Masanobu Awano

Impact of Anode Microstructure on Solid Oxide Fuel Cells

Porous Anodes for Solid Oxide Fuel Cells Fuel cells that use ion-conducting oxides as the electrolyte can be highly efficient and use hydrocarbon fuels directly. However, their very high operating temperatures (usually above 700°C) can lead to unwanted reactions with their electrode materials and premature degradation of their performance. In order to improve fuel-cell electrochemical performance, Suzuki et al. (p. 852) describe a route for increasing the porosity of the anode material, which contains nickel oxide and zirconia doped with scandium and cerium and is fabricated as a cylinder. Subsequent coating and firing steps added a layer of a zirconia-based electrolyte and the (La,Sr)(Co,Fe)O3 cathode. The resulting fuel-cell power density exceeded 1 watt per square centimeter at 600°C, and its performance improved as hydrogen fuel velocities were increased through the cell. A porous form of the oxide anode of a fuel cell with a zirconia-based electrolyte reduces its operating temperature. We report a correlation between the microstructure of the anode electrode of a solid oxide fuel cell (SOFC) and its electrochemical performance for a tubular design. It was shown that the electrochemical performance of the cell was extensively improved when the size of constituent particles was reduced so as to yield a highly porous microstructure. The SOFC had a power density of greater than 1 watt per square centimeter at an operating temperature as low as 600°C with a conventional zirconia-based electrolyte, a nickel cermet anode, and a lanthanum ferrite perovskite cathode material. The effect of the hydrogen fuel flow rate (linear velocity) was also examined for the optimization of operating conditions. Higher linear fuel velocity led to better cell performance for the cell with higher anode porosity. A zirconia-based cell could be used for a low-temperature SOFC system under 600°C just by optimizing the microstructure of the anode electrode and operating conditions.

Preparation and characterization of the Sb-doped TiO2 photocatalysts

Doped TiO2 photocatalysts have been prepared by a coprecipitation method. Uniformly doped nanocrystalline TiO2 of 10–20 nm sizes was synthesized by calcinating the coprecipitated gels at 400–650°C. Photocatalytic characterization along with the microstructural investigation for each catalyst provides better understanding of the photocatalytic behavior. It was found that the photodegradation of methylene blue (MB) was a complex function of the doping type and its concentration and the microstructural characteristics of the catalysts. Antimony doing significantly improved photocatalytic performance as compared to the undoped TiO2. Post-treatment of the as-precipitated wet doped Ti gels in an organic solvent also increased the surface area, forming approximately 8 nm size doped TiO2 with surface area ∼149 m2/g. Superior catalytic activity was observed in the Sb-doped TiO2 samples at a doping concentration ranging from 1 to 5 at%. Using the 5 at% Sb-doped TiO2 catalyst treated in butanol, 100 ppm of MB could be decomposed completely within 1 h, which was better than the commercial Degussa P-25.

Preparation of LaCoO3 catalytic thin film by the sol–gel process and its NO decomposition characteristics

LaCoO 3 and LaMnO 3 gel films were deposited by dip-coating technique on yttria-stabilized zirconia (YSZ) substrate using the precursor solution prepared from La(O-i-C 3 H 7 ) 3 , Co(CH 3 COO) 2 or Mn(O-i-C 3 H 7 ) 2 , 2-methoxyethanol, and polyethylene glycol. By heat-treating the gel films, electrochemical cells, LaCoO 3 YSZ LaCoO 3 were fabricated. The effect of polyethylene glycol on the microstructure evolution of LaCoO 3 and LaMnO 3 thin films was investigated and NO decomposition characteristics of LaCoO 3 powders and the electrochemical cells were investigated at 600-800°C. In the absence of oxygen, the LaCoO 3 powder was active both in decomposition of NO and reduction of NO by C 2 H 4 . It was found that they lost their catalytic activity in the presence of 2% oxygen. By applying a direct current to LaCoO 3 /YSZ/LaCoO 3 electrochemical cell, a good NO conversion rate could be obtained at relatively low current value even if excess oxygen is included in the reaction gas mixture.

Synthesis of nanocrystalline manganese oxide powders: Influence of hydrogen peroxide on particle characteristics

Nanocrystalline manganese oxide powders have been prepared at 25 °C by precipitation from Mn(NO 3 ) 2 aqueous solution. The presence and addition sequence of H 2 O 2 significantly influence particle characteristics of the resulting manganese oxides, including crystal structure, particle size and morphology, and surface area, depending upon molar ratio of H 2 O 2 with respect to Mn. The precipitation from preoxidized manganese solution by H 2 O 2 results in flakelike-shaped amorphous hydrous manganese oxide (MnO 2 x H 2 O). In the absence of H 2 O 2 , on the other hand, amorphous Mn(OH) 2 is obtained, and a part of Mn(OH) 2 subsequently transforms into crystalline Mn 3 O 4 by oxidation in air. Relative population of amorphous Mn(OH) 2 decreases by dissolution when post-treated with H 2 O 2 . At Mn:H 2 O 2 = 1:4, the well-defined 16-nm-sized nanocrystalline Mn 3 O 4 with homogenous particle morphology is prepared. The treatment with excess H 2 O 2 , however, destroys crystalline Mn 3 O 4 and leads to further oxidation of the aqueous manganese species. Under these conditions, a mixture of needlelike Mn 2 O 3 and cubelike Mn 3 O 4 , including amorphous MnO 2 x H 2 O, is obtained.

Evaluation of Micro LSM-Supported GDC/ScSZ Bilayer Electrolyte with LSM–GDC Activation Layer for Intermediate Temperature-SOFCs

A gadolinium-doped ceria (GDC)/scandia-stabilized zirconia (ScSZ) bilayer electrolyte on a microtubular (La, Sr) x MnO 3-δ (LSM) support was prepared via extrusion of a microtubular cathode support and subsequent surface coatings with electrolyte and anode slurries. The GDC/ScSZ bilayer electrolyte was obtained on the LSM support using a cosintering technique, and the LSM-supported microtubular solid oxide fuel cells (SOFCs) were evaluated using field-emission scanning electron microscopy, X-ray diffraction, and electrochemical measurements in wet hydrogen (3% H 2 O) atmosphere. An LSM-GDC activation layer was also introduced between the cathode tube and the electrolyte layers for improvement of cell performance. The micro-SOFC exhibited a stable open-circuit voltage above 1.03 V in the temperature range from 450 to 750°C, and the cell generated a maximum power density of 15, 73, 230, and 378 mW/cm 2 at 500, 600, 700, and 750°C, respectively. This result indicates that our developed cosintering fabrication technology can realize a stable and high-performance, LSM-supported micro-SOFC.

Densification and Grain-Orientation of Bi-Pb-Sr-Ca-Cu-O Superconductors by Hot-Pressing

The bulk density of a normal sintered sample of Bi1.6Pb0.4Sr1.6Ca2Cu2.8Oy with a Tc of 103 K was 4.81 g/cm3 and shrinkage of the sample was barely observable. By hot-pressing at a pressure of 39 MPa at 800°C in a vacuum of 5×10-5 Torr for 2 h, the bulk density of the hot-pressed sample reached 6.21 g/cm3, which was over 95% of the theoretical density, and the Tc was 72 K. The grains were oriented with the c-axis along the pressing axis. After annealing at 835°C in air for 40 h, the Tc was improved to 108 K and the critical current density at 77 K was found to be 731 A/cm2.

Cation Ordering in LaBa2Cu2TaO8+y

LaBa2Cu2TaO8+y crystallizes in a tetragonal unit cell with the dimensions a=3.9674(2) and c=12.052(1) A, showing semiconductivity. In this compound, the Ta ions were found to preferentially occupy the Cu1 site between the two Ba layers in the LaBa2Cu3O7-y-type structure.

Preparation and magnetic properties of Bi1.5Pb0.5Sr2Ca2Cu3Ox superconductors

New high-Tc superconductors were prepared by the partial substitution of Pb for Bi in Bi2Sr2Ca2Cu3Ox. The X-ray diffraction analysis of Bi1.5Pb0.5Sr2Ca2Cu3Ox showed that most of the sintered sample was in the 105 K phase. The lattice parameters were a=5.410 A and c=37.18 A. The transition temperature was 105 K. The lower critical field, Hc1, was 6 Oe at 77 K which was much smaller than that of YBa2Cu3O7-y (52 Oe). The volume fraction of the 105 K phase was estimated to be about 80%.

Synthesis of cordierite and cordierite-ZrSiO4 composite by colloidal processing

Cordierite powder of high purity and cordierite-ZrO2 composite powder were synthesized by colloidal processing. Cordierite was synthesized by calcining a precursor gel obtained by gelation and coprecipitation of AlOOH sol, SiO2 sol and Mg(NO3)2. The calcination temperature affected the constituent phases in the powders. The phase assemblage in calcined powder affected the sintering conditions. The optimum sintering temperature of the powder with cordierite single phase was 1440–1450 °C when it was calcined at 1270 °C. The sintered body had a dense microstructure with submicrometre grains. Addition of ZrO2 sol resulted in reaction with cordierite to form mostly ZrSiO4 at the sintering temperature. Several properties of cordierite and cordierite-ZrSiO4 composite, such as thermal expansion coefficient, bending strength, dielectric constant and insulation properties at high temperature, were investigated. Thermal expansion and electric properties were degraded by an increasing amount of ZrO2 additive, whereas the bending strength was improved by the addition of ZrO2.

Preparation and characterization of nanocrystalline doped TiO2

Abstract Nanocrystalline TiO 2 powders, doped in range of 0.05-5 mol% Nb and Ta, have been prepared using a chemical method. Synthesized powders have an anatase crystal structure and a fine particle size of 10-20 nm. Energy-dispersive X-ray microanalysis shows uniform dopant distribution in the powder. Temperature dependence of resistivity for both the Nb- and Ta-doped TiO 2 sintered at 1300°C exhibits n-type conductivity in the range 100-500°C with an activation energy of about 0.2 eV. It was found by transmission electron microscope analysis that the dopants in the doped TiO 2 sintered samples are segregated on grain boundaries after repeated high temperature treatment regardless of doping ranges. Addition of Zr in small amount (about 1 wt%) sufficiently suppresses such phase segregation, improving the high temperature stability.