Effects of Polymer Morphology on Proton Solvation and Transport in Proton-Exchange Membranes

Abstract

The effects of polymer morphology on proton solvation and transport in hydrated Nafion are investigated by using a novel reactive molecular dynamics approach. Three of the most significant morphological models of Nafion, the lamellar model, the cylinder model, and the cluster-channel model, are studied. The three models exhibit distinct proton transport (PT) patterns, which result in different proton diffusion rates. In both the lamellar and the cylinder models, the interaction between protons and the sulfonate groups is shown to be the key factor in determining PT behavior. For the cluster-channel model, the geometrical shape also plays an important role in influencing the PT behavior. The change in the excess proton solvation structure as a function of the distance between protons and sulfonate groups is also analyzed. It is found that the increase of the water cylinder radius or water layer height leads to the presence of more protons around the sulfonate groups, while, for the cluster model, an increase in the water sphere radius leads to the presence of fewer protons around the sulfonate groups. Furthermore, for the lamellar and cylinder models, the hydrated protons around the sulfonate groups consist of more Zundel-like (H 5 O 2 + ) structures when the hydration levels decrease, which is also influenced by the different morphological structures of Nafion.

Dependence of electroosmosis on polymer structure in proton exchange membranes

“…In this way, the significant changes in structures were not observed, testifying the negligible relationship between the direction of the cluster and the electric field. A similar approach was also used in previous work (Feng et al, 2012) to maintain the desired geometric shapes of water domains in Nafion membranes. This restraint has a negligible impact on the electroosmotic properties, which can be justified for two reasons: (i) because of the characteristic of hydrophobicity, the positions of the backbone carbon atoms are much further from the protons by comparison with the sulfonate groups; and (ii) our target system is a fuel cell in which the Nafion polymers are somehow tightened and fixed from both sides of the membrane.…”

“…An increase in EW results in the larger sulfonate-sulfonate separation, and thus hinders proton transport through hydrophilic channels (Allahyarov and Taylor, 2011). Theoretical and computational analyses, such as molecular dynamics (MD) simulations and ab initio simulations, have been used extensively to investigate proton transport properties including electroosmosis in Nafion membranes and thin films (Feng et al, 2012, Feng and Voth, 2011, Jorn et al, 2012, Petersen and Voth, 2006, Petersen et al, 2005, Tse et al, 2013, Seeliger et al, 2005, Spohr et al, 2002, Cui et al, 2007, Jang et al, 2004, Wang et al, 2011a, Karo et al, 2010, Knox and Voth, 2010, Komarov et al, 2013, Spohr, 2007, Mabuchi and Tokumasu, 2014, Wescott et al, 2006, Tuckerman et al, 1995, Eikerling et al, 2008, Kornyshev and Spohr, 2009, Choe et al, 2008, Yan et al, 2008, Aochi et al, 2016, Kurihara et al, 2017. However, extensive simulations using ab initio methods for a larger number of atoms, where systems reproduce nanostructured Nafion morphology and water domains with reliable statistics, have been limited because of the high computational cost of such simulations.…”

“…Yan et al (Yan et al, 2008) have studied electroosmotic drag using the classical hydronium cation model in the MD simulations and predicted a value of K drag about 2.5 times greater than the experimental data at 300 K in completely hydrated Nafion membranes. Although few studies incorporate the Grotthuss mechanism into MD simulations to develop reactive MD using, for example, empirical valence bond (EVB) approaches (Feng et al, 2012, Feng and Voth, 2011, Jorn et al, 2012, Petersen and Voth, 2006, Petersen et al, 2005, Tse et al, 2013, Seeliger et al, 2005, Spohr et al, 2002, Savage et al, 2014, they have focused only on the proton diffusion in the equilibrium polymer systems, and therefore the electroosmotic properties have not been fully understood.…”

Effects of water nanochannel diameter on proton transport in proton‐exchange membranes

“…Theoretical studies have been devoted to providing a detailed relationship between the morphological features and proton transport using both classical MD and RMD simulations . Liu et al .…”

First-Principles Molecular Dynamics Study of a Hydrocarbon Copolymer for Use in Polymer Electrolyte Membrane Fuel Cells

“…7,8 Over the past years, the structures and proton transport of many types of copolymers, including perfluorocarbon and aromatic polymers, have been simulated extensively using various techniques. [9][10][11][12][13][14][15] Choe et al reported the proton dynamics in Nafion and sulfonated polyethersulfone determined from first-principles molecular dynamics (FPMD) simulations. [16][17][18] Jorn et al used the multistate empirical valence bond model to investigate the proton conductivity of Nafion using classical molecular dynamics (MD) and coarse-grained descriptions.…”