- Email: [email protected]
Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Preparation and characterization of imidazoliumbased membranes for anion exchange membrane fuel cell applications Bencai Lin a,*, Gang Qiao a, Fuqiang Chu a,b, Juan Wang a, Tianying Feng a, Ningyi Yuan a, Shuai Zhang a, Xin Zhang a, Jianning Ding a,c,** a
School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou, 213164, Jiangsu, China b Jiangnan Graphene Research Institute, Changzhou 213100, Jiangsu, China c Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang, 212013, China
article info
abstract
Article history:
New anion exchange membranes (AEMs) with high conductivity, good dimensional and
Received 4 October 2016
alkaline stability are currently required in order to develop alkaline fuel cells into efficient
Received in revised form
and clean energy conversion devices. In this study, a series of AEMs based on 1, 2-
16 November 2016
dimethyl-3-(4-vinylbenzyl) imidazolium chloride ([DMVIm][Cl]) are prepared and investi-
Accepted 25 November 2016
gated. [DMVIm][Cl] is synthesized and used as ion carriers and hydrophilic phase in the
Available online xxx
membranes. The water uptake, swelling ratio, IEC and conductivity of the AEMs increase with increasing the [DMVIm][Cl]. The imidazolium-based AEMs show excellent thermal
Keywords:
stability, sufficient mechanical strength, the membrane which containing 30% mass frac-
Anion exchange membrane
tion of [DMVIm][Cl] shows conductivity up to 1.0 102 S cm1 at room temperature and
Fuel cell
good long-term alkaline stability in 1 M KOH solution at 80 C. The results of this study
Alkaline stability
suggest that this type of AEMs have good perspectives for alkaline anion exchange mem-
Imidazolium-based membranes
brane fuel cell applications.
High conductivity
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells, which convert chemical energy directly to electrical energy and avoid the limitations of Carnot cycle, are considered as one of the most promising power generation technologies [1e4]. Among various types of fuel cells, alkaline anion exchange membrane fuel cells (AEMFCs) which utilize anion exchange membranes (AEMs) as solid polymer
electrolytes have been extensively investigated over the past few decades. Compared with proton exchange membrane fuel cells with DuPont's Nafion® electrolyte membranes, AEMFCs which working under alkaline condition enable the use of non-precious metal catalysts instead of platinum catalysts [5,6]. In addition, the application of AEMFCs with solid electrolytes (AEMs) avoiding the leakage problem of traditional alkaline fuel cell with a KOH solution as
* Corresponding author. ** Corresponding author. School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou, 213164, Jiangsu, China. E-mail addresses: [email protected] (B. Lin), [email protected] (J. Ding). http://dx.doi.org/10.1016/j.ijhydene.2016.11.169 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
electrolyte. As the key component of AEMFCs, AEMs act both as a barrier to separate the fuel and an electrolyte to transport hydroxide anion from the anode to the cathode of fuel cells. An typical AEMs is composed of a polymer backbone onto which cationic groups are generally covalently bound, either as the side chains or onto the backbones (be an integral part of the backbone). It is demonstrated that the alkaline stability of the AEMs is fundamentally affected by the chemical structures of both polymer backbone and cationic groups [7,8]. To fulfill the application of AEMFCs, an ideal AEM should possess high conductivity, good thermal stability, sufficient mechanical properties, excellent alkaline stability and sufficient long-term durability at elevated temperatures under alkaline condition. However, the lack of commercially available AEMs with excellent alkaline stability has limited the application of AEMs in the AEMFCs, and the degradation of the polymer backbone and covalently tethered cationic groups, may be triggered by the high nucleophilicity and basicity of hydroxide ions [9]. Therefore, the major challenge in developing AEMFCs is to develop novel AEMs which maintain high conductivity, good alkaline stability, good thermal stability and sufficient mechanical strength under fuel cell operating conditions. The number of publications on AEMs has greatly increased from 2009, a variety of AEMs based on poly(sulfone)s, poly(arylene ether)s, poly(phenylene)s, poly(styrene)s, polypropylene, poly(phenylene oxide)s and poly(olefin)s, have been synthesized for the application of AEMs [10e20]. All of the AEMs mentioned above showed a similar chemical structure that a polymer backbone with pendent quaternary ammonium aliphatic side chains. Generally, once a chemical stable cationic group is identified, an appropriate backbone can then be rationally designed [21,22]. Therefore, cationic species, beside quaternary ammonium (QA), phosphonium [23], imidazolium [24e28], benzimidazolium [29], metal-cation [30], guanidinium [31], pyridinium [32], tertiary sulfonium [33], and pyrrolidinium [34] based AEMs also have been synthesized and investigated. Although, some of these polymeric AEMs showed relatively high conductivity and potential applications in AEMFCs, the alkaline stability of the AEMs should be enhanced for long-term application. Among the various cationic groups, imidazolium-based membranes have attracted much attention due to its robust alkaline stability resulted by the p-electrons in the imidazole rings. In 2010, Fang et al. [35] and Yan et al. [24] firstly reported imidazolium cationic groups-based AEMs, both of their results showed the imidazolium cations have excellent alkaline stability in high pH solution at 60 C. From then on, many publications of imidazolium-based AEMs had appeared. However, in Elabd's work, the degradation of imidazolium cations was observed under dry conditions, higher temperature and higher alkaline concentrations [36]. More recently, we found that the alkaline stability of imidazolium cations relate to its structure, and the C2-substituted imidazolium cations is much more stable than that of C2-unsubstituted imidazolium cations [37]. Among the C2-substituted imidazolium cations, methyl substituted imidazolium cations showed the best alkaline stability. However, to understand the effect of the content of C2-substituted imidazolium salts on the properties of AEMs, we need a systematic study.
In the present work, we describe the synthesis and characterization of C2-substituted imidazolium salt, 1, 2dimethyl-3-(4-vinylbenzyl) imidazolium chloride, and imidazolium-based AEMs. [DMVIm][Cl] was used as hydrophilic phase and anion carrier in the membrane because of its excellent alkaline stability at elevated temperature. The effect of the [DMVIm][Cl] content on the properties of AEMs was investigated with respect to their water uptake, swelling ratio, thermal stability, mechanical properties, alkaline stability and ionic conductivity.
Experimental Materials Styrene, 4-vinylbenzyl chloride, 1, 2-dimethylimidazole, acrylonitrile, benzoin ethylether, diethyl ether and divinylbenzene (DVB) were obtained from Aldrich. Potassium hydroxide, ethyl acetate, sodium hydroxide and hydrochloric acid were purchased from Alfa Aesar. 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, 2-dimethyl-3-(4-vinylbenzyl) imidazolium chloride ([DMVIm][Cl]) 1, 2-Dimethyl-3-(4-vinylbenzyl) imidazolium chloride 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 diethyl ether four times and then dried in dynamic vacuum at room temperature. 1H NMR (400 MHz, D2O): 7.50e7.53 (d, 2H), 7.27e7.34 (m, 4H), 6.73e6.76 (m, 1H), 5.83e5.88 (d, 1H), 5.36e5.37 (d, 1H), 5.31 (s, 2H), 3.76 (s, 3H), 2.55 (s, 3H).
Preparation of imidazolium-based 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 was stirred to obtain a homogeneous solution. Then, the mixture was casted onto a glass mold irradiation with UV light. The prepared membranes were immersed in 1 M KOH solution at 60 C for 24 h to convert the anion of membrane from Cl to OH. Then the membranes was immersed in deionized water and washed with deionized water until the pH of residual water was neutral.
Characterization Fourier transform infrared (FT-IR) spectra of the polymers were recorded on a Varian CP-3800 spectrometer in the range of 4000e400 cm1. 1H NMR spectra were recorded on a Varian 400 MHz spectrometer. Thermal analysis was carried out by Universal Analysis 2000 thermogravimetric analyzer (TGA), and samples were heated from 30 to 650 C at a heating rate of 10 C min1 under a nitrogen flow. The tensile properties of
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
3
membranes were measured by using an Instron 3365 at 25 C at a crosshead speed of 5 mm min1. AFM images were recorded using Agilent AFM with Pico plus molecular imaging system in the semi-contact mode at room temperature.
phenolphthalein as an indicator. The IEC value was calculated using the expression:
Conductivity
where V0,NaOH and Vx,NaOH are the volume of the NaOH solution consumed in the titration without and with membrane samples, respectively. CNaOH is the mol concentration of the NaOH solution, which are titrated by the standard oxalic acid solution, mdry is the mass of the dry membranes.
The resistance value of the membranes was measured by four-point probe alternating current (ac) impedance spectroscopy using an electrode system connected with an electrochemical workstation (Zahner IM6 EX). All the samples were fully hydrated in 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 in the temperature range 30e90 C. At a given temperature, the samples were equilibrated for at least 30 min before recording measurements. Repeated measurements were taken with 10 min interval until no more change in conductivity was observed. The conductivity s (S cm1) of the membrane can be calculated from: s¼
l RA
V0;NaOH CNaOH Vx;NaOH CNaOH mdry
Alkaline stability of imidazolium-based composite membranes The alkaline stability of the membranes was examined by immersing the membrane samples in 2 M KOH solution at 80 C for various time. The changes of hydroxide conductivity and IEC values of the membrane were measured to evaluate the degradation of membranes.
Results and discussion
where l is the distance (cm) between two inner electrodes, A is the cross-sectional area (cm2) of the membrane, obtained from the membrane thickness multiplied by its width, and R is the membrane resistance (U).
Water uptake and swelling ratio The membrane samples were immersed in deionized water at room temperature for 24 h. Then, the hydrated polymer membranes were taken out, and the excess water on the surface of membranes was removed by wiping with a tissue paper and weighed immediately, and a wet weight was obtained (Ww). Then the wet membrane was dried under vacuum at 80 C until a constant dry weight was obtained (Wd). The water uptake (WU) was calculated with the following equation: WUð%Þ ¼
ðWW Wd Þ 100% Wd
where Wd and Ww are the mass of the dry and wet membrane samples, respectively. The swelling ratio (SR) was characterized by linear expansion ratio, which was determined by the difference between wet and dry dimensions of a membrane sample. The calculation was based on the following equation: SRð%Þ ¼
IEC ¼
Xwet Xdry 100% Xdry
Preparation of imidazolium-based membranes In the present work, [DMVIm][Cl] with 1, 2-dimethylimidazolium cations was synthesized and used as d hydrophilic phase in the composite membranes, and the synthetic procedure for [DMVIm][Cl] as shown in Scheme 1. The purity and chemical structure of [DMVIm][Cl] were confirmed by 1H NMR measurements. Our previous work showed that 1, 2-dimethylimidazolium cations have excellent alkaline stability under high pH condition at elevated temperature [37], and the polymer containing 1, 2-dimethylimidazolium cations must be a good candidate as AEM materials. However, poly(1, 2dimethyl-3-(4-vinylbenzyl) imidazolium chloride), the homopolymer of [DMVIm][Cl], shows poor film forming properties, and it is very soluble in water. Therefore, styrene and acrylonitrile were chose to synthesize the copolymers with the ease of processing, good mechanical properties and chemical resistance. The copolymer membranes based on [DMVIm][Cl] were prepared by photo polymerization in a glass mold, and the preparation of imidazolium-based composite membranes is shown as Scheme 2. The produced membranes in Cl form were changed into OH form by immersed them in 1 M KOH solution, and the obtained membranes in OH form denoted as SANx-[DMVIm][OH]y, and x, y indicates the weight ratio of styrene/acrylonitrile and [DMVIm][Cl] in the membrane, respectively. Fig. 1 shows the photographs of SAN70-[DMVIm] [OH]30 with the thickness of about 100 mm. The membrane is transparent, yellow (in the web version), flexible, can be easily
where Xwet and Xdry are the lengths of wet and dry membranes, respectively.
Ion exchange capacity (IEC) Ion exchange capacities (IEC) of membranes were determined by a back-titration. The AEMs were immersed in 100 mL of 0.01 M HCl standard solution for 24 h. Then the solutions were titrated with a standardized NaOH solution using
Scheme 1 e The synthetic procedure for [DMVIm][Cl].
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
Scheme 2 e Reaction scheme for the preparation of SANx-[DMVIm][OH]y.
Fig. 1 e Photographs of SAN70-[DMVIm][OH]30 with the thickness of ~100 mm.
cut into any desired sizes, and it shows sufficient mechanical strength for the application as an AEM.
FT-IR spectra The chemical structure of styrene and SANx-[DMVIm][OH]y was confirmed by FT-IR spectroscopy, and the infrared spectra are shown in Fig. 2. For the spectrum of styrene, the strong absorption bond at 1630 cm1 are the characteristic absorption peaks of C]C, and the absorption bands at about 906 cm1 arose from the bending vibration of C]CeH. For all
the spectra of SANx-[DMVIm][OH]y, these absorption bands disappeared due to the polymerization of styrene. For all the membrane samples, the absorption bands at about 2234 cm1 arose from the stretching vibration of the cyano groups (C^N). Absorption bonds in the range of 3000e3500 cm1 was attributed to the stretching vibration of OeH. The absorption bands at about 2928 and 2848 cm1 arose from the stretching vibration of eCH3 and eCH2e. Absorption peaks at ~1583 cm1 and ~761 cm1 confirm the presence of imidazolium cations [24]. The content of [DMVIm][Cl] increases from SAN90[DMVIm][OH]10 to SAN50-[DMVIm][OH]50, and this trend was confirmed by a gradual increase in the intensity of the characteristic peaks of imidazolium cations. The results of FT-IR spectra confirm the chemical structure of SANx-[DMVIm] [OH]y.
Thermal analysis
Fig. 2 e FT-IR spectra of SANx-[DMVIm][OH]y.
The thermal stabilities of SANx-[DMVIm][OH]y were investigated by TGA (N2 at 10 C min1), and the results are showed in Fig. 3. Fig. 3 shows that all the membrane samples display three weight lose stages around 100, 320 and 380 C. The first weight loss (about 4 wt%) below 120 C due to the dehydration of membrane samples. The weight loss region at temperatures about 320 C due to the degradation of the side groups (such as imidazolium cations) of the copolymer. The third weight loss (about 50 wt%) of membranes, beginning at about 380 C, are considered to be caused by the degradation of main chain of the copolymer. These TGA curves reveal that this type of cross-linked membranes have high degradation temperatures. No noticeable decomposition was observed for SANx[DMVIm][OH]y till 200 C, which confirming that this type of
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
5
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
Fig. 3 e TGA curves of SANx-[DMVIm][OH]y under nitrogen flow. Heating rate: 10 C/min.
composite membranes indeed confers a high thermal stability for application in AEMFCs.
Mechanical properties The AEMs must have sufficient mechanical properties for the application using in AEMFCs. Here, the mechanical properties of SANx-[DMVIm][OH]y were measured by a universal testing machine with a crosshead speed of 5 mm min1, and the results are listed in Table 1. The tensile strength of SANx[DMVIm][OH]y is in the range of 18e52 MPa, with the tensile modulus of 410e1278 MPa, and the values of elongation at break of 10e37%. In the present work, [DMVIm][Cl] containing the exchangeable groups act both as hydrophilic phase and ion carriers in the membranes, and the hydrophilic phase of the membranes increased with increasing the [DMVIm][Cl] content. Therefore, the tensile strength and tensile modulus of membranes decrease with increasing the [DMVIm][Cl] content in the membranes due to its hydrophilic nature, and too high [DMVIm][Cl] content resulted in poor mechanical properties of membranes. Though the mechanical properties of membranes were decreased by increasing the content of [DMVIm][Cl], the mechanical properties of SAN70-[DMVIm] [OH]30 are equivalent to those of quaternary ammonia poly(arylene ether sulfone)s with aromatic main chain [9,38]. These results show that SANx-[DMVIm][OH]y membranes could possess sufficient mechanical strength for fuel cell applications by changing the content of [DMVIm][Cl] in the membranes.
Fig. 4 e Conductivity of SANx-[DMVIm][OH]y as a function of temperature.
IEC, water uptake and swelling ratio Table 1 lists the IEC values, water uptake and swelling ratio of SANx-[DMVIm][OH]y at room temperature. Here, the exchangeable groups of AEMs was provided by [DMVIm][Cl], and it is no surprise that the IEC values of the produced membranes increased with increasing the content of [DMVIm][Cl]. The IEC values of SANx-[DMVIm][OH]y samples determined by titrations are in range of 0.38e1.98 meq g1, and were found to be close to the theoretical IEC values (0.40e2.02 meq g1). AEMs with various IECs values could be easily obtained by adjusting the content of [DMVIm][Cl] in the membranes. Generally, the conductivity of AEMs largely depend on the IEC of membranes, and the conductivity of the composite membranes could be increased by increasing the [DMVIm][Cl] content in this work. Water uptake is another important parameter, which could affect the conductivity of membranes, it also have effect on the dimensional stability and mechanical properties of membranes. The water uptake of SANx-[DMVIm][OH]y samples increases with increasing the content of [DMVIm][Cl] in the membranes. For example, the water uptake of SAN90-[DMVIm] [OH]10 is 24%, and the value for SAN50-[DMVIm][OH]50 is 135%. The change of swelling ratio of SANx-[DMVIm][OH]y in accordance with that of the water uptake of the membranes. Generally, the high IEC values and high water uptake result in high conductivity, while reduce the mechanical properties of membranes. This phenomenon is already confirmed by the mechanical properties of SANx[DMVIm][OH]y listed in Table 1. Therefore, the content of
Table 1 e Mechanical properties of SANx-[DMVIm][OH]y membranes. Membranes SAN90-[DMVIm][OH]10 SAN80-[DMVIm][OH]20 SAN70-[DMVIm][OH]30 SAN60-[DMVIm][OH]40 SAN50-[DMVIm][OH]50
Tensile strength (MPa) 51.57 ± 43.44 ± 36.77 ± 28.13 ± 18.80 ±
0.41 1.60 0.53 1.51 2.55
Tensile modulus (MPa) 1278.07 ± 38.15 692.36 ± 20.36 543.41 ± 21.55 484.55 ± 30.50 410.33 ± 50.14
Elongation at break (%) 10.51 13.45 20.52 26.09 36.47
± 0.41 ± 0.83 ± 0.74 ± 1.03 ± 0.52
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
Fig. 5 e AFM height image (A) and phase image (B) of SAN70-[DMVIm][OH]30.
[DMVIm][Cl] in the membranes should be carefully controlled to balance the conductivity and mechanical properties of membranes.
The conductivity activation energy (Ea) of SANx-[DMVIm] [OH]y can be obtained from the conductivity of the membranes. Fig. 6 displays the plots of ln(s) vs 1000/T (T is the absolute temperature) for SANx-[DMVIm][OH]y. The Ea values
Conductivity The ionic conductivities of SANx-[DMVIm][OH]y at different temperatures are shown in Fig. 4. The composite membranes showed an increased conductivity of above 102 S cm1 when temperatures ranged from 30 C to 90 C. On the one hand, higher temperature always results in faster migration of ions. On the other hand, wider ion transport channels will be formed in the membranes by adsorbing more water at a higher temperature, which leading to a higher conductivity [25]. For example, the conductivity of SAN90-[DMVIm][OH]10 increased from 1.05 102 S cm1 at 30 C to 2.19 102 S cm1 at 90 C. At a given temperature, the conductivity of SANx-[DMVIm][OH]y increased with increasing the content of [DMVIm][Cl], which is like the changes of water uptake and swelling ratio. The density of ionizable functional groups link to the content of [DMVIm][Cl] in the membranes, the more [DMVIm][Cl] content results in higher water uptake and higher IEC of AEMs. The morphology of SAN70-[DMVIm] [OH]30 was studied using AFM, and the results showed as Fig. 5. A rough surfaces of SAN70-[DMVIm][OH]30 could been found in the height images (Fig. 5A), which caused by the rough surfaces of glass mold during the preparation process of membranes. The minimal correlation between phase images (Fig. 5B) and height images (Fig. 5A) indicates that phase images are unlikely to be an artifact of surface roughness [39], a microscopic phase separation structure could be observed. Compare with the imidazolium functionalized poly (ether sulfone) membranes [40], the AEMs produced in the present work showed much higher conductivity, probably due to their aliphatic chain which resulted in a higher water uptake and the formation of microscopic phase separation structure in the membranes. We believe that such a microscopic phase separation structure favor the formation of the ion transport channel, which therefore enhance the conductivity of the AEMs.
Fig. 6 e Arrhenius plots of SANx-[DMVIm][OH]y.
Fig. 7 e Conductivity of SAN70-[DMVIm][OH]30 after treatment with 2 M KOH solution at 80 C.
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
7
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
Table 2 e Water uptake, swelling degree, IEC and conductivity of SANx-[DMVIm][OH]y. IEC (meq g1)
Membrane
Theoretical SAN90-[DMVIm][OH]10 SAN80-[DMVIm][OH]20 SAN70-[DMVIm][OH]30 SAN60-[DMVIm][OH]40 SAN50-[DMVIm][OH]50
Water uptake (%)
Experimental
0.40 0.81 1.21 1.62 2.02
0.38 0.76 1.16 1.57 1.98
± 0.02 ± 0.04 ± 0.02 ± 0.04 ± 0.06
of SANx-[DMVIm][OH]y decreased with increasing the content of [DMVIm][Cl]. For example, the Ea of SAN90-[DMVIm][OH]10 decreased from 11.04 kJ mol1 to 6.21 kJ mol1 for SAN50[DMVIm][OH]50. SAN90-[DMVIm][OH]10 shows the highest Ea value but the lowest conductivity under the same conditions among these composite membranes, indicating that the transport of OHein SAN90-[DMVIm][OH]10 needs most energy due to its lowest density of ionizable functional groups and least free volume.
Alkaline stability From the point of view of practical applications, a major concern in AEMs is their alkaline stability at elevated temperature. To investigate the membrane alkaline stability at elevated temperature, SAN70-[DMVIm][OH]30 membranes were immersed in a hot alkaline solution (2 M KOH solution at 80 C) for various time. The alkaline stability of membrane was evaluated via conductivity and IEC measurements after stability testing. Fig. 7 shows the conductivity of SAN70-[DMVIm] [OH]30 after treatment with 2 M KOH solution at 80 C for various time. As shown in Fig. 7, no obviously decrease in conductivity was observed after stability test at 80 C, and SAN70-[DMVIm][OH]30 maintain more than 95% of the original conductivity even after 480 h, which indicating the excellent alkaline stability of SAN70-[DMVIm][OH]30 in 2 M KOH solution at 80 C. The IEC values of SAN70-[DMVIm][OH]30 were measured during the alkaline stability test in 2 M KOH aqueous solution, and the results summarized in Table 2. The IEC of SAN70-[DMVIm][OH]30 only changed slightly from 1.20 to 1.13 meq g1 even after 480 h alkaline stability test, further confirm the excellent alkaline stability of the composite membranes at elevated temperature. The quaternary ammonium based polymers can be degraded in alkaline solutions at high temperature by the nucleophilic attack reaction and bhydrogen (Hofmann or E2) elimination [6,37]. Here, the SANx[DMVIm][OH]y with 1, 2-dimethylimidazolium cations shows excellent alkaline stability due to the internal conjugation of
Table 3 e IEC values of SAN70-[DMVIm][OH]30 after treatment with 2 M KOH solution at 80 C. Time (h) 24 120 240 360 480
Swelling ratio (%)
IEC (meq g1) 1.20 1.18 1.20 1.16 1.13
± 0.06 ± 0.05 ± 0.07 ± 0.07 ± 0.06
24.49 ± 1.18 48.64 ± 2.48 63.00 ± 3.19 107.25 ± 3.25 135.08 ± 3.43
9.33 ± 1.69 23.64 ± 2.82 32.40 ± 3.98 38.57 ± 3.08 43.3 ± 1.88
Conductivity (102 S cm1) 30 C
90 C
1.05 1.66 2.01 2.45 3.30
2.19 2.82 3.32 4.02 5.03
the p-electrons in the imidazole rings and hyperconjugative effect between the CeH (s bond) and the p-conjugated imidazole ring (see Table 3).
Conclusions In summary, a series of composite anion exchange membranes were successfully designed and prepared via a simple synthetic strategy which also enabled straightforward control of the IEC of membrane by altering the content of [DMVIm][Cl]. The resultant composite AEMs demonstrated great potential for alkaline anion exchange membrane fuel cell applications based on its good thermal stability, sufficient mechanical properties and high conductivity. The water uptake, swelling ratio, IEC and conductivity of the composite membranes increased with increasing the content of [DMVIm][Cl]. At 80 C, showed high conductivity of SAN70-[DMVIm][OH]30 3.32 102 S cm1 with the relatively low IEC of 1.20 meq g1, and all the membranes showed conductivity up to 1 102 S cm1. Furthermore, all the AEMs synthesize in this work showed excellent long-term alkaline stability at elevated temperature.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51303017 and 21476031), Jiangsu Natural Science Foundation (BK20151187), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.11.169.
references
[1] Jacobson MZ, Colella WG, Golden DM. Cleaning the air and improving health with hydrogen fuel-cell vehicles. Science 2005;308:1901e5.
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
[2] Di Noto V, Negro E, Sanchez JY, Iojoiu C. Structure-relaxation interplay of a new nanostructured membrane based on tetraethylammonium trifluoromethanesulfonate ionic liquid and neutralized Nafion 117 for high-temperature fuel cells. J Am Chem Soc 2010;132:2183e95. [3] He G, Li Z, Zhao J, Wang S, Wu H, Guiver MD, et al. Nanostructured ion-exchange membranes for fuel cells: recent advances and perspectives. Adv Mater 2015;27:5280e95. [4] Elangovan M, Dharmalingam S. Preparation and performance evaluation of poly (ether-imide) based anion exchange polymer membrane electrolyte for microbial fuel cell. Int J Hydrogen Energy 2016;41:8595e605. [5] Lu S, Pan J, Huang A, Zhuang L, Lu J. Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts. Proc Natl Acad Sci 2008;105:20611e4. [6] Wang Y, Qiao J, Baker R, Zhang J. Alkaline polymer electrolyte membranes for fuel cell applications. Chem Soc Rev 2013;42:5768e87. [7] Mohanty AD, Tignor SE, Krause JA, Choe YK, Bae C. Systematic alkaline stability study of polymer backbones for anion exchange membrane applications. Macromolecules 2016;49:3361e72. [8] Si Z, Sun Z, Gu F, Qiu L, Yan F. Alkaline stable imidazoliumbased ionomers containing poly (arylene ether sulfone) side chains for alkaline anion exchange membranes. J Mater Chem A 2014;2:4413e21. [9] Han J, Liu Q, Li X, Pan J, Wei L, Wu Y, et al. An effective approach for alleviating cation-induced backbone degradation in aromatic ether-based alkaline polymer electrolytes. ACS Appl Mater Inter 2015;7:2809e16. [10] Gu S, Cai R, Yan Y. Self-crosslinking for dimensionally stable and solvent-resistant quaternary phosphonium based hydroxide exchange membranes. Chem Commun 2011;47:2856e8. [11] Zhao Z, Wang J, Li S, Zhang S. Synthesis of multi-block poly(arylene ether sulfone) copolymer membrane with pendant quaternary ammonium groups for alkaline fuel cell. J Power Sources 2011;196:4445e50. [12] Hibbs MR, Fujimoto CH, Cornelius CJ. Synthesis and characterization of poly(phenylene)-based anion exchange membranes for alkaline fuel cells. Macromolecules 2009;42:8316e21. [13] Zeng Q, Liu Q, Broadwell I, Zhu A, Xiong Y, Tu X. Anion exchange membranes based on quaternized polystyreneblock-poly(ethylene-ran-butylene)-block-polystyrene for direct methanol alkaline fuel cells. J Membr Sci 2010;349:237e43. [14] Luo Y, Guo J, Wang C, Chu D. Quaternized poly(methyl methacrylate-co-butyl acrylate-co-vinylbenzyl chloride) membrane for alkaline fuel cells. J Power Sources 2010;195:3765e71. [15] Zhang M, Liu J, Wang Y, An L, Guiver MD, Li N. Highly stable anion exchange membranes based on quaternized polypropylene. J Mater Chem 2015;3:12284e96. [16] Ma W, Zhao C, Lin H, Zhang G, Na H. Poly(aryl ether ketone)s with bromomethyl groups: synthesis and quaternary amination. J Appl Polym Sci 2011;120:3477e83. [17] Kim DJ, Park CH, Nam SY. Characterization of a soluble poly(ether etherketone) anion exchange membrane for fuel cell application. Int J Hydrogen Energy 2016;41:7649e58. ~ a HD, [18] Robertson NJ, Kostalik IVHA, Clark TJ, Mutolo PF, Abrun Coates GW. Tunable high performance cross-linked alkaline anion exchange membranes for fuel cell applications. J Am Chem Soc 2010;132:3400e4. [19] Noonan KJ, Hugar KM, Kostalik IV HA, Lobkovsky EB, ~ a HD, Coates GW. Phosphonium-functionalized Abrun
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
polyethylene: a new class of base-stable alkaline anion exchange membranes. J Am Chem Soc 2012;134:18161e4. Zhu L, Zimudzi TJ, Li N, Pan J, Lin B, Hickner MA. Crosslinking of comb-shaped polymer anion exchange membranes via thioleene click chemistry. Polym Chem 2016;7:2464e75. Meek KM, Elabd YA. Alkaline chemical stability of polymerized ionic liquids with various cations. Macromolecules 2015;48:7071e84. Meek KM, Nykaza JR, Elabd YA. Alkaline chemical stability and ion transport in polymerized ionic liquids with various backbones and cations. Macromolecules 2016;49:3382e94. Gu S, Cai R, Luo T, Chen Z, Sun M, Liu Y, et al. A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angew Chem Int Ed 2009;48:6499e502. Lin B, Qiu L, Lu J, Yan F. Cross-linked alkaline ionic liquidbased polymer electrolytes for alkaline fuel cell applications. Chem Mater 2010;22:6718e25. Feng T, Lin B, Zhang S, Yuan N, Chu F, Hickner MA, et al. Imidazolium-based organiceinorganic hybrid anion exchange membranes for fuel cell applications. J Membr Sci 2016;508:7e14. Li N, Zhang Q, Wang C, Lee YM, Guiver MD. Phenyltrimethylammonium functionalized polysulfoneanion exchange membranes. Macromolecules 2012;45:2411e9. Jheng L, Hsu SLC, Lin BY, Hsu YL. Quaternized polybenzimidazoles with imidazolium cation moieties for anion exchange membrane fuel cells. J Membr Sci 2014;460:160e70. Fang J, Lyu M, Wang X, Wu Y, Zhao J. Synthesis and performance of novel anion exchange membranes based on imidazolium ionic liquids for alkaline fuel cell applications. J Power Sources 2015;284:517e23. Thomas OD, Soo K, Peckham TJ, Kulkarni MP, Holdcroft S. A stable hydroxide-conducting polymer. J Am Chem Soc 2012;134:10753e6. Zha Y, Disabb-Miller ML, Johnson ZD, Hickner MA, Tew GN. Metal-cation-based anion exchange membranes. J Am Chem Soc 2012;134:4493e6. Zhang Q, Li S, Zhang S. A novel guanidinium grafted poly (aryl ether sulfone) for high-performance hydroxide exchange membranes. Chem Commun 2010;46:7495e7. Choi YJ, Park JM, Yeon KH, Moon SH. Electrochemical characterization of poly (vinyl alcohol)/formyl methyl pyridinium (PVA-FP) anion-exchange membranes. J Membr Sci 2005;250:295e304. Zhang B, Gu S, Wang J, Liu Y, Herring AM, Yan Y. Tertiary sulfonium as a cationic functional group for hydroxide exchange membranes. RSC Adv 2012;2:12683e5. € bbelin M, Azcune I, Bedu M, Ruiz de Luzuriaga A, Genua A, Do Jovanovski V, et al. Synthesis of pyrrolidinium-based poly (ionic liquid) electrolytes with poly (ethylene glycol) side chains. Chem Mater 2012;24:1583e90. Guo M, Fang J, Xu H, Li W, Lu X, Lan C, et al. Synthesis and characterization of novel anion exchange membranes based on imidazolium-type ionic liquid for alkaline fuel cells. J Membr Sci 2010;362:97e104. Ye Y, Elabd YA. Relative chemical stability of imidazoliumbased alkaline anion exchange polymerized ionic liquids. Macromolecules 2011;44:8494e503. Lin B, Dong H, Li Y, Si Z, Gu F, Yan F. Alkaline stable C2substituted imidazolium-based anion-exchange membranes. Chem Mater 2013;25:1858e67. Pan J, Zhu L, Han J, Hickner MA. Mechanically tough and chemically stable anion exchange membranes from rigidflexible semi-interpenetrating networks. Chem Mater 2015;27:6689e98.
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
[39] Lin B, Chu F, Ren Y, Jia B, Yuan N, Shang H, et al. Alkaline stable C2-substituted imidazolium-based cross-linked anion exchange membranes for alkaline fuel cell applications. J Power Sources 2014;266:186e92.
9
[40] Lu W, Shao Z, Zhang G, Zhao Y, Li J, Yi B. Preparation and characterization of imidazolium functionalized poly (ether sulfone) as anion exchange membrane and ionomer for fuel cell application. Int J Hydrog Energy 2013;38:9285e96.
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Preparation and characterization of imidazoliumbased membranes for anion exchange membrane fuel cell applications Bencai Lin a,*, Gang Qiao a, Fuqiang Chu a,b, Juan Wang a, Tianying Feng a, Ningyi Yuan a, Shuai Zhang a, Xin Zhang a, Jianning Ding a,c,** a
School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou, 213164, Jiangsu, China b Jiangnan Graphene Research Institute, Changzhou 213100, Jiangsu, China c Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang, 212013, China
article info
abstract
Article history:
New anion exchange membranes (AEMs) with high conductivity, good dimensional and
Received 4 October 2016
alkaline stability are currently required in order to develop alkaline fuel cells into efficient
Received in revised form
and clean energy conversion devices. In this study, a series of AEMs based on 1, 2-
16 November 2016
dimethyl-3-(4-vinylbenzyl) imidazolium chloride ([DMVIm][Cl]) are prepared and investi-
Accepted 25 November 2016
gated. [DMVIm][Cl] is synthesized and used as ion carriers and hydrophilic phase in the
Available online xxx
membranes. The water uptake, swelling ratio, IEC and conductivity of the AEMs increase with increasing the [DMVIm][Cl]. The imidazolium-based AEMs show excellent thermal
Keywords:
stability, sufficient mechanical strength, the membrane which containing 30% mass frac-
Anion exchange membrane
tion of [DMVIm][Cl] shows conductivity up to 1.0 102 S cm1 at room temperature and
Fuel cell
good long-term alkaline stability in 1 M KOH solution at 80 C. The results of this study
Alkaline stability
suggest that this type of AEMs have good perspectives for alkaline anion exchange mem-
Imidazolium-based membranes
brane fuel cell applications.
High conductivity
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells, which convert chemical energy directly to electrical energy and avoid the limitations of Carnot cycle, are considered as one of the most promising power generation technologies [1e4]. Among various types of fuel cells, alkaline anion exchange membrane fuel cells (AEMFCs) which utilize anion exchange membranes (AEMs) as solid polymer
electrolytes have been extensively investigated over the past few decades. Compared with proton exchange membrane fuel cells with DuPont's Nafion® electrolyte membranes, AEMFCs which working under alkaline condition enable the use of non-precious metal catalysts instead of platinum catalysts [5,6]. In addition, the application of AEMFCs with solid electrolytes (AEMs) avoiding the leakage problem of traditional alkaline fuel cell with a KOH solution as
* Corresponding author. ** Corresponding author. School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou, 213164, Jiangsu, China. E-mail addresses: [email protected] (B. Lin), [email protected] (J. Ding). http://dx.doi.org/10.1016/j.ijhydene.2016.11.169 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
electrolyte. As the key component of AEMFCs, AEMs act both as a barrier to separate the fuel and an electrolyte to transport hydroxide anion from the anode to the cathode of fuel cells. An typical AEMs is composed of a polymer backbone onto which cationic groups are generally covalently bound, either as the side chains or onto the backbones (be an integral part of the backbone). It is demonstrated that the alkaline stability of the AEMs is fundamentally affected by the chemical structures of both polymer backbone and cationic groups [7,8]. To fulfill the application of AEMFCs, an ideal AEM should possess high conductivity, good thermal stability, sufficient mechanical properties, excellent alkaline stability and sufficient long-term durability at elevated temperatures under alkaline condition. However, the lack of commercially available AEMs with excellent alkaline stability has limited the application of AEMs in the AEMFCs, and the degradation of the polymer backbone and covalently tethered cationic groups, may be triggered by the high nucleophilicity and basicity of hydroxide ions [9]. Therefore, the major challenge in developing AEMFCs is to develop novel AEMs which maintain high conductivity, good alkaline stability, good thermal stability and sufficient mechanical strength under fuel cell operating conditions. The number of publications on AEMs has greatly increased from 2009, a variety of AEMs based on poly(sulfone)s, poly(arylene ether)s, poly(phenylene)s, poly(styrene)s, polypropylene, poly(phenylene oxide)s and poly(olefin)s, have been synthesized for the application of AEMs [10e20]. All of the AEMs mentioned above showed a similar chemical structure that a polymer backbone with pendent quaternary ammonium aliphatic side chains. Generally, once a chemical stable cationic group is identified, an appropriate backbone can then be rationally designed [21,22]. Therefore, cationic species, beside quaternary ammonium (QA), phosphonium [23], imidazolium [24e28], benzimidazolium [29], metal-cation [30], guanidinium [31], pyridinium [32], tertiary sulfonium [33], and pyrrolidinium [34] based AEMs also have been synthesized and investigated. Although, some of these polymeric AEMs showed relatively high conductivity and potential applications in AEMFCs, the alkaline stability of the AEMs should be enhanced for long-term application. Among the various cationic groups, imidazolium-based membranes have attracted much attention due to its robust alkaline stability resulted by the p-electrons in the imidazole rings. In 2010, Fang et al. [35] and Yan et al. [24] firstly reported imidazolium cationic groups-based AEMs, both of their results showed the imidazolium cations have excellent alkaline stability in high pH solution at 60 C. From then on, many publications of imidazolium-based AEMs had appeared. However, in Elabd's work, the degradation of imidazolium cations was observed under dry conditions, higher temperature and higher alkaline concentrations [36]. More recently, we found that the alkaline stability of imidazolium cations relate to its structure, and the C2-substituted imidazolium cations is much more stable than that of C2-unsubstituted imidazolium cations [37]. Among the C2-substituted imidazolium cations, methyl substituted imidazolium cations showed the best alkaline stability. However, to understand the effect of the content of C2-substituted imidazolium salts on the properties of AEMs, we need a systematic study.
In the present work, we describe the synthesis and characterization of C2-substituted imidazolium salt, 1, 2dimethyl-3-(4-vinylbenzyl) imidazolium chloride, and imidazolium-based AEMs. [DMVIm][Cl] was used as hydrophilic phase and anion carrier in the membrane because of its excellent alkaline stability at elevated temperature. The effect of the [DMVIm][Cl] content on the properties of AEMs was investigated with respect to their water uptake, swelling ratio, thermal stability, mechanical properties, alkaline stability and ionic conductivity.
Experimental Materials Styrene, 4-vinylbenzyl chloride, 1, 2-dimethylimidazole, acrylonitrile, benzoin ethylether, diethyl ether and divinylbenzene (DVB) were obtained from Aldrich. Potassium hydroxide, ethyl acetate, sodium hydroxide and hydrochloric acid were purchased from Alfa Aesar. 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, 2-dimethyl-3-(4-vinylbenzyl) imidazolium chloride ([DMVIm][Cl]) 1, 2-Dimethyl-3-(4-vinylbenzyl) imidazolium chloride 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 diethyl ether four times and then dried in dynamic vacuum at room temperature. 1H NMR (400 MHz, D2O): 7.50e7.53 (d, 2H), 7.27e7.34 (m, 4H), 6.73e6.76 (m, 1H), 5.83e5.88 (d, 1H), 5.36e5.37 (d, 1H), 5.31 (s, 2H), 3.76 (s, 3H), 2.55 (s, 3H).
Preparation of imidazolium-based 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 was stirred to obtain a homogeneous solution. Then, the mixture was casted onto a glass mold irradiation with UV light. The prepared membranes were immersed in 1 M KOH solution at 60 C for 24 h to convert the anion of membrane from Cl to OH. Then the membranes was immersed in deionized water and washed with deionized water until the pH of residual water was neutral.
Characterization Fourier transform infrared (FT-IR) spectra of the polymers were recorded on a Varian CP-3800 spectrometer in the range of 4000e400 cm1. 1H NMR spectra were recorded on a Varian 400 MHz spectrometer. Thermal analysis was carried out by Universal Analysis 2000 thermogravimetric analyzer (TGA), and samples were heated from 30 to 650 C at a heating rate of 10 C min1 under a nitrogen flow. The tensile properties of
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
3
membranes were measured by using an Instron 3365 at 25 C at a crosshead speed of 5 mm min1. AFM images were recorded using Agilent AFM with Pico plus molecular imaging system in the semi-contact mode at room temperature.
phenolphthalein as an indicator. The IEC value was calculated using the expression:
Conductivity
where V0,NaOH and Vx,NaOH are the volume of the NaOH solution consumed in the titration without and with membrane samples, respectively. CNaOH is the mol concentration of the NaOH solution, which are titrated by the standard oxalic acid solution, mdry is the mass of the dry membranes.
The resistance value of the membranes was measured by four-point probe alternating current (ac) impedance spectroscopy using an electrode system connected with an electrochemical workstation (Zahner IM6 EX). All the samples were fully hydrated in 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 in the temperature range 30e90 C. At a given temperature, the samples were equilibrated for at least 30 min before recording measurements. Repeated measurements were taken with 10 min interval until no more change in conductivity was observed. The conductivity s (S cm1) of the membrane can be calculated from: s¼
l RA
V0;NaOH CNaOH Vx;NaOH CNaOH mdry
Alkaline stability of imidazolium-based composite membranes The alkaline stability of the membranes was examined by immersing the membrane samples in 2 M KOH solution at 80 C for various time. The changes of hydroxide conductivity and IEC values of the membrane were measured to evaluate the degradation of membranes.
Results and discussion
where l is the distance (cm) between two inner electrodes, A is the cross-sectional area (cm2) of the membrane, obtained from the membrane thickness multiplied by its width, and R is the membrane resistance (U).
Water uptake and swelling ratio The membrane samples were immersed in deionized water at room temperature for 24 h. Then, the hydrated polymer membranes were taken out, and the excess water on the surface of membranes was removed by wiping with a tissue paper and weighed immediately, and a wet weight was obtained (Ww). Then the wet membrane was dried under vacuum at 80 C until a constant dry weight was obtained (Wd). The water uptake (WU) was calculated with the following equation: WUð%Þ ¼
ðWW Wd Þ 100% Wd
where Wd and Ww are the mass of the dry and wet membrane samples, respectively. The swelling ratio (SR) was characterized by linear expansion ratio, which was determined by the difference between wet and dry dimensions of a membrane sample. The calculation was based on the following equation: SRð%Þ ¼
IEC ¼
Xwet Xdry 100% Xdry
Preparation of imidazolium-based membranes In the present work, [DMVIm][Cl] with 1, 2-dimethylimidazolium cations was synthesized and used as d hydrophilic phase in the composite membranes, and the synthetic procedure for [DMVIm][Cl] as shown in Scheme 1. The purity and chemical structure of [DMVIm][Cl] were confirmed by 1H NMR measurements. Our previous work showed that 1, 2-dimethylimidazolium cations have excellent alkaline stability under high pH condition at elevated temperature [37], and the polymer containing 1, 2-dimethylimidazolium cations must be a good candidate as AEM materials. However, poly(1, 2dimethyl-3-(4-vinylbenzyl) imidazolium chloride), the homopolymer of [DMVIm][Cl], shows poor film forming properties, and it is very soluble in water. Therefore, styrene and acrylonitrile were chose to synthesize the copolymers with the ease of processing, good mechanical properties and chemical resistance. The copolymer membranes based on [DMVIm][Cl] were prepared by photo polymerization in a glass mold, and the preparation of imidazolium-based composite membranes is shown as Scheme 2. The produced membranes in Cl form were changed into OH form by immersed them in 1 M KOH solution, and the obtained membranes in OH form denoted as SANx-[DMVIm][OH]y, and x, y indicates the weight ratio of styrene/acrylonitrile and [DMVIm][Cl] in the membrane, respectively. Fig. 1 shows the photographs of SAN70-[DMVIm] [OH]30 with the thickness of about 100 mm. The membrane is transparent, yellow (in the web version), flexible, can be easily
where Xwet and Xdry are the lengths of wet and dry membranes, respectively.
Ion exchange capacity (IEC) Ion exchange capacities (IEC) of membranes were determined by a back-titration. The AEMs were immersed in 100 mL of 0.01 M HCl standard solution for 24 h. Then the solutions were titrated with a standardized NaOH solution using
Scheme 1 e The synthetic procedure for [DMVIm][Cl].
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
Scheme 2 e Reaction scheme for the preparation of SANx-[DMVIm][OH]y.
Fig. 1 e Photographs of SAN70-[DMVIm][OH]30 with the thickness of ~100 mm.
cut into any desired sizes, and it shows sufficient mechanical strength for the application as an AEM.
FT-IR spectra The chemical structure of styrene and SANx-[DMVIm][OH]y was confirmed by FT-IR spectroscopy, and the infrared spectra are shown in Fig. 2. For the spectrum of styrene, the strong absorption bond at 1630 cm1 are the characteristic absorption peaks of C]C, and the absorption bands at about 906 cm1 arose from the bending vibration of C]CeH. For all
the spectra of SANx-[DMVIm][OH]y, these absorption bands disappeared due to the polymerization of styrene. For all the membrane samples, the absorption bands at about 2234 cm1 arose from the stretching vibration of the cyano groups (C^N). Absorption bonds in the range of 3000e3500 cm1 was attributed to the stretching vibration of OeH. The absorption bands at about 2928 and 2848 cm1 arose from the stretching vibration of eCH3 and eCH2e. Absorption peaks at ~1583 cm1 and ~761 cm1 confirm the presence of imidazolium cations [24]. The content of [DMVIm][Cl] increases from SAN90[DMVIm][OH]10 to SAN50-[DMVIm][OH]50, and this trend was confirmed by a gradual increase in the intensity of the characteristic peaks of imidazolium cations. The results of FT-IR spectra confirm the chemical structure of SANx-[DMVIm] [OH]y.
Thermal analysis
Fig. 2 e FT-IR spectra of SANx-[DMVIm][OH]y.
The thermal stabilities of SANx-[DMVIm][OH]y were investigated by TGA (N2 at 10 C min1), and the results are showed in Fig. 3. Fig. 3 shows that all the membrane samples display three weight lose stages around 100, 320 and 380 C. The first weight loss (about 4 wt%) below 120 C due to the dehydration of membrane samples. The weight loss region at temperatures about 320 C due to the degradation of the side groups (such as imidazolium cations) of the copolymer. The third weight loss (about 50 wt%) of membranes, beginning at about 380 C, are considered to be caused by the degradation of main chain of the copolymer. These TGA curves reveal that this type of cross-linked membranes have high degradation temperatures. No noticeable decomposition was observed for SANx[DMVIm][OH]y till 200 C, which confirming that this type of
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
5
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
Fig. 3 e TGA curves of SANx-[DMVIm][OH]y under nitrogen flow. Heating rate: 10 C/min.
composite membranes indeed confers a high thermal stability for application in AEMFCs.
Mechanical properties The AEMs must have sufficient mechanical properties for the application using in AEMFCs. Here, the mechanical properties of SANx-[DMVIm][OH]y were measured by a universal testing machine with a crosshead speed of 5 mm min1, and the results are listed in Table 1. The tensile strength of SANx[DMVIm][OH]y is in the range of 18e52 MPa, with the tensile modulus of 410e1278 MPa, and the values of elongation at break of 10e37%. In the present work, [DMVIm][Cl] containing the exchangeable groups act both as hydrophilic phase and ion carriers in the membranes, and the hydrophilic phase of the membranes increased with increasing the [DMVIm][Cl] content. Therefore, the tensile strength and tensile modulus of membranes decrease with increasing the [DMVIm][Cl] content in the membranes due to its hydrophilic nature, and too high [DMVIm][Cl] content resulted in poor mechanical properties of membranes. Though the mechanical properties of membranes were decreased by increasing the content of [DMVIm][Cl], the mechanical properties of SAN70-[DMVIm] [OH]30 are equivalent to those of quaternary ammonia poly(arylene ether sulfone)s with aromatic main chain [9,38]. These results show that SANx-[DMVIm][OH]y membranes could possess sufficient mechanical strength for fuel cell applications by changing the content of [DMVIm][Cl] in the membranes.
Fig. 4 e Conductivity of SANx-[DMVIm][OH]y as a function of temperature.
IEC, water uptake and swelling ratio Table 1 lists the IEC values, water uptake and swelling ratio of SANx-[DMVIm][OH]y at room temperature. Here, the exchangeable groups of AEMs was provided by [DMVIm][Cl], and it is no surprise that the IEC values of the produced membranes increased with increasing the content of [DMVIm][Cl]. The IEC values of SANx-[DMVIm][OH]y samples determined by titrations are in range of 0.38e1.98 meq g1, and were found to be close to the theoretical IEC values (0.40e2.02 meq g1). AEMs with various IECs values could be easily obtained by adjusting the content of [DMVIm][Cl] in the membranes. Generally, the conductivity of AEMs largely depend on the IEC of membranes, and the conductivity of the composite membranes could be increased by increasing the [DMVIm][Cl] content in this work. Water uptake is another important parameter, which could affect the conductivity of membranes, it also have effect on the dimensional stability and mechanical properties of membranes. The water uptake of SANx-[DMVIm][OH]y samples increases with increasing the content of [DMVIm][Cl] in the membranes. For example, the water uptake of SAN90-[DMVIm] [OH]10 is 24%, and the value for SAN50-[DMVIm][OH]50 is 135%. The change of swelling ratio of SANx-[DMVIm][OH]y in accordance with that of the water uptake of the membranes. Generally, the high IEC values and high water uptake result in high conductivity, while reduce the mechanical properties of membranes. This phenomenon is already confirmed by the mechanical properties of SANx[DMVIm][OH]y listed in Table 1. Therefore, the content of
Table 1 e Mechanical properties of SANx-[DMVIm][OH]y membranes. Membranes SAN90-[DMVIm][OH]10 SAN80-[DMVIm][OH]20 SAN70-[DMVIm][OH]30 SAN60-[DMVIm][OH]40 SAN50-[DMVIm][OH]50
Tensile strength (MPa) 51.57 ± 43.44 ± 36.77 ± 28.13 ± 18.80 ±
0.41 1.60 0.53 1.51 2.55
Tensile modulus (MPa) 1278.07 ± 38.15 692.36 ± 20.36 543.41 ± 21.55 484.55 ± 30.50 410.33 ± 50.14
Elongation at break (%) 10.51 13.45 20.52 26.09 36.47
± 0.41 ± 0.83 ± 0.74 ± 1.03 ± 0.52
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
Fig. 5 e AFM height image (A) and phase image (B) of SAN70-[DMVIm][OH]30.
[DMVIm][Cl] in the membranes should be carefully controlled to balance the conductivity and mechanical properties of membranes.
The conductivity activation energy (Ea) of SANx-[DMVIm] [OH]y can be obtained from the conductivity of the membranes. Fig. 6 displays the plots of ln(s) vs 1000/T (T is the absolute temperature) for SANx-[DMVIm][OH]y. The Ea values
Conductivity The ionic conductivities of SANx-[DMVIm][OH]y at different temperatures are shown in Fig. 4. The composite membranes showed an increased conductivity of above 102 S cm1 when temperatures ranged from 30 C to 90 C. On the one hand, higher temperature always results in faster migration of ions. On the other hand, wider ion transport channels will be formed in the membranes by adsorbing more water at a higher temperature, which leading to a higher conductivity [25]. For example, the conductivity of SAN90-[DMVIm][OH]10 increased from 1.05 102 S cm1 at 30 C to 2.19 102 S cm1 at 90 C. At a given temperature, the conductivity of SANx-[DMVIm][OH]y increased with increasing the content of [DMVIm][Cl], which is like the changes of water uptake and swelling ratio. The density of ionizable functional groups link to the content of [DMVIm][Cl] in the membranes, the more [DMVIm][Cl] content results in higher water uptake and higher IEC of AEMs. The morphology of SAN70-[DMVIm] [OH]30 was studied using AFM, and the results showed as Fig. 5. A rough surfaces of SAN70-[DMVIm][OH]30 could been found in the height images (Fig. 5A), which caused by the rough surfaces of glass mold during the preparation process of membranes. The minimal correlation between phase images (Fig. 5B) and height images (Fig. 5A) indicates that phase images are unlikely to be an artifact of surface roughness [39], a microscopic phase separation structure could be observed. Compare with the imidazolium functionalized poly (ether sulfone) membranes [40], the AEMs produced in the present work showed much higher conductivity, probably due to their aliphatic chain which resulted in a higher water uptake and the formation of microscopic phase separation structure in the membranes. We believe that such a microscopic phase separation structure favor the formation of the ion transport channel, which therefore enhance the conductivity of the AEMs.
Fig. 6 e Arrhenius plots of SANx-[DMVIm][OH]y.
Fig. 7 e Conductivity of SAN70-[DMVIm][OH]30 after treatment with 2 M KOH solution at 80 C.
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
7
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
Table 2 e Water uptake, swelling degree, IEC and conductivity of SANx-[DMVIm][OH]y. IEC (meq g1)
Membrane
Theoretical SAN90-[DMVIm][OH]10 SAN80-[DMVIm][OH]20 SAN70-[DMVIm][OH]30 SAN60-[DMVIm][OH]40 SAN50-[DMVIm][OH]50
Water uptake (%)
Experimental
0.40 0.81 1.21 1.62 2.02
0.38 0.76 1.16 1.57 1.98
± 0.02 ± 0.04 ± 0.02 ± 0.04 ± 0.06
of SANx-[DMVIm][OH]y decreased with increasing the content of [DMVIm][Cl]. For example, the Ea of SAN90-[DMVIm][OH]10 decreased from 11.04 kJ mol1 to 6.21 kJ mol1 for SAN50[DMVIm][OH]50. SAN90-[DMVIm][OH]10 shows the highest Ea value but the lowest conductivity under the same conditions among these composite membranes, indicating that the transport of OHein SAN90-[DMVIm][OH]10 needs most energy due to its lowest density of ionizable functional groups and least free volume.
Alkaline stability From the point of view of practical applications, a major concern in AEMs is their alkaline stability at elevated temperature. To investigate the membrane alkaline stability at elevated temperature, SAN70-[DMVIm][OH]30 membranes were immersed in a hot alkaline solution (2 M KOH solution at 80 C) for various time. The alkaline stability of membrane was evaluated via conductivity and IEC measurements after stability testing. Fig. 7 shows the conductivity of SAN70-[DMVIm] [OH]30 after treatment with 2 M KOH solution at 80 C for various time. As shown in Fig. 7, no obviously decrease in conductivity was observed after stability test at 80 C, and SAN70-[DMVIm][OH]30 maintain more than 95% of the original conductivity even after 480 h, which indicating the excellent alkaline stability of SAN70-[DMVIm][OH]30 in 2 M KOH solution at 80 C. The IEC values of SAN70-[DMVIm][OH]30 were measured during the alkaline stability test in 2 M KOH aqueous solution, and the results summarized in Table 2. The IEC of SAN70-[DMVIm][OH]30 only changed slightly from 1.20 to 1.13 meq g1 even after 480 h alkaline stability test, further confirm the excellent alkaline stability of the composite membranes at elevated temperature. The quaternary ammonium based polymers can be degraded in alkaline solutions at high temperature by the nucleophilic attack reaction and bhydrogen (Hofmann or E2) elimination [6,37]. Here, the SANx[DMVIm][OH]y with 1, 2-dimethylimidazolium cations shows excellent alkaline stability due to the internal conjugation of
Table 3 e IEC values of SAN70-[DMVIm][OH]30 after treatment with 2 M KOH solution at 80 C. Time (h) 24 120 240 360 480
Swelling ratio (%)
IEC (meq g1) 1.20 1.18 1.20 1.16 1.13
± 0.06 ± 0.05 ± 0.07 ± 0.07 ± 0.06
24.49 ± 1.18 48.64 ± 2.48 63.00 ± 3.19 107.25 ± 3.25 135.08 ± 3.43
9.33 ± 1.69 23.64 ± 2.82 32.40 ± 3.98 38.57 ± 3.08 43.3 ± 1.88
Conductivity (102 S cm1) 30 C
90 C
1.05 1.66 2.01 2.45 3.30
2.19 2.82 3.32 4.02 5.03
the p-electrons in the imidazole rings and hyperconjugative effect between the CeH (s bond) and the p-conjugated imidazole ring (see Table 3).
Conclusions In summary, a series of composite anion exchange membranes were successfully designed and prepared via a simple synthetic strategy which also enabled straightforward control of the IEC of membrane by altering the content of [DMVIm][Cl]. The resultant composite AEMs demonstrated great potential for alkaline anion exchange membrane fuel cell applications based on its good thermal stability, sufficient mechanical properties and high conductivity. The water uptake, swelling ratio, IEC and conductivity of the composite membranes increased with increasing the content of [DMVIm][Cl]. At 80 C, showed high conductivity of SAN70-[DMVIm][OH]30 3.32 102 S cm1 with the relatively low IEC of 1.20 meq g1, and all the membranes showed conductivity up to 1 102 S cm1. Furthermore, all the AEMs synthesize in this work showed excellent long-term alkaline stability at elevated temperature.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51303017 and 21476031), Jiangsu Natural Science Foundation (BK20151187), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.11.169.
references
[1] Jacobson MZ, Colella WG, Golden DM. Cleaning the air and improving health with hydrogen fuel-cell vehicles. Science 2005;308:1901e5.
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
[2] Di Noto V, Negro E, Sanchez JY, Iojoiu C. Structure-relaxation interplay of a new nanostructured membrane based on tetraethylammonium trifluoromethanesulfonate ionic liquid and neutralized Nafion 117 for high-temperature fuel cells. J Am Chem Soc 2010;132:2183e95. [3] He G, Li Z, Zhao J, Wang S, Wu H, Guiver MD, et al. Nanostructured ion-exchange membranes for fuel cells: recent advances and perspectives. Adv Mater 2015;27:5280e95. [4] Elangovan M, Dharmalingam S. Preparation and performance evaluation of poly (ether-imide) based anion exchange polymer membrane electrolyte for microbial fuel cell. Int J Hydrogen Energy 2016;41:8595e605. [5] Lu S, Pan J, Huang A, Zhuang L, Lu J. Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts. Proc Natl Acad Sci 2008;105:20611e4. [6] Wang Y, Qiao J, Baker R, Zhang J. Alkaline polymer electrolyte membranes for fuel cell applications. Chem Soc Rev 2013;42:5768e87. [7] Mohanty AD, Tignor SE, Krause JA, Choe YK, Bae C. Systematic alkaline stability study of polymer backbones for anion exchange membrane applications. Macromolecules 2016;49:3361e72. [8] Si Z, Sun Z, Gu F, Qiu L, Yan F. Alkaline stable imidazoliumbased ionomers containing poly (arylene ether sulfone) side chains for alkaline anion exchange membranes. J Mater Chem A 2014;2:4413e21. [9] Han J, Liu Q, Li X, Pan J, Wei L, Wu Y, et al. An effective approach for alleviating cation-induced backbone degradation in aromatic ether-based alkaline polymer electrolytes. ACS Appl Mater Inter 2015;7:2809e16. [10] Gu S, Cai R, Yan Y. Self-crosslinking for dimensionally stable and solvent-resistant quaternary phosphonium based hydroxide exchange membranes. Chem Commun 2011;47:2856e8. [11] Zhao Z, Wang J, Li S, Zhang S. Synthesis of multi-block poly(arylene ether sulfone) copolymer membrane with pendant quaternary ammonium groups for alkaline fuel cell. J Power Sources 2011;196:4445e50. [12] Hibbs MR, Fujimoto CH, Cornelius CJ. Synthesis and characterization of poly(phenylene)-based anion exchange membranes for alkaline fuel cells. Macromolecules 2009;42:8316e21. [13] Zeng Q, Liu Q, Broadwell I, Zhu A, Xiong Y, Tu X. Anion exchange membranes based on quaternized polystyreneblock-poly(ethylene-ran-butylene)-block-polystyrene for direct methanol alkaline fuel cells. J Membr Sci 2010;349:237e43. [14] Luo Y, Guo J, Wang C, Chu D. Quaternized poly(methyl methacrylate-co-butyl acrylate-co-vinylbenzyl chloride) membrane for alkaline fuel cells. J Power Sources 2010;195:3765e71. [15] Zhang M, Liu J, Wang Y, An L, Guiver MD, Li N. Highly stable anion exchange membranes based on quaternized polypropylene. J Mater Chem 2015;3:12284e96. [16] Ma W, Zhao C, Lin H, Zhang G, Na H. Poly(aryl ether ketone)s with bromomethyl groups: synthesis and quaternary amination. J Appl Polym Sci 2011;120:3477e83. [17] Kim DJ, Park CH, Nam SY. Characterization of a soluble poly(ether etherketone) anion exchange membrane for fuel cell application. Int J Hydrogen Energy 2016;41:7649e58. ~ a HD, [18] Robertson NJ, Kostalik IVHA, Clark TJ, Mutolo PF, Abrun Coates GW. Tunable high performance cross-linked alkaline anion exchange membranes for fuel cell applications. J Am Chem Soc 2010;132:3400e4. [19] Noonan KJ, Hugar KM, Kostalik IV HA, Lobkovsky EB, ~ a HD, Coates GW. Phosphonium-functionalized Abrun
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
polyethylene: a new class of base-stable alkaline anion exchange membranes. J Am Chem Soc 2012;134:18161e4. Zhu L, Zimudzi TJ, Li N, Pan J, Lin B, Hickner MA. Crosslinking of comb-shaped polymer anion exchange membranes via thioleene click chemistry. Polym Chem 2016;7:2464e75. Meek KM, Elabd YA. Alkaline chemical stability of polymerized ionic liquids with various cations. Macromolecules 2015;48:7071e84. Meek KM, Nykaza JR, Elabd YA. Alkaline chemical stability and ion transport in polymerized ionic liquids with various backbones and cations. Macromolecules 2016;49:3382e94. Gu S, Cai R, Luo T, Chen Z, Sun M, Liu Y, et al. A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angew Chem Int Ed 2009;48:6499e502. Lin B, Qiu L, Lu J, Yan F. Cross-linked alkaline ionic liquidbased polymer electrolytes for alkaline fuel cell applications. Chem Mater 2010;22:6718e25. Feng T, Lin B, Zhang S, Yuan N, Chu F, Hickner MA, et al. Imidazolium-based organiceinorganic hybrid anion exchange membranes for fuel cell applications. J Membr Sci 2016;508:7e14. Li N, Zhang Q, Wang C, Lee YM, Guiver MD. Phenyltrimethylammonium functionalized polysulfoneanion exchange membranes. Macromolecules 2012;45:2411e9. Jheng L, Hsu SLC, Lin BY, Hsu YL. Quaternized polybenzimidazoles with imidazolium cation moieties for anion exchange membrane fuel cells. J Membr Sci 2014;460:160e70. Fang J, Lyu M, Wang X, Wu Y, Zhao J. Synthesis and performance of novel anion exchange membranes based on imidazolium ionic liquids for alkaline fuel cell applications. J Power Sources 2015;284:517e23. Thomas OD, Soo K, Peckham TJ, Kulkarni MP, Holdcroft S. A stable hydroxide-conducting polymer. J Am Chem Soc 2012;134:10753e6. Zha Y, Disabb-Miller ML, Johnson ZD, Hickner MA, Tew GN. Metal-cation-based anion exchange membranes. J Am Chem Soc 2012;134:4493e6. Zhang Q, Li S, Zhang S. A novel guanidinium grafted poly (aryl ether sulfone) for high-performance hydroxide exchange membranes. Chem Commun 2010;46:7495e7. Choi YJ, Park JM, Yeon KH, Moon SH. Electrochemical characterization of poly (vinyl alcohol)/formyl methyl pyridinium (PVA-FP) anion-exchange membranes. J Membr Sci 2005;250:295e304. Zhang B, Gu S, Wang J, Liu Y, Herring AM, Yan Y. Tertiary sulfonium as a cationic functional group for hydroxide exchange membranes. RSC Adv 2012;2:12683e5. € bbelin M, Azcune I, Bedu M, Ruiz de Luzuriaga A, Genua A, Do Jovanovski V, et al. Synthesis of pyrrolidinium-based poly (ionic liquid) electrolytes with poly (ethylene glycol) side chains. Chem Mater 2012;24:1583e90. Guo M, Fang J, Xu H, Li W, Lu X, Lan C, et al. Synthesis and characterization of novel anion exchange membranes based on imidazolium-type ionic liquid for alkaline fuel cells. J Membr Sci 2010;362:97e104. Ye Y, Elabd YA. Relative chemical stability of imidazoliumbased alkaline anion exchange polymerized ionic liquids. Macromolecules 2011;44:8494e503. Lin B, Dong H, Li Y, Si Z, Gu F, Yan F. Alkaline stable C2substituted imidazolium-based anion-exchange membranes. Chem Mater 2013;25:1858e67. Pan J, Zhu L, Han J, Hickner MA. Mechanically tough and chemically stable anion exchange membranes from rigidflexible semi-interpenetrating networks. Chem Mater 2015;27:6689e98.
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
[39] Lin B, Chu F, Ren Y, Jia B, Yuan N, Shang H, et al. Alkaline stable C2-substituted imidazolium-based cross-linked anion exchange membranes for alkaline fuel cell applications. J Power Sources 2014;266:186e92.
9
[40] Lu W, Shao Z, Zhang G, Zhao Y, Li J, Yi B. Preparation and characterization of imidazolium functionalized poly (ether sulfone) as anion exchange membrane and ionomer for fuel cell application. Int J Hydrog Energy 2013;38:9285e96.
Please cite this article in press as: Lin B, et al., Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.169