For practical use of the MCFCs, the high
performance is required for long time operation. To obtain such a lifetime, the stabilities of the
materials must be improved.
The lifetime limiting factors are follows. I) NiO cathode dissolution/deposition that is the cause
of Ni short-circuit II)
Corrosion of the separator that is the cause of the electronic resistance
increase and the electrolyte loss
III) Pore structure change of the electrodes and the electrolyte matrix
that is the cause of the polarization increase for the reactions and the ionic
resistance.
In this study, the solubility of cathode
materials in molten carbonates and the corrosion behavior of metals with molten
carbonate have been investigated for the improvement of the materials for
molten carbonates fuel cells.
The NiO dissolution follows the acidic
dissolution mechanism under the operating condition of the molten carbonate
fuel cells. The solubility
of NiO decreases as the basicity of the molten carbonate change to basic, such
as Li/K carbonate changes to Li/Na carbonate. In this study, La2O3 was added
to the molten alkali carbonate and the solubility of NiO was compared with and
without addition. The NiO
solubility with La2O3 addition was lower than that
without addition. This
result would be caused by the basicity change to basic by the La2O3
addition.
The stainless steels (SUS316L and SUS310S) have the initial accelerated corrosion below 923 K. Especially, the corrosion is accelerated in basic carbonate melt such as Li/Na carbonate, which has lower solubility of NiO than the acidic carbonate melt. In this study, the corrosion behavior of the components of the stainless steels has been investigated by the thermo gravimetric analysis in order to elucidate the mechanism of the accelerated corrosion. The corrosion reaction of nickel with the lithium-sodium carbonate coating followed the liner rate low. The corrosion reaction of iron with lithium-sodium carbonate coating followed the liner rate low at the initial period and changed to the parabolic rate low in the later period. The change of the corrosion behavior of iron is similar to that of stainless steels. The initial accelerated corrosion of the stainless steels might strongly concern with the corrosion behavior of iron.