The National Institute of Advanced Industrial Science and Technology (AIST, President: Hiroyuki Yoshikawa) has developed a micro tubular solid-oxide fuel cell (SOFC) which is operable at low temperatures between 500 and 600°C (Figs. 1 and 2). Since SOFCs are fabricated from solid materials, they are highly reliable and easy to handle, in spite of high operating temperatures. On the other hand, since conventional SOFCs are used at temperatures of 800-900°C, their fields of application are limited. Thus, development of SOFCs operable at lower temperatures has been expected.

Using a ceria based material, which exhibits high oxygen-ion conduction at low temperatures, as an electrolyte, AIST has successfully enhanced the fuel reaction efficiency of the cell, and increased the thermal shock resistance to overcome the breakage problems caused by thermal distortion, which is serious especially for ceria based materials. Moreover, using the micro SOFC, we have dramatically enhanced the volumetric power density compared to that of conventional SOFCs. Once stacking technology for the micro SOFCs is developed, this micro tubular SOFCs will be widely applied, e.g., to distributed power sources for homes and portable electronic devices, and auxiliary power sources for vehicles.

This research work was carried out for the New Energy and Industrial Technology Development Organization (NEDO) project, "The Advanced Ceramic Reactor Project" and will be presented on January 26, 2006 at "FC EXPO2006", and on July, 2006 at the "7th EUROPEAN SOFC FORUM".

Figure 1 Images of micro tubular SOFC

Figure 2 Maximum power density of the micro tubular SOFC developed in this study.

Since fuel cells have high efficient power-generation performance, they can drastically reduce the generation of CO₂, which is considered to be a cause of global warming. Various types of fuel cells, such as polymer electrolyte fuel cells (PEFCs), molten carbonate fuel cells (MCFCs), phosphoric acid fuel cells (PAFCs), and solid oxide fuel cells (SOFCs), have been developed so far; of these, PEFC has gained attention as a fuel cell for home and vehicle use. SOFCs, on the other hand, have the highest efficiency for power generation, and utilize ceramics technologies, at which Japan excels. The operating temperatures of conventional SOFCs are high, i.e., 800-900°C, and their applications have been limited to large-scale power-generation facilities. Thus, to use SOFCs as distributed electric sources for homes, portable electronic devices, and auxiliary power sources for vehicles, the development of SOFCs which can be operated at temperatures of 500-600°C was necessary.

Ceramic reactors, which can electrochemically transfer mass and energy with high efficiency, can be applied to various applications such as SOFCs, and gas-purification filters for environmental pollutants. In particular, SOFC can exhibit high efficiency for power generation. But their usage has been restricted to large scale power-generation facilities because of their high operating temperatures of 800-900°C. However, since SOFCs have the high efficient performance for power-generation, it is more effective if they are utilized as distributed electric sources for homes and portable electronic devices, and auxiliary power sources for vehicles. Thus, for practical use, AIST has investigated SOFCs which can work at temperatures of 500-600°C, and have high power-generation performance and heat-shock resistance, as part of the NEDO project "The Advanced Ceramic Reactor Project" (for the fiscal years of 2005-2009).

To decrease the operating temperature, ceramic electrolyte materials with high ionic conductivity such as doped ceria and lanthanum-gallate have already been utilized, and in practice, the operating temperature has been reduced to 500-600°C. However, even though the operating temperature is reduced, thermal distortion still remains as a big obstacle for devices which need frequent startup and shutdown operations. Thus, AIST has developed a micro tubular cell and investigated methods of reducing thermal distortion of the cell.

AIST has successfully developed a high efficient micro tubular SOFC of millimeters to sub-millimeters in diameter (Fig. 1), which can solve the thermal distortion problem by such miniaturization of SOFC. The ceria based materials which enable the reduction in operating temperature down to 500-600°C are mechanically fragile, and thus their micro-fabrication has so far been considered to be difficult. In this study, however, using advanced micro tube processing technique which enables the control of microstructures and dense film-coating technique, we have succeeded in the fabrication of the micro tubular SOFCs. In addition, we have succeeded in optimizing electrode microstructures to greatly enhance the fuel reaction efficiency. In cell fabrication, nickel-ceria based materials and lanthanum cobalt-ceria based materials are used as the fuel and air electrodes, respectively.

The micro SOFC which was actually fabricated has a tubular structure of approximately 1 cm in length and 0.8-1.6 mm in diameter. For example, when hydrogen gas was flowed in a micro tube with 1.6 mm in diameter at 450-570°C, power densities of 0.17-1 W/cm² were obtained (Fig. 2). This value is on the world's highest level in SOFCs with ceria based electrolytes.

For the micro SOFC of 0.8 mm in diameter, a hundred micro SOFCs can be integrated in each 1 cm³, and thus power densities of 7 W/cm³ at 500°C, and 15 W/cm³ at 550°C can be expected in theory. This demonstrates that our technique can open the gateway for applications to distributed power sources for homes and portable electronic devices, and auxiliary power sources for vehicles.