Impact of Dispersion Solvent on Ionomer Thin Films and Membranes

Abstract

Perfluorosulfonic acid (PFSA) ionomers are an important class of materials that many electrochemical devices rely on as their ion-conducting electrolyte. Often, PFSA films are prepared through solution-processing techniques. Previous research has demonstrated that the solvent environment affects PFSA dispersion conformation, but it is not clear to what extent (if at all) these conformational effects persist for thin films and membranes upon casting, nor how dispersion solvent impacts film formation during the drying process. Here, we explore these questions by systematically examining the effect of water and n-propanol mixtures on PFSA thin-film formation and structure, membrane structure, and resulting properties. Using a combination of in situ, time-resolved grazing incidence X-ray scattering and tender wide-angle X-ray scattering, it is shown that films prepared from high-water-concentration dispersions exhibit stronger interactions and arrangement upon drying, and possess larger network domain sizes than those prepared from low-water-concentration dispersions. These stronger interactions likely manifest in greater network connectivity, as evidenced by enhanced conductivity for membranes and decreased water uptake for thin films. Significantly, these solvent-induced differences persist even after thermal annealing. It is clear that the dispersion solvent choice is a critical parameter controlling PFSA nano-and mesoscale structure and presents an important dial with which to direct PFSA macroscale properties.

Probing Ionomer Interactions with Electrocatalyst Particles in Solution

“…[5][6][7][8][9][10][11][12][13][14] These different conformations affect the self-assembly of ionomer aggregates and the properties of their films post-drying. [15][16] Furthermore, different conformations will alter how the ionomer adsorbs to catalyst particles. Changing the water:propanol ratio affects the acidity of dispersions, impacting electrostatic interactions between the ionomer and particles.…”

Structural Transitions During Formation and Rehydration of Proton Conducting Polymeric Membranes

“…At the beginning of the experiment, fitting results provide a core radius of ≈1 nm with a shell thickness of ≈1.5 nm, in close agreement with published results. [ 44 ] As the initial solid content is about 10 wt%, the micelles show some non‐negligible spatial correlation and a scattering bump associated with the presence of a structure factor is observed together with the micellar form factor (see Figure S2a, S2d, Supporting Information). With time, the micellar suspension rapidly evolves in the early stages of drying and the polymeric structures further concentrate as solvent molecules evaporate, which results in an increase of the term I polymer (0).…”

Linking Perfluorosulfonic Acid Ionomer Chemistry and High-Current Density Performance in Fuel-Cell Electrodes

“…To convert swelling data to water content, λ (moles of water per mole of sulfonate groups), one-dimensional swelling of the thin film was assumed. 20 Assuming that no significant excess free volume or macroscale voids exist and additive pure molar volumes are valid, λ can be estimated from 18,20,46…”

“…A dry density of 2.1 g/cm 3 was used for all ionomers. [46][47] GISAXS intensity distribution analysis: Using the data reported by Kusoglu et al, 22 we study the intensity distribution ( I ) of GISAXS patterns as a function of azimuthal angle ( χ ). The 2Dscattering pattern is divided into 2°-sector cuts over a χ-range of 6° to 68° after correcting for the geometry.…”