Membrane chemical degradation platinum

On the basis of results from micro-Raman spectroscopy, severe side degradation is found around the platimum redeposition line (Ohma et al. 2007a). The sulfate ion release rate on the cathode side is more than that on the anode side. A similar trend is also observed for the ERR. The results strongly support the role of the platinum line in membrane chemical degradation. 

Energy dispersive X-ray spectroscopy (EDX) is used predominantly for elemental analysis or chemical characterization. The advantage of this technique is that it can identify the elemental composition for a small area thus, a profile of element composition can be created across the sample. Since EDX tests are usually conducted in a scanning electron microscope, a composition profile can be directly overlaid onto a secondary scanning electron microscope micrograph. Major elements related to membrane chemical degradation such as fluorine, sulfur, platinum and other metals can be readily detected. 

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... 

As can be seen in Eig. 15, the relative concentration of the target element can be traced from the anode to the cathode side (Zhang et al. 2(X)7) thus, the chemical degradation can be monitored across the membrane. Similar to the study mentioned above, platinum and sulfur profiles were examined (Aoki et al. 2(X)6a Yu et al. 2(X)3). 

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. 

Typical examples of degradation phenomena include (1) carbon support oxidation/corrosion, (2) platinum dissolution/agglomeration, (3) chemical degradation of the electrolyte membrane, and (4) three-dimensional structural changes in the electrocatalyst layers, among others. These phenomena do not occur individually, but rather simultaneously and in a compound manner. 

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).

Catalyst materials. Especially for polymer electrolyte membranes (PEMs), high selectivity for hydrogen and a high tolerance of carbon monoxide are necessary [17]. Since noble metals such as platinum are used, high activity but also a reasonable distribution are needed to reduce the amount of catalyst required [18, 19]. Degradation due to chemical stabiHty and morphology is an issue. 

The reasons for the deterioration of ceU performance can be distinguished in reversible and irreversible power loss. Inevitable irreversible performance loss is caused by carbon oxidation, platinum dissolution, and chemical attack of the membrane by radicals [7]. Reversible power loss can be caused by flooding of the cell, dehydration of the membrane electrode assembly (MEA), or change of the catalyst surface oxidation state [8]. If corrective actions are not started immediately, reversible effects lead to irreversible power loss that we define as degradation. In this chapter, we focus on the degradation of the catalyst layer due to undesired side reactions. 

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