Journal of Membrane Science 585 (2019) 150–156
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Comb-shaped anion exchange membrane with densely grafted short chains or loosely grafted long chains?
T
Min Hu, Liang Ding, Muhammad A. Shehzad, Qianqian Ge, Yahua Liu, Zhengjin Yang∗, Liang Wu, Tongwen Xu∗∗ CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, School of Chemistry and Material Science, University of Science and Technology of China, Hefei, 230026, PR China
ARTICLE INFO
ABSTRACT
Keywords: Comb-shaped polymers Anion exchange membranes Chain length Graft density Alkaline stability
Anion exchange membranes (AEMs) are crucial components for advanced energy and environment processes including alkaline fuel cells, redox flow batteries, and industrial effluent treatment; while low anionic conductivity and poor stability remain the major challenges for the widespread implementation of AEMs. Through molecular engineering, comb-shaped AEMs have been proved possessing the capability of delivering both high conductivity and good alkaline stability. However, how to precisely control the side chain topology and how the chain topology would influence the membrane properties need to be further elucidated. We hereby propose a radically novel and readily scalable route towards the controllable synthesis of comb-shaped AMEs and we were able to determine the length of side chains and the number of ionic groups along the side chains. To probe the effect of side chain topology on membrane properties, two types of AEMs of densely grafted short side chains or loosely grafted long side chains with similar ion exchange capacity (IEC, ∼1.7 mmol/g) were synthesized and compared. We found that the comb-shaped AEMs with loosely grafted long chains (LG-LS-DIm), with higher hydroxide conductivity (55 mS cm−1 at 30 °C) and better alkaline stability (∼80 % of IEC retention after soaking in 2 mol L−1 NaOH solution at 60 °C for 25 days), outperform those with densely grafted short chains (HG-SSDIm) and the benchmark main-chain type AEMs (DIm-PPO), which is also superior to those of conventional linear AEMs with densely functionalized structure.
1. Introduction Currently, there is a growing demand for clean and efficient energy storage and conversion technologies, such as fuel cells, redox flow batteries, and water electrolysis process [1]. In particular, fuel cells have attracted great attention from scientists and technologists as an alternative energy generation technology due to their high conversion efficiency and environmental friendliness [2]. Compared with proton exchange membrane full cells (PEMFCs), the development of efficient alkaline fuel cells (AFCs) dominates the field as AFCs offer several advantages over the PEMFCs technology [3]. AFCs operating at high pH condition have enhanced oxygen reduction kinetics, allowing the use of less expensive, non-noble metal catalysts on the electrodes, which significantly lowers the cost of fuel cells [4,5]. However, the practical application of AFCs is impeded by the inferior properties of anion exchange membranes (AEMs). As one of the crucial components of AFCs, AEMs separate the anode reaction from the cathode reaction, while
∗
conducting OH- ions and transporting water molecules [6,7]. Due to the low mobility of OH- ions, which is around one half that of H+ [8,9], low ionic conductivity is one major obstacle that hinders the widespread application of AFCs. Another impediment is the poor chemical stability of AEMs under alkaline conditions, especially at elevated temperatures. It therefore remains a daunting challenge to develop highly conductive and highly stable AEMs [10]. AEMs consist of a polymer backbone and positively charged ion exchange groups, for instance quaternary ammonium (QA) [11], imidazolium [12,13], phosphonium [14] or guanidinium [15], which conduct OH- ions. These positively charged ion exchange groups can be directly or indirectly (via side chains) attached to the polymer backbone. Higher OH- conductivity can be achieved by increasing the content of ion exchange groups (IEC) in an intuitive way. However, this approach has been proved to cause excessive membrane swelling, further resulting in poor membrane stability [16]. Thus, the only target over the past few years is to devise an AEM that is highly conductive
Corresponding author. Corresponding author. E-mail addresses:
[email protected] (Z. Yang),
[email protected] (T. Xu).
∗∗
https://doi.org/10.1016/j.memsci.2019.05.034 Received 26 November 2018; Received in revised form 9 May 2019; Accepted 12 May 2019 Available online 16 May 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 585 (2019) 150–156
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and retains chemical stability over long operation period. To achieve this target, extensive research has been done on the investigation of highly conductive and chemically stable AEMs, and now it has been validated that design and optimization of polymer microstructures have significant impact on the resultant AEM properties [17–20]. Being state-of-the-art, AEMs with anionic groups attached to the polymer backbone through alkyl side chains have much higher conductivity than those conventional AEMs (where ion exchange groups are directly attached to the polymer backbone) because the flexible side chains facilitate micro-phase separation [21–24]. They also exhibit higher alkaline stability because of the long distance between ion exchange groups and polymer backbone. Within this framework, researchers have also developed densely functionalized AEMs, where multiple ion exchange groups are bounded to the specific repeating units of a polymer backbone via alkyl spacers and these AEMs are proved to be capable of conducting OH- ions more efficiently [25,26]. The reason can be explained by the formation of more distinct micro-phase separation. A more prominent version of these AEMs is the comb-shaped [27] AEMs, which has aroused tremendous research efforts and has delivered surprising performance. A critical controversy that needs to be further elucidated is how the polymer structure impacts the chemical stability and the OH- conductivity. For instance, there are reports suggesting these AEMs suffer poor alkaline stability because of the over-concentrated functional groups, while the long side chains keep ionic groups far away from polymer backbone thereby lowering the OHattack and mitigating the polymer backbone degradation. A recent report verified that the number and the distribution of the ion exchange groups on the side chain, and the side chain length played critical roles in determining the membrane stability and conductivity [16,28–36]. There is currently no definitive answer to this controversy because how to introduce a specific number of ion exchange groups on the side chains of comb-shaped AEMs and how to precisely control their distribution remain the biggest synthetic obstacle. Therefore, structure optimization of comb-shaped AEMs is fairly rare. To address the synthetic obstacle and elucidate how structure interacts with comb-shaped AEM properties, herein, we synthesized 1,2Dimethylimidazolium-based AEMs with ion exchange groups evenly distributed along the side chains, while the number of ion exchange groups can be accurately controlled. Two comb-shaped AEMs with similar IEC but of different side chain topology are synthesized via atom transfer radical polymerization (ATRP). AEMs with either densely grafted short side chains or loosely grafted long chains (Fig. 1) are generated. As the reference, a benchmark linear AEM from Poly (2,6dimethy-1,4-Phenylene Oxide) (PPO) to which 1,2-Dimethylimidazolium ion exchange groups are directly attached was selected [37]. We thoroughly characterized the membrane properties, morphologies, and how they were influenced by the side chain topology. We proposed that AEMs with loosely longer short chains outperform those main-chain type AEMs or highly grafted AEMs with shorter chains in term of OHconductivity and alkaline stability.
2. Experimental 2.1. Materials Brominated Poly (2,6-dimethy-1,4-Phenylene Oxide) (BPPO) was supplied by Tianwei Membrane Company (Shandong, PR China) and purified before using inN-methyl-2-pyrrolidolone (NMP), chlorobenzene, acetone, dimethyl sulfoxide (DMSO), diethyl ether were purchased from Shanghai-Sinopharm Chemical Corporation Ltd. and used without further purification. 4-Vinylbenzyl chloride (VBC) was washed with KOH solution to remove the polymerization inhibitor. 1,2Dimethylimidazole (DIm), cupric bromide (CuBr2), 2,2′-dipyridyl (bpy) and L (+)-ascorbic acid (AsAc) were all purchased from Aladdin Reagent Ltd. (PR China). Deionized water was used throughout the experiments. 2.2. Synthesis of VBC-DIm precursor DIm (20.55 g, 0.21 mol) was dissolved in 20 ml acetone and then VBC (20 ml, 0.14 mol) was added. The mixture was stirred under room temperature for 24 h before being washed with acetone for several times. The product was yielded as off-white solid after vacuum drying under room temperature (yield: 90%). 2.3. Synthesis of 1,2-Dimethylimidazolium-based comb-shaped polymers Atom transfer radical polymerization (ATRP) was used to obtain grafted polymers. A mixture of BPPO (1.0 g, bromination degree of 50%) and VBC-DIm (2 g) in NMP (40 mL) was stirred under N2 atmosphere for 40 min. A long neck round bottom tube was charged with CuBr2 (0.0112 g), bpy (0.0160 g) and AsAc (0.0886 g) and to which the fully dissolved mixture was transferred quickly. The tube was then frozen at 77 K (liquid N2 bath) and degassed by three freeze-pump-thaw cycles. The tube was sealed off and then heated at 120 °C for 24 h. A dark yellow solution was obtained and precipitated in excess diethyl ether, yielding yellow flocculent polymer with high grafted shorter side chains. For polymers with low grafted longer side chains, BPPO of 10% bromination degree was used and the CuBr2 (0.0451 g), bpy (0.0627 g) and AsAc (0.1769 g) were added. The two polymers were collected by filtration was then dried at 40 °C under vacuum overnight. 2.4. Fabrication of 1,2-Dimethylimidazolium-based comb-shaped polymeric AEMs 1 g high grafted shorter side chains was dissolved in 10 ml dimethyl sulfoxide (DMSO), affording a homogeneous solution of reddish brown (10 %). The solution was then cast on a clean glass plate, heated at 60 °C overnight to get brown transparent membrane HG-SS-DIm and similar treatment was applied to obtain membrane LG-LS-DIm.
Fig. 1. Synthesis and structures schematic of 1,2-Dimethylimidazolium-based comb-shaped polymers via ATRP; (a) high density grafted shorter side chain polymer membrane (HG-SS-DIm, DB=50 %); (b) low density grafted longer side chain polymer membrane (LG-LS-DIm, BD=10 %). 151
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2.5. Characterizations and measurements
WU =
2.5.1. Chemical structure, thermal stability, mechanical properties and morphology characterization 1 H NMR spectra were recorded on an Avance 400 spectrometer (Bruker, Germany). Thermal stability of membrane samples was characterized by Thermogravimetric Analysis (TGA, Shimadzu TGA-50H), and the test was performed at temperature ranging from 30 °C to 600 °C under nitrogen atmosphere with a heating rate of 10 °C min−1. Mechanical properties of samples in both wet and dry state were measured by a TA dynamic mechanical analyzer (DMA Q800), at a stretch rate of 0.25 N min−1. Atomic Force Microscopy (AFM, Veeco diInnova SPM) was utilized to observe the morphology of the AEMs, under tapping mode with a micro-fabricated cantilever of force constant 20 N m−1.
N % × 1000 28
Wdry
Wdry
× 100%
(2)
where, Wwet and Wdry are weight of the wet membrane samples and the dry membrane samples, respectively. 2.5.4. Hydroxide conductivity The hydroxide conductivity (σ) of HG-SS-DIm and LG-LS-DIm membranes was measured in a common four-point probe method at elevated temperatures (from 30 °C to 80 °C) reported previously in our lab [30]. The hydroxide conductivity was calculated by the following eq. (3):
=
L RWd
(3)
where R is the impedance of AEM samples and L is the distance between potential sensing electrodes on the test fixture, W and d represent the width (1 cm) and thickness of AEM samples, respectively.
2.5.2. Ion exchange capacity (IEC) IEC was determined by Elemental analysis. HG-SS-DIm and LG-LSDIm membranes in OH- form were washed with deionized water and dried at 60 °C under vacuum to get completely dry samples, which were then burned to collect the content of nitrogen (record as N%). The IEC was calculated by eq. (1):
IEC =
Wwet
2.5.5. Alkaline stability Alkaline stability of the membrane samples was determined by monitoring the change in IEC over time. HG-SS-DIm and LG-LS-DIm membrane samples were immersed in 2 M NaOH solution at 60 °C for 25 days. One sample was taken out at each time interval and thoroughly washed before IEC measurement.
(1)
3. Results and discussion
2.5.3. Water uptake HG-SS-DIm and LG-LS-DIm membrane samples in the OH- form were immersed in deionized water at a predetermined temperature for 24 h. Weight of the wet membranes was quickly measured after wiping out the surface water on the membrane samples. Then the membrane samples were dried at 60 °C under vacuum. The water uptake of membranes was calculated according to eq. (2):
3.1. Synthesis and characterization of 1,2-Dimethylimidazolium-based comb-shaped polymers Comb-shaped AEMs with densely grafted short chains or loosely grafted long chains were synthesized via ATRP, an efficient living polymerization that enables the preparation of polymers with precisely
Fig. 2. The 1H NMR spectra of 1,2-Dimethylimidazolium-based comb-shaped polymers (a) and BPPO (b). 152
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controlled molecular structure and distribution [38]. Brominated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO) was used as macro-initiators, and VBC-DIm was grafted onto the backbone as side chains, as shown in Fig. 1. By varying the amount of VBC-DIm and degree of bromination (DB, percentage of bromoalkylated benzylic methyl groups of PPO, DB=10% or 50%) in PPO backbone, ionomers with densely grafted short chains or loosely grafted long chains with similar IEC values were synthesized. In the 1H NMR spectra of BPPO and synthesized side chain polymer (Fig. 2), the peak at 4.3 ppm (-CH2Br) disappeared and a new peak at 5.5 ppm (-CH2- between the imidazole and the styrene benzene ring) emerged, indicating the success in grafting side chains. Accordingly, the graft length can be calculated from molar ratio of VBC-DIm to BPPO, turning out that HG-SS-DIm membrane has approximately 2 repeating units on each side chain, while LG-LS-DIm membrane has 6 repeating units. Both polymers showed acceptable mechanical strength for fuel cell applications and their tensile strength and elongation at break at corresponding IEC are summarized in Table 1.
(IEC=1.64 mmol/g) were less sensitive to temperature, especially the HG-SS-Dim membrane, which showed extremely low WU. We speculate that LG-LS-DIm membrane, with loosely grafted long side chains, could form larger free volume like a “water pool” [41,42] after aggregation, accommodating more water molecules. Moreover, ion groups tend to aggregate and form larger hydrophilic ion clusters due to the flexible long side chains in LG-LS-DIm membrane. As a result, we saw a sharp increase in WU as compared to the rest two membranes. Unexpectedly, with such high WU, LG-LS-DIm membrane showed acceptable dimension swelling, which is practically desired. As for HG-SS-DIm, although there were averagely 2 ionic groups on each side chain, the hydrophobicity of alkyl side chains still acted as a major role on the WU of HG-SS-DIm, similar to the AEMs with spacer between the polymer backbone and ion exchange groups [23,43]. 3.3. Membranes morphology The hydrophobic-hydrophilic phase separation is the key to improved AEM performance, especially to achieve high hydroxide conductivity. The conducting channels formed by aggregation of ion exchange groups promote the transfer of hydroxide ions, while the aggregation of the hydrophobic backbone maintains mechanical strength of AEMs. Clear phase separation of hydrophobic backbone and hydrophilic ion exchange groups were observed, as shown in Fig. 4, wherein the darker region represents the hydrophilic hydrated ion clusters and brighter domain, formed by the aggregation of hydrophobic polymer backbone. Both two side chains membranes showed well distributed and interconnected hydrophilic-hydrophobic phase separation. Obviously, the hydrophilic domains formed in the LG-LSDIm membrane were much wider and less tortuous than those in HGSS-Dim membrane, suggesting the effective formation of conduction channels and outstanding self-assembly ability. It is also noteworthy that these two AEMs with discriminative morphology have similar IEC (∼1.7 mmol/g), indicating that the topology structure is the main reason attributed to the micro-phase change, as well as some other properties of AEMs.
Table 1 IEC and mechanical properties of LG-LS-DIm and HG-SS-DIm. Membranes
LG-LS-DIm HG-SS-DIm
IEC(mmol g−1)
1.72 1.67
Tensile strength (MPa)
Elongation at break (%)
Wet
Dry
Wet
Dry
9.6 11.2
15.9 24.3
67.7 73.6
32.1 54.7
3.2. Water uptake Water uptake is an important parameter for better AEMs performance, as water molecules can facilitate ion dissociation and promote formation of hydrophilic domains with hydrated ionic groups. Nevertheless, excessive water uptake can result in severe swelling issues, weakening the mechanical strength of AEMs [39,40]. Here, HGSS-DIm and LG-LS-DIm membrane samples were fully hydrated (immersed in deionized water), and water uptake of these membranes were recorded at various temperatures, as shown in Fig. 3. We found all three AEMs showed increased water uptake as temperature climbed up from 30 °C to 80 °C, while for HG-SS-DIm and LG-LS-DIm membranes, even with similar IEC (∼1.7 mmol/g), their water uptake was significantly different. The WU of LG-LS-DIm membrane (IEC=1.76 mmol/g) was much higher than the other two membranes, increasing from 67 % to 179 %, almost linearly with temperature. In comparison, WU of HG-SSDIm (IEC=1.67 mmol/g) and the benchmark DIm-PPO-0.30 membrane
Fig. 4. AFM tapping phase images for two imidazole-functionalized side chain polymeric AEMs: HG-SS-DIm (a) and LG-LS-DIm (b); the dark areas correspond to the hydrophilic side chains and the bright areas correspond to the hydrophobic aromatic polymer backbones.
3.4. Thermal stability The operating temperature of AFCs is generally at 60 °C–80 °C and thus AEMs are required to be thermally stable under such condition. In the TGA curves of AEMs, the small weight loss before 100 °C (usually around 2 %) is generally due to the loss of absorbed water. Initial decomposition temperature (IDT), thermal degradation temperature (Td, defined as the temperature at 5% weight loss) and corresponding weight losses of HG-SS-DIm and LG-LS-DIm were determined and shown in Table 2. Both AEMs degraded from ∼200 °C, in which HG-SS-
Fig. 3. Water uptake of HD-SS-DIm (IEC=1.67 mmol/g), LG-SS-DIm (IEC=1.76 mmol/g) and DIm-PPO-0.30 (IEC=1.64 mmol/g) as a function of temperature in fully hydrated state. 153
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Table 2 Initial decomposition temperature (IDT), thermal decomposition temperature (Td) and weight loss of membrane HG-SS-DIm and LG-LS-DIm in TGA thermograms.
HG-SS-DIm LG-LS-DIm
IDT (oC)a
Td (oC)b
Weight loss (wt %)
202.8 305.2 376.5 202.1 324.9
305.2 376.5 699.6 324.9 687.2
12.5 11.9 39.5 14.0 54.1
a IDT is the initial decomposition temperature determined from TGA thermograms. b The thermal degradation temperatures (Td) are defined as the temperature at which the weight loss reaches 5 wt % in TGA thermograms.
DIm showed three weight loss stages, while LG-LS-DIm showed two weight loss stages starting from 202.1 °C with weight loss of 14.0 %. Our results suggest that both HG-SS-DIm and LG-LS-DIm are thermally stable for AFC applications.
Fig. 6. The alkaline stability of the HG-SS-DIm and LG-LS-DIm membranes in a 2 M aqueous NaOH solution at 60 °C.
3.5. Hydroxide conductivity
and on the other hand, more flexible movement of side chains accelerated the OH- conducting by increasing the solvation-shell fluctuations of hydrated OH-, achieving high transporting efficiency. For HG-SS-DIm and Dim-PPO-0.30, the ion conductivity of HG-SS-DIm was also higher than that of the main chain type Dim-PPO-0.30 (IEC=1.64 mmol/g) and that implies the advantage of comb-shaped AEM over main chain type AEM in transferring OH- ions. This is ascribed to the generally-accepted concept that comb-shaped AEM has well-interconnected ion channels due to more effective hydrophilichydrophobic aggregation of the flexible side chains [17,23]. Overall, both LG-LS-DIm and HG-SS-DIm maintain high hydroxide conductivity at temperatures above 60 °C, and no severe membrane swelling was observed, which exhibited great potential for future AFC applications.
To demonstrate the impact of structure design on AEMs properties, hydroxide conductivity of HG-SS-DIm, LG-LS-DIm and the benchmark DIm-PPO-0.30 was measured by standard four-point probe method at temperature ranging from 30 °C to 80 °C, as presented in Fig. 5. For all membranes, they manifest higher conductivity at elevated operational temperatures resulted from increased hydroxide mobility. It is worth noting that when temperature is higher than 50 °C, the conductivity rises sharply, which may result from the flexible movement of polymer side chain. We also found that for AEMs with similar IEC (∼1.7 mmol/ g), the polymer topology had a significant impact on membrane conductivity. LG-LS-DIm membrane showed much higher conductivity than the other two membranes, increasing significantly from 55 mS cm−1 at 30 °C to 100 mS cm−1 at 80 °C. Two main reasons are considered, as shown above, the much wider and less tortuous ionic channels shown in AFM provide more smooth OH- transporting path,
3.6. Alkaline stability Besides superior hydroxide conductivity, HG-SS-DIm and LG-LSDIm showed moderate alkaline stability under harsh conditions (2 mol L−1 NaOH, 60 °C). After soaking in alkaline solution for 25 days, the IEC loss of HG-SS-DIm and LG-LS-DIm were 21.1 % and 32.5 %, respectively while Dim-PPO-0.30 membrane exhibited very poor alkaline stability, almost losing all functional groups within 5 days. Among the three, LG-LS-DIm had a much slower degradation rate and showed improved chemical stability (Fig. 6). This is because the longer side chains of LG-LS-DIm rendered the side chains more hydrophobic, mitigating the attack of hydroxide ions. Meanwhile, the side chains with improved movement avoided the attack of hydroxide to some extent as well. In comparison, IEC of HG-SS-DIm dropped sharply within the first 10 days and this is caused by the fast degradation of dimethyl imidazolium cations due to heterocycle deprotonations and the following rearrangement reactions at unsubstituted C4 or C5 position under alkaline conditions [44]. Even though, HG-SS-DIm was more stable than DIm-PPO-0.30, probably due to the spacer between the cationic groups and polymer backbone. Moreover, the inferior polymer backbone stability resulted from the attack of aryl ether linkages by OH- should also be taken into consideration in further improving the chemical stability of the AEMs [45]. OH- conductivity and alkaline stability were compared with some reported AEMs (Table 3). Expectedly, our AEMs with
Fig. 5. Hydroxide conductivity of HG-SS-DIm, LG-LS-DIm and DIm-PPO-0.30 as a function of temperature.
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Table 3 IEC, OH− conductivity, and alkaline stability of different AEMs reported in the literature compared with those of grafted side chain membranes HG-SS-DIm and LGLS-DIm. Membrane
IEC (mmol g−1)
Conductivity (ms cm−1) 60 °C
80 °C
Alkaline stability
Configuration
Ref.
DIm-PPO-0.30 DIm-PPO-0.42 c4PAES-1.9Im c4PAES-1.8Q
1.74 2.16 1.9 1.8
43 65 10a 3a
/ / 15a 5a
/ 50 % IEC remain (2 M KOH, 60 °C, 216 h) / /
37
PPO-7Q-1.8
1.8
∼70
85
No degradation in NMR (1 M NaOH, 80 °C, 196 h)
23
PES-B100-C16
1.35
28
45
70% remain in σOH− (2 M KOH, 60 °C, 360 h)
22
ImPES-0.40
1.71
55
90
/
26
ImPES-0.45 PPO-PIm-31 PPO-PIm-40
1.84 1.59 1.85
75 49 68
113 64 96
68.2 % remain in σOH− (1 M KOH, 60 °C, 408 h) 88.2 % remain in σOH− (1 M NaOH, 60 °C, 168 h) 92 % remain in σOH− (1 M NaOH, 60 °C, 168 h)
31
3BQA-3 3BQA-4
1.59 1.95
15a 28a
25a 50a
/ 15.4 % IEC remain (1 M KOH, 60 °C, 240 h)
34
R4-C8 R4-C10 R4-C12
1.67 1.89 2.16
62 73 75
/ / 160
/ 78.8 % IEC remain (2 M NaOH, 60 °C, 552 h) /
30
HG-SS-DIm
1.67
46
64
67.5 % IEC remain (2 M NaOH, 60 °C, 600 h)
This work
LG-LS-DIm
1.72
82
96
78.9 % IEC remain (2 M NaOH, 60 °C, 600 h)
a
25
Conductivity in Br- form.
ion groups distributing uniformly along the side chains achieved both excellent hydroxide conductivity and good alkaline stability over other reported AEMs.
AEMs with longer-term alkaline stability. Acknowledgements This project has been supported by the National Natural Science Foundation of China (Nos. 21720102003, 91534203), K. C. Wong Education Foundation(2016-11)and International Partnership Program of Chinese Academy of Sciences (No. 21134ky5b20170010).
4. Conclusions In summary, we have synthesized AEMs with side chains of different topology and accurately controlled ion exchange group content via living polymerization, the atom transfer radical polymerization (ATRP). We demonstrate that for comb-shaped AEMs with similar IEC (∼1.7 mmol/g), the side chain topologies, including the number and distribution of ion exchange groups, can significantly alter the membrane performance. AEMs with loosely grafted long side chains exhibited high hydroxide conductivity (55 mS cm−1 at 30 °C), excellent alkaline stability (∼80 % IEC retention after soaking in 2 mol L−1 NaOH solution at 60 °C for 25 days), and much wider ion conducting channels as compared to the counterparts. This structure design combined the advantages of well distributed ion groups and long flexible side chains, providing comb-shaped AEMs with superior properties for alkaline fuel cells. Future work will focus on applying inert polymer backbones, such as ether-bond-free aryl polymers, and highly alkaline stable imidazolium groups to these stable polymer structure design for
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