Molybdenum Air batteries

I did a quick search on the net under the title molybdenum batteries and I found that the US government has a patent on molybdenum air batteries. See part of the article below.
If the US government have a patent on this type of batteries one can only assume it will take off soon.


Abstract
A metal-air battery has an anode in which the electrochemically active material is molybdenum. The molybdenum may be in the form of a bulk body of material or it may comprise a particulate material dispersed with or in another material. In some instances, the molybdenum may comprise a member of an alloy or mixture. Also disclosed is a modular battery system which may include the molybdenum-based anode material.

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Classifications
H01M12/06 Hybrid cells; Manufacture thereof composed of a half-cell of the fuel-cell type and of a half-cell of the primary-cell type with one metallic and one gaseous electrode
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US8148020B2
US Grant

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Inventor
Rongzhong Jiang
Charles W. Walker, Jr.
Deryn Chu

Current Assignee
US Secretary of Army
Original Assignee
US Secretary of Army

Priority date
2009-04-01
Family: US (1)
DateApp/Pub NumberStatus
2009-04-01US12416309Active
2010-10-07US20100255375A1Application
2012-04-03US8148020B2Grant
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Description
GOVERNMENT INTEREST
The invention described herein may be manufactured, used, and licensed by or for the United States Government.

FIELD OF THE INVENTION
This invention relates generally to electrochemical cells. More specifically, the invention relates to metal-air batteries, and in particular relates to a metal-air battery utilizing molybdenum as an anode material. The invention further relates to particular designs of modular electrochemical cells.

BACKGROUND OF THE INVENTION
Metal-air batteries are electrochemical cells wherein the oxidation of a metal anode by an oxidizing agent, such as air, creates a flow of electrical current. As such, metal-air batteries may also be considered to be a type of a fuel cell wherein the metal of the anode is considered to be the fuel.

A generalized metal-air battery includes an anode which is comprised of, or contains, an electrochemically active metal which is oxidized in the operation of the battery to generate electrical current. The metal-air battery further includes a cathode, and in the operation of the battery, oxygen is reduced at the cathode. Typically, the cathode is fabricated from an electrically conductive, air-permeable material such as a carbon fabric, porous metal or the like. The cathode may include a catalytic material therein to facilitate the reduction of oxygen, and such catalytic materials may include noble metals such as platinum and its compounds and alloys. The metal-air battery will also include an electrolyte which is ionically conductive and which is in contact with the anode and cathode. As is typical in batteries, a porous body of separator material may be disposed between the anode and cathode. As will be described in detail hereinbelow, such separators may comprise porous polymeric membranes as well as porous, fibrous materials such as glass, ceramic or cellulosic materials.

Metal-air cells typically have a high energy density as compared to other types of electrochemical cells. Heretofore, metal-air batteries have been fabricated utilizing relatively light metals such as magnesium, aluminum and zinc as electrochemically active anode materials. However, batteries utilizing magnesium or aluminum-based anodes have been found to exhibit problems of stability due to chemical reactions between the metals and electrode components and electrolytes. Zinc-air batteries have been found to be relatively stable when used in conjunction with alkaline electrolytes, and such batteries are in commercial use. Zinc-air batteries have a high theoretical voltage; however, their ultimate energy density is limited by the fact that the zinc comprising the anode undergoes only two-electron oxidation. The theoretical energy density of zinc-air batteries utilizing an alkaline electrolyte is only 489 Wh/Kg based upon the reaction of Zn+2 KOH. Other limitations upon the utility of zinc-air batteries involve the evolution of hydrogen at the anode causing fuel waste and increasing cell resistance. Additives such as mercury and lead may be utilized to retard hydrogen evolution; however, the poisonous nature of these additives does restrict their use. It has also been found that during storage, the electrolytes of zinc-air batteries tend to dry thereby decreasing battery performance. Storage can also cause carbonate formation in the zinc-air battery which can damage separator membranes and compromise cell performance.

As will be appreciated from this discussion, metal-air batteries have many potential advantages over conventional electrical power sources; however, battery structures and materials available to date have not fully exploited the advantages of metal-air battery systems. As will be explained in detail hereinbelow, the present invention provides a novel metal-air battery structure and chemistry which significantly improves upon metal-air battery performance characteristics. These and other advantages of the invention will be apparent from the drawings, discussion and description which follow.

BRIEF DESCRIPTION OF THE INVENTION
Disclosed herein is a metal-air battery. The battery has an anode which includes an electrochemically active metal which is oxidized when the battery is operative to generate electrical power. The electrochemically active metal comprises molybdenum. The battery further includes a cathode which is operative to reduce oxygen when the battery is generating electrical power. An electrolyte is disposed in contact with the anode and the cathode, and a separator may be disposed between the anode and the cathode. The electrolyte may comprise a base such as potassium or sodium hydroxide, or it may comprise an acid such as phosphoric acid. The molybdenum may be present in the anode as a unitary piece of solid or porous metal, or it may be present in a particulate form, and such particles may be intermixed with another material such as carbon. Further disclosed is a modular metal-air battery system. The modular system includes a cathode assembly which includes a case which houses the cathode and an associated electrical lead. The cathode assembly may further include electrode supports, current-collecting structures and the like. The cathode assembly may include an opening in the housing to permit passage of ambient air to the cathode. The modular system further includes an anode cartridge which is reversibly connectable to the cathode assembly. The anode cartridge houses the anode portion of the metal-air battery system and further includes a body of electrolyte material, which may be retained by a porous sponge or other such bibulous material. The anode cartridge will include current collectors and electrical leads. The anode cartridge may include a protective cover which closes off the active surface of the cartridge when it is not in use. A separator membrane is disposed in at least one of the cathode assembly or anode cartridge so that when the two are coupled together, the membrane will be disposed between the cathode and the anode. The modular system may be implemented with particular advantage in connection with the molybdenum-air battery of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a cathode assembly of a modular cell of the present invention;

FIG. 2 is a cross-sectional view of an anode cartridge which may be used in a modular battery system of the present invention;

FIG. 3 is a cross-sectional view of a modular metal-air battery of the present invention as comprised of the cathode assembly of FIG. 1 and the anode cartridge of FIG. 2;

FIG. 4 is a graph showing discharge voltage-current curves of molybdenum-air batteries of the present invention utilizing alkaline and acidic electrolytes;

FIG. 5 is a discharge power-current curve of the molybdenum-air batteries of FIG. 4;

FIG. 6 is a discharge voltage-current curve of molybdenum-air batteries utilizing 30 and 60 percent KOH electrolytes;

FIG. 7 is a graph showing discharge power-current curves of the molybdenum-air batteries of FIG. 6;

FIG. 8 is a graph showing the constant current discharge performance of a molybdenum-air battery of the present invention at two different discharge rates;

FIG. 9 is a voltage-time curve of a series of molybdenum-air batteries of the present invention utilizing different separator materials; and

FIG. 10 is a graph showing discharge current-time, electron number-time and electron number-capacity curves of a molybdenum-air battery of the present invention under constant voltage discharge.

DETAILED DESCRIPTION OF THE INVENTION
In accord with the present invention, the anode of a metal-air battery system includes an electrochemically active metal which comprises molybdenum. Within the context of this disclosure, an electrochemically active metal is understood to be a metal which, in a metal-air battery system, is oxidized when the battery is operative to generate power.

Molybdenum is a transition metal with an atomic weight of 95.9 and can provide up to 6 electrons upon oxidation. When molybdenum is used as the anode in a molybdenum-air battery having an acidic electrolyte, the electrode reactions are as follows:

At the anode,
Mo+4 H2O═H2MoO4+6 H++6e − E°=0.114V  (1)

At the cathode,
3/2 O2+6 H+6e −=3 H2O E°=+1.23 V  (2)

The overall reaction in the battery is
Mo+3/2 O2+H2O═H2MoO4 Cell voltage=1.12 V  (3)

When a molybdenum-air battery is operated utilizing an alkaline electrolyte electrode reactions are as follows:

At the anode,
Mo+8 KOH═K2MoO4+6 K++6e −+4 H2O E°=−0.913 V  (4)

At the cathode,
3/2 O2+3 H2O+6e −=6 OH− E°=+0.401 V  (5)

Overall reaction is
Mo+2 KOH+3/2 O2═K2MoO4+H2O Cell voltage=1.314 V  (6)

The theoretical energy density of a battery system of this type is 1581 Wh/Kg when utilizing an acidic electrolyte (calculated based upon the reactants of Mo+H2O) and 1012 Wh/Kg when utilizing a KOH electrolyte (calculated based upon the reactants of Mo+2 KOH). High power is achieved in such battery systems since the electrolytes are aqueous-based solutions having high ionic conductivities, as compared to organic electrolytes in other battery systems. Following the 6-electron oxidation of molybdenum in the presence of water, the final product at the anode is molybdic acid in an acidic electrolyte system, or a salt such as potassium molybdate in an alkaline electrolyte system. Thus, the battery system retains good ionic conductivity throughout its operational life thereby maintaining low cell resistance. Furthermore, spent electrolyte material may be readily recycled by a simple neutralization treatment. The high energy density of the molybdenum-air battery is particularly advantageous for high energy, portable power sources.

Various configurations of cells may be implemented in accord with the present invention. In general, a molybdenum-air cell of the present invention will include a cathode which is at least partially permeable to air or other such oxidizing gases. In one instance, the cathode is comprised of a porous, electrically conductive material such as a carbon cloth of the type which is known in the art. In other instances, porous metal bodies such as metal foams, meshes and the like may also be utilized to form the cathode. Typically, the cathode will include a catalytic material which facilitates oxygen reduction and such catalysts may include platinum group metals used either singly or in combination, although other catalytic materials may likewise be incorporated. In one specific instance, a cathode usable in the present invention comprises a thin layer of carbon cloth having a Pt/carbon catalyst coated thereonto at a density of approximately 0.2 mg/cm2. Depending upon cell configuration, the cathode will further include a current collector associated with the carbon cloth, and such a collector may comprise a body of foamed metal. The cathode may also include support structures, housings, casings and the like. The typical cell will include a molybdenum-containing anode structure. The anode typically does not need any catalyst because the electrochemical reaction of molybdenum can be carried out at the surfaces of the metal itself. In the molybdenum anode there are three-dimensional electrochemical reactions of molybdenum and the thickness of the anode layer can be as large as desired. This is in distinction to conventional cells where anode reactions are confined to a relatively thin surface. As will be described in detail hereinbelow, the molybdenum may be in the form of a fine metal powder interspersed with a conductive material such as carbon, or it may comprised a foamed body of metal, a roughened metal plate, or a smooth metal plate. Use of high surface area anodes allows for the fabrication of batteries which can deliver high levels of power.

As discussed above, the cell will include an








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