Contact between interconnect and cell in solid oxide cell stacks

Abstract

Improved contact between interconnect and oxygen electrode material in solid oxide cell (SOC) stacks is achieved through a contact point between the oxygen electrode or an oxygen-side contact layer of the SOC and a coated ferritic stainless steel interconnect in the SOC stack, where the coating on the metallic interconnect comprises Cu.

Claims

1. A contact point between a solid oxide cell and an interconnect of a solid oxide stack, said contact point comprising: a ferritic stainless steel interconnect substrate covered by a chromium oxide layer and at least one metallic layer comprising Co or Ni, the at least one metallic layer being coated by a coating comprising Cu having a thickness of approximately 100-200 nm, and an oxygen electrode or an oxygen-side contact layer of a solid oxide cell, wherein the Cu in the coating functions as a sintering aid towards the oxygen electrode or oxygen-side contact layer materials, and a fraction of the Cu in the coating diffuses into the oxygen electrode or oxygen-side contact layer of the solid oxide cell, thereby increasing the adhesion strength and lowering the electrical resistance of the contact point between the coated interconnect substrate and the oxygen electrode or oxygen-side contact layer of the solid oxide cell, such that the voltage drop across the contact point is less than 25 mV, when measured in air at 750° C., under a dc current density of 1.27 A/cm.sup.2, under a compressive load of 3 MPa.

2. A contact point according to claim 1, wherein the coating on the metallic interconnect comprises an oxide of Cu and Fe, an oxide of Cu and Ni, an oxide of Cu and Cu, or an oxide of Cu, Co and Ni, or an oxide of Cu, Co, Ni and Fe.

3. A contact point according to claim 1, wherein the oxygen electrode or oxygen-side contact layer material comprises a perovskite, a double perovskite, or a Ruddlesden-Popper phase material.

4. A contact point according to claim 1, wherein the adhesion strength of the contact point is of the same order of magnitude as the adhesion strength between the electrolyte and the barrier layer of the solid oxide cell.

5. A contact point according to claim 1, wherein the operating temperature of the solid oxide cell stack is between 500° C. and 900° C.

6. A method for creating a contact point between a coating on an interconnect and an oxygen electrode or oxygen-side contact layer of a solid oxide cell (SOC), comprising the steps of: providing a ferritic stainless steel interconnect substrate, depositing at least one layer of Co or Ni on an oxygen side of the interconnect; coating the layer of Co or Ni on the oxygen side of the interconnect with a coating comprising Cu having a thickness of approximately 100-200 nm, providing a solid oxide cell, and sintering the coated interconnect substrate and the solid oxide cell by heat treatment in air at a temperature exceeding 800° C., where the Cu in the coating functions as a sintering aid towards the oxygen electrode or oxygen-side contact layer materials, and a fraction of the Cu in the coating diffuses into the oxygen electrode or oxygen-side contact layer of the solid oxide cell, thereby increasing the adhesion strength and lowering the electrical resistance of the contact point between the coated interconnect substrate and the oxygen electrode or oxygen-side contact layer of the solid oxide cell, such that the voltage drop across the contact point is less than 25 mV, when measured in air at 750° C., under a dc current density of 1.27 A/cm.sup.2, under a compressive load of 3 MPa.

Description

(1) The present invention is described further in the examples which follow. In the examples, reference is made to the Figures, where

(2) FIGS. 1a, 1b and 1c illustrate a contact point, a scanning electron microscopy (SEM) image of the contact point and the voltage drop across the contact point, respectively, according to the prior art,

(3) FIGS. 2a, 2b and 2c illustrate a contact point, a scanning electron microscopy (SEM) image of the contact point and the voltage drop across the contact point, respectively, according to the present invention,

(4) FIG. 3a shows the deposition of a third metallic layer on top of the structure by ion exchange plating, further explained in FIGS. 3b and 3c, all according to the present invention, and

(5) FIGS. 4a and 4b illustrate an EDX (energy-dispersive X-ray analysis) line scan (4a) with point analysis (4b), both according to the present invention.

EXAMPLE 1 (COMPARATIVE ART)

(6) FIG. 1a presents a schematic drawing of a contact point 100 formed by a coated metallic interconnect and a solid oxide cell that can be considered prior art. The chromia forming ferritic stainless steel interconnect 101 is covered by a chromia layer 102 and an oxide coating 103 rich in Co, Mn, and Fe, but poor in Cr. The coated interconnect is in contact with the oxygen-side contact layer 104 of a solid oxide cell. FIG. 1b shows a scanning electron microscopy image of such a contact point. The adhesion strength of such a contact point is relatively low, as is evident from the micrograph, considering the interface between coating 103 and oxygen-side contact layer 104. The electrical properties of such a contact point were evaluated by exposing a structure consisting of a porous LSCF disk with a diameter of 10 mm, a 0.3 mm thick square piece of a coated stainless steel interconnect with a side length of 20 mm, and another porous LSCF disk with a diameter of 10 mm to elevated temperatures in air. A direct current of 1 A was applied through the structure, while a compressive loading of 3 MPa was applied via a load cell. Voltage drop through the structure is mostly governed by the resistance of the contact points, as the resistance of bulk interconnect steel and bulk LSCF is much lower than contact point resistance. According to FIG. 1c, voltage drop across such a contact point is approximately 5 mV at 900° C., 14.5 mV at 800° C., and 28 mV at 750° C. After measurement, it is relatively easy to remove the LSCF disks from the interconnect, indicating relatively low adhesion strength of contact point.

EXAMPLE 2

(7) FIG. 2a presents a schematic drawing of a contact point 200 formed by a coated metallic interconnect and a solid oxide cell according to the present invention. The chromia forming ferritic stainless steel interconnect 101 is covered by a chromia layer 102 and an oxide coating 203 rich in Co, Mn, Cu, and Fe, but poor in Cr. The coated interconnect is in contact with the oxygen-side contact layer 104 of a solid oxide cell. FIG. 2b shows a scanning electron microscopy image of such a contact point. The adhesion strength of such a contact point is expected to be significantly higher than that of Example 1, as is evident from the micrograph. It is noteworthy that the oxide coating 203 has partially diffused into the oxygen-side contact layer 104, and that several particles of the oxygen side contact layer 104 are partially or completely encapsulated by the coating.

(8) The electrical properties of such a contact point were evaluated using the same setup and under identical conditions as described in Example 1. According to FIG. 2c, voltage drop across such a contact point is approximately 4 mV at 900° C., 10.5 mV at 800° C., and 20 mV at 750° C. After measurement, it is relatively much more difficult to remove the LSCF disks from the interconnect, indicating a relatively high adhesion strength of the contact point compared to Example 1.

EXAMPLE 3

(9) A metallic coating on the surface of a ferritic stainless steel interconnect substrate 101 is formed by coating the oxygen side of the interconnect substrate first with a strike layer of Co or Ni 301 by electrodeposition, followed by electrodeposition of an additional layer 302 consisting of Co on top of the strike layer 301. A third metallic layer of Cu 303 is deposited by ion exchange plating on top of the structure comprising the interconnect substrate 101 and the coating layers 301 and 302 (FIG. 3a). The thickness of the Cu layer 303 is approximately 100-200 nm. To form the contact point 200, thus formed coated interconnect 304 is taken into contact with the oxygen-side contact layer 104 of a solid oxide cell at a temperature exceeding 800° C. This step is explained as A in FIG. 3b and FIG. 3c. At this temperature, the metallic coatings 301, 302 and 303 are oxidized, forming an oxide coating 203 rich in Co, Mn, Cu, and Fe in the case of a Co strike layer (FIG. 3b), and 204 rich in Co, Mn, Cu, Fe with small amounts of Ni in the case of a Ni strike layer (FIG. 3c). Both formed oxide coatings 203, 204 are thus poor in Cr. Simultaneously, a chromia layer 102 is formed between the interconnect substrate 101 and the oxide coatings 203 and 204. Also simultaneously, a fraction of the Cu in the oxide coatings 203 or 204 diffuses into the the oxygen-side contact layer 104 of a solid oxide cell, acting as a sintering aid. Hereby, the contact point 200 (FIG. 2a) is formed in the case where the oxide coating is 203. In FIGS. 4a and 4b, an EDX (energy-dispersive X-ray analysis) line scan with point analysis across the interface of the oxide coating 203 and the oxygen-side contact layer 104 is shown, indicating that a fraction of the Cu from the oxide coating 203 has diffused into the oxygen-side contact layer 104.