(486e) First-Principles Modeling of Proton Conductivity in Perfluorosulfonate Fuel Cell Membranes

Proton conductivity through a perfluorosulfonate polymer electrolyte membrane was investigated using an extension of a recent model developed for the purpose of quantitatively describing transport of charged species through the ionomer interface of a typical membrane [Kumar et al., JCP, 138, 064903, 2013].  In this study, the membrane was modeled as a collection of cylindrical pores with the anionic groups (i.e., –SO₃^–) assumed to be fixed at a prescribed periodic spacing along the pore wall, as determined by the material’s equivalent weight and surface charge density.  Microscopic potentials for the interactions between the various chemical species present within the pore were scaled up to a mesoscopic level, where transport equations were derived via the generalized bracket formulation of nonequilibrium thermodynamics in terms of the density and velocity of each species (hydronium ions and water) within the pore.  These coupled transport equations were solved based on reasonable boundary conditions, and an expression of conductivity was deduced in terms of the following quantities: water content, equivalent weight, temperature, density, and the monomeric structure of the perfluorosulfonate chain.  Theoretical predictions of this model were compared against experimental data of conductivity in three different membranes, Nafion 117 (EW = 1100), a 3M membrane (EW = 900), and a Dow membrane (EW = 800), for different values of water content and temperature. Theoretical predictions of the model matched quantitatively the experimental data without the necessity of using empirical fitting parameters.