Binary Protic Ionic Liquid Mixtures as a Proton Conductor - American

Nov 13, 2014 - diethylmethylammonium hydrogensulfate ([dema]HSO4) and ... using a binary electrolyte consisting of 56 wt % [dema][NTf2] was 1.03 V vs ...
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Binary Protic Ionic Liquid Mixtures as a Proton Conductor: High Fuel Cell Reaction Activity and Facile Proton Transport Muhammed Shah Miran,#,†,⊥ Tomohiro Yasuda,#,‡ Md. Abu Bin Hasan Susan,†,⊥ Kaoru Dokko,† and Masayoshi Watanabe*,† †

Department of Chemistry and Biotechnology, and ‡Cooperative Research and Development Center, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan S Supporting Information *

ABSTRACT: Binary mixtures of two protic ionic liquids (PILs), namely, diethylmethylammonium hydrogensulfate ([dema]HSO4) and diethylmethylammonium bis(trifluoromethanesulfonyl)amide ([dema][NTf2]), were prepared by mixing in various weight ratios for prospective use as fuel cell electrolytes. The binary mixtures showed significantly higher electrochemical activity compared with the constituent pure PILs, and the activity changed depending on the composition of the mixtures. Specifically, the open circuit potential (OCP) for a H2 | O2 cell using a binary electrolyte consisting of 56 wt % [dema][NTf2] was 1.03 V vs a reversible hydrogen electrode (RHE), whereas the values were 0.90 and 0.77 V for pure [dema]HSO4 and [dema][NTf2], respectively, under similar conditions. The electrochemical activity of the binary mixtures was interpreted by comparing their molecular characteristics inferred from Fourier transform infrared (FT-IR) and 1H NMR spectroscopy with those of the constituent PILs. The binary systems showed enhanced electrochemical activity, possibly due to anion/proton exchange through the formation of hydrogen bonds of varying strengths via the N−H bond. The anion/proton exchange appears to average the N−H bond strength to render it suitable for fuel cell reactions. Bulk physicochemical properties such as thermal properties, viscosity, ionic conductivity, and ionicity were also measured precisely. The results of the pulsed gradient spin echo (PGSE) NMR and Walden plot collectively suggest that the Grotthuss mechanism in addition to the vehicle mechanism contributes to proton transport in the binary systems, possibly due to the coexistence of [dema] cation and HSO4− anion, whereas the vehicle mechanism is dominant for pure [dema][NTf2].

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amines), PILs with ΔpKa of the constituent acid−base pair in the intermediate range of 16−18 exhibit high open circuit potentials (OCP) for H2 | O2 cells, whereas PILs with lower and higher ΔpKa show low OCP under similar conditions.22 Meager electrochemical activities for PILs with low ΔpKa appear to be a result of the existence of nonionic species and low proton activity.22 PILs with high ΔpKa exhibit promising bulk properties such as excellent thermal stability, high ionic conductivity, and high ionicity.2−4 However, only a few PILs exhibit both electrochemical activities and promising bulk properties as anhydrous proton conductors. Interestingly, PILs simply based on trifluoromethanesulfonic acid (TfOH) and nonafluorobutanesulfonic acid exhibited high OCP values comparable to the standard value of 1.15 V vs RHE at 150 °C, although PILs with weak acids (CF3CO2H, CH3CO2H and HCO2H) and strong as well as superstrong acids (H2SO4, CH3SO3H, and (CF3SO2)2NH (H[NTf2])) have also been extensively explored.22,23 For instance, the OCP of [dema][TfO] was 1.03 V, whereas the value observed for diethylmethylammonium bis(trifluoromethanesulfonyl)amide

INTRODUCTION Protic ionic liquids (PILs), prepared through neutralization reactions of Brønsted acids and bases, are promising materials and have diverse applications.1−4 Ionic liquids of this variety have properties similar to aprotic ionic liquids (AILs), such as high ionic conductivity, low vapor pressure, nonflammability, a wide liquid temperature range, and high thermal stability, if sufficiently large ΔpKa of the constituent acid−base pair is ensured.2−4 The ability of the PILs to conduct and/or donate protons offers them the potential to function as electrolytes or solvents for nonhumidifying fuel cells,5−9 Belousov−Zhabotinsky reactions,10double-layer capacitors,11 acid catalysis,12 pharmaceutical applications,13 and for stabilization and crystallization of proteins and viruses.14,15 Despite their very high technological importance, fundamental studies to identify factors governing physicochemical properties of PILs and to tune them for designing electrolytes or solvents with desirable properties for various applications have just commenced.16−21 While the role of active proton in the properties and applications of PILs is very interesting, especially for fuel cell electrolytes, it is yet to be systematically investigated. Specifically, its correlation with the structure of PILs is of much interest. Recently, we reported that for a series of PILs based on a superstrong base and conventional bases (tertiary © 2014 American Chemical Society

Received: July 12, 2014 Revised: November 10, 2014 Published: November 13, 2014 27631

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([dema][NTf2]) was only 0.77 V at 150 °C for Grove-type liquid fuel cells under similar conditions.5,22,23 This behavior could be explained by the fact that the strength of N−H bonds in the cationic structure increases with an increase in ΔpKa of the constituent acids and bases.22,24 The N−H bond strength, having an inverse relationship to hydrogen bond strength between the N−H group and anions, mainly relies on the anionic Lewis basicities24 and can be monitored through variation in N−H stretching frequency and N−H proton chemical shift for a series of PILs.24 Strong N−H bonds may slow proton donation to molecular oxygen (oxygen reduction reaction). Furthermore, since hydrophilic PILs may suffer from the drawback of being leached out when mixed with water during fuel cell operation,5 hydrophobic PILs having high electrochemical activity and capable of proton transfer through structural diffusion are critically sought.25,26 Since the addition of molecular solvents including water is discouraged from the point of view of thermal stability, the cation and anion structure of PILs may be functionalized to avoid problems associated with the use of a highly viscous or a high melting point system.27,28 In this study, it is presumed that enhanced electrochemical activity is achieved by incorporating both relatively weak and strong counteranions for a common cationic structure. This necessitates preparation of binary mixtures by a suitable combination of two PILs that have ΔpKa values lower and higher than 16−18. If anion/proton exchange occurs in such binary mixtures, the NH bond strength is averaged, which may lead to higher electrochemical activity for the fuel cell reactions. Investigation of physicochemical properties of such a binary system and their correlation with electrochemical behavior may assist in designing a suitable electrolyte for prospective use as fuel cell electrolytes under nonhumidifying conditions. In this study, we prepared binary mixtures of a relatively low ΔpKa PIL, diethylmethylammonium hydrogensulfate ([dema]HSO4), and a relatively high ΔpKa PIL, [dema][NTf2] (Figure 1). We

following the method reported earlier.4,23 The PILs were then mixed in different weight ratios inside an argon-filled glovebox and finally homogeneously mixed through sonication. The water content of all the PILs, as determined by Karl Fischer titration, was less than 100 ppm. Methodology and Measurements. The binary PILs were characterized by cyclic voltammetry measurements, vibrational stretching frequency determined from FT-IR spectra, and 1H NMR chemical shifts of the N−H proton, in addition to density, viscosity, ionic conductivity, self-diffusion coefficient, and thermal property measurements. Experimental conditions and methodology are summarized below. Thermal properties such as melting point, crystallization temperature, and glass transition temperature were determined using differential scanning calorimetry (DSC) with a Seiko Instruments differential scanning calorimeter (DSC6220) under a N2 atmosphere. The samples were hermetically sealed in aluminum pans under an Ar atmosphere in a dry glovebox. The samples were then heated to 150 °C, cooled to −150 °C, and then heated again to 150 °C at heating and cooling rates of 10 °C min−1. The DSC data were recorded during the reheating scans. Thermogravimetric measurements were conducted using a Seiko Instruments thermogravimetry/ differential thermal analyzer (TG-DTA 6200) by heating from room temperature to 550 °C at a rate of 10 °C min−1 under a N2 atmosphere in open aluminum pans. The temperature at which mass loss began was recorded as the decomposition temperature (Td). Cyclic voltammetry (CV) measurements were carried out for the binary PIL mixtures at different compositions using a standard three-electrode cell under dry N2, H2, or O2 gas bubbling atmospheres. Prior to conducting the experiments, the working electrode (WE) was treated overnight with a mixture of concentrated sulfuric and nitric acid to remove any impurities. The reversible hydrogen electrode (RHE) was used as a reference electrode (RE) and was prepared by placing the electrolyte in a glass tube, into which a platinum wire was inserted and hydrogen gas was bubbled. The RHE was kept close to the WE through a Luggin capillary. To eliminate junction potentials, the same PILs were used as test and reference electrolytes. A platinized Pt (platinum black) wire kept under a H2 atmosphere was used as the counter electrode (CE), while a Pt wire was used as the WE. The surface area of the WE was estimated from the hydrogen desorption peak of the cyclic voltammogram measured in a 1 M aqueous H2SO4 solution at 30 °C, with potential scanned at a rate of 0.01 V s−1. Potential difference between the WE and RE was considered as OCP with the WE and CE under O2 and H2 gas atmospheres, respectively. An FT-NMR spectrometer (Bruker AV-500) at 500 MHz was used to record 1H NMR spectra. Chemical shifts for the N−H proton were determined using a double tube (inner: PIL, outer: CDCl3 with TMS, Shigemi, Tokyo). FT-IR spectra were recorded on an Avatar 360 FT-IR spectrometer in the 1000− 4000 cm−1 range with a resolution of 1 cm−1 using CaF2 cells. Samples for IR spectra were prepared inside an inert atmosphere glovebox. A thermoregulated density/specific gravity meter DA-100 (Kyoto Electronics Manufacturing Co. Ltd.) was used to measure the density of the PIL mixtures in the temperature range of 15−40 °C. The viscosity was measured using a rheometer (Physica MCR 301, Anton Paar) under dry air conditions at controlled temperatures in the range of 30−150

Figure 1. Chemical structures of two acids and an amine utilized in this study. Data in parentheses indicate pKa values in aqueous solutions.

studied their electrochemical behavior as well as transport properties, and compared and contrasted various aspects of the binary mixtures with the constituent pure PILs, in order to gain a deep insight into the correlation between electrochemical activity and proton transport.

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EXPERIMENTAL SECTION Chemicals. For the preparation of PILs, diethylmethylamine (dema) (98%, TCI), H2SO4 (98%, Wako Pure Chemicals) and (CF3SO2)2NH (99%) (Kanto Chemical) were used as received without further purification. Preparation of Binary PIL Mixtures. PILs were prepared by a simple neutralization reaction of dema with two acids, 27632

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Figure 2. Cyclic voltammograms of [dema]HSO4, [dema][NTf2], and their binary mixtures: (a) [dema]HSO4 (0 wt % [dema][NTf2]), (b) 28 wt % [dema][NTf2], (c) 56 wt % [dema][NTf2], (d) 78 wt % [dema][NTf2], (e) 91 wt % [dema][NTf2], and (f) 100 wt % [dema][NTf2] using a Pt wire working electrode under H2, N2 (or Ar), and O2 gas bubbling atmospheres at 120 °C. Scan rate is 0.01 V s−1.

°C. The ionic conductivity was measured using a dip cell probe consisting of two platinum wires sheathed in glass. Conductivities were determined by measuring the complex impedance spectra in the frequency range of 10 MHz to 0.01 Hz using a potentiostat (Autolab, PGSTAT30). The temperature was controlled at 20 °C intervals in the range of 30−150 °C using a constant temperature oven (Yamato Scientific DKN 611), and the samples were thermally equilibrated at each temperature for at least 1 h prior to the measurements. The PGSE-NMR measurements were carried out using a JEOL JNM-AL 400 spectrometer with a 9.4 T narrow bore superconducting magnet, equipped with a JEOL pulse field gradient probe and a current amplifier. The self-diffusion coefficients were measured using a simple Hahn spin−echo sequence, incorporating a sine gradient pulse, Δ, set at 30−50 ms, with varying durations of the field gradient, δ.5 The samples were inserted into a 5 mm (outer diameter) NMR microtube (BMS-005J, Shigemi, Tokyo) under an Ar atmosphere, and the ionic self-diffusion coefficients of the PIL mixtures were

measured using 1H (399.7 MHz) and 19F (376.1 MHz) nuclei at 30 °C.

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RESULTS AND DISCUSSION Electrochemical Properties. Pure [dema]HSO4 and [dema][NTf2] exhibit very poor electrochemical activities (Figure 2a,f), which is apparent from their low OCP values during oxygen reduction reaction. Under identical conditions, the OCP values were 0.90 V vs RHE and 0.77 V vs RHE for [dema]HSO4 and [dema][NTf2], respectively, which is in agreement with our previous reports.22,23 In our recent study, it was clarified that PILs with ΔpKa less than 15 is not thermally stable at 130 °C.4 The thermal stability of [dema]HSO4 was also checked by measuring isothermal gravimetric (ITG) analysis at 130 °C (Figure S1, Supporting Information). Analysis of the ITG results inferred that a little amount (2%) of weight loss was observed for 2 h under a N2 atmosphere, possibly due to existence of neutral species including dema because of incomplete proton transfer and/or shifting of the equilibrium from the ionic to nonionic species at that 27633

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pure PILs are shown in Figure 3. The N−H stretching frequency peaks in the FT-IR spectra for pure [dema][NTf2]

temperature. Influence of neutral dema on the electrochemical properties (OCP) of PILs was also confirmed by chronopotentiometric OCP measurements, and a decrease in the OCP could be marked upon deliberate addition of dema.22 Therefore, the low OCP value exhibited by [dema]HSO4 is possibly due to the existence of neutral species caused by incomplete proton transfer, since the ΔpKa is 13.5. [dema][NTf2] also exhibits the very low OCP value despite very promising bulk properties.5 In this case, we expect that the strong N−H bond affects its electrochemical activity, as predicted for PILs with high ΔpKa.22 Figure 2 also shows electrochemical polarization curves for binary mixtures with various concentrations of [dema][NTf2] under H2, N2 and O2 gas atmospheres at 120 °C. Similar to other PIL systems, binary systems also show different current densities and polarities under different gas atmospheres.24 Therefore, it is assumed that hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) can be expressed as shown in Scheme 1. Scheme 1. Probable Electrochemical Reactions for PILs

Figure 3. FT-IR spectra for binary PIL ([dema]HSO4 and [dema][NTf2]) mixtures at room temperature. The contents of [dema][NTf2] are 100, 91, 78, 56, 28, and 0 wt % from top to bottom.

and [dema]HSO4 are located at 3159 and 3037 cm−1, respectively at room temperature, which agrees well with previous reports.22,24 The lower N−H stretching frequency for [dema]HSO4 compared to [dema][NTf2] indicates that the HSO4− anion forms stronger hydrogen bonds with [dema]+ cation than [NTf2]− anion does, through the N−H bond, owing to high anionic basicity. Upon close examination of the FT-IR spectra, two types of cations differing in N−H bond strength can be distinguished in the time scale of the IR measurements, although the cation in both PILs is common. The minor band is, however, hindered by the major one at some compositions. For instance, 28 wt % [dema][NTf2] shows the N−H stretching frequency at 3037 cm−1, whereas 91 wt % [dema][NTf2] exhibits a major band at 3159 cm−1 for the N−H proton. To obtain information about the chemical environment of the N−H bonds in the binary mixtures, 1H NMR measurements were also carried out. Figure S1, Supporting Information shows 1H NMR spectra for the binary PIL mixtures at 30 °C. The 1 H chemical shift (δ) for the N−H proton of [dema][NTf2] and [dema]HSO4 is located at 6.72 and 8.40 ppm, respectively. Variation of the δ values for the N−H proton in the cation as well as the O−H proton of HSO4− anion, as a function of composition, is summarized in Figure 4. The δ values of the N−H proton shift upfield with increasing [dema][NTf2] content in the mixtures, while the δ values for C−H proton remain constant for all composition ranges examined in this study (Figure S2, Supporting Information). Note that the N−H proton in the mixtures gives only one signal, although there are two anions that interact differently with the N−H proton, and two N−H bands appear in the FTIR spectra. This is due to the difference in time scales of the measurements; the IR time scale distinguishes between the two N−H bonds, whereas the NMR time scale cannot distinguish between them and averages the chemical shift of the N−H protons. The δ for the O−H proton also exhibits an upfield

Certain binary mixtures exhibit high electrochemical activities, evident from OCP values as high as 1.03 V vs RHE, as recorded for 56 wt % [dema][NTf2] (Figure 2c). Such values are similar to those measured for [dema][TfO],23 which is the most promising PIL ever reported. Interestingly, the OCP value measured for 56 wt % [dema][NTf2] is even higher than that exhibited by pure [dema]HSO4 (0.90 V vs RHE) or pure [dema][NTf2] (0.77 V vs RHE) under similar conditions. High electrochemical activity is also observed for 28 wt % [dema][NTf2] (Figure 2b) and 78 wt % [dema][NTf2] (Figure 2d). In the binary mixtures containing [dema]HSO4 as major component (Figure 2a,b), oxidative current was observed at high potentials (>0.8 V vs RHE). A similar observation has been reported for aqueous acid solution, and the observed current can be explained by oxidation of Pt surface in the presence of water, which hinders the fuel cell reaction.29 However, the exact reason for the oxidation current in the present PIL mixtures is still not clear. The HOR curves obtained for 91 wt % [dema][NTf2] (Figure 2e) are similar to those obtained for pure [dema][NTf 2 ] under similar conditions, and its HOR activity is low. It is surprising to have high electrochemical activities in binary mixtures, while the pure components show poor activity. While high OCP values reported so far in the literature for PILs are simply based on [TfO] and [NfO] anions,23,25 this is the first observation of remarkably high OCP values for binary mixtures of PILs based on [NTf2]− and HSO4− anions. Furthermore, electrochemical activities vary with compositions and exhibit a maximum at 56 wt % [dema][NTf2] content. In order to comprehend these high activities, molecular characterization was performed using FT-IR and 1H NMR spectroscopic measurements in addition to bulk transport property measurements. Finally, factors responsible for high electrochemical activities for binary mixtures are discussed (vide infra). FT-IR and NMR Analyses. FT-IR spectra comparing N−H stretching vibration frequency of the binary systems with that of 27634

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the presence of the [dema]+ cation with two different N−H bond strengths owing to the presence of two anions. DSC thermograms for binary systems and pure PILs are shown in Figure S3, Supporting Information, and thermal properties obtained from the TGA and DSC measurements are summarized in Table S1, Supporting Information. [dema][NTf2] exhibits a single melting point (Tm) at 25.0 °C, whereas [dema]HSO4 shows only a glass transition temperature (Tg) at −78.4 °C, perhaps due to its high viscosity (vide infra). For the binary mixtures, Tm, Tg, as well as the crystallization temperature (T c ) are observed at some intermediate compositions. It is interesting to note that both the Tm and the Tg for binary mixtures are lower than the corresponding values for the pure components. The mixing of [dema]HSO4 and [dema][NTf2] brings about a plasticizing effect to the counterparts. Transport Properties. Transport properties such as viscosity (η), ionic conductivity (σ), and molar conductivity (Λimp) as well as molecular characteristics such as density (ρ) and molar concentration (M), for binary mixtures of [dema]HSO4 and [dema][NTf2] and their pure components are summarized in Table 1. The dependence of these properties on

Figure 4. Variation of 1H chemical shift for N−H proton and HSO4 proton with [dema][NTf2] content in binary PIL ([dema]HSO4 and [dema][NTf2]) mixtures.

shift with increasing [dema][NTf2] content. However, the shift is negligible when compared to that of the N−H proton. The upfield shift for the N−H proton of [dema]+ cation can be attributed to weaker hydrogen bonding interaction with [NTf2]− anion than with HSO4− anion. The N−H bond is strengthened with increasing [dema][NTf2] content, due to weak anionic basicity as well as structural flexibility of the [NTf2]− anion.24 In other words, the N−H bond length becomes shorter as the [dema][NTf2] content is increased. It is worth mentioning again that the FT-IR spectra display two distinct N−H bands for the same cation due to the existence of two anions, whereas the NMR spectra show only a single peak for the N−H proton for all compositions. If anion/proton exchange occurred around the cation at a much higher time scale of the NMR measurements, an averaged chemical shift for the N−H proton would be observed for the binary systems. Thermal Properties. TGA curves for the binary mixtures and the pure PILs under a N2 atmosphere are shown in Figure 5. The TGA curves indicate that the pure PILs, [dema]HSO4

Table 1. Density, Molar Concentration, Viscosity, Ionic Conductivity, and Molar Conductivity for Binary PILs at 30 °Ca sample

ρ/g cm−3

M30/10−3 mol cm−3

η/m Pas

[dema]HSO4 l28 wt % [dema] [NTf2] 56 wt % [dema] [NTf2] 78 wt % [dema] [NTf2] [dema] [NTf2]

1.235 1.287

6.66 5.44

919.0 618.0

1.10 1.68

0.17 0.28

1.337

4.65

250.0

2.62

0.50

1.385

4.22

110.0

3.93

0.85

1.453

3.95

40.2

7.40

1.88

Λimp/S σ/mS cm−1 cm2 mol−1

ρ = density, M30 = molar concentration, η = viscosity, σ = ionic conductivity, and Λimp = molar conductivity.

a

temperature is shown in Supporting Information (Figures S4, S5, and S6 for ρ, η, and σ, respectively). The η and M values of [dema]HSO4 are comparatively high, due to the small anionic size and hydrogen bonding interactions not only between the cation and the anion but also between the anions. The η and M values decrease significantly with an increase in [dema][NTf2] content. In the case of [dema]HSO4, the system may contain dimeric anions via hydrogen bonding between two HSO4−anions.30 In fact, the η of HSO4 based PILs was found to decrease upon addition of water due to breaking of the hydrogen bond network in the system.28 A sharp decrease in the η of [dema]HSO4 with the addition of [dema][NTf2] is presumably due to the breaking of the dimeric anion. In other words, the interanionic interactions are suppressed upon the addition of [dema][NTf2]. It would be interesting to understand how the hydrogen bonded HSO4− anions take part in proton transport through the Grotthuss mechanism, and this will be discussed in the next section. Ionicity and Proton Transport Mechanism. Estimation of ionicity is crucial, since all of the unique properties of ILs are due to their ionic nature.2 However, understanding the ionicity

Figure 5. TGA curves for [dema]HSO4 and [dema][NTf2] binary PIL mixtures.

and [dema][NTf2], exhibit a single-step weight loss, whereas the binary PIL mixtures exhibit a two-step weight loss corresponding to their component PILs. [dema][NTf2] exhibits a higher decomposition temperature (Td) compared to [dema]HSO4, due to its high ΔpKa.4 The Td of the binary PIL mixtures correlates well with the N−H bond strength since thermal decomposition can be caused by a shift in equilibrium from salt to acid/base in the case of PILs.3,4 The two-step weight loss for the binary mixtures can also be an indication of 27635

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mole fraction of X ions. Here, Ndema is unity, since the [dema]+ cation is common for both PILs. DN−H and D HSO4 may include the contribution of proton hopping (transfer diffusion) in addition to hydrodynamic diffusion, because the N−H proton in [dema]+ and HSO4− proton are exchangeable. Figure 7

of PILs is not as straightforward as that of their aprotic analogues (AILs). In particular, in the case of PILs, free acid and base may exist in the system due to incomplete proton transfer, in addition to the possibility for formation of ionic aggregates.4,27 A plot of logΛimp (Λimp: molar conductivity) vs log(1/η) using the Walden rule has been used to roughly estimate ionicity, with data for 1 M aqueous KCl solution used as reference.2,31 An “ideal” line representing the properties of an ideal electrolyte has been drawn through data points for 1 M KCl and has a slope of unity.2,31 The deviation of Λimp from this ideal line (Λimp/Λideal) at a certain log(1/η) can be rough estimate of ionicity, which is utilized to classify PILs into superionic, good, poor, or nonionic categories.32 The validity of using aqueous KCl solution as reference for Walden plots, disregarding the hydrodynamic radii of various ions, is yet to be clarified.33 Figure 6 shows Walden plots for the binary PIL mixtures. While some of the plots for [dema]HSO4 and the binary

Figure 7. Variation of molar conductivity determined by conductometry (Λimp) and PGSE-NMR (ΛNMR), and their ratio (ionicity: Λimp/ΛNMR) as a function of [dema][NTf2] content at 30 °C in binary PIL ([dema]HSO4 and [dema][NTf2]) mixtures.

shows variation of Λimp and ΛNMR, and ionicity (Λimp/ΛNMR) as a function of [dema][NTf2] content in the binary mixtures. The ΛNMR values, including the contribution of proton hopping, was larger than the Λimp values, also including the contribution of proton hopping if any. Therefore, the ionicity values (Λimp/ΛNMR) were lower than unity for all the compositions, which contrasts with the behavior obtained by the Walden plots (Figure S7, Supporting Information). Since the ionicity determined from Λimp/Λideal is just a rough estimate and the physicochemical meaning is ambiguous, the ionicity determined from Λimp/ΛNMR is focused here. The Λimp/ΛNMR values less than unity indicates that all of the diffusive species cannot contribute to the ionic conduction, as widely seen in aprotic ILs;34,35 this is not surprising because ion pair interaction energy of protic ILs is much higher than that of aprotic ILs having isomer structures (vide infra).36 Figure 8 shows the ratio of diffusivity of C−H and N−H protons in [dema] cation (DN−H/DC−H). The difference between DC−H and DN−H may provide evidence for proton hopping,37 since the N−H proton is exchangeable. On the other hand, C−H proton can contribute to diffusion only through the vehicle mechanism since they are not exchangeable.7 The ratios of DN−H to DC−H shown in Figure 8 clearly differentiate the two diffusion coefficients for the same cation. N−H protons exhibit higher diffusion coefficients than C−H protons, and the difference in diffusion coefficients reaches a maximum at 28 wt % [dema][NTf2]. Here, it should be noted that proton hopping cannot contribute to proton conduction when back transfer occurs.37 However, note that the behavior observed in Figure 8 is akin to that observed in Figure S7, Supporting Information. The maximum seen in Figure 8 appears as a small hump in the Λimp/ΛNMR values in Figure 7. The ionicity in Figures 7 and S7 was estimated using molar conductivity of the mixtures, and all the existing ions in the

Figure 6. Walden plots for binary PIL ([dema]HSO4 and [dema][NTf2]) mixtures.

mixtures lie above the ideal 1 M KCl line, those for [dema][NTf2] are located beneath the ideal line. According to the classification by Angell et al.,32 [dema]HSO4 as well as the binary mixtures are super-PILs, whereas [dema][NTf2] is a good PIL. The ionicity values determined from the Walden plots for the binary mixtures as a function of [dema][NTf2] content at 30 °C are plotted in Figure S7, Supporting Information. The ionicity of [dema]HSO4 is higher than unity, increases with an increase in [dema][NTf2] content, and passes through a maximum at 28 wt % [dema][NTf2]. The ionicity becomes less than unity for pure [dema][NTf2]. Higher ionicity for the binary mixtures than unity might be attributed to the partial contribution of transport of proton via the Grotthuss mechanism. In order to discuss ionicity of the binary mixtures more quantitatively, we measured self-diffusion coefficients of C−H (DC−H) and N−H (DN−H) protons of [dema]+ cation, C−F (DC−F) fluorine of [NTf2]− anion, and HSO4− (DHSO4) proton individually, using the PGSE-NMR method (Figure S8, Supporting Information). The molar conductivity (ΛNMR) of the binary mixtures can be calculated from the self-diffusion coefficients and the Nernst−Einstein equation. ΛNMR = F 2(NdemaDN − H + NHSO4DHSO4 + NNTf 2DNTf 2)/RT

where F is the Faraday constant, R is the gas constant, T is the absolute temperature, DX (X = NH of [dema]+, HSO4−, or [NTf2]−) is the self-diffusion coefficient of X, and NX is the 27636

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Figure 9. Variation of OCP values and N−H proton chemical shift as a function of [dema][NTf2] content in binary PIL ([dema]HSO4 and [dema][NTf2]) mixtures.

Figure 8. Ratios of diffusion coefficients (D) of N−H to C−H protons in [dema] cation at 30 °C as a function of [dema][NTf2] content in binary PIL ([dema]HSO4 and [dema][NTf2]) mixtures.

to 28 wt % [dema][NTf2], beyond which it decreases to reach the value of pure [dema][NTf2] with further addition. Recently, we calculated the intermolecular interaction energies of PILs by ab initio method.36 The calculations show that the interactions between the ion pairs of PILs are definitely stronger than those between ion pairs in structurally similar AILs, due to the presence of hydrogen bond in the PILs. The anion in contact with the N−H bond is the most stable structure. The interaction energy for a given cation increases with a decrease in acidity of the conjugated acids of the anions because of increase in the hydrogen bonding interactions. Conversely, the N−H bond strength is weakened with an increase in the hydrogen bonding interactions. The 1H NMR chemical shift of the N−H proton can be used as a good metric to understand the hydrogen bonding interactions.24 For example, the upfield shift indicates weakening of the hydrogen bonds. The 1H NMR of the N−H proton of the present binary mixtures gives only one averaged chemical shift due to fast anion/proton exchange reactions. A decrease in the OCP values with an increase in [dema][NTf2] content can be ascribed to the strong N−H bond (weakening of hydrogen bond) with increase in [dema][NTf2] content.22 Notably, the hydrogen donating reaction (i.e., ORR) is governed by the average N−H bond strength. On the contrary, the decrease in OCP values for the binary mixtures with low [dema][NTf2] content may be ascribed to incomplete proton transfer and remaining trace amounts of neutral species, which can cause electrode surface poisoning as well as the low proton activity (vide supra).22

mixtures ([dema]+, [NTf2]−, HSO4−) can contribute to ionic conduction. The ionicity maxima seen in Figures 7 and S7 could not be reproduced, when considering the composition dependency of DC−F and DHSO4. It is implied that the disparity between DC−H and DN−H can be a reason for the ionicity maximum and indicates the presence of proton hopping. A plausible explanation of the disparity is the contribution of proton hopping between [dema]+ and the free amine, since a trace amount of free amine may exist in [dema]HSO4 due to the relatively low ΔpKa value (vide supra, Figure S1, Supporting Information). Figure S9, Supporting Information shows Walden plots of different PILs with a common [dema] cation. It is interesting to note that only PILs with HSO4− and H2PO4− anions exhibit Walden plots above the KCl ideal line. On the contrary, Walden plots of [dema][TFA] locate far below the KCl ideal line. [dema][FTA] has ΔpKa of 11.0 that is lower than that of [dema]HSO4 (ΔpKa = 13.5) and therefore may contain [dema] cation and the free amine, which may cause proton hopping. Therefore, we may need to consider the importance of HSO4− anion in proton transport by the Grotthuss mechanism. The HSO4− anion has both proton donating and proton accepting sites in the same ion and can form strong hydrogen-bonds, which allow proton transport via the Grotthuss mechanism.38 Angell et al. also found that delocalized proton hopping transport occurred in the case of a dihydrogen phosphate PIL, which was observed in Walden plots.39 This anion has a similar nature as the HSO4− anion. These experimental results may be understood if we presume that the Grotthuss mechanism of proton transport is caused not only by proton hopping between the [dema] cation and the free amine but also that between the HSO4−anions and the [dema]+ cation and the HSO4−anion. It should also be noted that the Λimp/ΛNMR values lower than unity (Figure 7) do not necessarily mean the absence of proton hopping in protic ILs. Correlation between Electrochemical Activity and Structure. The OCP values of the binary PIL mixtures are higher than those of their pure components (vide supra). The OCP measured is the potential difference between the WE and the RE when the WE and the CE are under O2 and H2 gas atmosphere, respectively. Figure 9 shows changes in the OCP value and the N−H proton chemical shift as a function of [dema][NTf2] content. The OCP values increase with an increase in [dema][NTf2] content, until they reach a maximum at intermediate compositions, beyond which they begin to decrease. The N−H proton chemical shift slightly increases up

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CONCLUSIONS Binary PIL mixtures consisting of [dema][NTf2] and [dema]HSO4 exhibit very promising electrochemical activities compared to the constituent pure PILs and are suitable for prospective use as fuel cell electrolytes. Appropriate N−H bond strength achieved through anion/proton exchange reactions in addition to the absence of nonionic species, which could cause electrode poisoning, leads to high electrochemical activities for the binary mixtures. PGSE-NMR and Walden plot results reveal that the proton is transported not only by hydrodynamic diffusion but also by proton hopping (Grotthuss mechanism), possibly owing to the formation of strong hydrogen bonding networks between the protonated [dema]+ cation and HSO4− anion as well as between HSO4− anions. The binary mixtures also exhibit a higher fluidity compared to pure [dema]HSO4, which have high viscosity and low electrochemical activity. While the correlation between high electrochemical activity and enhanced proton transport by the Grotthuss mechanism is a 27637

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(7) Noda, A.; Susan, M. A. B. H.; Mitsushima, S.; Hayamizu, K.; Watanabe, M. Brønsted Acid−Base Ionic Liquids as ProtonConducting Nonaqueous Electrolytes. J. Phys. Chem. B 2003, 107, 4024−4033. (8) Belieres, J.-P.; Gervasio, D.; Angell, C. A. Binary Inorganic Salt Mixtures as High Conductivity Liquid Electrolytes for >100 °C Fuel Cells. Chem. Commun. 2006, 4799−4801. (9) Ke, C.; Li, J.; Li, X.; Shao, Z.; Yi, B. Protic Ionic Liquids: An Alternative Proton-Conducting Electrolyte for High Temperature Proton Exchange Membrane Fuel Cells. RSC Adv. 2012, 2, 8953− 8956. (10) Ueki, T.; Watanabe, M.; Yoshida, R. Belousov−Zhabotinsky Reaction in Protic Ionic Liquids. Angew. Chem., Int. Ed. 2012, 51, 11991−11994. (11) Mayrand-P, L.; Lin, S.; Lazzerini, D.; Rochefort, D. Pyridiniumbased Protic Ionic Liquids as Electrolytes for RuO2 Electrochemical Capacitors. J. Power Sources 2010, 195, 5114−5121. (12) Greaves, T. L.; Mudie, S. T. Drummond, Effect of Protic Ionic Liquids (PILs) on The Formation of Non-ionic Dodecyl Poly(ethylene oxide) Surfactant Self-assembly Structures and The Effect of These Surfactants on The Nanostructure of PILs. C. J. Phys. Chem. Chem. Phys. 2011, 13, 20441−20452. (13) Stoimenovski, J.; Dean, P. M.; Izgorodina, E. I.; MacFarlane, D. R. Protic Pharmaceutical Ionic Liquids and Solids: Aspects of Protonics. Faraday Discuss. 2012, 154, 335−352. (14) Byrne, N.; Wang, L. M.; Belieres, J.-P.; Angell, C. A. Reversible Folding−unfolding, Aggregation Protection, and Multi-year Stabilization, in High Concentration Protein Solutions, Using Ionic Liquids. Chem. Commun. 2007, 2714−2716. (15) Byrne, N.; Rodoni, B.; Constable, F.; Varghes, S.; Davis, J. H., Jr. Enhanced Stabilization of The Tobacco Mosaic Virus Using Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2012, 14, 10119−10121. (16) Belieres, J.-P.; Angell, C. A. Protic Ionic Liquids: Preparation, Characterization, and Proton Free Energy Level Representation. J. Phys. Chem. B 2007, 111, 4926−4937. (17) Fumino, K.; Wulf, A.; Ludwig, R. The Potential Role of Hydrogen Bonding in Aprotic and Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2009, 11, 8790−8794. (18) Fumino, K.; Wulf, A.; Ludwig, R. Hydrogen Bonding in Protic Ionic Liquids: Reminiscent of Water. Angew. Chem., Int. Ed. 2009, 48, 3184−3186. (19) Burrel, G. L.; Burgar, I. M.; Separovic, F.; Dunlop, N. F. Preparation of Protic Ionic Liquids with Minimal Water Content and 15 N NMR Study of Proton Transfer. Phys. Chem. Chem. Phys. 2010, 12, 1571−1577. (20) Turton, D. A.; Sonnleitner, T.; Ortner, A.; Walther, M.; Hefter, G.; Seddon, K. R.; Stana, S.; Plechkova, N. V.; Buchner, R.; Wynne, K. Structure and Dynamics in Protic Ionic Liquids: A Combined Optical Kerr-effect and Dielectric Relaxation Spectroscopy Study. Faraday Discuss. 2012, 154, 145−153. (21) Mirjafari, A.; Pham, L. N.; McCabe, J. R.; Mobarrez, N.; Salter, E. A.; Wierzbicki, A.; West, K. N.; Sykora, R. E.; Davis, J. H., Jr. Building a Bridge Between Aprotic and Protic Ionic Liquids. RSC Adv. 2013, 3, 337−340. (22) Miran, M. S.; Yasuda, T.; Susan, M. A. B. H.; Dokko, K.; Watanabe, M. Electrochemical Properties of Protic Ionic Liquids: Correlation Between Open Circuit Potential for H2/O2 Cells under Non-humidified Conditions and ΔpKa. RSC Adv. 2013, 3, 4141−4144. (23) Nakamoto, H.; Watanabe, M. Brønsted Acid−base Ionic Liquids for Fuel Cell Electrolytes. Chem. Commun. 2007, 2539−2541. (24) Miran, M. S.; Kinoshita, H.; Yasuda, T.; Susan, M. A. B. H.; Watanabe, M. Hydrogen Bonds in Protic Ionic Liquids and Their Correlation with Physicochemical Properties. Chem. Commun. 2011, 47, 12676−12678. (25) Yasuda, T.; Ogawa, A.; Kanno, M.; Mori, K.; Sakakibara, K.; Watanabe, M. Hydrophobic Protic Ionic Liquid for Nonhumidified Intermediate-temperature Fuel Cells. Chem. Lett. 2009, 38, 692−693. (26) Yasuda, T.; Nakamura, S.; Lee, S.-Y.; Watanabe, M. Performance of Nonhumidified Intermediate-temperature Fuel Cells Based on

fascinating issue, the phenomenon is not entirely clear at present. This study would open up a novel route to improve the properties of PILs by mixing suitable PILs for task-specific applications, especially for fuel cell applications.

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ASSOCIATED CONTENT

* Supporting Information S

Figure S1. Time-dependent isothermal TG for [dema]HSO4. Figure S2. 1H NMR spectra for binary PIL. Table S1. Thermal behaviors for binary mixtures of [dema]HSO4 and [dema][NTf2]. Figure S3. DSC thermograms for binary PIL ([dema][NTf2] and [dema]HSO4) mixtures. Figure S4. Variation of density with compositions and temperatures for binary mixtures of [dema][NTf2] and [dema]HSO4. Figure S5. Variation of viscosity with temperature for binary mixtures of [dema][NTf2] and [dema]HSO4. Figure S6. Variation of ionic conductivity with compositions and temperatures for binary mixtures of [dema][NTf2] and [dema]HSO4. Figure S7. Ionicity of binary PIL ([dema]HSO4 and [dema][NTf2]) mixtures determined from Walden plot as a function of composition. Figure S8. Diffusion coefficients of cationic C-H and N-H protons and anionic C-F and HSO4. Figure S9. Walden plots for different PILs having common [dema] cation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +81-45-339-3955. E-mail: mwatanab@ynu. ac.jp. Present Address ⊥

Department of Chemistry, University of Dhaka, Dhaka 1000, Bangladesh. Author Contributions #

M.S.M. and T.Y. equally contributed to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Grants-in-Aid for Scientific Research of #A-23245046 and the Specially Promoted Research on “Iontronics” from MEXT of Japan.

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