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Highly Stable N3-Substituted Imidazolium-Based Alkaline Anion Exchange Membranes: Experimental Studies and Theoretical Calculations Fenglou Gu,† Huilong Dong,‡ Youyong Li,‡ Zhihong Si,† and Feng Yan*,† †

Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, and ‡Jiangsu Key Laboratory of Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P. R. China S Supporting Information *

ABSTRACT: Imidazolium cations with various N3-substituents (including methyl, butyl, heptyl, dodecyl, isopropyl, and diphenylmethyl groups) were synthesized and investigated in terms of their alkaline stability. The effect of the N3-substituent on the alkaline stability was studied by quantitative 1H NMR spectra and density functional theory (GGA-BLYP) calculations. The isopropyl substituted imidazolium cation ([DMIIm]+) with the highest LUMO energy value exhibited the highest alkaline stability in aqueous NaOH. The [DMIIm]+ cation also exhibited higher alkaline stability than that of a quaternary ammonium cation, benzyltrimethyl-ammonium ([BTMA]+), in CD3OD/D2O NaOH solution at elevated temperatures. This observation inspired the preparation of [DMIIm]+-based alkaline anion exchange membranes (AEMs) which showed high alkaline stability in alkaline solution.

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tional group for AEMs.28c The prepared AEMs exhibited excellent base stability when heated to 80 °C in 1 M NaOH over a 25 day period. Furthermore, the protection of the cations by steric hindrance and mesomeric stabilization have been studied and demonstarted to be effective.28a,b,32a,d Holdcroft and co-workers recently prepared sterically crowded benzimidazolium based AEMs.32a,d Crowding around the reactive benzimidazolium C2 position by installation of adjacent bulky groups would hinder nucleophilic attack by OH− anions and thus improve the stability of the hydroxide form. On the basis of a detailed analysis of the 1H NMR spectra, the degradation of imidazolium cations due to ringopening reaction mainly triggered by nucleophilic attack of OH− anions at the C2 position was observed by Elabd et al.21,22,33 Our group has previously reported that the substituent groups at C2 position of imidazolium rings could enhance alkaline stability of imidazolium cations at elevated temperatures.34 Results of this study suggest that imidazolium cations might be suitable for AEM applications. Although recent studies have demonstrated that both polymer backbone and cation chemistry play an important role in influencing the alkaline stability of AEMs,20c,29b,35a,b we believe that once an alkaline stable cationic-group is identified, the proper polymer backbones with high alkaline stability can be rationally designed.

INTRODUCTION The generation of clean, efficient and environmental-friendly energy is one of the major challenges for scientists and engineers.1 Proton exchange membrane fuel cells (PEMFCs) that convert chemical energy from a fuel directly into electrical work, are currently being investigated for a variety of applications because of their high energy-conversion efficiencies, high power density, and low operating temperature.2−4 However, widespread applications of PEMFCs are limited by the high cost of platinum (and its alloys) catalysts and polymer electrolyte membranes (such as Nafion membranes). Therefore, anion exchange membrane fuel cells (AEMFCs), which use nonplatinum metal (such as silver and nickel) electrocatalysts and carbon-free supports have been extensively investigated in recent years.5−8 The anion exchange membrane (AEM) is one of the key components of AEMFCs that transport anions from the cathode to the anode.9−11 Recently, polymers with pendant ammonium cations to facilitate hydroxide ion conduction have been extensively studied.12−20 However, it has been well demonstrated that quaternary ammonium cations are generally sensitive toward β-hydrogen (Hofmann or E2) elimination and direct nucleophilic substitution (SN2) under alkaline conditions.19−26 To circumvent this obstacle and to achieve highly stable alkaline AEMs, cationic groups other than quaternary alkylammonium, including guanidinium,25,26 metal−cation,27 phosphonium,28,29a imidazolium,30,31 and benzimidazolium32 cations have been recently investigated. For example, tetrakis(dialkylamino)phosphonium cation was evaluated as a func© 2013 American Chemical Society

Received: November 12, 2013 Revised: December 16, 2013 Published: December 19, 2013 208

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Synthesis of 1,2-dimethyl-3-Isopropylimidazolium Bromide ([DMIIm][Br]). [DMIIm][Br] was synthesized by stirring a mixture containing 1,2-dimethylimidazole and an equivalent molar amount of 2-bromopropane at room temperature under nitrogen atmosphere. Colorless viscous oil (yield: 73%). 1H NMR (400 MHz, D2O): 7.34− 7.44 (d, 1H), 7.22−7.30 (d, 1H), 4.5−4.64 (m, 1H), 3.64−3.74 (s, 3H), 2.46−2.60 (s, 3H), 1.34−1.48 (m, 6H). Synthesis of 1,2-Dimethyl-3-diphenylmethylimidazolium Bromide ([DMDPMIm][Br]). [DMDPMIm][Br] was synthesized by stirring a mixture containing 1,2-dimethylimidazole and an equivalent molar amount of bromodiphenylmethane at room temperature under nitrogen atmosphere. White solid (yield: 68%). 1H NMR (400 MHz, D2O): 7.37−7.45 (m, 6H), 7.27−7.30 (s, 1H), 7.12−7.20 (m, 4H), 6.92−6.95 (d, 1H), 6.87−6.91 (d, 1H), 3.70−3.76 (s, 3H), 2.43−2.48 (s, 3H). Synthesis of 1-(4-Vinylbenzyl)-2-methylimidazole. 1-(4-Vinylbenzyl)-2-methylimidazole was synthesized by stirring: a mixture containing 4.10 g (0.05 mol) of 2-methylimidazole, 7.63 g (0.05 mol) of 4-vinylbenzyl chloride, and 5.61 g (0.14 mol) NaOH in acetonitrile (40 mL) was stirred at room temperature for 36 h under an argon atmosphere. The solvent was removed under dynamic vacuum, and the crude product was extracted with CH2Cl2 three times. The combined organic phase was washed with distilled water and dried over anhydrous MgSO4, and the solvent was removed under vacuum. The resultant yellow oil was dried in dynamic vacuum at room temperature. 1H NMR (400 MHz, CDCl3): 7.35−7.39 (d, 2H), 6.98− 7.03 (d, 2H), 6.94−6.97 (s, 1H), 6.92−6.85 (s, 1H), 6.67−6.75 (m, 1H), 5.71−5.78 (d, 1H), 5.21−5.31 (d, 1H), 4.99−5.05 (s, 2H), 2.31− 2.34 (s, 3H). Synthesis of 1-(4-Vinylbenzyl)-2-methyl-3-butylimidazolium Bromide ([VMBIm][Br]) Monomer. [VMBIm][Br] was synthesized by stirring a mixture containing 1-(4-vinylbenzyl)-2-methylimidazole and an equivalent molar amount of 1-bromobutane for 24 h at 0 °C. The resultant viscous oil was washed with ethyl ether four times and then dried in dynamic vacuum at room temperature. 1H NMR (400 MHz, D2O): 7.46−7.51 (d, 2H), 7.32−7.39 (d, 2H), 7.19−7.24 (d, 2H), 6.73−6.80 (m, 1H), 5.77−5.87 (d, 1H) 5.30−5.38 (d, 1H), 5.25−5.29 (s, 2H), 4.02−4.12 (m, 2H), 2.49−2.57 (s, 3H), 1.68−1.80 (m, 2H), 1.22−1.32 (m, 2H), 0.82−0.91 (m, 3H). Synthesis of 1-(4-Vinylbenzyl)-2-methyl-3-isopropylimidazolium Bromide ([VMIIm][Br]) Monomer. [VMIIm][Br] was synthesized by stirring a mixture containing 1-(4-vinylbenzyl)-2-methylimidazole and an equivalent molar amount of 2-bromopropane for 24 h at 0 °C. The resultant viscous oil was washed with ethyl ether four times and then dried in dynamic vacuum at room temperature. 1H NMR (400 MHz, D2O): 7.50−7.53 (d, 2H), 7.31−7.37 (d, 2H), 7.16−7.25 (d, 2H), 6.73−6.76 (m, 1H), 5.83−5.88 (d, 1H), 5.36−5.43(d, 1H), 5.13−5.24 (s, 2H), 4.42−4.53 (m, 1H), 2.19−2.26 (s, 3H), 1.33−1.42 (m, 6H). Synthesis of Imidazolium and Quaternary Ammonium Cation-Based Polymers. The cation-based polymers were synthesized via free radical polymerization using azobis(isobutyronitrile) (AIBN) as thermal initiator. For example, poly(1-(4-vinylbenzyl)-2methyl-3-butylimidazolium bromide) ([PVMBIm][Br]) was synthesized by stirring a mixture containing [VMBIm][Br] and 1 wt % of AIBN dissolved in DMSO at 65 °C for 8 h under a nitrogen atmosphere. The polymer product was precipitated twice with acetone and then dried at 60 °C overnight. Poly(1-(4-vinylbenzyl)-2-methyl-3isopropylimidazolium bromide) ([PVMIIm][Br]) and poly(vinybenzyltrimethylammonium chloride) ([PVTMA][Cl]) were synthesized following the same procedure (for details, see Supporting Information). Preparation of Alkaline Anion-Exchange Membranes. Anionexchange membranes were prepared via photocross-linking of cationbased monomers with styrene and acrylonitrile using divinylbenzene as cross-inking agent. A homogeneous solution of styrene/acrylonitrile (1:8 weight ratio), [VMBIm][Br] (27.7 wt %), divinylbenzene (4 wt % based on the weight of monomer) and 2 wt % of benzoin ethyl ether (as a photoinitiator) was cast onto a glass mold and photocross-linked by irradiation with UV light of 250 nm wavelength for 40 min at room temperature. [VMIIm]+- and [VTMA]+-based polymeric membranes

However, it should be noted that the synthesis of C2substituted imidazolium cations is more difficult if compared with that of N3-substitutions because it needs the reaction procedure of the carbanion and halogenoalkane in alkaline condition, in which a number of side reactions could not be avoided and thus considerably reduced the yield of the desired products.35c,d Conversely, N3-substituted imidazolium cations can be synthesized in one-step by the simple nucleophilic substitution reaction between the lone pair electron located at nitrogen atom with halogenoalkane. On the basis of the results reported before, the alkaline stability of C2-substituted (with methyl group) imidazolium cations with various substituted groups on N3 position and their corresponding cationic polymers were studied. The influence of N3-substituents on alkaline stability of imidazolium cations and imidazolium-based alkaline AEMs was systematically studied by 1H nuclear magnetic resonance (NMR) analysis and density functional theory (GGA-BLYP) calculations. The differences between quaternary ammonium- and N3-substituted imidazolium-based AEMs with the same class of polymer backbone and identical ion-exchange capacities were investigated with respect to their ionic conductivity and alkaline stability.

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EXPERIMENTAL SECTION

Materials. Styrene, acrylonitrile, 2-methylimidazole, 1,2-dimethylimidazole, iodomethane, benzyltrimethylammonium chloride, (vinylbenyl)trimethylammonium chloride, divinylbenzene (DVB), 4vinylbenzyl chloride, benzoin ethyl ether, ethyl ether, ethyl acetate, acetonitrile, 2-bromopropane, 1-bromobutane, dichloromethane, 1bromoheptane, 1-bromododecane, bromodiphenylmethane, sodium hydroxide, and hydrochloric acid were used as purchased without further purification. All of the vinyl monomers were made inhibitorfree by passing the liquid through a column filled with basic alumina to remove the inhibitor and then stored at 5 °C before use. Distilled deionized water was used throughout the experiments. Synthesis and Characterization of Imidazolium Salts. Synthesis of 1,2,3-Trimethylimidazolium Iodide ([TMIm][I]). [TMIm][I] was synthesized by stirring a mixture of 1,2-dimethylimidazole with an equivalent molar amount of iodomethane at room temperature under nitrogen atmosphere. White solid (yield: 93%). 1H NMR (400 MHz, D2O): 7.27−7.290 (s, 2H), 3.75−3.77 (s, 6H), 2.57−2.54 (s, 3H). Synthesis of 1,2-Dimethyl-3-butylimidazolium Bromide ([DMBIm][Br]). [DMBIm][Br] was synthesized by stirring a mixture containing 1,2-dimethylimidazole and an equivalent molar amount of 1-bromobutane at room temperature under nitrogen atmosphere. Colorless viscous oil (yield: 88%). 1H NMR (400 MHz, D2O): 7.35− 7.37 (d, 1H), 7.31−7.33 (d, 1H), 4.09−4.15 (t, 2H), 3.76−3.79 (s, 3H), 2.58−2.61 (s, 3H), 1.75−1.85 (m, 2H), 1.30−1.40 (m, 2H), 0.92−0.97 (t, 3H). Synthesis of 1,2-Dimethyl-3-heptylimidazolium Bromide ([DMHIm][Br]). [DMHIm][Br] was synthesized by stirring a mixture containing 1,2-dimethylimidazole and an equivalent molar amount of 1-bromoheptane at room temperature under nitrogen atmosphere. White solid (yield: 76%). 1H NMR (400 MHz, D2O): 7.27−7.30 (d, 1H), 7.23−7.26 (d, 1H), 4.01−4.08 (t, 2H), 3.68−3.72 (s, 3H), 2.50− 2.54 (s, 3H), 1.70−1.80 (m, 2H), 1.16−1.3 (m, 8H) 0.76−0.83 (t, 3H). Synthesis of 1,2-Dimethyl-3-dodecylimidazolium Bromide ([DMDIm][Br]). [DMDIm][Br] was synthesized by stirring a mixture containing 1,2-dimethylimidazole and an equivalent molar amount of 1-bromododecane at room temperature under nitrogen atmosphere. Hazel solid (yield: 90%). 1H NMR (400 MHz, D2O): 7.40−7.42 (d, 1H), 7.43−7.45 (d, 1H), 4.10−4.17 (t, 2H), 3.75−3.80 (s, 3H),2.59− 2.63 (s, 3H), 1.73−1. 83 (m, 2H), 1.14−1.36 (m, 18H), 0.76−0.83 (t, 3H). 209

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Scheme 1. Molecular Structures of Imidazolium Salts Studied in This Work

were also prepared in the same way. The resulting polymeric membranes were immersed in 1 M NaOH solution at 60 °C to convert the membranes from Cl− to OH−. These converted membranes were then immersed in N2-saturated deionized water for 24 h until the pH of residual water was neutral. Characterization. 1H NMR spectra were carried out on a Varian 400 MHz spectrometer. Thermogravimetric analysis (TGA) was performed under nitrogen flow by Universal Analysis 2000 at a heating rate of 10 °C min−1 and samples were heated from 30 to 500 °C. Hydroxide Ion Conductivity. Hydroxide conductivity (σ, S cm−1) of each membrane sample was obtained using the following equation:

σ=

AEMs were soaked in 100 mL of 0.01 M HCl standard solution for 24 h to render the membrane to Cl− form. Then the solutions were backtitrated with a standardized NaOH solution using phenolphthalein as an indicator. The IEC was calculated as follows:

IEC =

where C1 and V1 are the concentration and volume of HCl solution before titration, respectively, C2 and V2 are the concentration and volume of NaOH solution consumed in the titration, m is the mass of the dried membrane. Alkaline Stability Measurements. Alkaline stability of imidazolium and ammonium cations was studied in D2O (or CD3OD/D2O) NaOH solutions. Test solutions were placed in polypropylene jars and heated at 80 °C for various times. Aliquots were sampled and imediately transferred into a standard NMR tube for 1H NMR spectrometry. The alkaline stability of imidazolium and ammonium cation based polymers were determined in the same way. Computational Details and Analysis. Theoretical analysis of N3-substituted imidazolium and quaternary-ammonium cations were performed by the Dmol3 density functional code as implemented in Materials Studio (Version 6.0).37,38 The GGA-BLYP functional39,40a and DNP basis set was used for all calculations. The single molecules were fully optimized with a self-consistent field (SCF) convergence value of 10−6 Ha. The convergence criteria for geometry optimizations used threshold values of 5 × 10−6 Ha for energy, 0.001 Ha/Å for gradient, and 0.005 Å for displacement convergence, respectively. Considering the solution environments, solvent effects were added into the calculations using ε = 78.54 for water and ε = 32.63 for methanol. Frequency analysis is performed on all the optimized single molecules to ensure there is no imaginary frequency. The complete linear synchronous transit/quadratic synchronous transit (LST/QST) method was then used in searching transition state (TS) structures.40b All the transition-state structures exhibit only one imaginary frequency along the reaction coordinate by frequency calculations. The nudgedelastic band available in the following transition state confirmation is carried out to ensure that the TS geometries directly connect the corresponding reactants and products.40c

l RA

where l is the distance (cm) between two electrodes, A is the crosssectional area (cm2) of the membrane. The resistance value (R) was measured over the frequency range from 1 Hz to 1 MHz by four-point probe alternating current (ac) impedance spectroscopy using an electrode system connected with an electrochemical workstation (Zahner IM6 EX). The choice of the four-probe instead of two-probe method for the through palne conductivity measurements is because the effect of contact resistance on the four-probe ionic conductivity is much lower than that on two-probe method, especially in high humidity.36 Prior to the conductivity measurement, all the membrane samples were soaked in N2 saturated deionized water for at least 24 h and rinsed repeatedly to remove free KOH. Conductivity measurements were carried out under fully hydrated conditions in a chamber filled with a N2 saturated deionized water to maintain the relative humidity at 100% during the experiments. All the samples were equilibrated for at least 30 min under a given temperature. Repeated measurements were then taken at that given temperature with 10 min interval until no more change in conductivity was observed. Water Uptake and Swelling Ratio. The membrane samples were dried under vacuum at 80 °C until a constant weight was obtained (Wd). The samples were then immersed in N2 saturated deionized water at room temperature for 24 h. Then the membranes were taken out, wiped with tissue papers to remove the excess water on the surface, and quickly weighed (Ww). The water uptake W was calculated with the following equation: W (%) =

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RESULTS AND DISCUSSION Alkaline Stability of N3-Substituted Imidazolium Cations. Scheme 1 shows chemical structure of the N3substituted imidazolium salts, inculding 1,2,3-trimethylimidazolium iodide ([TMIm][I]), 1,2-dimethyl-3-butylimidazolium bromide ([DMBIm][Br]), 1,2-dimethyl-3-heptylimidazolium bromide ([DMHIm][Br]), 1,2-dimethyl-3-dodecylimidazolium bromide ([DMDIm][Br]), 1,2-dimethyl-3-isopropylimidazolium bromide ([DMIIm][Br]), 1,2-dimethyl-3-diphenylmethylimidazolium bromide ([DMDPMIm][Br]), 1-(4-vinylbenzyl)2-methylimidazolium ([VMIm]), 1-(4-vinylbenzyl)-2-methyl-3butylimidazolium bromide ([VMBIm][Br]), and 1-(4-vinylbenzyl)-2-methyl-3-isopropylimidazolium bromide ([VMIIm][Br]) studied in this work. The chemical structure and purity of

Ww − Wd × 100% Wd

where Wd and Ww are the weight of dry and corresponding waterswollen membranes, respectively. The swelling ratio of the membranes was investigated by linear expansion ratio, determined by the difference between dry and wet dimensions of a membrane sample (3 cm in length and 1 cm in width). The calculation was based on the following equation:

swelling (%) =

X wet − Xdry Xdry

C1V1 − C 2V2 m

× 100%

Here Xwet and Xdry are the lengths of wet and dry samples, respectively. Ion Exchange Capacity (IEC). Ion exchange capacities (IEC) were measured using the conventional back-titration method. The 210

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Scheme 2. Ring-Opening Reaction Mechanism of [DMIIm]+ Cation in Alkaline Solution

Table 1. Degradation Degree of N3-Substituted Imidazolium Cations in Alkaline Solutions at 80 °Ca

a

Key: [a] In 1 M NaOH CD3OD/D2O solution.

these imidazolium salts were investigated by 1H NMR measurements (as shown in Supporting Information). The chemical stability of imidazolium cations in highly alkaline solution was first studied by NMR spectroscopy. The effects of N3-substitutions, alkaline concentration, and reaction temperature on the chemical stability of the imidazolium cations were evaluated. 1 H NMR spectra of synthesized N3-substituted imidazolium cations in 2 M NaOH solution at 80 °C were shown in Figure S1. It can be seen that the imidazolium cations reacted quickly with D2O undergoing hydrogen/deuterium (H/D) exchange of the ring protons, and thus the signals of C2, C4 and C5 protons disappeared (Figure S1A). However, a new weak peak at around 2.10 ppm was observed after being exposed to 2 M NaOH solution at 80 °C for 24 h, probably due to the ringopening reaction of imidazolium cation (Figure S1A). The intensity of the new peak increased with increasing reaction time, indicating a continuous degradation of the imidazolium cation. The degradation degree of N3-substituted imidazolium cations can be calculated by the relative integrated intensities of

the indicated 1H resonances. For example, the quantity 1′/(1 + 1′) provides such a measure (shown in Scheme 2 and in Table 1). The results shown in Table 1 indicate that about 36.4% of [TMIm]+ degraded in 2 M NaOH solution over a 432 h period, indicating poor alkaline stability. Similar results were observed for [DMDIm]+ and [DMDPMIm]+ which degraded about 47.8% and 48.3% in 2 M NaOH solution after 72 and 48 h testing, respectively. Among the N3-substituted imidazolium cations studied in this work, [DMBIm]+, [DMHIm]+, and [DMIIm]+ cations showed relatively better alkaline stability, and less than 4% degradation was observed after 432 h testing. These relatively alkaline stable imidazolium cations, including [DMBIm]+, [DMHIm]+, and [DMIIm]+ were further studied in 4 and 6 M NaOH aqueous solutions at 80 °C (see 1H NMR spectra shown in Figure 1). It can be clearly seen that [DMHIm]+ degraded quickly in 6 M NaOH solution after 8 h testing (about 98.5% degraded), while [DMIIm]+ showed the highest alkaline stability (22.5% degraded in 6 M NaOH solution after 156 h). 211

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Figure 1. 1H NMR spectra for (A) [DMBIm]+, (B) [DMHIm]+, and (C) [DMIIm]+ in 4 and 6 M NaOH solutions at 80 °C for 50 and 156 h, respectively.

Figure 2. 1H NMR spectra of (A) [BTMA]+, (B) [DMIIm]+, and (C) [DMBIm]+ in 1 M NaOH CD3OD/D2O solution (40 wt % of NaOH in D2O, [NaOH]/[cation] = 10/1, molar ratio) at 80 °C at various times.

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The alkaline stability order of the N3-substituted imidazolium cations studied in this work decreases in the order: [DMIIm]+ > [DMBIm]+ > [DMHIm]+ > [TMIm]+ > [DMDIm]+ > [DMDPMIm]+. Although the imidazolium ring undergoes ring-opening reactions mainly due to nucleophilic attack by OH− at the C2 position,21,33 it can be concluded that N3-substituents significantly affect the alkaline stability of these imidazolium cations. Once the stability of C2 substitution group is confirmed (such as methyl groups), the proper N3substituents could further improve the alkaline stability of the imidazolium cations. In addition, based on these results, it can be concluded that the degradation mechanisms (ring-opening reactions) of all the imidazolium cations studied in this work are the same. Quaternary ammonium cation-based polymers have been extensively studied and applied in formulating AEMs. Therefore, the alkaline stability of a typical quaternary ammonium cation, benzyltrimethylammonium ([BTMA]+), was studied and compared with N3-substituted imidazolium cations under the same experimental conditions. Here, the alkaline stability characterizations of all the cations were conducted in 1 M NaOH CD3OD/D2O solution (40 wt % of NaOH in D2O, [NaOH]/[cation] = 10/1, molar ratio) because the presence of methanol could accelerate the cation degradation and dissolve polyatomic cations better than pure aqueous solution.41 It can be clearly seen from Figure 2 that about 32.5% of quaternary ammonium cation ([BTMA]+) degraded after 168 h testing at 80 °C, while only 6.9% and 5.7% degradations of N3substituted imidazolium cations ([DMBIm]+ and [DMIIm]+) were observed under the same experimental conditions, respectively, indicating a better alkaline stability of N3substituted imidazolium cations. DFT Calculations. Figure 3 shows the LUMO energies of the cations and the HOMO energy of OH− in water and methanol evaluated via DFT calculations. As we can see that the LUMO energies of imidazolium cations, including [TMIm]+, [DMBIm]+, [DMHIm]+, [DMDIm]+, [DMIIm]+, and [DMDPMIm]+ in aqueous solution are determined to be −1.547, −1.527, −1.530, −1.533, −1.493, and −1.687 eV (Figure 3A), respectively. While the HOMO energy of OH− (in water) is calculated to be −3.081 eV which is lower than the LUMO energy of imidazolium cations. This leads to that the nucleophilic attack of OH− be restricted by the LUMO energy of imidazolium cation. The higher the LUMO energy, the more difficult it is for imidazolium cations to be attacked by OH− anions.42a−c To look insight of the role that LUMO energy played in nucleophilic attack of OH−, TS search was performed for the reaction. Figure S2 shows the calculated three reaction energy profiles, for DMIIm, DMBIm, and TMIm. Here, we set the relative energy of ion pair as 0 kcal/mol, and the ΔE represents the relative energy of other species against their corresponding initial ion pair. The sequence of activation barriers for TS1 shows good agreement with their LUMO energy sequence (DMIIm > DMBIm > TMIm). This result could explain why LUMO energy determines the alkaline stability of the imidazolium cations. The electroneutral alcohol compound (intermediate) is unstable and easy to form the energy-favorable carbonyl compound (product), which means the final degradation of the imidazolium cation. Similar reaction profile of C−N bond breaking is also reported,33 which supports the reliability of our calculations.42d−f Among the N3substituted imidazolium cations studied here, [DMIIm]+ shows the highest LUMO energy (−1.493 eV), which suggests it is the

Figure 3. Frontier molecular orbital energy of N3-substituted imidazolium and quaternary ammonium cations in (A) water and (B) methanol solution. The gray, blue, red and white balls represent C, N, O and H atoms, respectively. The black arrows indicate nucleophilic attacks by OH− on imidazolium and quaternary ammonium cations. Considering the solution environment, solvent effect is added into the calculations (ε = 78.54 in water, and ε = 32.63 in methanol).

most stable cation of those examined in alkaline solution at elevated temperatures. It should be noted that the LUMO energy value of [DMDIm]+ (−1.533 eV) is higher than that of [TMIm]+ (−1.547 eV). However, a poorer alkaline stability for [DMDIm]+ was observed in NMR test. It has already been demonstrated that the hyperconjugative effect between the N3substituent (for example, C−H (σ bond) of methyl group) and the π-conjugated imidazole ring could increase the electron density of imidazolium cations.34 In the case of [DMDIm]+, the neutral long tail groups (dodecyl chains) tend to aggregate due to collective short-range interaction.43 This tail aggregation may obstruct the formation of hypercongugative effects between dodecyl chains and imidazole rings, and thus facilitates [DMDIm]+ being more easily attacked by OH− as compared with [TMIm]+. Among the cations studied, [DMDPMIm]+ shows the worst alkaline stability, consistent with the lowest LUMO energy value (−1.687 eV) calculated, probably due to the electron-withdrawing effect of two benzene rings as well as the steric hindrance effect. In addition, two benzene rings decrease the hypercongugative effect between the C−H (σ bond) of the methyne group and the π-conjugated imidazole ring, which further lowers the alkaline stability of [DMDPMIm]+. Figure 3B shows the calculated energy diagram of [DMBIm]+, [DMIIm]+, and [BTMA]+ and the HOMO energy of OH− in methanol solution. The LUMO energy values of [DMBIm]+, [DMIIm]+, and [BTMA]+ in methanol were calculated to be −1.633, −1.597, and −1.776 eV, respectively. The HOMO energy of OH− (in methanol) is determined to be −3.283 eV and is also lower than the LUMO energy of imidazolium and quaternary ammonium cations. Therefore, the higher the LUMO energy, the more difficult it is for imidazolium or quaternary ammonium cations to be attacked by OH− anions.42a−c These results indicate that both 213

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[DMBIm]+ and [DMIIm]+ are more stable than [BTMA]+ in alkaline methanol solution, which is consistent with the alkaline stability results obtained earlier above. Synthesis characterization of AEMs. On the basis of the alkaline stability results of the N3-substituted imidazolium and quaternary ammonium cations above, polymers with pendant imidazolium and quaternary ammonium cations (Scheme 3)

[BTMA]+ cation-based monomers were photocross-linked with styrene and acrylonitrile using divinylbenzene as crossinking agent. It has been previously demonstrated that the ionexchange capacity (IEC) value which reflects the exchangeable groups of the polymeric membranes is closely related to water uptake, swelling degree, and ionic conductivity. Therefore, polymeric membranes with identical theoretical IEC value were synthesized for comparison. All the prepared AEMs are transparent, flexible, and could be easily cut into desired sizes even when dry (Figure S4, see Supporting Information). Figure S5 shows typical thermogravimetric analysis (TGA) curves of [PVMBIm][OH], [PVMIIm][OH], and [PVTMA][OH] membranes (see Supporting Information). All three polymeric membranes show a little weight loss due to the evaporation of absorbed moisture. The first degradative weight loss starts at about 150 °C and is associated with the degradation of functional groups, while the decomposition temperature at about 250 °C can be ascribed to the scission of copolymer backbone bonds. These results confirm that all three membranes possess high thermal stability, far beyond the range of interest for application in AEMFCs. Table 2 shows values of IEC, water uptake, swelling degree, and conductivity obtained for our synthesized [PVMBIm][OH], [PVMIIm][OH], and [PVTMA][OH] membranes. The IEC values measured by back-titration matched the theoretical values very well. All the fully hydrated polymeric membranes exhibited conductivities greater than 1.0 × 10−2 S cm−1 at 30 °C, which is competitive with typical anion conductivities in hydrated membranes found in the literature and fulfills the basic conductivity requirement for fuel cell applications.45 The hydroxide stability of these polymeric membranes was examined by monitoring the change in conductivity over time for membranes soaked in 1 M NaOH solution at 80 °C, and in 15 M NaOH solution at 30 °C, respectively (Figure 4). It can be seen that both N3-substituted imidazolium cation based polymeric membranes, [PVMBIm][OH] and [PVMIIm][OH], showed a slight loss of conductivity (less than 3%) after five days of immersion and stabilized at about 94% of the original conductivity over a 25-day immersion period in 1 M NaOH solution at 80 °C. However, a continuous decrease (∼41%) in conductivity was evident for immersion up to 25 days for the quaternary ammonium cation-based [PVTMA][OH] membrane. Similar alkaline stabiliy results were observed in 15 M NaOH solution at 30 °C. Figure S6 shows the FT-IR spectra of [PVMIIm][OH] and [PVTMA][OH] membranes in 1 M NaOH solution at 80 °C for various time. No obvious degradtion peaks of [PVMIIm][OH] membrane were observed. However, the absorption peak intensity of C−N at 1260 cm−1 of [PVTMA][OH] membrane decreased and two new absorption peaks at 1664 and 1180 cm−1 appeared, indicating the nucleophilic displacement of benzyltrimethylammonium cations. Therefore, the alkaline stabiliy of these

Scheme 3. Chemical Structures of Polymers with Pendant Cations Investigated in This Work

were synthesized via free radical polymerization of corresponding monomers. The alkaline stability of the synthesized polymers was also investigated by 1H NMR spectra as well in 1 M NaOH CD3OD/D2O solution (40 wt % of NaOH in D2O, [NaOH]/[cation] = 10/1, molar ratio) at 80 °C. Figure S3 shows the 1H NMR spectra of [PVMIIm][Br], [PVMBIm][Br] and [PVTMA][Cl]PVMBImBr, PVMIImBr and PVTMACl in CD3OD/D2O alkaline solution at 80 °C for 48 h (see Supporting Information). The proton signals belonging to the N1, C2, C4, and C5 protons disappeared due to the hydrogen/deuterium (H/D) exchange of the ring protons (Figure S3). A degradation of about 19.9%, 30.3% and 39.7% was observed, respectively, for [PVMIIm][Br], [PVMBIm][Br] and [PVTMA][Cl] in 1 M NaOH CD3OD/ D2O solution at 80 °C for 48 h. However it should be noted that the degradation degrees of polymers were higher than those of corresponding small molecular cations due to an effect of the polymer backbone,35a,44 which decreases the alkaline stability of polymers. However, only the stability of the cations and not the backbone chain of polymers was assessed in this work. The relative alkaline stability of these homopolymers is the same as that of the cations investigated above. We can conclude, therefore, that [PVMIIm][Br] has the most alkaline stability at elevated temperatures among the polymers we have investigated. The preparation of cation-based polymeric membranes is another useful tool for further evaluation of the alkaline stability of N3-substituted imidazolium and quarternary ammonium cation based AEMs. In order to prepare AEMs with high hydroxide conductivity, low swelling propensity, and good mechanical properties, N3-substituted imidazolium- and

Table 2. Ion Exchange Capacity (IEC), Water Uptake, Swelling Degree, and Conductivity of [PVMBIm][OH], [PVMIIm][OH], and [PVTMA][OH] membranes IEC value (meq g−1)

a

membrane

theoretical

[PVMBIm][OH] [PVMIIm][OH] [PVTMA][OH]

1.02 1.02 1.02

a

conductivity (×10−2 S cm−1)

experimental

water uptake (%)

0.97 ± 0.07 0.93 ± 0.05 0.94 ± 0.06

67.62 ± 7.82 63.51 ± 7.53 64.55 ± 7.11

b

swelling degree (%) 24.05 ± 2.98 20.73 ± 1.63 23.27 ± 1.86

b

30 °C

60 °C

1.09 ± 0.06 1.03 ± 0.04 1.02 ± 0.05

1.58 ± 0.08 1.53 ± 0.07 1.55 ± 0.08

Calculated from monomer ratio. bAfter immersion in water at 30 °C for 24 h; average of three trials. 214

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Figure 4. Conductivity of [PVMBIm][OH] (■), [PVMIIm][OH] (red ●), and [PVTMA][OH] (blue ▲) membranes as a function of time after immersion in N2 saturated (A) 1 M NaOH solution at 80 °C, and in (B) 15 M NaOH solution at 30 °C. (2) (a) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345−352. (b) Diat, O.; Gebel, G. Nat. Mater. 2008, 7, 13−14. (3) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Chem. Rev. 2004, 104, 4587−4612. (4) Whittingham, M. S.; Savinelli, R. F.; Zawodzinski, T. Chem. Rev. 2004, 104, 4243−4244. (5) Lu, S.; Pan, J.; Huang, A.; Zhuang, L.; Lu, J. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 20611−20614. (6) Ü nlü, M.; Zhou, J.; Kohl, P. A. Angew. Chem., Int. Ed. 2010, 49, 1299−1301. (7) Pan, J.; Chen, C.; Zhuang, L.; Lu, J. Acc. Chem. Res. 2012, 45 (3), 473 −481. (8) (a) Zhang, H.; Shen, P. Chem. Rev. 2012, 112, 2780 −2832. (b) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, K.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.; Iwashita, N. Chem. Rev. 2007, 107, 3904−3951. (9) Li, N.; Yan, T.; Li, Z.; Albrecht. T. T. Binder, W. H. Energy Environ. Sci. 2012, 5, 7888−7892. (10) Li, N.; Zhang, Q.; Wang, C.; Lee, Y. M .; Guiver, M. D. Macromolecules 2012, 45, 2411−2419. (11) Zhang, F.; Zhang, H.; Qu, C. J. Mater. Chem. 2011, 21, 12744− 12752. (12) Pan, J.; Lu, S.; Li, Y.; Huang, A.; Zhuang, L.; Lu, J. Adv. Funct. Mater. 2010, 20, 312−319. (13) (a) Wang, J.; Zhao, Z.; Gong, F.; Li, S.; Zhang, S. Macromolecules 2009, 42, 8711−8717. (b) Li, N.; Leng, Y.; Hickner, M. A.; Wang, C. -Y. J. Am. Chem. Soc. 2013, 135, 10124−10133. (14) Varcoe, J. R. Phys. Chem. Chem. Phys. 2007, 9, 1479−1486. (15) Varcoe, J. R.; Slade, R. C. T.; Yee, E. L. H.; Poynton, S. D.; Driscoll, D. J.; Apperley, D. C. Chem. Mater. 2007, 19, 2686−2693. (16) Danks, T. N.; Slade, R. C. T.; Varcoe, J. R. J. Mater. Chem. 2002, 12, 3371−3373. (17) Danks, T. N.; Slade, R. C. T.; Varcoe, J. R. J. Mater. Chem. 2003, 13, 712−721. (18) Wu, Y.; Wu, C.; Varcoec, J. R.; Poynton, S. D.; Xu, T.; Fu, Y. J. Power Sources 2010, 195, 3069−3076. (19) Wang, G.; Weng, Y.; Chu, D.; Xie, D.; Chen, R. J. Membr. Sci. 2009, 326, 4−8. (20) (a) Xiong, Y.; Liu, Q.; Zhu, A.; Huang, S.; Zeng, Q. J. Power Sources 2009, 186, 328−333. (b) Zhang, M.; Kim, H. K.; Chalkova, E.; Mark, F.; Lvov, S. N.; Chung, T. M. Macromolecules 2011, 44, 5937− 5946. (c) Fujimoto, C.; Kim, D. -S.; Hibbs, M.; Wrobleski, D.; Kim, Y. J. Membr. Sci. 2012, 423, 438−449. (d) Tsai, T. H.; Maes, A. M.; Vandiver, M. A.; Versek, C.; Seifert, S.; Tuominen, M.; Liberatore, M. W.; Herring, A. M.; Coughlin, E. B. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1751−1760. (21) Ye, Y.; Elabd, Y. A. Macromolecules 2011, 44, 8494−8503. (22) Singh, M. S. In Advanced Organic Chemistry: Reactions and Mechanisms, 2nd ed.; Dorling Kindersley: Delhi, India, 2008; p 159.

polymeric AEMs is consistent with the stability discussed above for the N3-substituted imidazolium cations, indicating that N3substituted imidazolium cation-based polymeric AEMs are more stable under alkaline condition than are quaternary ammonium cation-based membranes with the same polymer backbone and identical IEC values.

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CONCLUSIONS The alkaline stability of imidazolium cations with various N3substituents were investigated via quantitative 1H NMR spectra and DFT calculations. Compared with traditional quaternary ammonium cations (such as [BTMA]+), the N3-substituted imidazolium cations, [DMIIm]+ and [DMBIm]+ show higher alkaline stability in CH3OH/H2O NaOH solution. In addition, [DMIIm]+ and [DMBIm]+ cation-based polymeric membranes show conductivities comparable to that of [BTMA]+ cationbased membranes. The stability of [DMIIm]+ and [DMBIm]+ cation-based polymeric membranes in NaOH solutions further confirms the high alkaline stability of N3-substituted imidazolium cations. These results should impact the design of alkaline AEMs based on highly stable imidazolium cations and provide improved longevity and performance.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis, 1H NMR characterization of the synthesized compounds, reaction energy profiles, photographs of the membranes, TGA analysis of prepared AEMs, and FT-IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (F.Y.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (No. 21274101), the National Basic Research Program of China (973 Program) (No. 2012CB825800), the Natural Science Foundation of Jiangsu Province (No. BK2011274), and by project funding from the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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