Isamu Yasuda

The poisoning effect of sulfur-containing impurity gas on a SOFC anode: Part I. Dependence on temperature, time, and impurity concentration

An aqueous suspending agent solution or vehicle useful for suspension polymerization of a styrene-monomer such as styrene as well as derivatives of it and also mixtures with it to produce styrene-polymers. Also included is a suspension polymerization batch, as well as an improvement in the method, for the suspension polymerization of a styrene-monomer.

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.

Electrochemical properties of a SOFC cathode in contact with a chromium-containing alloy separator

Abstract Substrate-type solid oxide fuel cells (SOFCs) can reduce the operating temperature of SOFCs to enable alloys to be used as separators. When chromium-forming alloys are used as the separators, however, the performance of SOFC cathodes degrades rapidly. The degradation of a porous La 0.6 Sr 0.4 MnO 3+ δ (LSM)/YSZ electrode in contact with the alloy current-collector has been investigated using the frequency dispersion diagram of the complex impedance of the electrode. The equivalent circuit analysis based on a Randles-type equivalent circuit made clear that the Cr 2 O 3 deposition at the interface between the LSM and the YSZ electrolyte causes an increase in both diffusion and charge-transfer resistances.

Electrochemical Oxidation of H 2 and CO in a H 2 ‐ H 2 O ‐ CO ‐ CO 2 System at the Interface of a Ni‐YSZ Cermet Electrode and YSZ Electrolyte

The electrochemical oxidation of H{sub 2} and CO in a H{sub 2}-H{sub 2}O-CO-CO{sub 2} system at the interface of a porous Ni-yttria stabilized zirconia (YSZ) cermet electrode and YSZ electrolyte has been studied using complex-impedance spectroscopy and direct-current polarization measurements of 1,023 and 1,273 K under a constant oxygen partial pressure. The polarization resistance increased when the CO concentration ratio, p{sub CO}[p{sub H{sub 2}} + p{sub CO}], exceeded 0.2 and 0.5 at 1,023 and 1273 K, respectively. The electrochemical oxidation rate of H{sub 2} was 1.9--2.3 times and 2.3--3.1 times higher than that of CO at 1,023 and 1,273 K, respectively, and the water-gas shift reaction was found to be much faster than the electrode reaction at both temperatures. An equivalent-circuit analysis of the complex-impedance spectra suggested that the lower electrochemical oxidation rate of CO (compared to H{sub 2}) was caused mainly by the larger diffusion resistance of CO than H{sub 2} on the electrode surface at 1,023 K, and by both the larger surface diffusion resistance and charge-transfer resistance at 1,273 K.

Precise Determination of the Chemical Diffusion Coefficient of Calcium‐Doped Lanthanum Chromites by Means of Electrical Conductivity Relaxation

Chemical relaxation experiments were conducted on sintered samples of calcium-doped lanthanum chromites by abruptly changing the oxygen partial pressure in the atmosphere and following the time change of conductivity. The re-equilibration kinetics was analyzed by fitting the relaxation data to the solutions of Ficks second law for appropriate boundary conditions. The diffusion equation ignoring the effect of surface reaction failed to describe the transient behavior especially for the initial stage, while that taking the surface effect into account gave a satisfactory interpretation of the overall relaxation process and allowed a precise determination of the two kinetic parameters: oxygen chemical diffusion coefficient and surface reaction rate constant. The chemical diffusion coefficients increased with a decrease of the oxygen partial pressure due to the corresponding change in the concentration of the moving species. The activation energy was similar to that of oxygen vacancy diffusion coefficients in other monocrystalline perovskites, suggesting that the measured diffusion coefficients were attributable to lattice diffusion. The surface reaction rate constant increased with a decrease of the oxygen partial pressure similarly to the reported oxygen nonstoichiometry, which implies that the presence of oxygen vacancies plays an important role in the surface reaction kinetics.

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.

Electrical Conductivity and Defect Structure of Calcium‐Doped Lanthanum Chromites

Electrical conductivity of calcium-doped lanthanum chromites, La 1-x Ca x CrO 3-δ , was determined as a function of composition, temperature, and oxygen partial pressure, P o2 , to determine its defect structure and understand its redox behavior. The conductivity was independent of P o2 and was proportional to the dopant concentration at high P o2 . The activation energy of conductivity was 0.12 to 0.14 eV and the mobility was 0.066 to 0.075 cm 2 /V/s in the temperature range of 900 to 1050 o C, which was ascribable to small-polaron hopping. Under reducing conditions, the conductivity decreased exponentially with decreasing P o2 and asymptotically approached a P 02 1/4 relationship

Electrochemical properties of reduced-temperature SOFCs with mixed ionic-electronic conductors in electrodes and/or interlayers

Abstract We have investigated the electrochemical properties of two types of reduced-temperature solid oxide fuel cells (SOFCs) in which the mixed ionic–electronic conductors are used to improve their performances. Electrolyte-supported cells, in which doped LaGaO 3 strengthened by Al 2 O 3 dispersion is used as the electrolyte, were prepared and tested. Samaria-doped ceria (SDC) interlayers of 0.3-μm thickness were fired onto both surfaces of the electrolyte of 0.2-mm thickness at 1523 K, before firing the Ni–Sm 0.1 Ce 0.9 O 1.95 –(CeO 2 ) 0.1 [(Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 ] 0.9 (Ni–SDC–CeYSZ) (10 mol% ceria-doped yttria-stabilized zirconia [YSZ]) cermet anode at 1723 K and La 0.7 Sr 0.3 Co 0.2 Fe 0.8 O 3 –Sm 0.2 Ce 0.8 O 1.9 (LSCF–SDC) composite cathode at 1373 K. The cells have a nominal size of 60×60 mm 2 with an effective electrode area of 4 cm 2 . The single cell thus prepared showed a high power density of 0.67 W cm −2 at 1073 K and long-term stability during the operation time of 1000 h. Anode-supported cells with a thin YSZ electrolyte film approximately 30 μm thick were also prepared by co-sintering of screen-printed YSZ paste on a compacted anode substrate. The cells have a nominal size of 50×50 mm 2 with an effective electrode area of 4 cm 2 . The single cell with the LSCF–SDC composite cathode having SDC interlayer showed the maximum power density of 0.648 W cm −2 at 1023 K. The bilayer cathode also showed high resistance against degradation by Cr-poisoning.