U.S. patent application number 14/693213 was filed with the patent office on 2015-10-22 for material for an electrochemical device.
This patent application is currently assigned to CERAM HYD. The applicant listed for this patent is CERAM HYD. Invention is credited to Jean-Francois FAUVARQUE, Arash MOFAKHAMI.
Application Number | 20150299877 14/693213 |
Document ID | / |
Family ID | 39672615 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150299877 |
Kind Code |
A1 |
MOFAKHAMI; Arash ; et
al. |
October 22, 2015 |
MATERIAL FOR AN ELECTROCHEMICAL DEVICE
Abstract
The present invention relates to a material for an
electrochemical device, especially a fuel cell, an electrolyzer or
a storage battery, comprising a matrix and activated boron nitride
contained in the matrix.
Inventors: |
MOFAKHAMI; Arash; (Buthiers,
FR) ; FAUVARQUE; Jean-Francois; (Paris, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CERAM HYD |
Avon |
|
FR |
|
|
Assignee: |
CERAM HYD
Avon
FR
|
Family ID: |
39672615 |
Appl. No.: |
14/693213 |
Filed: |
April 22, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12920588 |
Dec 15, 2010 |
9105907 |
|
|
PCT/FR2009/050352 |
Mar 4, 2009 |
|
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14693213 |
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Current U.S.
Class: |
429/483 ;
204/296; 429/306; 429/313; 429/482; 524/404 |
Current CPC
Class: |
H01M 2/1646 20130101;
H01M 4/8668 20130101; H01M 4/622 20130101; H01M 8/1016 20130101;
H01M 8/124 20130101; Y02E 60/10 20130101; Y02E 60/50 20130101; H01M
2/1673 20130101; H01M 4/383 20130101; H01M 8/065 20130101; C01B
21/064 20130101; H01M 10/0565 20130101; H01M 4/38 20130101; H01M
8/1286 20130101; H01M 2008/1095 20130101; H01M 4/0411 20130101;
H01M 8/1226 20130101; C25B 11/04 20130101; H01M 4/9058 20130101;
H01M 8/1097 20130101; Y02P 70/50 20151101; C08J 2327/18 20130101;
C08J 2329/04 20130101; C25B 13/04 20130101; Y02E 60/32 20130101;
H01M 4/8864 20130101; H01M 4/905 20130101; H01M 2300/0082 20130101;
C08J 5/18 20130101; C08K 2003/385 20130101; H01M 8/0215 20130101;
H01M 2/145 20130101; H01M 8/1004 20130101; C25B 1/02 20130101; H01M
4/8652 20130101; C25B 13/08 20130101; H01M 2/166 20130101; C08K
3/38 20130101; H01M 8/0226 20130101 |
International
Class: |
C25B 13/08 20060101
C25B013/08; C08J 5/18 20060101 C08J005/18; C08K 3/38 20060101
C08K003/38; H01M 8/10 20060101 H01M008/10; H01M 10/0565 20060101
H01M010/0565 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 6, 2008 |
FR |
0851454 |
Claims
1. A material for an electrochemical device, comprising a matrix
and activated boron nitride contained in the matrix, wherein the
matrix comprises one of the following: PTFE, PVDF, PVDF-G-PSSA,
silica nanoparticles, polytetrafluoroethylene, polyvinylidene
fluoride.
2. The material as claimed in claim 1, in which the boron nitride
is in the form of percolated grains.
3. The material as claimed in claim 1, in which the boron nitride
is crystallized in a hexagonal form.
4. The material as claimed in claim 1, in which the boron nitride
is turbostratic.
5. The material as claimed in claim 1, comprising a mineral
matrix.
6. The material as claimed in claim 1, comprising a matrix
comprising at least one of an organic material and an inorganic
material.
7. The material as claimed in claim 1, in which the boron nitride
is present in the material in a mass proportion between 5 and
100%.
8. An extruded film made or that may be made from a material as
claimed in claim 1.
9. A casting film made from a material as claimed in claim 1.
10. A proton-exchange membrane for an electrochemical device,
especially a fuel cell, an electrolyzer or an accumulator,
comprising a layer of the material as claimed in claim 1.
11. A cell of a fuel cell, an electrolyzer or an accumulator,
comprising: an anode, a cathode, and a proton-exchange membrane
with or without catalytic layer, wherein at least one of the
cathode and the membrane comprising the material as claimed in
claim 1.
12. The cell as claimed in claim 11, in which the activated boron
nitride is pulverulent.
13. The cell as claimed in claim 11, wherein the proton-exchange
membrane is impermeable to hydrogen.
14. The cell as claimed in claim 11, wherein the proton-exchange
membrane has a thickness less than or equal to 500 .mu.m.
15. An electrolyzer comprising a cell as defined in claim 11.
16. A fuel cell comprising a cell as defined in claim and also a
circuit for conveying a fuel on the cathode side and a circuit for
conveying an oxidant on the anode side.
17. A process for activating a material for an electrochemical
device, comprising a matrix and boron nitride contained in the
matrix, comprising: exposing the material to a fluid , wherein the
fluid provides hydroxyl radicals --OH and/or H.sub.3O.sup.+ and/or
SO.sub.4.sup.2- ions and creates in the boron nitride B--OH,
B--SO.sub.4H, B--SO.sub.3H, N--SO.sub.4H, N--SO.sub.3H and/or
NH.sub.2 bonds.
18. The process as claimed in claim 17, in which the material is
exposed to an acidic solution.
19. The process as claimed in claim 17, in which the material is
exposed to a basic solution.
Description
[0001] This application is a continuation application of U.S.
application Ser. No. 12/920,588, which is a national stage
application of PCT/FR2009/050352, filed internationally on Mar. 4,
2009, the entire content of each of which is incorporated by
reference herein.
[0002] The present invention relates to materials used for the
preparation of a proton-exchange membrane comprising boron nitride,
intended for the manufacture of electrolyzers, fuel cells and/or
accumulators and in general any electrochemical device using
ion-selective membranes.
[0003] WO 2006/003 328 discloses the use of a boron nitride ceramic
for producing a collision between protons and electrons and storing
hydrogen.
[0004] Nafion.RTM. (from Dupont de Nemours) is commonly used for
producing membranes for fuel cells, for instance in patent
application WO 2004/067 611. This material has the drawback of
being very expensive and may become destructured at a working
temperature of greater than 80.degree. C. under normal
conditions.
[0005] There is thus a need to benefit from a material, which may
be less expensive, and which may be used at temperatures of up to
200.degree. C., for example about 95.degree. C., better still
120.degree. C., or even 150.degree. C., or even 180.degree. C.
[0006] The invention is especially directed toward proposing a
novel material for electrochemical devices, especially fuel cells,
electrolyzers or accumulators, which can afford good mechanical and
thermal stability and which can especially function at a relatively
high temperature just as at room temperature.
[0007] According to one of its aspects, one subject of the
invention is a material for an electrochemical device, especially a
membrane for a fuel cell, comprising a matrix and activated boron
nitride contained in the matrix.
[0008] According to one of its aspects, one subject of the
invention is a material for an electrochemical device, especially a
membrane for an electrolyzer, comprising a matrix and activated
boron nitride contained in the matrix.
[0009] The material comprising activated boron nitride may be used
to manufacture a fuel cell, an accumulator, an electrodialysis
device, a redox system for storing electrical energy, a
chlorine-sodium hydroxide electrolyzer or ion-selective
electrodes.
Activation of Boron Nitride
[0010] The term "activation of boron nitride" means a process for
promoting proton conduction in boron nitride. In activated boron
nitride, the number of B--OH, NH, B--SO.sub.4H, B--SO.sub.3H,
N--SO.sub.4H and N--SO.sub.3H bonds formed is sufficient to allow
the displacement of a proton from a group B--OH or BSO.sub.xH or
from a group NH or N--SO.sub.xH toward a lone pair of electrons on
a neighboring oxygen or nitrogen, or on a group OH or NH forming
groups NH.sub.2.sup.+, BOH.sub.2.sup.+, B--SO.sub.xH.sub.2.sup.+ or
N--SO.sub.xH.sub.2.sup.+.
[0011] Proton conduction may also be performed by means of lone
pairs on oxygen atoms inserted into nitrogen holes of the boron
nitride. Such nitrogen holes containing oxygen atoms may especially
be present when the boron nitride is obtained from B.sub.2O.sub.3
or from H.sub.3BO.sub.3.
[0012] Activated boron nitride may comprise B--OH, N--H,
B--SO.sub.4H, B--SO.sub.3H, N--SO.sub.4H and N--SO.sub.3H bonds
capable of being converted into BOH.sub.2.sup.+, NH.sub.2.sup.+,
B--SO.sub.xH.sub.2.sup.+ and N--SO.sub.xH.sub.2.sup.+, thus
allowing proton transfer via these sites created by means of the
activation.
[0013] Boron nitride may be in the form of grains stuck together,
for example percolated or sintered. The term "percolated" refers to
grains that are physically touching.
[0014] The material may comprise percolated grains of boron
nitride, for example maintained solidly attached to each other by
means of a matrix comprising a compound, for example a compound
from the following list: nickel, boron oxide, calcium borate, ethyl
cellulose, boric acid, polyvinyl alcohol (PVA), vinylcaprolactam,
PTFE (Teflon.RTM.), sulfonated polyether sulfone, this list not
being limiting.
[0015] The mass proportion of boron nitride in the material may be
between 5% and 100%, for example up to 70%. The material may be
made entirely of boron nitride powder sintered at high pressure. As
a variant, it may comprise boron nitride and a binder, being
manufactured, for example, via an HIP (Hot Isostatic Pressure,
high-pressure isostatic compression) process.
[0016] The boron nitride used may comprise at least one, for
example one or more, substituent components from the following
list: boron oxide, calcium borate, boric acid, hydrofluoric
acid.
[0017] The presence of such components may make it possible to
promote activation, especially when they are present in a mass
proportion of between 1% and 10%.
[0018] The presence of boric acid, for example present in the
porosities of the boron nitride or in amorphous form, may make it
possible to promote the creation of B--OH and NH bonds.
[0019] The material may comprise a mineral matrix, for example
comprising active charcoal or graphite, or alternatively boric
acid.
[0020] As a variant, the material may comprise organic matrix, for
example comprising at least one of the compounds from the following
list: polymer, fluorinated polymers, PVA, epoxy resin,
cellulose-based compounds. The material may be formed by boron
nitride, for example pulverulent, inserted, especially dispersed,
in a polymer matrix, which may afford the material very good proton
conductivity.
[0021] The polymer, for example PVA, may be used for plugging the
porosities present in the boron nitride. The addition of polymer
may be performed, for example, under vacuum, such that this polymer
is sucked into the porosities of the boron nitride.
[0022] The matrix may be made, for example, from a matrix precursor
that can be polymerized under the action of a stimulus, for example
by evaporation of one or more solvents, by a temperature increase
or by applying .gamma. radiation.
[0023] By way of example, PVA may be used in the matrix, by wetting
this mixture and then introducing the boron nitride. A crosslinking
agent is then added, for example glutaraldehyde, and a crosslinking
catalyst, for example acid, and the PVA is finally hot-crosslinked
at 40.degree. C. The PVA may preferably be hot-crosslinked and
immersed in a bath of acid. A BN membrane in a PVA matrix with a
conductivity of 10.sup.-1 S/cm may thus be obtained.
[0024] The matrix may itself be a proton conductor or, on the
contrary, may not be a proton conductor.
[0025] The matrix may comprise at least one of an organic material
and an inorganic material.
[0026] The matrix may comprise one or more of the compounds from
the following list, which is not limiting: inorganic compound, for
example silica, for example in the form of Aerosil.RTM., grafted
silica, fumed amorphous silica, organic silica with a thiol group,
silica with a phosphonic acid function, silica with
surface-anchored sulfonic acid, alumina, zirconia, sulfated
zirconia, titanium oxide, sulfonated titanium oxide, tungsten
trioxide, tungsten trioxide hydrate, heteropolyacid, for example
polytriacetylene (PTA), polymethacrylic acid (PTA), STA, SMA,
tungstophosphoric acid (TPA), molybdophosphoric acid (MBA),
disodium salt of tungstophosphoric acid (Na-TPA), phosphomolybdic
acid (PMA), hole heteropolyacid H.sub.8SiW.sub.11O.sub.39,
functionalized sulfonic heteropolyacid, PWA, silicotungstic acid,
PTA supported on SiO.sub.2, ZrO.sub.2 and TiO.sub.2, MCM-41 charged
heteropolyacid, mesoporous tungsten silicate material SI-MCM-41,
heteropolyacid charged with Y-zeolite, silicotungstic acid,
zirconium phosphate, zirconium sulfophenyl phosphate (ZRSPP),
hydrogenated zirconium phosphate Zr(HPO.sub.4).sub.2, zirconium
tricarboxybutyl phosphonate, zirconium sulfophenylene phosphonate,
Zr(HPO.sub.4).sub.10 (O.sub.3PC.sub.6H.sub.4SO.sub.3H).sub.10,
zirconium sulfophenylene phosphonate phosphate, sulfonated
zirconium phosphate, cesium salt of silicotungstic acid, silicated
multilayer nanoparticles, for example montmorillonite, laponite,
modified montmorillonite, for example sulfonated montmorillonite,
MCM-41, organic montmorillonite (OMMT), montmorillonite grafted
with organic sultones and with perfluorinated sultones,
phosphosilicates (P.sub.2O.sub.5-SiO.sub.2), phosphatoantimonic
acid, noble metals, for example platinum, ruthenium, platinum
silicate coated with Nafion.RTM., silver, zeolite, chabasite and
clinoptylolite, mordonite, phosphate, calcium phosphate, calcium
hydroxyphosphate, boron phosphate, organic compound, polymer,
Nafion.RTM., perfluorosulfonic acid, sulfonated polysulfone, PEO,
PTFE, polyaniline,
poly(vinylidene)fluoride-chlorotetrafluoroethylene, PEG, DBSA,
4-dodecylbenzenesulfonic acid, SEBSS (sulfonated styrene,
sulfonated styrene-(ethylene-butylene)), PVA, glutaraldehyde,
krytox, diphenyl silicate, diphenyl dimethoxysilicate, sulfonated
poly(ether sulfone), PVDF, Nafion.RTM. membrane NRE-212,
Cs.sub.2.5H.sub.0.5PWO.sub.40, PVDF-G-PSSA, polyvinylidene
fluoride, polyacrylonitrile, dodecatungstophosphoric acid,
sulfonated (poly)ether ether ketone (SPEEK), PEO, sulfonated
(poly(arylene ether sulfone), polyvinyl alcohol, PEEK (s-polyether
ether ketone), cardo sulfonated polyether sulfone, polyphenylene
oxide (PPO), polyethylene glycol, silica nanoparticles, divacant
tungstosilicate [.gamma.-SiW.sub.10O.sub.36].sup.8-, PWA, PBI, PEG,
polyethyleneimine (PEI), disulfonated poly(arylene ether sulfone),
Teflon.RTM., sulfonated divinylbenzene (crosslinked DVB),
polystyrene-grafted poly(ethylene-alt-tetrafluoroethylene),
poly(vinyl difluoride), polybenzimidazole, PVDF, cardo sulfonated
poly(ether ether ketone), poly(fluorinated arylene ether)S,
Nafion.RTM.115, polyimide, polyamideimide (PAI), polyvinylidene
fluoride (PVDF), styrene-ethylene-butylene-styrene elastomer
(SEVS), poly(sulfonated biphenyl ether sulfone),
polytetrafluoroethylene (PTFE), PBI.
[0027] The boron nitride may be activated during or at the end of
the membrane manufacturing process.
[0028] The production of a membrane with boron nitride may be
obtained, for example, via one or other of the following two
processes: the first process consists in manufacturing a membrane
by high-temperature sintering of a "raw" membrane prepared
according to the usual methods for manufacturing ceramic objects.
The second process consists in preparing a membrane by
incorporating boron nitride powder into an organic matrix serving
as binder, while ensuring percolation between the grains of boron
nitride.
[0029] For example, for the first process, boron nitride grains are
mixed with a polymeric binder in liquid form, this mixture being
poured onto a substrate, and then heated to a sufficient
temperature so as to cause calcination of the binder, for example
at a temperature of about 600 or 700.degree. C., such that the
boron nitride grains are mutually percolated on the substrate.
[0030] In an additional step, the result obtained is heated to a
temperature of about 1000-1500.degree. C. under a neutral
atmosphere, for example of nitrogen, ammonia or argon, causing
mutual sintering of the grains.
[0031] Finally, in an additional step, the substrate is removed to
give a rigid sheet of boron nitride composed of sintered grains,
for example between 50 .mu.m and 300 .mu.m thick. Activation is
performed after sintering, to avoid the risk of it being destroyed
by the sintering.
[0032] In another embodiment, it is also possible to incorporate
the pulverulent boron nitride into a molten inorganic binder, for
example boric acid. The activation may be performed under an
atmosphere of steam maintained at high temperature. The temperature
may be, for example, less than 600.degree. C., or even less than
500.degree. C., better still less than 400.degree. C., or even
about 300.degree. C.
[0033] The material may have sufficient mechanical cohesion to
allow it to be formed into a membrane, for example of between 50
.mu.m and 300 .mu.m thick.
[0034] According to the second process, the boron nitride may be
activated in its pulverulent form before insertion into the matrix,
for example into a polymer, or alternatively after insertion into
the matrix, for example as a function of the matrix used.
[0035] The material may be used in the form of a membrane whose
tensile strength, between 20.degree. C. and 180.degree. C., is
defined by an elastic modulus. The elastic modulus of a BN membrane
with a crosslinked PVA matrix may be, for example, between 10.sup.9
Pa and 10.sup.8 Pa.
[0036] The matrix may be water-insoluble. The term "insoluble"
should be understood as meaning that, after remaining for 600
seconds at 80.degree. C. in water, less than 10.sup.-2% of the
membrane passes into solution, even in the presence of an electric
field, for example an electric field of greater than 11 000 V/m,
i.e. a voltage of about 2.2 V applied to a membrane about 200 .mu.m
thick. The amount of boron nitride passed into solution may be less
than 10.sup.-8 mg/l.
[0037] The boron nitride may be in the form of grains stuck
together, for example percolated or sintered. The term "percolated"
should be understood as referring to grains that are physically
touching.
[0038] Commercially available boron nitride is electrically and
ionically insulating.
[0039] According to another of its aspects, a subject of the
invention is also an extruded, screen-printed film, membrane film
alone or with its electrodes, which is made or which may be made
from the material as defined above.
[0040] According to another of its aspects, a subject of the
invention is also a proton-exchange membrane for an electrochemical
device, especially a fuel cell, an electrolyzer or an accumulator,
comprising a layer of the material as defined above.
[0041] According to another of its aspects, a subject of the
invention is also a process for manufacturing such a membrane for
fuel cell or other applications, especially electrolyzer or
accumulator, in which the membrane is exposed to an acidic solution
and then rinsed.
[0042] Exposure of the membrane to acid may advantageously be
performed under an electric field of between 15 and 40 000 V/m, or
even of about 15 000 V/m, which may improve the activation
efficacy. A field of 15 000 V/m is equivalent to applying 1.5 V for
a boron nitride thickness of 100 .mu.m or 15 V for a thickness of 1
mm. The field may be at least 15 000 V/m, i.e. a voltage of about 3
V is applied to a membrane 200 .mu.m thick.
[0043] Total impermeability of the wall during the storage
production may make it possible to store within the electrode, for
example the cathode, the formed atomic and/or molecular hydrogen.
The adsorption of hydrogen necessary for this purpose may depend on
the nature of the electrode, for example the cathode. Specifically,
the presence of water in the electrode, for example the cathode,
may give rise to the risk of preventing the establishment of
molecular contact in the electrode, for example the cathode, thus
preventing the establishment of satisfactory electrical conduction,
and thus preventing the formation of hydrogen in the electrode, for
example the cathode, or at the interface. On the other hand, the
presence of water at the interface of the electrode and the
proton-exchange membrane may have no consequence on the system.
Specifically, water behaves like the continuity of the
ion-permeable wall due to its ion-conducting power. Moreover,
insofar as close to the electrode the medium is reductive by virtue
of the presence of hydrogen, the presence of water is not an
inconvenience for the storage.
[0044] At least one electrode, for example the anode, may be made
with any electrically conductive material that is compatible with
the Fr-ion donor, for example platinum, graphite, a thin layer of a
mixture of RuO.sub.2, IrO.sub.2 or RuO.sub.2, IrO.sub.2 and
TiO.sub.2 or RuO.sub.2, IrO.sub.2 and SnO.sub.2 lined with a plate
of porous titanium (for example 30% to 50%) or a conductive
polymer, inter alia. The thin layer may have a thickness of between
5 .mu.m and 20 .mu.m, for example about 10 .mu.m.
[0045] According to another of its aspects, a subject of the
invention is also an electrode for an electrochemical device,
especially a fuel cell, made at least partially of the material as
defined above. The electrode may be, for example, a cathode or an
anode. The material in accordance with the invention may be
metallized before activation.
[0046] At least one of the electrodes, for example the cathode, may
comprise activated boron nitride and active charcoal or graphite.
Said electrode, for example the cathode, may be embedded in a mass
of boron nitride. The electrode may comprise, for example, a metal
foam or any conductive material, for example active charcoal or
graphite, embedded in a mass of boron nitride.
[0047] At least one of the electrodes, for example the anode, may
comprise, for example, a thin layer of a mixture of RuO.sub.2,
IrO.sub.2 or RuO.sub.2, IrO.sub.2 and TiO.sub.2 or RuO.sub.2,
IrO.sub.2 and SnO.sub.2 lined with a plate of porous titanium (for
example 30% to 50%). The thin layer may have a thickness of between
5 .mu.m and 20 .mu.m, for example about 10 .mu.m.
[0048] One or other of the electrodes may be made in a pulverulent
form, being sprayed onto the membrane formed by the layer of boron
nitride mentioned above. After spraying, this layer may be
compressed in a press at a pressure of between 5 and 40 kg/m.sup.2,
for example about 20 kg/m.sup.2, at a temperature of between
15.degree. C. and 200.degree. C., for example between 25.degree. C.
and 150.degree. C., to improve the adhesion of the electrodes to
the membrane. The temperature depends on the nature of the layer,
depending, for example, on whether or not it comprises a polymer
that is sensitive to the applied maximum temperature.
[0049] According to another of its aspects, a subject of the
invention is also a cell of a fuel cell, an electrolyzer or an
accumulator comprising: [0050] a cathode, and [0051] a
proton-exchange membrane, at least one from among the cathode and
the membrane comprising, or even being formed from, the material as
defined above, especially the proton-exchange membrane.
[0052] The cell may also comprise an anode. The cathode of the cell
may be as defined above. As a variant, one or both of the
electrodes, anode and/or cathode, may comprise at least one of the
compounds of the following list, which is not limiting: platinum,
for example in the form of nanograins, active charcoal, a binder,
for example ethanol or a polymeric compound, for example PTFE, or
alternatively a mixture of these components. One or other of the
electrodes may be made in a pulverulent form, being sprayed onto
the proton-exchange membrane.
[0053] In another embodiment, the entire outer surface of the
cathode may be covered with a material comprising boron nitride,
for example activated boron nitride. The cathode may be immersed in
boron nitride, which may be pulverulent. The cathode may be rolled
upon itself, in a spiral form.
[0054] The cell may be of cylindrical general shape, the
proton-exchange membrane constituting a cylindrical envelope
surrounding the cathode.
[0055] The proton-exchange membrane may be impermeable to hydrogen.
The term "impermeable" should be understood as meaning that an
amount of hydrogen of less than 2% can cross a membrane 150 .mu.m
thick. The proton-exchange membrane may be non-porous. The term
"non-porous" should be understood as meaning that less than 2% of
gas can cross a membrane 150 .mu.m thick. The proton-exchange
membrane may have a nonzero surface porosity.
[0056] At least one of the electrodes, for example the anode, may
comprise at least one layer of a metallic compound in contact with
the proton-exchange membrane. The metallic compound may be chosen
from: platinum, gold, nickel, and alloys thereof.
[0057] One or both of the electrodes, anode and/or cathode, may be
as defined above. One or both of the electrodes, anode and/or
cathode, may comprise, for example, at least one of the compounds
from the following list, which is not limiting: platinum, for
example in the form of nanograins, boron nitride, especially
activated boron nitride as mentioned below, active charcoal, a
binder, for example ethanol or a polymeric compound, for example
PVA or PTFE, or alternatively a mixture of these components.
[0058] The thickness of the proton-exchange membrane may be less
than or equal to 1 mm, for example being between 50 .mu.m and 300
.mu.m.
[0059] The cell may also comprise a support substrate for the
proton-exchange membrane. The substrate may be chosen from:
alumina, zirconia, porous boron nitride, and mixtures thereof.
[0060] The substrate may comprise, for example, a grille, one or
more yarns, a foam, a film or a plate, for example a pierced plate.
The substrate may comprise, for example, a thin woven, made of a
polymer, for example polyamide, for example Nylon.RTM..
[0061] According to one of its aspects, a subject of the invention
is also an electrolyzer comprising a cell as defined above.
[0062] According to one of its aspects, a subject of the invention
is also a fuel cell comprising a cell as defined above, and also a
circuit for conveying a fuel on the cathode side and a circuit for
conveying an oxidant on the anode side.
[0063] The fuel may be hydrogen gas. The oxidant may be air or
oxygen. The hydrogen intended to feed the cell may be stored in
hydride form in the material defined above.
[0064] The use of the electrolyzer or of the fuel cell in
accordance with the invention may be performed at relatively high
temperatures, for example of about 80.degree. C. for the
electrolyzer and 150.degree. C. for the fuel cell.
[0065] According to one of its aspects, a subject of the invention
is also a process for activating a material for an electrochemical
device, comprising a matrix and boron nitride contained in the
matrix, in which the material is exposed to a fluid for providing
hydroxyl radicals --OH and/or H.sub.3O.sup.+ or SO.sub.4.sup.2-
ions and for creating in the boron nitride B--OH and/or
B--SO.sub.4H, B--SO.sub.3H, N--SO.sub.4H, N--SO.sub.3H and/or NH
bonds.
[0066] For this activation, the boron nitride, or the membrane, may
be exposed to a fluid for providing H.sub.3O.sup.+ or
SO.sub.4.sup.2 ions and for creating in the boron nitride B--OH
and/or B--SO.sub.4H, B--SO.sub.3H, N--SO.sub.4H, N--SO.sub.3H
and/or NH bonds. The fluid may be, for example, an acidic solution
containing H.sub.3O.sup.+ ions, for example strong acids such as
HCl, H.sub.2SO.sub.4, H.sub.3PO.sub.4, H.sub.2S.sub.2O.sub.7, or
weak acids, for example in the presence of an electric field, or
alternatively may not be an acidic solution, but, for example, a
basic solution containing OH.sup.- ions, for example a sodium
hydroxide or potassium hydroxide solution. The concentration of the
solution may have an influence on the rate and level of activation
obtained, i.e. on the level of proton conductivity obtained, but
not on the onset of activation itself. The acid concentration may
be, for example, between 1 and 18 mol/L and the sodium hydroxide
concentration may be between 0.5 and 1 mol/L.
[0067] In order to promote the creation of B--OH, B--SO.sub.4H,
B--SO.sub.3H, N--SO.sub.4H, N--SO.sub.3H and/or NH bonds, the
membrane containing the boron nitride may be subjected to an
electric field, for example an electric field of between 15 and 40
000 V/m, or even of about 15 000 V/m, in the presence of a 1 molar
H.sub.2SO.sub.4 solution, for example. A field of 15 000 V/m is
equivalent to applying a voltage of 1.5 V for a material thickness
of 100 .mu.m or alternatively 15 V for a thickness of 1 mm.
[0068] The electric field may be delivered by an external
generator.
[0069] The applied voltage may be, for example, between 1.5 V and
50 V, for example about 30 V. The voltage source may be constant
or, as a variant, inconstant. It may be configured to detect the
end of activation automatically, for example when the current
density in the material increases abruptly. The intensity of the
current circulating during activation in the material may be from
about 10 mA/cm.sup.2 to 1000 mA/cm.sup.2.
[0070] The electrodes used, cathode and anode, may be made, for
example, of platinum, or of platinized titanium at the cathode and
of titanium with IrO.sub.2 at the anode. The electrodes may be, for
example, of planar or, as a variant, nonplanar shape. They may be,
for example, in the form of a sinter.
[0071] The presence of an electric field may make it possible, for
example, to overcome the natural hydrophobicity of boron nitride.
Dihydrogen may be produced at the cathode during the activation,
followed by dioxygen at the anode. The dihydrogen formed at the
cathode and the dioxygen formed at the anode may be recovered in
reservoirs.
[0072] The fluid may be forced to circulate through the boron
nitride, for example by means of a pump.
[0073] The activation with a fluid may be performed at a
temperature of between 0 and 90.degree. C., for example about
60.degree. C., or even at room temperature.
[0074] The boron nitride may be activated in a basic solution, for
example sodium hydroxide, with or without application of an
electric field.
[0075] After having been exposed to the solution, the boron nitride
may be rinsed and optionally dried before being used to manufacture
a fuel cell, an electrolyzer or an accumulator. The fluid may be
removed such that its residual content is less than 2%.
[0076] The step of exposure to the fluid may have a duration of
between 10 and 50 hours, or even between 15 and 72 hours and better
still, for example, between 10 and 40 hours, and even better still
between 10 and 20 hours.
Boron Nitride
[0077] The boron nitride used may preferably be crystallized in a
hexagonal form. The material may, for example, comprise
turbostratic boron nitride, i.e. boron nitride in which the
crystallization planes may be slightly offset relative to the
theoretical position of crystallization, for example hexagonal
crystallization of the boron nitride, which leads to staggering of
the stacks of planes and poorer maintenance of planes to each
other, these planes being slightly more separated.
[0078] The material may comprise hexagonal boron nitride grains
stuck together, for example grains with a median size of greater
than 1 nm, or even greater than 10 nm, or even greater than 5
.mu.m, and less than 20 .mu.m. The boron nitride may be in the form
of grains, for example with a median size of between 5 and 15
.mu.m, or even between 7 and 11 .mu.m, or even of about 10 .mu.m.
The grains may themselves be composed of crystallites with an
average size of between 0.1 and 0.5 .mu.m.
[0079] When the material is in the form of a layer, the boron
nitride grains may preferably be oriented not all parallel to the
layer, but, for example, perpendicular to it, so as to ensure
better mechanical strength, or alternatively heterogeneously, so as
to ensure better proton conduction of the material.
Matrix
[0080] The matrix may comprise percolated boron nitride grains, for
example grains solidly attached to each other via a compound
forming a matrix. This matrix may be obtained from all kinds of
organic or inorganic materials, or hybrid materials that are both
organic and inorganic.
[0081] According to one particular embodiment of the invention, the
inorganic material may be a glass or a gel, obtained, for example,
from boric acid or borates, silica, silicates, silica gels,
alumina, alumina gels, clays or modified clays of zeolites, or from
any suitable combination, this list not being limiting.
[0082] The organic matrices may be formed from natural, artificial
or synthetic macromolecular compounds, for example cellulose,
modified celluloses, vinyl polymers, polyvinyl alcohol (PVA),
polyamides, polyesters, polyethers, epoxy resins, aromatic polymers
such as polyether ketones, polyether sulfones, sulfonated polyether
sulfones, polyoxyphenylene, polyphenylene sulfide, fluorinated and
perfluorinated polymers (PVDF, PTFE, Teflon.RTM., etc.) and any
organic material obtained from the preceding by chemical
modification, grafting and/or crosslinking, in order to obtain a
matrix with thermomechanical properties suited to the chosen
application.
[0083] The mass proportion of boron nitride in the material may be
between 5% and 100%, for example in the region of 70%, or even 80%,
or even 98%. A proportion of 100% or thereabouts refers to a
material entirely made of high-temperature-sintered boron nitride
powder comprising only a small proportion of possible binder. The
material may also be obtained from boron nitride and a binder. The
method for preparing the material will be chosen as a function of
the properties of the matrix material and of the desired
properties.
[0084] The material may be formed from boron nitride, for example
pulverulent, inserted, especially dispersed, in a polymer membrane,
which may afford the material very good proton conductivity. The
polymer, for example PVA, may also be used to plug the porosities
present in the boron nitride. The addition of the polymer may be
performed, for example, under vacuum, using a viscous solution,
such that the polymer is sucked into the porosities of the boron
nitride, and then stabilized by crosslinking according to a known
method. The matrix may also be made from a precursor that is
polymerizable under the action of a stimulus, for example by
evaporation of one or more solvents, an increase in temperature or
by applying gamma radiation, or thermal decomposition of the
polymerization initiator.
[0085] By way of example, PVA may be used for the matrix, by
dissolving this polymer in water at 80.degree. C., and then
introducing the boron nitride. A crosslinking agent is then added,
for example glutaraldehyde, and a crosslinking catalyst, for
example acid, and after pouring and evaporating, the PVA is
hot-crosslinked at 40.degree. C. The membrane obtained may
preferably be hot-crosslinked by being immersed in an acid bath. A
BN membrane in a PVA matrix with a conductivity of 10.sup.-1 S/cm
in an electrolyte of 0.5 molar sulfuric acid may thus be
obtained.
Additives
[0086] The matrix may itself be a proton conductor or, on the other
hand, it may not be a proton conductor, or it may contain a proton
conductor.
[0087] The matrix may comprise a mixture of organic and inorganic
compounds.
[0088] The matrix may thus comprise one or more of the compounds
from the following list, which is not limiting: inorganic compound,
for example silica, for example in the form of Aerosil.RTM.,
amorphous silica gel, silica grafted with organic groups with
phosphonic acid or sulfonic acid functions, alumina, zirconia,
zirconium hydrogen sulfate, titanium oxide, sulfonated titanium
oxide, tungsten trioxide, tungsten trioxide hydrate,
phosphomolybdic acid (PMA), tungstophosphoric acid (TPA),
molybdophosphoric acid, zeolite charged with heteropolyacid, hole
heteropolyacid H.sub.8SiW.sub.11O.sub.39, silicated multilayer
nanoparticles, for example montmorillonite, laponite, modified
montmorillonite, for example sulfonated montmorillonite, MCM-41,
montmorillonite grafted with organic sultones and with
perfluorinated sultones, phosphosilicates
(P.sub.2O.sub.5--SiO.sub.2), cellulose-based compounds, this list
not being limiting.
[0089] The matrix may also contain a proton-conducting organic
material, for example: Nafion.RTM., perfluorosulfonic acid,
sulfonated polysulfone, sulfonated polystyrene, sulfonated
polystyrene-(ethylene-butylene), cardo sulfonated poly(ether ether
ketone), polyvinylsulfonic acid,
polyamidomethylpropaneacrylosulfonic acid (PAMPS) and copolymers
thereof, this list not being limiting.
[0090] According to another of its aspects, a subject of the
invention is also an extruded film, which itself forms the membrane
used. The extrusion may be performed by means of an extruder fitted
with a head that leads to the formation of a film at the outlet.
The extruded material may be formed of a suspension of activated
boron nitride in a viscous solution of a suitable polymer, which
can be crosslinked after extrusion. The film obtained may be used
in unmodified form or modified. For example, electrodes may be
deposited on its surface by screen printing.
[0091] According to another of its aspects, a subject of the
invention is a proton-exchange membrane for an electrochemical
device, especially a fuel cell, an electrolyzer or an accumulator,
comprising a layer of the material as defined above and other
layers of material suited for the intended use.
[0092] The ion-permeable wall may preferably have a water
permeability of less than 5% of the mass of the transported ions,
for example of the mass of hydrogen produced, in the case where
H.sub.2 is produced.
[0093] The ion-permeable wall may have a zero water permeability,
measured under standard temperature and pressure conditions, with
water in liquid form, or even in the form of steam, at a
temperature of less than 600.degree. C. and a pressure difference
across the membrane not exceeding 4 bar.
[0094] The total water impermeability of the wall may be
particularly useful for hydride-treating water-sensitive metal
alloys. During the production and storage of hydrogen, it may allow
the storage in a cathode (electrode toward which the H.sup.+ ions
migrate) as a hydridable alloy the atomic and/or molecular hydrogen
formed during an electrolysis. Water-mediated corrosion of the
hydridable alloy is thus avoided. The hydride-treated material
obtained may in turn serve as an anode, by reversing the direction
of the current, and provide H.sup.+ ions that migrate without water
across the membrane containing the activated boron nitride.
[0095] On the other hand, the presence of water at the interface of
the electrode and the proton-exchange membrane may be beneficial
for other systems. Specifically, water containing acid, for example
at 0.5 mol, behaves like the continuity of the ion-permeable wall
due to its ion-conducting power.
[0096] The material of the catalyst may comprise, as a function of
the desired properties, a metallic or electrically conductive
inorganic material, for example active charcoal or graphite, or any
kind of material used in electrochemical devices: noble metals,
ruthenium, platinum, divided nickel, silver, cobalt, for example
platinum coated with Nafion.RTM., this list not being limiting,
making it possible to form the catalytic layers on either side of
the membrane in contact with the anode and cathode electrodes.
[0097] The anode may be made with any electrically conductive
material, for example platinum, graphite, for example deposited on
a plate of porous titanium (for example with a porosity of 30% to
50%) onto which is deposited a thin layer of catalysts such as
IrO.sub.2 or RuO.sub.2, a mixture of RuO.sub.2 and IrO.sub.2 and
TiO.sub.2, IrO.sub.2 and SnO.sub.2. The thin layer may have a
thickness of between 5 .mu.m and 50 .mu.m, or even between 5 and 20
.mu.m, for example about 10 .mu.m. The amount of catalyst per
cm.sup.2 may be from about 1 to 10 mg/cm.sup.2 and better still
between 1 and 3 mg/cm.sup.2, or even about 2 mg/cm.sup.2.
[0098] The cathode may also comprise activated boron nitride and
active charcoal or graphite. The cathode may be embedded in a mass
of boron nitride. The electrode may comprise, for example, a plate
of porous titanium (for example with a porosity of 30% to 50%)
containing a thin layer of catalysts such as platinum, palladium or
a mixture of platinum, palladium, nickel and cobalt. The thin layer
may have a thickness of between 5 .mu.m and 20 .mu.m, for example
about 10 .mu.m. The amount of catalyst per cm.sup.2 is about 0.1 to
1 mg, or even about 0.5 mg/cm.sup.2.
[0099] One or other of the catalytic layers may be made in a
pulverulent form, being sprayed onto the membrane formed by the
boron nitride layer mentioned above. After spraying, this layer may
be compressed in a press at a pressure of between 5 and 40
kg/cm.sup.2, for example about 20 kg/cm.sup.2, at a temperature of
between 15.degree. C. and 200.degree. C., for example between
25.degree. C. and 150.degree. C., to improve the adhesion of the
electrodes to the membrane. The temperature depends on the nature
of the layer, depending, for example, on whether or not it
comprises a polymer that is sensitive to the applied maximum
temperature.
[0100] According to another of its aspects, a subject of the
invention is also a cell of a fuel cell, electrolyzer or
accumulator comprising: [0101] a cathode, for example a catalytic
layer for the cathode, a proton-exchange membrane, for example a
catalytic layer for the anode and an anode, at least one from among
the cathode with its catalytic layer and the membrane comprising,
or even being formed from, the material as defined above,
especially the proton-exchange membrane.
[0102] The proton-exchange membrane may preferably be nonporous.
The proton-exchange membrane will have, for example, zero surface
porosity on at least one face. The proton-exchange membrane may
preferably be impermeable to hydrogen. The term "impermeable"
should be understood as meaning that an amount of hydrogen of less
than 2% of the amount produced or used can cross a membrane 150
.mu.m thick.
[0103] One or both of the catalytic thin layers of the anode and/or
the cathode may comprise, for example, at least one of the
compounds from the following list, which is not limiting: platinum,
for example in the form of nanograins, boron nitride, especially
activated boron nitride as mentioned below, active charcoal, a
binder, for example a polymeric compound, for example Nafion.RTM.,
PVA or PTFE, or a mixture of these components.
[0104] The thickness of the proton-exchange membrane may be less
than or equal to 1 mm, for example between 50 .mu.m and 300
.mu.m.
[0105] The cell may also comprise a support substrate for the
proton-exchange membrane. The substrate may be chosen from various
inorganic or organic materials that are compatible with the chosen
application: for example alumina, zirconia, porous boron nitride
and mixtures thereof, this list not being limiting.
[0106] The substrate may, for example, comprise a grille, one or
more yarns, nano yarns, a foam, a film or a plate, for example a
pierced plate. The substrate may comprise, for example, a thin
woven, made of a polymer, for example of polyamide, for example of
Teflon.RTM..
[0107] According to one of its aspects, a subject of the invention
is also an electrolyzer comprising a cell as defined above.
[0108] The invention may be understood more clearly on reading the
detailed description that follows, of a nonlimiting implementation
example thereof, and on examining the attached drawing, in
which:
[0109] FIG. 1 is a schematic partial view of a fuel cell comprising
activated boron nitride in accordance with the invention,
[0110] FIG. 2 is a schematic partial view of a proton-exchange
membrane for producing an electrolyzer membrane,
[0111] FIG. 3 is a schematic partial view of a proton-exchange
membrane for producing an accumulator,
[0112] FIG. 4 is a schematic partial perspective view of an
activation device,
[0113] FIG. 5 is a schematic partial top view of one variant of the
activation device,
[0114] FIG. 6 is a block diagram illustrating the process in
accordance with the invention,
[0115] FIG. 7 illustrates the variation in current density during
the activation of a membrane in accordance with the invention
according to one of the electrochemical activation methods,
[0116] FIG. 8 represents the infrared spectra of activated boron
nitride and of raw boron nitride,
[0117] FIG. 9 illustrates the X-ray diffractogram of a pellet of
activated boron nitride powder, and
[0118] FIGS. 10 and 11 illustrate the characteristic curve of the
membranes of examples 2 and 4.
[0119] In the drawing, the relative proportions of the various
components have not always been respected, for the sake of
clarity.
[0120] FIG. 1 represents, schematically and partially, a fuel cell
1 comprising a proton-exchange membrane 2 formed from a material
comprising a matrix and activated boron nitride contained in this
matrix. In the example described, they are grains of activated
hexagonal boron nitride h-BN linked via a polymer. The fuel cell 1
comprises an anode 3 on one side of the proton-exchange membrane 2
and a cathode 4 on the other side.
[0121] The anode 3 comprises, for example, a layer serving for the
oxidation reaction, for example a metallic compound such as
platinum or gold, or a composite such as graphite platinum or
graphite gold, and the cathode 4 comprises a layer of a catalyst
for the reduction reaction, for example a layer of platinum,
nickel, graphite nickel, graphite platinum, active-charcoal
platinum, active-charcoal platinum PVA or active-charcoal platinum
PVA with activated BN, each layer possibly being in contact with
the membrane 2.
[0122] The proton-exchange membrane 2 and the two layers arranged
on either side of it may be supported by a porous substrate 6, for
instance a layer of porous titanium acting as a current
collector.
[0123] Electrical conductors may contact the anode 3 and the
cathode 4.
[0124] The anode 3 may comprise, for example, on the layer serving
for the oxidation reaction, a deposit of gold, for example in the
form of a frame 10, so as to harvest the electrical current.
[0125] The thickness of the exchange membrane 2 is, for example,
100 .mu.m, and that of the layers serving for the oxidation
reaction and as catalyst ranges, for example, from 10 to 50 .mu.m,
or even to 30 .mu.m.
[0126] In one implementation example of the invention, the
proton-exchange membrane 2 is made from an h-BN boron nitride
ceramic of reference HIP from the company Saint-Gobain, activated
by exposure to sulfuric acid.
[0127] The exposure to acid may be performed, for example, for
several hours, for example at room temperature or at a higher
temperature, for example up to 300.degree. C., the sulfuric acid
being, for example, at a concentration of 0.1 M to 18 M, for
example 18 M. During this exposure, the membrane may, where
appropriate, be exposed to an electric field of about 30 000 V/m,
i.e. a voltage of 3 V when the membrane thickness is 100 .mu.m,
which may improve the quality of the activation, for example in the
case of exposure to an acid of low molarity. The ceramic is rinsed
after the exposure to acid. Without being bound by a theory, the
activation may make it possible to modify the side bonds of the
boron nitride grains.
[0128] When the membrane is activated in the presence of an
electric field, this electric field may be generated between two
electrodes. The anode may or may not be in contact with the
membrane and is in contact with the acidic electrolyte and water.
The cathode must be in contact only with the membrane, and not in
contact with the acidic electrode and water.
[0129] As a variant, the cathode may itself also be immersed in the
acid in a cathode compartment. In this case, there are two
compartments, an anode compartment and a cathode compartment,
separated in leaktight manner by a membrane. Each compartment
contains acid and the electrodes are or are not in contact with the
membrane.
[0130] As another variant, the boron nitride may be deposited in
powder form in a crucible 2 in which is also inserted the cathode
15, as illustrated in FIG. 4. The crucible may be made of boron
nitride, so as to promote the activation. The assembly is then
immersed in the electrolyte.
[0131] In another variant, the cathode 15 may have a spiral shape,
as illustrated in FIG. 5.
[0132] The electrodes may be electrodes that serve only for the
activation process and that are not useful thereafter, not being
present, for example, in the system using the membrane. They may
also be electrodes of which one is present in the final system,
especially the cathode, for example.
[0133] At least one of the electrodes serving for the activation,
or even both of them, may be in contact with the membrane and may,
for example, be permanently attached thereto. One of the electrodes
serving for the activation is, for example, a platinum anode, other
electrically conductive components possibly being used, provided
that they do not oxidize and do not degrade quickly.
[0134] The anode may also be made of porous platinum if it is in
contact with the membrane. The other electrode, which is also
porous, is a cathode made of a suitable electrically conductive
material. These electrodes may be plated, for example via processes
of deposition in thin layers, against the membrane.
[0135] In one variant, electrically conductive layers are deposited
on either side of the boron nitride layer, for example layers of
porous titanium containing platinum at the surface. The membrane
thus coated is then exposed to the acid to activate it, in the
presence of an electric field applied by means of the conductive
layers.
[0136] Once the exposure to acid has been performed, the membrane
may be rinsed.
[0137] Needless to say, it does not constitute a departure from the
context of the present invention to make modifications to the
examples that have just been given above.
[0138] It is especially possible to platinum-coat the exchange
membrane only on the cathode and to use a porous titanium plate as
current collector. It is also possible to coat the exchange
membrane only with an alloy of oxides on the anode and to use only
a porous titanium plate as current collector.
[0139] As another variant, one or both of the electrodes, anode
and/or cathode, may be at least partially made of a material
comprising a matrix and activated boron nitride contained in the
matrix.
[0140] The proton-exchange membrane may have various shapes, for
example a flat or cylindrical shape.
[0141] In the example of FIG. 2, the proton-exchange membrane 2 is
used in an electrolyzer comprising a cathode 20 made of porous
titanium surface-coated with platinum, the anode 30 being, for
example, porous titanium surface-coated with iridium oxide
IrO.sub.2.
[0142] In the example of FIG. 3, the exchange membrane 2 is used in
an accumulator, the anode 40 being made, for example, of porous
titanium surface-coated with iridium oxide IrO.sub.2 and platinum
and in contact with an aqueous acidic electrolyte, for example a
sulfuric acid solution, while the cathode 50 comprises a hydridable
material.
[0143] As another variant, at least one of the electrodes,
especially the cathode, may be at least partially made of a
material in accordance with the invention.
[0144] An example of an activation process in accordance with the
invention will now be described with reference to FIG. 6.
[0145] Use is made of boron nitride that may be hexagonal, or even
turbostratic hexagonal, and which, in one embodiment of the
invention, is in a pulverulent form. The activation may thereby be
facilitated. The boron nitride grains may be percolated. The boron
nitride may contain at least one additive that may promote the
activation, for example oxygen and boron oxide.
[0146] The selected boron nitride is, in a first step 61, exposed
to a fluid for providing hydroxyl radicals --OH, for example an
acidic or basic solution, or even quite simply water.
[0147] Activation of the boron nitride may, for example, be
performed as follows.
[0148] 18 M acid (99.99% pure) may be used. The boron nitride in
powder form is formed from polycrystalline grains of variable size,
which depends on the manufacturing methods used. Each BN crystal
contained in a polycrystalline BN grain is in contact with other
crystals at the edge of the leaflet. This crystalline structure may
be represented schematically in the form of stacks of leaflets.
[0149] In the case of activation of the BN powder with acid, the
infrared spectroscopic signature by FTIR spectroscopy (Fourier
Transformed Infrared spectroscopy) changes according to the degree
of activation.
[0150] The pulverulent boron nitride may be placed in a crucible
that is itself made of boron nitride, so as to ensure the proton
conductivity, or in another material.
[0151] In a second step 62, bonds are created between the boron
nitride and the hydroxyl radicals, especially B--OH bonds, or even
optionally bonds with nitrogen atoms of the boron nitride,
especially between the protons and the nitrogen atoms of the boron
nitride, for example N--H bonds, or even B--SO.sub.3H,
B--SO.sub.4H, N--SO.sub.3H and/or N--SO.sub.4H bonds.
[0152] The activation may be promoted by applying an electric field
in step 63, for example by means of a cathode and an anode soaked
in the solution, one or both of these electrodes possibly being
used and subsequently stored for the manufacture of the fuel
cell.
[0153] The activation may be promoted by increasing the temperature
of the boron nitride-acid mixture.
[0154] After activation, the activated boron nitride may be
recovered in a step 64, and may optionally be rinsed in a step 65
before performing the manufacture of a fuel cell in a step 66.
[0155] The boron nitride used may be combined with the matrix, for
example a polymer, before the activation, or after the activation
and before the manufacture of the fuel cell.
[0156] The result of the activation on the boron nitride will now
be described with reference to FIGS. 7 and 8.
[0157] FIG. 7 illustrates the change in current density and in
voltage during the activation of boron nitride. It is observed that
the current density D increases abruptly after a certain time,
namely about 30 hours in the example described, which illustrates
the fact that proton conduction in the boron nitride does indeed
take place.
[0158] FIG. 4 illustrates the infrared spectra of activated boron
nitride A and of raw boron nitride B, i.e. before activation.
[0159] It is found by observing these spectra A and B that they are
of different shape. The presence of two troughs in spectrum A that
are not present in spectrum B may be noted. These troughs
illustrate the presence of B--OH and N--H bonds, which result from
the activation of the boron nitride.
[0160] The activation of the boron nitride may also be observed by
measuring its proton conductivity. Raw, i.e. unactivated, boron
nitride may have a proton conductivity of about 10.sup.-5
Siemens/cm, whereas activated boron nitride may have a proton
conductivity of about 10.sup.-2 to 10.sup.-1 Siemens/cm, for
example 10.sup.-1 Siemens/cm.
[0161] For comparative purposes, Nafion.RTM. may have a proton
conductivity of about 8.6.times.10.sup.-2 Siemens/cm.
[0162] It is possible, for example, to use a boron nitride membrane
preparation process with a machine of SPS type (spark plasma
sintering). This technique consists in passing a high electric
current between two graphite electrodes, which, via the Joule
effect, will undergo a rapid and strong increase in temperature.
Between the two electrodes is placed a graphite mould containing
boron nitride powder, the powder-mould interface being made with
paper. When the current is applied, the powder is pressed.
[0163] In contrast with the usual press techniques, the SPS
technique makes it possible to dispense with the presence of a
sintering component, such that the result is thus a pellet formed
exclusively from the desired material, in this case boron
nitride.
[0164] In this example, a commercial HCV boron nitride powder is
used. Commercial HCV BN powder allows a pellet to be obtained.
After pressing, the pellet is polished to remove the paper
remaining at the surface of the sample.
[0165] The pellet obtained is analyzed by X-ray diffraction. The
diffractogram obtained is presented in FIG. 9 on curve C along with
the diffractogram of the commercial HCV powder that has not been
subjected to an SPS treatment, on curve D. It may be noted that the
pellet shows much higher crystallinity than the untreated HCV
powder. Families of planes such as the families (103) or (104) that
are not observable for the untreated HCV powder appear on the
diffractogram of the pellet. It may also be noted that the line
intensities are not in accordance with the standard intensities of
a hexagonal boron nitride powder. Such a difference may be
explained by the fact that, in the case of the pellet, the grains
possibly do not have a totally random orientation as in the case of
a powder, thus promoting the diffraction of certain families of
planes.
[0166] Thus, it may be observed in particular that the relative
intensities of the lines of the families (104), (110) and (112) are
not in accordance with the intensities normally expected for boron
nitride of hexagonal crystal structure. Specifically, the lines
(110), (112) are too intense relative to the line (104).
[0167] Examples of preparation of an activated boron nitride
membrane with PVA will now be described.
[0168] Among the various polymers proposed, polyvinyl alcohol (PVA)
is a cheap biocompatible polymer that has a good capacity to form
films, by pouring a solution of PVA (in water or in an organic
solvent), with excellent mechanical properties and good chemical
stability. It moreover has the advantage of being readily
crosslinkable, due to a high number of OH groups, which affords it
good mechanical strength in wet medium. The PVA used is supplied by
Celanese; it is the grade Celvol E 4/98 (degree of hydrolysis of
98.3 mol %) with a molecular mass of 31 000. Many methods are
proposed in the literature for the crosslinking of PVA: we opted
for chemical crosslinking with glutaraldehyde in acidic medium[8]
A. Martinelli, A. Matic, P. Jacobsson, L. Borjesson, M. A. Navarra,
A. Fernicola, S. Panero, B. Scrosati, Solid State Ionics, 177,
2431-2435 (2006).
EXAMPLE 1
[0169] We manufacture a membrane containing 20 g of activated BN
per 10 g of PVA.
[0170] 70 mL of water and 10 g of PVA are introduced into a 150 mL
beaker; the whole is brought to 90.degree. C. and stirred with a
paddle stirrer until dissolution of the polymer is complete (about
30 minutes), and then 20 g of activated BN powder are sprinkled in
(at 90.degree. C.), with continued stirring, until wetting of the
BN is complete.
[0171] The solution is cooled to room temperature and 0.2 mL of GA
(50% solution) is added with stirring. The solution is divided into
two parts. 0.5 mL of sulfuric acid (2 M) is added to the first
part.
[0172] The final mixture is rapidly poured into Petri dishes (10 cm
diameter). Drying is performed in a fume cupboard at room
temperature (12 hours). Membrane A is obtained.
[0173] Tests were performed in parallel, by pouring the mixture
PVA/BN/GA when the membrane is dry (about 12 hours at room
temperature or 3 hours in an oven at 60.degree. C.), it is dipped
in 2 M H.sub.2SO.sub.4 for about 1 hour to give membrane B.
[0174] The membranes obtained have thicknesses of 200 to 500 .mu.m.
The membrane obtained is not rigid, but it needs to be used
hydrated in order to have a soft consistency.
[0175] The permeability test with aqueous 0.2% erythrosine solution
is in accordance, the liquid does not pass through and does not
migrate at the surface for the membranes thus prepared.
[0176] The conductivity measurements on the two membranes are as
follows:
TABLE-US-00001 0.1M KCl 25 .OMEGA. Membrane A in 0.1M KCl 10 g BN
Thickness 250 .mu.m 42 .OMEGA. Membrane B in 0.1M KCl 10 g BN
Thickness 500 .mu.m 58 .OMEGA.
[0177] It is thus noted that these membranes are conductive.
[0178] The scanning electron microscopy examinations show that the
cast membranes have different surfaces as a function of the
contacts.
[0179] The surface dried in the open air is uniform with few
visible pores.
[0180] The surface dried in contact with glass (Petri dish) has
many pores, although each BN grain is coated with PVA and is bonded
together by the polymer.
EXAMPLE 2
[0181] Another manufacturing method is used to manufacture BN/PVA
membranes with a proportion of 35 g of activated BN per 15 g of
PVA.
[0182] Activation of the BN powder: 35 ml of 18 molar sulfuric acid
are added to 35 g of BN powder. 12 hours later, the mixture is
rinsed and filtered twice with 500 ml of water each time.
[0183] Preparation of the PVA: 10 g of PVA of molecular weight 186
000 (not limiting, other molecular weights may be used) is brought
to 80.degree. C. in 140 ml of demineralized water. 10 g of PVA of
molecular weight 31 000 (not limiting, other molecular weights may
be used) is brought to 80.degree. C. in 70 ml of demineralized
water.
[0184] Preparation of the membrane "casting" ink (manufacture): a
homogeneous mixture of activated BN powder is obtained with PVA 186
000 and 39 g of the solution of PVA 31 000. A PVA crosslinking
agent is added to this mixture. In this example, it is 0.7 ml of
glutaraldehyde with 7 ml of 1 molar sulfuric acid.
[0185] This final mixture allows casting of the membranes, which
may be crosslinked under various temperature and humidity
conditions. In the case of this example, the membranes were
crosslinked at a temperature of 18.degree. C. and 23% humidity.
[0186] The characteristics of these membranes are as follows:
conductivity in 1 molar acid: 2.times.10.sup.-1 S/cm.
[0187] They are flexible in water.
[0188] They are leaktight to hydrogen at a pressure of 1 bar. The
electrolysis behavior is illustrated in FIG. 10.
[0189] To obtain these characteristic curves, we deposited
nanostructured catalytic layers of iridium oxide (anode side, 3
mg/cm.sup.2) and of platinum (cathode side, 1 mg/cm.sup.2) in
accordance with the prior art on either side of the membrane.
[0190] In the context of using the membrane in a fuel cell, the two
catalytic layers are formed from nanostructured platinum.
EXAMPLE 3
Teflon-Coated BN Membrane
[0191] One of the techniques for manufacturing carbon electrodes
consists in using Teflon as a binder between the carbon grains.
This technique is applied to BN in order to obtain membranes that
are flexible and more heat-stable than PVA-based membranes.
[0192] Proportions of 7 g of BN and 8 g of an aqueous Teflon
suspension (60%), i.e. about 5 g of pure Teflon, are used.
[0193] The mixture is blended by hand while moistening with water
and ethanol until a very supple dough that can be spread using a
roll mill is obtained.
[0194] In a second series of tests, the amount of Teflon is reduced
so as to reduce the hydrophobicity of this membrane. Thus, the
formulation is produced with 7 g of activated BN and 3 g of Teflon
suspension (i.e. about 2 g of pure Teflon).
[0195] The membranes obtained in both cases may have thicknesses of
120 to 250 .mu.m.
[0196] The membranes obtained are porous, and the pores are plugged
with PVA. The deposition of PVA is then performed using a solution
containing 10 g in 70 mL with 0.2 mL of GA (see preceding
example).
[0197] The membrane is cut to size to be placed in a Buchner
funnel, the filtering part of which is covered with a nylon filter,
20 mL of this mixture are poured onto the BN/Teflon membrane,
turned over so as to thoroughly wet both faces, and the excess is
then removed by filtration under vacuum (water pump). In the
following step: [0198] either the acidic solution is poured into
the Buchner funnel, and after 10 minutes of reaction the excess is
filtered off and the membrane is dried in a fume cupboard (12
hours), [0199] or the membrane is removed from the Buchner funnel
and immersed in the acidic solution for at least 2 hours, and then
dried in the open air.
[0200] In both cases, the membranes are extremely supple and
fragile, because BN and Teflon are two materials used as
lubricants.
[0201] The conductivity in 1 M H.sub.2SO.sub.4 was measured: it is
between 0.1 and 0.2 S/cm.
[0202] Better mechanical properties are obtained by incorporating
silica into the mixture. Better properties were also obtained by
adding a small amount of sulfonated polyether sulfone.
EXAMPLE 4
[0203] The example that follows describes the preparation of
silica/BN or silica/PESS/BN membranes.
[0204] An agate mortar is used.
[0205] Quality of the tested silicas: [0206] Merck silica
gel--40-63 .mu.m pore 60 .ANG.--reference 9385 [0207] standard
grade TLC silica gel: 28 850 0--2 to 25 .mu.m--Sigma reference
[0208] Lichrosphere 8.7 to 15.4 .mu.m (average 12 .mu.m) pore 60
.ANG.--Merck reference 19654 [0209] Davisil grade 633 silica
gel--38 to 75 .mu.m pore 60 .ANG.--Sigma reference 236772 [0210]
Davisil grade 643 silica gel--38 to 75 .mu.m pore 150 .ANG.--Sigma
reference 236810 [0211] sulfonated polyether sulfone (PESS) ERAS
product reference UDEL P3500 (acid form) ion-exchange charge 0.76
meq./monomer unit.
[0212] The activated BN must be dry.
BN Silica Membrane
[0213] The required amount of silica is weighed out and introduced
into a mortar, followed by the required amount of BN.
[0214] Intimate mixing is performed using a pestle and a spatula.
The PTFE suspension is then introduced into the mixture, and about
2 ml of ethanol (60%) are then deposited.
[0215] The whole is mixed in the mortar until an ink and then a
soft film has formed.
[0216] This film is extracted from the mortar, 1 ml of 60% ethanol
is added and the dough is worked manually by folding and stretching
using a glass roller until a soft dough is obtained.
[0217] This dough is then worked using a roll mill (folding
stretching) a maximum of 4 times, taking care to reduce the
thickness gradually (150 .mu.m to 200 .mu.m to have a manipulable
membrane). The membrane is dried at 40.degree. C. for about 1
hour.
Silica/PESS/BN Membrane
[0218] Solutions of PESS in tetrahydrofuran (the solution is
slightly cloudy) at 2.5 g/25 ml or 2.5 g/50 ml are prepared in the
laboratory.
[0219] Preparation of Silica Impregnated with PESS
[0220] A known amount of silica is introduced into the mortar, a
sufficiently large volume (about 5 to 10 ml/g of SiO.sub.2) of PESS
solution (choose a concentration as a function of the desired final
percentage of PESS) is introduced onto the silica so as to wet it
and to form a crumbly dough.
[0221] In a fume cupboard, knead it in a mortar until the solvent
has evaporated off and a homogeneous, fine, non-agglomerated powder
is obtained. Complete the evaporation of the solvent by leaving the
powder in an oven at 40.degree. C. for 15 hours.
[0222] The required amount of silica/PESS mixture is weighed out
and introduced into a mortar, followed by the introduction of the
required amount of BN.
[0223] Intimate mixing is performed using a pestle and a spatula.
The PTFE suspension is then introduced into the mixture, followed
by about 2 ml of ethanol (60%).
[0224] The whole is mixed in the mortar until an ink and then a
soft film has formed.
[0225] This film is extracted from the mortar, 1 ml of 60% ethanol
is added and the dough is worked manually by folding and stretching
using a glass roller until a soft dough of smooth homogeneous
appearance is obtained.
[0226] This dough is then worked with a roll mill (folding
stretching) a maximum of 4 times, taking care to reduce the
thickness gradually (to 150 .mu.m to 200 .mu.m to have a
manipulable membrane). The membrane is dried at 40.degree. C. for
about 1 hour.
[0227] The electrolysis behavior is illustrated in FIG. 11.
Assembly and Determination of the Electrochemical Characteristics
of the Membranes
[0228] All the assemblies were made with membranes with a working
area of 2.25 cm.sup.2 in an H-TEK cell.
[0229] The membrane is moistened by dipping in a 1 M sulfuric acid
solution for at least 4 hours before mounting it in the cell. The
electrodes are made of E-TEK platinized carbon reference ELT 120EW.
The membrane is wetted with a few drops of 1 M sulfuric acid before
mounting the electrodes, and a plastic spring maintains the contact
between electrode-current collector.
[0230] The voltage at the terminals is recorded (fem), during
sweeping of the current (zero current up to maximum current for a
voltage of 0 mV), and the maximum power is defined (Pmax in
mW/cm.sup.2) with the corresponding current density (J
mA/cm.sup.2).
[0231] The membrane resistance is measured by impedance
measurement.
[0232] The first two sweeps are performed, and the cell is then
opened, a few drops of acid are again applied to the membrane and
the following sweep(s) are performed.
Cell Performance Results
TABLE-US-00002 [0233] Conductivity in Conductivity Thick- 0.01M
H.sub.2SO.sub.4 in 1M H.sub.2SO.sub.4 Ref. Composition Appearance
ness S cm.sup.-1 S cm.sup.-1 Cell Matrix 1 SiO.sub.2 Example 4
SiO.sub.2 (25 .mu.m) 38.5% Supple, and 248 .mu.m 1.6 .times.
10.sup.-3 1.2 .times. 10.sup.-1 Fem (i = 0) 752 mV BN A29 38.5%
good Pmax: 47.8 mW/cm.sup.2 PTFE 23.1% elasticity J/Pmax: 177.8
mA/cm.sup.2 Dried 40.degree. C. Rcell: 1.5 .OMEGA.cm.sup.2
Leaktightness: OK Control SiO.sub.2 (25 .mu.m) 66.6% Supple 198 8.1
.times. 10.sup.-4 7.2 .times. 10.sup.-2 Fem (i = 0) 776 mV without
BN PTFE 33.3% Difficult to Pmax: 7.5 mW/cm.sup.2 Dried 1 hour
40.degree. C. stretch for J/Pmax: 22.2 mA/cm.sup.2 small Rcell:
12.2 .OMEGA.cm.sup.2 thicknesses Leaktightness: good Exmaple 4
SiO.sub.2 23.5% Soft, but 310 1.4 .times. 10.sup.-3 1.20 .times.
10.sup.-1 Fem (i = 0) 770 mV BN A29 62.5% elastic, Pmax: 70.2
mW/cm.sup.2 PTFE 14.1% fragile J/Pmax: 289 mA/cm.sup.2 Silica
quality Rcell: 0.6 .OMEGA.cm.sup.2 50 .mu.m 150 .ANG.
Leaktightness: good grade 434 Matrix 1 SiO.sub.2/PESS Control
SiO.sub.2 (25 .mu.m) 55.5% 233 .mu.m 2.08 .times. 10.sup.-4 0.93
.times. 10.sup.-2 Fem (i = 0) 812 mV without BN PESS .sup. 11%
Pmax: 7.9 mW/cm.sup.2 PTFE 33.3% J/Pmax: 22.2 mA/cm.sup.2 Dried 1
hour 40.degree. C. Rcell: 15.7 .OMEGA.cm.sup.2 Leaktightness: good
Example 4 SiO.sub.2 25 .mu.m 22.6% Rigid wavy 208 1.25 .times.
10.sup.-3 0.85 .times. 10.sup.-1 Fem (i = 0) 720 mV PESS 4.3%
surface Pmax: 9.6 mW/cm.sup.2 BN A29 53.8% J/Pmax: 44.4 mA/cm.sup.2
PTFE 19.4% Rcell: 4.4 .OMEGA.cm.sup.2 17.5% Dried 1 hour
Leaktightness: good 40.degree. C. Exmaple 4 SiO.sub.2 25 .mu.m
37.0% Very supple 230 .mu.m 1.4 .times. 10.sup.-3 1.20 .times.
10.sup.-1* Fem (i = 0) 843 mV PESS 1.8% *by analogy Pmax: 44.6
mW/cm.sup.2 BN A29 43.7% relative to J/Pmax: 200 mA/cm.sup.2 PTFE
17.5% Rcell Rcell: 0.45 .OMEGA.cm.sup.2 2 mL 95.degree. ethanol
Leaktightness: good Matrix 3 PVA Control PVA (1 g).degree. + 50%
Supple, stable 284 .mu.m 1.8 .times. 10.sup.-5 Fem (i = 0) 977 mV
GA in H.sub.2SO.sub.4 Pmax: 8 mW/cm.sup.2 10 ml + 2M HCl 1 ml
J/Pmax: 6.7 mA/cm.sup.2 PrOH.sub.2 water Leaktightness: good
Examples 1 PVA 90M .sup. 50% Supple if 500 .mu.m 3.2 .times.
10.sup.-3 2.9 .times. 10.sup.-1 Fem (i = 0) 782 mV and 2 act. BN
batch 22 + .sup. 50% moisture Pmax: 76.8 mW/cm.sup.2 0.2 ml 5% GA +
content > 4% J/Pmax: 288.9 mA/cm.sup.2 0.2 ml 1M H.sup.+ Rcell:
not measured Dried 2 h30 R.T. Leaktightness: good Examples 1 PVA
30M .sup. 10% Supple if wet 1.6 .times. 10.sup.-3 1.2 .times.
10.sup.-1* Fem (i = 0) 893 mV and 2 186M 20.4% *by analogy Pmax:
52.3 mW/cm.sup.2 BN 69.6% relative to J/Pmax: 222 mA/cm.sup.2 5% GA
+ 1M H.sup.+ Rcell Rcell: 1.9 .OMEGA.cm.sup.2 Leaktightness: good,
membrane sticks to the electrodes
[0234] The tests with membranes without activated BN all lead to
high membrane resistances.
* * * * *