WO2002053798A1

WO2002053798A1 - Method for depositing thin layers on a porous substrate, fuel cell and fuel cell comprising such a thin layer

Info

Publication number: WO2002053798A1
Application number: PCT/FR2001/004102
Authority: WO
Inventors: Michel Cassir; Cécile BERNAY; Daniel Lincot; Fabrice Goubin
Original assignee: Renault S.A.S.
Priority date: 2000-12-28
Filing date: 2001-12-20
Publication date: 2002-07-11
Also published as: EP1356133A1; FR2818993B1; FR2818993A1

Abstract

The invention concerns a method for depositing on the surface of a porous substrate, thin layers of at least a solid ionic conductor comprising at least a base oxide and at least a doping agent. The deposition is carried out from precursor ln of the metal ion of one of the base oxides, precursor II oxidising the precursor In and, precursor IIIm of one of the oxides inputting one of the doping agents; n and m being each an integer not less than 1. Said method consists in providing at least a base oxide in a first sequence, and at least a doping agent in a second sequence. The first sequence is repeated from 1 to 10 times, the second only once; the whole process constitutes a cycle. The deposition is produced for a (first/second) sequence ratio determined on the basis on the (doping agent(s)/basic oxide(s)) atomic ratio and for a number of cycles determined on the basis of the thickness of the thin layer, and finally for a specific contacting duration between precursors and substrate. The invention also concerns a fuel cell, and a cell containing such a thin layer.

Description

PROCESS FOR DEPOSITING THIN FILMS ON A SUBSTRATE

POROUS, FUEL CELL AND CELL

FUEL COMPRISING SUCH A THIN FILM.

The invention relates to a method for depositing thin layers of at least one solid ionic conductor on a porous substrate, to a solid oxide fuel cell cell comprising such an ionic conductor, and to an oxide fuel cell. solid, comprising such a cell, and operating at a temperature below about 800 ° C.

In the field of fuel cells operating at high temperature, solid oxide fuel cells (SOFC) present prospects of yield and interesting power densities.

In addition, operating at temperatures close to those of the reforming process gives the possibility of directly oxidizing carbon onoxide.

Operation with conventional fuels is therefore achievable with simplified reforming compared to fuel cell technologies operating at a temperature below about 200 ° C.

However, an operating temperature between 850 ° C and 1000 ° C remains too high for correct thermal management of the system and obtaining a low cost of materials.

Regarding thermal management, there is, on the one hand, a drop in efficiency during thermal losses with the outside, and, on the other hand, the time required to get the operating temperature is too long.

Also, the desire to lower the operating temperature of SOFCs to a value between 600 ° and 700 ° C. essentially poses the problem of the ohmic drop linked to the electrolyte made of yttria zirconia (YSZ), the conductivity of which is insufficient in this temperature range.

A simple method for reducing this resistance consists in reducing the thickness of the electrolyte so as to obtain a thin layer.

There are already different techniques for producing thin layers of YSZ on a substrate, such as that of sputtering with several variations (see for example: LS WANG and SA BARNETT, Solide State Ionics, 61 (1993) 273), that of sol-gel processes and in particular the dip deposit

(see for example: C. SAKURAI, T. FUKUI and M. OKUYAMA,

J. Am. Ceram. Soc, 76 (4) (1993) 1061) or the deposit by rotation (see for example: H. NAGAMOTO and Z.H. CAI,

Proc 6 ^th Int. Symp. SOFC, 99-19 (1999) 163), that of colloid deposits (see for example: C. WANG, WL WORREL, S. PARK, JM VOHS ET RJ GORTE, Proc 6 ^th Int. Symp. SOFC, 99-19 (1999) 851) or suspensions (see for example: S. DE SOUZA, SJ VISCO and JL DE JONGHE, J. Electrochem. Soc 144 (3) (1997) L35) and 1 electrophoresis (see for example: RN BASϋ , CA RANDA L and MJ MAYO, Proc 6 ^zn Int. Sym. SOFC, 99-19 (1999) 153), that of strip casting (see for example: M.CHEN, TL. WEN, Z. HϋANG, PC. WANG, HY TU and ZY LU, Proc 6 ^th Int. Symp. SOFC, 99-19 (1999) 144), serigraphy, or even that of deposits chemical vapor phase (see for example: US 4, 831, 965 GZ. CAO, H .W. BRINIŒAN, K .J. DE VRIES and A .J. BURGGRAF, J. AM. Ceram. Soc, 76 (9) (1993) 2201).

The main drawback which arises, when producing thin layers on a substrate, relates to their densification rate, which remains insufficient vis-à-vis the passage of certain gases with which they are brought into contact.

Indeed, the electrolyte must be impermeable to the oxidizing gas and to the combustible gas (in particular to hydrogen), and consequently must be as dense as possible.

To reduce the porosity of the substrate in most of the studies already carried out, the deposition of the electrolyte is annealed at high temperature (about 1300 ° - 1500 ° C). However, this sintering step raises a problem due to the difference in coefficient of thermal expansion between the electrolyte in yttria zirconia and the support electrode of the deposit. This expansion gap can cause cracks to form in the deposit layer, making it unusable.

This annealing step can also be used to obtain the cubic phase of the crystalline state known to be the most conductive (see for example: KW CHOϋR, J. CHEN and R. XU, Thin Solid. Films, 304 (1997) 106) . However, such a step is not always sufficient to obtain a correct densification of the deposit of thin layers.

Also, there remains the need for a process, using the atomic layer epitaxy technique (in English "Atomic Layer Epitaxy" or ALE) such as described in documents US 4,058,430, and T. SUNTOLA, Handboo of Crystal Growth, Thin Film and Epitaxy, Elsevier (1994) for directly depositing, atomic layer by atomic layer, thin layers of at least one solid ionic conductor on the surface of a porous substrate, having a very high density thanks to the growth of the deposit by atomic layers, carried out at temperatures below approximately 800 ° C., without obstructing the internal pores of the substrate due to the limited penetration of the reactive gases, all by making a compromise between the ionic conductivity of the electrolyte ¹ and its mechanical strength.

The subject of the invention is therefore a process for depositing on the surface of a porous substrate, atomic layer by atomic layer, thin layers of at least one solid ionic conductor, said ionic conductor consisting of at least one oxide of base and at least one doping agent, all of said thin layers constituting an electrolyte, said deposit being produced from at least one precursor I _n of the metal ion of one of the base oxides, of a precursor II making it possible to produce an oxide from the precursor I _n , and at least one precursor III _m of one of the oxides providing one of the doping agents, said precursors being placed in the vapor state, n and m each being an integer greater than or equal to 1, characterized in that it consists in depositing at least one basic oxide according to at least a first sequence, and at least one doping agent according to at least a second sequence, each first sequence being repeated 1 to 10 times fa successive lesson before, after or in admixture, with or without contact with precursor II, with at least a second sequence, each second sequence being carried out only once, the whole comprising the first and second sequences constituting a cycle, the deposition of the base oxide (s) and of the doping agent (s) being carried out, on the one hand, for a sequence ratio (first / second) defined as a function of the atomic ratio (agent (s) dopant (s) / oxide (s) base) desired in each thin layer, on the other hand for a number of cycles defined according to the thickness of thin layers desired, and finally, for a setting time in contact with a precursor (I _n , IH _m ) with the defined substrate surface.

The ionic conductor, also called an oxide ion conductor, is defined as being a compound of at least one metal oxide doped with at least one other metallic element. The ionic conductor makes it possible to obtain the migration of the O ^2- oxide ions through its own structure.

The method according to the invention has the advantage of allowing a fine and dense deposit of electrolyte, in the form of at least one thin layer, having an average thickness ranging from about 0.1. at 10 μm, and having a shallow penetration depth in the porous substrate contrary to what one might expect by the use of the technique of atomic layer epitaxy.

Another subject of the invention is a solid oxide fuel cell cell comprising at least one solid ionic conductor present in at least one thin layer, and deposited by the process as defined above. A final object of the invention is a solid oxide fuel cell comprising at least one cell as defined above.

Preferably, the precursor I _n is chosen from zirconium hologenides and in particular zirconium chloride, cerium halides and in particular cerium chloride, cerium beta-diketonates and in particular tetrakis (2,2, 6,6- tetramethyl-3-5-heptanedionato) cerium, and their mixture.

The precursor II can be chosen from water, oxygen, ozone or an alcohol, and the precursor III _m can be chosen from: - yttriu halides and in particular yttrium chloride,

- yttrium beta-diketonates and in particular tris (2, 2, 6, 6-tetramethylheptan-3, 5-dionato) yttriu, - scandium halides and in particular scandium chloride, beta-diketonates scandium and in particular tris (2, 2, 6, 6-tetramethyl-3, 5-heptanedionato) scandium, - ytterbium halides and in particular ytterbium chloride, ytterbium beta-diketonates and in particular tris (2, 2, 6, 6-tetramethyl-3, 5-heptanedionato) ytterbium, - gadolinium halides and in particular gadolinium chloride,

- gadolinium beta-diketonates and in particular. tris (2, 2, 6, 6-tetramethyl-3, 5-heptanedionato) gadolinium, and samarium halides and in particular samarium chloride, samarium beta-diketonates and in particular tris, (2, 2, 6, 6-tetramethyl-3, 5-heptanedionato) samarium, and mixtures thereof.

The first sequence can be defined as desired by the successions of the following elements: first succession: a) precursor I _n b) nitrogen c) precursor II d) nitrogen

second succession: a) precursor I _n and I _{(n +} i ₎ b) nitrogen c) precursor II d) nitrogen

and, the second sequence can be defined as desired by the successions of the following elements:

nth succession: a) precursor III _m b) nitrogen c) precursor II d) nitrogen

second succession: a) precursors III _m and III _{(m + 1)} b) nitrogen c) precursor II d) nitrogen The process of the invention can make it possible to deposit at least one basic oxide and at least one doping agent simultaneously, with durations of contact with the surface of the substrate, identical or different for each precursor I _n , II or III _m .

The method can also make it possible to deposit, at first, at least one basic oxide according to a first sequence, then at least one doping agent, according to a second sequence, following the first sequence.

The ionic conductor is preferably chosen from zirconia doped with yttrium, zirconia doped with scandium, zirconia doped with ytterbium, cerine doped with gadolinium, cerine doped with samarium, and their mixtures.

Preferably, the duration during which a precursor in the vapor state is brought into contact with the surface of the substrate ranges from 0.1 to 20 seconds, and more preferably from 0.1 to 5 seconds.

The number of cycles can range from 500 to 50,000, and preferably from 500 to 5,000.

The atomic ratio of the doping agent (s) to the base oxide (s) can range from 2 to 25%.

The thickness of each thin layer can range from 0.1 to 10 μm.

Gradients of compositions in the ionic conductor can moreover be produced by varying the proportion of the sequences of synthesis of the base oxide (s) relative to the sequences of syntheses of the doping agent (s) during the deposition of thin layers.

The porous substrate can be chosen from a porous cathode material or a porous anode material for a solid oxide fuel cell and preferably from ₌ lanthanum anganite doped with strontium (LSM), lanthanum manganite doped with calcium, iron-doped lanthanum manganite, strontium-doped lanthanum cobaltate, and mixtures thereof, for the cathode, and among the nickel and zirconia cermet doped with yttrium (Ni / YSZ), the cermet of nickel and scandium-doped zirconia, nickel and zirconia cermet doped with ytterbium, nickel and cerine cermet doped with gadolinium, nickel and cerine cermet doped with samarium, zirconia stabilized with yttrium doped with l titanium oxide, and mixtures thereof for the anode.

The invention will now be described with the aid of the following examples, which are only given by way of illustration and which do not in any way limit the present invention.

- Figure 1 shows a schematic view, from above, obtained by the scanning electron microscopy technique of a substrate sample covered with thin layers according to the method of the invention, and - Figure 2 shows a schematic view, in perspective, obtained by the scanning electron microscopy technique of a sample of the substrate covered with two thin layers, according to the method of the invention. The reactor of the atomic layer epitaxy technique used is of the alternating flow type (F120 manufactured by ASM-Microche isty Ltd.). Nitrogen is used as the carrier gas.

In the following examples, the following precursors are used:

- ZrCl for zirconium,

- water, and

- Y (thd) ₃ or tris (2, 2, 6, 6-tetramethyl-3, 5-heptadionato) yttrium for yttrium,

The deposits are made on a porous substrate, such as a lanthanum manganite plate doped with strontium (La ₀ , 8, ^s ^ o, 2 ι Mn0 ₃ or LSM) which can be used as a SOFC cathode, or a plate of cermet of nickel and yttria zirconia (Ni / YSZ) which can serve as an anode of SOFC.

The precursors are heated to the following temperatures

. Y (thd) ₃ 110 ° to 150 ° C

. ZrCl ₄ 140 ° to 180 ° C

. water 10 ° to 30 ° C

The substrate is heated from 200 ° to 600 ° C before carrying out the process and is maintained at a temperature ranging from 205 ° to 600 ° C during the carrying out of the process of the invention.

To obtain thin layers of zirconia having the yttrium doping agent, the basic cycle is as follows: • n zirconia synthesis sequences, n being able to vary from 1 to 10. The sequence is defined as follows:

* - * - ZrCl: 1 to 10 seconds

"→ - nitrogen: 1 to 5 seconds * → - water: 0.5 to 10 seconds

* - * - nitrogen: 1 to 5 seconds

• 1 sequence of synthesis of yttrium oxide (dopant), as defined as follows:

^ Y (thd) ₃ 0.5 to 10 seconds ^ nitrogen 0.5 to 10 seconds * - * - water 0.5 to 10 seconds ^• → - nitrogen 0.5 to 10 seconds.

The number of yttrium oxide synthesis sequences relative to the number of zirconia synthesis sequences is adjusted as a function of the concentration of doping agent desired in the thin layer present on the surface of the substrates.

The combination of the “n” zirconia synthesis sequences and the unique yttrium oxide synthesis sequence constitutes a basic cycle for zirconia doped with yttrium.

The thickness of the thin layer depends on the number of cycles performed.

After the temperature rise of the reactor according to a temperature gradient from the heating temperature of the yttrium precursor (110 ° -150 ° C) to the temperature of the substrate (200 ° -600 ° C), the reaction is carried out between approximately 500 and 50,000 deposition cycles. Fyetnpl e 1: Preparing an on layer for

7, i rcone (env i on 1 7. um thickness i sseur) doped, with Y_ 0- ₃ _. present in a, a-Lian proportion of 2__ to 25 atomic%, on an ia ... e 8M. porous (2,5 x, .5cτn and 5 x

5om - pore size 0.5 to 1.5 dm).

1) Washing of the LSM plate in propane -2-ol, for 10 to 30 minutes, under ultrasound.

2) Rinsing with deionized water then with ethanol, drying with compressed air.

3) Introduction of the plate into the reactor.

4) Introduction of ZrCl ₄ - 0.5 to 1 gram in a nacelle in the atomic layer epitaxy reactor.

5) Introduction of Y (thd) ₃ - 0.5 to 1 gram into the nacelle in the reactor.

6) Waiting for the temperature rise of the reactor, and consequently of the precursor Y (thd) ₃ from 125 ° to 130 ° C, from _. ZrCl ₄ from 165 ° to 170 ° C and substrate from 350 ° to 410 ° C.

7) Implementation of 1800 to 2000 thin film deposition cycles.

A cycle consists of the following two sequences 1 ^st sequence

Contact for a period ranging from 1 to 2 s. substrate with vaporized ZrCl4;

Purge for a period of 1.5 to 2 s. with nitrogen;

Contact for a period of 0.5 to 1 s. substrate with water in vapor form;

Purge for a period of 1.5 to 2 s. with nitrogen.

This first sequence, which is repeated three times in succession, is followed by the following second sequence:

? ^th sequence:

Contact for a period of 1 to 2 s. substrate with vapors of Y (thd) ₃ ;

Purge for a period of 1.5 to 2 s. with nitrogen; - Contact for a period of 0.5 to 1 s. substrate with water in vapor form;

Purge for a period of 1.5 to 2 s. with nitrogen.

This second sequence is performed only once.

The cycle is performed from 1800 to 2000 times.

As can be seen in FIG. 2, the surface of porous substrate of doped lanthanum manganite 1 is covered by two thin layers 2, 3, arranged successively and each constituted by zirconia doped with yttrium. All of layers 2 and 3 form the top 4 of the sample obtained at the end of the process.

It can be noted that the set of these two layers is dense, and that it covers the pores of the substrate 1 well (see FIG. 1).

There is therefore no longer any apparent porosity. The first layer 2 has a thickness of approximately 1.4 μm, and the second a thickness of approximately 1.1 μm.

By.mpi p. ?. : Preparation of a thin layer (approximately

1 to 2 urn thick) having 2 to 2 atomic% in Y.2Q-2. on a plate çThe porous cermet (5 x 5cm - pore size 0.5 to 1.5 dm).

The same method is applied here as that detailed in Example 1.

Claims

1. Method for depositing on the surface of a porous substrate, atomic layer by atomic layer, thin layers of at least one solid ionic conductor, said ionic conductor consisting of at least one basic oxide and at least one at least one doping agent, all of said thin layers constituting an electrolyte, said deposit being produced from at least one precursor I _n of the metal ion of one of the base oxides, of a precursor II making it possible to produce an oxide from the precursor I _n , and from at least one precursor III _m of one of the oxides providing one of the doping agents, said precursors being brought into the vapor state, n and m each being a greater integer or equal to 1, characterized in that it consists in depositing at least one basic oxide according to at least a first sequence, and at least one doping agent according to at least a second sequence, each first sequence being repeated from 1 to 10 times successively before, after or as a mixture, with or without contacting the precursor II, with at least a second sequence, each second sequence being carried out only once, the assembly comprising the first and second sequences constituting a cycle, the deposition the base oxide (s) and the doping agent (s) being produced, on the one hand, for a sequence ratio (first / second) defined as a function of the atomic ratio (doping agent (s) / oxide (s)) base) desired in each thin layer, on the other hand for a number of cycles defined as a function of the thickness of thin layers desired, and finally, for a period of contacting a precursor (I _n , III _m ) with the defined substrate surface.

2. Method according to claim 1, characterized in that the precursor I _n is chosen from zirconium halides, and in particular zirconium chloride, cerium halides and in particular cerium chloride, cerium beta-diketonates and in particular the tetrakis (2, 2, 6, 6-tetramethyl- 3, 5, -heptanedionato) cerium and their mixture.

3. Method according to claim 1, characterized in that the precursor II is chosen from water, oxygen, ozone, or an alcohol.

4. Method according to claim 1, characterized in that the precursor III _m is chosen from: - yttrium halides and in particular yttrium chloride,

- yttrium beta-diketonates and in particular tris (2, 2, 6, 6-tetramethylheptan-3, 5-dionato) yttrium, - scandium halides and in particular scandium chloride,

- scandium beta-diketonates and in particular tris (2, 2, 6, 6-tetramethyl-3, 5-heptanedionato) scandium, - ytterbium halides and in particular ytterbium chloride,

- ytterbium beta-diketonates and in particular tris (2, 2, 6, 6-tetramethyl-3, 5-heptanedionato) ytterbium, - gadolinium halides and in particular gadolinium chloride,

- gadolinium beta-diketonates and in particular the tris (2, 2, 6, 6-tetramethyl-3, 5-heptanedionato) gadolinium, and - samarium halides and in particular samarium chloride, samarium beta-diketonates and in particular tris (2, 2, 6, 6-tetramethyl-3, 5-heptanedionato) samarium, and mixtures thereof.

5. Method according to claim 1, characterized in that the first sequence is defined as desired by the successions of the following elements: first succession: a) precursor I _n b) nitrogen c) precursor II d) nitrogen

second succession: a) precursors I _n and I ( _{n +} ι) b) nitrogen c) precursor II d) nitrogen

6. Method according to claim 1, characterized in that the second sequence is defined as desired by the successions of the following elements: first succession: a) precursor III _ra b) nitrogen c) precursor II d) nitrogen

second succession: a) Illm and III precursors _{(m +} i ₎ b) nitrogen c) precursor II d) nitrogen

7. Method according to claim 1, characterized in that the ionic conductor is chosen from zirconia doped with yttrium, zirconia doped with scandium, zirconia doped with ytterbium, cerine doped with gadolinium, cerine doped with samarium, and mixtures thereof.

8. Method according to claim 1, characterized in that the atomic ratio (agent (s) dopan (s) / oxide (s) base)) ranges from 2 to 25%.

9. Method according to claim 1, characterized in that the number of cycles ranges from 500 to 50,000, and preferably from 500 to 5,000.

10. Method according to claim 1, characterized in that the durations of bringing a precursor I _n , II or III _m into contact with the surface of the substrate are identical or different, and range from 0.1 to 20 seconds, and preferably from 0.1 to 5 seconds.

11. Method according to claim 1, characterized in that the thickness of each thin layer ranges from 0.1 to 10 μm.

12. Method according to claim 1, characterized in that the porous substrate is chosen from a porous cathode material or a porous anode material for solid oxide fuel cell (SOFC).

13. Method according to claim 12, characterized in that the cathode material is chosen from lanthanum manganite doped with strontium (LSM), lanthanum manganite doped with calcium, manganite from lanthanum iron-doped, lanthanum colbatate doped strontium, and mixtures thereof.

14. Method according to claim 12, characterized in that the anode material is chosen from nickel and zirconia cermet doped with yttrium (Ni / YSZ). , the cermet. of nickel and zirconia doped with scandium, the cermet of nickel and zirconia doped with ytterbium, the cermet of nickel and cerine doped with gadolinium, the cermet of nickel and cerine doped with samarium, stabilized zirconia 'yttrium doped with titanium oxide, and mixtures thereof.

15. Solid oxide fuel cell cell comprising at least one solid ionic conductor present in at least one thin layer and deposited by the process defined according to one of the preceding claims.

16. Solid oxide fuel cell comprising at least one cell as defined in claim 15.