The individual electrode processes from anode and cathode have been clearly identified from practical impedance spectra with high-frequency inductive impedances for the Mitsubishi segmented-in-series tubular solid oxide fuel cell by combining the distribution of relaxation time analysis and the complex nonlinear least square fitting. Anodic gas diffusion process and charge transfer reaction near anodic triple phase boundary appear at
∼0.3
and
1000Hz
, respectively. Cathodic oxygen reduction processes appear at
∼10Hz
at
700°C
, whereas the impedance arc over
10000Hz
is dominated by the cathodic oxygen ion transfer through the
(normalLa,normalSr)normalMnO3
(LSM)/
normalY2normalO3
-stabilized
normalZrO2
(YSZ) interface and YSZ of the composite. The results also exhibit an excellent agreement with those from the analysis of difference in impedance spectra.
Gas phase transport is a very important electrode process in practical solid oxide fuel cells. In this study, we have identified gas conversion impedance and gas diffusion impedance in the Mitsubishi segmented-in-series tubular solid oxide fuel cell. Gas conversion impedance is caused by the weak convection transport in the gas flow channel. It is observed that both the insufficient anode and cathode gas flow rates can result in the gas conversion impedance. Gas conversion impedance appears at less than 0.1 Hz, and its magnitude strongly depends on the gas flow rates. It disappears when the gas flow rates of both the anode and cathode are improved sufficiently. Anode gas diffusion through the porous substrate appears at ϳ0.5 Hz and dominates the overall diffusion impedance. Cathode gas diffusion through the porous current collecting layer appears at ϳ3 Hz, which significantly contributes to the overall gas diffusion impedance under low cathode oxygen partial pressures.
Segment-in-series cell fabricated by Mitsubishi Heavy Industries have been investigated from the various experimental approaches. Examined samples are type 1 cells which were operated for up to 10,000 h by CREIPI. Analyses were made by AIST on materials transport with dynamic SIMS, by CRIEPI on microstructure observation with EPMA, by Kyoto University on microstructure change with FIB-SEM, by Kyushu University on chemical states with S-TEM, and by Tohoku University on mechanical properties in the microscopic zone with micro indentation technique. On the bases of accumulated knowledge on these samples, the materials deterioration model has been established with a focus on the movement of the Ca component in the cathode layers. The degradation rate observed in the analyses by CRIEPI can be interpreted in terms of the Cr poisoning deposited on the three phase boundaries and also of the change in microstructure.
SOFC-gas turbine combined cycle system is able to attain extremely high efficiency over 70% (LHV). Mitsubishi Heavy Industries (MHI) has been developing SOFCs since 1984. Now, MHI is integrating SOFC with a gas turbine for a combined cycle power generation system. MHI has improved the components and the control technology for the combined cycle system; this includes improvement of the cell stack, verification of pressurized operation and control of SOFC module, and the combustor for low-calorie fuel. In 2006, MHI operated a combined cycle system with a maximum power output of 75 kW, for the first time in Japan. Now, MHI is ready to manufacture a 200 kW class SOFC-MGT combined cycle system, which will be operated in 2007.
Mitsubishi Heavy Industries (MHI) has developed SOFC with tubular type cell stack since 1984. In this study, we report the current status toward commercialization of SOFC-GTCC power generation system at MHI. To Redox robustness of a cell stack, the sintering characteristics of interconnect was improved. To improve durability of cell stack, long-term change of the electrochemical characteristics had been measured, and analyzed the interface of cathode and electrolyte by SIMS. For compact module, the packing density of the cell stack was increased from 400 to 700 stack/m2, and confirmed I-V characteristics and the temperature distribution of improved module. The generation performance of improved module is roughly comparable to that of conventional type module, and the temperature distribution is also roughly agreement of simulation result.
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