(344d) Co-Adsorption and Electrode-Electrolyte Interfacial Structure Effects on Oxygen Dissociation and Reduction Energetics
A majority of the performance losses in a proton exchange membrane fuel cell are allocated to processes at the oxygen reduction cathode, where power/efficiency losses, non-optimum utilization of expensive platinum catalyst, and sensitivity to the relative humidity of the reactant oxygen stream challenge the development of practical devices. These losses can, in part, be attributed to imperfect design of the Pt catalyst particle-polymer electrolyte interface. Additionally, the presence of other adsorbing species may alter the rates of elementary reaction steps within the oxygen reduction reaction (ORR). We describe the use of first principles methods to elucidate the effects of electrolyte structure and co-adsorbed species on the rates of molecular oxygen reduction and dissociation on the Pt(111) surface.
The ORR may initiate through either molecular oxygen dissociation or reduction elementary reactions. Though dissociation is a non-electrochemical reaction, variations in electrode potential alter the interactions of both the adsorbed state and the dissociation transition state with the catalyst surface, thereby making the barrier to dissociation dependent on the electrode potential. Results from density functional theory (DFT) calculations illustrate the coupling between the effects of potential variation and solvation on the dissociation barrier. Though electron transfer from the surface to O-O antibonding orbitals is enhanced at more negative (reducing) potentials, the dissociation barrier decreases at more positive potentials. This counterintuitive trend is due to enhanced stabilization of the dissociation transition state at more positive potentials. Co-adsorption of alkali species at the surface lowers the oxygen dissociation barrier at all potentials. The activation barrier to the initial reduction step, reducing adsorbed oxygen to OOH, is also dependent on the electrode potential, decreasing at more negative potentials. The relative rates of these two reactions is therefore potential dependent. Furthermore, the reduction energetics are dependent on the electrolyte structure at the interface. Dependence of reduction energetics on the interfacial water density and structure determined using DFT methods will be presented. Initial efforts in developing a multi-scale, integrated molecular dynamics and DFT approach to determining the interfacial structure dependence of ORR rates will be discussed.