DEVELOPMENT OF INEXPENSIVE PROTON EXCHANGE MEMBRANES FOR FUEL CELLS BY RADIATION INDUCED GRAFTING TECHNIQUE

Abstract

Fuel cells, devices transforming chemical energy directly into electricity, are regarded as one of the promising clean future power sources. Proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) using polymeric proton conductive membranes as one of key components are drawing more and more attention for their utility in automotive and portable electronic applications. A continuous effort is being made to develop new high-performance proton conductive membranes as an alternative to Nafion, which is the principal material used as the polymeric electrolyte in PEMFC systems because of its excellent chemical and mechanical stabilities and high proton conductivity. However, high cost, low operation temperature (≤80 °C), high methanol crossover, and environmental recycling uncertainties of Nafion and other similar perfluorinated membranes are limiting their widespread commercial applications in PEMFC and DMFC. The present research work is focused on the development of non-fluorinated, inexpensive proton exchange membranes (PEMs). The experimental membranes studied were prepared by gamma radiation induced grafting of styrene onto ultra-high molecular weight polyethylene (UHMWPE) powder, followed by film formation and subsequent sulfonation. Cobalt-60 was used as a source of gamma ray. The styrene monomer was selected for grafting due to the reason that styrene-grafted UHMWPE (UHMWPE-g-PS) films could be readily post-sulfonated to afford proton exchange membranes. Influence of various preparation conditions was investigated. Simultaneous as well as pre-irradiation grafting was performed in air and an inert atmosphere at room temperature. The effect of absorbed radiation dose and monomer concentration on the degree of grafting (DG) is discussed. It was found that the DG increases linearly with increase with the absorbed dose, grafting time and monomer concentration, reaching a maximum at a certain level. The order of rate dependence of grafting on monomer concentration was found to be 2.32. Furthermore, the apparent activation energy, calculated by plotting the Arrhenius curve, was 11.5 KJ/mole. The particle size of UHMWPE powder, measured before and after grafting, is found to increase linearly with DG. Attenuated total reflection Fourier transform infrared (FTIR-ATR) spectroscopic analysis confirmed that the styrene is successfully grafted onto UHMWPE powder. The relationship of DG with degree of Istyrene substitution (DSS) per UHMWPE repeat unit was also calculated. The UHMWPE- g-PS powder was then fabricated into films by compression moulding. The sulfonation of selective UHMWPE-g-PS films was carried out by chlorosulfonic acid and the product was evaluated as PEM for fuel cells. The range of Ion exchange capacity (IEC), 0.97 to 2.77 meq/g, obtained from styrene grafted and sulfonated UHMWPE (UHMWPE-g-PSSA) membranes with different DG realized that DG is an effective tool to control IEC. The water and methanol uptake of prepared membranes were studied on weight and volume basis. Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA), differential scanning calorimetery (DSC) and X-ray diffraction (XRD) analysis of pristine UHMWPE, UHMWPE-g-PS and UHMWPE-g-PSSA were performed to get the structural information. Mechanical properties of non-grafted film, grafted films, and grafted and sulfonated membranes were also investigated. An appropriate distribution and an excellent penetration of polystyrene (PS) to the UHMWPE backbone matrix were observed during morphological analysis using scanning electron microscope (SEM). Transmission electron microscopic analysis was performed to observe microstructure of UHMWPE-g-PSSA, for the evidence of micro phase separation of hydrophilic and hydrophobic domains. The sulfonation of UHMWPE-g-PS was also confirmed by x-ray photoelectron spectroscopy (XPS). A series of UHMWPE-g-PSSA membranes with different IEC were also investigated for their methanol permeability and proton conducting properties. The methanol permeability of UHMWPE-g-PSSA is found to be in the range of 4.86E-08 cm 2 /s for 12% grafting with corresponding proton conductivity (σ) of 16 mS/cm to 1.45E-06 cm 2 /s for 44% grafting with corresponding σ of 140 mS/cm. Hence, the methanol permeability is several times lower than that of Nafion 117 (1.65E-06 cm 2 /s) with corresponding σ of 45 mS/cm. Finally, a selected membrane was used to fabricate a membrane electrode assembly (MEA) which was tested in a single cell direct methanol fuel cell (DMFC). The DMFC performance confirmed the practicability of the developed PEM. Hence the UHMWPE-g-PSSA, owing to its fluorine free nature, low cost as compare to perfluorinated PEMs and other valuable characteristics as discussed in the present study, is found to be an extremely viable proton exchange membrane for low temperature PEMFC and DMFC applications.