U.S. patent application number 11/570859 was filed with the patent office on 2008-01-17 for process and device for water electrolysis comprising a special oxide electrode material.
This patent application is currently assigned to Electricite De France. Invention is credited to Jean-Marc Bassat, Jean-Claude Grenier, Cecile Lalane, Fabrice Mauvy, Philippe Stevens.
Application Number | 20080011604 11/570859 |
Document ID | / |
Family ID | 34946684 |
Filed Date | 2008-01-17 |
United States Patent
Application |
20080011604 |
Kind Code |
A1 |
Stevens; Philippe ; et
al. |
January 17, 2008 |
Process and Device for Water Electrolysis Comprising a Special
Oxide Electrode Material
Abstract
The invention relates to a method for water electrolysis
consisting in using an electrode containing at least one type of
oxide material of general formula
A.sub.2-x-yA'.sub.xA''.sub.yM.sub.1-zM'.sub.zO.sub.4+.delta.,
wherein A is a lanthanide and/or alkaline and/or alkaline earth
metal cation, A' is at least one lanthanide and/or alkaline and/or
alkaline earth metal cation, A'' is a cation gap, M is a transition
metal element, M' is at least one type of transition metal element,
wherein said metal is such that 0<y<0.30, preferably
0<y.ltoreq.0.20; -0 1.ltoreq..delta.<0 25, preferably
0.ltoreq..delta.<0.25, most preferably 0.ltoreq..delta.<0 10;
0.ltoreq.x.ltoreq.1 and 0.ltoreq.z.ltoreq.1 An associated device is
also disclosed
Inventors: |
Stevens; Philippe;
(Karlstrasse, DE) ; Lalane; Cecile; (Riviere,
FR) ; Bassat; Jean-Marc; (Cestas, FR) ; Mauvy;
Fabrice; (Canejan, FR) ; Grenier; Jean-Claude;
(Cadaujac, FR) |
Correspondence
Address: |
MILLER, MATTHIAS & HULL
ONE NORTH FRANKLIN STREET
SUITE 2350
CHICAGO
IL
60606
US
|
Assignee: |
Electricite De France
Paris
FR
75008
|
Family ID: |
34946684 |
Appl. No.: |
11/570859 |
Filed: |
June 21, 2005 |
PCT Filed: |
June 21, 2005 |
PCT NO: |
PCT/FR05/01556 |
371 Date: |
December 18, 2006 |
Current U.S.
Class: |
204/242 ;
204/293 |
Current CPC
Class: |
C25B 1/04 20130101; Y02E
60/36 20130101; H01M 4/9025 20130101; C25B 11/077 20210101; Y02E
60/50 20130101 |
Class at
Publication: |
204/242 ;
204/293 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 1/06 20060101 C25B001/06; C25B 9/00 20060101
C25B009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 23, 2004 |
FR |
04 06839 |
Claims
1. A process for electrolysis of water comprising the use of an
electrode comprising at least one oxide material of the following
general formula:
A.sub.2-x-yA'.sub.xA''.sub.yM.sub.1-zM'.sub.zO.sub.4+.delta.,
where: A is a metal cation belonging to the group formed by
lanthanides and/or alkali metals and/or alkaline-earth metals; A'
is at least one metal cation belonging to the group formed by
lanthanides and/or alkali metals and/or alkaline-earth metals; A''
is a cationic vacancy, that is to say a cation A and/or cation A'
vacancy; M is a metal belonging to the group formed by metals of
transition elements; M' is at least one metal belonging to the
group formed by metals of transition elements; said material being
such that 0.ltoreq.y.ltoreq.0.30, preferably
0.ltoreq.y.ltoreq.0.20; -0.1.ltoreq..delta.<0 25, preferably
0.ltoreq..delta.<0 25, more preferably 0.ltoreq..delta.<0.10;
0.ltoreq.x.ltoreq.1; and 0.ltoreq.z.ltoreq.1.
2. The process as claimed in the preceding claim 1, wherein: A and
A' are chosen independently from the group formed by lanthanum La,
praseodymium Pt, strontium Sy, calcium Ca and neodymium Nd,
preferably neodymium Nd, strontium Sr and calcium Ca, even more
preferably neodymium Nd, and wherein: M and Mt are chosen
independently from the group formed by chromium Cr, manganese Mn,
iron Fe, cobalt Co, nickel Ni and copper Cu, preferably nickel Ni
and copper Cu, even more prefer ably nickel Ni.
3. The process as claimed in claim 1, wherein: A is chosen from the
group formed by lanthanum La, praseodymium Pr, and neodymium Nd,
preferably neodymium Nd; and A is chosen from the group formed by
strontium Sr and calcium Ca, preferably calcium Ca; and wherein: M
is chosen from the group formed by chromium Cr, manganese Mn, iron
Fe, cobalt Co, nickel Ni and copper Cu, preferably nickel Ni; and
M' is chosen from the group formed by manganese Mn, iron Fe, copper
Cu or cobalt Co, preferably copper Cu or manganese Mn.
4. The process as claimed in claim 1, wherein said material has a
crystallographic structure of the K.sub.2NiF.sub.4 type.
5. The process as claimed in claim 1, wherein said material
possesses an oxygen surface exchange coefficient k greater than
1.times.10.sup.-8 cm/s at 500.degree. C. and greater than
2.times.10.sup.-6 cm/s at 900.degree. C. for oxygen.
6. The process as claimed in claim 1, wherein said material
possesses an electronic conductivity .sigma..sub.e at least equal
to 70 S/cm, preferably at least equal to 80 S/cm, even more
preferably greater than 90 S/cm at 700.degree. C.
7. The process as claimed in claim 1, wherein said material
possesses an oxygen diffusion coefficient greater than
1.times.10.sup.-9 cm.sup.2/s at 500.degree. C. and greater than
1.times.10.sup.-7 cm.sup.2/s at 900.degree. C.
8. The process as claimed in claim 1, wherein said material
possesses an oxygen surface exchange coefficient k greater than
1.times.10.sup.-8 cm/s at 500.degree. C. and greater than
2.times.10.sup.-6 cm/s at 900.degree. C. for oxygen, an electronic
conductivity .sigma..sub.e at least equal to 70 S/cm, preferably at
least equal to 80 S/cm, even more preferably greater than 90 S/cm
at 700.degree. C. and an oxygen diffusion coefficient greater than
1.times.10.sup.-9 cm.sup.2/s at 500.degree. C. and greater than
1.times.10.sup.-7 cm.sup.2/s at 900.degree. C.
9. The process as claimed in claim 1, wherein said material is such
that 6 is not equal to 0, preferably 0<.delta.<0.25, even
more preferably 0<.delta.<0.10.
10. The process as claimed in claim 1, wherein said process is
implemented at a temperature greater than or equal to 600.degree.
C.
11. A water electrolyzer type device comprising at least one
electrochemical cell comprising a solid electrolyte, a cathode and
an anode, which comprises at least one oxide material of the
following general formula:
A.sub.2-x-yA'.sub.xA''.sub.yM.sub.1-zM'.sub.zO.sub.4+.delta.,
where: A is a metal cation belonging to the group formed by
lanthanides and/or alkali metals and/or alkaline-earth metals; A'
is at least one metal cation belonging to the group formed by
lanthanides and/or alkali metals and/or alkaline-earth metals; A''
is a cationic vacancy, that is to say a cation A and/or cation A'
vacancy; M is a metal belonging to the group formed by metals of
transition elements; M' is at least one metal belonging to the
group formed by metals of transition elements; said material being
such that 0<y<0.30, preferably 0<y.ltoreq.0.20;
-0.1.ltoreq..delta.<0.25, preferably 0.ltoreq..delta.<0.25,
more preferably 0.ltoreq..delta.<0.10; 0.ltoreq.x.ltoreq.1; and
0.ltoreq.z.ltoreq.1.
Description
[0001] The invention relates to a process for water electrolysis
consisting in using an electrode comprising at least one special
oxide material, particularly at high temperature. The invention
also relates to a device for implementing such a water electrolysis
process.
[0002] The future use of hydrogen as an energy carrier requires the
development of a process that provides large-scale production at
low cost. The electrolysis of water enables hydrogen to be produced
from water by using electrical energy to break down the water. The
electrochemical reaction used is the reverse of the fuel cell
principle. The water electrolysis process used industrially today
is several decades old. It operates at a temperature of around 80
to 120.degree. C. and the electrical efficiencies of said process
are around 45 to 65%. This energy conversion efficiency is too low
for this technique to be able to be used for the production of
hydrogen as an energy carrier.
[0003] The low operating temperature is partly responsible for the
low energy conversion efficiency. A process operating at a high
temperature (i.e. generally at a temperature greater than or equal
to 600.degree. C.) has a better electrical efficiency since the
reactions at the electrodes are facilitated and because thermal
energy may also be used in the electrochemical conversion process.
Among the various techniques for producing hydrogen by
high-temperature electrolysis of water (described, for example, in
the article "High-temperature steam electrolysis", by E. Schouler,
E. Fernandez and H. Bernard, RGE, March 1982) none have succeeded
on an industrial scale, mainly due to the prohibitive cost, and
also for the reasons explained below.
[0004] The core of the electrochemical cell is composed of an anode
or positive electrode, a cathode or negative electrode and a solid,
ceramic-based electrolyte. The oxygen ions flow through the
electrolyte from the cathode toward the anode. The solid
electrolyte most commonly used is "yttria stabilized zirconia" or
YSZ, also known as yttriated zirconia. The cathode, which is the
site of the reduction of water to produce hydrogen and O.sup.2-
anions that will flow through the electrolyte, is most commonly a
cermet (ceramic/metal composite) of the type where nickel is
dispersed in the stabilized zirconia (YSZ), optionally doped with
ruthenium Ru. The anode, which releases the charges and which is
the site of the oxidation of O.sup.2- ions to oxygen, is most
commonly based on mixed-conducting oxides, such as SnO.sub.2-doped
In.sub.2O.sub.3 and lanthanum manganite LaMnO.sub.3 doped with
calcium or strontium.
[0005] The production of hydrogen requires the coupling, as a
battery, of many elementary cells, the creation of interconnections
that allow two contiguous cells to be connected electrically while
ensuring a seal between two anode and cathode compartments, is
generally achieved with a specific material generally known to a
person skilled in the art, for example with a
Cr--MgO--Al.sub.2O.sub.3 cermet-type material covered in platinum
on the oxygen (anode) side and with nickel on the hydrogen
(cathode) side. Lastly the geometry of such batteries is generally
of tubular or planar type, preferably tubular.
[0006] The only high-temperature process experimented with today
(the process known as "Hot Elly", called the Dornier System process
in the aforementioned article) relates to an electrolyzer that
operates at 1000.degree. C. Such an electrolyzer is of tubular
geometry and uses small concentric cylindrical tubes, predominantly
made of zirconia. The electrodes (nickel-yttriated zirconia cermet
for the cathode and lanthanum manganite for the anode) are
deposited by spraying. But the materials and the manufacturing
process used are too costly for a commercial application to be
reasonably envisaged.
[0007] It is in order to solve these problems of the prior art that
another type of oxide material must be used as the electrode of at
least one electrochemical cell for the electrolysis of water. It is
this that the process according to the invention achieves.
[0008] The process according to the invention is a water
electrolysis process comprising the use of an electrode containing
at least one oxide material of the following general formula:
A.sub.2-x-yA'.sub.xA''.sub.yM.sub.1-zM'.sub.zO.sub.4+.delta.,
where: (1)
[0009] A is a metal cation belonging to the group formed by
lanthanides and/or alkali metals and/or alkaline-earth metals;
[0010] A' is at least one metal cation belonging to the group
formed by lanthanides and/or alkali metals and/or alkaline-earth
metals;
[0011] A'' is a cationic vacancy, that is to say a cation A and/or
cation A' vacancy;
[0012] M is a metal belonging to the group formed by metals of
transition elements;
[0013] M' is at least one metal belonging to the group formed by
metals of transition elements;
[0014] said material being such that
[0015] 0.ltoreq.y<0.30, preferably 0.ltoreq.y.ltoreq.0.20;
[0016] -0.1.ltoreq..delta.<0.25, preferably
0.ltoreq..delta.<0.25, more preferably
0.ltoreq..delta.<0.10;
[0017] 0.ltoreq.x.ltoreq.1; and
[0018] 0.ltoreq.z.ltoreq.1.
[0019] The previous formula therefore includes the case where x is
equal to 0 or 2, that is to say the case where a single metal
cation is present, and also, independently or not of the previous
case, the case where z is equal to 0 or 1, that is to say the case
where a single metal is present,
[0020] A' may represent several metal cations and M' may also
independently represent several metals; a person skilled in the art
knows how to rewrite the formula (I) depending on the number of
components.
[0021] The previous formula therefore includes the case where
.delta. is equal to 0, that is to say the case where there is no
oxygen superstoichiometry or substoichiometry.
[0022] The previous formula therefore includes the case where y is
equal to 0, that is to say the case where there is no cation
vacancy.
[0023] In addition, the presence of an oxygen superstoichiometry or
substoichiometry coefficient .delta., preferably an oxygen
superstoichiometry coefficient .delta., with a value other than 0
may contribute advantageously to the ionic conductivity of the
material. In such a case, preferably 0<.delta.<0.25, more
preferably 0<.delta.<0.10.
[0024] According to a particularly preferred embodiment of the
invention, M and M' are of mixed valency, that is to say that
advantageously such metals contribute to the electronic
conductivity of the material.
[0025] Advantageously, such materials according to the invention
have good thermal stability in terms of composition. This has been
shown by TGA (thermogravimetric analysis) measurements in air, and
confirmed by X-ray diffraction at temperature, on two materials
which are Nd.sub.1 95NiO.sub.4+.delta. and Nd.sub.1
90NiO.sub.4+.delta.. Indeed, measurement of the oxygen
superstoichiometry or substoichiometry coefficient .delta. as a
function of the temperature, over a range from room temperature,
i.e. about 20.degree. C., to 1000.degree. C. does not show any
changes and confirms that the loss of mass is directly and solely
proportional to the variation of the oxygen content of the
material.
[0026] Advantageously, when according to an embodiment of the
invention y is other than 0, the A'' vacancies are randomly
distributed. Indeed, the electron diffraction pattern obtained by
transmission electron microscopy of the material that is Nd.sub.1
90NiO.sub.4+.delta. does not reveal any elongation or smearing of
the main (001) spots, which reveals a perfect order along the c
axis and the absence of intergrowths of the Ruddlesden-Popper type
within the A.sub.2MO.sub.4+.delta. stacks, thus confirming such a
random distribution of the neodymium vacancies.
[0027] The term "lanthanide" is understood according to the
invention to mean lanthanum La or an element of the lanthanides
group such as Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or
Lu and Y. The term "alkali metal" is understood according to the
invention to mean an element other than hydrogen from group 1
(IUPAC version) of the Periodic Table of the Elements. The term
"alkaline-earth metal" is understood according to the invention to
mean an element from group 2 (IUPAC version) of the Periodic Table
of the Elements. The term "transition metal" is understood
according to the invention to mean an element from groups 3 to 12
(IUPAC version) of the Periodic Table of the Elements, including of
course the elements from period 4 such as titanium Ti or gallium
Ga, the elements from period 5 such as zirconium Zr or tin Sn and
the elements from period 6 such as tantalum Ta or mercury Hg.
Preferably, according to the invention, the transition metal is an
element from period 4.
[0028] When y is other than 0, the material of the process
according to the invention is advantageously characterized by very
detailed measurements of (A and/or A')/(M and/or M') ratio(s) by
Castaing microprobe (or EPMA, the acronym for, "Electron Probe
Micxoanalysis"), which allows the exact composition of the material
to be established and optionally the cation vacancy structure of
said material to be evaluated.
[0029] In a preferred embodiment of the invention, said process is
such that:
[0030] A and A' are chosen independently from the group formed by
lanthanum La, praseodymium Pr, strontium Sr, calcium Ca and
neodymium Nd, preferably neodymium Nd, strontium Sr and calcium Ca,
even more preferably neodymium Nd, and such that:
[0031] M and M' are chosen independently from the group formed by
chromium Cr, manganese Mn, iron Fe, cobalt Co, nickel Ni and copper
Cu, preferably nickel Ni and copper Cu, even more preferably nickel
Ni.
[0032] In the particular cases according to the invention where x
is not equal to 0 or 2, and z is not equal to 0 or 1, the number of
type A cations is at least 2: A and A, and the number of type M
cations is at least 2: M and M'.
[0033] In such a case, and in a more general way, preferably:
[0034] A is chosen from the group formed by lanthanum La,
praseodymium Pr, and neodymium Nd, preferably neodymium Nd;
[0035] A' is chosen from the group formed by strontium Sr and
calcium Ca, preferably calcium Ca;
[0036] M is chosen from the group formed by chromium Cr, manganese
Mn, iron Fe, cobalt Co, nickel Ni and copper Cu, preferably nickel
Ni; and
[0037] M' is chosen from the group formed by manganese Mn, iron Fe,
copper Cu or cobalt Co, preferably copper Cu or manganese Mn.
[0038] In a particularly preferred embodiment according to the
invention, the material has a crystallographic structure of the
K.sub.2NiF.sub.4 type, as shown for example in "Inorganic Crystal
Structures", p. 30, by B. G. Hyde and S. Anderson, Wiley
Interscience Publication (1988). The structure is thus formed of
layers of oxygen-containing MO.sub.6 octahedra shifted with respect
to one another by 1/2 1/2 1/2, A atoms ensuring cohesion between
the layers. When additional oxygens O.sub.i are present, they may
be inserted between these layers in the vacant interstitial
sites.
[0039] In a preferred embodiment, the material of the process
according to the invention possesses an oxygen surface exchange
coefficient k greater than 1.times.10.sup.-8 cm/s at 500.degree. C.
and greater than 2.times.10.sup.-6 cm/s at 900.degree. C. for
oxygen. The variation in said coefficient follows an Arrhenius law,
which makes it easy to calculate this coefficient for another
temperature in the temperature range of interest in the
invention.
[0040] In a preferred embodiment, independently or not of the
previous embodiment, the material of the process according to the
invention possesses an electronic conductivity .sigma..sub.e at
least equal to 70 S/cm, preferably at least equal to 80 S/cm, even
more preferably greater than 90 S/cm at 700.degree. C.
[0041] In a preferred embodiment, independently or not of the
previous embodiment, the material of the process according to the
invention possesses an oxygen diffusion coefficient greater than
1.times.10.sup.-9 cm.sup.2/s at 500.degree. C. and greater than
1.times.10.sup.-7 cm.sup.2/s at 900.degree. C. The variation in
said coefficient follows an Arrhenius law, which makes it easy to
calculate this coefficient for another temperature in the
temperature range of interest in the invention.
[0042] In a preferred embodiment, the material of the process
according to the invention possesses an oxygen surface exchange
coefficient k greater than 1.times.10.sup.-8 cm/s at 500.degree. C.
and greater than 2.times.10.sup.-6 cm/s at 900.degree. C. for
oxygen, an electronic conductivity .sigma..sub.e at least equal to
70 S/cm, preferably at least equal to 80 S/cm, even more preferably
greater than 90 S/cm at 700.degree. C. and an oxygen diffusion
coefficient greater than 1.times.10.sup.-9 cm.sup.2/s at
500.degree. C. and greater than 1.times.10.sup.-7 cm.sup.2/s at
900.degree. C.
[0043] In an embodiment of the process according to the invention,
said material is such that .delta. is not equal to 0, preferably
0<.delta.<0.25, even more preferably
0<.delta.<0.10.
[0044] Preferably, the process according to the invention is
implemented at a temperature greater than or equal to 600.degree.
C.
[0045] The invention also relates to a device fox implementation of
the process according to the invention.
[0046] The invention thus relates to a water electrolyzer type
device comprising at least one electrochemical cell comprising a
solid electrolyte, a cathode and an anode, which comprises at least
one oxide material of the following general formula:
A.sub.2-x-yA'.sub.xA''.sub.yM.sub.1-zM'.sub.zO.sub.4+.delta.,
where: (1)
[0047] A is a metal cation belonging to the group formed by
lanthanides and/or alkali metals and/or alkaline-earth metals;
[0048] A' is at least one metal cation belonging to the group
formed by lanthanides and/or alkali metals and/or alkaline-earth
metals;
[0049] A'' is a cationic vacancy, that is to say a cation A and/or
cation A' vacancy;
[0050] M is a metal belonging to the group formed by metals of
transition elements;
[0051] M' is at least one metal belonging to the group formed by
metals of transition elements;
[0052] said material being such that
[0053] 0.ltoreq.y<0.30, preferably 0.ltoreq.y.ltoreq.0.20;
[0054] -0.1.ltoreq..delta.<0.25, preferably
0.ltoreq..delta.<0.25, more preferably
0.ltoreq..delta.<0.10;
[0055] 0.ltoreq.x.ltoreq.1; and
[0056] 0.ltoreq.z.ltoreq.1.
[0057] Said device also most often comprises at least one
interconnector between two electrolysis cells Besides the anode,
all the other parts of said device are generally components known
to a person skilled in the art.
[0058] Advantageously, the device according to the invention
allows, with the use of the anode according to the invention having
at the same time good electronic conductivity and good ionic
conductivity when 6 is other than 0, and also good thermal
stability and sufficient efficiency from an industrial point of
view. In such a case, preferably 0<.delta.<0.25 more
preferably 0<.delta.<0.10.
[0059] The invention will be better understood and other features
and advantages will appear on reading the description that follows,
given without limitation, with reference to FIGS. 1 to 3.
[0060] FIG. 1 is a graph showing, for the four materials used
according to the invention, the oxygen diffusion coefficient
D*(cm.sup.2/s) as a function of 1000/T (K.sup.-1), where T is the
temperature.
[0061] FIG. 2 is a graph showing, for the four materials used
according to the invention, the oxygen surface exchange coefficient
k (cm/s) as a function of 1000/T (K.sup.-1), where T is the
temperature.
[0062] FIG. 3 is a diagram of the principle for the electrolysis of
water, as observed by the process and device according to the
invention.
[0063] FIGS. 1 and 2 are explained below in the examples,
[0064] FIG. 3 is a diagram of the principle for the electrolysis of
water, as observed by the process and device according to the
invention. A device 1 can be seen therein, which is an
electrochemical cell for the electrolysis of water, consisting of
an anode 4, a cathode 2 and an electrolyte 3. An electrical
connection is provided by a device 5 made up of a generator 6 and
two electrical connecting wires 7 and 8.
EXAMPLES
[0065] The examples that follow illustrate the invention without in
any way limiting the scope thereof.
[0066] Two materials were synthesized:
Nd.sub.1.95NiO.sub.4+.delta., and Nd.sub.1.90NiO.sub.4+.delta.
having y values equal to 0.05 and 0.10 respectively. These
materials were synthesized indifferently by solid state reaction of
Nd.sub.2O.sub.3 and NiO oxides at 1100.degree. C. or by soft
chemistry or sol-gel methods, for example from the decomposition of
neodymium and nickel nitrates in solution with final annealing at
11000C. Their oxygen superstoichiometry values were equal to
.delta.=0.15 and .delta.=0.06 respectively, determined by chemical
analysis of Ni.sup.3+ (iodometry)
[0067] Their electronic conductivities .sigma..sub.e were measured
at 700.degree. C., equal to 100 S/cm and 80 S/cm respectively.
Their oxygen surface exchange coefficients k were equal to
5.5.times.10.sup.-8 cm/s and 1.7.times.10.sup.-8 cm/s respectively
at 500.degree. C. and to 5.5.times.10.sup.-6 cm/s and
1.7.times.10.sup.-6 cm/s respectively at 900.degree. C. Their
oxygen diffusion coefficients were equal to 3.2.times.10.sup.-9
cm.sup.2/s and 5.2.times.10.sup.-9 cm.sup.2/s respectively at
500.degree. C. and to 3.5.times.10.sup.-7 cm.sup.2/s and
2.5.times.10.sup.-7 cm.sup.2/s respectively at 900.degree. C. The
percentages of Ni.sup.3+ cations at 700.degree. C., determined by
TGA (thermogravimetric analysis) in air, were equal to 35% and 28%
respectively. The variation in the oxygen stoichiometry within this
temperature range, in which the operating temperature of the
electrolyzer lies, was small and had no influence over the thermal
expansion coefficient, which remained constant and equal to
12.7.times.10.sup.-6 K.sup.-1.
[0068] A material Nd.sub.2NiO.sub.4 was also used, having
respectively x, y and z values equal to 0. This material was
synthesized indifferently by solid state reaction of
La.sub.2O.sub.3 and NiO oxides at 1100.degree. C. or by soft
chemistry or sol-gel methods, for example from the decomposition of
lanthanum and nickel nitrates in solution with final annealing at
1100.degree. C. The oxygen superstoichiometry value was equal to
.delta.=0.22, determined by chemical analysis of Ni.sup.3+
(iodometry). The electronic conductivity .sigma..sub.e of said
material at 700.degree. C. was equal to 38 S/cm, and its oxygen
surface exchange coefficient k was equal to 3.times.10.sup.-13 cm/s
at 500.degree. C. and 1.8.times.10.sup.-6 cm/s at 900.degree. C.,
and its oxygen diffusion coefficient was 2.5.times.10.sup.-9
cm.sup.2/s and 2.times.10.sup.-7 cm.sup.2/s respectively at
500.degree. C. and 900.degree. C.
[0069] A material La.sub.2NiO.sub.4+.delta. was also used, having
respectively x, y and z values equal to 0. This material was
synthesized indifferently by solid state reaction of
La.sub.2O.sub.3 and NiO oxides at 1100.degree. C. or by soft
chemistry or sol-gel methods, for example from the decomposition of
lanthanum and nickel nitrates in solution with final annealing at
1100.degree. C. The oxygen superstoichiometry value was equal to
.delta.=0.16, determined by chemical analysis of Ni.sup.3+
(iodometry). The electronic conductivity .sigma..sub.e of said
material at 700.degree. C. was equal to 50 S/cm, and its oxygen
surface exchange coefficient k was equal to 5.times.10.sup.-7 cm/s
at 500.degree. C. and 5.times.10.sup.-6 cm/s at 900.degree. C., and
its oxygen diffusion coefficient was 4.5.times.10.sup.-9 cm.sup.2/s
and 2.2.times.10.sup.-1 cm.sup.2/s respectively at 500.degree. C.
and 900.degree. C. The percentage of Ni.sup.3+ cations at
700.degree. C., determined by TGA (thermogravimetric analysis) in
air, was equal to 26%. Its thermal expansion coefficient remained
constant with the temperature and was equal to 13.0.times.10.sup.-6
K.sup.-1.
[0070] The electrochemical properties of these four materials were
evaluated in a three-electrode assembly in a half-cell of electrode
material/YSZ/electrode material type, where the counterelectrode
and the working electrode are symmetrical, and are painted onto the
electrolyte and annealed at 1100.degree. C. for 2 hours. The
platinum reference electrode was placed far from the other two
electrodes. The behavior of this material was analyzed under
conditions close to those of high temperature water electrolysis,
that is to say under current and in a temperature range from 500 to
800.degree. C.
[0071] FIG. 1 is a graph showing, for the four materials used
according to the invention, the oxygen diffusion coefficient
D*(cm.sup.2/s) as a function of 1000/T (K.sup.-1), where T is the
temperature. Each curve is a straight line. The four materials
according to the invention were Nd.sub.2NiO.sub.4+.delta.,
La.sub.2NiO.sub.4+.delta., Nd.sub.1.95NiO.sub.4+.delta. and
Nd.sub.1.90NiO.sub.4+.delta.. It can be seen that in the
temperature range of interest in the invention, the materials used
according to the invention generally had, to within the measurement
error, a high, and therefore beneficial, coefficient D*.
[0072] FIG. 2 is a graph showing, for the four materials used
according to the invention, the oxygen surface exchange coefficient
k (cm/s) as a function of 1000/T (K.sup.-1), where T is the
temperature. Each curve is a straight line. The four materials
according to the invention were Nd.sub.2NiO.sub.4+.delta.,
La.sub.2NiO.sub.4+.delta., Nd.sub.1.95NiO.sub.4+.delta. and
Nd.sub.1.90NiO.sub.4+.delta.. It can be seen that in the
temperature range of interest in the invention, the materials used
according to the invention generally had a high, and therefore
beneficial, coefficient k.
* * * * *