Proton Hopping and Diffusion Behavior of Sulfonated Block

Jan 30, 2014 - (31-33) However, an in-depth analysis on the proton transport mechanism ... We observed the same intensity ratio of the characteristic ...
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Proton Hopping and Diffusion Behavior of Sulfonated Block Copolymers Containing Ionic Liquids Sung Yeon Kim,† Joungphil Lee,‡ and Moon Jeong Park*,†,‡ †

Division of Advanced Materials Science and ‡Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang, Korea 790-784 ABSTRACT: We have investigated the dependence of proton hopping and diffusion behavior of nanostructured poly(styrenesulfonate-b-methylbutylene) (PSS-b-PMB) block copolymers containing imidazolium-based ionic liquids (ILs) on the type and the composition of ILs. Pulsed-field gradient spin-echo nuclear magnetic resonance experiments suggest that fast proton hopping is facilitated by higher imidazolium contents in the ILs. On the contrary, the alkyl substituent at the protic position of imidazole results in a 2-fold reduction in the self-diffusion coefficient for proton hopping, becoming comparable to vehicle diffusion in the temperature window examined. The anion of ILs makes a significant impact on the activation barrier for proton conduction, leading to conductivities differing by a few orders of magnitude, depending on the type of anion in the ILs. Notably, a high proton transference number of 0.81 was achieved for the IL-containing PSS-b-PMB copolymers (compared to the low value of 0.52 for neat ILs) by optimizing the composition and the type of ILs. This increase is attributed to specific interactions between the ILs and the polymer matrix, affecting the nature of hydrogen bonds and ionic aggregate structures. This study provides valuable insights into the factors affecting the proton transport efficiency of IL-containing polymers, enabling the design of new polymer electrolytes characterized by both fast proton conduction and high proton transference numbers.

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INTRODUCTION Polymer electrolytes are of critical importance in advanced electrochemical devices. Accordingly, significant efforts have been devoted in recent years to explore new ion-containing polymers for efficient ion transport.1−3 Especially, the chemical4−7 or physical affixation8−10 of ionic liquids (ILs) into polymer matrices has emerged as a promising approach to improve the performance of electrochemical devices by achieving high ion conductivity and good electrochemical stability.11−13 A large number of studies on neat ILs have thus been performed to elucidate key factors affecting the ion transport properties of ILs.14−17 Interest in IL-containing polymers has been further escalated by the development of proton exchange membrane fuel cells (PEMFCs) that operate at elevated temperature and under anhydrous conditions.18 This is attributed to a unique physicochemical property of ILs, characterized by negligible vapor pressure.19,20 In particular, several studies have reported the use of Brönsted acid−base ILs in the high-temperature PEMFCs in order to achieve a high proton transference number.17,21−23 Imidazolium-based ILs have received the most attention in this context owing to the amphoteric nature of imidazole, which ensures high self-dissociation and efficient structural proton diffusion through intermolecular hydrogen bonding.21,24−26 Interestingly, pioneering work by Watanabe et al. demonstrated that proton shuttling could be facilitated in nonstoichiometric ILs containing excess imidazole, leading to enhanced proton conductivity and a high proton transference number.26 © 2014 American Chemical Society

While the benefits of Brönsted acidic ILs as proton solvents are well established, the majority of previous research efforts on ILcontaining polymers have focused on the use of quaternary alkylimidazolium ILs.4−7,9,27−29 A number of systematic investigations on the ion transport properties of polymer electrolytes containing ILs have demonstrated that the length of alkyl groups in the cation and the type of anion are key parameters in determining the ionic conductivity of ILcontaining polymers.6,28−30 Yet, despite these advances, a methodology for achieving substantial and efficient proton transport from such systems remains a significant challenge to be solved. To date, a few studies on the proton transport properties of Brönsted acid−base IL-containing polymers exist, which utilized a common matrix polymer, poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF−HFP).31−33 However, an in-depth analysis on the proton transport mechanism was not discussed in most cases. In this respect, recent work of Segalman et al. is worth noting, which elucidated proton hopping and diffusion behavior of poly(2-vinylpyridine) (P2VP)-based polymers containing imidazolium ILs.34 This work demonstrated that specific interactions between the imidazolium cations and the polymer matrix have a profound effect on the proton transport properties of the IL-containing polymer, attributed to the change in the number of effective proton carriers. Most strikingly, it has been Received: December 9, 2013 Revised: January 22, 2014 Published: January 30, 2014 1099

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[TFSI], and [Im][OTf] were synthesized at 110 °C with cation:anion molar ratios of 4:3, 3:2, and 2:1. Preparation of IL-Containing PSS-b-PMB Membranes. Inhibitor-free anhydrous tetrahydrofuran (THF, ≥99.9%) was used without further purification, and methanol was degassed two times prior to use. In an Ar-filled glovebox with oxygen and moisture concentration below 0.1 ppm, the predetermined amounts of the nonstoichiometric ILs were added into the PSS-b-PMB copolymer, and 5 wt % solutions were prepared using 80/20 vol % THF and methanol mixtures. The solutions were stirred overnight, and freestanding membranes were prepared by solvent casting under an Ar atmosphere for 2 days. The membranes were then exposed to vacuum at 70 °C for 7 days to remove any residual solvents. Concerning the evaporation of excess constituent of nonstoichiometric ILs, the final compositions of the ILs in IL-containing PSS-b-PMB membranes were confirmed by Fourier transform infrared (FT-IR) experiments. We observed the same intensity ratio of the characteristic peaks of C−N (from imidazole) and S−N bonds (from HTFSI) for neat ILs and IL-containing membranes, signaling nonsignificant changes in the IL compositions as a result of solvent casting. Small-Angle X-ray Scattering (SAXS). The IL-containing membranes were laminated into an airtight sample cell, which consists of an aluminum spacer, two Kapton windows, O-rings, and aluminum covers. Synchrotron SAXS measurements on these samples were performed using the 4C SAXS beamline at the Pohang Light Source (PLS). Sample temperature was controlled within ±0.2 °C using a sample stage provided by the PLS. The wavelength (λ) of the incident Xray beam was 0.15 nm (Δλ/λ = 10−4), and a sample-to-detector distance of 2.0 m was used yielding scattering wave vector q (q = 4π sin(θ/2)/λ, where θ is the scattering angle) in the range 0.03−2.4 nm−1. The twodimensional scattering data were averaged azimuthally to obtain intensity versus q. Transmission Electron Microscopy (TEM). The neat and ILcontaining PSS-b-PMB block copolymers were cryo-microtomed at −120 °C to obtain thin sections with thicknesses in the 80−120 nm range using a RMC Boeckeler PT XL ultramicrotome. The electron contrast in the samples was enhanced by exposure to ruthenium tetroxide (RuO4) vapor for 30 min. Imaging of stained samples was performed with a Hitach H-800 microscope operating at 80 kV. All data sets were acquired using Digital Micrograph (Gatan Inc.) software. Conductivity Measurements. The conductivities of IL-containing PSS-b-PMB membranes were measured using ac impedance spectroscopy (Model 1260A, Solartron) in a glovebox. The through-plane conductivity was measured using a home-built two-electrode cell with 1.25 cm × 1.25 cm stainless steel blocking electrodes, Kapton spacers, and 1 cm × 1 cm platinum working and counter electrodes. The counter electrode was engraved with 0.8 cm × 0.8 cm hole, and the samples (0.8 cm × 0.8 cm, 200 μm thick) were sandwiched between two platinum plates in the presence of spacers. Fourier Transform Raman Experiments (FT-Raman). Confocal Raman spectra were measured using a WITEC Alpha 300R Raman spectroscope (WITec, Ulm, Germany), equipped with a HeNe laser. The spatial resolution of the spectrometer was 250 nm. Laser excitation power was adjusted below 3 mW to reduce potential thermal damage caused by the laser source. For FT-Raman experiments, the ILcontaining PSS-b-PMB copolymer was solvent-cast onto a slide glass in an Ar-filled glovebox. After the removal of residual solvents, the sample was covered with a window glass and taken out of the glove box to exclude the issue of water contamination of hygroscopic samples during the measurements. Self-Diffusion Coefficient Measurements. The IL-containing PSS-b-PMB copolymers were loaded into 4 mm (o.d.) NMR microtubes in an Ar-filled glovebox and were sealed with caps. Self-diffusion coefficients of the samples were measured using 1H and 19F pulsed gradient spin echo (PGSE) NMR (Bruker AVB-300) in a 7 T and a 1.5T superconducting magnet, respectively, with z-axis gradient. Time interval of the field gradient and duration time between two gradient pulses was in the range 1−10 ms and 0.1−0.3s, respectively. The accessible temperature window of the NMR spectrometer was 25−90 °C, which was calibrated using ethylene glycol standard. At each

also demonstrated that the extent of proton hopping relative to vehicle diffusion could be significantly boosted when the polymer matrix contains nanoscale ionic domains, leading to lower activation energy for ion conduction.34 In fact, analogous results have been reported for a number of microphase-separated polymer electrolytes (not limited to IL-containing polymers) over the past decade given that well-defined phase boundaries between ionic domains and nonconducting phases could create a less tortuous pathway for ion transport by confinement.10,35−40 At this juncture, more comprehensive studies on the proton transport properties of self-assembled polymers composed of Brönsted acid−base ILs are needed to support new avenues in the development of new polymer electrolytes with high proton conductivity. Herein, we investigate the proton hopping and diffusion behavior of nanostructured polymer electrolytes containing nonstoichiometric ILs composed of imidazole (or alkylimidazole) and monoprotic acids. Poly(styrenesulfonate-b-methylbutylene) (PSS-b-PMB) block copolymer was employed as a matrix polymer to selectively confined the ILs within the PSS phases. Because the PSS chains of the PSS-b-PMB block copolymer contain chemically tethered sulfonic acid groups along the polymer backbone, they can also act as proton donors for imidazole in tandem with physically blended acids (from ILs). This is in sharp contrast to the range of nonionic polymer matrices that have been commonly employed for the incorporation of ILs.8,9,31−34 The main aim of this study is to propose a platform to improve the proton conductivity of polymer electrolytes containing ILs by clarifying the proton hopping and diffusion mechanisms. We show that the efficiency of long-range proton transport is governed by the extent of hydrogen bonding among imidazolium cations in interacting with the −SO3H groups within PSS phases. Compared to vehicle diffusion, the degree of proton hopping greatly increases with the use of unsubstituted imidazolium in the ILs owing to the formation of strong hydrogen-bonding networks, further confirmed by the high proton transference number achieved. In addition, the use of ILs composed of different anions suggests that the type of anion significantly impacts the activation barrier for proton conduction in ILcontaining PSS-b-PMB copolymers. We believe that our results would offer insights into achieving unprecedented levels of high proton transport, paving the way for the rational design of nextgeneration ion-containing polymers for future electrochemical devices.

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EXPERIMENTAL SECTION

Synthesis of PSS-b-PMB Copolymer. A poly(styrene-b-methylbutylene) (PS-b-PMB) block copolymer (3.8−3.8 kg/mol) with polydispersity index of 1.06 was synthesized by sequential anionic polymerization of styrene and isoprene, followed by selective hydrogenation of the polyisoprene.10 The synthesized PS-b-PMB block copolymer was characterized by combining 1H nuclear magnetic resonance (1H NMR, Bruker AVB-300) spectroscopy and gel permeation chromatography (GPC, Waters Breeze 2 HPLC). The styrene block of PS-b-PMB copolymer was sulfonated using procedures described in ref 10 to yield a poly(styrenesulfonate-b-methylbutylene) (PSS-b-PMB) block copolymer with a sulfonation level (SL) of 32 mol %. The SL value was determined by 1H NMR spectra with acetone-d6. Ionic Liquids (ILs). Imidazole (≥99.5%), 1-methylimidazole (≥99.0%), bis(trifluoromethane)sulfonimide (≥95%), and trifluoromethanesulfonate (≥97%) were purchased from Sigma-Aldrich and used as received. Nine different nonstoichiometric [Im][TFSI], [1-MIm]1100

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calculation using the Gaussian 03 software package.41 Density functional theory based on the B3LYP exchange-correlation functional was applied with 6-31G(d) basis set. As summarized in Table 1, the 0 K binding energies of [1-MIm][TFSI] and the parent [Im][TFSI] are similar, whereas that of the [OTf] anion to [Im] is noticeably higher, compared to those for [TFSI] anions. It is worthwhile to note that the binding energy of IL cations to the −SO3H group of polymer is the highest, implying strong interactions between ILs and polymer matrix. The conductivities of neat ILs increase in the order [Im][TFSI] > [1-MIm][TFSI] > [Im][OTf] at the same IL composition (Table 1). The higher conductivity is observed when the mole fraction of cations in the neat ILs increases, which is in good agreement with the results of Watanabe et al.26 The nonstoichiometric ILs were incorporated into the S37MB54(32) copolymer in cation:anion molar ratios of 4:3, 4:2, and 3:2 with respect to the moles of −SO3H in polymer. We first investigated the effects of the IL composition on the morphology and conductivity of [Im][TFSI]-containing S37MB54(32) copolymers. Figure 2 shows small-angle X-ray scattering (SAXS) profiles of the samples measured at 90 °C, which were unchanged in the temperature window of our interest. The S37MB54(32) copolymer shows lamellar (LAM) morphology in the absence of ILs, as evident from Bragg peaks (↓) at 1q*:2q*:3q*:4q* (q* = 2π/d100 and d100 = 12.6 nm). However, the addition of [Im][TFSI] to the S37MB54(32) copolymer resulted in morphological transformation from LAM to hexagonal cylinder (HEX), as confirmed by Bragg peaks (▼) at 1q*:√3q*:√4q*:√7q*:√9q*:√11q* (q* = 2π/d100), regardless of the IL composition. The LAM-to-HEX transition of the S37MB54(32) copolymer with the addition of 3:2 [Im][TFSI] was accompanied by 27% increase in domain size, as shown in the inset plot. Gradual increases in domain size were further observed upon changes in [Im]:[TFSI] from 3:2 to 4:2 and 4:3. The inset micrographs in Figure 2 show cross-sectional transmission electron microscopy (TEM) images acquired without ILs and with [Im]:[TFSI] = 4:2, confirming the LAM and HEX morphologies, respectively. Given that the PMB cylinders were dispersed in the PSS matrix, the appearance of the HEX structure can be attributed to the selective swelling of the PSS phases upon incorporation of [Im][TFSI], causing changes in the balance of interfacial area and the volume of PSS domains. Effects of the IL Composition on the Proton Hopping and Diffusion Behavior of IL-Containing S37MB54(32) Block Copolymers. To elucidate the effects of IL composition on the ion transport properties of [Im][TFSI]-containing S37MB54(32) copolymers, conductivity measurements were carried out with a use of a home-built two-electrode cell. Detailed illustration of the conductivity cell is provided

temperature, the samples were equilibrated for 1 h before the measurements.

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RESULTS AND DISCUSSION Morphology of IL-Containing PSS-b-PMB Copolymers. Figure 1a shows the molecular structure of a poly-

Figure 1. Molecular structures of (a) poly(styrenesulfonate-bmethylbutylene), S37MB54(32), (b) imidazolium, [Im], (c) 1-methylimidazolium, [1-MIm], (d) bis(trifluoromethane) sulfonimide, [TFSI], and (e) trifluoromethanesulfonate ([OTf]).

(styrenesulfonate-b-methylbutylene) (PSS-b-PMB, 4.8-b-3.8 kg/mol), hereafter referred to as S37MB54(32). The subscripts indicate the degree of polymerization of each block, and the number in parentheses presents the sulfonation level (SL) of the copolymer. The chemical structures of cations and anions of ILs used in this study are shown in Figures 1b−e. Among the many possible factors affecting proton transport properties in ILcontaining S37MB54(32) copolymers, particular emphasis has been placed on the type and composition of ILs. To that end, imidazolium ([Im]) and 1-methylimidazolium ([1-MIm]) were employed as the Brönsted bases in ILs in combination with two types of monoprotic acid counteranions: bis(trifluoromethane)sulfonamide ([TFSI]) and trifluoromethanesulfonate ([OTf]). A set of nonstoichiometric ILs composed of the aforementioned cations and anions, i.e., [Im][TFSI], [Im][OTf], and [1MIm][TFSI], were synthesized with cation:anion molar ratios of 4:3, 3:2, and 2:1. It should be noted that the ILs with nonstoichiometric compositions (particularly, acid-rich ones) are thermally unstable because of the evaporation of excess constituent.22 We have thus concentrated on limited base-rich compositions close to the equivalence ratio that reveal nonsignificant weight loss up to 200 °C from thermogravimetric analysis. The strength of the ionic interaction in the different ILs was deduced from binding energy between ionic pair by ab initio

Table 1. Molecular Properties of a Polymer and Ionic Liquids Used in the Present Study polymer

mol wt (kg/mol)

S37MB54(32)

4.8−3.8

sulfonation level (mol %)

ion exchange capacity (mmol/g)

32 conductivity (mS/cm)a

1.37

morphology

domain size (nm)

LAM

12.6

ionic liquids

4:3

3:2

2:1

binding energyb,c (kJ/mol)

[Im][TFSI] [1-MIm][TFSI] [Im][OTf]

9.3 8.1 4.6

10.8 9.4 8.7

14.6 12.6 11.2

378.4 378.9 417.2

At 70 °C, measured in our lab. bAll calculations were performed using a DFT exchange-correlation functional, B3LYP. c[Im][−SO3H] = 459.3 kJ/ mol; [1-MIm][−SO3H] = 439.4 kJ/mol.

a

1101

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Figure 2. SAXS profiles of [Im][TFSI]-containing S37MB54(32) copolymers, compared to that of neat S37MB54(32), by varying the molar ratio of [Im] and [TFSI]. The arrow (↓) for neat copolymer represents Bragg peaks at q*, 2q*, 3q*, and 4q*, indicative of LAM morphology. The 4:2 [Im][TFSI]-containing S37MB54(32) shows HEX morphology with Bragg peaks at q*, √3q*, √4q*, √7q*, √9q*, and √11q* (identified by ▼). Cross-sectional TEM micrographs in the inset confirm the LAM and HEX structures where PSS phases were stained by RuO4. The domain size of each sample is also plotted in the inset figure.

Figure 3. (a) Through-plane conductivities and (b) normalized conductivities of S37MB54(32) copolymers containing nonstoichiometric [Im][TFSI] with temperature. The molar ratios of [Im] and [TFSI] in the ILs are indicated in the figures. Solid lines in (a) and (b) were obtained by the VTF analysis.

elsewhere.39 As shown in Figure 3a, the conductivity of S37MB54(32) copolymers containing nonstoichiometric [Im][TFSI] was strongly influenced by the IL composition. In the 60−145 °C temperature range, the conductivity decreases in the order [Im]:[TFSI] = 4:3 > 4:2 > 3:2, most likely owing to the glass transition temperature (Tg) of the membranes, which is intimately related to the amount of ionic moieties. The conductivity is fitted using Vogel−Tamman−Fulcher (VTF) equation, as given below, to discover potential barriers to ion conduction in different samples.39 ⎛ −B ⎞ σ = σ∞ exp⎜ ⎟ ⎝ T − T0 ⎠

Table 2. VTF Fitting Parameters for Ionic Conductivities of [Im][TFSI]-Containing S37MB54(32) Copolymers [Im][TFSI]-containing S37MB54(32) copolymers

(1)

cation:anion molar ratios

Tga (K)

B (K)

4:3 4:2 3:2

275 322 341

608 356 279

a

Measured by differential scanning calorimetry (DSC) at a heating/ cooling rate of 5 °C/min.

where σ∞ is the infinite temperature conductivity, B is a fitting parameter related to the potential barriers to ion conduction, and T0 is the temperature at which the polymer relaxation time becomes infinite. The best fits are shown by solid lines in Figure 3a, and the VTF fitting parameters are summarized in Table 2. It is worthwhile to note that potential barriers to ion conduction in [Im][TFSI]-containing S37MB54(32) copolymers are largely deviated as 608, 356, and 279 K for [Im]:[TFSI] = 4:3, 4:2, and 3:2, respectively. The conductivity data given in Figure 3a were normalized by T0 of each membrane (50 K below Tg), and the results are plotted in Figure 3b. This enables us to evaluate the ion transport efficiencies while excluding the effects of segmental motions of the polymer chains on measured ionic conductivity. Interestingly, the normalized conductivity was the lowest for the S37MB54(32) copolymer containing [Im]:[TFSI] = 4:3 and the highest for [Im]:[TFSI] = 4:2. Given that the mole fractions of [Im] for [Im]:[TFSI] = 4:3, 3:2, and 4:2 are 0.57, 0.60, and 0.67,

respectively, the efficacy of ion conduction in [Im][TFSI]containing S37MB54(32) copolymers appears to increase with the content of [Im] in the [Im][TFSI]. 1 H NMR experiments on the [Im][TFSI]-containing S37MB54(32) copolymers were conducted to underpin the ILcomposition-dependent ion transport properties. Figure 4a shows representative NMR spectra measured at 90 °C for S37MB54(32) copolymers containing [Im][TFSI] with different molar ratios of [Im] and [TFSI], which exhibited three separate chemical shifts for the [Im] protons, i.e., one for the N−H protons (δN−H, downfield) and two for the C−H protons (δC−H, upfield). Interestingly, δN−H shifted toward lower magnetic field with the increase in the mole fraction of [Im] in the [Im][TFSI], implying that a larger percentage of [Im] is engaged in hydrogen bonding as the amount of [Im] increases. Simultaneously, δC−H 1102

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and C−H protons (DC−H) were measured by taking advantage of the distinct δN−H and δC−H values. A typical example of the I/I0 plots as a function of γ2g2δ2(Δ−δ/ 3) for the S37MB54(32) copolymer containing [Im]:[TFSI] = 4:2 is shown in Figure 5a, where the slopes of the plots provide the DN−H and DC−H values. Given that DN−H includes self-diffusion from both charged ([Im]) and neutral imidazole, the structural diffusion of the protons (DH+) through hydrogen-bonding networks can be determined from eq 3:43 D N−H = xD H+ + (1 − x)DC−H

(3)

where x is the mole fraction of [Im] in the ILs. In order to obtain x values, we assumed that imidazole is protonated at 1:1 ratio with respect to HTFSI owing to the superacid nature of HTFSI (pKa = −10).44 Protonation from the −SO3H groups, which are chemically tethered to the PSS chains of the S37MB54(32) copolymer, was also taken into account. The degree of proton dissociation (α) attributed to −SO3H groups in the PSS chains was measured by FT-Raman spectroscopy. Figure 5b shows the FT-Raman spectrum of the S37MB54(32) copolymer containing 4 equiv of [Im] to [−SO3H]. Because HTFSI absorption bands overlap with −SO3H bands, HTFSI was excluded from the Raman analysis under the assumption that the presence or absence of [TFSI] anions does not significantly alter the α value of PSS chains. As assigned in Figure 5b, the absorption band at 1127 cm−1 indicates the symmetric stretching of −SO3H, ν(SO2), whereas that at 1035 cm−1 denotes the symmetric stretching of deprotonated one, ν(SO3−).45 From the intensity ratio, I1035/(I1127 + I1035), α was calculated as 49 ± 4% for the [Im]-containing copolymer (under anhydrous conditions). A spectrum of a fully hydrated S37MB54(32) membrane (without [Im]) is also shown in Figure 5b for comparison, which indicates relatively small α value of 31 ± 2%. These results suggest that the deprotonation of the −SO3H groups of polymer increases in the presence of [Im] by virtue of the acid−base equilibrium. Note that α for PSS chains without ILs is reported in range 10−20%.45 Calculated DN−H and DC−H values in the temperature range 30−90 °C, displaying the proton hopping and diffusion behavior of S37MB54(32) copolymers containing [Im]:[TFSI] = 4:3 and 4:2, are plotted in Figure 5c. Whereas the C−H protons diffuse at similar rates for both samples, noticeably large DN−H values are observed for the polymer containing [Im]:[TFSI] = 4:2, particularly at high temperatures. Such difference becomes clear by comparing the DH+ value and the ratio of DH+ and DC−H of each sample. As shown in Figure 5d, a high DH+:DC−H ratio of 2 observed for the S37MB54(32) copolymer containing [Im]: [TFSI] = 4:2, the value was almost temperature-independent. In contrast, the DH+:DC−H ratio for the S37MB54(32) copolymer containing [Im]:[TFSI] = 4:3 was 1.7 at low temperature, which was rapidly reduced to 1.2 upon exposure to high temperature. These results indicate two things: (1) the higher amount of proton hopping than vehicle diffusion at increased [Im] content in the ILs, which is responsible for the high normalized conductivity seen with the [Im]:[TFSI] = 4:2 (Figure 3b); (2) the formation of strong hydrogen-bonding networks by incorporating the excess of [Im] in the ILs, which preserved the long-range proton transfer at elevated temperature. It is worthwhile to note that the DH+:DC−H ratios of the ILcontaining S37MB54(32) copolymers were noticeably higher than those of neat ILs. For example, the DH+:DC−H ratio of neat [Im][TFSI] = 4:2 was as low as 1.3 at 45 °C, which decreased to 1.1 at 90 °C. A similar trend was also seen for neat [Im][TFSI] =

Figure 4. (a) 1H NMR spectra of S37MB54(32) copolymers containing nonstoichiometric [Im][TFSI], measured at 90 °C. The molar ratios of [Im] and [TFSI] in the ILs are indicated in the figures. (b) Temperature-dependent δN−H values of the [Im][TFSI]-containing polymers, where solid lines indicate the linear regressions of the data.

shifted toward higher magnetic field because hydrogen bonding reduces the positive charge on the [Im] ring.26 It is noteworthy that heating of the sample shifted δN−H toward higher magnetic field, as shown in Figure 4b, indicative of weakened hydrogen bonding and/or decrease of the percentage of hydrogen-bonded [Im] molecules at elevated temperatures. While this behavior was consistently observed regardless of the IL composition, the temperature dependence of δN−H was steeper for the 4:3 [Im][TFSI]-embedded S37MB54(32) copolymer, in contrast to the more gradual changes seen with [Im]:[TFSI] = 4:2. This confirms that the nature of the hydrogen-bonding networks depends on the IL composition. This inspired us to further investigate the proton hopping and diffusion behavior of [Im][TFSI]-containing S37MB54(32) copolymers. Self-diffusion coefficients (D) of protons bound to [Im] species were determined from variable-temperature, 1H pulsed-field gradient spin-echo (PGSE) NMR experiments by eq 2:42 2 2 2 I = e−Dγ g δ (Δ− δ /3) I0 (2) where I/I0 is the spin-echo signal attenuation acquired with two equal gradient pulses of duration δ with separation Δ, γ is the magnetogyric ratio, and g is the magnitude of the gradient pulses. Two different D values associated with N−H protons (DN−H) 1103

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Figure 5. (a) I/I0 plots as a function of γ2g2δ2(Δ − δ/3) for N−H protons and C−H protons of the S37MB54(32) copolymer containing [Im]:[TFSI] = 4:2, measured at 60 °C. (b) FT-Raman spectrum of [Im]-containing S37MB54(32) with 4 equimolar [Im] to [−SO3H] in the wavenumber range 900− 1400 cm−1. A spectrum of a fully hydrated S37MB54(32) membrane (in the absence of [Im]) is also shown for comparison. (c) DN−H (filled symbols) and DC−H (open symbols) at different temperatures for the S37MB54(32) copolymers containing 4:3 [Im][TFSI] (squares) and 4:2 [Im][TFSI] (circles). (d) DH+ (filled symbols) and DH+:DC−H (open symbols) with temperature; 4:3 [Im][TFSI] (squares) and 4:2 [Im][TFSI] (circles).

4:3 with approximately 20% lower DH+:DC−H ratios than neat [Im][TFSI] = 4:2. These observations clearly indicate the importance of specific interactions between ILs and polymer matrix for enhancing proton hopping. It should be also noted here that there is a discrepancy between measured (Figure 3a) and analyzed (Figure 5c) transport properties of [Im][TFSI]-containing S37MB54(32) copolymers in that 1.8−2.8 times higher conductivities were obtained with [Im][TFSI] = 4:3 compared to [Im][TFSI] = 4:2. This is likely attributed to a combination of ion aggregation and differences in grain sizes because bulk conductivity measures ion transport over hundreds of micrometers. Effects of the IL Cation and Anion Type on the Proton Hopping and Diffusion Behavior of S37MB54(32) Block Copolymers Containing ILs. We extended our investigation to two additional nonstoichiometric ILs comprising different types of cation and anion: [1-MIm][TFSI] and [Im][OTf]. Using [1-MIm] cation with [TFSI] as the counteranion allows elucidation of the effects of alkyl substitution at the protic sites of [Im] on the proton transport mechanism of IL-containing polymers. A comparative study of the proton transport properties of S37MB54(32) copolymers containing either [Im][TFSI] or [Im][OTf] probes the influence of cation−anion interaction on the proton transport mechanism. SAXS experiments on the S37MB54(32) copolymers containing the aforementioned ILs were first conducted to examine their morphologies. Representative SAXS profiles at cation:anion:− SO3H molar ratio of 4:2:1, measured at 90 °C, are shown in Figure 6. Regardless of the methyl substituent of the [Im], a qualitatively similar HEX morphology was revealed with a concomitant small increase in domain size (Bragg peaks with q

Figure 6. SAXS profiles of the S37MB54(32) copolymers containing [Im][TFSI], [1-MIm][TFSI], and [Im][OTf] at cation:anion:−SO3H molar ratio of 4:2:1. The scattering profiles are vertically offset for clarity. The inverted arrows (↓), the inverted filled triangles (▼), and the inverted open triangles (▽) indicate HEX morphologies with Bragg peaks at q*,√3q*,√4q*,√7q*,√9q*, .... The domain size of each sample is plotted in the inset figure.

ratio of 1:√3:√4:√7:√9, identified by ▼ in Figure 6). Analogous HEX morphology was also determined for the S37MB54(32) copolymer containing [Im][OTf]. However, a substantial reduction in domain size (∼9%, see inset plot) and an 1104

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Figure 7. (a) Conductivities of the IL-containing S37MB54(32) copolymers after normalizing the temperature by T0 values. Solid lines were obtained by the VTF analysis, and the type of ILs is indicated in the figure. (b) DH+ (filled symbols) and DH+:DC−H ratios (open symbols) at different temperatures for the S37MB54(32) copolymers containing [Im][TFSI] (squares) and [1-MIm][TFSI] (circles). (c) Schematic illustrations of the different proton transport mechanisms for the [Im][TFSI]- and [1-MIm][TFSI]-containing S37MB54(32) copolymers. (d) DH+ (filled symbols) and DH+:DC−H ratios (open symbols) for the S37MB54(32) copolymers containing [Im][TFSI], [Im][OTf], and [1-MIm][TFSI], measured at 90 °C. In (a−d), the cation:anion molar ratio was fixed at 4:2.

copolymers containing [Im][TFSI], [1-MIm][TFSI], and [Im][OTf], the conductivities were plotted after normalizing the temperature by T0 values. As can be seen from the figure, the [Im][TFSI]-containing S37MB54(32) copolymer exhibits better ion transport efficiency than the [1-MIm][TFSI]-containing sample. Distinctly steep temperature dependence of the conductivity was evident for the [Im][OTf]-containing S37MB54(32); hence, the overall conductivity was low in the temperature window examined. The activation barrier for ion conduction was analyzed by the VTF equation, eq 1, and the best

improved phase separation with Bragg peaks (▽) at 1q*:√3q*:√4q*:√7q*:√9q*:√11q*:√12q* (q* = 2π/d100 and d100 = 15.0 nm) were apparent. This implies that the difference in ionic interactions within the PSS phases (see the respective binding energies in Table 1) impacts the thermodynamic properties of IL-containing S37MB54(32) copolymers.40 Next, we investigated the ion transport properties of S37MB54(32) copolymers containing different ILs. Figure 7a shows the ionic conductivity of the samples with IL cation:anion = 4:2. Considering the different Tg values of the S37MB54(32) 1105

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fits are shown by solid lines in Figure 7a. As summarized in Table 3, the activation barrier for the [Im][OTf]-containing

slow owing to the high Tg (85 °C); however, the DH+:DC−H ratio decreased in the order [Im][TFSI] > [Im][OTf] > [1MIm][TFSI]. This implies that while the nature of the IL anion is important for high absolute conductivity by influencing ionic cluster formation, the cation exerts stronger influence on the proton hopping and diffusion mechanism of IL-containing S37MB54(32) copolymers. Given that 1H NMR experiments indicate almost identical δN−H values for [Im][TFSI]- and [Im][OTf]-containing S37MB54(32) copolymers at the same temperature (data not shown here), it is inferred that the IL anion has a less impact on the nature of the hydrogen-bonded [Im] species. The efficacy of proton transport in S37MB54(32) copolymers integrated with different ILs was last quantified by analyzing the proton transference number (TH+) using eq 4:47

Table 3. VTF Fitting Parameters for Ionic Conductivities of IL-Containing S37MB54(32) Copolymers by Varying the Type of Cation and Anion in IL IL-containing S37MB54(32) copolymers

a

ionic liquids used

Tga (K)

B (K)

[Im][TFSI] = 4:2 [1-MIm][TFSI] = 4:2 [Im][OTf] = 4:2

322 306 358

356 453 663

Measured by DSC at a heating/cooling rate of 5 °C/min.

S37MB54(32) copolymer was approximately 2 times higher than those for [Im][TFSI] and [1-MIm][TFSI]. Note in passing that change of the IL composition to 4:3 and 3:2 produced very similar tendency as the 4:2-containing samples. This stimulated us to further investigate DN−H, DC−H, and DH+ for each of the above samples according to the procedures described in Figure 5. The direct comparison of the temperature dependence of the DH+ and DH+:DC−H ratio for [Im][TFSI]- and [1-MIm][TFSI]-containing S37MB54(32) copolymers is shown in Figure 7b. Distinctly high DH+ values were recorded for [Im][TFSI]-containing S37MB54(32) copolymer, and in particular, the high DH+:DC−H ratio was preserved up to 90 °C, as opposed to the low values (ca. 1) for [1-MIm][TFSI]. Given that the δN−H values were 0.5−0.8 ppm higher for [Im][TFSI]containing copolymers than the [1-MIm][TFSI]-containing polymers at the same temperature (data not shown here), we infer that a higher number of [Im] molecules than [1-MIm] molecules can participate in the hydrogen-bonding networks. This is intriguing since the deprotonation degree of −SO3H groups was not significantly altered by the alkyl substitution at the protic position of [Im] (α = 51 ± 3%, data not shown here), owing to similar pKa values of [Im] (7.0) and [1-MIm] (7.2).39 In addition, the elimination of one protic site of [Im] would not reduce the number density of protons by half because charged [Im] acts as monoprotic acids. From 1H NMR experiments on [1-MIm][TFSI]-containing S37MB54(32) copolymers, we found that the integral values of N−H protons (charged [1-MIm]) and C−H protons (charged and neutral [1-MIm]) are in good agreement with the predicted values based on the deprotonation degree of −SO3H groups (from FT-Raman) and 100% deprotonation from HTFSI moieties (by assumption). This implies that the number density of protonated species in [Im][TFSI]- and [1-MIm][TFSI]-containing copolymers is not markedly different as long as they contain the same amount of HTFSI. We thus infer that the slow proton transport in [1MIm][TFSI]-containing S37MB54(32) copolymers is owing to the need for rotation of [1-MIm] molecules to make the longrange structural diffusion of protons bound to [1-MIm]. This is considered to be a rate-determining step as the time scale of ∼20 ps (at 393 K) has been estimated for reorientation of [Im] ring.46 Schematic illustrations of the different proton transport mechanisms for the [Im][TFSI]- and [1-MIm][TFSI]-containing S37MB54(32) copolymers are depicted in Figure 7c. The effects of the IL anion on the proton hopping and diffusion behavior of IL-containing S37MB54(32) copolymers were also explored. Figure 7d compares DH+ value and the DH+:DC−H ratio for S37MB54(32) copolymers containing [Im][TFSI], [Im][OTf], and [1-MIm][TFSI] measured at 90 °C. The proton diffusion of the [Im][OTf]-containing polymer was

TH+ =

D H+ D H + + D F−

(4)

where DF− is the self-diffusion coefficient of the anion, as measured by 19F PGSE NMR experiments. Upon examining the [Im][TFSI]- and [1-MIm][TFSI]-containing S37MB54(32) copolymers, as plotted in Figure 8a, we found that the anion diffuses at similar rates regardless of the IL composition and the alkyl substituent in the IL cation. In addition, for all samples, the cation diffuses faster than the anion, presumably owing to the prevalence of anionic aggregates in the low-dielectric polymer matrix. The DF− value for 4:2 [Im][OTf]-containing sample is distinctly low because of the high Tg of the membrane.

Figure 8. (a) DF− values and (b) proton transference numbers of the ILcontaining S37MB54(32) copolymers as a function of temperature. The type and the composition of ILs are indicated in the figures. The proton transference numbers of neat ILs are shown by dotted lines in (b). 1106

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The calculated TH+ values of S37MB54(32) copolymers incorporated with different ILs are plotted in Figure 8b. The TH+ values for the 4:2 [Im][TFSI]-containing S37MB54(32) copolymer were high (0.73−0.81) within the experimental temperature window, whereas those for the 4:3 [Im][TFSI] were relatively low (0.64−0.71). Remarkably, the lowest and virtually temperature-independent TH+ = 0.6 was calculated for the 4:2 [1MIm][TFSI]-containing copolymer, indicative of predominant vehicle diffusion rather than proton hopping. It is noteworthy that the TH+ values of the IL-containing S37MB54(32) copolymers were larger and less sensitive to temperature compared to those of neat ILs, as shown by dotted lines in Figure 8b. For example, TH+ of neat [Im]:[TFSI] = 4:2 was as low as 0.65 at 45 °C, which decreased to 0.52 at 90 °C. While there may be discrepancies in the analyzed and actual TH+ values because NMR cannot discriminate between free protons and imidazolium, these observations clearly indicate the importance of specific interactions between ILs and polymer matrix for efficient proton transport.

System funded by the NRF under the Ministry of Education, Science and Technology.

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CONCLUSIONS We clarified the role of the IL cation and anion in proton hopping and diffusion behavior of Brönsted acid−base IL-containing polymers, aiming to improve proton conductivity. Not only did we explore the proton transport mechanism in a quantitative fashion but also achieved improved proton transport properties. A PSS-b-PMB block copolymer was employed as the matrix material to confine the ILs within well-defined nanoscale PSS domains in order to promote efficient ionic interactions between the IL cations and −SO3H groups of the polymer. We first demonstrated that the extent of proton hopping, compared to vehicle diffusion, increases with the content of [Im] in ILs owing to the formation of strong hydrogen-bonding networks. A comparison between two different ILs, [Im][TFSI] and [1MIm][TFSI], demonstrated that the alkyl substituent at the protic position of [Im] is not desirable for fast proton transport in the IL-containing polymers due to restrained long-range structural diffusion of protons bound to [1-MIm]. We also investigated the effect of cation−anion interactions on proton transport properties of IL-containing PSS-b-PMB copolymers by comparing [Im][TFSI] and [Im][OTf], where a distinctly high activation barrier for ion conduction was measured for [Im][OTf]. Finally, we demonstrated a large increase in the proton transference numbers for IL-containing PSS-b-PMB copolymers at elevated temperature, compared to those of neat ILs, indicating the importance of specific interactions between ILs and polymer matrix in achieving efficient proton transport.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.J.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Midcareer Researcher Program (Project No. 2013-023171) and National Nuclear R&D Program (Project No. 2011-0031931) through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology. We also acknowledge the Global Frontier R&D program on Center for Multiscale Energy 1107

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