First Principles Based Modeling of Diffusion and Interfacial Processes in SOFC Systems

We present quantum mechanical (QM) and molecular dynamics (MD) results on first-principles-based descriptions of the yttria-stabilized zirconia (YSZ) electrolyte structure, oxygen ion transport through the electrolyte, and chemical processes at the Ni-anode/YSZ-electrolyte interface. Theoretically, YSZ has been studied by various techniques such as empirical shell models, ab initio, pseudopotential-based molecular dynamics, and thermodynamic modeling. However, it turned out that the employed shell models was unable to adequately describe the different zirconia phases and ab initio calculations are effective for relatively small molecules and is not yet practical for studying larger molecules and solids. Therefore, it is very desirable and important to have a force filed that could, on the one hand, accurately reproduce QM data and, on the other hand, be used for MD simulations of diffusion processes, phase transitions, chemical reactions, catalysis, interfaces, etc. To reach this goal we have developed first-principles based reactive force fields (ReaxFF) that allow modeling of the above-mentioned properties and processes. We validated the use of the ReaxFF for fuel cell applications by using it in molecular dynamics simulations to predict the oxygen ion diffusion coefficient in YSZ as a function of temperature. These values are in excellent agreement with experimental results, setting the stage for the use of the ReaxFF to model the transport of oxygen ions through the YSZ electrolyte for a SOFC. Since ReaxFF descriptions are also available for some catalysts, including Ni which is widely used as anode in SOFCs, we have carried out ReaxFF MD simulations of the chemical processes at a triple phase boundary that includes Ni-anode, YSZ-electrolyte, and hydrogen and butane as fuel. The water formation reaction and butane conversion were successfully observed in the performed ReaxFF MD simulations. Analyzing products of these reactions, we found good agreement between our computational results and experimental data.
We can now consider fully first principles-based simulations of the critical functions in a SOFC, enabling the possibility of in silico optimization of these materials. That is, we can now consider using theory and simulation to examine the effect of materials modifications on both catalysis and transport processes in SOFCs.

Acknowledgement. This work was supported by the DoD Multidisciplinary University Research Initiative (MURI) program administered by the Office of Naval Research under grant N00014-02-1-0665 (Program manager Michele Anderson).