ELECTRODE FOR ELECTROCHEMICAL CELL AND METHOD OF
MAKING SUCH AN ELECTRODE
TECHNICAL FIELD
5 This invention relates to an electrode for an electrochemical cell, an
electrochemical cell comprising such an electrode and a method of making
such an electrode.
STATE OF PRIOR ART
10 An electrochemical cell used particularly for electrolysers or fuel cells
at medium and high temperatures usually comprises two electrodes between
which there is a solid electrolyte.
A solid electrolyte is usually formed by a doped ceramic oxide that at
the working temperature is in the form of a crystalline lattice with oxide ion
15 vacancies. The associated electrodes are usually made from cermets that
comprise ceramic and metal. More precisely, the cermets used in electrodes
are composed for example of a perovskite mixed with a metal. Perovskites
are materials with an AB03 or AA'BB'OB type crystalline structure in which A
and A' are lanthanides or actinides and B and B' are transition metals, based
20 on the natural perovskite CaTiOsstructure.
PRESENTATION OF THE INVENTION
The invention aims at disclosing an electrode with mixed electron and
proton conductivity, electron conductivity being better than with electrodes
25 according to prior art.
Another purpose of the invention is to disclose an electrode with good
adhesion to the solid electrolyte.
Another purpose of the invention is to disclose an electrode that can be
made at a lower temperature than electrodes according to prior art.
30 To achieve this, a first aspect of the invention discloses an electrode for
an electrochemical cell with mixed electron and proton conductivity, said
electrode comprising a ceramic, said ceramic being a perovskite doped with
a lanthanide with one or several degrees of oxidation, said ceramic being
doped with an additional doping element taken among the group composed
of niobium, tantalum, vanadium, phosphorus, arsenic, antimony, bismuth.
The fact that the ceramic is doped with niobium, tantalum, vanadium,
5 phosphorus, arsenic, antimony or bismuth makes the ceramic capable of
conducting electrons. The ceramic then conducts electrons and protons,
while if these doping elements are not present, the perovskite doped with a
lanthanide with a single degree of oxidation does not conduct electrons.
Therefore, the invention can be used to make an electrode from a
10 material with the same nature as the solid electrolyte that has good
conductivity of both protons and electrons, even when the ceramic is not
mixed with a metal.
The electrode according to the invention can also have one or several of
the following characteristics taken individually or in any technically possible
15 combination.
The lanthanide is preferably chosen from among lanthanides with one or
several degrees of oxidation: ytterbium, thulium, dysprosium, terbium,
europium, samarium, neodymium, praseodymium, cerium, promethium,
gadolinium and holmium.
20 According to one embodiment, the electrode also comprises a metal; the
metal and the ceramic then form a cermet. The presence of this metal can
further increase the electronic conductivity of the electrode.
Advantageously, the perovskite used is a zirconate.
The lanthanide used is preferably erbium due to its size and
25 monovalence 3.
A second aspect of the invention also relates to an electrochemical cell
comprising two electrodes according to a first aspect of the invention, and a
solid electrolyte placed between the two electrodes.
Advantageously, the perovskite used in the solid electrolyte is of the
30 same nature as that used in the electrodes, which can give better cohesion
between the electrodes and the electrolyte. However, the perovskite in the
electrolyte will be doped with a lanthanide element with a single degree of
oxidation, while the lanthanide in the electrodes may have one or several
degrees of oxidation.
The electrochemical cell is advantageously an electrochemical cell of an
5 electrolysis device such as high temperature electrolysers comprising a
membrane with ionic conductivity. The invention is also applicable to fuel
cells, typically of the SOFC or PCEC type to which technological
developments of high temperature electrolysers are directly applicable.
A third aspect of the invention relates to a method of making an electrode
10 based on the first aspect of the invention, the method comprising the
following steps:
- (a) Synthesis of a perovskite powder doped with a lanthanide with
one or several degrees of oxidation;
- (b) Synthesis of a powder of an additional compound comprising a
15 doping element taken among the group composed of niobium,
tantalum, vanadium, phosphorus, arsenic, antimony and bismuth, the
additional compound being such that the degree of oxidation of the
doping element in this additional compound is greater than or equal to
5;
20 - (c) Mix the doped perovskite powder and the additional compound;
- (e) Sinter this mix, the additional compound being such that the
degree of oxidation of the doping element can reduce during
sintering.
Advantageously, the lanthanide that dopes the perovskite has a single
25 degree of oxidation when the electrolyte is manufactured, and one or several
degrees of oxidation when the electrodes are manufactured.
This method is particularly advantageous because the additional
compound provides oxygen to the mix of powders during sintering due to the
reduction in the degree of oxidation of the doping element during sintering,
30 so that sintering can be done in atmospheres that are not or are only slightly
oxidising (i.e. an almost non-oxidising atmosphere) and at lower
temperatures than is possible in methods according to prior art.
A non-oxidising or slightly oxidising atmosphere means an atmosphere
with a dew point of less than -56°C and preferably -70°C. A dew point of -
5 70°C corresponds approximately to a pressure pH20 in H20 of 2.6x10-%tm
and a pressure PO2 in O2 of 2.3x10-~' atm corresponding to equilibrium at a
sintering temperature of 1540°C.
Advantageously, the perovskite powder and the powder of the additional
compound are mixed with a metallic powder or a metallic phase precursor so
10 as to make a cermet, which can give an electrode with very good electron
conductivity.
If the electrode has a metallic phase, sintering is done under a nonoxidising
atmosphere.
Therefore, the method is capable of sintering under a non-oxidising
15 atmosphere at temperatures less than temperatures used in methods
according to prior art. For example, the sintering temperature of a strontium
zirconate doped with erbium under hydrogenated argon can be reduced by
100°C by the addition of 0.4wt% of ZnNb208.
Advantageously, the method also comprises a step (d) for compaction of
20 the mix between the mixing step (c) and the sintering step (e).
The invention also relates to a method of making an electrochemical cell.
In this case, the method according to the third aspect of the invention also
comprises a step between steps (c) and (e), and preferably between steps
(c) and (d), in which a stack is made comprising at least two layers formed
25 from a mix of the doped perovskite powder and the additional compound,
between which there is an interlayer comprising a layer of perovskite
powder.
The stack may also comprise two intermediate layers, each intermediate
layer being located between the interlayer and one of the two layers formed
30 from the mix of the doped perovskite powder and the additional compound.
These intermediate layers will be used either as a protective layer of the
electrolyte to prevent the diffusion of species between the electrodes and the
electrolyte, or as accommodation layers if there are differences between the
coefficients of thermal expansion of the electrode and electrolyte layers,
particularly due to the presence of metal in the electrodes.
5 A fourth aspect of the invention relates to a method of manufacturing an
electrode based on the first aspect of the invention, the method comprising
the following steps:
- (a) Direct synthesis of a perovskite powder doped with a lanthanide
with one or several degrees of oxidation containing an additional compound
10 comprising a doping element taken among the group composed of niobium,
tantalum, vanadium, phosphorus, arsenic, antimony, bismuth, the additional
compound being such that the degree of oxidation of the doping element in
this additional compound is greater than or equal to 5;
- (b) Sintering of said powder, the additional compound being such that
15 the degree of oxidation of the doping element can reduce during sintering.
DESCRIPTION OF THE FIGURES
Other characteristics and advantages of the invention will become
clearer after reading the following detailed description given with reference to
20 the appended figures that show:
- Figure 1, a diagrammatic representation of an electrochemical cell
according to one embodiment of the invention;
- Figure 2, a diagrammatic representation of the steps in a method
according to the invention.
25 Identical or similar elements are marked by identical reference
symbols in all figures, to improve clarity.
DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT
Figure 1 shows an electrochemical cell according to one embodiment
30 of the invention. This electrochemical cell comprises two electrodes 1, 3
between which there is a solid electrolyte 2. Each electrode 1, 3 is an
electrode according to the first aspect of the invention.
Each electrode 1, 3 is made from a ceramic material that is a
perovskite doped with a lanthanide. In this example, the perovskite is a
zirconate with formula AZr03. The zirconate is dope by a lanthanide that in
this case is erbium. Furthermore, the perovskite doped with the lanthanide is
5 doped with a doping element from among the group composed of niobium,
tantalum, vanadium, phosphorus, arsenic, antimony and bismuth. These
doping elements are chosen to dope the ceramic because they can change
from a degree of oxidation equal to 5 to a degree of oxidation of 3, which
releases oxygen during sintering as we will see later. More precisely, the
10 doping element is preferably niobium or tantalum. Each electrode may also
comprise a metal mixed with ceramic to form a cermet.
In this example embodiment, the ceramic comprises between 0.1%
and 0.5% by mass of niobium, between 4 and 4.5% by mass of erbium and
the remainder in zirconate.
15 The electrochemical cell in figure 1 is manufactured according to the
method described with reference to figure 2. The first step is to synthesise a
perovskite powder doped with a lanthanide during a step 101. The ceramic
thus obtained is in the form of large aggregates composed of nanometric
grains. This ceramic is then formulated to reduce the size of its grains to
20 obtain a grain size distribution that will be conducive to compaction of the
powder.
A powder of an additional compound comprising a doping element
from among the group composed of niobium, tantalum, vanadium,
phosphorus, arsenic, antimony and bismuth, is also synthesised during a step
25 102, the additional compound being such that the degree of oxidation of the
doping element is greater than or equal to 5 in this additional compound. This
additional compound may for example by a niobiate, in other words a
compound comprising niobium, or a tantalate, in other words a compound
comprising tantalum. The niobiate used may for example be zinc niobiate with
30 formula ZnNb208.
The next step is to mix the doped perovskite powder obtained in step
101 and the powder of the additional compound obtained in step 102, in a
step 103. This mix may for example comprise between 0.1% and 0.5% by
mass of zinc niobiate.
The mix thus obtained can then be mixed with a metal powder so as to
form a cermet, during a step 104.
5 A step 105 can then be made to form a stack that will subsequently
form the electrochemical cell and that comprises two layers formed from the
mix of doped perovskite powder and the powder of the additional compound,
between which there is an interlayer comprising a layer of perovskite powder.
The two layers formed from the mix of doped perovskite powder and the
10 powder of the additional compound will each form the electrodes of the
electrochemical cell, while the interlayer will form the solid electrolyte. The
stack may also comprise two intermediate layers, each intermediate layer
being placed between the interlayer and one of the two layers formed from
the mix of the doped perovskite powder and the additional compound. These
15 intermediate layers will act either as the electrolyte protective layer to prevent
diffusion of species between the electrodes and the electrolyte, or as
accommodation layers if there are any differences between the coefficients of
thermal expansion of the electrode and electrolyte layers, particularly due to
the presence of metal in the electrodes.
20 The stack thus obtained can then be compacted during a step 106,
and then sintered during a step 107.
The manufacturing process is particularly advantageous because the
degree of oxidation of the doping element will reduce during sintering, usually
from +5 to +3, such that the additional compound releases oxygen.
25 It is thus possible to sinter at a lower temperature due to this added
oxygen. Thus for example, if the perovskite used is a zirconate doped with
erbium and mixed with zinc niobiate, sintering can take place at 1415°C.
Advantageously, sintering is done under a reducing atmosphere, in
other words an atmosphere of hydrogen (Hz) and argon (Ar).
30 The electrode thus obtained has good cohesion with the electrolyte.
The electrode thus obtained also has enhanced electron conductivity
and good proton conductivity. The ratio of electron conductivity to proton
conductivity of the electrode thus obtained is equal to approximately 100.
Naturally, the invention is not limited to the embodiments described
5 with reference to the figures, and variants could be envisaged without going
outside the scope of the invention. In particular, the proportions of the
different materials are given only for illustration. The geometry of the
electrochemical cell could also be different from the disclosed geometry.
CLAIMS
1. Electrode (1,3) for an electrochemical cell with mixed electron and
10 proton conductivity, said electrode (1,3) comprising a ceramic, said ceramic
being a perovskite doped with a lanthanide with one or several degrees of
oxidation, characterised in that said ceramic is doped with an additional
doping element taken among the group composed of niobium, tantalum,
vanadium, phosphorus, arsenic, antimony, bismuth.
15
2. Electrode (1,3) according to the previous claim, also comprising a
metal, the metal and the ceramic forming a cermet.
3. Electrode (1,3) according to one of the previous claims, in which the
20 perovskite used is a zirconate.
4. Electrochemical cell comprising two electrodes (1, 3) according to
one of the previous claims and a solid electrolyte (2) arranged between the
two electrodes (1, 3).
25
5. Electrochemical cell according to the previous claim, in which the
solid electrolyte (2) is made from a perovskite doped with a lanthanide with
one degree of oxidation, the perovskiteused in the solid electrolyte (2) being
of the same nature as that used in the electrodes (1, 3).
30
6. Method of making an electrode according to one of claims 1 to 3,
comprising the following steps:
- (a) Synthesis of a perovskite powder doped with a lanthanide (101)
35 with one or several degrees of oxidation;
- (b) Synthesis of a powder of an additional compound comprising a
doping element taken among the group composed of niobium, tantalum,
vanadium, phosphorus, arsenic, antimony and bismuth, the additional
compound being such that the degree of oxidation of the doping element in
5 this additional compound (102) is greater than or equal to 5;
- (c) Mix the doped perovskite powder and the additional compound
(103);
- (e) Sinter this mix (107).
10 7. Method according to the previous claim, in which sintering is done in
an almost non-oxidising atmosphere.
8. Method according to claim 6 or 7, in which the perovskite powder
and the powder of the additional compound are also mixed with a metallic
15 powder (104) or a metallic phase precursor.
9. Method according to one of claims 6 to 8, also comprising a step
between steps (c) and (e), in which a stack is made comprising at least two
layers formed from a mix of the doped perovskite powder and the additional
20 compound, between which there is an interlayer comprising a layer of
perovskite powdei (105).
10. Method according to the previous claim, in which the stack also
comprises two intermediate layers, each intermediate layer being located
25 between the interlayer and one of the two layers formed from the mix of the
doped perovskite powder and the additional compound.
11. Method of manufacturing an electrode according to one of claims 1
to 3, the method comprising the following steps:
30 - (a) Direct synthesis of a perovskite powder doped with a ianthanide
with one or several degrees of oxidation containing an additional compound
comprising a doping element taken among the group composed of niobium,
tantalum, vanadium, phosphorus, arsenic, antimony, bismuth, the additional
compound being such that the degree of oxidation of the doping element in
this additional compound is greater than or equal to 5;
(b) Sintering of said powder, the additional compound being such that
5 the degree of oxidation of the doping element can reduce during sintering.