WO2013050919A1

WO2013050919A1 - Mixed conductive titanates for a high-temperature electrochemical system

Info

Publication number: WO2013050919A1
Application number: PCT/IB2012/055249
Authority: WO
Inventors: Thibaud Delahaye; Charline ARRIVE; Maria-Teresa CALDES; Olivier Joubert; Gilles Gauthier
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives; Centre National De La Recherche Scientifique
Priority date: 2011-10-05
Filing date: 2012-10-01
Publication date: 2013-04-11
Also published as: FR2981063B1; FR2981063A1

Abstract

The invention relates to compounds of formula La_xSr_1-xTi_1-y-zMn_yNi_zO_3-δ and to the uses thereof. In particular, the invention is suitable for use in the following fields: electrodes, fuel cells, electrolysers and electrochemical cells.

Description

MIXED CONDUCTIVE TITANATES FOR HIGH TEMPERATURE ELECTROCHEMICAL SYSTEM

The invention relates to compounds of formula

as well as their use for the manufacture of electrodes.

It also relates to an electrode comprising such a compound as well as devices comprising such an electrode.

A major obstacle to the development of electrochemical cells incorporating a solid protonic conductive electrolyte lies in the choice of hydrogen electrode materials to be associated with it. Indeed, the current composite electrodes cermet type proton Ni-conductor have many disadvantages especially in terms of premature aging often related to the agglomeration of nickel in operation. Therefore, one of the solutions envisaged is to replace the classic cermet by oxide materials of mixed electron / proton conductors type. Indeed, these materials being single phase and having no metal phase are less subject to aging in operation. In addition, their active surface is no longer limited as in the case of cermets triple points (porosity / metal / ceramic) place of the chemical reaction but the entire free surface of the material. This delocalization thus favors the electrochemical performances.

In the literature very few materials have been identified as mixed electronic / protonic conductors. Matsumoto et al. [1] have in particular demonstrated a mixed conductivity in the compound

with x between 0 and 0.1 which is formed by substitution of cerium with ruthenium in BaCeo compound, 9Yo _, i0 _3- g. Jardiel et al. [2] also revealed the presence of this type of mixed conductivity in the compounds ₍₅ Lao Sro, 25) (Cro, 5Mno, 5 _x-xi) 0 ₃ -5 _(Lao> 75Sro _j2 5) (Cro _J 5 - _x Ni _x Mno _> 5) 0 ₃ -.6 T. Norby [3] in his state of the art presents a paragraph on materials likely to be mixed conductors. These materials are mainly derived from the compounds (BaCe0 ₃ ), (BaTb0 ₃ ), (BaPr0 ₃ ), (Sr ₂ TiO ₄ ).

In this context, the invention proposes a family of compounds with mixed ionic / electronic conductivity (La, Sr) (Ti, Mn Ni) (O ₃ ), also noted hereinafter LSTM. Thus, the invention provides compounds of formula I below

_X Sr _1-x Ti _1-yz Mn _y Ni _z 0 ₃ -s

in which

0.l <x <0.9;

0, Ky <0.6,

0.05 <z <0.3, and

0 <δ <0.5.

Preferably, these compounds have the formula I in which:

0.37 <x <0.57;

· 0.40 <y <0.55,

0.05 <z <0.1, and

δ = 0.

A group of preferred compounds according to the invention are those having the formula I in which:

0.47 <x <0.57;

0.40 <y <0.55,

• z- 0.1, and

• 6 = 0.

Another group of preferred compounds according to the invention are those having formula I in which:

0.37 <x <0.47;

0.40 <y <0.55,

Z = 0.05, and

· Δ = 0.

The compounds of the invention are preferably crystallized. In this case, they have a rhombohedral crystallographic structure of space group R3c.

The invention also provides an electrode comprising at least one compound of formula I according to the invention.

Preferably, this electrode is made of a material consisting of at least one compound of formula I according to the invention. The invention also proposes a proton conductor fuel cell comprising at least one electrode according to the invention.

The invention also proposes a high temperature steam electrolyser comprising at least one electrode according to the invention.

The invention also proposes an electrochemical cell comprising at least one electrode according to the invention.

The invention also proposes the use of a compound according to the invention for the manufacture of an electrode.

The invention finally proposes a method for manufacturing electrodes comprising a step of shaping a compound according to the invention.

The invention will be better understood and other characteristics and advantages thereof will appear more clearly on reading the explanatory description which follows and which is made with reference to the figures in which:

FIG. 1 is a schematic representation of the lattice of the crystallographic structure of the compounds of the invention,

- Figure 2 shows the X-ray diffraction pattern of a compound of the invention of formula _{Lao> 47} Sro, 53Tio, 4Mno, 55Ni _{0 (o} ₅ C * 3, also denoted LSTM55N5,

FIG. 3 shows the evolution of the mass as a function of temperature for the compound LSTM55N5 subjected to four repeated cycles,

FIG. 4 represents an enlargement of the last three heating cycles of FIG. 3,

FIG. 5 represents the variation of the electrical conductivity of the compound LSTM55N5 as a function of the temperature, under Ar / ¾ sec (- · - · · -) and under a mixture Ar / H ₂ load in water, that is, that is, wet in Figure 4 (- ■ - ■ -), and

FIG. 6 represents the X-ray diffraction pattern (XRD) made on the 8YSZ mixtures ([zirconia (ZrO ₂ ) stabilized at 8 mol% in Y ₂ O ₃ ]) and the compounds LSTM55N5, LSTM40N5, LSTM55N10 and LSTM40N10 respectively; (compounds of formulas Lao ^ _/ a _has Sro ^ -MaTio ^ - Nio _has _n ^ Oa or Laoj ₊ s _/ a _has Sro ^ io ^ s- -iea _has _Mn ^ Nio SOS wherein a = 0.55 or 0.4).

The invention provides compounds of formula I below: _X Sri, _x Tii _-yz Mn _y Ni _z 0 _3-5 in which:

• 0, l <x <0.9;

• 0, l <y <0.6,

• 0.05 <ζ <0.3, and

• 0 <δ <0.5.

These compounds are especially derived from the SrTiO ₃ compound by substitution of strontium by lanthanum and titanium by manganese and nickel. This triple substitution leads to a new family of lanthanates with mixed protonic and electronic conductivity.

Indeed, the catalytic and ionic conduction properties of the SrTiO compound are very low. It is the same compounds in which the strontium is partially substituted by lanthanum (LST), as explained in Q.X. Fu et al, Journal of the Electrochemical Society, 153 (4) D74-D83 (2006) and Olga A. Marina et al, Solid State Ionics, 149 (2002), 21-28.

As for the compounds described in T. Jardiel et al, they react with the usual electrolyte 8YSZ after a relatively low temperature treatment (1150 ° C. for 3 hours in air) to form a SrZrO ₃ phase, which is detrimental for the electrochemical performances as insulators. [T. Delahaye, T. Jardiel, O. Joubert, R. Laucournet, G. Gauthier, MT Caldes, Solid State Ionics, Volume 184, Issue 1, 3 March 201 I, Pages 39-41].

In addition, chromium poses toxicity problems.

The catalytic properties and ionic conductivity of LSTs can be improved by making site B more sensitive to reduction. The substitution by one or more element (s) having a lower degree of oxidation than Ti and accepting more easily a coordination number less than 6, in fact promotes the formation of oxygen vacancies and the migration of ions O ^" .

Thus, in the compounds of the invention, the substitution of Sr 2 by La ³⁺ makes it possible to increase the number of electronic charges. Substitution by Mn makes site B more flexible with respect to reduction, which facilitates the formation of oxygen vacancies and the establishment of ionic conductivity in these materials. Finally, the addition of Ni in the structure makes it possible to improve the properties electro-catalytic, in particular because it can be reduced to the metallic state in operation and forms nanoparticles of Ni.

These compounds have a perovskite crystal structure AB0 ₃ or A ₂ B0 derivative and are composed of five cations. The term perovskite refers to a family of minerals of the same structure whose general formula is ABO3. In the ideal cubic perovskite structure, the coordination of the A atoms is 12: they are on a site with a cubic oxygen environment. The coordination of B atoms is 6: they are on an octahedral oxygen site. Thus, the perovskite structure is constituted by octahedra BO "linked by the vertices along the three crystallographic axes, the atoms A being placed in the sites left vacant by the octahedra (Figure 1). Nevertheless, it is rare that the structure remains symmetrical and many distortions are generally observed (polar displacements, rational or under the Jahn-Teller effect of ions) as described by KS Aleksandrov et al. (KSAleksandrov and VV Berznosikov, Hierarchies of Perovskite-Like Crystals (review), Phys.25 Solid State, 39, (1997), 695-714).

The compounds (La, Sr) (Ti, Mn, Ni) O _{3 -} s of the invention were synthesized by combustion of a nitrate-citrate gel which is a variant of the Pechini method [MP Pechini, US Patent 3330697, July 11, 1967]. La ₂ 0 ₃ (Rhodia, 99.9%), SrC0 ₃ (Alfa Aesar, 99.99%), (CH ₃ C0 ₂₎ ₂ Ni (Alfa Aesar,> 99%), MnC0 ₃ (Alfa Aesar,> 99.985 %) and Ti {OCH (CH ₃ ) ₂ } ₄ (Alfa Aesar, 99.995%) were used as metal precursors. Titanium isopropoxide, Ti {OCH (CH ₃ ) 2} ₄ , is previously diluted in an ethylene glycol / citric acid mixture in order to limit the risks of precipitation during the syntheses. The titanium ion concentration of this solution was determined by thermogravimetric analysis for 10 hours at 1000 ° C.

First, the citric acid C ₆ H ₈ O is dissolved in a mixture of distilled water / nitric acid HN0 ₃ (65% m), then the metal precursors are added one by one in stoichiometric proportions with stirring and light heating ( 40-50 ° C). The volume of the mixture is then reduced by heating to 150 ° C until it starts to gel. An ammonia solution (NH ₄ OH at 28% vol.) Is then added hot to neutralize to pH = 8. The volume of the mixture is reduced again until gelation. The gel is then dried in an oven at 100 ° C. When drying, it swells and an aerated black "meringue" called xerogel is obtained. Pyrolysis of this xerogel under IR epiradiator makes it possible to continue the combustion reaction. This occurs with a large release of gas (C0 ₂ H2O, ...) and gives a very fine powder which is ground and calcined at 600 ° C to remove a major part of the organic compounds. The powder thus obtained is ground manually so as to homogenize it, then calcined at 1000 ° C. or 1200 ° C. to crystallize the phase. Longer heat treatments were carried out (ie at 1300 ° C for 12h) to obtain a better crystallization for the structural determination. X-ray diffraction patterns (XRD) are obtained using a Biuker D8 Advance diffractometer operating in Bragg-Brentano geometry. It is equipped with an anticathode of Cu generating X-rays, and a Ni filter placed upstream of the detector to eliminate Kp radiation from Cu. The experimental data were refined by the Rietveld method using the Fullprof2k program and its WinplotR GUI. The dilatometric measurements were performed using a Setaram DHT 2050K system, on pellets of low temperature powders (calcined at 1000 ° C or 1200 ° C for 3h) pressed uniaxially at 180MPa. The reduction behavior was studied by thermogravimetric analysis (Setaram TGA92) under Ar / H2 (2%) on the powders calcined at 1300 ° C. for 12 hours.

To prepare the conductivity bars, the low temperature powders (La, Sr) (Ti, Mn, Ni) 03-s were pressed uniaxially into pellets 1.5 mm thick and 10 mm in diameter and sintered between 1350 ° C and 1550 ° C. Fine pellets of compactness between 80 and 90% have thus been obtained. These pellets were cut, by means of a wire saw, into a bar 2 mm ² in section and 8 to 10 mm long. A gold lacquer was painted and sintered at 750 ° C for 15 min to provide electrical contacts. Electrical conductivity measurements were made by the 4-point method described in [RM. Smits, Measurements of sheet resistivities with the four-point probe. The Bell Sys. Tech. J., 37 (1958). pp. 711-718.] Under Ar (95% by volume) / H ₂ (5% by volume) and under Ar (95% by volume) / H ₂ (5 vol%) humidified (pH ₂ 0 ~ 0025 atm) of 200 ° C to 800 ° C.

Compounds of formula I in which: • 0.37 <x <0.57;

"0.40 <y <0.55,

• 0.05 <z <0, l, and

• δ = 0,

were synthesized and obtained pure.

More particularly, a first series of compounds of formula I comprising 10 atomic% of nickel in site B.

These compounds have formula I in which:

0.47 <x <0.57;

· 0.40 <y <0.55,

Z = 0, l, and

• 5 = 0.

Specifically, the formula for these compounds is: L¾2 + 2 / 3aSro 8-2 / 3a io, 9- _has _Mn Nio, _1˝ 03 wherein a = 0.55; 0.5; 0.45 and 0.4, respectively.

A second particular series of these compounds were synthesized and obtained pure, having 5 atomic% Ni in site B.

More specifically, these compounds have the following formula: Lao, i + 23aSro 9-2 3_ i _0j 95 _-a _Mn _Nio; o50 ₃ , wherein a = 0.55; 0.5; 0.45 and 0.4, respectively.

The compounds were then analyzed by X-ray diffraction analysis.

The refinement of the crystallographic structure of these compounds was carried out by the method of Rietveld described in "A Profile Processing Method for Nuclear and Magnetic Structures." Rietveld, HM (1969). J. Appl. Crystallogr., 2, 65-71, using the Fullprof program described in Juan Rodriguez-Carvajal, Thierry Roisnel, New Windows 95 / NT Applications for Diffraction Commission For Powder Diffraction, International Union for Crystallography, Newsletter No. 20 (May-August Summer 1998. This refinement has shown that the compounds of the invention have a rhombohedral symmetry (space group R3c). Structural model R-3c (n ° 167)

Site Atom Position x y z

To La / Sr 6a 0 0 0.25

B Ti / Mn / Nl 6b 0 0 0

O 0 18e -0.5 0 0.25

Where a _p ~ 3,905 Å is the mesh parameter of a cubic perovskite

In this regard, FIG. 2 shows the diffraction pattern X obtained on the compound of formula L o, 4 Sro, 53 10, 4 MnO, 55 NiO, o 503 (LSTM55N5).

Thermogravimetric analyzes (ATG) under an argon / hydrogen mixture containing 2% by volume of hydrogen were carried out on these compounds according to the following thermal cycle:

20 ° C ^{2K, whl} > 800 ° C / l / t ^{2K> min} > 20 ° C

In each case, mass catches are observed between 20 ° C and

600 ° C.

Figure 3 shows the evolution of the mass as a function of temperature for the compound LSTM55N5 subjected to 4 repeated cycles. For the first cycle, the irreversible mass loss observed during the rise in temperature corresponds to the reduction of Mn ⁺ in Mn ³⁺ and the partial reductions of Ni ²⁺ in Ni ⁰ and Mn ³⁺ in Mn ²⁺ , an increase in mass is observed while going down in temperature under 600 ° C.

For the other 3 cycles, of which FIG. 4 represents an enlargement of the relevant portion of FIG. 3, taps are systematically observed between 20 ° C. and 600 ° C. while rising in temperature and between 600 ° C. and 20 ° C. going down in temperature. These catches are attributed to the insertion of protons in the crystal structure of the compound.

This protonation was confirmed by comparing the 4-point electrical conductivity measurements under an argon / hydrogen mixture containing 5% by volume of dry hydrogen or under the same mixture loaded with water (3% by volume of water). The results of these measurements are shown in FIG. 4 for the compound LSTM55N5. These measurements show at the temperature rise a proton contribution between 200 and 600 ° C more marked under humidified gas. This higher level of conductivity is explained by the presence of additional water which, like hydrogen, is a source of protons that can be incorporated into the structure. This observation testifies well to the protonic nature of this low temperature electric contribution which is added to the electronic contribution or even to a much lower anionic contribution to such temperatures. During the descent in temperature this contribution is much less marked. This is explained by the partial dehydration of the compound during the incursion at high temperatures and the slow kinetics of hydration of the compound during the descent.

For comparison, the electrical conductivity was measured of SrTi0 ₃ or (La, Sr) Ti0 _3. These two compounds exhibit electrocatalytic activity with respect to the oxidation of very low hydrogen and low ionic conductivity. The advantages of LSTM over LST or SrTiO ₃ are already described in the literature, in particular by [QX Fu et al, Journal of the Electrochemical Society, 153 (4) D74-D83 (2006) and Olga A. Marina et al, Solid State Electronics, 149 (2002), 21-28].

In contrast to the LSCMNi compounds, the LSTMN compounds proposed here show no reactivity with the 8YSZ electrolyte after sintering in air at 1300 ° C. for 3 h and a 48 hour treatment at 800 ° C. under Ar / H ₂ (2%). / H ₂ O (3%).

The reactivity of the LSTMN compounds with the electrolyte material 8YSZ was tested by sintering powder mixtures at 50% by mass at 1300 ° C for 3h. These powder mixtures were then treated at 800 ° C under Ar / H ₂ (2%) / H ₂ O (3%) to verify the reactivity under standard operating conditions of an SOFC anode. Figure 6 shows the diffractograms (XRD) of the YSZ / LSTMN mixtures for the extreme compounds of the LSTMN5 and LSTMN10 series after these two treatments. No additional peaks appear on DRX diagrams following reactivity tests performed on LSTMN compounds. They are therefore chemically compatible with the 8YSZ electrolyte material. References ;

[1] H. Mats moto et al; Protonic-Electronic Mixed Conduction and Hydrogen Permeation in BaCe0.9- * Y0.1RuxO3-a; Journal of the Electrochemical Society, 152 (3) A488-A492 (2005).

[2] T. Jardiel et al; New SOFC electrode materials: The Ni-substituted LSCM-based compounds La0.75Sr0.25) (Cr0.5Mn0.5-xNix) O3-O and La0.75Sr0.25) (Cr0.5-xNixMn0.5) O3-O ; Solid State Ionics 181 (2010) 894-901.

[3] T. Norby; Solid-state protonic conductors: principles, properties, progress and prospects; Solid State lonics 125 (1 99) 1-11.

Claims

1. Compounds of formula I below:

The _x Sr! .χΤϊ] _-yz Mn _y Ni _z 03-5

in which

• 0, l <x <0.9;

• 0, Ky <0.6,

• 0.05 <z <0.3, and

• 0 <δ <0.5.

2. Compounds according to claim 1, characterized in that they have formula I in which:

• 0.37 <x <0.57;

0.40 <y <0.55,

· 0.05 <z <0, l, and

• 0 = 0.

3. Compounds according to claim 1 or 2, characterized in that they have the formula I in which:

0.47 <x <0.57;

· 0.40 <y <0.55,

• 0.1, and

• δ = ο.

4. Compounds according to claim 1 or 2, characterized in that they have formula I in which:

· 0.37 <x <0.47;

0.40 <y <0.55,

Z = 0.05, and

• δ = 0.

5. Compounds according to any one of the preceding claims, characterized in that they have a rhombohedral crystallographic structure of space group R3c.

6. An electrode characterized in that it comprises at least one compound according to any one of claims 1 to 5.

7. An electrode characterized in that it is a material consisting of at least one compound according to any one of claims 1 to 5.

8. A proton conductor fuel cell characterized in that it comprises at least one electrode according to claim 6 or 7.

9. Electrolyser of high temperature water vapor characterized in that it comprises at least one electrode according to claim 6 or 7.

10. Electrochemical cell characterized in that it comprises at least one electrode according to claim 6 or 7.

11. Use of a compound according to any one of claims 1 to 5 for the manufacture of an electrode.

12. A method of manufacturing electrodes characterized in that it comprises a step of shaping a compound according to any one of claims 1 to 5.