The outstanding properties of polymer electrolyte membranes (PEM) for energy applications are based on the microphase separation between the hydrophilic ionic material, which promotes ion conductivity, and the hydrophobic substance, which provides mechanical and chemical strength. NAFION, a perfluorinated sulfonic acid polymer (PFSA), is the benchmark for PEM fuel cell applications. However, PFSAs have several disadvantages, such as environmental hazards associated with the use of fluorinated compounds and their recovery, high costs and a limited operating temperature range due to unsatisfactory thermomechanical properties at high temperatures. Therefore, the current approach in proton exchange technology is to switch from PSAFs to environmentally friendly hydrocarbon materials while maintaining conductivity, mechanical stability and durability. Functionalized hydrocarbon polymers such as syndiotactic polystyrene (sPS) and polyether ether ketone (PEEK), which are less hazardous to humans and the environment than NAFION, are considered in this study. To design new PEM systems, one should not only consider the molecular architecture and composition, but also thoroughly understand the microphase-separated morphology of such materials at multiple structural levels from the local length scale (a few Å) to the mesoscopic length scale (hundreds of nm) under application-relevant conditions such as relative humidity (RH), temperature (T) and chemical exposure. Extended Q range neutron diffraction, combining small and wide-angle scattering (SANS & WANS), where Q is the scattering vector, is an experimental technique that can provide structural information over such a broad length scale (Solid State Ionics 320, 392, 2018; J. Appl. Cryst. 56, 947-960, 2023). Furthermore, D-labelling can be used to vary the scattering length density of either the crystalline region or the hydrated domains, helping to unambiguously characterize such semi-crystalline hydrocarbon membranes. Furthermore, recording the SANS and WANS data in time-of-flight (TOF) mode allows the separation of inelastically and elastically scattered neutrons from highly protonated samples (such as the real application membranes) and the possibility to remove the inelastic contribution from the data analysis, which would lead to a significant reduction of the incoherent background of the sample (J. Appl. Cryst. 54, 1217-1224, 2021).
The structural characterization of the water nanodomains and the nano-pathways that form and evolve in such membranes under different RH and T conditions will be interpreted in conjunction with the conductivity measurements under similar conditions. These experimental approaches with practical relevance for correlating different physical and transport properties with morphology and structure across length scales for different hydrocarbon PEMs in a common (hydrogenated) form suitable for energy applications will be tested and optimized within this project.