Microscopic Determination of the Local Hydration Number of Polymer Electrolyte Membranes Using SANS Partial Scattering Function Analysis

The conventional hydration number, λ, of a polymer electrolyte membrane (PEM) is estimated by the gravimetric measurement of the total water uptake in the membrane. This is the overall water molecules divided by the number of ionic groups covering the water distribution from the macroscale to the molecular level. For a more precise evaluation of the local ion-water interactions and ion transport efficiency in hydrophilic channels, in contrast to λ, we here propose an index, local hydration number (λ_local), defined as the number of water molecules located adjacent to and interacting with ionic groups in a PEM, per ionic group in the hydrophilic channels. We introduce an experimental method using partial scattering function analysis through a contrast-variation small-angle neutron scattering technique to precisely determine λ_local at the micro- (nano-) scale to the molecular level for PEMs. When applied to the benchmark material Nafion and the well-studied radiation-grafted PEM, consisting of poly(styrene sulfonic acid) grafted onto poly(ethylene-co-tetrafluoroethylene) (ETFE-g-PSSA), the obtained λ_local was close to the conventional λ. This means all water molecules interacted with sulfonic acid groups, which is much different from the case of anion-type grafted PEMs. Comparing the λ_local value obtained in this study with the conventional λ value confirmed that water existed only in nanostructured hydrophilic channels in these PEMs. The result of this work provides a route to understand local molecular structures of high-performing PEMs for energy conversion applications.