(190u) Multi-Scale Modeling of a Cathode/Electrolyte Interface In Proton Exchange Membrane Fuel Cells (PEMFCs)

The slow oxygen reduction reaction (ORR) at the cathode represents the largest performance loss of a PEMFC. Few studies have examined the impact of the cathode/electrolyte interfacial structure on surface reaction kinetics. We employed a solvated Pt(111) DFT model system to probe the dependence of ORR energetics on water density which represents interfacial structure variation. The variable electrode potential is modeled using double reference method (C. D. Taylor et al., 2006), which allows us to consider the coupled effect of solvent reorganization and electrode potential on reaction energetic. For comparison, a homogeneous electric field model perpendicular to the surface was applied on a vacuum slab model to evaluate trends in oxygen reduction under the influence of electric fields. Although the solvated model predicts a potential dependent rate limiting step of ORR, in agreement with experiment, the DFT conclusions are limited by the use of single water structure. Variations in DFT results due to the model choice emphasize the importance of using molecular dynamics (MD) to provide most probable interfacial structures. A reactive force field, based on the Central Force Model (F. H. Stillinger and A. Rahman, 1978), was used to model the electrolyte layer.  A polarizable and potential controlled electrode model, termed electrode charge dynamic (C. G. Guymon et al., 2005), was used to represent the electrode surface The MD simulations quantify the interfacial water and ion structure as a function of electrode potential and acid concentration. Significant structural difference of interfacial water between MD results and the bilayer water used in DFT methods are found. Representative interfacial structures from MD simulations will be used as the starting structure for DFT methods to study the influence of interfacial water structure on oxygen reduction kinetics.