(254b) Evaluation of Electronic Structure Models of the Electrocatalytic Interface

The sluggish oxygen reduction reaction (ORR) on the Pt electrode is the major performance loss of proton exchange membrane fuel cells (PEMFCs). ORR reaction kinetics are influenced by both electrode and electrolyte. The presence of adsorbing species from the electrolyte may block the reactive sites and or alter elementary step kinetics. The interfacial water structure affects the binding strength and extent of charge transfer to molecular oxygen adsorbed on the Pt(111) surface. Electronic structure models can provide atomic insight into these phenomena, directly evaluating the potential dependence of elementary reaction kinetics. Approaches developed to model electrocatalytic reaction kinetics from first-principles include extrapolating typical vacuum slab calculations with the electron energy dependent on potential, application of an external field, and direct tuning of the metal Fermi level by charging the system. We will evaluate these approaches to differentiate the importance of local electric field, through surface and through space interactions, coadsorption, and solvent effects including ion adsorption. Density functional theory will be used with various models to investigate oxygen adsorption and dissociation at the solvated, electrified Pt(111) surface.

We choose oxygen adsorption in the presence of sulfuric acid as our model system because oxygen adsorption is the first step of ORR and sulfate anions represent an initial model of the Nafion electrolyte. The adsorption energies at the solvated surface are calculated by replacement of an adsorbed water molecule with an oxygen molecule or ions in solution. The potential-dependent adsorption energies and dissociation barriers are the results of a compounded effect of the interactions between water-molecular oxygen, molecular oxygen-Pt(111), and sulfate-molecular oxygen, which not only depend on the spatial configuration, but are also affected by electronic interactions through the electrode surface. The dependence of molecular oxygen/water replacement energies on the interfacial water density supports the importance of the compounded through-space and through-surface interactions between water and oxygen. The influence of electrolyte structure on oxygen adsorption and dissociation will be presented. The use of different electric field models to clarify the factors attributing to the dependence of replacement energies on electrode potential will also be discussed.