Base Stable Pyrrolidinium Cations for Alkaline Anion Exchange

Article pubs.acs.org/Macromolecules

Base Stable Pyrrolidinium Cations for Alkaline Anion Exchange Membrane Applications Fenglou Gu,† Huilong Dong,‡ Youyong Li,‡ Zhe Sun,† 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: The synthesis and characterization of pyrrolidinium cation based anion exchange membranes (AEMs) are reported. Pyrrolidinium cations with various N-substituents (including methyl, ethyl, butyl, octyl, isopropyl, 2-hydroxylethyl, benzyl, and cyclohexylmethyl groups) were synthesized and investigated with respect to their chemical stability in alkaline media. The influence of substitutions on alkaline stability of pyrrolidinium cations was investigated by quantitative 1H nuclear magnetic resonance spectroscopy (NMR) and theoretical approaches. N,N-Ethylmethyl-substituted pyrrolidinium cation ([EMPy]+) exhibited the highest alkaline stability in this study. The synthesized AEMs based on [EMPy]+ show promising alkaline stability in strongly basic solution. The study of this work should provide a feasible way for improving the alkaline stability of pyrrolidinium cation based AEMs.

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INTRODUCTION Alkaline anion exchange membrane fuel cells (AEMFCs) have recently attracted a great deal of academic and industrial attention because of their high energy-conversion efficiencies, low starting temperature, and low formation of pollutants.1−3 Compared with proton exchange membrane fuel cells (PEMFCs), AEMFCs provide potential advantages, including the use of nonprecious metal (such as Co, Ag, and Ni) catalysts, carbonfree supports, and enhanced tolerances to CO2 impurities in gaseous feeds.4−8 The anion exchange membrane (AEM) has been considered as one of the key components of AEMFCs which transports hydroxide ion from the cathode to the anode. From the viewpoint of application, an ideal AEM should possess high hydroxide ion conductivity, limited swelling, and high alkaline stability under vigorous conditions (high pH environment and elevated temperatures).9,10 Recently, AEMs based on various cations, including ammonium,12−17 phosphonium,18,19 guanidinium,20,21 imidazolium,22,23 metal cation,24 and benzimidazolium cations,25,26 have been developed and extensively studied. These AEMs showed potential in AEMFCs; however, the chemical stability, especially alkaline stability, of these AEMs presents major challenges that limit the use of AEMs in practical applications.27,28 Primary degradation generally involves hydroxide ion attack upon the polymer backbone and the cationic groups of AEMs in alkaline media. It has been well demonstrated that cationic group chemistry plays a major role in providing alkaline stability in AEMs.27 Therefore, it is desirable to investigate AEMs with various cationic groups and polymer compositions. © 2014 American Chemical Society

Substituting (or functionalization of) cationic moieties has been proven to be an effective approach to increase the alkaline stability of cationic groups. For example, electron donors or highly sterically blocking groups could protect the quaternary phosphonium cations against hydroxide attack.18a,c The prepared AEMs exhibit high stability in strongly basic solution. Crowding the C-2 position of benzimidazolium by installation of adjacent bulky groups could hamper nucleophilic attack of OH− and thus enhance the chemical stability of the hydroxide form.29,25a Imidazolium cations have been recently investigated as promising cations for application in AEMs. In our previous studies, we have investigated that the alkaline stability of imidazolium cations to attack of OH− by both experimental and computational methods.30,31 In these imidazolium cation studies, the imidazolium cation was attacked by OH− to give ring-opened products.27,32 Substitutions at the C-2, N-1, and N-3 atom positions were found to have a significant and positive impact on the alkaline stability of those imidazolium cations.30,31 Compared with imidazolium cation, pyrrolidinium cation exhibits a higher electrochemical stability because of its nonaromatic character, which can be beneficial for electrochemical applications.33 AEMs based on pyrrolidinium cation have been recently developed. The resultant polymeric membranes exhibited relatively high alkaline durability.34 However, the relationships Received: July 23, 2014 Revised: September 24, 2014 Published: September 30, 2014 6740

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Synthesis of 1-Octyl-1-methylpyrrolidinium Bromide ([OMPy][Br]). [OMPy][Br] was synthesized by stirring a mixture containing 1-methylpyrrolidine and an equivalent molar amount of 1-bromooctane at room temperature under a nitrogen atmosphere for 48 h. The product was washed three times with ethyl ether. Colorless viscous oil (yield: 85%). 1H NMR (400 MHz, D2O): 0.778−0.894 (t, 3H), 1.187−1.395 (m, 10H), 1.699−1.810 (m, 2H), 2.108−2.241 (s, 4H), 2.933−3.037 (s, 3H), 3.231−3.322 (t, 2H), 3.391−3.522 (m, 4H). Synthesis of 1-Benzyl-1-methylpyrrolidinium Chloride ([BenMPy][Cl]). [BenMPy][Cl] was synthesized by stirring a mixture containing 1-methylpyrrolidine and an equivalent molar amount of benzyl chloride at room temperature under a nitrogen atmosphere for 48 h. The product was washed three times with ethyl ether. White solid (yield: 94%). 1H NMR (400 MHz, D2O): 2.063−2.303 (s, 4H), 2.806−2.983 (s, 3H), 3.306−3.446 (m, 2H), 3.482−3.676 (m, 2H), 4.393−4.535 (s, 2H), 7.412−7.593 (m, 5H). Synthesis of 1-Cyclohexylmethyl-1-methylpyrrolidinium Bromide ([CHMPy][Br]). [CHMPy][Br] was synthesized by stirring a mixture containing 1-methylpyrrolidine and an equivalent molar amount of cyclohexylmethyl bromide at room temperature under a nitrogen atmosphere for 48 h. The product was washed three times with ethyl ether. Yellow solid (yield: 96%). 1H NMR (400 MHz, D2O): 1.074−1.210 (m, 3H), 1.240−1.377 (m, 2H), 1.552−1.637 (m, 1H), 1.641−1.730 (m, 2H), 1.757−1.846 (d, 2H), 1.856−1.957 (s, 1H), 2.103−2.231 (s, 4H), 2.979−3.064 (s, 3H), 3.158−3.236 (d, 2H), 3.389−3.573 (m, 4H). Synthesis of 1,2,3-Trimethylimidazolium Iodide ([TMIm]). [TMIm] was synthesized by stirring a mixture of 1,2-dimethylimidazole with an equivalent molar amount of iodomethane at room temperature under a nitrogen atmosphere for 48 h. The product was washed three times with ethyl ether. 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 Diallyldimethylammonium Iodide ([DADMA][I]). [DADMA][I] was synthesized by stirring a mixture containing diallylmethylamine and an equivalent molar amount of iodomethane at room temperature under a nitrogen atmosphere for 48 h. The resultant colorless viscous oil was washed with ethyl ether three times and then dried in a dynamic vacuum at room temperature. Colorless viscous oil (yield: 87%). 1H NMR (400 MHz, D2O): 5.943−6.110 (m, 2H), 5.604−5.759 (m, 4H), 3.836−3.914 (d, 4H), 2.947−3.045 (s, 6H). Synthesis of Diallylethylmethylammonium Bromide ([DAEMA][Br]). [DAEMA][Br] was synthesized by stirring a mixture containing diallylmethylamine and an equivalent molar amount of bromoethane at room temperature under a nitrogen atmosphere for 48 h. The resultant colorless viscous oil was washed with ethyl ether three times and then dried in a dynamic vacuum at room temperature. Colorless viscous oil (yield: 82%) 1H NMR (400 MHz, D2O): 5.919−6.076 (m, 2H), 5.599−5.733 (m, 4H), 3.777−3.912 (d, 4H), 3.247−3.362 (t, 2H), 2.873−2.966 (s, 3H), 1.259−1.387 (t, 3H). Synthesis of Diallylbenzylmethylammonium Chloride ([DABenMA][Cl]). [DABenMA][Cl] was synthesized by stirring a mixture containing diallylmethylamine and an equivalent molar amount of benzyl chloride at room temperature under a nitrogen atmosphere for 48 h. The resultant colorless viscous oil was washed with ethyl ether three times and then dried in a dynamic vacuum at room temperature. Colorless viscous oil (yield: 86%) 1H NMR (400 MHz, D2O): 7.472−7.594 (m, 5H), 6.006−6.146 (m, 2H), 5.610− 5.761 (m, 4H), 4.409−4.461 (s, 2H), 3.856−3.965 (m, 2H), 3.749− 3.853 (m, 2H), 2.837−2.921 (s, 3H). Synthesis of Pyrrolidinium Cation Based Polymer. The pyrrolidinium cation based polymers were synthesized via free radical polymerization of cationic monomer in aqueous solution. For example, a solution of [DAEMA][Br] (0.33 g, 1.5 mmol) dissolved in water (5 mL) was purged with nitrogen for 20 min before the initiator 2,2′azobis(2-methylpropionamidine) dihydrochloride (AAPH, 22 mg, 0.08 mmol) was added. Then the mixture was stirred at 60 °C for 12 h under a nitrogen atmosphere. The obtained viscous liquid was washed with cold diethyl ether three times and then dried under reduced pressure. [DADMA]+ cation based polymer was synthesized in the same way.

between chemical structure and alkaline stability of pyrrolidinium cations have not been systematically investigated as far as we know. In this work, we report on the synthesis and characterization of pyrrolidinium-based AEMs. A series of pyrrolidinium cations with various substituents were first synthesized, characterized, and compared with imidazolium and benzyltrimethylammonium based cations. The influence of substituents on alkaline stability of pyrrolidinium cations was investigated by both experimental (via 1H nuclear magnetic resonance analysis) and theoretical approaches. Pyrrolidinium-based AEMs with enhanced alkaline stability were synthesized by free radical polymerization of a diallylmethylamine hydrochloride monomer. These AEMs were evaluated for their alkaline stability, thermal properties, and conductivity and also compared with benzyltrimethylammonium based AEMs.

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EXPERIMENTAL SECTION

Materials. 1-Methylpyrrolidine, iodomethane, bromoethane, 2-bromoethanol, 2-bromopropane, 1-bromobutane, 1-bromooctane, benzyl chloride, cyclohexylmethyl bromide, diallylmethylamine, styrene, acrylonitrile, benzyltrimethylammonium chloride, divinylbenzene (DVB), (vinylbenyl)trimethylammonium chloride, benzoin ethyl ether, ethyl ether, ethyl acetate, acetonitrile, sodium hydroxide, and hydrochloric acid were used as received without further purification. All of the vinyl monomers were made inhibitor-free by passing them 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 our experiments. Synthesis and Characterization of Pyrrolidinium Salts. Synthesis of 1,1-Dimethylpyrrolidinium Iodide ([DMPy][I]). [DMPy][I] was synthesized by stirring a mixture of iodomethane (2.84 g, 0.02 mol) and 1-methylpyrrolidine (1.72 g, 0.02 mol) in ethyl acetate (15 mL) at room temperature under a nitrogen atmosphere for 48 h. After the evaporation of solvent, the mixture was washed three times with ethyl ether to obtain [DMPy][I]. Yellow solid (yield: 93%). 1H NMR (400 MHz, D2O): 3.463−3.545 (t, 4H), 3.088−3.160 (s, 6H), 2.166−2.264 (m, 4H). Synthesis of 1-Ethyl-1-methylpyrrolidinium Bromide ([EMPy][Br]). [EMPy][Br] was synthesized by stirring a mixture containing 1-methylpyrrolidine and an equivalent molar amount of bromoethane at room temperature under a nitrogen atmosphere for 48 h. The product was washed three times with ethyl ether. Colorless viscous oil (yield: 73%). 1H NMR (400 MHz, D2O): 1.269−1.394 (s, 3H), 2.082−2.231 (s, 4H), 2.896−3.022 (s, 3H), 3.306−3.517 (m, 6H). Synthesis of 1-[(2-Hydroxyl)ethyl]-1-methylpyrrolidinium Bromide ([HEMPy][Br]). [HEMPy][Br] was synthesized by stirring a mixture containing 1-methylpyrrolidine and an equivalent molar amount of 2-bromoethanol at room temperature under a nitrogen atmosphere for 48 h. The product was washed three times with ethyl ether. Colorless viscous oil (yield: 64%). 1H NMR (400 MHz, D2O): 2.153−2.305 (s, 4H), 3.075−3.130 (s, 3H), 3.488−3.636 (m, 6H), 4.009−4.078 (m, 2H), Synthesis of 1-Isopropyl-1-methylpyrrolidinium Bromide ([IMPy][Br]). [IMPy][Br] was synthesized by stirring a mixture containing 1-methylpyrrolidine and an equivalent molar amount of 2-bromopropane at room temperature under a nitrogen atmosphere for 48 h. The product was washed three times with ethyl ether. Yellow solid (yield: 79%). 1H NMR (400 MHz, D2O): 1.291−1.433 (s, 6H), 2.110−2.257 (s, 4H), 2.779−2.885 (s, 3H), 3.303−3.436 (m, 2H), 3.496−3.636 (m, 3H). Synthesis of 1-Butyl-1-methylpyrrolidinium Bromide ([BMPy][Br]). [BMPy][Br] was synthesized by stirring a mixture containing 1-methylpyrrolidine and an equivalent molar amount of 1-bromobutane at room temperature under a nitrogen atmosphere for 48 h. The product was washed three times with ethyl ether. White solid (yield: 83%). 1H NMR (400 MHz, D2O): 0.874−0.976 (t, 3H), 1.299−1.427 (m, 2H), 1.693−1.809 (m, 2H), 2.118−2.248 (s, 4H), 2.973−3.039 (s, 3H), 3.255−3.338 (t, 2H), 3.413−3.531 (m, 4H). 6741

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Preparation of Alkaline Anion Exchange Membranes. Alkaline anion exchange membranes were prepared via photo-crosslinking of [DAEMA]-based monomer with styrene and acrylonitrile using divinylbenzene as cross-linking agent following a process previously reported.23a For example, a homogeneous solution of styrene/acrylonitrile (1:3 weight ratio), [DAEMA]+ (33 wt %), divinylbenzene (4 wt % of the weight of monomer), and 2 wt % of benzoin ethyl ether (as a photoinitiator) was cast onto a glass mold and photo-cross-linked by irradiation with UV light of 250 nm wavelength for 40 min at room temperature. The resultant polymeric membranes were immersed in N2-saturated 1 M NaOH solution at 60 °C to convert the membranes from Br− to OH−. To ensure a complete conversion displacement, such a process was repeated three times. The converted polymeric membranes were then immersed in N2-saturated deionized water for 24 h until the pH of residual water was neutral. [DADMA]+, [DABenMA]+, and vinybenzyltrimethylammonium cation ([VTMA]+) based AEMs were also prepared in the same way. Characterization. 1H NMR spectra were measured using a Varian spectrometer at 400 MHz to evaluate the alkaline stability of pyrrolidinium cations and pyrrolidinium-based polymers. Thermogravimetric analysis (TGA) was done under nitrogen flow using a Universal Analysis 2000 instrument. The samples were heated from 30 to 550 °C at a heating rate of 15 °C min−1. Fourier transform infrared (FTIR) spectra of the membranes were recorded on a Varian CP-3800 spectrometer in the range of 4000−400 cm−1. Hydroxide Ion Conductivity. Hydroxide surface conductivity (σ, S cm−1) of each polymeric membrane can be calculated from the equation

σ=

standard solution (100 mL) for 24 h. Then the resulting solution was back-titrated with a NaOH standard solution with phenolphthalein as an indicator. The IEC value can be calculated using the equation IEC =

where Wdry is the mass of the dried membrane, CHCl and CNaOH are the concentration of HCl and NaOH solution, respectively, and VHCl and VNaOH are the volume of HCl solution and NaOH solution consumed in the titration, respectively. Alkaline Stability Measurements. Alkaline stability of pyrrolidinium, ammonium, and imidazolium cations was studied by NMR spectroscopy in N2-saturated CD3OD/D2O NaOH solutions (1, 2, and 4 M, respectively). Test solutions were placed in sealed fluoropolymer lined vessels and heated at 80 °C for various times. The alkaline stability of pyrrolidinium cation based polymers was performed in the same way. Computational Details and Analysis. Theoretical analyses of pyrrolidinium cations were evaluated by Dmol3 density functional code35,36 available in Materials Studio (Version 6.0). All calculations were carried out by the generalized gradient approximation functional by Beck exchange and Lee, Yang, and Parr correlation (GGABLYP)37,38a as well as the double numerical plus polarization (DNP) basis set. The single molecule geometries were fully optimized until a self-consistent field (SCF) convergence value of 10−6 Ha was reached. The convergence criteria were 5 × 10−6 Ha for energy, 0.005 Å for displacement, and 0.001 Ha/Å for gradient during the geometry optimization. The solvent effect was taken into consideration in the calculations by using a dielectric constant ε = 32.63 for methanol and ε = 78.54 for water. To ensure the frequencies are all normal, a frequency analysis was applied to all the optimized single molecules. In searching transition state (TS) structures, a complete linear synchronous transit/quadratic synchronous transit (LST/QST) method was adopted.38b Along the reaction coordinate, only one imaginary frequency was observed in all the obtained TS structures. The nudged-elastic band (NEB) method embedded in the transition state confirmation was performed to make sure that the TS geometries were the direct connection between the corresponding reactants and products.38c

l RTW

where R is the resistance value (Ω) of the membrane, l is the interelectrode separation (cm), and T and W are the thickness (cm) and width (cm) of the membrane, respectively. The ionic conductivities of membranes were obtained by a typical four-point probe technique on an electrochemical workstation (Zahner IM6 EX) over the scanning frequency range of 1−105 Hz. All the membrane samples (1 cm × 4 cm) were completely immersed in N2-saturated deionized water and free KOH was removed by rinsing repeatedly prior to the conductivity measurement. Conductivity measurements were conducted under fully hydrated conditions, and the impedance spectrum was collected. At a given temperature, all the polymeric membrane samples were equilibrated for at least 30 min before recording results. Water Uptake and Swelling Ratio. The fully hydrated membrane samples (in OH− form) were taken out of water, wiped with tissue paper, and weighed immediately. Then the polymeric membranes were dried at 80 °C under a vacuum to obtain a constant weight. The water uptake values of samples could be calculated as follows:

W (%) =

Wwet − Wdry Wdry

C HClVHCl − C NaOHVNaOH Wdry

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RESULTS AND DISCUSSION Alkaline Stability of N-Substituted Pyrrolidinium Cation. It has already been acknowledged that the polymer Scheme 1. Molecular Structures of Pyrrolidinium Cations Studied in This Work

× 100%

where Wwet and Wdry are the weight of water-swollen and corresponding dried polymeric membranes, respectively. The swelling ratio of the polymeric membranes was characterized by a linear expansion ratio, evaluated by the difference between wet and dry dimensions of a membrane sample (4 cm in length and 1 cm in width). The swelling degree can be calculated from swelling (%) =

Lwet − Ldry Ldry

× 100%

where Ldry and Lwet are the lengths of dry and wet samples, respectively. Ion Exchange Capacity (IEC) Measurements. Ion exchange capacities (IEC) of membranes were determined by a conventional back-titration method. The samples were immersed in 0.01 M HCl 6742

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concentration on the chemical stability of the pyrrolidinium cations were evaluated. Figure S1 shows the 1H NMR spectra of synthesized pyrrolidinium cations in 1 M NaOH mixed solution (VCD3OD/ VD2O = 3:1) at 80 °C. The methanol was applied as a cosolvent because the presence of methanol could accelerate the cation degradation and dissolve polyatomic cations better than pure aqueous solution.18c After being exposed to 1 M mixed NaOH solution for 168 h, new weak peaks occurred indicating the occurrence of cation degradation. Scheme 2 shows a possible degradation mechanism of pyrrolidinium cations in alkaline media at elevated temperature.40 The degradation degree of cations can be calculated by the relative integrated intensities of corresponding 1H NMR spectra.30,31 Among the N-substituted pyrrolidinium cations studied in this work, [HEMPy]+, [CHMPy]+, and [BenMPy]+ showed relatively poor alkaline stability. For example, more than 13% degradation of [HEMPy]+ was observed after 168 h testing in 1 M NaOH solution (Figure 1A). In order to further test the stability of N-substituted pyrrolidinium cations, the alkaline stability test was conducted in 2 and 4 M NaOH mixed solution (VCD3OD/VD2O = 3:1; see 1H NMR spectra in Figures S2 and S3). Among the pyrrolidinium cations studied in this work, [EMPy]+ shows the highest alkaline stability, and about 2.3% and 7.3% degraded in 2 M NaOH and in 4 M NaOH mixed solution after 96 h stability test (Figure 1A,B), respectively. As a consequence, the alkaline stability order of the pyrrolidinium cations studied in

Scheme 2. Degradation Mechanism of Pyrrolidinium Cations in Alkaline Solution

backbone usually weakens the NMR signal and thus makes it difficult to determine the mechanism of cation degradation.39 To investigate the chemical stability of pyrrolidinium cation based AEMs, therefore, small molecular N-substituted pyrrolidinium cations, including 1,1-dimethylpyrrolidinium ([DMPy]+), 1-ethyl-1-methylpyrrolidinium ([EMPy]+), 1-[(2hydroxyl)ethyl]-1-methylpyrrolidinium ([HEMPy]+), 1-isopropyl1-methylpyrrolidinium ([IMPy]+), 1-butyl-1-methylpyrrolidinium ([BMPy]+), 1-octyl-1-methylpyrrolidinium ([OMPy]+), 1-benzyl1-methylpyrrolidinium ([BenMPy]+), and 1-cyclohexylmethyl-1methylpyrrolidinium ([CHMPy]+) were synthesized and evaluated (shown in Scheme 1). The chemical structure and purity of these organic cations were confirmed by 1H NMR measurements. The alkaline stability of all the pyrrolidinium cations was investigated by quantitative NMR spectroscopy. The effects of substituents, basic solvent, and alkaline

Figure 1. Degradation degree of pyrrolidinium cations under different test conditions: (A) in 1 M NaOH (■) and 2 M NaOH (●) mixed solution (VCD3OD/VD2O = 3:1) at 80 °C for 168 and 96 h, respectively; (B) in 4 M NaOH mixed solution (VCD3OD/VD2O = 3:1) at 80 °C for 96 h (⧫); and (C) in 1 M NaOH mixed solution (VCD3OD/VD2O = 9:1) at 80 °C for 96 h (▼). 6743

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this work was [EMPy]+ > [OMPy]+ > [BMPy]+ > [IMPy]+ > [DMPy]+ > [CHMPy]+ > [BenMPy]+ > [HEMPy]+. Both ammonium- and imidazolium-based AEMs have been recently extensively studied.13,22 Here, the chemical stability of 1,2,3-trimethyimidazolium ([TMIm]+) and benzyltrimethylammonium ([BTMA]+) cations was also investigated and compared with [EMPy]+ and [OMPy]+ cations under the same experimental conditions, as shown in Figure S4. In order to accelerate the degree of cation degradation, the volume ratio of CD3OD was raised, and the chemical stability studies was conducted in 1 M NaOH (VCD3OD/VD2O = 9:1) mixed solution. It is obvious that [EMPy]+ and [OMPy]+ cations showed much better alkaline stability than did the [BTMA]+ and [TMIm]+ cations. For example, [EMPy]+ and [OMPy]+ cations degraded about 8.2% and 11.5%, respectively, while [BTMA]+ and [TMIm]+ cations degraded 31.4% and 37.6%, respectively, under the same experimental conditions. These results further confirmed the highest alkaline stability of [EMPy]+ from this work. These results motivated us to prepare [EMPy]+-based AEMs for AEMFCs. DFT Calculations. The alkaline stability of pyrrolidinium cations was further investigated by a theoretical approach. Figure S5 shows the Mulliken charge population analysis on the pyrrolidinium cations which confirmed the reaction pathway from a computational view. Based on the charge population on α-C and β-C of the pyrrolidine ring, it is supposed that the ringopening substitution reaction is mainly happened on α-C (see Scheme 2). Therefore, the alkaline stability of pyrrolidinium cations was further investigated by a theoretical approach. Figure 2A schematically illustrates an energy barrier associated with a transition state during hydroxide nucleophilic attack on a pyrrolidinium cation. All the DFT calculations are based on an ideal condition, that is, 0 K and 1 atm. The relative energies of all pairs of pyrrolidinium cation and hydroxide anion in initial state were set as zero. Here, Ebarrier represents the energy that OH− needed to overcome the energy difference between transition state (TS) and initial state and be used to evaluate the alkaline stability of pyrrolidinium cations. The higher the Ebarrier is, the harder it is for pyrrolidinium cation to be attacked by OH−. The Ebarrier values of all the pyrrolidinium cations studied in work are shown in Figure 2B. Among the pyrrolidinium cations, [EMPy]+ shows the highest Ebarrier value of 36.11 kcal/mol, indicating the most stable cation in strongly basic solution at elevated temperatures. These results are consistent with the cation degradation degree observed in experimental study. It is noteworthy that [DMPy]+ shows the lowest Ebarrier value. This result contrasts with our experimental observations. It is supposed that the α-C of pyrrolidinium ring is the most favorable site for OH− attack. In the case of computational analysis, there is more chance for OH− attack α-C of [DMPy]+ since [DMPy]+ is a symmetric cation with two identical α-C positions. Furthermore, [HEMPy]+, which shows the poorest alkaline stability, exhibits an Ebarrier value of 33.05 kcal/mol, which is even higher than that of [CHMPy]+, [BenMPy]+, and [DMPy]+. The electronegativity of O is greater than that of C, and there is no conjugation effect between two lone pairs of electrons of O and any other functional group. Thus, the group of −OH mainly shows inductively effect toward the adjacent −CH2− which can account for the lowest alkaline stability of [HEMPy]+ in experiment. The steric hindrance and electron-donating effects enable [IMPy]+ to be more stable than [DMPy]+ while replacing H of methyl

Figure 2. (A) Energy diagram of pyrrolidinium cation during the reaction coordinates. (B) Energy barriers of (A) [EMPy]+, (B) [OMPy]+, (C) [BMPy]+, (D) [IMPy]+, (E) [HEMPy]+, (F) [CHMPy]+, (G) [BenMPy]+, and (H) [DMPy]+.

Scheme 3. Synthesis of Pyrrolidinium-Based Polymer

by cyclohexyl or phenyl makes pyrrolidinium cations degrade more quickly than [DMPy]+. The reason that [BenMPy]+ shows worse alkaline stability than [CHMPy]+ might be the electron-withdrawing effect of phenyl, while the steric hindrance effect enables [BenMPy]+ to be more stable than [HEMPy]+ in basic solutions. Based on the results of alkaline stability studies of pyrrolidinium cations, polymers based on the [EMPy]+ cation ([PEMPy]+) and the [DMPy]+ cation ([PDMPy]+) were synthesized via free radical polymerization (Scheme 3). Alkaline 6744

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Figure 3. 1H NMR spectra of (A) poly(dimethylpyrrolidinium) and (B) poly(1-ethyl-1-methylpyrrolidinium) in 1 M NaOH CD3OD/D2O solution (VCD3OD/VD2O = 9:1) at 80 °C for 96 h.

stability of the synthesized polymers was also studied in 1 M NaOH mixed solution (VCD3OD/VD2O = 9:1) at 80 °C, as shown in Figure 3. The degradation degree of [PDMPy]+ and [PEMPy]+ was calculated by the integration change of peak 3 with peak 2 as standard peak for normalization (see Figure 3). In Figure 3B, the peak densities increased at about 3.2 and 2.2 ppm after alkaline test, indicating the degradation of peaks 3 and 4, respectively. About 8.7% and 19.4% cation degradation was observed for cationic polymers [PEMPy]+ and [PDMPy]+, respectively. These results are consistent with the small molecule model compounds studied above, indicating that [PEMPy]+ is alkaline stable. Pyrrolidinium-based AEMs were also prepared by photocross-linking. In order to achieve AEMs with low swelling, good mechanical properties, and high hydroxide conductivity, polymeric membranes were fabricated by copolymerization of diallylethylmethylammonium bromide, styrene, and acrylonitrile with divinylbenzene as cross-linking agent (shown in Scheme 4). [DADMA]+ ([PDMPy][OH]), [DABenMA]+ ([PBenMPy][OH]), and [VTMA]+ ([PVTMA][OH]) based AEMs were prepared in the same way. Figure S6 shows a photo of such a pyrrolidinium-based membrane ([PEMPy][OH]), which is flexible, transparent, and could be cut into desired sizes easily. Figure S7 shows FTIR spectra of [PEMPy][OH] before and after the alkaline stability test. All the membranes show an absorption band due to cyano groups (CN) at about 2236 cm−1, while the absorption peak at 2934 cm−1 belongs to −CH3 and −CH2−. The absorption peaks at 1193 cm−1 are characteristic peaks of pyrrolidinium cations, and the absorption peaks at 1193 cm−1 belong to benzene rings. These results clearly confirm the successful synthesis of pyrrolidinium-based polymeric membranes. It should be noted that no new peaks and obvious shift of the absorption peaks were observed before

Scheme 4. Synthesis of Pyrrolidinium-Based Anion Exchange Membrane

and after alkaline stability test, indicating the high alkaline stability of the prepared [PEMPy][OH] polymeric membranes. Good thermal stability is required for AEMs used as electrolytes for AEMFCs. Figure S8 shows a typical thermogravimetric analysis (TGA) of pyrrolidinium-based polymer ([PEMPy][Br]) and membrane ([PEMPy][OH]). The pyrrolidinium-based polymer and membrane show less than 2.0% weight loss below 255 °C. The first degradative weight loss starts at about 260 °C is associated with the degradation of pyrrolidinium cations, while the decomposition temperature at about 350 °C can be ascribed to the degradtion of polymer backbone bonds. These results confirm that pyrrolidinium-based polymer and membrane possess high thermal stability, far beyond the range of interest for application in AEMFCs. Table 1 shows values of IEC, water uptake, swelling degree, and conductivity for our [PEMPy][OH] membranes. These fully hydrated polymeric membranes exhibited conductivities higher than 1.0 × 10−2 S cm−1 at 30 °C, which is competitive with typical anion conductivity in hydrated AEMs and fulfills

Table 1. Ion Exchange Capacity (IEC), Water Uptake, Swelling Degree, and Conductivity of [PEMPy][OH] Membranes IEC value (mequiv g−1)

a

a

membrane

theor

[PEMPy][OH] QPAE-c41 PNDB-OH-1042 PNDB-OH-2242

1.49 2.01

exptl

b

1.41 ± 0.08 1.45 1.13 1.43

conductivity (× 10−2 S cm−1) water uptake (%)

swelling degree (%)

30 °C

60 °C

87.07 ± 7.36 15.7 182.8 267.2

24.23 ± 3.67 10.2 ∼20.0 ∼25.0

1.27 ± 0.08 0.34 (20 °C) 0. 8 1.3

1.77 ± 0.13 1.54 (80 °C) 1.50 2.15

Calculated from monomer ratio. bAfter immersing in water at 30 °C for 24 h, average of three trials. 6745

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Table 2. Change of Mechanical Properties and Ion-Exchange Capacity (IEC) Values of AEMs after Immersion in 1 M NaOH Solution at 80 °C AEM [PEMPy][OH]

[PDMPy][OH]

[PBenMPy][OH]

[PVTMA][OH]

time (h) 0 72 168 0 72 168 0 72 168 0 72 168

tensile strength (MPa) 7.52 5.64 4.73 7.67 4.61 3.37 7.83 4.13 2.89 6.35 4.27 3.19

± ± ± ± ± ± ± ± ± ± ± ±

elongation at break (%)

1.14 0.77 0.68 1.09 0.63 0.52 1.23 0.56 0.44 0.90 0.59 0.45

8.26 6.14 6.53 7.93 5.88 4.71 7.67 4.51 3.63 6.73 4.09 3.78

± ± ± ± ± ± ± ± ± ± ± ±

1.32 0.93 0.96 1.14 0.82 0.69 1.12 0.63 0.52 1.04 0.60 0.56

IEC (mequiv/g) 1.41 1.44 1.40 1.43 1.27 1.13 1.47 1.02 0.75 0.96 0.76 0.63

± ± ± ± ± ± ± ± ± ± ± ±

0.042 0.045 0.040 0.046 0.039 0.037 0.051 0.037 0.033 0.035 0.030 0.025

Figure 4. (A) Conductivity of pyrrolidinium-based membranes as a function of time after immersion in N2-saturated 1 M NaOH aqueous solution at 80 °C. (B) Variation in conductivity of [PEMPy][OH] membranes as a function of time after immersion in 15 M NaOH aqueous solution at 30 °C.

after 240 h test in 1 M NaOH solution at 80 °C. The conductivity degradation degree of [PEMPy][OH] membrane is quite lower than the chemical degradation degree of [PEMPy]+ obtained in Figure 3. Such a difference might be due to the existence of CD3OD which could accelerate the degradation of [PEMPy]+ in an NMR test tube, while the AEMs are tested in NaOH aqueous solution, without the presence of methanol.18c

the basic conductivity requirement for fuel cell applications.23c,41,42 Compared with AEMs reported, [PEMPy][OH] membrane shows comparable conductivity while with relatively lower water uptake and swelling degree,41,42 indicating that the AEMs synthesized in this work are suitable for the AEMFCs applications. Table 2 shows the changes of mechanical properties and IEC values of AEMs before and after the alkaline stability test. It can be clearly seen that the mechanical properties of all the prepared AEMs slightly decreased. Such a decrement of mechanical properties is might due to the disentanglement of some un-cross-linked polymer chains in the membranes.23a However, the degradation of cations may also decrease the mechanical properties of AEMs. Further study is needed for better understanding. The IEC values of [PEMPy][OH] are almost unchanged before and after the test, which further confirms the high alkaline stability of the membranes. However, the IEC values of both [PBenMPy][OH] and [PVTMA][OH] were dramatically decreased after 168 h test probably due to the poor alkaline stability of cations. The hydroxide stability of the AEMs in strongly basic solution was examined by monitoring conductivity over time (Figure 4). The conductivity of [PEMPy][OH] decreased less than 3.0% over 18 days in 1 M NaOH solution at 80 °C and less than 4% in 15 M NaOH solution at 30 °C over the same period. These results further support the FTIR results, as shown in Figure S7, indicating excellent alkaline stability of [PEMPy][OH] membranes in strongly basic solution. On the other hand, more than 20.0% degradation degree of [PDMPy][OH], [PBenMPy][OH], and [PVTMA][OH] was observed

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CONCLUSIONS A series of N-substituted pyrrolidinium cations were synthesized and characterized. The effects of N-substituents on alkaline stability of the respective pyrrolidinium cations and corresponding AEMs were investigated by quantitative 1H NMR spectroscopy and computational analysis. The [EMPy]+ cation exhibited excellent alkaline stability in strongly basic solution. Furthermore, [EMPy]+ based AEMs possessed high chemical stability in alkaline solutions as well. We believe this work should provide means for improving the alkaline stability of pyrrolidinium cation based AEMs with improved longevity and performance.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR characterization of the synthesized compounds and TGA analysis of prepared AEMs. 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] (F.Y.). 6746

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Notes

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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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