Improvement Of The Long Term Stability In The High Temperature Solid Oxide Fuel Cell Using Functional Layers

B. Brückner*, C. Günther+, R. Ruckdäschef, E.Fendler0 and H. Schmidt*. * Siemens AG, Corporate Research and Development, 91050 Erlangen, Germany + Siemens AG, Power Generation Group KWU, 91050 Erlangen, Germany ° German Aerospace Research Establishment (DLR), 70569 Stuttgart, Germany

Introduction

In the planar Siemens design of the solid oxide fuel cell a metallic interconnector is used to seperate the ceramic single cells. A disadvantage of the metallic bipolar plate which consists of a chromium alloy (1) is the formation of high volatile Cr-oxides and -hydroxides at the surface at the cathode side. The reaction products evaporate and are reduced at the cathode/electrolyte interface to form new crystalline phases. This process gives rise to long term cell degradation (2). Protective coatings might be successful in preventing the chromium oxide evaporation. The required properties of the protective layers are (I) high electrical conductivity. (II) similar coefficients of thermal expansion to the bipolar plate (III), chemical compatibility to the bipolar plate and cathode material, (IV) a low diffusion coefficient of Cr and (V) chemical stability up to 1223K under oxygen atmosphere.

Furthermore, during operation at 1223K an electrical contact between the metallic plate and the electrodes has to be maintained. This problem could be solved using ceramic layer between the metallic plate and the single cells.

Experimental Procedure and Results For the deposition of protective coatings the vacuum spraying (VPS) has a high potential, differently doped LaCr03 powders were used for the coatings. Bipolar plates were first etched in order to remove oxide layers. In combination with a transferred arc a very good adhesion was attained. The effect of the transferred arc is the removal of impurities by gas discharge appearing due to negative polarity of the substrate with respect to the plasma torch (3). Furthermore, some samples were pre-heated just before starting the deposition process in order to increase the adhesion. The structured bipolar plates were deposited in a technical chamber with a robot handled torch and substrate movement which enables the deposition under different spray angles. The high velocity and laminarity of the plasma jet enables the production of very dense and well bonded layer. Phase stability and crystallinity of the layers were checked by x-ray diffraction (XRD) and energy dispersive x-ray analysis (EDX). The microstructure and porosity of the layers were investigated by optical and scanning electron microscopy. Optimizing the substrate pretreatment procedure and the plasma spray process homogenous layers were achieved (fig. la) (4,5).

30KU 57.8y. 173P 2383

Figure 1 a) VPS-deposition of perovskite layers on a structured bipolar plate

30KU 57.8y. 173P 2383

Figure 1 a) VPS-deposition of perovskite layers on a structured bipolar plate

Figure 1 b) Cross-section of a complete stack compound

As contact layer material between the metallic interconnector and the cathode LaCo03 and Sr-doped LaMnOj was investigated. The development of these layers was previously published (6). For the deposition of the contact layers an air - brushing technique was used. A good deformability of this porous layers is required in order to form the electrical contact between the bipolar plate and the cathode during the sealing procedure of the stack. Pieces of structured bipolar plates (1 cm2) were coated with several contact layer materials by wet powder spraying. To simulate the weight ratios in a real stack the layers were applied by two different weights, according to mechanical, forces of 3.3 and 13 N. Then the test samples were heated with a temperature program that simulates the sealing procedure in a stack. The thickness of the contact layer was measured before and after the temperature treatment. The difference was defined as a measure for the contact layer deformation. Dependent on the weight the layers were deformed by 10% and 20%, respectively. The protecting function of the plasma-sprayed layers was demonstrated with evaporation measurements. The detected chromium content of the investigated bipolar plates coated with LaosSro^MnOj were in the range of the detection accuracy. The values for Lao.9Sro.iCr03 layers on the bipolar plate are the same as for sintered Lao.9Sro.iCr03 material. No reaction products of evaporated chromium oxides could be detected after annealing coated bipolar plates in contact with cathodes.

Further characterization was maintained by contact resistance measurements and by single cell measurements.

The instrumental set-up of the contact resistance measurements was previously published (7). Two cylinders of bipolar material, 2 mm in diameter each, which were coated with the functional layers, were pressed with the coated area against a sintered sheet of a porous cathode foil. The time dependence of the resistance of this pile was measured by a 4-point technique under operating conditions of a SOFC cathode (1223K, air as ambient gas). For the determination of the contact resistance a constant current of 0.1 A was applied and the voltage drop was plotted as function of time. In each run 8 samples could be measured simultaneously. The time dependence of the contact resistance at 1223K in air of the several investigated layer combinations is illustrated in fig. 2. The gaps in the respective curves are caused by thermic cycles (cooling to room temperature and heating again to operating temperature). The thermic cycles were carried out in order to check the adhesive strength of the layers after this temperature treatment. If LaCo03 as contact layer was used without any protective layer, a continuos increase of the contact resistance could be observed. After a thermal cycle the contact resistance value increased by a factor of 2. The reason therefore are the different thermal expansion coefficients of LaCo03 (28 ppm/K) and the bipolar material (11.3 ppm/K) (6), so the temperature treatment leads to a

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