Yoshihiro Funahashi

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.

Design and Fabrication of Lightweight, Submillimeter Tubular Solid Oxide Fuel Cells

Here we report lightweight, high-performance tubular solid oxide fuel cells (SOFCs) with 0.8 mm diameter operable at/under 550°C. The SOFCs consist of ordinary materials with a well-designed anode microstructure with porosity of 46% (before reduction) and reduced anode thickness less than 200 μm. Thus, the weight of the SOFC was realized to be 0.015 g per 1 cm tube length (electrode area = 0.25 cm 2 ). The single-cell performance test showed that the peak power densities of the SOFC were 273, 628, and 1017 mW/cm 2 at 450, 500, and 550°C with wet H 2 fuel, respectively.

Development of Evaluation Technologies for Microtubular SOFCs Under Pressurized Conditions

We developed evaluation technologies for microtubular solid oxide fuel cells under pressurized conditions. The pressurized cell evaluation system for the single cell was produced. The chamber temperature of the evaluation system can be controlled up to 750 °C, and the maximum chamber pressure is 0.8 MPa. It was possible to manually control the pressure difference between air and fuel gas within ±3 kPa during the pressure increase. The hard sealing technique was introduced for the evaluation under pressurized conditions. Using two different types of commercial inorganic ceramic adhesives, the gas leakage was controlled at approximately 2%. Differential pressure control between fuel and air is effective for the stable open circuit voltage and power generation. The power generation under pressurized conditions was successful at 650°C, and the pressurized effect was clearly confirmed.

Development of Fabrication/Integration Technology for Micro Tubular SOFCs

Publisher Summary A typical Solid oxide fuel cell (SOFC) consists of doped zirconia for an electrolyte, Ni cermet for an anode and doped lanthanum manganite for a cathode, and it has shown its long-term stability over 20,000 h operation as well as high power output up to 2 W/cm2 at 800°C. SOFCs have been recognized as a keystone of the future energy economy, and the development of SOFC systems has been an important issue in recent years. This chapter discusses development of micro tubular SOFCs, summarizes recent development of fabrication/integration technology for micro tubular SOFCs, and examines characteristics of micro tubular SOFCs. High-performance SOFCs lies in the design of the cell/bundle/stack and novel fabrication technology that can realize optimized electrode structures. By using recent development of fabrication/integration technology for micro tubular SOFCs with diameters of 0.8–2.0 mm in diameter operable at at/under 550°C are fabricated. Characterization of micro tubular SOFC is discussed in detail, including current collecting loss due to the dimension of the cell. For practical realization of the micro tubular SOFC system, further investigations of the tubular cells and the bundles are necessary to gain the knowledge necessary for understanding the following issues: stack design (manifold design); thermal distribution in the stack and thermal management (simulation); gas pressure loss in the stack/system and optimization of gas flow; fuel utilization; current collecting loss from the cathode matrix; and sealing technology.

Simulation Study for the Optimization of Microtubular Solid Oxide Fuel Cell Bundles

Microtubular solid oxide fuel cells (SOFCs) are shown to be robust under rapid temperature changes and have large electrode area per volume (high volumetric power density). Such features are believed to increase a variety of application. Our study aims to establish a fabrication technique for microtubular SOFC bundles with the volumetric power density of 2 W cm -3 at 0.7 V. So far, we have succeeded to develop a fabrication technology for microtubular SOFC bundles using anode supported tubular SOFCs and cathode matrices with well-controlled microstructures. A key to improve the performance of the microtubular SOFC bundles is to optimize the microstructure of the cathode matrices because it influences a pressure loss for air and electric current collection. In this paper, a simulation study of an air flow, temperature, and potential distributions in the microtubular SOFC bundle was conducted in order to understand the characteristics of the present bundle design. In addition, operating conditions of the microtubular SOFC bundles was discussed for realizing the target power density of 2 W cm -3 .

Fabrication and Characterization of Microtubular SOFCs with Multilayered Electrolyte

Multilayered electrolytes for microtubular solid-oxide fuel cells (SOFCs) have been prepared and their electrochemical properties investigated. SC 2 O 3 -stabilized ZrO 2 (ScSZ) and Gd 2 O 3 -doped CeO 2 (GDC) were selected for the electrolyte materials; the multilayered electrolyte of GDC-ScSZ-GDC was successfully prepared on an anode tube (1.8 mm diameter) using multiple dip coating and a cofiring technique. The thickness of each electrolyte layer was about 3, 3, and 12 μm, respectively. The cell performance test showed an open-circuit voltage (OCV) of over 1 V at 600°C or higher temperature for the SOFC with a multilayered electrolyte, while a SOFC with a 12 μm thick GDC electrolyte showed an OCV of 0.84 V at 600°C. The cell performances of 0.35 W cm -2 at 0.7 V and 0.27 W cm -2 at 0.8 V were obtained for the SOFC with multilayered electrolyte at the operating temperature of 650°C.

Power Generation Properties of Microtubular Solid Oxide Fuel Cell Bundle Under Pressurized Conditions

A microtubular solid oxide fuel cell (SOFC) bundle was developed based on a new design. Anode-supported microtubular SOFCs with the cell configuration, La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF)-Ce 0.9 Gd 0.1 O 1.95 (CGO) cathode/CGO electrolyte/Ni-CGO anode were fabricated and bundled in a porous LSCF current-collecting cube with sides of 1 cm. The power generation of the fabricated SOFC bundle was measured under pressurized conditions. Using humidified 30% H 2 /N 2 mixture gas and air, the cubic power density of the bundle at 500°C under atmospheric pressure (0.1 MPa) was 0.47 W cm ―3 at 0.4 A cm ―2 . With increasing operating pressure, the performance increased, and the cubic power density reached 0.66 W cm ―3 at 0.6 MPa. The power enhancement brought about by pressurization was due to increased open circuit voltage and reduced polarization resistance. After comparing the power gain of the pressurized SOFC and the power consumption gain of the air compressor used for pressurization, it was found that pressurized cell operation exhibited the highest actual power gain at around 0.3 MPa.

Development of Microtubular SOFCs

Microtubular solid oxide fuel cells (SOFCs) have successfully demonstrated their advantages over conventional (planar) SOFCs, such as high thermal stability during rapid heat cycling and large electrode area per volume, which enables one to realize SOFC systems applicable to portable devices and auxiliary power units for automobile. In this study, the fabrication method of the microtubular SOFCs was examined. Shrinkage behavior and the microstructure of electrolyte/anode as a function of sintering temperature were shown and correlated with densification of the electrolyte during cosintering process.

Development and Characterization of Cathode-Supported SOFCs by Single-Step Cofiring Fabrication for Intermediate Temperature Operation

In this study, a single-step cofiring through the extrusion molding and wet-ceramic coating technique was developed to fabricate a cathode-supported microtubular cell. The cell is consisting of a Ce0.9Gd0.1O1.95 (GDC) electrolyte with a NiO–GDC anode on a porous La0.6Sr0.4Co0.2Fe0.8O3−δ∕GDC tube (460μm wall thickness and 2.26mm diameter). Densification of the ceria membrane (thickness <20μm) was successful by cosintering the laminated thin electrolyte and the anode with the cathode at 1200°C. Compared with that fabricated by the conventional two-step cofiring process, the cell showed an improved performance due to the increased anode sintering temperature, which leads to an improved anode∕electrolyte interfacial property. The cell having 2cm tube length fed with humidified 30vol%H2–Ar (3% H2O) produced the maximum power densities of 0.09Wcm−2, 0.08Wcm−2, and 0.05Wcm−2, at 600°C, 550°C, and 500°C, respectively.

Simulation Study for the Series Connected Bundles of Microtubular SOFCs

Solid oxide fuel cells (SOFCs) have the highest energy conversion efficiency among various power generators and expected to be earlier commercialization. Our study aims to develop fabrication techniques of microtubular SOFC bundles and establish realistic bundle structure for kilowatt class module. So far, we have succeeded to establish fabrication technology of the microtubular SOFC bundles using porous supporting matrices. In this study, the simulation study of the microtubular SOFC bundle was carried out to understand Joule heat and temperature distribution in the microtubular SOFC bundle during operation. The results indicated that the method of current collection had to be carefully considered, since the total output power loss of the bundle was estimated to be 27.8%. The temperature distribution of the bundle using porous MgO matrices turned out to be moderate compared with that in the previous bundle using porous (La, Sr) x(Co, Fe)O 3 matrices due to the difference in the thermal conductivity of each matrix constitute.