Excess Proton Solvation and Delocalization in a Hydrophilic Pocket of the Proton Conducting Polymer Membrane Nafion

Abstract

Solvation properties of the hydrated excess proton are studied in a hydrophilic pocket of Nafion 117 through a series of molecular dynamics simulations. The multistate empirical valence bond (MS-EVB) methodology, which enables the delocalization of the excess proton through the Grotthuss hopping mechanism, was employed for one of the excess protons in the simulation cell. Simulations were performed such that "classical" nondissociable hydronium cations and a single excess proton treated with the MS-EVB methodology were at a concentration ratio of 39:1. Two degrees of hydration of the Nafion polymer electrolyte membrane were simulated, each displaying the same marked difference between the solvation structures of the classical versus MS-EVB treated (Grotthuss shuttling) excess proton species. These differences are attributed to the solvent dynamics needed to transfer the cation between the solvent separated and contact pair positions about the sulfonic acid counterion. The results demonstrate that it is generally impossible to describe the low pH conditions in the hydrophilic domains of Nafion without the explicit treatment of Grotthuss delocalization in the underlying molecular dynamics model for the excess protons.

Modelling of morphology and proton transport in PFSA membranes

“…15) dependence of the mobility and showed that the dominant diffusion of the protons occurs in regions separated from the sulfonate groups. This latter result compares favourably with the earlier discussed EVB simulations of Petersen et al 51 where at l = 9 they observed maxima in the sulfonate-oxygen-hydroniumoxygen radial distribution function at 2.6 Å (contact ion pair) and 4.4 Å (solvent-separated ion pair).…”

Phase Separation and Water Channel Formation in Sulfonated Block Copolyimide

“…20,59 As discussed in the Introduction, many studies were performed to describe the hopping mechanism and obtained reasonable results. [26][27][28][29][30] However, because this study mainly focuses on the PEM morphology under hydrated conditions, such a model using excess protons is not applied. As a result, the diffusivity of Nafion in simulations is lower than that from the experimental results (about 7 × 10 -6 cm 2 s -1 at 303.15 K 60 ) due to the absence of the additional driving force of the hopping mechanism.…”

“Divide and conquer” semiclassical molecular dynamics: A practical method for spectroscopic calculations of high dimensional molecular systems

“…[1][2][3][4][5] Alternatively, perturbative quantum methods have also been successfully applied to many systems, but they are intrinsically limited to a single reference geometry. [6][7][8][9][10][11] High dimensional systems, such as peptides, are instead usually simulated through ad-hoc scaled harmonic approaches or by means of classical mechanics, either using force fields [12][13][14] or employing ab initio molecular dynamics (AIMD) [15][16][17][18][19][20][21] approaches in which the nuclear forces are calculated using electronic structure codes. In classical simulations the curse of dimensionality is significantly tamed with respect to quantum mechanical counterparts.…”