Masanobu Aizawa

Current-voltage characteristics and impedance analysis of solid oxide fuel cells for mixed H2 and CO gases

Current-voltage (I-V) characteristics and electrode impedance of a tubular-type solid oxide fuel cell (SOFC) were analyzed for mixed fuel gases, consisting mainly of CO, H 2 , H 2 O, and a carrier gas, as simulated reformed gas of hydrocarbons or coal gas. I-V characteristics of a single cell were measured as a function of various operational parameters including the H 2 -to-CO ratio, the type of carrier gas such as He, N 2 , and Ar, the temperature, the fuel-to-carrier gas ratio, and the water vapor concentration. It has been experimentally confirmed that the use of CO-rich gases results in comparable performance to that of H 2 -rich gases and thus mixed gas such as coal gas is useful as a SOFC fuel. We have found, for the first time, that the I-V characteristics depend on the carrier gas indicating the importance of gas transport in porous anodes for anodic polarization. The change in cell voltage hy varying fuel compositions was mainly caused by the change in anode impedance associated with a low frequency semicircle in a Cole-Cole impedance plot at 1000°C. The fuel gas compositions in thermodynamic equilibrium were calculated and compared with the initial gas compositions, suggesting the importance of water vapor concentration to control the equilibrium H 2 -to-CO ratio for CO-rich fuel gases.

Stability of Lanthanum Calcium Chromite‐Lanthanum Strontium Manganite Interfaces in Solid Oxide Fuel Cells

An investigation has been made on the chemical stability of the cathode-interconnect interface in solid oxide fuel cells. Lanthanum calcium chromite and lanthanum strontium manganite (a dense 10% A-site deficient manganite, a porous 10% A-site deficient one or a dense 1% A-site deficient one) were placed between air and fuel (hydrogen/water) at a selected electrical current density. The electrical conductivity across the interfaces was slightly increased for 300 h. Changes in morphology, chemical composition, and phases were examined by scanning electron microscopy/energy dispersive x-ray and x-ray diffractometry analysis. The dense 10% A-site deficient cathode gave rise to the precipitation of manganese oxide at the air-side surface as well as at the interface. The porous cathode enhanced chemical reactions between lanthanum calcium chromite and lanthanum strontium manganite. The dense 1% A-site deficient cathode was most stable. These phenomena have been thermodynamically analyzed in terms of (i) irreversible mass transfer under an oxygen potential gradient, (ii) changes of the stable composition region of the perovskite phases as a function of oxygen potential, and (iii) an enhancing effect of the liquid formation on reactions at interfaces.

Determination of the Oxygen Permeation Flux Through La0.75Ca0.25CrO3 − δ by an Electrochemical Method

The oxygen permeation flux measured as the oxide ion current density (J O 2-) through La 0.75 Ca 0.25 CrO 3-δ was determined at steady state by using an electrochemical method in the p O2 range from 10 5 to 10 -1 1 Pa at T = 1273 K. The oxygen vacancy diffusion coefficient (D v ) is estimated to be 1.5 X 10 -9 m 2 s -1 in the p O2 region from 1 to 10 -5 Pa. When p O2 is lower than 10 -5 Pa, J O2 - seems to depend on a surface reaction as the rate-determining step. The contribution of grain-boundary diffusion on permeation was considered in the oxidizing atmosphere. The oxygen permeation current density through a 1 mm thick plate is estimated as 0.03 A cm -2 in the solid oxide fuel cell operation.

Determination of the oxygen permeation flux through La{sub 0.75}Ca{sub 0.25}CrO{sub 3{minus}{delta}} by an electrochemical method

The oxygen permeation flux measured as the oxide ion current density (J O 2-) through La 0.75 Ca 0.25 CrO 3-δ was determined at steady state by using an electrochemical method in the p O2 range from 10 5 to 10 -1 1 Pa at T = 1273 K. The oxygen vacancy diffusion coefficient (D v ) is estimated to be 1.5 X 10 -9 m 2 s -1 in the p O2 region from 1 to 10 -5 Pa. When p O2 is lower than 10 -5 Pa, J O2 - seems to depend on a surface reaction as the rate-determining step. The contribution of grain-boundary diffusion on permeation was considered in the oxidizing atmosphere. The oxygen permeation current density through a 1 mm thick plate is estimated as 0.03 A cm -2 in the solid oxide fuel cell operation.