Method of fabricating contact elements in an electrochemical device such as SOFC or EHT

Abstract

The invention relates to a method of fabricating a contact element in an electrochemical device (9) such as an SOFC or an EHT which comprises the following steps: a) use is made of: at least one cell (8) consisting of an assemblage made up of an electrode to be hydrogenated (5)-electrolyte (4)-electrode to be oxygenated (3); at least one first interconnector (1); and at least one second interconnector (7); b) at least one layer of a conducting material is deposited on the first interconnector (1) and/or the second interconnector (7); c) an electrochemical device (9) is assembled; said method being characterized in that: d) a thermomechanical treatment is carried out on the electrochemical device obtained on completion of step c). The invention also relates to an electrochemical device (9) equipped with at least one contact element (2) obtained according to this fabrication method.

Claims

1. A method for manufacturing a contact element in an electrochemical device which comprises the following steps: a) there are disposed: at least one cell comprising a hydrogen electrode-electrolyte-oxygen electrode assembly; at least a first interconnector; at least a second interconnector; b) at least one layer of a conductive material is deposited over the first interconnector and/or the second interconnector; c) an electrochemical device is assembled by disposing the cell between the first interconnector and the second interconnector so that the conductive material layer is in contact with the oxygen electrode and/or the hydrogen electrode of the cell; and d) a thermo-mechanical treatment is carried out on the electrochemical device obtained at the end of step c) so as to form at least one contact element constituted of said conductive material and which ensures electrical contact and mechanical accommodation between said interconnectors and said electrodes, the thermo-mechanical treatment comprising concomitantly subjecting the electrochemical device to a temperature comprised between 850 C. and 1200 C. and applying thereto a mechanical stress comprised between 0.01 and 10 MPa, wherein at the end of the assembly step c) and prior to the step d) of thermo-mechanical treatment, the electrochemical device is heated to a nominal operating temperature comprised between about 600 C. and 900 C.

2. The manufacturing method according to claim 1, wherein the conductive material exhibits, at least for a period of time during the thermo-mechanical treatment of step d), a porosity comprised between 30% and 80%.

3. The manufacturing method according to claim 1, wherein, in step b), at least one conductive material layer is deposited over the first interconnector and in step c), an electrochemical device is constituted by disposing the cell between the first interconnector and the second interconnector so that the conductive material layer is in contact with the oxygen electrode of the cell.

4. The manufacturing method according to claim 1, wherein the conductive material has an electrical conductivity of at least 0.1 S.Math.cm.sup.1 under air at 800 C.

5. The manufacturing method according to claim 4, wherein the conductive material is selected from the group consisting of: La.sub.0.6Sr.sub.0.4Co.sub.0.8Fe.sub.0.2O.sub.3 (LSCF); La.sub.0.8Sr.sub.0.2Cu.sub.0.9Fe.sub.0.1O.sub.2.5 (LSCuF); La.sub.0.7Sr.sub.0.3CoO.sub.3 (LSC); Sm.sub.0.5Sr.sub.0.5CoO.sub.3 (SSC); SmBa.sub.0.5Sr.sub.0.5Co.sub.2O.sub.5 (SBSC); GdSrCo.sub.2O.sub.5 (GSC); La.sub.0.65Sr.sub.0.3MnO.sub.3 (LSM); LaBaCo.sub.2O.sub.5 (LBC); YBaCo.sub.2O.sub.5 (YBC); Nd.sub.1.8Ce.sub.0.2CuO.sub.4 (NCC); La.sub.0.8Sr.sub.0.2Co.sub.0.3Mn.sub.0.1Fe.sub.0.6O.sub.3 (LSCMF); La.sub.0.98Ni.sub.0.6Fe.sub.0.4O.sub.3 (LNF); La.sub.1.2Sr.sub.0.8NiO.sub.4 (LSN); La.sub.0.7Sr.sub.0.3FeO.sub.3 (LSF); and La.sub.2Ni.sub.0.6Cu.sub.0.4O.sub.4 (LNC).

6. The manufacturing method according to claim 1, wherein the at least one layer of the conductive material has a porosity comprised between 30% and 80%.

7. The manufacturing method according to claim 1, wherein the at least one layer of the conductive material comprises at least one pore-forming agent.

8. The manufacturing method according to claim 1, wherein the range of heating and/or cooling rate during the step d) of thermo-mechanical treatment is comprised between 0.5 C./min and 500 C./min.

9. The manufacturing method according to claim 1, wherein the range of the mechanical stress applied during the step d) of thermo-mechanical treatment is comprised between 0.05 MPa and 5 MPa.

10. An electrochemical device, equipped with at least one contact element obtained with the manufacturing method according to claim 1.

11. The electrochemical device according to claim 10, wherein the device comprises a SOFC or a HTE.

12. The manufacturing method according to claim 1, wherein the electrochemical device comprises a SOFC or a HTE.

13. The manufacturing method according to claim 1, wherein the nominal operating temperature is between about 600 C. and about 800 C.

14. The manufacturing method according to claim 1, wherein at the end of the step d) of thermo-mechanical treatment, the temperature of the electrochemical device is lowered to the nominal operating temperature of the electrochemical device comprised between about 600 C. and 900 C.

15. The manufacturing method according to claim 14, wherein the nominal operating temperature is between about 600 C. and about 800 C.

16. The manufacturing method according to claim 14, wherein before lowering the temperature of the electrochemical device, the mechanical stress is decreased at most 90%.

17. The manufacturing method according to claim 16, wherein concomitantly subjecting the electrochemical device to the temperature comprised between 850 C. and 1200 C. and applying thereto the mechanical stress comprised between 0.01 and 10 MPa comprises concomitantly subjecting the electrochemical device to a first temperature between 850 C. and 1200 C. and applying thereto the mechanical stress comprised between 0.01 and 10 MPa; and concomitantly subjecting the electrochemical device to a second temperature between 850 C. and 1200 C. and applying thereto the mechanical stress comprised between 0.01 and 10 MPa, the second temperature being less than the first temperature.

18. The manufacturing method according to claim 17, wherein before lowering the temperature of the electrochemical device, the mechanical stress is not decreased.

19. The manufacturing method according to claim 17, wherein at the end of the assembly step c) and prior to the step d) of thermo-mechanical treatment, the electrochemical device is heated to the nominal operating temperature comprised between about 600 C. and 900 C. while concomitantly subjecting the electrochemical device to the mechanical stress comprised between 0.01 and 10 MPa.

20. The manufacturing method according to claim 14, wherein before lowering the temperature of the electrochemical device, the mechanical stress is not decreased.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic longitudinal sectional view of an electrochemical device equipped with contact elements obtained according to the manufacturing method of the invention.

(2) FIG. 2 is a graph of the contact resistance as a function of time, of a deposit of 100 m of porous LSM under the effect of a mechanical stress of 0.1 MPa.

(3) FIG. 3 is a graph of the contact resistance as a function of time, of a deposit of 100 m of porous LSM whether under the effect or not of a mechanical stress of 0.1 MPa.

DETAILED DESCRIPTION

(4) In FIG. 1, there is schematically represented an electrochemical device 9 within the meaning of the present invention (namely it may consist of a SOFC or a HTE) which comprises: three cells 8 each comprising an oxygen electrode 3-electrolyte 4-hydrogen electrode 5 assembly; three first interconnectors 1; three second interconnectors 7; three contact elements 6 in form of nickel grids; three contact elements 2 obtained according to the method for manufacturing contact elements according to the invention.

(5) Experimental Part:

(6) Experiment No 1:

(7) There were provided: a cell comprising an oxygen electrode-electrolyte-oxygen electrode assembly. This type of cell allows obtaining a symmetrical configuration for the electrochemical system and simplifying the study of the contact resistances of the oxygen electrode. a first metallic interconnector; a second metallic interconnector.

(8) The electrodes were LSM electrodes, the electrolyte was a zirconia doped with 3% of yttrium and the interconnectors were made of Crofer.

(9) There were deposited by screen printing four layers of porous LSM with a porosity in the order of 50% having a total thickness of 100 m over each one of the two metallic interconnectors.

(10) An electrochemical device was constituted by disposing the cell between the first metallic interconnector and the second metallic interconnector so that the porous LSM deposit is in contact with the oxygen electrodes of the symmetrical cell.

(11) The following operations were carried out successively:

(12) 1) The device was heated up to 800 C., and this with a temperature gradient of 5 C./min with a mechanical stress of 0.1 MPa.

(13) 2) Next, the temperature was maintained at 800 C. for a time period of about 15 hours, and this while maintaining the mechanical stress of 0.1 MPa.

(14) 3) Next, a thermo-mechanical treatment was carried out on the electrochemical device as follows: A mechanical stress of 0.1 MPa was applied while increasing the temperature from 800 C. to 920 C., with a heating rate of 1 C..Math.min.sup.1; The temperature of 920 C. and the mechanical stress of 0.1 MPa were maintained for half an hour; The temperature was decreased from 920 C. to 860 C. with a cooling rate of 1 C..Math.min.sup.1 while maintaining the mechanical stress of 0.1 MPa; The temperature of 860 C. and the mechanical stress of 0.1 MPa were maintained for one hour. Finally, the temperature was decreased from 860 C. to 800 C. with a cooling rate of 1 C..Math.min.sup.1 while maintaining a mechanical stress of 0.1 MPa.

(15) The thermo-mechanical treatment lasted about 6 hours. In addition, this step 3) corresponded to a thermo-mechanical treatment as implemented in the method for manufacturing contact elements according to the invention.

(16) 4) At the end of this thermo-mechanical treatment, the electrochemical device was maintained at a temperature of 800 C. while maintaining the application of a mechanical stress of 0.1 MPa for about 3 hours.

(17) During these steps 1) to 4), the contact resistance of the porous LSM layer was constantly calculated from the high-frequency resistance measured by performing an electrochemical impedance spectroscopy.

(18) FIG. 2 is a graph expressing the thus measured contact resistance as a function of time.

(19) In this graph, the time interval during which the electrochemical device has been subjected to the thermo-mechanical treatment detailed above appears in the form of a shaded area.

(20) In view of the graph of FIG. 2, the following points are raised: The contact resistance decreased from 300 mOhm.Math.cm.sup.2 to 112 mOhm.Math.cm.sup.2 when the electrochemical device was maintained at 800 C. and under a mechanical stress of 0.1 MPa. The contact resistance was already almost constant after about ten hours. The contact resistance then decreased significantly during the thermo-mechanical treatment passing from a value of 112 mOhm.Math.cm.sup.2 to only 15 mOhm.Math.cm.sup.2.

(21) Thus, the thermo-mechanical treatment had the effect of significantly lowering the contact resistance from a value of about 100 mOhm.Math.cm.sup.2 to a value of about 15 mOhm.Math.cm.sup.2. Such a value of 15 mOhm.Math.cm.sup.2 would never have been reached without this thermo-mechanical treatment, because as explained above, at 800 C. and under the effect of a mechanical stress of 0.1 MPa, the contact resistance has become almost stationary after ten hours with a value in the order of 100 mOhm.Math.cm.sup.2.

(22) Moreover, in the graph of FIG. 2, it is noticed that by the end of the thermo-mechanical treatment, the contact resistance slightly fluctuates, then stabilizes at a value of 15 mOhm.Math.cm.sup.2. These slight fluctuations may be explained by the fluctuations of temperatures during the thermal stabilization after this thermo-mechanical treatment.

(23) Experiment No 2:

(24) There was provided an experimental device identical to that used for Experiment no 1.

(25) The following operations were carried out successively:

(26) 1) In 3 hours, the temperature of the electrochemical device was increased from a temperature of 20 C. to 800 C., and this without applying mechanical stress.

(27) 2) The electrochemical device was maintained at 800 C. for 15 hours, and this without applying mechanical stress.

(28) 3) The electrochemical device was subjected to the following temperature cycle, and this without applying mechanical stress: The temperature was increased from 800 C. to 920 C. in 2 hours; The temperature of 920 C. was maintained for half an hour; The temperature was decreased from 920 C. to 860 C. in 1 hour; The temperature of 860 C. was maintained for 1 hour; Finally, the temperature was decreased from 860 C. to 800 C. in 1 hour.

(29) 4) The electrochemical device was maintained at a temperature of 800 C. for one hour.

(30) 5) The electrochemical device was maintained at a temperature of 800 C. while concomitantly applying a mechanical stress of 0.1 MPa, and this for 8 hours.

(31) 6) While maintaining the mechanical stress of 0.1 MPa, the electrochemical device was subjected to the following temperature cycle: The temperature was increased from 800 C. to 920 C. in 2 hours; The temperature of 920 C. was maintained for half an hour; The temperature was decreased from 920 C. to 860 C. in 1 hour; The temperature of 860 C. was maintained for 1 hour; Finally, the temperature was decreased from 860 C. to 800 C. in 1 hour.

(32) In other words, this step 6) corresponded to a thermo-mechanical treatment as implemented in the method for manufacturing contact elements according to the invention.

(33) 7) While maintaining the mechanical stress of 0.1 MPa, the electrochemical device was subjected to a temperature of 800 C., and this for further 14 hours.

(34) The contact resistance was measured as soon as a mechanical stress of 0.1 MPa has been applied on the electrochemical device.

(35) In the graph of FIG. 3, the following elements are represented: The evolution of the temperature as a function of time during steps 1) to 7) as detailed above. The measured contact resistance as a function of time measured as soon as a mechanical stress has been applied on the electrochemical device (namely from step 5).

(36) According to the graph of FIG. 3, the following points are raised:

(37) The 1.sup.st contact resistance measurement which was taken as soon as a mechanical stress of 0.1 MPa is applied (hence at the beginning of step 5) when the temperature is of 800 C. has a value in the order of 312 mOhm.Math.cm.sup.2.

(38) After 8 hours, while maintaining a temperature of 800 C. and a mechanical stress of 0.1 MPa, the contact resistance decreases by 312 mOhm.Math.cm.sup.2 to stabilize at a value in the order of 220 mOhm.Math.cm.sup.2.

(39) This value of 220 mOhm.Math.cm.sup.2 is greater than the value of 112 mOhm.Math.cm.sup.2 which has been measured in Experiment no 1.

(40) Thus, the thermal treatment carried out in steps 3) and 4), hence prior to step 5) had detrimental consequences: the value of the contact resistance obtained after 8 hours while applying a mechanical stress of 0.1 MPa is about two times greater as if there were no this thermal treatment beforehand (the case of Experiment no 1).

(41) Moreover, when the thermo-mechanical treatment of step 6) was carried out on the electrochemical device, the value of the contact resistance obtained at the end of this treatment is in the order of 86 mOhm.Math.cm.sup.2. It is also much greater than that obtained at the end of the thermo-mechanical treatment of Experiment no 1.

(42) In other words, the thermo-mechanical treatment performed in step 6) did not allow compensating the consequences of the thermal treatment carried out beforehand on the measurement of the contact resistance.

(43) Thus, it appears from the results obtained during these two experiments that there are the coupled effects of the temperature and of the application of a mechanical stress that affect the value of the contact resistance of a contact element produced from LSM.

(44) In other words, it is thanks to a thermo-mechanical treatment that the LSM layer has been correctly deformed so that its contact resistance is as small as possible, which is the desired effect, because characteristic of a quality contact element.