A critical
issue to tackle before successful commercialization
of solid oxide fuel cells (SOFCs) can be achieved is the long-term
thermal stability required for SOFCs to operate reliably without significant
performance degradation despite enduring thermal cycling. In this
work, the impact of thermal cycling on the durability of NiO-yttria-stabilized
zirconia-based anode-supported cells is studied using three different
heating/cooling rates (1, 2, and 5 °C min–1) as the temperature fluctuated between 400 and 700 °C. Our
experiments simulate time periods when power from SOFCs is not required
(e.g., as might occur at night or during an emergency shutdown). The
decay ratios of the cell voltages are 8.8% (82 μV h–1) and 19.1% (187 μV h–1) after thermal cycling
testing at heating/cooling rates of 1 and 5 °C min–1, respectively, over a period of 1000 h. The results indicate SOFCs
that undergo rapid thermal cycling experience much greater performance
degradation than cells that experience slow heating/cooling rates.
The changes in total resistance for thermally cycled cells are determined
by measuring the R
pol of the electrodes
(whereas the ohmic resistances of the cells remain unchanged from
their initial value), signifying that electrode deterioration is the
main degradation mechanism for SOFCs under thermal cycling. In particular,
fast thermal cycling leads to severe degradation in the anode part
of SOFCs with substantial agglomeration and depletion of Ni particles
seen in our characterizations with field emission–scanning
electron microscopy and electron probe microanalysis. In addition,
the mean particle size in the cathode after thermal cycling testing
increases from 0.104 to 0.201 μm for the 5 °C min–1 cell. Further, the presence of Sr-enriched regions is more significant
in the La0.6Sr0.4Co0.2Fe0.8O3−δ cathode after fast thermally cycled
SOFCs.