PROTON CONDUCTING ELECTROCHEMICAL CELL AND
METHOD OF MAKING SUCH A CELL
TECHNICAL FIELD
5 The field of the invention is electrolysis devices such as high
temperature electrolysers comprising a proton conducting membrane.
The invention relates more particularly to electrochemical cells with
electron conducting electrodes (anode and cathode) bonded to a proton
conducting membrane by compaction and sintering.
10 The invention may also relate to fuel cells, to which technological
developments of high temperature electrolysers are directly applicable.
STATE OF THE ART
Current high temperature electrolyser technologies, for example of
15 the SOEC (Solid Oxide Electrolyser Cell) type, or fuel cells, for example of
the SOFC (Solid Oxide Fuel Cell) are based on the use of two electron
conducting electrodes, separated by an electrolyte with an electronically
insulating ionic (proton) conducting membrane and separating gases in
the anode and cathode compartments thus forming a structure called an
20 electrochemical cell or an elementary assembly.
Normally in high temperature electrolysers with a proton conducting
ceramic membrane, the cathode is formed for example by a zirconialnickel
or zirconialcobalt type cermet.
On the other hand, it is known that metallic oxide compounds can
25 be used, usually with a perovskite structure, for the anode that operates in
oxidising environment. It is also known that noble metals such as gold,
silver or even platinum can be used that resist corrosion and oxidation.
However, the use of noble metals increases the cost for
manufacturing these electrodes.
In order to avoid the use of relatively expensive noble metals for
making an electrode operating in an oxidising medium, patent
US7351488 disclosed the use of cermets for making the anode and the
cathode that resist oxidation in an oxidising atmosphere. These cermets
5 are advantageously formed by mixing an ion conducting ceramic
(identical to that used for making the electrolyte) and a transition metal
such as Chromium (Cr), Iron (Fe) or Copper (Cu).
However satisfactory cohesion with the electrolyte cannot be
achieved during manufacturing of an electrochemical cell using such
10 cermets. Furthermore, manufacturing of an electrochemical cell with such
cermets requires many operations and many thermal cycles.
Patent US6605316 discloses a method of manufacturing an
electrochemical cell by co-sintering the electrolytic membrane and a
cermet electrode in a single step at a sufficiently high temperature to
15 enable sintering of the electrode and densification of the electrolysis, so
as to improve cohesion between the electrolyte and electrodes.
However, the use of such a manufacturing method cannot
guarantee a densification of more than 90% despite the claims made in
the document, because the sintering temperature. of the cell is limited by
20 the melting temperature of the cermet transition metal.
PRESENTATION OF THE INVENTION
In this context, the invention aims at disclosing a proton conducting
electrochemical cell capable of solving the problems mentioned above,
25 the properties of which can improve densification of the electrolytic
membrane.
To achieve this, the invention discloses a proton conducting
electrochemical cell comprising an electrolytic membrane formed by a
ceramic and an electrode formed by a cermet; said electrochemical cell
30 being obtained directly by a method of co-sintering a ceramic layer
capable of forming the electrolytic membrane and a cermet layer capable
of forming the electrode, in a sintering tool at a sintering temperature of
the ceramic capable of making said ceramic layer designed to form the
electrolyte gas tight, said cell being characterised in that said cermet is
5 composed of a mix of a ceramic and a passivatable electron conducting
alloy comprising at least 40% by mole of Chromium capable of forming a
passive layer, the nature and Chromium content of said passivatable alloy
making it possible to co-sinter said electrochemical cell with densification
of the membrane to more than 90% without melting of said alloy.
10 Passivation or passivity represents a state of metals or alloys in which
their corrosion rate is significantly slowed due to the presence of a
passive film or a passive layer, which corresponds to the adsorption of
oxygen on the surface of the metal. Passive layers refers to thin passive
layers (i.e. with a thickness of a few atom layers) often based on
15 chromium as is the case for non-oxidisable alloys of transition metals
containing chromium, CrN, CrMo, CrTa, CrTi, CrW, CrNi, CrCo.
Thus, the melting temperature of the alloy may be modified due to the
nature and the metal content of passivatable alloy forming the Cermet so
that it remains higher than the sintering temperature, under a non-
20 oxidising (advantageously reducing) atmosphere of the ceramic of the
electrolytic membrane (to make it gas tight).
It is thus possible to make the electrochemical cell in a single cycle
sintering the different layers, at the sintering temperature of the
electrolytic membrane and without any prior sintering operation of the
25 electrodes.
This co-sintering can thus give a very good cohesion between the
different layers forming the electrochemical cell, while guaranteeing a
densification of the membrane of more than 90%, and preferably more
than 94%.
Advantageously, the metal element in the alloy must not degrade
the ion conduction of the ceramic by diffusion.
Advantageously, said passivatable alloy must remain electronically
conducting and maintain good mechanical strength, depending on the
5 atmosphere in the compartment (anode or cathode).
The proton conducting electrochemical cell according to the
invention may also have one or several of the foilowing characteristics
taken individually or in any technically possible combination:
- said passive protection layer is electronically conducting;
- said passivatable electronic conducting alloy forming the
cermet of said electrode is an alloy containing chromium and a
transition metal;
- the melting temperature of said alloy is higher than the sintering
temperature of said electrolytic membrane under a nonoxidising
atmosphere;
- the ceramic forming said cermet is of the same nature as the
ceramic forming said electrolytic membrane;
- said ceramic forming said cermet of said electrode and said
ceramic forming said electrolytic membrane are formed by a
perovskite structure based on zirconate or titanate or cerate or
silicate;
- the sintering temperature is higher than 1500°C.
A second aspect of the invention also relates to a high temperature
electrolysis device comprising a proton conducting electrochemical cell
25 according to the invention.
A third aspect of the invention also relates to a method of
manufacturing a proton conducting eiectrochemical cell according to the
invention characterised in that the method comprises:
- a placement step by superposition of
o a first cermet layer composed of the mix of a ceramic and a
passivatable electron conducting alloy comprising at least
40% by mole of chromium and capable of forming a first
electrode,
o a ceramic layer capable of forming said electrolyte,
o a second cermet layer formed by the mix of a ceramic and a
passivatable electron conducting alloy comprising at least
40% by mole of chromium capable of forming a second
electrode;
a co-sintering step of the different layers in a sintering tool at a
ceramic sintering temperature capable of making said ceramic
layer designed to form the electrolyte with a densification of
more than 90%, gas tight.
According to one advantageous embodiment, said co-sintering
15 step is done at a sintering temperature enabling densification of the
electrolyte to more than 94%.
DESCRIPTION OF THE FIGURES
Other characteristics and advantages of the invention will become
20 clearer after reading the following description given for information and
that is in no way limitative, with reference to the appended figures among
which:
- figure 1 shows a diagrammatic sectional view of an
electrochemical cell according to the invention;
25 - figure 2 shows a phase diagram of the cobalt-chromium (Co-Cr)
alloy;
- figure 3 shows a phase diagram of the chromium-nickel (Cr-Ni)
alloy;
- figure 4 shows a phase diagram of the chromium-iron (Cr-Fe)
30 alloy;
- Figure 5 shows a block diagram of the method of manufacturing
the electrochemical cell according to the invention.
DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT
5 The electrochemical cell 10, also called the elementary assembly, is
shown in figure 1.
The electrochemical cell is formed by a proton conducting
electrolytic membrane 13, along the sides of which the electrodes 11 and
12 (anode and cathode) lie.
10 The electrode 11, 12 in the electrochemical cell 10 according to the
invention is formed by a cermet composed of a mix of a ceramic and a
chromium-based metallic alloy.
The ceramic of the electrode 11, 12 is advantageously the same
ceramic as that used for making the electrolytic membrane 13.
15 According to a first advantageous embodiment of the invention, the
proton conducting ceramic used for making the cermet is a zirconate type
provskite ceramic with the general formula AZr03 that can
advantageously be doped by an element A chosen among the
lanthanides.
20 Therefore. the use of this type of ceramic to make the membrane
requires the use of a sintering temperature of more than 1500°C (sintering
under reducing atmosphere) in order to obtain sufficient densification to be
gas tight. The sintering temperature of the membrane 13 is defined more
particularly as a function of the nature of the ceramic but also as a function
25 of the required porosity ratio. The higher the sintering temperature, the
lower the porosity of the electrolytic membrane 13. Conventionally, it is
considered that the porostiy of the electrolytic membrane 13 must be less
than 10% and preferably less than 6% (or its density must be more than
90% and preferably more than 94%), in order to be gas tight.
30 Advantageously, the ceramic is sintered under a reducing
atmosphere to prevent oxidation of the metal at high temperature, in other
words under a hydrogen (H2) and argon (Ar) atmosphere, or even a
carbon monoxide (CO) atmosphere if there is no risk of carbonation.
Due to the particularly advantageous aspect of the method of
making the electrochemical cell according to the invention making it
5 possible to perform a single sintering operation in a single tooling, the
electrodes 11, 12 of the elementary assembly 10 are also sintered at a
temperature of more than 1500°C (in the example of sintering a zirconate
type ceramic).
The metallic alloy of the cermet is a passivatable electron
10 conducting alloy capable of forming a protective oxide layer so as to
protect it in an oxidising environment (i.e. at the anode of an electrolyser).
The passivatable alloy comprises chromium so as to have a cermet
with the special feature that it does not oxidise at high temperature. The
content by mole of chromium in the alloy is determined such that the
15 melting point of the alloy is higher than the sintering temperature of the
ceramic. Remember that the sintering temperature means the sintering
temperature necessary to sinter the electrolyte membrane so as to make it
gas tight.
Advantageously, the chromium alloy also comprises a transition
20 metal capable of maintaining an electron conducting nature of the passive
layer. Thus, the chromium alloy is an alloy of chromium and one of the
following transition metals: Cobalt, Nickel, Iron, Titanium, Niobium,
Molybdenum, Tantalum, Tungstene, etc.
Figure 2 shows the phase diagram of the cobalt-chromium alloy.
25 Thus, in order to obtain a melting point of the alloy exceeding the sintering
temperature of the zirconate type ceramic (i.e. 1500°C), the chromium
content must be higher than 70% (by mole) and advantageously 80% (by
mole).
When the chromium alloy is a chromium-nickel alloy, the chromium
30 content must be greater than 65% by mole (figure 3).
When the chromium alloy is a chromium-iron alloy, the chromium
content must be greater than 40% by mole (figure 4).
Due to the advantageous composition of the electrode 11, 12, the
electrochemical cell 10 can be made in a single sintering operation under
5 a non-oxidising (preferably reducing) atmosphere because the
composition of the electrode 11, 12 can resist high temperatures when
sintering the membrane 13 under a reducing atmosphere.
The method 100 of manufacturing the electrochemical cell is shown
particularly in figure 5.
10 The first step 110 in the method of manufacturing the
electrochemical cell 10 is a step in which a cermet layer, a ceramic layer
and a second cermet layer are superposed in a die, for example cylindrical
in shape.
The cermet and the ceramic are previously synthesised
15 conventionally either by band casting or by powder synthesis.
It is also possible to insert intermediate layers between the cermet
layers and the ceramic layer, forming the electrolytic membrane, that can
act as either:
- protective layers for the electrolytic membrane to prevent
20 . diffusion of species between the electrodes 11, 12 and the
electrolytic membrane 13, or
- accommodation layers to compensate for differences between
the coefficients of thermal expansion of the cermet layers and
the ceramic layer, particularly due to the presence of metal in
the cermet.
The second step 120 in the manufacturing method 100 is a step to
compact all the layers superposed during the previous step 110.
The third step 130 of the manufacturing method 100 is a sintering
step of the assembly under a reducing atmosphere so as to densify the
30 ceramic.
The invention has been described particularly with reference to a
zirconate type ceramic. However, the invention is also applicable with a
titanate, cerate or silicate type ceramic for which the sintering
temperatures, particularly under a reducing atmosphere, are more than
5 1500°C.
The invention has been described particularly for a high
temperature electolyser comprising a proton conducting membrane;
however, the invention is also applicable to fuel cells, typically SOFC type
cells, to which technological developments of high temperature
10 electrolysers are directly applicable.
Naturally, the invention is not limited to the embodiments described
with reference to the figures and variants could also be envisaged without
going outside the scope of the invention. In particular, the proportions of
the different materials are given only for illustrative purposes. Furthermore,
15 the electrochemical cell may have geometries different from the disclosed
geometry.
CLAIMS
1. Proton conducting electrochemical cell (10) comprising an
5 electrolytic membrane (13) formed by a ceramic and an electrode
(1 1, 12) formed by a cermet; said electrochemical cell (1 0) being
obtained directly by a method of co-sintering a ceramic layer capable
of forming the electrolytic membrane (13) and a cermet layer capable
of forming the electrode (11, 12), in a sintering tool at a sintering
10 temperature of the ceramic capable of making said ceramic layer
designed to form the electrolyte (13) gas tight so as, said cell (10)
being characterised in that said cermet is composed of a mix of a
ceramic and a passivatable electron conducting alloy comprising at
least 40% by mole of Chromium capable of forming a passive layer,
15 the nature and Chromium content of said passivatable alloy making it
possible to co-sinter said electrochemical cell with densification of the
membrane to more than 90% without melting of said alloy.
2. Proton conducting electrochemical cell (10) according to the
20 previous claim, characterised in that said protective passive layer is
an electron conducting layer.
3. Proton conducting electrochemical cell (10) according to one of the
previous claims, characterised in that said passivatable electron
25 conducting alloy forming the cermet of said electrode is an alloy
comprising chromium and a transition metal.
4. Proton conducting electrochemical cell (10) according to one of the
previous claims, characterised in that the melting temperature of said
30 alloy is higher than the sintering temperature of said electrolytic
membrane under a non-oxidising atmosphere.
5. Proton conducting electrochemical cell (10) according to one of the
previous claims, characterised in that the ceramic forming said
cermet is of the same nature as the ceramic forming said electrolytic
membrane (13).
5
6. Proton conducting electrochemical cell (10) according to one of the
previous claims, characterised in that said ceramic forming said
cermet of said electrode (11, 12) and said ceramic forming said
electrolytic membrane (13) are formed by a perovskite structure
10 based on zirconate or titanate or cerate or silicate.
7. Proton conducting electrochemical cell (10) according to one of the
previous claims, characterised in that the sintering temperature is
higher than 1500°C.
15
8. High temperature electrolysis device comprising a proton
conducting electrochemical cell (10) according to one of claims 1 to
9.
20 9. Method of manufacturing a proton conducting electrochemical cell
according to one of claims 1 to 7 characterised in that the method
comprises:
- a placement step by superposition of:
o a first cermet layer composed of the mix of a ceramic and a
passivatable electron conducting alloy comprising at least 40%
by mole of chromium and capable of forming a first electrode,
o a ceramic layer capable of forming said electrolyte,
o a second cermet layer formed by the mix of a ceramic and a
passivatable electron conducting alloy comprising at least 40%
by mole of chromium capable of forming a second electrode;
- a co-sintering step of the different layers in a sintering tool at a
ceramic sintering temperature capable of making said ceramic
layer designed to form the electrolyte with a densification of more
than 90%, gas tight.
5
10. Method of manufacturing a proton conducting electrochemical
cell according to the previous claim, characterised in that said cosintering
step is done at a sintering temperature enabling
densification of the electrolyte to more than 94%.