i. General Nature of the Invention
This invention relates to an oxygen electrode (i.e. a cathode) with performance characteristics which makes it suitable for coupling with either a fuel electrode (to form a fuel cell) or a metal anode (to form a metal-air battery).
II. Description of the Prior Art
Many metals and metal oxides may be used as catalysts in oxygen electrodes. To be effective, however, they must possess certain properties, namely, high surface area and high electrical conductivity. As most of the best catalysts, for example, platinum, palladium and silver, are also very expensive, numerous methods have been devised for depositing the catalyst on a suitable conducting substrate, such as carbon or nickel. The method used to impregnate the substrate is critical because it affects the distribution and the surface properties of the catalyst as well as the bonding of the catalyst to the substrate.
The limitations of existing oxygen electrodes are primarily of an economic nature. High costs are due not only to the use of precious metals but also to complicated production procedures.
U.S. Pat. No. 3,328,204 issued June 27, 1967 to W. T. Grubb describes a fuel cell capable of oxidizing fluid, saturated hydrocarbon fuels to carbon dioxide with the production of electrical energy at current densities up to and including the maximum power capability of the fuel cell under the cell operating conditions. The fuel cell described consisted of a pair of gas adsorbing, gas-permeable, electronically conductive electrode elements in direct contact with an aqueous phosphoric acid solution. The fuel cell was operated so that the electrolyte was at a temperature of at least about 130.degree.C. but no greater than about 250.degree.C. and also was in the range of from about 100.degree.C. below the boiling point up to about the boiling point of the particular concentration of electrolyte used at the ambient pressure above the electrolyte.
The patentee taught that, although a number of different types of electrode structures were suitable for use in such cells, the cathode electrode should be one which: is electronically conductive; will adsorb the fuel or oxidant employed; will act as a catalyst of the electrode reaction; and will not itself be oxidized or corroded severely under the operating conditions of the cell. Especially suitable materials disclosed included the noble metals, for example, gold and the noble metals of the Group VIII series of metals of the Periodic Table of Elements, namely, rhodium, ruthenium, palladium, osmium, iridium and platinum. Because of their ready availability and suitability, platinum and palladium were preferred. The patentee also taught that other metals included metals of the Group VIII series of metals, namely, nickel, iron, cobalt, etc., as well as other metals known catalytically to adsorb gases, for example, silver, copper, and metals of the transition series, for example, manganese, vanadium, rhenium, etc., were operative. However, such metals under normal cell operating conditions were severely attacked by the phosphoric acid. Therefore, in order to be used for long term cell operation, they would have to be protected, for example, electrolytically plated or alloyed with a non-corrodable metal, such as platinum, palladium, etc., prior to use. In addition, it was taught that the electrodes may be formed of carbon which has been activated with the noble metals of the Group VIII series, such as platinum and palladium. The patentee further taught that for maximum cell performance the electrodes should be made by using the very active noble metal powders of the Group VIII metals, for example, platinum black, palladium black, etc.
Many ways were disclosed by the patentee for constructing the catalytically active electrodes. For example, they could be of the skeletal type obtained by forming an alloy of two metals and dissolving one of the metals leaving the other metal in a porous sheet of sufficient rigidity to use as the electrode. The metal powders may be compacted and sintered to produce the suitable electrodes having a porous nature, and if desired, can be of a multi-porous nature whereby the pores in contact with the electrolyte are smaller than the pores in contact with the fuel or oxidant gas. They could also be made by mixing metal powders with an inert binder, for example, polytetrafluoroethylene. A disclosed very desirable electrode structure could be made by incorporating metal powders in polytetrafluoroethylene which has an additional film of polytetrafluoroethylene without metal particles on the electrode side in contact with the fluid fuel or oxidant gas.
Thus the patentee provided electrodes either made using expensive catalyst materials or catalyst materials which are admitted to be severely attacked during the use of the fuel cell.
U.S. Pat. No. 3,401,062 issued Sept. 10, 1968 to E. H. Lyons, Jr. provided a photoregenerative cell incorporating a photoreducible anode, and an oxygen cathode. The patentee taught that an oxygen cathode was in intimate contact with the electrolyte. The function of the cathode was to adsorb oxygen, ionize it and transfer it to the electrolyte. Any material suitable for carrying out these functions will suffice. Lithium-doped transition metal oxides, porous or sintered platinum, silver powder, nickel oxide doped with lithium, palladium and carbon impregnated with catalysts were disclosed as being just a few of the suitable materials for the cathode. The cathode could assume various physical forms; however, it must ordinarily be porous so that the necessary adsorption and ionization can take place.
The cathode could be prepared by sinter-casting techniques. This applies equally well to silver, nickel and lithiated nickel cathodes. The doped nickel could be prepared by either of two methods. On the one hand, an intimate mixture of Li.sub.2 0.sub.2 and NiO could be pressed into a pellet, sealed in a vessel and heated to about 900.degree.C. On the other hand, nickel could be reacted with a thermally decomposable salt of lithium, such as LiOH or LiNO.sub.3 by: (a) impregnating a sintered nickel electrode with a solution of the salt and firing to about 800.degree.C., or (b) impregnating nickel particles, firing, pressing and sintering the lithiated particles into a porous electrode body. Reinforcement of the lithiated nickel may be advisable.
In this patent, too, the disclosure of the suitable oxygen electrode involved the use of expensive or not readily available materials.
U.S. Pat. No. 3,432,355 issued Mar. 11, 1969 to L. N. Niedrach and H. R. Alford provided gas permeable, hydrophobic fuel cell electrodes. The electrodes comprise gas adsorbing metal particles bonded together into a cohesive mass with polytetrafluoroethylene and have a coating of polytetrafluoroethylene bonded to the electrode surface in contact with the gas phase.
The patentees taught that each electrode should be one which: is electronically conductive; will adsorb the fuel or oxidant employed; will act as a catalyst for the electrode reaction; and will not itself oxidize severely under the operating conditions of the cell. Suitable gas adsorbing metals are well known.
Suitable materials disclosed included the noble metals of Group VIII series of metals of the Periodic Table of Elements, which are rhodium, ruthenium, palladium, osmium, iridium, and platinum. Other suitable metals included the other metals of Group VIII, such as nickel, iron, cobalt, etc., as well as other metals known catalytically to adsorb gases, such as silver, copper, and metals of the transition series, such as manganese, vanadium, rhenium, etc. In addition to electrodes formed of these metals the electrodes can be formed of platinum or palladium black which has been deposited on a base metal such as stainless steel, iron, nickel and the like. In addition, suitable electrodes may be formed from metal oxides and carbon which have been activated with platinum or palladium, or from carbon which has been activated with oxides of iron, magnesium, cobalt, copper, etc.
For maximum cell performance, the patentees preferred to make the electrodes by using the very active metal powders of the Group VIII metals, for example, platinum black, palladium black, Raney nickel, and so forth. The noble metals of the Group VIII series of metals have the further advantage in that when the electrolyte is an acid, corrosion conditions exist at both the anode and cathode which shorten the life of the cells having electrodes incorporating metals such as nickel, iron, copper, etc. This effect was stated not to occur in cells having electrodes made from the noble metals of the Group VIII metals. The corrosive effect is not as pronounced in fuel cells using bases as the electrolyte. Long cell life may be obtained by using any metals which are resistant to bases, for example, the Group VIII metals, including nickel, cobalt, etc., as well as other known gas adsorbing metals, such as rhenium, in cells having an aqueous base electrolyte.
Many ways were disclosed for constructing the catalytically active electrodes. One means which could be used easily to construct these electrodes was to take an aqueous emulsion of polytetrafluoroethylene resin and form a thin film on a casting surface such as a sheet of metal foil, metal plate, etc., forming the final shape of the electrode, if desired, evaporating the water and wetting agent from the emulsion, followed by sintering of the polytetrafluoroethylene, under pressure if desired, at a temperature high enough to cause the sintering of the individual particles of polytetrafluoroethylene into a coherent mass, such as from about 325.degree. to about 450.degree.C., preferably from about 350.degree. to about 400.degree.C. The time of heating would be sufficient to insure that all particles of resin reach the desired temperature, usually about 1 to about 2 minutes. Thereafter, an aqueous emulsion of polytetrafluoroethylene resin would be mixed with sufficient metal particles that the final layer prepared from this mixture would be electronically conductive, for example, from about 2 to about 20 grams of the metal powder per gram of polytetrafluoroethylene resin in the emulsion. This mixture would be spread in a thin layer on the previously formed film of polytetrafluoroethylene resin followed by evaporation of the water and wetting agents from the emulsion and sintering of the polytetrafluoroethylene in the mix, preferably under pressure, for example, about 1000 to about 3000 p.s.i. at a temperature of about 350.degree. to about 400.degree.C. for about 2 to about 10 minutes. Thereafter, the electrode would be removed from the casting surface and would be cut to the desired shape if not so formed by the casting operation.
The patentees further disclosed that if a current collecting grid was to be incorporated into the electrode structure, such a current collecting grid, for example, metal wires, metal strip, metal wire mesh, sintered porous sheet, punched or expanded metal plates, porous metal sheet, etc., could be incorporated into the aqueous polytetrafluoroethylene metal mix before evaporation of the water. Alternatively, a sandwich-type of electrode could be made wherein a casting surface is first coated with polytetrafluoroethylene, followed by a coating of the polytetrafluoroethylene-metal mix which is dried but need not be sintered. The polytetrafluoroethylene-metal mix also could be used to cast a thin layer on a separate casting surface without first forming the polytetrafluoroethylene film. This would be dried but need not be sintered and a sandwich would then be made with the current collecting grid between the two layers still on the casting surfaces. This sandwich would be pressed and sintered, followed by removal of the casting surfaces to give an electrode in which the current collecting grid formed an integral part of the electrode.
The patentees further taught that filler such as fibrous cloth or mat, preferably made of fibers that are resistant to highly acidic or basic conditions which they will encounter in the fuel cell, for example, glass, asbestos, acrylonitrile, vinylidene chloride, polytetrafluoroethylene, etc., may be impregnated and surface coated with a mixture of polytetrafluoroethylene and metal powder. Such a technique was taught to be desirable if the current collecting grid was not incorporated as an integral part of the electrode, but was merely pressed to the surface of the electrode on the electrolyte side where it could make contact with the metal particles. Such a technique tended to decrease the effective surface area of the electrode in contact with the electrolyte and therefore it was preferred to incorporate the current collecting grid into the electrode structure.
The patentees also taught that although other materials such as polytrifluorochloroethylene, polyethylene, polypropylene, polytrifluoroethylene, etc., could conceivably be substituted for the polytetrafluoroethylene, the chemical resistance of these materials was inferior to polytetrafluoroethylene under the conditions encountered in the fuel cells and therefore such substitution could only be made with considerable sacrifice in the desired performance and stability of the electrodes.
Suitable electrolytes for use in the cells have been disclosed in the patents referred to above. These included a solid, a liquid, a liquid adsorbed upon a perforate solid matrix, a jelled-liquid or any other suitable physical form. The chemical constituency of the electrolyte may include, for example: a mixture of alkali carbonates contained in the capillary pores of a ceramic matrix; solid solutions and solidstate reaction products of selected, mixed conductive oxides, for example, ZrO.sub.2 --MgO, ZrO.sub.2 --CaO, HfO.sub.2 --CaO, ZrO.sub.2 --Y.sub.2 O.sub.3, ZrO.sub.2 --La.sub.2 O.sub.3 and similar systems; an aqueous caustic electrolyte solution which has been jelled by adding to it one or more of the following: carboxymethyl cellulose in very weak alkaline solution, guar gum, synthetic resin of various types, calcium stearate or other soaps, or a hydrous oxide, for example, Fe(OH).sub.3, Sc(OH).sub.3, Y(OH).sub.3, La(OH).sub.3 or other lanthanides; or an ion exchange membrane, a water solution of perhaps 5 - 60% NaOH or KOH absorbed on a matrix composed of a major amount of MgO and minor amounts of one or more additives, such as Al.sub.2 O.sub.3, SiO.sub.2, other refractories, CaO and BaO.
The aqueous electrolytes are usually aqueous solutions of strong acids or strong bases, but salt systems having buffering action may be used. Strong acids and strong bases are those having a high degree of ionization. Salt systems having buffering action are well known, for example, sodium dihydrogen phosphate-potassium monohydrogen phosphate, potassium carbonate-potassium bicarbonate, phosphoric acid-sodium dihydrogen phosphate, etc. The concentration of the electrolyte should be as high as can be tolerated by the materials of construction of the cell. Likewise, the electrolyte must be soluble in the aqueous phase and should have a low enough vapor pressure that it does not volatilize into the gaseous phase. Because of these limitations, the most desirable electrolytes are sulfuric acid, phosphoric acid, the aromatic sulfonic acids such as benzene, mono-, di- and trisulfonic acids, toluene mono-, di and trisulfonic acids, the naphthalene sulfonic acids such as the .alpha.- and .beta.-naphthalene monosulfonic acids and the various naphthalene disulfonic acids, etc. In general, acids and bases having a dissociation constant of at least about 1 .times. 10.sup..sup.-4 are satisfactory. Typical of the bases which may be used are sodium hydroxide, potassium hydroxide, lithium hydroxide, cesium hydroxide, rubidium hydroxide, etc. In view of their ready availability, stability under fuel cell operating conditions, low cost and high degree of ionization in aqueous solution, it is preferred to use inorganic acids, for example, sulfuric acid, phosphoric acid, etc., or inorganic bases, for example, sodium hydroxide, potassium hydroxide, etc. For effective operation, a three phase boundary should be set up between the catalyst, oxygen and the electrolyte.