Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Patent
1991-09-13
1993-11-09
Skapars, Anthony
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
429 27, 429 50, 429 72, H01M 804
Patent
active
052601447
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to metal/air batteries and methods for their operation, and particularly such batteries having recirculating electrolyte containing a particulate seeding agent.
Metal/air batteries produce electricity by the electro-chemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having opposite surfaces respectively exposed to the atmosphere and to the aqueous electrolyte of the cell. During cell operation oxygen is reduced within the cathode while metal of the anode is oxidized, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuitry can be connected. Present-day commercial air cathodes are commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) in association with a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element A variety of anode metals have been used or proposed; among them, zinc, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight, and ability to function as anodes in metal/air battery using a variety of electrolytes.
A typical aluminum/air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface.
Aqueous electrolytes for metal-air batteries consist of two basic types, namely a neutral-pH electrolyte and a highly alkaline electrolyte. The neutral-pH electrolyte usually contains halide salts and, because of its relatively low electrical conductivity and the virtual insolubility of aluminum therein, is used for relatively low power applications. The highly alkaline electrolyte usually consists of NaOH or KOH solution, and yields a higher cell voltage than the neutral electrolyte.
In neutral-pH electrolyte, the cell discharge reaction may be written:
In alkaline electrolyte, the cell discharge reaction may be written: solution), saturation level, by:
In addition to the above oxygen-reducing reactions, there is also an undesirable, non-beneficial reaction of aluminum in both types of electrolyte to form hydrogen, as follows:
There is a need for a metal-air battery which can be used as an emergency power source at locations where electric supply lines do not exist. Such a battery must have a high energy capacity and a high power density and be capable of running for a long period of time under high load, e.g. deliver 500 watts with an energy density in excess of 365 Wh/kg. During discharge of a battery containing aluminum anodes and caustic electrolyte, the concentration of dissolved aluminum in the electrolyte continues to build up until a limiting level of super-saturation is reached such that no more aluminum from the anode can enter into solution. At this point a film or scale of aluminum hydroxide forms on the anode surface causing passivation of the anode and collapse of the battery voltage.
The solubility of aluminum hydroxide increases with temperature and with caustic concentration. In metal-air batteries, caustic concentrations are chosen to maximize electrical conductivity and are typically in the range of 4-5 molar. At this caustic level the aluminum solubility at the prevailing battery temperature of 55.degree.-75.degree. C., corresponds to a molar ratio of dissolved Al to KOH or NaOH of roughly 0.40. Aluminum may continue to dissolve above this ratio into the supersaturated zone and even attain a ratio as high as 0.80. In the supersaturation zone the solution is in a metastable state an
REFERENCES:
patent: 4735872 (1988-04-01), Maimoni
patent: 4908281 (1990-03-01), O'Callaghan
patent: 4942100 (1990-07-01), Hunter et al.
Maimoni, Aluminum-Air Power Cell--A Progress Report, Aug. 1985, Society of Automotive Engineers, Inc. (Warrendale, Pa.).
Chemical Abstracts, vol. 92, No. 4, Jan. 1980, Drazic et al, Neutral Electrolyte Aluminum-Air Battery.
Alcan International Limited
Skapars Anthony
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