(339a) Molecular Modeling of Polymer Electrolyte Membrane for Fuel Cell

Jang, S. S., Georgia Institute of Technology

Molecular Modeling of Polymer Electrolyte Membrane for Fuel Cell

Seung Soon Jang

 Computational NanoBio Technology Laboratory, School of Materials Science and Engineering, Georgia Institute of Technology,771 Ferst Drive NW, Atlanta, GA 30332-0245, USA

We present what we have learned from our researches on polymer electrolyte membrane for fuel cell systems using molecular dynamics (MD) simulation method. Mostly our effort has been focused on the relationship between nanophase-segregation and transport properties. From our proton exchange membrane studies, it is clear that the interaction of polymer backbone with water molecules determines the nanophase-segregation, and such nanophase-segregation determines the internal structure of water phase as well as the distribution and dimension of the water phase. Based on these understanding on the proton exchange membranes, we pursue the working principles in anion exchange membranes. For fair comparison, we simulate proton and anion exchange membrane fuel cell using the same polymer backbone with the same molecular variables such as molecular weight, equivalent weight, water contents, and so on. The only difference is the ionic entities: the proton exchange membrane has sulfonate group with hydronium, whereas the anion exchange membrane has trimethyl ammonium group with hydroxide. Quantum mechanical density functional theory (DFT) is used to describe monomeric units with B3LYP and 6-31G**, from which Mulliken population charges are calculated for electrostatic interactions in MD simulations. We use Dreiding force field (FF) for polymers with F3C water model. In addition to the FF for hydronium published in 2004, we prepare the FF for hydroxide. From MD simulations, we investigate the relationship between nanophase-segregation and transport properties in the membranes. The nanophase-segregation is quantitatively evaluated by the structure factor analysis which characterizes the correlation length in reciprocal space. The transport of molecules such as water, hydronium and hydroxide is analyzed using the mean-square displacement.