On the Relationship between Polymer Electrolyte Structure and Hydrated Morphology of Perfluorosulfonic Acid Membranes

Abstract

Molecular dynamics simulations were performed to investigate the relationship between the molecular structure of perfluorosulfonic acid (PFSA) ionomers and the nanoscale morphology of the hydrated membranes. Three structural features are examined including (i) the length of the side chain to which the sulfonic acid group is attached, (ii) the equivalent weight (EW) of the electrolyte ionomer, and (iii) the molecular weight (MW) of the polymer electrolyte. Membrane morphologies are studied from the water content λ = 3 (λ represents number of water molecules per sulfonate group) to saturation (λ = 22). We find that with the longer side chain, there is more clustering of the sulfonate groups and more local water−water clustering, but a more poorly connected aqueous domain. When one decreases the equivalent weight (EW) in either the short side chain (SSC) PFSA or Nafion, there is more clustering of the sulfonate groups and more local water−water clustering and a better connected aqueous domain. Because connectivity enhances and confinement reduces water mobility, a decrease in EW, which enhances connectivity and reduces confinement, results in an increase in diffusivity. An increase in side chain length diminishes connectivity but reduces confinement, which together result in little change in the observed water diffusivity. For the short chains studied, we find these results to be independent of MW.

QENS investigation of proton confined motions in hydrated perfluorinated sulfonic membranes and self-assembled surfactants

“…Values of D lr are found to be similar in PFOS and Aquivion, and ∼ 3 to 10 times lower than D loc , as due to the nanoscale tortuosity effect. Interestingly, we can compare the values of the local and nanoscale diffusion coefficients to those extracted by Molecular Dynamics simulation performed on a short-side chain PFSA membrane [7] and Nafion pendant chain [35]. We notice that the water diffusion coefficients obtained by numerical approaches increase upon hydration, as expected.…”

“…EPJ Web of Conferences The diffusion coefficients computed from numerical simulations ( [7] and [35]) are also reported.…”

“…The PFSA proton conductivity is largely determined by the topology of the ionic phase and the proton-charges-water interactions. The morphology of the commercial membranes has been extensively studied in the last decades mostly by small-angle scattering techniques [1][2][3][4] and numerical simulations [5][6][7][8][9]. The swelling of percolating ionic nanodomains upon water loading ( Fig.…”

Hydrogen adsorption and diffusion in amorphous, metal-decorated nanoporous silica

“…The densities change, as the name implies, for a given eOTiCl 3 content, although there are examples where the density of a "low density" material with 100% eOTiCl 3 end groups is higher than a "medium density" material with 0% eOTiCl 3 end groups due to the high molecular weight of Ti. Accessible volume and surface area for each structure were calculated using geometrical methods that have been used for MOFs [27] and polymer membranes [28,29] in our previous studies. These geometrical methods are based on published approaches [30].…”

Molecular dynamics simulations of triflic acid and triflate ion/water mixtures: A proton conducting electrolytic component in fuel cells

“…[7][8][9][10][11][12] However, the conductivity of Nafion membrane is strongly dependent on its hydration. [13][14][15][16][17] The operating temperature of fuel cells using Nafion is limited to the boiling point of water, that is, 100 C, and at this temperature, there is low tolerance of the electro-catalyst like platinum against CO poisoning. However, a modifications of per-fluorosulfonated (PFSA) membranes like Nafion and other membranes were developed to sustain high CO tolerance and thermal, water management.…”