Hiroyasu Nishiguchi

Intermediate Temperature Solid Oxide Fuel Cells Using a New LaGaO3 Based Oxide Ion Conductor I. Doped as a New Cathode Material

‐based perovskite oxides doped with Sr and Mg exhibit high ionic conductivity over a wide range of oxygen partial pressure. In this study, the stability of ‐based oxide was investigated. The ‐based oxide was found to be very stable in reducing, oxidizing, and atmospheres. Solid oxide fuel cells (SOFCs) using ‐based perovskite‐type oxide as the electrolyte were studied for use in intermediate‐temperature SOFCs. The power‐generation characteristics of cells were strongly affected by the electrodes. Both Ni and (Ln:rare earth) were suitable for use as anode and cathode, respectively. Rare‐earth cations in the Ln site of the Co‐based perovskite cathode also had a significant effect on the power‐generation characteristics. In particular, a high power density could be attained in the temperature range 973–1273 K by using a doped for the cathode. Among the examined alkaline earth cations, Sr‐doped exhibits the smallest cathodic overpotential resulting in the highest power density. The electrical conductivity of increased with increasing Sr doped into the Sm site and attained a maximum at . The cathodic overpotential and internal resistance of the cell exhibited almost the opposite dependence on the amount of doped Sr. Consequently, the power density of the cell was a maximum when was used as the cathode. For this cell, the maximum power density was as high as 0.58 W/cm2 at 1073 K, even though a 0.5 mm thick electrolyte was used. This study revealed that a ‐based oxide for electrolyte and a ‐based oxide for the cathode are promising components for SOFCs operating at intermediate temperature.

Oxide ion conductivity in Sr-doped La10Ge6O27 apatite oxide

Abstract Oxide ion conductivity in La 10 Si 6 O 27 and La 10 Ge 6 O 27 -based apatite oxides was investigated in this study. In spite of the low symmetry of the crystal lattice, both oxides exhibited high oxide ion conductivity over a wide range of oxygen partial pressures. Oxide ion conductivity of La 10 Si 6 O 27 was increased by doping Sr for La sites. On the other hand, it was found that La 10 Ge 6 O 27 also exhibited high oxide ion conductivity which is comparable with that of La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3 perovskite oxide. Arrhenius plots of the oxide ion conductivity in La 10 Ge 6 O 27 show a knee around 1000 K. However, the knee in the Arrhenius plot of conductivity disappeared by doping Sr for La sites. Thus, the conductivity at low temperature was greatly enhanced by doping Sr for La sites in La 10 Ge 6 O 26 . This may be due to stabilization of the high temperature crystal phase. The electrical conductivities of Sr-doped La 10 Si 6 O 27 and La 10 Ge 6 O 27 were almost independent of the oxygen partial pressure from P O 2 =1 to 10 −21 atm. This suggests that these materials are ionic over a wide P O 2 range. This study revealed that apatite oxide of La 10 M 6 O 27 (M=Si, Ge) is a new class of the fast oxide ion conductors.

Mixed electronic–oxide ionic conductivity and oxygen permeating property of Fe-, Co- or Ni-doped LaGaO3 perovskite oxide

Abstract Mixed electronic–oxide ionic conductivity in LaGaO 3 -based oxide doped with Fe, Co or Ni was investigated in this study. The electric conductivity was greatly increased by doping Fe, Co, or Ni for the Ga site of La 0.8 Sr 0.2 GaO 3 . Compared to the conventional mixed electronic–ionic conductors such as LaCoO 3 , these LaGaO 3 -based oxides exhibited a lower electronic conductivity but a higher oxide ion conductivity. In particular, Fe-doped LaGaO 3 exhibited a high oxygen permeation rate and a stability against reduction. Consequently, higher CH 4 conversion in the partial oxidation of CH 4 was attained by using La 0.8 Sr 0.2 Ga 0.6 Fe 0.4 O 3 in the place of the conventional candidate material of La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3 for the oxygen permeation membrane. As a result, this study revealed that LaGaO 3 -based oxide doped with Fe, Co or Ni for Ga site is a new family of mixed electronic–oxide ionic conductors.

Intermediate Temperature Solid Oxide Fuel Cells Using LaGaO3 Electrolyte II. Improvement of Oxide Ion Conductivity and Power Density by Doping Fe for Ga Site of LaGaO3

Effects of small amounts of Fe doping for Ga site in LaGaO 3 -based oxide on oxide ion conductivity is investigated in this study. It is found that doping a small amount of Fe is effective for improving the oxide ion conductivity in La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3 (LSGM). The highest oxide ion conductivity was exhibited at x = 0.03 in La 0.8 Sr 0.2 Ga 0.8 Mg 0.2-x Fe x O 3 among the Fe-doped samples. Electron spin resonance (ESR) measurements suggest that Fe is trivalent in LaGaO 3 lattice. The application of the Fe-doped LaGaO 3 -based oxide for the electrolyte of solid oxide fuel cell was further investigated. Power density of the solid oxide fuel cell was increased by using Fe-doped LSGM for electrolyte. This can be explained by the decrease in electrical resistance loss by improving the oxide ion conductivity. A maximum power density close to 700 mW/cm 2 was obtained at 1073 K on the cell using 0.5 mm thick La 0.8 Sr 0.2 Ga 0.8 Mg 0-17 Fe 0.03 O 3 (LSGMF) and O 2 as the electrolyte and the oxidant, respectively. Therefore, close to the theoretical open-circuit potential was exhibited by the LSGMF cell. On the other hand, the power density was slightly smaller than that of the cell using Co-doped LSGM as electrolyte, especially, at temperatures lower than 973 K. This may result from the large activation energy for ion conductivity. However, the power density of the LSGMF cell was higher than that of the LSGM cell. Therefore, LSGM doped with a small amount of Fe is a promising electrolyte similar to Co-doped LSGM for the intermediate solid oxide fuel cell.

Photocatalytic reduction of CO2 with H2O on TiO2 and Cu/TiO2 catalysts

Photoinduced reduction of CO2 by H2O to produce CH4 and CH3OH has been investigated on wellcharacterized standard TiO2 catalysts and on a Cu2+ loaded TiO2 catalyst. The efficiency of this photoreaction depends strongly on the kind of catalyst and the ratio of H2O to CO2. Anatase TiO2, which has a large band gap and numerous surface OH groups, shows high efficiency for photocatalytic CH4 formation. Photogenerated Ti3+ ions, H and CH3 radicals are observed as reactive intermediates, by ESR at 77 K. Cu-loading of the small, powdered TiO2 catalyst (Cu/TiO2) brings about additional formation of CH3OH. XPS studies suggest that Cu+ plays a significant role in CH3OH formation.

Fe doped LaGaO3 perovskite oxide as an oxygen separating membrane for CH4 partial oxidation

Abstract Fe doped LaGaO3 based perovskite oxide was investigated in this study as an oxygen permeating membrane for CH4 partial oxidation. La0.7Sr0.3Ga0.6Fe0.4O3 exhibits a high oxygen permeation rate from air to He as high as 102 μmol/min cm2 (2.5 cm3-std/min·cm2) at 1273 K and 0.3 mm thickness. Application of LSGF membrane for CH4 partial oxidation was further studied. Since the PO2 difference became large, oxygen permeation rate drastically increased and it attained a value of 322 μmol/min cm2 (8 cm3/min cm2) at 1273 K and 0.5 mm thickness. Catalyst for CH4 partial oxidation has a great influence on oxygen permeation rate through membrane and it became clear that Ni and Ru are highly active for CH4 partial oxidation.

Nickel-Gd-doped CeO2 cermet anode for intermediate temperature operating solid oxide fuel cells using LaGaO3-based perovskite electrolyte

Abstract Ni–CeO 2 doped with 20 mol.% Gd 3+ (GDC) cermet was investigated as the anode of an intermediate temperature operating solid oxide fuel cell using LaGaO 3 -based oxide electrolyte. It was found that the anodic overpotential decreased by mixing Ni with mixed electronic-ionic conductor, in particular, mixing with doped CeO 2 is the most effective for decreasing the overpotential of the anode. Anodic overpotential became a minimum at 10 vol.% GDC to NiO. Using Ni–GDC cermet is also effective for improving the stability of the power density at the constant current output. Impedance analysis suggests that the improvement of anodic activity by mixing Ni with GDC was brought about by decreasing the diffusion overpotential, which was a result of the enlarged effective electrode area.

Preparation of CuO thin films on porous BaTiO3 by self-assembled multibilayer film formation and application as a CO2 sensor

Preparation of CuO thin films by decomposition of self-assembled multibilayer films as a molecular template was investigated. Furthermore, the CO2 sensing property of the resultant CuO thin films on a porous BaTiO3 was investigated as a capacitive type sensor. Self-assembled bilayer films of a few 1000 layers thickness can be readily obtained by casting an aqueous suspension composed of dimethyldihexadecylammoniun bromide (DC1–16), Cu(CH3CO2)2 , hexadecylethylenediamine and poly(vinyl alcohol). Divalent copper ions (Cu2+) which are associated with two hexadecylethylenediamine molecules were arranged in the hydrophobic layer of the multibilayer film. Rapid heating to the combustion temperature of DC1–16 was desirable for removing organic molecules in the multibilayer template. Thin films of CuO can be obtained by calcination at temperatures higher than 573 K. The resultant CuO thin films were porous and consisted of fine particles. The capacitance of CuO thin films prepared from self-assembled multibilayer films as a molecular template on the BaTiO3 porous substrate exhibited a high sensitivity to CO2 , which is twice that of a conventional mixed oxide capacitor of CuO–BaTiO3 . The capacitance of CuO thin films on BaTiO3 increases with increasing CO2 concentration in the range from 100 ppm to 50% at 873 K. Consequently, it is concluded that CuO thin films on BaTiO3 were appropriate capacitive type CO2 sensors.

La-Doped BaCoO3 as a Cathode for Intermediate Temperature Solid Oxide Fuel Cells Using a LaGaO3 Base Electrolyte

Cathodic behavior of BaCoO 3 doped with La was investigated in this study for an intermediate temperature solid oxide fuel cell (ITFC). Electrical conductivity of BaCoO 3 increased monotonically with an increasing amount of La doped for the Ba site at 1073 K po 2 = 10 5 atm. Cathodic overpotential at 1073 K decreased with increasing La content and attained a minimum at X = 0.3-0.5 in Ba 1-x La x CoO 3 . Since the cathodic overpotential of Ba 0.6 La 0.4 CoO 3 kept a small value at decreased temperature, Ba 0.6 La 0.4 CoO 3 is the optimum composition for the cathode of an ITFC among BaCoO 3 -based oxides When La 0.8 Sr 0.2 Ga 0.8 Mg 0.15 Co 0.05 O 3 was used for the electrolyte, the power density of the cell using Ba 0.6 La 0.4 CoO 3 for the cathode at 1073 K attained a value of 550 mW/cm 2 , which is slightly higher than that using Sm 0.5 Sr 0.5 CoO 3 for the cathode. In addition, a low cathodic overpotential of Ba 0.6 La 0.4 CoO 3 was also maintained in air. 18 O- 16 O exchange reaction was performed to estimate the surface activity for oxygen dissociation. It was found that BaCoO 3 exhibits high activity for the oxygen exchange reaction. Therefore, superior cathode property was assigned to the high surface activity of BaCoO 3 .

Oxidative dehydrogenation of iso-butane to iso-butene I. Metal phosphate catalysts

Abstract Metal pyrophosphates catalyse the oxidative dehydrogenation of iso-butane to iso-butene at 450–550°C using a feed gas of 75 mol% iso-butane and 5% O 2 . Ni 2 P 2 O 7 is the most selective catalyst with the iso-butene selectivity reaching to a maximum value of 82.2% at 550°C. Ag 4 P 2 O 7 and Zn 2 P 2 O 7 are also effective, but the iso-butene selectivities were slightly lower than that of Ni 2 P 2 O 7 . Pyrophosphates of Mg, Cr, Co, Mn, and Sn catalyse the oxidative dehydrogenation, but the iso-butene selectivity was 43.8–65.7% at the temperature where the maximum iso-butene yield is observed. The optimum oxygen concentration for iso-butene formation was 5–15 mol%, but the increase in O 2 concentration did not increase the iso-butene selectivity. No adsorbed oxygen species was found by means of TPD. The lattice oxygen of the pyrophosphates began to react with H 2 at 200–400°C. Reactivity of the lattice oxygen of pyrophosphates can be estimated from the value of ΔH f 0 for the corresponding oxide. More than 2 desorption peaks were observed in the TPD spectra of NH 3 adsorbed on the pyrophosphates, and a linear correlation was found between the concentration of acid amount of the catalysts and the specific rate of iso-butene formation. This strongly suggests that the acidic sites play a key role in the iso-butene formation.