Susumu Nagata

Performance analysis of planar-type unit SOFC considering current and temperature distributions

The solid oxide fuel cell (SOFC) is expected to be a candidate for distributed power sources in the next generation, due to its high efficiency, high-temperature waste heat utilization and low emission of pollutants to the environment. In this study, a quasi-two- (co- and counter-flow) and three- (cross-flow) dimensional simulation program for planar-type SOFC was made considering mass, charge and heat balances along the flow directions and perpendicular to the electrolyte membrane, in order to obtain temperature and current density distributions along the flow direction. Numerical results from this simulation with adiabatic boundary conditions show that the temperature increases along the flow direction in the co-flow case and the temperature profile has a maximum near the fuel inlet in the counter-flow case. The effects of the gas re-circulation ratio, operating pressure and physical properties on current and temperature distributions were also studied. The temperature distribution is uniform irrespective of flow type under the boundary condition of radiative exchange between the outer interconnector and the electric furnace surface with a realistic view factor. Temperature and current density profiles are discussed considering the Nernst potential and overvoltage changes along the flow direction.

Performance of a La0.6Sr0.4Co0.8Fe0.2O3–Ce0.8Gd0.2O1.9–Ag cathode for ceria electrolyte SOFCs

Abstract Performance of an La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3 –CGO–Ag cathode (where CGO is Ce 0.8 Gd 0.2 O 1.9 ) on a Ce 0.8 Sm 0.2 O 1.9 electrolyte was studied by the three-probe AC impedance method at the steady state of various DC bias currents. The electrode consisted of a porous layer of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3 –CGO (30 wt.%) and a layer of Ag particles coated on the surface. The Ag coating was used to improve the oxygen exchange reaction activity. As a result, an interface conductivity greater than 1 S/cm 2 was obtained at 600 °C with a low activation energy. The dependence of the AC impedance on the frequency was analyzed in order to clarify the reaction steps in the oxygen reduction mechanism.

Properties of oxides for high temperature solid electrolyte fuel cell

Abstract We present thermal expansion, electrical conductivities and structure transitions of the perovskite-type compounds based on LaCoO3, in the temperature range RT (room temperature) - 1100°C in air. They are described for a series of compositions made by substitution of the La3+ cation by the divalent cations Ca2+ and Sr2+. We reveal the transition of structure of the compounds at higher temperatures, and their effects to those properties. The Arrhenius energies obtained from the conductivity measurements fall in the low range 0.07 to 0.28 eV.

Electrodes and performance analysis of a ceria electrolyte SOFC

Abstract In the present work, the current–voltage characteristic at 700 °C across a thin film of Ce0.8Gd0.2O1.9 (CGO) was analyzed by local equilibrium theory. The performance of a Ni–Ce0.8Sm0.2O1.9 (CSO20) anode and a La0.6Sr0.4Co0.8Fe0.2O3–Ce0.8Gd0.2O1.9–Ag (LSCF–CGO–Ag) cathode was studied at 500–700 °C by three-probe complex impedance measurements. If the electrodes are used on a thin film of Ce0.8Gd0.2O1.9 (CGO20) electrolyte, the calculation showed that the maximum energy conversion efficiency at 700 °C could reach 0.43, where the power density was 0.62 W/cm2, which is large enough for practical operation.

Effect of electrode materials on the properties of high-temperature solid electrolyte fuel cells

Abstract A series of perovskite-type compounds Ln 1−x M ∗ x MO 3 (0 x ∗ : alkaline-earth metal, M: Co or Cr) were synthesized for the electrode materials of high-temperature solid electrolyte fuel cells. The electrical and mechanical properties of thin layers of these materials were examined at high temperatures in an atmospheric environment. The electrical conductivities of the compounds ranged from 10 −3 to 10 3 ω −1 cm −1 at 1100°C. The electrical properties of these compounds improved the output power characteristics and decreased the internal voltage drops of solid electrolyte fuel cells. The thermal expansion of the compounds was matched with other component materials of the fuel cells by selecting the value of x in Ln 1−x M ∗ x MO 3 .

Cycle Analysis of Combined Power Generation by Planar SOFC and Gas Turbine Considering Cell Temperature and Current Density Distributions

The planar solid oxide fuel cell (SOFC) with a Y 2 O 3 -stabilized ZrO 2 electrolyte is expected to be a candidate for distributed power sources in the next generation due to its high efficiency of power generation. In this study, we analyzed the system performance of a SOFC/gas turbine combined cycle of about 500 kW electrical output, using our two-dimensional simulation code for the planar SOFC with internal reformer. The effects of cell temperature, cell pressure, recirculation rations of fuel and air, utilization ratios of fuel and air, and average current density of SOFC on both the system efficiency and the cell temperature and current density distributions, were calculated under typical operating conditions taking account of realistic efficiencies and heat losses for auxiliary equipment. The addition of a Cheng cycle to the SOFC/gas turbine combined cycle improved system efficiency by 1-3%. The combined SOFC/gas turbine/Cheng cycle gave a high efficiency of 61.2% (based on a higher heating value) even under a small power generation scale of 500 kW class at 2.0 MPa SOFC pressure. We also discussed the possibility of carbon deposition at both external and internal reformers by calculating the chemical equilibrium carbon activities for estimated carbon deposition reactions.

Ni/Ceria Cermet as Anode of Reduced-Temperature Solid Oxide Fuel Cells

Anode characteristics of a Ce 0.8 Sm 0.2 O 1.9 /Ni cermet on the Ce 0.8 Sm 0.2 O 1.9 electrolyte were studied by a three-probe ac impedance method combined with dc polarization, instead of the conventional current-voltage curve measurement. The experimentally obtained Cole-Cole plots of ac impedance showed a shift with added dc voltage. Both bulk resistance and electrode reaction resistance increased with increasing anodic dc current. The shift of bulk resistance is explained by the electronic conduction of the electrolyte. The increase of electrode reaction resistance is thought to be a result of discrepancy between the ionic and the external current, differing from Ni/yttria-stabilized zirconia anode. An interface resistance lower than 0.1 Ω/cm 2 was obtained below 700°C by using a coprecipitation method to prepare the anode precursor and coliring it with the electrolyte to improve the contact.

Influence of cell configuration on measuring interfacial impedances between a solid electrolyte and an electrode

For AC impedance spectroscopy to measure interfacial impedances between an electrolyte and a working electrode in solid electrolyte cells with three electrodes, the experimental error that is caused by the cell configuration is analyzed using a numerical model. To simulate AC impedance spectra, potential and current distributions in the disk type cells are calculated for various frequencies, and the impedance between the reference electrode and the working electrode is estimated. From the calculation, it is clarified that the difference in the size between the working electrode and the counter electrode can cause the experimental error and the deformation of the impedance spectra from a simple semicircle. The influences of the material properties and the cell configuration on the error and the deformation of the impedance spectrum are discussed.

Cycle analyses of thermoelectric power generation and heat pumps using the β″-alumina electrolyte

Abstract Cycle analyses of the alkali-metal thermoelectric conversion (AMTEC) and the high-temperature heat pump using the β″-alumina electrolyte was performed. It is shown that the isothermal expansion or compression of the sodium ion flow through the electrolyte coincides with the expansion or compression process in the Ericsson cycle, respectively, when the internal losses by thin electrolyte and electrodes are small. The isothermal compression approximation in the heat-pump cycle is more consistent with the isothermal expansion in the AMTEC cycle than the adiabatic compression discussed in an earlier report. Typical voltage-current characteristics and thermal efficiencies for both the AMTEC and the heat pump are presented including not only the saturated liquid case, but also the superheated vapour case of sodium at the high temperature side. The operation in superheated vapour shows a decrease in the power-generation efficiency for the AMTEC and an increase in the coefficient of performance for the heat pump.

Trend of global warming and energy material related to reducing CO2 emission

Recently much attention has been paid to the climate change by greenhouse gases. Among these gases CO2 plays a main role. It is difficult to reduce CO2 emission because most of the energy sources for mankind are based on fossil fuel. To decrease CO2 emission various technical options are discussed, and various energy materials are required to realize the technical options. This paper concentrates on energy materials for advanced energy technologies such as solid oxide fuel cell, alkali metal thermo-electric converter, rechargeable lithium battery and latent heat storage, Kalina cycle of NH3-H2O mixture and coal-fired MHD power generation in which R and D programs we have participated.