Time [hour

Figure 3. Cell voltage vs. time of operation of H2/02 fuel cell at the current density of 0.7 A/cm2-

It is likely that the main chains of perfluorinated polymers such as FEP, PFA, and PTFE are subjected to chain scission at carbon-carbon bonding rather than the carbon-fluorine bonding upon the irradiation by gamma rays to form radicals, considering the bonding energies. Upon grafting reaction, styrene is grafted onto the low molecular weight fragments resulting from the chain scission for these perfluorinated matrix polymers. Thus resultant membranes lose mechanical strength and the low molecular main chain fragments connected with sulfonated polystyrene might be dissolved into water during cell operation. ETFE contains polyethylene structures in the main chain. It is well known that polyethylene is subjected to crosslinking upon the irradiation by gamma rays. Therefore, even if fluorinated portions in ETFE are subjected to chain scission, the main chains are interconnected through crosslinking at polyethylene portions. This gives better mechanical strength and stability in fuel cell operation.

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