Membrane chemical degradation catalyst

In several membrane chemical degradation mechanisms, such as the catalytic combustion mechanism, the catalyst plays an important role in HjOj or reactive radical formation. To improve the chemical durabihty of membranes in PEMFCs, it is thus important to develop new catalysts that produce fewer hydrogen peroxide/radical species. 

In addition to operating conditions, there are several other factors that have an impact on the membrane chemical degradation. Three major aspects, membrane thickness, catalyst type, and contamination, will be discussed here. In this section, general testing results are summarized. Extensive discussion on the mechanisms will be presented in later sections. 

The free radicals ( OH, OOH,. ..) from H2O2 decomposition are a primary cause of membrane and ionomer chemical degradation. The H202-related membrane degradation mechanism will be discussed in more detail in section 12.3.1. The remainder of section 12.2 is divided into four subtopics anode, cathode, catalyst support, and engineered nanostructured electrodes. 

Miyake et al. (2004, 2006) believed that their OCV test results could not be reasonably explained just by a chemical degradation mechanism and therefore proposed a thermal decomposition mechanism crossover hydrogen and oxygen react on the platinum catalyst to produce combustion heat, which can cause thermal decomposition or oxidation of the membrane. However small the combustion heat, it may still lead to gradual microscopic damage of the membrane. Inaba et al. (2006) agreed that gas... 

The chemical degradation of PEMFCs and PEMELCs is still under active study. The detailed mechanisms are still not well understood and there are contradictory observations that need to be clarified. As discussed previously, there are many correlations between the findings in PEMFCs and PEMELCs. Generally, PEMELCs demonstrate less chemical degradation than PEMFCs. Lower operating temperature, unsupported (pure) platinum catalyst, membrane stabilization, and significantly superior hydration contribute to the long lifetime of PEMELCs. 

Figure 10 shows TEM images of an MEA following an open-circuit endurance test in which was supplied to the anode and to the cathode. The test conditions were a cell temperature of 90 C, 30% relative humidity, anode atmosphere of H, and cathode atmosphere of O. Similar to the results of the load-cycling test, it was found that platinum from the cathode catalyst layer dissolved and was redeposited in the electrolyte membrane. Under these test conditions, redeposited platinum particles were observed near the center of the electrolyte membrane. The position of redeposited platinum particles is determined by a balance between the mixed potential of the electrolyte membrane and the partial pressures of the anode and cathode O. It was estimated that platinum particles would be redeposited near the center of the electrolyte membrane under the conditions used in this test (Fig. 11). Chemical degradation of the electrolyte membrane was observed centered on the band of redeposited platinum particles. An analysis was made of the drain water discharged from the MEA during the test and fluoride ions were detected, which suggests that the electrolyte manbrane was partially decomposed (Ohma et al. 2007).

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