sulphonated poly ether ether ketone/ polyvinyl alcohol ...

Chinese Journal of Polymer Science Vol. 27, No. 2, (2009), 157−164

Chinese Journal of Polymer Science ©2009 World Scientific

SULPHONATED POLY ETHER ETHER KETONE/ POLYVINYL ALCOHOL/PHOSPHOTUNGSTIC ACID COMPOSITE MEMBRANES FOR PEM FUEL CELLS* S. Guhan, N. Arun Kumar and D. Sangeetha** Department of Chemistry, Anna University, Chennai 600 025, India Abstract Composite membranes with polyvinyl alcohol (PVA), sulphonated poly ether ether ketone (SPEEK) and phosphotungstic acid (PWA) were prepared using solvent casting method. The proton conductivities of such membranes were found to be in the order of 10−3 S/cm in the fully hydrated condition at room temperature as measured by impedance spectroscopy. The crystalline properties were studied by X-ray diffraction analysis. The thermal properties were determined by TGA and DSC techniques. The tensile strength and percentage elongation were obtained from UTM studies. Water and methanol uptake of these membranes were studied. Keywords: PVA; SPEEK; PWA; Conductivity; Composite membranes.

INTRODUCTION The commercial proton exchange membranes are perfluorinated ionic polymers such as Dupont’s Nafion® and Asahi Chemical’s Aciplex®, because these materials have excellent proton conductivity, mechanical strength and thermal and chemical stability[1−5]. However, some disadvantages, such as their high cost, appreciable methanol permeability and decrease in ionic conductivity at high temperatures severely limit their commercialization in fuel cells. Therefore much effort has been taken to develop new membranes to overcome these disadvantages[6−10]. These efforts include the preparation of composite type membranes also[11−15]. Non-fluorinated membranes such as sulphonated phenol-formaldehyde membranes[16], vinyl polymers[17] and phosphazene based cation-exchange membranes[18] have been reported. Other alternative more economical non-perfluorinated polymers are based on aromatic thermo plastics such as poly(aryl ether ketone)s (PAEKs) (e.g. PEEK), poly(ether sulphone), polybenzimidazole (PBI) etc., which showed excellent chemical resistance, high thermo oxidative stability, good mechanical properties and low cost[19−23]. The proton conductivity was compared by Kreuer in his work[7]. Composites have become a good choice, as the properties of the membrane can be altered by varying the amount of the inorganic component. Kerres et al. synthesized and characterized novel acid-base polymer blend membranes based on PEEK, polysulphone (PSU) as acids and PBI, PEI as bases[21]. Jorissen et al. also discussed the same topic in their work[24]. PEK-PBI blends were discussed by Soczka-Goth et al.[25]. For composites, Murphy et al. discussed the effect of addition of heteropolyacid to perfluorinated polymers[26]. The effect of phosphotungstic acid (PWA) on PEEK[20] and PBI[27] composite membranes was studied by Zaidi et al. and Staiti et al., respectively. Hongwei et al. focused their study on sulphonated poly(phthalazinone ether ketone) doped with PWA[28]. Nafion doped with heteropolyacids and silica was primarily used to improve the mechanical strength[29, 30]. *

This work was financially supported by the Department of Science and Technology, India (SR/FTP/CS-33/2005 dated 1108-2005). ** Corresponding author: D. Sangeetha, E-mail: [email protected] Received November 2, 2007; Revised December 17, 2007; Accepted December 24, 2007

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Xu et al. prepared a composite of PVA:PWA:SiO2 in the composition 40:40:20 and proved it to be a better membrane for electrochemical applications[31]. Lin et al. doped PWA in PVA and studied its effect[32]. The work of Viswanathan et al. was on PVA matrix composite membranes doped with other inorganic contents like silicotungstic acid, zirconium phosphate[33, 34]. The heteropoly acids have different hydrated structures depending on the environment. In the dehydrated phase or in polar solvents, the primary structure is called a keggin unit. The keggin unit consists of a central atom in a tetrahedral arrangement of oxygen atoms surrounded by 12 oxygen octahedra connected with tungsten or molybdenum. There are four types of oxygen atoms found in the keggin unit ― the central oxygen atoms, two types of bridging oxygen atoms and the terminal oxygen atoms. In the hydrated phase, water moieties bridge the heteropoly acid molecules by forming hydronium ions[28]. In this paper, the preparation of composite membranes based on polyvinyl alcohol (PVA) which acts as a supporting medium, sulphonated poly ether ether ketone (SPEEK) which functions as a proton conductor and phosphotungstic acid (PWA) which exhibits the dual character of being both hydrophilic and enhanced proton conducting[34] is reported. The composite membranes showed proton conductivity in the order of 10−3 S/cm. Various other parameters like ion exchange capacity, water and methanol absorption, thermal stability, mechanical properties, etc were studied and reported. EXPERIMENTAL Materials The PEEK (polyoxy-1,4-phenylene oxy-1,4-phenylene-carbonyl-1,4-phenylene) in powder form was obtained from Victrex. Sulfuric acid, PVA, PWA and N-methyl pyrollidone (NMP) were obtained from SRL. Sulphonation of PEEK Sulphonation of PEEK was conducted by employing sulphuric acid as the sulphonating agent. The PEEK powder was dried overnight at 100°C to remove the moisture. Weighed amount of the PEEK polymer was transferred into a three necked round bottomed flask. Then the required amount of sulphuric acid was added. Continuous stirring was maintained during the course of the reaction. The reaction was allowed to proceed to the required time and was terminated by pouring the entire contents of the flask in a large excess of ice cold water. The sulphonated PEEK in the form of numerous fibers was filtered and washed several times with distilled water until the pH of the wash water was above 6. The product was then dried at 100°C for one day. The finally obtained product was the sulphonated PEEK (SPEEK). A number of experiments were performed to determine the optimum conditions for the sulphonation of PEEK, by varying the concentrations of the polymer, sulphonating agent and the reaction time. Preparation of Composite Membranes Required quantity of PVA was first dissolved in NMP at 90°C, and then SPEEK was added and dissolved. To this hot mixture, PWA was added slowly and magnetically stirred for two hours continuously at 80°C. Further, the mixture was kept in an ultrasonicator for thirty minutes to obtain uniform distribution of PWA. The solution was then cast on a glass petri dish and kept in an oven at 80°C for 15 h. The obtained membrane was pale brown in color. It was peeled from the petri dish at room temperature and stored for further analysis. Two sets of membranes were prepared with varying concentrations of SPEEK and PWA. The variations in the concentration are given in Tables 1 and 2. Ion Exchange Capacity The ion exchange capacity (IEC) indicates the number of milli equivalents of ions in 1 g of the dry polymer. It was determined by titration method. The membrane in its acid form was weighed and then soaked in an aqueous solution containing a large excess of KCl in order to extract all the protons from the membrane. The electrolyte solution was then neutralized using a very dilute Na2CO3 solution of known concentration. The EW (equivalent weight) values were calculated from the dry weight of the membrane divided by the volume and the normality of the Na2CO3 solution. The IEC values were expressed as number of meq. of sulphonic groups per gram of dry polymer.

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Swelling Studies The amount of solvent intake by the membranes was studied. The dried membranes were weighed and soaked in water and methanol separately and allowed to get equilibrated at room temperature for 24 h, above which the weight was uniform. The swollen membranes were then quickly weighed after blotting the surface water and the values noted. The swelling degree was determined using the formula, SW =

M wet − M dry M dry

× 100%

Thermal Studies TGA analysis is mainly carried out to determine the thermal stability of the membrane. The change in weight of the membrane with increase in temperature at a heating rate of 20 K/min in the range of the temperature between 30°C and 800°C is followed using an SDT Q600 US analyzer. The change in the amount of heat flow with increasing temperature is measured using the SDT Q10 US analyzer. A known weight of the dried membrane was heated at the rate of 20 K/min in the temperature range 30°C−400°C. Proton Conductivity The measurements of proton conductivity, σ (S/cm) of the membranes were carried out using an Autolab Potentiostat Galvanostat impedance analyzer. Membranes with required dimensions were cut and pretreated with 0.05 mol/L sulphuric acid and kept in water for 100% hydration. Then the membrane was placed between two silver electrodes with an area of 1.33 cm2 with a uniform pressure applied to hold the system. The cell set-up is Ag/PVA + PWA + SPEEK/Ag. The resistance offered by the membrane was calculated and then converted to conductivity values using the formula;

σ = L /( R × A) Where, σ is the conductivity in S/cm, R is the resistance offered by the membrane in ohms, L is the thickness of the membrane in cm and A is the area of the membrane in cm2. X-Ray Diffraction Studies To know the level of dispersion of the inorganic content in the polymer blend and to know the amount of crystallinity, XRD measurements were performed using a X′ Pert Pro diffractometer. The dried samples were mounted on an aluminium sample holder. The scanning angle ranged from 1° to 80° with a scanning rate of 2° per min. All the patterns were taken at ambient temperatures (25 ± 2)°C. Scanning Electron Microscope Images The surface morphology of the blend was investigated using a scanning electron microscope (SEM, JEOL JSM840A). A piece of membrane was vacuum sputtered with a thin layer of gold prior to SEM examination. The nature of pores and the level of dispersion of PWA in the blend were examined. Mechanical Tests The mechanical properties were obtained from a Hounsfield Universal Testing Machine. The samples were cut into a size of 5 mm × 50 mm as reported by Ding et al.[35]. The cross head speed was set at a constant value of 10 mm/min. For each testing reported, at least three measurements were taken and the average value was reported. RESULTS AND DISCUSSION Ion Exchange Capacity The results of IEC for the two sets are given in Tables 1 and 2. Set A shows a regular increase in the IEC values with increase in the concentration of PWA, whereas, set B shows a regular decrease in the IEC values with increase in the concentration of SPEEK. This confirms that the major proton carriers are the super acids in the membrane. This is in agreement with the report of Xu et al.[31]. The IEC values of the composite membrane were

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found to be better than the IEC of pure SPEEK membrane reported by Sangeetha[36]. Swelling Studies The percentages of swelling in water and methanol are given in Tables 1 and 2. It is observed that there is some negative absorption occurring which implies that the inorganic heteropolyacid leaches out of the membrane. The pH increase of the water confirmed the leaching[27]. In both the sets, the decrease in absorption may be attributed to the fact that with an optimal ratio of PWA to SPEEK, the PWA particles occupy the sites available in SPEEK that could be used to absorb water and also the interspace between the polymer chains that could be used to accommodate water[28] together with leaching of PWA particles. XRD Studies Generally PWA is crystalline in nature and is confirmed from the spectra. The XRD spectra of set A and set B are given in Fig. 1 and Fig. 2, respectively. The XRD pattern of pure PVA shows some crystalline behavior[31]. The peak at 72° is prominent in all the spectra. Other two peak planes are affected by the addition of SPEEK. The peaks at 42°, 44°, 49° and 51° may be due to development of new crystalline planes with the interaction of PVA and SPEEK. The increase in the concentration of PWA shows gradual increase in the disperse level, and the intense peaks confirm the existence of crystalline nature of PWA. The new peak at 32° in the PSW IV and PSW V membranes may be due to the interaction of PWA with PVA and SPEEK.

Fig. 1 XRD patterns of set A

Fig. 2 XRD patterns of set B

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In set B, PWA is dispersed well through out the membrane without losing its crystalline nature. Also the peaks at 32°, 44° and 51° are prominent in all the membranes confirming a good mix up between all the three components. The formation of new peaks when the concentration of PWA is increased reveals that the crystallinity increases with the concentration of PWA. SEM Images SEM photographs taken at two magnification levels (5 and 50 microns) are given in Fig. 3 and Fig. 4 and analyzed. At 50 μm the surface appeared to be smooth and homogeneous indicating the PWA particles are evenly distributed in the composite membrane, and at 5 μm micro pores were visible in PSW III, whereas in PWS III the inorganic content was well dispersed, and the membrane was devoid of pores.

Fig. 3 Scanning electron micrographs of PSW III membrane at (a) high and (b) low magnifications

Fig. 4 Scanning electron micrographs of PWS III membrane at (a) high and (b) low magnifications

Proton Conductivity The conductivity values obtained from the impedance measurements are shown in Tables 1 and 2. It is in accordance with the IEC values. In set A, the PSW II, III and IV showed a high conductivity due to the increase in the PWA content. PSW V was brittle and disintegrated in boiling water and hence its value could not be noted. In set B, PWS II and PWS III showed conductivity in the order of 10−3 S/cm. The conductivity of Nafion® was reported to be in the order of 10−2 S/cm[37]. Thus the conductivity decrease in set B is due to the decrease in the concentration of PWA content irrespective of the increase in the concentration of SPEEK[31]. Membrane code PS PSW I PSW II PSW III PSW IV PSW V

Table 1. Composition and properties of composite membranes (set A) Composition (wt%) IEC Absorption (wt%) Water Methanol (PVA (P):SPEEK (S):PWA (W)) (Milliequi/g) 50:50:00 0.9852 17.87 1.51 45:45:10 1.3108 17.33 4.11 40:40:20 1.8534 15.00 −16.66 35:35:30 2.9023 0.56 −22.41 30:30:40 3.3629 −13.13 −35.13 25:25:50 3.6607 −25.99 −47.11

Conductivity × 103 (S/cm) 0.59 0.61 0.67 0.68 0.68 −

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Membrane code PW PWS I PWS II PWS III PWS IV PWS V

Table 2. Composition and properties of composite membranes (set B ) Composition (wt%) IEC Absorption (wt%) (PVA (P):SPEEK (S):PWA (W)) (Milliequi/g) Water Methanol 50:50:00 3.5761 202.59 −33.56 45:45:10 3.3684 37.64 −37.86 40:40:20 3.1644 2.97 −33.69 35:35:30 2.5798 −1.45 −30.52 30:30:40 2.2925 −11.81 −35.35 25:25:50 2.1400 −16.03 −21.17

Conductivity × 103 (S/cm) 2.86 1.93 1.82 1.46 0.63 0.33

Thermal Studies The DSC curves of PSW II and PWS II are shown in Fig. 5. PEEK is a highly thermostable polymer with a Tg value of ca. 150°C. Sulphonated PEEK shows a glass transition temperature in the range of 200°C−220°C, depending on the degree of sulphonation. The increase of Tg is due to the strong interaction between sulphonic acid groups of SPEEK[23]. The Tg of pure PVA membrane is around 85°C[31]. The DSC result reveals that the PS membrane has a Tg at 195°C, and the Tg of PW is 196°C. In both sets, addition of the third component decreases the Tg in the range of 185°C−190°C. In set B, a regular decrease in the Tg is observed.

Fig. 5 DSC curves of PSW II (a) and PWS II (b)

The TGA curves of PSW II and PWS II are shown in Fig. 6. The PSW III membrane showed the initial weight loss due to water in the temperature region 80−100°C, and the next weight loss was due to decomposition of PVA in the temperature region 170−220°C. At around 270−300°C, the weight loss due to sulphonic groups in SPEEK occurs. The second major weight loss may be due to the conversion of phosphotungstic acid to respective metal oxide[33]. The decomposition of main chain of PEEK occurs in the range of 450–500°C[23]. The PW membrane shows two major weight losses, first due to decomposition of PVA, and the second due to conversion of phosphotungstic acid to tungstic oxide. Mechanical Tests The tensile strength and percentage elongation of the membranes showed a linear decrease with the decrease in the composition of PVA. The tensile strength and percentage elongation of the membranes obtained by mixing PVA, SPEEK and PWA are presented in Table 3.

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Fig. 6 TGA curves of PSW II (a) and PW (b) membranes S. No.

Set A

Set B

1 2 3 4 5 6 1 2 3 4 5 6

Table 3. Tensile strength and percentage elongation measured for set A and B Name of the membrane Tensile strength (MPa) Percentage elongation (%) PS 31 37 PSW I 27 24 PSW II 19 12 PSW III 16 10 PSW IV 12 3 PSW V 10 2 PW PWS I PWS II PWS III PWS IV PWS V

17 13 10 8 7 3

24 9 8 4 3 2

For set A as well as set B, both tensile strength and percentage elongation showed a linear response of decrease with decrease in the content of PVA. As these properties are strongly dependent on the interactions of the polymer chains, it could be presumed that PVA might be the maximum contributing component. As PVA chains can strongly associate through their ―OH groups by H-bonding, its contribution to tensile strength and percentage elongation therefore might be more significant than SPEEK. Hence it could be suggested that the content of PVA mainly decides the tensile strength as well as the percentage elongation of the membranes. The tensile strength of Nafion 117 was reported to be 21.88 MPa[35]. The tensile strength of PSW I was found to be better than Nafion 117. For both sets, irrespective of the content of PWA and SPEEK, the content of PVA decreases gradually. Hence the observed decrease of the tensile strength as well as the percentage elongation can be attributed to the percentage decreasing of PVA.

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CONCLUSIONS Two sets of composite membranes comprising of PVA, SPEEK and PWA were prepared by solution casting technique, using N-methyl pyrrolidone as the common solvent. Membranes were analyzed by XRD, TGA, DSC and SEM. The conductivity values were in the order of 10−3 S/cm at the fully hydrated state. The XRD patterns showed characteristic crystalline nature of PWA and its level of dispersion in the membranes. The membranes had their Tg values in the range of 180°C−200°C. The main and only drawback of such membranes was the leaching of PWA, when immersed in water for long hours. Overall, PSW IV and in Set B, PWS II and PWS III can act as better membranes for fuel cell application due to their better conductivity and stability. With these improved properties, the membranes will surely find better application in the electrochemical devices. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

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