Conductivity, Spectroscopic, and Computational Investigation of H3O+

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Conductivity, Spectroscopic, and Computational Investigation of H3O+ Solvation in Ionic Liquid BMIBF4 Lei Yu,* Jeremy Clifford, Toan T. Pham, Eduardo Almaraz, Fredrick Perry, III, Gregory A. Caputo, and Timothy D. Vaden* Department of Chemistry and Biochemistry, Rowan University, 201 Mullica Hill Road, Glassboro, New Jersey, United States S Supporting Information *

ABSTRACT: The hydrogen ion is one of the most important species in aqueous solutions, as well as in protic ionic liquids (PILs). PILs are important potential alternatives to H2O for swelling the proton exchange membranes (PEMs) and improving the high-temperature performance of fuel cells. The hydrogen ion (H+) or hydronium (H3O+) solvation mechanism is not only a fundamental principle of acid/base chemistry in ionic liquids but also key to understanding the charge- and proton-transport properties of the PIL solutions. In this paper, a PIL system was prepared by mixing 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIBF4) IL with an aqueous solution of a strong acid, HBF4. Water can be partially evaporated from the solution under a vacuum at room temperature. Conductivity and vibrational spectroscopy (IR and Raman) measurements were used in combination with density functional theory (DFT) calculations to characterize the molecular-level solvation of H+ and H2O in the IL solution. When water is present at high molar fraction, the cations (BMI+ and H+) and anions (BF4−) are both solvated by water and the solutions have high conductivity. After water evaporation, the PIL solution has excess H+ and reduced conductivity, which is still significantly higher than that of pure BMIBF4. Vibrational spectroscopy suggests that the BMI+ and BF4− ions are desolvated from water during the water evaporation. DFT calculations assist the interpretation of the vibrational spectroscopy and show that the remaining water is in the form of H3O+ solvated by the IL molecular ions. Hence, the species remaining after evaporation is a ternary PIL consisting of BMI+ cation, BF4− anion, and H3O+ cation. The H3O+ may be the principle charge carrier in the PIL solution and responsible for the high solution conductivity.

1. INTRODUCTION The aqueous hydronium ion, H3O+, is one of the most functionally important ions in chemistry and biochemistry because of its simple structure and importance for many chemical reactions and processes. H3O+ properties and solvation in aqueous solutions have been thoroughly investigated for more than a century and are still dynamic topics in current research. Hydronium ion chemistry has built the foundation of acid/base theories in aqueous and nonaqueous solutions. However, the properties of H3O+ in ionic liquids (ILs) have not been nearly as well characterized as H3O+ in aqueous solutions. Studies of room-temperature ILs have been vigorously ongoing for a decade due to their remarkable and unique properties such as negligible vapor pressures, low melting points, nonflammability, good solvation of many organic and inorganic chemicals, and high ionic conductivities.1 Due to these properties, ILs are considered excellent candidates, being solvents and electrolytes in fuel cells, lithium batteries, and solar cells.2−25 ILs offer excellent high-temperature performance, long life, and safety in these applications as a result of their negligible volatility, thermostability, and nonflammability. Currently, fuel cells based on a proton exchange membrane (PEM) are an excellent alternative energy source in the modern © XXXX American Chemical Society

sustainable energy economy. A typical PEM is a fully hydrated Nafion (perfluorosulfonic acid) membrane. Nafion is a thermally and chemically stable, conductive, and mechanically strong polymer material.26−30 In a PEM fuel cell, protons produced at the anode transfer through the PEM to the cathode. PEM fuel cells are usually operated at 100−150 °C, instead of room temperature, in order to reduce the catalyst poisoning as well as increase overall fuel cell efficiency.31 However, at operating temperatures close to or above 100 °C, water hydrating the PEM evaporates quickly, and as a result, proton conductivity in the fuel cell decreases, resulting in an overall loss of efficiency. Recently, protic ionic liquids (PILs, incorporating excess H+) have been found as promising alternatives to water for swelling Nafion as a fuel cell proton exchange membrane (PEM).16−22 For effective PIL performance in PEM systems, high proton concentration and high conductivity are essential. Addition of a Brønsted acid can directly increase the acidic proton in an IL solution to create the PIL. Several potential Brønsted acids such as EAN, hydrogen sulfate (R−SO3H), trifluoroacetic acid Received: March 14, 2013 Revised: May 18, 2013

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and IL BMIBF4 (density 1.21 g/mL) at appropriate volume ratios. The HBF4 molar concentrations reported below are computed from the number of moles of HBF4 divided by the total volume of solution containing both H2O and BMIBF4. Water was removed from the PIL mixtures by vaporization in a rotary evaporator under a vacuum at room temperature. The solution was analyzed gravimetrically to determine the percent water loss (assuming only water evaporates) and compute the molar fraction of water as a function of evaporation. 2.2. Ionic Conductivity Measurements. Conductivities of pure ILs and PIL solutions were measured by an AC Mode Traceable conductivity meter at a constant frequency of 3 kHz with a pair of parallel Pt plate electrodes at room temperature, approximately 23 °C in our laboratory. The cell constant is designed to be unity. The cell was calibrated by standard solutions with a conductivity of 1 and 10 mS/cm, respectively, prior to measurement. 2.3. Vibrational Spectroscopy. FTIR spectra of all samples were measured with a Varian FTS 7000 FTIR Spectrometer at 1 cm−1 resolution. Liquid samples were sandwiched between two TlBrI salt plates. Raman spectra were measured using a 17 mW HeNe laser source at 633 nm. Raman-scattered photons were detected and analyzed with a Horiba Jobin-Yvon microHR grating spectrometer with 50 μm slit entrance. The detector was an Andor Newton CCD camera, and the instrument resolution was approximately 5 cm−1. All Raman spectra reported were generated by coaddition of 15 individual scans, each corrected for background using standard baseline-adjustment software. 2.4. Density Functional Theory Simulation. Density functional theory (DFT) calculations of IL solvation structures were performed with the Schrodinger suite of programs (Schrodinger, Inc.) as described in our previous report.39 This method predicts the structures of small clusters (“miniclusters”) of PILs at the molecular level by assembling a cluster of a few IL molecular anions and cations and solutes (e.g., H3O+). This method does not attempt to model the bulk liquid phase or liquid properties such as conductivity and diffusion but rather utilizes structural predictions to correlate experimental vibrational spectra to molecular-level structures. Mini-clusters of several IL units were generated with molecular mechanics (MM) and computed with more accurate DFT quantum chemical calculations. For each species calculated, a comprehensive MM conformational search was performed with the OPLS-2005 force field. Geometries and vibrational frequencies of selected low-energy conformations were computed with DFT calculations using the M05-2X functional (chosen to treat dispersion) and the 6-31+G* basis set.

(TFA), and bis(trifluoromethanesulfonyl)imide (HTFSI) have been considered and investigated.32,33 However, the solvation and transportation mechanism of H+ in these systems is still an open question. Numerous groups have investigated this mechanism and found that in PILs a proton may be associated (solvated) with multiple anions to form a negatively charged cluster,34 the proton may be transported by molecular carriers such as protonated cations or anions,35 or proton hopping could occur between the proton donor and acceptor in aminecontaining PILs.33 One of the most common ILs is BMIBF4, which consists of butylmethylimidazolium (BMI +) and tetrafluoroborate (BF4−). A PIL solution may be prepared using aqueous fluoroboric acid (HBF4). In the current work, we characterize this particular PIL system. Vibrational spectroscopic methods are a powerful means for investigating the solvation process and molecular interactions, including the interactions of ILs with other molecules.36−38 The spectral peaks of IL molecular ions are significantly affected by the interactions of proton/hydronium and other IL ions. Raman and infrared spectroscopy (FTIR) together can offer complementary vibrational spectroscopic information. Protonsolvation structures can also be investigated by computational simulation, using a “mini-clusters” method according to the report of Angenendt and Johansson.9 This method models the IL with a few anions and cations to provide some structural clues that can be verified with vibrational spectroscopy. It neglects extended liquid intermolecular interaction networks, liquid effects, dynamics, and numerous other factors that would need to be properly modeled with molecular dynamics (MD) simulations. However, the method is useful for simple assignments of IL vibrational features and interpretation of experimental results in simple terms. In a previous work,39 we used FTIR and Raman spectroscopy, combined with computational simulations and conductivity measurements, to characterize the solvation structures of H+ in a PIL system. Here, we apply this methodology to deduce the H3O+ solvation in the HBF4/H2O/BMIBF4 PIL system. In this paper, we explore the conductivity and spectroscopic properties of a PIL system utilizing aqueous HBF4 as a proton source in BMIBF4 IL. BF4− is a typical noncoordination ion, and in the classic Brønsted−Lowry theory, BF4− is not considered a base that can associate a proton. As HBF4 molecule has not been isolated, protic BMIBF4 containing only excess H+ cannot be directly prepared. On the other hand, protons can be carried by another species as a conjugate base, such as H2O. This approach is problematic, as an appreciable concentration of contaminants, such as water (above 0.1 w/w %), can affect the mechanism of proton transportation and conductivity.40 The approach we have used in this work is to add excess water and subsequently remove “extra” H2O molecules, resulting in PIL consisting of BMI+ and H3O+ cations and BF4− anions. Here, we focus on the solvation of H+ and/or H3O+ in the PIL mixture.

3. RESULTS AND DISCUSSION Pure BMIBF4 has a conductivity of 3−4 mS/cm at room temperature. Values measured by different groups may vary slightly, most likely because of differences in temperature, relative humidity, and instrumentation (including glassware) used. The conductivity of ILs increases with the addition of pure water, due to decreased viscosity, which has been demonstrated at lower concentrations (97% pure) and tetrafluoroboric acid (HBF4, 48% w/w, 7.65 M) were purchased from Sigma-Aldrich Inc. BMIBF4 was dried in a vacuum oven overnight at 60 °C prior to use. We verified the absence of halide impurities in the IL with AgNO 3 precipitation, as shown in Figure S1 of the Supporting Information. HBF4/H2O/BMIBF4 solutions were prepared by mixing the 48% HBF4 solution (density 1.40 g/mL) in water B

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effects. However, further experimentation is required to clarify the ion transportation mechanisms and properties in H2O/ BMIBF4 solutions. In order to understand the ion transportation and solvation mechanism in this solution, we further measured the vibrational spectra of the HBF4/H2O/BMIBF4 solutions. The Raman spectra are shown in Figure 2. Spectra are shown as a function

Figure 1. Conductivity of HBF4 (aq)/H2O solutions (●) in BMIBF4 shown as a function of H2O concentration. Solutions were directly prepared by mixing 48% w/w HBF4 solution in water and IL BMIBF4. The molar ratio of HBF4:H2O is 1:5.28 in all solutions. Also shown are conductivity values for H2O only in BMIBF4 (■). A version of this figure with error bars included is shown in the Supporting Information as Figure S2. This conductivity of HBF4/H2O solutions in BMIBF4 is also shown vs HBF4 concentration in Figure S3 (Supporting Information).

Figure 2. (A) Raman spectra of pure BMIBF4 and HBF4/H2O/ BMIBF4 solutions with HBF4 concentration from 0.5 to 4.0 M. The molar ratio of HBF4:H2O is 1:5.28 in all solutions. (B) Comparison of the Raman spectra of 48% (7.65 M) HBF4 solution in H2O, BMIBF4 and 4.0 M HBF4 solution in H2O/BMIBF4.

and ion clusters. The formation of the ion pairs and ion clusters also results in a relatively high viscosity of ILs. The conductivity depends on the mobility of these giant ion clusters. Thus, pure ILs have conductivity values lower than typical aqueous solutions. When the IL is diluted by water, the viscosity of the solution decreases, and consequently, the conductivity increases with increasing water content. Above 15 M water concentration, the conductivity no longer increases linearly; it levels off because the overall ion concentration is diluted. When the aqueous HBF4 solution is added to the IL (forming the PIL system), the conductivity of the solution increases with increasing solute concentration. At higher water concentration (10 M or above), the conductivity increases more dramatically than when only H2O is added, as shown by the larger slope in Figure 1 for the data from the PIL solution. This is explained by the increase in proton charge carrier in solution. With addition of aqueous HBF4, the viscosity decreases (due to the water) and simultaneously the number of charge carriers increases due to addition of the H3O+ species. The charge carriers in solution may include BF4−, BMI+, and H3O+. At lower water concentration, 5 M or lower, the HBF4/ H2O/BMIBF4 solutions have similar or even lower conductivity values compared to the H2O/BMIBF4 solutions with the same H2O molarity. The difference between the two curves in the lower concentration range is not significant. Therefore, in the lower concentration range, the solution viscosity is still the most dominant factor that affects the solution conductivity. Figure 1 does show that the HBF4 increases the IL solution conductivity. In HBF4/H2O/BMIBF4 solutions, both HBF4 and H2O affect the conductivity of the solution but perhaps by different mechanisms. For a different perspective, the conductivity of HBF4/H2O solutions in BMIBF4 is also shown vs HBF4 concentration in Figure S3 (Supporting Information). As mentioned, water can affect the IL conductivity by reducing the solution viscosity; an alternative explanation may be based on the increase of ion mobility. The effects from HBF4 alone are not clear because pure HBF4 has not been prepared. HBF4 in solution may increase the charge carriers’ concentration and also change the viscosity. The solution conductivity is likely a combination of H2O and HBF4

of increasing HBF4 concentration in Figure 2A. It can be observed that a Raman band at about 800 cm−1 appears in the spectra from high-concentration solutions. The relative intensity of this band increases as the concentration increases. This band is not present in the Raman spectrum of water (not shown) nor is it present in the pure BMIBF4 spectrum (Figure 2B). However, this band is clearly present in the spectrum of the 48% w/w HBF4 aqueous solution, as shown in Figure 2B. In the HBF4 spectrum, the band at about 1700 cm−1 is attributable to water (the O−H bending mode) and the bands below 600 cm−1 are from intermolecular vibrations between H2O molecules in the water hydrogen-bond network. The 800 cm−1 band is particularly interesting. The Ramanactive symmetric BF4− stretch mode has been observed at 750 cm−1 in the absence of water and shifts to higher wavenumber upon addition of some water.41 Our results (Figure 2) show that the band at 800 cm−1 is most prominent at high HBF4 (aq) concentration, and is shifted dramatically to higher wavenumber relative to 750 cm−1. This band increases in intensity as the HBF4 (aq) concentration increases, which is interesting because adding the HBF4 (aq) necessarily also adds a significant amount of water to the IL system. Notably, the Raman intensity of this vibrational mode is significantly higher than the “desolvated” BF4− vibration at 750 cm−1. We therefore identify the 800 cm−1 band as the BF4− vibration in the BF4− molecular anions completely solvated by water. The amount of BF4− completely solvated by water must be proportional to the intensity of the 800 cm−1 band. Figure 2A clearly shows that the increasing HBF4 concentration results in an increase in the 800 cm−1 band intensity. From Figure 2B, it appears that the spectrum of the 4.0 M HBF4 solution could be considered as a linear combination of the HBF4 (aq) spectrum and the pure BMIBF4 IL spectrum. Each spectrum in Figure 2A could be considered as a dif ferent linear combination of the HBF4 (aq) and BMIBF4 spectral features from Figure 2B. As the BMIBF4 and HBF4 spectral features appear to be preserved in the mixed PIL system, the data in Figure 2 suggests that the C

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BMI+, BF4−, H2O, and H3O+ molecules behave more or less independent of one another in the PIL solution. That is, the vibrations of each molecule are unperturbed by the other species, and there are no special interactions between particular solutes, solvents, etc. This suggestion is also consistent with the FTIR data (shown in Figure S4 of the Supporting Information), which show that the FTIR spectra of HBF4/BMIBF4 solutions do not differ from the pure BMIBF4 spectrum. HBF4 is a very strong acid that exists only in solutions. Pure HBF4 has not been reported to have been isolated. When HBF4 is added to BMIBF4 to create the PIL, H2O and H3O+ must also be introduced to the IL system. The water from the HBF4 aqueous solution can be removed by evaporation, and must be removed for use of the PIL solution in “real-world” applications. The nature of the H+ or H3O+ interaction in the IL environment with reduced water content is an interesting question. We characterized the PIL system by slowly evaporating water under a vacuum (rotary evaporation) over a period of 3 weeks and measuring the conductivity and vibrational spectroscopy at multiple points during the evaporation. For each measurement during the evaporation, we quantify the amount of water remaining in the PIL solution by gravimetric analysis. We then calculate the molar fraction of water remaining, and hence the molar fraction of H+, under the assumption that BMI+, BF4−, and H+ are nonvolatile and only H2O evaporates. The conductivity of the HBF4/BMIBF4 solution is shown in Figure 3 as a function of increasing HBF4 molar fraction (i.e.,

solution conductivity clearly decreases as H2O is evaporated gradually. Therefore, within the molar fraction ranges investigated here, the H2O is very important for determining the bulk solution conductivity in the HBF4/H2O/BMIBF4 PIL system. The decrease in conductivity slows and approaches a constant conductivity value when the H2O molar fraction becomes lower than ∼57.1%, which corresponds to a HBF4 molar fraction of higher than ∼26.3% (Figure 3). By comparison with the conductivity values in Figure 1, it is clear that the constant conductivity at this point is much higher than that in the pure IL. We speculate that, at this point, there are insufficient H2O molecules to solvate the PIL molecular ions. In the final solution, the molar fraction of H2O is about 40.6%, while the molar fraction of HBF4 is about 36.4%. The number of H2O molecules is slightly larger than the number of HBF4. Therefore, all the H+ ions may have combined with H2O to form H3O+ ions. The solution may contain BMI+ and BF4− ion pairs (or ion clusters) and H3O+ solvated by the IL molecular ions. There could be a small amount of species such as H5O2+. This resulting mixture can be considered as a H3O+ solution in BMIBF4 solvent and hold PIL properties; e.g., the conductivity is much lower than the original solution. We further characterize the PIL solution during the evaporation process from a structural perspective using vibrational spectroscopy. Raman spectra in Figure 4 show the

Figure 4. Raman spectra of 4.0 M HBF4 in H2O/BMIBF4 solvent, shown as a function of water evaporation. (A) Full spectra. (B) Expanded region showing the disappearance of the band at 800 cm−1. The data beside each curve are the molar fractions of H2O.

Figure 3. Conductivity of HBF4/H2O/BMIBF4 solutions vs the molar fraction of HBF4 when water is gradually evaporated. The data in the frames are the molar fractions of water for each point.

evolution from the water-dominant solution (bottom trace) to the IL-dominant solution (top trace), corresponding to the proposed “de-solvation” process of BF4− from H2O. It is immediately clear from this data that the 800 cm−1 band, which arises from the H2O-solvated BF4−, decreases gradually and eventually disappears with water evaporation. Indeed, a shift to lower wavenumber of this band is clearly detectable, which is consistent with the results of Jeon et al. in that removing water molecules results in a shift to lower wavenumber of the BF4− symmetric stretch vibration.41 The other Raman bands, which arise primarily from the BMI+ molecular cation,42 exhibit no significant change. The Raman measurement results further support our speculation that BF4− is solvated by H2O molecules and then desolvated as H2O molecules are removed. In parallel to the Raman spectra, the FTIR absorbance spectra of the 4.0 M PIL solution during water evaporation were also collected, as shown in Figure 5. As the focus of the current work is on the solvation of H+ in PIL solutions, rather than a detailed spectroscopic analysis, we will only discuss the

increasing H+ concentration) achieved through evaporation. Initially, water is in significant excess in the PIL solution as the H2O molar fraction is 76.3% (76.3%:14.4%:9.2% H2O/HBF4/ BMIBF4). It is notable that in this solution the absolute conductivity is much higher than that in either bulk water or the binary H2O/BMIBF4 system due to the special properties of the IL system. In this solution, water should be considered as solvent and the other ions, including BMI+, BF4−, and H+, should be considered as solutes solvated by H2O molecules. These ions are current carriers in the solution and provide relatively high conductivity. After evaporation, the H2O:HBF4:BMIBF4 final ratio is 40.6%:36.4%:23.1%. Evaporation to even lower water ratios was not successfully achieved, and may not be possible in our current experimental design. Through the gradual evaporation process, the H2O molar fraction decreases, while the molar fractions of HBF4 and BMIBF4 increase accordingly. From the data in Figure 3, the D

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cations surround the anions from above and below. This was the lowest-energy structure predicted from the conformational searches, and also from the DFT energy calculations. Other conformations are predicted to be higher energy by at least 20 kJ/mol. The structure of pure BMIBF4 is rather simple. However, the analysis of the high-concentration HBF4/H2O/BMIBF4 PIL solution structure is more complex. When the H2O fraction is decreased, the role of H2O in the solution shifts from solvent to solute and BMIBF4 becomes the solvent. Meanwhile, the mixture also contains H+, which could be separate, and H+ and H2O may be separately solvated by BMI+ and BF4−. We considered this possibility in the DFT-computed mini-cluster in Figure 6B, in which the black H+ atom is at the center of the three BMI+ cations and four BF4− anions. On the other hand, the H+ may be carried by H3O+ and the latter may be solvated by the IL molecular anions, as considered in the structure in Figure 6C. This latter structure (Figure 6C) is more thermodynamically favorable due to the chemical protonation of H2O, and is reasonable considering the makeup of the HBF4/H2O/BMIBF4 system. We can directly compare the structural predictions in Figure 6 to the experimental results using the DFT vibrational frequencies and relative IR intensities (unfortunately, we cannot predict Raman intensities). This comparison is shown in Figure 7. The experiment−theory comparison for the pure IL system is shown in Figure 7A. The theoretical spectra were generated by using the harmonic vibrational frequencies, scaled by 0.97, and the relative IR intensities from the DFT calculations. These frequency−intensity values were used to generate simulated spectra by convolution with a Lorentzian

Figure 5. FTIR spectra of the PIL solution during evaporation. The data above each spectrum refers to the H2O molar fraction.

salient features of these IR spectra. The broad asymmetric IR band at 1640 cm−1, apparent as a shoulder on a sharp band at 1600 cm−1, arises from the H2O bending vibration and signifies the presence of water in the PIL solution. Notably, this band does not disappear during water evaporation. The O−H stretch bands, apparent at above 3200 cm−1, also do not disappear. However, more importantly, a new IR spectral feature appears to develop to the lower-frequency side of the large, intense band at 1000 cm−1. This new band is present as a weak shoulder in the solution before evaporation but increases in intensity as the H2O molar fraction decreases and is very clear in the spectrum of the final evaporation experiment (with the lowest H2O molar fraction, at the top trace in Figure 5) as a band at 890 cm−1. To gain insight into the interpretation of our spectroscopic results, we performed DFT computational simulations of the IL structures at the molecular level of solvation. The pure BMIBF4 IL system is calculated on the basis of the structure of a minicluster with three BMI+ cations and three BF4− anions (Figure 6A). The three BF4− anions are shown in an equatorial triangle (the “middle” BF4− is in the background), and the three BMI+

Figure 6. Mini cluster simulations of (A) three BMI+ and three BF4−, (B) H+ in three BMI+ and four BF4−, (C) H3O+ in three BMI+ and four BF4−. The BMI+ is generally in gray shade with two N atoms indicated by blue and purple colors, respectively. BF4− anions are indicated by green color. The oxygen atom in H3O+ is red, while the “free” proton is black.

Figure 7. (A) Comparison of simulated and measured IR spectra of pure IL BMIBF4, as a benchmark. (B) Experimentally measured IR spectrum of the H2O/HBF4/BMIBF4 solution after evaporation, simulated IR spectrum based on the structure in Figure 6C, and simulated spectrum based on the structure in Figure 6B. E

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line shape of full width half-maximum (fwhm) of 10 cm−1. From Figure 7A, the relative intensities of the bands around 1000 cm−1 and a few other experimental peaks are not accurately predicted by the theory, but for the most part, the theory agrees very closely with the experiment. This serves to validate our theoretical methodology for structural interpretation. The theory neglects several aspects of bulk-phase liquids, including intermolecular interactions that extend in a network beyond the mini-cluster, dynamic effects, temperature and entropy effects, conductivities, diffusion, etc. A full prediction of all our results, including conductivities, would require MD simulations, but as such simulations for PIL systems are at the forefront of theoretical chemistry in terms of development and computational expense,43 they are beyond the scope of the current work. The simulated IR spectrum of the structures in Figure 6B and C is shown in comparison to the spectrum of the HBF4/ BMIBF4 PIL solution in Figure 7B. The theoretical spectra correspond to the H+ solvated by IL molecular ions (labeled “H+ theory”, from Figure 6B) and the H3O+ solvated by IL ions (labeled “H3O+ theory”, from Figure 6C). The experimental spectrum shown is for the 4.0 M solution after water evaporation, where the H2O molar fraction is about 40.6%. The H3O+ theory clearly agrees with the experimental results more closely. The experimental IR band at 890 cm−1 is reproduced by a predicted band at 980 cm−1, as highlighted by the arrow in Figure 7B. This 980 cm−1 vibration is not predicted to be present in the pure BMIBF4 spectrum (Figure 7A), which is consistent with our experimental results. From the DFT calculation normal-mode analysis, the 980 cm−1 band corresponds to the asymmetric BF4− stretching vibration of the anions bound directly to H3O+. This same asymmetric BF4− stretch vibrational frequency is 1060 cm−1 in the pure BMIBF4 (Figures 6A and 7A), and hence, the 980 cm−1 band is only possible if a H3O+ cation interacts directly with a BF4− anion. Furthermore, from Figure 7B, we can see that the spectrum predicted for the H+ theory does not agree with the experiment particularly well. Most significantly, the theory does not predict the 890 cm−1 band and does predict an intense IR band at 1900 cm−1, corresponding to the shared-proton vibration of the H+ in solution, which is not detected. We feel it is relevant and noteworthy to include an additional result from this line of experimentation that was not directly related to the solvation. As mentioned previously in this paper, pure HBF4 has not been isolated. It is available only in aqueous or diethyl ether solutions. A HBF4 solution in diethyl ether has also been mixed with IL BMIBF4 in order to prepare the PIL solution. Evaporation of the ether generated HF gas which dissolved our glass flask and cuvettes. This finding further supports the hypothesis that H+ is simply not stable in BMIBF4 by itself and cannot be directly solvated by BMI+ and BF4− ions. A “proton carrier” such as H2O is required. This experimental observation also serves as a cautionary warning for investigators in the field. Due to the high level of toxicity of the HF gas, no further experiments were carried out under these conditions in our lab. For the aqueous HBF4/BMIBF4 PIL system, the conductivity and spectroscopy measurements elucidate the H3O+ solvation in the BMIBF4 IL system. When the PIL system is created with a high concentration of HBF4 (aq), the conductivity is greatly increased relative to pure BMIBF4. Both H2O and HBF4 contribute significantly to this increment of conductivity by reducing the solution viscosity and providing additional highly

mobile ions. However, this solution contains 76.3% (molar fraction) water, and exists as BF4− solvated by H2O and H3O+ (as evidenced by the Raman spectral band at 800 cm−1 in Figure 2). We evaporate the water in a vacuum, and the solution evolves from essentially H3O+ and BMIBF4 in water solution to HBF4 and H3O+ in BMIBF4 PIL solution with about 40.6% (mole) water. The H2O solvation of BF4− disappeared, evidenced by the disappearance of the 800 cm−1 band in the Raman spectra (Figure 4), and a new solution structure is generated, as evidenced by the appearance of a band in the FTIR spectra at 890 cm−1 (Figure 5). This is accompanied by a significant decrease in the conductivity, suggesting the high PIL conductivity measured in Figure 1 arises primarily from the water solution rather than special PIL charge mobility. The DFT calculations provide strong evidence that, in the evaporated PIL solution, the H+ is present as H3O+ (which also explains why the water cannot be completely evaporated). The H3O+ molecular cation is solvated by the IL molecular ions.

4. CONCLUSION The interest in PILs and their applications depend directly on the physical and chemical properties of these PILs. A thorough understanding of how complex mixtures of PILs behave is of great interest due to the wide range of potential applications and the relative novelty of the PIL field. Our conductivity, spectroscopy, and computational investigations of the HBF4/ H2O/BMIBF4 yielded several important findings that are directly relevant to the application of PILs. First, the conductivity of the solution is significantly higher than that of pure BMIBF4. When HBF4 and H2O are added into BMIBF4 at a fixed ratio (1:5.28), the solution conductivity increases as the HBF4 and H2O concentrations increase. However, when H2O is removed from the solution and left over a higher HBF4 concentration, the solution conductivity reduces. Second, when H2O is the dominant species (in molar fraction) in a HBF4/ H2O/BMIBF4 solution, the ions (BMI+, BF4−, and H+) are solvated by water molecules. This solution composition possesses a relatively higher conductivity than the original. Third, in a solution with very limited water, the remaining H2O is protonated to form H3O+ ions which are then solvated by the BMI+ cations and BF4− anions of the IL. Hence, a PIL of H 3O +/BMI+/BF 4− is the resulting solution. This PIL, containing “water” and possessing a high proton activity, might be a future candidate for a swelling solution of PEM in fuel cells. Accordingly, a free proton (H+ species) that is solvated by the ions of IL (BMI+ and BF4−) is not observed in the solution. Though water solvation in BMIBF4 has been investigated,44,45 to the authors’ best knowledge, this is the first report about H3O+ solvation in ILs. The results of this work can guide the future fundamental investigation of the acid/base properties in ILs, the proton transport properties in PIL swelling PEM, and the development of PILs with high proton activity and conductivity in ILs with noncoordination ions such as BF4−.

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ASSOCIATED CONTENT

S Supporting Information *

The AgNO3 precipitation tests to verify purity, HBF4/H2O/ BMIBF4 solution conductivity, and the FTIR spectra of HBF4/ BMIBF4 solutions before evaporation shown as a function of concentration. This material is available free of charge via the Internet at http://pubs.acs.org. F

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.Y.); [email protected] (T.D.V.). Phone: 856-256-5409 (L.Y.); 856-256-5457 (T.D.V.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported in part by the South Jersey Section of the American Chemical Society and the American Chemical Society Project SEED program. The authors would like to thank Mr. Fadi Elsmaily for performing the calibration IL− water conductivity measurements.

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REFERENCES

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