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Research paper : Challenge for the development of micro SOFC manufacturing technology (Y. Fujishiro et al.)−46−Synthesiology - English edition Vol.4 No.1 (2011) revolutionary micro SOFC manufacturing technology that uses the ceramic integration process technology.2 Status of the development of energy module technology ~ Expectation of industry for compact fuel cells with high power density and operable at low temperatureAs shown in Fig. 1, the industrial demand for highly efficient energy conversion at several W to several kW levels increased due to the diversified use of SOFC in various industries. The expectation is high for the compact SOFC technology that can be easily handled and is space-saving.In order to promote the use by expanding the operating condition of SOFC as an electrochemical module made of ceramic material, it was essential to develop the micro SOFC technology that would allow rapid startup/shutdown with equivalent performance at lower temperature range of 650 °C or less compared to the conventional operating temperature (700 °C~1000 °C). The micro SOFC is a power generation technology for palm-top cells that is smaller in size compared to the conventional ones, and it enables compact module design that also is space saving. Therefore, we embarked on the micro SOFC technology development to solve the various technological issues. Figure 2 shows the advantages of the micro SOFC. In general, since the oxide ceramic material had smaller heat conductivity compared to metals, a steep temperature gradient occurred throughout the cell during temperature increase (particularly during rapid increase) when the volumes of integrated module and ceramic electrochemical cell increased, and this could cause the destruction of the cell or the module parts. One of the technological solutions was to employ a design that reduced the volume of the cell or the module parts to decrease the relative temperature gradient, as shown in Fig. 2. This reduced the startup energy, and allowed the temperature distribution of the SOFC to be easily controlled at the same time. As a result, high thermal shock resistance was obtained for the cell and the module. In planar cell module, since the generation density per volume of the unit decreased by reducing the cell volume, it was necessary to increase the performances such as the generation efficiency and power density. To do so, it was necessary to develop a new high integration manufacturing technology that could be mass produced and could increase the generation performance per volume, by increasing the electrode surface area and by controlling the unit structure such as the diameter using small tubular cell as shown in Fig. 2. Setting several W to several kW level as our output target, it was important to develop the construction technology of the module that fit within 1 L size even at 2 kW level that surpassed the performance of the PEFC. As the output power increased, the temperature control of the module became difficult. Therefore, the cell integration module technology that enabled low temperature operation and easy control of startup/shutdown was demanded.The SOFC module is composed of dense oxide ion-conducting ceramics electrolyte, electrodes (anode and cathode) that enhance the electrochemical reaction, and Fig. 1 Industrial development of micro SOFCTable 1 Types and characteristics of fuel cellsReference: J. Larmine and A. Dicks: Fuel Cell Systems Explained, Wiley (2003) [H. Tsuchiya trans.: Kaisetsu Nenryo Denchi Shisutemu, Ohmsha (2004) (in Japanese)].ElectrolytematerialOperatingtemperatureCharacteristicsGenerationefficiencyOxideion-conductingceramics500-1000 ℃(point isachieving highperformanceat lowtemperaturerange)40-70 % Proton-conductingpolymer film~38 % Moltencarbonatefuel cells(MCFC)Moltencarbonate600-700 ℃45-60 % Phosphoricacidfuel cells(PAFC)Phosphoricacid160-220 ℃35-42 % Solid oxidefuel cells(SOFC)Polymerelectrolytefuel cells(PEFC)Roomtemperature~ about 90 ℃Electrode resistance is low since it operates at high temperature, and the cell performance is high. Major improvement in efficiency is possible by altering the fuel quality by using waste heat. It is expected to be future dispersed power source.Easy to handle due to low operating temperature. Research is active for use at home, in automobiles, and in portable devices, and commercialization has been achieved in some fields.Can be up-scaled easily. Biogas produced from garbage and wood can be used as fuel. Separation of CO2 is possible.Developed for commercial use among fuel cells currently available. It has also been used for dispersed power source at plants.High industrial demand (several W ~ kW)0.10.01Direct methanol (DMFC)Direct methanol (DMFC)Efficiency(%)Molten carbonate (MCFC)Solid oxide(SOFC)Phosphoric acid (PAFC)Polymer electrolyte (PEFC)SOFC+GTEngine1000100103050404020Gas turbine (GT)Lean burn engineScale of system (kW)1Electric vehicles, mobile devices, portable power sources for leisure and emergency, auxiliary power units (APU), small-scale fixed generators, etc.Fig. 2 Advantages of the micro SOFC module and its integration technologyIncreased electrode reaction surface per volumeVolume generation density(W/cm3)Small cellLarge cellLowHighStart-up energy (heating), temperature distribution● Downsizing and high performance of the module● Rapid start-up operationHigh thermal shock resistance0.01Tube diameter (mm)20151050.10High generation density per volume101

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