Chemical degradation proton exchange membrane

Xie, T., Hayden, C., Olson, K. and Healy, J. 2005. Chemical degradation mechanism of perfluorinated sulfonic acid ionomer. In Advances in materials for proton exchange membrane fuel cell systems, Pacific Grove, CA, Feb. 20-23, abstract 24. 

Other technical hurdles must be overcome to make fuel cells more appealing to automakers and consumers. Durability is a key issue and performance degradation is usually traceable to the proton exchange membrane component of the device. Depending on the application, 5,000 40,000 h of fuel cell lifetime is needed. Chemical attack of the membrane and electrocatalyst deactivation (due to gradual poisoning by impurities such as CO in the feed gases) are critical roadblocks that must be over come. 

Figure 18.3. A schematic showing the mechanism of particle growth by dissolution/precipitation. The chemical potential of smaller particles is higher than that of larger particles [32]. (Reproduced by permission of ECS— The Electrochemical Society, from Virkar AV, Zhou Y. Mechanism of catalyst degradation in proton exchange membrane fuel cells.)...

The sulfonic acid groups in the proton exchange membrane have a high affinity for many cationic species. Replacement of the protons by the contaminant cations will result in reduced conductivity of the membrane and performance loss. In addition, the membrane morphology and structure can be affected. An additional degradation issue associated with contaminant metal ions is the occurrence of Fenton s reactions to produce peroxy and hydroperoxy radicals, which in turn will chemically attack the membrane polymer structure. This mechanism is described in section 1.72.1 of this chapter and in chapter 5. The most common Fenton s metal of concern commonly found in the MEA is iron. 

A series of PFSA membranes were doped with various common metal ions and tested for chemical stability against solution (Kinumoto et al. 2006). It was found that alkali and alkaline metal ions do not accelerate membrane degradation in ex situ Fenton-style Yip tests (Fig. 8). The membrane degradation rate resembles that for protonated membranes even when the membranes are fully exchanged with the metal ions. On the other hand, certain transitional metal ions, such as iron and copper, will drastically increase the membrane degradation, while cobalt and chromium ions do not play a significant role. The iron-doped membrane degrades faster than copper-based membranes. 

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