Hydrogen Permeation in Hydrated Perfluorosulfonic Acid Polymer Membranes: Effect of Polymer Crystallinity and Equivalent Weight

Gas permeation through proton exchange membranes affects a fuel cell's electrochemical performance. To understand the gas permeation mechanism, gas permeability and wide-angle X-ray diffraction were measured for different equivalent weight (EW) perfluorosulfonic acid (PFSA) membranes at different water uptakes. A series of molecular dynamics (MD) simulations were also conducted. Through MD simulation, water sorption effects were found to result from an increased probability of dissolved H-2 molecules being near the water phase, where a higher H-2 diffusivity was observed. Furthermore, the local semicrystalline structure of the PFSA polymer and the morphology of the water clusters in the membrane were found to affect H-2 permeation in the high- and low-EW PFSA membranes, respectively. For the high-EW (EW > 909) membranes, the presence of the local crystalline structure decreased the void fraction in the structure and inhibits the H-2 diffusion across the aligned polymer chains. This effectively reduced H-2 solubility and diffusivity, thereby reducing H-2 permeability. The low-EW membranes contained a lower number of PTFE regions and exhibited a highly tortuous network in the aqueous domain. The former contributed to a reduction in H-2 solubility and the latter reduced the H-2 diffusivity. Therefore, H-2 permeability decreased with a decreased EW.