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 11- 08-2005). ** Corresponding author: D. Sangeetha, E-mail: [email protected] Received November 2, 2007; Revised December 17, 2007; Accepted December 24, 2007 158 S. Guhan et al. 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. SPEEK/PVA/PWA Composite Membranes for PEMFC 159 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, M − M SW = wet dry ×100% M dry 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 JSM- 840A).