Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1998-09-17
2001-09-18
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
Reexamination Certificate
active
06291090
ABSTRACT:
TECHNICAL FIELD
This invention relates to rechargeable metal-air cells, and more particularly, relates to the composition and manufacture of an air electrode for use in a metal-air cell.
BACKGROUND OF THE INVENTION
Metal-air cells have been recognized as a desirable means by which to power portable electronic equipment such as personal computers because such cells have a relatively high power output with relatively low weight as compared to other types of electrochemical cells. Metal-air cells utilize oxygen from the ambient air as a reactant in the electrochemical process rather than a heavier material such as a metal or metallic composition.
Metal-air cells use one or more air permeable cathodes separated from a metallic anode by an aqueous electrolyte. During the operation of a metal-air cell, such as a zinc-air cell, oxygen from the ambient air is converted at the cathode to hydroxide ions and zinc is oxidized at the anode and reacts with the hydroxide ions, such that water and electrons are released to provide electrical energy.
Recently, metal-air recharging technology has advanced to the point that metal-air cells are rechargeable and are useful for multiple discharge cycles. An electronically rechargeable metal-air cell is recharged by applying voltage between the anode and the cathode of the cell and reversing the electrochemical reaction. Oxygen is discharged back to the atmosphere through the air-permeable cathode and hydrogen is vented out of the cell.
Metal-air cells may be arranged in multiple cell battery packs to provide a sufficient amount of power output for devices such as computers. An example of a metal-air power supply is found in commonly owned U.S. Pat. No. 5,354,625 to Bentz et al., entitled Metal-Air Power Supply and Air Manager System, and Metal-Air Cell for Use Therein, the disclosure of which is incorporated herein by reference.
Attempts to increase even further the power output of metal-air cells have had mixed results. Increasing the power output of a cell usually involves operating the cell at a higher current drain. Such a higher load, however, can significantly decrease the total energy density of the system and greatly increase the production of heat, both of which are detrimental to efficiency and lifetime of the cell.
The composition of the air electrode or cathode is important in determining the power production capabilities and efficiencies of a metal-air cell. Air electrodes typically comprise carbon particles such as carbon black, an oxygen reduction catalyst, and a non-wetting binder such as tetrafluoroethylene. Secondary air electrodes also include an oxygen evolution catalyst. Oxygen reduction catalysts are also referred to as discharge catalysts and oxygen evolution catalysts are also referred to as recharge catalysts.
Suitable discharge catalysts include silver, cobalt oxides or spinels, transition metal macrocyclics, and perovskites. Suitable oxygen evolution catalysts include nickel, cobalt, iron, and tungsten compounds. These catalysts, with the exception of silver, are often added to the air electrode in the form of oxides in powder form. Generally described, an air electrode is formed by mixing carbon black, the catalyst powders and a non-wetting binder, forming this mixture into a sheet and adhering the sheet to a current collector.
Although the above described electrodes are effective in a metal-air cell, there remains a need for increased power output from a metal-air power supply without comprising the efficiency and lifetime of the cell.
SUMMARY OF THE INVENTION
The present invention fulfills the above described need by providing an air electrode made with water soluble catalyst precursors, a method for making such air electrodes, and a metal-air cell including such an air electrode. The water soluble catalyst precursors are applied to carbon particles in the form of an aqueous solution. The water soluble catalyst precursors desirably include one or more water soluble oxygen evolution catalyst precursors and can also include one or more water soluble oxygen reduction catalyst precursors. The resulting mixture of catalyst precursors and carbon particles is dried and then combined with other ingredients to form the air electrode. Metal-air cells containing such an air electrode made with water soluble catalyst precursors unexpectedly provide more power than metal-air cells including an air electrode made with catalyst powders instead of water soluble catalyst precursors.
More particularly, this invention encompasses a method for making a bifunctional air electrode for use in a rechargeable metal-air electrochemical cell. This method comprises wetting carbon particles with at least one water soluble oxygen evolution catalyst precursor in aqueous solution in aqueous solution to form a wetted carbon particle mixture. The mixture can also include one or more water soluble oxygen catalyst precursors. The wetted carbon particle mixture is dried so as to evaporate the water and form a carbon/catalyst mixture. The carbon/catalyst mixture is blended with a binder to form an active layer mixture and a layer of the active layer mixture is laminated onto a wet-proofing layer to form an active/wet-proofing layer. The active/wet-proofing layer is contacted with a current collector.
The air electrode of this invention, as set forth above, is useful in a metal-air cell. Generally described, such a metal-air cell comprises a cell case, an anode disposed in the cell case, the air electrode of this invention disposed in the cell case, and an electrolyte disposed in the cell case in contact with the anode and the air electrode.
The method of making an air electrode in accordance with this invention is simple and efficient because no high temperature treatment of the catalyst precursor is necessary and the entire process can be conducted at atmospheric pressure and in an air atmosphere. Desirably, the step of drying the wetted carbon particle mixture is conducted at a temperature and a pressure sufficient to evaporate the water in the wetted carbon particle mixture and uniformly distribute the at least one water soluble oxygen evolution catalyst precursor and the at least one water soluble oxygen reduction catalyst precursor throughout the carbon/catalyst mixture without alloying the at least one oxygen evolution catalyst or the at least one oxygen reduction catalyst and without substantially deteriorating the carbon particles. More particularly, the step of drying the wetted carbon particle mixture is conducted at a temperature from about 80 to about 150° C., and more desirably is conducted at a temperature from about 100 to about 150° C.
Alternatively, a bifunctional air electrode can be made according to this invention by wetting carbon particles comprising an oxygen reduction catalyst already deposited thereon with at least one water soluble oxygen evolution catalyst precursor in aqueous solution to form a wetted carbon particle mixture.
Accordingly, an object of the present invention is to provide a metal-air cell with enhanced power output.
Another object of the present invention is to provide a metal-air cell with enhanced power output and a lengthy cycle life.
Yet another object of the present invention is to provide a simple and efficient method for making air electrodes for use in a metal air cell.
Other objects, features and advantages of the present invention will become apparent from the following detailed description, drawings, and claims.
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Golovin Milton Neal
Kuznetsov Irena
AER Energy Resources Inc.
Chaney Carol
Sutherland Asbill & Brennan
Tsang Susy
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