U.S. patent application number 16/060257 was filed with the patent office on 2018-12-20 for composite air electrode and associated manufacturing method.
The applicant listed for this patent is ELECTRICITE DE FRANCE, SOLVAY SA. Invention is credited to Sophie Deshayes, Silvia Rita Petricci, Padmanabhan Srinivasan, Philippe Stevens, Gwenaelle Toussaint.
Application Number | 20180366737 16/060257 |
Document ID | / |
Family ID | 55236764 |
Filed Date | 2018-12-20 |
United States Patent
Application |
20180366737 |
Kind Code |
A1 |
Stevens; Philippe ; et
al. |
December 20, 2018 |
COMPOSITE AIR ELECTRODE AND ASSOCIATED MANUFACTURING METHOD
Abstract
A method for manufacturing a composite electrode for a metal-air
electrochemical cell with a liquid electrolyte of basic pH. A
liquid solution comprising a fluoropolymer suspended in a solvent
is synthesized, then deposited on the outer surface of a porous
structure forming an air electrode. The fluoropolymer comprises
SO.sub.2N groups suitable for conducting hydroxyl ions and is
capable of forming a membrane impermeable to at least the liquid
electrolyte of basic pH. When the liquid solution is applied to the
porous structure, the solvent flows through the porous structure
and the fluoropolymer is deposited by aggregating into a layer on
the outer surface of the porous structure.
Inventors: |
Stevens; Philippe; (Noisy
Rudignon, FR) ; Toussaint; Gwenaelle; (Nemours,
FR) ; Deshayes; Sophie; (Rampillon, FR) ;
Petricci; Silvia Rita; (Bresso Milano, IT) ;
Srinivasan; Padmanabhan; (Milan, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ELECTRICITE DE FRANCE
SOLVAY SA |
Paris
Brussels |
|
FR
BE |
|
|
Family ID: |
55236764 |
Appl. No.: |
16/060257 |
Filed: |
December 9, 2016 |
PCT Filed: |
December 9, 2016 |
PCT NO: |
PCT/EP2016/080444 |
371 Date: |
June 7, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/244 20130101;
H01M 4/881 20130101; H01M 4/8605 20130101; H01M 4/8657 20130101;
H01M 4/96 20130101; H01M 2004/8689 20130101; Y02E 60/10 20130101;
H01M 4/86 20130101; H01M 6/145 20130101 |
International
Class: |
H01M 4/86 20060101
H01M004/86; H01M 4/96 20060101 H01M004/96; H01M 4/24 20060101
H01M004/24; H01M 4/88 20060101 H01M004/88 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 11, 2015 |
FR |
15 62243 |
Claims
1: A method for manufacturing a composite electrode configured to
be used in a metal-air electrochemical cell with a liquid
electrolyte of basic pH, the method comprising: obtaining an air
electrode porous structure comprising an outer surface, the porous
structure being configured to facilitate an oxygen reduction
reaction into hydroxyl ions in the presence of an electric current;
synthesizing a first liquid solution comprising a fluoropolymer
suspended in a solvent, the fluoropolymer being capable of forming
a membrane impermeable to at least the liquid electrolyte of basic
pH, the fluoropolymer comprising SO.sub.2N groups capable of
conducting hydroxyl ions; applying the first liquid solution at
least once onto the outer surface of the porous structure, the
solvent flowing through the porous structure and the fluoropolymer
being deposited by aggregating into a layer on the outer surface of
the porous structure, thereby forming said membrane impermeable to
at least the liquid electrolyte of basic pH and conductive to
hydroxyl ions.
2: The method according to claim 1, further comprising: applying
the first liquid solution a second time onto the outer surface of
the porous structure, evaporating the solvent of the first liquid
solution applied a second time onto the outer surface of the porous
structure.
3: The method according to claim 1, further comprising, during the
obtaining of the porous structure: incorporating the fluoropolymer
comprising SO.sub.2N groups capable of conducting hydroxyl ions and
capable of forming a membrane impermeable to at least the liquid
electrolyte of basic pH, into a carbon powder used to prepare the
porous structure, obtaining the porous structure from the carbon
powder mixed with the fluoropolymer.
4: The method according to claim 1, wherein the electrode is
configured as a positive electrode of a metal-air battery, the pH
of the liquid electrolyte being about 14 or higher.
5: A composite electrode configured to be used in a metal-air
electrochemical cell with a liquid electrolyte of basic pH, the
composite electrode comprising: an air electrode porous structure
comprising an outer surface, the porous structure being configured
to facilitate an oxygen reduction reaction into hydroxyl ions in
the presence of an electric current; an impermeable membrane of
fluoropolymer, the fluoropolymer comprising SO.sub.2N groups
capable of conducting hydroxyl ions, the membrane being impermeable
to the liquid electrolyte of basic pH, the membrane being arranged
on the outer surface of the porous structure in the form of a
layer, the fluoropolymer having a polymerized structure adapted to
prevent penetration of said fluoropolymer into the porous
structure.
6: The composite electrode according to claim 5, wherein the
SO.sub.2N group is part of a SO.sub.2NRQ.sup.+ group where: Q.sup.+
is a group comprising at least one quaternary nitrogen atom, R is
selected from the group consisting of: hydrogen, an alkyl of the
C.sub.1-C.sub.20 group, a cyclic compound comprising the Q+ group
and between 2 and 20 carbon atoms, a cyclic compound comprising the
Q+ group, between 2 and 20 carbon atoms, and up to 4
heteroatoms.
7: The composite electrode according to claim 5, wherein the
fluoropolymer comprises a fluorinated backbone chain with polar
groups that are at least partially hydrogenated.
8: The composite electrode according to claim 7, wherein the
fluoropolymer further comprises at least one quaternary ammonium
group with no hydrogen in a beta-position of the at least one
quaternary ammonium group.
9: The composite electrode according to claim 5, wherein the
fluoropolymer comprises groups belonging to the family of
tetrafluoroethylene and sulfur groups.
10: The composite electrode according to claim 5, wherein a
thickness of the protective membrane is between 10 .mu.m and 100
.mu.m.
11: The composite electrode according to claim 5, wherein the
porous structure comprises a polymer-based material optimizing the
conduction of hydroxyl ions.
12: The composite electrode according to claim 11, wherein the
polymer-based material of the porous structure forms an
interpenetrating polymer network or a semi-interpenetrating polymer
network.
13: A metal-air battery comprising the composite electrode
according to claim 5.
14: The metal-air battery according to claim 13, further comprising
a metal negative electrode of zinc and a liquid electrolyte of a
basic pH of about 14 or higher.
Description
TECHNICAL AREA
[0001] The invention relates to the field of protecting the air
electrode of a metal-air electrochemical cell against the corrosive
effects of a liquid electrolyte of basic pH. It may have
applications in zinc-air batteries.
BACKGROUND
[0002] Electrochemical cells are generally composed of a negative
electrode, a positive electrode, and an electrolyte for the transit
of charge carriers from one electrode to the other.
[0003] Metal-air electrochemical cells generally comprise a liquid
electrolyte. The negative electrode, typically formed from a metal
compound M, decomposes into M.sup.n+ ions during discharge while
oxygen from the air is reduced at the positive electrode, called
the air electrode, in the following reactions:
Discharge at the negative electrode: M.fwdarw.M.sup.n++ne.sup.-
Discharge at the positive electrode:
O.sub.2+2H.sub.2O+4e.sup.-.fwdarw.4OH.sup.-
[0004] One of the advantages of metal-air systems is the use of a
positive electrode, also called an air electrode, of infinite
capacity. The oxygen consumed at the positive electrode does not
need to be stored in the electrode but can be taken from the
surrounding air Metal-air electrochemical generators are thus known
for their high specific energy, which can reach several hundreds of
Wh/kg.
[0005] Air electrodes are used for example in alkaline fuel cells,
which are particularly advantageous compared to other systems
because of the high reaction kinetics at the electrodes and the
absence of noble metals such as platinum.
[0006] Other metal-air cells are based on the use of a non-alkali
metal such as zinc in the negative metal electrode. A zinc
electrode is stable in an alkaline electrolyte. Metal-air
batteries, particularly zinc-air batteries, are found for example
in hearing aids.
[0007] An air electrode is a porous solid structure, usually of
carbon grains, in contact with the liquid electrolyte. The
interface between the air electrode and the liquid electrolyte is a
"triple contact" interface where the active solid material of the
electrode, the gaseous oxidant, meaning oxygen from the air, and
the liquid electrolyte are present simultaneously. A description of
the different types of air electrodes for zinc-air batteries is
provided for example by Neburchilov V. et al. in the article
entitled "A review on air cathodes for zinc-air fuel cells".
Journal of Power Sources 195 (2010) p 1271-1291.
[0008] The triple contact interface of an air electrode presents
several technical challenges. In particular, air electrodes degrade
quickly, even when not in operation, in particular because of the
corrosive effect of the liquid electrolyte of basic pH of the
electrochemical cells. However, despite this corrosive effect, it
is desirable to increase the concentration of the basic compound of
the electrolyte (potassium hydroxide, sodium hydroxide, lithium
hydroxide, etc.) to achieve even higher performance of the
metal-air cell.
[0009] Furthermore, the liquid electrolyte can dissolve the
constituent catalyst of the air electrode. The cations from the
dissolved catalyst may contribute to degrading the performance of a
metal-air cell, by facilitating an undesirable water reduction
reaction likely to interfere with the deposition of metal on the
negative electrode, reduce the Coulomb and energy efficiency, and
consume the water.
[0010] Indeed, in a zinc-air battery, metal zinc is deposited by
electrochemical reduction on the negative electrode of the battery,
from an aqueous solution of zincate ions. It turns out that the
theoretical reduction potential of zincate ion to metal zinc is
less negative than the theoretical reduction potential of water to
hydrogen. Both reactions can coexist at the negative electrode.
2H.sub.2O+2e.sup.-H.sub.2+2OH.sup.- E.degree./V=-0.8277
Zn(OH).sub.4.sup.2-+2e.sup.-Zn+4OH.sup.- E.degree./V=-1.199
[0011] An overvoltage at the zinc negative electrode facilitates
the zincate reduction reaction over the water reduction, but is
insufficient to prevent the water reduction, particularly in the
presence of ion impurities from a deteriorated air electrode.
[0012] At the same time, the reduction of oxygen in an aqueous
medium can occur according to several reactions:
O.sub.2+2H.sub.2O+4e.sup.-.fwdarw.4OH.sup.-.sub.(aq) reduction to 4
electrons
O.sub.2+H.sub.2O+2e.sup.-.fwdarw.HO.sub.2.sup.-+OH.sup.-.sub.(aq)
reduction to 2 electrons
HO.sub.2.sup.-+H.sub.2O+2e.sup.-.fwdarw.3OH.sup.-
2HO.sub.2.sup.-.fwdarw.2OH.sup.-+O.sub.2 disproportionation
reaction
[0013] The reduction to two electrons produces peroxide
intermediate. However, peroxide is known to degrade the polymer
electrolytes likely to be used in a fuel cell.
[0014] In addition to the corrosive effect of the basic compound of
the liquid electrolyte, the simple progressive wetting of the
porous structure of an air electrode until it is flooded eventually
renders such an electrode inoperative. This wetting is aggravated
during charging and discharging phases of the cell.
[0015] Furthermore, the carbon dioxide present in air diffuses into
the concentrated basic solution and is converted into carbonate
anion, which precipitates in the presence of cations. The low
solubility of the formed carbonates leads to progressive
carbonatation of the electrolyte.
[0016] These carbonates form primarily at the gas-liquid interface
in the air electrode pores, and are known for significantly
facilitating the flooding phenomenon. In addition, the
precipitation of carbonates in the pores of the air electrode
gradually destroys the air electrode and renders it
inoperative.
[0017] Because of these negative effects--flooding of the air
electrode by the electrolyte, carbonatation by CO.sub.2 dissolution
in the electrolyte, catalyst degradation by the electrolyte,
corrosion of the electrode by the electrolyte, all associated with
the use of a basic electrolyte--a solution is desired which
protects an air electrode from this basic electrolyte.
[0018] Document FR0953021 proposes protecting an air electrode of a
metal-air battery by using an interpenetrating polymer network
(IPN) or a semi-interpenetrating polymer network (semi-IPN) Uncured
network is deposited on the air electrode and then solidified in
situ by polymerization. The semi-IPN or IPN network is attached by
anchoring the polymer in the pores of the air electrode over a
thickness typically corresponding to 2% of the thickness of the
electrode. Although these networks form a membrane to protect the
electrode from the liquid electrolyte, the method of in situ
manufacturing and anchoring in the air electrode pores can be
further optimized. In addition, it has been noted that a membrane
of IPN or non-fluorinated semi-IPN is not impermeable to highly
concentrated basic electrolytes, and degrades in an electrolyte
comprising a hydroxyl ion concentration of around 8 mol/L or
higher.
DISCLOSURE OF INVENTION
[0019] In response to the problems described above, the present
invention provides a method for manufacturing a composite electrode
intended for use in a metal-air electrochemical cell with a liquid
electrolyte of basic pH, the method comprising: [0020] obtaining a
porous structure comprising an outer surface, the porous structure
being configured to facilitate an oxygen reduction reaction into
hydroxyl ions in the presence of an electric current; [0021]
synthesizing a first liquid solution comprising a fluoropolymer
suspended in a solvent, the fluoropolymer being capable of
conducting hydroxyl ions and of forming a membrane that is
impermeable to at least the liquid electrolyte of basic pH; [0022]
applying the first liquid solution at least once onto the outer
surface of the porous structure, the solvent flowing through the
porous structure and the fluoropolymer being deposited by
aggregating into a layer on the outer surface of the porous
structure, thereby forming said impermeable membrane of the porous
structure conductive to hydroxyl ions, said membrane being
impermeable to at least the liquid electrolyte of basic pH.
[0023] The invention relies on a method which does not perform in
situ polymerization in order to form a protective membrane on the
air electrode of a metal-air cell. The method of the invention also
does not require anchoring the polymer in the pores of an air
electrode. This reduces the thickness of the polymer membrane
deposited on the surface, without impacting its impermeability.
Indeed, as the fluoropolymer is already polymerized, in the form of
a suspension in the first liquid solution, it does not penetrate,
or barely penetrates, the pores of the air electrode and
advantageously remains trapped on the outer surface of the latter,
while the solvent of the first liquid solution flows through the
pores of the air electrode. The polymer trapped on the outer
surface of the air electrode will naturally aggregate to form a
thin protective layer which is impermeable to a basic liquid
electrolyte. The choice of fluoropolymer to obtain the desired
effect of hydroxyl ion conduction and impermeability can concern
compositions such as those described in WO2012/098146.
[0024] Furthermore, the method of the present invention allows
depositing a thinner polymer layer than methods of the prior art,
without losing impermeability to the liquid electrolyte. Typically,
the thickness of a protective layer for the electrode has a
thickness chosen as a compromise between acting as a barrier to the
liquid electrolyte and good conductance. A protective layer
according to the prior art is typically with a sufficient thickness
to avoid holes and to form an effective barrier against the liquid
electrolyte, although this is at the expense of conductance. The
resistivity of a protective layer increases with thickness. The
method of the invention facilitates obtaining a hole-free layer of
low thickness, thus forming an impermeable barrier to the liquid
electrolyte without increasing the resistivity of the protective
layer in comparison to methods of the prior art.
[0025] By forming such an impermeable membrane, the air electrode
is no longer directly exposed to the corrosive basic medium of the
liquid electrolyte. Thus, the phenomena of flooding the air
electrode and dissolution of the air electrode catalyst can be
avoided.
[0026] Another effect of this deposition of a layer is a reduction
of the carbonatation phenomenon in the composite electrode. It
seems that the presence of a fluoropolymer layer on the outer
surface of the air electrode contributes to slowing the release of
CO.sub.2 in the electrode. This also helps extend the service life
of the composite electrode in comparison to electrodes of the prior
art.
[0027] The method may further comprise: [0028] applying the first
liquid solution a second time onto the outer surface of the porous
structure. [0029] evaporating the solvent of the first liquid
solution applied a second time onto the outer surface of the porous
structure.
[0030] The implementation of a composite electrode according to the
method described above can be performed by a single application of
the first liquid solution, with no solvent evaporation step. When
the method for forming the composite electrode involves only a
single application of the first liquid solution, the solvent can
flow through the pores of the air electrode. However, when the
first liquid solution is applied multiple times onto the air
electrode, typically two or three times, it may be advantageous to
add a solvent evaporation step. The solvent can no longer flow
naturally through the air electrode pores, when access to these
pores is blocked by the previously applied polymer layer.
[0031] According to one embodiment, the method further comprises,
during the obtaining of the porous structure: [0032] incorporating
fluoropolymer capable of conducting hydroxyl ions and of forming a
membrane impermeable to at least the liquid electrolyte of basic
pH, into a carbon powder used to prepare the porous structure,
[0033] obtaining the porous structure from the carbon powder mixed
with the fluoropolymer.
[0034] According to this advantageous embodiment, the porous
structure of the air electrode comprises the polymer conductive to
hydroxyl ions that is impermeable to the liquid electrolyte,
distributed in the volume of the air electrode, preferably
uniformly. The porous structure of the composite electrode thus
obtained is effectively protected against any basic electrolyte
which successfully crosses the membrane covering the outer surface
of the air electrode. Furthermore, this embodiment also can
significantly reduce the carbonatation phenomenon in the composite
electrode. By incorporating the fluoropoly mer into the electrode
structure, the reduction of the carbonatation phenomenon is even
more pronounced.
[0035] The electrode may be intended for a metal-air battery as a
positive electrode, the pH of the liquid electrolyte being about 14
or higher.
[0036] In particular, it has been observed that the polymer
membrane obtained by the method described above forms an effective
seal against highly concentrated basic electrolytes having a pH
greater than 14 and which typically can reach hydroxyl ion
concentrations of around 8 mol/L or higher.
[0037] The invention also relates to a composite electrode intended
for use in a metal-air electrochemical cell with a liquid
electrolyte of basic pH, the composite electrode comprising: [0038]
a porous structure comprising an outer surface, the porous
structure being configured to facilitate an oxygen reduction
reaction into hydroxyl ions in the presence of an electric current,
[0039] an impermeable membrane of fluoropolymer, conductive to
hydroxyl ions, impermeable to the liquid electrolyte of basic pH,
the protective membrane being arranged on the outer surface of the
porous structure in the form of a layer, the fluoropolymer having a
polymerized structure adapted to prevent penetration of said
fluoropolymer into the porous structure.
[0040] According to one embodiment, the fluoropolymer of the
composite electrode comprises SO.sub.2N groups facilitating the
conduction of hydroxyl ions through the membrane.
[0041] In particular, the SO.sub.2N group may be part of a
SO.sub.2NRQ.sup.+ group where: [0042] Q.sup.+ is a group comprising
at least one quaternary nitrogen atom, [0043] R is selected from
the group consisting of: hydrogen, an alkyl of the C.sub.1-C.sub.20
group, a cyclic compound comprising the Q+ group and between 2 and
20 carbon atoms, a cyclic compound comprising the Q+ group, between
2 and 20 carbon atoms, and up to 4 heteroatoms.
[0044] The fluoropolymer described above may correspond to or
comprise a compound such as the one described in document
WO2012/098146.
[0045] According to one embodiment, the fluoropolymer comprises a
fluorinated backbone chain with polar groups that are at least
partially hydrogenated.
[0046] Surprisingly, it has been noted that a fluoropolymer
comprising a fluorinated backbone and at least partially
hydrogenated polar groups has the property of naturally forming
thin layers by the method described above. According to one
possible explanation of this phenomenon, provided as an
illustrative and non-limiting example, it seems that the at least
partially hydrogenated polar groups of the polymer contribute both
to binding the polymer aggregates together after application onto
the outer surface of the air electrode, and binding themselves to
the outer surface of the air electrode without significantly
penetrating into the pores of the surface.
[0047] According to one embodiment, the fluoropolymer further
comprises at least one quaternary ammonium group with no hydrogen
in the beta position.
[0048] It has been noted that by ensuring replacement of the
beta-position hydrogen of a quaternary ammonium group of the
fluoropolymer, a particularly high stability to strongly basic
electrolytes can be obtained.
[0049] According to one embodiment, the fluoropolymer comprises
groups belonging to the family of tetrafluoroethylene and sulfur
groups.
[0050] Advantageously, the thickness of the protective membrane can
be between 10 .mu.m and 100 .mu.m.
[0051] Such a thickness allows obtaining a composite electrode of
which the protective polymer membrane is simultaneously
sufficiently impermeable to a basic electrolyte, has sufficient
mechanical strength at the membrane, and sufficiently conducts
hydroxyl ions across the membrane. Good mechanical strength
combined with good ion conduction gives satisfactory electrical
performance to an electrochemical cell equipped with a composite
electrode according to the invention.
[0052] According to one embodiment, the porous structure comprises
a polymer-based material optimizing the conduction of hydroxyl
ions.
[0053] In particular, the polymer-based material of the porous
structure can form an interpenetrating polymer network or a
semi-interpenetrating polymer network.
[0054] By combining the hydroxyl ion conduction properties of the
polymer membrane with an optimized conduction of hydroxyl ions by
the porous structure of the air electrode, the hydroxyl ions can
move more easily from the electrolyte to the porous structure and
occupy as much of the volume of the air electrode as possible. It
is thus possible to obtain an air electrode providing particularly
efficient oxygen reduction, exploiting the full volume of the
electrode.
[0055] The invention also relates to a metal-air battery comprising
a composite electrode as described above.
[0056] The battery may further comprise a metal negative electrode
of zinc and a liquid electrolyte of a basic pH of about 14 or
higher.
DESCRIPTION OF FIGURES
[0057] The method that is the object of the invention will be
better understood from reading the following description of some
exemplary embodiments presented for illustrative purposes and in no
way limiting, and studying the following drawings in which.
[0058] FIG. 1 is a schematic representation of a metal-air
electrochemical cell comprising a composite electrode according to
the invention;
[0059] FIGS. 2a and 2b are schematic representations of the
application of a first liquid solution comprising a fluoropolymer
capable of forming a layer impermeable to a liquid electrolyte of
basic pH, onto a porous structure:
[0060] FIG. 3 is a schematic representation of a composite
electrode according to a first embodiment in a liquid electrolyte
of high basic pH, illustrating the conductive and barrier
properties of the electrode in discharge phase:
[0061] FIG. 4 is a schematic representation of a composite
electrode according to a second embodiment, in discharge phase.
[0062] FIG. 5 is a graph comparing the service life in hours with
untreated ambient air, of a standard air electrode of the prior
art, to that of a composite air electrode according to the present
invention, in two solutions of high hydroxyl ion
concentrations.
[0063] For clarity, the dimensions of the various elements
represented in these figures are not necessarily in proportion to
their actual dimensions. In the figures, identical references
correspond to identical elements.
DETAILED DESCRIPTION
[0064] The invention relates to a method for protecting an air
electrode from the negative effects of a liquid electrolyte of
basic pH FIG. 1 shows a metal-air electrochemical cell 10
comprising a composite electrode which is an object of the present
invention, obtainable by the method presented below. The cell
represented in FIG. 1 may be an integral part of a metal-air
battery comprising a plurality of electrochemical cells. It is also
possible for a battery to comprise only one cell.
[0065] As shown in FIG. 1, the electrochemical cell 10 comprises
two composite electrodes 1, corresponding to air electrodes having
a porous structure 7. The porous structure 7 of an air electrode of
an electrochemical cell can be obtained from carbon grains 6 joined
by a binder. The space between the carbon grains 6 allows air and
in particular oxygen contained in the air to flow through the
porous structure 7 to a triple interface of air/electrode/OH.sup.-
ions. This triple interface is the site of oxidation reactions.
[0066] The porous structure 7 comprises, on an outer surface facing
a liquid electrolyte, a membrane 8 impermeable to the liquid
electrolyte 3 of basic pH. The liquid electrolyte may
advantageously contain a low concentration of H.sup.+ ions and have
a pH of about 14 or higher. In particular, high pH values can be
expressed in concentrations of OH.sup.- ions. In the context of
metal-air electrochemical cells, a high concentration of OH.sup.-
ions can offer better electrical performance by allowing a larger
number of OH.sup.- ions to reach the reaction site at the triple
interface of the porous structure. The membrane 8 covering the
outer surface of the porous structure 7 of the composite electrode
1 in FIG. 1, facing the liquid electrolyte, protects the porous
structure from the liquid electrolyte even when the concentration
of OH.sup.- ions is about 8 mol/L or higher.
[0067] The liquid electrolyte 3 typically comprises a high
concentration of OH.sup.- ions, also known as hydroxyl ions 4, and
also comprises M.sup.n+ metal ions 5, as shown in FIG. 1. The
OH.sup.- ions can freely pass through the membrane 8. However, the
membrane 8 is impermeable to the other species present in the
electrolyte 3 and in particular can advantageously prevent direct
contact of the electrolytic liquid with the porous structure 7.
Thus, the flooding phenomenon observed in air electrodes can be
avoided as can the corrosive effect of the electrolyte 3 on the
porous structure 7.
[0068] The membrane 8 is composed of a fluoropolymer deposited on
the outer surface of the porous structure 7. FIGS. 2a and 2b
schematically represent the deposition of the polymer as a layer
22. This layer 22 is formed on the outer surface 25 of the porous
structure 7 without having to penetrate inside the pores.
[0069] As shown schematically in FIG. 2a, a first liquid solution
20 is prepared. This liquid solution comprises a fluoropolymer 9
suspended in a solvent 21. Although shown in FIG. 2a as being
suspended on the surface of the first liquid solution 20, it is
also possible for the fluoropolymer to be uniformly distributed in
the solvent 21 while being in suspension. The fluoropolymer 9 shown
in FIG. 2a is composed of molecules in the form of long polymer
chains. The representation as fibers in FIGS. 2a and 2b illustrates
the fact that the fluoropolymer 9 is present in the solvent 21 in
polymerized form. However, at this stage these polymer chains do
not yet form an assembled semi-rigid ensemble.
[0070] By applying the first fluid solution 21 onto the outer
surface 25 of the porous structure 7, the solvent 21 flows through
the pores of the porous structure 7 between the carbon grains 6,
for example following paths 23 as shown in FIG. 2b. The molecules
of elongate structure of the fluoropolymer 9 penetrate the porous
structure 7 with difficulty. As shown m FIG. 2b, the fluoropolymer
9 is retained on the outer surface 25 of the porous structure. This
retention on the outer surface without penetrating the pores of the
air electrode is related to the length of the fluoropolymer chains
9, already polymerized at the time of application of the liquid
solution onto the porous structure, and is related to the
composition of the porous structure 7 itself. Indeed, the porous
structures 7 used in metal-air electrochemical cells generally
comprise polytetrafluoroethylene (PTFE). It has been noted that
PTFE seems to have a repellent effect on the fluoropolymer 9,
facilitating its retention on the outer surface 25 of an air
electrode.
[0071] Deposition of the first fluid solution 20 onto the outer
surface 25 of the porous structure 7 may for example be done by
painting, transfer, and may involve the use of a scraper to spread
the polymer.
[0072] When the first fluid solution 20 is applied onto the porous
structure 7, the solvent 21 flows through the porous structure
while the polymer chains of the fluoropolymer 9 are filtered and
retained on the outer surface 25 of the porous structure. Due to
this filtering, the molecules of the fluoropolymer are no longer
dispersed in the solvent and their clumping on the outer surface 25
of the porous structure facilitates their aggregation to form an
impermeable structure in the form of a layer 22.
[0073] The process of consolidation of the layer 22 can occur
without outside intervention once the fluoropolymer 9 is deposited
on the porous structure 7. Aggregation of the polymer chains to
form the layer 22 may be accelerated, however, by exposing the
structure of FIG. 2b to a temperature higher than room
temperature.
[0074] It is generally accepted that obtaining a stable and
resistant membrane requires a particular mechanism for anchoring
the membrane to the air electrode. The method of the invention,
which provides no particular mechanism for anchoring the
fluoropolymer 9 in the pores of the porous structure 7, and which
applies to an air electrode an already polymerized fluoropolymer in
suspension m a solvent, provides a novel and counter-intuitive
solution to the problem of protecting an air electrode.
[0075] One possible explanation for the stability of the contact
between the layer 22 and the outer surface of the porous structure,
provided here for illustrative purposes and non-limiting, could be
attributed to adhesion of the fluoropolymer 9 to the outer surface
25 of the porous structure. Indeed, due to its porous structure,
the air electrode has an outer surface having numerous
irregularities which increase the surface area of this surface. Low
forces of adhesion, such as Van der Waals forces, can provide
sufficient attraction to maintain the layer 22 in a fixed position
in contact with the porous structure 7.
[0076] Moreover, it appears that the carboxyl groups typically
contained in the grains 6 of the porous structure 7 of an air
electrode also contribute to maintaining the fluoropoly mer in
contact with the outer surface 25.
[0077] The method for depositing the fluoropolymer 9 on the outer
surface of the porous structure 7 can be performed all at once as
schematically Illustrated in FIGS. 2a and 2b. However, it may also
be carried out in multiple applications, typically two or three, to
obtain a greater thickness of the membrane 8.
[0078] Particularly satisfactory properties of protection against a
liquid electrolyte 3 of basic pH, with hydroxyl ion concentrations
4 of about 8 mol/L or higher, have been observed for membrane 8
thicknesses between 10 .mu.m and 100 .mu.m. Such a thickness range
makes it possible to ensure sufficient impermeability to the
strongly basic liquid electrolyte 3, as well as effective
conduction of hydroxyl ions 4 through the membrane 8 and sufficient
mechanical strength of the membrane. It should be noted that the
method for forming a membrane 8 according to the invention has the
advantage of facilitating the formation of an impermeable layer of
low thickness, without holes. Thus, the obtained membrane 8 forms
an effective barrier to the liquid electrolyte while offering a
conductance which is not reduced by the thickness of the membrane.
Indeed, as mentioned above, membrane resistivity increases with
thickness. A reduction in the thickness therefore contributes to
increasing the conductance.
[0079] When the step of depositing the fluoropolymer 9 on the outer
surface 25 of the porous structure 7 is performed multiple times,
it may be that the solvent 21 cannot entirely flow through the
pores of the porous structure 7. More particularly, the solvent
will have difficulty flowing through the porous structure when a
layer 22 of polymer already covers the outer surface of the air
electrode. To overcome this difficulty in eliminating excess
solvent 21, an additional step of solvent 21 evaporation can be
implemented after a first application of the first liquid solution.
This evaporation may be achieved by heating the solvent, for
example at temperatures between room temperature and 100.degree.
C., preferably a temperature of 80.degree. C.
[0080] The electrode composite which is the object of the invention
can advantageously perform several functions to enhance the
performance and useful life of an air electrode of a metal-air
battery. FIG. 3 schematically shows a portion of a composite
electrode of a metal-air cell. The porous structure 7 comprises on
its outer surface a membrane 8 of fluoropolymer 9, aggregated into
a layer impermeable to the liquid electrolyte 3. However, although
the liquid electrolyte 3 is blocked by the membrane 8 and does not
come into contact with the porous structure 7, the hydroxyl ions
OH.sup.- can pass through the membrane 8.
[0081] In addition to the fact that the fluoropolymer, present as a
layer on the outer surface of the porous structure, forms a barrier
to the liquid electrolyte having a high concentration of hydroxyl
ions, it was found that this layer also reduces the carbonatation
phenomenon in the composite electrode by preventing direct contact
between the CO.sub.2 from the air and the liquid electrolyte.
Although the fluoropolymer does not appear to have the intrinsic
property of total impermeability to carbon dioxide, its use in
combination with the porous structure of the air electrode appears
to result in slowing down the carbon dioxide diffusion phenomenon
in the composite electrode. Without being tied to a particular
theory, and as a non-limiting and purely illustrative example, one
possible explanation for this observation could be a competition
between the diffusion of ions in the fluoropolymer and the
diffusion of carbon dioxide. When the hydroxyl ions diffuse into
the fluoropolymer, it is possible that the carbon dioxide has more
difficulty in simultaneously crossing the fluoropolymer layer,
which creates a synergistic effect between ion conduction and
reduction of the carbonatation phenomenon. In addition, the slowed
carbonatation of the liquid electrolyte, observed in the presence
of a layer of fluoropolymer, could be attributed to the presence of
the solid barrier constituted by this layer on the electrode.
[0082] The fluoropolymer is chosen so as to possess such multiple
properties combining impermeability to a liquid electrolyte 3 and
conduction of hydroxyl ions. In the family of fluoropolymers, it
has been noted that polymers comprising SO.sub.2N groups have
hydroxyl ion conduction properties enabling metal-air batteries to
provide satisfactory electrical performance. In particular, the
association of SO.sub.2N groups with a Q.sup.+ group comprising at
least one quaternary nitrogen atom, and a radical R, afforded
particularly effective conduction of hydroxyl ions through the
membrane 8. In the compound SO.sub.2NRQ.sup.+ associated with the
fluoropolymer 9, the radical R may typically be selected from
hydrogen, an alkyl of the C.sub.1-C.sub.20 group, a cyclic compound
comprising the Q+ group and between 2 and 20 carbon atoms, and a
cyclic compound comprising the Q+ group, between 2 and 20 carbon
atoms, and up to 4 heteroatoms. Examples of SO.sub.2NRQ.sup.+
compounds suitable for the composite electrode of the invention can
be obtained according to the methods described in WO
2012/098146.
[0083] As explained above, the deposition method of the invention
allows obtaining a protective membrane 8 which adheres to the outer
surface 25 of a porous structure 7 without having to penetrate the
pores of the air electrode to ensure stable anchoring to the
electrode. This surprising stability further enables oxygen to make
maximum use of the volume of the porous structure for oxidation
reactions. Indeed, the hydroxyl ions 4 can freely migrate through
the membrane 8 as indicated by the path 40 in FIG. 3, to reach the
pores between the grains 6 of the porous structure 7 where oxygen
from the air is also located. The oxygen O.sub.2 can also migrate
freely in the pores of the air electrode as indicated by the path
20 in FIG. 3. The hydroxyl ions and oxygen can thus interact in a
maximized number of reaction sites throughout the volume of the air
electrode. The reaction volume available for oxidation reactions is
not reduced by flooding phenomena, prevented by the presence of the
impermeable membrane, or by the fluoropolymer 9 which does not fill
the pores of the air electrode, even in the superficial areas of
the latter.
[0084] In addition, the adhesion of the membrane 8 to the outer
surface 25 of the porous structure 7 can be optimized when the
fluoropolymer 9 comprises a backbone chain with at least one polar
group and/or at least one group that is at least partially
hydrogenated. Polar groups can interact with the carbon contained
in an air electrode. Similarly, the presence of sulfur groups on
the backbone chain of the fluoropolymer 9 may also contribute to an
interaction with the air electrode in a way that facilitates
organization of the fluoropolymer 9 into a layer on the outer
surface 25 of a porous structure 7.
[0085] The membrane 8 comprising the fluoropolymer as a layer 9 has
a higher resistance to alkaline electrolytes with high hydroxyl ion
concentrations than membranes based on interpenetrating polymer or
semi-interpenetrating polymer networks such as those described in
document FR0953021. This improved resistance can be attributed to
the addition of a backbone chain comprising the fluoropolymer, in
the quaternary anmmonium group. In addition, whereas quaternary
ammonium groups are not known to be stable in strongly basic
solutions, it has been found that by using a diamine containing no
hydrogen in the beta position of the quaternary ammonium group, a
particularly satisfactory stability and resistance can be
obtained.
[0086] A non-limiting example of a fluoropolymer that can be used
in the invention is described below. Such a polymer consists of a
carbon backbone chain comprising groups associated thereto by
covalent bonds. These groups themselves comprise terminals of the
chemical formula SO.sub.2NRQ.sup.+ wherein Q, R are as defined
above.
[0087] The fluorinated backbone may be an arbitrary linear polymer
chain comprising, for example, repeating units of the chemical
formula:
##STR00001##
where R.sub.F is selected from F, Cl, or --CF.sub.3, and is
preferably selected as fluorine.
[0088] The element p of formula (II) above may be an integer
between 0 and 16. The element q may be an integer between 1 and 10,
and the ratio p'/q' may be between 0.5 and 16, p' denoting an
average value of all values of p in the repeating units and q'
denoting an average value of all values of q in the repeating
units.
[0089] More particularly, the fluoropolymer may comprise repeating
units of the chemical formula:
##STR00002##
where the terminals of chemical formula SO.sub.2NRQ.sup.+ are
covalently bonded to the backbone chain by groups of formula
--[O--(CF.sub.2CF(R.sub.F)O).sub.m--(CF.sub.2).sub.n]--.
[0090] The element R'.sub.F in formula (III) above may be selected
from F, Cl, or --CF.sub.3, and is preferably selected as being
fluorine or --CF.sub.3. The element m may be an integer equal to 0
or 1, n may be an integer between 0 and 10. R.sup.1 is as R defined
above, Q+ is as defined above, and X.sup.- may be a anion selected
from the group consisting of organic anions and lipophilic
inorganic anions. When m is 1, n is an integer between 0 and 10,
preferably between 0 and 6. Preferably, when m is 1, n is 2. When m
is 0, n is an integer between 0 and 10, preferably between 2 and 6,
more particularly between 2 and 4. Preferably, when m is 0, n is
2.
[0091] The fluoropolymer may be prepared by means of a method
comprising-reacting a fluoropolymer comprising moieties having
sulfonyl fluoride terminals with an amine, to obtain sulfonamide
terminal groups.
[0092] This reaction is followed by reacting the product of the
previous reaction with an alkylating agent to form an
ion-exchanging quaternary ammonium group.
[0093] The amine can be written in the generic formula
HNR.sup.1Q.sup.1 where R.sup.1 is as R defined above, and Q.sup.1
is a group comprising a precursor of the tertiary amino group of
the quaternary ammonium group Q.sup.+.
[0094] Copolymers comprising repeating units derived from
fluoroolefin of the chemical formula CF.sub.2.dbd.CFR.sub.F, where
R.sub.F is selected from F, Cl, and --CF.sub.3, as well as
copolymers comprising repeating units derived from at least one
functional monomer of chemical formula
CF.sub.2.dbd.CF--O--(CF.sub.2CF(R'.sub.F)O).sub.m--(CF.sub.2).sub.nSO.sub-
.2F, where m is an integer equal to 0 or 1, n is an integer between
0 and 10, are compounds suitable for preparation of the
fluoropolymer. The fluoroolefin is preferably
tetrafluoroethylene.
[0095] The group ensuring ion conduction in the polymer may have
the chemical formula:
##STR00003##
where Y is a C.sub.6-C.sub.10 aryl group, a heteroaryl group, or
CR.sup.7R.sup.8 where R.sup.7 is hydrogen, a halogen atom, or a
C.sub.1-C.sub.20 alkyl group, or forms a closed chain with R.sup.2,
R.sub.5, or R.
[0096] R.sup.8 is hydrogen, a halogen atom, or a C.sub.1-C.sub.20
alkyl group, or forms a closed chain with R.sup.3, R.sup.6, or
R.sup.7. Each chain formed by R.sup.7 or R.sup.8 contains from 2 to
10 carbon atoms and optionally 1 to 4 heteroatoms, and the
heteroaryl group comprises from 5 to 10 atoms m the closed
chain.
[0097] R.sup.1 is hydrogen, a C.sub.1-C.sub.20 alkyl group, or
forms a closed chain R.sup.2 or R.sup.5, the closed chain
comprising between 2 and 10 carbon atoms and between 1 and 4
heteroatoms.
[0098] R.sup.2 is a C.sub.1-C.sub.20 alkyl group or forms a closed
chain with R.sup.1, R.sup.3, R.sup.5, R.sup.7, or R.sup.9, the
closed chain containing between 2 and 10 carbon atoms and between 1
and 4 heteroatoms such as nitrogen atoms.
[0099] R.sup.3 is a C.sub.1-C.sub.20 alkyl group or forms a closed
chain with R.sup.2, R.sup.6, R.sup.8, or R.sup.10, the closed chain
containing 2 to 10 carbon atoms and 1 to 4 heteroatoms.
[0100] R.sup.4 is a C.sub.1-C.sub.20 alkyl group.
[0101] R.sup.5 is hydrogen, a halogen atom, or a C.sub.1-C.sub.20
alkyl group, or form a closed chain with R.sup.1, R.sup.2, R.sup.7,
or R.sup.9, the closed chain containing 2 to 10 carbon atoms and
optionally 1 to 4 heteroatoms.
[0102] R.sup.6 is hydrogen, a halogen atom, or a C.sub.1-C.sub.20
alkyl group, or form a closed chain with R.sup.3, R.sup.8, or
R.sup.10, the closed chain containing 2 to 10 carbon atoms and
optionally 1 to 4 heteroatoms.
[0103] Each R.sup.9 may be hydrogen, a halogen atom, a
C.sub.1-C.sub.20 alkyl group, or a closed chain with R.sup.2 or
R.sup.5, the closed chain containing 2 to 10 carbon atoms and
optionally 1 to 4 heteroatoms.
[0104] Each R.sup.10 may be hydrogen, a halogen atom, or a
C.sub.1-C.sub.20 alkyl group, or form a closed chain with R.sub.3
or R.sup.6, the closed chain containing 2 to 10 carbon atoms and
optionally 1-4 heteroatoms.
[0105] Z is an integer between 0 and 4, and the closed chain
structures in the group of chemical formula (IV) illustrated above
may be linked by bridges based on a C.sub.1-C.sub.4 alkyl
group.
[0106] Among the amines suitable for preparation of the
fluoropolymer, are compounds having the chemical formula:
##STR00004##
where R.sup.1, R.sup.2, R.sup.3, R.sup.5, R.sup.6, R.sup.9,
R.sup.10, Y and z are as defined above.
[0107] Examples of amines according to chemical formula (V) may be
taken from the following families of compounds: [0108]
.alpha.-(dimethylamino)- , -dialkyl-.omega.-aminoalkyls, for
example N,N,2,2-tetramethyl-1,3-propanediamine; [0109]
N-(.omega.-aminoalkyl) imidazoles; [0110]
2-alkyl-4-.omega.-aminoalkyl-N,N-dimethylaminobenzyls and
2,6-dialkyl-4-amino-N,N-dimethylaminobenzyls, for example
2,6-dimethyl-4-amino-N,N-dimethylbenzylamine; [0111]
1-methylpiperazines, mono- and/or disubstituted for alkyl groups in
position 2 and/or 6, for example 1,2,6-trimethylpiperazine; [0112]
1-(.omega.-aminoalkyl)piperazines, mono- and/or disubstituted for
alkyl groups in position 2 and/or 6; [0113] "bridged
aminopiperazine"; [0114] 1-methyl-4(.omega.-aminoalkyl)-3,5-alkyl
(mono, di) piperidines; [0115] 1-methyl (or H)-2,6 alkyl (mono, di,
tri or tetra)-4-aminopiperidines; [0116] "bridged aminopiperidine";
[0117] 1-methyl-3-aminopyrrolidines, optionally with alkyl
substituted in position 2- and/or 5-, for example
3-amino-1-methylpyrrolidine; [0118]
4-(.omega.-aminoalkyl)morpholine, with alkyl substituted in
position 3 and/or 5, for example 4-(2-aminoethyl) 2,6 dimethyl
morpholine; [0119] "aza-aminoadamantanes".
[0120] Preferably, the amine is
N,N,2,2-tetramethyl-1,3-propanediamine. Thus, in formula (V),
R.sup.1.dbd.H, R.sup.2.dbd.R.sup.3.dbd.CH.sub.3,
R.sup.5.dbd.R.sup.6.dbd.R.sup.9.dbd.R.sup.10.dbd.H, z=1 and
Y.dbd.CR.sup.8R.sup.9, with R.sup.8.dbd.R.sup.9.dbd.CH.sub.3.
[0121] The fluoropolymer of the membrane 8 of the composite
electrode 1 also provides stability and resistance to the corrosive
effect of the hydrogen peroxide present in a liquid electrolyte 3
of basic pH. It seems that resistance to the liquid electrolyte of
basic pH is associated with resistance to hydrogen peroxide,
particularly when hydrogen peroxide is present at a 5% mass
concentration.
[0122] Conduction of hydroxyl ions through the membrane 8 can be
supplemented by better conduction of hydroxyl ions in the porous
structure 7 by incorporating an anion-conducting fluoropolymer in
the structure of the air electrode at the time of manufacture.
Advantageously, the fluoropolymer used in the air electrode is the
same as the one present in the membrane 8. FIG. 4 illustrates a
portion of a composite electrode according to this second
embodiment, where the porous structure 7 further comprises
molecules 39 of a fluoropolymer 9, in order to improve the
conduction of hydroxyl ions in the volume of the air electrode and
render the porous structure 7 even more impermeable to any basic
liquid electrolyte m contact with said porous structure.
[0123] The molecules 39 of fluoropolymer are an integral part of
the porous structure 7. They may, for example, be incorporated into
the air electrode at the time of manufacture. The porous structure
7 is typically obtained from a carbon powder comprising grains 6
which are joined by a binder during a step which may be a sintering
step, for example. The fluoropolymer 9, in polymerized form and
suspended in the first liquid solution, can be mixed with the
carbon powder during manufacture of the air electrode to ensure
uniform distribution of the molecules 39 in the structure of the
air electrode, as is schematically represented in FIG. 4. Then a
fluoropolymer 9 is deposited as a layer on this air electrode as
described above.
[0124] Furthermore, although this additional step is not
represented, it is possible to further increase the ion conduction
properties of the porous structure 7, in both embodiments
represented in FIGS. 3 and 4. In particular, before application of
the first liquid solution to form the membrane 8, it is possible to
provide a step of applying a second liquid solution comprising an
interpenetrating polymer or semi-interpenetrating polymer network.
For example, the IPN or semi-IPN networks described in document
FR0953021 can be deposited on the porous structure before formation
of the membrane 8. This second liquid solution and the IPN and
semi-IPN networks are provided to penetrate the porous structure.
Once inside the air electrode, an evaporation step is carried out
in order to evaporate the solvent of the second liquid solution and
allow effective in-situ polymerization of the IPN or semi-IPN
network in the porous structure 7. This IPN or semi-IPN network,
integrated with the porous structure 7, allows better conduction of
hydroxyl ions in the air electrode so as to deliver these ions
throughout the volume of the porous structure.
[0125] It has been observed that a composite electrode 1 as
described above has a service life about seven times greater than
an air electrode according to the prior art, and in particular the
air electrodes of a zinc-air battery using ambient air from which
CO.sub.2 has not been filtered. In addition, the composite
electrodes of the invention are resistant to the electrolytes of
high alkalinity having hydroxyl ions in concentrations of about 8
mol/L or more encountered in metal-air batteries, particularly
zinc-air batteries.
[0126] FIG. 5 shows a graph of the results of an experiment
comparing the longevity of an air electrode according to the
invention to the longevity of an air electrode according to the
prior art.
[0127] The end of service life of electrodes of a battery is
characterized by a loss of impermeability of the electrode to the
electrolyte, which is indicated by the passage of liquid
electrolyte through the electrode.
[0128] On an air electrode operating at a constant current density
of 30 mA/cm.sup.2 with untreated ambient air in a liquid
electrolyte composed of an aqueous solution of KOH at a
concentration of 8 mol, we observe a loss of impermeability after
600 h of operation, as indicated by the white rectangle to the left
side of FIG. 5. The same electrode covered with the polymer of the
invention loses its impermeability after 3000 h under the same test
conditions, as indicated by the black rectangle to the left side of
FIG. 5.
[0129] The difference between the two types of electrodes is even
more pronounced in the right side of the graph of FIG. 5. The
results to the right side of the graph represent the service life
in hours of electrodes immersed in an electrolyte composed of an
aqueous solution of KOH at a hydroxyl ion concentration of 8 mol/L
saturated in ZnO. Under the same test conditions as those described
above (under untreated ambient air at 30 mA/cm.sup.2), the standard
electrode without polymer remains impermeable for only 50 h. while
an electrode according to the invention comprising a fluoropolymer
as described above on its outer surface remains impermeable for
2900 h.
[0130] An example of creating the fluoropolymer 9 described above
will now be presented. This example is given for illustrative
purposes. Alternative embodiments which maintain the beneficial
properties described above can be implemented without difficulty.
Additional elements may be used as inspired by the manufacturing
techniques described in WO 2012/098146.
[0131] Examples of Creating a Fluoropolymer
[0132] Synthesis of a polymer conductive to hydroxyl ions and
impermeable to a liquid electrolyte of basic pH may in particular
comprise the use of the following elements: [0133] a precursor of
perfluorosulfonic acid (PFSA) of the brand Aquivion.RTM.PFSA,
comprising --SO.sub.2F groups. The precursor is typically in the
form of powder dried in a vacuum oven at 70.degree. C. for 12 h.
[0134] a hydrofluoroether dried over molecular sieves having a pore
size of 3 Angstrom. [0135] N,N,2,2-tetramethyl-1,3-propanediamine,
[0136] ethanol, methanol, isopropanol, acetonitrile, [0137] methyl
iodide [0138] sodium tosylate [0139] sodium chloride.
[0140] In a first step, the compounds undergo an amidation
reaction.
[0141] 61 grams of Aquivion.RTM.PFSA precursor are placed in a
flask containing 350 g hydrofluoroether. The flask is equipped with
a stirrer, a dropping funnel, a nitrogen gas inlet, and a gas
outlet. A stream of nitrogen is maintained throughout the process
at a flow rate of about 1 liter per hour in order to keep the
mixture anhydrous. The polymer suspension is stirred at room
temperature for about 2 hours. The flask is then cooled to -20'C
using an external cold source and, while stirring the mixture, 60
grams of N,N,2,2-tetramethyl-1,3-propanediamine are slowly added
for about 20 minutes through the dropping funnel. The mixture is
stirred at a temperature of -20.degree. C. for an additional 6
hours. The polymer is then filtered, washed with 150 grams
hydrofluoroether for one hour while stirring, and then treated
twice with 200 grams of methanol for 1 hour while stirring. The
whole is then treated twice with 200 grams of a solution comprising
5% KOH by mass while stirring for one hour. The polymer is then
dried in a vacuum oven at 70.degree. C. for 4 hours. 73 grams of
dried polymer are thus obtained. Quantitation of the polymer
indicates a concentration of 2.0 meq/g.
[0142] Analysis of the obtained polymer compound confirms the
presence of a partially fluorinated backbone chain comprising
tetrafluoroethylene derivatives and sulfur compounds derived from
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2--SO.sub.2F. in other words
perfluoro-5-sulfonylfluoride-3-oxa-pentene. The formula shown below
illustrates a chemical composition of the resulting fluoropolymer.
The term Rf indicates the partially fluorinated backbone chain of
Aquivion.RTM.PFSA.
##STR00005##
[0143] Next, the resulting polymer undergoes an alkylation reaction
according to the example presented below.
[0144] 70 grams of a polymer prepared according to the previous
example and 350 grams of anhydrous acetonitrile are placed in a
flask equipped with a stirrer, a dropping funnel, a condenser, and
an inlet and outlet for nitrogen gas. A stream of nitrogen is
maintained throughout the process at a flow rate of about 1
liter/hour, in order to keep the mixture anhydrous. The flask is
heated to 65.degree. C. by an external heat source and the polymer
suspension is stirred for 1 hour. Next, 80 grams of methyl iodide
are added through the funnel and the mixture is stirred for 12
hours at 65.degree. C. After cooling the mixture to room
temperature, the polymer is filtered, washed with 250 ml
acetonitrile for one hour while stirring, then washed three times
with 250 ml methanol for 1 hour while stirring. The polymer is then
dried in a vacuum oven at 70.degree. C. for 4 hours. 73 grams of
dried polymer are thus obtained.
[0145] Quantitation of the polymer indicates an iodide
concentration of 0.85 meq/g.
[0146] The structure of the polymer obtained at the conclusion of
this preparation step is according to the formula shown below:
##STR00006##
[0147] The term Rf refers to the partially fluorinated backbone
chain of Aquivion.RTM.PFSA, described above.
[0148] Finally, the mixture undergoes an anion exchange reaction
(iodide replaced with tosylate).
[0149] 72 grams of quaternary ammonium iodide prepared according to
the protocol described above are stirred for 8 hours at 60.degree.
C. in an aqueous-alcohol mixture of salt of methyl tosylate sodium
(30 grams), ethanol (300 grams), and water (300 grams) This
operation is repeated twice. An argentometric assay carried out on
71 grams of polymer obtained after filtering and washing shows that
the compound does not have any residual iodide.
[0150] A first liquid solution of the fluoropolymer can then be
obtained using the protocol described below.
[0151] 51 grams of the polymer obtained in the step described above
are placed in a flask containing 200 grams N,N-dimethylacetamide
and exposed to a temperature of 80.degree. for 8 hours while
stirring vigorously. After a sonication step, the liquid
composition is placed in a centrifuge to rotate at 3000 rpm for 15
minutes. This causes a small amount of a transparent gel-like
composition to appear at the bottom of the flask. This polymer
compound represents a 19% polymer concentration by mass relative to
the solvent.
[0152] This example describes how to obtain the first liquid
solution used to manufacture a membrane 8 of the invention.
However, similar results can be obtained using other chemical
compositions possessing the chemical properties described above to
ensure both a good conduction of hydroxyl ions and an
impermeability to a basic liquid electrolyte. The impermeability
and conduction properties of a membrane obtained from the compound
synthesized according to the example described above have been
characterized in different basic solutions. In particular, a
membrane 50 .mu.m thick was prepared from the first liquid solution
on a support specially provided for experimental purposes.
[0153] The membrane was treated with a KOH solution at a
concentration of 1 mol/L, in a water ethanol mixture having a 1:1
mass ratio. The membrane is then washed with water and placed in a
flask containing a mixture as shown in the figure below, for the
durations and at the temperatures indicated in the table below. At
the end of each test, the membrane is placed for 24 hours in a
solution containing 100 grams of sodium chloride at a concentration
of 0.6 mol/L in a water-ethanol mixture having a 1:1 mass ratio,
then washed with water. The chloride content is measured and
indicated in the table below. The ion content of an untreated
membrane, serving as a control sample, is 0.8 meq/g.
TABLE-US-00001 TABLE 1 conduction and stability tests of a
fluoropolymer-based membrane Temperature Duration Ion content
solution (.degree. C.) (days) (meq/g) water 25 15 0.75 water 80 8
0.75 KOH 10 mol/L 25 30 0.72 KOH 8 mol/L 25 15 0.70 KOH 8 mol/L +
25 15 0.72 H.sub.2O.sub.2 5% LiOH 4 mol/L 25 15 0.75 LiOH 8 mol/L +
25 15 0.70 H.sub.2O.sub.2 5% KOH 8 mol/L 45 15 0.70 LiOH 4 mol/L 60
15 0.72 NaOH 2 mol/L 80 8 0.75
[0154] The ion contents indicated in this table reflect a stability
in the impermeability and conduction properties of the prepared
membrane over time, and good resistance in the presence of a
strongly alkaline liquid.
[0155] The present invention may find applications in all metal-air
batteries using a porous air electrode as a positive electrode. The
invention is of particular interest for protecting an air electrode
in a zinc-air battery, offering good electrical performance with
the general use of liquid electrolytes with a high concentration of
hydroxyl ions.
* * * * *