(159g) Ab Initio and MD Studies of the Catalyst – Nafion Interfacial Region in Pem Fuel Cells

Proton-exchange membrane fuel cells (PEMFC) are a promising technology under investigation for the replacement of traditional hydrocarbon fuels for automotive applications. The membrane electrode assembly, one of the components of the PEM fuel cells, is a complex system composed by an ensemble of carbon particles, metallic catalytic nanoparticles and NAFIONR polymer. This system should be designed to satisfy different opposite requirements as, for example, good contact between carbon particles to ensure good electron conductivity, large pore size to favor oxygen diffusion and high enough NAFIONR, and water contents to favor proton transport. Classical molecular dynamics simulations are applied to study the membrane conformation and attachment over a graphite surface under different conditions. NAFIONR solvation is studied under the presence of different water contents and, on representative systems, transport properties are studied and compared with well known bulk mechanisms and properties. The results obtained from the MD simulations are used to build a smaller representation of the system that is studied at a higher level of theory via an Ab Initio plane wave approach. Water and oxygen electroreduction intermediates are characterized in the proximities of the NAFION ? Pt(111) contact area. The unit cell is modeled as a three layer - 12 atoms per layer Pt (111) surface. Periodic boundary conditions are applied in the three spatial directions and 10 Å of vacuum space is left between periodic images in the z-direction, perpendicular to the surface. We adopted the PBE functional and Vanderbilt ultrasoft pseudopotentials to decrease the computational requirements associated with the description of inert core electrons. The plane wave cut off was set to 30Ry and the first Brillouin zone was sampled with a uniform mesh with 7x7x1 k-points. In order to facilitate convergence fractional occupancies were allowed, we applied Mazzari-Vanderbilt smearing with R = 0.0025 Ry. All MD simulations were carried out using DL_POLY while the Ab Initio simulations were performed using the quantum ESPRESSO package.