Characterization of porous SOFC electrodes

The objective of this work is to develop the AC impedance method as a tool to study the kinetic and mass transport processes in the porous electrodes of the solid oxide fuel cell (SOFC). In particular, the SOFC cathode is studied and simulated by mathematical models. Three goals are achieved in this work: (1) propose a new impedance model to describe the impedance responses for a wide range of overpotentials; (2) propose a new experimental method to study the electrode performance and monitor its degradation by means of the AC impedance method; (3) propose a new experimental method to determine the contact resistance and thereby help reduce the voltage loss of the fuel cell system.; The repeating unit cell of a SOFC stack consists of anode, cathode, electrolyte, interconnector, and current collector. The challenge is to reduce the voltage loss from each component. The major voltage loss usually comes from the cathode polarization. The cathode reaction is simply oxygen reduction, but its mechanism tends to be complicated. The cathode reaction involves a catalytic adsorption/desorption process and mass transfer processes, as well as the charge transfer reaction. The most widely used technique for the study of the SOFC is the AC impedance method, however, its effectiveness is limited by the difficulty of interpretation of the complicated impedance response. The present study was prompted by the success of the AC impedance method in the study of complicated corrosion behaviors, where mathematical impedance models have been shown to be effective in explaining the corrosion mechanism.; In this work, a surface-diffusion-controlled model is proposed which successfully fits the experimental results over a wide range of overpotentials with respect to 3% water/97% hydrogen as a fuel. The surface-diffusion-controlled model is also used to explore the impedance response of different electrodes used by other researchers and it shows sufficient flexibility to be able to simulate their impedance response patterns.; An important source of the voltage loss usually ignored by researchers is contact resistance, because it is very difficult to extract from the overall ohmic resistance which is measured by conventional current-interruption methods. Contact resistance usually increases with time due to the corrosion of the current collectors. By combining the AC impedance technique and a four-electrode cell design, it is also demonstrated how to extract the overall ohmic resistance. This new method is believed to be valuable for improving the electrochemical study of SOFC electrodes.; The AC impedance technique applied to porous electrode is still under development. However, it promises to yield valuable information. The results of this study may help improve and accelerate the design of fuel cells.