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735c

Kinetic Monte Carlo Simulation and Frequency Response Analysis of the Solid-Oxide Fuel Cell Cathode

Heath Turner, Chemical and Biological Engineering, University of Alabama, Box 870203, Tuscaloosa, AL 35487-0203, Kah Chun Lau, Department of Chemistry, George Washington University, Washington, DC 20052, and Brett Dunlap, Code 6189, Naval Research Laboratory, Washington, DC 20375.

A kinetic Monte carlo (KMC) model is developed to simulate the cathode side of a Yttria Stabilized Zirconia (YSZ) fuel cell, in order to translate experimental, and ultimately theoretical rates into an atomistic model of the cathode. The KMC model consists of a set of several electrochemical reaction rates, adopted from experiments and first-principles calculations. The KMC simulations are used to model these simultaneously occurring events, in order to determine potential limitations in solid-oxide fuel cell (SOFC) performance at different operating conditions. The main focus of this work is the ionic current density (J), studied as a function of various physical parameters: oxygen partial pressure (PO2), external applied bias voltage (Vext), temperature (T), dopant concentration (mol% Y2O3), relative permittivity of YSZ, and geometrical features of the YSZ electrolyte. This simple model can be used as a baseline to translate elementary chemical reaction rates into atomistic simulations of working SOFC cathodes, pertinent to the experimental operating conditions. As a supplement to conventional macroscopic scale modeling, atomistic modeling of fuel cell operation is important for understanding the underlying physical processes and developing a truly predictive model. Understanding SOFC performance at microscopic length and time scales (i.e., identifying the possible reaction pathways, as well as the effect of local electric fields on chemical reactions, ion and vacancy diffusion rates, and chemical interactions at the three-phase boundary) will be helpful for determining various boundary conditions in macroscopic studies for the general design of fuel cells.

The cathodic reaction can simultaneously occur via several competing paths, but here we assume that they can all be represented by a few well-defined reaction rates. For each of the allowed reaction pathways, all of the physical parameters influencing the relevant reactions and transport are incorporated within the framework of kinetic Monte Carlo simulations, including polarization resistance and electrostatic effects. In order to better understand the atomistic behavior of the YSZ cathode model, our simulated predictions of the ionic current density, J (mA/cm2), are divided into three categories, based upon the simulation parameters being investigated:

Materials independent: oxygen partial pressure (PO2), external applied bias voltage (Vext), and temperature (T)

Materials dependent: doping levels (i.e., concentration of Y in YSZ) and relative permittivity

Geometrical parameters: surface area (A) and electrolyte thickness (D)

To identify the influence of each of the parameters in a well-defined manner, we varied each of these parameters independently, while keeping all others fixed during the simulation. Moreover, some parameters (PO2, T, Vext, etc.) are rather easy to control experimentally, whereas other parameters (e.g., impurity segregation, thermally or electrically induced chemical and morphological changes of cathodes/YSZ interfaces), are experimentally ill-defined and a methodical variation and ultimate prediction of these influences is beyond the scope of this study.

Broadly speaking, all of the results obtained with our model are consistent with the experimental findings and previous theoretical predictions. Among the physical parameters that we studied, temperature, dopant fraction of Y2O3, and the relative permittivity of YSZ are found to have the most profound influence on the calculated ionic current density of the fuel cell cathode. Our most recent results from the KMC model include frequency-response analysis, generated from simulations of alternating applied voltages, over a range of frequencies. From these simulations, we can capture the properties of geometric and double-layer capacitance and resistance within the SOFC. Also, recent efforts will be presented that incorporate the anode-side of the SOFC.