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Preparation and characterization of quaternary ammonium functionalized poly(2,6-dimethyl-1,4-phenylene oxide) as anion exchange membrane for alkaline polymer electrolyte fuel cells
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 3 9 ( 2 0 1 4 ) 2 6 5 9 e2 6 6 8
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Preparation and characterization of quaternary ammonium functionalized poly(2,6-dimethyl-1,4phenylene oxide) as anion exchange membrane for alkaline polymer electrolyte fuel cells K. Hari Gopi, S. Gouse Peera, S.D. Bhat*, P. Sridhar, S. Pitchumani CSIR e Central Electrochemical Research Institute e Chennai Centre, CSIR Campus, Taramani, Chennai 600 113, India
article info
abstract
Article history:
Anion exchange membrane from poly(phenylene oxide) containing pendant quaternary
Received 31 July 2013
ammonium groups is fabricated for application in alkaline polymer electrolyte fuel cells
Received in revised form
(APEFCs). Chloromethylation of poly(phenylene oxide) (PPO) was performed by aryl sub-
27 November 2013
stitution and then homogeneously quaternized to form an anion exchange membrane
Accepted 1 December 2013
(AEM). The influence of various parameters on the chloromethylation reaction was
Available online 30 December 2013
investigated and optimized. The successful introduction of the above groups in the polymer backbone was confirmed by 1H NMR and FT-IR spectroscopy. Membrane intrinsic
Keywords:
properties such as ion exchange capacity, water uptake and ionic conductivity were
Alkaline polymer electrolyte fuel
evaluated. The membrane electrolyte exhibited an enhanced performance in comparison
cells
with the state-of-the-art commercial AHA membrane in APEFCs. A peak power density of
Anion exchange membrane
111 mW/cm2 at a load current density of 250 mA/cm2 was obtained for PPO based mem-
Polyphenylene oxide (PPO)
brane in APEFCs at 30 C.
Aryl substitution
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Homogeneous quaternization
1.
Introduction
Fuel cell is one of the best known power sources for many years to generate power for stationary and portable application as well as for transportation sector. Alkaline fuel cells (AFCs) are the oldest type of fuel cells used for space applications where the liquid electrolyte (aqueous KOH solution) played a major role for ion conduction [1]. However, the main limitation of using liquid electrolyte in AFCs was the precipitation of carbonate salts on the electrodes which block the pores of catalyst, thus reducing the efficiency of fuel cell [2].
reserved.
To eliminate carbonation, research is focused on developing Solid Polymer Electrolytes (SPEs) for its use in alkaline fuel cell. Due to this, polymer electrolyte membrane fuel cells (PEMFCs) started to evolve owing to their high efficiency at low temperature operation. One of the main components of PEMFCs is the use of solid electrolyte, usually Nafion, due to its superior proton conductivity, high mechanical and chemical stability and also long-term durability [3]. However, the major limitations of PEMFC are the utilization of expensive platinum based catalysts as also the cost of the membrane.
* Corresponding author. Tel.: þ91 44 22542068; fax: þ91 44 22542456. E-mail address: [email protected] (Santoshkumar D. Bhat). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.009
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Table 1 e Effect of CMEE & reaction temperature on chloromethylation. CMEE:PPO (mol:mol)
Polymer concentration (wt%)
Temperature ( C)
Substitution degreea (%)
% Gelationb
Membrane property
15 15 15 15 15 15
50 50 50 50 40 60
10 25 45 60 30 65
e e e 25 e 15
Stable Stable Swelling Brittle Stable Swelling
2:1 3:1 4:1 5:1 4:1 4:1 a b
Calculated from 1H NMR spectra. Gelation was determined by weighing.
Recently, alkaline polymer electrolyte fuel cells (APEFCs) have received an overwhelming attention, which replaces proton exchange membranes with OH conducting anion exchange membranes. The advantages of APEFCs lie in (1) lower activation loss in alkaline medium, (2) facile cathode kinetics permitting the use of abundant low cost metal catalysts like Ni, Fe, Ag, etc., (3) decreased oxidant reduction overpotential at high pH, thereby increasing the efficiency [4,5]. There have been several attempts to develop solid polymer electrolytes for APEFCs [6,7]. SPEs are the polyelectrolytes having charge carriers (usually quaternary ammonium or quaternary phosphonium type) grafted on the polymer backbone. Polymer electroneutrality was maintained by attaching a mobile counter-ion to each ionic functional group [8]. Polymers with this kind of anchored organic cation are promising candidates as anion exchange membranes. Several studies have been carried out to develop AEMs based on quaternized polymers by adopting different routes. One of the important routes involves the radiation grafting of monomers onto the polymers, followed by quaternization to form an AEM [9,10]. The other route is by attaching reactive chloromethyl group on the polymer backbone [11] followed by its reaction with tertiary amine for quaternization [12,13]. There have been significant advances in the synthesis of quaternary ammonium polymer as AEMs from variety of polymers like poly(ether-imide) (PEI) [14], polysulfone (PS) [15], poly(arylene ether sulfone) (PESF) [16], poly(phthalazinon ether sulfone ketone) (PPESK) [17], cardo polyetherketone (PEK-C) [18], poly(methyl methacrylate) [19], poly(phenylenes) [20], etc. Gu et al. prepared AEM using polysulfone with tris(2,4,6-trimethoxyphenyl)phosphine having quaternary phosphonium head-group [5]. Also Wang et al. reported AEMs containing guanidinium head-groups with high ionic conductivity [21]. However, the preparation of phosphonium and guanidinium based AEMs requires the use of expensive reagents like tris(2,4,6-trimethoxyphenyl)phosphine and pentamethylguanidine, thus increasing the cost. In order to prepare AEMs with cost effectiveness and also for high performance, our earlier reports suggested the use of trimethylamine for quaternization [22]. Poly(phenylene oxide) (PPO) has emerged as one of the most promising polymers for the fabrication of anion exchange membrane due to its excellent physicochemical properties. Wu et al. prepared AEMs by incorporating silica into PPO which exhibited high tensile strength and good
conductivity [23]. Recently, membranes with imidazolium groups (attached to benzyl position) have been developed from brominated PPO (BPPO) with good ionic conductivities [24]. But imidazolium cation is not advantageous over the benchmark quaternary ammonium cation due to its poor stability [25]. The present study reports the introduction of quaternary ammonium cation on the aryl carbon hitherto on benzyl carbon in the preparation of anion exchange membrane from engineering plastics namely poly(2,6-dimethyl1,4-phenylene oxide) via chloromethylation followed by a prefunctionalization methodology for quaternization. The influence of several parameters such as amount of chloromethylating agent, polymer concentration, reaction temperature, etc. was investigated and optimized. The results show that all these parameters have a significant impact on the extent of chloromethylation. The resulting membranes were studied by 1H NMR spectroscopy and thermogravimetric analysis (TGA), and also for ion exchange capacity (IEC), ion conductivity and APEFC performance.
2.
Experimental
2.1.
Materials
Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from SigmaeAldrich, India. Chloromethyl ethyl ether (CMEE) (80% tech stabilized), Zinc chloride (99.99% extra pure) and 1Methyl-2-pyrrolidone were procured from Acros Organics. Chlorobenzene was supplied by RANKEM India. Trimethylamine solution (30 wt% in water) was supplied by Loba Chemie. Toray TGP-H-120 carbon papers were procured from Nikunj Exim Pvt. Ltd., India. Vulcan XC-72R carbon and Pt/C (40 wt% Pt on carbon) were obtained from Alfa Aesar (Johnson Matthey company). All the chemicals were used as received. De-ionized water (18.4 MU cm) used for experiments was produced by Millipore system. AHA-NEOSEPTA membrane used in this study was purchased from ASTOM Corporation, Japan. The AHA membrane comprises tetra-alkyl ammonium as fixed cation groups bonded on to a polyolefin backbone chain.
2.2.
Chloromethylation of PPO
The preparation of anion exchange membrane (AEMs) involves chemical reactions such as chloromethylation,
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Table 2 e Impact of polymer weight on chloromethylation. Volume of CMEE (ml)
Weight of polymer (g)
CMEE:PPO (mol:mol)
Substitution degree (%)
% Gelation
1 2 3 2
1:0.25 1:0.5 1:0.75 1:0.5
45 40 30 60
e e e 10
3.1 3.1 3.1 a 3.1 a
Reaction temperature at 60 C.
quaternization followed by ion exchange. For chloromethylation, 2 g of PPO was dissolved in chlorobenzene (15 wt% solution) at 30 C in a three-neck round bottom flask fitted with reflux condenser and a mechanical stirrer. Required amount of zinc chloride catalyst (5 wt% with respect to the weight of polymer) dissolved in chloromethyl ethyl ether (CMEE) was added drop-wise into the flask containing PPO solution. The reaction mixture was then stirred vigorously for 5 h at 50 C. After cooling, the product chloromethylated PPO (CPPO) was precipitated with methanol. Finally, the precipitated CPPO was filtered, washed with distilled water and then dried in an oven for 24 h. Effects of different mole ratios of CMME and PPO on chloromethylation were studied and the results are given in Tables 1 and 2.
2.4.
Structural characterization
Proton (1H) NMR spectra were recorded on a Bruker spectrometer at 400 MHz using chloroform-d (CDCl3) as solvent and tetramethylsilane (TMS) as an internal standard. Fourier transform infrared (FT-IR) spectra were recorded using a Thermo Scientific Nexus 670 FT-IR spectrophotometer in the
CH3
O m
n
b
CH3
Membrane fabrication
Prior to the membrane fabrication, the solubility of polymers was examined by dissolving 0.05 g of PPO, CPPO in 5 mL of different solvents as represented in Table 3. The next step in the preparation of AEM involved quaternization through prefunctionalization methodology. Homogeneous amination was carried out by dissolving the dried CPPO in 1-Methyl-2pyrrolidone (NMP) to prepare 3 wt% solution. To this, trimethylamine solution (30 wt% in water) was added and the reaction mixture was stirred for 24 h at room temperature to introduce the quaternary ammonium groups. This solution was cast on a flat Plexiglas plate and dried at 75 C in a vacuum oven to form QPPO membrane. The obtained Cl form of membrane was immersed in 2.0 mol L1 KOH solution for 24 h to exchange it to OH form (Scheme 1). Finally, the membrane was washed repeatedly with de-ionized water to remove residual KOH and stored in de-ionized water for further studies.
CH3
5 h, 50°C ClCH2OCH2CH3 ZnCl2 CH3
CH3
d O
a
O m
b
n
c
CH3
ClH2C
Table 3 e Solubilities of PPO, CPPO and QPPO in various solvents. Chlorobenzene Chloroform Dichloroethane Cyclohexane Acetone Methanol 2-Propanol Dimethyl sulfoxide (DMSO) N,N-Dimethyl acetamide (DMAc) N-Methyl-2-pyrrolidone (NMP) (S) Soluble; (IS) Insoluble.
PPO
CPPO
QPPO
S S S IS IS IS IS IS IS IS
S S S IS IS IS IS IS S S
IS IS IS IS IS IS IS IS IS S
CH3
aq. N(CH3)3 CH3
CH3
O
O m
CH3
Solvent
b
O
a 2.3.
CH3
a
n CH3 N(CH3)3Cl-
KOH/ H2O CH3
CH3
O
O m
CH3
n CH3 N(CH3)3OH-
Scheme 1 e Synthetic method for the preparation of CPPO and QPPO.
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spectral range of 4000e400 cm1. The membrane samples were analysed using ATR mode of the instrument. Thermogravimetric analysis (TGA) and Differential scanning calorimetry (DSC) were carried out to analyse the thermal stability of membrane with NETZSCH Model STA 449 TG-DSC instrument under nitrogen atmosphere at a heating rate of 10 C/min. The surface morphologies of the membranes were observed using a scanning electron microscopy (SEM) with JEOL JSM 35CF microscope.
2.5.
Ion exchange capacity
Ion exchange capacity (IEC, in mmol/g) was determined using the Mohr’s titration method described elsewhere [13]. Membrane sample (Cl form) with known mass was immersed in Na2SO4 solution (0.5 M, 50.0 mL) for 8 h to convert the membrane from chloride (Cl) to sulphate (SO2 4 ) form. The chloride ions released from the membrane were titrated against 0.1 M AgNO3 solution using potassium chromate (0.25 M) as an indicator. Similarly a blank titration was also performed without the membrane. By measuring the amount of AgNO3 consumed in the titration, the molar quantity of chloride ions exchanged can be determined. The IEC was calculated according to the following equation, IEC ¼
2.7. Membrane-electrode assembly (MEA) fabrication and cell polarization Membrane-electrode assemblies were prepared as given below. Required amount of Vulcan XC-72R carbon powder dispersed in cyclohexane was mixed with polytetrafluoroethylene (15 wt% PTFE) to prepare the slurry and then coated on Teflonized Toray paper until the loading of 1.5 mg/cm2 is reached to form gas diffusion layer. Catalyst ink was prepared by dispersing carbon-supported catalyst (40% Pt) in de-ionized water followed by the addition of specific amount of Fumion (FAA-3) ionomer (polyaromatic polymer with quaternary ammonium ions, bromide counter-ion and NMP) (10 wt%) in ethanol with ultrasonic vibration for 30 min. The catalyst ink was then coated on the gas diffusion layer to make the electrodes. The desired Pt metal loading of both anode and cathode was kept at 0.5 mg/cm2. The two electrodes were dipped in KOH solution (1.0 mol L1) to exchange the binder to OH form. MEA was formed by sandwiching the membrane with above two electrodes at 80 C for 3 min at a pressure of 15e20 kg/cm2. MEA was then placed in a single-cell assembly with parallel serpentine flow-field machined on graphite plates. The active electrode area was 4 cm2. Measurement of cell potential as a function of current density were carried out galvanostatically by using electronic load (Model-LCN4-25-24/ LCN 50-24) from Bitrode Instruments (US) with humidified H2 and O2 flowing at 200 mL min1.
Volumeof AgNO3 consumedMolarityof AgNO3 ðmmol=gÞ Dryweightof Sample
3. 2.6.
Water uptake and ionic conductivity
Water uptake measurements were carried out by sorption method as reported in our earlier studies [26]. The conductivity of the membranes was measured by using two-probe AC impedance technique with Autolab PGSTAT 30 (Eco Chemie, Netherlands with FRA2 module) under potentiostatic mode over the frequency range from 104 Hz to 0.1 Hz. The conductivity cell comprised of two stainless-steel electrodes, each with 20 mm diameter. Before testing, the membranes were equilibrated in de-ionized water for 24 h. The membrane sample was sandwiched between the two electrodes mounted on a Teflon block and placed in a sealed glass chamber containing de-ionized water to provide 100% relative humidity (RH). The resistance (R) of the membranes was determined from the high-frequency intercept of impedance with the real axis and thus the conductivity can be calculated from the membrane resistance. The conductivity measurements were recorded at different temperatures ranging from 30 C to 70 C. The ionic conductivity for a given membrane can be calculated as: s¼
L RA
where s is the conductivity of the membrane in S/cm, L is the thickness of the membrane in cm, A is the cross-sectional area in cm2 and R is the membrane resistance obtained from impedance.
Results and discussion
3.1. Influence of reaction parameters on the chloromethylation reaction Chloromethylation is the crucial step in the synthesis of functionalized polymers due to its high reactivity, which determines the functional groups in polymer that can be directly quaternized in turn influencing the anion conductivity. In this study, chloromethylation is done on the aromatic ring of the polymer making it less susceptible for crowding unlike the conventional method of benzyl substitution [24] as depicted in Fig. 1, wherein the electronegative nature of oxygen presumably tends to withdraw electrons towards itself thereby reducing the electropositive character of nitrogen. Interestingly, Ran et al. [24] used brominated PPO as the base material in which the bromine atoms are attached to the side methyl
∗ ∗
Aryl position
Benyl position
Fig. 1 e Substitution of functional groups in PPO at aryl and benzyl positions.
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groups which are much closer to the electronegative oxygen atom. In this study, the quaternary ammonium groups are well shielded from the electronegative oxygen atom by the methyl group. The effects of important parameters like amount of CMEE, amount of polymer and reaction temperature on the chloromethylation reaction were studied. Tables 1 and 2 represent the influence of different parameters that affect the degree of chloromethylation. The 1H NMR spectrum was used as a tool to investigate the degree of chloromethylation. Fig. 2a shows the proton resonance signals of both PPO and chloromethylated PPO (CPPO). The peak at 6.4e6.5 ppm is assigned to the aryl protons of PPO (designated as ‘a’) and the protons due to the benzyl group (designated as ‘b’) appear at 2.0e2.1 ppm as multiplet. Two prominent peaks appear in the spectra of CPPO. The peak at 4.9 ppm is the characteristic peak of chloromethyl group (CH2Cl, designated as ‘c’), and because of this electrophilic substitution, aryl protons are shifted to a new position at 6.1 ppm (designated as ‘d’). The percentage of substitution (DS) of the chloromethyl groups was calculated using the following relation [27]: DSð%Þ ¼
100 d 0:5a þ d
where ‘a’ and ‘d’ are the integrals of protons represented in the spectra. The 14N NMR spectrum of QPPO was recorded to confirm the quaternization which showed a peak around 70 ppm corresponding to amine functionality as represented in Fig. 2b. Effect of the addition of chloromethylating agent with respect to polymer in different molar ratios (2:1, 3:1, 4:1 and 5:1) was investigated. As seen in Table 1, the degree of chloromethylation increases as the concentration of CMEE increased. The highest degree of substitution was attained for the molar ratio 5:1. However, the membrane formed was brittle, which may be due to the cross-linking reaction between the aromatic ring and the chloromethyl group that in turn increases the percentage of gelation [14]. Due to the above said reasons, the appropriate amount of CMEE to be added was optimized in the ratio of 4:1. It is noteworthy that because of the excessive use of CMEE, the formed membrane had high degree of swelling thereby reducing the stability of the polymer. To mitigate the swelling issues, the molar weight of PPO was increased by keeping the amount of CMEE constant (in the ratio of 4:1) and its effects are explored. By increasing the molar weight of the polymer, excess CMEE has access to active sites, which in turn increases the ionic conductivity. Also the formed membrane showed less swelling thus eliminating the stability issues. The optimum percentage of chloromethylation is 40% to form a stable membrane as seen in Table 2. As it is evident from the Table, higher the degree of chloromethylation, more is the gelation. Generally, increasing the reaction temperature can either accelerate the reaction by lowering the energy barrier and/or can lead to undesirable side reactions thus lowering the product yield. As shown in Table 1, an increase in temperature increased the number of chloromethyl groups. However, when the temperature reached 60 C, the percentage of gelation increased (Table 2) forming a cross-linked gel.
Fig. 2 e a e 1H NMR spectra of PPO and CPPO using CDCl3 as solvent. b e 14N NMR spectra of QPPO.
3.2.
Homogeneous amination
Chloromethylated PPO was further quaternized by homogeneous amination, in which PPO is directly converted to ionic form and is equally distributed over the polymer matrix
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because of the strong ionic interactions in comparison with heterogeneous amination [15]. During the quaternization reaction with trimethylamine, the amine acts as a nucleophile and reacts with the chloromethyl group of PPO via substitution reaction. A new carboneamine (CeN) bond is formed after the electrophilic carbon of the chloromethyl group is attacked by the amine and then chloride ion is displaced [28]. The degree of quaternization is calculated from the elemental analysis by using the equation reported in the literature [29].
polymer chains attached with quaternary ammonium groups as represented in Fig. 4d. To confirm the grafted domains of quaternary ammonium moiety, surface morphology of QPPO is taken at higher magnification and is represented in Fig. 5. The figure clearly shows the mounds of grafted quaternary ammonium groups on the QPPO membrane surface similar to the literature reported elsewhere [29].
3.5. 3.3.
FT-IR studies
The chemical structure of the functionalized polymers was further investigated by FT-IR. Fig. 3 shows the FT-IR spectra of PPO, CPPO & QPPO. The peak at 1600 cm1 corresponds to the C]C stretching and the band at 1190 cm1 is due to the CeOeC stretching of PPO [23]. In CPPO, the peak at 720 cm1 is assigned to the stretching vibration of CeCl bond, and the band at 1410 cm1 is the characteristic bending vibration of CH2 group [30]. Since 1H NMR spectra were not recorded for QPPO due to its insolubility in CDCl3, the introduction of quaternary ammonium group was confirmed by FT-IR. In QPPO, a new absorption peak at 930 cm1 due to CeN stretching indicates the characteristic peak of quaternary ammonium group [27]. In addition, the absence of peak at 720 cm1 (CeCl bond) also confirms the complete conversion of chloromethyl group to quaternary ammonium group. Also the band at 1410 cm1 (eCH2 group) is shifted to a higher wave number at 1440 cm1 because of quaternization as seen from Fig. 3.
3.4.
Morphology of membranes
Fig. 4 shows the surface morphologies of PPO, CPPO and QPPO membranes. The SEM image of PPO membrane (Fig. 4a) has a smooth, flat and homogeneous surface suggesting no phase separation, whereas Fig. 4b shows the phase separated morphology of CPPO. In contrast, the QPPO membrane surface is distributed with small grafted domains of quaternary ammonium groups as seen in Fig. 4c. The cross-sectional morphology of QPPO membrane shows well inter-connected
TG-DSC studies
TGA analyses were performed to evaluate the thermal properties of the membranes. The thermal stability curves of PPO and QPPO (in Cl form) are represented in Fig. 6a. In the TG curve of PPO, the weight loss occurring at 425 C corresponds to the degradation of the polymer backbone. Three decomposition steps are observed in the TG curve of QPPO. The initial weight loss observed between 90 and 120 C corresponds to the evaporation of absorbed water. The second weight loss above 210 C is due to the cleavage of quaternary ammonium groups from PPO [13]. The third weight loss observed at temperatures greater than 375 C corresponds to polymer backbone degradation. It is evident from TGA that the chloromethylation and quaternization reactions have slightly reduced the thermal stability of QPPO in comparison with pristine PPO membrane. The weight loss of QPPO membrane is about 10% from 200 to 350 C which suggests its good thermal stability suitable for use in APEFCs that operates at lower temperature. The glass transition of polymers was observed by DSC as a gradual increase in the heat capacity of sample during heating, due to the enhancement of molecular motion in the polymer. Fig. 6b shows the DSC thermograms of PPO and QPPO in N2 atmosphere. The curves indicate that the glass transition temperature (Tg) for PPO is 240 C while for QPPO, the Tg is 280 C. In addition, QPPO also showed high melting temperature (Tm) with the endotherm assigned at 660 C in comparison with the Tm of PPO at 580 C [31]. The increased Tg and Tm show that the glass transition and melting temperatures are higher for QPPO in comparison with PPO.
3.6.
Solubility of polymers
Transmittance (%)
PPO
CPPO
C-Cl streching
Bending of -CH2 group
QPPO
C-N streching
1800
1600
1400
1200
1000
Table 3 displays the solubility of PPO, CPPO and QPPO in different solvents. PPO is soluble only in non-polar solvents like chloroform, chlorobenzene and dichloroethane. After chloromethylation, CPPO is dissolved not only in the above mentioned solvents but also in some polar aprotic solvents like N,N-Dimethyl acetamide (DMAc) and 1-Methyl-2-pyrrolidone (NMP). The quaternized membrane QPPO is soluble only in 1Methyl-2-pyrrolidone (NMP) proving its chemical stability. QPPO membrane slightly swells in water due to the presence of hydrophilic quaternary ammonium groups that in turn are dependent on the degree of chloromethylation [16].
3.7. 800
Membrane conductivity
600
-1
Wavenumber (cm ) Fig. 3 e Typical FT-IR spectra of PPO, CPPO and QPPO.
Ionic conductivity is a vital property that influences the performance of APEFCs. Membrane conductivity is influenced by ion concentration, ion mobility and level of hydration. Fig. 7
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Fig. 4 e Surface morphology images of (a) PPO (b) CPPO (c) QPPO and (d) cross-sectional image of QPPO membrane.
represents the ionic conductivity of QPPO membrane in comparison with AHA membrane (in Cl form) at varying temperatures of 30e70 C. In general, the ion conductivity increases with increase in temperature due to the enhanced mobility of anion. Under fully hydrated conditions, the conductivity of QPPO membrane was in the range of 0.004e0.008 S/cm. Among the two membranes, QPPO reached a maximum conductivity of 8.3 103 S/cm at 70 C due to higher IEC and water uptake in comparison with AHA. It is noteworthy that higher water uptake promotes the transport of ions whereas higher IEC decreases the distance between ionic groups leading to faster ion conduction [32]. The ionic conductivity data for QPPO and AHA membranes and their corresponding IEC and water uptake values are shown in Table 4.
3.8.
Fig. 5 e Surface morphology image of QPPO at 15,000 X.
APEFC performance studies and stability test
Fig. 8 shows the cell polarization curves for MEAs containing QPPO and commercial AHA membranes at 30 C. A higher
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0.009
100 PPO QPPO
Ionic Conductivity (S/cm)
Weight (%)
80
AHA QPPO
0.008
a
60
40
0.007 0.006 0.005 0.004 0.003 0.002
20 100
200
300 400 500 Temperature (° C)
600
700
800
30
70
50 60 Temperature ( ° C)
70
80
b
50
Heat Flow (mW)
40
Fig. 7 e Ionic conductivity of QPPO & AHA membranes at temperatures ranging from 30 to 70 C.
PPO QPPO
60
0.001
40 30 20 10 0 -10 0
100
200
300
400
500
600
700
800
Temperature (° C) Fig. 6 e (a) TGA curves of PPO and QPPO at a heating rate of 10 C/min in N2 atmosphere and (b) DSC curves of PPO and QPPO in N2 atmosphere.
peak power density of 111 mW/cm2 at a load current density of 250 mA/cm2 is observed for QPPO based MEAs in comparison with a peak power density of 58 mW/cm2 at a load current density of 125 mA/cm2 for AHA based MEAs. The higher performance of QPPO at lower temperature is due to the substitution of chloromethyl groups in aryl position of polymer which increases the distance of interaction between the quaternary ammonium group and oxygen atom of polymer [23]. The cationic group in QPPO is well shielded from the
oxygen atom within the monomer by methyl group. The oxygen atom from the other monomer unit presumably may have less impact on the electropositive nature of nitrogen. However at higher temperatures, the performance of the membrane decreased probably due to the possible decomposition of quaternary ammonium groups because of the nucleophilic attack of OH ion by SN2 nucleophilic displacement [17,33], in turn reducing the ionic conductivity. The mechanism for possible degradation is represented in Scheme 2. Homogenously quaternized QPPO showed a much better electrochemical property in comparison with the fuel cell performance of the reported PPO based AEM that exhibited a peak power density of 30e32 mW/cm2 at 50 C [23,24]. The comparative performance of QPPO with similar kind of membranes is represented in Table 5. The stability of the cationic charge (nitrogen content) in the polymer is crucial for its use as an AEM in APEFCs. The stability test for the commercial AHA and QPPO membranes was studied under OCV condition at cell temperature of 30 C. The change in open circuit voltage (OCV) in relation to time is shown in Fig. 9. To start with, the cells comprising QPPO and AHA membrane had OCV values of 1.005 and 0.98 V, respectively. In both the cases, voltage drop is observed with time. It is noteworthy that QPPO has the better stability in comparison with AHA membrane as seen in Fig. 9. The above results show that the fabricated membrane is a suitable candidate as polymer electrolyte for APEFCs.
Table 4 e Characteristic properties of QPPO and AHA membrane. Sample name
QPPO membrane AHA membrane a
Nitrogen contenta (wt%)
7.565 1.798
Determined from CHNS analysis.
Degree of quaternization (%)
4.2 e
Conductivity (mS/cm) 30 C
70 C
4.3 1.6
8.3 2.6
IEC (mmol/g)
Water uptake (%)
0.70 0.35
30 15
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1.2
120
Table 5 e Comparative APEFC performance of QPPO along with other PPO based membranes studied in the literature.
100
0.8
80
0.6
60
0.4
40
0.2
20
Cell voltage (V)
2
1.0
Power density (mW/cm )
AHA at 30 °C QPPO at 30 °C
Membrane type
Peak power density (mW/cm2)
Operating temperature ( C)
Reference
111 30 30 32
30 50 50 50
Present study
QPPO BPPO-Im Silica/PPO
[24] [23]
1.0
0.0 0
50
100
150
200
250
300
0 350
Fig. 8 e Cell polarization curves of APEFCs comprising AHA and QPPO membranes at 30 C.
4.
Conclusion
Anion exchange membrane was prepared from poly(phenylene oxide) via chloromethylation and quaternization. The impact of important parameters on the chloromethylation reaction was investigated. The results show that the properties of the aforesaid membrane depend on the important reaction parameters like the effect of molar ratio of reactants, reaction temperature and polymer concentration. By optimizing the reaction parameters, the characteristics of the membrane were tailored with enhanced electrochemical properties. Chloromethylation on the aromatic ring and homogeneous quaternization have resulted in the enhanced
CH3
CH3
O
O m
Open Cell Voltage (V)
2
Current density (mA/cm )
0.8
QPPO AHA
0.6
0.4
0.2 0
10
20
30
40
50
Time (h)
Fig. 9 e Stability of QPPO and AHA under OCV conditions at 30 C.
ionic conductivity of anion exchange membrane. The QPPO membrane in alkaline polymer electrolyte fuel cell exhibited a higher power density of 111 mW/cm2 in comparison with the commercial AHA membrane. The optimized QPPO membrane also showed comparable stability in relation to AHA membrane at OCV conditions.
n
CH3
CH3
Acknowledgement
N(CH3)3 OH-
CH3
CH3
O
O m
N(CH3)3
Authors acknowledge CSIR for the financial support under 12th Five Year Plan e HYDEN programme (CSC 0122). Dr. Santoshkumar D. Bhat thank CSIR for the project under CSIR-Young Scientist Award Scheme. Authors thank Dr. Radhakrishnan and Mr. Ravishankar, CSIR-CECRI, Karaikudi for their help in NMR and SEM studies. Authors also thank Director, CSIR-CECRI for his kind support and valuable suggestions.
n
CH3
CH3
references
OH
Scheme 2 e Mechanism showing the degradation of quaternary ammonium groups.
[1] McLean GF, Niet T, Prince-Richard S, Djilali N. An assessment of alkaline fuel cell technology. Int J Hydrogen Energy 2002;27:507e26.
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[2] Varcoe JR, Slade RCT. Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells 2005;5:187e200. [3] Mauritz KA, Moore RB. State of understanding of Nafion. Chem Rev 2004;104:4535e86. [4] Lin BYS, Kirk DW, Thorpe SJ. Performance of alkaline fuel cells: a possible future energy system. J Power Sources 2006;161:474e83. [5] Gu S, Cai R, Luo T, Chen Z, Sun M, Liu Y, et al. A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angew Chem Int Ed Engl 2009;48:6499e502. [6] Merle G, Wessling M, Nijmeijer K. Anion exchange membranes for alkaline fuel cells: a review. J Membr Sci 2011;377:1e35. [7] Couture G, Alaaeddine A, Boschet F, Ameduri B. Polymeric materials as anion-exchange membranes for alkaline fuel cells. Prog Polym Sci 2011;36:1521e57. [8] Agel E, Bouet J, Fauvarque JF. Characterization and use of anionic membranes for alkaline fuel cells. J Power Sources 2001;101:267e74. [9] Varcoe JR, Slade RCT, Yee ELH. An alkaline polymer electrochemical interface: a breakthrough in application of alkaline anion-exchange membranes in fuel cells. Chem Commun 2006:1428e9. [10] Varcoe JR, Slade RCT, Yee ELH, Poynton SD, Driscoll DJ, Apperley DC. Poly(ethylene-co-tetrafluoroethylene)-derived radiation-grafted anion exchange membrane with properties specifically tailored for application in metal-cation-free alkaline polymer electrolyte fuel cells. Chem Mater 2007;19:2686e93. [11] Arges CG, Parrondo J, Johnson G, Nadhan A, Ramani V. Assessing the influence of different cation chemistries on ionic conductivity and alkaline stability of anion exchange membranes. J Mater Chem 2012;22:3733e44. [12] Li Y, Xu T, Gong M. Fundamental studies of a new series anion exchange membranes: membranes prepared from bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) and pyridine. J Membr Sci 2006;279:200e8. [13] Xu T, Liu Z, Li Y, Yang W. Preparation and characterization of Type II anion exchange membranes from poly(2,6-dimethyl1,4-phenylene oxide). J Membr Sci 2008;320:232e9. [14] Wang G, Weng Y, Zhao J, Chu D, Xie D, Chen R. Developing a novel alkaline anion exchange membrane derived from poly(ether-imide) for improved ionic conductivity. Polym Adv Technol 2010;21:554e60. [15] Yan J, Hickner M. Anion exchange membranes by bromination of benzylmethyl containing poly (sulfone). Macromolecules 2010;43:2349e56. [16] Zhou J, Unlu M, Vega JA, Kohl PA. Anionic polysulfone ionomers and membranes containing fluorenyl groups for anionic fuel cells. J Power Sources 2009;190:285e92. [17] Fang J, Shen PK. Quaternized poly(phthalazinon ether sulfone ketone) membrane for anion exchange membrane fuel cells. J Membr Sci 2006;285:317e22. [18] Xiong Y, Liu QL, Zeng QH. Quaternized cardo polyether ketone anion exchange membrane for direct methanol alkaline fuel cells. J Power Sources 2009;193:541e6. [19] Luo Y, Guo J, Wang C, Chu D. Quaternized poly(methyl methacrylate-co-butyl acrylate-co-vinylbenzyl chloride) membrane for alkaline fuel cells. J Power Sources 2010;195:3765e71.
[20] Hibbs MR, Fujimoto CH, Cornelius CJ. Synthesis and characterization of poly(phenylene) based anion exchange membranes for alkaline fuel cells. Macromolecules 2009;42:8316e21. [21] Wang J, Li S, Zhang S. Novel hydroxide-conducting polyelectrolyte composed of an poly(arylene ether sulfone) containing pendant quaternary guanidinium group for alkaline fuel cell applications. Macromolecules 2010;43:3890e6. [22] Hari Gopi K, Gouse Peera S, Bhat SD, Sridhar P, Pitchumani S. 3-methyl trimethylammonium poly(2,6-dimethyl-1,4phenylene oxide) based anion exchange membrane for alkaline polymer electrolyte fuel cells. Bull Mater Sci 2013 [in press]. [23] Wu Y, Wu C, Varcoe JR, Poynton SD, Xu T, Fu Y. Novel silica/ poly(2,6-dimethyl-1,4-phenylene oxide) hybrid anionexchange membranes for alkaline fuel cells: effect of silica content and the single cell performance. J Power Sources 2010;195:3069e76. [24] Ran J, Wu L, Varcoe JR, Ong AL, Poynton SD, Xu T. Development of imidazolium-type alkaline anion exchange membranes for fuel cell application. J Membr Sci 2012;415e416:242e9. [25] Deavin OI, Murphy S, Ong AL, Poynton SD, Zeng R, Herman H, et al. Anion-exchange membranes for alkaline polymer electrolyte fuel cells: comparison of pendent benzyltrimethylammonium and benzylmethylimidazoliumhead-groups. Energy Environ Sci 2012;5:8584e97. [26] Mohanapriya S, Bhat SD, Sahu AK, Pitchumani S, Sridhar P, Shukla AK. A new mixed-matrix membrane for DMFCs. Energy Environ Sci 2009;2:1210e6. [27] Wu L, Xu T, Yang W. Fundamental studies of a new series of anion exchange membranes: membranes prepared through chloroacetylation of poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) followed by quaternary amination. J Membr Sci 2006;286:185e92. [28] Ong AL, Saad S, Lan R, Goodfellow RJ, Tao S. Anionic membrane and ionomer based on poly(2,6-dimethyl-1,4phenylene oxide) for alkaline membrane fuel cells. J Power Sources 2011;196:8272e9. [29] Xiong Y, Fang J, Zeng QH, Liu QL. Preparation and characterization of cross-linked quaternized poly(vinyl alcohol) membranes for anion exchange membrane fuel cells. J Membr Sci 2008;311:319e25. [30] Lu C, Gao B, Liu Q, Qi C. Preparation of two kinds of chloromethylated polystyrene particle using 1,4-bis (chloromethoxy) butane as chloromethylation reagent. Colloid Polym Sci 2008;286:553e61. [31] Cao YC, Wang X, Mamlouk M, Scott K. Preparation of alkaline anion exchange polymer membrane from methylated melamine grafted poly(vinylbenzyl chloride) and its fuel cell performance. J Mater Chem 2011;21:12910e6. [32] Lin CW, Huang YF, Kannan AM. Cross-linked poly(vinyl alcohol) and poly(styrene sulfonic acid-co-maleic anhydride)-based semi-interpenetrating network as proton conducting membranes for direct methanol fuel cells. J Power Sources 2007;171:340e7. [33] Edson JB, Macomber CS, Pivovar BS, Boncella JM. Hydroxide based decomposition pathways of alkyltrimethylammonium cations. J Membr Sci 2012;399e400:49e59.
Available online at www.sciencedirect.com
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Preparation and characterization of quaternary ammonium functionalized poly(2,6-dimethyl-1,4phenylene oxide) as anion exchange membrane for alkaline polymer electrolyte fuel cells K. Hari Gopi, S. Gouse Peera, S.D. Bhat*, P. Sridhar, S. Pitchumani CSIR e Central Electrochemical Research Institute e Chennai Centre, CSIR Campus, Taramani, Chennai 600 113, India
article info
abstract
Article history:
Anion exchange membrane from poly(phenylene oxide) containing pendant quaternary
Received 31 July 2013
ammonium groups is fabricated for application in alkaline polymer electrolyte fuel cells
Received in revised form
(APEFCs). Chloromethylation of poly(phenylene oxide) (PPO) was performed by aryl sub-
27 November 2013
stitution and then homogeneously quaternized to form an anion exchange membrane
Accepted 1 December 2013
(AEM). The influence of various parameters on the chloromethylation reaction was
Available online 30 December 2013
investigated and optimized. The successful introduction of the above groups in the polymer backbone was confirmed by 1H NMR and FT-IR spectroscopy. Membrane intrinsic
Keywords:
properties such as ion exchange capacity, water uptake and ionic conductivity were
Alkaline polymer electrolyte fuel
evaluated. The membrane electrolyte exhibited an enhanced performance in comparison
cells
with the state-of-the-art commercial AHA membrane in APEFCs. A peak power density of
Anion exchange membrane
111 mW/cm2 at a load current density of 250 mA/cm2 was obtained for PPO based mem-
Polyphenylene oxide (PPO)
brane in APEFCs at 30 C.
Aryl substitution
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Homogeneous quaternization
1.
Introduction
Fuel cell is one of the best known power sources for many years to generate power for stationary and portable application as well as for transportation sector. Alkaline fuel cells (AFCs) are the oldest type of fuel cells used for space applications where the liquid electrolyte (aqueous KOH solution) played a major role for ion conduction [1]. However, the main limitation of using liquid electrolyte in AFCs was the precipitation of carbonate salts on the electrodes which block the pores of catalyst, thus reducing the efficiency of fuel cell [2].
reserved.
To eliminate carbonation, research is focused on developing Solid Polymer Electrolytes (SPEs) for its use in alkaline fuel cell. Due to this, polymer electrolyte membrane fuel cells (PEMFCs) started to evolve owing to their high efficiency at low temperature operation. One of the main components of PEMFCs is the use of solid electrolyte, usually Nafion, due to its superior proton conductivity, high mechanical and chemical stability and also long-term durability [3]. However, the major limitations of PEMFC are the utilization of expensive platinum based catalysts as also the cost of the membrane.
* Corresponding author. Tel.: þ91 44 22542068; fax: þ91 44 22542456. E-mail address: [email protected] (Santoshkumar D. Bhat). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.12.009
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Table 1 e Effect of CMEE & reaction temperature on chloromethylation. CMEE:PPO (mol:mol)
Polymer concentration (wt%)
Temperature ( C)
Substitution degreea (%)
% Gelationb
Membrane property
15 15 15 15 15 15
50 50 50 50 40 60
10 25 45 60 30 65
e e e 25 e 15
Stable Stable Swelling Brittle Stable Swelling
2:1 3:1 4:1 5:1 4:1 4:1 a b
Calculated from 1H NMR spectra. Gelation was determined by weighing.
Recently, alkaline polymer electrolyte fuel cells (APEFCs) have received an overwhelming attention, which replaces proton exchange membranes with OH conducting anion exchange membranes. The advantages of APEFCs lie in (1) lower activation loss in alkaline medium, (2) facile cathode kinetics permitting the use of abundant low cost metal catalysts like Ni, Fe, Ag, etc., (3) decreased oxidant reduction overpotential at high pH, thereby increasing the efficiency [4,5]. There have been several attempts to develop solid polymer electrolytes for APEFCs [6,7]. SPEs are the polyelectrolytes having charge carriers (usually quaternary ammonium or quaternary phosphonium type) grafted on the polymer backbone. Polymer electroneutrality was maintained by attaching a mobile counter-ion to each ionic functional group [8]. Polymers with this kind of anchored organic cation are promising candidates as anion exchange membranes. Several studies have been carried out to develop AEMs based on quaternized polymers by adopting different routes. One of the important routes involves the radiation grafting of monomers onto the polymers, followed by quaternization to form an AEM [9,10]. The other route is by attaching reactive chloromethyl group on the polymer backbone [11] followed by its reaction with tertiary amine for quaternization [12,13]. There have been significant advances in the synthesis of quaternary ammonium polymer as AEMs from variety of polymers like poly(ether-imide) (PEI) [14], polysulfone (PS) [15], poly(arylene ether sulfone) (PESF) [16], poly(phthalazinon ether sulfone ketone) (PPESK) [17], cardo polyetherketone (PEK-C) [18], poly(methyl methacrylate) [19], poly(phenylenes) [20], etc. Gu et al. prepared AEM using polysulfone with tris(2,4,6-trimethoxyphenyl)phosphine having quaternary phosphonium head-group [5]. Also Wang et al. reported AEMs containing guanidinium head-groups with high ionic conductivity [21]. However, the preparation of phosphonium and guanidinium based AEMs requires the use of expensive reagents like tris(2,4,6-trimethoxyphenyl)phosphine and pentamethylguanidine, thus increasing the cost. In order to prepare AEMs with cost effectiveness and also for high performance, our earlier reports suggested the use of trimethylamine for quaternization [22]. Poly(phenylene oxide) (PPO) has emerged as one of the most promising polymers for the fabrication of anion exchange membrane due to its excellent physicochemical properties. Wu et al. prepared AEMs by incorporating silica into PPO which exhibited high tensile strength and good
conductivity [23]. Recently, membranes with imidazolium groups (attached to benzyl position) have been developed from brominated PPO (BPPO) with good ionic conductivities [24]. But imidazolium cation is not advantageous over the benchmark quaternary ammonium cation due to its poor stability [25]. The present study reports the introduction of quaternary ammonium cation on the aryl carbon hitherto on benzyl carbon in the preparation of anion exchange membrane from engineering plastics namely poly(2,6-dimethyl1,4-phenylene oxide) via chloromethylation followed by a prefunctionalization methodology for quaternization. The influence of several parameters such as amount of chloromethylating agent, polymer concentration, reaction temperature, etc. was investigated and optimized. The results show that all these parameters have a significant impact on the extent of chloromethylation. The resulting membranes were studied by 1H NMR spectroscopy and thermogravimetric analysis (TGA), and also for ion exchange capacity (IEC), ion conductivity and APEFC performance.
2.
Experimental
2.1.
Materials
Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from SigmaeAldrich, India. Chloromethyl ethyl ether (CMEE) (80% tech stabilized), Zinc chloride (99.99% extra pure) and 1Methyl-2-pyrrolidone were procured from Acros Organics. Chlorobenzene was supplied by RANKEM India. Trimethylamine solution (30 wt% in water) was supplied by Loba Chemie. Toray TGP-H-120 carbon papers were procured from Nikunj Exim Pvt. Ltd., India. Vulcan XC-72R carbon and Pt/C (40 wt% Pt on carbon) were obtained from Alfa Aesar (Johnson Matthey company). All the chemicals were used as received. De-ionized water (18.4 MU cm) used for experiments was produced by Millipore system. AHA-NEOSEPTA membrane used in this study was purchased from ASTOM Corporation, Japan. The AHA membrane comprises tetra-alkyl ammonium as fixed cation groups bonded on to a polyolefin backbone chain.
2.2.
Chloromethylation of PPO
The preparation of anion exchange membrane (AEMs) involves chemical reactions such as chloromethylation,
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Table 2 e Impact of polymer weight on chloromethylation. Volume of CMEE (ml)
Weight of polymer (g)
CMEE:PPO (mol:mol)
Substitution degree (%)
% Gelation
1 2 3 2
1:0.25 1:0.5 1:0.75 1:0.5
45 40 30 60
e e e 10
3.1 3.1 3.1 a 3.1 a
Reaction temperature at 60 C.
quaternization followed by ion exchange. For chloromethylation, 2 g of PPO was dissolved in chlorobenzene (15 wt% solution) at 30 C in a three-neck round bottom flask fitted with reflux condenser and a mechanical stirrer. Required amount of zinc chloride catalyst (5 wt% with respect to the weight of polymer) dissolved in chloromethyl ethyl ether (CMEE) was added drop-wise into the flask containing PPO solution. The reaction mixture was then stirred vigorously for 5 h at 50 C. After cooling, the product chloromethylated PPO (CPPO) was precipitated with methanol. Finally, the precipitated CPPO was filtered, washed with distilled water and then dried in an oven for 24 h. Effects of different mole ratios of CMME and PPO on chloromethylation were studied and the results are given in Tables 1 and 2.
2.4.
Structural characterization
Proton (1H) NMR spectra were recorded on a Bruker spectrometer at 400 MHz using chloroform-d (CDCl3) as solvent and tetramethylsilane (TMS) as an internal standard. Fourier transform infrared (FT-IR) spectra were recorded using a Thermo Scientific Nexus 670 FT-IR spectrophotometer in the
CH3
O m
n
b
CH3
Membrane fabrication
Prior to the membrane fabrication, the solubility of polymers was examined by dissolving 0.05 g of PPO, CPPO in 5 mL of different solvents as represented in Table 3. The next step in the preparation of AEM involved quaternization through prefunctionalization methodology. Homogeneous amination was carried out by dissolving the dried CPPO in 1-Methyl-2pyrrolidone (NMP) to prepare 3 wt% solution. To this, trimethylamine solution (30 wt% in water) was added and the reaction mixture was stirred for 24 h at room temperature to introduce the quaternary ammonium groups. This solution was cast on a flat Plexiglas plate and dried at 75 C in a vacuum oven to form QPPO membrane. The obtained Cl form of membrane was immersed in 2.0 mol L1 KOH solution for 24 h to exchange it to OH form (Scheme 1). Finally, the membrane was washed repeatedly with de-ionized water to remove residual KOH and stored in de-ionized water for further studies.
CH3
5 h, 50°C ClCH2OCH2CH3 ZnCl2 CH3
CH3
d O
a
O m
b
n
c
CH3
ClH2C
Table 3 e Solubilities of PPO, CPPO and QPPO in various solvents. Chlorobenzene Chloroform Dichloroethane Cyclohexane Acetone Methanol 2-Propanol Dimethyl sulfoxide (DMSO) N,N-Dimethyl acetamide (DMAc) N-Methyl-2-pyrrolidone (NMP) (S) Soluble; (IS) Insoluble.
PPO
CPPO
QPPO
S S S IS IS IS IS IS IS IS
S S S IS IS IS IS IS S S
IS IS IS IS IS IS IS IS IS S
CH3
aq. N(CH3)3 CH3
CH3
O
O m
CH3
Solvent
b
O
a 2.3.
CH3
a
n CH3 N(CH3)3Cl-
KOH/ H2O CH3
CH3
O
O m
CH3
n CH3 N(CH3)3OH-
Scheme 1 e Synthetic method for the preparation of CPPO and QPPO.
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spectral range of 4000e400 cm1. The membrane samples were analysed using ATR mode of the instrument. Thermogravimetric analysis (TGA) and Differential scanning calorimetry (DSC) were carried out to analyse the thermal stability of membrane with NETZSCH Model STA 449 TG-DSC instrument under nitrogen atmosphere at a heating rate of 10 C/min. The surface morphologies of the membranes were observed using a scanning electron microscopy (SEM) with JEOL JSM 35CF microscope.
2.5.
Ion exchange capacity
Ion exchange capacity (IEC, in mmol/g) was determined using the Mohr’s titration method described elsewhere [13]. Membrane sample (Cl form) with known mass was immersed in Na2SO4 solution (0.5 M, 50.0 mL) for 8 h to convert the membrane from chloride (Cl) to sulphate (SO2 4 ) form. The chloride ions released from the membrane were titrated against 0.1 M AgNO3 solution using potassium chromate (0.25 M) as an indicator. Similarly a blank titration was also performed without the membrane. By measuring the amount of AgNO3 consumed in the titration, the molar quantity of chloride ions exchanged can be determined. The IEC was calculated according to the following equation, IEC ¼
2.7. Membrane-electrode assembly (MEA) fabrication and cell polarization Membrane-electrode assemblies were prepared as given below. Required amount of Vulcan XC-72R carbon powder dispersed in cyclohexane was mixed with polytetrafluoroethylene (15 wt% PTFE) to prepare the slurry and then coated on Teflonized Toray paper until the loading of 1.5 mg/cm2 is reached to form gas diffusion layer. Catalyst ink was prepared by dispersing carbon-supported catalyst (40% Pt) in de-ionized water followed by the addition of specific amount of Fumion (FAA-3) ionomer (polyaromatic polymer with quaternary ammonium ions, bromide counter-ion and NMP) (10 wt%) in ethanol with ultrasonic vibration for 30 min. The catalyst ink was then coated on the gas diffusion layer to make the electrodes. The desired Pt metal loading of both anode and cathode was kept at 0.5 mg/cm2. The two electrodes were dipped in KOH solution (1.0 mol L1) to exchange the binder to OH form. MEA was formed by sandwiching the membrane with above two electrodes at 80 C for 3 min at a pressure of 15e20 kg/cm2. MEA was then placed in a single-cell assembly with parallel serpentine flow-field machined on graphite plates. The active electrode area was 4 cm2. Measurement of cell potential as a function of current density were carried out galvanostatically by using electronic load (Model-LCN4-25-24/ LCN 50-24) from Bitrode Instruments (US) with humidified H2 and O2 flowing at 200 mL min1.
Volumeof AgNO3 consumedMolarityof AgNO3 ðmmol=gÞ Dryweightof Sample
3. 2.6.
Water uptake and ionic conductivity
Water uptake measurements were carried out by sorption method as reported in our earlier studies [26]. The conductivity of the membranes was measured by using two-probe AC impedance technique with Autolab PGSTAT 30 (Eco Chemie, Netherlands with FRA2 module) under potentiostatic mode over the frequency range from 104 Hz to 0.1 Hz. The conductivity cell comprised of two stainless-steel electrodes, each with 20 mm diameter. Before testing, the membranes were equilibrated in de-ionized water for 24 h. The membrane sample was sandwiched between the two electrodes mounted on a Teflon block and placed in a sealed glass chamber containing de-ionized water to provide 100% relative humidity (RH). The resistance (R) of the membranes was determined from the high-frequency intercept of impedance with the real axis and thus the conductivity can be calculated from the membrane resistance. The conductivity measurements were recorded at different temperatures ranging from 30 C to 70 C. The ionic conductivity for a given membrane can be calculated as: s¼
L RA
where s is the conductivity of the membrane in S/cm, L is the thickness of the membrane in cm, A is the cross-sectional area in cm2 and R is the membrane resistance obtained from impedance.
Results and discussion
3.1. Influence of reaction parameters on the chloromethylation reaction Chloromethylation is the crucial step in the synthesis of functionalized polymers due to its high reactivity, which determines the functional groups in polymer that can be directly quaternized in turn influencing the anion conductivity. In this study, chloromethylation is done on the aromatic ring of the polymer making it less susceptible for crowding unlike the conventional method of benzyl substitution [24] as depicted in Fig. 1, wherein the electronegative nature of oxygen presumably tends to withdraw electrons towards itself thereby reducing the electropositive character of nitrogen. Interestingly, Ran et al. [24] used brominated PPO as the base material in which the bromine atoms are attached to the side methyl
∗ ∗
Aryl position
Benyl position
Fig. 1 e Substitution of functional groups in PPO at aryl and benzyl positions.
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 3 9 ( 2 0 1 4 ) 2 6 5 9 e2 6 6 8
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groups which are much closer to the electronegative oxygen atom. In this study, the quaternary ammonium groups are well shielded from the electronegative oxygen atom by the methyl group. The effects of important parameters like amount of CMEE, amount of polymer and reaction temperature on the chloromethylation reaction were studied. Tables 1 and 2 represent the influence of different parameters that affect the degree of chloromethylation. The 1H NMR spectrum was used as a tool to investigate the degree of chloromethylation. Fig. 2a shows the proton resonance signals of both PPO and chloromethylated PPO (CPPO). The peak at 6.4e6.5 ppm is assigned to the aryl protons of PPO (designated as ‘a’) and the protons due to the benzyl group (designated as ‘b’) appear at 2.0e2.1 ppm as multiplet. Two prominent peaks appear in the spectra of CPPO. The peak at 4.9 ppm is the characteristic peak of chloromethyl group (CH2Cl, designated as ‘c’), and because of this electrophilic substitution, aryl protons are shifted to a new position at 6.1 ppm (designated as ‘d’). The percentage of substitution (DS) of the chloromethyl groups was calculated using the following relation [27]: DSð%Þ ¼
100 d 0:5a þ d
where ‘a’ and ‘d’ are the integrals of protons represented in the spectra. The 14N NMR spectrum of QPPO was recorded to confirm the quaternization which showed a peak around 70 ppm corresponding to amine functionality as represented in Fig. 2b. Effect of the addition of chloromethylating agent with respect to polymer in different molar ratios (2:1, 3:1, 4:1 and 5:1) was investigated. As seen in Table 1, the degree of chloromethylation increases as the concentration of CMEE increased. The highest degree of substitution was attained for the molar ratio 5:1. However, the membrane formed was brittle, which may be due to the cross-linking reaction between the aromatic ring and the chloromethyl group that in turn increases the percentage of gelation [14]. Due to the above said reasons, the appropriate amount of CMEE to be added was optimized in the ratio of 4:1. It is noteworthy that because of the excessive use of CMEE, the formed membrane had high degree of swelling thereby reducing the stability of the polymer. To mitigate the swelling issues, the molar weight of PPO was increased by keeping the amount of CMEE constant (in the ratio of 4:1) and its effects are explored. By increasing the molar weight of the polymer, excess CMEE has access to active sites, which in turn increases the ionic conductivity. Also the formed membrane showed less swelling thus eliminating the stability issues. The optimum percentage of chloromethylation is 40% to form a stable membrane as seen in Table 2. As it is evident from the Table, higher the degree of chloromethylation, more is the gelation. Generally, increasing the reaction temperature can either accelerate the reaction by lowering the energy barrier and/or can lead to undesirable side reactions thus lowering the product yield. As shown in Table 1, an increase in temperature increased the number of chloromethyl groups. However, when the temperature reached 60 C, the percentage of gelation increased (Table 2) forming a cross-linked gel.
Fig. 2 e a e 1H NMR spectra of PPO and CPPO using CDCl3 as solvent. b e 14N NMR spectra of QPPO.
3.2.
Homogeneous amination
Chloromethylated PPO was further quaternized by homogeneous amination, in which PPO is directly converted to ionic form and is equally distributed over the polymer matrix
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because of the strong ionic interactions in comparison with heterogeneous amination [15]. During the quaternization reaction with trimethylamine, the amine acts as a nucleophile and reacts with the chloromethyl group of PPO via substitution reaction. A new carboneamine (CeN) bond is formed after the electrophilic carbon of the chloromethyl group is attacked by the amine and then chloride ion is displaced [28]. The degree of quaternization is calculated from the elemental analysis by using the equation reported in the literature [29].
polymer chains attached with quaternary ammonium groups as represented in Fig. 4d. To confirm the grafted domains of quaternary ammonium moiety, surface morphology of QPPO is taken at higher magnification and is represented in Fig. 5. The figure clearly shows the mounds of grafted quaternary ammonium groups on the QPPO membrane surface similar to the literature reported elsewhere [29].
3.5. 3.3.
FT-IR studies
The chemical structure of the functionalized polymers was further investigated by FT-IR. Fig. 3 shows the FT-IR spectra of PPO, CPPO & QPPO. The peak at 1600 cm1 corresponds to the C]C stretching and the band at 1190 cm1 is due to the CeOeC stretching of PPO [23]. In CPPO, the peak at 720 cm1 is assigned to the stretching vibration of CeCl bond, and the band at 1410 cm1 is the characteristic bending vibration of CH2 group [30]. Since 1H NMR spectra were not recorded for QPPO due to its insolubility in CDCl3, the introduction of quaternary ammonium group was confirmed by FT-IR. In QPPO, a new absorption peak at 930 cm1 due to CeN stretching indicates the characteristic peak of quaternary ammonium group [27]. In addition, the absence of peak at 720 cm1 (CeCl bond) also confirms the complete conversion of chloromethyl group to quaternary ammonium group. Also the band at 1410 cm1 (eCH2 group) is shifted to a higher wave number at 1440 cm1 because of quaternization as seen from Fig. 3.
3.4.
Morphology of membranes
Fig. 4 shows the surface morphologies of PPO, CPPO and QPPO membranes. The SEM image of PPO membrane (Fig. 4a) has a smooth, flat and homogeneous surface suggesting no phase separation, whereas Fig. 4b shows the phase separated morphology of CPPO. In contrast, the QPPO membrane surface is distributed with small grafted domains of quaternary ammonium groups as seen in Fig. 4c. The cross-sectional morphology of QPPO membrane shows well inter-connected
TG-DSC studies
TGA analyses were performed to evaluate the thermal properties of the membranes. The thermal stability curves of PPO and QPPO (in Cl form) are represented in Fig. 6a. In the TG curve of PPO, the weight loss occurring at 425 C corresponds to the degradation of the polymer backbone. Three decomposition steps are observed in the TG curve of QPPO. The initial weight loss observed between 90 and 120 C corresponds to the evaporation of absorbed water. The second weight loss above 210 C is due to the cleavage of quaternary ammonium groups from PPO [13]. The third weight loss observed at temperatures greater than 375 C corresponds to polymer backbone degradation. It is evident from TGA that the chloromethylation and quaternization reactions have slightly reduced the thermal stability of QPPO in comparison with pristine PPO membrane. The weight loss of QPPO membrane is about 10% from 200 to 350 C which suggests its good thermal stability suitable for use in APEFCs that operates at lower temperature. The glass transition of polymers was observed by DSC as a gradual increase in the heat capacity of sample during heating, due to the enhancement of molecular motion in the polymer. Fig. 6b shows the DSC thermograms of PPO and QPPO in N2 atmosphere. The curves indicate that the glass transition temperature (Tg) for PPO is 240 C while for QPPO, the Tg is 280 C. In addition, QPPO also showed high melting temperature (Tm) with the endotherm assigned at 660 C in comparison with the Tm of PPO at 580 C [31]. The increased Tg and Tm show that the glass transition and melting temperatures are higher for QPPO in comparison with PPO.
3.6.
Solubility of polymers
Transmittance (%)
PPO
CPPO
C-Cl streching
Bending of -CH2 group
QPPO
C-N streching
1800
1600
1400
1200
1000
Table 3 displays the solubility of PPO, CPPO and QPPO in different solvents. PPO is soluble only in non-polar solvents like chloroform, chlorobenzene and dichloroethane. After chloromethylation, CPPO is dissolved not only in the above mentioned solvents but also in some polar aprotic solvents like N,N-Dimethyl acetamide (DMAc) and 1-Methyl-2-pyrrolidone (NMP). The quaternized membrane QPPO is soluble only in 1Methyl-2-pyrrolidone (NMP) proving its chemical stability. QPPO membrane slightly swells in water due to the presence of hydrophilic quaternary ammonium groups that in turn are dependent on the degree of chloromethylation [16].
3.7. 800
Membrane conductivity
600
-1
Wavenumber (cm ) Fig. 3 e Typical FT-IR spectra of PPO, CPPO and QPPO.
Ionic conductivity is a vital property that influences the performance of APEFCs. Membrane conductivity is influenced by ion concentration, ion mobility and level of hydration. Fig. 7
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 3 9 ( 2 0 1 4 ) 2 6 5 9 e2 6 6 8
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Fig. 4 e Surface morphology images of (a) PPO (b) CPPO (c) QPPO and (d) cross-sectional image of QPPO membrane.
represents the ionic conductivity of QPPO membrane in comparison with AHA membrane (in Cl form) at varying temperatures of 30e70 C. In general, the ion conductivity increases with increase in temperature due to the enhanced mobility of anion. Under fully hydrated conditions, the conductivity of QPPO membrane was in the range of 0.004e0.008 S/cm. Among the two membranes, QPPO reached a maximum conductivity of 8.3 103 S/cm at 70 C due to higher IEC and water uptake in comparison with AHA. It is noteworthy that higher water uptake promotes the transport of ions whereas higher IEC decreases the distance between ionic groups leading to faster ion conduction [32]. The ionic conductivity data for QPPO and AHA membranes and their corresponding IEC and water uptake values are shown in Table 4.
3.8.
Fig. 5 e Surface morphology image of QPPO at 15,000 X.
APEFC performance studies and stability test
Fig. 8 shows the cell polarization curves for MEAs containing QPPO and commercial AHA membranes at 30 C. A higher
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0.009
100 PPO QPPO
Ionic Conductivity (S/cm)
Weight (%)
80
AHA QPPO
0.008
a
60
40
0.007 0.006 0.005 0.004 0.003 0.002
20 100
200
300 400 500 Temperature (° C)
600
700
800
30
70
50 60 Temperature ( ° C)
70
80
b
50
Heat Flow (mW)
40
Fig. 7 e Ionic conductivity of QPPO & AHA membranes at temperatures ranging from 30 to 70 C.
PPO QPPO
60
0.001
40 30 20 10 0 -10 0
100
200
300
400
500
600
700
800
Temperature (° C) Fig. 6 e (a) TGA curves of PPO and QPPO at a heating rate of 10 C/min in N2 atmosphere and (b) DSC curves of PPO and QPPO in N2 atmosphere.
peak power density of 111 mW/cm2 at a load current density of 250 mA/cm2 is observed for QPPO based MEAs in comparison with a peak power density of 58 mW/cm2 at a load current density of 125 mA/cm2 for AHA based MEAs. The higher performance of QPPO at lower temperature is due to the substitution of chloromethyl groups in aryl position of polymer which increases the distance of interaction between the quaternary ammonium group and oxygen atom of polymer [23]. The cationic group in QPPO is well shielded from the
oxygen atom within the monomer by methyl group. The oxygen atom from the other monomer unit presumably may have less impact on the electropositive nature of nitrogen. However at higher temperatures, the performance of the membrane decreased probably due to the possible decomposition of quaternary ammonium groups because of the nucleophilic attack of OH ion by SN2 nucleophilic displacement [17,33], in turn reducing the ionic conductivity. The mechanism for possible degradation is represented in Scheme 2. Homogenously quaternized QPPO showed a much better electrochemical property in comparison with the fuel cell performance of the reported PPO based AEM that exhibited a peak power density of 30e32 mW/cm2 at 50 C [23,24]. The comparative performance of QPPO with similar kind of membranes is represented in Table 5. The stability of the cationic charge (nitrogen content) in the polymer is crucial for its use as an AEM in APEFCs. The stability test for the commercial AHA and QPPO membranes was studied under OCV condition at cell temperature of 30 C. The change in open circuit voltage (OCV) in relation to time is shown in Fig. 9. To start with, the cells comprising QPPO and AHA membrane had OCV values of 1.005 and 0.98 V, respectively. In both the cases, voltage drop is observed with time. It is noteworthy that QPPO has the better stability in comparison with AHA membrane as seen in Fig. 9. The above results show that the fabricated membrane is a suitable candidate as polymer electrolyte for APEFCs.
Table 4 e Characteristic properties of QPPO and AHA membrane. Sample name
QPPO membrane AHA membrane a
Nitrogen contenta (wt%)
7.565 1.798
Determined from CHNS analysis.
Degree of quaternization (%)
4.2 e
Conductivity (mS/cm) 30 C
70 C
4.3 1.6
8.3 2.6
IEC (mmol/g)
Water uptake (%)
0.70 0.35
30 15
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1.2
120
Table 5 e Comparative APEFC performance of QPPO along with other PPO based membranes studied in the literature.
100
0.8
80
0.6
60
0.4
40
0.2
20
Cell voltage (V)
2
1.0
Power density (mW/cm )
AHA at 30 °C QPPO at 30 °C
Membrane type
Peak power density (mW/cm2)
Operating temperature ( C)
Reference
111 30 30 32
30 50 50 50
Present study
QPPO BPPO-Im Silica/PPO
[24] [23]
1.0
0.0 0
50
100
150
200
250
300
0 350
Fig. 8 e Cell polarization curves of APEFCs comprising AHA and QPPO membranes at 30 C.
4.
Conclusion
Anion exchange membrane was prepared from poly(phenylene oxide) via chloromethylation and quaternization. The impact of important parameters on the chloromethylation reaction was investigated. The results show that the properties of the aforesaid membrane depend on the important reaction parameters like the effect of molar ratio of reactants, reaction temperature and polymer concentration. By optimizing the reaction parameters, the characteristics of the membrane were tailored with enhanced electrochemical properties. Chloromethylation on the aromatic ring and homogeneous quaternization have resulted in the enhanced
CH3
CH3
O
O m
Open Cell Voltage (V)
2
Current density (mA/cm )
0.8
QPPO AHA
0.6
0.4
0.2 0
10
20
30
40
50
Time (h)
Fig. 9 e Stability of QPPO and AHA under OCV conditions at 30 C.
ionic conductivity of anion exchange membrane. The QPPO membrane in alkaline polymer electrolyte fuel cell exhibited a higher power density of 111 mW/cm2 in comparison with the commercial AHA membrane. The optimized QPPO membrane also showed comparable stability in relation to AHA membrane at OCV conditions.
n
CH3
CH3
Acknowledgement
N(CH3)3 OH-
CH3
CH3
O
O m
N(CH3)3
Authors acknowledge CSIR for the financial support under 12th Five Year Plan e HYDEN programme (CSC 0122). Dr. Santoshkumar D. Bhat thank CSIR for the project under CSIR-Young Scientist Award Scheme. Authors thank Dr. Radhakrishnan and Mr. Ravishankar, CSIR-CECRI, Karaikudi for their help in NMR and SEM studies. Authors also thank Director, CSIR-CECRI for his kind support and valuable suggestions.
n
CH3
CH3
references
OH
Scheme 2 e Mechanism showing the degradation of quaternary ammonium groups.
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