Comparison of the Hydration and Diffusion of Protons in Perfluorosulfonic Acid Membranes with Molecular Dynamics Simulations

Abstract

Classical molecular dynamics (MD) simulations were performed to determine the hydrated morphology and hydronium ion diffusion coefficients in two different perfluorosulfonic acid (PFSA) membranes as functions of water content. The structural and transport properties of 1143 equivalent weight (EW) Nafion, with its relatively long perfluoroether side chains, are compared to the short-side-chain (SSC) PFSA ionomer at an EW of 977. The separation of the side chains was kept uniform in both ionomers consisting of -(CF 2) 15- units in the backbone, and the degree of hydration was varied from 5 to 20 weight % water. The MD simulations indicated that the distribution of water clusters is more dispersed in the SSC ionomer, which leads to a more connected water-channel network at the low water contents. This suggests that the SSC ionomer may be more inclined to form sample-spanning aqueous domains through which transport of water and protons may occur. The diffusion coefficients for both hydronium ions and water molecules were calculated at hydration levels of 4.4, 6.4, 9.6, and 12.8 H 2O/SO 3H for each ionomer. When compared to experimental proton diffusion coefficients, this suggests that as the water content is increased the contribution of proton hopping to the overall proton diffusion increases.

Molecular Simulations of Hydrated Proton Exchange Membranes: the Structure

“…Since we have attempted to limit the entanglement of the polymer chains when generating the systems, and since the least entangled systems by Knox and Voth also seem to be the ones closest to experiment, one might speculate that this is one key feature leading to the observed features in the scattering experiments, as expressed in the so-called ionomer peak. In contrast to some earlier work [54,69], we do not observe significant differences between Nafion and Hyflon. Since both the 'cylindrical' model and the 'random' model proposed here as well as most of Voth's model systems reproduce the experimental scattering data satisfactorily, we must conclude that the scattering data alone are not sufficient to elucidate the structure of systems as complex as proton exchange membranes in water, hence the ongoing debate over their morphology.…”

“…4). This is at variance with the work of Cui et al [68,69] who observed a less dispersed water distribution in Nafion as compared to Hyflon.…”

<i>Ab Initio</i> Study of Proton Transfer and Hydration in Phosphorylated Nata de Coco

“…The classical molecule dynamics (MD) and ab initio molecular dynamics (AIMD) methods were used to determine proton mobility as a function of hydration conditions and sulfonated function rings [14][15]. Studies on SSC PFSA membrane fragments show that the formation of hydrogen bonds between water molecules is highly dependent on acid ring connectivity.…”

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

“…[5] Among these membranes, Nafion is studied extensively by experiments [2,6] and computational methods. [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.…”