(373aa) Molecular Modeling of Nanostructure and Proton Transport Dynamics In Polymer Electrolyte Membranes

Devanathan, R., Pacific Northwest National Laboratory
Dupuis, M., Pacific Northwest National Laboratory

Multiscale modeling and simulation spanning multiple length and time scales were employed to understand proton dissociation and transport, membrane morphology in polymer electrolyte membranes (PEM) that are potential candidates for use in PEM fuel cells. At the most fundamental level, density functional theory was used to study proton dissociation in model acidic pendants that are building blocks for existing and proposed membranes, such as Nafion, sulfonated poly ether ether ketone (SPEEK) and sulfonyl imides. The sulfonyl imide acid group was observed to be the strongest, while the functional acid group of SPEEK membrane was found to be the weakest acid with the acidic proton dissociation requiring the addition of three and six water molecules respectively. Even though both sulfonyl imides and fluoroethyl sulfonic acids (functional acid group for Nafion) required only three water molecules to exhibit spontaneous proton dissociation, the largest possible solvent-separated hydronium ion was attained only for the former case [1]. At the next higher level, we used classical molecular dynamics to examine nanostructure, water network formation, and the transport of water molecules and hydronium ions at the membrane level in these PEMs [2-3]. Our results for proton diffusion coefficients, radial distribution functions for various interacting species and water percolation help interpret and compare experimental observations about the water sorption, conductivity of these membranes under varied hydration conditions. These observations provide molecular-level fundamental understanding of the desirable characteristics which can enable rational design of novel proton exchange membranes for fuel cell applications.

[1] Idupulapati N, Devanathan R and Dupuis M 2010 J. Phys. Chem. A. 114 6904. [2] Devanathan R, Venkatnathan A and Dupuis M 2007 J. Phys. Chem B. 111 8069. [3] Devanathan R, Venkatnathan A and Dupuis M 2007 J. Phys. Chem B. 111 13006.