A Molecular Dynamic Simulation of Hydrated Proton Transfer in Perfluorosulfonate Ionomer Membranes (Nafion 117)

Abstract

A molecular dynamic model based on Lennard-Jones Potential, the interaction force between two particles, molecular diffusion, and radial distribution function (RDF) is presented. The diffusion of the hydrated ion, triggered by both Grotthuss and vehicle mechanisms, is used to study the proton transfer in Nafion 117. The hydrated ion transfer mechanisms and the effects of the temperature, the water content in the membrane, and the electric field on the diffusion of the hydrated ion are analyzed. The molecular dynamic simulation results are in good agreement with those reported in the literature. The modeling results show that when the water content in Nafion 117 is low, H3O+is the main transfer ion among the different hydrated ions. However, at higher water content, the hydrated ion in the form of H+(H2O)2is the main transfer ion. It is also found that the negatively charged sulfonic acid group as the fortified point facilitates the proton transfer in Nafion 117 better than the free water molecule. The diffusion of the hydrated ion can be improved by increasing the cell temperature, the water content in Nafion, and the electric field intensity.

Simulation Study of Ion Diffusion in Charged Nanopores with Anchored Terminal Groups

“…The simulated axial diffusion coefficients have maximal values in the range D max x ≈(10-14)×10 −6 cm 2 /sec for water content λ≈6-8, and sidechain length L s ≈8-10. These diffusion rates are about 2-3 times larger than the diffusion rate D lr H3O (en-masse) in fully hydrated Nafion [59,104], and about 5-7 times larger than the diffusion rate D lr H3O (en-masse) in partly hydrated Nafion [63,69,103]. We believe that experimental realizations of our grafted pore set-up will produce even higher ion diffusion rates because of the Grotthuss structural diffusion contribution to D max x .…”

“…Our data for D max x corresponds to the long-range diffusion D lr of hydronium ions without the Grotthuss contribution. Therefore, we get a diffusion rate 2-3 times larger than D lr H3O (en-masse) in fully hydrated Nafion [59,104], and about 5-7 times larger in partly hydrated Nafion [63,103].…”

“…Additionally, one has to bear in mind that experimentally determined diffusion coefficients for hydronium ions include the Grotthuss proton hopping mechanism, which is not accounted for in our simulations. For example, D lr H3O in fully hydrated Nafion with the Grotthuss hopping accounted for reaches D lr H3O (Grotthuss + en-masse)≈(12-14)×10 −6 cm 2 /sec [59] which is more than twice as large as the en-masse diffusion D lr H3O (en-masse)≈(4-5)×10 −6 cm 2 /sec [59,104]. The same is valid in partly hydrated Nafion with λ≈6-10, where D lr H3O (Grotthuss + en-masse)≈(2-5)×10 −6 cm 2 /sec is larger than D lr H3O (enmasse)≈(1-2)×10 −6 cm 2 /sec [63,103].…”

Transmission of sodium chloride in PDMS membrane during Pervaporation based on polymer relaxation

“…5 that water flux and isobutanol flux gradually increase as the temperature rises from 30 • C to 60 • C. This is because temperature increases thermal movement of components, and also increases the swing of PDMS polymer chain, which promotes penetration of components [41]. In addition, temperature rise will increase diffusivity of hydrated ions [29,42], but it is found from Fig. 5 that PDMS membrane salt rejection rises from 99.34% to 99.92% with the rise of temperature.…”

Mass transport in PEM water electrolysers: A review

“…Contribution of different mechanisms is also dependent on hydration state. At low hydration, Nafion SO3 -… H + OH2 groupings occur with H + biased towards the water molecule [161], and when H2O/SO3H > 2, the excess protons exist in water hydrogen bonds instead of acid-water hydrogen bonds. The mechanism for proton hopping along the negatively charged surface has a significantly higher activation energy due to electrostatic attraction between the two entities.…”