Development of Oxygen Electrodes for Reversible Solid Oxide Fuel Cells
Abstract
The primary focus of this thesis has been to develop high performance stable oxygen electrodes, with a primary emphasis on a mixed conducting La0.3Ca0.7Cr0.3Fe0.7O3-δ (LCFCr) perovskite, for use in advanced reversible solid oxide fuel cells (RSOFCs). RSOFCs are single unit electrochemical devices that can operate in both the fuel cell (SOFC) and electrolysis (SOEC) mode, thus acting as flexible and efficient energy conversion systems.
The interest in LCFCr arose from earlier work with LSFCr (La0.3Sr0.7Cr0.3Fe0.7O3-δ), shown to be a very promising SOFC cathode. Thus, this thesis began with a rigorous study of the oxygen evolution (OER) and reduction (ORR) reactions at the LSFCr material, including understanding how to interpret the electrochemical results. However, LSFCr has a thermal expansion coefficient (TEC) that does not match with common solid electrolytes and thus its long time stability is questionable. For this reason, the Ca analogue (LCFCr), a new electrode material, was synthesized and shown to have a much more suitable TEC. Extensive 2-electrode and 3-electrode half-cell studies revealed two slow steps during the OER and ORR, including oxygen/LCFCr surface reactions and O2- transfer at the LCFCr/electrolyte interface. It was also shown that LCFCr is more active in the electrolysis mode (OER) than in the SOFC mode (ORR). Further, the infiltration of ceria or co-infiltration of ceria+LCFCr led to substantial improvements in LCFCr performance, also providing further insights into the processes that control the reaction rates.
It was shown that, after 200 h of operation in either the SOFC or SOEC modes, the main source of degradation was from the series resistance (the LCFCr/Au current collector interface), suggestive of Au sintering. The low frequency process, related to the LCFCr/air interface, also deteriorated somewhat, while the underlying LCFCr/electrolyte interface remained stable. This was confirmed from both electrochemistry and high-resolution TEM, showing the retention of a very high quality interface at the atomic level.
A novel and very promising SOC powder and cell sintering methodology was also developed using microwave technology, significantly lowering the manufacturing temperature, processing, and energy requirements, due also to the much lower energy demand of the MW furnace. This translates to significant manufacturing cost savings, making MW processing of complete solid oxide cells a promising route for the future.
Description
Keywords
Chemistry--Physical
Citation
Molero Sánchez, B. (2017). Development of Oxygen Electrodes for Reversible Solid Oxide Fuel Cells (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/25686