Achievements of NEDO durability projects on SOFC stacks in the light of physicochemical properties (diffusion and chemical reactions)

摘要

Achievements of NEDO durability projects on SOFC mode are summarized with a focus on the physicochemical mechanisms in terms of diffusion properties of cell components and chemical reactions of cell components with gaseous impurities. The conductivity degradation due to the transformation of the cubic YSZ electrolyte can be characterized in terms of two time-constants for the reductive and the oxidative regions to be determined by the Y-diffusivity, while observed gaps in conductivity degradation behavior between stacks behaviors and button cells can be ascribed to differences in cation diffusion and internal reduction of NiO inside YSZ electrolyte. The cathode performance degradation due to sulfur poisoning exhibits variety depending on the microstructure of doped-ceria interlayers, the thickness of YSZ electrolyte and the humidity in the anode atmosphere, suggesting effects of protons in the cathode vicinity and of the SrO activity after fabrication the LSCF/GDC/YSZ multilayers. Temperature dependence of the SrSO4 formation can be caused by increasing driving force for chemical reactions in contract to decreasing cation diffusivity with decreasing temperature. Overpotential values for the oxygen reduction reactions increase with decreasing temperature, affecting degradations in a sense that it can be accompanied with a large overpotential of cathodic water emission reactions; this causes enhancing driving forces for reactions of the SrO component with H～+ and O～(2-) to form volatile Sr(OH)2. This suggests that the proton reactivity is controlled by water vapor pressure in anode compartment, diffusivity of protons in YSZ/GDC, their thickness, and the electrochemical activity for water emission at cathodes. When the oxygen reduction overpotential decreases drastically, the sulfur poisoning of such cathodes becomes less sensitive, although the dense GDC interlayer was fabricated. Proton behaviors in the oxide-ion conductive electrochemical cells are hardly examined due to an extremely small contribution to the conductivity, but systematic considerations can be made to expect similar effects in SOEC mode or in the FC mode of the proton conductive solid oxide cells.