Correlating morphology, proton conductivity, and water transport in polyelectrolyte-fluoropolymer blend membranes

Obtaining detailed correlations among multi-scale morphology, ion conduction, and water transport will allow for informed design of novel membrane materials for alternative energy and sustainability applications. We compare morphological and performance features of polyelectrolyte-fluoropolymer blend membranes processed with different degrees of tetrabutylammonium hydroxide (TBA-OH) using the techniques of SEM, elemental analysis, proton conductivity, and water self-diffusion and spin relaxation by NMR. Increased TBA-OH significantly improves membrane phase homogeneity observed by SEM, consistent with observed enhancements in proton conductivity and water diffusion. We further observe drastic changes in local restrictions to water diffusion among the different blend membranes, and use theories of diffusion in porous media to understand improvements in characteristic domain size Rc and tortuosity ℑ with phase homogeneity. NMR relaxometry studies yield two-component T1 values, which further probe structural heterogeneities on smaller (1–10 nm) scales than diffusion experiments (1–10 μm). These multifaceted analyses linking morphology, chemistry, and transport properties provide critical information toward advanced membrane design.