Masakazu Hishinuma

3-D model calculation for planar SOFC

Abstract A three-dimensional mathematical model for a planar SOFC was constructed. The concentrations of the chemical species, the temperature distribution, the potential distribution, and the current density were calculated using a single-unit model with double channels of co-flow or counter-flow pattern. The finite volume method was employed for the calculation, which is based on the fundamental conservation laws of mass, energy, and electrical charge. The internal or external steam-reforming, the water-shift reaction, and the diffusion of gases in the porous electrodes were taken into the model. The effects of the cell size, the operating voltage and the thermal conductivity of the cell components on the calculated results were investigated. From the simulated temperature distributions in the electrolyte and the inter-connector, the stress distributions were calculated using the finite element method. The results demonstrated that the steam reforming would generate internal stresses of several tens MPa in an electrolyte.

Evaluation and modeling of performance of anode-supported solid oxide fuel cell

For an anode-supported planar SOFC, a single-unit with double channels was modeled for a counter-flow pattern, and the concentration polarization at the anode was estimated. The flow phenomena were simulated using the finite volume method and the distribution of the gaseous species was calculated. In the model, it was assumed that the gas flow in the porous anode is governed by Darcys Law, and the reactant species are transported to the electrolyte/anode interface mainly by diffusion in a multicomponent mixture system. For binary H2–H2O and CO–CO2 systems, the calculated concentration polarization was found comparable to the experimental results. As an example for a multicomponent system, a model using steam-reformed methane as a fuel was employed to simulate the concentration polarization at a high fuel utilization. From the simulated results, it was evident that the shift reaction effectively reduces the concentration polarization when the fuel utilization is high.

Oxygen tracer diffusion coefficient of (La, Sr)MnO3 ± δ

Abstract To understand the mechanism of oxygen diffusion, the oxygen tracer diffusion coefficient, D o ∗ , of strontium-doped lanthanum manganites, La1 − xSrxMnO3 ± δ (x = 0.05, 0.10, 0.15 and 0.20) was measured as a function of composition, temperature and oxygen partial pressure. The measured D o ∗ was as low as 10−12 to 10−11 cm2 s−1 at 1000 °C, from which the ionic conductivity of 10−7 to 10−6 S/cm was estimated. Because of such a poor ionic conductivity, the possibility for the bulk diffusion of oxide ions to contribute to the cathode reactions in solid oxide fuel cells is considered to be sparse. The negative dependence of D o ∗ on the oxygen partial pressure suggests the vacancy mechanism for oxygen diffusion.

Electrical conductivity and chemical diffusion coefficient of Sr-doped lanthanum chromites

Abstract The electrical conductivity and chemical diffusion coefficient of Sr-doped lanthanum chromites were measured as a function of oxygen partial pressure ( P o 2 ) and temperature, and the results were discussed in light of defect chemistry. The electrical conductivity was independent of P o 2 and in proportion to the Sr-content at high P o 2 , while at low P o 2 , the conductivity decreased exponentially with decrease of P o 2 . The P o 2 dependence was reasonably interpreted by the simple point defect model in which Sr′ La , Cr . Cr , and v .. o are assumed as predominant defect species. The chemical diffusion coefficient derived from time dependence of electrical conductivity after an abrupt change of P o 2 in the atmosphere increased with decrease of P o 2 , which was elucidated by a combination of the ambipolar diffusion theory and defect chemical analysis. The vacancy diffusion coefficient calculated from the measured chemical diffusion coefficient was independent of the vacancy concentration at not too high vacancy concentrations, suggesting that the vacancies are non-interactive and free.

Growth of yttria stabilized zirconia thin films by metallo-organic, ultrasonic spray pyrolysis

Abstract Randomly and preferentially oriented thin films of yttria stabilized zirconia (YSZ) have been prepared on fused quartz substrates by ultrasonic spray pyrolysis using zirconium octylate and yttrium octylate as metallo-organic precursors at a substrate temperature of 873–1023 K. The as-deposited films composed of fine columnar grains were found to be crystalline and transparent with a cubic fluorite structure. With increasing substrate temperature the growth rate and diameter of the columnar grains increased and the crystal habit showed a preferential orientation at the (111) plane.

Electrochemical Properties of Doped Lanthanum Chromites as Interconnectors for Solid Oxide Fuel Cells

To simulate the electrochemical properties of acceptor-doped lanthanum chromites as interconnectors for solid oxide fuel cells, theoretical analyses of the distribution of oxygen chemical potential, ionic leak current density, and area specific resistance are given. The results of the calculations using previously reported fundamental properties such as electrical conductivity as a function of oxygen partial pressure (P{sub O{sub 2}}), an equilibrium constant of a defect reaction for oxygen vacancy formation, and the diffusion coefficient of oxygen vacancies have shown that materials with a small amount of dopant can preferably be used from the viewpoint of energy conversion efficiency. The area specific resistance of interconnectors based on doped lanthanum chromites has been found to be sufficiently low for all the compositions and temperatures considered in the present study which include commonly used compositions and typical operating temperatures.

Electrical conductivity and chemical stability of calcium chromate hydroxyl apatite, Ca5(CrO4) 3OH, and problems caused by the apatite formation at the electrode/separator interface in solid oxide fuel cells

Abstract The calcium chromate hydroxyl apatite, Ca 5 (CrO 4 ) 3 OH, was synthesized, and its electrical conductivity and chemical stability were investigated. Based on the experimental results, detrimental effects of the apatite formation on the surface of lanthanum chromite separators in solid oxide fuel cells are discussed.

Photo-excitation effects on pyrolysis of metallo-organic precursors for yttria-stabilized zirconia thin films

Abstract Combination of pyrolysis and vacuum ultraviolet (VUV) photolysis under reduced pressure offered an effective method to prepare yttria-stabilized zirconia (YSZ) thin films from metallo-organic precursors at moderate temperatures. It became clear from IR and XPS analyses that thermal decomposition of metalloorganic precursors was accelerated by VUV excitation. The precursors dip-coated on a fused quartz substrate were found to decompose and crystallize into YSZ thin films as low as 603 K by thermal decomposition assisted by VUV irradiation. Without the VUV irradiation, however, the films were in an amorphous state at this temperature. It was also found that the VUV irradiation was more effective under air flow than under nitrogen flow, which suggests that active oxygen in the gas-phase generated by the VUV irradiation has an important role in the decomposition of the precursors.