- Email: [email protected]
Pendent piperidinium-functionalized blend anion exchange membrane for fuel cell application
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Pendent piperidinium-functionalized blend anion exchange membrane for fuel cell application Mengyao Niu a,b, Caimian Zhang a, Gaohong He a,b, Fengxiang Zhang a,*, Xuemei Wu b,** a
State Key Laboratory of Fine Chemicals, School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin, LN, 124221, China b Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian, LN, 116024, China
article info
abstract
Article history:
Alkaline anion exchange membrane fuel cell has fast cathode reactions and thus allows
Received 5 February 2019
the use of low cost electrocatalysts. However, its practical application is hindered by the
Received in revised form
low hydroxide ion conductivity and alkaline stability of AEM. In this study, pendent
8 April 2019
piperidinium functionalized polyetheretherketone is synthesized and blended with poly-
Accepted 17 April 2019
benzimidazole for fabrication of composite anion exchange membrane. The pendent
Available online 10 May 2019
piperidinium functionalized side chains can create well-connected ionic transporting channels and thus impart the blend membranes high hydroxide conductivity (61.5
Keywords:
e72.8 mS cm1 at 80 C) and good tensile strength (42.8e58.9 MPa). Due to the strong in-
Anion exchange membrane
teractions between polybenzimidazole and piperidinium groups of the polymers as
Blend polymers
confirmed by Fourier transform infrared spectroscopy, the piperidinium functionalized
Polybenzimidazole
blend anion exchange membrane can retain 95% of its original OH conductivity value
Piperidinium
when treated in 1 M KOH at 60 C for 576 h. The single fuel cell assembled with the
Inter-chain forces
membrane can yield a peak power density of 87 mW cm2 at 80 C. Our work provides a new and effective method to balance the hydroxide conductivity and alkaline stability of anion exchange membranes. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays the whole world is focusing on reducing the dependence on fossil fuels (such as coal, oil and natural gas) and finding the alternative renewable clean energies [1,2]. Polymer electrolyte fuel cell (PEFC) is regarded as one of the most promising environmentally benign energies and has been the focus of scientific research [3e6]. PEFC can be divided
into proton exchange membrane fuel cell (PEMFC) and anion exchange membrane fuel cell (AEMFC) according to the ions (Hþ or OH) transported in the membrane [7,8]. PEMFC has been widely studied and becomes rather mature through long-term efforts. However, due to the strong dependence on noble metal catalyst (such as Pt), high fuel permeability and high cost of proton exchange membranes (such as Nafion), large-scale commercialization has not been achieved for
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Zhang), [email protected] (X. Wu). https://doi.org/10.1016/j.ijhydene.2019.04.172 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
PEMFC [9e11]. As compared to PEMFC, AEMFC has several important advantages, including their compatibility with the use of non-noble metal catalyst (such as Ag, Ni and Co), ability to retain good stability of the catalyst, and faster oxygen reduction reaction [12e15]. As one of the key components of AEMFC, anion exchange membrane (AEM) acts as the barrier against fuel crossover and the medium for OH transport [16e18]. The main challenge of AEM toward practical application in fuel cells lies in the trade-off between OH conductivity and alkaline stability. The tradeoff is related to the inherently low OH diffusion rate (as compared to Hþ) and difficult dissociation of the cation-hydroxide ion pair [19], and therefore, high conductivity often depends on high degree of quaternization, rendering the membrane more vulnerable to OH attack. Over the past few years, several strategies have been proposed to alleviate the conductivity - stability contradiction, which include: (1) introducing long side chains or improving the concentration of ionic groups on the side chains in order to facilitate ionic cluster formation [20e23]; (2) synthesizing multi-block copolymers to promote the formation of wellconnected OH transporting channels [24e28], and (3) using polymer blends or constructing semi inter-penetrating networks to combine the merits of various functional polymers [29e32]. Importantly, the above approaches can work in a synergistic way to further improve the membrane performance, and there are reported examples of fabricating blend membranes containing long side chain polymers for PEMFC or vanadium redox flow battery [33,34]. Inspired by the above works, we design and synthesize a pendent piperidinium functionalized polymer and blend it with polybenzimidazole (PBI) for fabrication of high performance blend AEM. To the best of our knowledge, this is the first report of piperidinium functionalized blend AEM containing PBI. One advantage of this membrane design lies in the better alkaline stability of the piperidinium group relative to other common used cation structures for AEM like quaternary ammonium, imidazolium and guanidinium [35e39]. The long spacer between the polymer backbone and the cation center promotes microphase separation. Meanwhile, PBI possesses e NHe and C]N moieties, which allow it to interact with both electronegative and electropositive polymers so that robust blend membranes can be expected [40,41]. By changing the amount of PBI in the blend membrane, OH conductivity and alkaline stability can be well balanced. The obtained membrane shows a good hydroxide conductivity of 69 mS cm1 at 80 C for a measured ion exchange capacity of 1.49 mmol g1; it retains 95% of its original hydroxide conductivity after alkali-treated at 60 C for 576 h, which is superior among all blend AEMs.
15483
as PBI) was purchased from Shanghai Shengjun Plastics Technology Co.,Ltd. The average molecular weight is approximately 14000 Da. N-octyl alcohol, dichloromethane, trioxymethylene, anhydrous calcium chloride, sodium chloride, concentrated hydrochloric acid, concentrated sulfuric acid, absolute ethyl alcohol, 3-chloro-1-propanol, 1methylpiperidine (MPi), dimethyl formamide (DMF), dimethylsulfoxide (DMSO), sodium hydride (NaH) and tetrabutylammonium bromide (TBAB) were all purchased from J&K (China) Co. Ltd. and used as received.
Synthesis of chloromethylated polyetheretherketone Chloromethylated polyetheretherketone (CMPEEK) was synthesized by the reaction between CMOE and PEEK in concentrated sulfuric acid. Specifically, 2 g PEEK was dissolved in 120 ml sulfuric acid and 40 ml CMOE was added after complete dissolution. The reaction temperature was set as 10 C, and CMPEEK with different chloromethylation degrees (DC) were obtained via changing the reaction time. The CMPEEK was precipitated from ice water, washed with ice water a few times to remove the solvent, and then dried overnight at 25 C.
Synthesis of long side chain grafted polyetheretherketone The long side chain grafted polyetheretherketone (s-CMPEEK) was synthesized by the Williamson etherification reaction between 3-chloro-1-propanol and CMPEEK [43]. Firstly, 8 mmol 3-chloro-1-propanol was dissolved in 10 ml DMF; the resulting solution was then added into a mixture comprising 0.2 g TBAB, 8 mmol NaH (60 wt% dispersion in coal oil), and 20 ml DMF. The new mixture obtained was stirred at 40 C for 2 h with a continuous N2 purge. Then, a solution of 3 mmol CMPEEK (DC ¼ 1.02) in 10 ml DMF was added slowly and the mixture was stirred for 24 h. Finally, the s-CMPEEK was precipitated from ethanol, and washed with water for a few times to remove impurities, and then dried overnight at 50 C.
Synthesis of pendent piperidinium-functionalized polyetheretherketone The pendent piperidinium-functionalized polyetheretherketone (QHPEEK) was synthesized via the Menshutkin reaction between s-CMPEEK and 1-methylpiperidine. Firstly, 3 mmol s-CMPEEK and 8 mmol 1-methylpiperidine were dissolved in 60 ml DMF. Then, the mixture was reacted at 60 C for 36 h with N2 purge and QHPEEK was obtained by precipitation from ethanol, followed by washing with water several times to remove impurities, and then drying overnight at 50 C.
Experimental
Membrane fabrication
Materials
0.1 g blend material containing varied amounts of QH-1.89 (80, 90 and 95 wt%) and PBI was dissolved in 5 ml DMSO at room temperature; the resulting solution was casted onto a 4.5 cm 4.5 cm glass plate and dried at 60 C for 48 h to obtain the blend membrane, which is denoted as BQH-x, where x is the measured ion exchange capacity (IECm) of the
Polyetheretherketone (PEEK, VESTAKEEPVR 4000P) was purchased from Evonik Degussa (China) Co. Ltd and chloromethyl octyl ether (CMOE) was synthesized according to literature [42]. Poly (4,40 -diphenylether-5,50 -bibenzimidazole) (denoted
15484
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
membrane. The blend membrane was immersed in 1 M KOH solution at 25 C for 48 h to convert the counter ion from Cl to OH. The BQH membrane (ca. 40 mm thick) was sealed in a container full of deionized water for 48 h to completely remove the residual KOH. As a control, pure QHPEEK membranes were prepared using the same method as described above; the membranes are denoted as QH-x, where x is the IECm of the membrane.
Characterization and measurements 1
H NMR spectroscopy was recorded at a resonance frequency of 500 MHz using tetramethylsilane (TMS) as an internal standard substance and DMSO-d6 or CDCl3 as solvent on a Varian Unity Inova 500 spectrometer. FT-IR spectra were recorded in the wavelength range of 4000e400 cm1 with a resolution of 0.5 cm1 on a Bruker EQUINOX55 FTIR spectrometer. The mechanical strength and elongation at break of hydrated membrane were measured at a tensile rate of 5 mm min1 on SANS CMT8502 Universal Tensile Machine at room temperature. The hydrated membrane was cut into 0.5 cm 4.5 cm slices before the test. Thermal stability of the dry membrane sample was recorded on TGA analyzer (TAINC SDT Q600 thermogravimetric analyzer) from 25 C to 790 C under a nitrogen flow at a heating speed of 5 C$min1. A derivative thermogravimetry (DTG) curve is the first order differential of TGA as a function of temperature. Ion-exchange capacity (IEC) of the membrane in OH form was measured with the back titration method. The membrane sample was immersed in 25 ml 0.01 M HCl solution for 48 h, then the HCl solution was titrated by KOH standard solution (0.01 M) and the consumed volume of KOH was denoted as Va (L). The blank HCl solution (not used for membrane soaking) was titrated with the same method and the consumed volume of KOH was denoted as Vb (L). Then the membrane sample was vacuum dried at 50 C for 24 h and the weight of the dry membrane was denoted as Wdry (g). The measured IEC, or IECm (mmol/g) of the membrane was calculated by following Eq. (1). IECm ¼
ðVb Va ÞCKOH *1000 Wdry
(1)
The in-plane conductivity was measured on Ivium Technologies A08001 equipment by the typical four-electrode AC impedance method at a frequency of 1~105 Hz. The membrane was cut into 0.5 cm 4.5 cm slices and measured in 100% relative humidity. The in-plane conductivity (s) was calculated by Eq. (2). s¼
l R*S
(2)
where R is the high-frequency resistance, l is the distance between two electrodes, and S is the area of the membrane. The hydroxide ion transporting activation energy (Ea) is calculated by Eq. (3). Ea ¼ bR
(3)
where b is the slope of the regression line ln(s)~1000/T, T is the absolute temperature, and R is the gas constant.
Water uptake (WU), swelling ratio (SR) and the number of water molecules per piperidinium group (l) of the membrane were measured as follows. Firstly, the hydrated membrane with OH counter ion was soaked into deionized water for 12 h at different temperatures. After removing the residual water on the membrane surface, weight and size of the wet membrane were measured immediately. Then the membrane was vacuum dried at 50 C for 24 h and the weight and size of the dry membrane were measured again. WU, SR and l are calculated with Eqs. (4)e(6), respectively. WU ¼
SR ¼
l¼
Wwet Wdry *100% Wdry
lwet ldry *100% ldry
1000WUð%Þ MH2O IECm
(4)
(5)
(6)
In Eq. (4)-(6), Wwet and Wdry are the mass of the wet and the dried membrane, respectively. lwet¼(la*lb)^0.5 is the average size of the wet membrane, ldry¼(la’*lb’)^0.5 is the average size of the dried membrane. la, lb, la’, lb’ are the width and length of wet and dried membrane separately, and IECm is the measured ion exchange capacity. Membrane morphology was characterized with scanning electron microscopy (SEM, Quanta 450, FEI Co., USA), transmission electron microscopy (TEM, JEM-2000EX JEOL) and small angle X-ray scattering (SAXS, X'Pert Powder system). For SEM, the membrane samples were coated with gold before testing. For TEM, 4 mL of 3 wt% QHPEEK/NMP solution was added dropwise onto a carbon-coated copper grid and placed in the 60 C oven overnight. The resulting sample thickness was approximately 50 nm. Subsequently, it was stained by immersing the prepared sample in the 2.5 M sodium iodide solution at 80 C for 24 h, washed several times with deionized water, and then completely dried. SAXS spectra were recorded at room temperature with scattering angle 0e5 at a rate of 1 $min1. The membrane was dried under vacuum at 60 C for 24 h before testing. The Bragg spacing (d) was calculated by Eq. (7) d¼
2p q
(7)
Alkaline stability of the membrane was determined by monitoring the changes of its hydroxide conductivity after treating the membrane in 1 M KOH at 60 C for different times. Immediately after immersion, the membrane was immersed in deionized (DI) water for 24 h and then washed with DI water to remove the residual KOH; its hydroxide conductivity was measured at room temperature. For single fuel cell test, a commercial catalyst Pt/C (70 wt%, JM Co.) was mixed with the BQH-1.49 ionomer solution (1.5 mg ml1) to yield inks containing 20 wt% of ionomer and 80 wt% of Pt and then sonicated for half an hour to obtain a homogeneous ink. The Pt loading at the anode and the cathode were both controlled to be 0.4 mg cm2; the effective electrode area was 5 cm2. The feeding rate of H2 at the anode and O2 at the cathode was 100 and 200 ml min1, respectively.
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Results and discussion Synthesis and structure For the blend membrane fabrication, pendent piperidiniumfunctionalized polyetheretherketone (QHPEEK) was synthesized following the procedure presented in Scheme 1. The synthesis started with chloromethylation of PEEK, and the resulting chloromethylated PEEK (or CMPEEK) was reacted with 3-chloro-1-propanol via Williamson etherification to give side-chain grafted CMPEEK (or s-CMPEEK), which subsequently underwent a Menshutkin reaction with 1methylpiperidine (MPi) to give the final product (QHPEEK). In the substitution reaction of 3-chloro-1-propanol on phenyl rings, NaH was used for taking the hydrogen atom from 3chloro-1-propanol and TBAB was used as phase transfer catalyst: HOeCH2eCH2eCH2eCl þ NaH/ NaOeCH2eCH2e CH2eCl þ H2. Then NaOeCH2eCH2eCH2eCl reacts with e CH2Cl on phenyl ring: NaOeCH2eCH2eCH2eCl þ eCH2Cl/ e CH2eOeCH2eCH2eCH2eCl þ NaCl. 1 H NMR spectra of 3-chloro-1-propanol, CMPEEK, sCMPEEK, MPi and QHPEEK are shown in Fig. 1. In the spectrum of s-CMPEEK, the characteristic peaks of 3-chloro-1-propanol (at 3.78 ppm (Hk), 3.67 ppm (Hi) and 3.27 ppm (Hj)) move upshift (to 3.33 ppm (Hk’), 2.88 ppm (Hi’) and 2.72 ppm (Hj’), respectively), owing to the reduction of deshielding effect, which is attributed to the decline of the oxygen atom's electronegativity [44]. In the spectrum of CMPEEK, the peak at 4.91 ppm (Hd) is associated with the chloromethylene group, which confirms the successful synthesis of CMPEEK. The peak at 4.91 ppm (Hd) moves upshift to 4.70 ppm (Hd’), ascribed to the deshielding effect reduces and electron cloud density increases [45]. As for the spectrum of QHPEEK, all peaks of MPi (at 2.32 ppm (Hg), 2.23 ppm (Hh), 1.59 ppm (Hf) and 1.41 ppm (He)) move
15485
downshift (to 2.89 ppm (Hg’), 2.73 ppm (Hh’), 2.56 ppm (Hf’) and 1.82 ppm (He’), separately), which is ascribed to the deshielding effect of the piperidinium group. What's more, the disappearance of the peak at 4.70 ppm (Hd’) and the occurrence of a new peak at 4.46 ppm (Hd’’) indicate the full conversion from 3-chloro-1-propanol to QHPEEK. The successful synthesis of QHPEEK was also confirmed by the FT-IR spectra as shown in Fig. 2a. Peaks at 1226 cm1 and 1151 cm1 are ascribed to the asymmetric and symmetric Ce OeC stretching vibrations [46]. In addition, the broad peak at 3400 cm1 is attributed to the presence of water which is bound to the piperidinium group [47]. Blend membranes were fabricated using QH-1.89 and PBI in varied ratios, which gave different IECm values as indicated by the numbers in the membrane designations, BQH-1.64, 1.49, and 1.28; apparently, higher amount of PBI in the blend membrane will result in smaller IECm. FT-IR spectra of different blend membranes in comparison with a pure QHPEEK membrane (QH-1.66, the number also indicates IECm) are shown in Fig. 2b. As can be seen from the spectrum of QH1.66, the characteristic peak ascribed to the stretching vibration of CeNþ of piperidinium group has changed appreciably in the blend membranes. Clearly, with the increased amount of PBI, the CeNþ peak moves gradually to higher wavenumber (from 927 to 936 cm1). This is because the electrostatic interactions between eN-- of PBI and piperidinium groups may restrain the stretching vibration of CeNþ bond [48].
Ionic exchange capacity, water uptake, swelling ratio and hydration number The theoretical ionic exchange capacity (IECt), measured ionic exchange capacity (IECm) and hydration number (l) of the membranes are given in Table 1. The theoretical IEC (or IECt) values of the AEMs are calculated from 1H NMR spectra with the assumption that CMPEEK has been long side chain grafted
Scheme 1 e The synthesizing route of QHPEEK.
15486
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Fig. 1 e 1H NMR spectra of 3-chloro-1-propanol, CMPEEK, s-CMPEEK, MPi and QHPEEK.
Fig. 2 e a) FT-IR spectra of PEEK and QHPEEK materials; b) FT-IR spectra of QH and BQH membranes. and quaternized completely, and all the Cl is converted into OH. The measured IEC (or IECm) values are determined by the back titration method. As can be seen in Table SI, the yield of quaternization, also called degree of quaternization (DQ), which can be calculated from DQ¼(49.5 þ 288/DC)/(1000/IECm80.5)*100%, ranges from 72% to 74% for QH membrane. This means the difference between IECt and IECm of QH membrane
is ascribed to the incomplete quaternization process. As can be seen from Table 1, the difference between IECt and IECm of BQH membrane increases with the increase of PBI blending amount, and the difference between IECt and IECm of BQH membrane is much more larger than that of QH membrane, which means the difference between IECt and IECm of BQH membrane is not only due to the incomplete quaternization
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Table 1 e IECt, IECm and l of the membranes. Membrane
BQH-1.28 BQH-1.49 BQH-1.64 QH-1.30 QH-1.51 QH-1.66 QH-1.89 a
PBI (wt.%)
20 10 5 0 0 0 0
DC (%)
102 102 102 64 78 85 102
IEC (mmol$g1)
l
IECta
IECm
80 C
1.70 1.91 2.02 1.58 1.79 1.90 2.13
1.28 1.49 1.64 1.30 1.51 1.66 1.89
6.1 6.5 7 14.6 15.1 15.6 18.1
Take BQH-1.28 as an example, IECt ¼ 1000*DC*0.8/(288 þ 188*DC).
process but also due to the electrostatic interaction, which may bind some of the OH ions and the OH ions cannot be exchanged by Cl at the time of back titration [49]. The number of water molecules per piperidinium group (denoted as l) is calculated to investigate the microstructure-property relationship of the membranes. As can been seen from Table 1, the l value of the blend membrane decreases slightly when the PBI blending amount increases, which may be ascribed to the tough structure of PBI and the interactions existing in the blend membranes. Such hydrogen bonding, together with the electrostatic interactions between eN-- of PBI and the piperidinium groups, is shown in Scheme 2 [50]. These interactions will insure high robustness of the membranes as will be discussed in subsequent sections. It can be seen from Fig. 3 that WU and SR of all QH membranes are nearly constant from 20 C to 80 C except QH-1.89 membrane: this is because when IECm is appropriate, the wellordered microphase separation helps to form a continuous and solid hydrophobic matrix that prevents the excessive swelling of the membrane [51]; however, for QH-1.89 membrane, whose IECm is too high, the excessive ionic conducting
15487
groups will damage the continuous hydrophobic matrix and lead to higher water uptake and swelling ratio [52e54]. As for BQH membrane, when the blending amount of PBI increases, the impact of microphase separation is attenuated and the impact of hydrogen bonding and electrostatic interaction in blend membrane is enhanced. Maybe it is the hydrogen bonding and electrostatic interaction between two polymers that endow the blend membrane with good anti-swelling property and good dimensional stability.
Morphology of the membranes Scanning electron microscope (SEM) was used to investigate the morphology of membrane. As can be seen from Fig. 4, the surface of the blend membranes are quite smooth and have no obvious defects, which indicates a good compatibility between PBI and QHPEEK. What's more, the cross sections are dense and have no visible defects, which ensures excellent dimensional stability and mechanical properties of the blend membranes [55]. Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to investigate the microstructure-property relationship and to verify the microphase separation in membranes [56,57]. As can be seen in Fig. 5, bright and dark domains represent the hydrophobic and hydrophilic domains, respectively, formed by microphase separation. It is observed that both the pure (QH-1.66) and the blend (BQH-1.64) membranes show well defined ionic clusters and hydrophilic domains. Fig. 6 presents the SAXS of different membranes, where QH-1.66 and BQH-1.64 show scattering peaks at ca. 3.30 and 3.50 nm1, respectively, which confirm the presence of ion clusters created by microphase separation in both membranes; based on the scattering peak positions (q values), Bragg spacing (d ¼ 2p/q) of 1.92 and 1.80 nm are obtained for QH-1.66 and BQH-1.64, respectively, close to each other, implying that the presence of PBI and the strong interactions with the quaternized polymer in the BQH-1.64 membrane did not affect microphase separation. However, there is less significant scattering peaks in the SAXS of BQH1.49 and BQH-1.28 membranes, which means high PBI blending amount will adversely affect the microphase separation behavior. Therefore, it is vital to control the blending amount of PBI to obtain blend membranes with satisfying comprehensive performance.
Mechanical properties and thermal stability
Scheme 2 e The electrostatic interaction and hydrogen bonding in blend membrane.
Adequate mechanical properties are required for AEMs. The mechanical properties of membranes with different IECm values are listed in Table 2. As we can see, all blend membranes possess much higher tensile strength than the pure polymer membranes. In particular, the BQH-1.28 membrane shows a much higher tensile strength than QH-1.30 (58.9 vs 17.0 MPa) with similar IECm, which indicates the introduction of PBI can greatly enhance the mechanical properties of the membrane and therefore prevent the membrane from being damaged during the cell assembly and operation. Fig. 7 shows the TGA and DTG curves of the membranes in OH form, which was heated from 25 C to 790 C at a heating rate of 5 C$min1 in a nitrogen atmosphere. All membranes
15488
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Fig. 3 e a) Water uptake of synthesized membranes at 20e80 C, b) Swelling ratio of synthesized membranes at 20e80 C.
Fig. 4 e SEM surface images of a) BQH-1.28, b) BQH-1.49, c) BQH-1.64, and SEM cross-sectional images of d) BQH-1.28, e) BQH1.49, f) BQH-1.64.
Fig. 5 e TEM spectra of a) QH-1.66 and b) BQH-1.64 membrane.
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
15489
BQH-1.64 is much better than QH-1.66 with similar IECm, which implies the electrostatic interactions and hydrogen bonding between PBI and piperidinium groups can promote the formation of ionic-crosslinking structure, which can help improve thermal stability of blend membranes.
Hydroxide ion conductivity
Fig. 6 e Small angle X-ray scattering profiles of different membranes.
Table 2 e Mechanical strength of hydrated membranes at room temperature. Membrane BQH-1.28 BQH-1.49 BQH-1.64 QH-1.30 QH-1.51 QH-1.66 QH-1.89
PBI (wt%) 20 10 5 0 0 0 0
Tensile strength (MPa) 58.9 46.9 42.8 17.0 15.9 15.2 12.3
± 0.5 ± 0.7 ± 0.6 ± 0.4 ± 0.5 ± 0.4 ± 0.6
Elongation at break (%) 18.6 ± 0.3 22.1 ± 0.4 24.4 ± 0.2 8.3 ± 0.6 12.7 ± 0.4 16.0 ± 0.3 23.1 ± 0.2
Hydroxide ion conductivity is the paramount parameter of AEMs for fuel cells, and the conductivity data of different QH and BQH membranes are presented in Fig. 8a. When the temperature increases, the free volume and mobility of the ionic conducting groups are enhanced and thus the hydroxide ion conductivity is improved [60e62]. As we can see, the hydroxide ion conductivity of QHPEEK ranges from 23.7 to 29 mS cm1 at room temperature and from 80.8 to 91.5 mS cm1 at 80 C. With lower water uptake and swelling ratio, the blend membranes show hydroxide ion conductivity of 17e21.4 mS cm1 at room temperature and 61.5e72.8 mS cm1 at 80 C, which are comparable to the values of the pure membrane, meaning the introduction of PBI helps to inhibit swelling and improve dimensional stability but without compromising the hydroxide ion conductivity much. Peculiarly, the BQH-1.64 membrane has a superior hydroxide ion conductivity of 72.8 mS cm1 at 80 C and a moderate water uptake (21.5% at 80 C). The hydroxide ion transport activation energy (Ea) for different membranes can be calculated from Ea ¼ -bR, where b represents the slope of ln(s) vs (1/T) regression curve (Fig. 8b) and R is the gas constant [63]. As can be seen from Fig. 8b, the Ea values of the blend membranes obtained are just slightly higher than that of QH membranes with the similar IECm, which is possibly because the rigid structure of PBI inhibits the connectivity of hydroxide ion transport channels to some extent and thus leads to slightly decreased hydroxide ion conductivity of the blend membranes.
Alkaline stability
Fig. 7 e TGA and DTG curves of BQH-1.64 and QH-1.66 membranes.
have experienced three major stages of weight loss: the first stage is ascribed to the loss of piperidinium groups; the second stage corresponds to the loss of methylene groups of the side chains; and the final stage is attributed to the degradation of polymer matrix [58,59]. As we can see, the thermal stability of
As one of the most important properties of AEM for fuel cell application, long-term alkaline stability depends on the stability of the functional groups, the polymer frameworks and the linkage between them [64]. For alkaline stability assessment, the prepared membranes were immersed in 1 M KOH at 60 C for different times, during which hydroxide ion conductivity was measured at set intervals. As can be seen from Fig. 9a, the BQH-1.49 membrane retains 95% of its original conductivity after being alkali treated for 576 h while the QH1.51 membrane only retains 75% conductivity; the BQH-1.64 membrane retains 84% of its original conductivity after being alkali treated for 576 h while the QH-1.66 membrane only retains 73%. The above comparison confirms the contribution of PBI to the membrane's alkaline stability: the interaction between PBI and the quaternized PEEK may hinder the hydroxide ion attack on the piperidinium cation [65e69]. As we can see from Fig. 9b, in the FT-IR spectrum of the alkalitreated BQH-1.49 membrane, the characteristic peak at 928 cm1 assignable to CeNþ stretching vibration of piperidinium groups, remains virtually unchanged, indicating an excellent alkali stability of the membrane [70]. Such a stability
15490
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Fig. 8 e a) Temperature dependence of hydroxide ion conductivity of different synthesized membranes; b) Arrhenius plots of different synthesized membranes.
Fig. 9 e a) The conductivity of synthesized membranes after being alkali-treated for different time duration; b) the FT-IR spectrum of BQH-1.49 membrane before and after alkali-treated for 576 h
Fig. 10 e Polarization curves and power density curves of the BQH-1.49 and QH-1.51 membrane under operating temperature 30 C and 80 C.
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
may be attributed to the piperidinium group itself and the interaction between PBI and the quaternized PEEK.
Single fuel cell performance The single fuel cell performance was measured in air at atmosphere pressure with 100% relative humidity at 30 C and 80 C; the polarization and power density curves are shown in Fig. 10. The synthesized polymer was used as both anion exchange membrane and the electrode binder material, which may effectively reduce the ion transport resistance between the catalyst layer and the membrane [71]. The cell resistance of fuel cell using BQH-1.49 membrane at 30 C and 80 C is 22 mOhm and 26 mOhm, respectively. The area specific resistance of fuel cell using BQH-1.49 membrane at 30 C and 80 C is 86 mOhm$cm2 and 97 mU cm2, respectively. The open circuit potentials of all single cells are ca. 0.97 V, which means the membrane is compact enough and can prevent fuel crossover to the cathode. The peak power density of fuel cell using BQH-1.49 membrane increased from 65 mW cm2 at 30 C to 87 mW cm2 at 80 C, which means an elevated temperature accelerates the electrochemical reaction rate and leads to higher peak power density [72]. As can be seen in Fig. 10, the peak power density of BQH-1.49 membrane is higher than that of QH-1.51 membrane, which may be ascribed to the higher water uptake of QH-1.51 membrane. The above peak power density, although not high enough compared with the literature reported best results, demonstrates the potential of the BQH membrane to be used as an AEMFC electrolyte and encourages further optimization [73e81].
Conclusion We have successfully fabricated a series of novel blend membranes composed of synthesized side chain piperidinium functionalized polyetheretherketone and polybenzimidazole. The blend membranes exhibited lower swelling ratio, higher mechanical strength, and better alkaline stability than pure piperidinium functionalized polymer membranes. This is attributed to the existence of electrostatic interactions and hydrogen bonding between PBI and piperidinium polymers, which is confirmed via FT-IR. All blend membranes exhibit high hydroxide ion conductivity (61.5e72.8 ms cm1 at 80 C) with moderate water uptake (15.1e21.5% at 80 C) and good mechanical strength (42.8e58.9 MPa). Moreover, after immersed in 1 M KOH at 60 C for 576 h, the BQH-1.49 membrane still exhibits 95% of its original conductivity value. Therefore, the piperidinium functionalized, PBI-containing blend membranes can balance the hydroxide ion conductivity and alkaline stability well, and the universal synthesizing method can be applied to other polymers as well.
Acknowledgements We acknowledge the financial supports from the National Natural Science Foundation of China (Grant no. 21776042), the
15491
Science and Technology Innovation Fund of Dalian (2018J12GX052), the National Key Research and Development Program of China (Grant no. 2016YFB0101203), and China MOST (Ministry of Science and Technology) innovation team in key areas (No. 2016RA4053).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.172.
references
[1] Gottesfeld S, Dekel DR, Page M, Bae C, Yan Y, Zelenay P, et al. Anion exchange membrane fuel cells: current status and remaining challenges. J Power Sources 2018;375:170e84. [2] Jannasch P, Weiber EA. Configuring anion-exchange membranes for high conductivity and alkaline stability by using cationic polymers with tailored side chains. Macromol Chem Phys 2016;217:1108e18. [3] Zhang X, Li S, Chen P, Fang J, Shi Q, Weng Q, et al. Imidazolium functionalized block copolymer anion exchange membrane with enhanced hydroxide conductivity and alkaline stability via tailoring side chains. Int J Hydrogen Energy 2018;43:3716e30. [4] Wang XQ, Lin CX, Zhang QG, Zhu AM, Liu QL. Anion exchange membranes from hydroxyl-bearing poly(ether sulfone)s with flexible spacers via ring-opening grafting for fuel cells. Int J Hydrogen Energy 2017;42:19044e55. [5] Lin B, Qiao G, Chu F, Wang J, Feng T, Yuan N, et al. Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications. Int J Hydrogen Energy 2017;42:6988e96. [6] Dai J, He G, Ruan X, Zheng W, Pan Y, Yan X. Constructing a rigid crosslinked structure for enhanced conductivity of imidazolium functionalized polysulfone hydroxide exchange membrane. Int J Hydrogen Energy 2016;41:10923e34. [7] Cheng J, He G, Zhang F. A mini-review on anion exchange membranes for fuel cell applications: stability issue and addressing strategies. Int J Hydrogen Energy 2015;40:7348e60. [8] Zhu Y, He Y, Ge X, Liang X, Shehzad MA, Hu M, et al. A benzyltetramethylimidazolium-based membrane with exceptional alkaline stability in fuel cells: role of its structure in alkaline stability. J Mater Chem 2018;6:527e34. [9] Zhao B, He G, El Hamouti I, Gao L, Liu Y, Deng R, et al. A novel strategy for constructing a highly conductive and swellingresistant semi-flexible aromatic polymer based anion exchange membranes. Int J Hydrogen Energy 2017;42:10228e37. [10] Varcoe JR, Atanassov P, Dekel DR, Herring AM, Hickner MA, Kohl PA, et al. Anion-exchange membranes in electrochemical energy systems. Energy Environ Sci 2014;7:3135e91. [11] Rao AHN, Nam S, Kim T-H. Crosslinked poly(arylene ether sulfone) block copolymers containing pendant imidazolium groups as both crosslinkage sites and hydroxide conductors for highly selective and stable membranes. Int J Hydrogen Energy 2014;39:5919e30. [12] Hossain MA, Lim Y, Lee S, Jang H, Choi S, Jeon Y, et al. Comparison of alkaline fuel cell membranes of random & block poly(arylene ether sulfone) copolymers containing tetra quaternary ammonium hydroxides. Int J Hydrogen Energy 2014;39:2731e9.
15492
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
[13] Zhao CH, Gong Y, Liu QL, Zhang QG, Zhu AM. Selfcrosslinked anion exchange membranes by bromination of benzylmethyl-containing poly(sulfone)s for direct methanol fuel cells. Int J Hydrogen Energy 2012;37:11383e93. [14] Wang D, Wang Y, Wan H, Wang J, Wang L. Synthesis of gemini basic ionic liquids and their application in anion exchange membranes. RSC Adv 2018;8:10185e96. [15] Chen N, Long C, Li Y, Lu C, Zhu H. Ultrastable and high ionconducting polyelectrolyte based on six-membered Nspirocyclic ammonium for hydroxide exchange membrane fuel cell applications. ACS Appl Mater Interfaces 2018;10:15720e32. [16] Zhu H, Li R, Wang F, Chen N, Li Z, Wang Z. Poly tris (1imidazolyl) benzene ionic liquids/Poly (2,6-dimethyl phenylene oxide) composite membranes for anion exchange membrane fuel cells. J Mater Sci 2017;52:11109e19. [17] Wang J, Chen Y, Wei Q, Yang S, Fang H, Wei H, et al. Adamantammonium as a novel functional group for anion exchange membranes with excellent comprehensive performances. Polymer 2017;112:288e96. [18] Pham TH, Olsson JS, Jannasch P. N-spirocyclic quaternary ammonium ionenes for anion-exchange membranes. J Am Chem Soc 2017;139:2888e91. [19] Mo Z-H, Yang R, Hong S, Wu Y-X. In-situ preparation of cross-linked hybrid anion exchange membrane of quaternized poly (styrene- b -isobutylene- b -styrene) covalently bonded with graphene. Int J Hydrogen Energy 2018;43:1790e804. [20] Zhu L, Yu X, Hickner MA. Exploring backbone-cation alkyl spacers for multi-cation side chain anion exchange membranes. J Power Sources 2018;375:433e41. [21] Shukla G, Shahi VK. Poly(arylene ether ketone) copolymer grafted with amine groups containing a long alkyl chain by chloroacetylation for improved alkaline stability and conductivity of anion exchange membrane. ACS Appl Energy Mater 2018;1:1175e82. [22] Mahmoud AMA, Elsaghier AMM, Otsuji K, Miyatake K. High hydroxide ion conductivity with enhanced alkaline stability of partially fluorinated and quaternized aromatic copolymers as anion exchange membranes. Macromolecules 2017;50:4256e66. [23] Pan J, Chen C, Li Y, Wang L, Tan L, Li G, et al. Constructing ionic highway in alkaline polymer electrolytes. Energy Environ Sci 2014;7:354e60. [24] Zhu M, Zhang X, Wang Y, Wu Y, Wang H, Zhang M, et al. Novel anion exchange membranes based on quaternized diblock copolystyrene containing a fluorinated hydrophobic block. J Membr Sci 2018;554:264e73. [25] Mohanty AK, Devaraju S, Kim N, Paik H-j. Synthesis and characterization of poly(ether sulfone) block copolymers containing pendent quaternary ammonium- and imidazolium groups as anion exchange membranes. Solid State Ionics 2018;314:46e56. [26] Irfan M, Bakangura E, Afsar NU, Hossain MM, Ran J, Xu T. Preparation and performance evaluation of novel alkaline stable anion exchange membranes. J Power Sources 2017;355:171e80. [27] Dong X, Lv D, Zheng J, Xue B, Bi W, Li S, et al. Pyrrolidiniumfunctionalized poly(arylene ether sulfone)s for anion exchange membranes: using densely concentrated ionic groups and block design to improve membrane performance. J Membr Sci 2017;535:301e11. [28] Li L, Yue X, Wu W, Yan W, Zeng M, Zhou Y, et al. Multi-block copolymers with fluorene-containing hydrophilic segments densely functionalized by side-chain quaternary ammonium groups as anion exchange membranes. RSC Adv 2016;6:41453e64.
[29] Wang Y, Wan H, Wang D, Wang J, Wang L, Feng R. Preparation and characterization of A semi-interpenetrating network alkaline anion exchange membrane. Fibers Polym 2018;19:11e21. [30] Lin J, Yan X, He G, Chen W, Zhen D, Li T, et al. Thermoplastic interpenetrating polymer networks based on polybenzimidazole and poly (1, 2-dimethy-3allylimidazolium) for anion exchange membranes. Electrochim Acta 2017;257:9e19. [31] Gong Y, Liao X, Xu J, Chen D, Zhang H. Novel anionconducting interpenetrating polymer network of quaternized polysulfone and poly(vinyl alcohol) for alkaline fuel cells. Int J Hydrogen Energy 2016;41:5816e23. [32] He SS, Strickler AL, Frank CW. A semi-interpenetrating network approach for dimensionally stabilizing highlycharged anion exchange membranes for alkaline fuel cells. ChemSusChem 2015;8:1472e83. [33] Morandi CG, Peach R, Krieg HM, Kerres J. Novel imidazoliumfunctionalized anion-exchange polymer PBI blend membranes. J Membr Sci 2015;476:256e63. [34] Morandi CG, Peach R, Krieg HM, Kerres J. Novel morpholinium-functionalized anion-exchange PBIepolymer blends. J Mater Chem 2015;3:1110e20. [35] Olsson JS, Pham TH, Jannasch P. Poly(arylene piperidinium) hydroxide ion exchange membranes: synthesis, alkaline stability, and conductivity. Adv Funct Mater 2018;28:1702758. [36] Dang H-S, Jannasch P. A comparative study of anionexchange membranes tethered with different heterocycloaliphatic quaternary ammonium hydroxides. J Mater Chem 2017;5:21965e78. [37] Olsson JS, Pham TH, Jannasch P. Poly(N,Ndiallylazacycloalkane)s for anion-exchange membranes functionalized with N-spirocyclic quaternary ammonium cations. Macromolecules 2017;50:2784e93. [38] Pham TH, Jannasch P. Aromatic polymers incorporating bisN-spirocyclic quaternary ammonium moieties for anionexchange membranes. ACS Macro Lett 2015;4:1370e5. [39] Marino MG, Kreuer KD. Alkaline stability of quaternary ammonium cations for alkaline fuel cell membranes and ionic liquids. ChemSusChem 2015;8:513e23. [40] Zeng L, Zhao TS, An L, Zhao G, Yan XH. Physicochemical properties of alkaline doped polybenzimidazole membranes for anion exchange membrane fuel cells. J Membr Sci 2015;493:340e8. [41] Hou H, Wang S, Jiang Q, Jin W, Jiang L, Sun G. Durability study of KOH doped polybenzimidazole membrane for airbreathing alkaline direct ethanol fuel cell. J Power Sources 2011;196:3244e8. [42] Yan X, He G, Gu S, Wu X, Du L, Zhang H. Quaternized poly(ether ether ketone) hydroxide exchange membranes for fuel cells. J Membr Sci 2011;375:204e11. [43] Mohanty AD, Tignor SE, Krause JA, Choe Y-K, Bae C. Systematic alkaline stability study of polymer backbones for anion exchange membrane applications. Macromolecules 2016;49:3361e72. [44] Gong X, Yan X, Li T, Wu X, Chen W, Huang S, et al. Design of pendent imidazolium side chain with flexible ethercontaining spacer for alkaline anion exchange membrane. J Membr Sci 2017;523:216e24. [45] Chen W, Hu M, Wang H, Wu X, Gong X, Yan X, et al. Dimensionally stable hexamethylenetetramine functionalized polysulfone anion exchange membranes. J Mater Chem 2017;5:15038e47. [46] Hazarika M, Jana T. Novel proton exchange membrane for fuel cell developed from blends of polybenzimidazole with fluorinated polymer. Eur Polym J 2013;49:1564e76. [47] Vengatesan S, Santhi S, Sozhan G, Ravichandran S, Davidson DJ, Vasudevan S. Novel cross-linked anion
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
[62]
[63]
exchange membrane based on hexaminium functionalized poly(vinylbenzyl chloride). RSC Adv 2015;5:27365e71. Hazarika M, Arunbabu D, Jana T. Formation of core (polystyrene)-shell (polybenzimidazole) nanoparticles using sulfonated polystyrene as template. J Colloid Interface Sci 2010;351:374e83. Kosmala B, Schauer J. Ion-exchange membranes prepared by blending sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) with polybenzimidazole. J Appl Polym Sci 2002;85:1118e27. Maurya S, Shin S-H, Lee J-Y, Kim Y, Moon S-H. Amphoteric nanoporous polybenzimidazole membrane with extremely low crossover for a vanadium redox flow battery. RSC Adv 2016;6:5198e204. Weiber EA, Jannasch P. Anion-conducting polysulfone membranes containing hexa-imidazolium functionalized biphenyl units. J Membr Sci 2016;520:425e33. Li N, Leng Y, Hickner MA, Wang CY. Highly stable, anion conductive, comb-shaped copolymers for alkaline fuel cells. J Am Chem Soc 2013;135:10124e33. Yan X, Gao L, Zheng W, Ruan X, Zhang C, Wu X, et al. Longspacer-chain imidazolium functionalized poly(ether ether ketone) as hydroxide exchange membrane for fuel cell. Int J Hydrogen Energy 2016;41:14982e90. Ran J, Fu C, Ding L, Cao P, Xu T. Dual hydrophobic grafted chains facilitating quaternary ammonium aggregations of hydroxide conducting polymers: a theoretical and experimental investigation. J Mater Chem 2018;6:5714e23. Hao J, Jiang Y, Gao X, Lu W, Xiao Y, Shao Z, et al. Functionalization of polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with two cyclic quaternary ammonium cations for anion exchange membranes. J Membr Sci 2018;548:1e10. Dang H-S, Weiber EA, Jannasch P. Poly(phenylene oxide) functionalized with quaternary ammonium groups via flexible alkyl spacers for high-performance anion exchange membranes. J Mater Chem 2015;3:5280e4. Hu EN, Lin CX, Liu FH, Wang XQ, Zhang QG, Zhu AM, et al. Poly(arylene ether nitrile) anion exchange membranes with dense flexible ionic side chain for fuel cells. J Membr Sci 2018;550:254e65. Afsar NU, Erigene B, Irfan M, Wu B, Xu T, Ji W, et al. High performance anion exchange membrane with proton transport pathways for diffusion dialysis. Separ Purif Technol 2018;193:11e20. Xiao Lin C, Qin Wang X, Ning Hu E, Yang Q, Gen Zhang Q, Mei Zhu A, et al. Quaternized triblock polymer anion exchange membranes with enhanced alkaline stability. J Membr Sci 2017;541:358e66. Lin CX, Zhuo YZ, Hu EN, Zhang QG, Zhu AM, Liu QL. Crosslinked side-chain-type anion exchange membranes with enhanced conductivity and dimensional stability. J Membr Sci 2017;539:24e33. Lin CX, Wang XQ, Li L, Liu FH, Zhang QG, Zhu AM, et al. Triblock copolymer anion exchange membranes bearing alkyl-tethered cycloaliphatic quaternary ammonium-headgroups for fuel cells. J Power Sources 2017;365:282e92. Yang Z, Guo R, Malpass-Evans R, Carta M, McKeown NB, Guiver MD, et al. Highly conductive anion-exchange membranes from microporous troger's base polymers. Angew Chem 2016;55:11499e502. Lin CX, Zhuo YZ, Lai AN, Zhang QG, Zhu AM, Ye ML, et al. Side-chain-type anion exchange membranes bearing pendent imidazolium-functionalized poly(phenylene oxide) for fuel cells. J Membr Sci 2016;513:206e16.
15493
[64] Lai AN, Zhou K, Zhuo YZ, Zhang QG, Zhu AM, Ye ML, et al. Anion exchange membranes based on carbazole-containing polyolefin for direct methanol fuel cells. J Membr Sci 2016;497:99e107. [65] He Y, Si J, Wu L, Chen S, Zhu Y, Pan J, et al. Dual-cation combshaped anion exchange membranes: structure, morphology and properties. J Membr Sci 2016;515:189e95. [66] Yang Q, Lin CX, Liu FH, Li L, Zhang QG, Zhu AM, et al. Poly (2,6-dimethyl-1,4-phenylene oxide)/ionic liquid functionalized graphene oxide anion exchange membranes for fuel cells. J Membr Sci 2018;552:367e76. [67] Li G, Pan J, Han J, Chen C, Lu J, Zhuang L. Ultrathin composite membrane of alkaline polymer electrolyte for fuel cell applications. J Mater Chem 2013;1:12497. [68] Pan J, Li Y, Han J, Li G, Tan L, Chen C, et al. A strategy for disentangling the conductivityestability dilemma in alkaline polymer electrolytes. Energy Environ Sci 2013;6:2912. [69] Dang H-S, Jannasch P. Anion-exchange membranes with polycationic alkyl side chains attached via spacer units. J Mater Chem 2016;4:17138e53. [70] Ono H, Miyake J, Shimada S, Uchida M, Miyatake K. Anion exchange membranes composed of perfluoroalkylene chains and ammonium-functionalized oligophenylenes. J Mater Chem 2015;3:21779e88. [71] Li G, Pan J, Han J, Chen C, Lu J, Zhuang L. Ultrathin composite membrane of alkaline polymer electrolyte for fuel cell applications. J Mater Chem 2013;1:12497. [72] Hou H, Sun G, He R, Wu Z, Sun B. Alkali doped polybenzimidazole membrane for high performance alkaline direct ethanol fuel cell. J Power Sources 2008;182:95e9. [73] Yao Z, Zhang Z, Hu M, Hou J, Wu L, Xu T. Perylene-based sulfonated aliphatic polyimides for fuel cell applications: performance enhancement by stacking of polymer chains. J Membr Sci 2018;547:43e50. [74] Ran J, Fu C, Ding L, Cao P, Xu T. Dual hydrophobic grafted chains facilitating quaternary ammonium aggregations of hydroxide conducting polymers: a theoretical and experimental investigation. J Mater Chem 2018;6:5714e23. [75] Hu EN, Lin CX, Liu FH, Wang XQ, Zhang QG, Zhu AM, et al. Poly(arylene ether nitrile) anion exchange membranes with dense flexible ionic side chain for fuel cells. J Membr Sci 2018;550:254e65. [76] He Y, Ge X, Liang X, Zhang J, Shehzad MA, Zhu Y, et al. Anion exchange membranes with branched ionic clusters for fuel cells. J Mater Chem 2018;6:5993e8. [77] Hao J, Jiang Y, Gao X, Lu W, Xiao Y, Shao Z, et al. Functionalization of polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with two cyclic quaternary ammonium cations for anion exchange membranes. J Membr Sci 2018;548:1e10. [78] Hao J, Gao X, Jiang Y, Zhang H, Luo J, Shao Z, et al. Crosslinked high-performance anion exchange membranes based on poly(styrene- b -(ethylene- co -butylene)- b -styrene). J Membr Sci 2018;551:66e75. [79] Zhu L, Zimudzi TJ, Wang Y, Yu X, Pan J, Han J, et al. Mechanically robust anion exchange membranes via long hydrophilic cross-linkers. Macromolecules 2017;50:2329e37. [80] Wang L, Magliocca E, Cunningham EL, Mustain WE, Poynton SD, Escudero-Cid R, et al. An optimised synthesis of high performance radiation-grafted anion-exchange membranes. Green Chem 2017;19:831e43. [81] Lee KH, Cho DH, Kim YM, Moon SJ, Seong JG, Shin DW, et al. Highly conductive and durable poly(arylene ether sulfone) anion exchange membrane with end-group cross-linking. Energy Environ Sci 2017;10:275e85.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Pendent piperidinium-functionalized blend anion exchange membrane for fuel cell application Mengyao Niu a,b, Caimian Zhang a, Gaohong He a,b, Fengxiang Zhang a,*, Xuemei Wu b,** a
State Key Laboratory of Fine Chemicals, School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin, LN, 124221, China b Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian, LN, 116024, China
article info
abstract
Article history:
Alkaline anion exchange membrane fuel cell has fast cathode reactions and thus allows
Received 5 February 2019
the use of low cost electrocatalysts. However, its practical application is hindered by the
Received in revised form
low hydroxide ion conductivity and alkaline stability of AEM. In this study, pendent
8 April 2019
piperidinium functionalized polyetheretherketone is synthesized and blended with poly-
Accepted 17 April 2019
benzimidazole for fabrication of composite anion exchange membrane. The pendent
Available online 10 May 2019
piperidinium functionalized side chains can create well-connected ionic transporting channels and thus impart the blend membranes high hydroxide conductivity (61.5
Keywords:
e72.8 mS cm1 at 80 C) and good tensile strength (42.8e58.9 MPa). Due to the strong in-
Anion exchange membrane
teractions between polybenzimidazole and piperidinium groups of the polymers as
Blend polymers
confirmed by Fourier transform infrared spectroscopy, the piperidinium functionalized
Polybenzimidazole
blend anion exchange membrane can retain 95% of its original OH conductivity value
Piperidinium
when treated in 1 M KOH at 60 C for 576 h. The single fuel cell assembled with the
Inter-chain forces
membrane can yield a peak power density of 87 mW cm2 at 80 C. Our work provides a new and effective method to balance the hydroxide conductivity and alkaline stability of anion exchange membranes. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays the whole world is focusing on reducing the dependence on fossil fuels (such as coal, oil and natural gas) and finding the alternative renewable clean energies [1,2]. Polymer electrolyte fuel cell (PEFC) is regarded as one of the most promising environmentally benign energies and has been the focus of scientific research [3e6]. PEFC can be divided
into proton exchange membrane fuel cell (PEMFC) and anion exchange membrane fuel cell (AEMFC) according to the ions (Hþ or OH) transported in the membrane [7,8]. PEMFC has been widely studied and becomes rather mature through long-term efforts. However, due to the strong dependence on noble metal catalyst (such as Pt), high fuel permeability and high cost of proton exchange membranes (such as Nafion), large-scale commercialization has not been achieved for
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Zhang), [email protected] (X. Wu). https://doi.org/10.1016/j.ijhydene.2019.04.172 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
PEMFC [9e11]. As compared to PEMFC, AEMFC has several important advantages, including their compatibility with the use of non-noble metal catalyst (such as Ag, Ni and Co), ability to retain good stability of the catalyst, and faster oxygen reduction reaction [12e15]. As one of the key components of AEMFC, anion exchange membrane (AEM) acts as the barrier against fuel crossover and the medium for OH transport [16e18]. The main challenge of AEM toward practical application in fuel cells lies in the trade-off between OH conductivity and alkaline stability. The tradeoff is related to the inherently low OH diffusion rate (as compared to Hþ) and difficult dissociation of the cation-hydroxide ion pair [19], and therefore, high conductivity often depends on high degree of quaternization, rendering the membrane more vulnerable to OH attack. Over the past few years, several strategies have been proposed to alleviate the conductivity - stability contradiction, which include: (1) introducing long side chains or improving the concentration of ionic groups on the side chains in order to facilitate ionic cluster formation [20e23]; (2) synthesizing multi-block copolymers to promote the formation of wellconnected OH transporting channels [24e28], and (3) using polymer blends or constructing semi inter-penetrating networks to combine the merits of various functional polymers [29e32]. Importantly, the above approaches can work in a synergistic way to further improve the membrane performance, and there are reported examples of fabricating blend membranes containing long side chain polymers for PEMFC or vanadium redox flow battery [33,34]. Inspired by the above works, we design and synthesize a pendent piperidinium functionalized polymer and blend it with polybenzimidazole (PBI) for fabrication of high performance blend AEM. To the best of our knowledge, this is the first report of piperidinium functionalized blend AEM containing PBI. One advantage of this membrane design lies in the better alkaline stability of the piperidinium group relative to other common used cation structures for AEM like quaternary ammonium, imidazolium and guanidinium [35e39]. The long spacer between the polymer backbone and the cation center promotes microphase separation. Meanwhile, PBI possesses e NHe and C]N moieties, which allow it to interact with both electronegative and electropositive polymers so that robust blend membranes can be expected [40,41]. By changing the amount of PBI in the blend membrane, OH conductivity and alkaline stability can be well balanced. The obtained membrane shows a good hydroxide conductivity of 69 mS cm1 at 80 C for a measured ion exchange capacity of 1.49 mmol g1; it retains 95% of its original hydroxide conductivity after alkali-treated at 60 C for 576 h, which is superior among all blend AEMs.
15483
as PBI) was purchased from Shanghai Shengjun Plastics Technology Co.,Ltd. The average molecular weight is approximately 14000 Da. N-octyl alcohol, dichloromethane, trioxymethylene, anhydrous calcium chloride, sodium chloride, concentrated hydrochloric acid, concentrated sulfuric acid, absolute ethyl alcohol, 3-chloro-1-propanol, 1methylpiperidine (MPi), dimethyl formamide (DMF), dimethylsulfoxide (DMSO), sodium hydride (NaH) and tetrabutylammonium bromide (TBAB) were all purchased from J&K (China) Co. Ltd. and used as received.
Synthesis of chloromethylated polyetheretherketone Chloromethylated polyetheretherketone (CMPEEK) was synthesized by the reaction between CMOE and PEEK in concentrated sulfuric acid. Specifically, 2 g PEEK was dissolved in 120 ml sulfuric acid and 40 ml CMOE was added after complete dissolution. The reaction temperature was set as 10 C, and CMPEEK with different chloromethylation degrees (DC) were obtained via changing the reaction time. The CMPEEK was precipitated from ice water, washed with ice water a few times to remove the solvent, and then dried overnight at 25 C.
Synthesis of long side chain grafted polyetheretherketone The long side chain grafted polyetheretherketone (s-CMPEEK) was synthesized by the Williamson etherification reaction between 3-chloro-1-propanol and CMPEEK [43]. Firstly, 8 mmol 3-chloro-1-propanol was dissolved in 10 ml DMF; the resulting solution was then added into a mixture comprising 0.2 g TBAB, 8 mmol NaH (60 wt% dispersion in coal oil), and 20 ml DMF. The new mixture obtained was stirred at 40 C for 2 h with a continuous N2 purge. Then, a solution of 3 mmol CMPEEK (DC ¼ 1.02) in 10 ml DMF was added slowly and the mixture was stirred for 24 h. Finally, the s-CMPEEK was precipitated from ethanol, and washed with water for a few times to remove impurities, and then dried overnight at 50 C.
Synthesis of pendent piperidinium-functionalized polyetheretherketone The pendent piperidinium-functionalized polyetheretherketone (QHPEEK) was synthesized via the Menshutkin reaction between s-CMPEEK and 1-methylpiperidine. Firstly, 3 mmol s-CMPEEK and 8 mmol 1-methylpiperidine were dissolved in 60 ml DMF. Then, the mixture was reacted at 60 C for 36 h with N2 purge and QHPEEK was obtained by precipitation from ethanol, followed by washing with water several times to remove impurities, and then drying overnight at 50 C.
Experimental
Membrane fabrication
Materials
0.1 g blend material containing varied amounts of QH-1.89 (80, 90 and 95 wt%) and PBI was dissolved in 5 ml DMSO at room temperature; the resulting solution was casted onto a 4.5 cm 4.5 cm glass plate and dried at 60 C for 48 h to obtain the blend membrane, which is denoted as BQH-x, where x is the measured ion exchange capacity (IECm) of the
Polyetheretherketone (PEEK, VESTAKEEPVR 4000P) was purchased from Evonik Degussa (China) Co. Ltd and chloromethyl octyl ether (CMOE) was synthesized according to literature [42]. Poly (4,40 -diphenylether-5,50 -bibenzimidazole) (denoted
15484
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
membrane. The blend membrane was immersed in 1 M KOH solution at 25 C for 48 h to convert the counter ion from Cl to OH. The BQH membrane (ca. 40 mm thick) was sealed in a container full of deionized water for 48 h to completely remove the residual KOH. As a control, pure QHPEEK membranes were prepared using the same method as described above; the membranes are denoted as QH-x, where x is the IECm of the membrane.
Characterization and measurements 1
H NMR spectroscopy was recorded at a resonance frequency of 500 MHz using tetramethylsilane (TMS) as an internal standard substance and DMSO-d6 or CDCl3 as solvent on a Varian Unity Inova 500 spectrometer. FT-IR spectra were recorded in the wavelength range of 4000e400 cm1 with a resolution of 0.5 cm1 on a Bruker EQUINOX55 FTIR spectrometer. The mechanical strength and elongation at break of hydrated membrane were measured at a tensile rate of 5 mm min1 on SANS CMT8502 Universal Tensile Machine at room temperature. The hydrated membrane was cut into 0.5 cm 4.5 cm slices before the test. Thermal stability of the dry membrane sample was recorded on TGA analyzer (TAINC SDT Q600 thermogravimetric analyzer) from 25 C to 790 C under a nitrogen flow at a heating speed of 5 C$min1. A derivative thermogravimetry (DTG) curve is the first order differential of TGA as a function of temperature. Ion-exchange capacity (IEC) of the membrane in OH form was measured with the back titration method. The membrane sample was immersed in 25 ml 0.01 M HCl solution for 48 h, then the HCl solution was titrated by KOH standard solution (0.01 M) and the consumed volume of KOH was denoted as Va (L). The blank HCl solution (not used for membrane soaking) was titrated with the same method and the consumed volume of KOH was denoted as Vb (L). Then the membrane sample was vacuum dried at 50 C for 24 h and the weight of the dry membrane was denoted as Wdry (g). The measured IEC, or IECm (mmol/g) of the membrane was calculated by following Eq. (1). IECm ¼
ðVb Va ÞCKOH *1000 Wdry
(1)
The in-plane conductivity was measured on Ivium Technologies A08001 equipment by the typical four-electrode AC impedance method at a frequency of 1~105 Hz. The membrane was cut into 0.5 cm 4.5 cm slices and measured in 100% relative humidity. The in-plane conductivity (s) was calculated by Eq. (2). s¼
l R*S
(2)
where R is the high-frequency resistance, l is the distance between two electrodes, and S is the area of the membrane. The hydroxide ion transporting activation energy (Ea) is calculated by Eq. (3). Ea ¼ bR
(3)
where b is the slope of the regression line ln(s)~1000/T, T is the absolute temperature, and R is the gas constant.
Water uptake (WU), swelling ratio (SR) and the number of water molecules per piperidinium group (l) of the membrane were measured as follows. Firstly, the hydrated membrane with OH counter ion was soaked into deionized water for 12 h at different temperatures. After removing the residual water on the membrane surface, weight and size of the wet membrane were measured immediately. Then the membrane was vacuum dried at 50 C for 24 h and the weight and size of the dry membrane were measured again. WU, SR and l are calculated with Eqs. (4)e(6), respectively. WU ¼
SR ¼
l¼
Wwet Wdry *100% Wdry
lwet ldry *100% ldry
1000WUð%Þ MH2O IECm
(4)
(5)
(6)
In Eq. (4)-(6), Wwet and Wdry are the mass of the wet and the dried membrane, respectively. lwet¼(la*lb)^0.5 is the average size of the wet membrane, ldry¼(la’*lb’)^0.5 is the average size of the dried membrane. la, lb, la’, lb’ are the width and length of wet and dried membrane separately, and IECm is the measured ion exchange capacity. Membrane morphology was characterized with scanning electron microscopy (SEM, Quanta 450, FEI Co., USA), transmission electron microscopy (TEM, JEM-2000EX JEOL) and small angle X-ray scattering (SAXS, X'Pert Powder system). For SEM, the membrane samples were coated with gold before testing. For TEM, 4 mL of 3 wt% QHPEEK/NMP solution was added dropwise onto a carbon-coated copper grid and placed in the 60 C oven overnight. The resulting sample thickness was approximately 50 nm. Subsequently, it was stained by immersing the prepared sample in the 2.5 M sodium iodide solution at 80 C for 24 h, washed several times with deionized water, and then completely dried. SAXS spectra were recorded at room temperature with scattering angle 0e5 at a rate of 1 $min1. The membrane was dried under vacuum at 60 C for 24 h before testing. The Bragg spacing (d) was calculated by Eq. (7) d¼
2p q
(7)
Alkaline stability of the membrane was determined by monitoring the changes of its hydroxide conductivity after treating the membrane in 1 M KOH at 60 C for different times. Immediately after immersion, the membrane was immersed in deionized (DI) water for 24 h and then washed with DI water to remove the residual KOH; its hydroxide conductivity was measured at room temperature. For single fuel cell test, a commercial catalyst Pt/C (70 wt%, JM Co.) was mixed with the BQH-1.49 ionomer solution (1.5 mg ml1) to yield inks containing 20 wt% of ionomer and 80 wt% of Pt and then sonicated for half an hour to obtain a homogeneous ink. The Pt loading at the anode and the cathode were both controlled to be 0.4 mg cm2; the effective electrode area was 5 cm2. The feeding rate of H2 at the anode and O2 at the cathode was 100 and 200 ml min1, respectively.
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Results and discussion Synthesis and structure For the blend membrane fabrication, pendent piperidiniumfunctionalized polyetheretherketone (QHPEEK) was synthesized following the procedure presented in Scheme 1. The synthesis started with chloromethylation of PEEK, and the resulting chloromethylated PEEK (or CMPEEK) was reacted with 3-chloro-1-propanol via Williamson etherification to give side-chain grafted CMPEEK (or s-CMPEEK), which subsequently underwent a Menshutkin reaction with 1methylpiperidine (MPi) to give the final product (QHPEEK). In the substitution reaction of 3-chloro-1-propanol on phenyl rings, NaH was used for taking the hydrogen atom from 3chloro-1-propanol and TBAB was used as phase transfer catalyst: HOeCH2eCH2eCH2eCl þ NaH/ NaOeCH2eCH2e CH2eCl þ H2. Then NaOeCH2eCH2eCH2eCl reacts with e CH2Cl on phenyl ring: NaOeCH2eCH2eCH2eCl þ eCH2Cl/ e CH2eOeCH2eCH2eCH2eCl þ NaCl. 1 H NMR spectra of 3-chloro-1-propanol, CMPEEK, sCMPEEK, MPi and QHPEEK are shown in Fig. 1. In the spectrum of s-CMPEEK, the characteristic peaks of 3-chloro-1-propanol (at 3.78 ppm (Hk), 3.67 ppm (Hi) and 3.27 ppm (Hj)) move upshift (to 3.33 ppm (Hk’), 2.88 ppm (Hi’) and 2.72 ppm (Hj’), respectively), owing to the reduction of deshielding effect, which is attributed to the decline of the oxygen atom's electronegativity [44]. In the spectrum of CMPEEK, the peak at 4.91 ppm (Hd) is associated with the chloromethylene group, which confirms the successful synthesis of CMPEEK. The peak at 4.91 ppm (Hd) moves upshift to 4.70 ppm (Hd’), ascribed to the deshielding effect reduces and electron cloud density increases [45]. As for the spectrum of QHPEEK, all peaks of MPi (at 2.32 ppm (Hg), 2.23 ppm (Hh), 1.59 ppm (Hf) and 1.41 ppm (He)) move
15485
downshift (to 2.89 ppm (Hg’), 2.73 ppm (Hh’), 2.56 ppm (Hf’) and 1.82 ppm (He’), separately), which is ascribed to the deshielding effect of the piperidinium group. What's more, the disappearance of the peak at 4.70 ppm (Hd’) and the occurrence of a new peak at 4.46 ppm (Hd’’) indicate the full conversion from 3-chloro-1-propanol to QHPEEK. The successful synthesis of QHPEEK was also confirmed by the FT-IR spectra as shown in Fig. 2a. Peaks at 1226 cm1 and 1151 cm1 are ascribed to the asymmetric and symmetric Ce OeC stretching vibrations [46]. In addition, the broad peak at 3400 cm1 is attributed to the presence of water which is bound to the piperidinium group [47]. Blend membranes were fabricated using QH-1.89 and PBI in varied ratios, which gave different IECm values as indicated by the numbers in the membrane designations, BQH-1.64, 1.49, and 1.28; apparently, higher amount of PBI in the blend membrane will result in smaller IECm. FT-IR spectra of different blend membranes in comparison with a pure QHPEEK membrane (QH-1.66, the number also indicates IECm) are shown in Fig. 2b. As can be seen from the spectrum of QH1.66, the characteristic peak ascribed to the stretching vibration of CeNþ of piperidinium group has changed appreciably in the blend membranes. Clearly, with the increased amount of PBI, the CeNþ peak moves gradually to higher wavenumber (from 927 to 936 cm1). This is because the electrostatic interactions between eN-- of PBI and piperidinium groups may restrain the stretching vibration of CeNþ bond [48].
Ionic exchange capacity, water uptake, swelling ratio and hydration number The theoretical ionic exchange capacity (IECt), measured ionic exchange capacity (IECm) and hydration number (l) of the membranes are given in Table 1. The theoretical IEC (or IECt) values of the AEMs are calculated from 1H NMR spectra with the assumption that CMPEEK has been long side chain grafted
Scheme 1 e The synthesizing route of QHPEEK.
15486
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Fig. 1 e 1H NMR spectra of 3-chloro-1-propanol, CMPEEK, s-CMPEEK, MPi and QHPEEK.
Fig. 2 e a) FT-IR spectra of PEEK and QHPEEK materials; b) FT-IR spectra of QH and BQH membranes. and quaternized completely, and all the Cl is converted into OH. The measured IEC (or IECm) values are determined by the back titration method. As can be seen in Table SI, the yield of quaternization, also called degree of quaternization (DQ), which can be calculated from DQ¼(49.5 þ 288/DC)/(1000/IECm80.5)*100%, ranges from 72% to 74% for QH membrane. This means the difference between IECt and IECm of QH membrane
is ascribed to the incomplete quaternization process. As can be seen from Table 1, the difference between IECt and IECm of BQH membrane increases with the increase of PBI blending amount, and the difference between IECt and IECm of BQH membrane is much more larger than that of QH membrane, which means the difference between IECt and IECm of BQH membrane is not only due to the incomplete quaternization
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Table 1 e IECt, IECm and l of the membranes. Membrane
BQH-1.28 BQH-1.49 BQH-1.64 QH-1.30 QH-1.51 QH-1.66 QH-1.89 a
PBI (wt.%)
20 10 5 0 0 0 0
DC (%)
102 102 102 64 78 85 102
IEC (mmol$g1)
l
IECta
IECm
80 C
1.70 1.91 2.02 1.58 1.79 1.90 2.13
1.28 1.49 1.64 1.30 1.51 1.66 1.89
6.1 6.5 7 14.6 15.1 15.6 18.1
Take BQH-1.28 as an example, IECt ¼ 1000*DC*0.8/(288 þ 188*DC).
process but also due to the electrostatic interaction, which may bind some of the OH ions and the OH ions cannot be exchanged by Cl at the time of back titration [49]. The number of water molecules per piperidinium group (denoted as l) is calculated to investigate the microstructure-property relationship of the membranes. As can been seen from Table 1, the l value of the blend membrane decreases slightly when the PBI blending amount increases, which may be ascribed to the tough structure of PBI and the interactions existing in the blend membranes. Such hydrogen bonding, together with the electrostatic interactions between eN-- of PBI and the piperidinium groups, is shown in Scheme 2 [50]. These interactions will insure high robustness of the membranes as will be discussed in subsequent sections. It can be seen from Fig. 3 that WU and SR of all QH membranes are nearly constant from 20 C to 80 C except QH-1.89 membrane: this is because when IECm is appropriate, the wellordered microphase separation helps to form a continuous and solid hydrophobic matrix that prevents the excessive swelling of the membrane [51]; however, for QH-1.89 membrane, whose IECm is too high, the excessive ionic conducting
15487
groups will damage the continuous hydrophobic matrix and lead to higher water uptake and swelling ratio [52e54]. As for BQH membrane, when the blending amount of PBI increases, the impact of microphase separation is attenuated and the impact of hydrogen bonding and electrostatic interaction in blend membrane is enhanced. Maybe it is the hydrogen bonding and electrostatic interaction between two polymers that endow the blend membrane with good anti-swelling property and good dimensional stability.
Morphology of the membranes Scanning electron microscope (SEM) was used to investigate the morphology of membrane. As can be seen from Fig. 4, the surface of the blend membranes are quite smooth and have no obvious defects, which indicates a good compatibility between PBI and QHPEEK. What's more, the cross sections are dense and have no visible defects, which ensures excellent dimensional stability and mechanical properties of the blend membranes [55]. Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to investigate the microstructure-property relationship and to verify the microphase separation in membranes [56,57]. As can be seen in Fig. 5, bright and dark domains represent the hydrophobic and hydrophilic domains, respectively, formed by microphase separation. It is observed that both the pure (QH-1.66) and the blend (BQH-1.64) membranes show well defined ionic clusters and hydrophilic domains. Fig. 6 presents the SAXS of different membranes, where QH-1.66 and BQH-1.64 show scattering peaks at ca. 3.30 and 3.50 nm1, respectively, which confirm the presence of ion clusters created by microphase separation in both membranes; based on the scattering peak positions (q values), Bragg spacing (d ¼ 2p/q) of 1.92 and 1.80 nm are obtained for QH-1.66 and BQH-1.64, respectively, close to each other, implying that the presence of PBI and the strong interactions with the quaternized polymer in the BQH-1.64 membrane did not affect microphase separation. However, there is less significant scattering peaks in the SAXS of BQH1.49 and BQH-1.28 membranes, which means high PBI blending amount will adversely affect the microphase separation behavior. Therefore, it is vital to control the blending amount of PBI to obtain blend membranes with satisfying comprehensive performance.
Mechanical properties and thermal stability
Scheme 2 e The electrostatic interaction and hydrogen bonding in blend membrane.
Adequate mechanical properties are required for AEMs. The mechanical properties of membranes with different IECm values are listed in Table 2. As we can see, all blend membranes possess much higher tensile strength than the pure polymer membranes. In particular, the BQH-1.28 membrane shows a much higher tensile strength than QH-1.30 (58.9 vs 17.0 MPa) with similar IECm, which indicates the introduction of PBI can greatly enhance the mechanical properties of the membrane and therefore prevent the membrane from being damaged during the cell assembly and operation. Fig. 7 shows the TGA and DTG curves of the membranes in OH form, which was heated from 25 C to 790 C at a heating rate of 5 C$min1 in a nitrogen atmosphere. All membranes
15488
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Fig. 3 e a) Water uptake of synthesized membranes at 20e80 C, b) Swelling ratio of synthesized membranes at 20e80 C.
Fig. 4 e SEM surface images of a) BQH-1.28, b) BQH-1.49, c) BQH-1.64, and SEM cross-sectional images of d) BQH-1.28, e) BQH1.49, f) BQH-1.64.
Fig. 5 e TEM spectra of a) QH-1.66 and b) BQH-1.64 membrane.
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
15489
BQH-1.64 is much better than QH-1.66 with similar IECm, which implies the electrostatic interactions and hydrogen bonding between PBI and piperidinium groups can promote the formation of ionic-crosslinking structure, which can help improve thermal stability of blend membranes.
Hydroxide ion conductivity
Fig. 6 e Small angle X-ray scattering profiles of different membranes.
Table 2 e Mechanical strength of hydrated membranes at room temperature. Membrane BQH-1.28 BQH-1.49 BQH-1.64 QH-1.30 QH-1.51 QH-1.66 QH-1.89
PBI (wt%) 20 10 5 0 0 0 0
Tensile strength (MPa) 58.9 46.9 42.8 17.0 15.9 15.2 12.3
± 0.5 ± 0.7 ± 0.6 ± 0.4 ± 0.5 ± 0.4 ± 0.6
Elongation at break (%) 18.6 ± 0.3 22.1 ± 0.4 24.4 ± 0.2 8.3 ± 0.6 12.7 ± 0.4 16.0 ± 0.3 23.1 ± 0.2
Hydroxide ion conductivity is the paramount parameter of AEMs for fuel cells, and the conductivity data of different QH and BQH membranes are presented in Fig. 8a. When the temperature increases, the free volume and mobility of the ionic conducting groups are enhanced and thus the hydroxide ion conductivity is improved [60e62]. As we can see, the hydroxide ion conductivity of QHPEEK ranges from 23.7 to 29 mS cm1 at room temperature and from 80.8 to 91.5 mS cm1 at 80 C. With lower water uptake and swelling ratio, the blend membranes show hydroxide ion conductivity of 17e21.4 mS cm1 at room temperature and 61.5e72.8 mS cm1 at 80 C, which are comparable to the values of the pure membrane, meaning the introduction of PBI helps to inhibit swelling and improve dimensional stability but without compromising the hydroxide ion conductivity much. Peculiarly, the BQH-1.64 membrane has a superior hydroxide ion conductivity of 72.8 mS cm1 at 80 C and a moderate water uptake (21.5% at 80 C). The hydroxide ion transport activation energy (Ea) for different membranes can be calculated from Ea ¼ -bR, where b represents the slope of ln(s) vs (1/T) regression curve (Fig. 8b) and R is the gas constant [63]. As can be seen from Fig. 8b, the Ea values of the blend membranes obtained are just slightly higher than that of QH membranes with the similar IECm, which is possibly because the rigid structure of PBI inhibits the connectivity of hydroxide ion transport channels to some extent and thus leads to slightly decreased hydroxide ion conductivity of the blend membranes.
Alkaline stability
Fig. 7 e TGA and DTG curves of BQH-1.64 and QH-1.66 membranes.
have experienced three major stages of weight loss: the first stage is ascribed to the loss of piperidinium groups; the second stage corresponds to the loss of methylene groups of the side chains; and the final stage is attributed to the degradation of polymer matrix [58,59]. As we can see, the thermal stability of
As one of the most important properties of AEM for fuel cell application, long-term alkaline stability depends on the stability of the functional groups, the polymer frameworks and the linkage between them [64]. For alkaline stability assessment, the prepared membranes were immersed in 1 M KOH at 60 C for different times, during which hydroxide ion conductivity was measured at set intervals. As can be seen from Fig. 9a, the BQH-1.49 membrane retains 95% of its original conductivity after being alkali treated for 576 h while the QH1.51 membrane only retains 75% conductivity; the BQH-1.64 membrane retains 84% of its original conductivity after being alkali treated for 576 h while the QH-1.66 membrane only retains 73%. The above comparison confirms the contribution of PBI to the membrane's alkaline stability: the interaction between PBI and the quaternized PEEK may hinder the hydroxide ion attack on the piperidinium cation [65e69]. As we can see from Fig. 9b, in the FT-IR spectrum of the alkalitreated BQH-1.49 membrane, the characteristic peak at 928 cm1 assignable to CeNþ stretching vibration of piperidinium groups, remains virtually unchanged, indicating an excellent alkali stability of the membrane [70]. Such a stability
15490
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
Fig. 8 e a) Temperature dependence of hydroxide ion conductivity of different synthesized membranes; b) Arrhenius plots of different synthesized membranes.
Fig. 9 e a) The conductivity of synthesized membranes after being alkali-treated for different time duration; b) the FT-IR spectrum of BQH-1.49 membrane before and after alkali-treated for 576 h
Fig. 10 e Polarization curves and power density curves of the BQH-1.49 and QH-1.51 membrane under operating temperature 30 C and 80 C.
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
may be attributed to the piperidinium group itself and the interaction between PBI and the quaternized PEEK.
Single fuel cell performance The single fuel cell performance was measured in air at atmosphere pressure with 100% relative humidity at 30 C and 80 C; the polarization and power density curves are shown in Fig. 10. The synthesized polymer was used as both anion exchange membrane and the electrode binder material, which may effectively reduce the ion transport resistance between the catalyst layer and the membrane [71]. The cell resistance of fuel cell using BQH-1.49 membrane at 30 C and 80 C is 22 mOhm and 26 mOhm, respectively. The area specific resistance of fuel cell using BQH-1.49 membrane at 30 C and 80 C is 86 mOhm$cm2 and 97 mU cm2, respectively. The open circuit potentials of all single cells are ca. 0.97 V, which means the membrane is compact enough and can prevent fuel crossover to the cathode. The peak power density of fuel cell using BQH-1.49 membrane increased from 65 mW cm2 at 30 C to 87 mW cm2 at 80 C, which means an elevated temperature accelerates the electrochemical reaction rate and leads to higher peak power density [72]. As can be seen in Fig. 10, the peak power density of BQH-1.49 membrane is higher than that of QH-1.51 membrane, which may be ascribed to the higher water uptake of QH-1.51 membrane. The above peak power density, although not high enough compared with the literature reported best results, demonstrates the potential of the BQH membrane to be used as an AEMFC electrolyte and encourages further optimization [73e81].
Conclusion We have successfully fabricated a series of novel blend membranes composed of synthesized side chain piperidinium functionalized polyetheretherketone and polybenzimidazole. The blend membranes exhibited lower swelling ratio, higher mechanical strength, and better alkaline stability than pure piperidinium functionalized polymer membranes. This is attributed to the existence of electrostatic interactions and hydrogen bonding between PBI and piperidinium polymers, which is confirmed via FT-IR. All blend membranes exhibit high hydroxide ion conductivity (61.5e72.8 ms cm1 at 80 C) with moderate water uptake (15.1e21.5% at 80 C) and good mechanical strength (42.8e58.9 MPa). Moreover, after immersed in 1 M KOH at 60 C for 576 h, the BQH-1.49 membrane still exhibits 95% of its original conductivity value. Therefore, the piperidinium functionalized, PBI-containing blend membranes can balance the hydroxide ion conductivity and alkaline stability well, and the universal synthesizing method can be applied to other polymers as well.
Acknowledgements We acknowledge the financial supports from the National Natural Science Foundation of China (Grant no. 21776042), the
15491
Science and Technology Innovation Fund of Dalian (2018J12GX052), the National Key Research and Development Program of China (Grant no. 2016YFB0101203), and China MOST (Ministry of Science and Technology) innovation team in key areas (No. 2016RA4053).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.172.
references
[1] Gottesfeld S, Dekel DR, Page M, Bae C, Yan Y, Zelenay P, et al. Anion exchange membrane fuel cells: current status and remaining challenges. J Power Sources 2018;375:170e84. [2] Jannasch P, Weiber EA. Configuring anion-exchange membranes for high conductivity and alkaline stability by using cationic polymers with tailored side chains. Macromol Chem Phys 2016;217:1108e18. [3] Zhang X, Li S, Chen P, Fang J, Shi Q, Weng Q, et al. Imidazolium functionalized block copolymer anion exchange membrane with enhanced hydroxide conductivity and alkaline stability via tailoring side chains. Int J Hydrogen Energy 2018;43:3716e30. [4] Wang XQ, Lin CX, Zhang QG, Zhu AM, Liu QL. Anion exchange membranes from hydroxyl-bearing poly(ether sulfone)s with flexible spacers via ring-opening grafting for fuel cells. Int J Hydrogen Energy 2017;42:19044e55. [5] Lin B, Qiao G, Chu F, Wang J, Feng T, Yuan N, et al. Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications. Int J Hydrogen Energy 2017;42:6988e96. [6] Dai J, He G, Ruan X, Zheng W, Pan Y, Yan X. Constructing a rigid crosslinked structure for enhanced conductivity of imidazolium functionalized polysulfone hydroxide exchange membrane. Int J Hydrogen Energy 2016;41:10923e34. [7] Cheng J, He G, Zhang F. A mini-review on anion exchange membranes for fuel cell applications: stability issue and addressing strategies. Int J Hydrogen Energy 2015;40:7348e60. [8] Zhu Y, He Y, Ge X, Liang X, Shehzad MA, Hu M, et al. A benzyltetramethylimidazolium-based membrane with exceptional alkaline stability in fuel cells: role of its structure in alkaline stability. J Mater Chem 2018;6:527e34. [9] Zhao B, He G, El Hamouti I, Gao L, Liu Y, Deng R, et al. A novel strategy for constructing a highly conductive and swellingresistant semi-flexible aromatic polymer based anion exchange membranes. Int J Hydrogen Energy 2017;42:10228e37. [10] Varcoe JR, Atanassov P, Dekel DR, Herring AM, Hickner MA, Kohl PA, et al. Anion-exchange membranes in electrochemical energy systems. Energy Environ Sci 2014;7:3135e91. [11] Rao AHN, Nam S, Kim T-H. Crosslinked poly(arylene ether sulfone) block copolymers containing pendant imidazolium groups as both crosslinkage sites and hydroxide conductors for highly selective and stable membranes. Int J Hydrogen Energy 2014;39:5919e30. [12] Hossain MA, Lim Y, Lee S, Jang H, Choi S, Jeon Y, et al. Comparison of alkaline fuel cell membranes of random & block poly(arylene ether sulfone) copolymers containing tetra quaternary ammonium hydroxides. Int J Hydrogen Energy 2014;39:2731e9.
15492
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
[13] Zhao CH, Gong Y, Liu QL, Zhang QG, Zhu AM. Selfcrosslinked anion exchange membranes by bromination of benzylmethyl-containing poly(sulfone)s for direct methanol fuel cells. Int J Hydrogen Energy 2012;37:11383e93. [14] Wang D, Wang Y, Wan H, Wang J, Wang L. Synthesis of gemini basic ionic liquids and their application in anion exchange membranes. RSC Adv 2018;8:10185e96. [15] Chen N, Long C, Li Y, Lu C, Zhu H. Ultrastable and high ionconducting polyelectrolyte based on six-membered Nspirocyclic ammonium for hydroxide exchange membrane fuel cell applications. ACS Appl Mater Interfaces 2018;10:15720e32. [16] Zhu H, Li R, Wang F, Chen N, Li Z, Wang Z. Poly tris (1imidazolyl) benzene ionic liquids/Poly (2,6-dimethyl phenylene oxide) composite membranes for anion exchange membrane fuel cells. J Mater Sci 2017;52:11109e19. [17] Wang J, Chen Y, Wei Q, Yang S, Fang H, Wei H, et al. Adamantammonium as a novel functional group for anion exchange membranes with excellent comprehensive performances. Polymer 2017;112:288e96. [18] Pham TH, Olsson JS, Jannasch P. N-spirocyclic quaternary ammonium ionenes for anion-exchange membranes. J Am Chem Soc 2017;139:2888e91. [19] Mo Z-H, Yang R, Hong S, Wu Y-X. In-situ preparation of cross-linked hybrid anion exchange membrane of quaternized poly (styrene- b -isobutylene- b -styrene) covalently bonded with graphene. Int J Hydrogen Energy 2018;43:1790e804. [20] Zhu L, Yu X, Hickner MA. Exploring backbone-cation alkyl spacers for multi-cation side chain anion exchange membranes. J Power Sources 2018;375:433e41. [21] Shukla G, Shahi VK. Poly(arylene ether ketone) copolymer grafted with amine groups containing a long alkyl chain by chloroacetylation for improved alkaline stability and conductivity of anion exchange membrane. ACS Appl Energy Mater 2018;1:1175e82. [22] Mahmoud AMA, Elsaghier AMM, Otsuji K, Miyatake K. High hydroxide ion conductivity with enhanced alkaline stability of partially fluorinated and quaternized aromatic copolymers as anion exchange membranes. Macromolecules 2017;50:4256e66. [23] Pan J, Chen C, Li Y, Wang L, Tan L, Li G, et al. Constructing ionic highway in alkaline polymer electrolytes. Energy Environ Sci 2014;7:354e60. [24] Zhu M, Zhang X, Wang Y, Wu Y, Wang H, Zhang M, et al. Novel anion exchange membranes based on quaternized diblock copolystyrene containing a fluorinated hydrophobic block. J Membr Sci 2018;554:264e73. [25] Mohanty AK, Devaraju S, Kim N, Paik H-j. Synthesis and characterization of poly(ether sulfone) block copolymers containing pendent quaternary ammonium- and imidazolium groups as anion exchange membranes. Solid State Ionics 2018;314:46e56. [26] Irfan M, Bakangura E, Afsar NU, Hossain MM, Ran J, Xu T. Preparation and performance evaluation of novel alkaline stable anion exchange membranes. J Power Sources 2017;355:171e80. [27] Dong X, Lv D, Zheng J, Xue B, Bi W, Li S, et al. Pyrrolidiniumfunctionalized poly(arylene ether sulfone)s for anion exchange membranes: using densely concentrated ionic groups and block design to improve membrane performance. J Membr Sci 2017;535:301e11. [28] Li L, Yue X, Wu W, Yan W, Zeng M, Zhou Y, et al. Multi-block copolymers with fluorene-containing hydrophilic segments densely functionalized by side-chain quaternary ammonium groups as anion exchange membranes. RSC Adv 2016;6:41453e64.
[29] Wang Y, Wan H, Wang D, Wang J, Wang L, Feng R. Preparation and characterization of A semi-interpenetrating network alkaline anion exchange membrane. Fibers Polym 2018;19:11e21. [30] Lin J, Yan X, He G, Chen W, Zhen D, Li T, et al. Thermoplastic interpenetrating polymer networks based on polybenzimidazole and poly (1, 2-dimethy-3allylimidazolium) for anion exchange membranes. Electrochim Acta 2017;257:9e19. [31] Gong Y, Liao X, Xu J, Chen D, Zhang H. Novel anionconducting interpenetrating polymer network of quaternized polysulfone and poly(vinyl alcohol) for alkaline fuel cells. Int J Hydrogen Energy 2016;41:5816e23. [32] He SS, Strickler AL, Frank CW. A semi-interpenetrating network approach for dimensionally stabilizing highlycharged anion exchange membranes for alkaline fuel cells. ChemSusChem 2015;8:1472e83. [33] Morandi CG, Peach R, Krieg HM, Kerres J. Novel imidazoliumfunctionalized anion-exchange polymer PBI blend membranes. J Membr Sci 2015;476:256e63. [34] Morandi CG, Peach R, Krieg HM, Kerres J. Novel morpholinium-functionalized anion-exchange PBIepolymer blends. J Mater Chem 2015;3:1110e20. [35] Olsson JS, Pham TH, Jannasch P. Poly(arylene piperidinium) hydroxide ion exchange membranes: synthesis, alkaline stability, and conductivity. Adv Funct Mater 2018;28:1702758. [36] Dang H-S, Jannasch P. A comparative study of anionexchange membranes tethered with different heterocycloaliphatic quaternary ammonium hydroxides. J Mater Chem 2017;5:21965e78. [37] Olsson JS, Pham TH, Jannasch P. Poly(N,Ndiallylazacycloalkane)s for anion-exchange membranes functionalized with N-spirocyclic quaternary ammonium cations. Macromolecules 2017;50:2784e93. [38] Pham TH, Jannasch P. Aromatic polymers incorporating bisN-spirocyclic quaternary ammonium moieties for anionexchange membranes. ACS Macro Lett 2015;4:1370e5. [39] Marino MG, Kreuer KD. Alkaline stability of quaternary ammonium cations for alkaline fuel cell membranes and ionic liquids. ChemSusChem 2015;8:513e23. [40] Zeng L, Zhao TS, An L, Zhao G, Yan XH. Physicochemical properties of alkaline doped polybenzimidazole membranes for anion exchange membrane fuel cells. J Membr Sci 2015;493:340e8. [41] Hou H, Wang S, Jiang Q, Jin W, Jiang L, Sun G. Durability study of KOH doped polybenzimidazole membrane for airbreathing alkaline direct ethanol fuel cell. J Power Sources 2011;196:3244e8. [42] Yan X, He G, Gu S, Wu X, Du L, Zhang H. Quaternized poly(ether ether ketone) hydroxide exchange membranes for fuel cells. J Membr Sci 2011;375:204e11. [43] Mohanty AD, Tignor SE, Krause JA, Choe Y-K, Bae C. Systematic alkaline stability study of polymer backbones for anion exchange membrane applications. Macromolecules 2016;49:3361e72. [44] Gong X, Yan X, Li T, Wu X, Chen W, Huang S, et al. Design of pendent imidazolium side chain with flexible ethercontaining spacer for alkaline anion exchange membrane. J Membr Sci 2017;523:216e24. [45] Chen W, Hu M, Wang H, Wu X, Gong X, Yan X, et al. Dimensionally stable hexamethylenetetramine functionalized polysulfone anion exchange membranes. J Mater Chem 2017;5:15038e47. [46] Hazarika M, Jana T. Novel proton exchange membrane for fuel cell developed from blends of polybenzimidazole with fluorinated polymer. Eur Polym J 2013;49:1564e76. [47] Vengatesan S, Santhi S, Sozhan G, Ravichandran S, Davidson DJ, Vasudevan S. Novel cross-linked anion
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 4 4 ( 2 0 1 9 ) 1 5 4 8 2 e1 5 4 9 3
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[60]
[61]
[62]
[63]
exchange membrane based on hexaminium functionalized poly(vinylbenzyl chloride). RSC Adv 2015;5:27365e71. Hazarika M, Arunbabu D, Jana T. Formation of core (polystyrene)-shell (polybenzimidazole) nanoparticles using sulfonated polystyrene as template. J Colloid Interface Sci 2010;351:374e83. Kosmala B, Schauer J. Ion-exchange membranes prepared by blending sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) with polybenzimidazole. J Appl Polym Sci 2002;85:1118e27. Maurya S, Shin S-H, Lee J-Y, Kim Y, Moon S-H. Amphoteric nanoporous polybenzimidazole membrane with extremely low crossover for a vanadium redox flow battery. RSC Adv 2016;6:5198e204. Weiber EA, Jannasch P. Anion-conducting polysulfone membranes containing hexa-imidazolium functionalized biphenyl units. J Membr Sci 2016;520:425e33. Li N, Leng Y, Hickner MA, Wang CY. Highly stable, anion conductive, comb-shaped copolymers for alkaline fuel cells. J Am Chem Soc 2013;135:10124e33. Yan X, Gao L, Zheng W, Ruan X, Zhang C, Wu X, et al. Longspacer-chain imidazolium functionalized poly(ether ether ketone) as hydroxide exchange membrane for fuel cell. Int J Hydrogen Energy 2016;41:14982e90. Ran J, Fu C, Ding L, Cao P, Xu T. Dual hydrophobic grafted chains facilitating quaternary ammonium aggregations of hydroxide conducting polymers: a theoretical and experimental investigation. J Mater Chem 2018;6:5714e23. Hao J, Jiang Y, Gao X, Lu W, Xiao Y, Shao Z, et al. Functionalization of polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with two cyclic quaternary ammonium cations for anion exchange membranes. J Membr Sci 2018;548:1e10. Dang H-S, Weiber EA, Jannasch P. Poly(phenylene oxide) functionalized with quaternary ammonium groups via flexible alkyl spacers for high-performance anion exchange membranes. J Mater Chem 2015;3:5280e4. Hu EN, Lin CX, Liu FH, Wang XQ, Zhang QG, Zhu AM, et al. Poly(arylene ether nitrile) anion exchange membranes with dense flexible ionic side chain for fuel cells. J Membr Sci 2018;550:254e65. Afsar NU, Erigene B, Irfan M, Wu B, Xu T, Ji W, et al. High performance anion exchange membrane with proton transport pathways for diffusion dialysis. Separ Purif Technol 2018;193:11e20. Xiao Lin C, Qin Wang X, Ning Hu E, Yang Q, Gen Zhang Q, Mei Zhu A, et al. Quaternized triblock polymer anion exchange membranes with enhanced alkaline stability. J Membr Sci 2017;541:358e66. Lin CX, Zhuo YZ, Hu EN, Zhang QG, Zhu AM, Liu QL. Crosslinked side-chain-type anion exchange membranes with enhanced conductivity and dimensional stability. J Membr Sci 2017;539:24e33. Lin CX, Wang XQ, Li L, Liu FH, Zhang QG, Zhu AM, et al. Triblock copolymer anion exchange membranes bearing alkyl-tethered cycloaliphatic quaternary ammonium-headgroups for fuel cells. J Power Sources 2017;365:282e92. Yang Z, Guo R, Malpass-Evans R, Carta M, McKeown NB, Guiver MD, et al. Highly conductive anion-exchange membranes from microporous troger's base polymers. Angew Chem 2016;55:11499e502. Lin CX, Zhuo YZ, Lai AN, Zhang QG, Zhu AM, Ye ML, et al. Side-chain-type anion exchange membranes bearing pendent imidazolium-functionalized poly(phenylene oxide) for fuel cells. J Membr Sci 2016;513:206e16.
15493
[64] Lai AN, Zhou K, Zhuo YZ, Zhang QG, Zhu AM, Ye ML, et al. Anion exchange membranes based on carbazole-containing polyolefin for direct methanol fuel cells. J Membr Sci 2016;497:99e107. [65] He Y, Si J, Wu L, Chen S, Zhu Y, Pan J, et al. Dual-cation combshaped anion exchange membranes: structure, morphology and properties. J Membr Sci 2016;515:189e95. [66] Yang Q, Lin CX, Liu FH, Li L, Zhang QG, Zhu AM, et al. Poly (2,6-dimethyl-1,4-phenylene oxide)/ionic liquid functionalized graphene oxide anion exchange membranes for fuel cells. J Membr Sci 2018;552:367e76. [67] Li G, Pan J, Han J, Chen C, Lu J, Zhuang L. Ultrathin composite membrane of alkaline polymer electrolyte for fuel cell applications. J Mater Chem 2013;1:12497. [68] Pan J, Li Y, Han J, Li G, Tan L, Chen C, et al. A strategy for disentangling the conductivityestability dilemma in alkaline polymer electrolytes. Energy Environ Sci 2013;6:2912. [69] Dang H-S, Jannasch P. Anion-exchange membranes with polycationic alkyl side chains attached via spacer units. J Mater Chem 2016;4:17138e53. [70] Ono H, Miyake J, Shimada S, Uchida M, Miyatake K. Anion exchange membranes composed of perfluoroalkylene chains and ammonium-functionalized oligophenylenes. J Mater Chem 2015;3:21779e88. [71] Li G, Pan J, Han J, Chen C, Lu J, Zhuang L. Ultrathin composite membrane of alkaline polymer electrolyte for fuel cell applications. J Mater Chem 2013;1:12497. [72] Hou H, Sun G, He R, Wu Z, Sun B. Alkali doped polybenzimidazole membrane for high performance alkaline direct ethanol fuel cell. J Power Sources 2008;182:95e9. [73] Yao Z, Zhang Z, Hu M, Hou J, Wu L, Xu T. Perylene-based sulfonated aliphatic polyimides for fuel cell applications: performance enhancement by stacking of polymer chains. J Membr Sci 2018;547:43e50. [74] Ran J, Fu C, Ding L, Cao P, Xu T. Dual hydrophobic grafted chains facilitating quaternary ammonium aggregations of hydroxide conducting polymers: a theoretical and experimental investigation. J Mater Chem 2018;6:5714e23. [75] Hu EN, Lin CX, Liu FH, Wang XQ, Zhang QG, Zhu AM, et al. Poly(arylene ether nitrile) anion exchange membranes with dense flexible ionic side chain for fuel cells. J Membr Sci 2018;550:254e65. [76] He Y, Ge X, Liang X, Zhang J, Shehzad MA, Zhu Y, et al. Anion exchange membranes with branched ionic clusters for fuel cells. J Mater Chem 2018;6:5993e8. [77] Hao J, Jiang Y, Gao X, Lu W, Xiao Y, Shao Z, et al. Functionalization of polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with two cyclic quaternary ammonium cations for anion exchange membranes. J Membr Sci 2018;548:1e10. [78] Hao J, Gao X, Jiang Y, Zhang H, Luo J, Shao Z, et al. Crosslinked high-performance anion exchange membranes based on poly(styrene- b -(ethylene- co -butylene)- b -styrene). J Membr Sci 2018;551:66e75. [79] Zhu L, Zimudzi TJ, Wang Y, Yu X, Pan J, Han J, et al. Mechanically robust anion exchange membranes via long hydrophilic cross-linkers. Macromolecules 2017;50:2329e37. [80] Wang L, Magliocca E, Cunningham EL, Mustain WE, Poynton SD, Escudero-Cid R, et al. An optimised synthesis of high performance radiation-grafted anion-exchange membranes. Green Chem 2017;19:831e43. [81] Lee KH, Cho DH, Kim YM, Moon SJ, Seong JG, Shin DW, et al. Highly conductive and durable poly(arylene ether sulfone) anion exchange membrane with end-group cross-linking. Energy Environ Sci 2017;10:275e85.