U.S. patent application number 12/444976 was filed with the patent office on 2010-01-21 for electrochemical device comprising a proton-conducting ceramic electrolyte.
This patent application is currently assigned to Electricite De France. Invention is credited to Maria Teresa Caldes-Rouillon, Thibaud Delahaye, Olivier Joubert, Yves Piffard, Philippe Stevens.
Application Number | 20100015495 12/444976 |
Document ID | / |
Family ID | 37913637 |
Filed Date | 2010-01-21 |
United States Patent
Application |
20100015495 |
Kind Code |
A1 |
Stevens; Philippe ; et
al. |
January 21, 2010 |
Electrochemical Device Comprising a Proton-Conducting Ceramic
Electrolyte
Abstract
The invention relates to the use of a ceramic of formula
Ba.sub.2(1-x)M.sub.2xIn.sub.2(1-y)M'.sub.2yO.sub.4+.delta.(OH).sub..delta-
.' where M represents at least one metal cation with an oxidation
number II or III or a combination thereof, M' represents at least
one metal cation with an oxidation number III, IV, V or VI or a
combination thereof, 0<x<1, 0<y<1, .delta.<2 and
0<.delta.'<2, as solid proton-conducting electrolyte in an
electrochemical device, in particular a fuel cell, an electrolytic
cell, a membrane separating hydrogen from a gas mixture, or also a
hydrogen detector, at an operating temperature of said
electrochemical device preferably comprised between 200.degree. C.
and 600.degree. C.
Inventors: |
Stevens; Philippe;
(Noissy-Rudignon, FR) ; Joubert; Olivier; (Brains,
FR) ; Piffard; Yves; (La Chapelle Sur Erdre, FR)
; Caldes-Rouillon; Maria Teresa; (Nantes, FR) ;
Delahaye; Thibaud; (Blanquefort, FR) |
Correspondence
Address: |
MILLER, MATTHIAS & HULL
ONE NORTH FRANKLIN STREET, SUITE 2350
CHICAGO
IL
60606
US
|
Assignee: |
Electricite De France
Paris
FR
Centre Naional De La Recherche
Paris Ce'dex 16
FR
|
Family ID: |
37913637 |
Appl. No.: |
12/444976 |
Filed: |
October 5, 2007 |
PCT Filed: |
October 5, 2007 |
PCT NO: |
PCT/FR07/52085 |
371 Date: |
May 28, 2009 |
Current U.S.
Class: |
429/479 |
Current CPC
Class: |
C25B 13/04 20130101;
Y02E 60/50 20130101; B01D 2325/26 20130101; C25B 1/04 20130101;
Y02E 60/36 20130101; H01M 2008/1293 20130101; Y02P 70/50 20151101;
Y02E 60/366 20130101; Y02P 70/56 20151101; H01M 2300/0074 20130101;
H01M 8/1016 20130101; H01M 8/1246 20130101; B01D 71/024 20130101;
H01M 8/12 20130101; Y02E 60/525 20130101; H01M 2300/0077 20130101;
H01M 8/1253 20130101 |
Class at
Publication: |
429/33 |
International
Class: |
H01M 8/10 20060101
H01M008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 11, 2006 |
FR |
0608914 |
Claims
1-11. (canceled)
12. A fuel cell comprising an anode compartment, with an anode, fed
continuously with hydrogen or with a gas mixture containing
hydrogen, and a cathode compartment with a cathode, fed with oxygen
or air, the two compartments being separated by the
proton-conducting electrolyte of formula
Ba.sub.2(1-x)M.sub.2xIn.sub.2(1-y)M'.sub.2yO.sub.4+.delta.(OH).s-
ub..delta.' where M represents at least one metal cation with an
oxidation number II or III or a combination thereof, M' represents
at least one metal cation with an oxidation number III, IV, V or VI
or a combination thereof, 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1,
.delta..ltoreq.2 and 0<.delta.'.ltoreq.2, and an electrical
circuit connecting the anode to the cathode.
13-16. (canceled)
17. The fuel cell as claimed in claim 1, wherein said ceramic has a
proton conductivity, measured at 400.degree. C., greater than
10.sup.-3 S/cm.
18. The fuel cell as claimed in claim 1, wherein M represents a
cation of a metal selected from the group consisting of Sr, Ca and
the elements of the lanthanide series.
19. The fuel cell as claimed in claim 1, wherein M' represents a
cation of a metal selected from the group consisting of Ga, Sc, Y,
Ti, Zr, Hf, Nb, Ta, W, Mo and the elements of the lanthanide
series.
20. The fuel cell as claimed in claim 4, wherein M' represents
Ti(IV).
21. The fuel cell as claimed in claim 1, wherein
0.ltoreq.y.ltoreq.0.7.
22. The fuel cell as claimed in claim 6, wherein
0.ltoreq.y.ltoreq.0.3.
23. The fuel cell as claimed in claim 1, wherein the ceramic is
impermeable to gases.
24. The fuel cell as claimed in claim 1, wherein the ceramic is a
fritted ceramic material.
25. The fuel cell as claimed in claim 9, wherein the fritted
ceramic material has a closed porosity and a level of compactness
greater than 95%.
26. The fuel cell as claimed in claim 10, wherein the closed
porosity and the level of compactness greater than 95% can be
obtained by fritting at a temperature lower than or equal to
1400.degree. C.
Description
[0001] The present invention relates to an electrochemical device,
such as a fuel cell or an electrolytic cell, containing, as
electrolyte, a proton-conducting ceramic based on barium and indium
as well as the use of such a ceramic as proton conductor in an
electrochemical device at a temperature comprised between
200.degree. C. and 600.degree. C.
[0002] Fuel cells are electrochemical generators which
continuously, without direct combustion, convert gases such as
H.sub.2 and O.sub.2 into electricity and heat by means of the
electrochemical reactions taking place at electrodes separated by
an electrolyte. For reasons essentially linked to operating
reliability and to mass industrialization constraints, the choice
of fuel cells comprising a solid electrolyte is particularly
useful.
[0003] Currently, the two main types of solid-electrolyte fuel
cells are proton-exchange membrane fuel cells (PEMFCs) and solid
oxide fuel cells (SOFCs).
[0004] The main drawback of PEMFCs lies in the need to hydrate the
proton-conducting membrane and in the poor thermal stability of the
polymer materials, which limits the use of such cells to a
temperature range lower than 120.degree. C. and thus involves the
use, in the electrodes, of expensive platinum-based catalysts that
are susceptible to carbon monoxide poisoning.
[0005] The SOFC technology which involves operating at temperatures
generally higher than 700.degree. C. has numerous advantages
compared with PEMFCs, such as a high electrical efficiency, often
greater than 45%, the possibility of using carbon monoxide as fuel,
of direct reforming and the absence of expensive catalysts.
However, the high operating temperature of these cells induces a
loss of long-term stability, a long start-up time and a low
capacity for supporting thermal cycles. At issue for SOFCs is the
reduction in the operating temperature in order to limit the
degradation reactions at the interfaces, improve resistance to
thermal cycling and thus increase the life of the cells.
[0006] There is therefore a need for fuel cells allowing the
drawbacks of the systems described above to be overcome, i.e.
solid-electrolyte fuel cells capable of operating at lower
temperatures than SOFCs and less susceptible than PEMFCs to
dehydration and carbon monoxide poisoning.
[0007] Within the framework of his research aimed at developing
ever more efficient fuel cells, the Applicant discovered that one
particular family of ceramic materials, described in more detail
below, had excellent proton-conducting properties when they were
used in a temperature range from approximately 200.degree. C. to
600.degree. C., in other words in an intermediate temperature range
between the operating temperatures of SOFCs and PEMFCs.
[0008] The proton-conducting ceramics in question are known per se
and have been described in particular in the article by V.
Jayaraman et al., published in Solid State Ionics, 170, (2004),
pages 25-32. They are materials based on barium and indium oxide
having oxygen vacancies, the vacancies of which are at least
partially filled by hydroxyl groups reversibly fixed by hydration
of the materials. In the cited article, Jayaraman et al. assessed
the proton conductivity of the ceramic materials at temperatures
ranging from room temperature to approximately 180.degree. C. and
obtained unsatisfactory results, the maximum proton conductivity at
180.degree. C. not exceeding 10.sup.-6 Scm.sup.-1. These authors
did not take conductivity measurements at above 180.degree. C.
because they had observed that the dehydration of the ceramic
material started from approximately 200.degree. C., a dehydration
which should in principle be reflected in a reduction in the proton
conductivity following the disappearance of the hydroxyl
groups.
[0009] The present invention is based on the discovery that,
despite the phenomenon of dehydration of the ceramic material,
observed at above 200.degree. C., the materials described in this
article have a high proton conductivity at temperatures comprised
between 200.degree. C. and 600.degree. C., and may thus be used as
solid electrolyte in fuel cells, but also in other electrochemical
devices using proton conductors, such as electrolytic cells,
hydrogen separation membranes or hydrogen detectors.
[0010] As a result, the subject-matter of the present invention is
the use of a ceramic of formula (I)
Ba.sub.2(1-x)M.sub.2xIn.sub.2(1-y)M'.sub.2yO.sub.4+.delta.(OH).sub..delt-
a.' where (I)
M represents at least one metal cation with an oxidation number II
or III or a combination thereof, M' represents at least one metal
cation with an oxidation number III, IV, V or VI or a combination
thereof, 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, .delta..ltoreq.2
and 0<.delta.'.ltoreq.2, as solid proton-conducting electrolyte
in an electrochemical device, in particular a fuel cell, an
electrolytic cell, a membrane separating hydrogen from a gas
mixture, or also a hydrogen detector.
[0011] The ceramic material is preferably used in the temperature
range where its proton conductivity is at its maximum, i.e. at a
temperature comprised between 200.degree. C. and 600.degree. C., in
particular between 300.degree. C. and 500.degree. C.
[0012] The subject-matter of the present invention is moreover such
an electrochemical device containing a solid proton-conducting
electrolyte made of a ceramic of formula (I)
Ba.sub.2(1-x)M.sub.2xIn.sub.2(1-y)M'.sub.2yO.sub.4+.delta.(OH).sub..delt-
a.' where (I)
M represents at least one metal cation with an oxidation number II
or III or a combination thereof, M' represents at least one metal
cation with an oxidation number III, IV, V or VI or a combination
thereof, 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, .delta..ltoreq.2
and 0<.delta.'.ltoreq.2.
[0013] In a particular embodiment, the electrochemical device is a
fuel cell comprising an anode compartment, with an anode, fed
continuously with hydrogen or with a gas mixture containing
hydrogen and a cathode compartment, with a cathode, fed with oxygen
or air, the two compartments being separated by the
proton-conducting electrolyte of formula (I) above.
[0014] In another embodiment, the electrochemical device is an
electrolytic cell comprising a negative electrode, or cathode, and
a positive electrode, or anode, separated from each other by a
proton-conducting electrolyte of formula (I) above.
[0015] In a third embodiment, the device of the present invention
is a hydrogen purification membrane comprising a positive electrode
and a negative electrode separated by a proton-conducting ceramic
of formula (I) above.
[0016] In a fourth embodiment, the device of the present invention
is a hydrogen purification membrane formed by a fritted solid
containing a percolating mixture (i) of particles of at least one
electron-conducting material and (ii) of particles of at least one
proton-conducting ceramic of formula (I) above.
[0017] Finally, in a last embodiment, the device of the present
invention is a hydrogen detector comprising
[0018] a positive electrode,
[0019] a negative electrode,
the two electrodes being separated by a proton-conducting ceramic
of formula (I) above, and a perforated lid covering the whole of
the surface of the positive electrode.
[0020] Such an electrochemical device, when it is a fuel cell,
combines many advantages of SOFC or PEMFC type fuel cells described
in the introduction, but without suffering from their drawbacks.
Such a device can operate in mobile or stationary applications.
Like SOFCs, it has a high electrical efficiency and a high
co-generation temperature but it differs from SOFCs mainly by a
markedly lower optimum operating temperature, comprised between
200.degree. C. and 600.degree. C. This reduction in the operating
temperature leads to an increase in the life of the electrochemical
devices, to a better resistance to thermal cycling, and allows the
expensive ceramics and steels, used for SOFC interconnectors, to be
replaced by standard steels. Unlike PEMFCs, the electrochemical
devices of the present invention do not need expensive
platinum-based catalysts and are not susceptible to carbon monoxide
poisoning.
[0021] The proton-conducting ceramic material used as solid
electrolyte in the electrochemical device of the present invention
must have a negligible electronic conductivity, in order to avoid
short-circuits. It must moreover be impermeable to gases and
preferably be in the form of a fritted material with closed
porosity. This closed porosity is preferably minimized, in other
words the ceramic material advantageously has a high level of
compactness, preferably greater than 95%.
[0022] The ceramic materials of formula (I) above are distinguished
by an excellent suitability for fritting: they can be compacted to
the desired level of compactness, greater than 95%, by fritting at
a relatively low temperature, less than or equal to 1400.degree.
C., and typically comprised between 1300.degree. C. and
1400.degree. C.
[0023] Another advantage of the ceramic materials of formula (I)
lies in the fact that, unlike other ceramic materials, they do not
react, in the temperature range from 200.degree. C. to
approximately 550.degree. C., with the carbon dioxide likely to be
present in the anode compartment of the cell. The formation of
carbonates is one of the factors which limit the operating
temperature of the electrochemical device of the present invention
in the presence of CO.sub.2.
[0024] The proton-conducting ceramic material of the present
invention has a perovskite-type crystalline structure derived from
Ba.sub.2In.sub.2O.sub.5 by partial replacement of the barium and/or
indium ions and hydration of at least some of the oxygen vacancies,
forming hydroxyl groups, essential for the proton conduction of the
ceramic materials.
[0025] The proton conduction of the ceramic materials in fact stems
from the transfer of protons between (OH).sup.- and O.sup.2-
groups, known as the Grotthus mechanism. The level of conductivity
at a certain temperature is related to the mobility of the proton
and to its concentration. The proton concentration depends directly
on the density of the hydroxyl groups introduced into the material
by the hydration reaction of the oxygen vacancies. The
concentration of the oxygen vacancies can, in turn, be modulated by
replacing some of the barium and/or indium ions with metallic ions
(M or M') having a different oxidation number.
[0026] The mobility of the protons is dependent on the ease with
which they break their O--H bond with the oxygen of the hydroxyl
group bound to a metal cation. This mobility therefore depends on
the greater or lesser ionic or covalent nature of the oxygen-metal
cation bond. Thus, the level of proton conductivity strongly
depends on the composition and the concentration of metallic
elements.
[0027] The metals M capable of advantageously replacing some of the
Ba ions of the basic structure Ba.sub.2In.sub.2O.sub.5 are
preferably chosen from Sr, Ca and the elements of the lanthanide
series.
[0028] The metals M' capable of advantageously replacing some of
the In ions of the basic structure Ba.sub.2In.sub.2O.sub.5 are
preferably chosen from Ga, Sc, Y, Ti, Zr, Hf, Nb, Ta, W, Mo and the
elements of the lanthanide series. A particularly preferred metal
M' is Ti(IV).
[0029] The Applicant obtained excellent results, in particular in
terms of conductivity, with ceramic materials in which the barium
ions were not replaced by other metal cations (x=0) and only a
small fraction of the indium ions, less than 30%, was replaced by
Ti(IV) ions.
[0030] In a preferred embodiment of the electrochemical device of
the present invention, the solid proton-conducting electrolyte is
consequently a ceramic of formula (Ia)
Ba.sub.2In.sub.2(1-y)Ti.sub.2yO.sub.4+.delta.(OH).sub..delta.'
(Ia)
where 0.ltoreq.y.ltoreq.0.7, preferably 0.15.ltoreq.y.ltoreq.0.7,
in particular 0.15.ltoreq.y.ltoreq.0.7, .delta..ltoreq.2 and
0<.delta.'.ltoreq.2.
[0031] Of course, the performances of the ceramic materials used in
the present invention depend not only on the metal cation
composition and the number of oxygen vacancies, but also on the
level of hydration of the vacancies, each fixed water molecule
leading to the disappearance of a vacancy and the formation of two
hydroxyl groups. The hydration reaction is a reversible reaction,
the equilibrium constant of which depends directly on the operating
temperature of the electrochemical device. As already stated above,
Jayaraman et al. Solid State Ionics, 170, (2004), pages 25-32, had
in fact observed that a dehydration phenomenon occurred at
temperatures greater than approximately 200.degree. C. However, the
Applicant discovered that the reduction in proton conductivity due
to the dehydration of the ceramic material was largely compensated
for by a dramatic increase in proton mobility at high temperature.
The optimum operating temperature of a fuel cell according to the
present invention consequently equates to the best compromise
between the number of available hydroxyl groups, which decreases
with the temperature, and the mobility of each of the protons,
which increases with the temperature.
[0032] This optimum operating temperature, corresponding to a
maximum proton conductivity of the ceramic, is comprised between
200.degree. C. and 600.degree. C., preferably between 300.degree.
C. and 500.degree. C. The conducting ceramics used in the present
invention thus have, at a temperature in the range from 200.degree.
C. to 600.degree. C. and in a humidified air atmosphere containing
3% steam, a proton conductivity at least equal to 10.sup.-4 S/cm.
At temperatures close to 400.degree. C., this conductivity, also
measured in humidified atmosphere with 3% steam, exceeds 10.sup.-3
S/cm, or even 2.10.sup.-3 S/cm.
[0033] The conducting ceramics described above are moreover
distinguished by the fact that the hydroxylation reaction of the
oxygen vacancies by hydration is accompanied by a small increase in
volume. This property guarantees the mechanical stability of the
material and prevents it from breaking and cracking when it is
subjected to significant variations in temperature at the time of
start-up or shutdown of the electrochemical device of the present
invention.
[0034] The electrochemical device of the present invention may be a
standard fuel cell such as represented in FIG. 1, with an anode
compartment fed continuously with hydrogen or with a gas mixture
containing hydrogen and a cathode compartment fed with oxygen or
air, the two compartments being separated by the proton-conducting
electrolyte (1) of formula (1). The hydrogen, on contact with the
negative electrode (2) or anode, decomposes into electrons and
protons. The protons are conveyed through the proton-conducting
electrolyte (1) to the positive electrode (3) or cathode, creating
an electric current by transferring the electrons to the electrical
circuit (4) of the cell. At the cathode (3), the combination of the
protons with the oxygen and the electrons transported by the
electrical circuit leads to the formation of water. The anode may
be constituted for example by a percolating mixture, i.e. a mixture
allowing the electrons and protons to percolate, by a metal such as
nickel or platinum and by the proton-conducting ceramic of the
present invention. The cathode is constituted for example by a
percolating mixture of an electron-conducting ceramic (LaNiO.sub.4)
and of the proton-conducting ceramic of the present invention.
[0035] The electrochemical device of the present invention may of
course also be an electrolytic cell, as represented in FIG. 2, in
which the reaction opposite to that which takes place in the fuel
cell of FIG. 1 occurs. In such an electrolytic cell, a current is
applied between a negative electrode (5) or cathode and a positive
electrode (6) or anode, separated from each other by a
proton-conducting electrolyte (1) of formula (I) as described
above. Water introduced at the anode compartment is decomposed in a
known manner into protons, electrons and oxygen. The protons pass
through the proton-conducting electrolyte and combine in the
cathode compartment with the electrons led by the electrical
circuit (7) to form hydrogen.
[0036] The electrochemical device of the present invention may also
be a selective hydrogen purification membrane, one embodiment of
which is represented in FIG. 3. Such a membrane comprises a
positive electrode (8) and a negative electrode (9) separated by a
proton-conducting ceramic (1) of the present invention. The
hydrogen contained in a gas mixture (H.sub.2, N.sub.2, CH.sub.4,
CO) decomposes into protons and electrons on contact with the
positive electrode (8). While the protons formed migrate under the
influence of the electric potential through the proton-conducting
ceramic (1) to the negative electrode (9), the electrons enter the
electrical circuit (10). At the negative electrode, the protons and
electrons recombine to form pure hydrogen.
[0037] Another embodiment of a hydrogen purification membrane is
represented in FIG. 4. Such a membrane must contain, in addition to
the proton-conducting ceramic of formula (I), at least one
electron-conducting material, such as a metal. The
proton-conducting material and the electron-conducting material are
generally mixed in the form of powders and compacted and fritted
jointly so as to form a percolating mixture, i.e. a mixture
allowing the electrons and protons to percolate through a
continuous network of the electron- or proton-conducting material.
The operating principle of such a membrane is the following: one
side of the membrane is brought into contact with a pressurized gas
mixture containing hydrogen to be purified.
[0038] On contact with the membrane, the hydrogen decomposes into
protons and electrons. The former pass through the membrane via the
proton-conducting material of formula (I) while the latter are led
by the electron-conducting material. As the membrane is also
impermeable to the other gases of the mixture, the recombining of
the protons and electrons leads to the formation of pure hydrogen
on the other side of the membrane.
[0039] The only motive power of such a membrane is the partial
pressure difference of hydrogen on either side of the membrane. The
higher this is, the more effective the membrane is.
[0040] FIG. 5 represents a hydrogen detector using a
proton-conducting ceramic of the present invention. This detector
operates essentially according to the same principle as the
separation membrane represented in FIG. 3, except that the whole of
the surface of the positive electrode (8) is covered by a
perforated lid (11). When this lid is brought into contact with a
gas mixture containing hydrogen, some of this mixture passes
through the opening (12) of the lid (11) and diffuses towards the
positive electrode (8). The decomposition of the hydrogen on
contact with the positive electrode (8) gives rise to a current,
the intensity of which, measured by an ammeter (13) in the
electrical circuit (10), is directly proportional to the
concentration of H.sub.2 in the gas mixture.
EXAMPLE
Preparation of a Proton-Conducting Ceramic
[0041] Barium carbonate BaCO.sub.3, titanium oxide TiO.sub.2 and
indium oxide In.sub.2O.sub.3 are mixed in the suitable proportions
to obtain a material of formula (I) where M'=Ti, x=0 and y=0.2. The
powders are placed in a mortar, then mixed while grinding with
acetone. After evaporation of the acetone, the mixture of powders
is placed in a platinum crucible and heated at a rate of
400.degree. C./h to a temperature of 1200.degree. C., then kept at
this temperature for 24 hours. The material is then cooled to room
temperature at exactly the same rate at which it was heated, then
the product obtained is ground using a mortar so as to obtain a
fine powder. This powder is then compacted using a uniaxial press
and pressed into pellets. The pellets then undergo a thermal
treatment under air atmosphere at 1350.degree. C. for 24 hours
(rate of heating and cooling of 140.degree. C./h). This first stage
leads to a partially hydrated pure material corresponding to the
formula
Ba.sub.2In.sub.1.6Ti.sub.0.4O.sub.5.2-.delta.'/2(OH).sub..delta.'(.delta.-
'<0.8). This material is then raised to a temperature of
approximately 200.degree. C. under a humidified air atmosphere
(P.sub.H2O 3%) and kept in these conditions for one week. This hot
hydration leads to a material of formula
Ba.sub.2In.sub.1.6Ti.sub.0.4O.sub.4.4(OH).sub.1.6.
[0042] When this material is heated in a humidified CO.sub.2
atmosphere (P.sub.H2O 3%) a chemical stability vis-a-vis carbon
dioxide up to a temperature of approximately 550.degree. C. is
observed. Above this temperature, the material reacts with the
carbon dioxide at a rate proportional to the temperature.
[0043] The powder obtained at the end of the first stage is then
ground for 2 hours using a planetary mill (0.5 g powder in ethanol,
500 r.p.m., 3 beads per jar) then compacted using a uniaxial press.
The tablet is subjected to a thermal treatment under an air
atmosphere at 1350.degree. C. for 24 hours (rate of heating and
cooling 140.degree. C./h). Scanning electron microscopy of the
sample obtained reveals a closed porosity lower than 5%. The dense
sample, subjected to a thermal cycling between 30.degree. C. and
800.degree. C. under humid atmosphere, in other words to a
succession of alternating between hydration and dehydration, does
not show any sign of cracking or breaking.
[0044] The electrical characterization of the sample was obtained
by complex impedance spectroscopy under a controlled oxygen or
steam atmosphere.
[0045] FIG. 6 shows the conductivity of the compound
Ba.sub.2In.sub.1.6Ti.sub.0.4O.sub.5.2-.delta.'/2(OH).sub..delta.',
under humidified air (.box-solid.) at 3% in relation to the
temperature. The conductivity curve of the non-protonated compound
Ba.sub.2In.sub.1.6Ti.sub.0.4O.sub.5.2 under dry air is also given,
for information only. The conductivity of the hydrated material
reaches a value of 2.10.sup.-3 Scm.sup.-1 at approximately
400.degree. C., at which temperature .delta.' is approximately
equal to 0.25. This figure illustrates the optimum operating range
of the proton-conducting material of the present invention. In
fact, at temperatures lower than 200.degree. C., the material is
strongly hydrated but the limited mobility of the protons is
reflected in insufficient conductivities. Above approximately
550.degree. C. to 6000C, the material is almost completely
dehydrated and the conductivity curve determined under humid
atmosphere overlaps that determined under dry atmosphere. The
conductivity is thus essentially anionic (O.sub.2.sup.-)
* * * * *