Predicting Three Phase Boundary Density and Interfacial Areas through Mechanistic Modeling

With the world’s ever climbing energy needs and our current source of energy, fossil fuels, slowing eating away at the ozone layer; the world needs a source of clean renewable energy. One of the best options for this clean renewable energy is fuel cells. In this research we look at one type of fuel cell: the Solid Oxide Fuel Cell (SOFC). 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 total and active TPB density as well as electrical conductivity 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.    Highlighted results include (1) that among all the model inputs the scaffold:infiltrate size ratio has the greatest impact on TPB density (both active and total), (2) increasing scaffold:infiltrate size ratio decreases TPB density, and (3) TPB density shows a maximum when varying porosity. Each highlighted result is explained through comparison with the interfacial surface areas of the three phases involved in the TPB density. Composite results including both TPB density and effective conductivity are also presented.