Component Constituting an HTE Electrolyser Interconnector or SOFC Fuel Cell Interconnector and Associated Production Processes

Abstract

The invention relates to a component (8) comprising a substrate made of chromia-former metal alloy (82), the basic element of which is iron (Fe) or nickel (Ni), wherein the substrate has two main planar faces. According to the invention: one of the main planar faces is coated with a coating comprising a thick layer of ceramic (80), grooved to delimit channels (800) suitable for the distribution and/or collection of gases, such as H.sub.2O water vapour, H.sub.2 or air, and/or one of the main planar faces is coated with a thick metal layer (81), grooved to delimit channels (810) suitable for the distribution and/or collection of gases, such as H.sub.2O water vapour, H.sub.2, O.sub.2 or draining gas. The invention also relates to the associated production processes.

Claims

1.-14. (canceled)

15. A process a fuel cell (SOFC) or a high-temperature electrolyzer (HTE), comprising the following steps: a/ preparing a substrate made of metal alloy, of chromia-forming type, the base element of which is iron (Fe) or nickel (Ni), the substrate having two main flat faces; b/ coating one of the flat faces of the substrate with a thick ceramic layer in a green state and coating the other flat face of the substrate with a thick metallic layer in a green state; c/ grooving the thick ceramic layer and the thick metallic layer so as to delimit channels that are suitable for distributing and/or collecting gases; and d/ forming a stack of elementary electrolysis cells each formed from a cathode, an anode and an electrolyte intercalated between the cathode and the anode; wherein: to prepare an HTE reactor, the thick grooved ceramic layer is in contact with the anode of one of two adjacent elementary cells, and the thick grooved metallic layer is in contact with the cathode of the other of the two adjacent elementary cells; or to prepare an SOFC, the thick grooved ceramic layer is in contact with the cathode of one of two adjacent elementary cells, and the thick grooved metallic layer in contact with the anode of the other of the two adjacent elementary cells.

16. (canceled)

17. The process as claimed in claim 15, in which, prior to step b, the thick ceramic or metallic layer is obtained by tape casting, step b comprising hot-bonding or hot-pressing or chemical bonding of the strip to one or other of the faces of the substrate.

18. The process as claimed in claim 17, step b comprising hot-pressing or hot-bonding of the green ceramic strip at a temperature of between 60 and 130 C.

19. The process as claimed in claim 15, wherein step b comprises screen printing in thick layers of a ceramic or metallic paste onto one or other of the faces of the substrate.

20. The process as claimed in claim 15, wherein step c comprises calendaring a green ceramic strip obtained by tape casting between two rolls heated to the softening point of the polymers of the ceramic strip, at least one of the two rolls comprising ribs corresponding to the channels to be delimited.

21. The process as claimed in claim 15, wherein step c is performed by laser ablation once step b has been completed.

22. The process as claimed in claim 21, wherein step c is performed using a CO.sub.2 laser.

23. The process as claimed in claim 21, wherein step c is completed after several passes of the laser over the thick layer.

24. The process as claimed in claim 1, wherein the thick ceramic layer comprises a lanthanum manganite of formula La.sub.1xSr.sub.xMO.sub.3 with M (transition metals)=Ni, Fe, Co, Mn, Cr, alone or as a mixture, or materials of lamellar structure such as lanthanide nickelates of formula Ln.sub.2NiO.sub.4 (Ln=La, Nd, Pr), or another electrically conductive perovskite oxide.

25. The process as claimed in claim 15, wherein the thick metallic layer comprises nickel (Ni) or an alloy thereof or a chromia-forming alloy whose base element is iron (Fe).

26. The process as claimed in claim 15, wherein the thickness of the ceramic or metallic layer is between 60 and 500 m.

27. The process as claimed in claim 15, wherein the chromia-forming metal alloy of the substrate is chosen from ferritic (FeCr), austenitic (NiFeCr) stainless-steel alloys or superalloys based on nickel forming at the surface a layer of chromium oxide Cr.sub.2O.sub.3, known as the chromia layer.

28. The process as claimed in claim 15, wherein the substrate consists of at least one thin sheet.

29. The process as claimed in claim 28, wherein the thickness of a thin sheet is between 0.1 and 1 mm.

30. The process as claimed in claim 15, wherein the substrate consists of a single plate with flat main faces.

31. The process as claimed in claim 30, wherein the thickness of the plate is between 1 and 10 mm.

32. The process as claimed in claim 15, wherein the width of the channels is between 0.15 and 5 mm.

33. The process as claimed in claim 15, wherein the depth of the channels is between 0.1 and 0.5 mm.

Description

DESCRIPTION OF THE FIGURES

[0070] Other advantages and characteristics of the invention will emerge more clearly on reading the detailed description of implementation examples of the invention given as nonlimiting illustrations in reference to the following figures, among which:

[0071] FIG. 1 is a schematic front view of an interconnecting plate of an HTE electrolyzer according to the prior art,

[0072] FIG. 1A is a detailed view in cross section of an interconnecting plate according to FIG. 1,

[0073] FIG. 1B is a view similar to that of FIG. 1A showing the current lines passing through the plate,

[0074] FIG. 2 is a schematic front view of another interconnecting plate of an electrolyzer according to the prior art,

[0075] FIG. 3 is a photographic reproduction of a plate according to FIG. 1, obtained by mechanical machining,

[0076] FIG. 4 is a photographic reproduction of a plate according to FIG. 1, obtained by drawing,

[0077] FIG. 5 is an exploded schematic view of part of a high-temperature electrolyzer comprising interconnectors according to the prior art,

[0078] FIG. 6 is an exploded schematic view of part of an SOFC fuel cell comprising interconnectors according to the prior art,

[0079] FIG. 7 is a schematic view in cross section of an interconnector coated according to the invention,

[0080] FIG. 8 is a photographic reproduction of an example of a thick grooved ceramic layer hot-bonded onto a substrate made of ferritic alloy in accordance with the invention,

[0081] FIG. 8A is a schematic view in cross section of FIG. 8 showing the considered dimensions of the channels of the thick ceramic layer,

[0082] FIG. 9 shows the representative curves of the geometry of the grooves (furrows) obtained by CO.sub.2 laser ablation on a thick ceramic layer,

[0083] FIG. 10 shows the variation in profile of the grooves shown in FIG. 9 before and after testing under a compression load and at a temperature of 800 C.,

[0084] FIGS. 11A and 11B are photographic reproductions of a thick ceramic layer before and after testing under a compression load and at a temperature of 800 C.,

[0085] FIG. 12 illustrates the curves of serial resistance measurement of various thick ceramic layers according to the invention and, for comparative purposes, of gold grates, the gold grate and layers being in contact with a substrate made of metal alloy.

DETAILED DESCRIPTION

[0086] FIGS. 1 to 4 have already been commented on in the preamble. They are therefore not described in detail hereinbelow.

[0087] FIG. 5 represents an exploded view of elementary units of a high-temperature steam electrolyzer according to the prior art. This HTE electrolyzer comprises a plurality of elementary electrolysis cells C1, C2 . . . stacked alternatively with interconnectors 8. Each cell C1, C2, . . . consists of a cathode 2.1, 2.2, . . . and of an anode 4.1, 4.2, between which is placed an electrolyte 6.1, 6.2, . . . . The symbols and arrows indicating the path of steam, of dihydrogen and of oxygen, of the current are shown in this FIG. 5 for reasons of clarity.

[0088] In an HTE electrolyzer, an interconnector 8 is a component made of metal alloy which provides the separation between the anode 7 and cathode 9 compartments, defined by the volumes between the interconnector 8 and the adjacent anode 4.2 and between the interconnector 8 and the adjacent cathode 2.1, respectively. They also ensure the distribution of the gases to the cells. The injection of steam in each elementary unit takes place in the cathode compartment 9. The collection of the hydrogen produced and of the residual steam at the cathode 2.1, 2.2 is performed in the cathode compartment 9 downstream of the cell C1, C2 after dissociation of the steam therefrom. The collection of the oxygen produced at the anode 4.2 is performed in the anode compartment 7 downstream of the cell C1, C2 after dissociation of the steam therefrom.

[0089] The interconnector 8 ensures the passage of the current between the cells C1 and C2 by direct contact with the adjacent electrodes, i.e. between the anode 4.2 and the cathode 2.1 (FIG. 1).

[0090] FIG. 6 represents the same elementary units as those of FIG. 5, but for an SOFC fuel cell with elementary cells C1, C2 and the interconnectors 8. The symbols and arrows indicating the path of air, of dihydrogen and of oxygen, of the current are shown in this FIG. 6 for reasons of clarity.

[0091] The injection of air containing oxygen into each elementary unit takes place in the cathode compartment 9. The collection of the water produced at the cathode 2.1, 2.2 is performed in the cathode compartment 9 downstream of the cell C1, C2 after recombination of the water by the latter with the hydrogen H.sub.2 injected at the anode 4.2 is performed in the anode compartment 7 upstream of the cell C1, C2. The current produced during the recombination of the water is collected by the interconnectors 8.

[0092] According to the prior art, these interconnectors 8 are usually prepared by mechanical machining of thick plates or by using thin metal sheets, typically from 0.5 to 2 mm, drawn and then assembled by laser welding. The material and machining costs are high. The production technique has the advantage of limiting the cost of starting material, but does not make it possible to achieve a channel fineness as high as that by machining. Specifically, the production possibilities for the depth of the channels, the unit tooth width and the pitch between teeth are limited. Furthermore, the cost of the drawing tooling necessitates large-scale production. In addition, the electrical contact between the electrodes and the interconnector is not entirely satisfactory in particular due to the lack of planarity of the electrodes.

[0093] Thus, to simplify the techniques for producing interconnectors for SOFC fuel cells or HTE electrolyzers and to make them less expensive, the inventors propose a novel type of interconnector 8, an example of which is represented in FIG. 7.

[0094] The component 8 constituting the novel interconnector according to the invention comprises a substrate 82 made of metal alloy, the base element of which is iron (Fe) or nickel (Ni), the substrate having two main flat faces, one of the main flat faces being coated with a coating comprising a thick ceramic layer 80 and the other of the main flat faces being coated with a thick metallic layer 81, each of the thick layers being grooved, delimiting channels 800, 810 suitable for the distribution and/or collection of gases, such as H.sub.2O steam, draining gas, air, O.sub.2, H.sub.2.

[0095] Optionally, a thin protective ceramic layer 83 may be intercalated between the thick ceramic layer 80 and the substrate 82.

[0096] When functioning in a HTE electrolyzer or an SOFC fuel cell, the working conditions are the same as those conventionally used: the circulation of a reducing gaseous mixture is performed in the channels 810 of the thick metallic layer 81 and that of an oxidizing gaseous mixture takes place in the channels 800 of the thick ceramic layer 80.

[0097] The various steps in the production of an example of a thick ceramic layer 80 with its channels 800 and various tests proving the possibility of its use in the targeted applications, i.e. SOFC fuel cells and HTE electrolyzers, are described below.

[0098] The example below is performed starting with a substrate 82 consisting of a single thin metal sheet made of commercial ferritic alloy of the CROFER 22 APU type.

Step 1/: Manufacture of a Crude LSM Strip

[0099] A mixture is prepared between a compound with a weight of 60 g of lanthanum manganite of formula La.sub.0.8Sr.sub.0.2MnO.sub.3 with 0.8% by weight of oleic acid as dispersant, 15.7% of 2-butanone and 15.7% of ethanol as solvents.

[0100] The mixture is milled in a planetary mill. The operating cycle of the planetary mill is as follows: [0101] spin speed: 400 rpm; [0102] duration: 1 hour.

[0103] A weight of 3.2 g of polyvinyl butyral (PVB 90) and 5.5 g of polyethylene glycol (PEG 400) as solvent are then added to the milled mixture, and the whole is then mixed using a planetary mill. The operating cycle of the planetary mill is as follows: [0104] spin speed: 200 rpm; [0105] duration: 10 hours.

[0106] The mixture is then deaerated using a mixer of roll type. The operating cycle of the roll mixer is as follows: [0107] spin speed: 20 rpm; [0108] duration: 24 hours.

[0109] The suspension obtained after deaeration is then poured in a strip using a doctor blade. The active height of the blade is equal to 1000 m. The pouring speed is equal to 1.5 m/min. The pouring is performed onto a sheet of silicone-treated polymer (polyester) so as to promote the detachment of the strip once dried.

[0110] Next, drying of the crude strip obtained by pouring is performed, in ambient air for a duration of 3 hours.

[0111] The dried crude strip of LSM is finally chopped to the sizes corresponding to an air electrode in an SOFC cell, against which the strip is intended to bear. The cutting may be performed, for example, using a laser cutting table.

Step 2/: Hot-Pressing

[0112] The crude strip of LSM is then placed on a thin sheet of ferritic steel 1.5 mm thick and is then welded thereto by hot-pressing using a press. The thickness of the crude strip of LSM is 325 m. In the case where the thin protective ceramic layer 83 is added, the process is performed in an identical manner.

[0113] The operating cycle of the press is as follows: [0114] pressing force: 1 kg/mm.sup.2; [0115] pressing duration: 2 hours; [0116] regulated temperature of the two press plates: 80 C.

[0117] After cooling to room temperature, the assembly prepared between the crude strip of LSM and the thin sheet of ferritic steel is removed from the press.

Step 3/: Production of the Grooves

[0118] Grooving is performed by laser ablation of the crude strip of LSM. The ablation is performed using a flatbed plotter equipped with a CO.sub.2 laser of variable power up to a maximum power of 50 watts. The speed of movement of the laser is also variable, up to a maximum speed of 2 cm/s. The use of such a machine is particularly advantageous since it makes it possible by means of its variable operating characteristics to burn, i.e. to perform abrasion, more or less deeply the polymers constituting the crude strip, which thus releases the associated charge, the LSM. More or less deep grooves (furrows) may thus be dug. Where appropriate, several passes of the CO.sub.2 laser over the crude strip may be performed to increase the depth and/or width of the grooves to a greater or lesser extent.

[0119] FIG. 8 is a photographic reproduction of a thick layer of LSM grooved and hot-bonded onto a substrate consisting of a single sheet of CROFER 22 APU according to the invention. The photographed layer has, for example, a surface area of 100 cm.sup.2.

[0120] FIG. 9 shows the representative curves of the geometry of grooves (furrows), i.e. their height and their width, derived from a number, respectively, of two and four passes of the CO.sub.2 laser. Thus, the widths L1 of the teeth 801 obtained may be reduced to 150 m and those L2 of the channels 800 may be reduced to 150 m. It goes without saying that the widths L1, L2 may be greater than 150 m.

[0121] It is pointed out here that the height zero corresponds to the interface with the thin sheet of ferritic steel and that each of the geometries was obtained by adjusting the speed of movement of the laser to a value equal to 40% of the maximum speed indicated above and the power to a value equal to 50% of the maximum power indicated above (50 W).

[0122] During its use, i.e. when it constitutes an interconnector in a stack either of a high-temperature electrolysis (HTE) reactor or of an SOFC fuel cell, the metal component coated with the thick layer of LSM obtained in the example described above is placed under a compression load in the stack so as to ensure the electrical contact with the other elements of the stack and in particular the cathode of an SOFC cell. Moreover, under HTE conditions, it may be subjected to high temperatures of between 600 C. and 900 C.

[0123] To confirm the good thermomechanical behavior under a compression force, a test was performed according to which the LSM layer was subjected to a temperature of 800 C. and to a load of 0.2 MPa.

[0124] FIG. 10 shows the representative curves of the geometry of grooves (furrows) before and after the 0.2 MPa load. From this figure, it emerges that the crude layer of LSM is crushed by only about 15 m. This proves that the crude layer of LSM can be perfectly adapted mechanically to an electrochemical cell of an HTE reactor or SOFC fuel cell, under the high working temperature conditions. In other words, the crude nature of the LSM layer makes it possible to have a good thermomechanical behavior which implies a good mechanical contact with an electrochemical cell despite the possible surface imperfections of the latter.

[0125] FIGS. 11A and 11B are three-dimensional representations of the LSM layer before and after, respectively, the test at 800 C. under a 0.2 MPa load. It is clearly seen that the crude layer of LSM remains integral after use due to its sufficiently high mechanical strength.

[0126] In addition, measurements of the serial resistance with the component made of metal alloy were performed to characterize the electrical conductivity of the LSM layer obtained under representative operating conditions simulating the entry of cathode compartments 9 of an SOFC cell. The measuring method used is the four-point method as explained in publication [8].

[0127] To do this, several samples were prepared all from a stainless-steel component, the samples according to the invention Nos. 1 to 4 consisting of an assembly of the component with an LSM layer by hot-pressing, the comparative samples Nos. 5 to 6 consisting of an assembly of the component with a gold grate.

[0128] It is pointed out that in sample No. 1, the LSM layer lacks channels, whereas in samples Nos. 2 to 4, the LSM layer is grooved defining identical channels with a unit width L1 equal to 1 mm, two adjacent channels being spaced by a tooth or rib of unit width L2 equal to 0.25 mm.

[0129] FIG. 12 shows the results of measurement of the serial resistance of the various samples as a function of time. It is pointed out here that the measuring points between about 7 hours and 10 hours have not been reported in FIG. 12. The thickness of the LSM layers in samples Nos. 1 to 4 is 325 m, whereas the thickness of the grates in samples Nos. 5 and 6 is 500 m.

[0130] From these measurements, it is deduced that the serial resistances are virtually identical between all the samples Nos. 1 to 6. It may therefore be concluded that the contact resistance of a thick LSM layer according to the invention is close or even equal to that of a gold grate, which is the mechanical element known to date as having the lowest contact resistance. In other words, an LSM layer according to the invention has a negligible electrical contact resistance, of less than 10 m.Math.cm.sup.2.

[0131] Other measuring tests were performed beyond 24 hours, but with a stack of electrochemical cells, of interconnectors according to the invention and of contacts. These tests were satisfactory.

[0132] The production of a thick metallic layer according to the invention on the face of a metal substrate opposite that comprising the thick ceramic layer may be performed in a similar manner to that which has been described, i.e. with pouring in a strip, followed by hot-pressing and production of grooves by ablation using a CO.sub.2 laser.

[0133] The invention is not limited to the examples that have just been described; in particular, characteristics of the examples illustrated may be combined within variants not shown.

REFERENCES

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