Application of Ionic Liquids in Electrochemistry—Recent Advances

Review Application of Ionic Liquids in Electrochemistry—Recent Advances

Gonçalo A. O. Tiago 1, Inês A. S. Matias 2 , Ana P. C. Ribeiro 2,* and Luísa M. D. R. S. Martins 2,*

1 Instituto de Tecnologia Química e Biológica, Av. da República, 2780-157 Oeiras, Portugal; [email protected] 2 Centro de Química Estrutural and Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] * Correspondence: [email protected] (A.P.C.R.); [email protected] (L.M.D.R.S.M.); Tel.: +351-218419389 (L.M.D.R.S.M.) Academic Editor: César Augusto Correia de Sequeir Received: 12 November 2020; Accepted: 5 December 2020; Published: 9 December 2020

Abstract: In this review, the roles of room temperature ionic liquids (RTILs) and RTIL based solvent systems as proposed alternatives for conventional organic electrolyte solutions are described. Ionic liquids are introduced as well as the relevant properties for their use in electrochemistry (reduction of ohmic losses), such as diﬀusive molecular motion and ionic conductivity. We have restricted ourselves to provide a survey on the latest, most representative developments and progress made in the use of ionic liquids as electrolytes, in particular achieved by the cyclic voltammetry technique. Thus, the present review comprises literature from 2015 onward covering the diﬀerent aspects of RTILs, from the knowledge of these media to the use of their properties for electrochemical processes. Out of the scope of this review are heat transfer applications, medical or biological applications, and multiphasic reactions.

Keywords: ionic liquid; electrochemistry; electrolyte; electrodeposition; battery; cyclic voltammetry; tuneability

1. Introduction Working safety is nowadays considered a more important issue than performance, and the delivery of safe and efficient materials for practical uses has been taken into account in the research and development (R&D) efforts. Thus, new features in materials are expected to appear in the immediate future, aiming to boost the transition to greener technologies towards meeting the goals of the 2030 EU Agenda for Sustainable Development [1] and resolving the ambitious challenges endorsed by the EU 2050 Green Deal [2]. Room temperature ionic liquids (RTILs), or simply ionic liquids (ILs), are an example of this new type of development, providing solutions for practical applications [3–5]. Ionic liquids are (mainly organic) salts exhibiting a very low melting temperature (liquids below 100 ◦C) and extremely low (negligible) vapor pressure [4]. Lately, they have received significant attention from the scientific community due to such unusual properties as liquids, being considered as suitable substitutes for volatile organic solvents (VOC, one of the major sources of waste in chemical synthesis) [6]. In fact, due to the unique physicochemical properties of ionic liquids the last decade has seen a remarkable boost on their application in several fields (e.g., chemistry, materials science or chemical engineering) with a significant high number of scientific contributions (ca. 68,057 publications in the period November 2010–2020, Figure1).

Molecules 2020, 25, 5812; doi:10.3390/molecules25245812 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 28 Molecules 2020, 25, 5812 2 of 27

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Figure 1. Rising interest on the use of ionic liquids: yearly number of publications in the domain Figure 1. Rising interest on the use of ionic liquids: yearly number of publications in the domain of of ionic liquids in the 2010–2020 period (Database: Scopus, search terms: “ionic” AND “liquids”, ionic liquids in the 2010–2020 period (Database: Scopus, search terms: “ionic” AND “liquids”, search searchdate: date: 7 November 7 November 2020). 2020).

RoomRoom temperature temperature ionic ionic liquids liquids general general physicochemical physicochemical properties are are summarized summarized in in Table Table 1.1 .

TableTable 1. General1. General characteristics characteristics of of roomroom temperaturetemperature ionic ionic liquids liquids [7]. [7].

GeneralGeneral Properties Properties Features Features • TreatedTreated as liquid as liquidat ambient at ambient temperature$ temperature Low melting point • Low melting point$ • Wide temperatureWide temperature interval interval for applications$ for applications • $ Thermal stability Non volatility • Thermal• stability$ Non volatility$ Flame retardancy • Flame• retardancy$ High ion density Composed by$ ions • High• ion density$ High ion conductivity Composed by ions$ • • High ionDesignable conductivity$/Tuneable Organic ions$ • • Designable/Tuneable$Unlimited combinations possible Organic ions • Unlimited• combinations possible

Clearly, the physicochemical properties presented in Table1 for RTILs are very di fferent from those exhibitedClearly, bythe ordinary physicochemical molecular properties liquids. However,presented itin isTable worth 1 for mentioning RTILs are that very every different ionic from liquid doesthose not always exhibited show by ordinary the above molecular properties. liquids. However, it is worth mentioning that every ionic liquid does not always show the above properties. Ionic liquids are almost all times composed of organic ions, and therefore have unlimited Ionic liquids are almost all times composed of organic ions, and therefore have unlimited structural variations in view of the easy preparation of many different components. The tuneability of structural variations in view of the easy preparation of many different components. The tuneability combinations of cations and anions and the possibility to achieve modifications of the cation and/or of combinations of cations and anions and the possibility to achieve modifications of the cation and/or the anion part offer access to ionic liquids with targeted properties. This has led to an exponential the anion part offer access to ionic liquids with targeted properties. This has led to an exponential expansionexpansion of theof the number number of of multipurpose, multipurpose, interesting interesting ILsILs known in in recent recent years. years. Some Some of ofthe the most most commoncommon ionic ionic liquids liquids components components (cations (cations and and anions) anions) areare presentedpresented in in Table Table 22.. Imidazolium-based ILs, such as 1–butyl–3–methyl–imidazolium [bmim] and 1–ethyl–3– methyl–imidazolium [emim], are among the most studied due to their stability within oxidative and reductive conditions, low viscosity, and ease of synthesis [8]. The cationic component of ILs has been varied to include pyridinium, ammonium, phosphonium, thiazolium, and triazolium species [5] (Table2). In general, these cations have been combined with weakly coordinating anions, although not all weakly coordinating anions result in ILs [9]. Common examples include tetrafluoroborate, hexafluorophosphate, triflate, triflimide, and dicyanamide (Table2). The first two have been explored the most and must be treated with the greatest caution as they are fairly readily hydrolysed to boric acid and phosphate, respectively [4]. Indeed, various phosphate and phosphinate anions (Table2) have been employed bearing some advantages in RTILs [4,5]. Molecules 2020, 25, 5812 3 of 27 Molecules 2020, 25, x FOR PEER REVIEW 3 of 28 Molecules 2020, 25, x FOR PEER REVIEW 3 of 28

TableTableTable 2. 2. Structures Structures2. Structures of mostofmost most commoncommon common ions ions ions used used used in in designingin designingdesigning ionic ionicionic liquids. liquids. liquids. Adapted, Adapted, Adapted, with with permissionwith permission permission of ofref. ref.of [ 5ref.[5],], copyright, [5],copyright, copyright, 2020, 2020, 2020, Elsevier. Elsevier. Elsevier.

CationCation Cation

AnionAnion

Anion

Diﬀerent potential applications of ionic liquids rely on their physicochemical properties, which vary Imidazolium-based ILs, such as 1–butyl–3–methyl–imidazolium [bmim] and 1–ethyl–3–methyl– based on the structures of the cation and anion used to assemble them (Table2). A general understanding imidazolium [emim], are among the most studied due to their stability within oxidative and of these properties requires a detailed understanding of ionic liquid at the molecular level, namely the reductive conditions, low viscosity, and ease of synthesis [8]. The cationic component of ILs has been anion–cation interactions, which has been addressed in several previous reviews [5,10]. variedImidazolium-based to include pyridinium, ILs, such ammonium,as 1–butyl–3–methyl–imidazolium phosphonium, thiazolium, [bmim] and triazoliumand 1–ethyl–3–methyl– species [5] For instance, the ILs ﬂame retardancy, based on the non-volatility inherent to ion conductive imidazolium(Table 2). In [emim], general, are these among cations the have most been studiedcombined due with to weakly their coordinating stability within anions, oxidative although and liquids, opens up the possibility for their application in the energy sector. In fact, ionic liquids are reductivenot all conditions,weakly coordinating low viscosity, anions and result ease in of ILs synthesis [9]. Common [8]. The examples cationic include component tetrafluoroborate, of ILs has been being developed for energy devices that need to use safe materials (avoiding accidental explosion variedhexafluorophosphate, to include pyridinium, triflate, triflimide, ammonium, and phosphonium,dicyanamide (Table thiazolium, 2). The first and two triazolium have been explored species [5] or ignition) [7]. (Tablethe 2).most In andgeneral, must thesebe treated cations with have the greatestbeen combined caution as with they weakly are fairly coordinating readily hydrolysed anions, to although boric For electrochemical usage, the most important properties should be both electroconductivity and not acidall weakly and phosphate, coordinating respectively anions [4]. result Indeed, in ILs various [9]. Common phosphate examples and phosphinate include anionstetrafluoroborate, (Table 2) ion conductivity [11]. These are essentially the properties of advanced (and safe) electrolyte solutions hexafluorophosphate,have been employed triflate, bearing triflimide, some advantages and dicyanamide in RTILs [4,5]. (Table 2). The first two have been explored that are critical to energy devices. For example, currently, electrochemical capacitors are limited in the the mostDifferent and must potential be treated applications with the of greatest ionic liquids caution rely as on they their are physicochemical fairly readily hydrolysedproperties, which to boric cell voltage [12] due to the degradation of the applied electrolytes, based on organic solvents. Since the acidvary and basedphosphate, on the respectivelystructures of [4].the Indeed,cation and various anion phosphate used to assemble and phosphinate them (Table anions 2). A general (Table 2) storableunderstanding energy and of these power properties are dependent requires on a thedetailed square understanding of the cell voltage, of ionic it liquid is worth at the investing molecular in have(safer) been alternatives employed to bearing the state-of-the-art some advantages electrolytes. in RTILs [4,5]. Different potential applications of ionic liquids rely on their physicochemical properties, which vary based on the structures of the cation and anion used to assemble them (Table 2). A general understanding of these properties requires a detailed understanding of ionic liquid at the molecular

Molecules 2020, 25, 5812 4 of 27

Electrolytes should have high ionic conductivity to minimize ohmic losses, high salt concentration to prevent starvation effects, a high electrochemical stability window in an appropriate temperature range, as well as high safety and low environmental impact. Aqueous electrolytes exhibit very high ionic conductivities. However, their electrochemical stability window is very limited (the thermodynamic stability of water, at room temperature, is 1.23 V [13]), and therefore are not suitable to use at a wide temperature range or on a very small scale (due to the 100 ◦C boiling point of water) and are usually highly corrosive. Non-aqueous common electrolytes (e.g., tetraethylammonium or triethylmethylammonium tetrafluoroborate salts in acetonitrile) also display very limited potential windows mainly due to the decomposition of the used solvent (e.g., the potential window of commercially available devices based on acetonitrile is 2.7 V [14]). The use of ionic liquids as solvent free electrolytes might be one way to overcome the above potential window limitations. In fact, RTILs often display larger electrochemical stability windows (Table3)[ 15,16] as compared to common non–aqueous electrolytes, concomitant with high thermal stability, non-flammability and in certain cases high conductivity. 1 RTILs exhibit a broad range of conductivities spanning from 0.1 to 20 mS cm− [17]. In general, higher conductivities are found for imidazolium based ILs in comparison with the ammonium ones. Many factors can affect their conductivity, such as viscosity, density, ion size, anionic charge delocalization, aggregations and ionic motions [18]. Strong ion-pair associations have been invoked - in the case of bis(trifluoromethylsulfonyl)imide ([NTf2] , Tf = triflate) based ILs, to understand their lower conductivity in comparison with tetrafluoroborate based ionic liquids [18]. The applicable electrochemical potential window of ILs can be determined using well-known electrochemical methods, such as cyclic voltammetry, and is typically found to be similar to or slightly larger than that found in conventional organic solvents, but much larger than that of aqueous electrolytes (Table3). Imidazolium-based ILs display shorter electrochemical windows than the phosphonium ones indicating a higher electrochemical activity of the formers. In fact, by reduction, imidazolium ion can lead to the formation of N-heterocyclic carbenes [19]. Thus, the challenge is to design RTILs with a wide electrochemical window along with good electrical conductivity.

Table 3. Examples of electrochemical potential window of selected phosphonium- and imidazolium-based ionic liquids and standard aqueous and non-aqueous electrolytes.

Potential IL Solvent Salt Electrode Reference Window/V

[P2225][NTf2] 6.3 [P ][NTf ] 6.4 2228 2 Pt wire [15] [P222(1O1)][NTf2] 5.7 [P222(2O1)][NTf2] 5.4 -- [bmim][NTf ] 3 2 Carbon ﬁlm [16] [bmim][NO3] 2.8

[Pyr14][NTf2] TiC-CDC 2.5 [20] [Pyr14][NTf2] AC 3.5 [21] [EdMPN][NTf ] Sucrose 2.3 [22] 2 ± Acetone [NEt4][ClO4], [NBu4][PF6], NaClO4 3.5 CH3CN [NEt4][ClO4], [NBu4][PF6], LiClO4 4 CH2Cl2 [NBu4][PF6], [NBu4][ClO4], [NBu4][X] (X = Cl, Br, F, I) 3.7 - DMF [NEt4][ClO4], [NBu4][PF6], LiCl, NaClO4 Pt wire 4.3 [23] DMSO [NEt4][ClO4], [NBu4][PF6] 3.3 THF [NBu4][PF6], LiClO4, NaClO4 3.7 H2O NaClO4,KNO3 2

[P2225][NTf2] = triethyl-n-pentylphosphonium bis(trifluoromethylsulfonyl)amide; [P222(1O1)][NTf2] = triethyl (methoxymethyl)phosphonium bis(trifluoromethylsulfonyl)amide; [bmim][NTf2] = 1-butyl-3 methylimidazolium bis(trifluoromethylsulfonyl)imide; [Pyr14][NTf2] = 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide; [EdMPN][NTf2] = ethyldimethylpropylammonium bis(trifluoromethylsulfonyl) imide.

Therefore, ionic liquids having a large electrochemical potential window and particularly unique physicochemical properties (e.g., negligibly small vapor pressure), constitute promising (and safer) candidates for the substitution of currently used electrolyte solutions based on organic molecular Molecules 2020, 25, 5812 5 of 27 solvents. They have been applied as media for electrodeposition of metals [24], electrochemical biosensors [25], supercapacitors [26,27], batteries [28,29], and solar cells [30,31]. This review covers recent advances reported in the literature from 2015 to date, that have demonstrated a special focus on the application of ionic liquids as electrolytes for important electrochemical processes.

2. Imidazolium-Based Ionic Liquids The imidazolium based ILs constitute the class of ionic liquids most used in electrochemical applications. The ones bearing two to four carbon atoms in the cation chain length, such as 1–butyl–3–methyl–imidazolium [bmim] or 1–ethyl–3–methyl–imidazolium [emim], respectively, are preferred, namely for electrodeposition processes. Caporali et al. applied 1–Butyl–3–methyl–imidazolium bis(trifluoromethylsulfonyl)imide, [bmim][NTf2] (Tf = triflate) for the electrodeposition of a group of transition metals, such as silver, copper, cobalt, nickel, or zinc [32]. The metal was dissolved in such IL, under conditions suitable for industrial applications, i.e., an uncontrolled (moisture content) atmosphere, and electrochemically characterized by cyclic voltammetry and chronoamperometry. + Silver was present in [bmim][NTf2] in the form of uncoordinated or weakly coordinated Ag ions, and therefore represented the simplest electrochemical system. Homogeneous and crack-free silver coatings were potentiostatically obtained from this ionic liquid. The authors used a “dry” and a “wet” sample, were the water content was determined by Karl Fischer titration. The electroreduction of Ag+ was favoured in the “wet” system, but in both cases the reverse sweep showed a current crossover and a reduction peak that was dependent on the scan rate. Moreover, when they increased the scan rate, the peak potential value moved towards more negative values. Both of these observations are indicative that silver electroreduction follows a nucleation-growth mechanism. Thus, chronoamperometric measurements were carried out by stepwise variations of the potential of the working electrode from a value where no reduction of silver occurs to potentials sufficiently negative to induce the reduction process. In this manner, the electro-crystallization mechanism, by comparing experimental data with calculated values obtained from a theoretical model, was determined by the authors, that also compared their observations with previous published results obtained using other types of ionic liquids. They concluded that, in the “wet” system, the overpotential required to start the electroreduction is lower and the maximum current value is higher. This phenomenon was partially attributed to the enhanced Ag+ ion mobility due to the decreased viscosity of the electrochemical medium resulting from the presence of a larger amount of water. In the dry system, probably due to the lower mobility of the electroactive species, the effect of overpotential is less marked. In contrast, copper-, cobalt-, and zinc-bearing systems were highly moisture sensitive. The majority 1 of these metals were reduced at atmospheric pressure and at a scan rate of 100 mV.s− . The authors pointed out that the addition of the metal salts to the hydrophobic [bmim][NTf2] considerably increases the ability of this IL to absorb water from the atmosphere and, therefore, the mild dehydrating experimental conditions employed assured only partial water removal. Therefore, the voltammogram recorded in the “dry” and “wet” system could not be directly compared, leading to a determination of the growth mechanism only in the “wet” systems. The authors concluded that the presence of large amounts of water changed the nature of the coordination species present in the solution, facilitating the electroreduction of the metal centres, and had also effects on the displayed colours of the samples. Nevertheless, metallic deposits were obtained, and their morphologies investigated as a function of the deposition potential and water content. Only in the case of the Ni-bearing IL did the authors report that it didn’t form electroactive species. In this study, Caporali et al. [32] used very simple operating conditions that did not require a rigorously controlled atmosphere. This advantage made the process particularly easy for a potential upgrading for large low-cost applications. Molecules 2020, 25, x FOR PEER REVIEW 6 of 28 of water. In the dry system, probably due to the lower mobility of the electroactive species, the effect of overpotential is less marked. In contrast, copper-, cobalt-, and zinc-bearing systems were highly moisture sensitive. The majority of these metals were reduced at atmospheric pressure and at a scan rate of 100 mV.s−1. The authors pointed out that the addition of the metal salts to the hydrophobic [bmim][NTf2] considerably increases the ability of this IL to absorb water from the atmosphere and, therefore, the mild dehydrating experimental conditions employed assured only partial water removal. Therefore, the voltammogram recorded in the “dry” and “wet” system could not be directly compared, leading to a determination of the growth mechanism only in the “wet” systems. The authors concluded that the presence of large amounts of water changed the nature of the coordination species present in the solution, facilitating the electroreduction of the metal centres, and had also effects on the displayed colours of the samples. Nevertheless, metallic deposits were obtained, and their morphologies investigated as a function of the deposition potential and water content. Only in the case of the Ni- bearing IL did the authors report that it didn’t form electroactive species. In this study, Caporali et al. [32] used very simple operating conditions that did not require a rigorously controlled atmosphere. This advantage made the process particularly easy for a potential upgradingMolecules for large2020, 25 low-cost, 5812 applications. 6 of 27 The IL 1-butyl-3-methyl-imidazolium chloride, [bmim]Cl, was used as electrolyte in the study of the redox propertiesThe IL 1-butyl-3-methyl-imidazolium of Cu(I) and Cu(II) ions, chloride, showing [bmim]Cl, that wasthe usedCu(I) as ion electrolyte can be in oxidized the study to Cu(II) and reducedof the to redox Cu(0), properties and that of Cu(I) Cu(II) and can Cu(II) be ions, reduced showing to thatCu(I) the by Cu(I) the ion metallic can be oxidizedCu(0) [33]. to These Cu(II) and reduced to Cu(0), and that Cu(II) can be reduced to Cu(I) by the metallic Cu(0) [33]. electrochemical processes are depicted in Figure 2. These electrochemical processes are depicted in Figure2.

Figure 2. Cyclic voltammogram for Cu deposition on a Pt electrode exhibiting a nucleation overpotential Figure 2.(green); Cyclic Cyclic voltammogram voltammogram for obtained Cu deposition on a Pt electrode on in a the Pt [bmim]Cl electrode without exhibiting Cu(I) (blue). a nucleation overpotentialReproduced (green); with Cyclic permission voltammogram from [33], copyright obtained 2016, on Elsevier. a Pt electrode in the [bmim]Cl without Cu(I) (blue). Reproduced with permission from [33], copyright 2016, Elsevier. According to the authors, the experimental voltammograms suggested the presence of a nucleation phenomenon (Figure2), but mainly in a qualitative way. The electrodeposition of copper occurs, Accordingand the explanation to the authors, given is the that theexperimental formation of stablevoltammograms Cu nuclei on an suggested inert surface the requires presence a of a nucleationpotential phenomenon more negative (Figure than the2), reductionbut mainly of Cu(I) in a on qualitative a copper surface. way. Chronoamperometry The electrodeposition was also of copper occurs, andperformed the explanation to understand given the nucleation is that the/growth formation process of of stable copper onCu a nuclei Pt wire andon an a Pt inert disc electrode.surface requires a potential moreThe applicationsnegative than of rare the earth reduction metals inof technologicalCu(I) on a copper devices suchsurface. as cell Chronoamperometry phones, and other was electronic devices, as well as in permanent magnets leads to a pressing need to find a way to also performedrecycle them.to understand the nucleation/growth process of copper on a Pt wire and a Pt disc electrode. Gupta et al. [34], studied, by cyclic voltammetry and chronoamperometry at a glassy carbon The electrode,applications the electrochemical of rare earth properties metals in of technological europium(III) aiming devices at to such understand as cell the phones, oxidation and other electronicstate, devices, coordination as well geometry,as in permanent and physicochemical magnets leads behaviour to a pressing of the Eu(III) need to complex find a bearing way to recycle di-hexyl-N,N-diethylcarbamoylmethylphosphonate (DHDECMP) ligand (as complexing extractant) them. in [bmim][NTf2]. The reduction of Eu(III) to Eu(II) in [bmim][NTf2] has shown a quasi-reversible Guptareduction, et al. [34], at a peak studied, potential by of cyclic1.44 Vvoltammetry vs. Fc/Fc+ (Fc = andferrocene), chronoamperometry and was controlled byat dia ffglassyusion carbon − electrode,and the charge electrochemical transfer kinetics properties (Figure3). of europium(III) aiming at to understand the oxidation state, coordination Thegeometry, cyclic voltammogram and physicochemical of [bmim][NTf behaviour2], recorded of the at aEu(III) glassy carboncomplex working bearing electrode, di-hexyl-N,N- + is shown in the inset of Figure3. The reduction of the [bmim] cation occurs at a potential of 2.0 V + – + − (vs. Fc/Fc ), and the oxidation of the [NTf2] anion occurs at 1.0 V (vs. Fc/Fc ), allowing to detect the reduction of Eu(III) to Eu(II). Photoluminescence spectroscopy confirmed that the symmetry around the 3+ europium ions at Eu –DHDECMP in the [bmim][NTf2] was relatively low. By fluorescence lifetime measurements, the authors also determined that the number of water molecules in the inner sphere is six for non-complexed europium ions, and practically no water molecules was retained in the presence of the complexing extractant DHDECMP. Molecules 2020, 25, x FOR PEER REVIEW 7 of 28

diethylcarbamoylmethylphosphonate (DHDECMP) ligand (as complexing extractant) in [bmim][NTf2]. The reduction of Eu(III) to Eu(II) in [bmim][NTf2] has shown a quasi-reversible reduction, at a peak potential of −1.44 V vs. Fc/Fc+ (Fc = ferrocene), and was controlled by diffusion and charge transfer kinetics (Figure 3).

Molecules 2020, 25, x FOR PEER REVIEW 7 of 28 diethylcarbamoylmethylphosphonate (DHDECMP) ligand (as complexing extractant) in [bmim][NTf2]. The reduction of Eu(III) to Eu(II) in [bmim][NTf2] has shown a quasi-reversible reduction, at a peak potential of −1.44 V vs. Fc/Fc+ (Fc = ferrocene), and was controlled by diffusion and charge transferMolecules kinetics2020, 25, 5812 (Figure 3). 7 of 27

Figure 3. Cyclic voltammogram of 50 mM Eu(III) ions in [bmim][NTf2] at 20mV/s. Inset: Cyclic

The cyclic voltammogram of [bmim][NTf2], recorded at a glassy carbon working electrode, is shown in the inset of Figure 3. The reduction of the [bmim]+ cation occurs at a potential of −2.0 V (vs. Fc/Fc+), and the oxidation of the [NTf2]– anion occurs at 1.0 V (vs. Fc/Fc+), allowing to detect the reduction of Eu(III) to Eu(II). Photoluminescence spectroscopy confirmed that the symmetry around the europium ions at Eu3+–DHDECMP in the [bmim][NTf2] was relatively low. By fluorescence lifetime measurements, the authors also determined that the number of water molecules in the inner sphere is six for non-complexed europium ions, and practically no water molecules was retained in Figure 3. Cyclic voltammogram of 50 mM Eu(III) ions in [bmim][NTf2] at 20mV/s. Inset: Figurethe presence 3. Cyclic ofCyclic the voltammogram voltammogramcomplexing ofextractant [bmim][NTf of 50 mM 2DHDECMP.] at Eu(III) a glassy ionscarbon in electrode [bmim][NTf at 25 ◦C. Reproduced2] at 20mV/s. with Inset: Cyclic permission from [34], copyright 2015, Wiley. voltammogramAn IL with of [bmim][NTf a higher 2 cation] at a glassy chain carbon length, electrode [hmim][NTf at 25 2°C.] (hmimReproduced = 1–hexyl–3–methyl– with permission

fromimidazolium), [34], copyrightAn was IL with applied2015, a higher Wiley. in cation the study chain length, of the [hmim][NTf redox properties2] (hmim = 1–hexyl–3–methyl–imidazolium), of europium(III) for the recovery of europium wasby an applied extraction–electrodeposition in the study of the redox properties (EX-EL) of europium(III) procedure for [35]. the The recovery cyclic of europiumvoltammogram of by an extraction–electrodeposition (EX-EL) procedure [35]. The cyclic voltammogram of Eu(III) in TheEu(III) cyclic in [hmim][NTf voltammogram2], recorded of [bmim][NTf at a glassy carbon2], recorded electrode, at exhibited a glassy a carbonprominent working quasi-reversible electrode, is [hmim][NTf2], recorded at a glassy carbon electrode, exhibited a prominent quasi-reversible reduction + shownreduction in the inset wavewave of occurring occurring Figure at the 3. at onsetThe the of reductiononset0.25 Vof vs. −0.25 Fc of/Fc theV+ (Fc vs. [bmim]= Fc/Fcferrocene), (Fc+ cation culminating = ferrocene), occurs in a peak atculminating a at potential0.84 V vs. in ofa peak −2.0 atV (vs. − − −0.84 V vs.Fc Fc/Fc/Fc+ assigned+ assigned to the to reduction the reduction of Eu(III) toof Eu(II),Eu(III) as shownto Eu(II), in Figure as shown4. in Figure 4. Fc/Fc+), and the oxidation of the [NTf2]– anion occurs at 1.0 V (vs. Fc/Fc+), allowing to detect the reduction of Eu(III) to Eu(II). Photoluminescence spectroscopy confirmed that the symmetry around the europium ions at Eu3+–DHDECMP in the [bmim][NTf2] was relatively low. By fluorescence lifetime measurements, the authors also determined that the number of water molecules in the inner sphere is six for non-complexed europium ions, and practically no water molecules was retained in the presence of the complexing extractant DHDECMP. An IL with a higher cation chain length, [hmim][NTf2] (hmim = 1–hexyl–3–methyl– imidazolium), was applied in the study of the redox properties of europium(III) for the recovery of europium by an extraction–electrodeposition (EX-EL) procedure [35]. The cyclic voltammogram of Eu(III) in [hmim][NTf2], recorded at a glassy carbon electrode, exhibited a prominent quasi-reversible reduction wave occurring at the onset of −0.25 V vs. Fc/Fc+ (Fc = ferrocene), culminating in a peak at −0.84 V vs. Fc/Fc+ assigned to the reduction of Eu(III) to Eu(II), as shown in Figure 4. Figure 4. Cyclic voltammetry of [hmim][NTf2] (—) and Eu(III) in [hmim][NTf2] (—-) recorded at glassy Figure 4. carbonCyclic electrode. voltammetry [Eu(III)] = of100 [hmim][NTf mM, scan rate2=] 100(—) mV and/s, T =Eu(III)373 K. Reproducedin [hmim][NTf with permission2] (----) of recorded at glassy carbonref. [35 electrode.], copyright, 2015, [Eu(III)] Springer. = 100 mM, scan rate = 100 mV/s, T = 373 K. Reproduced with permission of ref. [35], copyright, 2015, Springer. The cathodic peak current was lowered, and the peak potential shifted cathodically in the presence of tri-n-butyl phosphate (TBP) and N,N-dihexyloctanamide (DHOA) as ligands, due to the

Figure 4. Cyclic voltammetry of [hmim][NTf2] (—) and Eu(III) in [hmim][NTf2] (----) recorded at glassy carbon electrode. [Eu(III)] = 100 mM, scan rate = 100 mV/s, T = 373 K. Reproduced with permission of ref. [35], copyright, 2015, Springer.

MoleculesMolecules 2020 2020, ,25 25, ,x x FOR FOR PEER PEER REVIEW REVIEW 8 8 of of 28 28

MoleculesThe2020The , cathodic25 cathodic, 5812 peak peak current current was was lowered, lowered, and and the the peak peak potential potential shifted shifted cathodically cathodically in in the the8 of 27 presencepresence of of tri-n-butyl tri-n-butyl phosphate phosphate (TBP) (TBP) and and N,N-dihexyloctanamide N,N-dihexyloctanamide (DHOA) (DHOA) as as ligands, ligands, due due to to the the co-ordinationco-ordination of of Eu(III) Eu(III) to to such such species species in in the the ionic ionic liquid liquid medium medium (Figure (Figure 5). 5). Moreover, Moreover, the the presence presence co-ordinationofof TBP TBP in in the the of ionic ionic Eu(III) liquid liquid to such mediummedium species shiftsshifts in the the Eu(III) ionicEu(III) liquid to to Eu(II) Eu(II) medium peak peak potential potential (Figure5 to ).to Moreover,moremore negative negative the values presencevalues of TBPasas compared compared in the ionic to to DHOA DHOA liquid (compare medium(compare shiftsFigure Figure the 5A,B). 5A,B). Eu(III) to Eu(II) peak potential to more negative values as compared to DHOA (compare Figure5A,B).

((AA)) ((BB)) Figure 5. Cyclic voltammograms of Eu(III) (100 mM) recorded at glassy carbon electrode at a scan rate Figure 5. Cyclic voltammograms of Eu(III) (100 mM) recorded at glassy carbon electrode at a scan rate Figure 5. Cyclic1 voltammograms of Eu(III) (100 mM) recorded at glassy carbon electrode at a scan rate of 100 mV s− −1and at 373 K, in (A) TBP/[hmim][NTf2] or (B) in DHOA/[hmim][NTf2]. Reproduced with ofof 100 100 mV mV s s−1 and and at at 373 373 K, K, in in ( (AA)) TBP/[hmim][NTf TBP/[hmim][NTf2]2] or or ( (BB)) in in DHOA/[hmim][NTf DHOA/[hmim][NTf22].]. Reproduced Reproduced with with permission of ref. [35], copyright, 2015, Springer. permissionpermission of of ref. ref. [35], [35], copyright, copyright, 2015, 2015, Springer. Springer. The stability constants of Eu(III)–ligand complex showed that the stability of Eu-DHOA complex TheThe stability stability constants constants of of Eu(III)–ligand Eu(III)–ligand complex complex showed showed that that the the stability stability of of Eu-DHOA Eu-DHOA complex complex in ionic liquid medium was lower than that of the Eu-TBP complex, and therefore the use of DHOA inin ionic ionic liquid liquid medium medium was was lower lower than than that that of of the the Eu-TBP Eu-TBP complex, complex, and and therefore therefore the the use use of of DHOA DHOA ligandligandligand for for for facilitating facilitating facilitating the the the electrodeposition electrodeposition electrodeposition ofof of europiumeuropium from from the the the ionic ionic ionic liquid liquid liquid during during during the the the recovery recovery recovery of of of europiumeuropiumeuropium by by by extraction–electrodeposition extraction–electrodeposition extraction–electrodeposition (EX-EL)(EX-EL) (EX-EL) procedureprocedure would would be be be the the the best best best option. option. option. KrishnaKrishnaKrishna et et et al. al. al. [ 36 [36], [36],], attached attached attached the the the neutral neutral neutral ligand ligand DHOA DHOA to to to [bmim][NTf [bmim][NTf [bmim][NTf22]] 2 to] to to study study study the the the electrodepositionelectrodepositionelectrodeposition of of of a a a seriesseries series of ofof lanthanides lanthanides (Ln), (Ln), (Ln), such such such as as europium, europium, as europium, neodymium, neodymium, neodymium, or or dysprosium. dysprosium. or dysprosium. The The Thereductionreduction reduction of of ofEu Eu Eu3+3+, ,Nd 3Nd+,3+ Nd3+, ,and and3+, Dy andDy3+3+ to Dyto their their3+ to respective respective their respective metallic metallic metallicforms forms was was forms successfully successfully was successfully achieved achieved (Figure (Figure achieved (Figure6).6). 6).

FigureFigureFigure 6. 6. 6.Cyclic Cyclic Cyclic voltammograms voltammograms of of of100 100 100 mM mM mM [Ln(NTf [Ln(NTf [Ln(NTf2)2)33]] in in2 )DHOA 3DHOA] in DHOA medium medium medium recorded recorded recorded at at glassy glassy carbon atcarbon glassy 1 carbonworkingworking working electrode electrode electrode at at the the at scan scan the raterate scan of of rate 50 50 mVs of mVs 50−1−1 mV at at 353 353 s− K. K.at Insets Insets 353K. show show Insets the the show photographs photographs the photographs of of the the of theelectrodepositselectrodeposits electrodeposits obtainedobtained obtained atat thethe at working working the working electrode electrode electrode afterafter electrolysiselectrolysis after electrolysis ofof 100100 mM mM of 100LnLn3+3+ mM in in the the Ln system system3+ in the system DHOA/[bmim][NTf ] at 3.0 V for about 2 h. Reproduced with permission from [36], 2 − copyright 2020, Elsevier.

To enhance their solubility, the investigation of the electrochemical behaviour of the lanthanide(III) ion present in neutral ligand-ionic liquid (NLIL) was performed by cyclic voltammetry. Figure6 shows Molecules 2020, 25, x FOR PEER REVIEW 9 of 28 Molecules 2020, 25, 5812 9 of 27 DHOA/[bmim][NTf2] at −3.0 V for about 2 h. Reproduced with permission from [36], copyright 2020, Elsevier. that the there is a two-step reduction observed for Eu3+, whereas the NLIL containing Nd3+ and Dy3+ To enhance their solubility, the investigation of the electrochemical behaviour of the underwent a single step 3-electron transfer reduction at the cathodic potential of 3.0 V. The authors lanthanide(III) ion present in neutral ligand-ionic liquid (NLIL) was performed by cyclic 0 0− also confirmedvoltammetry. that theFigure anodic 6 shows waves, that the corresponding there is a two-step to the reduction oxidation observed of Nd foror Eu Dy3+, towhereas higher the oxidation states, wereNLIL notcontaining observed Nd3+ duringand Dy3+ the underwent performed a single reversed step 3-electron scans. transfer reduction at the cathodic Senguptapotential etof al.,−3.0 [V.37 The] used authors [bmim][NTf also confirmed2] to that report the anodic a first-ever waves, cyclic corresponding voltammetric to the oxidation electrochemical characterizationof Nd0 or Dy of0 neptunium(IV)to higher oxidation complexes states, were bearing not observed the diglycolamideduring the performed ligand. reversed Np(IV) scans. was found to undergo mono-electronSengupta et al., [37] exchange used [bmim][NTf reactions.2] to The report activation a first-ever energy cyclic voltammetric values were electrochemical deduced using the characterization of neptunium(IV) complexes bearing the diglycolamide ligand. Np(IV) was found Arrhenius equation and thermodynamic parameters were derived using linear regression of the data. to undergo mono-electron exchange reactions. The activation energy values were deduced using the The redoxArrhenius reactions equation of the and Np(IV)thermodynamic complexes parameters were were exothermic. derived using In alinear further regression study, of thethe data. authors [38] detected,The by redox cyclic reactions voltammetry, of the Np(IV) a di complexesfferent electrochemical were exothermic. behaviourIn a further study, for neptunium(IV) the authors [38] bearing diphenyl-N,N-diisobutylcarbamoyl-methylphosphinedetected, by cyclic voltammetry, a different electrochemical oxide behaviour in [bmim][NTf for neptunium(IV)2] (CMPO-NTf bearing2): for the diphenyl-N,N-diisobutylcarbamoyl-methylphosphine oxide in [bmim][NTf2] (CMPO-NTf2): for the redox couples involving complexes of Np(IV) with CMPO or CMPO-NTf2, the enthalpy values were redox couples involving complexes of Np(IV) with CMPO or CMPO-NTf2, the enthalpy values were positive or endothermic. positive or endothermic. Later, Rama et al., [39] used [bmim][NTf ] to investigate the electrochemistry performance of Later, Rama et al., [39] used [bmim][NTf22] to investigate the electrochemistry performance of U(VI). TheU(VI). cyclic The cyclic voltammogram voltammogram of of U(VI) U(VI) inin [bmim][NTf[bmim][NTf2], 2 recorded], recorded at a at glassy a glassy carbon carbon working working electrodeelectrode at the at scan the scan rate rate of 100of 100 mV mV/s/s at at 373 373 K,K, exhibited a aprominent prominent quasi-reversible quasi-reversible reduction reduction of of U(VI) toU(VI) U(V) to (FigureU(V) (Figure7), where 7), where the the cathodic cathodic peak peak potential was was shifted shifted anodically anodically and the and cathodic the cathodic peak currentpeak current increased increased with with the the increase increase of of temperature.temperature. The The presence presence of tri-n-octyl of tri-n-octyl phosphate phosphate (TOP) (TOP) or tri-n-butyl phosphate (TBP, a reference extractant) ligands shifted the cathodic peak to more or tri-n-butylnegative phosphate potentials due (TBP, to athe reference formation extractant) of a U(VI) complex ligands (compare shifted theFigure cathodic 7A,B). peak to more negative potentials due to the formation of a U(VI) complex (compare Figure7A,B).

(A) (B)

Figure 7.FigureCyclic 7. Cyclic voltammograms voltammograms recorded recorded at at aa glassy carbon carbon electrode electrode ([U(VI)] ([U(VI)] = 100 =mM,100 scan mM, rate scan = rate = 100 1mVs−1, T = 373 K) of (A) [bmim][NTf2] (----) and U(VI) in [bmim][NTf2] (—); and (B) U(VI) in 100 mV s− ,T = 373 K) of (A) [bmim][NTf2] (—-) and U(VI) in [bmim][NTf2] (—); and (B) U(VI) in [bmim][NTf2], in TBP/[bmim][NTf2] and in TOP/[bmim][NTf2]. Reproduced with permission from [bmim][NTf ], in TBP/[bmim][NTf ] and in TOP/[bmim][NTf ]. Reproduced with permission from [39], [39], copyright2 2016, Elsevier. 2 2 copyright 2016, Elsevier. One year later, Krishna’ group extended the study of the redox properties of U(VI) by using One[bmim][Cl], year later, which Krishna’ increased group the cathodic extended current the density study and of consequently the redoxproperties the U(VI) reduction of U(VI) was by using [bmim][Cl],facilitated which [40]. increased the cathodic current density and consequently the U(VI) reduction was facilitated [40The]. above work was followed by the use of imidazolium-based IL bearing the dicyanamide 2+ Theanion above [bmim][DCA], work was by followed the same researchers by the use [41], of to imidazolium-based study the electrochemical IL bearingbehaviour the of UO dicyanamide2 . 2+ + The cathodic peaks corresponding to the reduction of UO2 to UO2 and then to UO2 were clearly 2+ anion [bmim][DCA],detected, indicating by that the [bmim][DCA] same researchers is a good [41 solvent], to study for this the lanthanide electrochemical cation. behaviour of UO2 . 2+ + The cathodic peaks corresponding to the reduction of UO2 to UO2 and then to UO2 were clearly detected, indicating that [bmim][DCA] is a good solvent for this lanthanide cation. Aiming at to accelerate the energy transition (urgent to be adopted in order to reduce anthropogenic causes for climate changes), an intense research is being developed in electrochemical processes of energy conversion and storage, through the design of new materials, namely ionic liquids, which would be able to increase the energetic eﬃciency of important processes. The suitability of room temperature ionic liquids as solvents for redox ﬂow batteries (RFBs) containing metal complexes was investigated by Ejigu et al. [42], using metal acetylacetonate (acac) Molecules 2020, 25, x FOR PEER REVIEW 10 of 28

Aiming at to accelerate the energy transition (urgent to be adopted in order to reduce anthropogenic causes for climate changes), an intense research is being developed in electrochemical Moleculesprocesses2020, 25 of, 5812 energy conversion and storage, through the design of new materials, namely ionic10 of 27 liquids, which would be able to increase the energetic efficiency of important processes. The suitability of room temperature ionic liquids as solvents for redox flow batteries (RFBs) complexescontaining [M(acac) metal 3complexes], (M = Mn, was Cr, investigated or V) in imidazolium by Ejigu et al. based[42], using RTILs. metal They acetylacetonate detected, by (acac) cyclic voltammetrycomplexes at [M(acac) a glassy3], carbon (M = Mn, electrode, Cr, or anV) irreversible in imidazolium behaviour based forRTILs. [Mn(acac) They 3 detected,] and [Cr(acac) by cyclic3] in 1-ethyl-3-methylimidazoliumvoltammetry at a glassy carbon bis(trifluoromethanesulfonyl)imide electrode, an irreversible behaviour [emim][NTffor [Mn(acac)2],3] and but the[Cr(acac) rate of3] in the 2+ 3+ Mn 1-ethyl-3-methylimidazolium/Mn reaction increased when bis(trifluoromethanesulfonyl)imide Au electrodes were used. [emim][NTf2], but the rate of the MnThe2+/Mn authors3+ reaction also increased observed when that Au electrodes V2+/V3+,V were3+ used./V4+ and V4+/V5+ redox couples were 2+ 3+ 3+ 4+ 4+ 5+ quasi-reversibleThe authors in 1-ethyl-3-methylimidazolium also observed that V /V , V /V bis(trifluoromethanesulfonyl)imide and V /V redox couples were quasi-reversible [emim][NTf2 ], by cyclicin 1-ethyl-3-methylimidazolium voltammetry at glassy carbon bis(trifluoromethanesulfonyl)imide electrode (with a high columbic [emim][NTf efficiency2], by of cyclic 72%). voltammetry at glassy carbon electrode (with a high columbic efficiency of 72%). Moreover, among Moreover, among the following RTILs studied, namely [bmim][BF4], [bmim][PF6], [emim][N(CN)2], the following RTILs studied, namely [bmim][BF4], [bmim][PF6], [emim][N(CN)2], and [emim][EtSO4], and [emim][EtSO4], [EtSO4]− is the ethylsulfate anion (Figure8), and only [emim][NTf 2] stabilized the − n+ various[EtSO Vn4]+ isspecies the ethylsulfate (Figure8A) anion [ 42 ].(Figure In fact, 8), in and Figure only8 [emim][NTfA, we can see2] stabilized clearly three the various well-defined V species redox (Figure 8A) [42]. In fact, in Figure 8A, we can see clearly three well-defined redox couples centred at couples centred at 1.37, 0.63, and 0.90 V corresponding to V2+/V3+,V3+/V4+ and V4+/V5+ oxidations, −1.37, 0.63, and −0.90 V corresponding to V2+/V3+, V3+/V4+ and V4+/V5+ oxidations, respectively. Also, the respectively. Also, the potential difference, ∆E, between the V2+/V3+,V3+/V4+ and V4+/V5+ couples is potential difference, ΔE, between the V2+/V3+, V3+/V4+ and V4+/V5+ couples is 2.0 V, which is comparable 2.0 V, which is comparable to several organic solvents. Such a decrease in ∆E for consecutive redox to several organic solvents. Such a decrease in ΔE for consecutive redox waves upon changing wavessolvents upon changingfrom acetonitrile solvents to fromRTILs acetonitrile seems to demonstrate to RTILs seems that tothe demonstrate RTIL are more that basic the RTILthan most are more of basic than most of the common organic solvents. Figure8B,C show that the [V(acac) ] redox couples the common organic solvents. Figures 8B and 8C show that the [V(acac)3] redox couples3 were not wereclearly not clearly discernible discernible in [bmim][BF in [bmim][BF4] and [bmim][PF4] and [bmim][PF6]. In [emim][N(CN)6]. In [emim][N(CN)2] and [emim][EtSO2] and [emim][EtSO4], the redox4 ], the redoxcouple couple at positive at positive potentials potentials was chemically was chemically irreversible irreversible (Figures (Figure8D and8 D,E).8E). Interesting Interesting isis in in the the particularparticular case case of [emim][N(CN) of [emim][N(CN)2],2 were], were a cathodic,a cathodic, return returnpeak peak diddid appear as as the the scan scan rate rate increased increased andand the ratiothe ratio of theof the cathodic cathodic to anodicto anodic peak peak currents, currents, also also increased. increased. This suggests suggests that that a afollowing following chemicalchemical reaction reaction occurred occurred after after oxidation. oxidation.

Figure 8. 2nd cycles of cyclic voltammograms recorded at a 5 mm diameter glassy carbon disk

electrode at 50 mV/s of (A) [V(acac)3] (10 mM) in [emim][NTf2], (B) [V(acac)3] (11 mM) in [bmim][BF4] (C) [V(acac)3] (12 mM) in [bmim][PF6], (D) [V(acac)3] (12 mM) in [emim][N(CN)2] and (E) [V(acac)3] (20 mM) in [emim][EtSO4]. All cyclic voltammograms began at the negative potential limit. Reproduced with permission from [42], copyright 2020, Springer. Molecules 2020, 25, x FOR PEER REVIEW 11 of 28

Figure 8. 2nd cycles of cyclic voltammograms recorded at a 5 mm diameter glassy carbon disk electrode at 50 mV/s of (A) [V(acac)3] (10 mM) in [emim][NTf2], (B) [V(acac)3] (11 mM) in [bmim][BF4]

Molecules 2020(C) [V(acac), 25, 58123] (12 mM) in [bmim][PF6], (D) [V(acac)3] (12 mM) in [emim][N(CN)2] and (E) [V(acac)3] 11 of 27 (20 mM) in [emim][EtSO4]. All cyclic voltammograms began at the negative potential limit. Reproduced with permission from [42], copyright 2020, Springer. In addition, they observed that the use of [bmim][NTf2] yields almost identical voltammetry to that observedIn addition, in [[emim][NTf they observed2], demonstrating that the use of that[bmim][NTf the RTILs2] yields anions almost played identical the most voltammetry significant rolesto n+ in stabilizingthat observed the in V [[emim][NTfspecies. They2], demonstrating also discussed that the RTILs importance anions ofplayed diffusion the most coe significantfficients. The roles low diffusionin stabilizing coefficients the V measured,n+ species. They which also shown discussed that [emim][NTf the importance2] are of an diffusion order of coefficients. magnitude lowerThe low than in acetonitrile,diffusion coefficients allied with measured, the relatively which high shown viscosity that [emim][NTf of RTILs can2] are suggest an order that of the magnitude RFBs containing lower [emim][NTfthan in acetonitrile,2] as the electrolyte allied with could the face relatively mass transport high viscosity issues. of RTILs can suggest that the RFBs containingBalo et [emim][NTf al. [43] applied2] as the electrolyte 1-ethyl-3-methylimidazolium could face mass transport bis(trifluoromethylsulfonyl)imide, issues. Balo et al. [43] applied 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][NTf2], for the recharge of lithium batteries, with promising results. They formed a gel polymer[emim][NTf electrolyte2], for (GPE) the recharge for high of performance lithium batteries, lithium with polymer promising batteries results. (LPBs). They The formed gel is a based gel on thepolymer polymer electrolyte polyethylene (GPE) for oxide high and performance the lithium lithium bis(trifluoromethylsulfonyl)imide polymer batteries (LPBs). The gel salt. is Bybased cyclic on the polymer polyethylene oxide and the lithium bis(trifluoromethylsulfonyl)imide salt. By cyclic voltammetric studies, the authors discovered that the gel exhibits promising characteristics suitable voltammetric studies, the authors discovered that the gel exhibits promising characteristics suitable for application in LPBs. The GPEs show high thermal stability, high ionic conductivity, high lithium for application in LPBs. The GPEs show high thermal stability, high ionic conductivity, high lithium transference number and high electrochemical stability window. One year later, Prasanna and transference number and high electrochemical stability window. One year later, Prasanna and collaborators [44] reported the use of [bmim][NTf2] incorporated in a polymeric matrix, containing collaborators [44] reported the use of [bmim][NTf2] incorporated in a polymeric matrix, containing polyvinylpolyvinyl chloride chloride (PVC) (PVC) andand poly(ethyl methacrylate) methacrylate) (PEMA), (PEMA), for the for electrochemical the electrochemical study of study zinc of zincion ion conducting conducting gel gel polymer polymer electrolytes electrolytes (GPEs). (GPEs). The prepared The prepared films of films gel polymer of gel polymer membranes membranes were werefully fully characterized characterized utilizing utilizing complex complex impedance impedance spectroscopy, spectroscopy, differential differential scanning scanning calorimetry calorimetry thermogravimetric,thermogravimetric, and and cyclic cyclic voltammetry voltammetry analyses. analyses. A similar trend trend was was observed observed for for the the dielectric dielectric constantconstant and and ionic ionic conductivity conductivity with with the the increase increase ofof [emim][NTf[emim][NTf22]] concentration. concentration. ZhangZhang group group reported reported that that the the use use of of 1-cyanopropyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide,bis(trifluoromethanesulfonyl)imide, [cpmim][NTf [cpmim][NTf22],], as as additive additive in in electrolyte electrolyte for for high-voltage high-voltage lithium-ionlithium-ion batteries batteries (LIBs), (LIBs), sharply sharply increasing increasing the thedischarge discharge capability capability of these of batteries these batteries[45]. Cyclic [45 ]. 0.5 1.5 4 Cyclicvoltammograms voltammograms of the of LiNi the LiNiMn0.5OMn/Li1.5 systemO4/Li system are presented are presented in Figure in 9, Figure displaying9, displaying two redox two redoxpeaks peaks on each on each curve. curve.

FigureFigure 9. Cyclic 9. Cyclic voltammograms voltammograms of of LiNi LiNi0.50.5MnMn1.51.5O44/Li/Li cells cells after after 30 30 cycles cycles with with different diﬀerent concentrations concentrations 1 of [cpmim][NTfof [cpmim][NTf2] at2] at 0.1 0.1 mV mVs s−−1. Reproduced Reproduced with with permission permission from from [45], [45 copyright], copyright 2020, 2020, Springer. Springer.

TheThe oxidation oxidation peaks peaks were were mainly mainly associated associated toto the redox reactions reactions undergone undergone by by the the transition transition metalsmetals present present at LiNiat LiNi0.5Mn0.5Mn1.51.5OO44//Li.Li. Moreover, the the electrolyte electrolyte with with 15 15 vol.% vol.% of of [cpmim][NTf [cpmim][NTf2] had2] had goodgood integrity integrity and and large large current current density. density. The The redox redox peaks peaks of 20 of vol.% 20 vol.% of [cpmim][NTf of [cpmim][NTf2] presented2] presented lower currentlower intensity current thanintensity the others,than the and others, the oxidation and the oxidation peak was peak anodically was anodically shifted. Theshifted. authors The authors presented twopresented possible reasonstwo possible for this reasons shift. for One this was shift. that One the was high that viscosity the high of 20 viscosity vol.% of of [cpmim][NTf 20 vol.% of 2] increased the internal resistance and the other reason may be related to the high concentration of ionic liquids that led to an enrichment phenomenon on the surface of cathode due to the charge of ionic liquid. As a consequence, the surface structure of the cathode was changed, and thus the peak had a slight shift. Even so, the authors concluded that the mixed electrolyte with [cpmim][NTf2] is promising to be used in the application of Li-ion batteries. Molecules 2020, 25, 5812 12 of 27

Recently, the chloroaluminate imidazolium ionic liquid AlCl3/[emim][Cl] was used to test the performance of titanium dioxide (TiO2) as electrode in rechargeable aluminum-ion batteries. TiO2 could be a promising electrode material for aluminum-ion battery but it is still quite understudied. Das et al., [46] demonstrate the rechargeability of an Al-ion cell with anatase TiO2 as cathode in the chloroaluminate ionic liquid electrolyte. The cell of Al-TiO2 was proven to have an excellent long-term stability. Also, the electroactive nature of TiO2 in the chloroaluminate electrolyte was studied both by cyclic voltammetry and galvanostatic cycling studies. The authors observed the existence of a synergistic effect of the current collector in improving the long-term stability of Al-TiO2 cell. In the same year, Huang and collaborators [47] also demonstrated that the use of pure 1-ethyl-3-methylimidazolium trifluoromethanesulfonate IL, [emim][OTf], as an ionic liquid electrolyte is a good choice for the design of a dual-graphite battery. Besides the usage of ILs themselves as electrolytes, some authors have applied them as electrolyte additives. For example, very recently [48], [emim][NTf2] was studied by cyclic voltammetry for usage as possible additive to the electrolyte in sodium ion batteries Electrochemical cycling of a hard carbon - electrode reveals that the [NTf2] anion, mainly coming from the added ionic liquid, can introduce new peaks in the cyclic voltammograms or new steps in the galvanostatic discharge. The products of such degradation can make an interface which determines the cycling properties. One very interesting work involving the application of ionic liquids as electrolyte additives was developed by Wong et al. [49], who studied the concentration of [emim][BF4] in an electrolyte to optimize the capacitive performance of high-energy density graphene supercapacitors. The authors reported that the electrolyte viscosity increased exponentially with the increase of IL concentration, while the ionic conductivity decreased with an increase in IL concentration. They also found a strong dependence of the specific capacitance not only on the electrolyte viscosity and ionic conductivity, but also in the maximum working voltage (MWV) where the electrode specific capacitance increases. The neat IL (i.e., [emim][BF4]) offered the largest specific capacitance and energy density among all IL concentrations for graphene-based electric double-layer capacitors (EDLC), despite having the highest viscosity and the lowest ionic conductivity. The large specific capacitance and energy density was also due to the largest MWV offered by this neat ILs. Another advantage reported by the authors was that, if the EDLC does not require to operate at the electrochemical stability window of the IL, diluted IL electrolytes with an optimized concentration/IL viscosity can be used instead. To sum up, they concluded that the concentration of IL electrolyte should be optimized according to the working voltage required. [Emim][N(Tf2)] was also used for studying the electrochemistry of I−,I2 and ICl, together with 3 [bmim][Cl], due to the integral role of the I−/I − couple in dye-sensitized solar cell technology [50]. Bentley et al., used the cyclic voltammetric technique at a platinum macrodisk electrode in neat + ILs, as well as a binary mixture of [bmim]Cl and [emim][N(Tf2)]. The neutral (I2) and positive (I ) + oxidation states of iodine are known to have strongly electrophilic behavior, and thus the I−/I2/I redox processes are sensitive to the presence of nucleophilic chloride or bromide. These anions are both commonly present as impurities in non haloaluminate RTILs. In the absence of chloride (e.g., in neat [emim][N(Tf2)], I− was oxidized in an overall one electron per iodide ion process to I2 via an - I3− intermediate, gave rise to two resolved I−/I3− and I3−/I2 processes. When Cl and I− were present, even in low concentrations (below 30 mM), an additional oxidation process appears at potentials less positive than the I3−/I2 process. This corresponded, according to the authors, to the oxidation of I3− to the inter-halide complex anion [ICl2]− in an overall two electrons per iodide ion process. Also, the electrochemistry of I−,I2, and ICl has been investigated by cyclic voltammetry in a binary IL mixture composed of [bmim]Cl and [emim][NTf2], using the same experimental conditions. The authors studied the effect of the presence of a large excess of Cl ([I ] 10 mM and [Cl ] 3.7 M) and − − ≈ − ≈ observed that I− was oxidized in an overall two electrons per iodide ion process to [ICl2]− via an [I2Cl]− + intermediate. Bentley and coworkers concluded that in the I−/I2/I processes, in non haloaluminate Molecules 2020, 25, x FOR PEER REVIEW 13 of 28

The authors studied the effect of the presence of a large excess of Cl− ([I−] ≈ 10 mM and [Cl−] ≈ 3.7 M) and observed that I− was oxidized in an overall two electrons per iodide ion process to [ICl2]− via an [I2Cl]− intermediate. Bentley and coworkers concluded that in the I−/I2/I+ processes, in non haloaluminate ILs, a complicated interplay between multiple electron transfer pathways and homogeneous chemical reactions is present, which may not be at equilibrium on the voltametric time scale. Molecules 2020, 25, 5812 13 of 27 A mixture of the IL [bmim][BF4] with acetonitrile as electrolyte was used by Lebedeva et al., to investigateILs, a complicated the electrochemical interplay between behavior multiple of polyaniline electron transfer composites, pathways and and homogeneous the author chemical proposed a mechanismreactions involving is present, [bmim] which may+ as protons not be at source equilibrium [51]. on the voltametric time scale. The searchA mixture for safer of the and IL [bmim][BF more sustainable4] with acetonitrile operation as conditions electrolyte washas usedbeen by also Lebedeva pursued et al.,in areas such asto investigate the mitigation the electrochemical of the greenhouse behavior gas of polyanilinecarbon dioxide composites, (a major and driver the author of climate proposed change) a mechanism involving [bmim]+ as protons source [51]. through its transformation into value-added products. In fact, CO2 conversion into valuable The search for safer and more sustainable operation conditions has been also pursued in areas chemicalssuch aswould the mitigation be the most of the promising greenhouse way gas carbonto reduce dioxide CO2 (a emissions, major driver making of climate it a change) part of through the solution as carbon feedstock. Moreover, such CO2 transformation would preferably occur in conditions that its transformation into value-added products. In fact, CO2 conversion into valuable chemicals would be allow theto avoid most promising the use of way volatile to reduce and CO toxic2 emissions, solvents making or catalysts it a part of(according the solution to as the carbon growing feedstock. demand for eco-friendlyMoreover, such synthetic CO2 transformation methodologies), would as preferably well as occur of any in conditions supporting that electrolyte allow to avoid (for the a use very of easy workupvolatile of the and reaction toxic solvents mixture). or catalysts (according to the growing demand for eco-friendly synthetic methodologies), as well as of any supporting electrolyte (for a very easy workup of the reaction mixture). Honores et al. [52] used [bmim][BF4] and [bmim][NTf2] to study the electrochemical reduction Honores et al. [52] used [bmim][BF ] and [bmim][NTf ] to study the electrochemical reduction of of carbon dioxide in the presence of cyclam4 Ni2+ and Co3+2 catalysts (Figure 10). carbon dioxide in the presence of cyclam Ni2+ and Co3+ catalysts (Figure 10).

Figure 10. Electrochemical behavior of the system in [bmim][BF4](A,B) and [bmim][NTf2] Figure 10. Electrochemical behavior of the system in [bmim][BF4] (A,B) and [bmim][NTf2] (C,D), (C,D), [Ni(cyclam)Cl2] and [Co(cyclam)Cl2]Cl at 9.7 mM and 2 mM, respectively, both saturated [Ni(cyclam)Cl2] and [Co(cyclam)Cl2]Cl at 9.7 mM and 2 mM, respectively, both saturated with N2 with N2 (black line) and CO2 (red line), at 100 mV/s. Reproduced with permission from [52], (blackcopyright line) and 2017, CO2 Elsevier. (red line), at 100 mV/s. Reproduced with permission from [52], copyright 2017, Elsevier. [Ni(cyclam)Cl2] exhibited a better performance than [Co(cyclam)Cl3]. Moreover, the solvent had a major impact on the catalytic performance of the systems: the hydrophilic ionic liquid [bmim][BF ] [Ni(cyclam)Cl2] exhibited a better performance than [Co(cyclam)Cl3]. Moreover, the solvent4 had promoted the catalytic activity, while [bmim][NTf2] led to very low values of TON. a major impact on the catalytic performance of the systems: the hydrophilic ionic liquid [bmim][BF4] A study using an ionic liquid bearing the triﬂate anion, [bmim][OTf], was also used to characterize promoted the catalytic activity,+ while [bmim][NTf2] led to very low values of TON. the cation bismuth/[bmim] , under conditions for CO2 reduction [53]. A study using an ionic liquid bearing the triflate anion, [bmim][OTf], was also used to Ratschmeier et al., [54] studied the CO2 reduction reactions (CO2RR) at Pt electrodes in four characterizeionic liquids. the cation These bismuth/[bmim] electrolytes were+, under 1-ethyl-3-methylimidazolium conditions for CO2 reduction dicyanamide, [53]. [emim][DCA], Ratschmeier[emim][BF4], et [bmim][BF al., [54] 4studied], and 1-ocytl-3-methylimidazolium the CO2 reduction reactions tetraﬂuoroborate, (CO2RR) at Pt [omim][BF electrodes4]. in It four was ionic liquids.found These that water electrolytes played an important were 1-ethyl-3-methylimidazolium role in CO2 reduction at a Pt electrode, dicyanamide, demonstrating [emim][DCA], that the formation of an imidazolium carboxylic acid intermediate occurs at electrode potentials of 0.4 V. [emim][BF4], [bmim][BF4], and 1-ocytl-3-methylimidazolium tetrafluoroborate, [omim][BF− 4]. It was found that water played an important role in CO2 reduction at a Pt electrode, demonstrating that the

Molecules 2020, 25, 5812 14 of 27 Molecules 2020, 25, x FOR PEER REVIEW 14 of 28

The authorsformation also of an reported imidazolium evidence carboxylic of the formationacid intermediate of CO. occurs As a consequence, at electrode potentials the presence of -0,4 of V. CO The led to deactivationauthors also of reported the Pt surface evidence and of tothe a formation decrease inof reductionCO. As a consequence, currents. the presence of CO led to deactivation of the Pt surface and to a decrease in reduction currents. On the other hand, [bmim][NTf2] was used for the study of adsorption and oxidation of CO. On the other hand, [bmim][NTf2] was used for the study of adsorption- and oxidation of CO. In In such work, beyond the CO oxidation, the oxidation of the [NTf2] anion and the release of a proton - weresuch observed work, [beyond55]. the CO oxidation, the oxidation of the [NTf2] anion and the release of a proton were observed [55]. The detection of gases has also been explored by cyclic voltammetry in ionic liquids. Yu’s group [56] The detection of gases has also been explored by cyclic voltammetry in ionic liquids. Yu’s group used [bmim][PF6] for this purpose. They modiﬁed this IL with a nickel oxide with reduced graphene [56] used [bmim][PF6] for this purpose. They modified this IL with a nickel oxide with reduced oxide to increase the sensitivity to detect O2. graphene oxide to increase the sensitivity to detect O2. Huang and collaborators [57] used the functionalized imidazolium IL 1-hydropropyl- Huang and collaborators [57] used the functionalized imidazolium IL 1-hydropropyl-3- 3-methylimidazolium tetraﬂuoroborate, [C OHmim][BF ], for the detection of H S. Due to the methylimidazolium tetrafluoroborate, [C3OHmim][BF3 4], 4for the detection of H2S. Due2 to the high highabsolute absolute values values of ofpotential potential needed needed to oxidize to oxidize or reduce or reduce H2S, monoethanolamine H2S, monoethanolamine (MEA) was (MEA) added was addedto facilitate to facilitate its detection its detection (Figure (Figure 11). 11).

A B

C D

FigureFigure 11. Cyclic 11. Cyclic voltammograms voltammograms of of H2 HS2 atS at the the Pt Pt microdisk microdisk electrode electrode (diameter(diameter = 100100 μm)µm) in in different diﬀerent electrolytes.electrolytes. (A )A is [bmim][BFis [bmim][BF4];4]; (B B) is is [C [C3OHmim][BF3OHmim][BF44]; (CC ) is is [C [C33OHmim][BFOHmim][BF4] 4 with] with MEA MEA and and atmosphericatmospheric gases gases and and (D) D is isa comparison a comparison of of cyclovoltamograms cyclovoltamograms between between [C [C33OHmim][BF4]4 ] and and [C3OHmim][BF4] with MEA. Scan rate: 50 mV s1−1; the circle indicates the open cell potential (OCP) of [C3OHmim][BF4] with MEA. Scan rate: 50 mV s− ; the circle indicates the open cell potential (OCP) of eacheach system, system, and and the the arrow arrow indicates indicates the the start start direction direction of of the the potentialpotential scan from from OCP. OCP. Reproduced Reproduced withwith permission permission from from [57 ],[57], copyright copyright 2018, 2018, Elsevier. Elsevier.

InIn both both [bmim][BF [bmim][BF4]4] (Figure (Figure 11A)11A) and and [C [C3OHmim][BF3OHmim][BF4] (Figure4] (Figure 11B) 11 media,B) media, the 2 the oxidationoxidation/reduction/reduction of of H HS 2occurredS occurred at a atvery a veryhigh/low high potential,/low potential, close to close the upper/lower to the upper limit/lower of limitthe of electrochemicalthe electrochemical window window of the of RTILs. the RTILs. This behaviour This behaviour led to led the to conclusion the conclusion that they that were they unsuitable for H2S sensing. Moreover, the addition of MEA to [C3OHmim][BF4] electrolyte (Figure were unsuitable for H2S sensing. Moreover, the addition of MEA to [C3OHmim][BF4] electrolyte 11D increases the solubility of H2S through chemical absorption, releasing electroactive HS-ions, (Figure 11D increases the solubility of H2S through chemical absorption, releasing electroactive which subsequently led to an additional anodic response at a potential appropriate for the detection HS-ions, which subsequently led to an additional anodic response at a potential appropriate for the of H2S. As a final remark, in this study it was possible to conclude that the detection is not affected detection of H2S. As a final remark, in this study it was possible to conclude that the detection is not by ambient gases such as CO2 or SO2. affected by ambient gases such as CO or SO . Recently, other functionalized 2imidazolium2 ILs were applied in electrochemistry. For example, 1-(2′,3′-dihydroxypropyl)-3-methylimidazoliumRecently, other functionalized imidazolium ILs hydroxide, were applied [dhpmim][OH], in electrochemistry. provided For a example, good 1-(20capacitance,30-dihydroxypropyl)-3-methylimidazolium to nickel oxide nanosheets as electrodes hydroxide, [58]. Further, [dhpmim][OH], the electrochemical provided stability a good capacitance to nickel oxide nanosheets as electrodes [58]. Further, the electrochemical stability windows of a series of sulfonic-functionalized ILs with trifluoroacetate anion in molecular solvents were investigated, presenting wide potential windows in acetonitrile [59]. Molecules 2020, 25, x FOR PEER REVIEW 15 of 28 windows of a series of sulfonic-functionalized ILs with trifluoroacetate anion in molecular solvents wereMolecules investigated,2020, 25, 5812 presenting wide potential windows in acetonitrile [59]. 15 of 27 In a completely different research area, the electrochemical behaviour of the nucleobases thymine and thymidine was investigated in [bmim][BF4], observing one- and two-electron reduction In a completely different research area, the electrochemical behaviour of the nucleobases thymine peaks, respectively [60], as it is shown in Figure 12. The chronoamperometric fit and the cyclic and thymidine was investigated in [bmim][BF ], observing one- and two-electron reduction peaks, voltammetry at different scan rates confirmed4 an irreversible one-electron transfer in both thymine respectively [60], as it is shown in Figure 12. The chronoamperometric fit and the cyclic voltammetry and thymidine. at different scan rates confirmed an irreversible one-electron transfer in both thymine and thymidine.

Figure 12. (A) Cyclic voltammogram of 5 M thymine at 400 mV/s in [bmim][BF4] depicting; (a) original Figure 12. (A) Cyclic voltammogram of 5 M thymine at 400 mV/s in [bmim][BF4] depicting; (a) original cyclic voltammogram, (b) convoluted cyclic voltammogram. (B) Cyclic voltammogram recorded cyclic voltammogram,1 (b) convoluted cyclic voltammogram. (B) Cyclic voltammogram recorded at at 400 mV s− for 5 mM thymidine in [bmim][BF4], (a) original (b) convoluted. Reproduced with 400 mV s−1 for 5 mM thymidine in [bmim][BF4], (a) original (b) convoluted. Reproduced with permission from [60], copyright 2019, Elsevier. permission from [60], copyright 2019, Elsevier. 1 The corrected cyclic voltammogram of thymine in [bmim][BF4] at 400 mV s− (Figure 12A), exhibitsThe the corrected reduction cyclic peak voltammogram at 0.67 V vs. Fc of/ Fc thymine+ redox in couple. [bmim][BF The authors4] at 400 were mV able s−1 to (Figure elucidate 12A), − exhibitsthe electrochemical the reduction reductive peak at −0.67 mechanism V vs. Fc/Fc involved+ redox in couple. the reduction The authors of thymine were able and to elucidate thymidine the electrochemicalin [bmim][BF4] reductive by convolutive mechanism semi-integral involved analysis in the of reduction the cyclic of voltametric thymine and data thymidine produced. in [bmim][BFThe electrochemical4] by convolutive reduction semi-integral of thymine followed analysisa of one-electron the cyclic transfer voltametric at the data electrode produced. surface The electrochemicalresulting in the formationreduction of of electron thymine adducts followed of thymine a one-electron at the cathode transfer peak. at the This electrode mechanism surface is resultingsupported in by the theoretical formation simulations of electron of theadducts variation of thymine of bond dissociationat the cathode free energypeak. This via the mechanism concerted is supportedsticky dissociative by theoretical model [60 simulations]. of the variation of bond dissociation free energy via the concertedFrom sticky the examples dissociative presented model above, [60]. it is clear that imidazolium based ILs are very promising electrochemicalFrom the examples electrolyte presented media, mainlyabove, it due is clear to their that electrochemical imidazolium based stability ILs and are versatilityvery promising for electrochemicalnumerous applications. electrolyte media, mainly due to their electrochemical stability and versatility for numerous applications. 3. Ammonium-, Pyrrolidinium-, Phosphonium- and Sulfonium-Based ILs 3. Ammonium-,In this section Pyrrolidinium-, we will present progresses Phosphonium- in electrochemistry and Sulfonium-Based using ILs other than ILs imidazolium-based ones. Out of the several ILs, the quaternary ammonium salts are an economically advantageous class In this section we will present progresses in electrochemistry using ILs other than imidazolium- of ionic liquids used in different applications as they exhibit better thermal and chemical stability based ones. compared to pyridinium- and imidazolium-based compounds. Out of the several ILs, the quaternary ammonium salts are an economically advantageous class Sultana et al. [61] used N-trimethyl-N-hexylammonium bis(trifluoromethylsulfonyl)amide, of ionic liquids used in different applications as they exhibit better thermal and chemical stability [N ][NTf ], to study the electrochemical behaviour of an acetylacetonate (acac) platinum(II) complex, compared1116 to2 pyridinium- and imidazolium-based compounds. [Pt(acac)2], as function of temperature (Figure 13). A cathodic peak corresponding to the reduction of Sultana et al. [61] used N-trimethyl-N-hexylammonium bis(trifluoromethylsulfonyl)amide, Pt(II) to Pt(0) and consequent electrodeposition of this metal on the electrode was found to depend on [N1116][NTf2], to study the electrochemical behaviour of an acetylacetonate (acac) platinum(II) the diffusion of Pt(acac)2 in the [N1116][NTf2] IL. complex, [Pt(acac)2], as function of temperature (Figure 13). A cathodic peak corresponding to the The ammonium-based ionic liquid tri-n-butylmethylammonium chloride, [N1444]Cl, was selected reduction of Pt(II) to Pt(0) and consequent electrodeposition of this metal on the electrode2+ was found by Bhujbal et al. [62] to study the electrochemical behaviour of uranyl ion, UO2 (Figure 14). toAn depend irreversible on the reduction diffusion of of U(VI) Pt(acac) to U(IV)2 in the was [N observed,1116][NTf resulting2] IL. in the electrodeposition of uranium oxide (UO2) over the glassy carbon working electrode.

Molecules 2020, 25, x FOR PEER REVIEW 16 of 28

Molecules 2020, 25, 5812 16 of 27 Molecules 2020, 25, x FOR PEER REVIEW 16 of 28

Figure 13. Cyclic voltammograms of a glassy carbon electrode in [N1116][NTf2] containing 5 mM [Pt(acac)2] at various temperatures. Scan rate: 50 mV/s. Reproduced with permission from [61], copyright 2016, RSC.

The ammonium-based ionic liquid tri-n-butylmethylammonium chloride, [N 1444]Cl, was selected 2+ by BhujbalFigureFigure et 13. 13.al. Cyclic Cyclic[62] to voltammograms voltammograms study the electrochemical of a glassy glassy carbon carbon behaviour electrode electrode of in uranyl in [N [N11161116][NTf ion,][NTf 2UO]2] containing containing2 (Figure 5 5 mM 14). mM An irreversible[Pt(acac)[Pt(acac) reduction2]2 ] at at various various of U(VI) temperatures. temperatures. to U(IV) was Scan Scan observed, rate: rate: 50 50 mVmV/s. resulting/s. Reproduced in the electrodeposition with with permission permission from from of uranium [ 61 [61],], oxide (UOcopyrightcopyright2) over 2016, 2016, the RSC.glassy RSC. carbon working electrode.

The ammonium-based ionic liquid tri-n-butylmethylammonium chloride, [N1444]Cl, was selected by Bhujbal et al. [62] to study the electrochemical behaviour of uranyl ion, UO22+ (Figure 14). An irreversible reduction of U(VI) to U(IV) was observed, resulting in the electrodeposition of uranium oxide (UO2) over the glassy carbon working electrode.

Figure 14. Cyclic voltammogram of [N ]Cl and UO 2+ in [N ]Cl recorded at glassy carbon 1444 2+2 1444 Figure 14. Cyclic2 + voltammogram of [N1444]Cl and UO2 in [N1444]Cl1 recorded at glassy carbon electrode. [UO2 ] = 0.07 M, temperature = 383 K, scan rate = 100 mV s− . Reproduced with permission electrode. [UO22+] = 0.07 M, temperature = 383 K, scan rate = 100 mVs−1. Reproduced with permission from [62], copyright 2020, Elsevier. from [62], copyright 2020, Elsevier.

As depicted in Figure 14, [N1444]Cl exhibits the electrochemical window of 2.8 V with the cation 1444 reductionAs depicted potential in Figure of 1.914, V[N and]Cl the exhibits anion oxidation the electrochemical potential of window+0.9 V. Theof 2.8 use V with of [N the1444 ]Clcation as −2+ 2+ reductionelectrolyteFigure potential 14. for Cyclic the of UO −1.9 voltammogram2 / VUO and2 reduction the of anion [N couple1444 oxidation]Cl and allowed UO potential2 a in fast [N (401444 of ]Cl +0.9 min) recorded V. deposit The at use of glassy a of considerable [N carbon1444]Cl as 2+ electrolyteamountelectrode. of for uranium the[UO UO22+] in=2 0.07 the/UO M, form2 reductiontemperature of uranium couple = 383 dioxide. K, allowed scan rate a = fast100 mVs(40 −1min). Reproduced deposit withof a permissionconsiderable amountfromArkhipova of uranium[62], copyright and in coworkersthe 2020, form Elsevier. of [63 uranium] employed dioxide. tetraethylammonium bis (triﬂuoromethylsulfonyl)imide, [NArkhipova2222][NTf2], to study and the redox coworkers properties of[63] mesoporous employed graphene nanoﬂakes, tetraethylammonium showing a capacitance bis (trifluoromethylsulfonyl)imide,aboveAs 100depicted F/g, while in Figure M. Pajaket 14, [N al.14442222 [64]Cl][NTf], have exhibits2], demonstratedto study the electrochemical the redox that some properties protic window ammonium of mesoporousof 2.8 V basedwith graphene the ILs withcation reduction potential of −1.9 V and the anion oxidation potential of +0.9 V. The use of [N1444]Cl as nitrate anion (ethylammonium nitrate and propylammonium nitrate) can act as proton donors in the electrolyte for the UO22+/UO2 reduction couple allowed a fast (40 min) deposit of a considerable amount of uranium in the form of uranium dioxide. Arkhipova and coworkers [63] employed tetraethylammonium bis (trifluoromethylsulfonyl)imide, [N2222][NTf2], to study the redox properties of mesoporous graphene

Molecules 2020, 25, x FOR PEER REVIEW 17 of 28 nanoflakes, showing a capacitance above 100 F/g, while M. Pajaket al. [64], have demonstrated that some protic ammonium based ILs with nitrate anion (ethylammonium nitrate and propylammonium nitrate)Molecules can act2020 as, 25 proton, 5812 donors in the electrosorption of H2 in palladium. This result is quite17 of relevant 27 because most of the “first generation” ionic liquids were discarded over the years. Pyrrolidinium-based ionic liquids are considered among the most promising electrolytes for the electrosorption of H2 in palladium. This result is quite relevant because most of the “first generation” developmentionic liquids of novel were discardedand sustainable over the portable years. energy devices. Therefore, very recently, a significant increase onPyrrolidinium-based the reported electrochemical ionic liquids are studies considered in such among ILs was the most observed. promising electrolytes for the Indevelopment 2016, Bouvet of novel and andKrautscheid sustainable [65] portable verified energy that devices. the redox Therefore, potentials very recently,of a Fe(II)/Fe(III) a significant system usingincrease pyrrolidinium-based on the reported electrochemicalILs with L-proline studies and in such ferrocene ILs was as observed. building blocks are independent on the IL anionIn 2016,and Bouveton the andalkyl Krautscheid chain length. [65] verified that the redox potentials of a Fe(II)/Fe(III) system using pyrrolidinium-based ILs with L-proline and ferrocene as building blocks are independent on the After, a series of pyrrolidinium-based ILs (with different cation chain lengths) bearing the [NTf2]- IL anion and on the alkyl chain length. anion has been used for the electrochemical synthesis of 2–3 nm Pt nanoparticles [66]. The After, a series of pyrrolidinium-based ILs (with different cation chain lengths) bearing the [NTf2]− electrochemicalanion has been reduction used for the electrochemical of [Pt(acac) synthesis2] in ofthe 2–3 nmaprotic Pt nanoparticles pyrrolidinium-based [66]. The electrochemical ILs, 1-R-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (R = butyl, hexyl or decyl) was characterized reduction of [Pt(acac)2] in the aprotic pyrrolidinium-based ILs, 1-R-1-methylpyrrolidinium by cyclicbis(trifluoromethylsulfonyl)amide voltammetry and suggested (R =thatbutyl, the hexyl reduction or decyl) of was[Pt(acac) characterized2] to metallic by cyclic platinum voltammetry occurred throughand a suggested two-electron that the transfer reduction process of [Pt(acac) without2] to the metallic formation platinum of any occurred intermediate through a species. two-electron M.transfer Manjun process et al. without [67], theexplored formation the of electrochemical any intermediate reaction species. of samarium species in 1-butyl-1- M. Manjun et al. [67], explored the electrochemical reaction of samarium species methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, [bmpy][NTf2], at different temperatures. The Smin 1-butyl-1-methylpyrrolidinium system was electrochemically bis(trifluoromethylsulfonyl)amide, active, exhibiting a cathodic [bmpy][NTf peak corresponding2], at different to the temperatures. The Sm system was electrochemically active, exhibiting a cathodic peak corresponding Sm(III)/Sm(II) reduction and an anodic one corresponding to the Sm(II) oxidation formed during the to the Sm(III)/Sm(II) reduction and an anodic one corresponding to the Sm(II) oxidation formed during prior thecathodic prior cathodic scan (Figure scan (Figure 15). 15).

Figure 15. Figure 15. CyclicCyclic voltammetry voltammetry of of [Sm(NTf [Sm(NTf22))33]] (10 (10 mM)mM) in in [bmpy][NTf [bmpy][NTf2] at2] aat glassy-carbon a glassy-carbon electrode electrode at variousat various scan rates, scan rates,25 °C 25(left◦C() andleft )100 and °C 100 (right◦C().right Reproduced). Reproduced with permission with permission from from[67], copyright [67], copyright 2019, ECS. 2019, ECS.

The cathodic peak current density at 100 ◦C was higher than that at 25 ◦C, reflecting a decrease in Thethe IL cathodic viscosity peak at 100 current◦C. Moreover, density the disappearanceat 100 °C was of higher the anodic than current that peakat 25 at °C, 100 reflecting◦C was observed a decrease in theand IL consideredviscosity at to 100 be caused °C. Moreover, by a chemical the reactiondisappearance of Sm(II) of in the ionicanodic liquid. current The proportionation peak at 100 °C was observedand disproportionation and considered toequilibrium be caused among by a Sm, chemical Sm(II), and reaction Sm(III) of also Sm(II) led to in the the formation ionic liquid.of Sm The proportionationnanoparticles and in the disproportionation ionic liquid at 100 ◦ C.equilibrium among Sm, Sm(II), and Sm(III) also led to the formationAn of ILSm with nanoparticles the same cation in the but ionic with [DCA]liquid− at(DCA 100 =°C.dicyanamide) anion was used in a hybrid electrolyte containing a redox additive, improving the specific capacitance of N-doped reduced An IL with the same cation but with [DCA]− (DCA = dicyanamide) anion was used in a hybrid graphene aerogel capacitors [68]. The electrochemical reduction mechanism of NbF5 and NbCl5 in the electrolyteionic liquid containing 1-butyl-1-methylpyrrolidinium a redox additive, improving trifluoromethanesulfonate, the specific capacitance [bmpy][OTf], ofwas N-doped also reported reduced graphenevery recentlyaerogel [capacitors69]. [68]. The electrochemical reduction mechanism of NbF5 and NbCl5 in the ionic liquidLahiri 1-butyl-1-methylpyrrolidinium et al. [70] synthesised germanium-tin trifluoromethanesulfonate, alloys with two different [bmpy][OTf], ionic liquids, was also reported1-butyl-1-methylpyrrolidinium very recently [69]. trifluoromethylsulfonate ([Py1,4]TfO) and 1-butyl-1- methylpyrrolidinium Lahiribis(trifluoromethylsulfonyl)amide et al. [70] synthesised germanium-tin ([Py1,4]Tf2N). The alloys authors with performed two different voltammetric ionic studiesliquids, of both1-butyl-1- methylpyrrolidinium trifluoromethylsulfonate ([Py1,4]TfO) and 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]Tf2N). The authors performed voltammetric studies of both ionic liquids and conclude that the best ionic liquid to be used as an electrolyte, to prepare Ge1-

Molecules 2020, 25, 5812 18 of 27 Molecules 2020, 25, x FOR PEER REVIEW 18 of 28 ionicxSnx liquids, is [Py1,4 and]TfO. conclude Cyclic voltametric that the best experiments ionic liquid run to with be used the precursors as an electrolyte, (SnCl2 and to prepareGeCl4) allowed Ge1-xS nx, is [Pyfor1,4 the]TfO. detection Cyclic of voltametric a peak associated experiments to the formation run with of the the precursors germanium-tin (SnCl alloys.2 and GeCl4) allowed for the detectionGligor of a et peak al. associated [71] prepared to the formation and characterized of the germanium-tin new films alloys. based on poly-3,4- ethylenedioxythiopheneGligor et al. [71] prepared (PEDOT) and characterized for nitrite detection new films on two based substrate on poly-3,4-ethylenedioxythiophene materials (glassy carbon and (PEDOT)gold) for and nitrite four detection electrolytes on two substrate (water, materials acetonitrile, (glassy 1-butyl-1-methylpyrrolidinium carbon and gold) and four electrolytesbis(trifluoromethylsulfonyl)imide (water, acetonitrile, 1-butyl-1-methylpyrrolidinium and bis(trifluoromethylsulfonyl)imide 1-ethyl-3-methylimidazolium andbis(trifluoromethylsulfonyl)amide). 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide). The authors observed that the electrodes The authors prepared observed in that ionic the electrodesliquids led prepared to higher in ionicelectrocatalytic liquids led activity to higher and electrocatalyticbetter sensitivities. activity and better sensitivities. PhosphoniumPhosphonium cation-based cation-based ILs ILs are are a a readily readily available available family family of of ionic ionic liquids liquids that, that, for for some some applications,applications, off offerer superior superior properties properties as as compared compared toto nitrogennitrogen cation-based cation-based ILs. ILs. Recent Recent applications applications cover their use as extraction and synthetic solvents, as well as in electrochemical processes, such as cover their use as extraction and synthetic solvents, as well as in electrochemical processes, such as corrosion protection or electrolytes in batteries and super capacitors. Relevant recent examples corrosion protection or electrolytes in batteries and super capacitors. Relevant recent examples follow. follow. Girard et al. [72], studied the electrochemical performance of trimethyl(isobutyl)phosphonium Girard et al. [72], studied the electrochemical performance of trimethyl(isobutyl)phosphonium bisfluorosulfonyl)imide, [P ][NTf ], by varying the concentration of Li[NTf ]. The results are bisfluorosulfonyl)imide, [P111i4111i4][NTf22], by varying the concentration of Li[NTf2].2 The results are presentedpresented in Figurein Figure 16 .16. The The voltammograms voltammograms indicateindicate a quasi-reversible Li Li deposition–dissolution deposition–dissolution process.process. A smallA small shift shift of of the the onset onset potential potential towardstowards fewer negative values values was was observed observed with with the the increaseincrease in lithium-ionin lithium-ion concentration concentration in in accord accord with with the the Nernst Nernst equation. The The oxidation oxidation current current densitiesdensities for for the the reversible reversible peak peak decreased decreased significantly significantly with increasing increasing lithium lithium salt salt concentration concentration as as expectedexpected from from the the transport transport property property trends.trends. The The authors authors concluded concluded that that the thesolutions solutions of Li[NTf of Li[NTf2] in 2] in thethe [P [P111i4111i4][NTf][NTf2]2 ] IL IL exhibited exhibited reasonable reasonable transport transport properties, properties, and and therefore therefore could could be be promising promising electrolyteselectrolytes for for lithium lithium batteries. batteries.

FigureFigure 16. 16.Linear Linear sweep sweep voltammograms voltammograms (1st (1st cycle) cycle) for for neat neat [P[P111i4111i4][][ NTf NTf2]2 and] and solutions solutions of of0.5, 0.5, 3.2 3.2 and and 1 1 3.8 mol3.8 mol kg− kgLi[NTf−1 Li[NTf2]2 in] in [P [P111i4111i4][][ NTfNTf22] at a Ni working electrode electrode with with a a scan scan rate rate of of 20 20 mVs mV−1 s −at 25at 25°C.◦ C. ReproducedReproduced with with permission permission from from [72 [72],], copyright copyright 2015,2015, RSC.RSC.

KhrizanforovKhrizanforov et et al. al. [73 [73]] applied applied the the phosphonium-RTIL phosphonium-RTIL dodecyl(tri-tert-butyl)phosphonium tetrafluoroborate,tetrafluoroborate, [(t-Bu) [(t-Bu)3PH][BF3PH][BF44],], in new new carbon carbon paste paste electrode electrode to study to study the redox the redox properties properties of an of 2 II 5 5 III an ironiron complex complex({ µ ({μ-[Fe2-[Fe(IIη(η5-C-C5H4–P(PhOO)(η–P(PhOO)(5η-C-C5H4H–P(PhOOH))]–P(PhOOH))]3FeIII}·THF).Fe } THF). The The authors authors could could 5 4 5 4 3 · concludeconclude that that the the prepared prepared electrode shows shows high high conductivity, conductivity, a large a large electrochemical electrochemical window window (5.6 (5.6V), V), stability stability in intime, time, and and reproducibility. reproducibility. The cyclic The cyclic voltammetric voltammetric analysis analysis of the iron of thecomplex iron complexusing usingthe thenew new carbon carbon paste pasteelectrode electrode ionic liquid ionic confirmed liquid confirmed two different two iron diff erentredox ironcenters redox in oxidation centers in oxidationstates states(II) and (II) (III).and (III). Also, Also, the the electrodeposition electrodeposition of of Nd(III) Nd(III) in in triethylpentylphosphonium triethylpentylphosphonium bis(trifluoromethanesulfonyl)imide, [P2225][NTf2], at high temperature performed by Ota et al. [74] bis(trifluoromethanesulfonyl)imide, [P2225][NTf2], at high temperature performed by Ota et al. [74] was confirmed by the cathodic peak detected by cyclic voltammetry. was confirmed by the cathodic peak detected by cyclic voltammetry. Sulfonium-based ILs, namely ferrocenyl-sulfonium ILs were applied by Venkeret et al. [75]. to Sulfonium-based ILs, namely ferrocenyl-sulfonium ILs were applied by Venkeret et al. [75]. investigate the effect of the sulphur atom substituent in redox behaviour of the ionic liquid. They to investigate the effect of the sulphur atom substituent in redox behaviour of the ionic liquid.

Molecules 2020, 25, 5812 19 of 27 MoleculesMolecules 2020 2020, ,25 25, ,x x FOR FOR PEER PEER REVIEW REVIEW 19 19 of of 28 28

Theyreportedreported reported that that that electron electron electron withdrawing withdrawing withdrawing substituents substituents substituents cause cause cause a a positive positive a positive shift shift shift of of the ofthe the Fe(II)/Fe(III) Fe(II)/Fe(III) Fe(II)/Fe(III) ferrocene ferrocene ferrocene redoxredoxredox potential potential potential (Figure (Figure (Figure 17 17). ).17). The The The study study study concludedconcluded concluded that thatthat this thisthis class class of of room room room temperature temperature temperature ionic ionic ionic liquids liquids liquids mightmightmight be be usefulbe useful useful as as redoxas redox redox mediators mediators mediators forfor for dye-sensitizeddye-sensitized dye-sensitized solar solarsolar cells cells (DSSCs), (DSSCs), as as as redox redox redox electrolytes electrolytes electrolytes in in in supercapacitors,supercapacitors,supercapacitors, or or asor as overchargeas overcharge overcharge protection protection protection additivesadditives additives in in batteries. batteries.

Figure 17. Cyclic voltammograms of selected new ferrocenyl sulfonium compounds plotted against FigureFigure 17. 17. Cyclic Cyclic voltammograms voltammograms of of selected selected new new ferrocenyl ferrocenyl sulfonium sulfonium compounds compounds plotted plotted against against Fc/Fc+. The measurements were performed in [emim][NTf2] with an Ag/Ag NTf2 reference electrode. Fc/Fc+.Fc/Fc+. The The measurements measurements were were performed performed in in [emim][NTf [emim][NTf2]2 ]with with an an Ag/Ag Ag/Ag NTf NTf2 2reference reference electrode. electrode. Reproduced with permission from [75], copyright 2018, RSC. ReproducedReproduced with with permission permission from from [75], [75], copyright copyright 2018, 2018, RSC. RSC.

The redox properties of diethylmethylsulfonium bis(triﬂuoromethylsulfonyl)imide, [S222][NTf2], TheThe redox redox properties properties of of diethylmethylsulfonium diethylmethylsulfonium bis(trifluoromethylsulfonyl)imide, bis(trifluoromethylsulfonyl)imide, [S [S222222][NTf][NTf2],2], together with diﬀerent lithium salts were electrochemically characterized by Rangasamy [76]. togethertogether with with different different lithium lithium salts salts were were electrochemically electrochemically characterized characterized by by Rangasamy Rangasamy [76]. [76]. The The The system [S222][NTf2]-LiNTf2 appears stable in a wide potential range and exhibits a reversible systemsystem [S [S222222][NTf][NTf2]-LiNTf2]-LiNTf2 2appears appears stable stable in in a a wide wide potential potential range range and and exhibits exhibits a a reversible reversible redox redox redoxbehaviourbehaviour behaviour (Figure (Figure (Figure 18), 18), which 18which), which are are excellent excellent are excellent features features features to to be be applied applied to be appliedas as an an electrolyte electrolyte as an electrolyte for for practical practical for battery battery practical batteryapplications.applications. applications.

FigureFigureFigure 18. 18. Cyclic18. Cyclic Cyclic voltammetric voltammetric voltammetric measurements measurements measurements ofof of thethe the [S[S [S222222222][NTf][NTf22]-LiNTf2]-LiNTf2 22IL ILIL electrolyte. electrolyte. electrolyte. Reproduced Reproduced Reproduced withwithwith permission permission permission from from from [76 [76], [76],], copyright copyright copyright 2019, 2019, 2019, Elsevier. Elsevier. Elsevier.

FigureFigureFigure 18 shows 18 18 shows shows the cyclic the the cyclic voltammetrycyclic voltammetry voltammetry of the [S of of222 the the][NTf [S [S2222222]-LiNTf][NTf][NTf2]-LiNTf2]-LiNTf2 ionic2 liquid2 ionic ionic electrolyte liquid liquid electrolyte electrolyte measured at 60measuredmeasured◦C. As reportedat at 60 60 °C. °C. As byAs reported thereported authors, by by the the both authors, authors, forward both both and forward forward reverse and and scan reverse reverse redox scan scan peaks redox redox have peaks peaks been have have observed been been in bothobservedobserved cycles. in in both Oneboth cycles. iscycles. the lithiumOne One is is the the deposition lithium lithium deposition deposition process, and process, process, the other and and the the is relatedother other is is torelated related the lithium to to the the lithium strippinglithium process.strippingstripping The process. currentprocess. The densityThe current current of the density density both ofthe of the the peaks both both are the thein peaks peaks the sameare are in in range the the same same indicating range range indicating indicating that the ionic that that theliquid the electrolyteionicionic liquid liquid exhibits electrolyte electrolyte a good exhibits reversibleexhibits a a good redoxgood reversible reversible behaviour. redox redox This behaviour. behaviour.more evidence This This of more more the evidence potential evidence of role of the the that thispotentialpotential ionic liquid role role electrolytethat that this this ionic ionic could liquid liquid have electrolyte electrolyte as a potential could could candidatehave have as as a a forpotential potential a “workable” candidate candidate electrolyte for for a a “workable” “workable” to be used in a lithium cell. A wide electrochemical stability window is one of the most desired properties of an

Molecules 2020, 25, x FOR PEER REVIEW 20 of 28 electrolyte to be used in a lithium cell. A wide electrochemical stability window is one of the most desired properties of an “ideal” electrolyte because the chemistry of the two electrode-electrolyte interfaces involved in the battery depends on the properties of the electrolyte. Usually, it is the oxidative stability of the anion that determines the anodic limit of the ILs. Parameters such as being a weak Lewis acidMolecules result2020, 25, 5812in weak interactions with the weakly Lewis-acidic organic20 of 27 cations in the liquids, making them have relatively good anodic stability. Because such cations and anions have difficulty to discharge“ideal” electrolyte on the because electrodes, the chemistry mainly of the twoat lower electrode-electrolyte potentials, interfaces the result involved is in a the large window of the electrochemicalbattery stability. depends on the properties of the electrolyte. Usually, it is the oxidative stability of the anion that determines the anodic limit of the ILs. Parameters such as being a weak Lewis acid result in weak Klein et al. interactions[77] selected with the different weakly Lewis-acidic families organic of ILs cations (ammonium, in the liquids, making imidazolium them have relatively and pyrrolidinium) to measure theirgood differential anodic stability. Because capacitances. such cations and Thus, anions butyl-trimethylammonium have diﬃculty to discharge on the electrodes, bis(trifluoromethyl mainly at lower potentials, the result is a large window of the electrochemical stability. sulfonyl)imide, Klein [N1114 et][NTf al. [77]2 selected], differentethyl-methylimidazolium families of ILs (ammonium, imidazolium bis(trifluoromethylsulfonyl)imide, and pyrrolidinium) [emim][NTf2], andto measure methyl-propylpyrrolidiniumtheir differential capacitances. Thus, bis(trifluoromethylsulfonyl)imide, butyl-trimethylammonium bis(trifluoromethyl [Pyr13][NTf2], sulfonyl)imide, [N ][NTf ], ethyl-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][NTf ], were analysed by electrochemical1114 2 impedance spectroscopy (EIS) over the entire2 electrochemical and methyl-propylpyrrolidinium bis(triﬂuoromethylsulfonyl)imide, [Pyr13][NTf2], were analysed by window determinedelectrochemical by cyclic impedance voltammetry spectroscopy (EIS)(with over Fc|Fc the entire+ internal electrochemical reference, window determined as shown by in Figure 19). cyclic voltammetry (with Fc|Fc+ internal reference, as shown in Figure 19).

Figure 19. Cyclic voltammograms of [emim][NTf2], [N1114][NTf2], and [Pyr13][NTf2] with 10 mM Figure 19. Cyclicferrocene voltammograms (Fc). Scan rate = 20 mV of/ s. [emim][NTf Reproduced with2], permission [N1114][NTf from [772], copyright and [Pyr 2018,13 RSC.][NTf2] with 10 mM ferrocene (Fc). Scan rate = 20 mV/s. Reproduced with permission from [77], copyright 2018, RSC. In this work, the quaternary ammonium-based IL displayed a different electrochemical potential window and the difference to the previous publications was attributed by the authors [77] to a In this work,diff theerence quaternary in water content, ammonium-based trace impurities, or a difference IL displayed in current cut oaff .different One of the most electrochemical important potential conclusions from this work is related to the factors for the increase in capacitance. It was expected window and the difference to the previous publications was attributed by the authors [77] to a that [N1114] would display crowding behaviour at the interface while [Pyr13] and [emim] formed an difference in waterinterface content, structure with trace an over-screened impurities, surface. or These a difference findings support in the current fact that the cut bulk off.measure One of the most of molecular interactions, such as viscosity, do not influence the behaviour of the IL at the nanoscale important conclusionslengths of thefrom electrode–electrolyte this work is interface. related to the factors for the increase in capacitance. It was - expected that [N1114V]é lezwould applied dicationicdisplay pyrrolidinium crowding and behaviour piperidinium ILsat withthe [NTf interface2] anion as while electrolytes [Pyr in 13] and [emim] formed an interfacelithium structure batteries [78]. with Later, a an list ofover-screened imidazolium-, pyridinium-, surface. pyrrolidinium-, These findings and piperidinium-based support the fact that the ILs was used together with a carbon paste electrode to study the electrochemical response of bulk measure ofdopamine molecular [79], andinteractions, the results showed such that as theviscosity, sizes and do types not of bothinfluence cation and the anion behaviour have of the IL at the nanoscale lengthsan influence of onthe that electrode–electrolyte response. interface. Overall, all the reported IL classes can be used in a large number of electrochemical applications - Vélez appliedwith promisingdicationic results pyrrolidinium and their usage as electrolytesand piperidinium or electrolyte additives ILs with is expanding [NTf2] thisanion area. as electrolytes in lithium batteriesIn the [78]. future, Later, other ILs a familieslist of shouldimidazolium-, be tested, namely pyridinium-, guanidinium-based pyrrolidinium-, ILs. and piperidinium- based ILs was used together with a carbon paste electrode to study the electrochemical response of dopamine [79], and the results showed that the sizes and types of both cation and anion have an influence on that response. Overall, all the reported IL classes can be used in a large number of electrochemical applications with promising results and their usage as electrolytes or electrolyte additives is expanding this area. In the future, other ILs families should be tested, namely guanidinium-based ILs.

4. General Remarks Considering ILs Purification and Viscosity From the above-mentioned published contributions, it is clear that impurities, such as water content or contamination with halides, can lead to significant changes in the expected electrochemical results. Therefore, purifying the ILs is a basic requirement to obtain reliable cyclic voltammograms. Several technologies, i.e., adsorption, aqueous two-phase extraction, crystallization, distillation,

Molecules 2020, 25, 5812 21 of 27

4. General Remarks Considering ILs Purification and Viscosity From the above-mentioned published contributions, it is clear that impurities, such as water content or contamination with halides, can lead to significant changes in the expected electrochemical results. Therefore, purifying the ILs is a basic requirement to obtain reliable cyclic voltammograms. Several technologies, i.e., adsorption, aqueous two-phase extraction, crystallization, distillation, extraction, membrane separation, and external force field separation, have been investigated for the recovery and purification of ILs either in the lab or at pilot scale. A small survey of the techniques in use nowadays (Table4) ilustrates the plethora of possibilities regarding the recovery and purification of ILs.

Methods Characteristics Distillation of volatile compounds ILs are remained as residue. Distillation Distillation through reactions of ILs ILs are distilled as neutral species or intact ion pairs. Extraction with water Extract hydrophilic solutes from hydrophobic ILs. Extraction Extraction with organic solvents Extract hydrophobic solutes from ILs.

Extraction with scCO2 Both hydrophobic and hydrophilic ILs can be separated. Adsorption by ACs Affected by pore structure and surface chemistry of ACs. Adsorption Adsorption by soils and sediments Affected by TOC and CEC of soils. AAdsorption by ion exchange resins Affected by functional group and ionic form of resins. Pressure-driven membrane techniques ILs can be either permeated or rejected. Membrane separation Pervaporation ILs are rejected while volatile species are permeated. Membrane distillation ILs are rejected while water vapor is permeated. Electrodialysis Cations and anions of ILs cross ion exchange membranes.

Aqueous two-phase ATPE based on chemicals addition Formation of ATPS by adding salts, carbohydrates or CO2. extraction (ATPE) ATPE based on changing temperature Formation of ATPS or LCST-type phase separation. Solution crystallization ILs are crystallized from solution. Crystallization Melt crystallization ILs are crystallized from melt. Pressure-induced crystallization ILs are crystallized under high pressure. Gravity separation ILs are separated from immiscible liquids. Force ﬁeld Centrifugation ILs emulsion were separated by centrifugation. Magnetic separation ILs are separated by magnetic ﬁeld.

Several solvents, such as water, organic solvents, and supercritical carbon dioxide (scCO2), have been tested for ILs extraction processes. Aqueous two-phase extraction (ATPE) is used when two immiscible phases (both soluble in water), e.g., polymer/polymer, polymer/salt, or salt/salt, are placed into contact with each other above the critical concentration at a specific temperature. This type of extraction is recognized since it is a rapid, low-cost, and scalable technology for separation and purification of antibiotics, enzymes, therapeutic proteins, etc. In this method, the ILs can be concentrated and recovered in the IL-rich phase. The employment of gravity field, as well as centrifugation and/or magnetic fields have been proposed for recovery of hydrophobic ILs or solutions of magnetic ILs [80]. Recently, the use of membrane processes for the recovery and purification of ILs, taking advantage of the selective permeability of the membranes, has been employed. One of its features is the relatively low energy consumption and simple operation procedures required. Table5 ilustrates the advantages and disadvantages of the above methods. Molecules 2020, 25, 5812 22 of 27

Methods Advantages Disadvantages Distillation Simple to operate Energy consuming Low impurity Extraction Simple, low cost Limited application Cross-contamination Requirement of special apparatus Non-destructive, suitable for diluted Adsorption Insuﬃcient desorption data solutions Membrane separation Low energy demand, selective permeability Concentration polarization Large membrane area Membrane fouling Membrane fouling Aqueous two-phase extraction Rapid, low-cost, scalable High concentration of salts or organics Limited application Crystallization High purity Energy consuming Force ﬁeld Simple, low energy demand Low separation rate Small throughput Limited application

Another factor to be considered is the viscosity of ILs. They are more viscous than common organic solvents, and therefore exhibit a diferent behaviour when submitted to a potential. Recently, Margulis et al. [80] classified ILs structure in three possible structural motifs associated with (i) vicinal interactions, (ii) formation of positive–negative charge-alternating chains or networks, and (iii) alternation of these networks with apolar domains. They concluded that friction and mobility of Ils are nowhere close of being spatially homogeneous, and called “mechanical heterogeneity” to this phenomem, where charge networks are intrinsically stiff and charge-depleted regions are softer, flexible, and mobile. Moreover, they proposed that charge blurring associated with the loss of memory of where charges (positive and negative) are within networks is the key mechanism associated with viscosity in ILs. An IL will have low viscosity if a characteristic charge-blurring decorrelation time is low [81]. Lower viscosity implies higher conductivity and more efficient mass transport for the applications of electrochemical systems. The anion is responsible for viscosity, so its proper selection would have advantageous implications in the system environment where transport properties are pivotal. Therefore, the physicochemical properties and structure–property relationships of the ionic species in ILs, especially the effects of cationic structures on the transport properties, including viscosity, conductivity, and electrochemical properties, are still in need of more studies.

5. Conclusions There are endless combinations of mixed salts that can be designed on demand. Already, novel ionic liquids are being vigorously developed by the scientific community, but the next step should be the fine-tuning of their functionalities, e.g., by mixing two or more ionic liquids. Even with all of the synthetic advances, one of the biggest limitations to the use of RTILs is their cost. Common RTILs remain quite expensive, particularly when compared to conventional organic solvents. For example, the price of one liter of THF is ca. $60, whereas butylmethylimidazolium tetrafluoroborate ([bmim][BF4], one of the least expensive RTILs at the moment) is over $2000 per liter [82]. As a result, a very important research area is the design of much less expensive, equally stable ILs. Several examples presented in this work mention the economic impact or the potential aspects of producing an ionic liquid that could effectively extract, e.g., rare earth elements. Over the years, ionic liquids have exhibited excellent properties as electrolytes, not only because of the absence of an organic solvent, but also due to their electrochemical stability, which allow them to Molecules 2020, 25, 5812 23 of 27 be used in a wide electrochemical potential window without being oxidized or reduced. In addition, they allow for the study of the redox behaviour of organic and inorganic compounds by a simple technique such as cyclic voltammetry. ILs can also be used as additives, as well as mixed with inorganic salts, which has also presented promising results. In this contribution, several examples were given whereby the role of the IL to dissolve or to crystalize materials can be used for energy or sensing applications, making them a brand-new field to discover. It is expected that the electrochemical characteristics of ionic liquids will continue to find meaningful applications. Several examples were given in the production of new materials or methods. They show physicochemical properties, such as polarity, basicity, acidity, solution temperature, and even reaction step by a colour change. A conductive liquid can transport not only ions but also electrons, microchips, and so on. A liquid battery can be prepared by coating three layers of electrodes and electrolytes. In general, the electrochemical field in ILs is gaining space in chemical, biological, and energy areas, and it deserves to be explored further. The ILs are offering significant advantages over the conventional solvents, but due to the huge number of ILs and parameters to be optimized, more tests should be done. Most of the published work presented in this review are ground-breaking articles, such that now more contributions are required to be able to develop new fields in chemistry. With advances in the science of ionic liquids, there are bound to be some excellent treatments for ionic liquid decomposition. Ionic liquids could be developed to have electrolytes that carry a decomposition switch. Both developments would make ionic liquids more “green” and useful electrolyte materials. The recovery and recycling processes and other surrounding technologies should be developed along with the progress made in the functional design of ionic liquids. It is also important to mention that the complexity of most chemical reactions or separation systems makes the use of a single method unable to reach the required purity for ILs electrochemical studies. To improve the purification efficiency, the conjugation of several techniques may be needed.

Author Contributions: Writing—original draft preparation, G.A.O.T., I.A.S.M., A.P.C.R., and L.M.D.R.S.M.; writing—review and editing, I.A.S.M., A.P.C.R., and L.M.D.R.S.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was partially funded by Fundação para a Ciência e Tecnologia through UIDB/00100/2020 project of Centro de Química Estrutural. APCR thanks Instituto Superior Técnico for the Scientific Employment contract IST-ID/119/2018. IASM is thankful to FCT for her PhD fellowship (SFRH/BD/146426/2019). Conflicts of Interest: The authors declare no conflict of interest.

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