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Plasma-grafted anion-exchange membrane preparation and process analysis
Electrochimica Acta 204 (2016) 218–226
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Plasma-grafted anion-exchange membrane preparation and process analysis Chengxu Zhanga,b , Jue Hub,** , Wenguang Fana , Michael K.H. Leunga,* , Yuedong Mengb a b
Ability R&D Energy Research Centre, School of Energy and Environment, City University of Hong Kong, China Institute of Plasma Physics, Chinese Academy of Sciences, China
A R T I C L E I N F O
Article history: Received 16 January 2016 Received in revised form 22 March 2016 Accepted 16 April 2016 Available online 19 April 2016 Keywords: Anion exchange membrane Plasma-grafting Vinylbenzyl trimethylammonium chloride Grafting efficiency Plasma bombardment efficiency
A B S T R A C T
A green and simplified plasma grafting approach was adopted for anion exchange membranes (AEMs) synthesis based on polyvinyl chloride powders in three steps: plasma bombardment, grafting and alkalization, avoiding the use of toxic chloromethyl ether and quaternization reagents. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses demonstrate the successful grafting of benzyltrimethylammonium groups into the polyvinyl chloride matrix. The plasmagrafted AEM exhibits satisfactory thermal stability, chemical stability and ionic conductivity, suggesting potential application in fuel cell. The plasma bombardment efficiency reaches up to 50% based on the FTIR and XPS results, meaning that one in every three syndiotactic CH2–CHCl units is bombarded in the polyvinyl chloride matrix. But only a small part of plasma bombarded CH2–CHCl units are grafted with (vinylbenzyl)trimethylammonium chloride (VBTAC) monomer, indicating that the grafting efficiency is the performance limiting factor in the plasma-grafted AEM synthesis. We anticipate this finding will provide guidance of plasma grafting technique for AEM preparations. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Although proton exchange membrane fuel cell (PEMFC) is a superior method to supply clean electricity at high conversion efficiency for stationary, electric vehicles and portable applications, the high cost of platinum-based catalysts is the primary obstacle to the commercialization of PEMFC technology [1]. Anion exchange membrane fuel cell (AEMFC) has attracted growing interest in the last decade due to the prospect of using non-noble metal electro-catalysts [2]. Different from traditional alkaline fuel cell, AEMFC uses solid anion exchange membranes (AEMs) instead of aqueous metal hydroxide solutions leading to an improved tolerance to CO2 and elimination of electrolyte leakage problems during fuel cell operation [3]. As one of the key components of AEMFC, AEM serves dual functions of hydroxide ion transportation and reactant separation, and as a result, seriously affects the performance of AEMFC. Great efforts have been devoted to the preparation of highperformance AEMs [4–8]. Among them, the grafting technique
* Corresponding author. Tel.: +852 3442 4626; fax: +852 3442 0688. ** Corresponding author. Tel.: +86 551 65591378; fax: +86 551 65591310. E-mail addresses: [email protected] (J. Hu), [email protected] (M.K.H. Leung). http://dx.doi.org/10.1016/j.electacta.2016.04.078 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
has attracted great attention owing to the easy preparation of various pre-formed commercial polymers and monomers to yield the desired membranes. Furthermore, the highly toxic chloromethyl ether can be avoided when the chloromethylation reaction is displaced by grafting process. Some noteworthy developments came from Varcoe et al. who prepared AEMs by radiation-grafting of vinylbenzyl chloride or vinylbenzyl methylimidazolium chloride monomers onto fluorocarbon membranes, such as fully fluorinated poly(tetrafluoroethene-cohexafluoropropylene) (FEP), partially fluorinated poly(vinylidene fluoride) (PVDF) and poly(ethylene-co-tetrafluoroethylene) (ETFE), followed by quaternization [2,9–12]. Radiation-grafting approach has been also used to fabricate AEMs based on nonfluorinated polyethylene (PE) [13]. Another important AEM system was synthesized by plasma grafting approach. The plasma grafting approach as an effective and facile way can successfully introduce functional groups into the polymer matrix and at the same time has little impact on the stability of AEMs. Recently, we have reported a series of stable AEMs prepared by the plasma grafting methods [14–16]. The success of these grafted AEMs has inspired increasing interest in the grafting technique. It is known that in the grafting process, high-energy electron bombardment leads to the formation of reactive specie in the polymer matrix which acts as the initial active sites for grafting functional groups [17]. Although
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there are some insights into how AEMs be fabricated by the grafting method, the study on grafting mechanism is limited. As one of the most important parameters, the efficiency of bombardment process is still unknown. More careful studies are still necessary to deeply understand the whole grafting process. Therefore, in this study we focus on the plasma grafting process of the plasma-grafted AEM preparation and the characterization of the plasma-grafted AEM. The goal of the present work is to prepare AEMs by plasmagrafting approach using polyvinyl chloride (PVC) powders as the substrate polymer matrix and (vinylbenzyl)trimethylammonium chloride (VBTAC) as the grafting monomer. This synthetic procedure can directly introduce ionic exchange groups (benzyltrimethylammonium groups) into the PVC matrix, leading to a simplified preparation procedure, and at the same time, avoiding the use of toxic chloromethyl ether and quaternizaiton reagents. The chemical structure, ionic conductivity and stability of the plasma-grafted AEM were characterized to evaluate its potential application in fuel cell. The efficiencies of plasma bombardment and grafting processes are also investigated to gain a better understanding of the whole plasma-grafting process. 2. Experimental section 2.1. Preparation of plasma-grafted anion exchange membranes PVC powders with a particle size of approximately 50 mm were used as the polymer substrate to prepare AEMs. There are three steps in preparation of plasma-grafted AEMs, as shown in Scheme 1: plasma bombardment of PVC substrate, grafting of VBTAC (Sigma-Aldrich1) monomer with N+(CH3)3Cl groups onto PVC matrix, and alkalizing of N+(CH3)3Cl groups into N+(CH3)3OH groups. The plasma bombardment process (step 1) was carried out in a low-pressure plasma discharge system which consisted of a glass vacuum bottle, an inductively coupled coil electrode, a radio frequency power supply with corresponding power coupling, a stirring device and a vacuum pump. The inductively coupled plasma sustained by a 13.56 MHz power supply outside the glass bottle using high purity argon (99.999% purity) as the working gas. PVC powders were placed in the plasma discharge centre with continuous magnetic stirring during this process. The experimental parameters were 100 W for the input power, 20 Pa for the total pressure and 30 min for the treatment time. In the next step (step 2), the plasma treated PVC powders were rapidly immersed in 10 wt% VBTAC aqueous solution with excess VBTAC monomer and stirred at 60 C for 48 h. The obtained PVC-gPVBTAC (PVC-TAC) powders were dissolved in
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dimethylformamide (DMF, Shanghai Chemical Reagent Store) to form 10 wt% solution, and then cast onto a flat and clean glass plate. After drying in vacuum oven for 12 h at 60 C, a polymer film was formed. In the last step (step 3), the PVC-TAC film was soaked in 1 M potassium hydroxide (KOH) aqueous solution for 48 h to alkalized the N+(CH3)3Cl groups (trimenthylammonium chloride, TAC) into N+(CH3)3OH groups (trimenthylammonium hydroxide, TAH). Then, the alkalized PVC-g-PVBTAH membrane (PVC-TAH) was washed with deionized water and soaked in deionized water with frequent water changes for at least 48 h to remove the trapped KOH. 2.2. Characterization of membranes To analyzing the effect of plasma bombardment on materials structure, PVC and plasma bombarded PVC powders were tested by Fourier transform infrared spectroscopy (FTIR). The chemical structure of PVC and PVC-TAH membrane were further analyzed by attenuated total reflection FTIR (ATR-FTIR). The FTIR spectra were recorded on a Nicolet NEXUS 870 spectrometer after 40 scan in the range of 4000400 cm1. The ATR-FTIR spectra were presented after 256 scans with spectral resolution of 2 cm1 in the range of 4000670 cm1. All the spectra were subtracted the contribution of CO2 and H2O. The chemical structure of PVC and PVC-TAC membrane were also tested by X-ray photoelectron spectroscopy (XPS) to analyse the plasma-grafting process. The XPS analysis was conducted using a Thermo ESCALAB 250 spectrometer using an Al Ka X-ray source (1486.6 eV). The pass energies were set at 70 eV and 20 eV for survey spectra and core level spectra, respectively. All of the binding energies were calibrated with respect to the C 1s band of graphitized carbon at 284.6 eV and the spectrometer energy scale calibration was checked by setting Ag 3d5/2 at 368.26 eV. Thermogravimetric analysis (TGA) measurements were carried out using a DTG-60H analyzer. Accurately weighted (about 10 mg) samples, which placed in a Pt crucible, were heated to 800 C under constant nitrogen purging at 10 ml min1 at a constant heating rate of 10 C min1. 2.3. Water uptake, swelling ratio, ion-exchange capacity and ionic conductivity measurements Water uptake (WU) was determined gravimetrically by soaking the dry weighted samples separately into deionized water for 24 h. Then the samples were taken out and weighted after wiping away the surface water. Water uptake was calculated by: WUðwt%Þ ¼
mwet mdry 100% mdry
Scheme 1. Synthesis of plasma grafted anion-exchange membrane based on PVC powder substrate and monomer containing quaternary ammonium groups.
ð1Þ
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where mdry and mwet are the weight of the wet and dry membrane samples, respectively. The average value of the three samples calculated from Equation (1) is the WU of the measured membrane. Swelling ratio was calculated from the change of film thickness by: SRð%Þ ¼
T wet T dry 100% T dry
ð2Þ
where T wet and T dry are the thicknesses of the wet and dry membrane samples, respectively. The average value of the three samples calculated from Equation (2) is the swelling ratio of the measured membrane. The ion-exchange capacities (IECs) of the plasma-grafted PVC-TAH membranes were determined by the back-titration method [18]. Three dried samples of the measured membrane (about 100 mg) were accurately weighted, and then immersed in 25 mL of 0.05 M hydrochloric acid (HCl) standard aqueous solution for 48 h. After that, the HCl solution was back-titrated by a standardized KOH solution (0.05 M) using phenolphthalein as the indicator. The IEC values of the samples could be calculated from the titration results via Equation (3). IECðmmol g1 Þ ¼
n1;HCl n2;HCl mdry
ð3Þ
Here n1;HCl and n2;HCl are the amount (mmol) of HCl required before and after equilibrium, respectively. mdry is the mass (g) of the dried membrane sample. The average value of the three samples calculated from Equation (3) is the IEC value of the measured membrane. The hydroxide conductivity of the PVC-TAH membrane was measured at room temperature in deionized water on an IM6e (Zahner, Germany) electrochemical workstation using threeelectrode AC impedance spectroscopy method with frequency ranging from 1 MHz to 0.1 Hz [14]. To measure the hydroxide conductivity, the fully hydrated PVC-TAH membrane in the OH form was assembled in two polytetrafluoroethylene plates. The hydroxide conductivity was calculated as:
s OH ðS cm1 Þ ¼
l Rm A
ð4Þ
where l and A are the distance (cm) between the working electrode and reference electrode and the cross-sectional area (cm2) of the membrane, respectively. Rm (V) is the AC impedance of the measured membrane.
3. Results 3.1. Chemical structure characterization The chemical structure of the original PVC and plasma-grafted PVC-TAH membrane were studied by ATR-FTIR. Fig. 1 shows the ATR-FTIR spectra of a typical PVC (curve (a)) with three dominant bands of C Cl stretching vibrations, trans C H wagging and C H rocking at about 844 cm1, 961 cm1 and 1254 cm1, respectively [20,21]. The ATR-FTIR spectrum of PVC-TAH membrane (curve (b) in Fig. 1) shows the similar bands to that of PVC. New signals at 828 cm1 and 1164 cm1 are assigned to C H deformation for para-substituted aromatics and ring C H in plane bending vibration, respectively, indicating the grafting of VBTAC monomer into the PVC matrix [4,22]. The ATR-FTIR spectrum of PVC-TAH membrane presents two other bands at 1380 cm1 and 1484 cm1, which are related to the CH3 groups and C H bending of trimethylammonium groups, respectively, confirming the successful grafting of benzyltrimethylammonium groups in the PVC-TAH membrane [23,24]. The broad band appearing over 3100–3600 cm1 which ascribes to OH stretching vibration of the PVC-TAH membrane indicates successful alkalization of N+(CH3)3Cl groups into N+(CH3)3OH groups [23]. The broad peak of PVC-TAH has stronger signal than that of PVC. In order to investigate the grafting process, the chemical structure of the original PVC and plasma-grafted PVC-TAC membrane were analyzed by XPS under the same condition, shown in Fig. 2. As regards to the C 1s band decomposition, the two spectra are composed of two peaks: one peak at binding energy of 284.6 eV attributed to the sp3 and sp2 carbon atoms (C C, C H and C¼C); the other peak at about 286.1 eV related to the sp3 carbon bonded to one chlorine atom (C Cl), nitrogen (CN) or oxygen atom (C O) [18,25]. It is clear that the grafting of VBTAC monomer in the PVC matrix leads to a decrease of C Cl, CN and C O moieties, and on the contrary, an increase in the C C, C H and C¼C components. This may attribute to the high percentage of sp3 and sp2 carbon atoms in the VBTAC monomer. As shown in Fig. 3, the Cl 2p spectrum of the original PVC is composed of one spin-orbit-split doublet at the binding energy of 199.9 eV corresponding to the covalently bonded chlorine species (C Cl) [26]. As for the Cl 2p spectrum of PVC-TAC membrane, a new spinorbit-split doublet emerging at around 196.8 eV is assigned to the ionic chloride (Cl) [26]. These changes confirm that plasma-
2.4. Chemical stability test The chemical stability of the obtained PVC-TAH membrane was determined by immersing the membrane samples separately in 2 M KOH aqueous solution at 60 C for 4 days and comparing the changes of IEC values at certain time intervals [19]. 2.5. Mechanical properties The mechanical properties of the plasma-grafted PVC-TAH membranes were measured at 20 C and relative humidity (RH) of 0% and 100% on a dynamic mechanical thermal analyzer (Q800) at a cross-head speed of 0.5 N min1. All of the membrane samples were 20 mm in length and 5 mm in width. The initial grip distance was 10 mm. The samples in 0% RH state were obtained in vacuum oven at 60 C for at least 12 h and the fully hydrated samples were obtained by immersing them in deionized water at 20 C for more than 12 h.
Fig. 1. ATR-FTIR of (a) original PVC and (b) PVC-TAH membrane.
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Fig. 2. XPS spectra of C 1s for (a) original PVC and (b) PVC-TAC membrane.
Fig. 3. XPS spectra of Cl 2p for (a) original PVC and (b) PVC-TAC membrane.
grafting is an effective way to graft VBTAC monomer into the PVC matrix. In the plasma-grafted PVC-TAC membrane, the formation of benzyltrimethylammonium groups is the key structural feature. Therefore, we employed XPS to ascertain the existence of the benzyltrimethylammonium groups in the PVC-TAC membrane. As revealed in Fig. 4, the XPS signal can be clearly deconvolved into two peaks. The peak with lower binding energy (at about 399.2 eV) is corresponding to the C N moieties and the peak with higher binding energy (at about 401.7 eV) is related to the quaternary nitrogen cation (N+) [15,27]. This may further confirm the introduction of functional benzyltrimethylammonium groups into the PVC backbone. According to the quantitative analysis of the N 1s spectrum in Fig. 4, the percentage of the quaternary nitrogen (N+) with respect to the total atom quantity is 1.21 at% for PVC-TAC membrane. 3.2. Plasma-grafting process analysis As mentioned above, there are two steps in preparation of plasma-grafted PVC-TAC membrane: plasma bombardment and
Fig. 4. XPS spectrum of N 1s for the PVC-TAC membrane.
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grafting. The efficiencies of plasma bombardment and grafting are the key parameters to affect the membrane performance. In the plasma bombardment process, exciting species, such as electrons, ions and neutral particles generated in plasma discharge, bombard with the polymer substrate and at the same time create active sites in the polymer matrix [14]. The amount of functional groups on the plasma-grafted membrane strongly depends on the amount of these active sites in the polymer matrix [28]. Therefore, plasma bombardment is one of the most critical steps in fabricating plasma-grafted AEMs. Most studies suggest that free radicals are the most likely reactive species in polymerization and grafting reactions under plasma conditions [26]. It has been reported in the literature that based on the electron spin resonance spectra the plasma treatment indeed promoted radicals on the PVC surface which could act as the initial active sites for grafting [29]. However, the study on the plasma bombardment mechanism is still limited. To understand the plasma bombardment process, it is crucial to investigate the effect of plasma on the chemical structure of polymer matrix. FTIR is a widely used analysis technique to provide qualitative and semi-quantitative information on the chemical state of the substance. FTIR spectra of the original PVC and plasma bombarded PVC powders can be seen in Fig. 5. The FTIR spectra for both original PVC and plasma bombarded PVC show similar shape and present typical strong C H rocking at 1254 cm1 and asymmetrical double bands of C Cl stretching in syndiotactic CH2–CHCl at about 610 cm1, demonstrating the preservation of PVC backbone after plasma treatment [30]. It is believed that free radicals are generated firstly on the dissociation of C Cl bonds in PVC backbone due to their relatively low bond dissociation energy [31]. The strength of the C Cl stretching vibration at about 840 cm1 attenuates after plasma bombardment, suggesting that CCl bonds dissociation is occurred. Chlorine radicals (Cl) existed in plasma atmosphere may react with the PVC backbone leading to the production of new C Cl bonds. Also we can see that the C¼C band at around 1625 cm1 gains strength after plasma bombardment, indicating more C¼C fraction is generated [23,32]. A possibility of C¼C formation may be the dehydrochlorination involved b-hydrogen atom abstraction by Cl from the syndiotactic CH2–CHCl unit [33]. The attenuated CH2 deformation band at 1330 cm1 and CCl stretching vibration band at 840 cm1 verify the dehydrochlorination reactions [34]. The band at 1254 cm1 for after plasma bombarded PVC, which corresponds to the overlap of
Fig. 5. FTIR of the (a) original PVC and (b) plasma bombarded PVC powders at discharge power of 100 W for 30 min.
C H rocking in PVC and CO stretching in COC group, is markedly enhanced relative to the original PVC powders, suggesting the interaction betweent free radicals and oxygen during or after plasma bombardment [34,35]. Moreover, a new band at 1030 cm1, not present in the spectrum of the original PVC, is assigned to the CO stretching in C OH confirming the generation of hydroxyl group in the plasma bombarded PVC powders [23]. The enhanced band at 1648 cm1 attributed to N H deformation suggests the existence of the NH2 groups in the plasma bombarded PVC powders [36]. The generation of oxygen and nitrogen containing groups in the plasma bombarded PVC may be attributed to the air contamination during and after plasma bombardment process. The efficiency of the plasma bombardment can be well reflected by the ratio of the plasma bombarded CH2–CHCl units to nonbombarded CH2–CHCl units. According to the FTIR analysis, the free radicals generated from C Cl dissociation in the plasma bombarded PVC matrix may act as the initial active sites for grafting, suggesting a possible chemical structure of the PVC-TAC (Fig. 6). The amount of the active sites in the plasma bombarded PVC matrix determines the amount of the quaternary ammonium groups on the plasma-grafted membrane. Based on the quantitative analysis of the XPS results, the percentages of the quaternary nitrogen (N+), carbon (C) and covalently bonded chlorine (Cl) in the PVC-TAC membrane with respected to total atoms are 1.21 at%, 72.04 at% and 19.15 at%, respectively. It can be deduced that the relationships between non-bombarded CH2–CHCl units (x) and the plasma bombarded CH2–CHCl units (y + z) are as the follows: Nþ yi 1:21 ¼ ¼ 2x þ 2z þ ð2 þ 12iÞy 72:04 C
ð5Þ
Cl x 19:15 ¼ ¼ C 2x þ 2z þ ð2 þ 12iÞy 72:04
ð6Þ
Nþ yi 1:21 ¼ ¼ x 19:15 Cl
ð7Þ
The value of yþz x is approximately equal to 1:2, which means that one in every three syndiotactic CH2–CHCl units is bombarded by plasma in the PVC matrix, indicating a great efficiency of the plasma bombardment. Although plasma bombardment mainly occurred on the material surface, the other comprehensive function of plasma including ultraviolet radiation, shockwave, pyrolysis, and so on, can generate various chemical and physical effects on the substance, resulting in a high efficiency of plasma treatment [37,38]. Furthermore, in order to understand the grafting process, the grafting efficiency, which is defined as the ratio of grafted CH2– CHCl units (y) to non-grafted CH2–CHCl units (z) of is well considered. According to Equations (4)–(6), it is easy to deduce the
Fig. 6. Possible chemical structure of the plasma-grafted PVC-TAC membrane.
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relationship between y and z. When i = 1, the value of yz approximately equals to 1:7, which means that one in every eight plasma bombarded CH2–CHCl units is grafted with VBTAC monomer. 3.3. Water uptake and swelling ratio It is well known that water uptake of AEMs has a significant effect on the ionic transport behavior through AEMs. The water uptake (26.4 wt% for PVC-TAH and 3.4 wt% for PVC) increases greatly after plasma-grafting owing to the introduction of hydrophilic quaternary ammonium groups into the polymer matrix. The water uptake and swelling ratio of the plasma-grafted PVC-TAH membrane were measured from 20 C to 60 C. As shown in Fig. 7, the water uptake of the PVC-TAH membrane increases with the temperature. The swelling ratios in different temperature were obtained in through-plane direction. From Fig. 7, the swelling ratio of the PVC-TAH membrane increase with the increasing temperature and water uptakes. The variation in swelling ratio with temperature as an important parameter can evaluate the thermal dimensional stability of the AEMs under working conditions. The variation in swelling ratio from is only 3.5%, indicating an excellent thermal dimensional stability for the plasma-grafted PVC-TAH membrane [39]. 3.4. IEC and ionic conductivity The ion-exchange capacity, hydroxide conductivity and stability are crucial performance parameters for AEMs. IEC value for AEMs is a common index for appraising the degree of ionic functionalization of the polymer. The IEC value for PVC-TAH membrane is 0.86 mmol g1 versus to original PVC material (IEC = 0) due to the presence of quaternary ammonium groups. The quaternary ammonium groups on the polymer backbone also increase the hydrophilicity of the PVC-TAH membrane relative to the unfunctionalized PVC, leading to the increase of water uptake for PVC-TAH membrane. The improved IEC and water uptake of the synthesized PVC-TAH membrane indicate more water molecules in the OH transportation, leading to the enhancement of the ionic conductivity [22]. The hydroxide conductivity of PVC-TAH membrane is 4.6 mS cm1 at 20 C under the membrane thickness of 83 mm. By comparison, Chen’s PEK-based HEMs exhibited a conductivity of 0.8 mS cm1 at 20 C in deionized water, Wang’s poly(ether-imide) based AEMs showed conductivity from 2.28 mS cm1 to 3.51 mS cm1 at room temperature, Wu’s PEO-SiO2 hybrid AEMs possessed
Fig. 7. The water uptake and swelling ratio of the plasma-grafted PVC-TAH membrane as a functional of temperature.
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conductivity about 3 mS cm1 at room temperature, and Xiong et al. observed a hydroxide conductivity of 1.6 mS at 20 C in deionized water for their chloromethyl PEK-C-based AEMs [40– 43]. The hydroxide conductivity of the plasma-grafted PVC-TAH membrane is much higher than that of commercial AEMs, which are 2.5 mS cm1 and 1.5 mS cm1 at 20 C for commercial ACS membrane (Tokuyama Soda Co.) and AW membrane (Solvay), respectively, indicating that the hydroxide conductivity of plasmagrafted PVC-TAH membrane is acceptable [44]. Fig. 8 shows the hydroxide conductivities of the plasma-grafted PVC-TAH membrane and the commercial AHA membrane (in OH form, NEOSEPTA1) as a function of temperature. As seen in Fig. 8, the hydroxide conductivities for both PVC-TAH and AHA membranes increase with temperature, indicating that the membrane conductivity followed the Arrhenius law. The hydroxide transfer barrier through the membranes as reflected by activation energy (Ea) can be calculated using the Arrhenius equation. From the slopes of the fitted lines in Fig. 8, the plasma-grafted PVC-TAH membrane shows lower activation energy than commercial AHA membrane. For comparison, some important properties of the studied PVC-TAH membrane and the commercial AHA membrane, including the thickness, IEC value, conductivity and activation energy were tested under the same conditions and then listed in Table 1. The plasma-grafted PVC-TAH membrane possesses smaller thickness and activation energy, higher IEC and ionic conductivity than the commercial AHA membrane, indicating a great potential for application in AEMFCs. 3.5. Chemical stability and thermal stability As one of the most important properties, chemical stability of AEMs, especially alkaline stability under high pH environment and elevated temperatures is the major challenge for fuel cell applications due to the attachment of the cationic functional groups. The quaternary ammonium cations, as the conductive functional groups in AEMs, are likely to degrade mainly via three degradation pathways: Hofmann elimination (E2), ylide formation and nicleophilic substitution (SN2) under the attack of OH [45–47]. In the present work, the chemical stability of the PVC-TAH membrane was evaluated in 2 M KOH solution at elevated temperature for 96 h. The IECs of the PVC-TAH membrane at certain time intervals were examined to see the changes of the quaternary ammonium groups under such rigorous condition. As
Fig. 8. Arrhenius plot of hydroxide conductivity as a function of temperature for the plasma-grafted PVC-TAH membrane and commercial AHA membrane.
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Table 1 Thickness, IEC value, conductivity and activation energy of the plasma-grafted PVC-TAH membrane and the commercial AHA membrane. Membrane
Thickness (mm)
IEC (mmol g1)
Conductivity (mS cm1)
Ea (kJ mol1)
PVC-TAH AHA
83 210
0.86 0.47
4.6 3.5
16.03 17.56
Fig. 9. Chemical stability tests for PVC-TAH membrane in 2 M KOH solution at 60 C.
shown in Fig. 9, the IEC of the PVC-TAH membrane slightly decreases with the immersion time and only drops by 12.8% in 96 hours. The decrease of the ionic exchange groups may be attributed to the attack of the benzyltrimethylammonium groups by OH via the nucleophilic substitution reaction (SN2 pathway) rather than Hofmann elimination pathway due to the absence of b-hydrogen atoms [48]. After immersion in 2 M KOH solution for 4 days, the PVC-TAH membrane shows no observable changes in shape and morphology. Even after immersion in 4 M KOH solution for 2 day, the residual IEC value of PVC-TAH membrane (0.76 mmol g1) is still up to 88% of the initial value. The high preservation of quaternary ammonium groups and main-chain structure under the rigid basic environment indicates a good alkaline stability of the PVC-TAH membrane. The thermal stability of the AEMs is always a concern for fuel cells because of the enhanced reaction kinetics and reduced voltage losses in fuel cells when operating at high temperature. The
thermal stability of the plasma-grafted PVC-TAH membrane was analysed by recording its TGA curves from room temperature to 800 C under flowing N2. For comparison, the original PVC powders were also measured. According to the TGA and its differential (DrTGA) profiles shown in Fig. 10, the PVC-TAH membrane mainly decomposed in three steps. The first step is the loss of the benzyltrimethylammonium groups started from the temperature of 120 C which means a steady operation of PVC-TAH membrane below 100 C, indicating a good thermal stability of the PVC-TAH membrane. The second and third decomposition steps of PVC-TAH membrane are the dehydrochlorination of main-chain in the range of 217 to 380 C, and the thermal cracking of carbonaceous conjugated polyene sequences in the range of 380 to 537 C, respectively [49]. These two steps can also be found in the degradation of the original PVC, confirming the decomposition of the main-chain of the PVC matrix. It is worth to notice that the degradation of the benzyltrimethylammonium groups includes two phases according to the DrTGA curve of the PVC-TAH membrane. Since Hofmann elimination is avoided owing to the lack of b-hydrogen, nucleophilic substitution reaction (SN2 pathway) is the main degradation pathway for benzyltrimethylammonium groups [48]. According to the reaction of SN2 pathway, methanol is generated at the very start, then the rest parts degrade [50]. As shown in Fig. 10a, the weight loss for methanol generation in the range of 120 to 188 C is about 3.0 wt%, which is close to the value (2.8 wt%) calculated from IEC. 3.6. Mechanical properties The mechanical properties of the plasma-grafted PVC-TAH membrane with different relative humidity (RH = 0% and RH = 100%) were investigated by tensile traction tests at about 20 C. As listed in Table 2, the tensile strength, Young modulus and the breaking elongation for the PVC-TAH membrane at 0% RH were 17.4 MPa, 917.4 MPa and 3.2%, respectively. In comparison, the tensile strength, Young modulus and breaking elongation for fully hydrated (100% RH) PVC-TAH membrane were 18.6 MPa, 787.6 MPa and 3.8%, respectively. The slightly inferior mechanical properties
Fig. 10. (a) TGA and (b) DrTGA of the PVC and PVC-TAH membrane.
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Table 2 Mechanical properties of PVC-TAH membranes with relative humidity (RH) of 0% and 100%. Membrane
Thickness (mm)
Tensile strength (MPa)
Young modulus (MPa)
Breaking elongation (%)
PVC-TAH 0% RH PVC-TAH 100% RH
77 83
17.4 18.6
917.4 787.6
3.2 3.8
of the fully hydrated PVC-TAH membrane may be attributed to the combination of the hydrophilic anion exchange groups in PVC-TAH membrane matrix with water molecules, leading to the membrane swelling (7.8% according to the change of thickness) and a less compact membrane structure. Compared to the literature values, the mechanical properties of the PVC-TAH membrane are acceptable, indicating that the plasma-grafted PVC-TAH membrane is strong enough to be AEMs [12,51,52]. 4. Discussion As mentioned above, the amount of active sites in the plasma bombarded polymer matrix determines the amount of functional groups on the plasma-grafted membrane, and as a result, the conductivity of the plasma-grafted AEM. The generation of those active sites strongly depends on the plasma bombardment. The plasma bombardment efficiency in the present study is as high as 50%, which means one in every three syndiotactic CH2–CHCl units is bombarded by plasma in the PVC matrix. However, only a small part of plasma bombarded CH2–CHCl units are grafted with VBTAC monomer, indicating that the grafting efficiency is the performance limiting factor in the plasma-grafted AEM preparation. The low grafting efficiency may be attributed to: (1) the elimination of the free radicals by dehydrochlorination and air contamination during and after plasma bombardment process; (2) the extinction of the shot-lived free radicals in the plasma bombarded PVC matrix; (3) low monomer concentration in the grafting liquid leading to a low contact probability between free radicals and monomer; and so on. Therefore, in order to improve the grafting efficiency, one important way is to adjust the plasma condition to decrease the free radicals elimination. As shown in Fig. 11, there is less C¼C, CO and CN moieties generated in plasma bombarded PVC at plasma discharge power of 50 W. Moreover, optimization of the grafting operation is also necessary. Further researches are in progress.
Fig. 11. FTIR of plasma bombarded PVC powders at the discharge power of (a) 50 W and (b) 100 W for 30 min.
5. Conclusions In summary, plasma-grafted PVC-TAH AEM was prepared by directly introducing benzyltrimethylammonium groups into the PVC matrix. ATR-FTIR and XPS analyses confirm the successful introduction of benzyltrimethylammonium groups into the PVC matrix. The produced AEM shows the satisfactory chemical stability in strong basic solution at 60 C, thermal stability above 120 C, excellent thermal dimensional stability and acceptable anion conductivity, indicating that the plasma-grafted AEM fulfills the requirements for fuel cell applications. The plasma-grafting process analysis indicates that the plasma treatment efficiency is as high as 50%, but the grafting efficiency is quite low. It reveals that the grafting efficiency is the performance limiting factor in plasmagrafted AEM synthesis. Further research focusing on increasing the grafting efficiency is required for enhancing the overall performance of plasma-grafted AEMs. Acknowledgements This research was financially supported by the National Nature Science Foundation of Anhui Province (No. 1508085QA10), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. 2015265), China Postdoctoral Science Foundation Funded Project (No. 2015M582021), the Hong Kong Scholars Program, and the Hong Kong General Research Fund (GRF Project 9042044) and CityU Strategic Research Grant (Project 7004476). References [1] R. Borup, J. Meyers, B. Pivovar, Y.S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J.E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K.C. Kimjima, N. Iwashita, Scientific aspects of polymer electrolyte fuel cell durability and degradation, Chem. Rev. 107 (2007) 3904–3951. [2] O.I. Deavin, S. Murphy, A.L. Ong, S.D. Poynton, R. Zeng, H. Herman, J.R. Varcoe, Anion-exchange membranes for alkaline polymer electrolyte fuel cells: comparison of pendent benzyltrimethylammonium- and benzylmethylimidazolium-head-groups, Energy Environ. Sci. 5 (2012) 8584–8597. [3] L.A. Adams, S.D. Poynton, C. Tamain, R.C.T. Slade, J.R. Varcoe, A carbon dioxide tolerant aqueous-electrolyte-free anion-exchange membrane alkaline fuel cell, Chemsuschem 1 (2008) 79–81. [4] C. Zhang, J. Hu, Y. Meng, M. Nagatsu, H. Toyoda, Preparation of highperformance hydroxide exchange membrane by a novel ablation restriction plasma polymerization approach, Chem. Commun. 47 (2011) 10230–10232. [5] J. Pan, C. Chen, Y. Li, L. Wang, L.S. Tan, G.W. Li, X. Tang, L. Xiao, J.T. Lu, L. Zhuang, Constructing ionic highway in alkaline polymer electrolytes, Energy Environ. Sci. 7 (2014) 354–360. [6] S. Gu, R. Cai, T. Luo, Z.W. Chen, M.W. Sun, Y. Liu, G.H. He, Y.S. Yan, A soluble and highly conductive Ionomer for high-performance hydroxide exchange membrane fuel cells, Angew. Chem. Int. Ed. 48 (2009) 6499–6502. [7] Q. Zhang, S. Li, S. Zhang, A novel guanidinium grafted poly(aryl ether sulfone) for high-performance hydroxide exchange membranes, Chem. Commun. 46 (2010) 7495–7497. [8] Z. Jiang, Z.J. Jiang, Plasma techniques for the fabrication of polymer electrolyte membranes for fuel cells, J. Membr. Sci. 456 (2014) 85–106. [9] T.N. Danks, R.C.T. Slade, J.R. Varcoe, Alkaline anion-exchange radiation-grafted membranes for possible electrochemical application in fuel cells, J. Mater. Chem. 13 (2003) 712–721. [10] H. Herman, R.C.T. Slade, J.R. Varcoe, The radiation-grafting of vinylbenzyl chloride onto poly(hexafluoropropylene-co-tetrafluoroethylene) films with subsequent conversion to alkaline anion-exchange membranes: optimisation of the experimental conditions and characterisation, J. Membr. Sci. 218 (2003) 147–163.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Plasma-grafted anion-exchange membrane preparation and process analysis Chengxu Zhanga,b , Jue Hub,** , Wenguang Fana , Michael K.H. Leunga,* , Yuedong Mengb a b
Ability R&D Energy Research Centre, School of Energy and Environment, City University of Hong Kong, China Institute of Plasma Physics, Chinese Academy of Sciences, China
A R T I C L E I N F O
Article history: Received 16 January 2016 Received in revised form 22 March 2016 Accepted 16 April 2016 Available online 19 April 2016 Keywords: Anion exchange membrane Plasma-grafting Vinylbenzyl trimethylammonium chloride Grafting efficiency Plasma bombardment efficiency
A B S T R A C T
A green and simplified plasma grafting approach was adopted for anion exchange membranes (AEMs) synthesis based on polyvinyl chloride powders in three steps: plasma bombardment, grafting and alkalization, avoiding the use of toxic chloromethyl ether and quaternization reagents. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses demonstrate the successful grafting of benzyltrimethylammonium groups into the polyvinyl chloride matrix. The plasmagrafted AEM exhibits satisfactory thermal stability, chemical stability and ionic conductivity, suggesting potential application in fuel cell. The plasma bombardment efficiency reaches up to 50% based on the FTIR and XPS results, meaning that one in every three syndiotactic CH2–CHCl units is bombarded in the polyvinyl chloride matrix. But only a small part of plasma bombarded CH2–CHCl units are grafted with (vinylbenzyl)trimethylammonium chloride (VBTAC) monomer, indicating that the grafting efficiency is the performance limiting factor in the plasma-grafted AEM synthesis. We anticipate this finding will provide guidance of plasma grafting technique for AEM preparations. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Although proton exchange membrane fuel cell (PEMFC) is a superior method to supply clean electricity at high conversion efficiency for stationary, electric vehicles and portable applications, the high cost of platinum-based catalysts is the primary obstacle to the commercialization of PEMFC technology [1]. Anion exchange membrane fuel cell (AEMFC) has attracted growing interest in the last decade due to the prospect of using non-noble metal electro-catalysts [2]. Different from traditional alkaline fuel cell, AEMFC uses solid anion exchange membranes (AEMs) instead of aqueous metal hydroxide solutions leading to an improved tolerance to CO2 and elimination of electrolyte leakage problems during fuel cell operation [3]. As one of the key components of AEMFC, AEM serves dual functions of hydroxide ion transportation and reactant separation, and as a result, seriously affects the performance of AEMFC. Great efforts have been devoted to the preparation of highperformance AEMs [4–8]. Among them, the grafting technique
* Corresponding author. Tel.: +852 3442 4626; fax: +852 3442 0688. ** Corresponding author. Tel.: +86 551 65591378; fax: +86 551 65591310. E-mail addresses: [email protected] (J. Hu), [email protected] (M.K.H. Leung). http://dx.doi.org/10.1016/j.electacta.2016.04.078 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
has attracted great attention owing to the easy preparation of various pre-formed commercial polymers and monomers to yield the desired membranes. Furthermore, the highly toxic chloromethyl ether can be avoided when the chloromethylation reaction is displaced by grafting process. Some noteworthy developments came from Varcoe et al. who prepared AEMs by radiation-grafting of vinylbenzyl chloride or vinylbenzyl methylimidazolium chloride monomers onto fluorocarbon membranes, such as fully fluorinated poly(tetrafluoroethene-cohexafluoropropylene) (FEP), partially fluorinated poly(vinylidene fluoride) (PVDF) and poly(ethylene-co-tetrafluoroethylene) (ETFE), followed by quaternization [2,9–12]. Radiation-grafting approach has been also used to fabricate AEMs based on nonfluorinated polyethylene (PE) [13]. Another important AEM system was synthesized by plasma grafting approach. The plasma grafting approach as an effective and facile way can successfully introduce functional groups into the polymer matrix and at the same time has little impact on the stability of AEMs. Recently, we have reported a series of stable AEMs prepared by the plasma grafting methods [14–16]. The success of these grafted AEMs has inspired increasing interest in the grafting technique. It is known that in the grafting process, high-energy electron bombardment leads to the formation of reactive specie in the polymer matrix which acts as the initial active sites for grafting functional groups [17]. Although
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there are some insights into how AEMs be fabricated by the grafting method, the study on grafting mechanism is limited. As one of the most important parameters, the efficiency of bombardment process is still unknown. More careful studies are still necessary to deeply understand the whole grafting process. Therefore, in this study we focus on the plasma grafting process of the plasma-grafted AEM preparation and the characterization of the plasma-grafted AEM. The goal of the present work is to prepare AEMs by plasmagrafting approach using polyvinyl chloride (PVC) powders as the substrate polymer matrix and (vinylbenzyl)trimethylammonium chloride (VBTAC) as the grafting monomer. This synthetic procedure can directly introduce ionic exchange groups (benzyltrimethylammonium groups) into the PVC matrix, leading to a simplified preparation procedure, and at the same time, avoiding the use of toxic chloromethyl ether and quaternizaiton reagents. The chemical structure, ionic conductivity and stability of the plasma-grafted AEM were characterized to evaluate its potential application in fuel cell. The efficiencies of plasma bombardment and grafting processes are also investigated to gain a better understanding of the whole plasma-grafting process. 2. Experimental section 2.1. Preparation of plasma-grafted anion exchange membranes PVC powders with a particle size of approximately 50 mm were used as the polymer substrate to prepare AEMs. There are three steps in preparation of plasma-grafted AEMs, as shown in Scheme 1: plasma bombardment of PVC substrate, grafting of VBTAC (Sigma-Aldrich1) monomer with N+(CH3)3Cl groups onto PVC matrix, and alkalizing of N+(CH3)3Cl groups into N+(CH3)3OH groups. The plasma bombardment process (step 1) was carried out in a low-pressure plasma discharge system which consisted of a glass vacuum bottle, an inductively coupled coil electrode, a radio frequency power supply with corresponding power coupling, a stirring device and a vacuum pump. The inductively coupled plasma sustained by a 13.56 MHz power supply outside the glass bottle using high purity argon (99.999% purity) as the working gas. PVC powders were placed in the plasma discharge centre with continuous magnetic stirring during this process. The experimental parameters were 100 W for the input power, 20 Pa for the total pressure and 30 min for the treatment time. In the next step (step 2), the plasma treated PVC powders were rapidly immersed in 10 wt% VBTAC aqueous solution with excess VBTAC monomer and stirred at 60 C for 48 h. The obtained PVC-gPVBTAC (PVC-TAC) powders were dissolved in
219
dimethylformamide (DMF, Shanghai Chemical Reagent Store) to form 10 wt% solution, and then cast onto a flat and clean glass plate. After drying in vacuum oven for 12 h at 60 C, a polymer film was formed. In the last step (step 3), the PVC-TAC film was soaked in 1 M potassium hydroxide (KOH) aqueous solution for 48 h to alkalized the N+(CH3)3Cl groups (trimenthylammonium chloride, TAC) into N+(CH3)3OH groups (trimenthylammonium hydroxide, TAH). Then, the alkalized PVC-g-PVBTAH membrane (PVC-TAH) was washed with deionized water and soaked in deionized water with frequent water changes for at least 48 h to remove the trapped KOH. 2.2. Characterization of membranes To analyzing the effect of plasma bombardment on materials structure, PVC and plasma bombarded PVC powders were tested by Fourier transform infrared spectroscopy (FTIR). The chemical structure of PVC and PVC-TAH membrane were further analyzed by attenuated total reflection FTIR (ATR-FTIR). The FTIR spectra were recorded on a Nicolet NEXUS 870 spectrometer after 40 scan in the range of 4000400 cm1. The ATR-FTIR spectra were presented after 256 scans with spectral resolution of 2 cm1 in the range of 4000670 cm1. All the spectra were subtracted the contribution of CO2 and H2O. The chemical structure of PVC and PVC-TAC membrane were also tested by X-ray photoelectron spectroscopy (XPS) to analyse the plasma-grafting process. The XPS analysis was conducted using a Thermo ESCALAB 250 spectrometer using an Al Ka X-ray source (1486.6 eV). The pass energies were set at 70 eV and 20 eV for survey spectra and core level spectra, respectively. All of the binding energies were calibrated with respect to the C 1s band of graphitized carbon at 284.6 eV and the spectrometer energy scale calibration was checked by setting Ag 3d5/2 at 368.26 eV. Thermogravimetric analysis (TGA) measurements were carried out using a DTG-60H analyzer. Accurately weighted (about 10 mg) samples, which placed in a Pt crucible, were heated to 800 C under constant nitrogen purging at 10 ml min1 at a constant heating rate of 10 C min1. 2.3. Water uptake, swelling ratio, ion-exchange capacity and ionic conductivity measurements Water uptake (WU) was determined gravimetrically by soaking the dry weighted samples separately into deionized water for 24 h. Then the samples were taken out and weighted after wiping away the surface water. Water uptake was calculated by: WUðwt%Þ ¼
mwet mdry 100% mdry
Scheme 1. Synthesis of plasma grafted anion-exchange membrane based on PVC powder substrate and monomer containing quaternary ammonium groups.
ð1Þ
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where mdry and mwet are the weight of the wet and dry membrane samples, respectively. The average value of the three samples calculated from Equation (1) is the WU of the measured membrane. Swelling ratio was calculated from the change of film thickness by: SRð%Þ ¼
T wet T dry 100% T dry
ð2Þ
where T wet and T dry are the thicknesses of the wet and dry membrane samples, respectively. The average value of the three samples calculated from Equation (2) is the swelling ratio of the measured membrane. The ion-exchange capacities (IECs) of the plasma-grafted PVC-TAH membranes were determined by the back-titration method [18]. Three dried samples of the measured membrane (about 100 mg) were accurately weighted, and then immersed in 25 mL of 0.05 M hydrochloric acid (HCl) standard aqueous solution for 48 h. After that, the HCl solution was back-titrated by a standardized KOH solution (0.05 M) using phenolphthalein as the indicator. The IEC values of the samples could be calculated from the titration results via Equation (3). IECðmmol g1 Þ ¼
n1;HCl n2;HCl mdry
ð3Þ
Here n1;HCl and n2;HCl are the amount (mmol) of HCl required before and after equilibrium, respectively. mdry is the mass (g) of the dried membrane sample. The average value of the three samples calculated from Equation (3) is the IEC value of the measured membrane. The hydroxide conductivity of the PVC-TAH membrane was measured at room temperature in deionized water on an IM6e (Zahner, Germany) electrochemical workstation using threeelectrode AC impedance spectroscopy method with frequency ranging from 1 MHz to 0.1 Hz [14]. To measure the hydroxide conductivity, the fully hydrated PVC-TAH membrane in the OH form was assembled in two polytetrafluoroethylene plates. The hydroxide conductivity was calculated as:
s OH ðS cm1 Þ ¼
l Rm A
ð4Þ
where l and A are the distance (cm) between the working electrode and reference electrode and the cross-sectional area (cm2) of the membrane, respectively. Rm (V) is the AC impedance of the measured membrane.
3. Results 3.1. Chemical structure characterization The chemical structure of the original PVC and plasma-grafted PVC-TAH membrane were studied by ATR-FTIR. Fig. 1 shows the ATR-FTIR spectra of a typical PVC (curve (a)) with three dominant bands of C Cl stretching vibrations, trans C H wagging and C H rocking at about 844 cm1, 961 cm1 and 1254 cm1, respectively [20,21]. The ATR-FTIR spectrum of PVC-TAH membrane (curve (b) in Fig. 1) shows the similar bands to that of PVC. New signals at 828 cm1 and 1164 cm1 are assigned to C H deformation for para-substituted aromatics and ring C H in plane bending vibration, respectively, indicating the grafting of VBTAC monomer into the PVC matrix [4,22]. The ATR-FTIR spectrum of PVC-TAH membrane presents two other bands at 1380 cm1 and 1484 cm1, which are related to the CH3 groups and C H bending of trimethylammonium groups, respectively, confirming the successful grafting of benzyltrimethylammonium groups in the PVC-TAH membrane [23,24]. The broad band appearing over 3100–3600 cm1 which ascribes to OH stretching vibration of the PVC-TAH membrane indicates successful alkalization of N+(CH3)3Cl groups into N+(CH3)3OH groups [23]. The broad peak of PVC-TAH has stronger signal than that of PVC. In order to investigate the grafting process, the chemical structure of the original PVC and plasma-grafted PVC-TAC membrane were analyzed by XPS under the same condition, shown in Fig. 2. As regards to the C 1s band decomposition, the two spectra are composed of two peaks: one peak at binding energy of 284.6 eV attributed to the sp3 and sp2 carbon atoms (C C, C H and C¼C); the other peak at about 286.1 eV related to the sp3 carbon bonded to one chlorine atom (C Cl), nitrogen (CN) or oxygen atom (C O) [18,25]. It is clear that the grafting of VBTAC monomer in the PVC matrix leads to a decrease of C Cl, CN and C O moieties, and on the contrary, an increase in the C C, C H and C¼C components. This may attribute to the high percentage of sp3 and sp2 carbon atoms in the VBTAC monomer. As shown in Fig. 3, the Cl 2p spectrum of the original PVC is composed of one spin-orbit-split doublet at the binding energy of 199.9 eV corresponding to the covalently bonded chlorine species (C Cl) [26]. As for the Cl 2p spectrum of PVC-TAC membrane, a new spinorbit-split doublet emerging at around 196.8 eV is assigned to the ionic chloride (Cl) [26]. These changes confirm that plasma-
2.4. Chemical stability test The chemical stability of the obtained PVC-TAH membrane was determined by immersing the membrane samples separately in 2 M KOH aqueous solution at 60 C for 4 days and comparing the changes of IEC values at certain time intervals [19]. 2.5. Mechanical properties The mechanical properties of the plasma-grafted PVC-TAH membranes were measured at 20 C and relative humidity (RH) of 0% and 100% on a dynamic mechanical thermal analyzer (Q800) at a cross-head speed of 0.5 N min1. All of the membrane samples were 20 mm in length and 5 mm in width. The initial grip distance was 10 mm. The samples in 0% RH state were obtained in vacuum oven at 60 C for at least 12 h and the fully hydrated samples were obtained by immersing them in deionized water at 20 C for more than 12 h.
Fig. 1. ATR-FTIR of (a) original PVC and (b) PVC-TAH membrane.
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Fig. 2. XPS spectra of C 1s for (a) original PVC and (b) PVC-TAC membrane.
Fig. 3. XPS spectra of Cl 2p for (a) original PVC and (b) PVC-TAC membrane.
grafting is an effective way to graft VBTAC monomer into the PVC matrix. In the plasma-grafted PVC-TAC membrane, the formation of benzyltrimethylammonium groups is the key structural feature. Therefore, we employed XPS to ascertain the existence of the benzyltrimethylammonium groups in the PVC-TAC membrane. As revealed in Fig. 4, the XPS signal can be clearly deconvolved into two peaks. The peak with lower binding energy (at about 399.2 eV) is corresponding to the C N moieties and the peak with higher binding energy (at about 401.7 eV) is related to the quaternary nitrogen cation (N+) [15,27]. This may further confirm the introduction of functional benzyltrimethylammonium groups into the PVC backbone. According to the quantitative analysis of the N 1s spectrum in Fig. 4, the percentage of the quaternary nitrogen (N+) with respect to the total atom quantity is 1.21 at% for PVC-TAC membrane. 3.2. Plasma-grafting process analysis As mentioned above, there are two steps in preparation of plasma-grafted PVC-TAC membrane: plasma bombardment and
Fig. 4. XPS spectrum of N 1s for the PVC-TAC membrane.
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grafting. The efficiencies of plasma bombardment and grafting are the key parameters to affect the membrane performance. In the plasma bombardment process, exciting species, such as electrons, ions and neutral particles generated in plasma discharge, bombard with the polymer substrate and at the same time create active sites in the polymer matrix [14]. The amount of functional groups on the plasma-grafted membrane strongly depends on the amount of these active sites in the polymer matrix [28]. Therefore, plasma bombardment is one of the most critical steps in fabricating plasma-grafted AEMs. Most studies suggest that free radicals are the most likely reactive species in polymerization and grafting reactions under plasma conditions [26]. It has been reported in the literature that based on the electron spin resonance spectra the plasma treatment indeed promoted radicals on the PVC surface which could act as the initial active sites for grafting [29]. However, the study on the plasma bombardment mechanism is still limited. To understand the plasma bombardment process, it is crucial to investigate the effect of plasma on the chemical structure of polymer matrix. FTIR is a widely used analysis technique to provide qualitative and semi-quantitative information on the chemical state of the substance. FTIR spectra of the original PVC and plasma bombarded PVC powders can be seen in Fig. 5. The FTIR spectra for both original PVC and plasma bombarded PVC show similar shape and present typical strong C H rocking at 1254 cm1 and asymmetrical double bands of C Cl stretching in syndiotactic CH2–CHCl at about 610 cm1, demonstrating the preservation of PVC backbone after plasma treatment [30]. It is believed that free radicals are generated firstly on the dissociation of C Cl bonds in PVC backbone due to their relatively low bond dissociation energy [31]. The strength of the C Cl stretching vibration at about 840 cm1 attenuates after plasma bombardment, suggesting that CCl bonds dissociation is occurred. Chlorine radicals (Cl) existed in plasma atmosphere may react with the PVC backbone leading to the production of new C Cl bonds. Also we can see that the C¼C band at around 1625 cm1 gains strength after plasma bombardment, indicating more C¼C fraction is generated [23,32]. A possibility of C¼C formation may be the dehydrochlorination involved b-hydrogen atom abstraction by Cl from the syndiotactic CH2–CHCl unit [33]. The attenuated CH2 deformation band at 1330 cm1 and CCl stretching vibration band at 840 cm1 verify the dehydrochlorination reactions [34]. The band at 1254 cm1 for after plasma bombarded PVC, which corresponds to the overlap of
Fig. 5. FTIR of the (a) original PVC and (b) plasma bombarded PVC powders at discharge power of 100 W for 30 min.
C H rocking in PVC and CO stretching in COC group, is markedly enhanced relative to the original PVC powders, suggesting the interaction betweent free radicals and oxygen during or after plasma bombardment [34,35]. Moreover, a new band at 1030 cm1, not present in the spectrum of the original PVC, is assigned to the CO stretching in C OH confirming the generation of hydroxyl group in the plasma bombarded PVC powders [23]. The enhanced band at 1648 cm1 attributed to N H deformation suggests the existence of the NH2 groups in the plasma bombarded PVC powders [36]. The generation of oxygen and nitrogen containing groups in the plasma bombarded PVC may be attributed to the air contamination during and after plasma bombardment process. The efficiency of the plasma bombardment can be well reflected by the ratio of the plasma bombarded CH2–CHCl units to nonbombarded CH2–CHCl units. According to the FTIR analysis, the free radicals generated from C Cl dissociation in the plasma bombarded PVC matrix may act as the initial active sites for grafting, suggesting a possible chemical structure of the PVC-TAC (Fig. 6). The amount of the active sites in the plasma bombarded PVC matrix determines the amount of the quaternary ammonium groups on the plasma-grafted membrane. Based on the quantitative analysis of the XPS results, the percentages of the quaternary nitrogen (N+), carbon (C) and covalently bonded chlorine (Cl) in the PVC-TAC membrane with respected to total atoms are 1.21 at%, 72.04 at% and 19.15 at%, respectively. It can be deduced that the relationships between non-bombarded CH2–CHCl units (x) and the plasma bombarded CH2–CHCl units (y + z) are as the follows: Nþ yi 1:21 ¼ ¼ 2x þ 2z þ ð2 þ 12iÞy 72:04 C
ð5Þ
Cl x 19:15 ¼ ¼ C 2x þ 2z þ ð2 þ 12iÞy 72:04
ð6Þ
Nþ yi 1:21 ¼ ¼ x 19:15 Cl
ð7Þ
The value of yþz x is approximately equal to 1:2, which means that one in every three syndiotactic CH2–CHCl units is bombarded by plasma in the PVC matrix, indicating a great efficiency of the plasma bombardment. Although plasma bombardment mainly occurred on the material surface, the other comprehensive function of plasma including ultraviolet radiation, shockwave, pyrolysis, and so on, can generate various chemical and physical effects on the substance, resulting in a high efficiency of plasma treatment [37,38]. Furthermore, in order to understand the grafting process, the grafting efficiency, which is defined as the ratio of grafted CH2– CHCl units (y) to non-grafted CH2–CHCl units (z) of is well considered. According to Equations (4)–(6), it is easy to deduce the
Fig. 6. Possible chemical structure of the plasma-grafted PVC-TAC membrane.
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relationship between y and z. When i = 1, the value of yz approximately equals to 1:7, which means that one in every eight plasma bombarded CH2–CHCl units is grafted with VBTAC monomer. 3.3. Water uptake and swelling ratio It is well known that water uptake of AEMs has a significant effect on the ionic transport behavior through AEMs. The water uptake (26.4 wt% for PVC-TAH and 3.4 wt% for PVC) increases greatly after plasma-grafting owing to the introduction of hydrophilic quaternary ammonium groups into the polymer matrix. The water uptake and swelling ratio of the plasma-grafted PVC-TAH membrane were measured from 20 C to 60 C. As shown in Fig. 7, the water uptake of the PVC-TAH membrane increases with the temperature. The swelling ratios in different temperature were obtained in through-plane direction. From Fig. 7, the swelling ratio of the PVC-TAH membrane increase with the increasing temperature and water uptakes. The variation in swelling ratio with temperature as an important parameter can evaluate the thermal dimensional stability of the AEMs under working conditions. The variation in swelling ratio from is only 3.5%, indicating an excellent thermal dimensional stability for the plasma-grafted PVC-TAH membrane [39]. 3.4. IEC and ionic conductivity The ion-exchange capacity, hydroxide conductivity and stability are crucial performance parameters for AEMs. IEC value for AEMs is a common index for appraising the degree of ionic functionalization of the polymer. The IEC value for PVC-TAH membrane is 0.86 mmol g1 versus to original PVC material (IEC = 0) due to the presence of quaternary ammonium groups. The quaternary ammonium groups on the polymer backbone also increase the hydrophilicity of the PVC-TAH membrane relative to the unfunctionalized PVC, leading to the increase of water uptake for PVC-TAH membrane. The improved IEC and water uptake of the synthesized PVC-TAH membrane indicate more water molecules in the OH transportation, leading to the enhancement of the ionic conductivity [22]. The hydroxide conductivity of PVC-TAH membrane is 4.6 mS cm1 at 20 C under the membrane thickness of 83 mm. By comparison, Chen’s PEK-based HEMs exhibited a conductivity of 0.8 mS cm1 at 20 C in deionized water, Wang’s poly(ether-imide) based AEMs showed conductivity from 2.28 mS cm1 to 3.51 mS cm1 at room temperature, Wu’s PEO-SiO2 hybrid AEMs possessed
Fig. 7. The water uptake and swelling ratio of the plasma-grafted PVC-TAH membrane as a functional of temperature.
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conductivity about 3 mS cm1 at room temperature, and Xiong et al. observed a hydroxide conductivity of 1.6 mS at 20 C in deionized water for their chloromethyl PEK-C-based AEMs [40– 43]. The hydroxide conductivity of the plasma-grafted PVC-TAH membrane is much higher than that of commercial AEMs, which are 2.5 mS cm1 and 1.5 mS cm1 at 20 C for commercial ACS membrane (Tokuyama Soda Co.) and AW membrane (Solvay), respectively, indicating that the hydroxide conductivity of plasmagrafted PVC-TAH membrane is acceptable [44]. Fig. 8 shows the hydroxide conductivities of the plasma-grafted PVC-TAH membrane and the commercial AHA membrane (in OH form, NEOSEPTA1) as a function of temperature. As seen in Fig. 8, the hydroxide conductivities for both PVC-TAH and AHA membranes increase with temperature, indicating that the membrane conductivity followed the Arrhenius law. The hydroxide transfer barrier through the membranes as reflected by activation energy (Ea) can be calculated using the Arrhenius equation. From the slopes of the fitted lines in Fig. 8, the plasma-grafted PVC-TAH membrane shows lower activation energy than commercial AHA membrane. For comparison, some important properties of the studied PVC-TAH membrane and the commercial AHA membrane, including the thickness, IEC value, conductivity and activation energy were tested under the same conditions and then listed in Table 1. The plasma-grafted PVC-TAH membrane possesses smaller thickness and activation energy, higher IEC and ionic conductivity than the commercial AHA membrane, indicating a great potential for application in AEMFCs. 3.5. Chemical stability and thermal stability As one of the most important properties, chemical stability of AEMs, especially alkaline stability under high pH environment and elevated temperatures is the major challenge for fuel cell applications due to the attachment of the cationic functional groups. The quaternary ammonium cations, as the conductive functional groups in AEMs, are likely to degrade mainly via three degradation pathways: Hofmann elimination (E2), ylide formation and nicleophilic substitution (SN2) under the attack of OH [45–47]. In the present work, the chemical stability of the PVC-TAH membrane was evaluated in 2 M KOH solution at elevated temperature for 96 h. The IECs of the PVC-TAH membrane at certain time intervals were examined to see the changes of the quaternary ammonium groups under such rigorous condition. As
Fig. 8. Arrhenius plot of hydroxide conductivity as a function of temperature for the plasma-grafted PVC-TAH membrane and commercial AHA membrane.
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Table 1 Thickness, IEC value, conductivity and activation energy of the plasma-grafted PVC-TAH membrane and the commercial AHA membrane. Membrane
Thickness (mm)
IEC (mmol g1)
Conductivity (mS cm1)
Ea (kJ mol1)
PVC-TAH AHA
83 210
0.86 0.47
4.6 3.5
16.03 17.56
Fig. 9. Chemical stability tests for PVC-TAH membrane in 2 M KOH solution at 60 C.
shown in Fig. 9, the IEC of the PVC-TAH membrane slightly decreases with the immersion time and only drops by 12.8% in 96 hours. The decrease of the ionic exchange groups may be attributed to the attack of the benzyltrimethylammonium groups by OH via the nucleophilic substitution reaction (SN2 pathway) rather than Hofmann elimination pathway due to the absence of b-hydrogen atoms [48]. After immersion in 2 M KOH solution for 4 days, the PVC-TAH membrane shows no observable changes in shape and morphology. Even after immersion in 4 M KOH solution for 2 day, the residual IEC value of PVC-TAH membrane (0.76 mmol g1) is still up to 88% of the initial value. The high preservation of quaternary ammonium groups and main-chain structure under the rigid basic environment indicates a good alkaline stability of the PVC-TAH membrane. The thermal stability of the AEMs is always a concern for fuel cells because of the enhanced reaction kinetics and reduced voltage losses in fuel cells when operating at high temperature. The
thermal stability of the plasma-grafted PVC-TAH membrane was analysed by recording its TGA curves from room temperature to 800 C under flowing N2. For comparison, the original PVC powders were also measured. According to the TGA and its differential (DrTGA) profiles shown in Fig. 10, the PVC-TAH membrane mainly decomposed in three steps. The first step is the loss of the benzyltrimethylammonium groups started from the temperature of 120 C which means a steady operation of PVC-TAH membrane below 100 C, indicating a good thermal stability of the PVC-TAH membrane. The second and third decomposition steps of PVC-TAH membrane are the dehydrochlorination of main-chain in the range of 217 to 380 C, and the thermal cracking of carbonaceous conjugated polyene sequences in the range of 380 to 537 C, respectively [49]. These two steps can also be found in the degradation of the original PVC, confirming the decomposition of the main-chain of the PVC matrix. It is worth to notice that the degradation of the benzyltrimethylammonium groups includes two phases according to the DrTGA curve of the PVC-TAH membrane. Since Hofmann elimination is avoided owing to the lack of b-hydrogen, nucleophilic substitution reaction (SN2 pathway) is the main degradation pathway for benzyltrimethylammonium groups [48]. According to the reaction of SN2 pathway, methanol is generated at the very start, then the rest parts degrade [50]. As shown in Fig. 10a, the weight loss for methanol generation in the range of 120 to 188 C is about 3.0 wt%, which is close to the value (2.8 wt%) calculated from IEC. 3.6. Mechanical properties The mechanical properties of the plasma-grafted PVC-TAH membrane with different relative humidity (RH = 0% and RH = 100%) were investigated by tensile traction tests at about 20 C. As listed in Table 2, the tensile strength, Young modulus and the breaking elongation for the PVC-TAH membrane at 0% RH were 17.4 MPa, 917.4 MPa and 3.2%, respectively. In comparison, the tensile strength, Young modulus and breaking elongation for fully hydrated (100% RH) PVC-TAH membrane were 18.6 MPa, 787.6 MPa and 3.8%, respectively. The slightly inferior mechanical properties
Fig. 10. (a) TGA and (b) DrTGA of the PVC and PVC-TAH membrane.
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Table 2 Mechanical properties of PVC-TAH membranes with relative humidity (RH) of 0% and 100%. Membrane
Thickness (mm)
Tensile strength (MPa)
Young modulus (MPa)
Breaking elongation (%)
PVC-TAH 0% RH PVC-TAH 100% RH
77 83
17.4 18.6
917.4 787.6
3.2 3.8
of the fully hydrated PVC-TAH membrane may be attributed to the combination of the hydrophilic anion exchange groups in PVC-TAH membrane matrix with water molecules, leading to the membrane swelling (7.8% according to the change of thickness) and a less compact membrane structure. Compared to the literature values, the mechanical properties of the PVC-TAH membrane are acceptable, indicating that the plasma-grafted PVC-TAH membrane is strong enough to be AEMs [12,51,52]. 4. Discussion As mentioned above, the amount of active sites in the plasma bombarded polymer matrix determines the amount of functional groups on the plasma-grafted membrane, and as a result, the conductivity of the plasma-grafted AEM. The generation of those active sites strongly depends on the plasma bombardment. The plasma bombardment efficiency in the present study is as high as 50%, which means one in every three syndiotactic CH2–CHCl units is bombarded by plasma in the PVC matrix. However, only a small part of plasma bombarded CH2–CHCl units are grafted with VBTAC monomer, indicating that the grafting efficiency is the performance limiting factor in the plasma-grafted AEM preparation. The low grafting efficiency may be attributed to: (1) the elimination of the free radicals by dehydrochlorination and air contamination during and after plasma bombardment process; (2) the extinction of the shot-lived free radicals in the plasma bombarded PVC matrix; (3) low monomer concentration in the grafting liquid leading to a low contact probability between free radicals and monomer; and so on. Therefore, in order to improve the grafting efficiency, one important way is to adjust the plasma condition to decrease the free radicals elimination. As shown in Fig. 11, there is less C¼C, CO and CN moieties generated in plasma bombarded PVC at plasma discharge power of 50 W. Moreover, optimization of the grafting operation is also necessary. Further researches are in progress.
Fig. 11. FTIR of plasma bombarded PVC powders at the discharge power of (a) 50 W and (b) 100 W for 30 min.
5. Conclusions In summary, plasma-grafted PVC-TAH AEM was prepared by directly introducing benzyltrimethylammonium groups into the PVC matrix. ATR-FTIR and XPS analyses confirm the successful introduction of benzyltrimethylammonium groups into the PVC matrix. The produced AEM shows the satisfactory chemical stability in strong basic solution at 60 C, thermal stability above 120 C, excellent thermal dimensional stability and acceptable anion conductivity, indicating that the plasma-grafted AEM fulfills the requirements for fuel cell applications. The plasma-grafting process analysis indicates that the plasma treatment efficiency is as high as 50%, but the grafting efficiency is quite low. It reveals that the grafting efficiency is the performance limiting factor in plasmagrafted AEM synthesis. Further research focusing on increasing the grafting efficiency is required for enhancing the overall performance of plasma-grafted AEMs. Acknowledgements This research was financially supported by the National Nature Science Foundation of Anhui Province (No. 1508085QA10), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. 2015265), China Postdoctoral Science Foundation Funded Project (No. 2015M582021), the Hong Kong Scholars Program, and the Hong Kong General Research Fund (GRF Project 9042044) and CityU Strategic Research Grant (Project 7004476). References [1] R. Borup, J. Meyers, B. Pivovar, Y.S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J.E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K.C. Kimjima, N. Iwashita, Scientific aspects of polymer electrolyte fuel cell durability and degradation, Chem. Rev. 107 (2007) 3904–3951. [2] O.I. Deavin, S. Murphy, A.L. Ong, S.D. Poynton, R. Zeng, H. Herman, J.R. Varcoe, Anion-exchange membranes for alkaline polymer electrolyte fuel cells: comparison of pendent benzyltrimethylammonium- and benzylmethylimidazolium-head-groups, Energy Environ. Sci. 5 (2012) 8584–8597. [3] L.A. Adams, S.D. Poynton, C. Tamain, R.C.T. Slade, J.R. Varcoe, A carbon dioxide tolerant aqueous-electrolyte-free anion-exchange membrane alkaline fuel cell, Chemsuschem 1 (2008) 79–81. [4] C. Zhang, J. Hu, Y. Meng, M. Nagatsu, H. Toyoda, Preparation of highperformance hydroxide exchange membrane by a novel ablation restriction plasma polymerization approach, Chem. Commun. 47 (2011) 10230–10232. [5] J. Pan, C. Chen, Y. Li, L. Wang, L.S. Tan, G.W. Li, X. Tang, L. Xiao, J.T. Lu, L. Zhuang, Constructing ionic highway in alkaline polymer electrolytes, Energy Environ. Sci. 7 (2014) 354–360. [6] S. Gu, R. Cai, T. Luo, Z.W. Chen, M.W. Sun, Y. Liu, G.H. He, Y.S. Yan, A soluble and highly conductive Ionomer for high-performance hydroxide exchange membrane fuel cells, Angew. Chem. Int. Ed. 48 (2009) 6499–6502. [7] Q. Zhang, S. Li, S. Zhang, A novel guanidinium grafted poly(aryl ether sulfone) for high-performance hydroxide exchange membranes, Chem. Commun. 46 (2010) 7495–7497. [8] Z. Jiang, Z.J. Jiang, Plasma techniques for the fabrication of polymer electrolyte membranes for fuel cells, J. Membr. Sci. 456 (2014) 85–106. [9] T.N. Danks, R.C.T. Slade, J.R. Varcoe, Alkaline anion-exchange radiation-grafted membranes for possible electrochemical application in fuel cells, J. Mater. Chem. 13 (2003) 712–721. [10] H. Herman, R.C.T. Slade, J.R. Varcoe, The radiation-grafting of vinylbenzyl chloride onto poly(hexafluoropropylene-co-tetrafluoroethylene) films with subsequent conversion to alkaline anion-exchange membranes: optimisation of the experimental conditions and characterisation, J. Membr. Sci. 218 (2003) 147–163.
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