Ex-situ study of PEMFC membrane degradation under coupled chemical/mechanical stresses

Increasing the operating temperature of proton exchange membrane fuel cells (PEMFC) is an important step in accelerating their large-scale deployment, especially in heavy-duty transport. Higher temperature is expected to improve the reaction rates and, mostly, makes it possible to use smaller heat exchangers. However, higher material degradation rates are also expected, as the operating conditions are different. For these reasons, it is necessary to study the impact of elevated temperature on the properties and durability of the membrane in PEMFC. 

An ex-situ approach has been employed to study the evolution of the structure and properties before and after degradation. The aim of this approach is to evaluate the membrane state by decoupling the effect of other components. To do so, a home-made bench was developed, allowing us to induce either coupled or uncoupled chemical/mechanical degradation. The sample is placed in a cell through which a vapor-phase hydrogen peroxide (H₂O₂) is introduced, while simultaneously being mechanically stressed to induce coupled chemical/mechanical degradation. After exiting the cell, the vapor flux is condensed for analysis. The bench enables us to control and therefore vary parameters such as the H₂O₂ concentration, the relative humidity of the vapor flux, the frequency and the amplitude of the compression cycle. Structural characterization with Fourier-transform infrared (FTIR) spectroscopy on the sample and analyses of the degradation solution by UV spectroscopy are performed after the degradation protocol. To begin, several tests have been performed on pretreated sulfonated (Poly ether ether ketone) (sPEEK) with various H₂O₂ concentrations and no constrain applied.

After degradation, sPEEK sample were macroscopically brittle and the presence of structural damages have been observed by FTIR spectroscopy. However, no degradation products have been detected by UV spectroscopy, possibly indicating that the degradation products are trapped in the sample. The next step is the characterization of sPEEK samples after mechanical and coupled chemical/mechanical degradation. Other similar tests on PFSA membrane such as Nafion 211 are incoming to reach a better understanding of the difference between the two kinds of polymer in terms of durability. The aim of this study is to evaluate the impact of a vapor environment at 95°C, coupled or not with mechanical stresses, on the chemical and functional properties of different membranes.