Toshio Suzuki

Nanocrystalline undoped ceria oxygen sensor

Abstract The results of a study of the structure and the electrical properties of nanocrystalline cerium oxide sensor are presented. The relationship between the resistance of the thin film ceria and oxygen partial pressure is shown. In the range of oxygen concentration from 10xa0ppm to 100% the conductivity of the ceria follows ( P O 2 ) −1/4 behavior. The response time of the sensor and its cross-sensitivity to nitrogen dioxide and sulfur dioxide is investigated.

Impact of Anode Microstructure on Solid Oxide Fuel Cells

Porous Anodes for Solid Oxide Fuel Cells Fuel cells that use ion-conducting oxides as the electrolyte can be highly efficient and use hydrocarbon fuels directly. However, their very high operating temperatures (usually above 700°C) can lead to unwanted reactions with their electrode materials and premature degradation of their performance. In order to improve fuel-cell electrochemical performance, Suzuki et al. (p. 852) describe a route for increasing the porosity of the anode material, which contains nickel oxide and zirconia doped with scandium and cerium and is fabricated as a cylinder. Subsequent coating and firing steps added a layer of a zirconia-based electrolyte and the (La,Sr)(Co,Fe)O3 cathode. The resulting fuel-cell power density exceeded 1 watt per square centimeter at 600°C, and its performance improved as hydrogen fuel velocities were increased through the cell. A porous form of the oxide anode of a fuel cell with a zirconia-based electrolyte reduces its operating temperature. We report a correlation between the microstructure of the anode electrode of a solid oxide fuel cell (SOFC) and its electrochemical performance for a tubular design. It was shown that the electrochemical performance of the cell was extensively improved when the size of constituent particles was reduced so as to yield a highly porous microstructure. The SOFC had a power density of greater than 1 watt per square centimeter at an operating temperature as low as 600°C with a conventional zirconia-based electrolyte, a nickel cermet anode, and a lanthanum ferrite perovskite cathode material. The effect of the hydrogen fuel flow rate (linear velocity) was also examined for the optimization of operating conditions. Higher linear fuel velocity led to better cell performance for the cell with higher anode porosity. A zirconia-based cell could be used for a low-temperature SOFC system under 600°C just by optimizing the microstructure of the anode electrode and operating conditions.

RAMAN SCATTERING AND LATTICE DEFECTS IN NANOCRYSTALLINE CEO2 THIN FILMS

Abstract The results of Raman scattering studies of nanocrystalline CeO 2 thin films are presented. The spectra have been described using the spatial correlation model from which the correlation length has been determined as a function of grain size. A direct comparison between the concentration of defects related to correlation length and CeO 2 non-stoichiometry has been achieved. The relationship between the lattice disorder and the form of the Raman spectra in nanocrystalline CeO 2 is discussed.

Electrical conductivity of nanocrystalline ceria and zirconia thin films

Abstract The results of studies of the preparation, structure and electrical conductivity of ZrO 2 :16% Y and CeO 2 thin films are presented. Dense films with grain size controlled in the region of 1–400 nm have been obtained on monocrystalline sapphire and polycrystalline Al 2 O 3 substrates using a polymeric precursor spin coating method. The electrical conductivity of nanocrystalline thin films has been studied as a function of oxygen activity and temperature and correlated with the microstructure. Nanocrystalline specimens are characterized by enhanced electrical conductivity and different stoichiometry compared with microcrystalline material.

Microstructure–electrical conductivity relationships in nanocrystalline ceria thin films

A study of nanocrystalline oxide thin film processing and influence of microstructure on the electrical properties of nanocrystalline Gd3+-doped CeO2 thin films was reported. Nanocrystalline films on sapphire substrate were prepared using a polymeric precursor spin coating technique. The grain size of these films depends upon the annealing temperatures and the dopant content, where higher content of dopant realized smaller grain size. The electrical conductivity of nanocrystalline Gd3+-doped CeO2 thin films was studied as a function of temperature and oxygen activity, and correlated with the grain size. The results show that the electronic conductivity of CeO2 increases, whereas the ionic conductivity increases in doped samples as the grain size decreases. From these results, the enthalpy of oxygen vacancy formation was determined as a function of grain size. For CeO2 sample, an enhancement of electronic conductivity was observed with decreasing grain size below 100 nm. In the case of Gd3+-doped CeO2, the electrical conductivity results show that an increase of the ionic conductivity was observed as the grain size decreased, which is related to a decrease in the activation energy for the ion mobility.

Performance of a Porous Electrolyte in Single-Chamber SOFCs

A cell which consists of a porous 18 μm thick Y-doped ZrO 2 (YSZ) electrolyte (23 ′ 3 vol % open porosity) on a NiO-YSZ anode substrate and a cathode using (La, Sr)(Co, Fe)O 3 has been investigated in the single-chamber configuration. The cell performance and catalytic activity of the anode was measured in a flowing air-methane gas mixture with various flow rates. The results showed that the open-circuit voltage and the power density increased as the gas flow rate increased. The cell generated an open-circuit voltage of about 0.78 V, which was only about 0.1 V lower than that observed with dense electrolyte specimens. A maximum power density of 660 mW cm - 2 (0.44 V) was obtained at set temperature = 606°C (cell temperature = 744°C) in the flow rate of 900 cm 3 min - 1 , where the current efficiency was about 5% determined from fuel consumption.

Anode Supported Single Chamber Solid Oxide Fuel Cell in CH 4-Air Mixture

In this study, yttrium-stabilized zirconia (YSZ) thin films 1-2 μm thick electrolyte have been prepared using NiO-YSZ anode as substrates. Fuel cell test was conducted with the single chamber configuration in methane-air gas mixture using (La,Sr)(Co,Fe)O 3 (LSCF) as the cathode. Test results showed that the open-circuit voltage to be >0.8 V, with power density as high as 0.12 W cm -2 . It was also shown that gas flow rate has a large influence on the performance of the fuel cell, which indicates the importance of the geometrical design for anode support fuel cell system.

Role of Composite Cathodes in Single Chamber SOFC

The influence of cathode composition on the open-circuit voltage (OCV) and the performance of a single chamber solid oxide fuel cell (SC-SOFC) was investigated. Because the SC-SOFC is operated using a mixture of air and fuel, the OCV of the cell is determined by the difference of catalytic activity between the cathode and anode. Due to its catalytic activity with fuel, Ni-cermet has proven to be an excellent anode. Typical cathode materials such as perovskite oxides also have catalytic activity with fuel, which can influence the OCV. In this study, cells with composite cathode materials (a mixture of Sm-doped ceria (SDC) and (La, Sr)(Co, Fe)O 3 (LSCF)) and reference electrode (Ag) on SDC electrolyte were prepared and tested in a mixture of propane and air. Several cathode compositions yielded a voltage as high as 0.1 V against the Ag reference electrode and the voltage decreased as the SDC content in the LSCF increased. These cathodes were also tested in the SC-SOFC configuration with Ni-cermet anode on an SDC electrolyte (0.5 mm thick). A cell using a cathode composition of 30 vol % SDC-LSCF showed the lowest overpotential among those tested.

Impedance Studies of Diffusion Phenomena and Ionic and Electronic Conductivity of Cerium Oxide

In this study, the electrical conductivity of undoped cerium oxide is evaluated using impedance spectroscopy. We have observed a new phenomenon, which is not related to electrode response but probably to diffusion processes in the grain boundary. An equivalent circuit has been constructed based on the observation, and the electronic bulk, ionic grain, and ionic grain boundary conductivities are calculated. Evaluation of fitting results to the proposed equivalent circuit suggests that diffusion-related phenomena are caused by blocking effects in the grain boundaries due to impurity segregation. Investigation of the grain boundary composition by energy dispersive X-ray spectroscopy shows that it contains detectable amounts of impurities in comparison to the grain, which supports the suitability of the proposed equivalent circuit.

Evaluation of Micro LSM-Supported GDC/ScSZ Bilayer Electrolyte with LSM–GDC Activation Layer for Intermediate Temperature-SOFCs

A gadolinium-doped ceria (GDC)/scandia-stabilized zirconia (ScSZ) bilayer electrolyte on a microtubular (La, Sr) x MnO 3-δ (LSM) support was prepared via extrusion of a microtubular cathode support and subsequent surface coatings with electrolyte and anode slurries. The GDC/ScSZ bilayer electrolyte was obtained on the LSM support using a cosintering technique, and the LSM-supported microtubular solid oxide fuel cells (SOFCs) were evaluated using field-emission scanning electron microscopy, X-ray diffraction, and electrochemical measurements in wet hydrogen (3% H 2 O) atmosphere. An LSM-GDC activation layer was also introduced between the cathode tube and the electrolyte layers for improvement of cell performance. The micro-SOFC exhibited a stable open-circuit voltage above 1.03 V in the temperature range from 450 to 750°C, and the cell generated a maximum power density of 15, 73, 230, and 378 mW/cm 2 at 500, 600, 700, and 750°C, respectively. This result indicates that our developed cosintering fabrication technology can realize a stable and high-performance, LSM-supported micro-SOFC.