Proton mechanical degradation

Modeling and experimental studies [131-133] have also indicated that when other cations (except LT) replaced protons in the polymer, less water was coordinated and the amount of water in the cell was affected. For example, the exchange of Ca + for H+ can cause a 19% reduction in membrane water content. Less water may result in a greater concentration of H2O2 and, therefore, a greater concentration of destructive hydroxyl radicals [131,134,135]. Moreover, low water content might accelerate mechanical degradation due to membrane dehydration. 

The best compromise of proton conductivity and stability is achieved at intermediate lEC and moderate water uptake. Insufficient water content extinguishes the bulk-water-like proton mobility. Excessive swelling is a problem, as well. It reduces the steady-state performance as a result of proton dilution. Moreover, excessive swelling increases the microscopic stress on polymer fibrils, rendering PEMs with high lEC more prone to mechanical degradation. 

Fig. 3. Mechanisms for polymer degradation. The illustration is a schematic representation of three degradation mechanisms I, cleavage of cross-links II, hydrolysis, ionisa tion, or protonation of pendent groups III, backbone cleavage. Actual biodegradation may be a combination of these mechanisms.

A detailed investigation by Groenewegen et al. (1990) has examined the uptake of 4-chlorobenzoate by a coryneform bacterium that degraded this compound. The uptake was inducible and occurred in cells grown with 4-chlorobenzoate but not with glucose. A proton motive force (Ap)-driven mechanism was almost certainly involved, and uptake could not take place under anaerobic conditions unless an electron acceptor such as nitrate was present. 

Virkar AV, Zhou YK. 2007. Mechanism of catalyst degradation in proton exchange membrane fuel cells. J Electrochem Soc 154 B540-B547. 

ESR spectroscopy, used in the direct detection or spin trapping modes, is a sensitive method for the detection of polymer fragments and for determining the degradation mechanism. Recent applications for the study of stability in ionomer membranes used as proton exchange membranes in fuel cells demonstrate the capability of ESR to detect details that cannot be obtained by other methods. 

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. 

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