Systematic Characterization and Analysis of Resistance from Conductors and Electrodes for Solid Oxide Fuel Cells

Solid oxide fuel cells are all ceramic devices that generate electricity by direct electrochemical reactions of a fuel and oxidizer. Recent efforts are underway to reduce the operating temperature of solid oxide fuel cells which allow these devices to become more economically competitive. However, at decreased temperatures the resistance from key electrochemical processes greatly increases. The presented work encompasses the characterization and analysis of resistances from conductors and electrodes in solid oxide fuel cells. Ionic conductivity is a thermally activated process; therefore, the conductivity of the ion conducting phase must be improved for suitable operation at lower temperatures. Ionic transport along and across grain boundaries differ distinctly between polycrystalline solids with convention and nanometers sized grains. Ionic conductivity is often greater in the grain boundaries than compared to the grain bulk due to an accumulation of charge carriers. The Van der Pauw technique was leveraged in this worked to measure the conductivity of thin films with thicknesses on the order of nanometers. The results showed that ionic conduction within nanostructured thin films exceeds that of conventional polycrystalline materials. Furthermore, there is a need to identify the resistance that arises from individual electrochemical processes. Electrochemical impedance spectroscopy (EIS) is a technique regularly employed to analyze the resistance from electrochemical processes in the electrodes. Distribution of relaxation times has been applied to the impedance spectrum obtained through EIS. This high resolution plot allowed for the identification of resistances from individual electrochemical impedance processes. The resistances from gas diffusion in the anode and cathode, electrical charge transfer, and transport of ions through the ionic phase have been identified through distribution of relaxation times