Nanoscale Distribution of Sulfonic Acid Groups Determines Structure and Binding of Water in Nafion Membranes

Abstract

Abstract:The connection between the nanoscale structure of two chemically equivalent, yet morphologically distinct Nafion fuel-cell membranes and their macroscopic chemical properties is demonstrated. Quantification of the chemical interactions between water and Nafion reveals that extruded membranes have smaller water channelsw ith ar educed sulfonic acid head group density compared to dispersion-cast membranes.A saresult, ad isproportionally large amount of nonbulk water molecules exists in extruded membranes,whichalso exhibit larger proton conductivity and larger water mobility compared to cast membranes.T he differences in the physicochemical properties of the membranes,that is,the chemical constitution of the water channels and the local water structure, and the accompanying differences in macroscopic water and proton transport suggest that the chemistry of nanoscale channels is an important, yet largely overlooked parameter that influences the functionality of fuel-cell membranes.

Raman Under Water – Nonlinear and Nearfield Approaches for Electrochemical Surface Science

“…The relative higher amount of non‐bulk water in N117 than in N212 can be explained by smaller nanopores in N117 that lead to more widely spaced SO 3 − head groups, more surface area, and thus to more non‐bulk water compared to N212 (Figure c). The CARS results further suggest that non‐bulk water molecules diffuse an order of magnitude faster ( D nb ≈ 16×10 −10 m 2 /s) than bulk‐water molecules ( D b ≈ 10 −10 ) inside the membrane nano‐channels, which could lie at the origin of the faster macroscopic water and proton transport observed in N117 compared to N212 …”

“…The nanoscale chemical interactions in two differently processed Nafion membranes, N117 and N212, of identical chemical composition but distinct proton transport properties has been elucidated with CARS . While the Nafion signatures of both N117 and N212 are identical, distinct spectral shapes of the water OH stretch signature for the two membranes were observed (Figure a) that can be fitted with two types of water signatures (Figure b), namely bulk water (water molecules coordinated to other water molecules, blue) and under‐coordinated non‐bulk water (orange), located in the center of the ionic channels and at the interface between the hydrophobic channel walls and bulk water, respectively . The relative higher amount of non‐bulk water in N117 than in N212 can be explained by smaller nanopores in N117 that lead to more widely spaced SO 3 − head groups, more surface area, and thus to more non‐bulk water compared to N212 (Figure c).…”

“…b) Fitting with bulk (blue) and non‐bulk (orange) water spectra show that N117 contains more non‐bulk water than N212 which can be explained by c) the different nanoscale chemical constitution of the membrane channels of N117 (smaller channels, less densely packed SO 3 − groups) compared to N112. Reproduced from Ref …”

In Situ Growth and Characterization of Metal Oxide Nanoparticles within Polyelectrolyte Membranes

“…Nafion serves as the archetype polyelectrolyte,d ue to its commercial availability and widely investigated properties. [15,16] An instructive image of nanoscale segregation in amodel Nafion film simulated with the coarse-grained dissipative particle dynamics [14] is given in Scheme 1. Upon hydration, Nafion nanosegregates into hydrophobic and hydrophilic subphases.…”

“…[14] Thel atter represents ac ontinuous 3D network of water domains of the nanometer size. [15,16] An instructive image of nanoscale segregation in amodel Nafion film simulated with the coarse-grained dissipative particle dynamics [14] is given in Scheme 1. Thec ontinuity and restricted geometry of the hydrophilic subphase allows for uniform nucleation and controlled growth of MONP.T his approach, that has been earlier demonstrated with examples of Fe 2 O 3 , [17] ZnO, [18,19] and from zirconium phosphate, [20,21] has particular advantages over other efforts to incorporate MONP into Nafion, which have largely focused on traditional techniques to fabricate the nanoparticles beforehand then incorporating them into the polymer,such as dropcasting with apolymer resin via the doctor blade method [22] or with the use of organic salts as the precursor.T hese traditional methods suffer from the disadvantage of larger than desired MONP, and the need for an additional capping agent.…”