Phase segregation of sulfonate groups in Nafion interface lamellae, quantified via neutron reflectometry fitting techniques for multi-layered structures

Abstract

Neutron reflectometry analysis methods for under-determined, multi-layered structures are developed and used to determine the composition depth profile in cases where the structure is not known a priori. These methods, including statistical methods, sophisticated fitting routines, and coupling multiple data sets, are applied to hydrated and dehydrated Nafion nano-scaled films with thicknesses comparable to those found coating electrode particles in fuel cell catalyst layers. These results confirm the lamellar structure previously observed on hydrophilic substrates, and demonstrate that for hydrated films they can accurately be described as layers rich in both water and sulfonate groups alternating with water-poor layers containing an excess of fluorocarbon groups. The thickness of these layers increases slightly and the amplitude of the water volume fraction oscillation exponentially decreases away from the hydrophilic interface. For dehydrated films, the composition oscillations die out more rapidly. The Nafion-SiO2 substrate interface contains a partial monolayer of sulfonate groups bonded to the substrate and a large excess of water compared to that expected by the water-to-sulfonate ratio, λ, observed throughout the rest of the film. Films that were made thin enough to truncate this lamellar region showed a depth profile nearly identical to thicker films, indicating that there are no confinement or surface effects altering the structure. Comparing the SLD profile measured for films dried at 60 °C to modeled composition profiles derived by removing water from the hydrated lamellae suggests incomplete re-mixing of the polymer groups upon dehydration, indicated limited polymer mobility in these Nafion thin films.

Plasmonic Gas Sensing Using Nanocube Patch Antennas

“…For a 100 nm NC, with 5 nm corners above a 5 nm Nafi on fi lm, parameters which have been shown to be achievable, [ 57,58 ] a peak sensitivity to thickness change of around 43 nm nm −1 is expected, which translates to a humidity sensitivity of 1.3 nm/% RH, over three times better than the best average from this investigation, and more than double the best individual result.…”

Uncovering the Structure of Nafion–SiO2 Hybrid Ionomer Membranes for Prospective Large‐Scale Energy Storage Devices

“…One possibility for the slight decrease in peak intensity with the addition of the amine‐functionalized nanoparticles is the formation of a water‐rich layer at the Nafion/amine‐functionalized SiO 2 nanoparticle interface. Both past and current neutron reflectivity work at the National Institute of Standards and Technology (NIST) suggests that a water‐rich layer forms at the interface between Nafion and silicon surfaces functionalized with native oxide and/or acid‐forming amines (NH 3 + ). The slight reduction of the ionomer peak intensity observed with loading of amine‐functionalized nanoparticles might be consistent with an interaction between the sulfonic groups and the surface of these particles.…”

Influence of Water and Temperature on Ionomer in Catalytic Layers and Membranes of Fuel Cells and Electrolyzers Evaluated by AFM

“…Interestingly, the thickness of water layers, i.e., on oxides or ice, measures approximately 0.8 nm to 1 nm, although effects of surfaces are important [68,69]. A varying water layer thickness of approximately 0.6-0.9 nm within an ultrathin Nafion film on SiO 2 at 68% RH was reported by DeCaluwe et al, resulting from modeling of neutron reflectometry measurements [41]. Recent measurements by Berrod et al derived from combined small-angle X-ray scattering, quasielastic neutron scattering, and pulsedfield gradient NMR show a thickness of enclosed water layers of approximately 0.8 nm for the used range of humidities [24].…”