Abstract
Nano-scale analyses were performed to investigate the ion transport phenomena inside fuel cell electrolytes. Molecular dynamics (MD) techniques were employed to carry out the ionic dynamics simulation. Example fuel cells involved in this thesis are solid oxide fuel cells (SOFCs) and proton exchange membrane fuel cells (PEMFCs). The chosen electrolytes for the SOFC are the traditional yttria stabilized zirconia (YSZ) and the modern yttria doped ceria (YDC), while the Nafion® polymer is for the PEMFC. The transport mechanism of oxygen ions inside the SOFC electrolyte is proved to be through non-continuous hopping between oxygen vacancies. Influences of Y2O3 concentrations and operation temperatures on the ionic conductivity were studied. Simulation results show that there exists an optimal concentration (8.0 mol% for YSZ and 10.2 mol% for YDC) for the nano-scale transport. Also higher operation temperature promotes the oxygen ion move-ability that increases the ionic conductivity. An investigation of proton dynamics at various hydration levels and thermal conditions inside the Nafion membrane has been carried out also based on the molecular dynamics technique. Semi-empirical quantum mechanics calculations were performed to optimize the complex molecular structure of the polymer. The atomistic simulation was conducted at four different hydration levels (3, 6.125, 9 and 15.375 H2O/SO3-) and three different thermal conditions (333 K, 343 K and 353 K). Simulation results show that different ionic segregations toward the hydrophobic (near fluorocarbon) and hydrophilic (sulfonate acid groups) regions. It is also found that higher temperature enhances the size of the hydrophilic phase. The diffusion coefficients of protons (or hydroniums) at various conditions have been evaluated and the comparison with experimental data shows good agreements.