Issue 20, 2009

Nature of proton dynamics in a polymer electrolyte membrane, nafion: a first-principles molecular dynamics study

Abstract

First-principles molecular dynamics simulations have been carried out to investigate the nature of proton dynamics in Nafion, a representative polymer electrolyte membrane (PEM) widely used in PEM fuel cells. From the trajectories of the simulations, diffusion coefficients for the protonic defects were calculated to be 0.3 × 10−5 cm2 s−1 and 7.1 × 10−5 cm2 s−1 for λ = 4.25 and 12.75, respectively, where λ denotes hydration levels inside Nafion defined as a number of water molecules per sulfonic group. Our simulations show that proton hopping probability does not depend much on the water content inside Nafion. This finding indicates that the classical vehicular (or en masse) diffusion model, which has been employed to account for the slow diffusion process of protons in low water-content Nafion, is an oversimplification and does not correctly describe proton dynamics. Furthermore, it is found that difference in the value of the proton diffusion coefficient with respect to water content inside Nafion is related to the different character of proton hopping occurring in the water hydrogen bond network. When the water content is low, the proton hopping occurs in a manner that does not contribute constructively to proton mobility, while when the water content is high, it occurs in a manner which is beneficial to overall proton mobility. Such a different nature of proton hoppings arises mainly from the difference in the connectivity of water hydrogen bond network. Our results broadly support earlier simulation studies and provide the molecular level origin of properties arising from the proton dynamics in Nafion.

Graphical abstract: Nature of proton dynamics in a polymer electrolyte membrane, nafion: a first-principles molecular dynamics study

Supplementary files

Article information

Article type
Paper
Submitted
04 Nov 2008
Accepted
23 Feb 2009
First published
14 Mar 2009

Phys. Chem. Chem. Phys., 2009,11, 3892-3899

Nature of proton dynamics in a polymer electrolyte membrane, nafion: a first-principles molecular dynamics study

Y. Choe, E. Tsuchida, T. Ikeshoji, S. Yamakawa and S. Hyodo, Phys. Chem. Chem. Phys., 2009, 11, 3892 DOI: 10.1039/B819535H

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