Method of preparing an electrochemical half-cell

Abstract

The present invention relates to a method for preparing an electrode-supported electrochemical half-cell including a step consisting in subjecting a green electrode layer on which a precursor gel of the electrolyte or a precursor thereof is deposited to sintering at a temperature of less than or equal to 1350° C.

Claims

1. A method for preparing an electrode-supported electrochemical half-cell including a single step of sintering, said step consisting of subjecting a green electrode layer on which a precursor gel of an electrolyte or a sol precursor of said gel is deposited to sintering at a temperature of less than or equal to 1350° C., said gel comprising a solid network wherein metallic cations are uniformly distributed in an organic matrix, and said sol precursor of said gel comprising a colloidal or polymer suspension wherein the metallic cations contribute to forming a colloidal or polymer network, and wherein said green electrode layer is obtained from a ceramic ink corresponding to a dispersion (or suspension) of powders of the electrode material in an aqueous or organic solvent.

2. A method according to claim 1, wherein said powder is a powder of a material chosen from among perovskite materials of structure ABO.sub.3, where A represents a rare earth and B a transition metal; stabilised zirconias, optionally blended with NiO; substituted cerias, optionally blended with NiO, and blends thereof.

3. A method according to claim 1, wherein said ceramic ink includes at least one element chosen from among a solvent, a dispersing agent, a binder, a plasticiser and a pore-forming agent.

4. A method according to claim 1, wherein the gel or its sol precursor is prepared by a sol-gel process, by a polymer process derived from the Péchini method, or by an NPG (nitrate polyacrylamide gel) synthesis process.

5. A method according to claim 4, wherein the gel or its sol precursor is prepared by a sol-gel process and said sol-gel process uses at least one organometallic precursor.

6. A method according to claim 5, wherein R has 1 to 10 carbon atoms.

7. A method according claim 1, wherein the sintering temperature is between 1000 and 1350° C., and wherein the green electrode layer on which a precursor gel of the electrolyte or a sol precursor of said gel is deposited is maintained at the sintering temperature for a period of between 1 and 5 h.

8. A method according to claim 7, wherein the sintering temperature is between 1000 and 1300° C.

9. A method according to claim 1, wherein before being brought to the sintering temperature, the green electrode layer on which a precursor gel of the electrolyte or a sol precursor of said gel is brought to an intermediate temperature of between 400 and 800° C. and wherein it is maintained at this intermediate temperature for a period of between 15 and 120 min.

10. A method according to claim 9, wherein said green electrode layer on which a precursor gel of the electrode or a sol precursor of said gel is deposited is brought to an intermediate temperature by means of a slow rising temperature gradient of between 10 and 50° C./h[L].

11. A method according to claim 10, wherein the temperature gradient is between 15 and 40° C./h.

12. A method according to claim 9, wherein said green electrode layer on which a precursor gel of the electrode or a sol precursor of said gel is deposited is brought from the intermediate temperature to the sintering temperature by means of a rising temperature gradient of between 25 and 100° C./h.

13. A method according to claim 9, wherein the intermediate temperature is between 500 and 700° C.

14. A method according to claim 1, including the steps consisting of: a) preparing the green electrode layer; b) depositing, on said green electrode layer, the precursor gel of the electrolyte or the sol precursor of said gel and c) subjecting said green electrode layer on which said precursor gel of the electrolyte or the sol precursor of said gel is deposited to a sintering temperature of less than or equal to 1350° C.

15. A method according to claim 14, wherein, in step (a), the green electrode layer is obtained by deposition of a ceramic ink corresponding to a dispersion (or suspension) of powders of the electrode material in an aqueous or organic solvent by tape casting.

16. A method according to claim 14, wherein, in step (b), the deposition is accomplished by screen printing.

17. A method according to claim 14, wherein, following step (c), the sintered material is subjected to a reduction.

18. A method according to claim 17, wherein the electrode-supported electrochemical half-cell obtained following the step of reduction has a porous support electrode and a thin, dense electrolyte.

19. A method according to claim 1, wherein the electrode-supported electrochemical half-cell obtained following the step of sintering has a porous support electrode and a thin, dense electrolyte.

20. A method according to claim 5, wherein the at least one organometallic precursor comprises at least one metallic alkoxide selected from the group consisting of zirconium methoxide, zirconium ethoxide, zirconium n-propoxide, zirconium butoxide, zirconium pentoxide, cerium methoxide, cerium ethoxide, cerium n-propoxide, cerium butoxide and cerium penoxide.

Description

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

(1) FIG. 1 is a micrograph obtained by optical microscopy of a green electrode tape as implemented in the present invention.

(2) FIG. 2 shows two micrographs in secondary electron mode of the fracture surface of the electrochemical half-cell (NiO/8YSZ//8YSZ) heat treated at 1200° C. for 3 h in accordance with the method of the invention. FIGS. 2A and 2B are micrographs of the same fracture surface, taken at two different enlargements.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(3) I. Preparation of a Green Tape of Composite NiO/8YSZ.

(4) Production of a electrode-supported electrochemical cell according to the invention involves, initially, the production of a green tape of composite NiO/8YSZ. It is produced by tape casting of a slurry.

(5) This suspension consists of a powdery blend of 60% by mass of NiO and 40% of 8YSZ constituting 69% of the final mass of the slurry, of CP213 constituting 0.6% by mass, of a blend of MEK and of ethanol constituting 23% by mass, of PVB constituting 3.4% by mass and of PEG constituting 4% by mass.

(6) When the tape has been produced and dried for 24 h at 20° C. it is cut into the required electrode shapes and dimensions, taking account of shrinkage during sintering.

(7) II. Preparation of Electrolyte 8YSZ.

(8) In the case of an electrolyte 8YSZ, a sol obtained from a system involving zirconium n-propoxide, yttrium nitrate, acetylacetone (acac.), n-propanol and water was used. The synthesis parameters chosen for the sol are: concentration: C=[Zr]=0.5 mol.Math.L.sup.−1 complexing rate R′

(9) where R′=[acac.]/([Zr]+[Y])=0.7 hydrolysis rate W′

(10) where W′=[H.sub.2O]/([Zr]+[Y])=10.

(11) The yttrium precursor was dissolved in 1-propanol at a concentration of 1 mol.Math.L.sup.−1. The zirconium n-propoxide is introduced in the 1-propanol/acac blend whilst it is being stirred. Yttrium nitrate dissolved in 1-propanol and water is then added.

(12) Under these synthesis conditions, the sol consists of colloidal particles created at the start of the hydrolysis and condensation reactions. The elementary particles, moving randomly, aggregate when they come into contact with one another. This mechanism continues and leads to the formation of objects (aggregates), which themselves diffuse and aggregate. This is the step of gelification or sol-gel transition. When this process of aggregation is finished the gel is obtained. It is this which was deposited on the surface of the green tape of NiO/8YSZ (FIG. 1) by means of a screen printing machine.

(13) As shown by the photograph of the surface of the green tape in FIG. 1, the impermeable microstructure of this tape (polymer+powder) enables penetration of the sol or the gel into the final microstructure of the electrode to be prevented.

(14) III. Heat Treatment.

(15) When the gel has been deposited the [tape+gel] system is heat treated to give sufficient mechanical properties to the support electrode and to densify the electrolyte. The thermal cycle used in this example is as follows:
25° C..fwdarw.25° C./h.fwdarw.600° C./1h.fwdarw.50° C./h.fwdarw.1200° C./3h.fwdarw.100° C./h.fwdarw.25° C.

(16) A microstructural observation of the fracture surface of the heat treated surface was accomplished by scanning electron microscopy. These observations are presented in FIG. 2.

(17) In these micrographs one can see in the upper part the dense electrolyte which is ˜5 μm thick, and in the lower part the composite NiO/8YSZ electrode which will lead, by in situ reduction (i.e. when the cell is put into service), to the cermet Ni/8YSZ.

REFERENCES

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