nafion membranes for fuel cells studied by using the ...

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Polymer electrolyte fuel cells (PEFC's) are attracting wide attention as a clean, high efficiency, low pollution and low temperature power generation technology.
The 2nd Asia-Pacific Symposium on Radiation Chemistry (APSRC-2008) 29/8 – 1/9, 2008, Waseda University, Tokyo, Japan

NAFION MEMBRANES FOR FUEL CELLS STUDIED BY USING THE POSITRON ANNIHILATION TECHNIQUE Hamdy F. M. Mohamed1-3,*, Libin Wu2, A.Ohira1 and Y. Kobayashi2 1

FC-Cubic, National Institute of Advanced Industrial Science and Technology (AIST), Tokyo 135-0064, Japan 2 NMIJ, AIST, Tsukuba, Ibaraki 305-8565, Japan 3 Physics Department, Faculty of Science, Minia University, B.O. 61519 Minia, Egypt * E-mail: [email protected]

Keywords: positron annihilation, positronium, free volume, nafion, membrane, fuel cell Polymer electrolyte fuel cells (PEFC’s) are attracting wide attention as a clean, high efficiency, low pollution and low temperature power generation technology. They have wide range of applications in areas such as transportation, portable power sources, and distributed power supplies. Membrane electrode assembly (MEA) is the heart of PEFC and the performance of PEFC is largely dependent on the microstructure of MEA. For example, oxygen permeation through the cathode layer of the MEA is important for fuel cell operation. Increased oxygen permeation enhances the activation rate of the oxygen reduction reaction, thereby increasing PEFC efficiency. Conventionally, MEA has been prepared by sandwiching the electrolyte membrane such as Nafion with electro-catalyst and gas diffusion layers by hot press. However, the heating of the electrolyte membrane could influence its microstructure and consequently PEFC performance. The free volume model of permeation states a close relationship between the gas permeability and free volume; so that we studied the effect of heating of Nafion (NRE212) in acidic form on the free volume by using the positron annihilation lifetime (PAL) technique. The measurements were conducted over a temperature range from -110 to 120 oC. Temperature dependence of the free volume showed α-relaxation at 80 - 110 oC and β-transition at - 20 - 10 oC. After annealing at 120 oC (above α-relaxation), it was observed that the free volume size and fraction became smaller. Decreasing of the free volume fraction is due to increased crystallinity as confirmed by wide angle x-ray diffraction etc. Possible impact of heating on the relation between the free volume and gas permeation will be discussed.