Membrane chemical degradation hydroxyl radicals

Metal ions mainly come from fuel cell components and system accessories, and cause a decrease in membrane conductivity by replacing H+. Metal ions also cause membrane decay by catalyzing the formation of hydroxyl radicals, which are the most active chemicals involved in chemical degradation of membranes. 

The chemical degradation of the membrane can occur due to the action of hydroxyl ( OH) and peroxyl ( OOH) radicals that are most commonly formed from reactions involving H2O2. H2O2 can be produced as an intermediate during oxygen reduction [14] ... 

The chemical degradation of PFSA membranes is mainly caused by the gas crossover, forming hydrogen peroxide, which reacts with impurities (e.g., iron contaminations) producing hydroxyl and hydroperoxy radicals that could attack (decompose) the membrane (refer to Chapter 4 for detailed degradation mechanisms). 

On the basis of XPS results, it is found that chemical degradation during fuel cell operation occurs mainly on the hydrocarbon fraction of the radiation-grafted ionomer membranes (Nasef and Saidi 2002). It is believed that tertiary hydrogen of the a-carbon is most vulnerable to hydroxyl radical attacks. 

Figure 3 indicates the ESR spectra of the Pt/C catalyst dispersed solution. Figure 4 indicates the ESR spectra of Ketjenblack-dispersed solution. Both spectra indicate the formation of radicals, which corresponded with the hydroxyl radical. From these experimental results, the degradation mechanism of the MEA under the OCV condition can be determined. At the cathode, is chemically formed, and a hydroxyl radical is generated by the decomposition of the H O. At the anode, is chemically or electrochemically generated, and a hydroxyl radical is similarly generated by the decomposition of the H O. The hydroxyl radical will attack the ionomer, membrane, and carbon support of the catalyst. 

For NPC MEA-I, the NPC manbrane was iQiplied. The thickness of the membrane was 40 (im. The conventional MEA failed within lOh of operation, releasing a high amount of fluoride ion. On the other hand, NPC MEA-I showed excellent stability over l,000h, and the fluoride ion release rate was less than 1% of that of the conventional MEA. This result indicates the exceptional chemical stabihty of the NPC MEA against degradation caused by the hydroxyl radical at 120 C and low humidity. 

In addition to the radical attack of the end group, hydroxyl radical OH will attack the C-S and C-O-C bond in the copolymer side chain. Loss of a great number of sulfonate gronps or whole side chains would affect the proton conductivity of the membranes. Ghassemzadeh et al. studied chemical degradation of PFSA after fuel cell in sitn tests by solid-state NMR spectroscopy. The NMR spectra prove that degradation mostly takes place within the polymer side chains. ... 

Ex situ accelerated methods were applied to study the degradation of PFS A membranes. Ex situ accelerated chemical degradation experimentation of PFSA most commonly employs Fenton s testing. Fenton s reagents include hydrogen peroxide with Fe ions in order to produce hydroxyl radicals as follows ... 

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