Perfluorinated polymer electrolyte membrane durability

Durability is a critical issue in all fuel cell systems, certainly so for PEM membrane polymers. Perfluorinated ionomers are generally favored in PEM systems at the present time; the useful lifetimes of such systems can be limited by damage including membrane thinning, weight loss, and redistribution of catalyst materials during fuel cell operation. Hydrogen peroxide remains the most likely culprit for membrane degradation, being readily produced as a by-product under fuel cell catalysis conditions. Once generated, hydrogen peroxide can be readily homolysed into peroxide radicals capable of breaking of polymer constituent bonds. The leading mechanism for degradation of commercially-available PEM membranes is initiated by abstraction of a hydrogen atom from residual carboxylic acid ends on PTFE backbones. Such atom abstraction initiates a systematic chain oxidation to carbon dioxide and hydrogen fluoride, which is detected in the effluent water. Reduction of the carboxylic acid ends by fluorination substantially reduces degradation and increases durability. Under these conditions, polymer degradation appears to proceed by peroxide attack at less active positions as well, perhaps 2-3 orders of magnitude slower to react than at carboxylic acid groups; this lower reactivity can be balanced by the considerably higher concentration of non-carboxylate positions on the polymer. In the years to come, critical questions to be answered include determination of actual peroxide radical concentrations within operating fuel cells under steady state, start up/shut down and upset conditions, a better understanding of specific sites of attack beyond carboxylic acid end groups, and the methods for reducing the extent and the impact of polymer degradation in order to prolong PEM fuel cell lives.