U.S. patent application number 14/126555 was filed with the patent office on 2014-05-01 for membrane/electrode assembly for an electrolysis device.
The applicant listed for this patent is Commissariat a l'energie atomique et aux energies alternatives. Invention is credited to Nicolas Guillet, Eric Mayousse.
Application Number | 20140116877 14/126555 |
Document ID | / |
Family ID | 46420075 |
Filed Date | 2014-05-01 |
United States Patent
Application |
20140116877 |
Kind Code |
A1 |
Guillet; Nicolas ; et
al. |
May 1, 2014 |
MEMBRANE/ELECTRODE ASSEMBLY FOR AN ELECTROLYSIS DEVICE
Abstract
A membrane-electrode assembly for an electrolysis device
includes a proton-exchange membrane, an anode and a cathode
disposed on either side of the proton-exchange membrane, a first
conductive catalyst disposed within the proton-exchange membrane,
and a first conductive junction linking the first conductive
catalyst and the cathode. The first conductive junction has an
electrical resistance greater than a proton resistance of the
membrane between the first conductive catalyst and the cathode.
Inventors: |
Guillet; Nicolas; (Pizancon,
FR) ; Mayousse; Eric; (Grenoble, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Commissariat a l'energie atomique et aux energies
alternatives |
Paris |
|
FR |
|
|
Family ID: |
46420075 |
Appl. No.: |
14/126555 |
Filed: |
June 12, 2012 |
PCT Filed: |
June 12, 2012 |
PCT NO: |
PCT/EP2012/061118 |
371 Date: |
December 16, 2013 |
Current U.S.
Class: |
204/282 |
Current CPC
Class: |
C25B 13/00 20130101;
Y02E 60/50 20130101; Y02E 60/36 20130101; H01M 8/0656 20130101;
C25B 1/10 20130101; C25B 9/10 20130101 |
Class at
Publication: |
204/282 |
International
Class: |
C25B 9/10 20060101
C25B009/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 17, 2011 |
FR |
1155351 |
Claims
1-10. (canceled)
11. An apparatus comprising a membrane-electrode assembly for an
electrolysis device, said membrane-electrode assembly comprising a
proton-exchange membrane, an anode and a cathode disposed on either
side of said proton-exchange membrane, a first conductive catalyst
disposed within said proton-exchange membrane, and a first
conductive junction linking said first conductive catalyst and said
cathode, said first conductive junction having an electrical
resistance greater than a proton resistance of said membrane
between said first conductive catalyst and said cathode.
12. The apparatus of claim 11, wherein said electrical resistance
of said first conductive junction is at least twenty times greater
than a proton resistance between said first conductive catalyst and
said cathode.
13. The apparatus of claim 11, wherein said first conductive
junction forms a peripheral frame maintaining said proton-exchange
membrane in position.
14. The apparatus of claim 11, wherein said first conductive
junction comprises a structural part having electrical resistivity
at 293.15K greater than 20 .mu..OMEGA.cm.
15. The apparatus of claim 11, wherein said first conductive
catalyst is selected to be is capable of oxidizing molecular
hydrogen.
16. The apparatus of claim 15, wherein said first conductive
catalyst comprises titanium fixed on a conductive graphite support,
said conductive graphite support being fixed to a first layer of
said proton-exchange membrane fixedly attached to said cathode and
to a second layer of said proton-exchange membrane fixedly attached
to said anode.
17. The apparatus of claim 16, wherein a proton resistance of said
first layer is lower than a proton resistance of said second
layer.
18. The apparatus of claim 11, wherein said proton-exchange
membrane comprises first, second and third proton-exchange layers,
said cathode being fixed to said first proton-exchange layer, and
said anode being fixed to said third proton-exchange layer, wherein
said first conductive catalyst is disposed between said first and
second proton-exchange layers, wherein said membrane-electrode
assembly further comprises a second catalyst disposed between said
second and third proton-exchange layers, and a second conductive
junction connecting said second catalyst and said anode.
19. The apparatus of claim 11, further comprising an electrical
power supply configured to apply a difference in potential between
said anode and said cathode of said membrane-electrode assembly,
said difference in potential being selected for hydrolyzing water
in contact with said anode.
20. The apparatus of claim 19, wherein a resistance of said
junction between said catalyst and said cathode is configured in
such a way that said voltage of said catalyst is below 0.8V (RHE).
Description
RELATED APPLICATIONS
[0001] This applicant is the national stage under 35 USC 371 of
PCT/EP2012/061118, filed on Jun. 12, 2012, which claims the benefit
of the priority date of French application FR 1155351, filed on
Jun. 17, 2011, the contents of which are herein incorporated by
reference.
FIELD OF DISCLOSURE
[0002] The invention pertains to the production of gas by
electrolysis and especially to devices for producing hydrogen using
a proton-exchange membrane to implement electrolysis at low
temperature of water.
BACKGROUND
[0003] Fuel cells are envisaged as an electric power supply system
for future mass-produced motor vehicles as well as for a large
number of applications. A fuel cell is an electrochemical device
that converts chemical energy directly into electrical energy.
Hydrogen (H2) or molecular hydrogen is used as a fuel for the fuel
cell. The molecular hydrogen is oxidized on an electrode of the
cell and oxygen (O2) or molecular oxygen from the air is reduced on
another electrode of the cell. The chemical reaction produces
water. The great advantage of the fuel cell is that it averts
emissions of atmospheric pollutant compounds at the place where
electricity is generated.
[0004] One of the major difficulties in the development of such
fuel cells lies in the synthesis and supply of dihydrogen (or
molecular hydrogen). On earth, hydrogen does not exist in great
quantities except in combination with oxygen (in the form of
water), sulphur (in the form of hydrogen sulphide) and nitrogen (as
ammonia) or carbon (fossil fuels such as natural gas or petroleum).
The production of molecular hydrogen therefore requires either the
consumption of fossil fuels or the availability of large quantities
of low-cost energy in order to obtain this hydrogen from the
decomposition of water, by thermal or electrochemical means.
[0005] The most widespread method for producing hydrogen from water
consists of the use of the principle of electrolysis. To implement
such methods, electrolyzers provided with proton-exchange membranes
(PEMs) are known. In such an electrolyzer, an anode and a cathode
are fixed on either side on the proton-exchange membrane and put
into contact with water. A difference in potential is applied
between the anode and the cathode. Thus, oxygen is produced at the
anode by oxidation of water. The oxidation at the anode also gives
rise to H+ ions that pass through the proton-exchange membrane up
to the cathode, and electrons that are sent back to the cathode by
the electrical supply unit. At the cathode, the H+ ions are reduced
at the level of the cathode to generate molecular hydrogen.
[0006] Such an electrolysis device comes up against undesirable
effects. The proton-exchange membrane is not perfectly impermeable
to gas. A part of the gases produced at the anode and the cathode
thus passes through the proton-exchange membrane by diffusion. This
induces problems of purity of the gas produced but also induces
problems of security. The proportion of hydrogen in oxygen must
especially remain absolutely below 4%, such a proportion being the
lower limit of the explosivity of hydrogen in oxygen.
[0007] The permeability of the membranes to gas can be reduced by
increasing the thickness of the proton-exchange membrane. This,
however, causes an increase in the electrical resistance by making
it more difficult for the H+ ions to pass through, and lowers the
performance of the systems.
[0008] To limit the permeability of a proton-exchange membrane to
gases, certain developments suggest a depositing of catalyst
particles inside the proton-exchange membrane. The catalyst
particles seek to recombine the molecular hydrogen passing through
the membrane with molecular oxygen passing through the membrane.
The quantities of molecular oxygen that reach the cathode and of
molecular hydrogen that reach the anode are thus reduced.
[0009] However, the recombination reaction of the catalyst
particles is exothermal and induces a loss of energy. Furthermore,
such a solution is not optimized for industrial-scale applications
since a part of the molecular hydrogen generated at the cathode is
nevertheless lost inside the proton-exchange membrane. Furthermore,
the permeability of the proton-exchange membrane to molecular
hydrogen is greater than its permeability to molecular oxygen.
Consequently, a part of the molecular hydrogen nevertheless reaches
the anode since the quantity of molecular oxygen is insufficient in
the catalyst particles disposed in the membrane.
SUMMARY
[0010] The invention seeks to resolve one or more of these
drawbacks. The invention thus pertains to a membrane-electrode
assembly for an electrolysis device comprising: [0011] a
proton-exchange membrane; [0012] an anode and a cathode disposed on
either side of the membrane; [0013] a conductive catalyst disposed
within the proton-exchange membrane; [0014] a conductive junction
linking the catalyst and the cathode, the conductive junction
having electrical resistance greater than the proton resistance of
the membrane between the catalyst and the cathode.
[0015] According to one variant, the electrical resistance of the
junction is at least 20 times greater than the proton resistance
between the catalyst and the cathode.
[0016] According to another variant, the junction forms a
peripheral frame maintaining the proton-exchange membrane in
position.
[0017] According to yet another variant, the junction comprises a
structural part having electrical resistivity at 293.15K greater
than 20 .mu..OMEGA.cm.
[0018] According to yet another variant, the catalyst is capable of
oxidizing molecular hydrogen.
[0019] According to one variant, the catalyst comprises titanium
fixed to a conductive graphite support, the conductive graphite
support being fixed to a first layer of the proton-exchange
membrane fixedly attached to the cathode and to a second layer of
the proton-exchange membrane fixedly attached to the anode.
[0020] According to another variant, the proton resistance of the
first proton-exchange layer is lower than the proton resistance of
the second proton-exchange layer.
[0021] According to yet another variant, the proton-exchange
membrane comprises first, second and third proton-exchange layers,
the cathode being fixed to the first proton-exchange layer and the
anode being fixed to the third proton-exchange layer, said catalyst
being a first catalyst, disposed between the first and second
proton-exchange layers, the assembly furthermore comprising: [0022]
a second catalyst disposed between the second and third
proton-exchange layers; [0023] another conductive junction
connecting the second catalyst and the anode.
[0024] The invention also pertains to a device for the electrolysis
of water, comprising a membrane-electrode assembly as described
here above and an electrical power supply applying a difference in
potential between the anode and the cathode of the
membrane-electrode assembly, this difference in potential being
appropriate for hydrolyzing water in contact with the anode.
[0025] According to one variant, the values of resistance of the
junction between the catalyst and the cathode are configured in
such a way that the voltage of the catalyst is below 0.8V.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] Other features and advantages of the invention shall appear
more clearly from the following description given by way of an
indication that is in no way exhaustive, with reference to the
appended drawings, of which:
[0027] FIG. 1 is a schematic view in section of an electrolysis
device incorporating a membrane-electrode assembly according to a
first embodiment of the invention;
[0028] FIG. 2 is a schematic view in section of an electrolysis
device incorporating a membrane-electrode assembly according to a
second embodiment of the invention.
DETAILED DESCRIPTION
[0029] The invention proposes to place a catalyst within the
proton-exchange membrane of a membrane-electrode assembly. An
electronic conductive junction links the catalyst to the cathode,
with electric resistance 2 to 500 times greater than the proton
resistance of the membrane between the catalyst and the
cathode.
[0030] The invention enables the oxidation of the molecular
hydrogen diffusing through the membrane from the cathode in order
to limit the quantity of molecular hydrogen reaching the anode. The
invention also enables molecular hydrogen to be reformed at the
cathode by reducing protons with the electrons that come from the
oxidation of the hydrogen and are collected by the catalyst. The
energy efficiency of the catalyst is thus improved.
[0031] FIG. 1 is a view in section of an example of an electrolysis
device 1 according to one embodiment of the invention. The
electrolysis device 1 comprises an electrochemical cell 2 and an
electrical supply 3.
[0032] The electrochemical cell 2 comprises a membrane-electrode
assembly 4, electrical power supply plates 203 and 204, porous
current collectors 205 and 206 and seals 201 and 202.
[0033] The membrane-electrode assembly 4 comprises a
proton-exchange membrane as well as a cathode and an anode fixed to
either side of this proton-exchange membrane. The proton-exchange
membrane comprises a first layer 401 to which the cathode 403 is
fixed. The proton-exchange membrane comprises a second layer 402 to
which the anode 404 is fixed. A catalyst in the form of a catalytic
layer or catalyst layer 410 is disposed within the proton-exchange
membrane between the first layer 401 and the second layer 402. The
membrane-electrode assembly 4 thus comprises a stack formed by the
cathode 403, the first layer 401, the catalyst layer 410, the
second layer 402 and the anode 404. The membrane-electrode assembly
4 also comprises an electronically conductive junction 411
connecting the cathode 403 to the catalyst layer.
[0034] The porous current collector 205 is interposed between the
cathode 403 and the power supply plate 203. The porous current
collector 206 is interposed between the anode 404 and the power
supply plate 204.
[0035] The electrical supply plate 203 has a water supply conduit,
not shown, communicating with the cathode 403 by means of the
porous current collector 205. The electrical power supply plate 203
also has a conduit for removing molecular hydrogen, not shown, in
communication with the cathode 403 by means of the porous current
collector 205.
[0036] The electrical power supply plate 204 has a water supply
conduit, not shown, in communication with the anode 404 by means of
the porous current collector 206. The electrical power supply plate
204 also has a conduit for removing molecular oxygen, not shown, in
communication with the anode 404 by means of the porous current
collector 206.
[0037] The electrical power supply 3 is configured to apply a DC
voltage generally ranging from 1.3V to 3.0V with a current density
at the power supply plates ranging from 10 to 40000 A/m.sup.2, and
advantageously from 500 to 40000 A/m.sup.2. By applying such a
voltage, a reaction of oxidation of the water at the anode produces
molecular oxygen and, simultaneously, a proton reduction reaction
at the cathode produces molecular hydrogen.
[0038] The reaction at the anode 404 is the following:
2H.sub.2O.fwdarw.4H.sup.++4e.sup.-+O.sub.hd 2
[0039] The protons generated by the anode reaction pass through the
proton-exchange membrane up to the cathode 403. The power supply 3
conducts the electrons generated by the anode reaction up to the
cathode 403.
[0040] The reaction at the cathode 403 is thus as follows:
2H.sup.++2e.sup.-.fwdarw.H.sub.2
[0041] The proton-exchange membrane has the function of being
crossed by protons coming from the anode 404 towards the cathode
403 while at the same time blocking the electrons as well as the
molecular oxygen and the molecular hydrogen generated. However, the
prior-art proton-exchange membrane structures undergo a phenomenon
of diffusion by a part of the gases produced at the cathode and at
the anode.
[0042] The first function of the catalyst layer 410 is to oxidize
the molecular hydrogen passing through the membrane to form
protons. The protons thus formed return under the effect of the
electrical field to the cathode 403. The quantity of molecular
hydrogen that reaches the anode 404 is thus reduced. The second
function of the catalyst layer 410 is to reduce the molecular
oxygen passing through the membrane to form water. This reaction of
reduction brings into play especially the protons present in the
proton-exchange membrane.
[0043] The third function of the catalyst layer 410 is to collect
the electrons generated by the oxidation of molecular hydrogen not
compensated for by the reduction of molecular oxygen. For this
purpose, the catalyst layer 410 is conductive.
[0044] The electrons collected by the catalyst layer 410 are
conducted up to the cathode 403 by means of the conductive junction
411. These electrons enable an additional reduction of protons at
the cathode 403. Thus, the efficiency of generation of molecular
hydrogen by electrolysis is increased while, at the same, an
appreciable reduction is obtained in the diffusion of molecular
hydrogen up to the anode 404.
[0045] Advantageously, the electrical resistance of the junction
411 is at least two times greater than the proton resistance of the
membrane between the layer 410 and the cathode 403, advantageously
at least 20 times greater, by preference at least 50 times greater
and preferably at least 100 times greater. With such values, the
creation of an excessively great leakage current is prevented.
[0046] The SHE standard potential (at 100 kPa and 298.15 K) of the
pair H.sup.+/H.sub.2 is equal to 0V. The SHE standard potential of
the pair O.sub.2/H.sub.2O is equal to 1.23V.
[0047] The potential of the layer 410 must therefore be greater
than 0 to enable the oxidation of the molecular hydrogen and must
advantageously be lower than 0.8V (RHE) to ensure optimal reduction
of molecular oxygen.
[0048] The permeation of hydrogen measured on materials
conventionally used as membranes corresponds to a maximum current
density of 10 mA cm.sup.-2 (as a function of the thickness and
conditions of temperature, pressure, etc.).
[0049] This value of current density is the maximum value that can
pass through the junction 411. Indeed, a part of the hydrogen
passing through the membrane is directly recombined at the layer
410 with the oxygen (reduction) to form water.
[0050] The following notations will be used:
[0051] Ucat is the cathode potential, Ra is the proton resistance
between the layer 410 and the cathode 403, Rsa is the resistance of
the junction 411, Sa is the cross-section of the junction 411,
j.sub.jonc is the density of current passing through the junction,
and Ucou is the potential of the layer 410.
[0052] Ucou-Ucat=Sa.times.Rsa.times.j.sub.jonc therefore
Ucou=Sa.times.Rsa.times.j.sub.jonc+Ucat
For Ucou>0
[0053] It is necessary for Ucou to be greater than -Ucat (Ucat zero
or negative). This is verified if Rsa>Ra.
For Ucou<0.8 V(RHE)
[0054] Ucat is zero or negative (potential of reduction of the
proton). Therefore, it is necessary to compute Rsa for the maximum
value of Ucou, i.e. when Ucat=0.
[0055] Thus: Ucou=Sa.times.Rsa.times.j.sub.jonc whence
Rsa=Ucou/j.sub.jonc/Sa
[0056] For Ucou=0.8 V(ERH), Sa=10 cm.sup.2 and j.sub.jonc=10 mA
cm.sup.-2, we obtain Rsa=8.OMEGA..
[0057] The maximum value of the resistance of the junction 411 is
thus 8.OMEGA..
[0058] The proton resistance of the membrane between the layer 410
and the cathode 403 could, in this case, advantageously range from
6 to 32 m.OMEGA. according to its nature, its thickness, and the
conditions of measurement (temperature, pressure), taking for
example a cross-section of 25 cm.sup.2 for the anode 404.
[0059] Finally, the electrical resistance of the junction 411 is at
least equal to twice the proton resistance of the membrane between
the layer 410 and the cathode 403 and at most 1400 times greater
than this resistance (when Ra=6 m.OMEGA.).
[0060] The junction 411 can be obtained by means of a material with
high resistivity such as a semi-conductive metal oxide (SnO.sub.2,
oxide combined with antimony or indium for example) or an
electronic conductive polymer. The junction 411 can for example be
obtained by means of a structural element having electrical
resistivity at 293.15K greater than 20 .mu..OMEGA.cm. The junction
411 can also be obtained by means of a resistive electronic
component connected to the layer 410 and the cathode 403 by means
of electrical cables. Advantageously, as illustrated in FIG. 1, the
junction 411 forms a peripheral frame holding the cathode 403 or
the first layer 401 in position.
[0061] The cathode 403 can advantageously be formed by using an
electronic conductive material formed by platinum particles
supported by carbon. The anode 404 can advantageously be formed by
using noble metal oxides such as iridium oxide or ruthenium oxide
in order to resist high potentials.
[0062] The layer 410 is advantageously formed by a porous
electronic conductive support on which a catalyst material such as
platinum is fixed. This layer 410 is configured in a known manner
to enable the passage of the protons. The layer 410 can be obtained
in the form of a conductive carbon screen to which platinum
particles are fixed. The layer 410 can also be made in the form of
a carbon layer coated with a layer of platinum particles.
[0063] The layer 410 can be formed by the application of ink
containing catalyst material on the conductive support. The layer
410 formed can be assembled with the layers 401 and 402 by any
appropriate method such as a hot pressing.
[0064] The layer 410 can also be formed by the application of this
ink directly on the first layer 401 or on the second layer 402 of
the proton-exchange membrane. The application of ink can be
obtained by any appropriate method, for example spraying, coating,
silk-screen printing. The deposit of the layer 410 can also be
obtained by any other technique such as physical vapor deposition
(PVD) or by metal-oxide chemical vapor deposition (MOCVD).
[0065] The thickness of the layer 410 can, for example, be limited
so as not to induce excessive resistance to the diffusion of
protons through the membrane-electrode assembly 4.
[0066] The layers 401 and 402 can be formed out of materials
usually selected by those skilled in the art for proton-exchange
membranes. A material such as the one commercially distributed
under the reference Nafion 211 or the reference Nafion 212 can for
example be used.
[0067] The permeability of the proton-exchange membrane to
molecular hydrogen is greater than its permeability to molecular
oxygen. The goal is to limit the direct recombination of hydrogen
with oxygen at the layer 410. The use of the junction 411 enabling
the retrieval of permeation hydrogen at the cathode 403 can be
preferred.
[0068] The quantity of oxygen present at the layer 410 must be
limited by the sizing of the layers 401 and 402. Advantageously,
the thickness of the layer 402 is greater than the thickness of the
layer 401.
[0069] Using layers 401 and 402 made out of material commercially
distributed under the reference Nafion 211, it would be appropriate
for these layers 401 and 402 to have respective thicknesses of 25
.mu.m and 75 .mu.m.
[0070] Most of these cases will use a layer 401 whose proton
resistance is smaller than the proton resistance of the layer
402.
[0071] FIG. 2 is a view in section of an example of an electrolysis
device 1 according to another embodiment of the invention. As in
the example of FIG. 1, the electrolysis device 1 comprises an
electrochemical cell 2 and an electric power supply 3. The electric
power supply 3 is identical to that of the previous embodiment and
shall not be described in further detail.
[0072] The electrochemical cell 2 comprises electrical power supply
plates 203 and 204, porous current collectors 205 and 206, and
seals 201 and 202. These are components whose structure and
configuration are identical to those described with reference to
FIG. 1. The electrochemical cell 2 also comprises a
membrane-electrode assembly 4.
[0073] The membrane-electrode assembly 4 comprises a
proton-exchange membrane as well as a cathode and an anode fixed on
either side of this proton-exchange membrane. The cathode 403 and
the anode 404 are identical to those of the previous
embodiment.
[0074] The proton-exchange membrane comprises a first layer 421 to
which the cathode 403 is fixed. The proton-exchange membrane
comprises a second layer 422. A first catalyst in the form of a
catalyst layer 431 is disposed within the proton-exchange membrane
between the first layer 421 and the second layer 422. The
membrane-electrode assembly 4 furthermore comprises a conductive
junction 441 connecting the cathode 403 to the catalyst layer
431.
[0075] The proton-exchange membrane comprises a third layer 423 to
which the anode 404 is fixed. A second catalyst in the form of a
catalyst layer 432 is disposed within a proton-exchange membrane
between the second layer 422 and the third layer 423. The first
catalyst layer 431 and the second catalyst layer 432 are thus
separated by the third layer 423. The membrane-electrode assembly 4
furthermore comprises a conductive junction 442 connecting the
anode 404 to the catalyst layer 432.
[0076] As in the above embodiment, the proton-exchange membrane has
the function of being crossed by protons of the anode 404 going to
the cathode 403 while at the same time blocking the electrons as
well as the molecular oxygen and the molecular hydrogen
generated.
[0077] The catalyst layer 431 has a function of oxidizing the
molecular hydrogen passing through the membrane to form protons.
The protons thus formed return to the cathode 403. The quantity of
molecular hydrogen reaching the anode 404 is thus reduced.
[0078] The catalyst layer 431 also has the function of collecting
electrons generated by the oxidation of the molecular hydrogen
passing through the proton-exchange membrane. To this end, the
catalyst layer 431 is conductive.
[0079] The electrons collected by the catalyst layer 431 are
conducted up to the cathode 403 by means of the conductive junction
441. These electrons make it possible to obtain an additional
reduction of protons at the cathode 403. Thus, the efficiency of
generation of molecular hydrogen by electrolysis is increased while
at the same time an appreciable reduction is fostered in the
diffusion of molecular hydrogen up to the anode 404.
[0080] The catalyst layer 432 has the function of conducting
electrons coming from the anode 404. To this end, the catalyst
layer 432 is conductive.
[0081] The catalyst layer 432 also has the function of reducing the
molecular oxygen passing through the membrane to form water. This
reaction of reduction especially brings into action protons present
in the proton-exchange membrane and electrons generated by the
oxidation of the molecular oxygen at the anode 404 and conducted up
to the catalyst layer 432 by means of the conductive junction
442.
[0082] In this embodiment, a direct reaction at the catalyst layers
431 or 432 between the molecular hydrogen and the molecular oxygen
is almost non-existent because they are separated by the second
layer 422. Thus, the major part of the molecular hydrogen diffused
through the proton-exchange membrane is oxidized before reaching
the catalyst layer 432 and, conversely, the major part of the
molecular oxygen diffused through the proton-exchange membrane is
reduced before it reaches the catalyst layer 431. The gases
diffusing through the proton-exchange membrane are thus oxidized or
reduced at an early stage of their diffusion.
[0083] The catalyst layers 431 and 432 can have the same structure
as the catalyst layer 410 of the previous embodiment. Methods of
manufacture equivalent to those described for the catalyst layer
410 can also be used for these catalyst layers 431 and 432.
[0084] The junctions 441 and 442 can have appreciably the same
structure as the junction 411 of the previous embodiment.
[0085] The SHE standard potential (at 100 kPa and 298.15 K) of the
pair H.sup.+/H.sub.2 is equal to 0V. The SHE standard potential of
the pair O.sub.2/H.sub.2O is equal to 1.23V.
[0086] The potential U1 of the layer 431 must therefore be greater
than 0 to enable the molecular hydrogen to be oxidized.
[0087] The potential U2 of the layer 432 must advantageously be
lower than 0.8 V(SHE) to ensure optimum reduction of molecular
oxygen.
[0088] The permeation of hydrogen measured on materials
conventionally used as membranes corresponds to a maximum density
of current j.sub.jonc H2 of 10 mA cm.sup.-2 (depending on the
thickness and conditions of temperature, pressure, etc.). The
permeation of oxygen is half as great and corresponds to j.sub.jonc
O2.
[0089] It is possible to carry out the same type of evaluation of
the values of resistances of the junctions as for the previous
embodiment.
[0090] Rsa is defined as the resistance of the junction 441, Rsb
the resistance of the junction 442, Ra the proton resistance
between the layer 410 and the cathode, Rb the proton resistance
between the layer 432 and the anode, Uan the anode potential and
Ucat the cathode potential, Sa the cross-section of the junction
441 and Sb the cross-section of the junction 442
U1-Ucat=Sa.times.Rsa.times.j.sub.jonc H2
Uan-U2=Sb.times.Rsb.times.j.sub.jonc O2
For U1>0
[0091] It is necessary that U1 should be greater than -Ucat (Ucat
is zero or negative). This is verified if Rsa>Ra.
[0092] Advantageously, the electrical resistance of the junction
441 is greater than the proton resistance of the membrane between
the layer 421 and the cathode 403. Such values prevent the creation
of a short circuit and limit the deterioration of the potential
within the proton-exchange membrane.
For U2<0.8 V(ERH)
[0093] For a polarization curve classically encountered in PEM
electrolysis, Uan is around 1.8 V(RHE).
[0094] Thus, for an anode voltage 1.8 V, a value of Sb of 10
cm.sup.2, Rsb=(Uan-U2)/j.sub.jonc O2/Sb giving Rsb=24.OMEGA..
[0095] The proton resistance of the membrane 423 between the layer
432 and the anode 404 advantageously ranges from 6 to 32 m.OMEGA.
depending on its nature, its thickness and the conditions of
measurement (temperature, pressure), in taking for example a
cross-section of 25 cm.sup.2 for the cathode 403.
[0096] Finally, in this example, the electrical resistance of the
junction 442 is at least 750 times greater than the proton
resistance of the membrane 423 between the layer 432 and the anode
404 and at most 4000 times greater than this resistance (when Rb=32
mQ).
[0097] The layers 421, 422 and 423 can be made out of a material
distributed under the trade reference Nafion 211. Here, the
presence of two junctions makes the two sides independent since
there is no longer any direct recombination between hydrogen and
oxygen on the central catalyst layer unlike in the previous
embodiment. There is no longer any ratio of flow of diffusion
(related to the thickness of the layers) to be complied with
between the two gases as above. Respective thicknesses of 25, 25
and 75 .mu.m can be proposed for the layers 421, 422 and 423.
[0098] The invention has been described with reference to a device
for the electrolysis of water. It is however also possible to
envisage a case where such a device is configured to carry out
other types of electrolysis resulting in a generation of gases for
which it is desirable to prevent their diffusion through a
proton-exchange membrane.
* * * * *