No abstract

A series of density functional theory (DFT) based electrochemical models are applied to systematically examine the effect of solvent, local electric field, and electrode potential on oxygen reduction reaction (ORR) kinetics. Specifically, the key elementary reaction steps of molecular oxygen dissociation, molecular oxygen protonation, and reduction of a hydroxyl adsorbate to water over the Pt(111) surface were considered. The local electric field has slight influence on reaction energetics at the vacuum interface. Solvent molecules stabilize surface adsorbates, assisting oxygen reduction. A collective solvation-potential coupled effect is identified by including long range solvent-solvent interactions in the DFT model. The dominant path of the ORR reaction varies with electrode potential and among the modeling approaches considered. The potential dependent reaction path determined from the solvated model qualitatively agrees with experiment ORR kinetics.

The adsorption of molecular oxygen is the first step in the oxygen reduction reaction. Influences of interfacial water structure and electrode potential on oxygen adsorption to the Pt(111) surface were evaluated using density functional theory. Two approaches were used to model the electrification of the interface, an applied homogeneous electric field and the double-reference method of Filhol, Taylor, and Neurock. The free energy change for molecular oxygen replacement of water at the surface shows qualitatively different trends between the two models. The inclusion of solvation effects and direct control of the electrode potential offered by the double-reference method lead to the conclusion that O(2) replacement of water is favorable at all potentials studied, and O(2) binding becomes more favorable with increasing potential. This trend is contrary to that observed using an external electric field model to represent the electrochemical double layer, and arises due to the compounded effect of potential on water-surface, oxygen-surface, and water-molecular oxygen interactions. These results indicate that oxygen replacement of adsorbed water does not limit the overall oxygen reduction reaction rate at a proton-exchange membrane fuel cell cathode. The impacts of aspects of model construction (number of water layers, water density) and variation of electrode potential on the O(2)-Pt(111) interaction are described.