Patent GB-1443354-A (GB)
Title: Hydrophobic mats for gas diffusion electrodes
IPC: H01M4/86
Filed: 1973-08-28
Abstract: 1443354 Laminates WESTINGHOUSE ELECTRIC CORP 28 Aug 1973 [31 Aug 1972] 40526/73 Heading B5N [Also in Divisions D1 and H1] One layer 14 of a gas diffusion electrode comprises an air permeable hydrophobic mat of pressed, interlocked fibrillated strands and irregular shaped particles of polyethylene having a weight average molecular wt. of 1750,000 to 5,000,000 and a melt flow index of 0 to 0À02. It is formed by mixing 4-35% by wt. of polyethylene co-ordination polymerized in particles of average diameter 100 to 400 microns, 40-80% of a solvent extractable thermoplastic polymer (polycarbonate, polymethylmethacrylate, polystyrene or polyethyleneoxide) and 0-40% of a solvent extractable plasticizer (dioctylphthalate, dicyclohexylphthalate, a dibenzoate or an adipate). The mixture is blended in a twin shell dry blender, milled on a 2 roll mill at 85-225‹ C., an aqueous dispersion containing 0-10% of non-wettable polytetrafluoroethylene polyamide, polyester, polypropylene or polyethylene fibres 0À01-2 inches long and 0-5% p.t.f.e. or p.t.f.e./hexafluoropropane copolymer particles applied and the milling continued to form a rubbery sheet. The sheet is cut, rotated through 90 degrees and fed back through the nip. The sheet is ground optionally extruded through a ribbon die and reground and the ground material hot pressed at 125-180‹ C. and 300-2000 p.s.i. and cooled to form a 0À02-0À06 inch thick sheet. The sheet is placed between bronze screens and immersed in an organic solvent to dissolve out the extractable thermoplastic polymer and the plasticizer. The product 14 has a porosity (void space) of 20-50% and 15-85% of the polyethylene particles have been sheared into fibrils. The air permeable hydrophobic layer is laminated to a catalytic layer 13 by calendering them together at 90-200‹ C. and 170-375 lb./ linear inch pressure. Catalytic layer 13 comprises a porous electrical conductor support such as a wire mesh or expanded metal member, a perforated metal sheet or preferably a metal "wool" mat in which the metal fibres are nickel or nickel coated iron or steel and are diffusion bonded at points of contact. A mixture of carbon, graphite and/or boron carbide particles, a polysulphone, polyethylene, polypropylene fluorocarbon or chlorofluorinated resin and a catalyst of platinum, iridium, ruthenium, rhodium, palladium, gold mercury or silver. is applied to the porous support as a paste. The resin comprises 20-50% by weight of layer 15. The catalyst is present in the amount of 0À1-10 mg. 1 sq. cm. electrode area. The electrode may include an optional support layer 11 composed of carbon, graphite and/or boron carbide particles and 5 to 50% by wt. of resin. The electrode is used as an air electrode 10 in a metal air battery. An air chamber 25 and a NaOH or KOH electrolyte chamber 26 contact electrode 10. Electrode 28, is composed of an oxidizable metal (iron, cadmium, zinc) is encased in open topped envelope 30 of cellophane sandwiched between fibrous polypropylene sheets. Charging electrode 29 is of nickel or other inert metal. The electrode may be tested against an inert counterelectrode 40 (Fig. 4, not shown) in a driven circuit.