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

Abstract

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.

Effects of Functional Graft Polymers on Phase Separation and Ion-Channel Structures in Anion Exchange Membranes Analyzed by SANS Partial Scattering Function

“…Note that a relatively smaller D f of both AEM_IS64 and AEM_IS37 than that of previously reported ETFE- g -PSSA ( D f = 2.4) indicates less compact packing density in ETFE-based AEMs, probably because the phase separation tendency between ETFE and Im-St copolymers is weaker than that between ETFE and PSSA. The different separation tendency should originate from the fact that the Im-St copolymer has a weak base (Im) with a hydrophobic comonomer unit (St) while PSSA has a strong acid group without a hydrophobic comonomer unit.…”

“…Following the strategy that we applied to the hydrated Nafion and grafted PEM (ETFE- g -PSSA), − the hydrated AEM_IS64 and AEM_IS37 are partitioned into separate components of the ETFE base polymer (BP), graft polymer (GP), and water, as shown in Scheme .…”

“…It is worthy to show the mathematical foundation of the decomposition of I ( q ) in CV-SANS experiments into PSFs, corresponding to the abovementioned components before the experimental results. We here omit the detailed description that can be found in the Supporting Information S2 and our previous work − and only give the main equation on the basis of the incompressibility assumption that I ( q ) is described using three PSF self-terms I ( q ) = ( b BP − b GP ) ( b BP − b normalW ) S BP − BP ( q ) + ( b GP − b BP ) ( b GP − b normalW ) S GP − GP ( q ) + ( b normalW − b BP ) ( b normalW − b GP ) S normalW − normalW ( q ) where b i and S ii ( q ) are the scattering length density (SLD) and PSF self-term of the i component ( i = BP: base polymer, GP: graft polymer, and W: water). S ii is defined as S i i ( q ) = 1 V < ∬ δ φ i ( r⃗ ) δ φ…”

“…can be deduced from S ii using eqs – under the incompressibility assumption − S BP − GP = 1 2 ( S W − W − S BP − BP − S GP − GP ) S GP − normalW = 1 2 ( S BP − BP − S GP − GP − S W − W ) S BP − normalW = 1 2 ( S GP − GP − S BP − BP − S W − W ) …”

“…In this study, we applied the PSF analysis to elucidate the detailed hierarchical structure of each component in hydrated graft-type AEM_IS64 and AEM_IS37, representing two typical ion-channel structures shown in Scheme b, in multiple length scales and to provide mechanistic insights into graft hydrophobicity and structure correlations. As PSF was for the first time used to characterize AEM structures, we would specify the typical and unique structures by comparing to PEMs such as Nafion and graft-type ETFE- g -PSSA. − …”