Effect of Aromaticity in Anion on the Cation–Anion Interactions and Ionic Mobility in Fluorine-Free Ionic Liquids

Ionic liquids (ILs) composed of tetra(n-butyl)phosphonium [P4444]+ and tetra(n-butyl)ammonium [N4444]+ cations paired with 2-furoate [FuA]−, tetrahydo-2-furoate [HFuA]−, and thiophene-2-carboxylate [TpA]− anions are prepared to investigate the effects of electron delocalization in anion and the mutual interactions between cations and anions on their physical and electrochemical properties. The [P4444]+ cations-based ILs are found to be liquids, while the [N4444]+ cations-based ILs are semi-solids at room temperature. Thermogravimetric analysis revealed higher decomposition temperatures and differential scanning calorimetry analysis showed lower glass transition temperatures for phosphonium-based ILs than the ammonium-based counterparts. The ILs are arranged in the decreasing order of their ionic conductivities as [P4444][HFuA] (0.069 mS cm–1) > [P4444][FuA] (0.032 mS cm–1) > [P4444][TpA] (0.028 mS cm–1) at 20 °C. The oxidative limit of the ILs followed the sequence of [FuA]−> [TpA]−> [HFuA]−, as measured by linear sweep voltammetry. This order can be attributed to the electrons’ delocalization in [FuA]− and in [TpA]− aromatic anions, which has enhanced the oxidative limit potentials and the overall electrochemical stabilities.


■ INTRODUCTION
Ionic liquids (ILs), especially room-temperature ionic liquids (RTILs), as emerging electrolytes have been extensively studied in numerous electrochemical applications owing to their exceptional physicochemical and electrochemical properties. 1−4 Some of the adoptable properties, which are common to ILs, are negligible vapor pressure, nonflammability, wide temperature range of liquid phase, high thermal stability, high solvating behavior, high ionic conductivity, and wide electrochemical stability window. ILs composed of bis-(trifluoromethyl sulfonyl)imide (TFSI) anion with different nitrogen-based aromatic and aliphatic cations such as imidazolium, pyridinium, pyrrolidinium, piperidinium, and quaternary ammonium derivatives are widely used in different electrochemical applications. 5−10 The quaternary ammonium cations and their derivatives coupled with TFSI anion have good electrochemical stability at room temperature but are thermally less stable than quaternary phosphonium counterparts, and they would not be good electrolytes at high temperatures. 11,12 There has been considerable interest in the synthesis and electrochemical characterization of phosphonium-based RTILs due to their advantages in simultaneously providing chemical, thermal, and electrochemical stabilities. 13−17 The phosphonium ILs have been used as electrolytes for various applications such as electrochemical sensing, 18 electro-polymerization of organic monomers, 19 and electrolytes in supercapacitors 20,21 and in dye-sensitized solar cells. 22,23 Many of these reported phosphonium ILs are based on large molecular weight cations derived from tris(n-hexyl)phosphine, 14,24 and the resulted ILs have relatively high viscosities and low ionic conductivities. Low viscosity is a main requisite for electrochemistry since viscosity pointedly affects the ionic conductivity, diffusivity, dispersion, mixing, filtration, and even equipment selections. Despite the importance of lower viscosity and higher ionic conductivity, there have been limited attempts on lowering liquid viscosities of phosphonium ILs. 25 In this regard, we have designed tetra(n-butyl)ammonium and tetra(n-butyl)phosphonium-based ILs containing relatively smaller heterocyclic anions with a motivation to achieve the following desirable properties: (i) enhanced thermal stability (phosphonium-based ILs are known for their high chemical and thermal stabilities and comparatively low toxicity than their ammonium analogues); 11,13,26 (ii) faster diffusivity of ions (the investigation of ions self-diffusions is of great importance for ILs as electrolytes for electrochemical devices); 27,28 (iii) higher electrochemical stability (phosphonium-based ILs have wide electrochemical windows and higher ionic conductivities 13,29−31 and in some cases better electrochemical properties than their ammonium counterparts). 32 The lowest-viscosity phosphonium-based IL for electrochemical application is tri(n-butyl)octylphosphonium TFSI IL; 33 however, the viscosity is still higher in comparison to conventional TFSI-based ILs. Tsunashima and Sugiya have reported a series of ILs comprising triethylphosphine (TEP)based cations and TFSI anions having relatively higher ionic conductivity and lower viscosity. 11 The main limitation of ILs with TFSI anions in electrochemical systems is their high fluorine content, which would lead to toxicity and environmental concerns. There have been a number of salts developed 34−37 to substitute the LIBs' electrolyte salt LiPF 6 , but many of these salts were thermally or electrochemically unstable. In the mid-1990s, a successful attempt was made toward the synthesis of salts based on aromatic anions (following the Huckel rule) as potential battery electrolytes. This concept was strongly supported by molecular modeling. 38−40 Among the possible aromatic anions using the cyclization synthesis approach, LiTDI salt (lithium 4,5dicyano-2-(trifluoromethyl)imidazolate) was made at a large scale. 41 Similar aromatic analogs, LiPDI (lithium 4,5-dicyano-2-(pentafluoroethyl)imidazolate) and LiHDI (lithium 4,5dicyano-2-(n-heptafluoropropyl)imidazolate), 42,43 were also synthesized. The newly synthesized unique salts were thermally stable, and unlike LiPF 6 , they were stable in the presence of moisture as well. Further, the aromatic salts (LiTDI and LiPDI) electrolytes were also highly stable on the surface of Pt electrode even in a wide potential window of 4.8 V versus Li + / Li as confirmed by voltammetric measurements. 44 Sẃiderski et al. studied the effect of metal cations, including Li and Na, on the electronic system of 2-furoate and thiophene-2-carboxylate. 45 Experimental and theoretical calculations showed perturbation (a disturbance of the regular elliptical motion) of the aromatic electron in the ring, which depends on the type of cations. The perturbation and polarization of the aromatic electrons in the ring further strengthened the coordination and electrostatic interaction with metal ions. In this work, the idea of aromatic anions functionalized with an electron delocalized carboxylic functional group is explored in halogen-free and highly stable ionic liquids as future potential electrolytes for electrochemical devices. The structural design of the thermally and electrochemically stable ILs is the motivation of this study, in which we focused on small phosphonium and ammonium cations and fluorine-free aromatic heterocyclic anions such as furoate [FuA] − and thiophene-2-carboxylate [TpA] − that can potentially be used as future electrolytes for electrochemical applications. Both experimental and theoretical studies have been performed in order to understand the effect of functional aromatic anions on the physical and electrochemical properties of these ILs.
Synthesis of Ionic Liquids. Detailed description of the synthesis of these ILs is given in the Supporting Information (SI). In short, the ILs were synthesized by the neutralization reaction of phosphonium or ammonium hydroxide base with the corresponding cyclic carboxylic acid in a 1:1 molar ratio. After purification and drying, phosphonium-based ILs were obtained as room-temperature liquids and ammonium-based ILs were found to be semi-solids at room temperature. NMR chemical shifts, FTIR bands, and MS (ESI) data of the synthesized ionic liquids are presented in the SI.
Characterization. Bruker Ascend Aeon WB 400 (Bruker BioSpin AG, Fallanden, Switzerland) spectrometer was used for NMR analysis. The analysis medium was CDCl 3 with 0.03% (v/v) TMS. The working frequencies were 400.21 MHz for 1 H, 100.64 MHz for 13 C, and 162.01 MHz for 31 P. Bruker Topspin 3.5 software was used for data processing.
Fourier transform infrared (FTIR) analysis was performed on a Bruker IFS 80v vacuum FTIR machine, which is equipped with an attenuated total reflection (ATR) (liquid analysis) and with a deuterated triglycine sulfate (DTGS) detector. Forward−backward acquisition modes were used for recording spectra at room temperature. A total of 256 scans were coadded and signal-averaged at an optical resolution of 4 cm −1 .
Densities of the samples were measured using an Anton Paar (DMA 4100M) density meter over the range of 20−60°C. A Lovis 2000 ME Automated Microviscometer (an Anton-Paar falling ball type viscometer with a 2.50 mm glass capillary, the viscosity range 10−10 000 cP) was used for viscosity measurements.
Thermogravimetric analysis (TGA), using a PerkinElmer 8000 TGA machine, was performed with about 2 mg of IL sample from 30 to 600°C at 10°C min −1 under nitrogen. A PerkinElmer 6000 differential scanning calorimetry (DSC) machine was used for DSC measurements. For experiments, about 3−5 mg of the IL sample was packed in an aluminum pan and heating/cooling DSC scans were recorded from −80 to 100°C at 10°C min −1 . The onset of the DSC peak was considered as the glass transition temperature (T g ). Nitrogen gas was supplied at 20 mL min −1 to maintain a dry environment inside the sample chamber.
Electrochemical Measurements. Metrohm Autolab (PGSTAT302N) electrochemical workstation, with FRA32 M module for impedance, was used for the electrochemical studies. About 80 μL of the electrolyte was placed in the TSC 70 closed cell. The cell was mounted onto the Microcell HC cell stand, which is connected to a temperature controller. In the Microcell HC setup, the temperature is controlled through a Peltier element which enables adjusting the temperature from −20 to 100°C. A two-electrode cell assembly, glassy carbon electrode as working and Pt cup as counter electrodes, was used for electrochemical impedance to measure IL ionic conductivity. Impedance measurements were performed in the frequency range of 0.1 Hz to 1 MHz with an alternating current (AC) amplitude of 10 mV. Heating and cooling impedance responses were recorded from −20 to 100°C. The cell was thermally equilibrated for at least 10 min before each measurement. The cell constant of the Pt cup was determined by means of a 100 μS cm −1 KCl standard solution from Metrohm (K cell = 1.486 cm −1 ). Prior to each experiment, the working electrode was polished with a Kemet diamond paste (0.25 μm). For data acquisition, Nova 2.02 software was used. In case of the ILs electrochemical stability windows (ESW) tests, linear sweep voltammetry (LSV) was run at different temperatures from −20 to 80°C using a three-electrode cell assembly: glassy carbon (GC) as working, Ag wire (coated with AgCl) as pseudoreference, and Pt cup as counter The Journal of Physical Chemistry B pubs.acs.org/JPCB Article electrodes, respectively. The LSV scan rate was 20 mV s −1 , and the applied potential was from −3 to +3 V. Ferrocene was used as an internal reference, and the potentials were converted to Fc/Fc + couple standard. 46 Ferrocene was poorly soluble in the IL, and an ultrasonic bath (VWR USC-TH) was employed to assist the dissolution. The current was converted to current density using the working electrode surface area (2 mm diameter). The electrochemical window of the IL was considered as the potential at 0.10 mA cm −2 cutoff current density. NMR Diffusometry. Pulsed gradient spin−echonuclear magnetic resonance (PGSE-NMR) measurements were performed on a Bruker Avance III (Bruker BioSpin) NMR spectrometer. The working frequency for 1 H analysis was 400.27 MHz. Data were processed using Bruker Topspin 3.1 software. NMR self-diffusion measurements were performed on 1 H with a PGSE-NMR probe Diff50 (Bruker) with a maximum amplitude of the magnetic field gradient pulse of 29.73 T m −1 . The sample was placed in a standard 5 mm glass sample tube and closed with a plastic stopper to avoid air contact. Prior to measurements, the samples were equilibrated at a specific temperature for 30 min.
Simulation Methodology. Density functional theory (DFT) calculations were first performed to obtain optimized molecular geometries of three anions, and thereafter their tightly bound ion pair structures with tetra(n-butyl)ammonium and tetra(n-butyl)phosphonium cations, respectively, using the Gaussian 16 package (Revision B.01) 47 at the B3LYP/6-311+g(d,p) level of theory 48 with Grimme's-D3 dispersion correction. 49 Only molecular geometries with all positive vibration frequencies were taken into consideration to ensure that the obtained ion and ion pairing structures are in fact true minima in the corresponding energy landscapes.
Atomistic force field parameters for three anions based on the AMBER framework were developed in a procedure similar to that described in previous publications. 50 The CHELPG atomic partial charges for three anions were determined by fitting molecular electrostatic potentials generated from DFT calculations of individual anions at the same level of theory with B3LYP hybrid functional and 6-311+g(d,p) basis set, and thereafter uniformly down-scaled using an empirical scaling factor of 0.8 from unity ion charges to account for polarization and charge transfer effects between ion species. The crossinteraction parameters between different atom types were obtained from the Lorentz−Berthelot combination rules.
In the current work, all simulation systems are composed of 400 ion pairs having ∼26000 atoms. Atomistic molecular dynamics simulations were performed using the GROMACS package with cubic periodic boundary conditions. 51 The equations of motion were integrated using a classical velocity Verlet leapfrog integration algorithm with a time step of 1.0 fs. A cutoff distance of 1.6 nm was set for short-ranged van der Waals interactions and real-space electrostatic interactions between atomic partial charges. The particle-mesh Ewald summation method with an interpolation order of 5 and a Fourier grid spacing of 0.2 nm was employed to handle longrange electrostatic interactions in reciprocal space.
All simulation systems were first energetically minimized using a steepest descent algorithm and thereafter annealed gradually from 800 to 300 K within 20 ns. All annealed simulation systems were equilibrated in an isothermal−isobaric ensemble for 40 ns maintained using the Nose−Hoover thermostat and the Parrinello−Rahman barostat with time coupling constants of 0.5 and 0.2 ps, respectively, to control the temperature at 300 K and the pressure at 1 atm. Atomistic simulations were further performed in a canonical ensemble for 100 ns, and simulation trajectories were recorded at an interval of 100 fs for further microstructural and dynamical analyses.

■ RESULTS AND DISCUSSION
The synthesized ILs, namely, tetra(n-butyl)phosphonium furoate ( have three resonance lines associated with the alkyl chains of the phosphonium cation; a triplet in the range from 0.90 to 0.95 ppm is for the terminal methyl groups, a multiplet between 1.46 and 1.54 ppm is for all of the methylene protons, and another multiplet between 2.30 and 2.46 ppm is due to the 8 α-protons. The downfield resonance lines of varying multiplicity in the ranges of 6.32−6.34 ppm (multiplet) and 6.88−6.90 ppm (multiplet) and at 7.34 ppm (singlet) are assigned to the three non-equivalent protons of the furoate anion ( Figure S1). Similar varying multiplicities were also observed for the three non-equivalent protons of thiophenecarboxylate anion at 6.97−6.99 pm (triplet), 7.32−7.33 pm (doublet), and 7.66−7.67 ppm (doublet), as shown in Figure  S4  It means an increase in electronic charge density around C3, C4, and C5 carbon atoms and decrease around C1 and C2 carbon atoms in the ILs compared with free acid. 45 This is due to the electron aromaticity and delocalization from the carbonyl group toward the furan ring. In the spectra of [P 4444 ][TpA], signals arising from C1, C3, C4, and C5 carbons (in thiophene-2-carboxylate anion) are shifted upfield while the signal from the C2 carbon (in thiophene-2carboxylate anion) is shifted downfield in comparison with the chemical shifts present in the spectra of thiophene-2-carboxylic acid, Figure 2c,d. It means higher electronic densities on C1, C3, C4, and C5 carbon atoms and lower electronic density on C2 carbon atom. 45 The C2 carbon in thiophene-2-carboxylate anion is more deshielded due to the ring aromaticity along with electron-withdrawing effect of the carbonyl group. The difference in chemical shifts of C1 carbon signals (Figure 2b,d) is due to the presence of oxygen in furoate anion ring and sulfur in thiophene-2-carboxylate anion ring. The 31 P NMR spectra ( Figure S3, Figure S6, and Figure S9 (Figure 4c).
The charge delocalization effects on these three anions are further manifested on the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energies. Figure 5  The properties of ILs such as thermal behavior and stability, viscosities, ionic conductivity, diffusivity, and electrochemistry are significantly affected by impurities of IL samples. 50,53,54 Among all possible impurities, water holds a peculiar significance. 50 50 The cation−cation RDFs (Figure 6b) exhibit a broad first maxima with some distance shoulder which is presenting an ionic system dominated by a long-range electrostatic interaction. 50 This is specially observed for tetra(n-butyl)ammonium cations having stronger interactions with neighboring cations than their phosphonium counterparts (Figure 6b) when they are coordinated with the same anion. This supports the semi-solid nature of the tetra(n-butyl)ammonium ILs at room temperature. The RDFs calculations clearly show that the local ionic structures of the ILs are affected by the nature of the cations and anions. Thermal properties of the ILs are determined using TGA and DSC. The most significant descriptor in TGA analysis is the decomposition onset temperature (T d ), which is the intersection of the baseline and the tangent of the weight versus temperature ( Figure S17). The T d depends on the intrinsic stability and mutual interactions between ions. 55 The TGA curves of the ILs shown in Figure 7a indicate that these ILs have two inflection points, corresponding to two decomposition steps.  (Table 1).
Conversely, the ammonium-based ILs exhibit single-step decomposition TGA curves (Figure 7a) with T d at about 200°C. Ohno and co-workers have reported a series of tetra(nbutyl)phosphonium-based amino acid ILs having higher thermal stabilities than imidazolium-and ammonium-based ILs. 26 57 respectively. On the other hand, cholinebased ILs with a range of amino acids anions are known to decompose at temperatures < 200°C. 58 The effect of cations on the thermal decomposition of furoate-based ILs have also been reported. 59 59 The DSC heating scans at a rate of 10°C min −1 are shown in Figure 7b, where we can see that each IL exhibits a heat capacity change that corresponds to a glass transition temperature (T g ). In comparison to the phosphonium counterparts, the T g values are positively shifted with ammonium cations, Figure 7b and Table 1. The subzero T g values of the ILs indicate the very low crystallization rate and fairly stable supercooling state. The thermal analysis shows that the synthesized ILs have relatively high thermal stability and lower glass transition temperatures. The high thermal stability and subzero glass transition temperatures of these ILs make them excellent electrolytes for electrochemical devices operating at variable temperatures. Figure 8 shows temperature-dependent ionic conductivities of the phosphonium-based ILs. The original ionic conductivity data as a function of temperature are provided in Figure S18   . The difference in ionic conductivities of these ILs for the same cation is attributed to the variations in microstructures of the three anions and their varied charge delocalization effect, which in turn affect their ionic mobility.
The Vogel−Fulcher−Tamman (VFT) equation (eq 1) was used to fit the temperature (K) versus ionic conductivity plots of the phosphonium-based ILs in the temperature range from −20°C (253 K) to 100°C (373 K).
where σ o (S cm −1 ), B (K), and T o (K) are the fitting parameters. σ o is a pre-exponential factor, B is a factor related to the activation energy (E), and T o is the ideal glass transition temperature, respectively.  Figure 8. The best-fit parameters of eq 1 are given in Table 2. T o is found to be about 60 K below the experimental T g values, which is typical for ionic liquids. 60 There is a direct relation between B and activation energy for ion conduction (E σ = BR). As can be seen in Table 2, the E σ value is increasing from 12.    Figure 9.
where D o (m 2 s −1 ), T o (K), and B (K) are adjustable parameters. Energy of activation for diffusion is related to B (E D = BR). The values of the best-fit parameters are tabulated in Table 3. The solid and dashed lines in Figure 9 are the calculated curves using eq 2. It is worth mentioning that the experimental and simulated diffusion coefficients for the anions in each IL are larger than those for the corresponding cations. This is due to the smaller sizes of these anions as compared with the [P 4444 ] + cation.  Table 3). The ESW of an IL is the potential range in which it does not get oxidized or reduced, which can be calculated by subtracting the reduction potential limit (cathodic limit, E C ) from the oxidation potential limit (anodic limit, E A ). In order to make a base for the ESW determination, a current density called cutoff current density must be selected to evaluate the redox potentials. Water content and the operating temperatures are among the main factors affecting the ESW of an IL. Generally, increasing temperature and water content results in narrowing of both E C and E A and subsequently reducing the ESW of ILs. 54,61 The LSV characterization at variable temperatures is carried out to evaluate the thermo-electrochemical stabilities of the synthesized ILs. The LSV curves of the phosphonium-based ILs at 20°C are shown in Figure 10a. Both the [P 4444 ][FuA]  Figure 9. Temperature-dependent self-diffusion coefficients of the cations (a) and anions (b) in tetra(n-butyl)phosphonium-based ILs.  Figure 2, stretching shifts of CO in the FTIR spectra in Figure 3 and the molecular electrostatic potential contours of these three anions shown in Figure 4 and Figure 5.
The temperature-dependent LSV profiles of these ILs in the temperature from −20 to 80°C are shown in Figure S19 and the ESW values are given in  Figure 10b shows the decrease in ESW as a function of temperature, with a large decrease observed between 10 and 70°C. The decrease in ESW with temperature can be associated with the decrease in viscosity resulting in a higher ionic conductivity and diffusivity of ions and thus frequent interactions of ions with the electrode surface. Both the E A and E C are affected with temperature, showing that both anions and cations are sensitive to the temperature increase. 63 Traces of water can also decrease the ESW of ILs. 64 It is wellknown that water molecules present heterogeneous structures and dynamics in the cavities of IL matrices and are strongly bounded to neighboring ionic groups via preferential hydrogen bonding interactions. 65 In most cases, such interactions are so strong that it is impossible to completely remove traces of these dispersed water molecules even by using high temperature and vacuum. Hence, a shoulder around 1.  Figure S19a,c) that is more obvious at 60 and 80°C might be due to the trapped water molecules' electrolysis at the electrode surface. 66 The LSV thermo-electrochemical stabilities are in very good agreement with the TGA thermal stabilities of the synthesized ILs. On the basis of these physicochemical and electrochemical investigations, it is worth mentioning that the halogen-free phosphonium-based ILs of this study can be promising electrolytes for high-temperature electrochemical devices in particular to supercapacitors and batteries.

■ CONCLUSIONS
A combined experimental and theoretical study has found that the nature of cation and anion, and an electron delocalization in the anion, has significant effect on the physical and electrochemical properties of ILs. The thermal analysis revealed that the tetra(n-butyl)phosphonium-based ILs are thermally more stable and have lower glass transition temperatures than the tetra(n-butyl)ammonium-based ILs having common anions. The aromatic structure of 2-furoate and thiophene-2-carboxylate anions enhanced the electrostatic interaction with cations that resulted in highly stable ILs. The aromatic anions ([FuA] − and [TpA] − ) based ILs are more thermally stable and presented a wider electrochemical window than nonaromatic anion ([HFuA] − ) based IL. This is probably due to the aromatic stability of the 2-furaote and thiophene-2carboxylate anions that enhanced the overall stability of the ILs. As expected, the mobility of anion containing sulfur atom, [TpA] − , is found to be slower than the anion with an oxygen atom, [FuA] − . The anion structures have shown a pronounced effect on the oxidative limit of these ILs. In addition, [FuA] − and [TpA] − anions-based ILs have relatively wider electrochemical windows than the IL with [HFuA] − due to their distinct aromatic structural effect. The new anions presented in this study will help researchers to develop new ionic liquids with promising physicochemical and electrochemical properties that can be utilized as electrolytes for energy applications.