Anion-Exchange Hydroxyl Ion-Conducting Membranes

It is natural then to ask why one could not make OH/on-exchange (hydroxyl ion-conducting) membranes having properties similar to those of Nafion but because they are alkaline in nature, leading to a marked acceleration of the electrode reactions. 

Despite great efforts, this has not been achieved until now. Existing types of anion-exchange membranes (used for electrodialysis) are far inferior to Nafion, both in their conductivity and in the chemical and thermal stability. Therefore, one cannot so far meaningfully discuss their potential as substitutes for the circulating or matrix electrolyte in alkaline hydrogen-oxygen fuel cells. 

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New versions of anion-exchange membranes were prepared via chlormethy-lation and amination of polysulfone. Amination using different diamines made it possible to investigate the effect of the length of the alkyl chain on membrane properties. The chain with the longest alkyl chain showed better hydroxyl ion conductivity and thermal stability than did those with a shorter chain (Park et al., 2008). 

Solid alkaline membrane fuel cell (SAMFC) working at moderate temperatures (20-80 °C) for which an anion-exchange membrane (AEM) is the electrolyte, electrically conducting by, for example, hydroxyl ions (OH ). 

Stoica et al. [140,141] developed and characterized another anion-conducting membrane based on a poly(epichlorohydrin) copolymer using allyl glycidyl ether as cross-linking agent. In order to introduce anionic properties, two cyclic diamines were incorporated DABCO and l-azabicyclo-[2.2.2]-octane (quinuclidine) (Fig. 5.13). To stabilize the membrane, the film was thermally or photochemically cross-linked. High conductivities were obtained without any KOH addition. At 60 °C and 98% relative humidity, the membrane exhibited a hydroxyl conductivity of 1.3 X 10 S/cm and an ion exchange capacity of 1.3 X 10 equiv./g. The performance of this MEA in H2/O2 AFC operation was 100 X 10 W/cm for 270 X 10 A/cm [142]. 

The proton exchange membrane can be a source of fluoride ions as well [143]. Hydroxyl radicals, formed via crossover gases or reactions of hydrogen peroxide with Fenton-active contaminants (e.g., Fe +), could attack the backbone of Nafion, causing the release of fluoride anions these anions in turn promote corrosion of the fuel cell plates and catalyst, and release transition metals into the fuel cell [143]. Transition metal ions, such as Fe, then catalyze the formation of radicals within the Nafion membrane, resulting in a further release of fluoride anions. On the other hand, transition metal ions also can cause decreased membrane and ionomer conductivity in catalyst layers, as discussed in section 2.4 of this chapter. 

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