Crosslinked Poly(Vinylbenzyl Chloride) Anion Exchange Membranes

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Crosslinked Poly(Vinylbenzyl Chloride) Anion Exchange Membranes with Long Flexible Multi-Head for Fuel Cells En Ning Hu, Chen Xiao Lin, Fang Hua Liu, Qian Yang, Ling Li, Qiu Gen Zhang, Ai Mei Zhu, and Qing Lin Liu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00698 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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ACS Applied Energy Materials

Crosslinked Poly(Vinylbenzyl Chloride) Anion Exchange Membranes with Long Flexible Multi-Head for Fuel Cells En Ning Hu, Chen Xiao Lin, Fang Hua Liu, Qian Yang, Ling Li, Qiu Gen Zhang, Ai Mei Zhu, Qing Lin Liu*

Department of Chemical & Biochemical Engineering, College of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China

*Corresponding author:

E-mail: [email protected]. Tel: 86-592-2188072 Fax: 86-592-2184822

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Abstract

To enhance the ionic conductivity and mechanical property of the chemically stable aliphatic anion exchange membranes (AEMs), crosslinked poly(vinylbenzyl chloride) AEMs (PVBC-xQ4) bearing long flexible crosslinker chains with multiple quaternary ammonium (QA) head were prepared via a facile one-step process. The AEMs exhibit high conductivity and excellent mechanical property. The crosslinked AEMs excluding any other heteroatomic bond especially the sulfone linkage or ether bond in the backbone have robust alkaline stability,

low

water

uptake

and

good

dimensional

stability.

A well-developed

hydrophilic/hydrophobic phase separated morphology was observed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The PVBC-50Q4 membrane with ionic exchange capacity (IEC) of 1.87 meq g-1 displays a highest ionic conductivity of 74.3 mS cm-1 at 80 °C. After tested in a 2 M aqueous KOH solution at 80 °C for 1300 h, 88% of the conductivity and 89% of the IEC were remained. Furthermore, a single cell using the PVBC-50Q4 membrane has a maximum peak power density of 109.2 mW cm-2 at 60 °C.

KEYWORDS: Anion exchange membranes; poly(vinylbenzyl chloride); alkaline stability; phase separation; fuel cells

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1. Introduction Fuel cells are regarded as one of the most promising new energy technologies and have been applied in many fields including stationary power generation, portable equipment and new energy automobile because of high efficacy.1-4 Though proton exchange membrane fuel cells (PEMFCs) show high performance,5-6 the high cost, dependency of precious metal catalysts and serious fuel permeation limit their further commercial applications.7-9 Anion exchange membrane fuel cells (AEMFCs) exhibit higher electrode reaction rate, independence on precious metal catalysts, lower fuel permeation and more options for fuels than PEMFCs.9-13 AEMs, as the key component of AEMFCs, play an important role in ion conduction and the separation of the anode from the cathode of fuel cells, and have attracted broad attention in the past decades. However, two major challenges still need to be addressed. One is the trade-off between the ionic conductivity and swelling which are both closely related to IEC and/or water uptake. Crosslinking, an effective and handy strategy, has come up to improve the dimensional stability. Crosslinked membranes show better thermal and alkaline stability.14-18 Other strategies including bearing side chain19 and dense functional groups20-22 were also reported to enhance the ionic conductivity without increasing the IEC and swelling. The other challenge is to improve the alkaline stability of the AEMs which is essential for the performance and lifetime of fuel cells. Besides the type of cation, the type of backbone, the side chain and the substituent group on cation all have an appreciable influence on the alkaline stability of the AEMs.23-24 The copolymer AEMs mainly contain the aromatic and aliphatic backbones.25 Of the aromatic copolymers, quaternized poly(ether sulfone) (PES) has been proved to be 3



unstable in alkaline condition where the sulfone linkage and ether bond bring the risk of degradation under the attack from OH-.26-29 Aliphatic backbones without any other heteroatomic bond except C-C covalent bond are believed to be the most promising alternatives for AEMs due to their long-term alkaline stability. 30-31 Wu et al. prepared a series of crosslinked AEMs based on chloromethylated SIBS32 and SEBS.33 The membranes showed pretty low IEC (< 1.0 meq g-1) and conductivity (< 12 mS cm-1 at 30 °C) because of the low functionalization degree. Our group prepared a series of side chain type SEBS-CH2-QA-x AEMs.34 The membranes (IEC =1.23 meq g-1) are chemically stable in an alkaline environment (a decrease by 7.7% in conductivity in 1 M KOH at 60 °C), but the high WU (89.9%) and SR (58%) at 80 °C limit the further improvement of the IEC and ionic conductivity. The crosslinked HMTA/PVBC AEMs prepared by Vasudevan et al.35 were quite stable in alkaline condition (> 93% IEC remained in 5 M KOH at 30 °C) but their poor mechanical property from the rigid aromatic backbone and low conductivity from high crosslinking degree must be improved before available. Therefore, though aliphatic polymers like PVBC have tough alkali resistance, their high WU and SR, low conductivity and poor mechanical property hampered the application in AEMs. Here we designed a series of PVBC based AEMs crosslinked by a long flexible ionic liquid (IL) crosslinker containing multiple QA cations to enhance the ionic conductivity and mechanical property. Crosslinked structure was adopted to constrain the swelling and degradation of the AEMs.14-16 The alkyl chain length between the cation groups is controlled at 6 because of their advantages in providing higher performance and stability.36-37 The morphology of the as-prepared AEMs was observed by SEM, AFM and TEM. The 4


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performance of the AEMs with various crosslinker contents was investigated in detail. 2. Experimental 2.1. Materials N,N,N,N-tetramethyl-1,6-hexanediamine (TMHDA, 99%) was purchased from J&K Scientific Ltd. Poly(vinylbenzyl chloride) (PVBC, 60/40 mixture of 3- and 4- isomers), with average molecular weight of Mn 55,000 and Mw 10,000 from GPC, was obtained from Sigma Aldrich. 1,6-Dibromohexane (99%) and dimethyl sulfoxide (DMSO, 99.8%) were purchased from Aladdin Industrial Inc. and were used as received. All other reagents were purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd. (China), and used without further purification. 2.2. Preparation of AEMs 2.2.1. Synthesis of IL Crosslinker 1,18-(N’,N’-dimethylamino)-6,12-(N,N-dimethylammonium) octodecane bromine ionic liquid crosslinker (NQQN) was synthesized according to the literature.38 The synthetic method is shown in Scheme S1 (Supporting Information). Typically, 1.26 mL of 1,6-dibromohexane was dissolved in ethanol to form a 2% v/v solution. 15 mL of N,N,N,N-tetramethyl-1,6-hexanediamine (TMHDA) was added into a 150 mL three-necked round-bottomed flask with a magnetic stirrer. Then the 1,6-dibromohexane solution was added dropwise into the flask under stirring. The mixture was heated to 50 °C and stirred for 5 h. After cooling to room temperature (RT), the resulting solution was transferred into 400 mL of anhydrous ether to precipitate a white powder solid. After cooled in a refrigerator at -18 °C for 12 h, a white precipitate was obtained via vacuum filtration. The crude product was 5



purified with anhydrous ether for three times followed by vacuum drying at RT for 24 h. 2.2.2. Fabrication and Alkalization of Flexible Crosslinked Membranes

Scheme 1 Illustration of the one-step preparation of the flexible crosslinked membranes. Scheme 1 depicts the procedure for preparation of the flexible crosslinked membranes. PVBC was dissolved in DMSO (5% w/v) to form a colorless transparent solution. After filtration through a 0.45 µm PTFE syringe filter, a certain amount of NQQN IL was added into the solution with vigorous shaking until the IL was dissolved and mixed completely. 6


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After removing the bubbles by ultrasound, the solution was cast onto a clear glass plate, and vacuum dried at 60 °C for 24 h. The resulting membrane was taken off the glass plate and immersed into a 1 M KOH solution for 48 h to convert the Cl- and Br- into OH-. Finally, the AEMs were washed with deionized water to remove the residual alkali and stored in deionized water. 2.3. Characterization Characterization via 1H NMR, FT-IR, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) is detailed in Section S1 (Supporting Information). The determination of ionic exchange capacity (IEC), degree of crosslinking (DC), gel fraction, water uptake (WU), swelling ratio (SR), ionic conductivity, mechanical property, thermal stability, alkaline stability and single cell performance is included in Section S2 (Supporting Information).

3. Results and discussion 3.1. Synthesis and Characterization of IL Crosslinker and the Crosslinked Polymer 1,18-(N’,N’-dimethylamino)-6,12-(N,N-dimethylammonium)octodecane

bromine

ionic

liquid (NQQN) was synthesized by the reaction of 1,6-dibromohexane and excess N,N,N,N-tetramethyl-1,6-hexanediamine (TMHDA) in ethanol at RT without any catalyst (a pretty mild condition). The 1H NMR spectrum (Fig. S1) is consistent with the literature.35 The peak around 2.16 ppm corresponds to the proton resonance from methyl in the tertiary amine groups (-N(CH3)2). The peak around 2.32 ppm is assigned to methylene (-CH2-) next to the tertiary amine groups and the peaks appeared at 3.23 and 2.99 ppm are ascribed to the proton resonance of methyl in the QA groups (-N+(CH3)3-) and the methylene (-CH2-) beside the QA 7



groups, respectively. The signals for the methylene (-CH2-) in other positions range from 1.3 to 1.7 ppm. Preparation of the crosslinked membranes by a facile one-step process includes in situ functionalization and crosslinking of PVBC via the SN2 substitution reaction. The NQQN IL plays the part of crosslinker and quaternary ammonium reagent for PVBC as well. There are four QA groups in one crosslinker chain after crosslinking reaction, so the membranes are termed PVBC-xQ4, in which x is the weight ratio of NQQN to PVBC. In this work, the PVBC-30Q4, PVBC-40Q4 and PVBC-50Q4 membranes were made. PVBC-20Q4 was too brittle to form film and PVBC-60Q4 became jelly in a very short period of time. This just shows the feasibility of the molecular design.

Fig. 1 Images of the PVBC-50Q4 membrane (a) EDX spectra, (b, c) compositional mapping. 8


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FT-IR spectra of the crosslinked PVBC-50Q4 membrane and pristine PVBC powder are shown in Fig. S2. PVBC-50Q4 (curve b) has some new absorption peaks against PVBC (curve a). The new peaks at 956, 1042 and 1642 cm-1 are associated with the C-N bond, and the new vibration peak at 1471 cm-1 is attributed to the C-H bond in the QA groups. Furthermore, the absorption peak at 1266 cm-1 for the C-Cl bond was significantly reduced after crosslinking. This indicates the partial conversion of the benzyl chloride groups. 39-40 The EDX spectra and the mapping of the Cl and N elements of the PVBC-50Q4 membrane are shown in Fig. 1. The N content was detected but the proportion of Cl is a little higher than the theoretical value (7.7% of atom). This indicates that the actual functionalization degree is slightly lower than expected. The element mapping images for Cl and N reveal a uniform distribution of the Cl and N elements in the crosslinked membrane (Fig. 1b and 1c).

XPS (N 1s)

Experimental data Fitting curve

QA-1

QA-2

PVBC-50Q4

406

405

404

403

402

401

400

399

398

Binding energy (eV) Fig. 2 XPS signals for N 1s electron in the PVBC-50Q4 membrane. According to the molecular structure of the crosslinked polymer, there are two different types of ammonium groups (QA-1, the quaternary ammonium cation attached to the PVBC 9



backbone, and QA-2, the quaternary ammonium cation on the chain) in the crosslinked membranes. This is identified by XPS. As shown in Fig. 2, a single peak assigned to both the QA-1 (402.3 eV) and QA-2 (401.8eV) groups38 confirmed the successful introduction of the flexible crosslinker. 3.2. Morphology From the photo of the PVBC-30Q4 membrane (Fig. S3a and Scheme 1), the transparent membrane exhibits an excellent flexibility which can be folded for four times without any crack or break. This is the benefit from the flexible crosslinker, which greatly increased the flexibility of the PVBC backbone. We will talk about this in detail later in this article. Fig. S3b and c shows the SEM images of the surface and the cross-section of the PVBC-30Q4 membrane. A homogeneous and compact structure without visible defect can be observed and the membrane thickness is approximately 40.6 µm. AFM in a tapping mode was used to investigate the microphase structure of the crosslinked AEMs. As displayed in Fig. S4, all the samples exhibited a clear bright/dark region separation, where the bright regions represent the hydrophobic domains composed of the PVBC main chain, and the dark regions stand for the hydrophilic domains formed by the multi-head crosslinker chain. 41-42 With the increase of the NQQN content, the microphase separation became more significant and the hydrophilic domain developed into ionic transport network (PVBC-50Q4). The dense cation distribution on the crosslinker chains and the strong hydrophobicity of the PVBC backbone facilitated the formation of microphase separation. All of these indicate that the as-designed structure could improve the ionic conductivity. 10


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The morphology of the membranes was also observed by TEM. Fig. 3 shows the TEM images of PVBC-40Q4 and PVBC-50Q4. It can be seen that the formation of ion cluster provides a hydrophilic domain (the dark region) for ion conduction. The ion cluster size and the hydrophilic domain increased significantly by increasing the content of multi-head crosslinker. A similar result with AFM can be concluded that the ion clusters in PVBC-50Q4 are larger than those in PVBC-40Q4. This means that increasing the content of flexible crosslinker would facilitate the formation of microphase separation and the ion transport channels.

Fig. 3 TEM images of (a) PVBC-40Q4 and (b) PVBC-50Q4. 3.3. Ionic Exchange Capacity (IEC), DC and Gel Fraction Table 1 IEC, DC and gel fraction of the PVBC-xQ4 AEMs. IEC (meq g‒1) Membranes Cal.

a

a

Exp.

DC a (%)

Gel fraction (%)

b

PVBC-30Q4

1.55

1.30±0.03

15.4

98.81±0.5

PVBC-40Q4

1.92

1.61±0.04

20.5

98.85±0.5

PVBC-50Q4

2.24

1.87±0.03

25.7

98.75±0.4

Calculated from the feed monomer ratio, b Estimated from back titration. 11



IEC reflects the number of exchangeable ions per unit mass of the membranes which affects almost all the properties of AEMs especially water uptake, swelling and ionic conductivity. In previous reports,31-34,43 many aliphatic backbone AEMs showed a pretty low IEC (

Crosslinked Poly(Vinylbenzyl Chloride) Anion Exchange Membranes

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