A Review of Ionic Liquids and Their Composites with Nanoparticles for Electrochemical Applications

Abstract

The current study focuses on reviewing the actual progress of the use of ionic liquids and derivatives in several electrochemical application. Ionic liquids can be prepared at room temperature conditions and by including a solution that can be a salt in water, or a base or acid, and are composed of organic cations and many charge-delocalized organic or inorganic anions. The electrochemical properties, including the ionic and electronic conductivities of these innovative fluids and hybrids, are addressed in depth, together with their key influencing parameters including type, fraction, functionalization of the nanoparticles, and operating temperature, as well as the incorporation of surfactants or additives. Also, the present review assesses the recent applications of ionic liquids and corresponding hybrids with the addition of nanoparticles in diverse electrochemical equipment and processes, together with a critical evaluation of the related feasibility concerns in different applications. Those ranging from the metal-ion batteries, in which ionic liquids possess a prominent role as electrolytes and reference electrodes passing through the dye of sensitized solar cells and fuel cells, to finishing processes like the ones related with low-grade heat harvesting and supercapacitors. Moreover, the overview of the scientific articles on the theme resulted in the comparatively brief examination of the benefits closely linked with the use of ionic fluids and corresponding hybrids, such as improved ionic conductivity, thermal and electrochemical stabilities, and tunability, in comparison with the traditional solvents, electrolytes, and electrodes. Finally, this work analyzes the fundamental limitations of such novel fluids such as their corrosivity potential, elevated dynamic viscosity, and leakage risk, and highlights the essential prospects for the research and exploration of ionic liquids and derivatives in various electrochemical devices and procedures.

1. Introduction

The innovative fluids designated as ionic liquids can be defined as low-melting salts composed of organic cations and organic or inorganic anions. Ionic liquids have several useful characteristics like enhanced ionic conductivity and thermal stability, minor vapor pressure, wide electrochemical potential windows, and high levels of solubility [1]. Also, ionic liquids usually contain asymmetric ions presenting delocalized electrostatic charges [2]. The combination of strong coulombic interactions and weak directional interactions like cation-π, hydrogen bonding, and van der Waals interactions promote the creation of nanostructures in the ionic liquids and in mixtures of ionic liquids and solvents or ionic liquid and solutes [3]. In the ionic liquids with cations containing long alkyl side chains, the alkyl chains may aggregate, forming non-polar domains, whilst the other ionic liquid parts produce polar domains [4]. The highly hydrogen-bonded networks of the ionic liquids enable them to dissolve diverse substances in which the solutes have the tendency to localize in the domains to which they possess greater affinity [5]. Ionic liquids can also act as solvents in certain reactions [6]. The polyionic liquids, also known as polymerized ionic liquids, can be used as reaction media [7] and in the fields of energy harvesting, catalysis, and separation procedures because of the combined polymers’ and ionic liquids’ properties [8]. The nature of ionic liquids also leads to many useful physicochemical features. For instance, ionic liquids have minor vapor pressures [9], making them effectively non-volatile under harsh scenarios. Such beneficial features, along with their considerable competence for dissolving several solutes, has led to their vivid investigation as environmentally benevolent substitutes for the classic volatile organic compounds [10]. The characteristics of ionic liquids can also be tuned by altering the anion–cation combination and the alkyl chains length, or through the addition of other functional groups. For this reason, ionic liquids can be taken as customized solvents or electrolytes. Because of their valuable characteristics, ionic liquids find suitability for different energy, materials, and environmental fields of research including catalysis [11], absorption of gas [12], membranes [13], heat transport [14], oil refining [15], production of nanomaterials [16], and lubricants [17]. These neat ionic liquids can conduct electricity through the motion of charged ions in the liquid phase. This makes them distinct from conventional ionic compounds, which, caused by their solid nature, must be initially dissolved in a solvent to become electrolytes. Such a valuable property convinced researchers to study the usage of ionic liquids in a wide range of energy applications such as electrolytes for rechargeable batteries [18], fuel cells [19], and supercapacitors [20]. The possibility of exploring ionic liquids to store hydrogen through hydrogenation and dehydrogenation processes has also been receiving great attention from researchers [21]. Ionic liquids have also been examined in hydrocarbon fuel cells [22] and spacecraft propellants [23]. Apart from the use of ionic liquids in the liquid phase, these novel fluids can also be explored to improve the conductivity of solid and polymer electrolytes [24]. Ionic liquids were also used in conjunction with surfactants to produce electrolytes with increased energy density [25]. The improved thermal stability, non-volatility, and non-flammability of ionic liquids entail appreciable advantages over traditional electrolytes like the ones based on carbonates because of the inherent flammability that contributes to safety issues over the lithium-ion area [26]. Moreover, ionic liquids with embedded nanoparticles can create various hybrid structures, depending on the balance between the intramolecular and intermolecular interactions. For instance, ionic liquids can be the solvent media for colloidal suspensions [27], facilitating the dispersion of metal nanoparticles [28], structured inorganic nanoparticles [29], graphene [30], and carbon nanotubes [31], aiding in the cellulose dissolution [32] and dispersing the nano-scaled cellulose [33]. Ionic liquids, contributing electrostatic forces on the surface of nanoparticles, can also be used as stabilizers of nanoparticles produced in aqueous solutions [34]. Through the incorporation of nanoparticles in ionic liquids, the elevated electrical conductivity of the nanoparticles conjugates with the improved proton transport capability of the ionic liquids, with extra beneficial features for electrochemical devices. Figure 1 presents the fundamental properties and most promising applications of ionic liquids. The properties of ionic liquids are directly related to their applications. For example, the low melting point allows devices that need to operate at relatively low temperatures to efficiently release thermal energy. Low volatility is very useful in lubrication, decreasing the fluid loss due to evaporation. Additionally, for carbon capture, reduced volatility minimizes the loss of liquid during the carbon dioxide absorption process. To maintain the fluidity and structural stability of the crystalline liquid phases of liquid crystals, moderate viscosity is a promising alternative. Chemical and thermal stability is useful in harsh environments, preventing corrosion, and in electrolytic cells and capacitors, enhancing the longevity and efficiency of the equipment. Being an excellent ionic conductor allows the efficient transport of ions, contributing to the performance and storage capacity in devices. The high polarity of ionic liquids facilitates the dissolution and capture of carbon dioxide molecules, which is useful in carbon capture. In liquid crystals, high polarity helps in the formation of ordered structures, essential when these devices need to respond to applied electric fields. The applications of ionic liquids are not limited to those mentioned. Throughout the present work, other alternatives are discussed, although the focus is on electrochemical applications.

Remarkable synergistic effects have been reported in nanomaterials combining the inclusion of ionic liquids and nanoparticles or the so-called ionic liquid hybrids. The properties of the nanoparticles, including enhanced catalytic and adsorption efficiency, thermal stability, and electrochemical and electrical behavior, may be improved by physical or chemical surface modification with ionic liquids [35]. Also, because of the interaction between the nanoparticles and ionic liquids, the electrochemical features of the ionic liquid and nanoparticle hybrids may be higher than the ones of the neat ionic liquids alone [36]. In the case of titania nanotube dispersions over an ionic liquid-impregnated polymer matrix, the composite electrolytes showed a considerable ionic conductivity of 10⁻³ S.cm⁻¹ at 120 °C, elevated thermal stability above 250 °C, and improved morphology, showing a reduction in the volume of crystalline domains and increased amorphous phase, offering great potential for diverse device purposes [37]. The present review discusses the potential ends of the recent hybrids composed of nanoparticles within ionic liquids, bearing always in mind the possible correlations between structure and function of the nanomaterials. The present review provides an updated assessment of the main electrochemical applications of ionic liquid and nanoparticle hybrids, focusing on their structural properties. It will address various forms of ionic liquid and nanoparticle hybrid materials such as gels, colloidal glasses, and suspensions, and ionic liquid-grafted nanoparticles. In addition, the advanced applications of the nanoparticle and ionic liquid hybrids in the electrochemistry area were already addressed. To the best of the knowledge of the authors of this work, and in comparison to already published reviews on the electrochemical applications of ionic liquids [38,39,40], the present overview is more complete in terms of the description of the different types and properties of ionic liquids and their derivatives. Even though the referred works are much-improved reviews of the devices and processes linked with the electrochemical exploration of ionic liquids, the present study also summarizes a more comprehensive survey of possible applications. It covers seven different types of rechargeable metal batteries operating with ionic liquids, passes through the assessment of purposes rarely detailed such as low-heat harvesting with ionic liquids, and ends with the very specific application of ionic liquids in the production of refence electrodes for electrochemical sensing. Also, the more than two hundred and fifty references on the matter makes the current work a very considerable gathering of information on ionic liquids operating in electrochemical devices. Finally, the limitations and challenges herein described and associated particularly with the difficulties of the in situ characterization and theoretical studies of ionic liquids as electrodes and electrolytes, emphasize once more the uniqueness of this review work.

The catalytic properties of nanoparticles and the electron and ion conductivities of ionic liquids and nanoparticles favor all the electrochemical processes. The intermolecular interactions in ionic liquid and nanoparticle hybrids are involved in the organization of materials with distinct structures. The suspensions of colloids with enhanced stability, colloidal glasses, or gels can be obtained through dispersing the nanoparticles in ionic liquids [1], forming a protective layer around the nanoparticles, which enhances their stability. The ionic liquids can bond covalently on the surface of the nanoparticles, combining the useful features of the ionic liquid and nanoparticles. Other types of nanoparticle and ionic liquid hybrids are the nanoparticles added to ionic liquid preformed structures, and nanoparticle-stabilized ionic liquid emulsions [1]. Nanoparticles and ionic liquids are also produced into other materials like membranes or films to ameliorate the corresponding performance. The development of ionic liquids entails the design of the counter-cations and the selection of the anion lithium salts or acids. Amongst the different commercial anions in the form of lithium salts, the bis(trifluoromethanesulfonyl)-imide [TFSI] and hexafluorophosphate anions [PF₆]⁻ are most suitable to be applied in batteries. In the form of ionic liquids, the former possesses the ability for enhancing the ionic conductivity, and the latter has improved stability. The [TFSI]⁻ anions usually serve to synthesize ionic liquids due to their usual trend of expanding the range of temperatures in which the ionic liquids remain in the liquid state. Apart from these, the bis(fluorosulfonyl)imide ([FSI]⁻) anions have also been increasingly considered as an electrolyte potential anion. The ionic liquids containing the anions [FSI]⁻ exhibit decreased viscosity and increased ionic conductivity, despite the [FSI]⁻ anions having a decreased ion size and elevated density of charge. These facts are due to the negative charge of [FSI]⁻ being delocalized through fluorine atoms [41]. Also, the ionic liquids with [FSI]⁻ anions create solid-electrolyte interphases, allowing a sufficient charge transport of lithium and protecting both the electrode and electrolyte from decomposition [42]. Nevertheless, a major part of the ionic liquids with [FSI]⁻ will crystallize at sub-zero points and they happen to be not sufficiently ion conductive at temperatures inferior to those [43]. To eliminate the crystallizing evolution of the [FSI]⁻, the anions should be combined with cations with a delocalized charge or versatile substituents. Concerning the structures of the cations, the aliphatic cations like pyrrolidinium and phosphonium are preferable for batteries, given that they have improved thermal and electrochemical stability due to the absence of unsaturated bonds [44]. However, such cations have the tendency to augment the melting point of the ionic liquids in comparison to the imidazolium cations in the case where they possess the same counter anions and alkyl chains. This effect is caused by the π-electrons in the imidazolium cations that contribute to reducing the melting point [45]. To avoid the crystallization of ionic liquids without the use of unsaturated bonds, the functionalization of the aliphatic cations via alkoxy groups is an adequate approach [46]. Figure 2 summarizes the fundamental applications of ionic liquids with nanoparticles or ionic liquid hybrids.

It is also usual for the addition of surfactants to the ionic liquids to form electrolytes presenting increased energy density even at high temperatures [47,48], hence possessing the ability to overcome part of the drawbacks of common electrolytes. Specifically, the fast-growing need for portable devices and vehicles using lithium-ion technology has inspired significant research on the employment of ionic liquids as electrolytes in advanced batteries. In energy harvesting electrochemical devices like fuel and solar cells and batteries, the electrolyte as a charge transfer medium regulates how rapidly energy is delivered during equipment operation [49]. To function in a proper manner, the electrolytes should exhibit electrochemical stability, being resistant to electrochemical oxidation and reduction, and requiring extended electrochemical stability windows to avert deterioration in the operating potential range. Apart from this, the electrolyte should possess improved insulation and ionic conductivity. Having smaller investment costs and inferior environmental risks than the common disposable batteries, the need for rechargeable batteries exhibiting enhanced energy harvesting capability and charge–discharge cycling stability has clearly increased in the recent decades [1]. Nonetheless, after the repletion of discharge and charge cycles, the dendrites of metal will grow in the electrolyte and bridge the space between the electrodes, leading to short-circuits [1]. Traditional electrolytes are composed of salts dissolved in an organic or aqueous solvent heaving an elevated dielectric constant. The polymer electrolytes are nowadays a promising category of electrolytes, given that they are intrinsically safer than the traditional electrolytes, which is the case for the highly flammable organic solvents. Nevertheless, a great part of the polymers having increased dielectric constants is the possession of low ion transport capabilities [50]. The ionic liquids can be used as electrolytes because of their intrinsic ionic conductivity and extended electrochemical stability windows. An electrochemical stability window of approximately 4.5 V and an electrical conductivity in the range from 0.1 to 18 mS.cm⁻¹ was already observed for electrolytes based on ionic liquids [51]. Nevertheless, the significant viscosity of ionic liquids lowers their ionic conductivity. The hybrid electrolytes with a significant concentration of nanoparticles in an insoluble phase are prepared with an ionic conductivity which is higher than that of an electrolyte with a single phase [1,52,53]. Considering such interactions, the nanoparticles and ionic liquid composites are found to be enhancers of the performance of electrolytes with higher electrochemical and thermal stabilities, electrical conductivity, and diffusion coefficients, making them very suitable electrolytes for batteries and cells. For such purpose, many nanoparticle and ionic liquid hybrid structures are synthesized considering the electrodes and electrolytes, among which the dispersions of nanoparticles in ionic liquids and ionic liquid-grafted nanoparticles are the most adopted approaches. Ionic liquid-functionalized nanoparticles have been adopted to be additives to expand the electrochemical stability of electrolytes [1]. For instance, the structure of the brush-shaped form of the ionic liquid tethered to the nanoparticles facilitated the silica nanoparticle dispersion in a traditional LiTFSI-propylene carbonate electrolyte by providing double-layer and steric interactions [54]. The hydrophobic nanoparticles of silica formed a robust and tortuous porous mesh to avert dendrite creation, which might bridge the anode and cathode leading to eventual thermal runaway and short-circuits. Ionic liquid-functionalized nanoparticles are also studied for acting as lithium-ion battery electrolytes given their surface functionality, elevated particle dispersion, and enhanced ability for salt dissolution [55]. Furthermore, Xu et al. [56] added 10% vol. of 1-methyl-3-propyl-imidazolium bis(trifluoromethanesulfonyl)-imide-tethered silica nanoparticles in an electrolyte propylene carbonate NaTFSI and observed an increased rechargeability superior to twenty cycles of the cell Na–CO₂/O₂ at potentials reaching 5 V, which was obtained without any reported electrolyte degradation. In conclusion, the use of electrolytes based on ionic liquids has the beneficial features of decreased oxidation/decomposition of the electrolytes, augmented ion transference numbers, reduced dendrite formation, and absence of the creation of any uneven SEI (solid-electrolyte interphase). Figure 3 summarizes the main drawbacks of lithium batteries, which can be suppressed by employing electrolytes based on ionic liquids. The formation of dendrites in the lithium-ion batteries can be prevented with the ionic liquid production of a solid-electrolyte interphase layer that enables the smooth plating/stripping of the lithium-ion on the anode surface, in this way averting the dendrite production. The decomposition and oxidation of the operating electrolyte can be relieved by the usage of ionic liquids, given that the side-reactions within the charge and discharge processes are decreased, which will ameliorate the cycling behavior of the cell. The generation of an uneven solid-electrolyte interphase layer can be prevented with the creation of a more uniform and denser SEI layer via the ionic liquids. The effect of the low lithium-ion transference number can be decreased through the reduction in the activation barrier for the transport of the lithium ion given the superior coefficient of diffusion and ionic conductivity. Berginc et al. [57] analyzed the thermal performance of a dye-sensitized solar cell using a propyl-methyl-imidazolium iodide ionic liquid electrolyte with the inclusion of nanoparticles of silica. The authors found that the silica nanoparticles in the propyl-methyl-imidazolium iodide increased the charge transport of the redox species in the electrolyte and, hence, enhanced the solar cell efficiency by up to 20%. Also, the short-circuit current density and efficiency of the dye-sensitized solar cell were increased by 20% compared to the inclusion of nanoparticles of silica at a concentration of 2% wt. in the electrolyte. The redox couple charge transport in the electrolytes hinders the short-circuit current density of the dye-sensitized solar cells. The temperature of the cell had strongly influenced its thermal performance using propyl-methyl-imidazolium iodide electrolytes independently of the silica nanoparticles’ concentration. In terms of ionic liquid hybrids for antibacterial purposes, the work conducted by Patil et al. [58], who prepared 1-butyl-3-methylimidazolium halide ionic liquids through anion exchange reactions and metathesis, should be highlighted. The authors also synthesized silver nanoparticles in ionic liquids in a pressurized reactor via reduction of silver nitrate using hydrogen. Antibacterial activity of the ionic liquids and silver nanoparticles in ionic liquids were observed via the diffusion method for gram-positive Bacillus cereus NCIM-2155 and gram-negative Escherichia coli NCIM-2931 bacteria. The researchers argued that the antibacterial activity of the ionic liquid and silver nanoparticle composites was regulated by the anions of the ionic liquids and the size of the silver nanoparticles. Also, the ionic liquid 1-butyl-3-methylimidazolium iodide showed the highest antibacterial activity amongst all the tested ionic liquids. Additionally, the silver nanoparticles in the 1-butyl-3-methylimidazolium iodide ionic liquid increased its antibacterial activity for Bacillus cereus and Escherichia coli bacteria.

Julio et al. [59] argued that the permeability and low solubility of the drugs are some of the fundamental limitations linked with the modern drug delivery systems, and, as such, the ionic liquid and nanoparticles carrier systems should be taken seriously for the delivery process of poorly soluble drugs. An ionic liquid–nanocarrier composite loading the poorly soluble drug rutin, was synthesized via modified double-emulsion. 2-hydroxyethyl-trimethylammonium-l-phenylalaninate [Cho][Phe] or 2-hydroxyethyl-trimethylammonium-l-glutaminate [Cho][Glu] ionic liquids were prepared and applied to produce hybrid ionic liquid–polylactic-co-glycolic acid systems, at sufficient concentrations for cell viability. The hybrid nano systems had a diameter inferior to 500 nm, having improved stability and polydispersity index. An association efficiency of the drug ranging from 35% to 50% was found, a considerable result for a poorly soluble drug, and in formulations with pH equal to 6.7, this factor enhanced substantially. A durable ionic liquid–polymer nanoparticle system with the polyvinyl alcohol at 2% wt. was produced, exposing its ability to include greater quantities of poorly soluble drugs. The reason behind the research was to form particles having the smallest size possible and an elevated association degree for rutin, examining diverse polylactic-co-glycolic acid ratios and concentrations of polyvinyl alcohol. The content of the surfactant used changed its properties, with the polyvinyl alcohol enhancing the stability. The different concentrations of the polyvinyl alcohol showed good physicochemical properties, but, in the options with polyvinyl alcohol at 2% wt., such factors became improved. Consequently, the ionic liquid–nanoparticle hybrid with polyvinyl alcohol at 2% (w/v) exhibited a more acceptable diameter between 220 nm and 340 nm, with a polydispersity index from 0.2 to 0.4 and a colloidal stability between −35 and −45 mV. Moreover, in the presence of the ionic liquids in the internal phase, the characteristics of the nanoparticles were similar for the distinct polylactic-co-glycolic acid ratios. Yet, the hybrid systems allowed an increase in the drug loading, as proven by the determination of the association efficiency of rutin being in between 35% and 50%, and it was not possible to achieve this in the absence of the ionic liquid. Regarding the ionic liquid–polylactic-co-glycolic acid nanoparticle system obtained with the inner phase at a pH equal to 6.7, the pH adjustment promoted the enhancement of the association efficiency. Nonetheless, it changes appreciably the physicochemical factors of the hybrid nano-scaled systems, and it is not the ideal route to follow to incorporate a greater amount of rutin in these formulations. In terms of association efficiency and physicochemical characteristics, the formulations with [Cho][Phe] demonstrated better rutin delivery than those with [Cho][Glu].

The use of nanoparticles and ionic liquids composites in biosensors was studied by Franzoi et al. [60]. The authors evaluated novel biosensors based on gold or silver nanoparticles dispersed in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF₆] and laccase from Aspergillus oryzae immobilized in chitosan chemically crosslinked using cyanuric chloride. This enzyme catalyzes the oxidation of luteolin to the corresponding o-quinone, which is electrochemically reduced back to luteolin at 0.35 V vs. Ag/AgCl. The best performance was achieved with 50:20:15:15% wt. for the graphite powder: Chi-CC: silver-[BMIM][PF₆] or gold-[BMIM][PF₆] composition (Lac 0.29 units mL⁻¹) in 0.1 M acetate buffer solution with adjusted pH to 4 with frequency, pulse amplitude, and scan increment at 50 Hz, 100 mV, and 5 mV, respectively. The cathodic currents augmented linearly with increasing luteolin concentrations ranging between around 0.01 and 5.8 µM, with limits of detection of 0.054 ± 0.004 µM silver-[BMIM][PF₆] and 0.028 ± 0.002 µM gold-[BMIM][PF₆]. The biosensors demonstrated enhanced sensitivity, reproducibility, and stability with a 13% decrease in response over 70 days. The recovery study for luteolin in chamomile tea samples yielded values from around 92% to 105%. The effect of the laccase immobilized in the chitosan–cyanuric acid and nanoparticles-[BMIM][PF₆] promoted the performance of the biosensors.

Teixeira et al. [61] developed indium-doped tin oxide substrates modified with gold nanoparticle–polyionic liquid composite films for triclosan detection in commercial toothpaste and samples. Triclosan is a strong bactericide employed in personal care products, and it is currently recognized as a concerning contaminant presenting endocrine disrupting chemical activity. The electroanalytical performance was evaluated by the authors through electrochemical impedance spectroscopy, cyclic voltammetry, and differential pulse voltammetry techniques. The analyses revealed the modified electrode oxidizes the triclosan at higher oxidation currents than the indium-doped tin oxide. The triclosan determinations were carried out in spiked lake water samples showing that the response of the electrode was not affected. The concentration of triclosan found in a commercially available toothpaste, simultaneously determined using voltammetry and high-performance liquid chromatography, was similar between both techniques and comparable to that described in the product package. The poly(1-vinyl-3-carboxymethylimidazolium)-chloride polymeric ionic liquid made the chemical reduction of the AuCl₄⁻ by citrate more effective and contributed to a greater yield of gold nanoparticles in the composite. Apart from this, the protonic ionic liquid ensured improved stability for the nanocomposite. The gold–polymeric ionic liquid nanocomposite was anionic and could be layer-by-layer assembled with cationic polydiallyldimethylammonium chloride (PDAC) into ultrathin films. The ITO substrates modified with gold–polyionic liquid/PDAC films showed improved electrocatalytic activity for the oxidation of triclosan. Under optimized differential pulse voltammetry conditions, the ITO/gold–polyionic liquid/PDAC electrode permits the detection of triclosan in lake water and toothpaste with low limit of detection and high sensitivity.

Lin et al. [62] prepared proton-conducting hybrid membranes via in situ crosslinking of mixtures of polymerizable oils with the n-ethylimidazolium trifluoromethanesulfonate [EIm][TfO] protic ionic liquid and nanoparticles or mesoporous nanospheres of silica. The developed membranes showed good thermal stability and tunable mechanical properties. Also, the authors found that the addition of silica enhanced considerably the proton conductivity of the membranes, which was probably due to the ion transport channel or network structures created in the membranes. Nonetheless, additional incorporations of silica could block the channels for ion transport and reduce the conductivity of the membranes. Also, in comparison with the silica nanoparticles, the mesoporous silica nanospheres were more effective in increasing the conductivity and in suppressing the release of ionic liquid from the hybrid membranes. The membranes exhibited a proton conductivity reaching 10⁻² S.cm⁻¹ at 160 °C, making them appropriate for fuel cells with high-temperature polymer electrolyte membranes.

In the field of innovative catalysts, Xu et al. [63] produced in situ palladium nanoparticles in couplings between allylic carbonates and arylboronic acids in ionic liquids. That is the case when using phenylboronic acid in the presence of 1% mol. Pd(OAc)₂ in [C₄MIM][PF₆]. It may also be performed in [C₄MIM][SbF₆], [C₄MIM][NTf₂] or a two-phase [C₄MIM][PF₆]–water system, while, when using [C₄MIM][OTf], the conversion is low, and in [C₄MIM][BF₄], a mixture of products is obtained. Following optimized factors enabled the production of palladium nanoparticle dispersions with average particle sizes of 3.2 nm in [C₄MIM][PF₆]–water and 2.8 nm in [C₄MIM][SbF₆]. In a two-phase [C₄MIM][PF₆]–water system, the reaction proceeds with full conversion and elevated selectivity. The palladium nanoparticles were immobilized in the ionic liquid, and the system was reusable for three repeated cycles without losses in catalytic activity. At the 4th cycle, the reaction was complete with the reaction time increasing to 18 h. The catalytic activity reduction was caused by leaching rather than nanoparticle aggregation. The substrate scopes were appreciable when exploring allyl–aryl couplings in pure [C₄MIM][SbF₆] and [C₄MIM][PF₆]–water. The arylboronic acids exhibiting electron-donating groups reacted with increased yields and selectivities. The 4-(Trifluoromethyl)phenylboronic acid reacts with low yields in both systems, whereas the 1-nafthylboronic acid reacts with high yields. The phenylboronic acid reacts with high yields unless the carbonate is sterically hindered via, for instance, groups of isopropyl.

In the technological area of the novel heat transfer fluids, El-Maghlany and Minea [64] studied the natural convection behavior of water and [C₂MIM][CH₃SO₃] mixtures and alumina nanoparticles. The researchers considered different aspect ratio cavities up to 4 and a Rayleigh number between 10³ and 10⁵. The authors concluded that the inclusion of nanoparticles reduced the heat transfer capability and enhanced entropy generation. The heat transfer decrease was provoked by the viscosity increase with merely a little increase in the thermal conductivity. The aspect ratio enhancement increased the upward flow resistance to the heat transfer via natural convection. The heat transfer decreases, and the generation of entropy in the system augmented with the incorporation of nanoparticles. As such, it is suggested to use this innovative kind of fluid in forced convection instead of natural convection.

Figure 4 presents the fundamental advantageous features of employing ionic liquid electrolytes in various electrochemical devices.

The gas crossover is an inevitable phenomenon in proton-exchange fuel cell membranes. The oxygen and nitrogen from the cathode pass through the membrane to the anode, whilst the hydrogen crosses from the anode to the cathode. The hydrogen crossover leads to an efficiency decrease caused by parasitic hydrogen consumption and mixed potentials on the cathode electrode. Also, the hydrogen crossover provokes degradation and pinhole formation, and that is why it represents a key factor for the durability of a fuel cell, and the quantification of the crossover is a major parameter for membrane qualification. Concerning this, Neves et al. [65] altered membranes of Nafion through the addition of ionic liquid cations to infer the impact of such an alteration in gas and methanol crossover. The action of employing diverse amounts of ionic liquid cations and types of ionic liquid cations in the transport of methanol and gas was investigated by the authors. The findings were compared with the ones achieved with an unchanged membrane made of Nafion. Depending on the included ionic liquid cation, a decrease in the methanol crossover ranging between 60 and 600 times was obtained in comparison with that of a membrane made of Nafion-112. The decrement was closely linked with the added amount and nature of the cation that defines the quantity of water retained by the membrane and its structuring level within the membrane. The hydrogen, oxygen, nitrogen, and carbon dioxide permeabilities were determined, and it was verified that a lower gas crossover resulted from the changed membranes of the Nafion–ionic liquid cation in comparison to the ones achieved with the unchanged membrane Nafion-H⁺. The research team stated that the electric properties, gas and methanol crossover, and high-temperature stability of the developed membranes could be adjusted by regulating the type and level of incorporation of the cation. The ideal balance between characteristics must be found to consider their use in direct methanol fuel cells. Yamanaka et al. [66] evaluated dye-sensitized titanium dioxide solar cells with electrolytes based on imidazolium ionic liquid crystals. A 1.1- to 1.2-times higher ratio of Im⁻/I³⁻ in C12-MImI/I₂ to C11MImI-I₂ was effective at enhancing Dex; the Dex value of C12 MImI/I₂ should range from 1.1 to 1.2 times as high as C11MImI-I₂. However, the Dex value of C12MImI-I₂ was 1.7 times as high as that for C11MImI-I₂. Apart from the effect of the increase in Im⁻, the researchers also proposed another effect of the promotion, which was with the two-dimensional electron conductive pathways. It was already reported that the charge transport of an I⁻/I³⁻ redox couple in ionic liquid and molecular liquid was different. The redox couple in the ionic liquids exists in a strong ionic strength field. Anions I⁻, I³⁻, and Im⁻ collide with each other more easily in the ionic liquids than in the molecular liquids through the kinetic salt effect. Each one of the redox couples can react in the electron conductive two-dimensional paths of the ionic liquid crystal. In the conductive paths of C12-MImI/I₂, a greater number of collision frequencies between the iodide species I⁻, I³⁻, and Im^- could be achieved than in the three-dimensional space of C11MImI-I₂. An ionic liquid crystal C12MImI/I₂ with a smectic A phase with the imidazolium cations showed high conductivity, despite its high viscosity, and greater short-circuit current and light-to-electricity conversion than that using the non-liquid crystalline ionic liquid, C11MImI/I₂, as an electrolyte. In C12MImI/I₂, the two-dimensional electron conductive pathways organized by the localizing of I³⁻ and I⁻ at the smectic A phase layers induced by a self-assembled structure of the imidazolium cations augmented the iodide species among the redox couple, which promoted the exchange reactions through their collision frequencies. The conductivity increase caused by the promotion of the exchange reactions will lead to an enhancement in the short-circuit current of dye-sensitized solar cells. It was solidified as an ionic liquid crystal via the gelator providing a hydrogen-bonded fiber structure and was applied to a quasi-solid-state dye-sensitized solar cell. When a little amount of the gelator was included in the ionic liquid crystal, the electron conductive pathways between the smectic A phase layers would be organized in a more effective mode because of the ameliorated molecular order of the imidazolium cations producing smectic A phase layers, and the exchange reaction would be favored. This would lead to the enhancement of the conductivity of the C12MImI/I₂ gel. A quasi-solid-state dye-sensitized solar cell using C12MImI/I 2 gel exhibited a higher short-circuit current and light-to-electricity conversion efficiency than those attained with C12-MImI/I₂ without a gelator. Qin et al. [67] stated that the attainment of ideal ionic liquid–polymer electrolytes lies in the selection of the polymer itself and suitable device fabrication methods. Overall, the ionic liquid–polymer gel electrolytes can be obtained through mixing a preformed polymer at room temperature with ionic liquids to create the gel and then filling it into the dye-sensitized solar cells at high temperatures or, alternatively, via in situ polymerization where the precursors with low viscosity are preinjected into cells and, after that, the gelation process occurs inside the cell under heating or ultraviolet irradiation. The in-situ polymerization exhibited advantages, such as easy treatment and good interfacial contact between the electrolyte and the titanium dioxide photoanode. The most important thing to retain is that this process is adequate for large-scale dye-sensitized solar cells. Currently, the electrolytes based on in situ chemically crosslinked gel were already produced via thermal initiation, where the gelation process is carried out at temperatures varying between 80 °C and 90 °C. Nevertheless, a problem often occurs when, once the formation of the precursor is complete, its viscosity gradually augments even at ambient conditions. Lin et al. [68] developed iodide-free polymeric ionic liquid for quasi-solid-state dye-sensitized solar cells. These groups are characterized using FTIR to reveal a successful synthesis of POEI-IS. The POEI-I segment acted as a gelling agent and provided a good retardation effect for the recombination reactions. The selenocyanate anion promoted the interfacial contacts between the POEI-IS electrolytes and the electrodes without the self-aggregation of the polymer and possessed a reversible redox couple of SeCN⁻/(SeCN)³⁻ with a more positive standard potential than that of iodide species. The POEI-IS showed only a 5% wt. loss at a temperature of 340 °C, demonstrating its increased thermal stability, and thus afforded the best POEI-IS solar cell an extended longevity, since the cell efficiency maintained 95% of its initial value after one thousand hours. It was found that the addition of 10 to 30% wt. of POEI-IS to the electrolyte was suitable for application in dye-sensitized solar cells. Finally, the best solar cell attained a power conversion efficiency of around 8%, an open-circuit voltage of 826 mV, a short-circuit current of nearly 14 mAcm⁻², and a filling factor of 0.7 via employing an electrolyte with 30% wt. of POEI-IS. Also, when compared to the cell with an electrolyte without the inclusion of the POEI-IS, a greater power conversion efficiency was attained. Chi et al. [69] produced a polymeric ionic liquid of poly(oxyethylene)-imide-imidazolium selenocyanate to act as multifunctional gel electrolyte in a quasi-solid-state dye-sensitized solar cell. The developed polymeric ionic liquid acted as a gelling agent for the cell electrolyte and had a redox mediator of [SeCN]⁻ for producing a SeCN⁻/(SeCN)³⁻ redox couple exhibiting a more positive redox potential than the one of the classic I⁻/I³⁻. The prepared ionic liquid also chelated the potassium cations via the lone-pair electrons of the oxygen atoms of its polyoxyethylene-imide-imidazolium segments and obstructed the recombination of photoinjected electrons with [SeCN]³⁻ ions in the electrolyte through its POEI-I parts. Consequently, the POEI-IS rendered a high open-circuit voltage to the quasi-solid-state dye-sensitized solar cell. The cell with the gel electrolyte containing 30% wt. of the POEI-IS in liquid selenocyanate electrolyte exhibited a high open-circuit voltage of around 826 mV and a high-power conversion efficiency of nearly 8%. The quasi-solid-state dye-sensitized solar cell with 30% wt. of POEI-IS kept up to 95% of its initial efficiency after a stability test for more than one thousand hours. The authors prepared a co-polymer composed of a polymerized ionic liquid, poly(1-((4-ethenylphenyl)methyl)-3-butyl-imidazolium iodide) [PEBII], and an amorphous rubbery poly(oxyethylene methacrylate) [POEM], synthesized and employed as a solid electrolyte in an I₂-free dye-sensitized solar cell. The copolymer electrolytes infiltrated into the nanopores of mesoporous titanium dioxide films, leading to an enhanced interfacial contact between the electrolyte and electrode. The solar cell efficiency of PEBII–POEM of 4.5% at 100 mW/cm² was inferior to that of PEBII of around 6%, demonstrating that the ion concentration was dominant in reference to the chain flexibility. With the inclusion of the 1-methyl-3-propylimidazolium-iodide ionic liquid, the efficiency of PEBII kept practically the same, while the one of the PEBII–POEM was improved, reaching 7% due to increased I⁻ ion concentration, which is one of the highest values for I₂-free dye-sensitized solar cells. The ionic conductivities between the PEBII (2 × 10⁻⁴ S.cm⁻¹) and PEBII–POEM (2.2 × 10⁻⁴ S.cm⁻¹) were not much different from each other, but the I₂-free dye-sensitized solar cell efficiency of PEBII was larger than that of PEBII–POEM. These results revealed that the polymer electrolyte ion concentration is critical for determining the performance of I₂-free dye-sensitized solar cells. The efficiency of the solar cell using PEBII–POEM copolymer was profoundly ameliorated by 7% through the incorporation of the MPII. Hence, MPII played a vital role for increasing the I⁻ ion concentration in PEBII–POEM, leading to a better performance of the solar cell.

2. Ionic Liquid Forms

Ionic liquids possess enhanced electrical conductivity, specific heat, thermal conductivity, and melting points, despite being too viscous. Figure 5 summarizes the possible ways ionic liquids can be used.

2.1. Neat Ionic Liquids

Ionic liquids can be obtained from molten organic salts at ambient conditions and are composed of organic cations and inorganic or organic anions, presenting melting temperatures below 100 °C [70]. In respect to the inorganic type of salts, the anion and cation asymmetry in the ionic liquids causes the intermolecular interaction shielding and avoids the ion clustering in the low-energy crystalline state [55]. Owing to their natural enhanced thermophysical characteristics, ionic liquids impact the performance of devices like heat exchangers, reactors, and distillation columns [71]. Ionic liquids exhibit a very-low vapor pressure, enhanced electrochemical and thermal stabilities, and a broad solubility in many chemical compounds. Such improved properties can also be adjusted by the coupling of anions and cations through regulating the van der Waals interactions and Lewis basicity or acidity, among other factors. Considering the anion and cation combinations and their ameliorated electrochemical, physicochemical, and biological properties, like the miscibility in aqueous solvents, ionic conductivity, and acidity or basicity levels, ionic liquids can be differently classified. Nevertheless, ionic liquids can be classified according to the protic, aprotic, and zwitterionic fundamental types. Apart from this, it is noteworthy to state that not all the ionic liquids are presented in a fluidic state at temperature values inferior to 100 °C, and some of these fluids only have a relatively low glass transition point and slow kinetics of crystallization. Chen et al. [72] verified that ionic liquids are very promising for electrochemical devices for improving the reaction of the hydrogen evolution. The researchers stated that ionic liquids exhibit low vapor pressure and increased electrical conductivity and electrochemical stability, which derive, among other characteristics, from many suitable functional groups. However, the enhanced viscosity degrees of the common ionic liquids can hinder their high ionic conductivity. Also, the electrolytes with solid nanoparticles show an ameliorated ionic conductivity when compared to that of single-phase electrolytes. Through such interactions, the composites composed of nanoparticles in ionic liquids have been found to improve the electrochemical behavior of electrolytes with increased thermal and electrochemical stabilities, electrical conductivity, and coefficients of diffusion that enabled these composite fluids to work very well as fluidic electrolytes in batteries and cells. In this direction, diverse combined formulations of nanoparticles in ionic liquids have been adopted, with the ionic liquid-grafted nanoparticles and the dispersions of nanoparticles in ionic liquids being the most usually employed. A successful case is the electrolyte composed of 0.5% wt. of gold nanoparticles in the ionic liquid [EMIM][EtSO₄] [73], which increased significantly the ionic conductivity and made the electrode capacitance reach a 190% enhancement. The verified electrochemical beneficial features were attributed by the researchers to the energy of interaction between the particles and ionic constituents, resulting from the attraction of the anions of the [EtSO₄] to the surface of the nanoparticles of gold nanoparticles, causing changes in the ionic structure and charge distribution.

2.2. Room Temperature Ionic Liquids

The authors Earle and Sedom [74] proposed to distinguish between ionic liquids with a melting temperature inferior to 100 °C and those with that melt at room temperature. After that, the room temperature ionic liquids gained increasing attention from researchers, given that their best specification is that these ionic liquids are made of ions of equal and opposite polarity, making them electrically neutral. In general, these ionic liquids have negligible vapor pressure and a density superior to that of water. They are not volatile in comparison to other organic solvents and are stable up to 400 K, indicating their suitability for electrochemical sensing and catalysis. The structure of the room temperature ionic liquids comprises a large asymmetric organic cation and a small organic or inorganic anion. The most usual cations reported for electrochemical sensing, solvent extraction, and synthesis contained imidazolium or pyridinium rings with one or more alkyl chains attached to a nitrogen or carbon center. The quaternary ammonium salts were also usual options for cations for electrochemical ends. The halides, tetrafluoroborate [BF]⁻⁴, tetrachloroaluminate [AlCl]⁻⁴, hexafluorophosphate [PF]⁻⁶, bis(perfluoromethylsulphonyl)imide-bistriflate imide [NTF2] and trifluoromethanesulphonate [F₃MeS]⁻ anions are the common ones present in the room temperature ionic liquids [75]. 1- ethyl-3-methyl imidazolium cation [EMIM]⁺, when combined with tetrafluoroborate [BF]⁻⁴, yields the final [C2MIM][BF4] room temperature ionic liquid with a hydrophilic character [76], while the same cation when combined with bis(perfluoromethylsulphonyl)imide-bistriflate imide [NTF]⁻², produces the [C2MIM][NTF₂] ionic liquid with a hydrophobic nature [77]. Even though there is still a limited number of room temperature ionic liquids for electrochemical applications, electrochemical sensors using room temperature ionic liquids to detect metals such as lead, cadmium, copper, and phenolic compounds have already been used. The focus was on the different works exploring gas sensors based on room temperature ionic liquids for applications ranging from environmental monitors to detectors of several relevant gases. Room temperature ionic liquids can be functionalized to enable selective recognition of gas molecules, which will avoid unnecessary interferences. Currently, there is the practical possibility of combining microelectrodes with room temperature ionic liquids for the replacement of the common electrode–membrane–electrolyte interface. This may lead to electrochemical gas sensors without the use of any membrane where multiple room temperature ionic liquids can be evaluated. They have been intensively investigated as electrolytes, solvents, and lubricants. The widespread appeal of room temperature ionic liquids relies somewhat on their environmental benevolence. The inclusion of this type of ionic liquids into industrial processes and their organic character motivated the study of their interaction with biomolecules and bio-organisms. Toxicity is also a measure of the high affinity between room temperature ionic liquid and biosystems. Such affinity in conjunction with the very adjustable chemical nature of the room temperature ionic liquids is the future pathway for pharmacology and biomedicine applications. It has been already shown, for example, that room temperature ionic liquids could stabilize proteins and enzymes [78] and penetrate, create pores, and destroy bio membranes [79]. Considering these facts, the interactions of room temperature ionic liquids with bio membranes is an important research topic. Given that the first encounter of a chemical species with a living cell is most likely to happen at its protective plasma membrane, these subjects merge the assessment and reduction in the cytotoxicity of the room temperature ionic liquids.

2.3. Chelate-Based Ionic Liquids

The chelate-based ionic liquids have gradually gained the attention of researchers. The ligands with nitrogen and oxygen atoms like polyethylene glycol, alkanolamines, and crown ethers will chelate with alkali metal ions. Some alkali metal chelated ionic liquids like [Li(TDA-1)][SCN] and [Li-tetraglyme][NTf2] have been developed using this method for sulfur dioxide capture. Nonetheless, it is not the best method for the desulfurization in flue gas since the interaction between the chelate-based ionic liquids and the low-concentration sulfur dioxide is weak.

Jiang et al. [80] proposed the synthesis of chelate-based ionic liquids via simple chelation in which chelate-based ionic liquids with tunable functionalized anions (azole), PEG 400 ligand, and lithium, sodium, and potassium cations were developed for sulfur dioxide capture. A strategy of tuning the anions was used to promote the action of the metal ions, anions, and ligands. The analysis retrieved from absorptions and spectroscopic examination and density functional theory determinations showed that the sulfur dioxide capture derived mainly from the nucleophilic action of the electronegative N site toward sulfur dioxide. Also, it was verified that the regulation of alkali metal ions demonstrated only minor influence on the sulfur dioxide absorption process. A preparation route for the chelate-based ionic liquids via facile chelation was implemented in which functionalized chelate-based ionic liquids were produced through the reaction between the ligands and the alkali metal salts of azoles. PEG400 proved to be a very suitable ligand for alkali metal salts of azoles to produce chelate-based ionic liquids, and [Na(PEG400)][Tetz] exhibited excellent desulfurization performance: a high available absorption capacity of 0.57 mol SO₂/mol ionic liquid, enhanced selectivity of 63 under 2000 ppm SO₂/15% CO₂, and improved reversibility.

The high-energy-density metal–air rechargeable batteries, like zinc–air batteries, attracted great research interest because of their prominent role in energy harvesting applications including hybrid and electric vehicles. Ionic liquids offer several properties as electrolytes in such applications. Kar et al. [81] prepared and characterized ‘‘chelating’’ ionic liquids for solubilizing and chelating the zinc cations to produce electrolytes for zinc cells. These are composed of quaternary alkoxy alkyl ammonium cations with diverse oligo-ether side chains and anions like p-toluene sulfonate, bis(trifluoromethylsulfonyl)amide, and dicyanoamides. It was shown that enlarging the chain length of the ether of the cation from two to four oxygens increased the ionic conductivity and reduced the melting temperature of the tosylate system from 671 to 151 °C. The alteration of the anion also has a strong influence in the deposition of the zinc process. It was shown that zinc can be reversibly deposited from [N222(20201)][NTf2] and [N222(202020201)][NTf2] beginning at 1.4 V and 1.7 V vs. SHE, respectively, but not in the case of tosylate ionic liquids. This demonstrates that [NTf₂] is a weaker coordinating anion with the zinc cation, compared to the tosylate anion, enabling the coordination of the ether chain to determine the stripping and deposition of the ions of zinc. The ionic liquids containing ether side chain cations, [N222(20201)]⁺, [N222(2020201)]⁺, and [N222(202020201)]⁺, demonstrate that an increase in the tosylate ether chain length decreased the melting temperature derived from the asymmetry addition in the cation. The flowability of the ionic liquids for a given anion augments with increasing ether chain length, and the associated increase in conductivity is minor. A Walden plot indicated that the tosylates are the least dissociated for a given cation and present evolutions in reference to the chain length in contrast to [N(CN)₂] and [NTf2]. The longer ether chains for [N(CN)₂] and [NTf2] ionic liquids tended towards the optimum Walden plot potassium chloride line. This shows that the anion has great impact on the alkoxy ammonium ionic liquids’ properties. The ability to deposit and strip zinc from these ionic liquids is also dependent on the anion: when using [N222(20201)] [NTf2], zinc is easily deposited from the ionic liquid, while the addition of water is required to favor reversible zinc in the tosylate ionic liquid. Comparing the alkoxy ammonium [NTf2] ionic liquids to the non-alkoxy ammonium [NTf2] ionic liquids such as [C2mpyr] [NTf2], where zinc cannot be reversibly deposited, suggests that the cation plays a significant role in the deposition of the metal ion. Even though it was observed that there is a lower viscosity in the [N222(202020201)] [NTf2] ionic liquid, the zinc electrochemical performance was more favored using the shorter chain ionic liquid, [N222(20201)][NTf2]. This suggests that this ionic liquid with the longer ether chain complexes stronger to the zinc cations.

Shang et al. [82] studied the chelated orthoborate ionic liquid-capped carbon quantum dot hybrid nanomaterials playing the role of friction reducers and anti-wear additives in the lubrication area of research. The carbon quantum dots were synthesized and, after that, covalently grafted with the ionic liquid 3-(hydroxypropyl)-3-methyl imidazolium bis(salicylate)borate (OHMimBScB) to produce carbon quantum dot (OHMimBScB) nanomaterials. The authors found that the prepared nanomaterials improved the lubricating action under a wide range of loads and boundary lubrication in reference to those of the polyethylene glycol, carbon quantum dots, ionic liquid, and their combination, reducing the friction coefficient by approximately 75%, 74.5%, 35%, and 38%, respectively, and the wear volume by 92%, 57%, 52.5%, and 51%, respectively. The combined effect of the carbon quantum dots’ lubricity, adsorption of the ionic liquid, and co-deposition of the borate and the carbon at the interface after the tribo-chemistry reactions protected the surface from wear and friction.

2.4. Ionic Liquid Hybrids

Diverse types of ionic liquids possessing embedded nanoparticles, also categorized as ionic liquid hybrids have been designed in the recent years [1]. These hybridized formulations put together the innovative characteristics of the nanoparticles and ionic liquids, favoring certain emerging potential applications in catalysis, separation, and electrochemical devices and systems. Stable suspensions of nanoparticles in the ionic liquids are obtained in the cases where the repulsive intermolecular interactions are dominant relative to the attractive intermolecular interactions between adjacent particles [83]. The ionic liquid may create a layer in the vicinity of the nanoparticles to avoid agglomeration and corrosivity. The nanoparticle suspensions in ionic liquids are commonly employed in electrochemical, catalytic, and separation processes. Regarding the catalytic ones, the metallic nanoparticles usually serve as catalysts, whereas the ionic liquids are explored for stabilization and protection of the nanoparticles to ameliorate the efficiency of catalysis and their recycling capability [84]. The well-developed ionic liquids with the addition of nanoparticle nanocomposites are adopted as particularly performant catalysts used in multiphase catalytic reactions [85]. The combined effect of the improved proton transport feature of the ionic liquids and the increased electron conductivity of the metallic nanoparticles or carbon nanotubes makes the nanoparticle suspensions within ionic liquids most adequate for electrochemical purposes including electrochemical sensors and batteries. The incorporation of nanoparticles in ionic liquids as electrolytes in batteries has already been demonstrated to enhance the cation coefficient of diffusion in the electrolyte [86], electrical conductivity [87], electrochemical and thermal stabilities [88], and to decrease the corrosivity potential [89]. The addition of nanoparticles with catalytic characteristics may reduce the overpotential of electrochemical reactions [1], in this way making them very suitable to electrochemical sensors with increased sensibility [90]. With specific intermolecular interactions and distinctive characteristics of developable structures, the nanoparticle suspensions within ionic liquids have been explored in separation processes, particularly when dealing with solid-phase extraction. The ionic liquids improved low volatility and thermal stability makes them a very promising option for use as volatile organic solvents, whereas the considerable surface-area-to-volume ratio of the nanoparticles promotes enhanced efficiency in separation processes [91]. Figure 6 schematically illustrates the ion clusters surrounding the nanoparticles and the generation of a protective electric double layer.

The modification of the surface of the nanoparticles can be accomplished via chemical bonding using ionic liquids. The final ionic liquid-grafted nanoparticles will exhibit the features of both ionic liquids and nanoparticles, and usually ameliorate the characteristics of isolated nanoparticles and pure ionic liquids [1]. The grafted ionic liquid can yield stable dispersions of nanoparticles in the liquid media that prolongs their lifespan in purposes including rechargeable electrolytes and catalysts. The ionic liquid-grafted nanoparticles gather the benefits of heterogeneous and homogeneous catalysts and aid in the separation of the catalyst after a reaction occurs [92]. The catalysts composed of ionic liquid-grafted nanoparticles having considerable reusability and recoverability may further enhance the yield and rate of reaction. The ionic liquid-functionalized nanoparticles can be adopted as electrolyte additives in which the ionic liquid-grafted nanoparticle structure enhances the electrochemical stability via double layer and steric interactions [93]. Figure 7 illustrates schematically the network structure of the ionic liquid 1,3-dialkyl-imidazolium with the incorporation of metallic particles in the ionic liquid network [94], in which steric and electrostatic stabilization was suggested through the production of an anion layer around the particles.

Apart from the dispersion of nanoparticles and surface functionalization, ionic liquids incorporating particles are produced in other materials to take advantage of the synergistic benefits of the final composite. Colloidal gels can be obtained when the particles interconnect with each other to create three-dimensional networks, whilst colloidal glasses can be formed in the case where the dispersed particles are trapped in the neighboring particles [94]. Also, other kinds of ionic liquid and nanoparticle composite structures embrace nanoparticles and ionic liquid crystalline hybrids and nanoparticle-stabilized ionic liquid microemulsions. Ionic liquid and nanoparticle composites are manufactured into the form of films or membranes for coating electrodes [95], gas separation, and for use as catalysts. An enhanced membrane performance has been attained following the inclusion of nanoparticles and ionic liquids. Membranes incorporating nanoparticles and ionic liquids have already been examined for gas separation. The observed ameliorated gas separation selectivity was attributed to the interactions between the nanoparticles and ionic liquids and the reaction media. The structure of the films enabled additional electrostatic interactions between charged objects and the nanoparticles, improving thermal and electrochemical stability and conductivity [96]. Enhanced response and sensibility were confirmed in the case of electrochemical sensors with films based on nanoparticles and ionic liquids [97]. A better understanding of the correlations involving structures, features, and interactions of nanoparticle and ionic liquid hybrids would be a valuable tool for the development of these nanomaterials according to different intended purposes. In this sense, and for an intended purpose, an optimized behavior at specific working conditions can be obtained through the precise selection of the ionic liquids and nanoparticles. The nanoparticles can be prepared and stably suspended in the ionic liquids without any clustering because of the stabilization induced by the ionic liquid [98]. Ionic liquids can create anionic–cationic layers in the vicinities of the nanoparticles inducing electrostatic forces [99]. The ionic liquids containing imidazolium have the tendency to produce clusters having the formula [(Im)X(X)X − n]n+ for the cationic cluster and the formula [(Im)X−n(X)X]n− for the anionic aggregate in which Im is the imidazolium cation and X represents the anion [1,100]. The double layer of the ionic liquid results in electrostatic interactions, which, with high efficiency, avoid the aggregation of the nanoparticles by balancing with the van der Waals forces between the particles according with the Derjaguin–Landau–Verwey–Overbeek model [101]. Apart from these interactions, the ionic liquids’ alkyl side chains stretch out from the surface of the nanoparticles, providing a secondary steric stabilization to prevent the particles being near to each other and agglomerating [101]. The hydrogen bonds in the ionic liquid structure and between the ionic liquid and the nanoparticle surface can also provide stable nanoparticles. In a work on gold nanoparticles synthesized and stabilized in the [BMIM][PF₆] ionic liquid, the supramolecular aggregates of the ionic liquid were observed to be generally coordinated with the gold particles [102]. Overall, it is hard to achieve stable dispersions of graphene and other carbon-based nanomaterials in water with any surface functionalization or introduction of surfactants. Nonetheless, reduced graphene oxide dispersions stable over time and with a thickness of nearly 0.9 nm were produced at a comparatively elevated fraction of 7 mg/mL in alkyl pyridinium and ionic liquids of 1-alkyl-3-methyl-imidazolium without the inclusion of any surfactant and/or stabilizer [103]. The carbon nanomaterials rich in π electrons like carbon nanotubes and graphene interact with the ionic liquids via cation-π short-range interactions and dispersion long-range interactions that may trap adjacent graphene nanoplates in the bi-layer structure of graphene without assembling in the graphite. Also, the stabilization provided by the ionic liquids and compatible polymers and surfactants can chemically graft or physically adsorb onto the surface of nanoparticles and aid in their stability [104]. The surfactant layer on the surface of the nanoparticles and the polymer will create repulsive steric interactions in the cases where it is under compression [105]. An innovative magnetic fluid was synthesized through the dispersion of magnetite nanoparticles having a diameter of 7.4 nm and coated with oleate in the 1-ethyl-3-methyl-imidazolium-ethyl-sulfate ionic liquid, including oleic acid, which generated double layers through hydrophobic interactions and extended out from the magnetite nanoparticles surface with steric repulsion to obtain an effective stabilization [106]. The polyionic liquids display the properties of both the polymer and ionic liquid, and can stabilize the dispersions of nanoparticles and, hence, can be used in nanoparticle production [107]. Via coating of their surface, the particles can be modified with polyionic liquids to prevent the nanoparticle clustering and to obtain extra features [108,109]. One triblock co-polymer from the ionic liquid imidazolium bromide was found to stabilize multi-walled carbon nanotubes and graphene in water [110]. The nanoparticles of nickel, silver, and gold nanoparticles were stable for periods superior to 75 days in poly-1-vinyl-3-alkyl-imidazolium polymer ionic liquids [111], which interacted electrostatically with the nanoparticles and adjusted their size, depending on the chain length of the polymer ionic liquids that synergistically enhances the nanoparticles’ stability in time. Figure 8 summarizes the possible interaction of the ionic liquids with ionic polymers or/and polymerized ionic liquids.

In specific cases, the nanoparticles are not dispersed in a stable mode in a solvent, because of the interactions and excessive concentration, and form solid soft materials called colloidal glasses and gels [1]. A colloidal gel is produced from the suspended nanoparticles generating a structural interconnected network [112], whilst a colloidal glass can be obtained in the case where high fractions of dispersed nanoparticles are impeded from getting in motion by the adjacent nanoparticles via the cage effect. The colloidal gels presenting a three-dimensional network of polymer and nanoparticles percolated over the ionic liquid are known as ionogels [35]. As an example, one ionogel may be synthesized by suspending silica nanoparticles at a weight concentration of 3% and with surface silanol Si-OH hydrophilic groups in 1-ethyl-3-methyl-imidazolium-bis-trifluoromethane-sulfonyl-amide ionic liquid because of the creation in the ionic liquid of interconnected networks of silica [113]. When dealing with surface-modified nanoparticles with grafted polymers, the affinity between the ionic liquid solvents and the grafted polymers defines the structure of the nanoparticle suspension. In an unsuitable ionic liquid solvent for the grafted polymer, the steric forces between the nanoparticles reduce and the polymer-grafted nanoparticles attach to each other and form ionogels [114]. Additionally, the formation of the colloidal glasses happens when the grafted polymers present good affinity to the base ionic liquid that provides intense repulsive interactions. At elevated concentration values, and depending on their surface tension and dimension, the nanoparticles may generate colloidal glasses [115]. That is the case in the suspension of the polymethyl methacrylate-grafted nanoparticles of silica in [EMIM][NTf₂], which form a colloidal glass having from 0.70% vol. to 0.74% vol. concentrations [116]. Playing the role of structuring media for the inorganic ionogel production, the natural ionic liquids’ properties and structure affect the nanoparticles interconnected network structure [117], resulting in colloidal gels based on ionic liquids for batteries because of their enriched ionic conductivity, while keeping a solid-like rheological behavior [118].

2.5. Ionic Liquid-Grafted Nanoparticles

Ionic liquids have the possibility to be grafted chemically onto nanoparticles’ surface. The so-called supported ionic liquids are immobilized ionic liquids on the surface of solid nanostructures such as nanoparticles [119]. These formulations retain the beneficial features of both ionic liquids and dispersed nanoparticles resulting in an advanced performance [120]. Also, the supported ionic liquids exhibit adjustable properties including superior thermal stability. This is the case for carbon nanotubes modified with ionic liquids of imidazolium having [Cl]⁻ or [Br]⁻ counter-anions, which were able to be dispersed in water, whereas the ones modified with imidazolium ionic liquids containing [BF₄]⁻ or [PF₆]⁻ hydrophobic counter-anions were dispersible in organic solvents like CHCl₃ [121]. The intense alkyl interactions between the ionic liquids and the graphitic support enabled the ionic liquids to be trapped on the graphene or carbon nanotubes, making possible the modification of the surface properties of the carbon materials by changing the synthesis temperature [122]. The supported ionic liquids can also alter the properties of the ionic liquids. This is the case for imidazolium ionic liquids immobilized on an SiOx nanoparticle surface, which were reported to have inferior melting temperatures to those of the ionic liquids themselves [123]. The van der Waals interactions between the nanoparticles and the creation of intermolecular bonds of hydrogen between the silanol groups present in the surface of SiOx nanoparticles and anions of the ionic liquids reduce the cation transport in the ionic liquids at the interface nearby region, trapping the cations in a superior entropy state, and, hence, result in a reduction in the melting point. Another type of grafted nanoparticles are the nano-scaled ionic materials made of a nanoparticle core and an oppositely charged canopy created via ionic pairing [1], which constitute a kind of grafted nanoparticle that has received major interest form the research community. The nanoscale ionic materials’ core can be functionalized with a charged corona, which can be an ionic liquid fraction. The nano-scaled ionic materials are also categorized as nano-scaled ionic liquids, since these materials share characteristics with the ionic liquid, like an ionic attraction between the nanoscale ionic material components and a vapor pressure approaching zero. Also, the nano-scaled ionic materials show fluid-like characteristics with solvent absence at ambient conditions, and for this reason they are also usually known as solvent-free nanofluids [124]. As an example, via surface functionalization using a thiol-containing ionic liquid followed by the bromide for sulfonate anion exchange [1], metallic nanoparticles such as palladium and gold can flow almost like a fluid in ambient conditions [125]. The nanoscale ionic materials produced through silica core surface modification with a monolayer of an alkyl silane coupled with amine-terminated polyethylene oxide-polypropylene oxide block co-polymer [126] canopy showed fluidic behavior at room temperature and solvent absence that was interpreted based on the fast exchange of the co-polymer canopy with the silica altered cores by the ions. Furthermore, the nano-scaled ionic materials composed of multi-walled carbon nanotubes had improved flowability with a viscosity ranging from 20 to 110 Pa∙S at a temperature of 30 °C [127], depending on the organic moiety molecular weight. Through altering the organic modifiers’ polyethylene glycol-substituted tertiary amines which have distinct molecular weights, the core–shell structure of the nanoscale ionic materials may be adjusted and the physical properties, including the rheological ones, can be tuned in accordance [1]. The modification of the surface of the nanoparticles with an electrostatically interacting layer may be further adopted to synthesize fluidic protein from solid-state proteins in the absence of a solvent. The fluid-like nano-scaled ionic materials may serve as functional ionic liquids and dispersions of surface-functionalized nanoparticles, which are very promising in applications like lithium-ion cells, catalysis, and carbon capture. Geng et al. [128] prepared silica nanoscale ionic materials—NIMs-silica—for obtaining enhanced mechanical characteristics indicated by the proton conductivity increase and improved fuel cell efficiency. Apart from being dispersed or grafted, the nanoparticles can also be added to long-range organized liquid crystals and emulsions. Luo et al. [129] synthesized an ionic liquid with the incorporation of clusters of polymethyl methacrylate-grafted iron oxide nanoparticles at low fractions, and it was observed to have distinct ionic conductivity from that of the pure ionic liquid. Also, the diffusivity degree of the 1-hexyl-3-methyl-imidazolium bis-trifluoro-methyl-sulfonyl-imide [HMIM][TFSI] imidazolium cations and polymer-grafted iron oxide nanoparticles was addressed via confinement. It was argued that the confinement facilitated by the elevated grafted nanoparticles’ concentration decreased the restricted volume of the HMIM-TFSI. The researchers also proposed that such increased diffusivity was due to the cation local ordering within the poly(methylmethacrylate)-grafted iron oxide nanoparticle clusters. Kubisa [130] argued that the usage of solvents based on ionic liquids for polymer-grafted nanoparticles or polymers is the route to follow for the full exploration of the interactions between the electrolyte and polymer. Furthermore, [HMIM][TFSI] may present specific interactions with the polymer via hydrogen bonding or ion–dipole forces. Li et al. [131] produced polymethyl methacrylate-b-polystyrene sulfonate PMMA-b-PSS copolymer-grafted iron oxide nanoparticles having diverse sulfonation degrees between 4.9% and 10.9% mol. SS, and corresponding ionic conductivity was observed in the mixtures of acetonitrile and HMIm–TFSI-acetonitrile. It was reported that the conductivity enhanced with an increasing fraction of nanoparticles in the acetonitrile which was caused by the clustering of the grafted nanoparticles, leading to the connectivity of the sulfonic domains. The ionic conductivity was reported to be associated with the hopping transport in the ion conductive channels. Oppositely, the ionic conductivity diminished or remained the same value with a growing nanoparticle fraction in the HMIm–TFSI-acetonitrile mixture. The research team attributed these findings to the ion coupling between the copolymer domains and ionic liquids. PMMA-b-PSS copolymer-grafted iron oxide nanoparticles at different sulfonation degrees were synthesized to determine the ionic conductivity of the nanoparticles solved in [HMIm][TFSI]. It was verified that the network of polymer-grafted particles formed ion pathways in which there is ion hopping due to the percolating sulfonated polymer in acetonitrile, which appreciably augments the conductivity with an increasing fraction of particles. Electrochemical impedance spectroscopy analysis confirms that the percolating structures run the ion transfer vua hopping instead of free diffusion. The nanoparticles in the [HMIM][TFSI] displayed the opposite tendency with a reduction in the conductivity with an increasing fraction of grafted polymer. Figure 9 shows the scheme of the [EMIM][PF₆] ionic liquid immobilized on silica nanoparticles’ surface [123].

2.6. Embedded Nanoparticles in Ionic Liquid with Preformed Structures

Nanoparticles can also be embedded in preformed structures composed of ionic liquids, including structures of long-range ordered liquid crystalline, films, and microemulsions. Also, ionic liquids with extended side alkyl chains exhibit an amphiphile character because of the polar cation head and non-polar part of the alkyl chain combined arrangement. This results in the formation of molten-state long-range ordered liquid crystalline structures via the long chain ionic liquids. These fluids can produce lyotropic liquid crystalline phases [132,133] with the aid of solvents like other ionic liquids [134]. Alternatively, the lyotropic liquid crystalline structures can be obtained through self-assembly processes in the ionic liquids of block co-polymers, lipids, and surfactants’ concentrated amphiphiles [135]. The nanoparticles can be included in the preformed structures of the ionic liquids and generate nanoparticle and liquid crystal composites due to the balance between the interactions of the selective nanoparticle–molecule, particle–particle excluded, and stretching in the molecules of the polymer [136]. Ionic liquids can be explored as oil and water substitutes and as surfactants to produce microemulsions [137]. Nanoparticles can accumulate at the interfaces between the ionic liquid and water or oil microemulsions [138]. The emulsions’ outer and inner droplets, like ionic liquid in oil, oil in ionic liquid, and ternary microemulsions, may be adjusted via modifying the volumetric concentrations of the fluid components and stabilizing nanoparticles’ concentrations [139]. On the other hand, it has been noted that the nanoparticles of copper in the 1-butyl-3-methylimidazolium tetrafluoro borate [BMIM][BF₄] ionic liquid and castor oil emulsion [140] had increased stability even after half a year of storage at ambient conditions, which offers benefits in certain purposes such as highly performant lubricants. Ionic liquids with embedded nanoparticles can be introduced into other materials. As an example, a film can be produced from ionic liquid-functionalized solid nanomaterials and nanoparticles [141] or, alternatively, through nanoparticle dispersions in polymer and ionic liquids mixtures [142]. Via the solid materials’ ionic liquid immobilization procedures that are usually designated by supported ionic liquids, the ionic liquid intended features can be imparted to the solid material substrate [143]. The nanoparticles can also be added to micelles, liquid crystals, and microemulsions produced from surface-active ionic liquids containing alkyl chains with increased length which have simultaneously polar and non-polar behaviors. Such categories of amphiphiles form self-assembled molten-state liquid crystals and aggregation of solvents such as glycerol, water, and ionic liquids in this complex, providing a strongly organized lyotropic liquid crystalline structure. Also, the localization of the nanoparticles in the lyotropic liquid crystalline structures formed the nanoparticle–liquid crystal–lyotropic liquid crystalline nanocomposites [144]. In this scope, Zhao et al. [145] produced lyotropic liquid crystalline phases via mixing the room temperature ionic liquid EAN (ethylammonium nitrate) and [C₁₄MIM][Cl], in which the latter played the role of surfactant. This hexagonal liquid crystal phase was explored for multi-walled carbon nanotube dispersions and formed an extremely high-conductive nanocomposite. The ionic liquids possess viscosities between 10 cP and 500 cP, which are much higher in comparison to the nanocomposite with water with a value of 0.89 cP, resulting in a decrease in the mass transfer rate. Therefore, it is very challenging to employ ionic liquids in emulsion systems as the aqueous or oil phase, as a surfactant, or in mixtures of two of such components [146]. According to Kataria et al. [147], the ionic liquids’ micellar surface areas relate to the corresponding alkyl chain length: the size of the micelles decreased with growing chain length. Guo et al. [148] prepared a polyaniline core decorated with the PANI-titania composite in an ionic liquid [C₄MIM][PF₆] and water emulsion. It was reported with cyclic voltammetry curves that the nanocomposite PANI-titania presented higher electrochemical catalytic activity in comparison to that of PANI. Pei et al. [149] fabricated high-temperature emulsions having ionic liquids to produce platinum nanoparticles with high porosity. It was observed that the nano-scaled droplets could be kept stable at 473 K for more than two and a half years in ambient conditions. Also, the hybrid films are synthesized by the inclusion of ionic liquids with nanoparticle dispersions to other surfaces. As an example, Shahrokhian and Rastgar [150] manufactured a film through the coverage of glassy carbon electrodes’ surfaces using multi-walled carbon nanotubes decorated with [C₁₂MIM][Br]⁻ gold nanoparticles. The resultant electrode was applied in the aqueous electro-oxidation processes of dihydroxy benzene isomers.

2.7. Quasi-Solid-State Ionic Liquids

The electrolytes composed of quasi-solid-state ionic liquid are very suitable for supercapacitor applications to accomplish the ever-increasing power density requirements of electronics. The usage of the different ionic liquid quasi-solid-state electrolytes is an adequate one to reduce the inherent environmental risk [151] related with the eventual corrosion of the components, fluid leakage, and easiness of assembly for energy storage systems with reduced dimensions [151,152]. The ionogels comprising ionic liquids with polymer matrices such as polyethylene oxide, polymethyl methacrylate, and polyvinyl alcohol, among others, are most promising considering the advantages of leakage avoidance, enlarged electrochemical stability, and ion transport at elevated temperatures [153]. Rana et al. [154] synthesized an ionogel electrolyte made of cellulose by mixing and phosphorylating a crystalline scaffold made of cellulose in the ionic liquid 1,3-dimethyl-imidazolium methyl phosphite, with a following polymerization process [151]. They reported a peak toughness of around 1.5 MJ.m⁻³ and significant mobility of the ions from 2.6 to 22.4 mS.cm⁻¹ in the 30 to 120 °C working temperature range with the developed ionogel electrolyte, providing the final activated carbon supercapacitor with excellent 174 F.g⁻¹ of specific electrode capacitance at 2.5 V [151] and a temperature of 120 °C. The polyionic liquids manufactured through polymerization in situ of monomeric ionic liquids are considered solid-state electrolytes with great potential, as the weakly bound ions in the polyionic liquids leads to an excellent compatibility with the ionic liquids and an increased ionic conductivity in comparison with that of the usual polymer matrices [152]. Additionally, Li et al. [155] produced polyionic liquid with the incorporation of halloysite nanotube electrolytes from the polymerization in situ of ionic liquid monomers and charged halloysite nanotubes. Taking advantage of the polyionic liquid nanocomposite electrolyte exhibiting increased mechanical stability and modulus up to 4.4 MPa and 26.7 MPa, respectively, the produced solid-state supercapacitor had noticeable electrode capacitance and improved cycling stability. Li et al. [156] incorporated the n-methyl-n-propyl-pyrrolidinium bis(fluorsulfonyl)imide-Pyr₁₃FSI ionic liquid into a hybrid network to produce gel polymer electrolytes. The properties of the gel polymer electrolytes were built by varying the network structure and ionic liquid fractions. The investigation team obtained an ionic conductivity greater than 1.0 mS.cm⁻¹ in ambient conditions. The obtained gel polymer electrolytes possessed improved electrochemical and thermal stabilities and were stable with the lithium anode. Also, the symmetrical lithium cells employing the gel polymer electrolytes exposed stable cycling for a time superior to 6800 h at a current density of 0.1 mA.cm⁻² and plating and stripping of lithium at a current density of 1 mA.cm⁻², being the highest confirmed current density until now using an ionic liquid gel polymer electrolyte. The Li-LiFePO₄ rechargeable batteries employing the developed electrolyte provided enhanced rate and cycling stability from 0 to 90 °C.

3. Properties of the Ionic Liquids

3.1. Melting Point

Tuning the chemical composition of the ionic liquids could permit operations at high temperature or other designated scenarios. In the stage of the component selection, the mixture of multiple inorganic salts with, for instance, lithium and potassium cations decreased the melting temperatures of ionic liquids according to the mixing energy of Gibbs. Similarly, combinations of the binary salts M [TFSA]-M′ [TFSA] and M[FSA]-M′ [FSA], where [TFSA] is the bis(trifluoromethylsulfonyl)amide salt and [FSA] is the bis(fluorosulfonyl)amide salt, resulted in a melting point inferior to those of single salts [157]. Moreover, salt systems with three components possess inferior eutectic temperatures than those of the salt systems composed of two components, which is confirmed in the Na[FSA]_0.4K[FSA]_0.25Cs[FSA]_0.3 ternary salt mixture that exhibited a 36 °C eutectic point [158]. Also, the use of inorganic salts has the extra beneficial characteristic of an elevated fraction of charged ions, which is truly an adequate route to produce even highly concentrated aqueous and organic electrolytes. Figure 10 presents the summary of the melting point influencing factors of the ionic liquids.

The main findings concerning the influencing factors of melting points of ionic liquids can be gathered in the next points:

• The ionic liquids with [NTf₂]⁻ anions have lower melting points in reference to the Br⁻, [BF₄]⁻, and [PF₆]⁻ anions [159];
• In general, the inclusion of an alkoxy group typically reduces the ionic liquids’ melting point [160];
• The ionic liquids presenting unsaturated alkyl tails possess low melting points [161];
• The melting point of hex isothiocyanate complexes can be decreased by enlarging the alkyl chains in the imidazolium cation [162];
• The balance between the van der Waals and coulombic interactions regulates the melting temperature of the ionic liquids presenting distinct lengths of the alkyl chain [163];
• The hydrogen bonding of the ionic liquids augments their melting point [164];
• A dense molecular packing may induce a relatively high melting point of ionic liquids [165];
• The polar C-I bond generates a molecular dipole that promotes a denser packing, enhancing the melting point of ionic liquids [166];
• A larger anion volume in ionic liquids results in lower melting points [167].

3.2. Viscosity

The ionic liquids are considerably viscous fluids, and their viscosity depends fundamentally on the cation–anion interactions. The viscosity of the ionic liquids is very superior to that of, for instance, water. The viscosity increases with increasing salt concentration and decreasing ionic conductivity and operating temperature. An alkyl chain increase usually leads to enhanced viscosity levels. Nonetheless, it is agreed in the ionic liquids field of research that this high viscosity is undesirable and hinders the wide application of these fluids in many applications like, for instance, separation processes. To diminish the viscosity of ionic liquids, some solutions have been studied like the one of Yang et al. [168], who used 2,2,2-trifluoroethanol as a solvent. The authors analyzed the effect of the solvent on three distinct ionic liquids, [EMIM][BF₄], [EMIM][Tf₂N], and [EMIM][OAC], and found a decrease in the hybrid liquid viscosity. The researchers argued that the main factor affecting the viscosity of binary mixtures of ionic liquids and trifluoroethanol was the hydrogen bonding, which decreased with increasing temperature of the solutions.

Li et al. [169] synthesized the 1-butyl-3-methylpyridinium (rFeCl₃/[BMP]Cl ionic liquid for removing hydrogen sulfide. Hydrogen sulfide gas is a toxic pollutant produced during the use of fossil resources, causing damage to human health and industrial infrastructure [170]. This ionic liquid, with different iron–pyridine ratios of 0.6, 0.8, and 1, showed low viscosity, enhanced hydrogen sulfide solubility, and improved thermal stability. Its low viscosity facilitates the handling and absorption of hydrogen sulfide, while its hydrophobicity prevented dilution through the water by-product.

To achieve efficient carbon dioxide capture, Liu et al. [171] developed a solvent of the ionic liquid diethylenetriamine serine-polyethylene glycol dimethyl ether-water [DETA][SER]-[NHD]-water. The authors attained a low viscosity through the weakening of the hydrogen bonds and van der Waals interactions in the ionic liquid by including the dimethyl ether and water to [DETA][SER]. The ideal mass ratio of [DETA][SER]/NHD-water was of 20:40:40 wt. The viscosity of the solvent was approximately 7.8 mPa.s, with a total absorption capacity of around 1.3 mol·mol⁻¹ ionic liquid. The dimethyl ether promoted phase separation. The authors highlighted the suitability of the solvent for carbon dioxide capture.

In summary, although ionic liquids possess higher viscosity in comparison to that of other organic solvents, this is most likely caused by the strong electrostatic interactions between the anions and cations that ultimately contribute to increased hydrogen bonding of the liquid. As demonstrated by these researchers, efficient solutions to reduce viscosity have been achieved by adding different types of solvents. Figure 11 summarizes the fundamental influencing factors on the ionic liquid viscosity.

3.3. Electrical Conductivity

The neat ionic liquids can conduct electricity by the motion of the ions in the liquid phase. Such a fact makes them different from the solid ionic compounds that must first be dissolved in a solvent to act as electrolytes.

This distinctive feature conducted in the investigation of the exploration of the ionic liquids in a broad range of energy storage ends entailing electric conduction like, for instance, electrolytes for electrochemical cells, supercapacitors, and hydrogen storage materials. Liu et al. [172] determined the electrical conductivity of hydrophobic functionalized ionic liquids. The functionalized ionic liquids were 1-(cyanomethyl)-3-methylimidazolium bis-(trifluoromethyl)sulfonyl-imide [MCNMIM][NTf₂] and 1-(2-hydroxyethyl)-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [EOHMIM][NTf₂]. The Vogel–Fulcher–Tammann and Arrhenius equations were used for fitting the electrical conductivity temperature dependence. The activation of electrical conductivity was calculated using the final version of the Vogel–Fulcher–Tammann equation. The molar conductivities were determined from the density, and electrical conductivity was determined from the empirical equation. The Walden equation was employed for determining the electrical conductivity, density, and viscosity. The incorporation of the functional group hydroxy and cyano in the imidazolium ring enabled hydrogen bonding. The functionalized ionic liquids showed greater viscosity and density and lower electrical conductivity than the non-functionalized ionic liquids. Moreover, Liu et al. [173] examined the effects of temperature, addition of the benzene and toluene solvents, and halogen salts on the electrical conductivity of quaternary ammonium salt-type AlCl₃ ionic liquids. The studied ionic liquids were phenyltrimethylammonium chloride, benzyltrimethylammonium chloride, benzyltrimethylammonium chloride, benzyltributylammonium chloride, and tetramethylammonium chloride. The authors found that the electric conductivity of the ionic liquids increased with increasing temperature. Phenyltrimethylammonium chloride-AlCl₃ showed the highest electric conductivity and smallest activation energy determined in accordance with the Arrhenius equation of around 19.3 kJ.mol⁻¹. Furthermore, the authors found that the incorporation of the mentioned solvents into the ionic liquids promoted significant enhancements in the electric conductivity with increasing volumetric portion of the solvents. In the case where the mixed systems had roughly 50% vol. of organic solvents, their electrical conductivity reached the maximum of 14.5 mS.cm⁻¹. Among all the alkaline halides tested, the solubility of LiCl in phenyltrimethylammonium chloride-AlCl₃ was the highest, presenting a value of around 50 g.L⁻¹. At temperature values superior to 60 °C, the electrolyte containing 2 g LiCl exhibited the highest electrical conductivity of nearly 7.5 mS.cm⁻¹, when the concentration of LiCl in the electrolyte was close to saturation. Zhang et al. [174] prepared stimulus-responsive ionic liquids composed of azobenzimidazole ionic liquids presenting reversible photo-induced electrical conductivity adjustment. The electrical conductivity alteration under ultraviolet–visible irradiation in aqueous solution was examined, and the influence of certain factors like the concentration and chemical structure of the ionic liquids with azobenzene to the regulation of photo-responsive electrical conductivity was also addressed. The obtained results confirmed that the exposition of the ionic liquid aqueous solution to the ultraviolet light increased considerably their electrical conductivity. Also, the researchers stated that the ionic liquids containing longer alkyl chains had the greatest electrical conductivity enhancements, which reached 75.5%. Also, under visible light irradiation, the electrical conductivity of the solution returned to its starting value. More investigation on the reversible photo-induced conductivity regulation of azobenzene ionic liquids’ aqueous solution suggested that this fact could be due to the production of the ionic liquid aggregates in aqueous solution by the isomerization of azobenzene under ultraviolet–visible irradiation and resulted in reversible conductivity regulation. Also, the electrical conductivity of the composites made of ionic liquids and nanoparticles has a great deal of importance. The electrical conductivity of the ionic liquid hybrids is affected by the free electron/ion and, consequently, the ions and nanoparticles in the ionic liquid composites contributed to an increased electrical conductivity of their own. Alizadeh et al. [175] obtained the electrical conductivity of hybrid ionic liquids of germanium nanoparticles suspended in [Bmim][PF₆] and confirmed that their electrical conductivity augmented with a rising fraction of nanoparticles and temperature. The peak electrical conductivity was found to be approximately 25 mS/cm at a temperature of 30 °C. Chereches and Minea [176] obtained the electrical conductivity of hybrid ionic liquids composed of a mixture of [C₂mim][CH₃SO₃] ionic liquid and water with dispersions of nanoparticles made of alumina. The researchers confirmed a huge electrical conductivity enhancement close to 300% in comparison to that achieved with the pure ionic liquid. The research team noted that the enhancements in the nanoparticle concentration diminished the electrical conductivity of the ionic liquid hybrids that was attributed to the change in the viscosity induced by the growing effect of the nanoparticles in the ionic liquid. Additionally, consistent results were observed in the case where the [MMIM][DMP] aqueous environment increased the electrical conductivity of the hybrid ionic liquids [177]. Deb et al. [178] determined the electrical conductivity of zinc sulfide nanoparticles dispersed in the ionic liquid [Deim][NTf₂] at various fractions and temperatures. The investigation team reported that the hybrid ionic liquids’ electrical conductivity was enhanced with increasing concentration of the nanoparticles up to a particular limit and beyond this limit it strongly reduced. Figure 12 gathers the fundamental influencing factors of the electrical conductivity of ionic liquids.

3.4. Ionic Conductivity

The study of the correlation between the ionic conductivity and the structure of the ionic liquids suggests that the core types of the anion and cation and hydrocarbon chain length seriously impact the ionic conductivity trend of the ionic liquids. Also, it was already found that the interactions between anions and cations are relevant factors for finding the ionic conductivity, with the greater ionic conductivity of some type of cations merely resulting from their combination with specific anion types. The relationship between the ionic conductivity and other ionic liquid properties showed that the ionic conductivity markedly correlates with the other transfer properties including the diffusion coefficient and viscosity, while it was not verified that there were any correlations with volume characteristics like density. Such facts suggested that ion migration is the most influential parameter determining the ionic conductivity of ionic liquids. Figure 13 summarizes the fundamental influencing factors of the ionic liquid ionic conductivity.

3.5. Electrochemical Stability

Several factors influence the electrochemical stability of ionic liquids, namely the chemical structure defined by the type of anions and cations, the functional groups, C2 methylation, alkyl chain length, the position of the substituents, and non-structural parameters like reference and working electrodes, dynamic viscosity, conductivity, melting temperature, cut-off current density, operating temperature, and pH. The importance of the structural parameters can be ordered as follows: anion type > cation type > functional groups > alkyl chain length > C2 methylation > position of the substituents. The ionic liquids with the anion of tris(perfluoroalkyl)trifluorophosphate [FAP] or bis(trifluoromethylsulfonyl)imide [Tf₂N] tend to have increased electrochemical stability. Piperidinium, phosphonium, pyrrolidinium, and azepanium show higher electrochemical stability than ammonium, imidazolium, and pyridinium anions. A decreased functionalization, C2 methylation, and alkyl chain length do not imply a higher electrochemical stability, which is dependent of the nature of cations, type of anions, and interaction between cations and anions. The effect of non-structural factors on electrochemical stability of the ionic liquids appears to be more complex to interpret. The oxygen and moisture in the air would generally narrow the electrochemical window, whereas the addition of water in different ionic liquids can also enlarge the electrochemical window. The melting temperature, viscosity, ionic conductivity, and pH could also be considered useful in predicting the electrochemical stability. Nonetheless, the wettability, junction potential, and eventual leakage of the solvents affect the electrochemical stability. The ionic liquid electrolytes in lithium batteries are mainly the ones containing the anion of [Tf₂N][FSI] or cation of pyrrolidinium/piperidinium/imidazolium. Also, apart from the electrochemical window, the anodic and cathodic limits and electrochemical decomposition mechanisms should be considered to evaluate electrochemical stability. The exploration of stable reactants will increase the electrochemical stability of the ionic liquids. Oppositely, the reactants having electrochemical decomposition tendency will lead to the production of ionic liquids with narrow electrochemical windows. For instance, using Tf₂N or FAP salts as reactants of ionic liquids would make them more stable, whereas when using the iodide anion prepared from iodide salts, the electrochemical stability of the ionic liquids will be poor. Moreover, the non-structural parameters like temperature, pressure, time, impurities, cut-off current density, and pH should also be considered. The cut-off current density should be 1 mA.cm⁻² to obtain an improved comparison. The processing of ionic liquids with high electrochemical stability should consider the ions and their interactions. The more stable anions and cations will increase the stability of the ionic liquids. The strengthening of the interaction between the anion and the cation could be favorable to producing ionic liquids with elevated electrochemical stability. The strong intermolecular interactions associated with ionic liquids would protect the ionic liquids from being oxidized or reduced by electricity. For instance, polymerization of ionic liquids is a beneficial approach for strengthening the interaction between the cations and anions, which could enlarge the electrochemical stability of the electrolytes based on ionic liquids. Figure 14 summarizes the fundamental influencing factors of the electrochemical stability of ionic liquids.

3.6. Hygroscopicity

Hygroscopicity is also a relevant property of ionic liquids, and it should be properly addressed. As any charged system is hygroscopic, so are the ions in an ionic liquid. The study of hygroscopicity of ionic liquids entails certain research issues like the great difficulties associated with the production and maintenance of totally dry ionic liquids. If the ionic liquids are not completely dry, then the water can be easily decomposed in electrochemical applications at much lower potentials than the electrochemical stability window of the ionic liquids themselves. Also, the molecules of water can cause the ions in the ionic liquid to degrade (hydrolyze) as described by Freire et al. [179]. The authors also addressed the main influencing factors of the hygroscopicity of the ionic liquids like the cation side alkyl chain length, pH, and temperature of the solutions. Figure 15 summarizes the main influencing factors of the hygroscopicity of ionic liquids.

3.7. Polarity

A major part of the published works reported that ionic liquids had intermediate- to-high polarity. The hydrogen bond interactions between the measuring probe and the ionic liquids or between the ionic liquids themselves enhanced the polarity, whereas the polarity was reduced with increasing alkyl chain length. However, the effect of the cations and anions are still not yet completely clarified. The key limitation related to the polarity of the ionic liquids is the absence of a theoretical model to describe in detail the solvent effect of the ionic liquids. It is of importance to detail the correlation between the distinct polarities of the ionic liquids and the interactions between the ions of the probe and the ionic liquids. Moreover, the equilibrium and kinetic rate constants of the chemical reactions also have an influence. Numerous types of functionalized radicals can be employed to evaluate the different solvent–solute interactions in the ionic liquids as they are sensitive to the polarity of the ionic liquids. It may be more practical to determine the rotation correlation times simultaneously. Like the electron paramagnetic resonance probes of the transition metals, the metals in the chelate-based ionic liquids are suitable probes for the polarity of the ionic liquids by using their intrinsic electron paramagnetic resonance signals. Finally, molecular simulations could be used to determine the hydrogen bond length and the interaction energy of the ionic liquids, enabling the determination of the polarity of the ionic liquids. Figure 16 presents the main influencing factors of the polarity of ionic liquids.

3.8. Thermal Stability

The thermal stability of ionic liquids at room temperature, atmospheric pressure, and short operating time could be considerable for some sensitive ionic liquids. Factors like cation and anion nature, chain length, C2 methylation, and functional group are the fundamental influencing factors of the thermal stability of ionic liquids. Also, the operating parameters should be seriously considered including temperate, time, and pressure. Many routes can be followed for the thermal stability enhancement of ionic liquids via changing their chemical structure. Also, the thermal stability of ionic liquids is partially defined by the thermal stability of their components: the choice for raw materials with high thermal stability usually augments the ionic liquids’ thermal stability. The selection of adequate cations and anions, C2 methylation, alkyl chain length, and functional groups regulate the thermal stability of ionic liquids. Moreover, a higher bond strength between the anions and cations of the ionic liquids contributes to an increased thermal stability. In addition, there are several types of stability to consider when dealing with ionic liquids including thermal, electrochemical, chemical, and radiolytic, and all these must be balanced. Amongst all the existing types of ionic liquids, ionic liquid gel/polymer thermal stability is especially high, which is caused by their strong interactions, apart from the ionic bond between the constitutive anions and cations. Figure 17 summarizes the fundamental influencing factors of the thermal stability of ionic liquids. Figure 18 summarizes the fundamental routes for thermal stability enhancement of ionic liquids.

4. Electrochemical Applications of the Ionic Liquids

The next sections of this work will address the main electrochemical purposes of the ionic liquids, for which suitability is dependent fundamentally on the ionic liquids’ nature as shown in Figure 19.

4.1. Rechargeable Metal Batteries

4.1.1. Lithium Batteries

In recent years, ionic liquids have been investigated for their potential to be adopted as novel electrolytes. Nonetheless, the significant ionic conductivity feature of these fluids may be mitigated through their high viscosities, hindering the electrochemical analysis of batteries working at medium-to-high temperatures. Figure 20 presents the possible functions of the ionic liquids in rechargeable metal batteries. Figure 21 summarizes the main benefits and disadvantages of using electrolytes based on ionic liquids in rechargeable metal batteries.

In the review work performed by Chen et al. [180], the safety issues that have limited the design and implementation of lithium cells were discussed. The research team highlighted that the inclusion of little amounts of charges or additives into the classic fluidic electrolytes could avoid misuse without compromising the electrochemical behavior. The electrolyte can be considered the main component in a battery as it aids in the ion transport between the electrodes of the battery. In these circumstances, ionic liquids can replace traditional fluidic electrolytes, increasing the efficiency, safety, and reliability of lithium-ion batteries. Rana et al. [181] investigated the use of ionic liquids as electrolytes operating in lithium rechargeable batteries. The authors noted that ionic liquids became more competitive electrolytes when compared to those composed of volatile organic compounds. Tsurumaki et al. [182] prepared ionic liquids containing [FSI]⁻ anions and ether oxygens without any crystallization, exhibiting increased ionic conductivity. The authors prepared a molar fraction 9:1 mixture electrolyte of [P1,2O₂][FSI]-LiFSI and [M1,2O₂][FSI]-LiFSI. [P1,2O₂][FSI]-LiFSI had the best ionic conductivity of 2.4 × 10³ S.cm⁻¹ at room conditions, a small interfacial resistance on surfaces of lithium, and an extended electrochemical stability window in comparison to those of the electrolyte [M1,2O₂][FSI]-LiFSI. Derived from these beneficial features, the Li-LiFePO₄ battery working with the electrolyte [P1,2O₂][FSI]-LiFSI revealed a capacity up to 150 mAh.g⁻¹ when the cell was cycled at C/10. Nirmale et al. [183] showed that a modification with an anion of the imidazolium cation enhanced the diffusion and, consequently, the cyclability and ionic conductivity of the system. The researchers synthesized [C6MIM₂][TFSI]₂ ionic liquid and inferred its potential to be applied in lithium rechargeable batteries. The maximum ionic conductivity of approximately 1 × 10⁻³ S.cm⁻¹ at a temperature of 30 °C and an electrochemical stability window reaching 5.3 V constitute themselves as considerable marks by far. A lithium-LiFePO₄ battery employing a dicationic ionic liquid at a 0.1 C rate exposed 133 mA.h.g⁻¹ of capacity after completing 100 cycles, presenting a coulombic efficiency of around 98.8%. Even under greater current rates, the battery retained the starting capacity of discharge, which indicated enhanced stability and reversibility. The batteries exhibited discharge capacities of approximately 113 mAh.g⁻¹ and 74 mAh.g⁻¹ at rates of 0.2 C and 0.5 C [183], respectively. The dicationic ionic liquid behavior at ambient conditions showed that the aggregate production and mobility influenced the lithium-ion diffusion and coordination. The imidazolium dicationic ionic liquids were produced, characterized, and employed as electrolytes for lithium-ion rechargeable batteries. It was found that the ionic conductivity was reduced with the increasing alkyl chain length that was caused by the viscosity enhancement with growing alkyl chain length of the dicationic ionic liquids. The peak conductivity of the [C6MIM₂][TFSI]₂ ionic liquid at a temperature of 30 °C was nearly 1 × 10⁻³ S.cm⁻¹ that augments to around 6.6 × 10⁻³ S.cm⁻¹ at a temperature of 70 °C. The observed electrochemical stability window reached 5.3 V, which was much greater than that of other commercially available electrolytes and certain ionic liquids based on imidazolium. Having such greater anodic stability, the dicationic ionic liquid may be explored for high-voltage cathode materials like, for instance, LiCoO₂ and LiMn₂O₄. A lithium-ion cell using [C6MIM₂][TFSI]₂ was cycled at a 0.1 C rate and was demonstrated to possess a capacity of 133 mA.h,g-1 after the completion of one hundred cycles having a coulombic efficiency of 98.8%. Upon the completion of one hundred cycles using a greater current rate, the battery kept its initial discharge capacity. In essence, the [C₆MIM₂][TFSI]₂ dicationic ionic liquid proved to be a very suitable choice instead of flammable electrolytes for lithium rechargeable batteries and can be tuned for high-temperature and flexible lithium cells [183]. It was verified that the electrochemical and thermal characteristics of the ionic liquids come directly from the structural arrangements of anions and cations. The formation and diffusion of clusters alter the structures of the ionic liquids and the structural alteration may induce slight variations in the physicochemical characteristics and interactions [183]. Regarding the anion [TFSI]⁻, four anions can coordinate one ion of lithium. A negatively charged aggregate hinders the lithium-ion transfer, deteriorating the cell performance. Such aggregates charged negatively are repelled by the electrodes hindering the diffusion of the ions of lithium [184]. Some investigation teams have employed organic solvent diluents to decrease the aggregation [184], but these compounds possess organic flammable fluid components. The most adequate substitutes for such solvents are the rationally developed ionic liquids, since with the specific design of ionic liquids the agglomerates can be broken to achieve an advanced electrochemistry of the lithium [183]. It was confirmed earlier by molecular dynamics simulation that the inclusion of LiTFSI in ionic liquids resulted in the segregation of the ionic liquid anions in the vicinity of the lithium ion, forming an extra-rigid structure [183]. Figure 22 shows the time evolution of the lithium-ion rechargeable batteries employing electrolytes of ionic liquids.

Also, the dicationic ionic liquid structure remained unchanged with the incorporation of the lithium cation in comparison to a monocationic ionic liquid. Furthermore, when dealing with the higher alkyl chain dicationic ionic liquids [C₁₂MIM₂][TFSI]₂ and [C₉MIM₂][TFSI]₂, the ion conduction of such ionic liquids was inferior to that of the [C₆MIM₂][TFSI]₂ ionic liquid. Hence, the researchers chose the [C₆MIM₂][TFSI]₂ dicationic ionic liquid for future battery studies. Regarding the imidazolium ionic liquids, the hydrodynamic radius reduces with increasing length of the alkyl chain, being also the alterations in the hydrophobic interactions of relevance [183]. The dicationic ionic liquids ionic conductivity was less influenced through the addition of the lithium salt when compared to what happened to a monocationic ionic liquid that was caused by the structure arrangement during the lithium-ion coordination [185]. The dicationic ionic liquids can produce diverse lithium-ion aggregates including [LiTFSI₂]⁻ and [Liy/2(TFSI)y](y/2). The crosslinking of various anions [TFSI]⁻ in the second structure may impact the properties. There are also viable configurations such as [LiTFSI₄]³⁻, despite possessing a short lifespan. The triplet structure of [LiTFSI₂]⁻ involves less crosslinking [183] because of the larger size of the cation [186]. The structural combinations in the vicinities of the lithium ion were not observed with monocationic ionic liquids and, thus, it was found that the dicationic ionic liquid offered a greater ionic conductivity to the lithium-ion and was proven to be a very suitable option against the conventional electrolytes made of organic fluids. Additionally, ionic liquids can interact with the solvent molecules that can diminish the interactions between the water and solvents, in this way averting the electrolyte deterioration and enlarging its stability. The C₁₄-Im-C₁₄-Br ionic liquid crystal molecules preferred to absorb on the surface of the electrodes, which is demonstrated by the shifted peak to decreased potentials in cyclic voltammetry, and, thus, could decrease the activation energy of the oxidation and reduction reactions at the electrode [187]. The ionic liquid crystal can increase the inclusion of magnesium cations through the adsorption of counter-ions in the electrolyte solution, in this way enhancing the capacity of the magnesium cell without any loss at 300 mA.g⁻¹ after two hundred and fifty cycles [187,188]. Liu et al. [189] evaluated the influence on the electrochemical efficiency of concentrated organic cation ionic liquid electrolytes. The authors examined the behavior of lithium rechargeable cells with the comparison of electrolytes with the cation 1-butyl-1-methyl-pyrrolidinium [Pyr14]⁺ or 1-ethyl-3-methyl-imidazolium [EMIM]⁺ cation. It was confirmed that the organic cation structure in the electrolytes of locally concentrated ionic liquids had only a little impact on the coordination of the lithium cation bis-(fluorosulfonyl]imide anion FSI⁻. Nonetheless, the coordination between the organic cations and the [FSI]⁻ anions was distinct. The lower coordination of the [FSI]⁻ anion to [EMIM]⁺ than to [Pyr14]⁺ cations lead to a decreased viscosity and faster mobility of the ions of lithium in the [EMIM]⁺ electrolyte than in the electrolyte [Pyr14]⁺. A more stable interphase between the electrolyte and the solid was grown in the presence of the [EMIM]⁺ cation and resulted in a higher lithium stripping and plating coulombic efficiency of 99.2%. Hence, the cells of litium-EmiBE-LiNi0.8Mn0.1Co0.1O₂ were demonstrated to have 185 mAh.g⁻¹ of capacity at 1 C discharge and kept 96% of the initial capacity after two hundred cycles. At similar conditions, the cells of PyrBE exhibited merely a capacity of 34 mAh.g⁻¹ with around 40% retention. The elevated amount of the [EMIM]⁺ cation with reference to the [Pyr14]⁺ cation led to a solid electrolyte interface with increased stability. In essence, the exploration of the organic cation [EMIM]⁺ was very efficient for the regulation of the transport of the lithium cations and chemical composition of the solid-electrolyte interface generated on the surface of the lithium metal anode, resulting in improved rate capability and cyclability of the lithium batteries. Swiderska-Mocek [190] developed an electrolyte to be used in lithium cells composed of a lithium salt solution in an imidazolium cation with vinyl group ionic liquid. The electrolyte was synthesized by the dissolution of solid lithium bis-trifluoro-methane-sulphonyl-imide [Li][NTf2] in 1-ethyl-3-vinyl-imidazolium bis-trifluoro-methane-sulphonyl-imide EVImNTf2. Moreover, 1 molar of [Li][NTf2] in the electrolyte [EVIm][NTf2] exhibited enhanced cathodic stability promoted by the vinyl group. The LiFePO₄ olivine lithium iron phosphate, graphite-lithium anode, LiFePO₄ cathode, and electrolyte were evaluated with electrochemical impedance spectroscopy, galvanostatic charge and discharge cycling, and cyclic voltammetry. The discharge and charge experiments of LiFePO₄-1 molar LiNTf2 in the cell Lithium-EVImNTf2 at diverse C rates showed increased specific capacities of 115 and 110 mA.h.g⁻¹ at 0.5 and 1 C, respectively. The anode of graphite revealed improved cyclability exposing 307 mA.hg⁻¹ after the completion of forty cycles and a coulombic efficiency of 94%. The efficiency of the cell LiFePO₄-electrolyte charging–discharging was 125 mA.h.g⁻¹ after the completion of 30 cycles.

4.1.2. Sodium Batteries

The rechargeable ambient temperature sodium metal batteries can be relevant to many energy harvesting ends. The implicit requirement for superior energy density must be accompanied by an increased durability and safety, which is rather complex to obtain using electrolytes of organic solvents and facilitated when using electrolytes based on ionic liquids. Especially challenging in the sodium cells field is the achievement of enhanced stability at elevated discharge and charge rates.

Makhlooghiazad et al. [191] examined polymerized ionic liquid to be applied as sodium batteries’ solid polymer electrolytes, namely a di-block copolymer, polystyrene–polyethylene adipate [BuI][TFSI]. The authors studied binary and ternary electrolyte systems by combining the polymer with salt and the ionic liquid [C₃mpyr][FSI]. The inclusion of the salt caused a plasticizing action in the polyionic liquid phase, producing binary electrolytes with enhanced ionic conductivity, and the incorporation of the ionic liquid increased the plasticizing effect, enlarging the ionic conductivity. Also, the researchers reported that the inclusion of [Na][FSI] and ionic liquid impacted the [TFSI]^– anion conformation and decreased the interactions between the polymer and the [TFSI]^– anion. The solid polymer electrolyte operating in sodium–sodium symmetrical cells showed stable sodium stripping/plating at elevated densities of current that reached 0.7 mA.cm^–2, maintaining its integrity at a temperature value of 70 °C. Moreover, it was inferred that the electrochemical performance of batteries composed of sodium-NaFePO₄, which was cycled at C/10 and C/5 rates at 50 °C and 70 °C, demonstrated enhanced coulombic efficiency and capacity retention.

Also, Forsyth et al. [192] prepared mixtures of inorganic–organic cation fluorosulfonamide [FSI] ionic liquids exhibiting elevated sodium cation transference numbers derived from the structural diffusion mechanism. The researchers studied the influence of the concentration of the NaFSI salt in the methylpropylpyrrolidinium [C₃mpyr][FSI] ionic liquid on the reversible plating and dissolution of sodium, both on a copper electrode and in a symmetric sodium cell. Even though the ion diffusivity decreased considerably with increasing alkali salt concentration, it was found that these elevated sodium cation-containing electrolytes can bear greater current densities of 1 mA.cm⁻² and exhibit greater stability upon cycling. The electrochemical impedance spectroscopy technique showed that the interfacial impedance is reduced in the high-concentration systems that provides for a low-resistance solid-electrolyte interphase, resulting in a faster interfacial charge transport. The techniques of PFG diffusion, NMR spectroscopy, and the molecular dynamics simulations revealed the change to an easy structural diffusion mechanism for sodium ion transport at elevated concentrations in these electrolytes. It was verified that a [Na][C3mpyr][FSI] electrolyte could exhibit a sodium cation transference number superior to 0.3, which can be compared to the electrochemical performance of a lithium-ion electrolyte. The EIS findings showed that a larger sodium ion concentration diminished the surface impedance of the electrode, accounting for the lower polarization potential electrochemical cycling of sodium between 50 °C and 75 °C, despite a reduced ionic conductivity. Such cycling performance was similar to that of an already reported lithium FSI system, suggesting that a similar structural diffusion mechanism is also dominant in those mixed ion electrolytes. It was also verified that higher salt concentrations resulted in a lower surface impedance, which resulted in lower polarization potentials and a higher current charging of 1 mA.cm⁻² in comparison to that of previous works, which promoted lower concentrations of NaFSI. The highest salt concentrations led to stable symmetric cell cycling at higher current densities and indicated that the cyclic voltammetry cannot predict the performance of a given electrolyte in a device configuration. It also indicated that the interfacial layer formed when using the most concentrated electrolytes was dominant in relation to the ionic conductivity of the electrolyte, considerably enhancing the sodium cycling behavior. The molecular dynamics simulations indicated that for a highly concentrated sample of 50% mol. clustering of the sodium cation and [FSI]⁻ ions occurred with one FSI anion coordinating with two sodium ions. This enabled a site exchange or structural diffusion mechanism for the sodium cation considering its higher transference number. The data retrieved from the NMR spectra also supported a fast exchange for the sodium ion between different coordination environments, especially at higher temperature values and concentrations.

Wang et al. [193] prepared butylmethylpyrrolidinium-bis(trifluoromethanesulfonyl)imide [BMP][TFSI] ionic liquid containing NaClO_4. NaTFSI, NaPF₆, and NaBF₄ sodium solutes as electrolytes in sodium–Na_0.44MnO₂ batteries. The battery with the NaClO₄ containing ionic liquid electrolyte showed the best charge–discharge behavior because of the smallest electrolyte–electrode interface and charge transfer resistances at the electrodes of sodium and Na_0.44MnO₂. This last electrode showed an optimal 97 mAh.g⁻¹ of discharge capacity at 25 °C and 0.05 C. At a temperature of 75 °C, the capacity of the Na_0.44MnO₂ in the ionic liquid electrolyte containing NaClO₄ was high and equal to 115 mAh.g⁻¹ at 0.05 C, approaching the theoretical capacity value of 121 mAh.g⁻¹. Additionally, 85% of this capacity was retained in the case where the charge–discharge rate was augmented to 1 C. After the completion of one hundred cycles, it retained 83% of its initial capacity.

Bellusci et al. [194] produced sodium ion conducting mixtures of 1-ethyl-3-methyl-imidazolium, trimethyl-butyl-ammonium, and N-alkyl-N-methyl-piperidinium ionic liquids. The sodium-bis(trifluoromethylsulfonyl)imide, [Na][TFSI], was selected for salt. The NaTFSI-ionic liquid electrolytes were examined, and their stability and ionic conductivity were compared as a function of the working temperature, the anion type, and the cation aliphatic side chain length. The 0.1NaTFSI-0.9ionic liquid mixtures showed beneficial features as ionic liquid electrolytes for sodium cells. The typical room temperature conductivities for electrochemical devices, overcoming by far 10⁻⁴ or 10⁻³ S.cm⁻¹, were accomplished by all the tested ionic liquid mixtures. The imidazolium cation-containing electrolytes exhibited the best ion transfer features, particularly at low temperatures, for instance, at a negative temperature of −20 °C, values between 10⁻⁴ and 10⁻³ S.cm⁻¹ were reported. The [FSI] mixtures exposed faster transport properties than those of the [TFSI]⁻ and IM14. A conductivity fair superior to 10⁻² S.cm⁻¹ was reported at 80 °C. The 0.1NaTFSI-0.9IL electrolytes were reported to be thermally stable up to 275 °C for the [EMI][TFSI] electrolyte apart from the [FSI] ones that are stable up to 150 °C. This enabled these electrolytes to be employed in cells working above 100 °C that are not allowed in standard organic solutions, increasing the safety and reliability of the equipment, particularly when extended overheating occurs.

4.1.3. Aluminum Batteries

The aluminum-ion batteries are a very attractive energy storage media derived from their intrinsic characteristics like enhanced safety, energy density, recyclability, and the natural abundance of the aluminum metal. Nonetheless, this type of cell poses important challenges including a better knowledge of the aluminum oxide passive layer on the aluminum metal anode, narrow electrolyte electrochemical windows, and difficulty associated with adequate cathodes.

Currently, the research community pays great attention to the anhydrous organic electrolytes. Some of these electrolytes are the acidic chloroaluminate ionic liquids with nitrogen-centered cations including imidazolium and pyridinium that have already demonstrated their usefulness as electrolytes for aluminum-ion batteries. Thus, the ionic liquids based on 1-ethyl-3-methylimidazolium chloride and aluminum trichloride salt [EMIM]Cl: AlCl₃ in a 1:1.3 molar ratio entail enhanced electrical conductivity and aluminum reduction and oxidation reactions due to the Al₂Cl₇ ions present in this ionic liquid. Nonetheless, this electrolyte is costly, and exhibits high viscosity and corrosivity. Aluminum-containing liquid coordination complexes can be produced by mixing of anhydrous AlCl₃ with neutral donor molecules with lone electron pairs and are capable of coordinating on the Al3þ ion. It is hypothesized that the donor coordination favors the asymmetric cleavage of the Al₂Cl₆ dimer furnishing aluminum-containing species, namely cations [AlClm(L)n], chloro aluminum anions, mostly AlCl₄ and Al₂Cl₇, and [AlCl3(L)x] complexes. Several donor molecules including dialkylsulfones, dipropylsulfide, 4-propylpyridine, and 1-butylpyrrolidine, have been explored to produce aluminum with liquid coordination complexes for electrochemical applications. The chloroaluminate ionic liquid analogues composed of amides of fluorinated carboxylic acids, namely 1-trifluoroacetyl piperidine [TFAP], have been described. The application of these electrolytes has been studied for [TFAP][AlCl₃], [TFAP][LiCl][AlCl₃], [TFAP][EMIM][Cl][AlCl₃], and [EMIM][Cl][AlCl₃] to be applied in an aluminum cell. The measured conductivities at 30 °C using the current interruption and impedance spectroscopy techniques were approximately 0.5, 0.3, 1.4, and 20.2 mS.cm⁻¹, respectively. The exchange current densities at the aluminum–electrolyte interface were examined through impedance spectroscopy and were equal to around 0.2, 0.03, 0.4, and 1100 mA.cm², respectively. The presence of the molecular [AlCl₃][TFAP], and the ionic [AlCl₄], [AlCl₂(TFAP)₄]þ, and [Al(TFAP)₆]3þ species was proposed by the authors according to the NMR spectra of the investigated ionic liquid analogues.

Elterman et al. [195] developed electrolytes of 1-trifluoroacetyl piperidine with the addition of aluminum trichloride and of 1-ethyl-3 methylimidazolium chloride with the addition of lithium chloride to be applied in aluminum batteries. The proposed electrolytes did not crystallize when cooled to 60 °C. Proceeding from 1-trifluoroacetyl piperidine as an elucidative case, several chloroaluminate-containing liquid coordination complexes of different composition were obtained and characterized with NMR spectroscopy; the formation of aluminum-containing cations and anions has been described. The authors proposed a model for the coordination of the aluminum trichloride in 3,3,3-trifluoropropyl acetate [TFAP] liquid mixtures. The reduction in the conductivity of the [TFAP] electrolytes appears to be linked to the generation of low mobility [AlCl(nTFAP)]2þ and molecular species [AlCl₃(TFAP)] that decrease the concentration of the Al₂Cl₇ and AlCl₄ in comparison to the electrolyte imidazolium chloride. The incorporation of the lithium chloride reduced the conductivity because of the production of the weakly dissociating LiAlCl₄ compound. The exchange current densities at the aluminum–electrolyte interface changed to 0.18, 0.03, 0.42, 1100 mA.cm² due to the significant alteration in the concentration of Al₂Cl₇. It has been confirmed that the presence of AlCl₄ in the ionic liquids enhances the conductivity, but considering the data retrieved from NMR spectra and electrochemical measurements, the Al₂Cl₇ in the ionic liquids provoked the electrochemical reaction at the interface between the electrolyte and the aluminum electrode that considerably enhanced the aluminum exchange current density.

Zhu et al. [196] developed electrolytes to be used in aluminum–graphite cells composed of mixtures of 1-methyl-1-propylpyrrolidinium chloride [Py13Cl] having different aluminum chloride ratios. The researchers compared the characteristics of the ionic liquid with the ones of the commonly employed ionic liquids 1-ethyl-3-methylimidazolium aluminum chloride [EMIC][AlCl₃]. The ionic liquid [Py13][Cl][AlCl₃] had higher viscosity, and lower conductivity and density in respect to its counterpart [EMIC][AlCl₃]. The Raman scattering spectroscopy technique was used for probing the ionic liquids over a silicon substrate, and via normalization of the silicon Raman scattering peak, the speciation including AlCl₄, Al₂Cl₇, and larger AlCl₃ related species with the formula [AlCl₃]n in distinct ionic liquid electrolytes was determined. It was reported that larger [AlCl₃]n species were present merely in the [Py13][Cl][AlCl₃] cell. It was proposed that the larger cationic size of the [Py13]⁺ against the [EMI]⁺ determined the differences in the chemical and physical properties of the ionic liquids. The chloroaluminate anion graphite charging capacity and cycling stability of the two batteries were similar. The [Py13][Cl][AlCl₃] cell exhibited a slightly greater overpotential than [EMIC][AlCl₃], leading to lower energy efficiency, which was the result of a lower conductivity and higher viscosity. At the same [AlCl₃]–organic chloride ratio, the [Py13][Cl] [AlCl₃] system had lower density, higher viscosity, and lower conductivity than the [EMIC][AlCl₃] counterpart. Using Raman spectroscopy, it was revealed that the monomeric AlCl₄ and dimeric Al₂Cl₇ were present in both ionic liquids, having their concentrations diminished and augmented, respectively, as the amount of AlCl₃ was augmented. The amount of Al₂Cl₇ and AlCl₄ was lower in the [Py13][Cl][AlCl₃] ionic liquid, which agreed with its lower conductivity. The large polymeric [AlCl₃]n species were merely present in the ionic liquid [Py13][Cl][AlCl₃]. The batteries had similar capacity and similar stability. Nonetheless, the Py₁₃Cl-AlCl₃ electrolyte cell exposed a greater overpotential, which was caused by its increased viscosity and decreased conductivity. The cation/anion size in an ionic liquid may determine its properties like density, viscosity, and conductivity, and cell performance indicators including efficiency, rate capability, and overpotential. All these factors derived from the solvation and coordination of counter ions in the ionic liquid.

Lahiri et al. [197] clarified the role of ionic liquids in changing the aluminum solvation dynamics that influence the performance of magnesium cells. The investigation team demonstrated that the incorporation of 1-ethyl-3-methylimidazolium trifluoromethylsulfonate [EMIM][TfO] ionic liquid modified the solvation structure of aluminum in the aqueous electrolyte Al(TfO)₃ to lower coordinated solvation shells, thus enhancing the stripping and deposition of the aluminum onto the surface of the zinc–aluminum anode. The performed DFT calculations showed that the aluminum coordination was altered with the ionic liquid, making the aluminum intercalation into MnOx, and, hence, imposing a lower amount of strain within the magnesium oxide matrix and enlarged storage ability of the tested battery. Nonetheless, with increased cycling, the strain within the magnesium oxide matrix led to crack formation and loss of capacity retention. Through the optimization of the composition of the electrolyte, it a potential above 1.7 V was obtained with the zinc–aluminum–MnO_x cell.

4.1.4. Magnesium Batteries

The magnesium-ion batteries have attracted great research interest due to the high abundance of magnesium and improved safety. Nonetheless, the design of magnesium-ion cells still faces many challenges such as maintaining an elevated Mg/Mg²⁺ reversible chemistry, eliminating the side reactions, and enhancing the electrochemical stability window of the electrolyte. Oppositely to the rechargeable lithium metal batteries, the produced passivation layer onto the surface of the magnesium anode is inactive toward the magnesium cation and, hence, this cation cannot deposit reversibly on the anode. The electrolytes for the magnesium-ion batteries must be compatible with the anodes of magnesium, have increased ionic conductivity, and exhibit an adequate electrochemical stability window. The magnesium batteries are very promising alternatives for energy harvesting applications, given that magnesium possesses an increased charge capacity of approximately 2200 Ah.kg⁻¹, benefiting from its redox chemistry, and entails less dendritic formation than other metals such as lithium and sodium.

Gao et al. [18] made serious attempts to better understand the magnesium-ion speciation in ionic liquid electrolytes via the development of alkoxy-functionalized cations having distinct alkoxy substituents. Oppositely to the reported works on the coordination sphere of the magnesium cation, the comparison of spectroscopy and electrochemical calculation results of different ionic liquid electrolytes indicated that the coordination sphere of the magnesium cation influenced the magnesium reversible deposition and dissolution processes. Also, the researchers developed a magnesium–vanadium oxide cell using the non-corrosive ionic liquid electrolyte that had 140 mA.h.g⁻¹ of initial discharge capacity and 100 mA.h.g⁻¹ of reversible capacity.

Ren et al. [198] designed an electrolyte composed of magnesium bis(diisopropyl)amide, a non-nucleophilic organic magnesium salt, for rechargeable magnesium batteries. Hence, the 1-butyl-1-methylpiperidinium bis(trifluoromethyl sulfonyl)imide [PP₁₄TFSI] ionic liquid as the co-solvent of tetrahydrofuran in chlorine-free magnesium bis(diisopropyl)amide electrolytes improved considerably the ionic conductivity and oxidative potential of 2.2 V vs. Mg/Mg²⁺ on stainless-steel. The authors achieved reversible magnesium electrochemical plating and stripping with an overpotential inferior to 200 mV and approximately 90% of coulombic efficiency. The current density of the magnesium plating and stripping was enlarged by almost 240 times after the [PP₁₄TFSI] inclusion, due to proposed mechanism of competitive coordination of [TFSI]^–, theoretically facilitating magnesium plating/stripping. The magnesium bis(diisopropyl)amide-2AlF₃ electrolyte with a ratio-optimized tetrahydrofuran-PP₁₄TFSI co-solvent presented compatibility with the Mo₆S₈ cathode. In addition, the Se@pPA-magnesium cell showed an initial capacity of around 450 mA.h.g^–1 and a capacity decay per cycle of approximately 0.7% for more than 70 cycles at 0.2 C with Li-TFSI additives.

Vardar et al. [199] evaluated the suitability for magnesium batteries of inclusion of magnesium salts in ionic liquids, exploring plating/stripping voltammetry. The authors used borohydride [BH₄]^–, trifluoromethanesulfonate [TfO^–], and bis(trifluoromethanesulfonyl)imide [Tf₂N]^– salts of magnesium and the l-n-butyl-3-methylimidazolium [BMIM][Tf₂N], n-methyl-n-propylpiperidinium [PP₁₃][Tf₂N], and n,n-diethyl-N-methyl(2-methoxyethyl)ammonium [DEME]⁺ tetrafluoroborate [BF₄]^– ionic liquids. In solutions with [BMIM]⁺, the oxidative activity around 0.8 V vs. Mg/Mg²⁺ was most probably linked with the cation [BMIM]⁺, rather than with magnesium stripping. The absence of magnesium plating from the ionic liquids with [Tf₂N]^– and [BF₄]^– suggested that the strong magnesium/anion coulombic attraction inhibited the electrodeposition process. The co-solvent incorporations into Mg(TNf₂)₂/PP13-TNf₂ were explored but did not result in enhanced plating/stripping activity. The electro-chemistry of electrolytic solutions of the different magnesium salts Mg(TfO)₂, Mg(TNf2)₂, and Mg(BH₄)₂, and ionic liquids [BMIM][TNf2], [PP₁₃][Tf2N], and [DEME][BF4], and organic co-solvents DME and ACN on platinum electrodes was examined. Opposite to known reports, the reversible magnesium plating was not confirmed for the studied salt–ionic liquid combinations. The results indicated that when evaluating a magnesium-containing ionic liquid solution, it is essential to determine the redox activity of the pure ionic liquid. It was reported that the anodic peak at around 0.8 V vs. Mg/Mg²⁺ in the voltammetry of Mg(TfO)₂ dissolved in [BMIM][BF₄], which was earlier ascribed to the stripping of the magnesium, was probably derived from the redox activity linked with BMIM + Mg(TfO)₂ dissolved in [PP₁₃][Tf2N]. It has been reported to exhibit reversible magnesium plating, but voltammetry of three magnesium salts in [PP₁₃][Tf2N] did not show evidence of reversible magnesium plating in the present investigation. The incorporation of two organic solvents to Mg(Tf2N)₂-[PP₁₃][Tf2N] also did not give rise to signatures of magnesium plating/stripping. Mg(BH₄)₂ in [DEME][BF₄] was also considered, but like the other systems, no evidence of magnesium plating or stripping was observed. The findings suggested that the failure of the magnesium to plate from electrolytes with [BF₄]⁻ and [Tf2N]⁻ was due to the great coulombic attraction between these anions and the very-charge-dense magnesium cation. The strong association of [Tf2N]⁻ and the magnesium cation could not be overcome through lowering the ion solvation energies through the inclusion of co-solvents with high polarity. Thus, it appears very difficult for simple magnesium salts to be used as magnesium suppliers in the ionic liquid electrolytes for magnesium batteries, unless specific measures are taken to increase the dissociation process of the magnesium salt and decrease the ion solvation energies. It is predicted that a magnesium-containing ionic liquid cation with lower charge density than the magnesium cation or an anion shared by the salt and ionic liquid could enable magnesium plating. Also, it is possible that the incorporation of strong Lewis bases to ionic liquid solutions could overcome the attraction between ions in simple magnesium salts, facilitating the dissociation and the magnesium plating from ionic liquids.

4.1.5. Potassium Batteries

The potassium-ion batteries exhibit increased potential to be explored in energy harvesting applications. The non-aqueous potassium-ion cells define an innovative complementary technology to lithium-ion cells caused by the large affordability and availability of potassium metal. Also, the smallest charge density of the potassium-ion in comparison to that of the lithium-ion favors the ion-transport characteristics in the liquid electrolytes, consequently enabling the rechargeable potassium batteries to present enhanced rate and low-temperature performances. Nowadays, the most promising potassium-ion batteries are based on potassium manganese hexacyanoferrate. However, these cells pose certain limitations such as the formation of dendrites at the potassium anode surface, affecting the safety and longevity issues of the batteries, and the exploration of the ionic liquids as electrodes may overcome such challenge.

Yoshii et al. [200] investigated the ionic liquids composed of potassium bis(trifluoromethanesulfonyl)amide [K][TFSA] in 1-methyl-1-propyl-pyrrolidinium bis(trifluoromethanesulfonyl)amide [Pyr13][TFSA] to act as electrolytes for high-voltage cathode materials of potassium-ion batteries. The 0.5M KTFSA in [Pyr13][TFSA] ionic liquid electrolyte exhibited lower redox potential than those of, for example, lithium. The suitability of the [K][TFSA] ionic liquids was demonstrated by matching with innovative high-voltage layered cathode materials. However, the 6 V electrochemical window suggested that a 5 V potassium cell might be achievable. Nonetheless, the relatively high viscosity of the ionic liquids at room temperature of around 120 mPa.s at 25 °C for 0.5M KTFSA in [Pyr13][TFSA] can deteriorate the power density of the cell.

Jeon et al. [201] developed an electrolyte composed of potassium bis-fluorosulfonyl-imide [KFSI] and 1-methyl-1-propyl pyrrolidinium bis-fluorosulfonyl-imide [Pyr13][FSI] for potassium batteries. Compared to the carbonate KPF₆EC-PC and dilute potassium ionic liquid electrolytes, the developed potassium salt-concentrated ionic liquid electrolyte showed benefits such as improved oxidation stability. The produced electrolyte promoted the creation of a potassium fluoride-enriched solid-electrolyte interface at the anode, preventing the production of dendrites and augmenting the potassium anode cyclability. Posing such benefits, the K-KVPO₄F batteries exhibiting a capacity of 0.8 mA.h.cm⁻² for the KVPO₄F-containing ionic liquid electrolytes presented a significant cycle stability with nearly 75% of capacity retention after three hundred cycles and elevated cell efficiency of 99.6% at a temperature of 25 °C.

Dhir et al. [202] characterized the ionic conductivity and other factors of a potassium-ion electrolyte composed of potassium bis(fluorosulfonyl)imide K-[FSI] salt and 1,2-dimethoxyethane solvent. The researchers also compare the electrolyte with the [LiFSI][DME] electrolyte with the concentration ranging from 0.25 to 2 mol. The results confirmed that the diffusion coefficient of the salt and the transference number of the cation of the [KFSI][DME] electrolyte were substantially larger than those of the electrolyte [Li][FSI] considering concentration values inferior to 2 mol. The greater transference numbers of the cations and salt diffusion coefficients reduced the ionic concentration gradient creation and related concentration overpotentials, hence making the potassium cells present an enhanced rate capability and electrochemical performance at low temperatures. The ionic conductivities were almost the same at a temperature of 20 °C, with the LiFSI increasing to approximately 1.7 m, which was probably derived from an inadequate [K][FSI] salt dissociation. The thermodynamic factor evolution with the concentration suggested weaker ion-to-ion and solvent interactions of the potassium cation with respect to those attained with, for instance, a lithium cation.

4.1.6. Calcium Batteries

Stettner et al. [203] argued that the electrolytes based on aprotic and protic ionic liquids have improved transport characteristics and electrochemical stability comparable with those of the ionic liquid electrolytes proposed for lithium and sodium devices. Exploring these electrolytes in electrochemical double-layer capacitors enables the development of cells having improved capacitance, reversibility, and stability. Their combined usage with titanium disulfide cathodes appears to be more difficult, given that the cation of the ionic liquids is inserted in the layered structure of this material during charging. The electrolytes based on mixtures of polycarbonate and aprotic ionic liquid–protic ionic liquid with the [Ca][TFSI]₂ salt had enhanced transport features and electrochemical stability windows comparable with the ones exhibited by electrolytes with similar composition. The use of these electrolytes in electric double-layered capacitors with activated carbon electrodes demonstrated potential to be a promising approach to fabricate batteries with improved capacitance, stability, and reversibility. Using aprotic ionic liquid electrolytes, it is possible to construct high-voltage batteries. Considering these results, activated carbon appears to be a very suitable material for calcium cells. To fabricate a calcium battery, the active material should either insert the calcium cation selectively or accommodate the large cation of the ionic liquid. It was demonstrated that this was not the case for the combined usage of the [Pyr14]⁺ cation with titanium disulfide, which can host many different species in its structure but gets amorphized after the inclusion of the [Pyr14]⁺ cation.

Biria et al. [204] developed an ionic liquid polymer gel membrane as a separator and electrolyte for room temperature rechargeable calcium batteries. The membrane was prepared through polyethylene glycol diacrylate photo-crosslinking in the presence of calcium salt dissolved in an ionic liquid. The membrane exhibited a room temperature ionic conductivity from 10^–4 to 10^–3 S.cm⁻¹, around 4 V stability vs. Ca/Ca²⁺, a cationic transference number of 0.17, increased thermal stability up to a temperature of 300 °C, and complete dissociation of the calcium salts in the ionic liquid. The fabrication of the ionic liquid polymer gel membrane was produced with 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, a calcium tetrafluoroborate Ca(BF₄)₂, calcium perchlorate Ca(ClO₄)₂, and calcium bis-(trifluoromethylsulfonyl)imide Ca(TFSI)₂. The electrolyte displayed leakage avoidance and increased conductivity and thermal stability and enabled a performant calcium-ion battery functioning in room conditions. Full cell battery configurations were assembled by employing calcium cobalt oxide and vanadium oxide as positive and negative electrodes, respectively. The cell achieved 1.2 V of open-circuit voltage and reached 140 mAh.g⁻¹ of specific capacity.

4.1.7. Zinc Batteries

Rechargeable zinc batteries have significant potential for several energy storage applications. However, the selection of the most adequate electrolytes for zinc cells is still challenging due to the passivating effect and generation of dendrites related with the zinc metal, limiting the durability of the batteries. Furthermore, the intense reactivity of water in aqueous electrolytes toward zinc under harsh working conditions remains the main drawback related to the large-scale usage of zinc cells. In this scope, the relevance of ionic liquids as potential electrolytes for zinc batteries is still growing. However, the formation of dendrites and the parasitic side reactions somewhat impede the development of rechargeable zinc cells. Though the reported attempts to synthesize advanced hydrogel electrolytes, it is still challenging to obtain hydrogel electrolytes with increased durability and ionic conductivity.

Yu et al. [205] developed an ionic liquid diluent 1-ethyl-3-methylimidazolium bis-fluorosulfonyl-amide [EMIM][FSI], which could eliminate the water activity of aqueous electrolytes, enveloping the active water-dominated zinc cation solvates and protecting them from parasitic side reactions. During the zinc deposition process, the [EMIM]⁺ cation and [FSI]⁻ anion inhibited the tip effect and regulated the solid-electrolyte interphase, respectively, thereby aiding the growth of a smooth zinc deposition layer, which was protected by a solid-electrolyte interphase enriched with inorganic species and with improved stability. In conjunction with the improved stability provided by the ionic liquid, the developed ionic liquid-including aqueous electrolyte enabled the stable functioning of Zn-Zn_0.25V₂O₅·nH₂O cells at 60 °C having a capacity retention superior to 85% after the completion of four hundred cycles.

Liu et al. [206] examined the electrochemical behavior of a zinc ion secondary battery composed of a bio-ionic liquid–water electrolyte, a zinc metal anode, and a structured Prussian blue analogue cathode. The researchers found that the zinc anode and Prussian blue analogue cathode presented excellent compatibility with the bio-ionic liquid–water electrolyte, enabling the dissolution and/or deposition of zinc at the surface of the zinc anode and reversible insertion or extraction of the zinc cations at the cathode. The zinc battery showed a discharge voltage plateau of nearly 1.1 V having nearly 120 mA.h.g^–1 of specific capacity at 0.1 C. The zinc anode exhibited a high reversibility, and zinc deposits without the formation of dendrites were achieved upon the completion of one hundred deposition and dissolution cycles.

Zhang et al. [207] synthesized a hydrogel electrolyte to be applied in zinc cells containing an ionic crosslinked network based on an imidazole ionic liquid and a carboxylic bacterial cellulose fiber, followed by a covalent network of polyacrylamide. The hydrogel electrolyte exhibited a remarkable ionic conductivity of around 43.8 mS.cm⁻¹, resulting in a zinc cation transference number of 0.45, and enhanced mechanical stress of approximately 175 kPa with an elastic modulus of 3.5 GPa. Also, under the anion-coordination action of the carboxyl groups present in the bacterial cellulose and [BF₄]⁻ anions in the imidazole ionic liquid, the solvation sheath of the hydrated zinc cations and the nucleation overpotential of the zinc plating were controlled. The experimental results on cycling revealed that it mitigated the creation of zinc dendrites, and parasitic side reactions with irreversible by-products were indeed reduced. Employing the carboxylic bacterial cellulose hydrogel electrolyte, the zinc–zinc symmetric batteries presented improved cyclability. It achieved a breaking elongation of around 38% by the dual crosslinked network using [EMIM][BF₄] and polyacrylamide. Under the anion-coordination of the [BF₄]⁻ anions and carboxyl groups, the solvated structure could be defined. Thus, the gel electrolyte could provide a stable zinc deposition and low polarization potentials.

4.2. Fuel Cells

The conversion of chemical energy into electrical power is accomplished with fuel cells providing a power production potential with the exploration of renewable energy resources [187]. The ionic liquid crystals presenting regulated structures constitute themselves as very suitable electrolytes to improve the ion conductive paths for the hopping of the protons, achieving an elevated ionic conductivity [208]. Only 2.8 molecules of water per group of sulfonic acid were required for the ions to hop in the ionic liquid crystal electrolytes, and such quantity was even less than what is observed with organic-based electrolytes. However, the ionic conductivity of aqueous electrolytes could undergo an appreciable drop when heated, which might be linked to the [HTNf2] or water evaporation and subsequent suppression of the hydrogen bonded networks [187]. The polyionic liquid complex electrolyte can be prepared through UV radiation [209]. An ionic conductivity between one and two order of magnitude higher could be obtained using the crosslinking polymeric films synthesized from the 4-dodecyl benzenesulfonic acid and 3-1-vinyl-3-imidazolium propane sulfonate in comparison with the Nafion 117 conductivity. The increased ion conduction ability could be attributed to the transfer of ions through hopping and water diffusion [210]. Luo et al. [211] developed a quasi-solid-state electrolyte polypropylene membrane wet with an electrolyte based on ionic liquid crystal. The researchers reported with this system 210 mS.cm^{− 1} at 25 °C of proton conductivity. Such a value can be compared to that attained with a membrane composed of alkyl sulfonic acid where the protons conducted via hopping through the hydronium ion diffusion and hydrogen bonding. The research team attributed the result to the free [HSO]⁻ anion in the two-dimensional water layer. The fuel cell open-circuit voltage using the ionic liquid crystal electrolyte applied on the porous membrane made of polypropylene was 0.84 V, which is a high value that indicates its capability to eliminate the crossover of the oxygen and hydrogen. Koboyashi et al. [212] confirmed the capability of the ionic liquid crystal electrolytes to suppress the permeability of hydrogen. A low ohmic loss was observed for the [C₁₄MIm][HSO₄]-15 cell, and the peak polarization current was four-fold greater than the [C₂MIm][HSO₄]-15 cell. Merely small weight changes below 5% were reported after the characterization of the fuel cell, indicating its considerable electrochemical stability.

4.3. Dye-Sensitized Solar Cells

The distinctive features of ionic liquids including elevated thermal and electrochemical stabilities, the capability to dissolve several inorganic and organic compounds, and negligible volatility attracted considerable interest from the researchers investigating dye-sensitized solar cells. In general, the physicochemical characteristics of the ionic liquids may be adjusted through the alteration of the cationic and anionic components. Relatively low viscosities and hydrophobic character, which are very much desired for solar-cell purposes, could be obtained following this route. It has already been demonstrated that the ionic liquids are ameliorated solvents to be applied in dye-sensitized solar cells’ fluid electrolytes and are the most adequate components of quasi-solid-state electrolytes and gel electrolytes. Via the condition optimization of the efficiency finding of the dye-sensitized solar cells, their efficiency could be enhanced, and factors including type of conducting glass, overall solar cell area, light filtering, and counter electrodes type are some of the main performance influential factors. Generally, the verified instability over time of the dye-sensitized solar cells may be caused by the inadequate encapsulation of the cell that leads to the premature evaporation of the electrolytes and by the chemical processes occurring in the electrolyte and solid–liquid interface, which may result in the degradation of the electrolyte, dye, and counter-electrode. The utilization of ionic liquids may soften some of these limitations. Concerning this, Lee et al. [213] verified that a dispersion of nanoparticles of copper at 0.08% wt. encapsulated in a shell of carbon in the [BMIM][PF₆] ionic liquid was favorable to the dye-sensitized solar cells’ functioning. The electrolyte displayed an increase of 65% and 35% in the electrical conductivity and diffusion coefficient, respectively, when compared to the neat ionic liquid. The augmented electrical conductivity of the ionic liquid electrolyte was ascribed to the interactions between the [Bmim]⁺ cations and encapsulated nanoparticles of copper. The addition of nanoparticles into the ionic liquid provoked the reduction in the ionic bonding energy between the [PF₆]⁻ ionic liquid anions and [BMIM]⁺ ionic liquid cations, enhancing the coefficient of self-diffusion of the cations [Bmim]⁺ and decreasing the viscosity of the ionic liquid. Neo and Ouyang [214] prepared multi-walled carbon nanotubes at a weight concentration of 0.1% in the ionic liquid 1-propyl-3-methyl-imidazolium iodide for electrolyte purposes. Such a formulation produced bonds of hydrogen between the nanotube carboxylic groups and the [Pmim]⁺ cations and decreased the coulombic interactions between the iodide anions and [Pmim]⁺ cations. It induced a reduction in the ionic liquid viscosity and an enhancement in the thermal stability of the produced electrolyte. Apart from the thermophysical and electrochemical characteristic enrichment provided by the ionic liquid electrolytes with embedded nanoparticles, the ionic liquids usually enhance the electrolyte stability through the generation of ionic double layers, protecting the particles against corrosivity effects. Additionally, Adhyaksa et al. [215] examined a plasmonically ameliorated dye-sensitized solar cell employing nanoparticles of silver embedded in imidazolium-dicyanamide ionic liquids as the electrolyte. It was observed that the solar cell exhibited enhanced stability, since, without the ionic liquid, the acetonitrile electrolyte having redox compounds that corrode the particles made of silver would deteriorate the solar cell performance and stability. Nevertheless, the ionic liquid with negligible volatility protected the surface of the silver nanoparticles from direct contact with the existent redox compounds, producing an ionic double layer around the nanoparticles and, consequently, decreasing the corrosivity. Sharma et al. [216] designed and tested a dye-sensitized solar cell using an electrolyte made of polymethyl methacrylate doped with 1-ethyl-3-methylimidazoliumtricyanomethanide ionic liquid, which provided a short-circuit current density of nearly 7.2 mA/cm², nearly 0.6 V of short-circuit current voltage, and around 3.5% of overall efficiency at 1 sun. Chen et al. [33] used the [BMIM][PF₆] ionic liquid and nanoparticles made of copper at 0.08% wt. encapsulated in a carbon shell Cu@C and confirmed that this alternative was an improved electrolyte to be applied in a dye-sensitized solar cell, exhibiting a 35% greater diffusion coefficient and 65% higher electrical conductivity with respect to the pure ionic liquid. The increased electrical conductivity was ascribed by the research team to the interactions between the cations [BMIM]⁺ and Cu@C particles. The incorporation of Cu@C nanoparticles in the ionic liquids may provoke the decrease in the ionic bonding energy between the [PF₆]⁻ anions and cations [BMIM]⁺ of the ionic liquid, promoting an enhancment of the coefficient of diffusion of the cations [BMIM]⁺ and a drop in the ionic liquid viscosity [1]. Figure 23 presents the scheme of one typical dye-sensitized solar cell using ionic liquids.

4.4. Thermo-Electrochemical Cells

The rate of conversion heat to electricity of the ionic liquids electrolytes is fundamentally decided by the ions Soret effect [217]. Specifically, under the influence of a temperature gradient, the ions migrate and aggregate between the cold and hot cell sides. Thus, the ion transport within the electrolyte seriously affects its heat conversion rate. In the case where there is not much difference in the cation’s and anion’s mobility in the electrolyte, it is predicted that both anions and cations reach the cold electrode at the same time, and under such circumstances there is no thermovoltage or thermopower generation by the thermo-electrochemical cells [55]. Hence, to obtain the elevated thermopower values needed for large thermovoltage production, the thermo-electrochemical cells should have a considerable mobility gradient between electrolyte cations and anions. The cations and anions aggregate at the cold and hot electrodes, respectively, conducting excessive thermopower and thermovoltage for the thermo-electrochemical cells to bare [55]. Additionally, the ion transport within the electrolyte can be regulated through determining the interactions between the ions and their vicinities in accordance with the Soret effect. Abraham et al. [218] verified that thermo-electrochemical cells using electrolytes based on ionic liquid were most adequate for temperature values above 100 °C in thermal energy storage. The I⁻/I₃⁻ dispersed in [C₂mim][BF₄] electrolytes achieved a power density that reached 29 mW.m⁻² in unoptimized cells with their hot side working at a temperature of 130 °C. The researchers obtained a tuned thermoelectric figure of merit to characterize the electrolytes, and this required the measurement of the electrolyte redox couple diffusivity, Seebeck coefficient, and thermal conductivity. Cheng et al. [219] defined the interactions of the ion–dipole between a gel matrix and an ionic liquid and achieved an extremely high thermopower of 26.1 mV.K⁻¹. The generated thermopower can also be enhanced by tuning the interactions of the small molecules of the electrolytes with their surroundings. As an example, the fixation of sodium cations in the polyvinyl alcohol hydrogel through coordinating interaction with the groups of hydroxyls led to the transport of the OH^- anions in the polyvinyl alcohol, and an elevated thermopower of approximately 19.7 mV.K⁻¹ was confirmed [220].

4.5. Electrochemical Low-Grade Heat Harvesting

The Thermally Regenerative Electrochemical Cycle (TREC) is a technological solution with great potential for low-grade heat harvesting taking advantage of electrodes’ thermo-galvanic action. The electrolytes employed in TREC systems possess merely a negligible response to alterations in the operating temperature. Wu et al. [221] included a thermo-responsive ionic liquid in an electrolyte to induce a phase change depending on temperature. The electrolyte was explored in a TREC system based on copper hexacyanoferrate to be employed for low-grade heat harvesting purposes. The TREC system functioned from 10 °C to 30 °C across the phase change critical point, changing the ions’ solvation states in the charge and discharge cycles and attaining an increased energy density of 1.3 J.g⁻¹ and a significant energy conversion rate of nearly 1.3%. The energy efficiency was 10-fold superior to the one of a conventional system without the use of a thermo-responsive ionic liquid at similar operating conditions. In addition, the phase change vital point of the thermo-responsive ionic liquid can be adjusted in reference to the concentration and components of the electrolyte salt. A thermo-responsive ionic liquid was used in a CuHCFe TREC system to yield low-grade heat sources having narrow temperature gradients. The electrolyte phase change impacted the solvation state of the solved ions. The TREC configuration exhibited a considerable efficiency in the cases where a phase change occurred during the temperature changes. The system worked from 10 to 30 °C in an electrolyte mixture of 0.6 molar-NaTFSI-water-HbetTFSI or HOMO phase and attained an energy density close to 1.1 Jg^{− 1}, having a heat-to-electricity conversion efficiency and relative efficiency of around 1% and 15%, respectively. The ameliorated behavior could be interpreted based on the increased cycling coulombic efficiency linked with the phase change behavior and the comparatively low specific heat of the electrolyte. At lower current densities, a higher heat-to-electricity conversion efficiency rate of approximately 1.3% and a relative efficiency of 20% were achieved at 0.2 C given the lower voltage decrease and hysteresis. At similar conditions, the system displayed a greater energy harvesting efficiency in comparison to that of TREC systems without the exploration of non-thermo-responsive electrolytes. As a successful example, the mixture of 0.4 molar-NaTFSI-water-HbetTFSI, having a critical point of 33 °C, functioned from 25 °C to 45 °C and presented an energy density of around 0.8 J.g⁻¹, which was four-fold the energy density of the HOMO phase systems operating in the same temperature range. The usage of adequate counter electrodes in the operating cell may further enhance the efficiency rate of energy conversion.

4.6. Supercapacitors

Supercapacitors exhibit the considerable advantages of rapid charging and much prolonged cycling lifespan with simple design and affordable maintenance [222]. The employment of ionic liquid crystal electrolytes may decrease the resistance of charge transfer and increase the time of discharge and the charge accumulated in the capacitor, thus promoting an enhanced performance. Supercapacitors can store energy through the existing interactions at the electrolyte–electrode interface and through ion mobility [223]. The double-layer charging in the ionic liquids can be physically realized through capacitive electro-sorption between the surface of the electrode and the electrolyte ions [224]. The improved electrochemical stability, wide liquid phase range of temperatures, and intrinsic non-flammability of ionic liquids are explored to be included in electrolytes to provide the charge segregation at the interface at increased operating voltages [151,225]. The thermal and electrochemical stabilities of the ionic liquids can also be controlled through multiple cation–anion couplings to adjust the temperature and voltage of the supercapacitors [226]. Comparing electrochemical stability windows from distinct experiments under distinct experimental conditions is not the better path to follow. Another inconsistency in defining electrochemical stability windows arises from the usual concept that the cathodic limits are completely determined by the cationic species, while the anodic limits are affected by the anionic species. Nevertheless, some works confirmed that the anodic and cathodic limits can be determined through the cationic species. Other discrepant results may come from the distinct ion couplings and stabilization that have also been reported to promote the global ionic liquids’ electrochemical stability. To evaluate the electrolyte electrochemical stability, cyclic and linear sweep voltammetry are commonly used within a range of measurement electrodes and conditions. Also, the [TFSA]^– anion-based ionic liquids exhibit greater anodic stability when compared to those of the ionic liquids [C₂C₁im][OTf], [C₂C₁im][BF₄], and [C₂C₁im][TFSA]. The cationic species are the main cathodic stability influencers of the ionic liquids. The cathodic stability is usually superior in the case of saturated cations with quaternary ammonium structures, whereas the aromatic cations in ionic liquids are less stable, caused by their lowest unoccupied molecular orbit levels in comparison to those of the saturated cations. The levels of energy of the lowest unoccupied molecular orbits of such cations can be ordered as [C4pyr]⁺<[C2C1im]⁺<[N4441]⁺ in concordance with cathodic stabilities measured experimentally [227]. The enhanced chain length of the organic cations’ aliphatic substituents increases the cathodic stability. It was already reported that the quaternary ammonium ions having cyclic structures showed enhanced cathodic stability in comparison to that of their non-cyclic counterparts [228]. A linear sweep voltammetry analysis on the electrolyte oxidation stability confirmed that the temperature enhancement led to a sharp reduction in the electrolyte oxidation stability. At a temperature of 25 °C, the 0.01 mA.cm^–2 anodic limits above a 5.5 V threshold were obtained in the electrolytes 1 molar [PF₆]-PC Na, Na[PF₆]-EC-DMC, and Na[FSA][C₃C₁pyrr][FSA] [228], whilst the 1 molar Na[ClO₄]-PC achieved 4.7 V. Nonetheless, in the case where the temperature was enhanced to 45 °C and 60 °C, only the electrolyte Na[FSA]-[C₃C₁pyrr][FSA] maintained a wide oxidative potential limit since the electrolytes of organic solvents showed narrow anodic limits defined through the oxidative decomposition occurring in values superior to 4V. To accomplish an evaluation model of an actual battery electrode, linear sweep voltammetry experiments were also performed on carbon electrodes along with electrolytes of Na[FSA]-[C₃C₁pyrr][FSA], Na[PF₆]-EC-DMC, and Na[ClO₄]-PC [20,228]. The additives based on fluoroethylene carbonate and vinylene carbonate were found to augment the electrolyte Na[PF₆]-EC-DMC anodic current over the operating temperature range. Nonetheless, the energy difference between the highest occupied molecular orbitals and Na[FSA]-[C₃C₁pyrr][FSA] was reported to achieve the greatest anodic stability, independently of the temperature alterations. The ionic liquids employed in supercapacitors are commonly made of the combined arrangement of pyridinium, pyrrolidinium, tetraalkylammonium, and imidazolium cations and flexible anions [151] covering bis(trifluoromethanesulfonyl)amide TFSI⁻, bis(fluorosulfonyl)imide [FSI]⁻, hexafluorophosphate [PF₆]⁻, tetrafluoroborate [BF₄]⁻, and [Cl]⁻ [229]. Wang et al. [230] proposed a theory for confined ion packing in micro–meso–cylindrical pores for electrodes made of carbon, which confirmed the impact of the pore textures on the capacitive behavior of 1-ethyl-3-methyl-imidazolium-tetrafluoro borate (EMIMBF₄) and 1-butyl-3-methyl-imidazolium hexafluoro-phosphate (BMIMPF₆) ionic liquids. Exploring a denser surface packing and under 4V, the electrode capacitance of the double layer reached 387 F.g⁻¹ and 290 F.g⁻¹ at 60 °C and 20 °C, respectively, and consequently a 204 W.h.kg⁻¹ extremely high energy capacity was transferred into the BMIMPF₆ electrolyte. Also, Pan et al. [231] studied the sub-zero electrochemical behavior of symmetric supercapacitors assembled with nitrogen-doped carbon porous electrodes and ionic liquid electrolytes. Taking advantage of the intrinsic adaptability and wide temperature range, the studied equipment delivered, under 3.5 V, a high energy density of around 48 and 19 Wh·kg⁻¹ at 1.8 and 35 kW·kg⁻¹ [151] and at a negative temperature value of −40 °C. Also, Bi et al. [232] studied the charging procedure of the double layer in supercapacitors using rich conduction electrodes of metal−organic frameworks and the ionic liquid EMIMBF₄ as the electrolyte. Given that the metal-organic frameworks are faradically inert in aprotic ionic liquid electrolytes, they may offer valuable insights for the development of metal–organic framework ionic liquid supercapacitors with enhanced energy and power densities. Pseudo-capacitors are another type of supercapacitors, where the electrochemical performance is dominated through faradaic redox reactions on the surface and vicinities of the pseudo-capacitive electrodes. Sharma et al. [216] fabricated a polymer electrolyte of polymethyl methacrylate doped with 1-ethyl-3-methylimidazoliumtricyanomethanide ionic liquid via a solution cast procedure. The researchers stated that the inclusion of the ionic liquid augmented the ionic conductivity up to around 8.8 × 10⁻⁵ S.cm⁻¹ at a weight fraction of 30% wt. of the ionic liquid. The research team employed an ionic liquid doped with 30% wt. of polymer matrix electrolyte and placed it between carbon electrodes to produce a supercapacitor exhibiting 130 mF/g of specific electrode capacitance at 50 mV.s⁻¹ scan rate. Also, Rochefort and Pont [233] examined the pseudocapacitive performance of a ruthenium oxide electrode in a proton-exchange ionic liquid electrolyte. The redox signals, suggesting the electrode pseudo-capacitance ability, might be produced by the exchange of protons between the protic ionic liquid to the ruthenium oxide redox sites. However, such signals were absent in aprotic ionic liquid electrolytes such as [EMIM][BF₄], where only a double-layer charging took place. Shen et al. [234] evaluated the supercapacitor composed of the electrode γ-FeOOH and ionic liquid electrolyte 1-ethyl-3-methyl-imidazolium bis-trifluoro-methyl-sulfonyl-imide, in which the pseudo-capacitive response was fundamentally derived from the diffusion-driven intercalation of the cations [EMIM]⁺. Due to the increased pseudo-capacitive activity of γ-FeOOH and the beneficial characteristics of the electrolytes made of ionic liquids, the system attained an elevated energy output of around 1.4 mW.h.cm⁻³ at a temperature of 200 °C. Molinari et al. [235] confirmed that the charge/discharge procedures at the ionic liquid–La_0.74Sr_0.26MnO₃ interface resulted in electrostatic segregation and electrochemical reactions in a supercapacitor hybrid. In this case, the 10 μF.cm⁻² capacitive offer was, in time, substituted with a pseudo-capacitive contribution reaching 180 μF.cm⁻² at growing voltages. The cyclic voltammetry analysis of the 1-cholesteryloxy-carbonyl decyl-3-methyl-imidazolium bromide-containing smectic C phase 0.5% in ethanol exposed a quasi-rectangular shape, corresponding to a much-improved capacitive performance. Consequently, a specific electrode capacitance of around 158 F.g^{− 1} was observed at 0.5 A.g^{− 1}, together with the use of the electrode of manganese–graphene oxide–dioxide–carbon fibers. The imidazolium ionic liquid crystal having the smectic A phase could also provide a rapid exchange at the mesoporous interface between the carbon electrode and the electrolyte and rapid interfacial charge transport [162], as suggested by the 251 Hz knee frequency and around 13 s of response period. Hence, an increased specific electrode capacitance, energy density, and power density of approximately 131 F.g^{− 1}, 34 Wh.kg⁻¹, and 1033 W.kg⁻¹, respectively, at 0.37 A.g^{− 1}, which were higher than the ones of other capacitors, were measured. The nearly rectangular-shaped cyclic voltammetry curves with the absence of redox peaks were maintained with the growing scan rate, indicating a much-improved capacitive performance. On the other hand, the ionic liquid electrolytes’ modification with conductive salts, organic solvents, and other ionic liquids has gained the recent attention from the research community to ameliorate the comparatively elevated viscosity and poor electrical conductivity for pure ionic liquids such as, for example, 0.014 S.cm⁻¹ at a temperature of 25 °C using [EMIM][BF₄] [236]. A vigorous anion−cation attraction in the pure ionic liquids confines the ion mobility, resulting in sluggish ion transport dynamics in media with increased viscosity [151,237]. Wang et al. [238] conducted molecular dynamics simulations to study the performance of ionic liquid supercapacitors and anion-type correlation. The researchers concluded that the charging time and thickness of the double layer augmented with the growing diameter of the anions [BF₄]⁻ < [TFSI]⁻. Tsai et al. [239] examined the super-capacitive behavior of electrodes made of porous graphite oxide in the n-butyl-n-methyl pyrrolidinium-bis-fluorosulfonyl-imide [PYR14][FSI] and n-methyl-n-propyl piperidinium bis-fluorosulfonyl-imide [PIP13][FSI] ionic liquid mixture at a molar ration of 1:1. The ionic liquid eutectic mixture electrolyte kept fluidic between −50 °C and 80 °C, and the electrodes of the final system composed of carbon and ionic liquid showed an enhanced electrode capacitance of 180 F.g⁻¹, despite a 3.5 V to 2.8 V potential decrease. Wang et al. [240] adopted an ionic liquid eutectic mixture with diverse ion interactions to diminish the anion−cation attraction in pure ionic liquids for a dense ionic structure and high electrochemical rate, which revealed useful insights for coupling electrolytes based on ionic liquid eutectic mixture with porous electrodes to achieve greater power and energy when using supercapacitors. Additionally, Dou et al. [241] introduced the fully beneficial route for carbon supercapacitors through the inclusion of a silica-grafted ionic liquid in the electrolyte made of propylene carbonate-1-butyl-3-methyl-imidazolium-bis-trifluoro-methane-sulfonyl-imide. The working potential of the equipment increased from 2.8 V to 3.2 V, promoting an approximately 39% energy capacity increase and prolonged cycling longevity. Moreover, the redox-active mediators mixed with electrolytes of ionic liquids are most promising for applications including hybrid capacitors and pseudo-capacitors [242]. The incorporation of pseudo-capacitive species into the medium could provide more electrode capacitance and reduction in the charge transport resistance, resulting in improved energy storage ability. In this sense, Navalpotro et al. [243] adopted a solution of para-benzoquinone in an ionic liquid electrolyte as a redox ionic liquid electrolyte for hybrid capacitors. In the case of a carbon electrode exhibiting considerable surface area, the inclusion of quinone redox molecules enhanced the electrode capacitance up to 156 F.g⁻¹ and the energy density up to 30 Wh.kg⁻¹. Moreover, Fleischmann et al. [244] used the ionic liquid electrolyte 1-methyl-1-propyl pyrrolidinium bis(trifluoromethylsulfonyl)imide with soluble alkali ions in an asymmetric hybrid supercapacitor. This equipment obtained elevated voltage cycling tolerance with an energy density of 100 W.h.kg⁻¹, maintaining at the same time up to 2 kWh.kg⁻¹ specific power and a stable cycling behavior at a temperature of 80 °C.

In the field of proton-storage organic electrodes, it is noteworthy to mention the work performed by Zhu et al. [245], who produced a novel manganese oxide electrode presenting improved electrochemical performance in an ionic liquid electrolyte. Remarkably, the manganese oxide nanoneedles were wrapped with porous n-doped carbon derived from graphitic carbon nitride playing the role of a sacrificial template. Benefiting from numerous active sites, increased electrical conductivity, and adequate porous structure, the final manganese oxide porous n-doped carbon electrode had improved rate capability and specific capacitance in the ionic liquid electrolyte. Also, a supercapacitor presenting extended cycling stability has been assembled based on the manganese oxide-porous n-doped carbon electrode and ionic liquid electrolyte, indicating its suitability for electronics. The coupling between the n-doped carbon nanosheets and manganese dioxide nanoneedles resulted in enhanced charger transfer and electrical conductivity to the electrode. Also, the electrode MnO₂-PNC delivered a high specific capacitance of approximately 188 F.g⁻¹ and increased rate capability in the ionic liquid electrolyte.

In the research area of imine organic compounds, Zhu et al. [246] argued that the even though manganese dioxide possesses improved pseudocapacitive storage behavior in certain electrolytes based on ionic liquids, it still presents decreased specific capacitance and electrochemical kinetics. The researchers proposed graphene quantum dots with manganese dioxide nanosheets to produce the composite electrode GQDs@MnO₂ for supercapacitors using ionic liquids. The combined effect of the graphene quantum dots and manganese dioxide provided a morphology having enhanced surface area, structural integrity, and kinetics. The graphene quantum dots played an important role in altering the energy band gap and density of state and increasing the electrical conductivity of the GQDs@MnO₂ electrode. Such features resulted in enhanced capacitive storage, charge–discharge response, and reversibility to the GQDs@MnO₂ electrode in the ionic liquid electrolyte. A high-performance ionic liquid supercapacitor was constructed with the electrode GQDs@MnO₂, delivering enhanced energy density of around 82 Wh.kg⁻¹, high-power density of nearly 12 kW·kg⁻¹, and extended cycle performance.

4.7. Electrochemical Sensors Reference Electrodes

Considering the type of ion to which the presented electrodes were sensitive, most electrodes are cation-selective electrodes, primarily sensitive to heavy and transition metal ions, lanthanide cations, and imidazolium cations of ionic liquids [247]. Two papers concerned sodium electrodes and one concerned a calcium electrode. A smaller number of electrodes have been developed for anions. This rule applies to all ion-selective electrodes. Among the anionic electrodes, electrodes sensitive to halide ions, nitrate ions, thiocyanate ions, and atorvastatin have been described. Considering the type of ionic liquid, liquids containing an imidazolium cation with two allyl chains and non-organic anions such as chloride were used in electrodes with a polymer membrane (mainly PVC-based) and in carbon paste electrodes. In polymer membranes, ionic liquids were a component of the membrane performing specific functions, most often as an ionic additive lowering the membrane resistance or an ionophore/ion exchanger providing the membrane with ion sensitivity. In some cases, the ionic liquid served as a plasticizer or membrane matrix. In paste electrodes, ionic liquids were a component of the paste used as a binder material instead of mineral oil or as an additional ingredient modifying the paste. In both electrode designs, the use of ionic liquids in combination with other substances in the form of composites with carbon nanomaterials, metal nanoparticles, or in the form of connections with a redox buffer is also shown. In each case, the use of the ionic liquid allowed the obtention of a sensor with improved parameters. Finally, it should be highlighted that the use of ionic liquids allowed the development of many new reference electrodes without a salt bridge. In this case, the most frequently used compound was 1-methy-3-octylimidazolium bis(trifluoromethylsulfonyl)imide. A promising application area of ionic liquids are the composite materials, the use of which in ion-selective electrodes has considerably enlarged. Considering the limitations of using ionic liquids in ion-selective electrodes, it is relevant to consider the purity of these compounds, which is often insufficient for this type of application, and their toxicity, especially when designing sensors dedicated to biomedical applications. The interfaces of ionic liquids and aqueous solutions have stable electrical potentials within a wide concentration range of aqueous electrolytes. This makes ionic liquids suitable as bridge materials that separate, in electroanalytical measurements, the reference electrode from samples with low and/or unknown ionic strengths. Nevertheless, the preparation methods of ionic liquids reference electrodes should be explored more.

All-solid-state ionic liquid membrane reference electrodes were developed presenting internal solid layers like silver–silver chloride, three-dimensionally ordered macroporous carbon, and colloid-imprinted mesoporous carbon. The saturated silver chloride in the ionic liquid prevented the dissolution of silver chloride covering silver and established the phase-boundary potential between the silver chloride and the ionic liquid. To improve the potential reproducibility of the ionic liquid membrane reference electrode containing the colloid-imprinted mesoporous carbon, a redox buffer was added to the polyvinyl chloride membrane, and a chloride sensor was manufactured, including the ionic liquid membrane reference electrode and a silver–silver chloride electrode. The additives in the ionic liquid, silver chloride, and redox buffer were dissolved in the aqueous sample solution, leading to a potential change in the ionic liquid membrane reference electrodes. Kojima et al. [248] proposed an all-solid ionic liquid membrane reference electrode with the inner solid electrode covered with an insertion material composed of Na_0.33MnO₂ and an alumina solid electrolyte. The inorganic insertion material, Na_0.33MnO₂, in the paste had a redox reaction together with the insertion and disinsertion of the sodium cations and acts as an ion-to-electron transducer. In addition, the alumina acted as a supplier of cations of sodium for Na_0.33MnO₂. The insertion material paste stabilizes the phase boundary potential of the inner solid electrode in the membrane reference electrode, even if the ionic liquid membrane presents no inserted sodium cations. The developed ionic liquid-based membrane reference electrode exhibited increased reproducibility. The membrane reference electrode was integrated into a potassium-ion sensor chip with the all-solid potassium-ion selective electrode K+-ISE for calibration-free measurements. The potassium-ion sensor chip was developed to avoid the action of the ionic liquid eluted from the membrane refence electrode on the K+-ISE potential by regulating the concentration of the ionic liquid in the membrane and the distance between the K+-ISE and the reference electrode. The produced potassium-ion sensor chips had elevated potential reproducibility of ±1.3 mV in potassium chloride solutions and ±2.2 mV in blood serum. The all-solid-state membrane with an inorganic insertion material paste had a membrane reference electrode applied to the ion sensor chip containing the reference electrode and the K+-ISE for calibration-free ion sensing. The insertion material paste layer was composed of the inorganic insertion material Na_0.33MnO₂, and β″-alumina functioned as an ion-to-electron transducer on the inner solid electrode not only in the K ± ISE but also in the reference electrode in the absence of the insertion of sodium cations in the sensor membranes. The elution of the ionic liquid from the membrane reference electrode into a sample solution can be considered unavoidable. Nevertheless, the influence of the eluted ionic liquid on the K ± ISE integrated into a sensor chip was reduced by optimizing the ionic liquid concentration in the membrane of the reference electrode, the distance between the reference electrode and K ± ISE in the sensor chip, and the measurement time. The K ± ISE and reference electrode integrated into the sensor chip are made of the same insertion material paste layer, which facilitates the mass production of disposable all-solid-state ion sensor chips. Due to the formation of stable interfacial potentials in solutions of different composition of the electrolyte and ionic strength, the ionic liquid reference electrodes offer advantageous features in comparison to the commercially available electrodes made of potassium salts and porous-frit materials. The ionic liquid reference electrodes have been demonstrated to possess stable potentials even at extreme conditions including the use of weakly buffered and low-ionic-strength solutions, while porous-frit reference electrodes may have reference potentials dependent on the sample solutions’ ionic strength and the size of the pores of the frit material. There are considerable changes in the potentials for reference electrodes having classic porous glass frits at low ionic strengths, in which the Debye length is similar to the size of the pores of the material, resulting in the electrostatic screening of the ions of the electrolyte. Polymerization-induced microphase separation was recently explored in the synthesis of nanostructured polymers and to produce mechanically robust solid-state ion-conducting polymer composites for fuel cells and rechargeable metal batteries. The nanostructured-polymer–ionic liquid composites synthesize by adding an ionic liquid to one domain of a crosslinked and microphase-separated co-polymer, resulting in enhanced ionic conductivity and thermal and mechanical robustness. The polymerization-induced microphase separation is a simple one-pot route, which transforms a homogeneous liquid precursor consisting of monofunctional and bifunctional monomers, an ionic liquid-miscible polymer macro-chain-transfer agent, and an ionic liquid into a solid, robust monolith with bi-continuous morphology. The easy-to-process liquid reaction precursor with a viscosity around 25 cP at room temperature undergoes polymerization and simultaneous in situ crosslinking, hence solidifying and adopting the reactor shape. The interfaces of the ionic liquids and aqueous solutions exhibited stable electrical potentials over a broad concentration range of aqueous electrolytes. This evidence suggests ionic liquids can serve as adequate bridge materials to separate the reference electrode from samples presenting low ionic conductivity in electrochemical measurements.

Chopade et al. [249] designed reference electrodes based on ionic liquids via polymerization-induced microphase separation. The reference electrodes incorporated ion-conducting channels that were filled with the ionic liquids 1-octyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide [C₈PIMS] or 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₁₂PIMS], supported by crosslinked polystyrene. The authors stated that the electrodes had low resistance and improved reference potential stability and reproducibility varying less than 3 mV when used together with the different aqueous solutions from deionized water to 100 mM potassium salt. The ionic liquid reference electrodes demonstrated reproducible potentials with variations inferior to 3 mV. The results indicated that the electrodes were very suitable for electrochemical measurements in solutions having hydrophilic electrolytes and in solutions of low ionic strength. The [C₈PIMS] and [C₁₂PIMS] reference electrodes showed reproducible potentials for replicates of 0.262 V and 0.313 V vs. the standard hydrogen electrode, respectively.

Wardak et al. [250] developed a solid-contact lead ion-selective electrode produced via the modification of a polymer membrane using a composite of 1-hexyl-3-methylimidazolium hexafluorophosphate ionic liquid and nanofibers made of carbon. The research team experimented with electrodes with distinct nanocomposite content in the membrane up to 9% wt. utilizing a carbon electrode or a platinum wire. The authors verified that the inclusion of the composite imparted a reduction in the resistance and enhancements in the capacitance and hydrophobicity of the membrane. As a result, the electrodes with the modified membrane had a decreased limit of detection and an increased measuring range and selectivity in comparison to the unchanged electrode. Also, remarkable improvements in the reversibility and electrochemical stability of the electrode potential were reported, and, in addition, the electrodes were resistant to alterations in the sample redox potential. The best performance was achieved with the electrode produced with a platinum wire as the inner electrode with a membrane containing the composite at 6% wt. The manufactured electrode had a Nernstian response to lead over a broad range of concentrations exhibiting a slope of 31.5 mV/decade and a detection limit of 6 × 10⁻⁹ mol.L⁻¹. Additionally, the sensor exhibited an improved stability and operated properly for four months after its preparation. The CNFs-HMImPF₆ nanocomposite performed several functions in the electrode, benefiting its initial characteristics. The alteration of the membrane with the composite improved its extraction capability toward lead, due to which the produced electrodes displayed a lower detection limit, wider measuring range, and more suitable values for the selectivity coefficient. Benefiting from this, the leaching of active ingredients from the membrane phase was much decreased, resulting in excellent electrode stability. Furthermore, the altered membranes exhibited increased capacitance and decreased resistance. Because of these facts, the developed electrodes displayed a short response time and stable and reversible potential.

5. Challenges and Recommendations for Future Research Works

The challenges and recommendations for future research works regarding the exploration of ionic liquids and their derivatives in electrochemical equipment and systems can be summarized in the next points:

• Further experimental investigations on the use of electrolytes composed of ionic liquids are recommended as it is still limited by the marked scarcity of ionic liquid electrolytes under room temperature conditions. Furthermore, more research work is required to attain an ameliorated knowledge of the interfacial compatibility, electrochemical stability, ionic conductivity, and working safety of the electrolytes composed of ionic liquids;
• Further molecular dynamics simulations are required to better comprehend the motion of the ions and the interactions at the ionic liquid electrolyte–electrode interface. The numeric studies and numeric simulations should be carried out according to a form to attain more valuable data on the molecular interactions and guidelines to benefit from the generalized use of the ionic liquids as fluidic electrolytes;
• More research efforts should be made to study the best possible way to integrate the different ionic liquid electrolytes and electrode components in just one single equipment for the rapid development of integrated supercapacitors operating with ionic liquids. The existing asymmetry between the anion and cations and inherent adaptability of the ionic liquids must deal with very complex van der Waals and electrostatic interactions and hydrogen bonding, to generalize the broad ionic liquid use in the manufacturing of electrodes and electrolytes to be applied in supercapacitors. Through the regulation of Lewis basicity and acidity, substituents of the alkyl chains, and other ion factors, the phase transition and thermal stability of the ionic liquids can be performed and are suitable to integrate the component requirements in only one piece of equipment [126];
• The correlation between structures and physiochemical characteristics of the ionic liquids requires further investigation to obtain a safe and improved behavior over a wide variety of temperature values. Also, some fundamental parameters need to be fully addressed like, for instance, the reduced low-temperature conduction and the high-temperature structure-driven modification of the migration model;
• Only small research attempts have been performed for the obtention of optimized dyes and electrode materials to be employed in conjunction with the ionic liquid electrolytes in dye-sensitized solar cells. Considering these facts, more and better studies are most welcome to infer the interactions between the electrolytes of ionic liquids and electrodes. These investigations should present innovative dyes, additives, and dopants for electrolytes, redox agents, and electrode materials to indeed take advantage of the beneficial features of the ionic liquid electrolytes and derivatives when employed in such types of solar cells;
• The use of in situ characterization techniques is vital to infer the transient functional and structural characteristics of the ionic liquids. To better know the electrochemical performance of the ionic liquid electrolytes and SEI formation, composition, and structure, it is critical to create and implement innovative analytical methodologies for non-invasive information retrieval during practical cell operation. The impact of temperature on the decomposition of the electrolytes and the generation of diverse inorganic species requires more profound analysis to form an SEI that can achieve an improved passivation of the anodes. Given the advancements verified with other alkali-ion batteries, modern characterization techniques should be used to better understand the influencing factors and their impact on the composition and structure of the SEI layers generated in the cells. The different characterization tools being placed into one integrated system are most welcome, since this option will provide a more comprehensive examination of the ongoing electrochemical processes in the cells. The techniques of Fourier transform spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy can identify the chemical composition of the SEI species, whereas atomic force microscopy can reveal the structural alteration to evaluate the thickness and strain at the electrolyte–electrode interface. These techniques can be combined to collect information on SEI, which will offer more insightful evidence to understand the correlation between the structure and the properties. Indeed, the integration of characterization devices has already made significant progress. Also, real-time examination of the growth of sodium dendrites using simultaneous mechanical measurements was already performed through in situ tunneling electron microscopy–atomic force microscopy dual characterization. Gathering data from different characterization methodologies will aid in the development of innovative ionic liquid electrolytes. To obtain accurate findings under in situ working conditions, it is essential to develop very-well-designed electrochemical cells. The associated difficulties with inaccurate and inconsistent results can be derived from lose contact between the current collector and the particles of the electrode, the presence of air and moisture in the electrolytes and electrodes of the cell, poor electrical contact, and excessive background noise. All these factors should be satisfied for the gathering of reliable data. The development of more powerful computational methods is required to more accurately estimate the underlying mechanisms of the creation, growth, and functional and structural characteristics of the SEI. Though diverse theoretical routes have been extensively followed for advanced battery research, the actual numeric simulations are elaborated with over-simplified models to avert prolonged computational times. An adequate modeling of the characteristics of the electrolytes and SEI may simulate accurately the electrochemical processes closely linked with the SEI layers. The machine learning algorithms have also demonstrated the capability of obtaining the relationship between the characteristics and structures of the ionic liquid electrolytes, and an increased throughput computation is very useful for the development of electrolytes. The computational analysis may also better elucidate the impact of the solvation sheath structure of alkali ions in the electrolyte on the SEI and may also explain the contribution of the ionic liquids to the stabilization of the interfaces between the electrolyte and the electrode in the electrochemical cells. The integration of theoretical analysis and numeric simulations with experiments is required to attain a more profound understanding of the solid-electrolyte interface to obtain guidelines for the clever design of the SEI layers on the electrodes by adjusting the main impacting factors on the SEI layers.

6. Conclusions

The main concluding remarks of this review about the usage of ionic liquids in electrochemical equipment and systems are gathered in the following points:

• Ionic liquids have great potential for electrochemical processes such as those discussed in the present review since they possess enriched electrochemical characteristics including increased electrical conductivity and electrochemical stability;
• The properties of the electrolytes based on ionic liquids should be optimized through the customized preparation of the constitutive ionic liquids, regulating the type and concentration of the components, and adjusting the temperature in which the intermolecular interactions have a critical role. The characteristics of the ionic liquid electrolytes, including the wide operating temperature range are extremely promising for applications where the goal is safe energy storage and conversion capabilities under extreme conditions;
• The correlation between the characteristics and structure of the ionic liquid precursors and carbon materials is of great relevance to improve the behavior of the electrode through anion and cation coupling;
• The different cation and anion possible arrangements in the ionic liquids embraces the interactions of van der Walls, hydrogen bonding, and electrostatic attractions, and generalizes the adoption of the ionic liquids to produce electrodes and electrolytes to be used in supercapacitors;
• There is an increasing usage of ionic liquids in the development of supercapacitors given their very attractive features including vapor pressure with negligible values and improved thermal and chemical stabilities;
• For pseudocapacitive electrodes, the energy storage ability may be considerably enhanced using novel electrolytes based on protic ionic liquids together with flexible redox mediators derived from processes of pseudo-capacitive hybrid energy storage. Furthermore, the porous quasi-solid-state ionic matrices can be adopted to mitigate the inherent environmental risks closely linked with fluid leakage, corrosivity, and assembly easiness for improved energy storage ability [126];
• Although ionic liquid electrolytes exhibit inherent safety, the leakage avoidance issue still must be assumed in quasi-solid-state electrolytes [162]. Therefore, the exploration of solid-state ionic liquid electrolytes is advisable. Nevertheless, the ion mobility can be minorly decreased. In this sense, the coordination of sites and ion conductive channels at the interfaces of the phase domains is of importance [162];
• The inclusion of ionic liquid crystal electrolytes at separators has already been demonstrated to be very suitable for the manufacturing of ionic liquid crystal membranes. Apart from this, the blend of ionic liquid crystals with polymers or the polymerization in situ [162] of the ionic liquid crystals with unsaturated bonds can also be a promising strategy. The direct contact between the ionic liquid crystal electrolytes and the electrodes, parasitic side reaction avoidance, and generation of ion conduction interphase layers could be achieved with in situ polymerization to reduce the interfacial resistance, inclusion of functional groups interacting with the solvents, and film-manufacturing additives;
• To the best of the author’s knowledge, ionic liquid reference electrodes are not yet available at a large-scale, despite their high stability and reproducibility that outperform the commercially available frit reference electrodes in some cases. It is believed that this is partially caused by the complicated and somewhat not well-defined preparation methods of the reported ionic liquid reference electrodes to date.