Note: Descriptions are shown in the official language in which they were submitted.
lZ99936
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CURRENT COLLECTOR BONDED TO A SOLID POLYMER MEMBRANE
The invention relates to an improved method of
manufacturing a current collector/catalyst
electrode/membrane assembly which has increased elec-
~- trical conductivity in the area between the catalyst
electrode and the current collector. Such assemblies
are useful in a variety of applications including, for
example, fuel cells, water electrolysis cells, chlor-
alkali cells, and the like. The assembly produced
according to the present invention is substantially
structurally stable which allows the membrane portion
to be substantially thinner than those presently
available, thereby reducing the ionic resistance of the
membrane.
It is highly desirable, given the harsh
conditions of many of the applications for the
membrane, that the membrane portion of the assembly
have substantial structural integrity. Thinner
membranes have been viewed as fragile and yet thinner
membranes are desirable due to their reduced ionic
resistance.
33,070A-F -1-
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This requires a balance between providing adequate
structural support for the assembly and yet reducing
the membrane's thickness to reduce the ionic resistance
of the membrane without a sacrifice in the structural
integrity.
References which have a bearing on this
invention include U. S. Patent No . 4, 272,353, which
discloses a surface abrading technique for scratching a
solid polymer electrolyte (SPE) base member in
preparation for subsequent treatment. U. S. Patent No.
4,272,560 describes a membrane having a cathode made of
multiple coatings with a backing fabric; a dissolved
copolymer is used in the fabrication of this electrode.
U. S. Patent No. 4,182,670 discloses a combined cathode
and diaphragm utilizing a spray coating of a metal
substrate with powdered metal; a polymer impregnated
diaphragm is also described. An electrode body having
impregnated powdered metal (typically noble metals) is
described in U. S. Patent No. 3,276,911, and it also
mentions a permeable ionic electrolytic material.
U. S. Patent No. 4,364,813 discloses catalytic
particles deposited on an ion exchange material with a
SPE membrane; additionally, this patent has an ion
exchange feature mentioning a sulfonic group. U. S.
Patent No. 4,366,041 describes a cathode and diaphragm
assembly with a sacrificial film made of wax.
~he present invention particularly describes a
structurally stable electrode assembly which has lower
ionic resistance in the membrane portion and which has
higher electrical conductivity in the catalyst
electrode and current collector portions. Membrane
thinness is achieved without sacrifice of structural
33,070A-F -2-
1Z!~9936
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integrity and yet resistance to ionic movement through
the membrane is reduced.
While the foregoing refers in general terms to
the present assembly, the structure thereof and the
method of manufacture are exempliied in the detailed
description of the preferred embodiments following.
The invention particularly resides in a method
of forming an assembly of an ion permeable membrane,
electrode, and current collector, comprising the steps
of:
(a) forming the current collector from a
foundation layer of a porous, electrically conductive
material;
(b) at least partially coating a fluoro-
polymer binder on at least one surface of the foundation
layer;
(c) forming the electrode by applying a
particulate catalyst material over the fluoropolymer
binder on the foundation layer;
(d) applying a fluoropolymeric material as a
solution or dispersion over the catalyst material in a
manner to obtain penetration of the fluoropolymeric
material into the porous foundation layer to form a
substantially continuous coating on the catalyst
material and the at least partially coated foundation
layer; said fluoropolymeric material forming the ion
permeable membrane, and
(e) applying heat and/or pressure to the
assembly to enhance the flow of the fluropolymeric
material into the foundation layer and around the
catalyst material to obtain adherence of the catalyst
material to the foundation layer and to sinter the
33,070A-F -3-
~...,'
. ~. ~....
l;~g9936
.
-3a-
fluoropolymeric material into a substantially non-porous
layer around the catalyst material.
33,070A-F -3a-
A~
, ~
-
~ 9 ~6
--4--
The foundation layer is an electrically
conductive, hydraulically permeable matrix which acts
as a current collector to transmit electrical energy to
or from the electrode. It may be composed of a variety
of substances, including carbon cloth, carbon paper,
carbon felt, metallic screens, metallic felt, and
porous metallic sheets. Preferably, however, the
foundation layer is a carbon paper, which is readily
available, performs well, is easily handled, and is
relatively inexpensive.
The paper most preferably used in this
invention is also one having low electrical
resistivity, possessing sufficient strength for
fabrication, and having adequate surface properties,
such as roughness, to provide good bonding between the
fluoropolymer binder and the foundation layer. It is
also preferable to provide good electrical contact
between the carbon paper and the catalytically active
particles of the electrode.
As a beginning step, the foundation layer is at
least partially coated with a suitable polymer binder.
This polymer binder can be a fluorocarbon polymer, such
as polytetrafluoroethylene sold under the trademark of
Teflon. Other suitable polymers can include thermo-
plastic, non-ionic forms of sulfonic acid copolymers;
thermoplastic, non-ionic forms of carboxylic acid
copolymers; and the like.
Particularly preferred as the fluoropolymer
binder are thermoplastic, non-ionic forms of perfluor-
inated polymers described in the following U. S. Patent
Nos. 3,282,875; 3,909,378; 4,025,405; 4,065,366;
4,116,888; 4,123,336; 4,126,588; 4,151,052; 4,176,215;
33,070A-F -4-
1~9936
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4,178,218; 4,192,725; 4,209,635; 4,212,713; 4,251,333;
4,270,996; 4,329,435; 4,330,654; 4,337,137; 4,337,211;
4,340,680; 4,357,218; 4,358,412; 4,358,545; 4,417,969;
4,462,877; 4,470,889; 4,478,695; and published European
Patent Application 0,027,009. Such polymers usually
have equivalent weights of from 500 to 2000.
Particularly preferred for use as the fluoro-
polymer binder are copolymers of monomer I with monomer
II (as defined below). Optionally, a third type of
monomer may be copolymerized with I and II.
The first type of monomer is represented by the
general formula:
CF2-CZZ' (I)
where:
Z and Z' are independently selected from -H,
-Cl, -F, and -CF3
The second monomer consists of one or more
monomers selected from compounds represented by the
general formula:
(II)
(CF2)a (cFRf)b-tcFRf.)c-o-[cF(cF2x)-cF2-o]n-cF=cF2
where:
Y is selected from -S02Z, -CN, -COZ, and
-C(R3f)(R4f)OH;
Z is selected from -I, -Br, -Cl, -F, -OR and
-NRlR2;
33,070A-F -5-
~.ZW936
--6--
R is selected from a branched or linear alkyl
radical having from 1 to 10 carbon atoms or an aryl
radical;
R3f and R4f are independently selected from
perfluoroalkyl radicals having from 1 to 10 carbon
atoms;
R1 and R2 are independently selected from -H, a
branched or linear alkyl radical havin~ from 1 to 10
carbon atoms or an aryl radical;
a is o-6;
b is 0-6;
c is 0 or 1;
provided a+b+c is not equal to 0;
,- 15 X is selected from -Cl, -Br, -F, or mixtures
thereof when n~1;
n is 0 to 6; and
Rf and Rf~ are independently selected from -F,
-Cl, perfluoroalkyl radicals having from 1 to 10 carbon
atoms, and fluorochloroalkyl radicals having from 1 to
10 carbon atoms.
Particularly preferred is when Y is -S02F or
-CooCH3; n is 0 or 1; Rf and Rf~ are -F; X is -Cl or
-F; and a+b+c is 2 or 3.
The third and optional monomer is one or more
monomers selected from the compounds represented by the
general formula:
(III)
(CF2)a' (CFRf)b'-(CFR f)cl-o-[cF(cF2xl)-CF2-O]n,-CF=CF2
33,070A-F -6-
9936
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where:
Y' is selected from -F, -Cl, or -Br;
a' and b' are independently 0-3;
c' is 0 or 1;
provided a'+b'+c' is not equal to 0;
n' is 0-6;
Rf and R'f are independently selected from -8r,
-Cl, -F, perfluoroalkyl radicals having from 1 to lO
carbon atoms, and chloroperfluoroalkyl radicals having0 from 1 to 10 carbon atoms; and
X' is selected from -F, -Cl, -Br, or mixtures
thereof when n'>1.
The binder is typically applied in a solution
or dispersion to at least partially coat the foundation
layer. The solution or dispersion can be applied to
the foundation layer using a variety of methods well
known in the art.
When the electrode is to be used in a fuel
cell, preferably the binder is a hydrophobic material
like polytetrafluoroethylene. When, however, the
electrode is to be used in an electrolytic cell, such
as a chlor-alkali cell, the binder is preferably a
hydrophilic material like the copolymers formed from
monomers I, II and, optionally III (described above).
The preferred loading, i.e. amount of
application of the binder, is from 0.50 to 50 mg/cm2 of
foundation area with a preferred range of from 2.5 to
30 mg/cm2 of foundation area.
When the binder is applied as a solution or a
dispersion, the solvent/dispersant can be a variety of
materials including, for example, water, methanol,
33,070A-F -7-
~&
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ethanol, and compounds represented by the general
formula:
XCF2-CYZ-X '
wherein:
X is selected from F, Cl, Br, and I;
X' is selected from Cl, Br, and I;
Y and Z are independently selected from H, F,
Cl, Br, I and R'; and
R' is selected from perfluoroalkyl radicals and
chloroperfluoroalkyl radicals having from 1 to 6 carbon
atoms.
The most preferred solvents or dispersants are
,- 15 1,2-dibromotetrafluoroethane (commonly known as Freon
114 B 2)
BrCF2-CF2Br
and 1,2,3-trichlorotrifluoroethane (commonly known as
Freon 113):
ClF2C-CC12F
Of these two materials, 1,2-dibromotetrafluoroethane is
the most preferred solvent or dispersant.
The solution or dispersion used to apply the
binder to the foundation layer may have a concentration
of from 2 to 30 weight percent of polymer in the
3 solvent/dispersant. Preferably, the concentration is
from 8 to 20 weight percent of polymer in the
solvent/dispersant.
After the solution or dispersion has been
applied to the foundation layer, the solvent can then
be driven off using heat, a vacuum, or a combination of
33,070A-F -8-
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heat and a vacuum. Optionally, the solvent/dispersant
may be allowed to evaporate under ambient conditions.
Preferably, the solvent is removed at an elevated
temperature. In addition to removing the
solvent/dispersant, the heat sinters the binder and
causes it to more completely penetrate and surround the
foundation layer. As an example, when
polytetrafluoroethylene is used as the binder, exposure
at a temperature of from 300C to 340C for about 20
minutes will suffice to remove the solvent/dispersant
and to sinter the polytetrafluoroethylene.
The next step in the method of the present
invention is the application of catalytically active
and electrically conductive particles to the coated
foundation layer. The composite structure will,
ultimately, form what is commonly referred to as a
solid polymer electrolyte, or SPE, when the composite
is used in an electrochemical cell. The electrode can
be ultimately used as either a cathode or as an anode.
Materials suitable for use as electro-
catalytically active anode materials include, for
example, metals or metal oxides of platinum group
metals, such as ruthenium, iridium, rhodium, platinum,
palladium, either alone or in combination with an oxide
of a film-forming metal such as Ti or Ta. Other
suitable activating oxides include cobalt oxide, either
alone or in combination with other metal oxides, such
as those described in U. S. Patent Nos. 3,632,498;
4,142,005; 4,061,549; and 4,214,971.
Materials suitable for use as electro-
catalytically active cathode materials include, forexample, platinum group metals or metal oxides, such as
33,070A-F -9-
lZ99936
--1 o
ruthenium or ruthenium oxide. U. S. Patent No.
4,465,580 describes such cathodes.
The catalytic particles used in the present
invention are preferably finely divided and have a
preferred range of from 270 to smaller than 400 mesh
size (U. S. Standard) (53 to less than 37 microns).
The metal powder is applied to the binder-coated
foundation layer by methods known to those skilled in
the art including, for example, spraying, forming a
sheet of catalytic particles and pressing the sheet
onto the foundation layer, or forming and applying the
particles in the form of liquid dispersion, for
example, an aqueous dispersion. A suitable loading of
catalyst particles has been found to be from 0.2 to 20
mg/cm2 of foundation area with a preferred range of
from 1.5 to 5.0 mg/cm2 of foundation area.
Separately, a copolymer is formed. One such
suitàble polymer is the polymer formed from monomers I,
II, and optionally III, as defined above. The polymer
may be a thermoplastic, non-ionic precursor of a
sulfonic acid copolymer or a thermoplastic, non-ionic
precursor of a carboxylic acid copolymer, or a variety
of other polymers as defined for use as the binder.
Preferably, the copolymer is formed into a solution or
a dispersion with a solvent for application to the
catalytically active particles. On mixing with a
suitable solvent or dispersant, the polymer is applied
to the catalyst particle coated foundation layer.
Utilizing a vacuum on one side of the foundation layer,
the polymer in the solvent or dispersant is pulled onto
the catalyst and into the foundation layer. While in
one sense it can be described as coated on one side,
33,070A-F -10-
1299936
1 1
the coating nevertheless sufficiently penetrates into
the porous sheet.
In the step of bonding a fluoropolymer ontc the
surface of the catalytic particle coated foundation
layer, the most convenient procedure is the use of
conventional organic solvents. Typical solvents used
are 1,2-dibromotetrafluoroethane, methanol, ethanol,
and the like. The polymeric material which is applied
forms a substantially non-porous ion exchange layer.
The next step is the application of heat and/or
pressure to remove the solvent/dispersant and to sinter
the polymer, thereby forming the polymer into a
substantially continuous sheet. In addition, the heat
and/or pressure enhance the coating of the polymer
around the catalyst particles and the foundation layer.
For example, exposure to a temperature in the range of
from 260 to 320C is generally suitable to bond the
polymer to the particles and the foundation layer. The
temperature range is limited primarily by the onset of
thermal degradation of the polymer caused by excessive
heat. The pressure is preferably sufficiently high and
sustained for an interval to achieve bonding. In one
example, pressure may be applied up to about 5 kg/cm2
for about one minute at elevated temperature.
The next step in the manufacture of the
improved electrode structure is to hydrolyze the
structure from the non-ionic to the ionic form.
Hydrolysis may be accomplished by treating the polymer
with a basic solution if the polymer is a thermo-
plastic, non-ionic precursor of a sulfonic acid polymer
or a thermoplastic, non-ionic precursor of a carboxylic
acid polymer. In addition, if the polymer is a
33,070A-F -11-
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-12-
thermoplastic, non-ionic precursor of a carboxylic acid
polymer, an acid solution may be used to hydrolyze the
polymer. For example, in a thermoplastic, non-ionic
precursor of a sulfonic acid polymer, the completed
structure may be hydrolyzed in 25 weight percent sodium
hydroxide for 16 hours at an elevated temperature of
80C.
The completed article is then ready for use.
As an example of typical size, it is not uncommon to
encounter a membrane which is in a range of from 5 to
10 mils (0.125 to 0.25 mm) thick due to the need for
structural integrity. The finished product can yield a
membrane with a thickness in a range of from 1 to 2
mils (0.025 to 0.05), or even less. The resistance of
ionic movement through the membrane is thus lowered by
a significant amount.
In an alternate application, two similar sheets
of equal size are positioned in contact with one
another in a manner so that the foundation layers face
toward the outside of the combination and the polymer
layer on each sheet is contacted against the polymer
layer on the other sheet. The coterminous sheets are
then placed into a press and on the application of
suitable pressure and/or heat, they are joined
together.
33,070A-F -12-
