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Effect of cesium salt of tungstophosphoric acid (Cs-TPA) on the properties of sulfonated polyether ether ketone (SPEEK) composite membranes for fuel cell applications
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Effect of cesium salt of tungstophosphoric acid (Cs-TPA) on the properties of sulfonated polyether ether ketone (SPEEK) composite membranes for fuel cell applications an a,*, Tu¨lay Y. Inan a,*, Elif Unveren a, Metin Kaya b Hacer Dog a
The Scientific and Technological Research Council of Turkey (TU¨BI˙TAK), Marmara Research Center, Chemistry Institute, P.K. 21, 41470 Gebze-Kocaeli, Turkey b _ _ ¨nu¨ Cad. No:245 Bozu¨yu¨k/Bilecik, Turkey DEMIRDO¨KU¨M A.S. 4 Eylu¨l Mah, Ismet Ino
article info
abstract
Article history:
We have prepared composite membranes for fuel cell applications. Cesium salt of tung-
Received 22 January 2010
stophosphoric acid (Cs-TPA) particles was synthesized by aqueous solutions of tung-
Received in revised form
stophosphoric acid and cesium hydroxide and, Cs-TPA particles and sulfonated (polyether
25 March 2010
ether ketone) (SPEEK) with two sulfonation degrees (DS), 60 and 70%have been used. We
Accepted 8 May 2010
examined both the effects of Cs-TPA in SPEEK membranes as functions of sulfonation
Available online 11 June 2010
degrees of SPEEK and the content of Cs-TPA. The performance of the composite membranes was evaluated in terms of water uptake, ion exchange capacity, proton
Keywords:
conductivity, chemical stability, hydrolytic stability, thermal stability and methanol
Sulfonated polyether ether ketone
permeability. The morphology of the membranes was investigated with SEM micrographs.
(SPEEK)
Increasing sulfonation degree of SPEEK from 60 to 70 caused agglomeration of the Cs-TPA
Cesium salt of tungstophosphoric
particles. The methanol permeability was reduced to 4.7 107 cm2/s for SPEEK (DS: 60%)/
acid
Cs-TPA membrane with 10 wt.% Cs-TPA concentration, and acceptable proton conductivity
Direct methanol fuel cell
of 1.3 101 S/cm was achieved at 80 C under 100% RH. The weight loss at 900 C increased
Proton conductivity
with the addition of inorganic particles, as expected. The hydrolytic stability of the SPEEK/ Cs-TPA based composite membranes was improved with the incorporation of the Cs-TPA particles into the matrix. We also noted that SPEEK60/Cs-TPA composite membranes were hydrolytically more stable than SPEEK70/Cs-TPA composite membranes. On the other hand, Methanol, water vapor, and hydrogen permeability values of SPEEK60 composite membranes were found to be lower than that of Nafion. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The proton conducting membrane which is the key component of a fuel cell system allows the transport of protons and water through it, but remains mostly impermeable to hydrogen and oxygen for proton exchange membrane fuel cell (PEMFC) and to the methanol for direct methanol fuel cell (DMFC) [1]. Commercially available Nafion membrane,
a perfluorosulfonic acid solid polymer electrolyte membrane produced by the Dupont Co., is known to be so far the best performing polymer electrolyte [2]. Recently, alternative membranes, especially partially-fluorinated and non-fluorinated ionomer membranes are currently under study. The following polymers have been regarded as possible candidates for new proton-conductive polymers: Polyamide-imide (PAI), Polybenzimidazole (PBI), Polyether ether ketone (PEEK),
* Corresponding authors. Tel.: þ90 262 677 2913; fax: þ90 262 641 2309. an), [email protected] (T.Y. Inan). E-mail addresses: [email protected] (H. Dog 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.05.045
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Polyether Sulfone (PES) and Polyimide (PI) [1,2]. Among them, the sulfonated poly(ether ether ketone) has been studied extensively due to some apparent advantages such as low cost, high proton conductivity and low methanol permeability [3e7] Inorganic compounds have been incorporated into the polymer matrix in order to enhance the proton conductivity [8]. Heteropoly acids (HPA) with Keggin anion structures have received attention due to their simple preparation and strong acidity in development of composite membranes for PEMFCs [9,10]. High solubility of HPA additive in aqueous media is a limiting factor for the membrane performance, but HPA can be immobilized by the formation of insoluble salts, by imbedding in a silicate matrix, or by covalently linking an organic moiety to a coordination site in a lacunary HPA [11]. The most studied insoluble salt of HPA is cesium salt of tungstophosphoric acid (Cs-TPA, Cs2.5H0.5PW12O40), a well known acidic catalyst in which the residual protons are more acidic than the homogenous acid catalysts (e.g., H2SO4, H3PW12O40 and ptoluenesulfonic acid). It has been reported that the acidity per unit acid site of Cs-TPA is superior to those of Amberlyst-15 and Nafion-H and Cs-TPA has been reported as a “watertolerant” solid acid catalyst desirable for environmentally benign industrial processes [12]. Recently, acidic Cs-TPA has been synthesized as an additive used in PEMFC [13e15] resulting in improvement of conductivity and possible increase in the thermal stability of the membrane. Ramani et al. [13] reported that Nafion/salt (Csþ, NH4 þ, þ and Tlþ) of tungstophosphoric acid composite Rb membranes demonstrate low H2 crossover currents and similar proton conductivity in comparison to pure Nafion membrane in the order of 1.6 102 S/cm at 120 C and 35% relative humidity. Cs-TPA/Nafion/PTFE self-humidifying composite membrane for PEMFC was prepared by recasting the Cs-TPA/Nafion self-humidifying layer onto the two side of the Nafion/PTFE composite membrane with a solution recast membrane and it was shown that the Cs-TPA/Nafion selfhumidifying layer not only contributed to humidify the membrane but also to the improvement of the open-circuit voltage of the fuel cell [14]. Wang et al. [15] impregnated CsTPA onto silicon dioxide and prepared Cs-TPA eSiO2/Nafion self-humidifying composite membrane. Their work compared the performance of the fuel cell with commercial Nafion NRE-212 membrane and reported that the cell performance with the self-humidifying composite membrane was obviously improved under both humidified and dry conditions at 60 and 80 C. SPEEK (sulfonated polyether ether keton)/Pt-Cs2.5 self-humidifying membrane developed by Zhang et al. [16] exhibited higher water absorbing and proton-conductive properties relative to the plain SPEEK membrane. In the present study, we synthesized Cs-TPA particles from aqueous solutions of tungstophosphoric acid and cesium hydroxide. We used SPEEK polymer as organic matrix and proton conductor with the sulfonation degrees (DS) of 60 and 70%. In this study, we changed the contents of Cs-TPA particles in the composite membranes between 5 wt.% and 20 wt.% and we used Cs-TPA particles as proton conducting site of the membrane as SPEEK. We later investigated the effect of CsTPA particles and its content on the membrane performance by X-ray power diffraction (XRD), Fourier Transform Infrared (FTIR), scanning electron microscopy (SEM), thermal
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gravimetric analyzer (TGA) and differential scanning calorimeter (DSC). We also investigated the ion exchange capacity (IEC) value, water uptake, proton conductivity and methanol permeability of these membranes.
2.
Experimental
2.1.
Materials
Polyether ether ketone (PEEK) 450 PF obtained in powder form (particle size of 25 mm) from Victrex was dried in vacuum at 100 C overnight. Sulfuric acid (95e97 wt.%), hydrogen peroxide (H2O2) and dimethylacetamide (DMAc, for synthesis) were obtained from Fluka. Tungstophosphoric acid, H3PW12O40$29H2O (TPA) and cesium hydroxide (99.9 wt.%) were received from Aldrich. All chemicals were used as received without further purification.
2.2.
Sulfonation of PEEK
PEEK was sulfonated by using concentrated sulfuric acid (conc. H2SO4) following the same procedure reported by Zaidi [17]. Five g dried PEEK was dissolved completely in 100 ml concentrated sulfuric acid (95e97%) under nitrogen at 25 C by strong mechanical agitation at least 800 rpm to maintain a homogenous reaction media. Later, the temperature was increased to the desired reaction temperature. The desired sulfonation degrees of 60, 70, and 82% were achieved within 90e130 h, and then the reaction solution was gradually precipitated into cold deionized water under agitation. The precipitate was washed with deionized water and filtered at several times until the pH value was approximately between 6 and 7. The sulfonated polymer samples were dried in vacuum at room temperature for 12 h and then at 60 C for 12 h. SPEEK with 60%, 70% and 82% degree of sulfonation (DS) was used in the membrane preparation.
2.3.
Cs-TPA preparation
Three g of dry tungstophosphoric acid was dissolved in 10 ml of water and stirred for 15 min. As a stoichiometric ratio, 0.8 g of Cs(OH) solution was added drop-wise to tungstophosphoric acid solution and kept at the room temperature for 24 h [18]. The resulting precipitate was recovered from the solution by evaporation at 100 C. The inorganic powder was calcined at 200 C for 3 h.
2.4.
Membrane preparation
Membranes were prepared by casting 20% (w/v) N,N-Dimethylacetamide (DMAc) solutions of SPEEK containing Cs-TPA particles on a clean glass plate. Membranes were dried under vacuum for 8 h at 30e60 C and another 15 h at 60e80 C. Then, membranes were soaked and kept in deionized water until testing. The membranes were analyzed by Headspace GC/MS and NMR that assured no solvent residue remained in the membranes. The membrane thickness was measured at dry state using a digital micrometer. The thickness of prepared membranes ranged from 80 to 100 mm.
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2.5.
Characterization of Cs-TPA
2.5.1.
Particle size measurement
Particle size measurement of the cesium salt of tungstophosphoric acid was performed by Laser Beam Scattering Technique (Malvern Hydro 2000M/MU Particle Size Analyzer).
solution at 50 C for 48 h to exchange Hþ with Naþ. The polymer solution containing released Hþ was titrated with a 0.1 N NaOH solution by using bromothymol blue as an indicator. IEC values for both sulfonated polymers (Ep) and membranes (Em) were calculated by using the following equation: IEC ¼
2.5.2.
X-ray diffraction (XRD)
Tungstophosphoric acid and cesium salt of tungstophosphoric acid were analyzed by X-ray diffraction (XRD, 600 SHIMADZU). The radiation was CuKa of wavelength 1.5405 A.
2.6.
Characterization of composite membranes
2.6.1.
Morphological characterization
Membrane morphology was investigated under a scanning electron microscope (JEOL JSM-6335 F) and Transmission Electron Microscope (JEOL JEM-2100 HRTEM). The TEM analysis was operated at 20 kV accelerating voltage and at 200 kV with LaB6 filament carbon coated with 200 mesh copper grid.
2.6.2.
Thermogravimetric analysis (TGA)
The thermal stability of the membranes was determined by using Perkin Elmer PYRIS 1 TGA. The samples were heated, under nitrogen atmosphere, up to 150 C and kept at this temperature for half an hour to remove moisture completely. The samples were cooled down to 80 C and then re-heated up to 900 C with a heating rate of 10 C/min.
2.6.3.
Differential scanning calorimeter (DSC)
DSC analyses were performed with Perkin Elmer Jade DSC at a heating rate of 10 C/min under nitrogen atmosphere. All data were collected from a second heating cycle and the glass transition temperatures (Tg) were calculated as a midpoint of thermogram.
2.6.4.
Fourier transform infrared spectroscopy (FTIR)
FTIR spectra were recorded for membranes and heteropoly acid salt using Perkin Elmer Pyris 1 FTIR (Fourier Transform Infrared) Spectrophotometer. Membrane samples were prepared by making KBr pellets.
2.6.5.
Water uptake
Water uptake value (WU %) was determined according to the ASTM D 570-98. SPEEK composite membranes were conditioned in an oven at 105 C for 1 h and then at 50 C for 24 h. Conditioned samples were cooled to room temperature in desiccators, immediately weighed (Wdry), and then immersed in water at room temperature for 24 h. The membranes were removed from the water, wiped free of the moisture with a dried cloth and then weighed immediately (Wwet) again. Percentage increase in weight during immersion was calculated as follows;
VNaOH NNaOH FNaOH Wp
where VNaOH is titrated volume of NaOH solution (L), NNaOH is the normality of NaOH solution and Wp is the dried weight of polymer or membrane (g). SD (%) for SPEEK was calculated by using the following equation: SDð%Þ ¼
288 IEC 100 1000 102 IEC
2.6.7.
Proton conductivity
The proton conductivity of the SPEEK composite membranes was determined by using BT-512 BekkTech membrane test system at varying relative humidity (30e100%) for the fuel cell working temperatures of 60 and 80 C.
2.6.8.
Chemical stability tests
Fenton test (chemical stability test) was carried out according to the procedure described by Zhang and Mukerjee [20]. Test specimens were cut into 2 2 cm pieces. All samples were dried in vacuum at 68 C for 12 h before test. Well-dried membrane samples were treated in 3 wt.% H2O2 which contained 4 ppm Feþ2. Then the time ruptured or weight loss for a certain time was reported.
2.6.9.
Methanol permeability
Methanol permeability of the membranes was measured by using jacketed diffusion glass cell as shown in Fig. 1. The cell is composed of a two-compartment glass cell with the membrane separating the two compartments. The membrane was clamped vertically between two glass compartments. For solution agitation, magnetic stirring bar was used in each compartment. One of the compartments was filled with 70 ml, 2 M methanol solution and equal volume of distilled water was added to the other compartment after the desired temperature (25 C) was reached. Both compartments were magnetically stirred during the permeation experiments. Methanol concentrations in the water-filled cell were measured on samples picked periodically and analyzed by using a gas chromatography. The methanol diffusion coefficient was calculated with the following equation. from heater-circulator
A
B
15 w/v % methanol solution
WUð%Þ ¼ Wwet Wdry 100=Wdry
2.6.6.
to heater-circulator
Membrane
Water
Ion exchange capacity (IEC)
Ion exchange capacity (IEC) of SPEEK composite membranes was determined quantitatively by the titration method according to the previously described procedure [19]. 0.2e0.3 g of dried samples was immersed into 50 ml of saturated NaCl
Clamp Stir bar
Effective membrane area
Fig. 1 e Schematic representation of the diffusion cell.
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2CA 2AM DAB t ln 1 ¼ CAo VO DX
5 4 Volume (%)
where AM, DX and Vo are the effective area (7.6 cm2), the thickness of membrane and the volume of permeated compartment, respectively. CAo and CA are the methanol concentration in methanol chamber and water chamber. DAB is the methanol diffusion coefficient [21].
6
2
2.6.10. Gas permeability
1
The gas permeability properties of the blend membranes were evaluated according to ASTM D1434e82 and H2 flow rate was calculated according to the following equation:
0 0,01
2.6.11. Water vapor permeability Water vapor permeability of the membranes was determined by means of water vapor transmission rate (WVTR). This test has been standardized by ASTM E 96-/E-96M-05. Membrane specimens (ø: 11 mm) were placed on the glass cups containing a certain amount of desiccant (anhydrous calcium chloride) and the membrane was secured in place with the help of clamp. The cups were placed in a desiccators containing sulfuric acid solution. The test was conducted with 45e50% relative humidity outside the cup and negligible relative humidity inside the cup. The value of relative humidity outside the cup was measured by hygrometer at 19e21 C. The mass of the cup was recorded at 6 h interval for 7 days. Mass loss was plotted versus time and a straight line was obtained. Linear regression (linear regression coefficient >0.993) was used to estimate the slope of the line to calculate water vapor transmission rate as follows: WVTR ¼
Slope A
Where WVTR is the water vapor transmission rate (g/m2day) and A is the test cup mouth area (m2). Water vapor permeability is calculated by means of WVTR. WVP ¼
WVTRxL SxðR1 R2 Þ
0,1
1
10
Fig. 2 e Particle size distribution of Cs-TPA.
was applied using conc. sulfuric acid (95e97%). It is known that the DS can be controlled by changing reaction time, acid concentration and temperature, which can provide a sulfonation range of 30e100% [17]. The PEEK was sulfonated for different reaction times ranging from 3 to 112 h and temperatures from 23 to 60 C to produce polymers of various DS. In this study, SPEEK with sulfonation degree of 60 and 70% was prepared at temperature of 60 C for three and four hours, respectively. The produced sulfonated PEEK polymer was denoted as SPEEKX where X represents sulfonation degree of sulfonated poly(ether ether ketone).
3.2.
Cs-TPA preparation
Elemental analysis of Cs-TPA (Cs2.5H0.5PW12O40) for cesium and tungsten was performed by means of Inductively Coupled Plasma-Atomic Emmision Spectroscopy (ICP-AES). The composition determined by ICP analysis of the Cs/W atomic ratio, agreed with the composition calculated from the stoichiometric amount of reagents added to prepare Cs-TPA. The production of Cs-TPA was accomplished by using CsOH instead of CsCO3 and CsCl2. The particle size distribution of Cs-TPA dispersed in water has varied from 0.2 to 2 mm as shown in Fig. 2. Patterns of pure tungstophosphoric acid and cesium salt of tungstophosphoric acid are illustrated in Fig. 3. It can be seen that the location of XRD peaks of Cs-TPA is very similar to that
Where S is saturation vapor pressure at test temperature, R1 and R2 relative humidity at the desiccators and vapor sink respectively.
3.
Results and discussion
3.1.
Sulfonation of PEEK
Different sulfonation agents (conc. sulfuric acid, chlorosulphonic acid and fumed sulfuric acid) have been used in previous works for the PEEK sulfonation [22,23]. In this study, an easy and relatively cheap method for the PEEK sulfonation
100
Particle Size (micron)
VL Gas Permeability cm3STP xcm=cm2 xsxcmHg ¼ AxtxDP where V is the volume of the permeated gas, A the area of the membrane, t the time, 6P the partial pressure difference of a gas between the feed and the permeating sides of a membrane and L the membrane thickness.
3
Fig. 3 e XRD patterns of TPA and Cs-TPA samples.
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of pure TPA; indicating that the Cs salt has similar crystalline structure as the pure TPA. However, the main peaks depicting TPA are shifted toward slightly higher angles in Cs-TPA as shown in Fig. 3. We used infrared spectroscopy to observe the structural variations of tungstophosphoric acid (TPA) (Fig. 4). The Keggin anion structure of H3PW consists of a PO4 tetrahedron surrounded by twelve WO6 octahedral, which share edges in W3O13 triad groups and corners between each triad through oxygen atoms. It is possible to deduce four types of oxygen, which provides four characteristic bands in the range 1200e700 cm1 [24]. As shown in Fig. 4a, for bulk acid H3PW12O40, six characteristic peaks of its Keggin structure were observed at 1080 cm1 (PeO in central tetrahedral), 982 cm1 (W ¼ Of terminal oxygen in the Keggin structure), 888 cm1 and 800 cm1 (WeOceW), 595, and 523 cm1, which coincides with those referred in the literature [24e28]. The FTIR spectra of the Cs-TPA salt showed bands at 1081, 985, 890,
800, 595 and 524 cm1, which are also characteristic bands of bulk acid TPA. However, the typical vibration of the W ¼ Of ¼ 982 cm1 on acidic form splits into two components at 992 and 985 cm1 as Dias et al. [24] and Essayem et al. [26] observed. This shows the interaction of Csþ ions with W ¼ O. All results depicted that the production of Cs-TPA was accomplished by using CsOH instead of CsCO3 and CsCl2. The thermo-gravimetric curves of TPA and Cs-TPA are shown in Fig. 5a. It was observed that the TPA showed a weight loss in two steps. The first weight loss occurred below 100 and 200 C as a result of loss of water of crystallization (about 4% mass loss up to 200 C); whereas, the second weight loss occurred at about 500 C probably as a result of the decomposition of the Keggin anion [29]. The Csþ salt showed a weight loss below 100 C and around 400e500 C. The weight loss of Cs-TPA was less than TPA as expected and it has also higher residual weight than TPA. All results proved that CsTPA was synthesized successfully.
Fig. 4 e IR spectra of pure TPA and Cs-TPA samples a) and Cs-TPA/SPEEK60 composite membranes b).
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Fig. 5 e Thermograms of TPA and Cs-TPA samples a) Cs-TPA/SPEEK60 composite membranes b) and the composite membranes of 5% Cs-TPA/SPEEK with the different sulfonation degrees c).
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Table 1 e Glass transition temperatures (Tg) for Cs-TPA/ SPEEK composite membranes. Membrane SPEEK60
SPEEK70
Cs-TPA Content wt.%
Tg ( C)
0 5 10 15 20 0 5 10 15 20
191.2 190.5 189.9 190.6 190.1 195.6 194.2 194.6 194.4 193.8
3.3.
Composite membrane characterization
3.3.1.
FTIR studies
IR spectrums of the composite membranes with different amount of Cs-TPA are given in Fig. 4b. Most of the characteristic peaks of Cs-TPA were blocked due to the interference by SPEEK matrix. With the increase in the amount of Cs-TPA, the characteristic peaks of the Keggin structure started to appear at 1080, 982, 888 and 800 cm1. The presence of these peaks confirmed the existence of the additive in the composite membrane.
3.3.2.
TGA and DSC studies
The TGA thermograms of SPEEK60 composite membranes are displayed in Fig. 5b. It is seen that all composite membranes started to decompose in a rapid manner above 300 C. The first stage that was located between 300 and 400 C can be
Fig. 6 e Cross-sectional SEM photographs of composite SPEEK60, SPEEK70 and SPEEK82 membranes with 5% (a), 10% (b), 15% (c) and 20% (d) of Cs-TPA (330,000).
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Fig. 7 e TEM image of the SPEEK60 composite membrane containing 20 wt.% Cs-TPA particles.
3.3.3.
Membrane morphology
The morphology of SPEEK composite membranes containing Cs-TPA powders was studied by SEM. Fig. 6 shows the cross-
sectional SEM images of SPEEK60 and SPEEK70 at various CsTPA content ranging from 5 to 20%. Increase in the content of Cs-TPA in SPEEK70 caused agglomeration after 10% Cs-TPA addition. To justify the effect of sulfonation degree clearly, 5% Cs-TPA was added to SPEEK matrix with 82% degree of sulfonation. We speculate then that the increase in the sulfonic groups that portray more hydrophilic character in SPEEK might have caused the agglomeration of Cs-TPA particles. In this study, SPEEK82 was eliminated as polymer matrix in the composite membrane since the chemical stability was insufficient to be used in fuel cell system. The distribution of the Cs-TPA particles in SPEEK matrix was investigated by TEM. Fig. 7 shows the SPEEK composite membrane 50 45 40 Wat er Up t ake, %
attributed to the decomposition of sulfonic acid groups. The weight loss at 900 C decreased with the addition of inorganic particles, as expected. Fig. 5c shows the thermal stability of the membranes with different degree of sulfonation for the content of 5 wt.% Cs-TPA. Just as predicted the weight loss increases with the increase of sulfonic acid groups. Also, increasing the content of Cs-TPA from 5 to 20 wt.% enhanced the residual weight of the membranes; as expected. The DSC results are given in Table 1. A single glass transition temperature (Tg) was observed for SPEEK polymers at 191.2 C and 195.6 C for SPEEK60 and SPEEK70 respectively. The Tg’s of the polymers increased 10 C with the increase of sulfonation degree from 60% to 70%. This was attributed to the incorporation of larger sulfonyl groups that increases Tg as a result of the restrictions on the segmental movements in the PEEK .The addition of inorganic particles to the SPEEK60 and SPEEK70 membranes has no significant effect on glass transition temperature.
35 30 25 20 15
SPEEK 60
10
SPEEK 70
5 0 0
5
10
15
20
25
Cs-TPA content in membrane, wt.%
2 1,8 1,6 IEC (meq/g)
1,4 1,2 1 0,8 0,6 0,4
SPEEK 60
0,2
SPEEK 70
0 0
5
10
15
20
25
Cs-TPA content in membrane, wt. %
Fig. 8 e Proton conductivity at different Cs-TPA content.
Fig. 9 e Change of water uptake and ion exchange capacity with Cs-TPA content.
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containing 20% Cs-TPA. It can be said that Cs-TPA particles are generally homogenously dispersed in the membrane, but some particles are clearly aggregated in the polymer matrix with aggregate sizes of several hundred nanometers even in the case of the highest concentration of the particles.
3.3.4.
Proton conductivity
The proton conductivity is an important characterization method for the appropriate membrane selection in the fuel cell applications. Nafion, well known membrane, is taken as a standard material in our study. Conductivity data for SPEEK/ Cs-TPA composite membranes, taken at 80 C under 100% RH, as a function of Cs-TPA weight ratio for two different sulfonation degrees of PEEK (60 and 70) are shown in Fig. 8 where the increase in the degree of sulfonation of PEEK polymer seem to cause an increase in the conductivity value as expected. However they showed almost the same trend with the increase in the content of the Cs-TPA. Proton conductivity of SPEEK60/ Cs-TPA composite membranes slightly increases as the content of the Cs-TPA changes from 5 to 15 wt.%. No significant change was observed with the addition of Cs-TPA to SPEEK70 matrix. When chemical stability of SPEEK70 was considered, it was concluded that the best results came from the membrane with 5e10 wt.% Cs-TPA in SPEEK60, whose conductivities are 125 mS/cm and 130 mS/cm, respectively. We measured the conductivity of Nafion 117 with our test system for comparison and found a value of 133 mS/cm. Incorporation of Cs-TPA increased conductivity of the membranes about 10% of the pure SPEEK60 membranes. The incorporation of Cs-TPA increased the proton conductivity because of the acidity of the Cs-TPA as Zhang et al. reported [16]. The proton conductivity measurements were performed on four separate membranes (pristine SPEEK60 and SPEEK70) produced at different times and the average of the three proton conductivity values, and the repeatability standard deviation were calculated according to ASTM E 691e09. One sample for each membrane with different degree of sulfonation (SPEEK60 and SPEEK70) exhibited lower proton conductivity than others. For three separate membranes the repeatability standard deviation was calculated as 12% and 2% for pristine SPEEK60 and SPEEK70, respectively. We argue that the repeatability of the preparation of high sulfonation degree SPEEK membranes was higher than that of low sulfonation degree SPEEK membranes. The error bars for pristine SPEEK membranes are given in Fig. 8.
3.3.5.
Water uptake and IEC properties
The water sorption by the sulfonated polymers is known to have a profound effect on the membrane conductivity and mechanical stabilities. Fig. 9a shows the water uptake of the membranes at room temperature. With the increase of sulfonic acid groups, as a result, the water uptake values increases from 27 to 32 wt.% for pristine SPEEK60 and pristine SPEEK70, respectively. It has been reported in the literature [30] that sulfonation of the PEEK polymer results in more hydrophilic character by increasing the protonated sites (eSO3H). In our study, incorporation of Cs-TPA content from 5 to 20 wt.% caused an increase in the water uptake of the SPEEK60 composite membranes as seen from Fig. 9a; however, 10 wt.% Cs-TPA additions did not indicate significant effect.
Fig. 10 e Membrane pictures after the hot water treatment.
The addition of Cs-TPA into SPEEK70 composite membranes, which have agglomerated particles as seen from SEM and TEM images (Figs. 6 and 7), had almost no effect on the water uptake. Li et al. [31] reported the increase in water uptake after incorporation of Cs-TPA into Nafion and the reason for this was interpreted to be the hydrophilic property of Cs-TPA for the strong interaction with water. The IEC values of SPEEK/Cs-TPA composite membranes are shown in Fig. 9b. It is seen that the addition of Cs-TPA into SPEEK60 polymer in the amount of 5 and 10 wt.% led to an increased IEC value. However, incorporation of Cs-TPA particles to the SPEEK70 decreased the IEC values of the membrane. It is presumed in this case that the polymers with higher sulfonation degree interacted with the inorganic particles. The repeatability standard deviation for pristine SPEEK membranes was calculated to be 0.5% for IEC and 2% for water uptake values, respectively. The results suggest that the membrane preparation procedure is repeatable.
3.3.6.
Chemical stability
Fenton tests of the membranes showed the chemical durability of the membranes in 3 wt.% hydrogen peroxide solution which contained 4 ppm Feþ2 at 68 C. In the evaluation of the polymer when subjected to peroxide radical attack, SPEEK60/ Cs-TPA and SPEEK70/Cs-TPA composite membranes broke down completely into pieces after 6 h and 3 h, respectively. On the other hand, 0.3% weight loss was observed for Nafion 117 after 24 h at the same test conditions. The addition of the CsTPA particles in the amount of 5 and 10% did not show important difference compared with original polymer matrix in the Fenton tests; however higher sulfonation degree of the polymers led to decreased chemical stability. SPEEK70 and 60 composite membranes produced in this study ruptured in three to 6 h; whereas at the same conditions, sulfonated polyether ether ketone (SPEEK) and sulfonated polyether sulfone (SPES) with 40% DS were ruptured in only 1 h in the Fenton test [32]. On the other hand, it has been reported that fuel cell lifetime of membranes based on polybenzimidazoles (PBI) was over 5000 h whereas average rupture time of these membranes
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determined by Fenton test was only 30 min [20,33]. Since Fenton test exposes membrane to an unrealistic amount of radicals, it acts only as a fast preview test and therefore a longterm durability test in fuel cell must be also done substantially for determination of the chemical stability of its membrane. Consequently, chemical stability of the SPEEK membranes with the addition of Cs-TPA particles was improved when compared with values reported in the literature [20,33]
3.3.7.
Hydrolytic stability
We investigated the hydrolytic stability of the pristine SPEEK60 and SPEEK60/Cs-TPA with 15% and 20% concentrations of Cs-TPA composite membranes at 80 C for 48 h in water. The pristine SPEEK60 membrane was completely disintegrated at 80 C after 24 h as shown in Fig. 10, whereas 15
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and 20% Cs-TPA containing composite SPEEK60 based membranes kept the membrane form throughout 48 h with suffering only 3e3.5 wt.% weight losses indicating the improvement of hydrolytic stability of the membranes by the incorporation of the Cs-TPA particles into the matrix. In order to observe the amount of tungsten extracted into the water phase, tungsten (W) analysis was performed in the water phase by using ICP and only 1% of the Cs-TPA was observed in the water phase; indicating the chemical interaction of the CsTPA particles with the polymer. TGA results of the extracted and unextracted SPEEK/Cs-TPA based membranes are given in Fig. 11 where first decomposition temperature of the extracted SPEEK/Cs-TPA is seen to increase from 300 to 320 C because of the inter/intra-molecular interaction of the (eSO3H) due to the heat treatment of membrane at 80 C for 48 h. Due to the
Fig. 11 e Thermograms of untreated and treated a) SPEEK60/15 wt.% and b) SPEEK60/20 wt.% composite membranes.
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partial crosslinking of the (eSO3H) groups, proton conductivity of the treated membrane decreased about while membrane thermal stability increased [34] as shown in Fig. 11. On the other hand, the membranes prepared by using SPEEK70/Cs-TPA were ruptured after 24 h treatment with water at 80 C. This means that membranes prepared by using SPEEK60/Cs-TPA were more hydrolytically stable than the membranes prepared by using SPEEK70/Cs-TPA.
3.3.8.
SPEEK60 SPEEK60/10% Cs-TPA SPEEK60/15% Cs-TPA Nafion [35]
WVTR (g/m2 h)
WVP (1010 g/m.s. Pa)
6.4 6.9 5.8 e
1.7 1.78 1.3 13 (at 80 C)
Methanol permeability
For the methanol permeability measurements, SPEEK60/CsTPA 10 wt.% membrane was taken because of its higher proton conductivity and higher chemical stability. The methanol permeability of the membranes was measured at ambient temperatures. The methanol permeability of Nafion 117, SPEEK60, SPEEK70, SPEEK60/Cs-TPA 10 wt.% are, respectively, 1.2 106, 2.7 107, 5.8 107, 4.7 107 cm2/s; all much lower than that of the Nafion 117. SPEEK60/Cs-TPA 10 wt.% composite membrane showed resistance to methanol permeability with higher proton conductivity compared to Nafion 117. However, methanol permeability of SPEEK60/CsTPA 10 wt.% composite membrane was slightly higher than SPEEK60 membrane. The repeatability standard deviation between methanol permeability of SPEEK60/Cs-TPA 10 wt.% membranes that we produced in different times confirms the repeatability of membrane preparation.
3.3.9.
Table 2 e Water vapor transmission and permeability of SPEEK60 composite membranes.
Gas permeability
Hydrogen permeability of SPEEK60 composite membranes and Nafion was measured at 25 C because gas permeability could greatly affect the performance and the durability of PEMFCs and DMFCs. SPEEK60 composite membranes containing 5 wt.% and 10 wt.% Cs-TPA showed lower hydrogen permeability than that of Nafion membrane (Fig. 12).
3.3.10. Water vapor permeability Water vapor transmission through proton exchange membranes is important for fuel cell operation and the water vapor permeance can give an indication of how facile water is transported through the membrane A high water vapor permeance signifies higher diffusion of the water through the membrane during fuel cell operation, which could lead to a more uniform distribution of water during fuel cell operation [35].
Fig. 12 e Hydrogen permeability of the membranes.
The incorporation of Cs-TPA particles into SPEEK membranes did not seem to have any significant effect on the water vapor transmission and water vapor permeability values at 20 C (Table 2). However, increasing Cs-TPA content from 10 to 15% gave only slightly lower WVTR and WVP values. As expected, the pristine SPEEK and composite membranes gave much lower water vapor permeability and we suggest that the membranes produced in this study are moderate water vapor barriers.
4.
Conclusion
In this study, Cs-TPA was prepared and structurally characterized by FTIR and XRD that proved showing similar properties to those reported in the literature. We used Cs-TPA and SPEEK to prepare composite membranes which are the first attempt of composite membrane with Cs-TPA and SPEEK with different sulfonation degree (60 and 70) as an organic polymeric matrix and proton conducting polymer as well. SPEEK was easy to prepare and was added to Cs-TPA by low cost process. The main conclusions of this study are as follows; 1) Weight loss at 900 C increased with the addition of inorganic particles, as expected. The hydrolytic stability of the SPEEK/Cs-TPA based composite membranes was improved with the incorporation of the Cs-TPA particles into the matrix. 2) Addition of Cs-TPA into SPEEK60 polymer in the amount of 5 and 10% increased IEC value while addition of Cs-TPA to the SPEEK70 does not have any affect on the IEC values of the membrane. Thus, we suggest that the high sulfonation degree in the polymers caused the interaction with Cs-TPA particles and agglomeration. 3) Membranes prepared by using SPEEK60/Cs-TPA were hydrolytically more stable than SPEEK70/Cs-TPA composite membranes. SPEEK60 composite membranes containing 5 wt.% and 10 wt.% Cs-TPA showed lower hydrogen and water vapor permeability than that of Nafion membrane. 4) The proton conductivity values for SPEEK60/Cs-TPA composite membranes slightly increased with the addition of the Cs-TPA between 5 and 10 wt.%. We obtained the best results from the membrane 10 wt.%. Cs-TPA in SPEEK60 with conductivity value of 1.3 101 S/cm. 5) The observed low methanol, water vapor, and hydrogen permeability, promising proton conductivity value, improved chemical stability, thermal properties and hydrolytic stability of SPEEK60/Cs-TPA composite membranes suggest that these membranes appeared to be highly potential candidate for fuel cell applications
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Acknowledgements [17]
This work is financially supported by Tu¨rk Demir Do¨ku¨m Fab. A.S. The authors are pleased to acknowledge to Victrex for the donation of Polyetheretherketone (PEEK). The authors thank to Mrs. Handan Karakale, Nevin Bekir, Zekayi Korlu and Mustafa Candemir for valuable technical assistance in laboratory experiments. Operation of SEM was possible with help _ of Orhan Ipek and Cem Berk.
[18]
[19]
[20]
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Effect of cesium salt of tungstophosphoric acid (Cs-TPA) on the properties of sulfonated polyether ether ketone (SPEEK) composite membranes for fuel cell applications an a,*, Tu¨lay Y. Inan a,*, Elif Unveren a, Metin Kaya b Hacer Dog a
The Scientific and Technological Research Council of Turkey (TU¨BI˙TAK), Marmara Research Center, Chemistry Institute, P.K. 21, 41470 Gebze-Kocaeli, Turkey b _ _ ¨nu¨ Cad. No:245 Bozu¨yu¨k/Bilecik, Turkey DEMIRDO¨KU¨M A.S. 4 Eylu¨l Mah, Ismet Ino
article info
abstract
Article history:
We have prepared composite membranes for fuel cell applications. Cesium salt of tung-
Received 22 January 2010
stophosphoric acid (Cs-TPA) particles was synthesized by aqueous solutions of tung-
Received in revised form
stophosphoric acid and cesium hydroxide and, Cs-TPA particles and sulfonated (polyether
25 March 2010
ether ketone) (SPEEK) with two sulfonation degrees (DS), 60 and 70%have been used. We
Accepted 8 May 2010
examined both the effects of Cs-TPA in SPEEK membranes as functions of sulfonation
Available online 11 June 2010
degrees of SPEEK and the content of Cs-TPA. The performance of the composite membranes was evaluated in terms of water uptake, ion exchange capacity, proton
Keywords:
conductivity, chemical stability, hydrolytic stability, thermal stability and methanol
Sulfonated polyether ether ketone
permeability. The morphology of the membranes was investigated with SEM micrographs.
(SPEEK)
Increasing sulfonation degree of SPEEK from 60 to 70 caused agglomeration of the Cs-TPA
Cesium salt of tungstophosphoric
particles. The methanol permeability was reduced to 4.7 107 cm2/s for SPEEK (DS: 60%)/
acid
Cs-TPA membrane with 10 wt.% Cs-TPA concentration, and acceptable proton conductivity
Direct methanol fuel cell
of 1.3 101 S/cm was achieved at 80 C under 100% RH. The weight loss at 900 C increased
Proton conductivity
with the addition of inorganic particles, as expected. The hydrolytic stability of the SPEEK/ Cs-TPA based composite membranes was improved with the incorporation of the Cs-TPA particles into the matrix. We also noted that SPEEK60/Cs-TPA composite membranes were hydrolytically more stable than SPEEK70/Cs-TPA composite membranes. On the other hand, Methanol, water vapor, and hydrogen permeability values of SPEEK60 composite membranes were found to be lower than that of Nafion. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The proton conducting membrane which is the key component of a fuel cell system allows the transport of protons and water through it, but remains mostly impermeable to hydrogen and oxygen for proton exchange membrane fuel cell (PEMFC) and to the methanol for direct methanol fuel cell (DMFC) [1]. Commercially available Nafion membrane,
a perfluorosulfonic acid solid polymer electrolyte membrane produced by the Dupont Co., is known to be so far the best performing polymer electrolyte [2]. Recently, alternative membranes, especially partially-fluorinated and non-fluorinated ionomer membranes are currently under study. The following polymers have been regarded as possible candidates for new proton-conductive polymers: Polyamide-imide (PAI), Polybenzimidazole (PBI), Polyether ether ketone (PEEK),
* Corresponding authors. Tel.: þ90 262 677 2913; fax: þ90 262 641 2309. an), [email protected] (T.Y. Inan). E-mail addresses: [email protected] (H. Dog 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.05.045
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Polyether Sulfone (PES) and Polyimide (PI) [1,2]. Among them, the sulfonated poly(ether ether ketone) has been studied extensively due to some apparent advantages such as low cost, high proton conductivity and low methanol permeability [3e7] Inorganic compounds have been incorporated into the polymer matrix in order to enhance the proton conductivity [8]. Heteropoly acids (HPA) with Keggin anion structures have received attention due to their simple preparation and strong acidity in development of composite membranes for PEMFCs [9,10]. High solubility of HPA additive in aqueous media is a limiting factor for the membrane performance, but HPA can be immobilized by the formation of insoluble salts, by imbedding in a silicate matrix, or by covalently linking an organic moiety to a coordination site in a lacunary HPA [11]. The most studied insoluble salt of HPA is cesium salt of tungstophosphoric acid (Cs-TPA, Cs2.5H0.5PW12O40), a well known acidic catalyst in which the residual protons are more acidic than the homogenous acid catalysts (e.g., H2SO4, H3PW12O40 and ptoluenesulfonic acid). It has been reported that the acidity per unit acid site of Cs-TPA is superior to those of Amberlyst-15 and Nafion-H and Cs-TPA has been reported as a “watertolerant” solid acid catalyst desirable for environmentally benign industrial processes [12]. Recently, acidic Cs-TPA has been synthesized as an additive used in PEMFC [13e15] resulting in improvement of conductivity and possible increase in the thermal stability of the membrane. Ramani et al. [13] reported that Nafion/salt (Csþ, NH4 þ, þ and Tlþ) of tungstophosphoric acid composite Rb membranes demonstrate low H2 crossover currents and similar proton conductivity in comparison to pure Nafion membrane in the order of 1.6 102 S/cm at 120 C and 35% relative humidity. Cs-TPA/Nafion/PTFE self-humidifying composite membrane for PEMFC was prepared by recasting the Cs-TPA/Nafion self-humidifying layer onto the two side of the Nafion/PTFE composite membrane with a solution recast membrane and it was shown that the Cs-TPA/Nafion selfhumidifying layer not only contributed to humidify the membrane but also to the improvement of the open-circuit voltage of the fuel cell [14]. Wang et al. [15] impregnated CsTPA onto silicon dioxide and prepared Cs-TPA eSiO2/Nafion self-humidifying composite membrane. Their work compared the performance of the fuel cell with commercial Nafion NRE-212 membrane and reported that the cell performance with the self-humidifying composite membrane was obviously improved under both humidified and dry conditions at 60 and 80 C. SPEEK (sulfonated polyether ether keton)/Pt-Cs2.5 self-humidifying membrane developed by Zhang et al. [16] exhibited higher water absorbing and proton-conductive properties relative to the plain SPEEK membrane. In the present study, we synthesized Cs-TPA particles from aqueous solutions of tungstophosphoric acid and cesium hydroxide. We used SPEEK polymer as organic matrix and proton conductor with the sulfonation degrees (DS) of 60 and 70%. In this study, we changed the contents of Cs-TPA particles in the composite membranes between 5 wt.% and 20 wt.% and we used Cs-TPA particles as proton conducting site of the membrane as SPEEK. We later investigated the effect of CsTPA particles and its content on the membrane performance by X-ray power diffraction (XRD), Fourier Transform Infrared (FTIR), scanning electron microscopy (SEM), thermal
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gravimetric analyzer (TGA) and differential scanning calorimeter (DSC). We also investigated the ion exchange capacity (IEC) value, water uptake, proton conductivity and methanol permeability of these membranes.
2.
Experimental
2.1.
Materials
Polyether ether ketone (PEEK) 450 PF obtained in powder form (particle size of 25 mm) from Victrex was dried in vacuum at 100 C overnight. Sulfuric acid (95e97 wt.%), hydrogen peroxide (H2O2) and dimethylacetamide (DMAc, for synthesis) were obtained from Fluka. Tungstophosphoric acid, H3PW12O40$29H2O (TPA) and cesium hydroxide (99.9 wt.%) were received from Aldrich. All chemicals were used as received without further purification.
2.2.
Sulfonation of PEEK
PEEK was sulfonated by using concentrated sulfuric acid (conc. H2SO4) following the same procedure reported by Zaidi [17]. Five g dried PEEK was dissolved completely in 100 ml concentrated sulfuric acid (95e97%) under nitrogen at 25 C by strong mechanical agitation at least 800 rpm to maintain a homogenous reaction media. Later, the temperature was increased to the desired reaction temperature. The desired sulfonation degrees of 60, 70, and 82% were achieved within 90e130 h, and then the reaction solution was gradually precipitated into cold deionized water under agitation. The precipitate was washed with deionized water and filtered at several times until the pH value was approximately between 6 and 7. The sulfonated polymer samples were dried in vacuum at room temperature for 12 h and then at 60 C for 12 h. SPEEK with 60%, 70% and 82% degree of sulfonation (DS) was used in the membrane preparation.
2.3.
Cs-TPA preparation
Three g of dry tungstophosphoric acid was dissolved in 10 ml of water and stirred for 15 min. As a stoichiometric ratio, 0.8 g of Cs(OH) solution was added drop-wise to tungstophosphoric acid solution and kept at the room temperature for 24 h [18]. The resulting precipitate was recovered from the solution by evaporation at 100 C. The inorganic powder was calcined at 200 C for 3 h.
2.4.
Membrane preparation
Membranes were prepared by casting 20% (w/v) N,N-Dimethylacetamide (DMAc) solutions of SPEEK containing Cs-TPA particles on a clean glass plate. Membranes were dried under vacuum for 8 h at 30e60 C and another 15 h at 60e80 C. Then, membranes were soaked and kept in deionized water until testing. The membranes were analyzed by Headspace GC/MS and NMR that assured no solvent residue remained in the membranes. The membrane thickness was measured at dry state using a digital micrometer. The thickness of prepared membranes ranged from 80 to 100 mm.
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2.5.
Characterization of Cs-TPA
2.5.1.
Particle size measurement
Particle size measurement of the cesium salt of tungstophosphoric acid was performed by Laser Beam Scattering Technique (Malvern Hydro 2000M/MU Particle Size Analyzer).
solution at 50 C for 48 h to exchange Hþ with Naþ. The polymer solution containing released Hþ was titrated with a 0.1 N NaOH solution by using bromothymol blue as an indicator. IEC values for both sulfonated polymers (Ep) and membranes (Em) were calculated by using the following equation: IEC ¼
2.5.2.
X-ray diffraction (XRD)
Tungstophosphoric acid and cesium salt of tungstophosphoric acid were analyzed by X-ray diffraction (XRD, 600 SHIMADZU). The radiation was CuKa of wavelength 1.5405 A.
2.6.
Characterization of composite membranes
2.6.1.
Morphological characterization
Membrane morphology was investigated under a scanning electron microscope (JEOL JSM-6335 F) and Transmission Electron Microscope (JEOL JEM-2100 HRTEM). The TEM analysis was operated at 20 kV accelerating voltage and at 200 kV with LaB6 filament carbon coated with 200 mesh copper grid.
2.6.2.
Thermogravimetric analysis (TGA)
The thermal stability of the membranes was determined by using Perkin Elmer PYRIS 1 TGA. The samples were heated, under nitrogen atmosphere, up to 150 C and kept at this temperature for half an hour to remove moisture completely. The samples were cooled down to 80 C and then re-heated up to 900 C with a heating rate of 10 C/min.
2.6.3.
Differential scanning calorimeter (DSC)
DSC analyses were performed with Perkin Elmer Jade DSC at a heating rate of 10 C/min under nitrogen atmosphere. All data were collected from a second heating cycle and the glass transition temperatures (Tg) were calculated as a midpoint of thermogram.
2.6.4.
Fourier transform infrared spectroscopy (FTIR)
FTIR spectra were recorded for membranes and heteropoly acid salt using Perkin Elmer Pyris 1 FTIR (Fourier Transform Infrared) Spectrophotometer. Membrane samples were prepared by making KBr pellets.
2.6.5.
Water uptake
Water uptake value (WU %) was determined according to the ASTM D 570-98. SPEEK composite membranes were conditioned in an oven at 105 C for 1 h and then at 50 C for 24 h. Conditioned samples were cooled to room temperature in desiccators, immediately weighed (Wdry), and then immersed in water at room temperature for 24 h. The membranes were removed from the water, wiped free of the moisture with a dried cloth and then weighed immediately (Wwet) again. Percentage increase in weight during immersion was calculated as follows;
VNaOH NNaOH FNaOH Wp
where VNaOH is titrated volume of NaOH solution (L), NNaOH is the normality of NaOH solution and Wp is the dried weight of polymer or membrane (g). SD (%) for SPEEK was calculated by using the following equation: SDð%Þ ¼
288 IEC 100 1000 102 IEC
2.6.7.
Proton conductivity
The proton conductivity of the SPEEK composite membranes was determined by using BT-512 BekkTech membrane test system at varying relative humidity (30e100%) for the fuel cell working temperatures of 60 and 80 C.
2.6.8.
Chemical stability tests
Fenton test (chemical stability test) was carried out according to the procedure described by Zhang and Mukerjee [20]. Test specimens were cut into 2 2 cm pieces. All samples were dried in vacuum at 68 C for 12 h before test. Well-dried membrane samples were treated in 3 wt.% H2O2 which contained 4 ppm Feþ2. Then the time ruptured or weight loss for a certain time was reported.
2.6.9.
Methanol permeability
Methanol permeability of the membranes was measured by using jacketed diffusion glass cell as shown in Fig. 1. The cell is composed of a two-compartment glass cell with the membrane separating the two compartments. The membrane was clamped vertically between two glass compartments. For solution agitation, magnetic stirring bar was used in each compartment. One of the compartments was filled with 70 ml, 2 M methanol solution and equal volume of distilled water was added to the other compartment after the desired temperature (25 C) was reached. Both compartments were magnetically stirred during the permeation experiments. Methanol concentrations in the water-filled cell were measured on samples picked periodically and analyzed by using a gas chromatography. The methanol diffusion coefficient was calculated with the following equation. from heater-circulator
A
B
15 w/v % methanol solution
WUð%Þ ¼ Wwet Wdry 100=Wdry
2.6.6.
to heater-circulator
Membrane
Water
Ion exchange capacity (IEC)
Ion exchange capacity (IEC) of SPEEK composite membranes was determined quantitatively by the titration method according to the previously described procedure [19]. 0.2e0.3 g of dried samples was immersed into 50 ml of saturated NaCl
Clamp Stir bar
Effective membrane area
Fig. 1 e Schematic representation of the diffusion cell.
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2CA 2AM DAB t ln 1 ¼ CAo VO DX
5 4 Volume (%)
where AM, DX and Vo are the effective area (7.6 cm2), the thickness of membrane and the volume of permeated compartment, respectively. CAo and CA are the methanol concentration in methanol chamber and water chamber. DAB is the methanol diffusion coefficient [21].
6
2
2.6.10. Gas permeability
1
The gas permeability properties of the blend membranes were evaluated according to ASTM D1434e82 and H2 flow rate was calculated according to the following equation:
0 0,01
2.6.11. Water vapor permeability Water vapor permeability of the membranes was determined by means of water vapor transmission rate (WVTR). This test has been standardized by ASTM E 96-/E-96M-05. Membrane specimens (ø: 11 mm) were placed on the glass cups containing a certain amount of desiccant (anhydrous calcium chloride) and the membrane was secured in place with the help of clamp. The cups were placed in a desiccators containing sulfuric acid solution. The test was conducted with 45e50% relative humidity outside the cup and negligible relative humidity inside the cup. The value of relative humidity outside the cup was measured by hygrometer at 19e21 C. The mass of the cup was recorded at 6 h interval for 7 days. Mass loss was plotted versus time and a straight line was obtained. Linear regression (linear regression coefficient >0.993) was used to estimate the slope of the line to calculate water vapor transmission rate as follows: WVTR ¼
Slope A
Where WVTR is the water vapor transmission rate (g/m2day) and A is the test cup mouth area (m2). Water vapor permeability is calculated by means of WVTR. WVP ¼
WVTRxL SxðR1 R2 Þ
0,1
1
10
Fig. 2 e Particle size distribution of Cs-TPA.
was applied using conc. sulfuric acid (95e97%). It is known that the DS can be controlled by changing reaction time, acid concentration and temperature, which can provide a sulfonation range of 30e100% [17]. The PEEK was sulfonated for different reaction times ranging from 3 to 112 h and temperatures from 23 to 60 C to produce polymers of various DS. In this study, SPEEK with sulfonation degree of 60 and 70% was prepared at temperature of 60 C for three and four hours, respectively. The produced sulfonated PEEK polymer was denoted as SPEEKX where X represents sulfonation degree of sulfonated poly(ether ether ketone).
3.2.
Cs-TPA preparation
Elemental analysis of Cs-TPA (Cs2.5H0.5PW12O40) for cesium and tungsten was performed by means of Inductively Coupled Plasma-Atomic Emmision Spectroscopy (ICP-AES). The composition determined by ICP analysis of the Cs/W atomic ratio, agreed with the composition calculated from the stoichiometric amount of reagents added to prepare Cs-TPA. The production of Cs-TPA was accomplished by using CsOH instead of CsCO3 and CsCl2. The particle size distribution of Cs-TPA dispersed in water has varied from 0.2 to 2 mm as shown in Fig. 2. Patterns of pure tungstophosphoric acid and cesium salt of tungstophosphoric acid are illustrated in Fig. 3. It can be seen that the location of XRD peaks of Cs-TPA is very similar to that
Where S is saturation vapor pressure at test temperature, R1 and R2 relative humidity at the desiccators and vapor sink respectively.
3.
Results and discussion
3.1.
Sulfonation of PEEK
Different sulfonation agents (conc. sulfuric acid, chlorosulphonic acid and fumed sulfuric acid) have been used in previous works for the PEEK sulfonation [22,23]. In this study, an easy and relatively cheap method for the PEEK sulfonation
100
Particle Size (micron)
VL Gas Permeability cm3STP xcm=cm2 xsxcmHg ¼ AxtxDP where V is the volume of the permeated gas, A the area of the membrane, t the time, 6P the partial pressure difference of a gas between the feed and the permeating sides of a membrane and L the membrane thickness.
3
Fig. 3 e XRD patterns of TPA and Cs-TPA samples.
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of pure TPA; indicating that the Cs salt has similar crystalline structure as the pure TPA. However, the main peaks depicting TPA are shifted toward slightly higher angles in Cs-TPA as shown in Fig. 3. We used infrared spectroscopy to observe the structural variations of tungstophosphoric acid (TPA) (Fig. 4). The Keggin anion structure of H3PW consists of a PO4 tetrahedron surrounded by twelve WO6 octahedral, which share edges in W3O13 triad groups and corners between each triad through oxygen atoms. It is possible to deduce four types of oxygen, which provides four characteristic bands in the range 1200e700 cm1 [24]. As shown in Fig. 4a, for bulk acid H3PW12O40, six characteristic peaks of its Keggin structure were observed at 1080 cm1 (PeO in central tetrahedral), 982 cm1 (W ¼ Of terminal oxygen in the Keggin structure), 888 cm1 and 800 cm1 (WeOceW), 595, and 523 cm1, which coincides with those referred in the literature [24e28]. The FTIR spectra of the Cs-TPA salt showed bands at 1081, 985, 890,
800, 595 and 524 cm1, which are also characteristic bands of bulk acid TPA. However, the typical vibration of the W ¼ Of ¼ 982 cm1 on acidic form splits into two components at 992 and 985 cm1 as Dias et al. [24] and Essayem et al. [26] observed. This shows the interaction of Csþ ions with W ¼ O. All results depicted that the production of Cs-TPA was accomplished by using CsOH instead of CsCO3 and CsCl2. The thermo-gravimetric curves of TPA and Cs-TPA are shown in Fig. 5a. It was observed that the TPA showed a weight loss in two steps. The first weight loss occurred below 100 and 200 C as a result of loss of water of crystallization (about 4% mass loss up to 200 C); whereas, the second weight loss occurred at about 500 C probably as a result of the decomposition of the Keggin anion [29]. The Csþ salt showed a weight loss below 100 C and around 400e500 C. The weight loss of Cs-TPA was less than TPA as expected and it has also higher residual weight than TPA. All results proved that CsTPA was synthesized successfully.
Fig. 4 e IR spectra of pure TPA and Cs-TPA samples a) and Cs-TPA/SPEEK60 composite membranes b).
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Fig. 5 e Thermograms of TPA and Cs-TPA samples a) Cs-TPA/SPEEK60 composite membranes b) and the composite membranes of 5% Cs-TPA/SPEEK with the different sulfonation degrees c).
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Table 1 e Glass transition temperatures (Tg) for Cs-TPA/ SPEEK composite membranes. Membrane SPEEK60
SPEEK70
Cs-TPA Content wt.%
Tg ( C)
0 5 10 15 20 0 5 10 15 20
191.2 190.5 189.9 190.6 190.1 195.6 194.2 194.6 194.4 193.8
3.3.
Composite membrane characterization
3.3.1.
FTIR studies
IR spectrums of the composite membranes with different amount of Cs-TPA are given in Fig. 4b. Most of the characteristic peaks of Cs-TPA were blocked due to the interference by SPEEK matrix. With the increase in the amount of Cs-TPA, the characteristic peaks of the Keggin structure started to appear at 1080, 982, 888 and 800 cm1. The presence of these peaks confirmed the existence of the additive in the composite membrane.
3.3.2.
TGA and DSC studies
The TGA thermograms of SPEEK60 composite membranes are displayed in Fig. 5b. It is seen that all composite membranes started to decompose in a rapid manner above 300 C. The first stage that was located between 300 and 400 C can be
Fig. 6 e Cross-sectional SEM photographs of composite SPEEK60, SPEEK70 and SPEEK82 membranes with 5% (a), 10% (b), 15% (c) and 20% (d) of Cs-TPA (330,000).
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Fig. 7 e TEM image of the SPEEK60 composite membrane containing 20 wt.% Cs-TPA particles.
3.3.3.
Membrane morphology
The morphology of SPEEK composite membranes containing Cs-TPA powders was studied by SEM. Fig. 6 shows the cross-
sectional SEM images of SPEEK60 and SPEEK70 at various CsTPA content ranging from 5 to 20%. Increase in the content of Cs-TPA in SPEEK70 caused agglomeration after 10% Cs-TPA addition. To justify the effect of sulfonation degree clearly, 5% Cs-TPA was added to SPEEK matrix with 82% degree of sulfonation. We speculate then that the increase in the sulfonic groups that portray more hydrophilic character in SPEEK might have caused the agglomeration of Cs-TPA particles. In this study, SPEEK82 was eliminated as polymer matrix in the composite membrane since the chemical stability was insufficient to be used in fuel cell system. The distribution of the Cs-TPA particles in SPEEK matrix was investigated by TEM. Fig. 7 shows the SPEEK composite membrane 50 45 40 Wat er Up t ake, %
attributed to the decomposition of sulfonic acid groups. The weight loss at 900 C decreased with the addition of inorganic particles, as expected. Fig. 5c shows the thermal stability of the membranes with different degree of sulfonation for the content of 5 wt.% Cs-TPA. Just as predicted the weight loss increases with the increase of sulfonic acid groups. Also, increasing the content of Cs-TPA from 5 to 20 wt.% enhanced the residual weight of the membranes; as expected. The DSC results are given in Table 1. A single glass transition temperature (Tg) was observed for SPEEK polymers at 191.2 C and 195.6 C for SPEEK60 and SPEEK70 respectively. The Tg’s of the polymers increased 10 C with the increase of sulfonation degree from 60% to 70%. This was attributed to the incorporation of larger sulfonyl groups that increases Tg as a result of the restrictions on the segmental movements in the PEEK .The addition of inorganic particles to the SPEEK60 and SPEEK70 membranes has no significant effect on glass transition temperature.
35 30 25 20 15
SPEEK 60
10
SPEEK 70
5 0 0
5
10
15
20
25
Cs-TPA content in membrane, wt.%
2 1,8 1,6 IEC (meq/g)
1,4 1,2 1 0,8 0,6 0,4
SPEEK 60
0,2
SPEEK 70
0 0
5
10
15
20
25
Cs-TPA content in membrane, wt. %
Fig. 8 e Proton conductivity at different Cs-TPA content.
Fig. 9 e Change of water uptake and ion exchange capacity with Cs-TPA content.
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containing 20% Cs-TPA. It can be said that Cs-TPA particles are generally homogenously dispersed in the membrane, but some particles are clearly aggregated in the polymer matrix with aggregate sizes of several hundred nanometers even in the case of the highest concentration of the particles.
3.3.4.
Proton conductivity
The proton conductivity is an important characterization method for the appropriate membrane selection in the fuel cell applications. Nafion, well known membrane, is taken as a standard material in our study. Conductivity data for SPEEK/ Cs-TPA composite membranes, taken at 80 C under 100% RH, as a function of Cs-TPA weight ratio for two different sulfonation degrees of PEEK (60 and 70) are shown in Fig. 8 where the increase in the degree of sulfonation of PEEK polymer seem to cause an increase in the conductivity value as expected. However they showed almost the same trend with the increase in the content of the Cs-TPA. Proton conductivity of SPEEK60/ Cs-TPA composite membranes slightly increases as the content of the Cs-TPA changes from 5 to 15 wt.%. No significant change was observed with the addition of Cs-TPA to SPEEK70 matrix. When chemical stability of SPEEK70 was considered, it was concluded that the best results came from the membrane with 5e10 wt.% Cs-TPA in SPEEK60, whose conductivities are 125 mS/cm and 130 mS/cm, respectively. We measured the conductivity of Nafion 117 with our test system for comparison and found a value of 133 mS/cm. Incorporation of Cs-TPA increased conductivity of the membranes about 10% of the pure SPEEK60 membranes. The incorporation of Cs-TPA increased the proton conductivity because of the acidity of the Cs-TPA as Zhang et al. reported [16]. The proton conductivity measurements were performed on four separate membranes (pristine SPEEK60 and SPEEK70) produced at different times and the average of the three proton conductivity values, and the repeatability standard deviation were calculated according to ASTM E 691e09. One sample for each membrane with different degree of sulfonation (SPEEK60 and SPEEK70) exhibited lower proton conductivity than others. For three separate membranes the repeatability standard deviation was calculated as 12% and 2% for pristine SPEEK60 and SPEEK70, respectively. We argue that the repeatability of the preparation of high sulfonation degree SPEEK membranes was higher than that of low sulfonation degree SPEEK membranes. The error bars for pristine SPEEK membranes are given in Fig. 8.
3.3.5.
Water uptake and IEC properties
The water sorption by the sulfonated polymers is known to have a profound effect on the membrane conductivity and mechanical stabilities. Fig. 9a shows the water uptake of the membranes at room temperature. With the increase of sulfonic acid groups, as a result, the water uptake values increases from 27 to 32 wt.% for pristine SPEEK60 and pristine SPEEK70, respectively. It has been reported in the literature [30] that sulfonation of the PEEK polymer results in more hydrophilic character by increasing the protonated sites (eSO3H). In our study, incorporation of Cs-TPA content from 5 to 20 wt.% caused an increase in the water uptake of the SPEEK60 composite membranes as seen from Fig. 9a; however, 10 wt.% Cs-TPA additions did not indicate significant effect.
Fig. 10 e Membrane pictures after the hot water treatment.
The addition of Cs-TPA into SPEEK70 composite membranes, which have agglomerated particles as seen from SEM and TEM images (Figs. 6 and 7), had almost no effect on the water uptake. Li et al. [31] reported the increase in water uptake after incorporation of Cs-TPA into Nafion and the reason for this was interpreted to be the hydrophilic property of Cs-TPA for the strong interaction with water. The IEC values of SPEEK/Cs-TPA composite membranes are shown in Fig. 9b. It is seen that the addition of Cs-TPA into SPEEK60 polymer in the amount of 5 and 10 wt.% led to an increased IEC value. However, incorporation of Cs-TPA particles to the SPEEK70 decreased the IEC values of the membrane. It is presumed in this case that the polymers with higher sulfonation degree interacted with the inorganic particles. The repeatability standard deviation for pristine SPEEK membranes was calculated to be 0.5% for IEC and 2% for water uptake values, respectively. The results suggest that the membrane preparation procedure is repeatable.
3.3.6.
Chemical stability
Fenton tests of the membranes showed the chemical durability of the membranes in 3 wt.% hydrogen peroxide solution which contained 4 ppm Feþ2 at 68 C. In the evaluation of the polymer when subjected to peroxide radical attack, SPEEK60/ Cs-TPA and SPEEK70/Cs-TPA composite membranes broke down completely into pieces after 6 h and 3 h, respectively. On the other hand, 0.3% weight loss was observed for Nafion 117 after 24 h at the same test conditions. The addition of the CsTPA particles in the amount of 5 and 10% did not show important difference compared with original polymer matrix in the Fenton tests; however higher sulfonation degree of the polymers led to decreased chemical stability. SPEEK70 and 60 composite membranes produced in this study ruptured in three to 6 h; whereas at the same conditions, sulfonated polyether ether ketone (SPEEK) and sulfonated polyether sulfone (SPES) with 40% DS were ruptured in only 1 h in the Fenton test [32]. On the other hand, it has been reported that fuel cell lifetime of membranes based on polybenzimidazoles (PBI) was over 5000 h whereas average rupture time of these membranes
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determined by Fenton test was only 30 min [20,33]. Since Fenton test exposes membrane to an unrealistic amount of radicals, it acts only as a fast preview test and therefore a longterm durability test in fuel cell must be also done substantially for determination of the chemical stability of its membrane. Consequently, chemical stability of the SPEEK membranes with the addition of Cs-TPA particles was improved when compared with values reported in the literature [20,33]
3.3.7.
Hydrolytic stability
We investigated the hydrolytic stability of the pristine SPEEK60 and SPEEK60/Cs-TPA with 15% and 20% concentrations of Cs-TPA composite membranes at 80 C for 48 h in water. The pristine SPEEK60 membrane was completely disintegrated at 80 C after 24 h as shown in Fig. 10, whereas 15
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and 20% Cs-TPA containing composite SPEEK60 based membranes kept the membrane form throughout 48 h with suffering only 3e3.5 wt.% weight losses indicating the improvement of hydrolytic stability of the membranes by the incorporation of the Cs-TPA particles into the matrix. In order to observe the amount of tungsten extracted into the water phase, tungsten (W) analysis was performed in the water phase by using ICP and only 1% of the Cs-TPA was observed in the water phase; indicating the chemical interaction of the CsTPA particles with the polymer. TGA results of the extracted and unextracted SPEEK/Cs-TPA based membranes are given in Fig. 11 where first decomposition temperature of the extracted SPEEK/Cs-TPA is seen to increase from 300 to 320 C because of the inter/intra-molecular interaction of the (eSO3H) due to the heat treatment of membrane at 80 C for 48 h. Due to the
Fig. 11 e Thermograms of untreated and treated a) SPEEK60/15 wt.% and b) SPEEK60/20 wt.% composite membranes.
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partial crosslinking of the (eSO3H) groups, proton conductivity of the treated membrane decreased about while membrane thermal stability increased [34] as shown in Fig. 11. On the other hand, the membranes prepared by using SPEEK70/Cs-TPA were ruptured after 24 h treatment with water at 80 C. This means that membranes prepared by using SPEEK60/Cs-TPA were more hydrolytically stable than the membranes prepared by using SPEEK70/Cs-TPA.
3.3.8.
SPEEK60 SPEEK60/10% Cs-TPA SPEEK60/15% Cs-TPA Nafion [35]
WVTR (g/m2 h)
WVP (1010 g/m.s. Pa)
6.4 6.9 5.8 e
1.7 1.78 1.3 13 (at 80 C)
Methanol permeability
For the methanol permeability measurements, SPEEK60/CsTPA 10 wt.% membrane was taken because of its higher proton conductivity and higher chemical stability. The methanol permeability of the membranes was measured at ambient temperatures. The methanol permeability of Nafion 117, SPEEK60, SPEEK70, SPEEK60/Cs-TPA 10 wt.% are, respectively, 1.2 106, 2.7 107, 5.8 107, 4.7 107 cm2/s; all much lower than that of the Nafion 117. SPEEK60/Cs-TPA 10 wt.% composite membrane showed resistance to methanol permeability with higher proton conductivity compared to Nafion 117. However, methanol permeability of SPEEK60/CsTPA 10 wt.% composite membrane was slightly higher than SPEEK60 membrane. The repeatability standard deviation between methanol permeability of SPEEK60/Cs-TPA 10 wt.% membranes that we produced in different times confirms the repeatability of membrane preparation.
3.3.9.
Table 2 e Water vapor transmission and permeability of SPEEK60 composite membranes.
Gas permeability
Hydrogen permeability of SPEEK60 composite membranes and Nafion was measured at 25 C because gas permeability could greatly affect the performance and the durability of PEMFCs and DMFCs. SPEEK60 composite membranes containing 5 wt.% and 10 wt.% Cs-TPA showed lower hydrogen permeability than that of Nafion membrane (Fig. 12).
3.3.10. Water vapor permeability Water vapor transmission through proton exchange membranes is important for fuel cell operation and the water vapor permeance can give an indication of how facile water is transported through the membrane A high water vapor permeance signifies higher diffusion of the water through the membrane during fuel cell operation, which could lead to a more uniform distribution of water during fuel cell operation [35].
Fig. 12 e Hydrogen permeability of the membranes.
The incorporation of Cs-TPA particles into SPEEK membranes did not seem to have any significant effect on the water vapor transmission and water vapor permeability values at 20 C (Table 2). However, increasing Cs-TPA content from 10 to 15% gave only slightly lower WVTR and WVP values. As expected, the pristine SPEEK and composite membranes gave much lower water vapor permeability and we suggest that the membranes produced in this study are moderate water vapor barriers.
4.
Conclusion
In this study, Cs-TPA was prepared and structurally characterized by FTIR and XRD that proved showing similar properties to those reported in the literature. We used Cs-TPA and SPEEK to prepare composite membranes which are the first attempt of composite membrane with Cs-TPA and SPEEK with different sulfonation degree (60 and 70) as an organic polymeric matrix and proton conducting polymer as well. SPEEK was easy to prepare and was added to Cs-TPA by low cost process. The main conclusions of this study are as follows; 1) Weight loss at 900 C increased with the addition of inorganic particles, as expected. The hydrolytic stability of the SPEEK/Cs-TPA based composite membranes was improved with the incorporation of the Cs-TPA particles into the matrix. 2) Addition of Cs-TPA into SPEEK60 polymer in the amount of 5 and 10% increased IEC value while addition of Cs-TPA to the SPEEK70 does not have any affect on the IEC values of the membrane. Thus, we suggest that the high sulfonation degree in the polymers caused the interaction with Cs-TPA particles and agglomeration. 3) Membranes prepared by using SPEEK60/Cs-TPA were hydrolytically more stable than SPEEK70/Cs-TPA composite membranes. SPEEK60 composite membranes containing 5 wt.% and 10 wt.% Cs-TPA showed lower hydrogen and water vapor permeability than that of Nafion membrane. 4) The proton conductivity values for SPEEK60/Cs-TPA composite membranes slightly increased with the addition of the Cs-TPA between 5 and 10 wt.%. We obtained the best results from the membrane 10 wt.%. Cs-TPA in SPEEK60 with conductivity value of 1.3 101 S/cm. 5) The observed low methanol, water vapor, and hydrogen permeability, promising proton conductivity value, improved chemical stability, thermal properties and hydrolytic stability of SPEEK60/Cs-TPA composite membranes suggest that these membranes appeared to be highly potential candidate for fuel cell applications
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Acknowledgements [17]
This work is financially supported by Tu¨rk Demir Do¨ku¨m Fab. A.S. The authors are pleased to acknowledge to Victrex for the donation of Polyetheretherketone (PEEK). The authors thank to Mrs. Handan Karakale, Nevin Bekir, Zekayi Korlu and Mustafa Candemir for valuable technical assistance in laboratory experiments. Operation of SEM was possible with help _ of Orhan Ipek and Cem Berk.
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