CA1173790A

CA1173790A - Asymmetric current distributor

Info

Publication number: CA1173790A
Application number: CA000387764A
Authority: CA
Inventors: Frank Solomon
Original assignee: Diamond Shamrock Corp
Current assignee: Diamond Shamrock Corp
Priority date: 1980-10-31
Filing date: 1981-10-13
Publication date: 1984-09-04
Also published as: US4354917A; EP0051437A1; JPS57108282A

Abstract

ASYMMETRIC CURRENT DISTRIBUTOR

ABSTRACT OF THE DISCLOSURE

This disclosure is directed to an asymmetric woven wire mesh current distributor for an electrode containing more conductive elements (portions) in one direction than in the other direction when installed in use in an electrode so that the greater number of wires is perpendicular to the major current feeder bars and span the narrow part (conductive path) of a rectangular air cathode. Inaccordance with the preferred embodiment of this invention, the current distributor is a woven wire mesh wherein the numerical ratio between the number of said perpendicular wires to said parallel wires ranges from about 1.5 to about 3:1.

Description

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ASYMMETRIC CURRENT DISTRIBUTOR
'~ BACKGROUND OF THE INVENTION

In chlor-alkali cells, an electric current is passed through a saturated brine (sodium chloride salt) solution to produce chlorine gas and caustic soda (sodium hydroxide). Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane, ; such as the commercially available NAFIONR manufactured by the E. I.
du Pont de Nemours ~ Company. Alternatively, the separator can be a porous diaphragm, e.g., asbestos, which can be in the form of vacuum ` ~ deposited fibers or asbestos paper sheet as are well known in the art.
The anode can be a valve metal, e.g., titanium, provided with a noble ~; ~ metal coating to yield what is known in the art as a dimensionallystable anode. Steel cathodes are presently used for the cathodes. A
large portion of the chlorine and caustic soda for the chemical and plastics industries is produced in chlor-alkali cells.
- One of the unwanted by-products present in existing commercial chlor-alkali cells is hydrogen which forms at the cell cathode. It has been :~ ~

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estimated that approximately 25 percent of the electrical energy required to operate a chlor-alkali cell is utilized due to the forma-tion of hydrogen at the cathode. Hence, the elimination of hydrogen formation can lead to substantial energy savings and cost savings with respect to the electrical power required to operate such cells. Recently there has been considerable interest in oxygen (air) cathodes. These cathodes prevent the formation of molecular hydrogen at the cathode ; and enhance the formation of hydroxyl groups which, in turn, assist in the preparation of alkali which can ~e readily removed as a product.
Savings in the cost of electrical energy are thereby achieved. Such oxygen (air) cathodes can utilize wire mesh current distributors as disclosed and claimed herein. Of course, the present invention is applicable to other electrodes in addition to oxygen cathodes and chlor-alkali cells.
FIELD OF THE INVENTION
The asymmetric woven wire mesh current distributors of the present invention are particularly useful in serving as a current dis-tributor in an oxygen (air) cathode useful in chlor-alkali cells. Due to their asymmetric structure, substantial economies in material and weaving costs for the current distributor and control of the direction of current travel (with its resulting control of current path) can be achieved. The present invention is particularly useful when serving as the current distributor in three~layer laminated electrodes.
PRIOR ART
_ The use of precious metals in conJunction with less noble metal electrode bodies is already known in the field of fuel cell gas electrodes. For example, U.S. Patent 3,368,950, to Levine, et al., disclosed producing fuel cell electrodes by electrochemically depositing a uniform precious metal coating on a thin, less noble metal body, e.g., platinum on gold; platinum on silver; palladium on silver; gold on silver, gold on copper; silver on copper; nickel on iron or platinum on iron.
U.S. Patent 3,352,719 is directed to a method of making a silver-catalyzed fuel cell electrode vy plating a silver catalyst on a carbon or nickel substrate.

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British Patent 1,222,172 utilizes silver-coated nickel particles in combination with silver-coated carbon particles disposed in a PTFE
(polytetrafluorethylene) matrix in which there is embedded a nickel or other wire mesh screen (35).
U.S. Patent 3,598,657, to Barber, discloses current collector screens (4) and (5) which can be made of tantalum for the acid electrolyte and nickel for the basic electrolyte.
French Patent 1,520,791, to Gove, is directed to 10 an electroconductive support containing an active catalyst constituted of a dispersion of thorium in nickel.
U.S. Patent 4,191,618 shows an oxygen depolarized cathode wherein a mass of noble metal 15 catalytic particles and particulate binder is bonded to the surface of a NAFION membrane.
In general, little attention has been directed to the particular configuration of the current ; distributor in relation to achieving economies and improvements in chlor~alkali cells containing oxygen (air) cathodes.

DETAILED DESC~IPTIO~ OF THE INVENTION
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The present invention provides an improved gas electrode having an active surface layer of activated 25 carbon particles bound together by a carbon black-fibrillated polytetrafluoroethylene mixture with a current conductor in contact therewith, said current conductor comprising an asymmetric woven wire mesh having more conductive wire strands in the direction 30 generally perpendicular to the major current feed to said conductor than in the direction generally parallel to said direction of major current feed. The present invention also provides an improved oxygen 3~

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cathode having an active surface layer of activated carbon particles bound together by a carbon black-fibrillated polytetrafluoroethylene mixture with a current collector in contact therewith, said current 5 collector comprising a asymmetric woven wire mesh current collector having more conductive wire strands in the direction generally perpendicular to the direction of major current feed than in the direction generally parallel thereto, said generally lO perpendicular wire spanning the shorter conductive path of said cathode.
The invention will be described in greater detail in conjunction with Figures l and 2 of the drawings. Figure l is a frontal view of an electrode 15 incorporating an asymmetric woven wire mesh current distributor in accordance with this invention. Figure

2 is a cross sectional view thereof taken along the line 2-2 of ~igure 1.
As will be apparent from the drawings, oxygen (air) cathode (2) is mounted in a channel (1) which is comprised of upper and lower peripheral current feeder bars ~6) and (7), which are the major current feeder bars. Said cathode has an active layer (3), a backing layer (4) and a current distributor (5). Mesh wires 25 or elements (8) axe arranged generally perpendicular to the major current eeder bars which are in turn connected to a current take-off means (10), whereas wires or elements (9) are positioned generally parallel thereto. These latter wires (9) traverse the 30 length of the oxygen (air) cathode. According to this invention, there are more wires (8) perpendicular to the major current feeder bars than are parallel thereto. The asymmetric woven wire mesh current distributor as shown prefereably has from 1.5 to 3 35 times as many such perpendicular wires (8) as parallel wires (9).

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~, Such wires can be made of a variety of materials, inluding, but not necessarily limited to, nickel; nickel-plated copper; nickel-plated iron; silver-plated nickel; silver-plated, nickel-plated copper and like materials. The diameter of the wire can characteristically range from 0.003 to 0.007 inch (with plating in the case of plated wires). Acco,rding to one preferred embodiment of this invention, the ratio between such perpendicular wires (8) and parallel wires (9) is approximately 2:1 which reflects cost economies of 25 percent in material and 50 percent in weaving time, e.g., with asymmetric woven wire mesh having a wire diameter of 0.005 inch with 50 strands/inch of perpendicular wires and 25 strands/inch of parallel wires versus conventional symmetrical woven wire mesh having a wire thickness of 0.005 inch and 50 strands of perpendicular wires/inch and 50 strands of parallel wires/inch.
Preferably, the wire material is selected from the group consisting of nickel; nickel-plated copper; silver-plated nickel and silver-plated, nickel-plated copper.

EXAMPLE
An asymmetric woven wire mesh of the basic type illustrated in Figure 1 was formed using nickel wire strands having a diameter of approximately 0.005 inch with approximately 50 wires per inch generally perpendicular to the ma~or current feeder bars which were in the hori-zontal dimension and approximately 25 wires per inch generally pa~allel thereto and in the horizontal dimension. This nickel wire cloth was incorporated as the asymmetric woven wire mesh current distributor into a three-layer laminated electrode in accordance with the following procedure.
An active layer of catalyzed or uncatalyzed active carbon particles present within an unsintered network of fibrillated carbon black-polytetrafluoroethylene was placed on one side of said nickel asymmetric woven wire mesh and a wetproofing layer was assembled on the other surface of the active layer, viz., the nonworking surface, thereof. The active layer contained silver-catalyzed active carbon.
The commecially available carbon used "RB carbon" was found to have an ash content of approximately 12 weight percent as received. This "RB" carbon ~ ~7379~
was treated 38 percent KOH for 16 hours at 115C and found to contain 5.6 percent ash content after a subsequent furnace operation. The alkali ` treated RB carbon was then treated (immersed) for 16 hours at room temp-,., erature in 1:1 aqueous hydrochloric acid (20 percent concentration). The resulting ash content had been reduced to 2.8 percent. RB carbon, deashed as above, was silvered in accordance with the following procedure:
Twenty (20 g) grams of deashed RB carbon was soaked in 500 ml of 0.161N (normal) aqueous AgN03 with stirring for two hours; the excess solution was filtered off to obtain a filter cake. The retrieved filtrate was 460 ml of 0.123N AgN03. The filter cake was rapidly stirred into an 85C alkaline aqueous formaldehyde solution to ppt. Ag in the pores of the active carbon.
Calculation indicated that 79 percent of the silver in the catalyst was derived from adsorbed silver nitrate. The resulting ratio of silver to RB carbon was 0.13:1.
; Separately, "Shawinigan Black," (a trademark of Midwest Carbide Corp.) a commercially available acetylene carbon black, was teflonated with "Teflon 30" (du Pont trademark for a polytetrafluoroethylene disper-; sion), 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 minutes. Upon removal and cooling, it was once again high speed chopped, this time for 10 seconds. Then 18 grams of the classified sllvered active carbon was added to the 7.2 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 Brabender Prep Center, ~odel D101, with an attached measuring head REO-6 on 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 30/70 (by volume) mixture of isopropyl alcohol in water as a lubricant to aid in fibrillating. The mixer 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 was high speed chopped in preparation for rolling.
The chopped particulate material was then passed through a rolling mill, a Bolling rubber mill. The resulting matrix active layer sheet had an area density of 22-1/2 milligrams per square centimeter and was ready for lamination.
A polytetrafluoroethylene (PTFE) containing wetproofing (backing) - layer was prepared by the following procedure.

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Two hundred cubic centimeters of isopropyl alcohol were poured into an "Osterizer" (a trademark of the Oster Division of Sunbeam) 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 con-sistency. Then another 100 cc of isopropyl alcohol were added in the blender and the mixture was blended (again at the "blend" position) for an additional 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 determined by a 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 three 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 Buchner funnel and filtered and then placed in an oven at 80C
where it was dried for three 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 Brabender Prep Center (Model DlOl) with attached Sigma (a trademark of Sigma Engineering Co.) Mixer (Model 02-09-000) having a volume cavity of 650 ml with a charge of approximately 140 g of mix for 10 to 209 e.g., 15 minutes, at 100 rpm at ambient room temperature.
AEter fibrillating which compresses and greatly attenuates the PTFE, the fibrillated material is chopped to a fine, dry powder using a coffee blender, i.e., Type Varco, 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 30 material is finely chopped.
The chopped PTFE-Na2C03 mix is fed to 6-inch diameter nickel 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 layer in forming electrodes, 35 e.g., oxygen cathodes, with no further processing beyond cutting, trim-ming to size and the like.
The thus formed layer (after removal of the pore-forming agent subsequent to lamination) is characterized as porous, self-sustaining, coherent9 unsintered, uniaxially oriented backing (wetproofing) layers 40 of fibrillated polytetrafluoroethylene having pore openings of about 0.1 to 40 microns (depending on the size of the pore former used).

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Two ~hree-layer laminates were formed either by roll bonding at roll temperatures above 90C, e.g., 90 ~o 200C, or by hydraulically pressing the three assembled layers at 4 to 8.5 tons/in2 pressure and 90 to 200C for sufficient time to effect consolidation thereof. The asym-metric current distributor was positioned on the one (active layer) sidewhile the backing layer was placed on the other side of the active layer.
These laminates were then hot soaked in ethylene glycol at 75C
for 20 minutes before water washing at 65C for 15 hours and then dried.
The purpose of this ethylene glycol hot soak is to reduce or eliminate blistering during water washing.
These laminated three-layer cathodes were tested in half cells against counter electrodes using 38 percent aqueous NaOH at 300 milliamps per cm current density to simulate the corrosive catholyte environment of a chlor-alkali cell. These cells were fed CO2-free air at 3 to 5 times the theoretical rate of oxygen needed to operate an oxygen cathode in a chlor-alkali cell. The result was that the asymmetric woven wire mesh current distributors of this invention distributed the current in an efficient manner yet saved on material and weaving.
As will be noted from the testing conducted herein, the asym-metric woven wire mesh current distributors of this invention performedwell when incorporated with an active layer and backing layer wherein said asymmetric woven wire mesh current collector was laminated to the "working" active layer side.
It is also within the purview of this invention to laminate the asymmetric current distributors of this invention on the air side, viz., the side containing the PTFE hydrophobic wetproofing (backing) material, when conductive material, e.g., highly porous carbon black particles, are incorporated with the PTFE in a porous electrically conductive backing layer.

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BRIEF SUMUiARY OF THE INVENTION

There has been disclosed an asymmetric woven wire mesh current distributor, especially for an oxygen (air) or other gas electrode having more conductive wires in the direction generally perpendicular to the major current feed to said distributor than in the direction generally parallel to said direction of major current feed. These generally perpendicular wires span the narrow (shorter) conductive path of said electrode.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved gas electrode having an active surface layer of activated carbon particles bound together by a carbon black-fibrillated polytetrafluoroethylene mixture with a current conductor in contact therewith, said current conductor comprising an asymmetric woven wire mesh having more conductive wire strands in the direction generally perpendicular to the major current feed to said conductor than in the direction generally parallel to said direction of major current feed.

2. An improved gas electrode as stated in claim 1 wherein said generally perpendicular wires span the shorter conductive path to said electrode.

3. An improved gas electrode as stated in claim 1 wherein the numerical ratio of generally perpendicular wires to generally horizontal wires range from about 1.5 to 3:1.

4. An improved oxygen cathode having an active surface layer of activated carbon particles bound together by a carbon black-fibrillated polytetrafluoroethylene mixture with a current collector in contact therewith, said current collector comprising a asymmetric woven wire mesh current collector having more conductive wire strands in the direction generally perpendicular to the direction of major current feed than in the direction generally parallel thereto, said generally perpendicular wire spanning the shorter conductive path of said cathode.