The effect of temperature gradients on thermal cycling and isothermal ageing of micro-tubular solid oxide fuel cells

Isothermal ageing and thermal cycling are performed on micro-tubular solid oxide fuel cells (SOFCs) in-order to understand degradation and failure mechanisms in micro-tubular SOFCs. For isothermal ageing, the effect of temperature gradients is investigated at 800 degrees C on two micro-tubular SOFC samples (1) 25 mm long and (2) 55 mm long. A temperature gradient is induced across the cells by passing 25% excess fuel for combustion at the cell outlet, thereby raising the temperature at this end to 950 degrees C. 25 mm long samples presented higher power density than the later ones and their rates of degradation were similar. Also, the effect of temperature gradients is investigated during the thermal cycling of micro-tubular SOFC to understand their contribution to electrochemical performance degradation. Two micro-tubes were characterized; one with optimum hydrogen flow rate and the other with 25% excess. No micro-cracking or de-lamination was observed in the micro-tube without a temperature gradient, whereas severe delaminations and micro-cracking were observed when 25% excess hydrogen flow was used during thermal cycling. In conclusion, the effect of temperature gradients during isothermal ageing was marginal, and Ni sintering was found to dominate the degradation mechanism. On the other hand during thermal cycling, the temperature gradients were found to be contributive to degradation by opening micro-cracks and de-laminations. (C) 2009 Elsevier B.V. All rights reserved.