode/electrolyte interface, as can be judged from the impedance spectra (Figure 14c). The specific capacitance of Co(OH)2 has been doubled because of the role played by the IL (Figure 14b). Another important issue is the natural increase in the charge transfer resistance due to the surface restructuring or the formation of blocking films on the electroactive material in the course of cycling. This phenomenon is almost absent when the IL has modified the CO(OH)2 surface (Figure 14d).

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Figure 14. Cyclic voltammogram and charge/discharge profiles for Co(OH)2/IL and Co(OH)2/IL in a 3 M KOH aqueous electrolyte. The scan rates are 50 mV s–1 and 1 A g–1, respectively. (c) Impedance spectra of the fresh electrodes and after 1000 charge/discharge cycles. (d) A schematic of the role of IL intermediating hydrogen adsorption on Co(OH)2 lattice. Reproduced with permission from Ref.[233]. Copyright 2013, American Chemical Society.

Similar to wettability, the functionalized ions have straightforward interactions with the electrolyte leading to a higher electrochemical stability. A pseudocapacitor made of ZrO2 modified by an IL showed a good performance over a wider potential window [234]. This is also accompanied by better thermal and mechanical stabilities. Bag et al. reported that functionalizing reduced graphene oxide with hydrophilic ILs can widen the stable electrochemical window in an aqueous electrolyte to 2 V [235]. This wrapping conductive agent somehow facilitates ion exchange process [236].

In a similar fashion, ILs can functionalize carbon materials for double layer supercapacitors. This improves the wettability of carbon nanomaterials, as the anions adsorbed on the surface serve as intermediator for the interaction with electroactive species [236]. A composite made of CNT (originally having a specific capacitance of 20 F g–1), activated carbon (originally having a specific capacitance of 90 F g–1) and an IL delivered a specific capacitance of 188 F g–1 [237]. The role of the IL in this composite structure is different from the functional groups of graphene, which have redox systems. Instead, the native presence of IL ion within the electrode matrix facilitates interactions at the electrode/electrolyte interface.

IL somehow acts as a binder in the composite as the surface interactions are solidified [49]. Figure Page 35 of 72

15 illustrates the solidification of an IL in the carbon composite. This guarantees the mechanical stability of the electrode. On the other hand, the electrode permeability is excellent to achieve extremely high capacitance. A hybrid composite of IL/graphene/CNT delivered a specific capacitance above 200 F g–1 [49].

Figure 15. The structure of a hybrid composite of multi-walled carbon nanotubes, graphene, and an IL (BMI-TFSI). Reproduced with permission from Ref.[49]. Copyright 2012, American Chemical Society.

Due to the high viscosity and larger ions, the rate capability of the IL-based supercapacitors is usually poor in comparison with their performance in conventional electrolytes, as the diffusion is slower. Functionalizing graphene by ILs can improve the rate capability of its corresponding supercapacitor [138] because the electroactive species quickly interact with the electrode surface.

As mentioned earlier, overcoming the van der Waals forces between the graphene sheets to keep them separated by small nanoparticle is a practical approach for preserving the high surface area of graphene for supercapacitor applications. Bozym et al. used sucrose nanoparticles along with EMIBF4 for this purpose and could achieve an energy density of 13.3 Wh kg–1, which is the highest among similar systems [238].

In a similar fashion, 2D electrode materials need intercalating species to keep the nanosheets separated. For instance, the 2D ordered structure of Ti3C2Tx is transformed to a hydrogel with the intercalating species in an aqueous solution. The resulting hydrogel displays a well-defined capacitive behaviour [239]. ILs can also transform Ti3C2Tx to an ionogel with the same capacitive Page 36 of 72

behaviour but over a broad range of potential over 3.0 V [90]. In this case, the unlimited choices of the IL provide a rare opportunity for controlling the internal structure of the ionogel.

Owing to the electromechanical properties of polymerised ILs or IL-based hybrid composites, they are promising materials for fabricating actuators. Interestingly, a hybrid composite of carbon/IL can serve as both actuator and supercapacitor [21,240]. This can pave the path for a new design in innovative electronic devices.

6.2. ILs As Precursors or Soft Templates for the Synthesis of Carbon Nanomaterials

Owing to the popularity of carbon nanomaterials, various precursors have been utilised for controlling the morphology and chemical composition of the resulting carbon. In a sense, the starting compound serves as a soft template during the carbonisation process shaping the arrangement of carbon atoms and other elements acting as dopants. ILs have a versatile architecture with the possibility of including a wide range of elements. Since doping has an enormous impact on the supercapacitor performance of carbon nanomaterials, the IL-derived carbon nanomaterials have been examined as potential electrode materials in supercapacitors. Sun et al. prepared a carbon precursor by a simple reaction of phenothiazine and H2SO4 forming a protic IL [241]. The carbon electrode derived by carbonisation of this IL could deliver an extremely high specific capacitance of 440 F g–1 at low rates while preserving at least half of this value when increasing the applied current to 10 A g–1. Further carbonisation of the precursor at higher temperatures resulted in a massive decrease in the capacitive behaviour. Zhu et al. employed a series of IL-doped alkali organic salts as small molecular precursors for the preparation of porous carbon [242]. They could tune the synthesis procedure to achieve a specific capacitance of 287 F g–1 at 1 A g–1. The IL concentration can control the reaction adjusting the dopant concentration. It should be considered that the excess amount of IL can have a severely harmful effect on the synthesis resulting in a much lower specific capacitance [64]. In a similar context, Shao et al. reported that modifying the reduced graphene oxide with an IL could slightly improve the specific capacitance at low charge/discharge rates, but it had a huge impact on the supercapacitor rate capability, which significantly improved the power density [243].

In a more sophisticated approach, ILs can be employed as soft templates restructuring the carbon precursor. Chen et al. synthesised nitrogen-doped hollow mesoporous carbon spheres from Page 37 of 72

resorcinol/formaldehyde resin in a hydrothermal process in which an IL was utilised as the soft template [244]. The particle size and structure could be controlled by the IL concentration. More uniform particles tend to form in the higher concentration of the IL, though, the dependence of the specific capacitance was not linear with respect to the IL concentration. Further, the length of the IL alkyne chain played a substantial impact on the morphology.

7. Performance Under Extreme Conditions

Some applications of supercapacitors specifically need performance at high temperatures, because quick energy delivery of supercapacitors might be required in emergency situations. Conventional electrolytes are not suitable for the high-temperature performance due to instability and safety risks. Key issues are inevitable side chemical reactions [245] and thermal instability of electrolytes at high temperatures [246]. Current candidates are mainly gel polymer electrolytes, but aqueous or protic gels are subject to gradual loss of proton media [247-248].

Excellent thermal stability and inflammability of ILs have named them as promising candidates (if not the best possible choice) for the high-temperature supercapacitors [146-147,216]. Tsai et al. reported that a conventional activated carbon supercapacitor in a eutectic mixture of ILs can deliver a specific capacitance of about 180 F g–1 over a wide potential window of 3.5 V over a broad range of temperature from–50 to 80 °C [146]. IL-based ultracapacitors can well perform at 60 °C for over 20,000 cycles [201]. Zhang et al. compared the capacitive performance of a series of ILs in acetonitrile at different temperatures in the range of 20–80 °C [249]. The temperature dependency of these electrodes was straightforward with no non-linear behaviour.

An advantage of performance at high temperature is that the electrical and ionic conductivities of metal oxides and similar materials are quite good at elevated temperatures. This is also accompanied by faster diffusion. It has been suggested that some problematic electrode materials show ideal behaviour for the electrochemical energy storage at high temperatures [250]. Pseudocapacitor made of FeOOH, and an IL can have a high energy density of 1.44 Wh L–1 performing at 200 °C [216].

In a similar fashion, supercapacitors should be able to operate at extremely low temperatures. Conventional electrolytes are subject to freezing, low conductivity, and slow diffusion at low Page 38 of 72

temperatures. The problem of ILs is usually high viscosity, but still, the ionic mobility is preserved. On the other hand, it is a characteristic feature of almost all ILs, which can form supercooled liquids below their melting points. Although supercooled liquids are viscous, they can provide an ordered structure for the diffusion. An electrolyte of pyrrolidinium nitrate has a conductivity of 9 mS cm–1 at –40 °C, and the corresponding ultracapacitor (using activated carbon) could deliver a specific capacitance of 117 F g–1 at this temperature [251]. Generally speaking, conventional electrolytes can be used at temperatures as low as –25 °C, whereas ILs are not suitable at temperatures lower than 20 °C [147]. However, new designs of ILs employing eutectic mixtures have provided a new opportunity for successful performance of ILbased supercapacitors as low as –50 °C [146-147]. This suggests that ILs are among the best choices for fabricating supercapacitors for extreme temperatures, but a particular attention should be paid to tailor ILs for specific applications.

8. Summary and Outlook

ILs have enormous potentials in electrochemical systems as provide an ion-rich matrix. However, their applicability has been limited by the general perception that they are ionic electrolytes to replace the conventional electrolytes. Instead, they should be considered for the source of mobile ions, which can mediate various charge transfer processes in the electrochemical systems. The applications and performance of ILs in supercapacitors is somewhat different from similar systems such as batteries. In the case of pseudocapacitors, which are based on the redox systems similar to batteries, the major difference comes from the importance of surface reactions in a supercapacitor, while the solid-state diffusion is the key process in a battery (such as the lithium-ion battery). In the latter, the role of the external electrolyte is not as crucial as the surface reactions. In double layer supercapacitors, the formation of a double layer in an IL is significantly different from that in an aqueous medium. Therefore, understanding the surface architecture at the IL electrode/electrolyte interface is of vital importance in designing the corresponding IL supercapacitors, but our knowledge is still limited in this context.

In any case, the applications of ILs are not limited to the conventional architecture of supercapacitors. ILs should be considered as an alternative to solid electrolytes rather than the conventional electrolytes. The importance of this standpoint is that a broad range of ILs, which Page 39 of 72

have been totally excluded because of high viscosity, might be reconsidered as potential electrolytes in the realm of electrochemistry. The viscosity is indeed a privilege to avoid the problematic leakage of the electrochemical energy storage devices. This standpoint can pave the path for relevant comparisons to gain the knowledge reflected in the literature. For instance, the diffusion of IL ions within the carbon pores in an ultracapacitor resemble the diffusion of electroactive species within the solid electrode (e.g., in lithium-ion batteries), because of interactive ions diffusing through an ion-rich medium.

IL-based gel polymer electrolytes seem to be among the most promising choices for the future flexible supercapacitors. However, the current research is still focused on popular ILs whose popularities are because of meeting the requirement of standalone ILs. Moreover, owing to the time-consuming purification process, many researchers prefer to use commercial ILs rather than customising the ILs for specific systems. When ILs are employed as additives (in small amounts), the purity is not as critical as standalone IL electrolytes. In any case, it is vitally necessary to gain the versatility of ILs in design to customise the electrochemical systems under consideration. Note that it is not a matter of choice but designing the ILs for a specific supercapacitor. On the other hand, the polymer matrix can be prepared from polymerised ILs, which could have a better compatibility (not necessarily) with the IL agent.

Furthermore, ILs are potential candidates for designing supercapacitors for extreme conditions such as high- or low-temperature performance. In general, the applicability of ILs should not be limited to alternative electrolytes for replacing organic solvents. The characteristic feature of ILs is the flexibility in design due to the numerous possible anion and cation combination. This flexibility should be gained in a manner to design novel types of supercapacitors by thinking outside the box, way beyond the available cases.

In summary, the future outlines of research on ILs in designing supercapacitors can be listed as:

(i) The choice of ILs is not as simple as the conventional electrolytes. While changing the inorganic ions in the conventional electrolytes salts is accompanied by a gradual change in the electrochemical behaviour due to the size and charge density of the diffusing ion, the organic ions are too complicated as the electrochemical behaviour can be crucially changed because of their asymmetrical shape. Thus, limiting to the popular ILs is not a safe strategy of research but losing the potential opportunities. In fact, new ILs should be considered for developing the future Page 40 of 72

electrochemical systems, but if simply limiting ourselves to standalone IL electrolytes, it is likely to end up with the same old common ILs.

(ii) Although ILs have the capability to be used as standalone electrolytes, it is not a reasonable choice for several reasons. The high viscosity of ILs is a major obstacle in the commercial development. On the other hand, the cost of highly pure ILs is too high to be the sole component of the electrolyte. Instead, it is more reasonable to utilise ILs as a component in the electrolyte to provide mobile ions.

(iii) PILs or ILs trapped within the polymeric matrixes seem to be better options for designing the gel-like electrolyte. In this case, the high viscosity of ILs is gained as an advantage to tackle the leakage issue of electrochemical energy storage devices. Moreover, gel polymer electrolytes are ideal components for fabricating the flexible supercapacitors, which are highly in demand.

(iv) Although the ionic mobility of ILs makes them ideal media for the charge transfer between the electrodes, they can play similar roles within the electrodes. Therefore, they can be as modifiers in designing the electrodes. In this case, the cost or viscosity of ILs is not an issue, and thus, a wider range of ILs can be utilised in addition to the common ones, which have become popular because of their better electrochemical behaviours as electrolytes.

Acknowledgements

The author would like to thank Dr Francesca Soavi (University of Bologna) for her invaluable comments.

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Figure Captions

Figure 1. The molecular structure of typical ILs based on common anions and cations as employed in the electrochemical systems. (a) EMI-BF4, (b) BMI-PF6, and (c) Et4N-TFSI.

Figure 2. Electrochemical stability of typical ILs on a Pt working electrode. The reference electrode is Ag/AgCl and the scan rate 1 mV s–1. Reproduced with permission from Ref.[28]. Copyright 2004, Elsevier.

Figure 3. Electrochemical behavior of Pt and carbon electrodes in typical ILs. Reproduced with permission from Ref.[29]. Copyright 2016, Elsevier.

Figure 4. Comparison of the capacitive behavior of a graphene electrode in aqueous, organic, and IL media. Reproduced with permission from Ref.[46]. Copyright 2013, Royal Society of Chemistry.

Figure 5. (a) Cyclic voltammograms and (b) charge/discharge profiles of a carbon electrode. Reproduced with permission from Ref.[252]. Copyright 2013, American Chemical Society.

Figure 6. (a) Impedance spectra of a carbon electrode in various ILs, and (b) specific capacitance as a function of discharge current. Reproduced with permission from Ref.[81]. Copyright 2013, American Chemical Society.

Figure 7. The schematic diagram of the proposed charge storage mechanism of the FeOOH electrode in the EMI-TFSI IL. Reproduced with permission from Ref.[216]. Copyright 2016, Royal Society of Chemistry.

Figure 8. (a) Cyclic voltammograms and (b) impedance spectra of a carbon electrode in various ILs (different anions for the same cation). (c) The differences in electrical conductivity and viscosity of the corresponding ILs. Reproduced with permission from Ref.[133]. Copyright 2014, Wiley-VCH.

Figure 9. Distributions of cations (red line for EMI) and anions (green line for BF4 and blue line Page 67 of 72

for TFSI) in EDLs of pure and mixed RTILs near a positive surface with ψ − PZC = 1.5V: (a) x = 0, (b) x = 0.25, and (c) x = 1. Inserts are schematics of the EDL structures. Reproduced with permission from Ref.[144]. Copyright 2016, American Chemical Society.

Figure 10. (a) Galvanostatic charge–discharge profiles for the RILSC with 80 wt.% of [EMI][FcNTf] in acetonitrile at 25 °C and a 2 mA current. (b) The charging potential profile of the positive electrode showing the processes during charge. Reproduced with permission from Ref.[139]. Copyright 2016, Elsevier.

Figure 11. Concentration dependency of an midazolium-based ionic liquid crystal formed in acetonitrile. AFM images of (a) Nematic batonnet phase (concentration: 0.1 M, conductivity: 3-6 mS cm–1), (b) columnar phase (concentration: 0.3 M, conductivity: 19.4 mS cm–1). PLM images of (c) smectic A phase (concentration: 0.4 M, conductivity: 33.10 mS cm–1), and (d) smectic C phase (concentration: 0.6 M, conductivity: <40 mS cm–1). Reproduced with permission from Ref.[163]. Copyright 2016, American Chemical Society.

Figure 12. (a) The cross-linked structure between poly-4-vinylphenol (c-P4VPh) and 1-ethyl-3-TFSI). (b) The dependency of the ionic conductivity to the IL/polymer ratio. (c) The stable electrochemical windows of the EMI-TFSI and a series of the IL/polymer electrolytes. Reproduced with permission from Ref.[171]. Copyright 2016, Royal Society of Chemistry.

Figure 13. (a) Chemical structure of PILTFSI and different ILs (PYR14TFSI, PYR14FSI, PYR14DCA and HEMi-TFSI) and the respective photo for each membrane. The mass ratio of ILs to PILTFSI for IL-b-PE1, IL-b-PE2 and IL-b-PE3 is 60:40 whereas for IL-b-PE4 is 30:70. (b) Arrhenius plots of ionic conductivities for different free standing IL-b-PE membranes at different temperatures from 30 to 120 °C. (c) The stable electrochemical windows at 25 °C and 10 mV s–1. (d) Charge/discharge profiles recorded at 2 mA cm–2 Reproduced with permission from Ref.[193]. Copyright 2016, Elsevier.

Figure 14. Cyclic voltammogram and charge/discharge profiles for Co(OH)2/IL and Co(OH)2/IL in a 3 M KOH aqueous electrolyte. The scan rates are 50 mV s–1 and 1 A g–1, respectively. (c) Impedance spectra of the fresh electrodes and after 1000 charge/discharge cycles. (d) A schematic of the role of IL intermediating hydrogen adsorption on Co(OH)2 lattice. Reproduced with permission from Ref.[233]. Copyright 2013, American Chemical Society.

Figure 15. The structure of a hybrid composite of multi-walled carbon nanotubes, graphene, and an IL (BMI-TFSI). Reproduced with permission from Ref.[49]. Copyright 2012, American Chemical Page 68 of 72

Society.

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Table 1.

Type

Material

IL

Specific Specific Specific Rate / Window Energy Power / Category Capacitance A g-1 /V / Wh kW kg– / F g-1 kg-1 1

Cyclability / Retention Ref. (Number of Cycles)

11.4

98

90% (10,000) @ 30 100 A g–1

20

3.1

97% (1,000)

Double layer

Porous carbon

EMI-BF4

Pure 147 electrolyte

1

Double layer

Porous carbon

EMI-BF4

Pure 147 electrolyte

2 mA 4.0 cm–2

Double layer

SiC-derived carbon EMI-BF4

Pure 170 electrolyte

0.1

Double layer

Porous carbon

EMI-TFSI

Gel polymer 172 electrolyte

1 mA 4.0 cm–2

72

Double layer

Carbon nanofibers EMI-TFSI

Pure 161 electrolyte

1

246

Double layer

Ti3C2Tx

EMI-TFSI

Pure 70 electrolyte

1 mV 3.0 s–1

Double layer

Carbon

EMI-TFSI

Pure 160 electrolyte

1

3.0

20

42

150

Double layer

Porous carbon nanofiber

EMI-TFSI

Pure 180 electrolyte

0.5

3.5

80

0.4

256

Double layer

Graphene-based carbon

EMI-TFSI/AN

Mixed 174 electrolyte

2

3.5

74

338

94% (1,000)

252

Double layer

Activated carbon

BMI-Cl

Gel polymer 136 electrolyte

1.5

10.6

3.4

90% (3,000)

199

Double layer

Activated carbon

BMI-BF4

Gel polymer 138 electrolyte

2.5

36

24.5

80% (10,000) @ 215 1.5 A g–1

Double layer

Si nanowires

BMI-TFSI

Pure 0.7 electrolyte

1.6

0.23

0.65

257

Double layer

Activated carbon

PYR13-FSI

Gel polymer 21 electrolyte

2.5

16

1.1

Double layer

Activated carbon

PYR14-FSI

2 mA 3.5 cm–2

36

1.17

Gel polymer

150

8

1

3

3.6

3.5

253

254

171

30

255

90

100% (2,500)

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258

193

Type

Material

IL

Specific Specific Specific Rate / Window Energy Power / Category Capacitance A g-1 /V / Wh kW kg– / F g-1 kg-1 1

Cyclability / Retention Ref. (Number of Cycles)

electrolyte

Double layer

Carbonized cellulose/Activated BMPY-TFSI carbon

Pure 84 electrolyte

0.1

3.0

21

41.6

92% (10,000)

259

Double layer

N-doped reduced graphene oxide aerogel

BMP-DCA

Pure 765 electrolyte

1

4

245

6.53

86% (3,000)

96

Double layer

Graphene nanosheets

BMP-DCA

Pure 330 electrolyte

3.3

140 at 60 °C

52.5 at 60 °C

Double layer

Ionic Mesoporous carbon Imidazolium-based liquid crystal

0.37

2.5

38

3.58

3

3.8

108

131

Pseudocapacitive C/RuO2

EMI-BF4

Pure 52 electrolyte

Pseudocapacitive FeOOH

EMI-TFSI

Ionogel

Pseudocapacitive K10 clay

Et4N-BF4/AN

Mixed 36 electrolyte

2

MMI-Br

Electrode material

0.005 1 mA

BMI-SCN

Aqueous 781 electrolyte

Pseudocapacitive

Poly(orthoaminophenol)

Pseudocapacitive ZnFe2O4

84

80% (2,000)

163

98.5% (100,000)

260

216

489

2.7

1.2

171

156

1.98

7.11

90% (5,000)

261

82% (1,000)

262

95% (3,000)

113

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