Nanophase segregation and water dynamics in hydrated Nafion: Molecular modeling and experimental validation

Abstract

Reported results of coarse-grained molecular dynamics simulations rationalize the effect of water on the phase-segregated morphology of Nafion ionomers. We analyzed density maps and radial distribution functions and correlated them with domain structures, distributions of protogenic side chains, and water transport properties. The mesoscopic structures exhibit spongelike morphologies. Hydrophilic domains of water, protons, and anionic side chains form a random three-dimensional network, which is embedded in a matrix of hydrophobic backbone aggregates. Sizes of hydrophilic domains increase from 1 to 3 nm upon water uptake. At low water content, hydrophilic domains are roughly spherical and poorly connected. At higher water content, they convert into elongated cylindrical shapes with high connectivity. Further structural analysis provides a reasonable estimate of the percolation threshold. Radial distribution functions from coarse-grained and atomistic molecular dynamics models exhibit a good agreement. Water cluster size distributions from coarse-grained molecular dynamics and dissipative particle dynamics are consistent with small angle x-ray scattering data. Moreover, we calculated the water diffusivity by molecular dynamics methods and corroborated the results by comparison with pulsed field gradient NMR.

Ionomer Self-Assembly in Dilute Solution Studied by Coarse-Grained Molecular Dynamics

“…Atomistic molecular dynamics (MD) simulations [27][28][29][30][31][32]34,37 have served as the focal point of membrane simulation studies. Other approaches such dissipative particle dynamics (DPD) simulations 50,53,55 and mean-field approaches 56 have helped expanding the time and length scale of atomistic simulations.…”

Correlating Humidity-Dependent Ionically Conductive Surface Area with Transport Phenomena in Proton-Exchange Membranes

“…These phenomena are critical for high membrane proton conductivity and high PEMFC power density. 15,16 A wealth of analytical techniques has been used to obtain morphological and structural information about Nafion  , including X-ray and neutron scattering, 13,14,[17][18][19][20][21][22] electron microscopy, 23 atomic force microscopy (AFM), [24][25][26][27][28][29] numerical modeling, 30,31 etc. Numerous studies of the ionic conductivity of Nafion ® have been reported in the literature.…”

Modeling and Simulation for Fuel Cell Polymer Electrolyte Membrane

“…Equations (15)(16)(17)(18) with T in Kelvin were used for diffusion in the Nafion phase [26] and water phase [27]. Water diffusion D'…”

“…These jumping rules ensured that the diffusion of the gas particles within both phases was well described, and that during the simulations, the gas particle concentration within each phase matched exactly the experimental pure phase component solubility. The gas diffusion constants within the membrane were obtained by multiplication of the slopes derived from the MSD curves with the diffusion constant of oxygen and hydrogen in the pure water phase given by Equations (17) and (18), respectively. The permeability K was calculated by multiplying of the thus-obtained diffusion constant with the solubility values given by Equation (12).…”

“…Another coarse-graining strategy, coarse-grained molecular dynamics (CGMD), was applied to understand the microstructure within the membrane [17] and catalyst layer [18]. These studies compared water cluster size with small angle X-ray scattering data.…”