Predicting the proton conductivity of perfluorosulfonic acid membrane via combining statistical thermodynamics and molecular dynamics simulation

Abstract

The electrochemical properties of a perfluorosulfonic acid (PFSA) membrane are estimated using a combination of molecular dynamics simulation and statistical thermodynamic model. We obtain all parameters in an ionic conductivity model from an atomistic simulation and remove all adjusted model parameters. From a microscopic point of view, the hydrated PFSA membrane shows micro-phase segregation which separated into hydrophilic and hydrophobic phases. Our present work originates with this phenomenon and we treat this phase segregation as if it is a continuous phase for each of which the proton (H þ ) is transported inside the PFSA membrane/solvent (water and alcohols) mixture. The chemical potential for a given system is estimated using a molecular simulation technique to predict the van der Waals interaction energy between the polymer and solvent. In addition, the self diffusion coefficients are calculated from the molecular dynamics simulation. We study various polymer/solvent compositions to understand the concentration dependence of self diffusion coefficient. Our self diffusion coefficients and also the predicted final ionic conductivity agree well with previously reported experimental data.

New Insights into Perfluorinated Sulfonic-Acid Ionomers

“…70,71,73,449,455,456 Once an equilibrium morphology is obtained, self-diffusivities of water and hydronium are calculated from their mean-square displacement, which increases with hydration, similar to the e x p e r i m e n t a l o b s e r v a t i o n s ( s e e s e c t i o n 4.1.3). 65,72,357,443,446,448,449,452,456,464,465,533,626,627 Also, these studies consistently report lower diffusion for H 3 O + than H 2 O diffusion, 72,357,446,448,449,451,452,464,465 which can be explained by the stronger interactions H 3 O + has with the SO 3 − groups, compared to H 2 O, which is, however, slowly released with hydration due to increasing solvation of hydronium ions and their increased lability. 357,448 Despite variations in reported data, the difference in self-diffusion between various PFSAs (Nafion vs SSC vs 3M) 449,456 is much lower compared to the difference in their measured conductivities, which implies that the effect of PFSA chemistry and EW on conductivity is more pronounced for the hopping mechanism.…”

“…Finally, it should be noted that there have also been studies in which multiple physical phenomena were combined (such as equilibrated mesoscale structures from DPD with kinetic Monte Carlo, or thermodynamic data and chemical potentials , ) to study various other aspects of transport, including electro-osmosis, conductivity, ,, gas permeability, ,, sorption, , or glass transition temperature …”

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

“…The proton-exchange membrane (PEM) is one of the primary components used in fuel cell technologies. Extensive experimental − and computational studies − have been conducted with a goal of helping to improve the performance of PEMs, including the operational temperature range, mechanical properties, water management, selectivity, gas permeation, and proton conductivity. For example, composite electrolytes were found to contribute to the improvement of proton-exchange membrane fuel cell (PEMFC) performance at high temperatures (130 °C) .…”

A Thermodynamic Approach to Model Proton Conductivity of Nafion-117 Membranes: Temperature and Water Content Effects

“…22,23 Utilizing the thermodynamic approach enables the comprehensive analysis of the effects of the molecular weight and volume fraction of the components, interaction parameters between the components and the operational temperature on the ion conductivity. The above-mentioned model has recently been used by Bae et al, 24,25 for z E-mail: abdollahim@modares.ac.ir; asharif@modares.ac.ir predicting the proton conductivity in the Nafion 117 membranes, in which the concentration of hydronium ions has been considered to be the concentration of all the solvent molecules in the Nafion/water system. Furthermore, they ignored the vital role of hopping mechanism as well as the temperature dependence of the self-diffusion coefficient of hydronium ions in predicting the proton conductivity.…”

“…Furthermore, they ignored the vital role of hopping mechanism as well as the temperature dependence of the self-diffusion coefficient of hydronium ions in predicting the proton conductivity. 25 All of these issues resulted in a disagreement between the model prediction and experimental data. 25 The present study aims to further elaborate the proton conductivity in Nafion 117 membranes by modifying the total ion concentration expression and considering both the hopping and vehicle mechanisms in the afore-mentioned thermodynamic model-combined ionic conductivity equation.…”