Kinetic Modeling of Chemical Degradation Phenomena in Fuel Cell Ionomers

A kinetic model has been developed to describe the chemical degradation of Nafion-type polymer electrolyte membranes. The model accounts for pathways of radical formation and consumption along with mechanisms of ionomer degradation through radical attack. Systematic simplifications on the set of model equations lead to analytical expressions for concentration of aggressive hydroxyl radical as a function of initial iron content and hydrogen peroxide concentration. The ionomer degradation model employs a coarse-grained structure of Nafion ionomer. . It distinguishes units corresponding to head groups and trunk segments of sidechains. The backbone segment between two sidechains is the third unit considered in the degradation model.  A set of differential equations is formulated to describe the consumption of these units and determine the associated rate of fluorine loss and the change in the ion exchange capacity (IEC). A parametric study was performed to study the impact of different degradation mechanisms, degradation conditions, and chemistries of the PFSA on fluorine loss and IEC change. The rate constants of degradation processes are found through comparison with the experimental degradation data. Results will be discussed in view a structural stabilization of PEMs and the mitigation of chemical degradation.