Modelling the Chemical Degradation of PFSA Membranes in Proton Exchange Membrane Fuel Cells – Effect of Low Relative Humidity on the Degradation of the Sulfonic Acid Group

The chemical membrane degradation of automotive proton exchange membrane (PEM) fuel cells gains significant importance due to the increased stress caused by high temperature and low relative humidity (RH) operation required to reduce humidification and cooling system complexity. While several models have been introduced describing the ether cleavage and main chain unzipping mechanisms, the degradation of the sulfonic group has not been modeled yet. This is critical, because at low RH, the degradation of the sulfonic acid group, followed by the unzipping along the backbone, is expected to be the dominant chemical degradation mechanism. In this study, the acid-base equilibrium of the sulfonic acid groups was solved as a function of the membrane water content to derive the number of reactive, hydrated sulfonic acid groups and to calculate the reaction rate of the initial radical attack. The simulation was combined with a Fenton model and a coarse grain approach to derive the fluoride emission rate as a function of RH. It was found that the hydration ratio of the sulfonic acid group changes by six orders of magnitude between 25% and 95% RH, leading to an almost two orders of magnitude higher degradation rate at dry conditions.