Modeling Three Phase Boundary Density of Infiltrated Solid Oxide Fuel Cell Electrodes

Solid Oxide Fuel Cells (SOFCs) are electrochemical devices, characterized by their solid, ceramic electrolytes that directly oxidize a fuel to produce electricity with low emissions and high efficiency. For the fuel cell to be functional the electrodes must have porosity for fuel transport through the cell, oxygen ion conductivity to transport the oxidizing agent to the fuel, electrical conductivity to transport electrons from the reaction, and three phase boundary (TPB) sites to enable chemical reaction. TPB sites occur at the intersections of electrical conductor particles, ceramic particles and empty space so that the oxidation reaction may occur at a site with the electrons, oxidizing agent and fuel all present.  Recent work has shown that electrode fabrication by infiltration of electronic conductor/oxidation catalyst particles into an oxygen ion rich porous backbone provides excellent performance while decreasing the amount of conductor material necessary as compared with other fabrication methods. Infiltrated electrodes also allow for flexibility in composition and structure of the electrodes. This fabrication method, however, has many design parameters whose impact on the resulting cell’s performance is not fully understood. Experimentally exploring these parameters is very time consuming, thus a modeling effort can help to guide the desired experiments.

In this work, we show a model for the design process of a SOFC and the corresponding determination of both electrical conductivity and TPB density for the formulated electrodes. The model is based on the underlying physics of the electrode fabrication process. The fabrication process begins with the slurry formation and film creation, followed by pore former combustion, ceramic particle sintering and conducting particle infiltration.  After formulation, results can be determined from cells with appropriate size to mimic an infinitely sized cell to get consistent data.  Some highlighted results include a TPB density decrease with increasing ceramic particle size.  Additionally, for equal sized pore, ceramic and conductor particles the TPB density shows an optimum near 1/3 occupation of each phase in the cell.   Future opportunities for model development also are considered.