Achievements of NEDO durability projects on SOFC mode are summarized with a focus on the physicochemical mechanisms characterized by diffusion properties of cell components and chemical reactions of ...cell components with gaseous impurities. Ni sintering and depletion including impurity (P, B, S) effects have been examined in terms of the surface/interface energies of Ni/oxide cermet anodes. The conductivity degradation due to the transformation of the cubic YSZ electrolyte was found to be characterized in terms of two time constants for the reductive and the oxidative regions to be determined by the Y‐diffusivity and its enhancement on NiO internal reduction in YSZ, while observed gaps in conductivity degradation behavior between stacks and button cells were ascribed to differences in those physicochemical properties involved, namely cation diffusion and kinetics associated with NiO internal reduction. The cathode performance degradation due to sulfur poisoning exhibits a variety of dependences on the microstructure (dense or porous) 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 the SrO activity changes during fabrication the LSCF/GDC/YSZ multilayers. Some defect chemical considerations were made on how such defects are affected by fabrication processes.
In order to introduce molten carbonate fuel cells (MCFCs) in commercial applications, the target lifetime of a MCFC has been set at 40,000
h. We have carried out long-term operation tests on several ...bench-scale MCFCs, which include a 66,000-h continuous operation, and clarified the question of voltage degradation in relation to operating time. We have also carried out post-test analyses on the long-term operated cell components including the electrodes, the electrolyte matrix and the current collectors. The results of the long-term operation and the post-test analyses are described in this paper. The degradation mechanisms of voltage and components are discussed.
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.
Recent progress of the NEDO project on durability/reliability of SOFC stacks is reported with an emphasis on the achievement of Mitsubishi Heavy Industries' segment‐in‐series cells in which the ...lanthanum manganite cathode has been improved. Durability tests were made by CRIEPI on their cells with/without doped ceria interlayer to check plausible effects of microstructure change and of chromium poisoning. Improved cells exhibit essentially no degradation for 10,000 h and also strong tolerance against the Cr contamination from stainless steel tubes. These features are discussed within the generalized degradation model developed inside the NEDO project. In particular, the extremely small overpotential can be considered to be effective in lowering the Cr poisoning by reducing the driving forces for the electrochemical Cr deposition at active sites. Insertion of doped ceria is also useful in preventing the Cr deposition or enhancing the volatilization of deposited Cr with cathodically emitted water vapors from ceria. Thermodynamic considerations reveal that the initial composition of LSM cathode is important to determine the microstructure change due to the chromium dissolution into the B‐sites in the perovskite lattice. Discussions are made on a role of doped ceria to prevent deterioration of Mn‐dissolved electrolyte by lowering the Mn dissolution into YSZ.
Several years ago, Li/Na carbonate (Li
2CO
3/Na
2CO
3) was developed as the electrolyte of molten carbonate fuel cells (MCFCs) in place of the usual Li/K carbonate (Li
2CO
3/K
2CO
3) to the advantage ...of a higher ionic conductivity and lower rate of cathode NiO dissolution. To estimate the potential of Li/Na carbonate as the MCFC electrolyte, the dependence of the cell performance on the operating conditions and the behavior during long-term performance was investigated in several bench-scale cell operations. The obtained data on the performance of Li/Na cells was analyzed to estimate the impact of voltage losses by using a performance model and discussed in comparison with the data of conventional Li/K cell performance.
Abstract
Recent progress of the NEDO project on durability/reliability of SOFC stacks is reported with an emphasis on the achievement of Mitsubishi Heavy Industries' segment‐in‐series cells in which ...the lanthanum manganite cathode has been improved. Durability tests were made by CRIEPI on their cells with/without doped ceria interlayer to check plausible effects of microstructure change and of chromium poisoning. Improved cells exhibit essentially no degradation for 10,000 h and also strong tolerance against the Cr contamination from stainless steel tubes. These features are discussed within the generalized degradation model developed inside the NEDO project. In particular, the extremely small overpotential can be considered to be effective in lowering the Cr poisoning by reducing the driving forces for the electrochemical Cr deposition at active sites. Insertion of doped ceria is also useful in preventing the Cr deposition or enhancing the volatilization of deposited Cr with cathodically emitted water vapors from ceria. Thermodynamic considerations reveal that the initial composition of LSM cathode is important to determine the microstructure change due to the chromium dissolution into the B‐sites in the perovskite lattice. Discussions are made on a role of doped ceria to prevent deterioration of Mn‐dissolved electrolyte by lowering the Mn dissolution into YSZ.
To evaluate the effects of fluoride on the performance of molten carbonate fuel cells (MCFCs), bench-scale cell tests and half-cell experiments have been performed with fuel gas containing HF. The ...cell voltage does not change due to the addition of HF. According to the following reaction, most of the added HF reacts with carbonate ion to form fluoride ion, which accumulates in the electrolyte.
2
HF+
CO
3
2−→2
F
−+
H
2
O+
CO
2
Hence, the carbonate loss and the decrease of the electrolyte volume have a negative effect on the cell-life expectancy. Moreover, a large number of fluoride ions causes corrosion on the cathode current collector. However, the MCFCs can generate electricity even if the electrolyte contains 40
mol% of LiF. There are no obvious effects of fluoride ion on the anode and cathode reaction.
To examine the correlations between ras oncogene expression and the development of cervical cancer, the authors studied the reactivity of cervical intraepithelial neoplasia (CIN) and microinvasive ...lesions of the human uterine cervix by using anti‐ras p21 mouse monoclonal antibody rp35. The frequency of positive p21 staining increased with increased grades of malignancy from 17.9% in CIN 1 to 28.9% in CIN 2 and 53.9% in CIN 3, whereas in microinvasive carcinoma it was 50.0%. Furthermore, ten cases of lesions that regressed during a 1‐year follow‐up period were positive for ras p21 in 20% of cases, but 14 cases of lesions that progressed and developed into higher graded lesions during the 2‐ to 5‐year follow‐up period had a 50.0% rate of positive p21 staining. It was concluded that ras oncogene product p21 correlates with the early phase of carcinogenesis of squamous cells of the uterine cervix.
Polarization of the nickel oxide (NiO) cathode limits the performance of the state-of-the-art MCFC. It is therefore important to clarify the phenomena which occur when, as is usually the case, the ...NiO cathode is formed in situ in the MCFC. This occurs by chemical or electrochemical reactions between nickel, which is the base material of the cathode, and molten carbonate (usually Li
2CO
3:K
2CO
3=62:38 mol%), which is the electrolyte. To clarify these formation phenomena, a direct observation method involving a telescope and CCD (charge coupled device) camera, in combination with potential measurements, is applied to the oxidation of a nickel sheet which is partially immersed in molten carbonate. In an atmosphere of pure CO
2, a partially immersed nickel sheet is relatively stable, as is a gold foil even in oxidant gas. In the case of nickel exposed to oxidant gas, however, the area exposed directly to the oxidant gas is rapidly covered by an electrolyte film, and undergoes intensive chemical or electrochemical reactions with CO
2 gas generation during oxidation and lithiation. As a consequence, a progressively rougher NiO surface develops over the entire sheet. After oxidation and lithiation, the non-immersed part of the sheet remains covered with electrolyte. Although the oxygen reduction current at the in situ lithiated NiO is over one order of magnitude higher than that at a gold electrode at the same applied potential, the extended meniscus region is the dominant reaction site for oxygen reduction. The same is true for the much more limited meniscus region of the gold electrode.
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