Akihiro Sawata

Kinetic studies on the reaction at the La0.6Ca0.4MnO3/YSZ interface, as an SOFC air electrode

To determine the rate equation for the reaction at the SOFC (Solid Oxide Fuel Cell) air electrode of the type O2(g)/porous La0.6Ca0.4MnO3/YSZ, measurements were made on the impedance due to the electrode reaction and the steady-state polarization in the oxygen partial pressures, PO2, between 1 and 10−4 atm at temperatures, T, between 700 and 1000°C. At T > 900°C and in PO2<10−3 atm, the electrode conductivity (reciprocal of the electrode impedance) was proportional to PO2. The rate of reaction for this condition was found to be controlled by the oxygen gas diffusion in the electrode pores. At higher PO2 or at the lower temperatures, the steady-state current, i, was found to obey the rate equation, i=k[aO−PO2a−1O], where k is the rate constant and aO is the oxygen activity in YSZ at the La0.6Ca0.4MnO/YSZ interface. Here, aO is related to the electrode potential, E , by 2FE = RT 1n aO. The electrode conductivity for this condition was proportional to P12O2.

Crystallization behavior of amorphous solid solutions and phase sepration in the Cr2O3-Fe2O3 system

Fine particles of the amorphous Cr2O3-Fe2O2 solid solutions were prepared by dehydration of coprecipitated hydroxides and their crystallization behavior was studied by differential thermal analysis and X-ray diffraction. The peak temperature for crystallization attained a maximum at a composition near Fe2O3 content of about 60 mol % and the activation energy for crystallization attained a minimum at a composition near Fe2O3 content of about 50 mol % in this quasibinary system. Phase separation occurred in a range of Fe2O3 content from about 35 to 80 mol % in the corundum-type solid solutions heat treated at 600 °C for 2 h. Crystallization behavior was discussed briefly related with phase separation and diffusion in fine particles.

Oxygen chemical potential profile in a solid oxide fuel cell and simulation of electrochemical performance

Abstract Based on the reported kinetic equations on the air and fuel electrodes, simulation was made on the oxygen potential profile in a solid oxide fuel cell (SOFC) and the electrochemical performance. As a model for simulation, we considered the SOFC of the type, H 2 -H 2 O/porous Pt electrode/YSZ/porous La 0.6 Ca 0.4 MnO 3 /O 2 (g). In the previous papers, we reported the empirical rate equations calculated from the results of the complex impedance due to electrode reaction and the steady-state polarization for the O 2 (g)/porous La 0.6 Ca 0.4 MnO 3 /YSZ electrode and for the H 2 -H 2 O/porous Pt/YSZ electrode. They were expressed using oxygen activity, a O , in YSZ at the electrode/YSZ interface and the partial pressures of gaseous components: I = k O ( a O − a −1 O P O 2 ) for the air electrode, and I=k H {P H 2 P − 1 2 H 2 Ka O −P − 1 2 H 2 P H 2 O (K a O ) − 1 2 for the fuel electrode, where I is the steady-state current density, k O and k H are the rate constants and K is the equilibrium constant of H 2 -H 2 O-O 2 system. Here, k O and k H were related to the electrode interface conductivities σ E (O) and σ E (H) by σ(O)=4Fk O P 1 2 O 2 /RT and σ E (H)=3Fk H P 1 2 H 2 O /RT . Using the above equations and the relationship of μ O =( RT /2) ln P O 2 , the μ O profiles in SOFC were calculated with current density and the interface conductivities as parameters. Also, I–V curves of SOFC were simulated with the electrolyte thickness as the additional parameter to predict the performance of SOFC with any cell construction.

Metastable phase relation and phase equilibria in the Cr2O3-Fe2O3 system

In order to ascertain the metastable phase relation in the Cr2O3-Fe2O3 system, the existing phases were investigated by X-ray analysis using samples obtained by heating the coprecipitated powders for 1 h at 600–1000°C. There was a metastable two-phase region of Cr2O3-rich (CC) and Fe2O3-rich (FC) phases below about 940°C. Equilibrium state of 1:1 composition at 600–900°C was considered to be a single phase of the corundum solid solution. The metastable two-phase CC + FC region was suggested to appear probably due to the compositional inhomogeneity in the coprecipitated powders.