Relating Conductivity and Tortuosity through Modeling of Infiltrated Solid Oxide Fuel Cells | AIChE

Relating Conductivity and Tortuosity through Modeling of Infiltrated Solid Oxide Fuel Cells

As the worldâ??s energy needs continue to increase and the supply of fossil fuels continues to decrease, the demand for a clean energy source is greater than ever. One promising source of clean energy is hydrogen fuel cells. In this research we focus specifically on Solid Oxide Fuel Cells (SOFCs). SOFCs work by directly producing electricity from oxidizing a fuel. Some of the unique benefits of SOFCs include high efficiencies, low emissions, and fuel flexibility. These benefits come at the cost of high operating temperatures and cells sensitive to their surroundings. In order for the fuel cell to produce an electrical current, the electrodes must have sufficient porosity (empty space) for fuel to transport through the cell, oxygen ion conductivity to allow the oxidizing agent to react with the fuel, and electron conductivity to transport electrons created when a reaction occurs. Additionally, reactions can only take place at three phase boundary (TPB) sites, which are defined as the intersections of electrically conducting particles, oxygen ion conducting particles, and empty space. In this work, we focus on infiltrated SOFCs, which involves in extra step in the creation process of adding a material with high electron conductivity after the backbone of the cell has already been created. Infiltrated electrodes allow for greater flexibility in the design of SOFCs and have been shown to increase performance and allow for lower operating temperatures. The creation process for infiltrated SOFCs is time consuming, expensive, and involves many design parameters many of which are poorly understood. By using a model to recreate this design process, this research hopes to shape future experiments that can be run in a laboratory setting.

The model works by taking in a variety of design parameters and closely follows the anode fabrication process in order to randomly generate a cell. The fabrication process begins with the slurry formation and film creation, followed by the combustion of pore forming particles, ceramic sintering and infiltration. A variety of calculations are then performed on this cell such as effective conductivity and TPB density. This work focuses specifically on the path tortuosity of each phase within a cell and uses this information to better understand and predict the performance of each phase as well as overall cell performance. Although tortuosity has been linked with performance in past research, due to the difficulty of measuring tortuosity, studies are extremely limited and the results have varied. Highlighted results include that (1) as ceramic:infiltrate radius ratio increases in a two phase system, the ceramic tortuosity increases while pore tortuosity is unaffected, (2) as infiltrate loading (%) increases, the ceramic tortuosity is unaffected, the pore tortuosity increases, and infiltrate tortuosity decreases.