Abstract

A 2 k\V class combined cell stacked module (182 cm2 X 4X 17) was examined. An output power of 2.47 kW and output power density of 0.20 \Y/cm2 were obtained at the current density of 0.3 A/cm2.

The temperature uniformity is an important factor to develop large scale SOFC modules. Therefore, in this 2 k\V class module, one cell was divided into four smaller unit cells to decrease temperature difference across these cells. Moreover, an internal heat-exchanging duct was arranged to spend the surplus heat effectively in the middle of the module.

As for the basic research, the followings were investigated to improve thermal cycle characteristics. One was to adopt a silica/alumina-based sealing material in order to absorb the thermal expansion difference between the electrolyte and the separator. Deterioration was quite small after 12 thermal cycles with a 150 by 150 mm single cell. The other was to use a heat-resisting ferritic alloy as a separator in a 50 by 50 mm single cell in order to decrease the thermal expansion coefficient of the separator. High performance was obtained for 2000 hours at 900 "C in an endurance test and deterioration was quite small after a thermal cycle.

EXPERIMENTAL Module

Figure 1 shows the schematic diagram of the combined cell stacked module. The single combined cell consisted of four unit cells, and active electrode area of each unit cell was 182 cm2. A 2 kW class combined cell stacked module (182 cm2 X 4X 17) was examined. The each size of the electrolyte (3 mol% Y2O3 partially stabilized ZrOa) was 150 mm X 200 mm X 0.2 mm'. A heat-resisting alloy (Inconel 600) was used for the separators. The separators were 350 mm X 350 mm in size. A

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form of a gas flow, which consisted of an inner manifold, a counter-flow and an oxidant gas flow with side exhaust, was adopted to the combined cell as well as a 1 kW class module'). An internal heat-exchanger duct was arranged at the center in order to decrease the temperature differences within face of combined four unit colls. The temperature profile was measured by thermocouples inserted into the separators. A mixture of 8 mol% Y2O3-stabilized ZrOa (8YSZ) and NiO was used for the anode material. It was applied to the electrolyte by screen printing and sintered for 2 hours at 1300 °C. A mixture of Lao.sSro.iMnOs (LSM) and 8YSZ was used for the cathode. It was applied to the electrolyte and sintered for 4 hours at 1100 cC. A mixture of LSM and LasOs was applied to the cathode similarly to form the cathode second laver.

Internal

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Internal

Oxidant

heat-exchanger duct

Oxidant

Figure 1

The schematic diagram of the combined cell stacked module

Thermal cycle test of the single cell

Thermal cycles were carried out for a 150 by 150 mm single cell using different sealing materials. A silica/alumina-based sealing material was adopted in order to absorb the thermal expansion difference between the electrolyte and the separator. A heat-resisting alloy (Inconel 600) was used for the separators.

Endurance test of ferrite alloy separator

In order to improve the thermal cycle characteristics, a ferrite alloy was examined for the separator material because -its thermal expansion coefficient is smaller than that of nickel-based alloys. A heat-resisting ferrite alloy containing a small amount of rare earth metals was used for the separator and a 50 by 50 mm single cell test was carried out at 900 °C.

RESULTS AND DISCUSSION Module

Figure 2 shows V-I and power characteristics of this module. The fuel utilization (Uf) and the oxidant utilization (Uox) were 15 % and 30 % at 0.3 A/cm2 , respectively. An output power of 2.47 kW and output power density of 0.20 \V/cm2 were obtained at the current density of 0.3 A/cm2. The output power density was equivalent to that of the 1 kW class module (160 cm2 X 30) which was previously reported1'2). It was therefore considered that the combined cell structure was effective for higher output power SOFC modules, because there was no reduction of

Output power density by adopting this structure.

Good performance was obtained up to Uf = 80 % of operation for a single combined cell, so the uniformity of the gas distribution was confirmed.

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