Membrane chemical degradation

Figure 8.24 shows the model capabilities to address competitive degradation phenomena membrane chemical degradation makes the proton conductivity decrease and thus potential cathode decreases which in turn decreases the kinetics of the cathode Pt degradation. In other words, membrane degradation can mitigate Pt degradation in the cathode, under specific operation conditions. 

Corns F D, Liu H and Owejan J E (2008), Mitigation of perfluorosulfonic acid membrane chemical degradation using cerium and manganese ions , ECS Trans, 16 1735-1747. 

Corns FD (2008) The chemistry of fuel cell membrane chemical degradation. ECS Trans 16 235-255... 

This mechanism can produce H2O2 at the cathode side under normal fuel cell operating conditions where current is generated with an external load. Since membrane chemical degradation is more severe... 

Membrane chemical degradation directly results from attack by active radicals. Hydrogen peroxide formed in fuel cells can result in radical formation. Some researchers add radical or HjOj scavengers to eliminate radicals or reduce the formed HjOj to harmless species. Aoki et al. (2006) confirmed that an appreciable fraction of HjOj or HO radicals was easily scavenged at Pt particles in the CL or in Pt-dispersed Nafion membranes. Danilczuk et al. (2009) found that Ce(III) in Nafion membranes with low concentrations can efficiently scavenge HO radicals because of the Ce(lll)/Ce(lV) couple redox... 

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. 

Membrane failure is believed to be the result of chemical and mechanical effects acting together (Liu et al., 2001). Membrane chemical degradation can result in altered stress-strain behavior and loss of... 

Fundamentals Governing the Mechanism for Membrane Chemical Degradation... 

Before discussing membrane chemical degradation in detail, the factors governing the degradation mechanism must be identified. Among three major types of membrane materials, hydrocarbon, partially fluorinated, and perfluorinated ionomers, perfluorinated sulfonic acid (PFSA) is the most widely used membrane material owing to its high chemical stability (Schiraldi 2006). 

A comprehensive study of the key elements of PEMFC membrane chemical degradation was recently conducted for PFSA-type ionomers (Table 1 Liu et al. 2005). 

Table 1 Key elements for membrane chemical degradation. All tests employed Nafion 112 with 0.4mgpjCm per electrode and were conducted at 95°C, 300kPa, 100% relative humidity, 525 seem gas flows, open-circuit voltage...

To summarize this section, or gas crossover and reaction of the crossed-over gas on a catalytic surface are the most fundamental governing mechanisms for membrane chemical degradation. This lays a foundation before experimental parameters, other impacting factors, detailed degradation mechanisms, and potential mitigation methods can be discussed in later sections. 

Higher operating temperature will lead to more severe chemical degradation (Healy et al. 2005 Curtin et al. 2004). The kinetics of chemical reactions inevitably increases with temperature. Since membrane chemical degradation is highly irreversible, the degradation rate will increase with operating temperature. 

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. 

To maximize performance, efficiency, and lower material cost, thinner membranes have been the focus of recent fuel cell development (Mathias et al. 2005). The impact of membrane thickness on membrane chemical degradation is, thus, of great importance. 

As discussed earlier, is one of the main reactants for membrane chemical degradation. When the membrane is contaminated by metal ions, these ions can catalyze decomposition and generate radicals. It is important to evaluate which metal ions can effectively convert to radicals, i.e., which are Fenton-active. 

Detailed mechanisms of pinhole formation, resulting from both mechanical and chemical degradation, can be very complicated. In this section, the discussion will focus on several aspects of membrane chemical degradation. 

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. 

With distributed current collection hardware, current distribution during cell operation can be evaluated (Yoshioka et al. 2005). It is found that when the inlet RH is low, the highest current density is observed near the gas outlet, where the humidity is relatively high. Thus, the membrane chemical degradation can be accelerated in the gas inlet region since lower RH usually accelerates degradation. 

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