Haruo Kishimoto

Effect of SO2 Concentration on Degradation of Sm0.5Sr0.5CoO3 Cathode

A trace amount of SO2 in air is not a negligible factor on degradation of SOFC cathode. SO2 acts like Cr vapors which cause Cr-poisoning, and easily reacts with the strontium component in lanthanum-cobaltite based SOFC cathodes. Effects of SO2 concentration on degradation of an Sm0.5Sr0.5CoO3 (SSC) cathode were investigated in order to evaluate degradation of SSC cathode in an accelerated mode. Three different concentrations of SO2 (100 ppm, 1 ppm, and 5 ppb) were supplied to the cathode with air during cell tests at T = 1073 K Degradation of SSC cathode was accelerated with increasing SO2 concentration. In the thin SO2 conditions (1 ppm and 5 ppb SO2), SO2 in air was captured on the surface of the SSC cathode. SO2 reacted with the strontium component in the SSC cathode to form SrSO4. In the thick SO2 condition (100 ppm SO2), SO2 reacted with the strontium and samarium components. Too excess SO2 concentration in air was not suitable to examine cathode degradation in an accelerated mode because other degradation modes were appeared. One ppm - SO2 in air was applicable to accelerate the degradation of SSC cathode.

Sulfur Poisoning on SOFC Ni Anodes: Thermodynamic Analyses within Local Equilibrium Anode Reaction Model

Possible effects of sulfur on solid oxide fuel cell (SOFC) Ni anodes have been analyzed from the thermodynamic considerations of Ni―S―C―O―H systems by constructing the predominant area diagrams as the Ellingham diagram (oxygen potential vs temperature plot) with a parameter of partial pressure (10 ―4 or 10 ―6 atm) of sulfur-containing gaseous species such as H 2 S. Focus was made on the sulfur potential and its role on the sulfur adsorption on Ni, the sulfur dissolution into Ni, or the Ni sulfide formation, particularly on the eutectics formation. The important features of sulfur poisoning have been extracted and discussed as follows: (i) Under equilibrated conditions with 1-100 ppm contamination of H 2 S around 1073 K, Ni anodes are stable against sulfide formation up to a high utilization such as 90%. (ii) When the anode overpotential is strongly related to the change in oxygen potential in the vicinity of three-phase boundaries (TPBs), Ni can be sulfurized; in worse cases, Ni-S eutectic liquids can be formed. (iii) Several factors that can lead to serious damages are discussed in terms of diffusion of sulfur inside nickel, enhancement accumulation of sulfur at TPBs due to the electrochemical oxidation of H 2 S, and enrichment of sulfur in the lower temperature region in stacks.

Interface Stability of Perovskite Cathodes and Rare-Earth Doped Ceria Interlayer in SOFCs

The reaction and interdiffusion characteristics at the interface of rare-earth doped ceria (M 0.2 Ce 0.8 O 1.9 , M = Gd, La) and perovskite-type cathode materials (La 0.8 Sr 0.2 CoO 3 , and La 0.8 Sr 0.2 FeO 3 ) were investigated and their effect on the long-term reliability of solid oxide fuel cell (SOFC) performance are discussed. Secondary phase formation was observed at the ceria side of the interface by the reaction of the rare-earth component in ceria and transition metal components in perovskite. A significant diffusion of transition metals (cobalt or iron) and strontium from perovskite to ceria was observed. The iron content in perovskite and the type of rare earth in ceria strongly affect the stability of secondary phase and extent of transition metal diffusion in ceria. The bulk diffusivity was determined as a function of temperature. The iron component exhibited a fast and inhomogeneous migration in ceria, forming the iron-rich inclusions. The strontium component exhibited the tendency of the fast grain boundary diffusion in ceria. The effects of mass transport on the SOFC materials durability are discussed.

Effect of Water on Oxygen Transport Properties on Electrolyte Surface in SOFCs I. Surface Reaction Mechanism of Oxygen Isotope Exchange on Solid Oxide Electrolytes

The effect of water vapor on the oxygen isotope exchange properly was investigated for yttria-stabilized zirconia (YSZ) oxide ion conducting ceramics, which is widely used as an electrolyte material for solid oxide fuel cells (SOFCs). A slight amount of water vapor addition made a great enhancement of surface exchange rate of oxygen isotope ( 18 O) on the surface of YSZ single crystal. The surface exchange rate constant increases with partial pressure of water vapor, whereas the oxygen isotope diffusivity has no dependence on water vapor. The temperature and water vapor pressure dependences of the enhancement of surface exchange rate are in good agreement with the dependences of water coverage on YSZ surface. Hence, the fast interaction between H 2 O molecule and oxygen vacancy in electrolytes was considered to be dominant in a humid atmosphere, and it resulted in the apparent high surface exchange rate constant of oxygen isotope. Relatively lower oxygen isotope diffusivity was obtained in humid atmospheres as compared to those measured in dry oxygen in the literature. The low diffusion coefficient was observed only for YSZ but not for other electrolytes such as gadolinium-substituted ceria. This is probably due to a temperature gradient between the sample and the thermocouple which is caused from low adsorption of YSZ single crystal for infrared. The effects of heating procedure, preannealing conditions, and materials used for sample holders were investigated.

Reaction Process in the Ni–ScSZ Anode for Hydrocarbon Fueled SOFCs

Hydrocarbon fueled cell reactions, which consist of thermal decomposition, reforming, and CO shift reactions, and electrochemical oxidation in the Ni-ScSZ anode of solid oxide fuel cells, were examined by electrochemical measurements and outlet gas analysis for several hydrocarbon fuels (CH 4 , C 3 H 8 , C 8 H 18 , C 12 H 26 ). Examinations on the anode potential and its relation to observed outlet gas compositions have revealed that hydrogen molecules were mainly electrochemically oxidized even for hydrocarbon fuels. The electrode conductivities can be well correlated with the water vapor partial pressure for both hydrogen and hydrocarbons. Water vapor produced as a result of the electrochemical oxidation of hydrogen promotes the hydrogen-producing reactions in the vicinity of the electrochemically active site. A large difference in the thermal decomposition of hydrocarbons appeared in the anode potential and also in the maximum fuel utilization between methane and other hydrocarbons. A simple reaction model has been proposed to interpret consistently the electrode reactions as well as the reforming and the CO shift reactions with a focus on the oxygen atoms on Ni surfaces as a catalyst for both the electrode and the reforming reactions.

Determination of Chromium Concentration in Solid Oxide Fuel Cell Cathodes: ( La , Sr ) MnO3 and ( La , Sr ) FeO3

Chromium poisoning was investigated at the cathode/Gd 0.2 Ce 0.8 O 19 electrolyte interfaces under cathodic polarization by using a preoxidized Cr metal. The concentration levels of Cr in the porous cathodes [La 0.8 Sr 0.2 MnO 3 (LSM) and La 0.8 Sr 0.2 FeO 3 (LSF)] was first determined by secondary ion mass spectrometry. Maxima of Cr concentration were observed at the LSM/electrolyte interfaces (about 1000 ppm). On the other hand, no maxima of Cr concentration were observed with homogeneous distribution of Cr (1000-1700 ppm). The amount of Cr deposited in the LSF cathode is estimated to be about 21% of the supplied Cr and is about 1.6 times higher in the LSM.

Oxide scale formation and stability of Fe-Cr alloy interconnects under dual atmospheres and current flow conditions for SOFCs

Oxide scales formed on Fe-Cr alloys were characterized by electrochemical resistance measurements and microstructure observation. Fe-Cr alloy samples were exposed to air on one side and fuel gas (dilute CH 4 -H 2 O with N 2 ) on the other side under current flow (0.3 Acm -2 ) (dual atmospheres and current flow). The alloys were connected with Pt-mesh cathode and Ni-mesh anode at 1073 K. The interface resistance changes at the Ni-mesh/Fe-Cr alloy interfaces were monitored as a function of time, which increased with time in a parabolic relationship. The observed interface resistance after 800 h operation was less than 0.015 fl cm 2 at 1073 K. The oxide scale phases formed were Cr-Mn-Fe spinel, Cr 2 O 3 , and SiO 2 layer from surface to inner alloy, which is almost the same with the oxide scales formed in a single atmosphere. No significant effects of dual atmosphere and current flow was observed in the formed oxide scale phases of both anode and cathode side on Fe-Cr alloys. Around the glass-sealing/alloy contact interfaces, a thick oxide scale was observed on the alloy. The reaction of glass components with alloy can affect the stability of oxide scales and alloys in the long-term operation.

Feasibility of n-Dodecane Fuel for Solid Oxide Fuel Cell with Ni-ScSZ Anode

Direct feeding of n-dodecane (C 1 2 H 2 6 ), a model liquid fuel of kerosene, was carried out using a cell consisting of ScSZ electrolyte with Ni-ScSZ cermet anode and Ni mesh current collector. The influence of steam to carbon ratio (S/C) and fuel utilization on the cell performance was examined. At S/C = 1.0-2.0 and low fuel concentration (<2%), stable operation was achieved at 127 mA/cm 2 (∼45% fuel utilization) with an anode potential of about -0.77 V. There was no degradation of the anode and current collector. At S/C = 0.5, a stable operation was also achieved, but the current collector was partially disintegrated. The thermal decomposition of n-dodecane into mainly C 2 H 4 and H 2 occurred without serous carbon deposition and subsequent steam reforming reactions of C 2 H 4 with water vapor produced hydrogen within the Ni-ScSZ anode and onto the Ni current collector. Increasing fuel utilization from 44 to 66% resulted in promoting the H 2 oxidation and the shift reaction of CO. A small amount of methane and C 2 compounds remained and, as a result, the partial pressure of hydrogen was lower than the calculated equilibrium composition. The observed lower open-circuit voltage is due to the incomplete equilibration for internal reforming reaction of n-dodecane.

Electronic Conductivity of CeO2: Its Dependence on Oxygen Partial Pressure and Temperature

The electronic conductivity of pure ceria was successfully measured by a dc polarization technique based on the Hebb-Wagner ion blocking method at T = 873-1273 K. The electronic conductivity increased with increasing temperature at given P(O 2 ) values. It also increased with decreasing P(O 2 ) following slope values of -1/6 to -1/4. At 873-1073 K, no P(O 2 ) dependent region was observed at higher P(0 2 ) more than 10 -2 , 10 -4 , and 10 -5 MPa at 1073, 973, and 873 K, respectively. Activation energies calculated from the Arrhenius plots for those different regions were 2.2, 2.9, and 1.9 eV for P(O 2 ) dependences of -1/6, -1/4, and 0 regions, respectively.

Deposition of Platinum Particles at LSM/ScSZ/Air Three-Phase Boundaries Using a Platinum Current Collector

Deposition of platinum particles at lanthanum manganite cathode (LSM)/scandia-stabilized zirconia (ScSZ)/air three-phase boundaries (TPBs) was investigated. Button-type half-cells consisting of a strontium-doped LSM and a ScSZ electrolyte were operated at T = 1073 K using platinum (Pt) or gold (Au) components as current collectors and lead wires. When Pt components were used, Pt particles were observed only in the vicinity of three-phase boundaries of the cathode after a power generation test. No particle was observed around TPBs of the cathode when Au components were used.