Membranes made from polymers without fluorine

PFSA-based membranes have a combination structure built up from hydrophilic segments associated with the sulfonic acid groups and a backbone of hydrophobic, perfluorinated hydrocarbon within which the sulfonic acid groups are distributed uniformly. The branched network of interconnected hydrophilic segments provides the protonic conduction path of the membrane but at the same time offers methanol molecules a chance to diffuse from the anodic to the cathodic side of a DMFC. Therefore, anything done to reduce methanol crossover will cause some decrease in protonic conductivity. 

Methanol crossover was reduced markedly when a barrier layer of benzimidazole was apphed to the surface of a Nafion membrane while the protonic conductivity of the original Nafion membrane was preserved (Hobson et al., 2002). A similar effect was attained when sulfonated poly(vinyl alcohol) was introduced into the membrane (Shao and Hsing, 2002). 

Choi et al. (2001) applied plasma etching and plasma palladium sputtering to Nafion membranes. This treatment depressed methanol crossover by about 35%. The current-voltage curves for fuel cells with treated membrane were better than those for fuel cells with an untreated membrane (data concerning the conductivity change were not reported in the paper). 

Kim et al. (2003) suggested impregnating the Nafion membrane with palladium nanoparticles. According to their data, this membrane modification produced only a small conductivity decrease (by about 35%) but a very strong drop in methanol crossover (by a factor of 7). 

4 MEMBRANES MADE FROM POLYMERS WITHOUT FLUORINE 

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