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Dual-cation comb-shaped anion exchange membranes: Structure, morphology and properties
Journal of Membrane Science 515 (2016) 189–195
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Dual-cation comb-shaped anion exchange membranes: Structure, morphology and properties Yubin He a, Jiaojiao Si b, Liang Wu a, Shengli Chen b, Yuan Zhu a, Jiefeng Pan a, Xiaolin Ge a, Zhengjin Yang a, Tongwen Xu a,n a CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Centre of Chemistry for Energy Materials, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, PR China b College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 February 2016 Received in revised form 28 May 2016 Accepted 31 May 2016 Available online 3 June 2016
Anion exchange membranes (AEMs) are employed as the gas separator and hydroxide ion conductor between the anode and the cathode of alkaline polyelectrolyte fuel cell (APEFC). Highly conductive and stable AEMs are urgently needed in order to achieve satisfactory fuel cell performance. Nevertheless, the low hydroxide conductivity remains major challenge that limits the development and application of APEFC. In order to improve the hydroxide conductivity of AEMs, highly ordered ion conducting channels must be constructed within the membrane matrix. In this study, an AEM with particular polymer structure was designed to achieve fine ion conducting channels by facilitating the hydrophilic-hydrophobic phase separation. Concretely, the side chain of this polyelectrolyte contains two quaternary ammonium groups and terminates with one long hydrophobic tail. Ascribing to the enhanced nanophase separation ability, inter-connected ion conducting channels were observed by AFM as well as SAXS. A high hydroxide conductivity of 47 mS/cm at 25 °C and 85 mS/cm at 80 °C was achieved. In addition, when applied in an APEFC system, the synthesized AEM showed an outstanding peak power density of 369.3 mW/cm2. Good alkaline tolerance, lowered water uptake and swelling ratio were also observed attributing to its carefully designed polymer structure. & 2016 Elsevier B.V. All rights reserved.
Keywords: Anion exchange membranes Phase separation Hydroxide conductivity Fuel cell
1. Introduction Polyelectrolyte fuel cell (PEFC) is recognized as one promising energy generating technology for its distinct advantages like high conservation of energy, low operation temperature, fuel diversity and so on [1]. For the past decades, the research interests of fuel cell community were mainly focused on the development of the acidic polyelectrolyte fuel cell which depends on the precious metal catalyst (Pt) and the expensive perfluorinated sulfonic acid membrane. In order to lower the equipment cost and further broaden the application of fuel cell technology, significant advance has been made by employing an anion exchange membrane (AEM) instead of a cation exchange membrane between the cathode and the anode, namely alkaline polyelectrolyte fuel cell (APEFC) [2]. Because of the faster kinetics of oxygen reduction under alkaline environment, Pt is possible to be replaced by other non-precious catalyst like Ni and Co [3]. However, the development and application of APEFC were still n
Corresponding author. E-mail address: [email protected] (T. Xu).
http://dx.doi.org/10.1016/j.memsci.2016.05.058 0376-7388/& 2016 Elsevier B.V. All rights reserved.
challenged by the insufficient hydroxide conductivity of the anion exchange membrane. This has evoked extensive research interests towards exploring the structure-property relationship of AEM in order to achieve high conductivity through polymer structure optimization [4]. It has been well documented that formation of well-connected anion conducting channels is the key factor that dominated the conductivity of AEM [5]. Because of the hydrophilic-hydrophobic discrimination between the water repellent polymer backbone and the hydrophilic cationic segments, the anion conducting groups of AEM tend to aggregate together, allowing phase separation in nano-scale. The ionic domains formed during the phase separation process contain high density of ion conducting groups, thus are considered to be the main pathway for anion conduction. Over the last decade, numerous strategies [6–11] have been developed aiming to achieve good nano-phase separation and well-connected ion conducting channels. For example, Jannasch and co-workers [9] found that by densely grafting the quaternary ammonium groups (QA groups) onto one particular segment of polymer backbones, the nano-phase separation ability and the hydroxide conductivity could be enhanced because of the increased hydrophilic-hydrophobic discrimination [9,12–16]. Bai and
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co-workers also reported one highly conductive AEM with three QA groups locating on one pendent phenyl ring [17]. There is a very unusual phenomenon that high hydroxide conductivity was achieved while no nano-phase separation was observed by SAXS. The author considered that the decreased distance between QA groups, which can efficiently facilitate the hopping conduction of hydroxide ions, is the main reason for the observed high conductivity. Another strategy to improve the nano-phase separation of AEM is to enhance the flexibility of the functional side chain. Previously in our lab [18], one side chain type AEM with a flexible aliphatic spacer between ion conducting group and polymer main chain have been prepared and proved to possess enhanced hydroxide conductivity. Recently, a series of side chain type AEMs have been reported [19–21], all demonstrating that the flexibility of side chain, which decides the mobility of ion conducting group, also has profound influence on the nano-phase separation ability and the anion conductivity of AEM. As an upgrade for the side-chain type AEM, Zhuang's research group introduced two QA groups into one same side chain of anion exchange membrane [22]. Improved alkaline stability was observed due to the decreased grafting ratio of cationic group on the polymer backbone. Afterwards, our group reported AEM with the dual-cation [23] and tri-cation containing side chain [24]. Excellent nano-phase separation as well as high hydroxide conductivity were observed because of the increased polar difference between the hydrophilic side chain and the hydrophobic polymer backbone. Yan and co-workers further increased the number of cation on the side chain. Changing trend of the conductivity with the side chain functionalization degree was investigated [25]. Hickner's research group evaluated the properties of a series of AEMs with the multi-cation functionalized side chain, excellent fuel cell performance was observed for the tri-cation based AEM [26]. Hickner and co-workers [27] also prepared the comb-shaped AEM by reacting brominated poly (2,6-dimethyl-1,4-phenylene oxide) with one tertiary amine bearing long aliphatic chain. Excellent nano-phase separated morphology was observed by small angle X-ray scattering (SAXS) due to the unique polymer structure with the hydrophilic main chain and the hydrophobic side chain [10,28,29]. Zhuang and co-workers [6] reported another combshaped AEM prepared by attaching the aliphatic chain onto the polymer backbone but separately from QA group, outstanding high temperature hydroxide conductivity even comparable to the Nafions was achieved. Inspired by these works, we have designed one dual-cation
comb-shaped polyelectrolyte which is expected to possess the excellent nano-phase separation ability and the resultant high hydroxide conductivity. As depicted in Fig. 1b, a long aliphatic tail was introduced to the end of the side chain in order to ensure the good nano-phase segregation. The high mobility of QA group resulting from the flexibility of the side chain can also facilitate the aggregation of the ionic segments. Besides, two QA groups were inserted between the polymer main chain and the aliphatic tail in order to increase both the side chain hydrophilicity and the functional group density in the ion conducting channels. Experimentally, two steps of Menshutkin reactions were employed to prepare this dual-cation comb-shaped polyelectrolyte. Afterwards, AEMs with this unique polymer structure were investigated by the atomic force microscopy (AFM) and the small angle X-ray scattering (SAXS) to detect their nano-phase separation. Hydroxide conductivities, fuel cell performance, water uptake, swelling ratio, mechanical properties as well as alkaline tolerance were also characterized.
2. Experimental section 2.1. Materials Poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) with an intrinsic viscosity of 0.57 dl/g in chloroform at 25 °C was manufactured by Asahi Kasei Chemicals Corporation (Japan) and kindly supplied by Tianwei Membrane Company (Shandong, P.R. China). PPO was brominated as previously reported [23,24] via free radical bromination. Bromination degree of brominated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO) was calculated according to 1 H NMR Spectrum (Fig S1). N-bromosuccinimide (NBS), 2,2′-azobis (2-methylpropionitrile) (AIBN), N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA), 1-bromooctadecane were purchased from Energy Chemical Co. Ltd. (Shanghai, P.R. China) and used as received. N-methyl-2-pyrrolidolone (NMP, AR), diethyl ether, chlorobenzene, ethanol, chloroform, toluene, acetonitrile, Hydrochloric acid (HCl) aqueous solution (37% AR), sodium chloride (AR), sodium hydroxide (AR) and sodium sulfate (Na2SO4, AR) were purchased from Sinopham Chemical Reagent Co. Ltd. Deionized water was used throughout. 2.2. Synthesis of N-(6-(dimethylamino)hexyl)-N,N-dimethyloctadecan-1-aminium (DMAQA-C18) DMAQA-C18 was prepared according to the previously reported
Fig. 1. Schematic illustration of (a): conventional main chain type AEMs. (b): dual-cation comb-shaped AEMs prepared in this work.
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literature [30] and briefly described as follow: to a stirred solution of N,N,N′,N′-tetramethyl-1,6-hexanediamine (25 mmol) in 50 mL toluene-acetonitrile mixture (v/v ¼1:1) was added 1-bromooctadecane (2.5 mmol) and then heated at 60 °C for 24 h. Afterwards, the solvent was removed under vacuum and the resulting solid was collected by filtration and followed by washing with diethyl ether. The resulting white solid was added to THF and filtrated, the solvent was removed under vacuum and then dried in vacuum at 50 °C for 10 h. NMR spectra of DMAQA-C18 was obtained employing D2O as solvent (Fig S2). 2.3. Synthesis of comb-shaped dual-cation polyelectrolytes (C18BQAPPO) To a stirred solution of BPPO (1 g) in NMP (15 mL) was added excess amount of DMAQA-C18 (1.5 equiv) then stirred at room temperature for 24 h to ensure complete conversion of the benzyl bromide groups. The IEC values of synthesized AEMs were controlled by varying the brominated ratio of BPPO. The resulting C18BQAPPO polyelectrolytes were obtained by precipitating in diethyl ether followed by filtration and dried in vacuum at 60 °C for 24 h. The NMR spectrum of C18BQAPPO was obtained employing DMSO-d6 as solvent. 2.4. Fabrication of C18BQAPPO AEMs 1 g of C18BQAPPO-x polyelectrolyte was dissolved in 15 mL NMP to form a homogeneous solution and then cast onto a clean glass plate. After heating at 60 °C for 24 h, a transparent membrane of C18BQAPPO-x was peeled off and stored in deionized water. Three AEMs with different IEC values were prepared and x was named to be 1, 2 and 3 according to the ranking of IEC values (Table 1).
3. Results and discussion 3.1. Synthesis of C18BQAPPO The synthetic procedure for an ideal AEM should be facile and easy to scale up for the practical applications. Besides, the employed starting material must be inexpensive and low-toxic. Thus in this study, two steps of Menshutkin reactions were employed to prepare the dual-cation, comb-shaped AEMs. As depicted in Fig. 2, the reaction between TMHDA and 1-bromooctadecane performed under mild condition and gave the pure product of DMAQA-C18 after simple purification process. The NMR spectrum of DMAQAC18 is depicted in Fig S2 and all signals were well-assigned. Afterwards, the C18BQAPPO was obtained via the reaction between BPPO and DMAQA-C18 under mild conditions. Table 1. The key properties of synthesized C18BQAPPO-x membranes and QPPO-x AEMs. Membrane
Bromination degreea
IEC (mmol/ g)
Water uptakeb (%)
Swelling ratiob (%)
Conductivityb (mS/cm)
C18BQAPPO-1 C18BQAPPO-2 C18BQAPPO-3 QPPO-1 QPPO-2 QPPO-3 QPPO-4
0.12 0.19 0.26 0.25 0.27 0.33 0.36
1.37 1.66 1.98 1.47 1.62 1.88 1.99
10.4 25.1 54.5 48.7 73.8 99.5 143.6
5.6 13.2 21.9 9.1 15.5 21.1 30.3
14.2 23.3 47.0 12.9 18.6 29.5 32.9
a b
Caculated based on the NMR spectrum of BPPO. Measured in the OH form at 25 °C.
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Characteristic signals at 3.1 ppm and 3.4 ppm were assigned to the protons of N þ -CH3 and pH-CH2-N þ (CH3)3 respectively (Fig. 3), verifying the as designed polymer structure of C18BQAPPO. 3.2. Morphology and hydroxide conductivity The nano-phase separation ability has long been considered as the dominating factor that decides the hydroxide conductivity of AEMs. Thus, in order to ensure good nano-phase separation of the prepared membranes, a dual-cation containing side chain was designed in order to enhance the hydrophilicity of ionic side chain. Besides, high mobility of QA group was ensured by inserting a flexible aliphatic spacer between the two quaternary ammonium groups. Further, a long hydrophobic tail of eighteen carbon atoms was introduced as the termination of this side chain. As depicted in Fig. 4, clearer phase separation of C18BQAPPO-3 was observed compared with C18BQAPPO-2 because of the increased grafting ratio of functional side chains as well as the higher ion exchange capacity. SAXS was conducted in order to further detect the phase separation of C18BQAPPO-3 AEM. As depicted in Fig. 5, a clear characteristic ionomer peak was observed. The corresponding periodic structure with length scale d value (d ¼2 π/q) was about 3.9 nm, which can be roughly recognized as the size of ionic clusters. The distinct nano-phase separation of C18BQAPPO-3 polyelectrolyte resulting from its unique polymer structure is one of the main reasons for its high hydroxide conductivity (Table 1). On the other hand, the conventional quaternized poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO) AEM prepared by the reaction between BPPO and trimethylamine exhibits very poor nanophase separation which could hardly be detected by AFM or SAXS (Figs. 4 and 5). As a result of its excellent nano-phase separation, good hydroxide conductivity up to 47 mS/cm was observed for C18BQAPPO-3 AEMs at 25 °C (Fig. 6). Besides, considering the practical operation conditions in a fuel cell system, the temperature dependent hydroxide conductivity of C18BQAPPO-3 membrane was also measured (Fig. 7). With increasing temperature, a hydroxide conductivity of 85 mS/cm was observed at 80 °C ascribing to the enhanced mobility of hydroxide ions. Previously, we have reported dual-cation AEM without the long aliphatic tail (BQAPPO-4, see Supporting information) [24]. In this work, one long aliphatic tail was introduced as the termination of the side chain. From Fig S6 and Fig. 4, we can see that C18BQAPPO3 AEM exhibits much more obvious nano-phase separated morphology because of the introduction of the long aliphatic tail. However, hydroxide conductivity of it was not enhanced. We consider that nano-phase separation ability of AEM is not the only factor that decides the hydroxide conductivity of AEM. Bai's research group previously reported AEM with tri-functionalized moieties [31]. High conductivity was achieved while no detectable nano-phase separation was observed. As they suggest, we also consider that the density of ion conducting groups in the ion conducting channels also have profound influence on the hydroxide conductivity of AEM. After introduction of the long aliphatic tail, although better nano-phase separation was achieved, the cation density in the ionic cluster decreased as well, thus resulting slightly lower hydroxide conductivity. Besides, considering its much lower water uptake (54.5%) compared with BQAPPO-4 (120.4%), its comparable conductivity with BQAPPO-4 suggests the high-efficiency utilization of quaternary ammonium groups and the formation of well-connected ion conducting channels. 3.3. Alkaline tolerance The alkaline tolerance is another key property of AEMs which decides the lifetime of the AEMs-based electro-chemical devices.
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Br C18H37
+
CH3CN/Toluene
N
N
60 oC
N
N
C18H37
Br
DMAQA-C18
CH3
CH3 N
N
C18H37
O
+
Br X X=H or Br
NMP n
O
R.T.
n
C18BQAPPO
Y Y=H or
Br N
N
C18H37
Br
Fig. 2. Synthetic procedure of DMAQA-C18 and C18BQAPPO.
Fig. 3. The NMR spectrum of C18BQAPPO polyelectrolyte.
Fig. 4. The AFM images of QPPO-4, C18BQAPPO-2 and C18BQAPPO-3 (500 nm 500 nm).
In this work, the hydroxide stability was investigated by immersing the membrane in 1 M KOH aqueous solution at 60 °C for increased length of time and then measuring the change of hydroxide conductivity. As depicted in Fig. 8, C18BQAPPO-3 membrane retained 80.7% of its initial hydroxide conductivity even after 380 h while the conventional QPPO membrane degenerated quickly under similar conditions. To further evaluate the hydroxide stability of prepared AEMs, small molecular model compound
(BnBQAC18) was synthesized (Fig S4) and alkaline stability of it was tested in 2 M NaOD solution at 60 °C. After 7 days, the NMR spectrum shows negligible changes (Fig S5), suggesting its good alkaline stability. The degeneration of polymer backbone triggered by the electro-withdrawing effect of the attached QA group has been revealed by many researchers [32–35]. One strategy to minimize this effect is to decrease the grafting ratio of cations on the polymer
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Fig. 8. Hydroxide stability of QPPO-4 and C18BQAPPO-3 membranes.
Fig. 5. The SAXS profiles of QPPO-4 and C18BQAPPO-3 membranes.
backbone by allocating two QA groups in one side chain [11,22]. In this study, the good alkaline stability of C18BQAPPO can also be attributed to its particular structure with the dual-cation containing side chains. Besides, in order to minimize the effect of Hoffman elimination, the two QA groups were separated by a long spacer with six carbon atoms [36]. 3.4. Water uptake, swelling ratio and mechanical properties
Fig. 6. Room temperature hydroxide conductivity of C18BQAPPO-x and QPPO-x membranes.
The water uptake and swelling ratio of synthesized membranes were measured at 25 °C in OH form. As depicted in Fig. 9, both the water uptake and swelling ratio increase with enhanced IEC values. At IEC values of 1.98 mmol/g, the C18BQAPPO-3 AEM exhibits low water uptake of 54.5% and swelling ratio of 21.9%. While the conventional QPPO membrane suffers from excess absorption of water (143.6%) at similar IEC value (1.99 mmol/g). This could be explained by the hydrophobic nature of the long aliphatic tail which can effectively decrease the absorption of water by the ion conducting groups. Besides, a well-connected hydrophobic matrix resulting from its good nano-phase separation is also considered to act as a robust network which can maintain the dimensional stability of membrane. In order to evaluate the feasibility of C18BQAPPO-3, the water
Fig. 7. Temperature dependence of hydroxide conductivity for the C18BQAPPO-3 membrane and the QPPO-4 membrane.
Fig. 9. The Water uptake and swelling ratio of C18BQAPPO-x and QPPO-4 membranes at 25 °C.
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Fig. 10. The temperature dependence of water uptake and swelling ratio for the C18BQAPPO-3 and QPPO-4 membrane. Fig. 11. Mechanical properties of C18BQAPPO-x and QPPO-4 membranes in wet state.
uptake and swelling ratio of C18BQAPPO-3 AEM was evaluated at different temperature ranging from 25 °C to 80 °C (Fig. 10). We found that water uptake and swelling ratio only increased a little even at 80 °C. This could be explained by the good dimensional stability attributing to the hydrophobic nature of the long aliphatic tail. On one hand, the long aliphatic chain can better restrict the absorption of water at high temperatures. On the other hand, the resultant good nano-phase separation lead to more continuous and cohesive hydrophobic matrix which can keep the dimensional stability of membranes at high temperature. Compared with our previously reported dual-cation AEM (BQAPPO-4), the introduction of one long aliphatic tail can efficiently decrease the water uptake and swelling ratio of the AEM (Table 1 and Table S1). This advantage of the comb-shaped, dualcation AEM is much more obvious when the temperature is high. At 80 °C, the swelling ratio of C18BQAPPO-3 AEM (24.8%, Fig. 10) is only half the value of BQAPPO-4 AEM (49.2%, Table S3). The mechanical properties including tensile strength (TS) and elongation at break (Eb) of the prepared AEMs were also measured under hydrated state in OH form. As depicted in Fig. 10, the TS values of C18BQAPPO membranes decrease from 20.1 MPa to 8.4 MPa with increasing IEC values while the Eb values show contrary trends (8.5–27.5%). This was due to the increased water uptake and swelling ratio at high IEC values. Interestingly, we found that Eb of C18BQAPPO-3 is much larger than that of QPPO-4 membrane, while the tensile strengths are almost same. This phenomenon could be explained by the formation of the better nano-phase separated morphology. The well-connected three-dimensional hydrophobic network may lead to higher flexibility of the AEM. Furthermore, the tensile strength of C18BQAPPO-3 AEM (8.4 MPa) is much higher than that of BQAPPO-4 AEM (5.5 MPa, Table S2). This could be explain by its better nano-phase separation ability. The hydrophobic phase is mainly composed of the aromatic main chain of PPO, thus the formation of well-connected hydrophobic network may enhance the mechanical properties of AEM. In a word, these results suggest that the C18BQAPPO membranes are tough and ductile enough for application in the alkaline polyelectrolyte fuel cell system.
cathode and anode was 0.4 mg/cm2, and the gas flow rates of O2 and H2 was 200 cc/min with back-pressure of 0.1 MPa. At a cell operation temperature of 60 °C, the peak power density of C18BQAPPO-3 AEM reached 369.3 mW/cm2 (Fig. 11). This excellent fuel cell performance can be attributed to its good hydroxide conductivity as well as well-connected ion-conducting channels (Fig. 12).
4. Conclusions In conclusion, we have prepared the dual-cation comb-shaped AEMs with long aliphatic chains to facilitate the formation of the excellent nano-phase separation. Particularly, two QA groups were inserted into the side chains of the AEMs in order to enhance the hydrophilicity of the functional side chains. As a result, the synthesized membranes exhibit a good hydroxide conductivity of 47 mS/cm at 25 °C and 85 mS/cm at 80 °C. An outstanding peak power density of 369.3 mW/cm2 was achieved when applied in the APEFC system. Besides, ascribing to the low grafting ratio of cationic groups on the polymer backbones, good hydroxide stability was also observed. The low water uptake and swelling ratio of prepared AEMs were also attributed to the carefully designed polymer structure. Also considering its facile synthetic procedure
3.5. Fuel cell performance In order to investigate the suitability of synthesized dual-cation, comb-shaped membranes in real devices, the C18BQAPPO-3 AEM with IEC values of 1.98 mmol/g and thickness of 90 mm was evaluated in the APEFC system. Specifically, the Pt loading on both
Fig. 12. Fuel cell performance of C18BQAPPO-3 membrane.
Y. He et al. / Journal of Membrane Science 515 (2016) 189–195
and the low manufacture cost, the C18BQAPPO AEMs are considered to possess good potential for the practical applications.
Acknowledgments This research is supported in part by the National Natural Science Foundation of China (Nos. 91534203 and 21490581), National Basic Research Program of China (No. 2012CB932800) and One Hundred Person Project of the Chinese Academy of Sciences (No. 2014type D).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2016.05. 058.
Nomenclature Abbreviations Interpretation PPO poly(2,6-dimethyl-1,4-phenylene oxide) BPPO brominated poly(2,6-dimethyl-1,4-phenylene oxide) DMAQA-C18 N-(6-(dimethylamino)hexyl)-N,N-dimethyloctadecan-1-aminium C18BQAPPO dual-cation comb-shaped polyelectrolytes QA groups Quaternary ammonium groups NMP N-methyl-2-pyrrolidolone AEM anion exchange membrane AFM atomic force microscope TMHDA N,N,N′,N′-Tetramethyl-1,6-hexanediamine SAXS Small angle X-ray scattering LER linear expansion ratio IEC ion exchange capacity WU Water uptake QPPO quaternized poly(2,6-dimethyl-1,4-phenylene oxide) Ts tensile strength Eb elongation at break
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A 3 (2015) 8571–8580. [30] B. Liu, L. Zheng, Z. Zhu, K. Zhang, H. Xi, Y. Qian, Effect of synthesis conditions on the structural and catalytic properties of hierarchically structured ZSM-5 Zeolites, RSC Adv. 4 (2014) 13831. [31] (Q. Li, L. Liu, Q. Miao, B. Jin, R. Bai, A novel poly(2,6-dimethyl-1,4-phenylene oxid)e) (with trifunctional ammonium moieties for alkaline anion exchange membranes), Chem. Commun. 50 (2014) 2791–2793. [32] C.G. Arges, L. Wang, J. Parrondo, V. Ramani, Best practices for investigating anion exchange membrane suitability for alkaline electrochemical devices: case study using quaternary Ammonium Poly(2,6-dimethyl 1,4-phenylene) oxide anion exchange membranes, J. Electrochem. Soc. 160 (2013) F1258–F1274. [33] S.A. Nuñez, M.A. Hickner, Quantitative1H NMR analysis of chemical stabilities in anion-exchange membranes, ACS Macro Lett. 2 (2013) 49–52. [34] J. Parrondo, C.G. Arges, M. Niedzwiecki, E.B. Anderson, K.E. Ayers, V. 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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Dual-cation comb-shaped anion exchange membranes: Structure, morphology and properties Yubin He a, Jiaojiao Si b, Liang Wu a, Shengli Chen b, Yuan Zhu a, Jiefeng Pan a, Xiaolin Ge a, Zhengjin Yang a, Tongwen Xu a,n a CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Centre of Chemistry for Energy Materials, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, PR China b College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 February 2016 Received in revised form 28 May 2016 Accepted 31 May 2016 Available online 3 June 2016
Anion exchange membranes (AEMs) are employed as the gas separator and hydroxide ion conductor between the anode and the cathode of alkaline polyelectrolyte fuel cell (APEFC). Highly conductive and stable AEMs are urgently needed in order to achieve satisfactory fuel cell performance. Nevertheless, the low hydroxide conductivity remains major challenge that limits the development and application of APEFC. In order to improve the hydroxide conductivity of AEMs, highly ordered ion conducting channels must be constructed within the membrane matrix. In this study, an AEM with particular polymer structure was designed to achieve fine ion conducting channels by facilitating the hydrophilic-hydrophobic phase separation. Concretely, the side chain of this polyelectrolyte contains two quaternary ammonium groups and terminates with one long hydrophobic tail. Ascribing to the enhanced nanophase separation ability, inter-connected ion conducting channels were observed by AFM as well as SAXS. A high hydroxide conductivity of 47 mS/cm at 25 °C and 85 mS/cm at 80 °C was achieved. In addition, when applied in an APEFC system, the synthesized AEM showed an outstanding peak power density of 369.3 mW/cm2. Good alkaline tolerance, lowered water uptake and swelling ratio were also observed attributing to its carefully designed polymer structure. & 2016 Elsevier B.V. All rights reserved.
Keywords: Anion exchange membranes Phase separation Hydroxide conductivity Fuel cell
1. Introduction Polyelectrolyte fuel cell (PEFC) is recognized as one promising energy generating technology for its distinct advantages like high conservation of energy, low operation temperature, fuel diversity and so on [1]. For the past decades, the research interests of fuel cell community were mainly focused on the development of the acidic polyelectrolyte fuel cell which depends on the precious metal catalyst (Pt) and the expensive perfluorinated sulfonic acid membrane. In order to lower the equipment cost and further broaden the application of fuel cell technology, significant advance has been made by employing an anion exchange membrane (AEM) instead of a cation exchange membrane between the cathode and the anode, namely alkaline polyelectrolyte fuel cell (APEFC) [2]. Because of the faster kinetics of oxygen reduction under alkaline environment, Pt is possible to be replaced by other non-precious catalyst like Ni and Co [3]. However, the development and application of APEFC were still n
Corresponding author. E-mail address: [email protected] (T. Xu).
http://dx.doi.org/10.1016/j.memsci.2016.05.058 0376-7388/& 2016 Elsevier B.V. All rights reserved.
challenged by the insufficient hydroxide conductivity of the anion exchange membrane. This has evoked extensive research interests towards exploring the structure-property relationship of AEM in order to achieve high conductivity through polymer structure optimization [4]. It has been well documented that formation of well-connected anion conducting channels is the key factor that dominated the conductivity of AEM [5]. Because of the hydrophilic-hydrophobic discrimination between the water repellent polymer backbone and the hydrophilic cationic segments, the anion conducting groups of AEM tend to aggregate together, allowing phase separation in nano-scale. The ionic domains formed during the phase separation process contain high density of ion conducting groups, thus are considered to be the main pathway for anion conduction. Over the last decade, numerous strategies [6–11] have been developed aiming to achieve good nano-phase separation and well-connected ion conducting channels. For example, Jannasch and co-workers [9] found that by densely grafting the quaternary ammonium groups (QA groups) onto one particular segment of polymer backbones, the nano-phase separation ability and the hydroxide conductivity could be enhanced because of the increased hydrophilic-hydrophobic discrimination [9,12–16]. Bai and
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co-workers also reported one highly conductive AEM with three QA groups locating on one pendent phenyl ring [17]. There is a very unusual phenomenon that high hydroxide conductivity was achieved while no nano-phase separation was observed by SAXS. The author considered that the decreased distance between QA groups, which can efficiently facilitate the hopping conduction of hydroxide ions, is the main reason for the observed high conductivity. Another strategy to improve the nano-phase separation of AEM is to enhance the flexibility of the functional side chain. Previously in our lab [18], one side chain type AEM with a flexible aliphatic spacer between ion conducting group and polymer main chain have been prepared and proved to possess enhanced hydroxide conductivity. Recently, a series of side chain type AEMs have been reported [19–21], all demonstrating that the flexibility of side chain, which decides the mobility of ion conducting group, also has profound influence on the nano-phase separation ability and the anion conductivity of AEM. As an upgrade for the side-chain type AEM, Zhuang's research group introduced two QA groups into one same side chain of anion exchange membrane [22]. Improved alkaline stability was observed due to the decreased grafting ratio of cationic group on the polymer backbone. Afterwards, our group reported AEM with the dual-cation [23] and tri-cation containing side chain [24]. Excellent nano-phase separation as well as high hydroxide conductivity were observed because of the increased polar difference between the hydrophilic side chain and the hydrophobic polymer backbone. Yan and co-workers further increased the number of cation on the side chain. Changing trend of the conductivity with the side chain functionalization degree was investigated [25]. Hickner's research group evaluated the properties of a series of AEMs with the multi-cation functionalized side chain, excellent fuel cell performance was observed for the tri-cation based AEM [26]. Hickner and co-workers [27] also prepared the comb-shaped AEM by reacting brominated poly (2,6-dimethyl-1,4-phenylene oxide) with one tertiary amine bearing long aliphatic chain. Excellent nano-phase separated morphology was observed by small angle X-ray scattering (SAXS) due to the unique polymer structure with the hydrophilic main chain and the hydrophobic side chain [10,28,29]. Zhuang and co-workers [6] reported another combshaped AEM prepared by attaching the aliphatic chain onto the polymer backbone but separately from QA group, outstanding high temperature hydroxide conductivity even comparable to the Nafions was achieved. Inspired by these works, we have designed one dual-cation
comb-shaped polyelectrolyte which is expected to possess the excellent nano-phase separation ability and the resultant high hydroxide conductivity. As depicted in Fig. 1b, a long aliphatic tail was introduced to the end of the side chain in order to ensure the good nano-phase segregation. The high mobility of QA group resulting from the flexibility of the side chain can also facilitate the aggregation of the ionic segments. Besides, two QA groups were inserted between the polymer main chain and the aliphatic tail in order to increase both the side chain hydrophilicity and the functional group density in the ion conducting channels. Experimentally, two steps of Menshutkin reactions were employed to prepare this dual-cation comb-shaped polyelectrolyte. Afterwards, AEMs with this unique polymer structure were investigated by the atomic force microscopy (AFM) and the small angle X-ray scattering (SAXS) to detect their nano-phase separation. Hydroxide conductivities, fuel cell performance, water uptake, swelling ratio, mechanical properties as well as alkaline tolerance were also characterized.
2. Experimental section 2.1. Materials Poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) with an intrinsic viscosity of 0.57 dl/g in chloroform at 25 °C was manufactured by Asahi Kasei Chemicals Corporation (Japan) and kindly supplied by Tianwei Membrane Company (Shandong, P.R. China). PPO was brominated as previously reported [23,24] via free radical bromination. Bromination degree of brominated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO) was calculated according to 1 H NMR Spectrum (Fig S1). N-bromosuccinimide (NBS), 2,2′-azobis (2-methylpropionitrile) (AIBN), N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA), 1-bromooctadecane were purchased from Energy Chemical Co. Ltd. (Shanghai, P.R. China) and used as received. N-methyl-2-pyrrolidolone (NMP, AR), diethyl ether, chlorobenzene, ethanol, chloroform, toluene, acetonitrile, Hydrochloric acid (HCl) aqueous solution (37% AR), sodium chloride (AR), sodium hydroxide (AR) and sodium sulfate (Na2SO4, AR) were purchased from Sinopham Chemical Reagent Co. Ltd. Deionized water was used throughout. 2.2. Synthesis of N-(6-(dimethylamino)hexyl)-N,N-dimethyloctadecan-1-aminium (DMAQA-C18) DMAQA-C18 was prepared according to the previously reported
Fig. 1. Schematic illustration of (a): conventional main chain type AEMs. (b): dual-cation comb-shaped AEMs prepared in this work.
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literature [30] and briefly described as follow: to a stirred solution of N,N,N′,N′-tetramethyl-1,6-hexanediamine (25 mmol) in 50 mL toluene-acetonitrile mixture (v/v ¼1:1) was added 1-bromooctadecane (2.5 mmol) and then heated at 60 °C for 24 h. Afterwards, the solvent was removed under vacuum and the resulting solid was collected by filtration and followed by washing with diethyl ether. The resulting white solid was added to THF and filtrated, the solvent was removed under vacuum and then dried in vacuum at 50 °C for 10 h. NMR spectra of DMAQA-C18 was obtained employing D2O as solvent (Fig S2). 2.3. Synthesis of comb-shaped dual-cation polyelectrolytes (C18BQAPPO) To a stirred solution of BPPO (1 g) in NMP (15 mL) was added excess amount of DMAQA-C18 (1.5 equiv) then stirred at room temperature for 24 h to ensure complete conversion of the benzyl bromide groups. The IEC values of synthesized AEMs were controlled by varying the brominated ratio of BPPO. The resulting C18BQAPPO polyelectrolytes were obtained by precipitating in diethyl ether followed by filtration and dried in vacuum at 60 °C for 24 h. The NMR spectrum of C18BQAPPO was obtained employing DMSO-d6 as solvent. 2.4. Fabrication of C18BQAPPO AEMs 1 g of C18BQAPPO-x polyelectrolyte was dissolved in 15 mL NMP to form a homogeneous solution and then cast onto a clean glass plate. After heating at 60 °C for 24 h, a transparent membrane of C18BQAPPO-x was peeled off and stored in deionized water. Three AEMs with different IEC values were prepared and x was named to be 1, 2 and 3 according to the ranking of IEC values (Table 1).
3. Results and discussion 3.1. Synthesis of C18BQAPPO The synthetic procedure for an ideal AEM should be facile and easy to scale up for the practical applications. Besides, the employed starting material must be inexpensive and low-toxic. Thus in this study, two steps of Menshutkin reactions were employed to prepare the dual-cation, comb-shaped AEMs. As depicted in Fig. 2, the reaction between TMHDA and 1-bromooctadecane performed under mild condition and gave the pure product of DMAQA-C18 after simple purification process. The NMR spectrum of DMAQAC18 is depicted in Fig S2 and all signals were well-assigned. Afterwards, the C18BQAPPO was obtained via the reaction between BPPO and DMAQA-C18 under mild conditions. Table 1. The key properties of synthesized C18BQAPPO-x membranes and QPPO-x AEMs. Membrane
Bromination degreea
IEC (mmol/ g)
Water uptakeb (%)
Swelling ratiob (%)
Conductivityb (mS/cm)
C18BQAPPO-1 C18BQAPPO-2 C18BQAPPO-3 QPPO-1 QPPO-2 QPPO-3 QPPO-4
0.12 0.19 0.26 0.25 0.27 0.33 0.36
1.37 1.66 1.98 1.47 1.62 1.88 1.99
10.4 25.1 54.5 48.7 73.8 99.5 143.6
5.6 13.2 21.9 9.1 15.5 21.1 30.3
14.2 23.3 47.0 12.9 18.6 29.5 32.9
a b
Caculated based on the NMR spectrum of BPPO. Measured in the OH form at 25 °C.
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Characteristic signals at 3.1 ppm and 3.4 ppm were assigned to the protons of N þ -CH3 and pH-CH2-N þ (CH3)3 respectively (Fig. 3), verifying the as designed polymer structure of C18BQAPPO. 3.2. Morphology and hydroxide conductivity The nano-phase separation ability has long been considered as the dominating factor that decides the hydroxide conductivity of AEMs. Thus, in order to ensure good nano-phase separation of the prepared membranes, a dual-cation containing side chain was designed in order to enhance the hydrophilicity of ionic side chain. Besides, high mobility of QA group was ensured by inserting a flexible aliphatic spacer between the two quaternary ammonium groups. Further, a long hydrophobic tail of eighteen carbon atoms was introduced as the termination of this side chain. As depicted in Fig. 4, clearer phase separation of C18BQAPPO-3 was observed compared with C18BQAPPO-2 because of the increased grafting ratio of functional side chains as well as the higher ion exchange capacity. SAXS was conducted in order to further detect the phase separation of C18BQAPPO-3 AEM. As depicted in Fig. 5, a clear characteristic ionomer peak was observed. The corresponding periodic structure with length scale d value (d ¼2 π/q) was about 3.9 nm, which can be roughly recognized as the size of ionic clusters. The distinct nano-phase separation of C18BQAPPO-3 polyelectrolyte resulting from its unique polymer structure is one of the main reasons for its high hydroxide conductivity (Table 1). On the other hand, the conventional quaternized poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO) AEM prepared by the reaction between BPPO and trimethylamine exhibits very poor nanophase separation which could hardly be detected by AFM or SAXS (Figs. 4 and 5). As a result of its excellent nano-phase separation, good hydroxide conductivity up to 47 mS/cm was observed for C18BQAPPO-3 AEMs at 25 °C (Fig. 6). Besides, considering the practical operation conditions in a fuel cell system, the temperature dependent hydroxide conductivity of C18BQAPPO-3 membrane was also measured (Fig. 7). With increasing temperature, a hydroxide conductivity of 85 mS/cm was observed at 80 °C ascribing to the enhanced mobility of hydroxide ions. Previously, we have reported dual-cation AEM without the long aliphatic tail (BQAPPO-4, see Supporting information) [24]. In this work, one long aliphatic tail was introduced as the termination of the side chain. From Fig S6 and Fig. 4, we can see that C18BQAPPO3 AEM exhibits much more obvious nano-phase separated morphology because of the introduction of the long aliphatic tail. However, hydroxide conductivity of it was not enhanced. We consider that nano-phase separation ability of AEM is not the only factor that decides the hydroxide conductivity of AEM. Bai's research group previously reported AEM with tri-functionalized moieties [31]. High conductivity was achieved while no detectable nano-phase separation was observed. As they suggest, we also consider that the density of ion conducting groups in the ion conducting channels also have profound influence on the hydroxide conductivity of AEM. After introduction of the long aliphatic tail, although better nano-phase separation was achieved, the cation density in the ionic cluster decreased as well, thus resulting slightly lower hydroxide conductivity. Besides, considering its much lower water uptake (54.5%) compared with BQAPPO-4 (120.4%), its comparable conductivity with BQAPPO-4 suggests the high-efficiency utilization of quaternary ammonium groups and the formation of well-connected ion conducting channels. 3.3. Alkaline tolerance The alkaline tolerance is another key property of AEMs which decides the lifetime of the AEMs-based electro-chemical devices.
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Br C18H37
+
CH3CN/Toluene
N
N
60 oC
N
N
C18H37
Br
DMAQA-C18
CH3
CH3 N
N
C18H37
O
+
Br X X=H or Br
NMP n
O
R.T.
n
C18BQAPPO
Y Y=H or
Br N
N
C18H37
Br
Fig. 2. Synthetic procedure of DMAQA-C18 and C18BQAPPO.
Fig. 3. The NMR spectrum of C18BQAPPO polyelectrolyte.
Fig. 4. The AFM images of QPPO-4, C18BQAPPO-2 and C18BQAPPO-3 (500 nm 500 nm).
In this work, the hydroxide stability was investigated by immersing the membrane in 1 M KOH aqueous solution at 60 °C for increased length of time and then measuring the change of hydroxide conductivity. As depicted in Fig. 8, C18BQAPPO-3 membrane retained 80.7% of its initial hydroxide conductivity even after 380 h while the conventional QPPO membrane degenerated quickly under similar conditions. To further evaluate the hydroxide stability of prepared AEMs, small molecular model compound
(BnBQAC18) was synthesized (Fig S4) and alkaline stability of it was tested in 2 M NaOD solution at 60 °C. After 7 days, the NMR spectrum shows negligible changes (Fig S5), suggesting its good alkaline stability. The degeneration of polymer backbone triggered by the electro-withdrawing effect of the attached QA group has been revealed by many researchers [32–35]. One strategy to minimize this effect is to decrease the grafting ratio of cations on the polymer
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Fig. 8. Hydroxide stability of QPPO-4 and C18BQAPPO-3 membranes.
Fig. 5. The SAXS profiles of QPPO-4 and C18BQAPPO-3 membranes.
backbone by allocating two QA groups in one side chain [11,22]. In this study, the good alkaline stability of C18BQAPPO can also be attributed to its particular structure with the dual-cation containing side chains. Besides, in order to minimize the effect of Hoffman elimination, the two QA groups were separated by a long spacer with six carbon atoms [36]. 3.4. Water uptake, swelling ratio and mechanical properties
Fig. 6. Room temperature hydroxide conductivity of C18BQAPPO-x and QPPO-x membranes.
The water uptake and swelling ratio of synthesized membranes were measured at 25 °C in OH form. As depicted in Fig. 9, both the water uptake and swelling ratio increase with enhanced IEC values. At IEC values of 1.98 mmol/g, the C18BQAPPO-3 AEM exhibits low water uptake of 54.5% and swelling ratio of 21.9%. While the conventional QPPO membrane suffers from excess absorption of water (143.6%) at similar IEC value (1.99 mmol/g). This could be explained by the hydrophobic nature of the long aliphatic tail which can effectively decrease the absorption of water by the ion conducting groups. Besides, a well-connected hydrophobic matrix resulting from its good nano-phase separation is also considered to act as a robust network which can maintain the dimensional stability of membrane. In order to evaluate the feasibility of C18BQAPPO-3, the water
Fig. 7. Temperature dependence of hydroxide conductivity for the C18BQAPPO-3 membrane and the QPPO-4 membrane.
Fig. 9. The Water uptake and swelling ratio of C18BQAPPO-x and QPPO-4 membranes at 25 °C.
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Fig. 10. The temperature dependence of water uptake and swelling ratio for the C18BQAPPO-3 and QPPO-4 membrane. Fig. 11. Mechanical properties of C18BQAPPO-x and QPPO-4 membranes in wet state.
uptake and swelling ratio of C18BQAPPO-3 AEM was evaluated at different temperature ranging from 25 °C to 80 °C (Fig. 10). We found that water uptake and swelling ratio only increased a little even at 80 °C. This could be explained by the good dimensional stability attributing to the hydrophobic nature of the long aliphatic tail. On one hand, the long aliphatic chain can better restrict the absorption of water at high temperatures. On the other hand, the resultant good nano-phase separation lead to more continuous and cohesive hydrophobic matrix which can keep the dimensional stability of membranes at high temperature. Compared with our previously reported dual-cation AEM (BQAPPO-4), the introduction of one long aliphatic tail can efficiently decrease the water uptake and swelling ratio of the AEM (Table 1 and Table S1). This advantage of the comb-shaped, dualcation AEM is much more obvious when the temperature is high. At 80 °C, the swelling ratio of C18BQAPPO-3 AEM (24.8%, Fig. 10) is only half the value of BQAPPO-4 AEM (49.2%, Table S3). The mechanical properties including tensile strength (TS) and elongation at break (Eb) of the prepared AEMs were also measured under hydrated state in OH form. As depicted in Fig. 10, the TS values of C18BQAPPO membranes decrease from 20.1 MPa to 8.4 MPa with increasing IEC values while the Eb values show contrary trends (8.5–27.5%). This was due to the increased water uptake and swelling ratio at high IEC values. Interestingly, we found that Eb of C18BQAPPO-3 is much larger than that of QPPO-4 membrane, while the tensile strengths are almost same. This phenomenon could be explained by the formation of the better nano-phase separated morphology. The well-connected three-dimensional hydrophobic network may lead to higher flexibility of the AEM. Furthermore, the tensile strength of C18BQAPPO-3 AEM (8.4 MPa) is much higher than that of BQAPPO-4 AEM (5.5 MPa, Table S2). This could be explain by its better nano-phase separation ability. The hydrophobic phase is mainly composed of the aromatic main chain of PPO, thus the formation of well-connected hydrophobic network may enhance the mechanical properties of AEM. In a word, these results suggest that the C18BQAPPO membranes are tough and ductile enough for application in the alkaline polyelectrolyte fuel cell system.
cathode and anode was 0.4 mg/cm2, and the gas flow rates of O2 and H2 was 200 cc/min with back-pressure of 0.1 MPa. At a cell operation temperature of 60 °C, the peak power density of C18BQAPPO-3 AEM reached 369.3 mW/cm2 (Fig. 11). This excellent fuel cell performance can be attributed to its good hydroxide conductivity as well as well-connected ion-conducting channels (Fig. 12).
4. Conclusions In conclusion, we have prepared the dual-cation comb-shaped AEMs with long aliphatic chains to facilitate the formation of the excellent nano-phase separation. Particularly, two QA groups were inserted into the side chains of the AEMs in order to enhance the hydrophilicity of the functional side chains. As a result, the synthesized membranes exhibit a good hydroxide conductivity of 47 mS/cm at 25 °C and 85 mS/cm at 80 °C. An outstanding peak power density of 369.3 mW/cm2 was achieved when applied in the APEFC system. Besides, ascribing to the low grafting ratio of cationic groups on the polymer backbones, good hydroxide stability was also observed. The low water uptake and swelling ratio of prepared AEMs were also attributed to the carefully designed polymer structure. Also considering its facile synthetic procedure
3.5. Fuel cell performance In order to investigate the suitability of synthesized dual-cation, comb-shaped membranes in real devices, the C18BQAPPO-3 AEM with IEC values of 1.98 mmol/g and thickness of 90 mm was evaluated in the APEFC system. Specifically, the Pt loading on both
Fig. 12. Fuel cell performance of C18BQAPPO-3 membrane.
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and the low manufacture cost, the C18BQAPPO AEMs are considered to possess good potential for the practical applications.
Acknowledgments This research is supported in part by the National Natural Science Foundation of China (Nos. 91534203 and 21490581), National Basic Research Program of China (No. 2012CB932800) and One Hundred Person Project of the Chinese Academy of Sciences (No. 2014type D).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2016.05. 058.
Nomenclature Abbreviations Interpretation PPO poly(2,6-dimethyl-1,4-phenylene oxide) BPPO brominated poly(2,6-dimethyl-1,4-phenylene oxide) DMAQA-C18 N-(6-(dimethylamino)hexyl)-N,N-dimethyloctadecan-1-aminium C18BQAPPO dual-cation comb-shaped polyelectrolytes QA groups Quaternary ammonium groups NMP N-methyl-2-pyrrolidolone AEM anion exchange membrane AFM atomic force microscope TMHDA N,N,N′,N′-Tetramethyl-1,6-hexanediamine SAXS Small angle X-ray scattering LER linear expansion ratio IEC ion exchange capacity WU Water uptake QPPO quaternized poly(2,6-dimethyl-1,4-phenylene oxide) Ts tensile strength Eb elongation at break
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