Dikwal, Chinnan Maclean (2009)
Ph.D. thesis, University of Birmingham.
A major problem of solid oxide fuel cells (SOFCs) is their long term durability under cyclic operation, for example during start-up and shutdown, where cracking can occur. The objective of this project is to understand these mechanisms of cyclic degradation for micro-tubular SOFC, then to set-up experiments to measure the degradation in terms of the drop in electrochemical performance and subsequently confirming the theories by dilatometry and scanning electron microscopy (SEM). In conclusion, the methods and conditions for minimizing degradation in SOFC have been put forward. First, a theory of degradation based micro-crack propagation due to severe expansion and direct oxidation of the Ni anode were propounded. Several experiments were designed to illustrate the degradation phenomena. The first was isothermal (steady state) ageing, which was performed to provide a benchmark of degradation for easy comparison with degradation under transient conditions (i.e. thermal, redox and load cycling). During this operation, sintering was found to dominate the degradation mechanism, causing irreversible deformation and decreasing the power output without obvious micro-cracking taking place. With the bench mark for steady state degradation established, thermal cycling was performed by rapidly alternating between peak temperature and 200⁰C. This was found to have a marginal effect on the electrochemical performance and no microcracking was observed. However, thermal cycling with a temperature gradient imposed across the tubes was found to cause electro-chemical performance decrease and micro-cracking and de-laminations were observed. Thirdly, redox cycling was performed by changing the fuel flow between 20mL/min and no-flow. This was essential to deprive the anode of fuel in order to allow for oxidation of the Ni anode to occur. Redox cycling was found to have an adverse effect on the electro-chemical performance of micro-tubes. Severe micro-cracking and delaminations were observed. Lastly, thermal and electrical shock test were performed. The critical fracture temperature (ΔTC) was established by thermal shock at approximately 180⁰C. Under electrical shock testing, the tubes were found to fail after 13 – 15 electrical shock cycles, depending on the shock temperature. In conclusion, theories of micro-tubular SOFC cycling degradation have been proposed for thermal cycling, redox cycling and isothermal ageing and several experiments have provided confirmation.
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