Solid Oxide Fuel Cells (SOFCs) show a great promise as new power generators with high energy conversion efficiency. Decreasing operating temperature is critically requested for the further improvement in SOFC performance, in particular, for improving stability. Perovskite oxide of LaGaO3, doped with La and Mg exhibits the high oxide ion conductivity over wide PO2 range; the value is higher than that of Y2O3 stabilized ZrO2 by an order of magnitude at 773 K and useful for electrolyte of SOFC operating at intermediate temperature range. When LaGaO3-based oxide was used as the electrolyte of SOFC, the power density could be improved greatly, in particular in low temperature range. In this study, power generation property of SOFC using LaGaO3 electrolyte and also the improvement in power density of the cell by using a LaGaO3 thin film is also introduced. The maximum power density of the cell using LaGaO3 based oxide is as high as 0.6 W/cm2 at 773 K. This suggests that SOFC could be operable at temperature lower than 773 K. Tubular type cell stack using LSGM thin film is also introduced.
The stability of the anode-supported solid oxide fuel cell consisting of Ni-YSZ|YSZ|LSM was examined at 800°C by monitoring the current density at a constant terminal voltage of 0.5 V. Although stable power generation was possible within initial 60 min, gradual decrease and subsequent sudden drop in current density were observed: a series of this behavior was different from that observed in limiting current density. This behavior is suggested to be one of the degradation factors related with transport of gas species in the pores of the electrode. In response to performance deterioration, a drastic increase in ohmic loss and impedance was confirmed soon after the discharge operation. It is noted, however, that the deteriorated performance was reversibly recovered depending on the holding time under the open-circuit condition. Analogous phenomena occurred with the cell consisted of Ni-YSZ|YSZ|Ce0.8Sm0.2O1.9|La0.6Sr0.4Co0.2Fe0.8O3, indicating that the cause of the sudden deterioration was in the anode side.
Crystal structure and thermal expansion behavior at high temperatures of La0.7Sr0.3Ga0.7Fe0.2Mg0.1O3−δ have been investigated. Its crystal structure was cubic without structural phase transition. At about 500°C, variation of thermal expansion due to generation of oxide ion vacancy was observed. The behavior of δ can be explained assuming that Fe valence decreased from 3.230+ to 3+ with increase of temperature and that stable phase with Fe3+ exists. La0.7Sr0.3Ga0.7Fe0.2Mg0.1O3−δ has high potential for electrolyte material for low temperature SOFC since increase of oxide ion conductivity and decrease of hole conduction by oxide ion vacancy can be expected above 500°C.
Fe-Cr ferritic alloys are applied for interconnects of solid oxide fuel cells (SOFCs) operated at around medium temperatures. The oxidation behavior of sheets with various thicknesses of Fe-22% Cr ferritic alloy, ZMG232L, was investigated. The oxidation rate of thin sheet was much higher than that of thick one because Cr content decreased under oxide layer of edge part of thin sheet therefore Cr oxide became unstable and Fe was mainly oxidized. Such accelerated oxidation was improved mainly by reducing Mn, additionally by increasing Cr and adding W.
Ceria based electrolyte is one of candidates for low temperature SOFCs operated under 600°C, since it has reasonably high ionic conductivity, and moderate material cost, in spite of the drawback, lower open circuit voltage. In this study, the effect of fabrication process of the cell on the cell performance was examined using micro tubular, anode-supported cell design. Two samples were prepared by co-sintering anode supported tube and electrolyte (Gd doped CeO2), using the sintering temperatures of 1260 and 1400°C, respectively. The cell performance test showed that both samples had advantages depending up on the operating temperatures. The sample sintered at 1260°C showed better cell performance operated at 450°C, as high as 0.15 Wcm−2, while the sample sintered at 1400°C showed the power density of 0.49 Wcm−2 at 550°C, 1.4 times better than that of the sample sintered at 1260°C. This behavior is resulted from the difference in the conductivity of the electrolyte and the microstructure of the anode, which were prepared form different sintering temperatures.
We have studied the design and fabrication of an innovative cathode-supported honeycomb SOFCs, which can generate high volumetric power density. Among various cell designs, cathode-supported honeycomb SOFCs are suitable for compact SOFC modules operated at intermediate temperature since they have large effective electrode area per unit volume. In this report, we summarized the material system for the honeycomb SOFC, and we selected (LaSr)MnO3 (LSM), Sc2O3-doped zirconia and NiO-Ce0.9Gd0.1O1.95 (GDC) as cathode, electrolyte and anode materials, respectively. The LSM-supported SOFC with a LSM-GDC activation layer can generate maximum power densities of 47, 79 and 163 mW cm−2 at 550, 600 and 650°C, respectively. The results indicate that the honeycomb SOFC with a volumetric electrode area of 40 cm2 cm−3, which we have successfully fabricated, could generate a volumetric power density of 2 W cm−3 at 600°C.
A-site deficiency was introduced into the perovskite-type oxides in the compositions (La1−xSrx)1−z(Co1−yFey)O3−δ (x=0.4; y=0.8) with z=0, 0.02 and 0.04. Tolerance limit with the A-site deficiency for single perovskite phase was rather small, approximately 0.02. The electrical conductivity of the perovskites was investigated in a porous body at elevated temperatures in air with the pressures of 0.1–0.7 MPa. The pressurization slightly increases the conductivity. The A-site deficient material exhibits a higher physical reactivity than the stoichiometric one under the pressurized conditions.
The electrochemical performances of Proton-conducting SOFC with La0.7Sr0.3FeO3 (LSF) cathode fabricated by the electrophoretic deposition (EPD) technique were investigated. The EPD technique provided the uniform layer of LSF cathode with constant thickness and can easily control the thickness by changing an applied voltage. The power density of the SOFC cell was dependent on the thickness of LSF cathode. The activation energy was measured to elucidate the rate-determining step for LSF cathode reaction.
Dense and high oxide ion conducting solid electrolytes, Al-doped lanthanum silicate [La9.33+x(Si6−yAly)O26+1.5x−0.5y] were fabricated and their electrical conducting properties were studied. Among the specimens tested, the electrical conductivity of the slight Al-doped one (x=0.67, y=0.2) was the highest though that was the weighed composition. Its conductivity was almost P(O2)-independent, while a slight increase in conductivity with increasing P(O2) was observed in the high temperature region. The current-voltage characteristic of a fuel cell utilizing the novel electrolytes was also investigated employing platinum electrodes as both anode and cathode.
The Fe, Ni-BCY20 and Fe-Ni catalysts were used as the materials for anode of an intermediate-temperature Solid Oxide Fuel Cell (SOFC) for direct utilization of dimethylether(DME) gas fuel. A proton conducting oxide BaCe0.8Y0.2O3−δ was employed as the electrolyte(thickness 500 µm) and a Pt was applied as the cathode. The cell was operated at 600°C. The maximum power density of the cell with the Fe9-Ni1 anode that has 9/1 molar ratio of Fe to Ni was 46.4 mW cm−2 at 80 mA cm−2. The Fe9-Ni1 anode showed remarkable improvement of cell performance in comparison with the Fe anode that we reported before.
We prepared La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM) and La0.8Sr0.2Ga0.8Mg0.115Co0.085O3−δ (LSGMC) and studied their oxygen nonstoichiometries with a microvalance, and crystal structures and conduction paths of oxide ions using a neutron source. From Rietveld analyses, it was indicated that these samples had a single phase of an orthorhombic structure represented as Pnma at 298∼680 K. It was also suggested that oxygen content in LSGMC decreased with increasing temperature and decreasing oxygen partial pressure although that in LSGM was almost independent of them. Such oxygen content behaviors were also demonstrated by nonstoichiometry measurements using a microvalance. Nuclear densities calculated based on MEM technique suggested faster migration of oxide ions in LSGMC than LSGM and the different conduction paths between them.
Cermet anode of NiFe(9:1)-La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) was studied for direct CH4 fueled SOFC operating at intermediate temperature. In case of NiFe bimetal anode, power density of the cell decreased drastically in CH4. On the other hand, mixing Sm doped CeO2, MgO, or LSGM is effective for improving the long-term stability. The cell with NiFe-LSGM anode exhibited much stable power density under CH4 feeding condition. At 1073K, the maximum power density of the cell using NiFe-LSGM anode is achieved a value of 0.48 W/cm2, and after 15 hours operation, degradation in the power density is slightly observed. However, stability in the power density is much improved. Deposition of coke is also hardly detected by Raman spectroscopy, and so it was found that NiFe-LSGM10 shows a high tolerance against the coke deposition.
In this work, we synthesized LaBaGaO4 and LaBaGa0.95Mg0.05O4−δ, and investigated their crystal and electronic structures and electrical conduction properties. As a result, it was indicated that LaBaGaO4 had an orthorhombic structure (S.G.; P212121) at 298 and 900 K. The orthorhombic structure was kept even after Mg-doping although the electronic structure seemed to be varied maybe due to a partial substitution of Mg2+ for Ga3+. From an H/D isotope effect on conductivity, it was confirmed that LaBaGa0.95Mg0.05O4−δ exhibited protonic conduction at elevated temperatures under a moisturized condition.
In this study, we have analyzed the mechanism of mixed conduction due to electrons and oxygen ions in (La0.75Sr0.25)MnO3.00 (LSM) and (Ba0.5Sr0.5)(Co0.8Fe0.2)O2.33 (BSCF) by using the Rietveld refinement technique and the maximum entropy method (MEM). Using neutron diffraction measurements, we showed that the site occupancies of the O1 (4c) and O2 (8d) sites in BSCF are 0.59 and 0.87, respectively. Further, the MEM analysis of synchrotron X-ray diffraction measurements revealed that the Mn-O plane in LSM has a strong, isotropic covalent bond that enables conduction of electrons. The (Co,Fe)-O2 plane with a covalent bond and a low concentration of oxygen vacancies in BSCF can also conduct electrons. On the other hand, the (Ba,Sr)-O1 plane in BSCF, which has a high concentration of oxygen vacancies, large isotropic displacement parameter (Uiso) values, and a strong ionic bond, is responsible for the diffusion of oxygen ions.
Construction of structural phase diagram of LaGa1−xMgxO3−δ has been attempted by using synchrotron X-ray diffraction, high temperature X-ray diffraction, dilatometry and differential scanning calorimetry. Existence of long-period anti-phase domain structure(LPAPDS) observed in the specimens with x larger than 0.10 by using electron diffraction analysis was considered for the phase diagram construction. Existence of four phases has been proposed and their phase relationship has been evaluated. For the specimen with x≈0.00, orthorhombic phase free from LPAPDS and rhombohedral phase without LPAPDS were detected. In LaGa1−xMgxO3−δ with x larger than 0.10, orthorhombic phase and rhombohedral phase with LPAPDS were proposed. It was suggested that the slow kinetics of the phase transition from orthorhombic phase with LPAPDS to rhombohedral with LPAPDS was the origin of region, where mixture of orthorhombic phase and rhombohedral phase was observed. Existence of two-phase region, where phase free from LPAPDS and one with LPAPDS coexist, has been clarified. The compositional region, where x is around 0.05, is revealed to be the two-phase region from room temperature to 800°C.
In this work, current leakage of the micro-tubular cells with a 10-µm Ce0.9Gd0.1O1.95 (GDC) electrolyte based on cathode- and anode-supported configurations was investigated at 500 and 550°C. It was found that the open circuit voltage (VOC) of the GDC membrane cell greatly corresponds to the cell structure, particularly influenced by the electrode polarization. Calculations derived from the experimental results showed that the leak current through the GDC electrolyte gradually vanished as the external current increased to certain degree, which accordingly related to the polarizations, due to the cell structures. The anode-supported cell can be capable of generating a high power density with a reasonable maximum-efficiency. On the other hand, not only VOC but also cell output and efficiency were significantly lowered for the cathode-supported cell. It suggests that an appropriate cell structure is necessary to make the doped ceria a promising electrolyte for low-intermediate temperature power-generation applications.
The compatibility of bismuth compounds as a sintering aid for (ZrO2)0.89(ScO1.5)0.1(CeO2)0.01 (ScSZ) electrolyte in intermediate-temperature solid oxide fuel cells (IT-SOFCs) has been discussed. Addition of 3 mol% BiO1.5 considerably decreased the sintering temperature of ScSZ of approximately 200°C and achieved the densification with a relative density above 95% at 1200°C. The solid solubility limit of BiO1.5 in ScSZ was less than 2 mol%. Dispersion of Bi2O3 at the grain boundaries correspondingly accelerated the growth of ScSZ grains and increased the grain boundary resistance at temperatures below 450°C. At elevated temperatures of 450–750°C, the electrical conductivity of the ScSZ after doping remained almost unchangeable in air and under reducing atmospheres. The results suggested that bismuth addition is promising to be applicable to a low-temperature co-firing fabrication of ScSZ based components for IT-SOFCs.
The reaction process for the Ni-SDC cermet anode on the ScSZ electrolyte was examined by impedance spectrometry with wet hydrogen fuels and results were compared with previously obtained data for the Ni-ScSZ anode. Area specific conductivities (σSDC) for each rate controlling steps were obtained from the impedance spectra. Hydrogen and water vapor pressure (PH2 and PH2O, respectively) dependencies and temperature (T) dependence of σ for the Ni-SDC are compared with those of the Ni-ScSZ. Three rate controlling steps were observed for the Ni-SDC anode. The σSDC obtained at the lowest frequency region (σLSDC) shows slightly negative dependence on the T and it was proportional to the PH2O. The σMSDC obtained in the middle frequency region is activated with increasing PH2, PH2O and T. This process should be associated with the electrochemical oxidation reaction step accompanied with charge transfer. The σHSDC obtained at the highest frequency region is independent on both PH2 and PH2O, but it is activated with increasing temperature. This process appears only in the Ni-SDC anode, and therefore, it is reasonable to assume formation of proton at the Ni/SDC interface. The difference in reaction processes between the Ni-SDC anode and the Ni-ScSZ anode is discussed.
We developed environmental-friendly co-sintering processing technology of electrolyte/anode layers in solid oxide fuel cells (SOFCs) using several different types of nano-powders and water-based slurry. The shrinkages of Ce0.9Gd0.1O1.95(CGO) nano-powder for electrolyte and NiO-CGO nano-powder for anode substrate matched each other after the appropriate material selection and the optimization of pre-treatments. Next, the water-based CGO slurry was developed for electrolyte thin film formation. By effect of an emulsion type binder and lemon pectin, the water-based CGO slurry which has the both adhesive strength and stability was obtained. Finally, a dense and thin CGO film was formed on the NiO-CGO anode substrate by co-sintering process at 1400°C, and the prepared film thickness was ca. 8 µm.