Sulfonic acid groups fuel cell membrane stability

One of the technically and commercially most important cation-exchange membranes developed in recent years is based on perfluorocarbon polymers. Membranes of this type have extreme chemical and thermal stability and they are the key component in the chlorine-alkaline electrolysis as well as in most of today s fuel cells. They are prepared by copolymerization of tetrafluoroethylene with perfluorovinylether having a carboxylic or sulfonic acid group at the end of a side chain. There are several variations of a general basic structure commercially available today [11]. The various preparation techniques are described in detail in the patent literature. 

The composite membranes exhibit thermal decomposition temperatures greater than 250 °C, which is caused by the eHmination of sulfonic acid groups in SPI. This thermal stability is sufficient for intermediate fuel cell operations. 

Similar to pure hydrocarbon membranes discussed previously, both the aromatic rings and the SOj" groups are lost after the durability test (Yu et al. 2003). No preferential cleavage of sulfonic acid groups was observed. On the basis of XPS results, it also is found that chemical degradation during fuel cell operation occurs mainly on the hydrocarbon fraction of the radiation-grafted ionomer membranes (Nasef and Saidi 2002). Thus, the hydrocarbon fraction has less chemical stability than the fluorinated part. 

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