DE1805019A1 - Oxygen electrode for fuel cells and process for their manufacture - Google Patents

Description

5716

Pioneer Research., Inc., Manchester, Connecticut, USA

Oxygen electrode for fuel cells and process for their manufacture

· The invention "refers to an active material for m oxygen electrodes and in particular to oxygen electrodes for fuel cells and a method for their preparation..

There have already been proposed fuel cells with porous cathodes, on the one hand with the Electrolytes and on the other hand with oxygen (or air) are in contact, so that oxygen (or Air) is transported from the cathode through the electrolyte to the anode, where it is reacted with the fuel ™

will. In general, the oxygen (the air) is forced through the porous wall of the cathode with excess pressure, whereby care must be taken that the overpressure is not so great that the oxygen (air) reaches the electrolyte side of the The cathode bubbles before it is chemisorbed or absorbed on the cathode surface. As long as the oxygen supply is maintained at the cathode surface and sufficient fuel is available at the anode, takes place in a chemical conversion takes place in the cell, by means of which a current of electrons is generated. The porous cathode can be made of carbon,

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sintered Raney nickel, silver or other cathode material exist.

The previously proposed porous cathodes still have certain disadvantages. In particular, the metal cathodes are relatively expensive and must be handled very carefully if they are to have the desired porosity. she are generally made by sintering together metal particles and therefore have a relatively low mechanical strength. In addition, these cathodes are often poisoned by the nitrogen in the air, whereby their efficiency is adversely affected. If potassium hydroxide is used as the electrolyte, then there is also the possibility that the carbon dioxide in the air through the cathode into the electrolyte diffuses, which increases the carbonate content, which is undesirable. It is because of these disadvantages that one is there switched to using pure oxygen instead of air. However, this reduces the unit cost of the Electric current generated increased so much that such cells are unusable for most commercial purposes. Finally, the aforementioned air cathodes usually have maintenance difficulties.

The object of the invention is therefore to propose active electrode materials or electrodes for fuel cells which require little maintenance. In particular, devices for transferring the atmospheric oxygen into a

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Solution, e.g. air cathodes, can be created that can be produced easily and cheaply and during their production not so complicated stages of manufacture as in manufacture of sintered cathodes are necessary. The cathodes should also be of any shape and size in fuel cells, e.g. rechargeable secondary batteries, which be operated with metal, gaseous or hydrocarbon fuels. Finally supposed to a method for producing such electrodes can be specified.

According to the invention, an electrically semiconducting mixture is used as the active electrode material for air or oxygen electrodes, which consists of a fluorocarbon resin, for example polytetrafluoroethylene, and a catalytic silver material dispersed therein. An air or oxygen electrode is produced according to the invention characterized in that the catalytically ^ silver material mixed ά with the Pluorkohlenstoffharz, this mixture is then brought into the desired electrode shape or applied as a coating to a substrate and then baked, the formed electrode at elevated temperature.

The invention will now also be made with reference to the "accompanying figures described in detail, with all details or features emerging from the description and the figures can contribute to the solution of the problem within the meaning of the invention

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and were included in the application with the intention of being patented.

The Pig. 1 and 2 show exemplary embodiments according to the invention for air cathodes in section.

The Pig. 3 schematically shows a fuel cell with an air cathode according to the invention and a catalytic one Anode.

The Pig. 4 shows a further exemplary embodiment for an air cathode according to the invention.

The Pig. 5 shows a fuel cell with a consumable metal anode and a cathode according to the invention.

The Pig. 6 shows a fuel cell according to the invention, whose anode consists of a liquid mercury / sodium alloy.

If the device for converting the atmospheric oxygen into a solution is used as an air cathode, no special ones are required Precautions to be taken. For example, it can be porous or practically non-porous without that their property of converting the oxygen from the air into the solution is adversely affected.

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In contrast to the usual air cathodes, the oxygen or the air cannot be diffused through the center of the cathode, so that a gas circulation system cannot necessary is. The cathode, which is at least semiconducting from an electrical point of view, is characterized by the property that the oxygen selectively from the air side of the cathode as ionic hydrogen to the electrolyte side the cell is transported.

According to the invention, an active material for electrodes of fuel cells contains an electrically semiconducting material

Mixture of a fluorocarbon resin and a catalytic Silver material uniformly dispersed in the resin. Used as a "silver catalytic material" here a type of silver denotes which is indifferent whether it is metallic silver or a simple silver Silver oxide acts, to the oxygen catalytically "behaves.

A decomposable, oxygen-containing silver compound is preferably used to produce the active material such as silver carbonate, which is uniformly coated with a fluorocarbon resin, for example an aqueous dispersion of Polytetrafluoroethylene resin with a resin content of about 60 percent by weight, is mixed. The mixture is then brewed into the desired shape, dried and

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baked at such a high temperature that the silver carbonate is converted into a catalytic silver material and the resin flows and a silver compound around ~ giving membrane forms ·. After cooling, the result is a coherent structure that resists oxygen behaves actively and catalyzes.

W When producing means for forming the desired geometry of a deformable mixture into a flat shape pressed or = transfer of oxygen such as air cathodes for fuel cells, can first a slurry is prepared, which is then, or just such a usually porous support Substrate is applied. The coated substrate is then dried and then baked out. Fluorocarbon resins are particularly useful because they are extremely hydrophobic and

fe, if necessary, can have a very fine porosity, without letting the electrolyte seep through. Examples of fluorocarbon resins in addition to polytetrafluoroethylene are Hexafluoropropylene, chlorotrifluoroethylene and copolymers of tetrafluoroethylene and chlorotrifluoroethylene suitable.

The heat treatment, i.e. baking out, is important in order to consolidate the product, the catalyst particles from one another to isolate and further densify the structure. In contrast to the well-known air cathodes, the

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inventive cathode not to be porous, i.e. no special controls are necessary during manufacture, if one of the maintenance of the bake-out temperature

rature in a preselected area. "|

A cathode according to the invention can be manufactured in the following way:

An active material is produced that is about 62.5 percent by weight Silver carbonate and about 37 »5 percent by weight of a dispersion of polytetrafluorethylene resin,

calculated on the dry weight. To one of the-

To obtain like composition, a part of silver carbonate are mixed with a part of a liquid resin dispersion which contains about 60 percent by weight of polytetrafluoroethylene. ' The silver carbonate corresponds to a silver content of about 49 percent by weight in the mixture, calculated on the dry weight. %

The ingredients are uniformly mixed with one another and then applied to a substrate, for example a finely woven glass fabric impregnated with polytetrafluoroethylene. The coated glass fabric is then baked for about ten minutes ^{at a temperature of 375 ° C. + 5 #.} The machine size of the glass fabric is not essential for the puncture of the electrode.

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A tight resin impregnated fabric is preferred, however. The cathode and the substrate underneath are hydrophobic, so that no further leak-proof pad is required will. If necessary, however, a porous base made of polytetrafluoroethylene (Teflon) can also be provided without adversely affecting the output power of the electrode. When using such a cathode in Connection to a zinc anode in a cell with a 25% aqueous potassium hydroxide solution as the electrolyte in steady-state operation with an open circuit, 1,400 millivolts are obtained and with a load of 650 milliamperes a work potential of about 1,000 millivolts, what a

• Output power of approximately 14 milliwatts per cm (13 watts per square foot) corresponds to the electrode furnace area at room temperature. The air cathode or electrode can be operated at room temperature and atmospheric pressure without the need for compressors or pressure regulators. If necessary, however, a pump (plumbing) can be provided. In addition, it was found that the output power per unit of cathode surface by increasing the temperature from 20 to 80 ⁰ O may be reduced to 200 # and more increased. In a cell with a zinc anode and potassium hydroxide electrolyte, the output current increases with an electrode surface area of 100 cm ² from 600 milliamps at 20 ^° C. to 1,800 milliamps at 80 ^° C.

Although the mechanism for oxygen transfer cannot yet be fully explained, it is believed that,

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when one side of the electrode is exposed to air, the Oxygen is selectively removed from the> air and converted into ionic Form is brought into the electrolyte without nitrogen and carbon dioxide entering the cell at the same time. Since no pump circuit is necessary for effective operation of the cathode, although you do so if necessary Oxygen or air can be supplied, the cathode is completely maintenance-free and does not have the janigen Disadvantages inherent in the rather complex, porous carbon or metal electrodes. There is no pump is necessary to bring the air to the cathode surface, the output power of the fuel cell is also not partially consumed for the pump circuit. A fuel cell with the air cathode according to the invention can When connected to a radio, they can be operated completely maintenance-free for more than 2,000 hours, with only 500 g of hydrazine hydrate be consumed as fuel.

The output power of a fuel cell with an air cathode according to the invention can be further improved by changing the surface properties of the cathode by making a number of crater-shaped recesses on the electrolyte side. This can be done by mixing the active material with a polyalcohol, such as glycerine, before baking. At the beginning of the heating, a reaction then takes place in which gases are released so that crater-shaped recesses return.

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stay. As a starting material is suitable is a composition comprising from about 38.5 $ silver carbonate, about 23 i * contains polytetrafluoroethylene and about 38.5 ° 1 glycerol. The amount of polyalcohol or glycerine that can be added is between about 1 and percent by weight. Other suitable polyalcohols are, for example, ethylene glycol, diethylene glycol, propylene glycol and pentamethylene glycol.

An exemplary embodiment of an air cathode according to the invention is shown in FIG. 1. The air cathode 10 includes a Base 11 made of a finely woven glass fabric with Teflon impregnation (Polytetrafluoroethylene), one surface of which is coated with an active material 12, which from the catalytic silver material and polytetrafluoroethylene consists. The glass fabric provides the necessary stiffening and enables the production of a thin, lightweight electrode. A metal lead 13 is crimped onto the top of the electrode, by means of which the electrode is connected to an electrical circuit.

The electrode according to FIG. 2 is similar to the electrode according to FIG. 1, but the base or substrate exists from sintered together tetrafluoroethylene particles, so that

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This results in a rigid, leak-proof, porous substrate on one surface of which a coating 15 is applied which consists of the active material and which has subsequently been baked at about 375 ± 5 % in order to encapsulate the catalytic silver in the coating. As in FIG. 1, the electrode is provided with a metal contact 16 so that it can be connected to a circuit.

The Pig. 3 schematically shows a fuel cell with an air cathode according to the invention. It contains a vessel 17 that is filled with an electrolyte 17a, which consists for example of a 25 $ strength potassium hydroxide solution and has a pH of more than 7. An anode 18 and an air cathode 19, which have feed lines 21, are immersed in the electrolyte. Although only a single cell is shown, multiple cells can be used and electrically interconnected so that one receives the performance required for the individual case.

The vessel has an opening 22 in one wall, into which the air cathode 19 is inserted in a sealed manner. The air cathode has essentially the same appearance as the Pig cathode. 2, but its air side is almost leakproof, although the polytetrafluoroethylene sintered substrate may be porous. The pore size and the strength of the hydrophobicity of the substrafss are chosen in such a way that the electrolyte is prevented from flowing out. The side 23 of the cathode is exposed to the air. -11-

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The anode 18 can depending on the fuel used

an extensive sheet of nickel 25 or other conductive substrate with a porous, solid, continuous layer 26 of Raney nickel or cobalt. A Raney Kobaite anode (RaCo) is particularly suitable for the dissociation of fuels such as hydrazine hydrate (N _{2 H,).} A method for applying Raney metals to substrates has already been proposed elsewhere (application P 39 169, filed on April 7, 1966).

If a very stiff electrode is desired, a metal mesh or cloth can be embedded in the substrate. A An exemplary embodiment of an electrode stiffened with a metal is given in Pig. 4 shown. It contains a substrate made of sintered polytetrafluorethylene resin, in which a Nickel mesh 28 is embedded and on one surface of which a layer 29 of active material is applied. That Nickel mesh is connected to a metal contact 30.

In a fuel cell according to FIG. 3, which has a Raney nickel anode and containing potassium hydroxide as the electrolyte, various combustible hydrocarbons can be used as the fuel will. For example, ethyl alcohol, ■ methyl alcohol, gasoline (gasoline) or ethylene are suitable. Though some These hydrocarbons are insoluble in the electrolyte, they can be dispersed in the electrolyte and form an emulsion form. If, for example, gasoline (gasoline) is used as fuel

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is used, a highly dispersed emulsion can be produced thereby that this is in a commercial detergent such as a sodium lauryl sulphate based detergent is dispersed. One such emulsion is made up of the potassium hydroxide electrolyte easily picked up. When using methyl alcohol as fuel, this can be used in a proportion from .about 1 to 10 percent by weight are dissolved in the electrolyte.

The chemical behavior of hydrocarbon fuels in a typical fuel cell is, for example, published in Chapter 3 of the book "Fuel Cells" by George J. Young ( by Reinhold Publishing Corporation, '1960ei). The power generated in such cells results from the Consumption of the hydrocarbon fuel at the anode, i.e. from the conversion with ionic oxygen, which on the electrolyte side the air cathode is brought into the solution. However, the invention is not limited to this type of reaction mechanism limited.

The air cathode according to the invention can, for example, also be used in cells in which the fuel source a consumable metal anode such as a sodium amalgam anode is. The amalgam can be in the liquid or solid phase, depending on the composition used.

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Zinc and cadmium are also suitable for making consumable metal anodes. In the pig. 5 shows a fuel cell ^ which contains an air cathode 32 according to the invention and a consumable metal anode 31 made of, for example, zinc. Like in the example after Pig. 3, the air side of the cathode 32 consists of sintered Teflon particles (polytetrafluoroethylene), so that the atmospheric oxygen penetrates through this substrate into the cathode.

fe An advantage of fuel cells with consumable metal anodes is that they can be used as secondary elements, i.e. such elements can be used as conventional ones Secondary batteries are recharged. Have attempts show that secondary elements with the inventive air cathodes Bring about six times the power that would normally be taken from a conventional lead-acid battery when the values to be compared are related to weight. A secondary element according to the invention provides about 132 watt hours per kilogram (60 watt hours per US pound) of the element compared to about 22 watt hours per pound Kilograms (1Ό watt-hours per US pound) of a lead-acid battery.

A single fuel cell with an air cathode according to the invention and an organic fuel provides about 15 mA

per cm of the electrode surface at about 0.5 V. In contrast to the known fuel cells, the inventive Fuel cell has a higher efficiency and is almost maintenance-free. In cases where there is a very high instantaneous output is required, a primary element can be used together with a se-

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secondary element according to Pig. 5 can be used. With normal load the primary element with its organic fuel can then be used to recharge the secondary element which can then be used as the primary element "at peak loads.

The Pig. 6 shows a further exemplary embodiment for a fuel cell with a consumable anode. In a vessel 33 there is a lake 34 made of mercury-sodium-amal- % gam (H-1 $ Na), which is covered with an aqueous solution 35 which contains some sodium hydroxide. The Amalgaaianode 34 is in contact with a supply line 36, while the aqueous solution 35 is in contact with a floating air cathode 37, the bottom 38 of which has approximately the appearance of the cathode according to Pig. 2 and includes a substrate 39 made of sintered Teflon and an attached layer 40 of active material which is in contact with the solution. A lead 41 is connected to the cathode. When this cell is operated, the oxygen converted into the λ solution is reacted with the sodium in the mercury, producing sodium oxide, which dissolves in the aqueous solution with the formation of sodium hydroxide. A current of electrons flows as the pole of this chemical reaction.

The proportion of the catalytic silver material in the active Mass can correspond to a silver content of about 8 to 70 percent by weight, while the remainder is 92 to 30 percent by weight of fluorine

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carbon resin is used. The corresponding silver content is preferably between 20 and 60 percent by weight, which corresponds to a resin content of about 80 to 40 percent by weight.

The electrolytes that are used in connection with the electrodes according to the invention have a pH value of at least seven. Alkali metal hydroxide electrolytes are preferably used, in particular electrolytes from Potassium, sodium or lithium hydroxide. The concentration of such electrolytes can be between about 5 and 50 percent by weight lie.

In addition to the specified materials, extenders such as carbon, manganese dioxide, cadmium oxide and the like can be added. When these or similar extenders are used, the silver catalyst content is as indicated above based on the content of the resins used.

Although the oxygen electrodes according to the invention in connection are described with fuel cells, they can also be used as devices for converting the oxygen in the air into a solution can be used wherever chemical reactions with ionic oxygen play a role. For example, can they are used in chemical sales in organic chemistry, in which hydrocarbons are used to produce oxygenated compounds are partially oxidized.

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Claims (16)

Claims 1. Method of manufacturing an oxygen electrode for Conversion of atmospheric oxygen into a solution, for example in the electrolyte of a fuel cell, thereby characterized in that a catalytic silver material in (or with) an e-fluorocarbon resin containing Substrate is dispersed (or mixed) in such an amount that it is embedded in the substrate and a electrically semiconducting mass is created. 2. The method according to claim 1, characterized in that ' draws that the dispersion or mixture is brought into the desired shape and then baked at a temperature in which the silver material, for example silver carbonate, is converted into a catalytic silver material and the resin fuses around the silver material. 3. The method according to claim 1 or 2, characterized in that the fluorocarbon resin a polytetrafluoroethylene resin is used. 4. The method according to claim 2 or 3, characterized in that the silver material in one Proportion is used which corresponds to a silver content in the mixture of about 8 to 70 $, while the remainder is essentially a polytetrafluoroethylene resin is used. -17- 90982 4/1096 5. The method according to claim 4, characterized in that the silver content in the mixture is approximately Is 20 to 60 percent by weight. 6. The method according to any one of claims 2 to 5, characterized in that it is baked out at a temperature of 375 ⁰ C + 5 $. 7. The method according to any one of claims 2 to 6, characterized in that up to about 30 minutes is baked out. 8. The method according to any one of claims 1 to 7, characterized characterized in that the mixture is placed on a substrate made of sintered polytetrafluoroethylene prior to baking is applied. 9. The method according to any one of claims 1 to 8, characterized in that the mixture before baking a preselected amount of a polyalcohol is added, through which a large number of crater-shaped recesses during heating is formed in the electrode. 10. The method according to claim 9, characterized that glycerol is used as the polyalcohol. -18- 90982 4/1096 11. The method according to any one of claims 1 to 10, characterized in that in the Substrate for stiffening a metal mesh is embedded. 12. Fuel cell with an air cathode made according to the method is produced according to one or more of claims 1 to 11, characterized in that that the electrolyte has a pH of at least 7. 13. The fuel cell according to claim 12, characterized% in that the electrolyte is an alkali metal hydroxide. 14. Fuel cell according to one of claims 12 or 13, characterized in that the anode consists of mercury / sodium amalgam, zinc or cadmium and used as a fuel source. 15. Fuel cell according to claim 12, characterized characterized in that the anode is made of Raney nickel exists and a combustible hydrocarbon is used as fuel. 16. Fuel cell according to claim 12, characterized in that the anode made of Raney cobalt. and hydrazine hydrate is used as fuel. -19- 90982A / 1096 Blank page