Kinetics Of Oxygen Reduction In Perovskite Cathodes For Solid Oxide Fuel Cells: A
Combined Modeling And Experimental Approach
Committee Members: Advisor: Srikanth Gopalan, MSE/ME; Uday Pal, MSE/ME; Soumendra
N. Basu, MSE/ME; Karl Ludwig, MSE/Physics; Assigned Reader: Michael Gevelber,
Abstract: Solid oxide fuel cells (SOFCs) have the potential to replace conventional stationary power generation technologies; however, one of the major obstacles to commercialization is poor cathode performance. Unfortunately, a dearth of knowledge about the mechanisms occurring at the cathode is one of the major factors preventing commercialization of SOFCs.
In this work, electrochemical impedance spectroscopy (EIS) was performed on patterned half-cells that consisted of a cathode on an electrolyte. In contrast to conventional cathodes that are porous and have ill-defined geometrical features such as surface area, porosity, and tortuosity, patterned cathodes have a tightly controlled geometry. With controlled geometry, identification of reaction mechanisms responsible for sluggish kinetics attributable to material properties and environmental conditions, separate from geometric effects such as surface area and triple phase boundary (TPB) length is possible.
Patterned cathodes were fabricated through a combination of photolithography and rf-magnetron sputter deposition, to grow patterned dense thin films. The patterns were generated to maintain a constant interfacial area between the cathode and electrolyte, but with increasing TPB length. Films of Ca-doped LaMnO3 (LCM) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF-6428) that are two different, but commonly used cathode materials were fabricated. EIS data were collected from both sets of samples over an operating temperature pO2 range of 600-800 °C and 10-3- 1.00 atm respectively.
To understand the mechanisms of oxygen reduction occurring in each of these samples, EIS spectra were simulated from an assumed reaction pathway. A 1-D state-space model (SSM) was used to interpret the LCM patterned samples, while a 2-D numerical model was used to model the more complicated LSCF-6428 system. Based on fitting the model predictions to the experimental results, conclusions were drawn about the oxygen reduction reaction mechanisms. Further, estimates of parameter values that are difficult to obtain directly from experiments, such as surface diffusivity, were made.
Finally, the models predicted a strong dependence of oxygen reduction on the specific surface chemistry. This was explored experimentally by correlating changes in electrochemical performance, after applying bias to a thin film, with surface chemical changes observed by soft x-ray techniques such as x-ray absorption/emission spectroscopy (XAS/XES) and x-ray photoemission spectroscopy (XPS).