Molecular Dynamics Simulations of Electroosmosis in Perfluorosulfonic Acid Polymer

Abstract

An atomistic MD simulation method has been developed to study the electroosmotic drag in the hydrated perfluorosulfonic acid polymer. The transport characteristics of the hydroniums and water molecules are evaluated from their velocity distribution functions with an electric field applied. It is shown that the microstructure of the hydrated perfluorosulfonic acid polymer is not perturbed significantly by the electric field up to 2 V/microm, and the velocity distribution functions obey the peak shifted Maxwell velocity distribution functions. The evaluated peak shifting velocities are only about 1% of the average thermal motion. The hydronium flow and water flow are evaluated from the average transport velocities or the peak shifting velocities. The electroosmotic drag coefficients from the MD simulations are in good correspondence with the experimental values. It is also shown that the electroosmotic drag coefficient has no or weak temperature dependence.

Dependence of electroosmosis on polymer structure in proton exchange membranes

“…Mabuchi and Tokumasu, Mechanical Engineering Journal, Vol.4, No.5 (2017) [DOI: 10.1299/mej.17-00054] Figure 7 shows K drag in the 1100 EW membrane for each λ and the results are compared with the available experimental values in Nafion 117 (~1100 EW) membranes (Fuller and Newman, 1992, Ise et al, 1999, Ren and Gottesfeld, 2001, Zawodzinski et al, 1995, Zawodzinski et al, 1993. The results are in good agreement with the experimental data in comparison with the previous K drag prediction of ~6.0 at λ = 20 using the classical hydronium cation model, which is about 2.5 times greater than the experimental data (Yan et al, 2008). This suggests the importance of incorporating the Grotthuss contribution to the dynamics of protons to estimate K drag accurately.…”

“…Furthermore, most MD simulation studies have employed classical hydronium cation models that do not take into account the Grotthuss mechanism, resulting in an overestimation of K drag . 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.…”

“…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.…”

Evaluation of electroosmotic drag coefficient of water in hydrated sodium perfluorosulfonate electrolyte polymer

“…1). 26,28 This hydration degree is reasonably compared to commercial PFSA, for example, preswollen NAFION membrane (in acidic form) in contact with water has a water content as high as 22. 29 Although the Na 1 -form PFSA may have a lower water content 4,30,31 ; higher hydration is still preferred because the simulation cell is much smaller than the real hydrated membrane, and it could get a large hydrophilic subphase if higher hydration degree is employed in the MD simulation.…”

“…26 We firstly evaluated the molecular velocity distribution functions from the trajectory file recorded during the MD simulation. And then, we fitted the molecular velocity distribution functions to the Maxwellian distribution function or the peak shifted Maxwellian distribution function.…”