Alkaline Stable C2-Substituted Imidazolium-Based Anion-Exchange

Mar 27, 2013 - ACS Applied Energy Materials 2018 1 (3), 1175-1182. Abstract | Full Text .... and Vijay K. Ramani. ACS Applied Materials & Interfaces 0...
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Alkaline Stable C2-Substituted Imidazolium-Based Anion-Exchange Membranes Bencai Lin,†,§ Huilong Dong,‡ Youyong Li,‡ Zhihong Si,† Fenglou Gu,† 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), Soochow University, Suzhou, 215123, China § Center for Low-Dimensional Materials, Micro-Nano Devices and Systems, Changzhou University, Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou 213164, China S Supporting Information *

ABSTRACT: The alkaline stability of imidazolium salts and imidazoliumbased alkaline anion-exchange membranes (AEMs) was investigated in this work. C2-substituted (with methyl, isopropyl or phenyl groups) imidazolium salts, 3-ethyl-1,2-dimethyl imidazolium bromine ([EDMIm][Br]), 3-ethyl-2-isopropyl-1-methylimidazolium bromine ([EIMIm][Br]), and 3-ethyl-1-methyl-2-phenyl- imidazolium bromine ([EMPhIm][Br]), were synthesized and characterized. The effect of the C2-substitution on the alkaline stability of imidazolium salts was investigated by 1H and 13C NMR spectroscopy. Compared with the C2-unsubstituted imidazolium salt, 3-ethyl-1-methylimidazolium bromine ([EMIm][Br]), the alkaline stability of C2-substituted imidazolium salts is significantly enhanced at elevated temperatures, probably due to the steric hindrance of the substituents, which protected the imidazolium cations against the hydroxide attack. Moreover, the higher LUMO energies may also improve the alkaline stability of imidazolium salts. The alkaline stability of C2-substituted imidazolium salts was found to be in the order [EDMIm][Br] > [EIMIm][Br] > [EMPhIm][Br]. This work provides a feasible approach for enhancing the chemical stability of C2-substituted imidazolium salts, which has potential applications for alkaline anion-exchange membranes. KEYWORDS: anion-exchange membranes, imidazolium salts, alkaline stability, C2-substitution, energy level

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INTRODUCTION With the increasing demand for clean and efficient energy worldwide, fuel cells are attracting more and more attention as an environmentally friendly power source and substitute for conventional fossil fuels.1−4 Proton exchange membrane fuel cells (PEMFCs), which use a solid polymer membrane as the electrolyte, have been considered as renewable and portable energy devices due to their high power density, high energyconversion efficiencies, low starting temperature, and low formation of pollutants.2−4 However, the high cost of noble metal catalysts (such as platinum) and electrolyte membranes hindered the commercialization of PEMFCs. Compared with PEMFCs, catalysts based on non-precious metals such as silver and nickel could be used in alkaline anion-exchange membrane fuel cells (AEMFCs), the reactions of which proceed in alkaline environments.5−9 The most commonly used anion-exchange membranes (AEMs), containing quaternary ammonium cationic groups, have been extensively studied.10−18 However, it has been demonstrated that quaternary ammonium-based AEMs are generally sensitive toward β-hydrogen (Hofmann or E2) elimination19,20 and direct nucleophilic substitution (SN2)21,22 in a high pH environment at elevated temperature.17,18 In © 2013 American Chemical Society

addition, the fabrication of quaternary ammonium-based AEMs often involves carcinogenic chemicals. Therefore, it is desirable to prepare the AEMs with novel cationic groups rather than quaternary ammonia groups. To circumvent the drawbacks of quaternary ammoniumbased membranes, alternatives to quaternary ammonium cations including guanidinium,23,24 phosphonium,25,26 imidazolium,27,28 and benzimidazolium29 and metal-cation30 based AEMs have been investigated. Recently, imidazolium-based AEMs have been studied by several groups.19,27,28 The resultant imidazolium-based AEMs show good alkaline stability, probably due to the presence of the π-conjugated imidazole ring, which reduced the SN2 substitution and Hofmann elimination reactions and improved the alkaline stability of the imidazolium cations.19,28a,b However, degradation was also observed for imidazolium-based AEMs under dry conditions at 80 °C and/ or at higher alkaline concentrations. Based on a detailed analysis of the 1H NMR spectra, an imidazolium ring-opening mechanism as the primary degradation pathway was Received: February 6, 2013 Revised: March 21, 2013 Published: March 27, 2013 1858

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suggested,19 indicating that the imidazolium ring is mainly triggered by the nucleophilic attack of OH− at the C2 position. The study by Zhang and co-workers found that the conductivity of an imidazolium-functionalized polysulfonebased AEM decreased by 23.3% after treatment with 3 M NaOH at 60 °C for 24 h, and the cation degradation is probably affected by the main chain backbone structure of the polymers.26 All of these results suggest that imidazolium cations have considerable chemical stability in alkaline condition and a promising future for the development of imidazolium-based AEMs for using in AEMFCs. However the stability of imidazolium cations under much more vigorous conditions (such as higher temperature and alkaline concentrations) still needs to be improved. The protection of cationic groups by steric hindrance and/or mesomeric stabilization might be a prospective way to get more stable AEMs under alkaline conditions. Yan and co-workers synthesized tris(trimethoxyphenyl) phosphonium based AEMs. The resultant polymeric membranes showed high alkaline stability because the trimethoxyphenyl groups are electron donors and take part in conjugation, which enhances the stability of the quaternary phosphonium group. In addition, the steric bulk effect of trimethoxyphenyl phosphine also protects the core phosphorus atom.25a In the present work, C2-unsubstituted and C2-substituted imidazolium salts and imidazolium-based polymers were synthesized and characterized. The influence of C2-substituted groups on the alkaline stability of imidazolium cations and imidazolium-based polymers was systematically studied. In addition, the effect of C2-substituted groups on the properties of imidazolium-based AEMs was investigated with respect to their thermal stability, chemical stability, and ionic conductivity.

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Synthesis of 3-Ethyl-1-methylimidazolium Bromine ([EMIm][Br]). [EMIm][Br] was synthesized by stirring a mixture of N-methylimidazole with an equivalent molar amount of bromoethane at room temperature under nitrogen atmosphere. The resultant viscous oil was washed with ethyl ether three times and then dried in dynamic vacuum at room temperature for 24 h. 1H NMR (400 MHz, D2O): 8.73 (s, H), 7.49 (d, 1H), 7.42 (d, 1H), 4.20−4.25 (m, 2H), 3.89 (s, 3H), 1.47−1.51(t, 3H). Synthesis of 3-Ethyl-1,2-dimethylimidazolium Bromine ([EDMIm][Br]). [EDMIm][Br] was synthesized by stirring a mixture containing 1,2-dimethylimidazole and an equivalent molar amount of bromoethane at room temperature under nitrogen atmosphere. The resultant white solid was washed with ethyl ether three times and then dried in dynamic vacuum at room temperature for 24 h. 1H NMR (400 MHz, D2O): 7.28−7.30 (d, 1H), 7.23−7.25(d, 1H), 4.06−4.08 (m, 2H), 3.69−3.70 (s, 3H), 2.51−2.58 (m, 3H), 1.35−1.36(t, 3H). Synthesis of 3-Ethyl-2-isopropyl-1-methylimidazolium Bromine ([EIMIm][Br]). [EIMIm][Br] was synthesized by stirring a mixture containing 1-methyl-2-isopropylimidazole and an equivalent molar amount of bromoethane at room temperature under nitrogen atmosphere. The resultant yellow solid was washed with ethyl ether three times and then dried in dynamic vacuum at room temperature for 24 h. 1H NMR (400 MHz, D2O): 7.25 (d, 1H), 7.18 (d, 1H), 4.11−4.17 (m, 3H), 3.79 (s, 3H), 3.51−3.59 (m, 1H), 1.36−1.38(m, 6H). Synthesis of 3-Ethyl-1-methyl-2-phenylimidazolium Bromine ([EMPhIm][Br]). [EMPhIm][Br] was synthesized by stirring a mixture containing 1-methyl-2-phenylimidazole and an equivalent molar amount of bromoethane at 60 °C under nitrogen atmosphere. The resultant yellow solid was washed with ethyl ether three times and then dried in dynamic vacuum at room temperature for 24 h. 1H NMR (400 MHz, D2O): 7.62−7.71 (m, 3H), 7.55−7.57 (m, 2H), 7.46− 7.47(m, 2H), 3.95−4.03(m, 2H), 3.63(s, 3H), 1.27−1.31(m, 3H) Synthesis of 1-Methyl-3-(4-vinylbenzyl) Imidazolium Chloride ([MVIm][Cl]). [MVIm][Cl] was synthesized by stirring a mixture containing 1-methylimidazole and an equivalent molar amount of 4vinylbenzyl chloride 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 before polymerization.31 1H NMR (400 MHz, DMSO-d6): 9.0 (1H, s), 7.8 (1H, s), 7.6 (2H, d), 7.5(1H, s), 7.2 (2H, d), 6.75 (1H, m), 5.9 (2H, s), 5.6 (1H, s), 5.2 (1H, d), 3.8 (3H, s). Synthesis of 1,2-Dimethyl-3-(4-vinylbenzyl) Imidazolium Chloride ([DMVIm][Cl]). [DMVIm][Cl] was synthesized by stirring a mixture containing 1,2-dimethylimidazole and an equivalent molar amount of 4-vinylbenzyl chloride 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), 7.27−7.34 (m, 4H), 6.73−6.76 (m, 1H), 5.83− 5.88 (d, 1H), 5.36−5.37 (d, 1H), 5.31 (s, 2H), 3.76 (s, 3H), 2.55 (s, 3H). Synthesis of 2-Isopropyl-1-methyl-3-(4-vinylbenzyl) Imidazolium Chloride ([IMVIm][Cl]). [IMVIm][Cl] was synthesized by stirring a mixture containing1-methyl-2-isopropylimidazole and an equivalent molar amount of 4-vinylbenzyl chloride for 24 h at 0 °C. The resultant viscous oil was washed with ethyl ether and then dried in dynamic vacuum at room temperature before polymerization. 1H NMR (400 MHz, D2O): 7.38−7.39 (d, 2H), 7.10−7.21 (m, 4H), 6.63−6.64 (m, 1H), 5.70−5.76 (d, 1H), 5.29 (s, 2H), 5.20−5.23 (d, 1H), 3.75 (s, 3H), 3.42−3.46 (m, 1H), 1.14−1.16 (m, 6H). Synthesis of 1-Methyl-2-phenyl-3-(4-vinylbenzyl) Imidazolium Chloride ([MPhVIm][Cl]). [MPhVIm][Cl] was synthesized by stirring a mixture containing 1-methyl-2-phenylimidazole and equivalent molar amount of 4-vinylbenzyl chloride 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 before polymerization. 1H NMR (400 MHz, D2O): 7.65−7.70(d, 2H), 7.51−7.58 (m, 4H), 7.30−7.35 (m, 1H), 6.92−6.96 (m, 2H), 6.72− 6.73 (m, 3H), 5.73−5.79 (d, 1H), 5.25−5.30 (d, 1H), 5.13 (s, 2H), 3.63 (s, 3H). Synthesis of Imidazolium-Based Polymers. Imidazolium-based polymers were synthesized with free radical polymerization by using

EXPERIMENTAL SECTION

Materials. Styrene, acrylonitrile, n-methylimidazole, 1,2-dimethylimidazole, 2-isopropylimidazole, 2-phenylimidazole, iodomethane, bromoethane, benzyltrimethylammonium chloride, divinylbenzene (DVB), 4-vinylbenzyl chloride, benzoin ethyl ether, ethylether, ethyl acetate, acetonitrile, potassium hydroxide, sodium hydroxide, and hydrochloric acid were used as purchased. All of the vinyl monomers were made inhibitor-free 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 of 1-Methyl-2-isopropylimidazole. A mixture containing 2-isopropylimidazole (5.46 g, 0.05 mol), iodomethane (7.10 g, 0.05 mol), and KOH (5.61 g, 0.10 mol) in acetonitrile (40 mL) was stirred at room temperature for 4 h under an argon atmosphere. The solvent was removed under dynamic vacuum, and the crude product was extracted with CHCl3 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): δ 6.89 (d, H), 6.73(d, H), 3.56 (s, 3H), 2.94−3.00 (m, 1H), 1.28−1.30 (m, 6H). Synthesis of 1-Methyl-2-phenylimidazole. A mixture containing 2-phenylimidazole (7.16 g, 0.05 mol), iodomethane (7.10 g, 0.05 mol), and KOH (5.61 g, 0.10 mol) in acetonitrile (40 mL) was stirred at room temperature for 4 h under an argon atmosphere. The solvent was removed under dynamic vacuum, and the crude product was extracted with CHCl3 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): δ 7.61−7.63 (m, 2H), 7.37−7.46 (m, 3H),7.11(d, 1H), 6.96 (d, 1H), 3.74 (s, 3H). 1859

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where Wd and Ww are the mass of the dry and water-swollen samples, respectively. The swelling ratio was characterized by linear expansion ratio, which was determined by the difference between wet and dry dimensions of a membrane sample (3 cm in length and 1 cm in width). The calculation was based on the following equation:

azobisisobutyronitrile (AIBN) as the radical initiator. In brief, poly(1methyl-3-(4-vinylbenzyl) imidazolium chloride) (PMVImCl) was synthesized by stirring a mixture containing [MVIm][Cl] and 1 wt % of AIBN at 65 °C for 8 h under a nitrogen atmosphere. The polymer was precipitated, purified twice with acetone, and then dried at 60 °C overnight. Poly(1,2-dimethyl-3-(4-vinylbenzyl) imidazolium chloride) (PDMVImCl), poly(2-isopropyl-1-methyl-3-(4-vinylbenzyl) imidazolium chloride) (PIMVImCl), and poly(1-methyl-2-phenyl-3(4-vinylbenzyl) imidazolium chloride) (PMPhVImCl) were synthesized in the same way. Preparation of Alkaline Imidazolium-Based Copolymer Membranes. A mixture of styrene/acrylonitrile, [DMVIm][Cl], divinylbenzene (2 wt % to the formulation based on the weight of monomer), and 1 wt % of benzoin ethyl ether (photoinitiator) was stirred and ultrasonicated to obtain a homogeneous solution, which was then cast into a glass mold and photo-cross-linked by irradiation with UV light of 250 nm wavelength at room temperature. The resultant polymeric membranes were immersed in N2-saturated 1 M KOH solution at 60 °C for 24 h to convert the membrane from Cl− to OH− form.32 This process was repeated three times to ensure a complete conversion displacement. Then the converted membranes was immersed in N2-saturated deionized water for 24 h and washed with deionized water until the pH of residual water was neutral. Characterization. NMR spectra were recorded on a Varian 400 MHz spectrometer. Fourier transform infrared (FT-IR) spectra of the polymers were recorded on a Varian CP-3800 spectrometer in the range of 4000−400 cm−1. Thermal analysis was carried out by Universal Analysis 2000 thermogravimetric analyzer (TGA). Samples were heated from 30 to 500 °C at a heating rate of 10 °C min−1 under a nitrogen flow. Energy dispersive X-ray spectroscopy (EDX) measurements were performed with the spectrometer attached on the HITACHI S-4700 FESEM. Hydroxide Ion Conductivity. The resistance value of the membranes 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 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.33 All samples were fully hydrated in N2-saturated deionized water for at least 24 h prior to the conductivity measurement. Conductivity measurements under fully hydrated conditions were carried out in a chamber filled with a N2-saturated deionized water to maintain the relative humidity at 100% during the experiments. All samples were equilibrated for at least 30 min at a given temperature. Repeated measurements were taken with 10 min interval until no additional change in conductivity was observed. The ionic conductivity σ (S cm−1) of a given membrane can be calculated from

σ=

swelling (%) =

Xdry

× 100

where Xwet and Xdry are the lengths of wet and dry membranes, respectively. Ion Exchange Capacity (IEC). Ion exchange capacities (IEC) were determined by a back-titration. The AEMs were immersed in 100 mL of 0.01 M HCl standard solution for 24 h. The solutions were then titrated with a standardized NaOH solution using phenolphthalein as an indicator. The IEC value was calculated using the expression

IEC =

V0,NaOHC NaOH − Vx ,NaOHC NaOH mdry

where V0,NaOH and Vx,NaOH are the volumes of the NaOH consumed in the titration without and with membranes, respectively, CNaOH are the molar concentrations of NaOH, which were titrated by the standard oxalic acid solution, and mdry is the mass of the dry membranes. Three replicates were conducted for each sample. Chemical Stability in Alkaline Solution. The chemical stability of imidazolium salts and the imidazolium-based polymers was examined by NMR spectroscopy using D2O as the solvent. The chemical stability study under alkaline conditions was performed in NMR tubes. The stability of the membranes in alkaline solution was examined by immersing the membrane samples in N2-saturated 1 M KOH solution at 80 °C. The degradation of polymer membranes was evaluated by measuring the changes of hydroxide conductivity and IEC values. Computational Details and Analysis. Theoretical analysis on the single molecules of imidazolium salts was carried out by the DMol3 density functional code as implemented in Materials Studio (Version 6.0).34 The all-electron GGA-PW91 functional, along with a DNP basis set, was used for all the geometry optimizations.35 A 3.7 Å real space cutoff of atomic orbital was specified. Spin-restricted selfconsistent field calculations were converged with a threshold value of 10−6 Ha. Default convergence criteria of 1× 10−5 Ha for energies, 2 × 10−3 Ha/Å for gradient, and 5 × 10−3 Å for displacement were employed in geometry optimizations. Solvent effect was included by the usage of a conductor-like screening model (COSMO) (water, ε = 78.54).36,37 All single molecules are fully optimized to ensure there is no imaginary frequency.

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RESULTS AND DISCUSSION In order to investigate the alkaline stability of imidazoliumbased AEMs, the small molecule model compounds of C2-

l RA

Scheme 1. Molecular Structure of C2-Unsubstituted and C2Substituted Imidazolium Salts Studied in This Work

where l is the distance (cm) between two electrodes, A is the crosssectional area (cm2) of the membrane, obtained from the membrane thickness multiplied by its width, and R is the membrane resistance value from the AC impedance data (Ω). Water Uptake and Swelling Ratio. The membrane samples were soaked in the N2-saturated deionized water at room temperature for 24 h. The hydrated polymer membranes were taken out, the excess water on the surface was removed by wiping with a tissue paper, and the membranes were weighed immediately (Ww). Then the wet membrane was dried under vacuum at a fixed temperature of 80 °C until a constant dry weight was obtained (Wd). The water uptake W was calculated with the following equation:

W (%) =

X wet − Xdry

unsubstituted [EMIm][Br] and C2-substituted imidazolium salts [EDMIm][Br], [EIMIm][Br], and [EMPhIm][Br] were first studied (Scheme 1). The purity and chemical structure of these imidazolium salts were confirmed by 1H NMR spectra.

(Ww − Wd) × 100 Wd 1860

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Figure 1. Photographs of (A) N2-saturated 1 M KOH solution, (B) [EMIm][Br], (C) [EDMIm][Br], (D) [EIMIm][Br], and (E) [EMPhIm][Br] in N2 -saturated 1 M KOH solution in glass bottles, before (top) and after (bottom) treatment at 80 °C for 60 h, respectively.

Figure 3. (A) 1H NMR spectra and (B) 13C NMR spectra for [EDMIm][Br] in 1 M KOH solution at 80 °C at various times.

that no obvious differences were observed between the precipitates formed in pure KOH solution (Supporting Information, Figure S1A) and in imidazolium salt/KOH solutions (see Supporting Information, Figure S1B−E). The results of FTIR spectra indicate that the white precipitates formed in the imidazolium salt/KOH solutions might not be the degradation products of the imidazlium salts but could be potassium silicates formed via the reaction of the glass bottles and KOH solution. The energy dispersive X-ray (EDX) spectra (see Supporting Information, Figure S2) of all of the precipitates showed that no nitrogen and carbon elementals could be detected in the precipitate samples, which again confirms that the white precipitates formed in the glass bottles are not the degradation products of the imidazlium salts. The results of the EDX spectra also imply that the possible degradation products of the imidazlium salts might be soluble in KOH aqueous solution. Furthermore, no color changes of [EDMIm][Br], [EIMIm][Br], and [EMPhIm][Br] solution were observed throughout the experiment, while the [EMIm][Br] solution changed to be yellow, probably due to the degradation of the compound. The alkaline stability of imidazolium salts was further characterized by NMR spectroscopy, and the effects of substituent groups at the C2 position of imidazole ring, the alkaline concentration, and the reaction temperature on the chemical stability of the imidazolium cations were investigated. Figure 2 shows the 1H NMR spectra of the C2-unsubstituted imidazolium salt, [EMIm][Br], after the exposure to 1 M KOH solution at 30 °C for 480 h and at 80 °C for 12 and 60 h,

Figure 2. 1H NMR spectra for [EMIm][Br] in 1 M KOH solution at 30 or 80 °C at various times.

Scheme 2. Ring-Opening Reaction of [EMIm][Br]

Figure 1 shows photographs of [EMIm][Br], [EDMIm][Br], [EIMIm][Br], and [EMPhIm][Br] dissolved in N2-saturated 1 M KOH solution in glass vials. As controls, pure N2-saturated 1 M KOH solution was treated under the same experimental conditions. It can be seen that all of the imidazolium salts dissolve well in KOH solution and form a colorless solution (Figure 1, top). However, white precipitates formed and settled in the glass bottles after the solution was heated at 80 °C for 60 h. Very similar precipitates were observed by Varcoe and coworkers, which were considered to be the degradation products of organic molecules.38 In this work, all of the precipitates were isolated, purified, and characterized by FT-IR spectra, as shown in Figure S1 (see Supporting Information). It should be noted 1861

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Figure 4. 1H NMR spectra for (A) [EDMIm][Br], (B) [EIMIm][Br], and (C) [EMPhIm][Br] in 2 and 6 M KOH solution at 80 °C at various times.

Table 1. Degree of Ring-Opening Degradation of Imidazolium Salts after Exposure to Alkaline Solutions at 80 °C imidazolium salts [EMIm][Br] [EDMIm][Br] [EIMIm][Br] [EMPhIm][Br] [EDMIm][Br] [EIMIm][Br] [EMPhIm][Br] [EDMIm][Br] [EIMIm][Br] [EMPhIm][Br] benzyltrimethylammonium bromide25c

KOH solution

time (h)

degradation (%)

1 1 1 1 2 2 2 6 6 6 1

60 168 168 168 168 168 168 10 10 10 96

47.6 0 0 0 0 10.2 25.7 5.4 20.2 32.8 20

M M M M M M M M M M M NaOHa

A mixture of CD3OD and 40 wt % NaOD/D2O solution (1 M NaOD, [NaOD]/[BTMA] = 10).

Figure 5. LUMO energy and isosurface of imidazolium salts. The gray, blue, red, and white balls represent C, N, O, and H atoms, respectively. The black arrows indicate the nucleophilic attacks from OH− to imidazolium salts.

respectively. It can be found that [EMIm][Br] reacts rapidly with D2O under hydrogen/deuterium (H/D) exchange of the ring protons, and thus the proton signals belonging to the C2, C4, and C5 protons disappeared (Figure 2B).39 However, no new peaks were observed after the exposure to 1 M KOH solution at 30 °C for 480 h, indicating that the C2unsubstituted [EMIm][Br] was quite stable in 1 M KOH solution at 30 °C. Figure 2C,D shows the 1H NMR spectra of

[EMIm][Br] after exposure to 1 M KOH solution at 80 °C for 12 and 60 h, respectively. New peaks at around 1.0, 2.1, and 2.5 ppm probably due to the ring-opening reaction of [EMIm][Br] were observed. Scheme 2 shows the possible ring-opening degradations of [EMIm][Br] in alkaline condition at elevated temperature.19 The chemical shifts observed at 1.0, 2.1, and 2.5 ppm (see Supporting Information, Figure 1C,D) are attributed

a

1862

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The chemical stability of C2-substituted imidazolium salts in higher concentration (2 and 6 M) KOH solutions at 80 °C was further investigated. Figure 4 shows the 1H NMR spectra of C2-substituted imidazolium salts in 2 and 6 M KOH solutions at 80 °C, respectively. It can be seen that [EDMIm][Br] was stable in 2 M KOH solution at 80 °C for 168 h (Figure 4A). However, new peaks appeared in the 1H NMR spectra of [EIMIm][Br] and [EMPhIm][Br] under the same experimental conditions, indicating the degradation of [EIMIm][Br] and [EMPhIm][Br] in 2 M KOH solution at 80 °C (Figure 4B,C). Furthermore, the alkaline degradation of [EDMIm][Br] was observed in 6 M KOH solution at 80 °C for 10 h (Figure 4A). On the basis of the detailed analysis of the 1H NMR spectra, it is supposed that the alkaline degradation of imidazolium cations is mainly due to the nucleophilic attack of OH− on the imidazole ring at the C2 position. The degree of the ringopening reaction of the imidazolium cations can be estimated by the relative integrations of the indicated 1H resonances (i.e., 1′/(1 + 1′), Scheme 2). The degradation degree of C2substituted imidazolium cations was calculated from 1H NMR spectra, and the results are summarized in Table 1. It can be clearly seen that all of the C2-substituted imidazolium salts studied in the present work are quite stable in 1 M KOH solution at 80 °C for 168 h. Furthermore, C2-substituted imidazolium salts show a better alkaline stability if compared with the quaternary ammonium salt.25c In the case of [EIMIm][Br] and [EMPhIm][Br], about 10.2% and 15.7% of the compounds, respectively, were degraded in 2 M KOH solution at 80 °C after 168 h test, while no degradation was observed for [EDMIm][Br] under the same experimental conditions. The degradation degree was calculated to be 5.4%, 20.2%, and 32.8% for [EDMIm][Br], [EIMIm][Br], and [EMPhIm][Br] in 6 M KOH solution at 80 °C for 10 h, respectively. Therefore, it can be concluded that the C2substituted imidazolium salts are stable in 1 M KOH solutions even at 80 °C. Under high temperature and a high alkaline concentration ([KOH] > 1 M), the alkaline stability order of the imidazolium salts studied in this work could be determined to be [EDMIm][Br] > [EIMIm][Br] > [EMPhIm][Br] > [EMIm][Br]. It has been reported that the nucleophilic reaction of imidazolium salts is affected by the lowest unoccupied molecular orbital (LUMO) of the compound. The lower the LUMO energy of nucleophiles (imidazolium cations here), the easier they can be attacked by OH−.42 Therefore, the alkaline stability of small molecule imidazolium salts was further evaluated via density functional theory (DFT/B3LYP)

Scheme 3. Structures of Polymers Studied in the Present Work

to the proton of 6′, 1′, and 3′ as shown in Scheme 2. The ringopening reaction of [EMIm][Br] became more pronounced with increasing reaction time, from 12 to 60 h (Figure 2D). These results indicated that [EMIm][Br] was stable in 1 M KOH solution at 30 °C but degraded quickly at elevated temperatures (e.g., 80 °C). Figure 3A shows the 1H NMR spectra of C2-substituted imidazolium salt [EDMIm][Br] after exposure to 1 M KOH solution at 30 °C for 480 h and at 80 °C for 60 and 168 h, respectively. Similarly, the proton peaks associated with the C2, C4, and C5 positions of imidazole rings disappeared due to the H/D exchange reaction. No new peaks of [EDMIm][Br] were observed after exposure to 1 M KOH solution at 30 °C for 480 h and even at 80 °C for 168 h, indicating that [EDMIm][Br] was quite stable in 1 M KOH solution. Similar results were observed in 1H NMR spectra of C2-substituted [EMPhIm][Br] and [EIMIm][Br] under the same experimental conditions (see Supporting Information, Figure S3). These results indicated that C2-substitution is an effective method to improve the alkaline stability of imidazolium cations. The enhanced chemical stability of C2-substituted imidazolium cations is probably due to the steric hindrance effect of methyl, isopropyl, and phenyl groups that protect the imidazolium cations against hydroxide attack.25a,40 The enhanced alkaline stability of C2-substituted [EDMIm][Br] was further confirmed by 13C NMR spectra as shown in Figure 3B. The peaks at 9, 119, and 121 ppm attributed to C3, C4, and C5 were weakened after exposure to 1 M KOH solution at 80 °C for 168 h, if compared with the imidazolium salts before the alkaline treatment. In addition, split peaks were observed at the corresponding positions due to the C−2H coupling.41 Similar results were also obtained for [EMPhIm][Br] and [EIMIm][Br] (see Supporting Information, Figure S4). All of these results further confirmed the alkaline stability of C2-substituted imidazolium cations in 1 M KOH solution at 80 °C.

Figure 6. Photographs for imidazolium-based polymers in 1 M KOH solution (in polypropylene bottles) at 80 °C at (A) 0 h and (B) 60 h. 1863

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Figure 7. Photographs for C2-substituted imidazolium-based polymers at 80 °C in 2 M KOH solution (in polypropylene bottles) at various times: (A) 0 h, (B) 10 h, and (C) 60 h).

Figure 9. Conductivity Arrhenius plots of [PDMVIm]40[OH] and [PDMVIm]30[OH] as a function of temperature.

lium cations, [EDMIm]+ shows the highest LUMO energy (−1.688 eV), which thus enables it be the most stable cation in alkaline solution at elevated temperatures. In addition, the hyperconjugative effect between the C−H (σ bond) of methyl group and the π-conjugated imidazole ring could increase the electron density of imidazolium cations and enlarge the region of conjugation, which thus enhances the alkaline stability of [EDMIm][Br].40,43 Among the small molecule model compounds studied, [EMPhIm][Br] shows the lowest LUMO energy (−2.369 eV) probably due to the delocalization of the π-orbital in [EMPhIm]+. Therefore, it is not surprising that the

Figure 8. Photographs of [PDMVIm]40[OH] membrane prepared via photo-cross-linking of a mixture containing [DMVIm][Cl] (40 wt %), and styrene/acrylonitrile (1:3 weight ratio).

calculation to obtain the LUMO energy level for comparison. Figure 5 shows the LUMO energy level of the C2unsubstituted and C2-substituted imidazolium cations. As can be seen, the LUMO energy of [EDMIm]+, [EIMIm]+, [EMPhIm]+, and [EMIm]+ are −1.688, −1.778, −2.369, and −1.804 eV, respectively. Among the C2-substituted imidazo-

Table 2. Ion Exchange Capacity (IEC), Water Uptake, Swelling Degree, and Conductivity of [PDMVIm]40[OH] and [PDMVIm]30[OH] IEC (mequiv g−1)

a

conductivity (×10−2 S cm−1)

membrane

theoreticala

experimental

water uptake (%)b30 °C

swelling degree (%)b 30 °C

30 °C

60 °C

[PDMVIm]40[OH] [PDMVIm]30[OH]

1.61 1.22

1.52 ± 0.11 1.15 ± 0.07

63.0 ± 7.19 49.6 ± 6.09

32.4 ± 3.98 27.5 ± 4.26

1.41 ± 0.16 1.30 ± 0.08

2.59 ± 0.13 1.99 ± 0.12

Calculated from the monomer ratio. bAfter immersing in water at 30 °C for 24 h; average of three trials. 1864

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imidazolium cations were observed, which further confirmed the alkaline stability of C2-substituted imidazolium cations in 1 M KOH solution at elevated temperature. The chemical stability of C2-substituted imidazolium-based polymers in 2 M KOH solutions at 80 °C was further investigated. Figure 7 shows photographs of PDMVImCl, PIMVImCl, and PMPhVImCl in 2 M KOH solution at 80 °C at 0, 10, and 60 h (Figure 7), respectively. It can be seen that all polymers dissolved well in 2 M KOH solution at room temperature. However, floccules were observed in PMPhVImCl after being heated at 80 °C for 10 h (Figure 7B), and the same result was observed for PIMVImCl after 60 h, indicating the degradation of PIMVImCl and PMPhVImCl (Figure 7C). The good liquidity of PDMVImCl solutions was maintained under the same experimental conditions, indicating a better alkaline stability of PDMVImCl. The alkaline stabiliy of polymers is well consistent with the small molecule model compounds studied above, indicating that PDMVImCl is the most stable polymer in alkaline condition at elevated temperature. C2-substituted imidazolium-type AEMs were further prepared in this work. However, the polymers containing imidazolium cations are generally water-swollen or even soluble in water, which retarded the potential application for fuel cells. This obstacle could be overcome to a certain degree by copolymerization of imidazolium monomers with suitable monomer oils (such as styrene and acrylonitrile), and thus membranes with high hydroxide conductivity, low swelling degree, and good mechanical properties could be achieved. Herein, a homogeneous solution containing 1,2-dimethyl-3-(4vinylbenzyl) imidazolium chloride ([DMVIm][Cl]), styrene/ acrylonitrile (1:3 weight ratio), benzoin ethyl ether, and DVB (2 wt % and 4 wt % to the formulation based on the weight of monomer, respectively) was photo-cross-linked by irradiation with UV light. The resultant copolymer membranes were immersed in N2-saturated 1 M KOH solution at 60 °C for 24 h to convert the membrane from Cl− to OH− form. The resultant copolymers were denoted as [PDMVIm]x[OH] (the subscript x indicates the weight ratio of [DMVIm][Cl]). All of the membranes were transparent and flexible and could be easily cut into any desired sizes even under dry conditions (Figure 8). Figure S6 in Supporting Information shows the typical thermogravimetric analyzer (TGA) curves for [PDMVIm]x[OH]. The temperature at 5% weight loss was higher than 300 °C with a heating rate of 10 °C/min, and only about 2% weight loss was observed at 120 °C for 12 h, which confirmed that [PDMVIm]x[OH] indeed confers a high thermal stability, far beyond the range of interest for application in AEMFCs.

Table 3. Ion Exchange Capacity (IEC) of [PDMVIm]40[OH] and [PDMVIm]30[OH] after Immersion in N2-Saturated KOH Solution at 60 °C IEC (mequiv g−1)

a

KOH (M)

time (h)

1 1 1 1 2

0a 24 60 96 60

[PDMVIm]40[OH] 1.61 1.52 1.58 1.55 1.56

± ± ± ±

0.11 0.09 0.07 0.11

[PDMVIm]30[OH] 1.22 1.15 1.17 1.14 1.12

± ± ± ±

0.07 0.04 0.07 0.08

Theoretical IEC calculated from monomer ratio.

alkaline stability of [EMPhIm][Br] is poorer than that of C2substituted [EDMIm][Br] and [EIMIm][Br]. However, it should be noted that the alkaline stability of [EMPhIm][Br] is still better than that of C2-unsubstituted [EMIm][Br] even though the LUMO energy of [EMPhIm]+ is the lowest among the four small molecule model compounds investigated. Here, it is supposed that the steric hindrance of the C2-substituted phenyl groups could also enhance the alkaline stability of the imidazolium cations if compared with C2-unsubstituted [EMIm][Br]. On the basis of the alkaline stability of the small molecule model compounds studied above, polymers containing C2unsubstituted and C2-substituted imidazolium cations were synthesized via the free radical polymerization of corresponding imidazolium-type monomers (Scheme 3). The alkaline stability of the synthesized polymers was also characterized by NMR spectra. Figure 6 shows the photographs of the polymer solutions before and after the alkaline stability test. It can be seen that all of the polymers dissolved well in 1 M KOH solution at room temperature (Figure 6A). However, the solution of PMVImCl was gelatinized after being heated at 80 °C for 60 h (Figure 6B). Although the mechanism of gelatinization is still unclear, our understanding is that the ring-opening reaction of imidazolium cations yielded the polymer gels. The good liquidity of PDMVImCl, PIMVImCl, and PMPhVImCl solutions was maintained under the same experimental conditions, indicating the good alkaline stability of synthesized polymers. Figure S5 in Supporting Information shows the 1H NMR spectra of PDMVImCl, PIMVImCl, and PMPhVImCl in 1 M KOH solution at 80 °C for 60 h, respectively. Chemical shifts attributed to the attached imidazolium groups were observed at about 3.6 (methyl at N1 of the imidazole ring) and 7.0−8.0 ppm (the proton of C4 and C5 of the imidazole ring). No additional chemical shifts associated with the degradation of

Figure 10. Conductivity Arrhenius plots of (A) [PDMVIm]40[OH] and (B) [PDMVIm]30[OH] after immersion in N2-saturated 1 and 2 M KOH solution at 80 °C for various times. 1865

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practical applications of alkaline anion-exchange membranes (AEMs).

The ion-exchange capacity (IEC) value is generally considered to be responsible for ion transfer and thus is an indirect and reliable approximation of the ion conductivity. The IEC value of the membranes was controlled by varying the ratio of [DMVIm][Cl] to styrene and acrylonitrile in the original membrane. Table 2 shows the values of IEC, swelling degree, water uptake, and conductivity of [PDMVIm]40[OH] and [PDMVIm]30[OH]. The IEC values of produced membranes are calculated to be 1.52 and 1.15 mequiv g −1 for [PDMVIm]40[OH] and [PDMVIm]30[OH], which are close to the theoretical value of 1.61 and 1.22 mequiv g−1. Swelling behavior of the membrane is an essential factor influencing the mechanical properties and the morphologic stability of membranes. Generally, the water uptake and swelling degree increase with the IEC values. In the present work, the water uptake and swelling degree of [PDMVIm]30[OH] are 49.6% and 27.5% and increased to 63.0% and 32.4% for [PDMVIm]40[OH], respectively. The conductivity of AEMs is significantly influenced by the IEC values and water uptake. Here, the conductivity of the AEMs was measured after the membranes were fully hydrated in deionized water at room temperature. Figure 9 shows the ionic conductivity as a function of the temperature of the membranes. The conductivities of the membranes increase with the increase in temperature because the free volume in favor of ion transport and the mobility of anions are increased as the temperature increases. The conductivities of all of the membranes are about 1.0 × 10−2 S cm−1 at 30 °C and increase to 2.0 × 10−2 S cm−1 at 90 °C, which fulfills the basic conductivity requirement of fuel cells. The chemical stability of [PDMVIm]x[OH] in high pH environments at elevated temperature was also studied. Here, the long-term alkaline stability of produced AEMs was also investigated by immersing membrane samples in N2-saturated KOH solution at 80 °C. The changes of hydroxide ion conductivity and the IEC values of the tested membranes were measured. The IEC values of [PDMVIm] 40[OH] and [PDMVIm]30[OH] also remained constant during the stability test in 1 and 2 M KOH solution, indicating good chemical stability of the membranes in alkaline solution (Table 3). Figure 10 shows the conductivity Arrhenius plots of [PDMVIm]40[OH] and [PDMVIm]30[OH] membranes after certain testing times in N2-saturated 1 and 2 M KOH solution at 80 °C. All membranes maintained the hydroxide conductivity during the testing time, which further confirms the excellent alkaline stability of C2-substituted imidazolium cations.

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ASSOCIATED CONTENT

S Supporting Information *

Characterization of the synthesized compounds and TGA analysis of prepared AEMs (PDF). 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: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of China (No. 21274101), Research Fund for Ph.D. Programs Foundatio n of Ministry of Education of China (20103201110003), Program for Scientific Innovation Research of College Graduate in Jangsu Province (CXZZ11-0103), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

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CONCLUSION In summary, a series of imidazolium salts and imidazoliumbased alkaline anion-exchange membranes were synthesized and characterized. Compared with C2-unsubstituted imidazolium salt, the alkaline stability of C2-substituted imidazolium salts are significantly enhanced at elevated temperature, probably due to the steric hindrance effect and the σ−π hyperconjugative effect of the C2-substituted groups, which are effective in stabilizing the imidazolium cations. Moreover, the higher LUMO energy of the C2-substituted imidazolium salts makes the imidazolium cations more difficult to be attacked nucleophilicly by OH− anions. 1H NMR and 13C NMR results demonstrated an excellent chemical stability of [EDMIm][Br] in high pH solution at elevated temperatures. These results of the study suggest a feasible approach for enhancing the chemical stability of imidazolium salts and the synthesis and 1866

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