Realisation Of An Anode Supported Planar Sofc System

H.P. Buchkremer*. U. Diekmann»*, L.G.J. de Haart***, H. Kabs****, U. Stimming*** and D. Stöver* •Institut für Werkstoffe der Energietechnik, "Zentralabteilung Technologie, ***Institut fOr Energieverfahrenstechnik, ****Programmleitung SOFC Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany

Introduction

Lowering the operating temperature of SOFCs to below 800 °C potentially lowers production costs of a SOFC system because of a less expensive periphery and is able to guarantee sufficient life time of the stack. One way of achieving lower operating temperatures is the development of new high conductive electrolyte materials. The other way, still based on state-of-the-art material, i.e. yttria-stabilized zirconia (YSZ) electrolyte, is the development of a thin film electrolyte concept.

In the Forschungszentrum Jülich a program was started to produce a supported planar SOFC with an YSZ electrolyte thickness between 10 to 20 pm. One of the electrodes, i.e. the anode, was used as support, in order not to increase the number of components in the SOFC. The high electronic conductivity of the anode-cermet allows the use of relatively thick layers without increasing the cell resistance. An additional advantage of the supported planar concept is the possibility to produce single cells larger than 10x10 cmxcm, that is with an effective electrode cross area of several hundred cm2.

Materials and Component Manufacturing

NiO-YSZ substrates, with a standard composition of 44 \vt% Ni, were manufactured by a Coat-Mix® process. The YSZ electrolyte layer was applied on the pre-sintered anode substrate by vacuum slurry coating. After sintering at 1400 °C, a gas tight electrolyte layer of approximately 15 - 20 pm thickness was formed. The thickness of the substrate was in between 1.5 and 2.0 mm. The substrates were manufactured as large as 25x25 cm2. A composite cathode layer (La0 65Sr0ji)MnOj/8YSZ) of typically 40 pm was finally applied on the electrolyte by Wet Powder Spraying®. A detailed description of the component manufacturing is published elsewhere [1,2].

Metal ODS-alloy plates (94Cr5Fe 1Y203; Plansee AG, Austria) were used as interconnectors and end-plates. Gas channels were mechanically machined into the plates in a cross-flow configuration. Contact layers in order to decrease the electrical resistance at the interface electrode/interconnector were applied to both the anode and cathode side of the interconnector during stack assembly. A composite glass was used as sealing material [I].

Initial Cell and Stack Performance

Single cells and stacks were tested using humidified hydrogen as fuel gas and dry air as the oxidant gas. Fig. 1 shows the initial performance of a 10-cell (10x10 cm2) stack at 950 and 800 °C. At 950 "C and 7.0 V stack voltage (i.e. 0.7 V per cell) a current density of 420 mA/cm2 was reached. This corresponds to a power density of 0.28 W/cm2. The fuel utilization was 37% at this current density. Lowering the operating temperature to 800 °C the current density decreased to around 270 mA/cm2 at the the same stack voltage of 7.0 V (0.20 W/cm2) [3].

Figure 1. Performance of a 10-cell (10x10 cm3) stack with humidified H2/air at 950 °C

Current Density / (mA/cm2)

Figure 1. Performance of a 10-cell (10x10 cm3) stack with humidified H2/air at 950 °C

The separate cells in the stack showed an almost uniform potential distribution, differences not exceeding ±20 mV. The area (81 cm2 per cell) specific resistance amounts 0.73 f2«cm2 at 950 °C and 1.05 n»cm= at 800 °C.

A single cell as large as 25x25 cm2, with an effective electrode area of 576 cm2 was also manufactured and tested with humidified H2 and air using metallic endplates for current collection. At 950 °C and a working voltage of 0.7 V a current density of 300 mA/cm2 was reached, corresponding to a power density of 0.21 W/cm2. The area specific resistance was 1.00 n-cm2 [3],

Both the 10-cell (10x10 cm2) stack and the single cell (25x25 cm2) showed at 950 °C a rapid decrease of the performance, which is most likely due to Cr-oxide deposition in the cathode region, the Cr originating from the unprotected interconnector and endplates used.

Improved Cell and Stack Performance

Materials innovations with respect to the metallic interconnector, the glass-ceramic sealant and the anode substrate facing the electrolyte layer have a pronounced effect both on the performance as well as on the stability of the cells and stacks. These investigations were conducted using small short-stacks, each containing 2 cells of 5x5 em2 with an effective electrode area of 16 cm2. As an example Fig. 2 shows the performance of such a 2-cell (5x5 cm2) stack as function of the operating temperature. At the working voltage of 1.4 V (0.7 V per cell) the current density reached 1,195 mA/cm2 at 954 °C and 475 mA/cm2 at 811 °C. Corresponding power densities are 0.84 W/cm2 and 0.33 W/cm2, respectively. In comparison with the results obtained for the 10-cell stack this means an improvement in power density by a factor of 3 at 950 "C and 1.7 at around 800 °C. The difference in these factors is caused by a difference in the temperature dependence of both stacks, for which no clear explanation can be given yet.

Fig. 3 shows the time dependent performance of same the 2-cell (5x5 cm2) stack. After the initial recording of the current-voltage characteristics shown in Fig. 2 during the first 32 hours of operation, the stack was operated at 800 °C at 313 mA/cm2 for a period of 357 hours. The stack voltage remained constant at around 1.64 V (0.26 W/cm3) without showing any noticablc degradation. A failure in the gas supply system caused irreversible damages to the stack.

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 Current Density / (A/cm2)

Figure 2. Performance of a 2-cell (5x5 cm2) stack with humidified Hj/air as function of operating temperature between 954 °C and 755 °C

Figure 3. Time dependence of the performance of the 2-cell (5x5 cm2) stack starting at 32

h at 800 °C under constant load (311 mA/cm2) with humidified Hj/air.

Figure 3. Time dependence of the performance of the 2-cell (5x5 cm2) stack starting at 32

h at 800 °C under constant load (311 mA/cm2) with humidified Hj/air.

References

[1] H.P. Buchkremer, U. Diekmann and D. Stöver, "Components Manufacturing and Stack Integration of an Anode Supported Planar SOFC System", in: Proc. 2nd Europ. SOFC Forum (B. Thorstensen, Ed.), pp. 221-228, U. Bossel, Morgenachcrstrasse 2F, CH-5452 Oberrohrdorf, Switzerland, 1996.

[2] R. Wilkenhöner, W. Mallener, H.P. Buchkremer, Th. Hauber and II. Stimming, "Cathode Processing by Wet Powder Spraying", in: Proc. 2nd Europ. SOFC Forum (B. Thorstensen, Ed.), pp. 279-288, U. Bossel, Morgenachcrstrasse 2F, CH-5452 Oberrohrdorf, Switzerland, 1996.

[3] L.G.J, de Haart, Th. Hauber, K. Mayer and U. Stimming, "Electrochemical Performance of an Anode Supported Planar SOFC System", in: Proc. 2nd Europ. SOFC Forum (B. Thorstensen, Ed.), pp. 229-235, U. Bossel, Morgenacherstrasse 2F, CH-5452 Oberrohrdorf, Switzerland, 1996.

Acknowledgements

The authors would like to thank all members of the SOFC-Substratc-Concept Team for their technical support and helpful discussions.

LaCrOj-DISPERSED Cr FOR METALLIC INTERCONNECT OF PLANAR SOFC

Rak-Hyun Song, Dong Ryul Shin and *Masayuki Dokiya Korea Institute of Energy Research, P.O.Box 103, Yusong, Taejon, 305-600, Korea ♦National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan

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