Wednesday, October 19, 2011: 4:35 PM
200 A (Minneapolis Convention Center)
Multi-scale modeling, using both density functional theory (DFT) and molecular dynamics (MD), are applied to the electrode/electrolyte interface in a proton exchange membrane fuel cell (PEMFC). The complexity in the electrochemical interface and the difficulties to characterize the interface hinder the fundamental understanding of the oxygen reduction reaction (ORR) under fuel cell operating conditions. Slow ORR kinetics represent a major performance loss in a PEMFC. To capture ORR energetics under different electrochemical conditions, various DFT-based electrochemical models were employed. We determined rate limiting steps in both dissociative and associative ORR paths as a function of electrode potentials and hydration levels. The overall reaction barrier at PEMFC operation potential agrees well with the experimental data. In addition, a solvated model predicts the potential-dependent rate limiting step, qualitatively consistent with the experimental results. However, the influence of the water structure (hydration level) on ORR energetics emphasizes the needs to consider the structure and dynamics of the interfacial water. To extend time and length scales, we performed MD simulations in systems of pure water and 1M H2SO4 aqueous solution, which represents a simplified electrolyte solution in PEMFCs, at a potential controlled Pt (111) surface. Results indicate that the electrode potential affects the orientation and the ordering of interfacial water molecules, and the packing of counter ions as well. The structure of the electric double layer is more complicated than that derived from classical-continuum models. Characteristics of the double layer region from MD simulations will provide insights for an advanced DFT-based electrochemical model for a further investigation of the ORR energetics.