Long-term performance testes by CRIEPI (Central Research Institute for Electric Power Industry) on six industrial stacks have revealed an interesting correlation between cathode polarization loss and ohmic loss. To make clear the physicochemical meaning of this correlation, detailed analyses were made on the conductivity degradation of YSZ electrolyte in button cells and then on the ohmic losses in the industrial cells in terms of time constants which are determined from speed of the tetragonal transformation through the Y diffusion from the cubic phase to the tetragonal phase. In some cases, shorter time constants (faster degradations) were detected than those expected from the two-time-constant (with and without NiO reduction effects) model, suggesting that additional ohmic losses after subtracting the contribution from the tetragonal transformation must be caused from other sources such as cathode-degradation inducing effects. Main cathode degradations can be ascribed to sulfur poisoning due to contamination in air in the CRIEPI test site. An important feature was extracted as this cathode degradations became more severe when the gadolinium-doped ceria (GDC) interlayers were fabricated into dense film. Plausible mechanisms for cathode degradations were proposed based on the Sr/Co depletion on surface of lanthanum strontium cobalt ferrite (LSFC) in the active area. Peculiar cathode degradations found in stacks are interpreted in term of changes in surface concentration by reactions with sulfur oxide, electrochemical side reactions for water vapor emission or Sr volatilization, and diffusion of Sr/Co from inside LSCF.

We have been developing a rapid evaluation method for durability of SOFC stacks for thermal cycles during their lifetimes based on the assumption of residential use. The durability for thermal cycles is expected to be affected by the degradation during their long-term operation. In order to accelerate the evaluation, some treatments to intentionally cause the degradation were investigated. A degradation factor was determined depending on the SOFC stacks with different structures respectively because each degradation mechanism during their long-term operation also depends on them. The SOFC stacks were supplied by four SOFC stack manufactures in Japan. In this work, we investigated the Cr poisoning treatment to tubular SOFC (TOTO) and the S poisoning treatment to single step co-fired planar SOFC (Murata Manufacturing). As results of both cases, 10 years’ worth of degradation was successful to be intentionally caused in short period.

Performance of the SOFC single cell with the CO 2 reforming catalyst was investigated utilizing CH 4 /CO 2 . It was found that the voltage in CH 4 /CO 2 was comparatively high against conventional steam reforming conditions even in high fuel utilization ranges. The process simulation was conducted to the model of 1 kW CO 2 reforming SOFC system with anode off gas recycling. The simulation results such as heat balance and recycling procedure suggested that the CO 2 reforming could be one of the feasible approach for high electrical efficiency SOFC system.

The performance and heat balance of a solid oxide fuel cell (SOFC) hot box under dry and steam reforming fed by city gas, CO 2 and H 2 O was investigated. From the results, the heat balances of a dummy stack hot box showed an increase in stack temperature and heat loss in dry reforming compared to that of steam reforming. The evaluation of the electrical performance of SOFC hot box under dry reforming showed an electrical efficiency and power of 62.2% (LHV) and 853.1 W with satisfactory thermal selfsustainability, respectively. Its efficiency was 1.6% lower than that of steam reforming. There was no difference in stack temperature during dry reforming compared to that of steam reforming. The differences in stack temperature trends between the dummy hot box and actual hot box may have been due to an increase in the internal endothermic heat from the reformate and reduction in combustion in the actual hot box.

No abstract