Simulations of Solid Oxide Fuel Cell Electrodes with Complex Microstructures.

Abstract

The goal of this research was to determine the relationship between microstructure and properties to aid microstructure analysis and design. A series of continuum modeling frameworks were developed to study microstructural effects in solid oxide fuel cell electrodes. Microstructures experimentally obtained by focused ion beam (FIB)-scanning electron microscopy (SEM) were employed.

The first study identified the characteristics that optimize competing transport in two-phase composites. A model was developed to evaluate tortuosity. It is based on the finite-difference method with regular Cartesian grids. The spinodally-decomposed two-phase composite was found to exhibit simultaneous transport properties comparable to those of optimized minimal-surface structures.

The second study focused on the development of phase-field frameworks to quantitatively model Ni coarsening in a Ni-yttria stabilized zirconia (YSZ) anode. Through asymptotic analysis, two models (A and B) differing in the treatment of YSZ were linked to experiments without fitting parameters. In both models, the contact angles at a triple junction were demonstrated to obey Young's equation. The TPB reduction predicted by Model B, which employs the smoothed boundary method (SBM), was in reasonable agreement with experimental results, whereas Model A, which couples two Cahn-Hilliard equations, overestimated TPB reduction. The modeling results indicate that reducing the contact angle of Ni on YSZ can enhance anode stability.

In the third study, the SBM was introduced to implement a dual-transport-path electrochemical model for mixed-conducting cathodes. The simulation results were validated against the analytical solution in a cylindrical cathode with bulk transport. The utilization length of an experimentally obtained microstructure was examined. This length was found to be locally affected by microstructures. A Nyquist plot generated using a dual-transport-path simulation for the microstructure is presented. The result shows that this approach can aid the interpretation of electrochemical impedance spectroscopy results, which are usually convoluted with multiple mechanisms and microstructural effects.

These models were implemented into large-scale simulations using state-of-the-art numerical schemes and parallel computing platforms and can be employed to examine the effects of microstructures on the performance of various porous electrodes and other materials for electrochemical devices.