Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
PRODUCING ELECTRODE ACTIVE LAYER FROM
ACTIVE CARBON PARTICLES AND FIBRILLATED POLYTETRAFLUOROETHYLENE
-. COATED CARBON BLACK
BACKGROUND OF THE INVENTION AND PRIO~ ART
- - Within the field of electrochemistry~ there is a well-known type of
electrolytic cell known as a chlor-alkali cell. Basically this is a cell wherein5 chlorine gas and caustic soda, viz., sodium hydroxide, are produced by passing an
electric current through a concentrated salt (brine) solution containing sodium
chloride and water. A large portion of the chlorine and caustic soda for the
chemical and plastic industries is produced in chlor-alkali cells.
Such cells are divided by a separator into anode and cathode
10 compartments. The separator characteristically can be a substantially hydrau- lically impermeable membrane, e.g., a hydraulically impermeable cation
exchange membrane such as the commercially available NA~ION~) manufactured
by E.l. duPont de Nemours and Co. Alternatively, the separator can be a porous
diaphragm, e.g., asbestos, which can be in the form of vacuum deposited fibers
15 or asbestos paper sheet as are well known in the art. The anode can be a valve
metal, e.g., titanium, provided with a precious metal coating to yield what is
known in the art as a dimensionally stable anode.
~ he cathodes employed in such chlor-alkali cells are subjected to thecorrosive environment of the caustic soda and so special precautionary measures
20 and techniques have been employed in an attempt to reduce damage and
deactivation of the active layer particles contained in the cathodes used in
chlor-alkali cells.
One of the unwanted byproducts present in a chlor-aJkali cell is
hydrogen which forms at the cell cathode. This hydrogen increases the power
25 requirement for the overall electrochemical process and eliminating its
production is one OI the desired results in chlor-alkali cell operation. Fairly
recently, attention has been directed in chlor-alkali cell technology to variousforms of oxygen (air) cathodes. Such cathodes can result in significant savings in
the cost of electrical energy employed to operate chlor-alkali cells. Estimates
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indicate that there is a theoretical savings of about 25% of the total electrical
energy required to operate chlor-alkali cells provided that the formation of
molecular hydrogen gas at the cathode can be prevented. In other words, about
25% of the electrical energy employed in a chlor-alkali cell is used to form
5 hydrogen at the cathode. Hence, the prevention of hydrogen formation at the
cathode can lead to significant savings in the cost of electrical power. This isone of the major benefits of and purposes for oxygen (air) cathodes. However,
such cathodes, being in contact with the electrolyte caustic soda, are subjectedto the corrosive action thereof.
10One known form of oxygen (air) cathode involves use of an active
cathode layer containing porous active carbon particles whose activity in
promoting the formation of hydroxide may or may not be cataly~ed (enhanced)
- using precious metal catalyst materlals, such as silver, platinum, etc. The active
carbon particles become wetted (flooded) by the caustic soda thereby
15 significantly reducing their ability to eliminate the formation of hydrogen at the
cathode and resulting in a loss of activity of the air cathode. Some attempts toovercome this difficulty involve incorporating hydrophobic materials, e.g.,
polytetrafluoroethylene ~PTFE) in such active layers in particulate or fibrillated
(greatly attenuated and elongated ) fonn tc impart hydrophobicity to the active
20 carbon layer, per se. With the PTFE, however, comes the problem of reduced
electrical conductivity in the cathode active layer in as much as PTFE, per se, is
nonconductive when compared with the porous active carbon particles. Such
active carbon/PTFE-containin~ elec trode active layers are subject to loss of
strength resulting in failure combined with blisterin~ thereof when the chlor-
25 alkali cell is operated at high current densities, viz., current densities rangingfrom about 250 milliamperes/cm2 and higher for prolon~ed time periods.
Some oxygen (air) ca~hodes contain PTFE in both the active layer and
in a backing sheet laminated thereto. The PTFE has been employed in
particulate or fibrillated (greatly attenuated and elongated) form to impart
30 hydrophobicity to the desired layer. Thus it can be seen that the develpment of
corrosion resistant oxygen (air) cathodes of improved durability for use in
conjunction with chlor-alkali cells is an overall objective in the newlv developin~
oxygen (air) cathode field.
FIELD OF THE INVENTION
35The present invention is particularly directed to an improved fibril-
lated matrix electrode active layer, the gas, e.g., oxygen (air) electrode
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containing it and a process for forming the active layer and electrode such thatthe resulting coherent, self-sustaining active layer sheet can be employed as ~he
active layer when lamina~ed to a backing (wetproofing) sheet and current
collector to form an oxygen (air) cathode having high durability and resistance to
5 degradation due to the corrosive environment present in a chlor-alkali cell, fuel
cell, etc. In other words, the fibrillated, matrix active layer produced in
~ccordance wi~h this invention is capable of long life with a lower rate of decline
in operating voltage. The ~erm "matrix" is employed herein in as much as it is
believed that in an electrode of this type, the catalyzed active carbon is
10 thoroughly involved with assisting the reduction of oxygen within the cathodeactive layer while the carobn black and the PTFE act in one or more ways; (a) asa hydrophobic gas path, (b) as a conductive agent, which lowers the electrical
resistance of the mixture from about 2 to 3 times, resulting in a better currentdistribution to the current collector, and (c) as a hydrophobic binder,
15 incorporating the wet active carbon in a matrix of Tef l~n*/carbon black . Ofcourse, however, the present invention is not dependent upon this or any theory
for the operation thereof.
U.S. Patent 4,058,482 discloses a sheet material principally
comprised of a polymer such as PTFE and a pore-forming material wherein the
20 sheet is formed of co-agglomerates of the polymer and the pore former. This
patent teaches mixing polymer particles with posi~ively charged particles of a
pore former, e.g., 7inc oxide, to form co-agglomerates thereo~ followed by
mixin~ same with a catalyst suspension so as to form co-agglomerates of
catalyst and polymer pore-former agglomerates followed by pressing, drying, and
25 sintering these co-agglomerates. Subsequent to this sintering, the pore iormer
can be leached out of the electrodes.
U.S. Patent 4,1509076 (a division of U.S. Patent 4,058,482) is directed
to the process for forming the sheet of U.S. Patent 4,058,482, said process
involving formation of polymer pore-former co-agglomerates, distributing same
30 as a layer on a suitable electrode support plate, for example, a carbon paper, to
form a fuel cell electrode by a process which includes pressing, drying, sintering,
and leaching.
U.S. Patent 4,170,540 to Lazarz et al discloses microporous
membrane rnaterial suitable for electrolytic cell utilization and formed by
35 blending particulate polytetrafluoroethylene, a dry pore-forming particulate
material, and an organic lubricant. These three materials are milled and formed
into a sheet which is rolled to the desired thickness, sintered, and subjected to
leaching of the pore-forming material. The present invention avoids the use of
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Iubricants and similarly avoids the necessity of removing same. Additionally,
- according to the present invention, when forming the sheet by passing the
fibrillated mixture of PTFE-particulate pore-forming agent through the rollers,
special care is taken to avoid conditions which would cause the PTFE~ to sinter.The present invention is clearly distinguishable from U.S. Patent 4,170,540 in
respect of preparation of the backing sheet.
British Patent 1,284,054 to Boden et al is directed to forming an air-
breathing electrode containing an electrolyte within an air-depolarized cell.
This air-breathing electrode is made by hot pressing a fluoropolymer sheet
containing a pore-forming agent onto a catalyst composition (containing silver)
and a metallic grid member. According to page 3 of said British patent, the
PTFE-pore-forming agent-paraffin wax containing sheet, is subjected to a
solvent wash to remove the paraffin wax and then sintered in a sintering furnaceat the appropriate temperatures for sintering the fluorocarbon polymer. After
the PTFE-containing sheet is sintered and while it still contains the pore-forming
particles, it is then ready for application to the catalyst composition of the air
electrode for the hot pressing operation. Hot pressing involves the use of
pressures ranging from about 5,000 to about 30,000 psi in conjunction with
temperatures ranging from about 200 to 400F. The process of the present
20 invention is r eadily distinguishable from British Patent 1,284~054 in that the
present invention avoids the use of wax, avoids the trouble and expense of
removing the wax with a solvent wash and does not use sintering thereby
imparting greater porosity to the PTFE in fibrillated form in the finished
electrode. Additionally the present invention avoids the repeated stripping-
25 ~olding over-rolling again procedures required in all the examples of BritishPatent 1,284,054. It will be observed that one of the backing layers which can be
laminated according to the present invention surprisingly allows the formation of
a porous, self-sustaining, coherent backing sheet or layer of PTFE using only a
single pass through rollers.
U.S. Patent 3,385,780 to l-Ming Feng discloses a thin, porous
electrode consisting of a thin layer of a polytetrafluoroethylene pressed against
a thin layer of polytetrafluoroethylene containing finely divided platinized
carbon, the platinum being present in amounts of 1.2 to 0.1 mg/cm2 in the
electrically conductive face of the thin electrode, viz., the side containing the
35 platinized carbon, viz., the active layer. A thermally decomposable filler
material can be used, or the filler can be a material capable of being leached out
by either a strong base or an acid~ U.S. Patent 3,385,780 also mentions a singleunit electrode involving finely divided carbon in mixture with PTFE.
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- In accordance with one embodiment of this invention in respect of
the backing layer, partially fluorinated acetylene black carbon par-ticles are
incorporated with the PTFE in the backing layer thereby resulting in improved
electrical conductivity in the backing layer combined with balanced
5 hydrophobici ty.
U.S. Patent 4,135,995 to Cletus N. Welch is directed to a cathode
having a hydrophilic portion formed of a solid intercalation compound of fluorine
and carbon of the emperical formula CFX, where x ranges from about 0.25 to l
and preferably ranges from about 0.25 to 0.7. The intercalation compounds of
10 carbon and fluorine are referred to as hydrophilic, fluorinated graphites andgraphite fluorides characterized by an infrared spectrum showing an absorption
band at 1220 cm~l. A layer of hydrophobic material, such as polyperfluoro-
ethylene (polytetrafluoroethylene) can be utilized in a hydrophobic portion of the
same layer or it can be utilized in the form of a different layer which can be
15 associated with a current carrier layer. The \Velch cathode may be utilized as an
oxygen (air) cathode.
The present inventior. in respect of the backing layer is readily
distinguishable from that of the Welch patents (when incorporating partially
fluorinated acetylene carbon black particles) in several respects. First, the
20 partially fluorinated compounds utilized in accordance with this invention have a
hydrophobicity greater than that of the acetylene carbon black prior to partiai
fluorination. Secondly, the partially fluorinated compounds which can be utilized
in accordance with one embodiment of this invention are acetylene carbon blacks
of the formula CFx, wherein x ranges from about 0.1 to 0018. Hence, the extent
25 Of fluorination is markedly less in these partially fluorinated compounds as
compared with those disclosed by said Welch patent. Thirdly, it will be observedthat the Welch intercalation compounds are fluorinated graphites or graphite
fluorides. The partially fluorinated acetylene carbon balck compounds which can
be used in the laminates of this invention are partially fluorinated carbon black,
30 e.g., acetylene black, which acetylene blacks are produced by the explosive or
thermal cracking of acetylene or by corresponding electrical procedures. Such
acetylene carbon blacks show significant differences when compared with
graphitic blacks and active carbons due to their structure and history of
production.
U.S. Patent 3,838,064 to John W. Vogt et al is directed to a process
for dust control involving mixing a finely divided fibrillatable polytetrafluoro-
ethylene with a material which characteristically forms a dust ~o form a dry
mixture followed by sufficient working to essentially avoid dusting. Very small
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concentrations oE PTFE, e.g., from about 0.07 to abDut 3% by weight, are
employed ~o achieve the dust con~rol. Corresponding U.S. Patent 3,838,092 also
to Vogt et al is directed to dustless compositions containing fibrous polytetra-fluoroethylene in concentrations of about 0.02% to less than 1%, e.g., about
0O75~6 by weight, of PTFE based on total solids.
The active layers whose use is contemplated to form the laminated
three-layer electrodes in accordance with this invention are readily
distinguishable from both the John W. Yogt et al patents (U.S. Paten~s 3,838,064and 3,838,092) employ much higher concentrations of PTFE and for different
purposes than are taught by said Vogt et al patents.
An article entitled "On the Effect of Various Active Carbon
Catalysts on- the Behavior of Carbon Gas-Diffusion Air Electrodes: 1. Alkaline
Solutions" by I. Iliev et al appearing in the Journal of Power Sources, 1
(1976tl977) 35, 46, Elsevier Sequoia S.A.~ Lausanne-printed in the Netherlands,
at pages 35 to 46 of said Journal there are described double layer fixed-zone,
Teflon-bonded carbon eiectrodes having a gas supplying layer of carbon black
IxC*" (not further defined by the authors) wetproofed with 35% Te~lon ancl an
active layer consisting of a 30 mg/cm2 mixture of the same wetproof material
"XC-35" and active carbon "weight ratio of 1:2.5." These electrodes were
sintered at 350C under a pressure of 200 kg/cm2 and employed as oxygen (air)
cathodes in alkaline test environments.
The active layers and laminates of this invention are also readiy
distinguishable from the oxygen (air) cathodes described in Iliev et al. In
accordance with this invention, the active layer is a "matrix" layer prepared
esentially by shear blending (fibrillating) a cornbined mixture of two separately
formed mixes which are in turn mixed, chopped and then fibrillated to result in a
coherent, self-sustaining sheet having a ~ensile strength characteristically
exceeding 100 psi. Such active layers, when laminated, yield a matrix electrode
having an unusual combination of high tensile strength with resistance to
blistering under high current densities in use. It will be observed that the
conditions employed in formation of the two separately formed mixtures and
fibrillation thereof are insufficient tc effect sintering of the PTFE contained in
s~id matrix electrode.
The publication "Advances in Chemistry Series,'' copyright 1969,
Robert F. C;ould (Editor), American Chemical Society Publications, contains at
pages 13 to 23 an article entitled "A Novel Air Electrode" by H. P. Landi et al.The electrode described contains 2 ~o 8% PTF, is produced wi~hout sintering
and is composed of graphltic carbon (ACCO Graphite*) or metallized graphitic
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carbon particles blended with a PTFE latex and a thermoplastic molding
compound to form an interconnected network which enmeshes the filter
particles. This blend is molded into a flat sheet and the thermoplastic is then
extracted. The present process employs nongraphitic active carbons,
5 significantly higher concentrations of PTFE in the active layer while avoidingthe use of thermoplastic molding compound and avoiding the necessity to remove
same. Also, the active layer used according to this invention is formed by rolling
a prefibrillated granular mix and no molding step is necessary. No indication isgiven by Landi et al as to the stability and/or durability of their air electrode,
10 and no life testin~ or data is included in said article.
U.S. Patent 3,368,950 discloses producing fuel cell electrodes by
electrochemically depositing a uniform noble metal coating on a thin less noble
metal body, for example, platinum on gold; platinum on silver; palladium on
silver; gold on silver; rhodium on silver; gold on copper; silver on copper; nickel
15 on iron or platinum on iron.
U.S. Patent 3,352,719 is directed to a method making silver-
catalyzed fuel cell electrodes by plating a silver catalyst on a carbon or nickel
substrate.
British Patent 1,222,172 discloses use of an embedded conductive
~0 metal mesh or screen 35 withn a formed electrode 30 containing a particulate
matrix 34 of polytetrafluoroethylene polymer particles 21 in which there are
located dispersed electrically conductive catalyst particles 24 which can be
- silver-coated nickel and silver-coated carbon particles, viz., two diferent types
of silver-coated particles in the PTFE particulate matrix if it is desired to
25 overcome an increase in resistance as silver is consumed in the gas diffusion fuel
cells to which said British patent is directed.
U.S. Patent 3,539,469 is directed to the use of silver-coated nickel
particles (powder) in a fuel cell catalyst to economize on the use of silver. This
patent states that silver9 as an oxygen activation catalyst, has been known and
30 heretofor used.
Of course, none of these current distributor patents disclose an
asymmetric woven wire mesh current distributor which can be used in
accordance with this invention.
/
BRIEF SUIAMARY OF THE INVENTION
The active layer of this invention is comprised of active carbon
particles present within an unsintered network (matrix) of f ibrillated carbon
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black/PTFE. The laminated gas electrodes of this invention are
comprised of said active layer laminated on the electrode working
surface to a current distributor and on its opposite surface to a
porous coherent, hydrophobic, polytetrafluoroethylene containing
wetproofing layer.
The active carbon particles of the active layer prefer-
ably are catalyzed to contain silver or platinum and range in
size from about 1 to 30 microns. The unsintered network (matrix)
contains from about 25 to 35 weight parts of polytetrafluoro-
ethylene and about 75 to 65 weight parts of carbon black having asurface area ranging from about 25 to 300 m2 per gram and particle
sizes ranging from about 50 to 3000 angstroms. The active layer
contains a pore-forming agent and the concentration of active
carbon therein ranges from about 40 to 80 weight ~.
DFTAILED D~SCRIPTION OF THE INYENTION
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The Backing (~etproofing) Layer
The three-layer laminated electrodes produced in accord-
ance with this invention contain an outer wetproofing or backing
layer the purpose of which is to prevent electrolyte from coming
through the active layer and wetting the gas side of the active
layer and th~reby impeding access of the oxygen (air) gas to the
active layer. According to one preferred embodiment of this
invention, the backing layer is a porous one made by one-pass
process, viz., wherein it is formed as a coherent, self-sustaining
backing layer sheet by a single pass through hea-ted rollers.
In accordance with another embodiment of this invention,
the porous backing layer contains not only a pore former and
polytetrafluoroethylene particles, but also contains either
electroconductive carbon black particles, per
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se, or carbon black particles which have been partially fluorinated to cer~ain
extents of fluorination, as will be poin~ed out in more detail hereinafter.
When i~ is desired to employ a porous PTFE backing layer made by
the single-pass procedure and containing chiefly only a pore former and PTFE,
5 the backing layer can be prepared in accordance with the process described in
U.S. Patent 4,339,325 entitled "One Pass Process For Formin~
Electrode sacking Sheet" f iled in the names of Frank Solomon
and Charles Grun. When using such a backing layer, the Tef-
lon particles are usually employed ln the form of a nonaqueous
10 disp~rsion, e.g.3 the duPont Teflon 6A series. Teflon 6A, for
example, consists of coagulates or agglomerates having a particle size of about
500 to 550 microns which were made by coagulating (a~glomeratin~) PTFE
dispersed particles of abou~ 0.05 to 0.5 micr~n and having an avera~e particle
size of about 0.2 micron. These agglomerates are dispersed in an organic liquld
15 medium, usually a lower alkyl alcohol, such as isopropanol, and broken down by
beating, e.g., in a high speed Waring* blender for about 3 minutes to redispersesame and break up the larger particles into smaller Teflon particles in
isopropanol.
Then pulverized sodium carbonate particles, having particle sizes
20 ranging from about 1 to about 40 microns, and more usually from about 5 to 20microns, and preferably having an average ~Fisher Sub-Sieve Sizer*) p~rticle size
of 3 to 4 microns, are added ~o the alcohol dispersion of the blended PTFE
particles in a weight ratio ranging from about 30 to 40 weight parts of PTFE to
about 60 to about 70 weight parts of sodium carbonate to result in an intimate
25 dispersion of PTFE with pore former~ Then the alcohol is removed and the
PTFE-Na2CO3 mix particles are dried.
Subsequent to dryin~, the particulate PTFE-sodium carbonate
mixture is subjected to sigma mixing under condi~ions which mildly "fiberize'`
~fibrillate) ~he PTFE. The sigma mixing is conducted in a Brabender* Prep Center3~ Model D101 with attached Sigma Mixer* with a charge of approximately 140
grarns of mix. This fibrilla~ion is performed for approximately 10 to 20, e.g., 15,
minutes at 100 rpm and 15 to 25C, e.g., 20C.
After fibrillating and before passing the mix between rolls, the
fibrillated PT~E-pore former mix is chopped for 1 to 20 seconds, e.~., 5 ~o 10
33 seconds.
The mildly "fiberized" chopped mixture of PTFE--sodium carbonate
is then dry rolled into sheet form using a single pass through one or more sets of
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metal, e.g., chrome plated steel rolls. Temperatures of about 70 to about 90C
and roll gaps ranging from about 5 to about 15 mlls are customarily employed.
The conditions employed in the dry rolling are such as to avoid sintering of thePT~E particles.
~hroughout this disclosure there appear examples. In such examples,
all parts, percents and ratios are by weight unless otherwise indicated.
PREPARATION OF NONCONI:)UCTIVE ~ACKING LAYERS
EXAMPLE 1
(Single-Pass Procedure)
Two hundred cubic centimeters of isopropyl alcohol were poured into
an "Osterizer*"blender. Then 49 grams of duPont 6A polytetrafluoroethylene
were placed in the blender and the PTFE--alcohol dispersion was blended at the
"blend" position for approximately one minute. The resulting slurry had a thick,pasty consistency. Then another 100 cc of isopropyl alcohol were added in the
blender and the mixture was blended (again at the "blend" position) for an
addi~ional two minutes.
Then 91 grams of particulate sodium carbonate in isopropanol (ball
milled and having an average particle size of approximately 3.5 microns as
measured by Fisher Sub-Sieve Sizer) were added to the blender. This PTFE--
sodium carbonate mixture was ~hen blended at the "blend" position in the
"Osterizer" blender for 3 minutes followed by a higher speed blending at the
"liquefying" position for an additional one minute. The resulting PTFE--sodium
carbonate slurry was ~hen poured from the blender onto a Buchner funnel
filtered and then placed in an oven at 80C where it was dried for 3 hours
resultlng in 136.2 grams yield of PTFE-sodium carbonate mixture. This miXture
con~ained approximately 35 weight parts of PTFE and 65 weight parts of sodium
carbonate.
This material was then fibrillated mildly in a Brabender Prep Center
D101 for 15 minutes at 100 rpm and 20C using the Sigma Mixer Blade Model 02-
û9-000 as described above. The thus fibrillated mixture was then chopped for S
to 10 seconds in a coffee blender (i.e., Type Varco, Inc. Model 228.1.00 made inFrance) to produce a fine powder.
The chopped, fibrillated mixture was then passed through 6-inch
diameter rolls, heated to about 80C and using a roll gap typically 0.008 inch (8
mils). The sheets are formed directly in one pass and are ready for use as
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backing layers in forming electrodes, e.g., oxygen cathodes, with no further
processing beyond cutting, trimming to size and the like.
The thus formed layers (after removal of the pore-forming agent) are
characterized as porous, self-sustaining, coherent, unsintered uniaxially oriented
5 backing (wetproofing) layers of fibrillated polytetrafluoroethylene having pore
openings of about 0.1 to 40 microns (depending on the size of the pore-former
used) and exhibit air permeability particularly well suited for oxygen (air)
cathodes.
EXAMPLE 2
1 0 (Re-Rolling)
The procedure of Example 1 was repeated with the exception that
after the PTFE/~a2C03 sheet was passed through the rollers once it was folded
in half and re-rolled in the same direction as the original sheet. A disc of this
material was pressed at 8.5 tons per square inch and 115C and then washed with
water to remove the soluble pore former. Permeability tests conducted on this
sample resulted in a permeability of 0.15 ml of air/minute/cm~ at a pressure of
one cm of water as compared to a test sample prepared according to Example 1
- and pressed and washed as above which gave a permeability of 0021 ml of
air/minute/cm-/cm of water. The permeability test was done according to th~
method of A.S.T.M. designation E 128-61 (Maximum Pore Diameter and
Permeability of Rigid Porous Filters for Laboratory Use) in which the test
equipment is revised to accept discs for test rather than the rigid filters for
which the test was originally designed. The revision is a plastic fixture for
holding the test disc in place of the rubber stopper shown in Figs. 1 and 2 of said
2~ A.S.T.M. standard. Apparently folding and re-rolling are counter productive to
air permeability, an important and desired property in a backing layer for an
oxygen cathode. Moreover, folding and re-rolling may form laminae which give
rise to delamination of the backing layer in use, e.g., in a chlor-alkali cell.
EXAMPLE 3
- 30 (Single-Pass `~ith Volatile Pore Former)
A porous Teflon sheet was fabricated using a mixture of 40 weight %
ammonium benzoate (a volatile pore former) and 60 weigh~ % PTFE prepared as
in Example 1. The sheets were fabricated by passing the above mix (fibrillated
and chopped) through the 2 roll mill once. The rolled sheet was then pressed at
8.5 tons per square inch and 65C. The volatile pore former was then removed
by heating the sheet in an oven at 150c. Substantially, all of the volatile pore
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former was thus sublimed leaving a pure and porous PTFE sheet. Permeability of
these sheets average 0.2.
PREPARATION OF CONDUCTIVE BACIClNG LAYERS
On the other hand, when the laminate has a backing layer containing
carbon particles to enhance the conductivity thereof, either unmodified carbon
blacks or partially fluorinated carbon blacks, e.g., partially fluorinated acetylene
black particles, can be utilized to impart conductivity to the backing layer.
When utilizing unfluorinated carbon black particles to impart ~he
conductivity to the PTFE-containing porous backing layer, carbon blacks can be
employed which are electrically conductive. The term carbon black is used
~enerically as defined in an article entitled "Fundamentals of Carbon Black
Technology" by Frank Spinelli appearing in the August 1970 edition of American
Print Maker to include carbon blacks of a particulate nature within the size
range of 5 to 300 millimicrons which includes a family of indus~rial carbons such
as lampblacks, channel blacks, furnace blacks, thermal blacks, etc.
A preferable form of unmodified (unfluorinated) carbon black is
acetylene carbon black, e.g., made from acetylene by continuous thermal
decomposi~ion, explosion, by combustion in an oxygen-deficient atmosphere, or
by various electrical processes. Characteristically, acetylene black contains
99.5+ weight percent carbon and has a particle size ranging from about 50 to
about 2000 angstrom units. The ~rue density oE the acetylene black rnaterial is
approximately 1.95 ~rams per cubic centimeter. More preferably, the acetylene
black is a commercially available acetylene black known by the designa~ion
"Shawini~an Black"* and has a mean particle size of 425 angstroms with a
standard deviation of about 250 angstroms. Such acetylene blacks are sornewhat
hydrophobic, e.g., as demonstrated by the fact that the par~icles thereof float on
cold water but quickly sink in hot water.
The hydrophobic electroconductive electrode backing layers were
prepared in accordance with this invention by combining the PTFE in particulte
form as a dispersion with the carbon black particles as described above.
According to a preferred embodiment of this invention, the acetylene carbon
black employed is ~hat having an average particle size of approximately 435
angstrom units with the remainder having a standard deviation of 250 angstrom
units. The range of particle size is from about 50 to about 2000 angstroms.
These acetylene black particles are mixed with PTFE particles by
adding a commercially available aqueous dispersion, e.g., duPont "Teflon 30" to
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the carbon black, also dispersed in water to form an intimate mixture thereof.
The "Teflonated" mix can contain from about 50 to about 80 weight % carbon
black and from about 20 to about 50 weight '?~ PTFE. Water is removed and the
mix is dried. The dried Teflonated mix can then be heated at 275 to 300C for
5 10 to 80 minutes to remove a substantial portion of the wetting agent used to
disperse the PTFE in water. Approximately 50 weight % of this mix is fibrillated(as described above in relation to the "one-pass" process) and then mixed with
the remaining unfibrillated mix. A water soluble pore-forming agent, e.g.,
sodium carbonate, can be added thereto and the "Teflonated" carbon black and
10 pore former mixed.
Such conductive PTFE/carbon black-containing backing layers
characteristic:ally have thicknesses of 5 to 15 mils and may be produced by
filtration or by passing the aforementioned acetylene black-PTFE mixes through
heated rollers at temperatures of 65 to 90C or by any other suitable technique.
Then these backing layers are laminated with a current distributor
and the aclive layer as disclosed herein.
EXAMPLE 4
(Preparation of PTFE/Carbon Blacl~)
One -and one-h~lf ~1.5) grams of "Shawinigan Black,'' hereinafter
20 - referred to as "SB," were suspended in 30 mls of hot water (80C) and placed in a
srnall ultrasonic bath (Model 250, RAI Inc.) where it was simultaneously stirredand ultrasonically agitated.
Sixty-eight one hundredths (0.68) ml of duPont "Teflon 30" aqueous
PTFE dispersion was diluted with 20 mls of water and added dropwise from a
25 separatory funnel to the SB dispersion gradually, over approximately a 10-minute
- period with stirring, followed by further stirring for approximately one hour.
This material was then filtered, washed with water and dried at 110C. The
dried material ws spread out in a dish and heated at 300C in air for 20 minutesto remove the PTFE wetting agent (employed to stabilize PTF~ in water
30 dispersion in the first instance).
EXAMPLE 5
(PTFE/SB Wetproofing Layer by Filtration Method)
A PTFE/SB conductive, hydrophobic wetpoofing layer or sheet was
prepared by the filtration method as follows: 225 milligrams of the PTFE
35 discontinuously coated SB, prepared in accordance with Example 1, were chopped
in a small high speed co-ffee grinder (Varco Model 228-1, made in France) for
J~ L7
- 14-
about 30 to 60 seconds and then dispersed in 250 mls of isopropyl alcohol in a
- Waring blender. This dispersi~n was then filtered onto a "salt paper,'' viz., NaCI
on filter paper, of 17 cm2 area to form a cohesive, self-sustaining wetproofing
layer having 10.6 mg/cm? by weight (20 mg total).
Resistivity o~ this wetproofing layer was measured and found to be
0.53 ohm-centimeters. The resistivity of pure PTFE (from "Teflon 30") is
greater than 10 ohm-cm by way of comparison.
The resistivity of the PTFE/SB carbon black wetproofing layer
illustrates that it is still low enough to be useful in forming electrodes when in
intimat~ contact with a current distributor.
Permeability is an important factor in hi~h current density operation
of a gas electrode having hydrophobic (conduc~ive or nonconductive) backin~,
viz., a wetproofing or liquid barrier layer.
The wetproofing layers employed in forming laminates according to
this invention have adequate permeability to be comparable to that of pure PTFE
backings (even when pressed at up to 5 tons/in2) yet ha~e far superior
electroconductivity. The active carbon can be conditioned and used with or
without a precious metal catalyst, e.~ ., platinum, silver, etc., on andlor within
the pores thereof,
The testing of air elcctrodes employing such backing layers in the
corrosive alkaline environment present in a chlor-alkall cell has revealed a
desirable combination of electroconductivity with balanced hydrophobicity and
said layers are believed to have achieved a desired result in the oxygen (air)
cathode field.
CONDUCTIVE BACKING J AYER
CONTAINING PARTIALLY FLUORINATED CARBON BLA~K
When in accordance with this invention conductive backing layers are
employed, it is also contemplated to use partially fluorina~ed carbon black, e.g.,
the partially fluorinated carbon black backing layers as disclosed in
U.S. Patent 4,382,904 filed in the names of
- 15-
Frank Solomon and Lawrence J. (:;estaut and entitled "Electrode Backing Layer
and Method of Preparing". Such partially fluorinated carbon
blacks are preferably acetylene blacks which are subjected to
par~ial fluorination to arrive a~ compounds having the formula CFx, wherein x
ranges from about 0.1 to about 0.18.
The hydrophobicity of the already hydrophobic acetylene black
par~icles is enhanced by such partial fluorination as was observed from
comparative experiments wherein the unfluorinated acetylene black particles
floated on cold water but quickly sank in hot water versus the partially
fluorinated acetylene blacks, fluorinated to the extent of x being about 0.1 ~o
about 0.18, which floated on hot wa~er virtually indefinitely and could not be
made to pierce the meniscu~ of the water.
Such hydrophobic electrode backing layers (containing CFx=O. 1 to
0.18 partially fluorinated carbon black) were prepared by combining the PTFE in
particulate form as a dispersion with the partially fluorinated acetylene black
particles. According to a preferred embodiment, the acetylene black employed
is that having an average particle s;ze of approximately 425 angstrom units witha standard deviation of 250 angstrom units. The range of particle size is from
about 50 to about 2000 angstrorns.
2Q The partially fluorinated carbon black particles are suspended ir.
isopropyl alcohol and a dilute aqueous dispersion of PTFE ~2 weight % PTFE) is
added gradually thereto. This dilute dispersion is made from PTFE dispersion of
60 weight parts of PTFE in 40 weight parts of water to ~orm an intimate mixture
of CFx=O.l to 0.18/PTFE. The PTFF/CFo 1 to 0 18 mix was then filtered, dried,
treated ~o remove the PTFE wetting agent ~by heatlng at 300C for 20 minutes
in air or extractig it with chloroform), and briefly chopped to form a granular
mix and then fabricated into sheet form either by (a) passing between heated
rollers (65 to 90C)9 or (b) by dispersion of said PTFE/CFx-O.I to 0.18 particles
in a liquid dispersion medium capable sf wetting said particles and filtration on a
salt (NaCI) bed previously deposited on filter paper or like filtration media, or (c)
by 5praying the CFo 1 to o 18/PTFE mix in a mixture of water and alcohol, e.g.,
isopropyl alcohol~ on an electrode active layerIcurrent distributor composite
assembly and drying to yield a fine-pore vetproofing layer. The "Teflonated"
mix can contain from about 50 to 80 weight 96 CFo.1 to 0.18 and about 20 to 50
weight % PTFE.
In any case, a pore former can be incorporated into the CFo 1 to
o 18/PTFE mix prior to forming the wetproofing layer or shee$. The pore former
~ ' .
53
- 16-
can be of the soluble type, e.g., sodium carbonate or the like, or the volatile
type, e.g., ammonium benzoate or the like.
Whether the wetproofing sheet is formed by rolling, filtration or
spraying, the pore former can be removed by washing (if a soluble one) or heating
5 (if a volatile one) either prior to laminating the wetproofing layer to the current
distributor (with the distributor on ~he gas side) and active layer, or ater
lamination thereof. ln cases where a soluble pore former is used, the laminate is
preferably given a hot (5D to about 100C~ soak in an alkylene polyol, e.g.,
ethylene glycol or the like, prior to water washing for 10 to 60 minutes. The
10 ethylene glycol hot soak combined wlth water washing imparts enhanced
resistance of such laminated electrodes to blistering during water washing and is
the subject matte~ described in U.S. Patent 4,357,262 en-
titled "Electrode Layer Treating Process" in the name of
. Frank Solomon.
When the wetproofing layer is formed by filtration, it can be released
from the filter media by washing with water to dissolve the salt bed, drying andpressing lightly to consolidate same, followed by laminating to the current
distributor and active layer. Alternatively, the filter paper/salt/wetproofing
layer can be laminated to the current distributor and active layer (with the filter
20 paper side away from the current distributor and the wetproofing layer side incontac~ with the current distributor~ followed by dissolving the salt away.
The testing of the electroconductive, hydrophobic backing layers of
this invention in the corrosive environment of use of a chlor-alkali cell has
revealed a desirable combination o~ electroconductivity with balanced hydro-
25 phobicity and said layer is believed to have achieved a much desired result in theoxygen (air) cathode field.
The testing of such partially fluorinated backing Iayers in the
corrosive alkaline environment of use in a chlor-alkali cell has revealed a
desirable combination of electroconductivity with balanced hydrophobicity and
30 said layers are belieYed to have achieved a desired result in the oxygen (air)
cathode field.
The formation and testing of the partially fluorinated carbon-
containing backing layers will be described in greater detail in the examples
which follow. The term "SBF" as used herein means partially fluorinated
35 "Shawinigan Black."
--....
ii3
EXAMPLE 6
(Preparation of SBF 0.17/PTPE Mix)
One and one-half (1.5) grams of SBFo 17 were suspended in 30 ml of
isopropyl alcohol (alcohol wets 513F~. The mixture was placed in a small
5 ultrasonic bath, Model 250, RAI, Inc. and was silultaneously stirred and subjected
to ultrasonic agitation.
Sixty-eight one hundredths (0.68) ml of duPont "Teflon 30" dispersion
were diluted with 20 ml H2O and added dropwise from a separatory funnel to the
SBF 0.17, slowly (i.e., 10 minutes). After further stirring (1 hr), the material was
10 filtered, washed and dried at 110C.
A layer was made by a filtration method. Of the above material9 225
mg was chopped in a small high speed coffee grinder, then dispersed in 250 ml
isopropyl alcohol in a Waring blender and filtered onto a sodium chloride (salt)layer deposited on a filter paper of 19 cm area to form a layer having an area
15 density of 10.6 mg/cm2. Resistivity was measured and found to be 8.~ ohm-cm.
The SEi control strip was prepared in accordance with E:xamples 4 and
5 above. ~esistivity of this S~ control strip was found to be 0.53 ohm-cm.
Although the resis.ivity of the SBF strip is 16.6 times as great as that of saidcontrol strips, i. is still low enough to be useful when a mesh conductor is
- 20 embedded in ~e h3~rophobic backing. Pure PTFE has a resistivity of greater
than i ol5 ohm-cr~L oy way of comparison.
Gas permeability is an important property for high current density
operation of a gas electrode having a hydrophobic conductiYe or nonconductive
backing layer. ~ ll~e SBF-PTE~ backing layer prepared as above had adequate air
25 permeability comparable to the "one pass" PTFE backings of Examples 1 and 3
above even when p~ssed to, tons per square inch.
~= THE ACTlVE LAYER
In forming the three-layer laminate electrode of this invention, there
is employed a "matrix" active layer as an essential component. This matrix
30 active layer com~rises active carbon particles present within an unsintered
network (matrix) o$ fibrillatei carbon black/polytetrafluoroethylene.
One s~ream (mixture), the matrixing mix component, is obtained by
adding a dilute dispersion containing polytetrafluoroethylene (PTFE), e.g.,
duPont "Teflon 30" having a particle size of about from 0.05 to 0.5 micron in
35 water to mix of ~ carbon black, e.g., an acetylene black, and water in a weight
ratio of from about 25 to 35 weight parts of PTFE to from about 65 to 75 weight
parts of carbon black to "Teflonate" the carbon black, viz., form an intimate mix
- of PTFE/carbon black particles, drying the aforementioned mixture and heat
treating it to remove the PTFE wetting agent thereby resulting in a f irst
component mix.
The second component, the active carbon-containing catalys~
component, is comprised of an optionally catalyzed, preferably previously
deashed and optionally particle size classified active carbon, having a particlesize ranging from about 1 to about 30 microns and more usually from about lU to
about 20 microns.
Deashing can be done by pretreatment with caustic and acid to
remove a substantial amount of ash from the active carbon prior to catalyzing
same. The term ash refers to oxides principally comprised of silica, alumina, and
iron oxides. The thus deashed, classified, active carbon particles
can then be catalyzed with a precious metal7 e.g., by contac~ing with a silver or
15 platinum precursor, followed by chemical reduction with or without heat to
deposit silver, platinum or other respective precious metal on the active carbon.
The catalyzed carbon can be filtered, dried at temperatures ranging frGm abou~
80 to 150C, with or without vacuum, to pcoduce a second (active carbon
catalyst) component or mixture.
~0 This mixtures are then chopped together, with or without the addi~ion
of a particulate, subsequently remoYable (fugitive) pore-forming agent and then
shear blended (fibrillated) at temperatures ranging from abou~ 40 $o about 60Cfor 2 to 10 minutes, e.g.9 4 to 6 minutes, in the presence of a processing aid or
lubricant, e.g., a 50:50 mixture (by weight) of isopropyl alcohol and water, viz.,
25 when no pore former is used as bulking agent. e~lhen a water-soluble pore former
is used, the lubricant can be isopropyl alcohol. The previously chopped mixture
can be fibrillated using a mixer having a Sigma or similar blade. During this
fibrillation step, the chopped mixture of ~he two-component mixes is subjected
to shear blending forces, viz., a combination of compression and attenuation
30 which has the effect of substantially lengthening the PTFE in the presence of the
remaining components. This fibrillation is believed to substantially increase the
strength of the resulting sheets formed from the fibrillated mixed components.
After such fibrillating, the mixture is noted to be fibrous and hence the term
"fiberizing" is used herein as synonymous with fibrillating.
,~;
`~
i3
- 19 -
Subsequent to fibrillation, the mixture is dried, chopped for from 1 t~3
10 seconds into a fine powder and formed into a sheet by rolling at 50 to 100C-
or by deposition on a filter. A pore former, if one is employed as a bulking
agent, can be then removed prior to electrode fabrication. In the event no pore
5 former is employed, the matrix active layer sheet can be used (as is) as the
active catalyst-containing layer of an oxygen (air~ cathode, e.g., for use in a
chlor-alkali cell fuel cell, etc.
In forming the active layers and laminates of the present invention,
the aforementioned blistering and structural strength problems encountered at
10 high current densities in the active layers of gas electrodes can be substantially
overcome by a process involving: forming two separate components, one a
matrixing mix component containing carbon black with polytetrafluoroethylene
particles and heat treating this PTFE-carbon black mix at given temperature
conditions; separately forming an active carbon-containing catalyst component;
15 combining these two separate components into a mix; chopping the mix and
shear-blending the chopped mix (fibrillating same) in order to arrive at a readily
formable matrix which can be formed, e.g., pressed betwèen rolls, or deposited
upon a filter paper as a forming medium, pressed and then used as the active
layer in an oxygen tair) cathode. Such process results in active layers having
20 reduced carbon corrosion, higher conductivity and air-transport combined wi~h strength when compared with prior structures. This results in electrodes which
can be used longer and are more stable in use.
Tensile strength tests of the coherent, self-sustaining active layer
sheets rolled from the fiberized material characteristically displayed approxi-
25 mately 50% greater tensile strength than unfiberized sheets. Life testing ofelectrodes employing the fibrillated (fiberized) active layer sheets of this
invention resulted in approximately 8900 hours life at 300 milliamps/cm2 in 309~hot (60 to 80C) aqueous sodium hydroxide before failure. In addition to the
advantages of longevity and strength, this process is easy to employ in making
30 large batches of active layer by continuous rolling of the fibrillated mix resulting
in a material uniform in thickness and composition. Furthermore, the process is
easy to administer and control.
In accordance with one preferred embodiment of this invention, a
water-soluble pore-forming agent, e.g., sodium carbonate, is employed in the
35 mixing step wherein the polytetrafluoroethylene dispersion is mixed with carbon
black. Alternatively, the pore-forming agent can be added later, when the
carbon black-PTFE mix and the catalyzed ac-tive carbon particles are mixed
together and chopped.
5~
- 20 -
In forming an initial mixture of carbon black and polytetrafluoro-
ethylene, the usual particle size of the carbon black ranges Erom about 50 to
about 3000 angstroms and it has a surface area rangin~ from about 25 to about
30Q m2/gram. The PTFE is preferably employed in aqueous dispersion form and
5 the mixture of carbon black and polytetrafluoroethylene can contain from about65 to about 75 weight par~s of carbon black and about 35 to about 25 weight
parts of PTFE. After mixing, the carbon black and PTFE are dried and then the
dried initial mix is heated in air at temperatures ranging from about 250 to
325C, and more preferably 275 to 300C, for time periods ranging from 10
minutes to 1.5 hours and more preferably from 20 minutes to 60 minutes. This
heating removes the bulk of the PTFE wetting agent.
The other component of the matrix electrode, viz., the active carbon,
preferably"Rs*" carbon manufactured by Calgon*, a division of Mer~k, is deashed
by contact with an aqueous alkali, e.g., sodium hydroxide, or equivalent alkali,15 and more usually aqueous sodium hydroxide having a sodium hydroxide concen-
tration of about 2~ to about 55 weight 96 for 0.5 to 25 hours. After washing, the
active carbon is ~hen contacted with an acid, which can be hydrochloric acid,
phosphoric acid, sulfuric acid, hydrobromic acid, etc., at ambient temperatures
using aqueous acid solutions havlng from about 10 to about 30 weight % acid,
20 based on total solution for comparable time periods. Subsequent to the contact
with acid, the deashed activP carbon particles are preferably catalyzed. The
deashed particles are preferably catalyzed as by contact with a precursor of a
precious metal catalyst. In the event that silver is desired to be deposited within
the pores of the active carbon, it is preferred to use silver nitrate as the catalyst
25 precursor followed by removal of excess silver and chemical reduction with
alkaline forrnaldehyde.
On the other hand9 in the event that it is desired to deposit platinum
within the pores of the acti~e carbon material9 chloropla~inic acid can be used as
a precursor followed by removal of excess chloroplatinic acid and chemical
30 reduction using sodium borohydride or formaldehyde as a reducing agen~.
According to a preferred embodiment, the platinum is derived from
H3Pt(5O3)20H by the procedure set forth in U.S. Patent 4,044,193. The
reduct;on can be accompanied with the use of heat or it can be done at ambient
room temperatures. After catalysis, the active carbon particles are filtered and
*Trademark
7S3
- 21 -
vacuum dried as the active carbon-containing catalyst component in preparation
for comoination with the acetylene black/PTFE matrixing component mix.
The carbon black/PTFE matrlxing component mix preferably in a
weight ratio ranging from about 65 to 75 weight parts of carbon black to 25 to 35
weight parts of PTFE is mixed with the catalyzed deashed active carbon-
containing component and subjected to chopping to blend the carbon black/PTFE
matrixing component with the catalyst component in the manner set forth above~
This mix is then subjected to fibrillation (shear blending or fiberizing), for
example, in a mixer with appropriate blades at approximately 50C. This shear
blended material has a combination of good conductivity and high tensile
strength with low Teflon content resulting in extraordinarily long life in use at
high current densities in the corrosive alkaline environment present in a chlor-alkali cell.
The active layers employed in this invention can contain (af ter
removal of any pore-forming bulking agent therefrom) from about 40 to 80
weight % of active carbon, the remainder being the matrixing materials1 carbon
black and PTFE.
Subsequent to the fibrillation step, the fibrillated material is dried,
chopped and rolled at approximately 75C yielding the resulting coherent, self-
sustaining and comparatively high tensile strength active layer sheet. Active
carbon-containing active layer sheets produced in accordance with this inventioncharacteristically have thicknesses of 0.010 to 0.025 inch (10 to 25 mils) with
corresponding tensile strengths ranging from about 75 to 150 psi (measured afterpressing in a hydraulic press at 8.5 Tlin2 and 112C for 3 minutes).
,
EXAMPLE 7
IA matrix active layer containing silver-catalyzed active carbon
particles)
Commercially available ball milled "RB carbon" was found to have an
ash content of approximately 12% as received. This "RB carbon" was treated in
30 38% KOH for 16 hours at 115C and found to contain 5.6% ash content after a
subsequent furnace operation. The alkali treated "RB carbon" was then treated
(immersed) for 16 hours at room temperature in 1:1 aqueous hydrochloric acid
(20% concentration). The resulting ash content had been reduced ~o 2.8%. "RB
carbon," deashed as above, was silvered in accordance with the following
35 procedure:
5;:~ ,
20 grams o~ deashed "RB carbon" were soaked in 500 ml of 0.161 N
(normal) aqueous AgN03 with stirring for 2 hours. The excess solution was
filtered off to obtain a filter cake. The retrieved filtrate was 460 ml of 0.123 N
AgN03. The filter ca~e was rapidly stirred into an 85C alkaline formaldehyde
solution, prepared usirg 300 cc (cubic centirneters) svater, and 30 cc of 30%
aqueous NOH and 22 cc of 37% aqueous CH20, to ppt. Ag in the pores of the
active carbon.
Calculation indicated that 79% of the 2.58 grams o~ retained silver in
the catalyst was derived from adsorbed silver nitrate.
Separately, "Shawinlgan Black,'' a commercially available acetylene
carbon black was teflonated with ~'Teflon 30" (duPont polytetrafluoroethylene
dispersion) using an ultrasonic generator to obtain intimate mixture. 7.2 grams
of the carbon black/PTFE mix was high speed chopped, spread in a dish, and then
heat treated at 525F for 20 minu~es. Upon removal and cooling, it was once
15 again high speed chopped, this time for 10 seconds. Then 18 grams of the
classified silvered active carbon was added to the 702 grams of carbon black-
Teflon mix, high speed chopped for 15 seconds, and placed into a fiberizing
(fibrillating) apparatus. The apparatus used for fiberizing consists of a
Brabiender Prep Center, Model D101, with an attached measuring head REO 6 on
20 the Brabender Prep Center and medium shear blades were used. The mixture
was added to the cavity of the mixer using 50 cc of a 3D/70 (by volume) mixture
of isopropyl alcohol in water as a lubricant to aid in fibrillating. The mlxer was
then run for 5 minutes at 30 rpm at 50C, after which the material was removed
as a fibrous coherent mass. This mass was then oven dried in a vacuum oven and
25 was lligh speed chopped in preparation for rolling.
The chopped particulate material was then passed through a rolling
mill~ a BQlling* rubber mill. The resulting matrix active layer sheet had an area
density of 22.5 milligrams per square centimeter and was ready for lamination.
EXAMPLE 8
(A matrix active layer containing platinum-catalyzed active carbon
particles)
The proceaure of Example 7 was repeated except that platinum was
deposited on the deashed active ("RB"3 carbon instead of silver. The 10 to 20
micron classified deashed "RB" carbon had platinum applied thereto in
*Trademark
,,,~'
53
accordance with the procedure described in U.S. Patent ~,044,193 using
~13Pt(S03)20H to deposit 1 weight part platinum per 34 weight parts of deashed
active carbon.
After fibrillation and upon rolling, the area density of the active
5 layer was de~ermined to be 22.2 milligrams per cm2. This matrix active layer
was then ready for lamination.
EXAMPLE 9
(A matrix active layer containing silver-catalyzed active carbon
particles without heat treatment before fibrillation)
An active layer containing deashed, silvered "RB" active carbon was
prepared as in Example 7 with the exception that the 70/30 (by weight)
"Shawinigan Black"/"Teflon 30" matrixing material was not heat treated before
fibrillating. This matrix active layer was heavier than those prepared accordingto Example 7 and 8. It had an area density of 26.~ milligrams per cm2 and was
15 ready for lamination.
EXAMPLE 10
(A matrix active layer containing platinum-catalyzed active carbon
particles incorporating a pore former and heat treated9 as in Examples 7 and 89
before fibrillation)
This matrix active layer was made according to the basic procedure
of Example 7 using deashed "R~" active carbon platinized by the method of U.S.
Patent 4,044,193 to a level of 19 weight parts of deashed "RB" active carbon perweight part platinum. Six grams of ultrasonically teflonated (70:30, "ShawiniganBlack":PTFE) carbon black were heat treated for 20 minutes at 525F prior to
25 addition thereto of 15 grams of said active carbon along wi~h 9 grams of sodium
carbonate, which had been classified to the particle size range of +5 to -10
microns. This material was fibrillated and rolled out as in Example 1 and
extracted by water (to remove the sodium carbonate) after first hot soaking it in
ethylene glycol at 75C for 20 minutes. The resulting active layer sheet was a
30 very porous and lightweight material.
'7~i3
~ 24 -
THE CURRENl DISTRIBUTOR LAYER
The current distributor layer, which is usually positioned next to and
laminated to the working surface of the active layer o~ the three-layer laminate,
can be an asymmetric woven wire mesh wherein the material from which the
5 wire is made is selected from the group consis~ing of nickel, nicl:el-plated
copper, nickel-plated iron, silver-plated nickel, and silver-plated, nickel-plated
copper and like materials. In such asymmetric woven wire mesh current
distributs)rs, there are more wires in one direction than in the other direction.
The current dis~ributor or collector utilized in accordance with this
10 invention can be a woven or nonwoven, symmetrical or asymmetric wire mesh or
grid. Generally there is a preferred current carrying direction. When the
current distributor is a copper wire mesh, there should be as few wired as
feasible in the noncurrent carrying direction. There will be found to be a
minimum required for fabrication of a stable wire cloth. A satisfactory
15 asymmetric wire cloth configuration may consist of, e.g., 50 wires/inch in the
warp direction but only 25 wires per inch in the fill, thus enhancing the economy
and utility o~ the wire cloth, simultaneously.
These asymmetric woven wire current distributors reIerred to herein-
ab~ve are described in U.S. Patent 4,354,917 filed in the
name of Frank Solomon, and entitled "Asymmetric Current
Distributor." Such asymmetric, woven wire mesh current
distributors are useful as the current dis~ribu~or in the three-layer
laminates of this invention which are useful as oxygen cathodes in chlor-alkali
cells.
Alternatively the current distribuor can be of the plaque type, viz., a
comparatively compact yet porous layer, having porosities ranging from about 30
to abou~ 80% and made of powders of Ni, Ag or the like.
FORMING_THE THREE-LAYER LAMINATES
The three-layer laminates produced in accordance with $his invention
usually have the active layer centrally located, viz., positioned in the middle
between the backing layer on the one side and the current distributor (collector)
layer on the other side. The three layers arranged as described are laminated
using heat and pressure at temperatures ranging from about 100 to about 130C
and pressures of 0.5 to 10 Tlin2 followed by removal from the pressing device.
The laminates are preferably then subjected to a hot soaking step in e~hylene
9~
- 25 -
glycol or equivalent polyol to enhance removal of the pore-forming agent(s)
employed to form the aforementioned backing (wetproofing) layer and any
bulking and/or pore-forming agent optionally included in the active layer, upon
subsequent washing(s) with water.
The laminating pressures used will depend on whether or not electro-
conductive (carbon black) particles have been included in the backing layer along
with the PTFE. Thus when using pure Teflon, viz., Teflon with pore former only,
pressures of 4 to 8 T/in2 and temperatures of 90 to 130C are customarily
employed. Upon lamination, the current collector is deeply embedded in the
10 active layer.
On the other hand, when using the electroconductive carbon black
particles in the backing layer, pressures as low as 0.5 to 2 T/in2, and more
characteristically as low as l T/in2 have been determined to be adequate to
effect the bonding of the conductive backing to the active layer. Of course,
15 higher laminating pressures can be employed so long as the porosity of the
structure is not destroyed.
The three-layer laminates of this invention can be formed using a
variety of the aforementioned backing layers and current distributors. The
following examples further illustrate their preparation and actual testing in
20 corrosive alkaline environments and at current densities such as are employed in
chlor-alkali cells, fuel cells, batteries, etc.
EXAMPI E l l
(Forming laminated electrodes from the matrix active layers of
Examples 7 to 9 and testing them in alkaline media at current densities o~ 250
25 milliamperes per square centimeter and higher)
The active layers prepared in accordance with Examples 7 to 9,
respectively, were each laminated to a current distributor and a backing sheet of
sodium carbonate-loaded PTFE prepared as follows:
Two hundred cubic centimeters of isopropyl alcohol were poured into
30 an "Osterizer" blender. Then 49 grams of duPont 6A polytetrafluoroethylene
were placed in the blender and the PTFE--alcohol dispersion was blended at the
"blend" position for approximately one minute. The resulting slurry had a thick9pasty consistency. Then another 100 cc of isopropyl alcohol were added in the
blender and the mixture was blended (again at the "blend" position) for an
35 additional two minutes.
7S~
- 26 -
Then ~1 grams of particulate sodium carbonate in isopropanol (ball
milled and having an average particle size o-f approximately 3.5 microns as
measured by Fisher Sub-Sieve Sizer) were added to the blender. This PTFE--
sodium carbonate mixture was then blended at the "blend" position in the
"Osterizer" blender for 3 minutes followed by a higher speed blending at the
"liquefying" position for an additional one minute. The resulting PTFE--sodium
carbonate slurry was then poured from the blender onto a ~uchner funnel,
filtered and then placed in an oven at 80C where it was dried for 3 hours
resulting in 136.2 grams yield of PTFE--sodium carbonate mixture. This mixture
contained approximately 35 weight parts of PTFE and 65 weight parts of sodium
carbonate.
This mixture was mildly fibrillated in a ~rabender Prep Center with
attached Sigma mixer as described above.
After fibrillating, which compresses and greatly attenuates the
PTFE, the fibrillated material is chopped to a fine dry powder using a coffe
blender, i.e., Type ~arco, Inc. Model 228.1.00 made in France. Chopping to the
desired extent takes from about 5 to 10 seconds because the mix is friable. The
extent of chopping can be varied as long as the material is finely chopped.
The chopped PTFE-NaCO3 mix is fed to 6-inch diameter chrome-
plated steel rolls heated to about 80C. Typically these rolls are set at a gap of
0.008 inch (8 mils) for this operation. The sheets are formed directly in one pass
and are ready for use as backing layers in forming electrodes, e.g., oxygen
cathodes, with no further processing beyond cutting, trimming to size and the
like.
The current distributor was a 0.004 to 0.005 inch diameter nickel
woven wire mesh having a 0.0003 inch thick silver plating and the woven strand
arrangement tabulated below. The distributor was positioned on one active layer
side while the backing layer was placed on the other side of the active layer.
The lamination was performed in a hydraulic press at 100 to 130C
30 and using pressures of 4 to 8.5 T/in2 for several minutes. These laminates were
then hot soaked in ethylene glycol at 75C for 20 minutes before water washing
at 65C for 18 hours and then dried.
The laminates were then placed in respective half cells for testing
against a counter electrode in 30% aqueous sodium hydroxide at temperatures of
35 70 to 80C with an air flow of 4 times the theoretical requirement for an air
cathode and at a current density of 300 milliamperes per cm2. The testing
results and other pertinent notations are given below.
- 27 -
TABLE 1
ActiveType of AGInitial VoltageUseful Life
LayerPlated vs. Hg/HgO of Matrix Voltage at
Examp.Ni MeshlRef. ElectrodeElectrode ~hrs)Failure
_
758x60x0.004-0.265volt 8,925 -0.395 volt ~1)
850x50x0.005-0.201 volt 3,512* N.A. ~2)
958x60x0.004-0.282 volt 3,861 -0.509 volt ~3)
(1) Shortly after 8,925 hours, there was a steep decline in potential, and
the electrode was judged to have failed~
10 ~2) A$ter 188 days9 its voltage was -0.246 volts compared to the Hg/HgO
reference electrode (a very slight decline in potential), and this matrix electrode
-is still on life testing. After being started at 300 milliamperes per cm2, the test
current density was changed to 250 milliamperes/cm2.
(3) The final failure was caused by separation of the current distribu~or
15 from ~he face of the electrode.
It should be noted here that in each of these "matrix" electrodes the
approximate concentration of PTFE in the active layer mix is only about 1296 by
weight. Prior to these "matrix" active layers used according to ~his invention,
PTFE concentrations in active layers of approximately 20% were usually
20 considered mandatory to obtain satisfactory electrodes. For example, prior tothis invention, PTFE concentrations in active layers of below about 18 weight %
yielded completely unsatisfactory electrodes. Hence it will be recognized that
the "matrix" active layers o~ this invention enable considerably less Teflon to be
used while still achieving the combined requirements of conductivity, strength,
25 permeability and longeYity, long sou~ht in air-breathing electrodes.
E~MPLE 12
A laminated electrode was Eormed using the PTFE/sodium carbonate
one pass backing layer of Example 1, the active layer sf Example 7 and a prior
art type porous sintered nickel plaque current distributor (Dual Porosity Lot No~
30 502-62-46). The matrix active layer was positioned on ~he coarse side of saidplaque and the PTFE/sodium carbonate backing layer was placed on top of the
other surface of the active layer. This sandwich was pressed at 8.5 Ttin2 and
115C for 3 minutes after which it was hot soaked in ethylene glycol at 75C for20 minutes followed by water washing at 65C for 18 hours. This air elec~rode
35 was operated at 4 times theoretical air and 250 milliamperes/cm2 in 30% NaOH
at 70C and operated satisfactorily for 17 days before failure.
