Abstract:
The three electrode zinc-air cell with replaceable and rechargeable zinc anode has been improved to increase battery performance. The electroactive composition of the replaceable zinc anode includes expanded graphite and a conjugated polymer, and the sizes of zinc particles are in the range 30nm-5000nm. The current collector of the zinc anode includes alloys of lead, bismuth or indium. The replaceable zinc cartridge can be used with a metal-air battery to produce electricity, and a hydrogen-on-demand system to generate hydrogen.

Description:
This application claims benefit to U.S. provisional application 61/530,508 filed Sep. 2, 2011, and to U.S. provisional application 61/530,428 filed Sep. 2, 2011, and to U.S. provisional application 61/531,446 filed Sep. 6, 2011, and to U.S. provisional application 61/541,033 filed Sep. 29, 2011, and to U.S. provisional application 61/543,328 filed Oct. 5, 2011 all of the subject matter has been incorporated by reference. 
     REFERENCES CITED 
     
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
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                 Harats and 
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                 U.S. Pat. No. 
                 April/2008 
                 Burchhardt 
                 ReVolt Technology Ltd 
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     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to the energy storage systems based on zinc-air batteries with rechargeable/replaceable zinc anodes. 
     (2) Description of the Prior Art 
     Metal-air batteries are considered prospective devices for energy storage because of low cost, high energy density and no risk of fire or explosion. However zinc-air batteries have numerous problems: electrolyte drying, materials degradation, air electrode clogging, self-discharge, flooding, metal corrosion, growing resistance and poor rechargeability. Many of the mentioned problems have been addressed in this invention. 
     Zinc, iron, magnesium, aluminum or their alloys can be used in metal-air batteries with replaceable metal anodes. At present only zinc electrodes and their alloys are suitable for practical implementation as components of electrochemically rechargeable metal-air batteries. Zinc is prospective anode material because of sufficiently high battery voltage and rechargeability. An iron-air battery has too small voltage to be considered viable for use in a metal-air battery. 
     The word “battery” in this application is equivalent to the word “cell” when the number of elementary cells is not specified. 
     A zinc-air rechargeable battery with a bifunctional air electrode was proposed by Shepard et al., U.S. Pat. No. 4,842,963. The air electrode included oxygen reduction and oxygen evolution catalysts. This application uses a bifunctional air electrode, which is expensive and difficult to optimize for long term operation. 
     A battery with a replaceable metal anode was developed by Koral et al. U.S. Pat. No. 5,418,090 who suggested a mechanically rechargeable zinc-air battery. The cell could be recharged quickly, however the zinc anode required a complex procedure of recovery. 
     A battery with dual air electrodes was proposed by Harats and Goldstein U.S. Pat. No. 5,318,861 and Tsai et al. U.S. Pat. No 20050123815. A three electrode battery included an air reduction and oxygen evolution electrodes. The oxygen evolution electrode was positioned inside the cell. 
     The most attractive property of a zinc-air cell is low cost, however the complexity of the design of the discussed above batteries increases the cost essentially, and makes zinc-air devices less competitive in comparison with lithium batteries. 
     Burchardt U.S. Pat. No 20080096061 proposed to include metal hydrates as the components of the air electrodes to eliminate zinc-air cell drying. 
     Yager U.S. Pat. No 20120082907 suggested that anode metal could be deposited on the metal core (for example wire) to form a bundle preferably in form of coil. The presence of an adhesive or a binder as a component of a metal anode results in the increased resistance of the metal anode. A metal anode in form of coil is difficult to arrange in a flat battery to form a storage device with high energy density. The use of current collectors, which are expensive, or catalyze hydrogen evolution, or form insulating oxide layer, or form hydride layer is ineffective from the point of view of price or stability. 
     Objectives of the Invention 
     The principal objective of this invention is to propose a low cost, electrochemically rechargeable and replaceable zinc anode for zinc-air batteries. 
     A further objective of the invention is to improve and simplify the zinc-air cell by elimination of the non-functional construction elements. 
     A further objective of the invention is to provide a new electroactive composition of the replaceable zinc anode, which is rechargeable, stable in operation, suitable for fast and low cost production. 
     A further objective of the invention is to provide a non-toxic additive to the electroactive composition of the metal anode to further eliminate hydrogen evolution in the rechargeable zinc-air cell 
     A further objective of the invention is to eliminate the growth of electrical resistance of the metal anode by changing the composition of its current collector. 
     A further objective of the invention is to eliminate drying of a rechargeable metal-air battery. 
     A further objective of the invention is to use a metal anode as a universal energy storage module that can produce electricity in the zinc-air cells or hydrogen in the rechargeable hydrogen generators. 
     A further objective of the invention is to propose a rechargeable hydrogen generator with the replaceable and rechargeable zinc anode of this invention. 
     SUMMARY OF THE INVENTION 
     A three electrode zinc-air cell with mechanically replaceable and electrochemically rechargeable zinc anode was proposed. The functions of oxygen reduction and oxygen evolution are split between two air electrodes that form the cell walls. The zinc-air cell includes an additional air tight ion conduction membrane at the interface between the oxygen evolution air electrode and the electrolyte that prevents the penetration of oxygen inside the metal-air cell. The metal air cell further includes two layers of the hydroxide hydrate composition to decrease drying. 
     A replaceable metal anode includes a zinc composition, a current collector that has the shape of a spiral spring, and a porous jacket with ion selective properties. The metal anode composition includes expanded graphite. The anode uses a low cost small particle size metal dust to realize maximum possible power, and a conjugated polymer to inhibit hydrogen evolution. The current collector to the metal anode is made of zinc, lead, bismuth and indium alloys. 
     This application proposes a replaceable zinc anode as a universal energy storage unit to produce electricity as an anode of a zinc-air battery or hydrogen as the anode of a hydrogen-on-demand generator. 
     A hydrogen-on-demand generator suitable for using a replaceable metal anode of this invention includes a cathode, an oxygen evolution electrode, and an electrolyte. The cathode is covered by a hydrogen evolution catalyst, while the oxygen evolution electrode with an oxygen evolution catalyst. When the zinc anode is connected to the cathode through a variable resistor the hydrogen is produced on the cathode while the zinc anode is oxidized. The variable resistor controls the rate of hydrogen production. Zinc electrode can be electrochemically recovered by applying DC voltage to the oxidized zinc anode and oxygen evolution electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows exploded view of the electrochemically rechargeable zinc-air cell with a replaceable zinc anode. 
         FIG. 2  shows a replaceable zinc anode 
         FIG. 3  shows a replaceable zinc anode, which is a set of the elementary zinc anodes 
         FIG. 4  shows exploded view of the electrochemically rechargeable zinc-air cell and the position of the hydroxide hydrate layers 
         FIG. 5  shows a rechargeable hydrogen generator with a replaceable zinc anode 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention proposes the electrochemically rechargeable zinc-air cell with dual air electrodes, and a replaceable zinc anode. The charge and discharge functions are split between the air electrode optimized for oxygen evolution, and the air electrode for oxygen reduction. The discharge of the cell takes place by connection a load to the air electrode and the zinc anode. Oxygen evolution air electrode is disconnected during discharging. The charge of the cell takes place by connection of a DC or pulse power supply to the oxygen evolution electrode and the zinc anode. The oxygen reduction air electrode is disconnected during charging. 
     Some of technical solutions, excluding recharging, can be applied to iron, magnesium, silicon and aluminum electrodes or their alloys. The list of alloys includes MgZn 2 , Mg 2 Zn 3 , Mg 2 Zn 11 , MgZn, Mg 48 Zn 52 , or Mg 2 Zn 11    
     The expression “replaceable zinc anode” can be used interchangeably for zinc cartridge, zinc module, zinc cassette of other inventions when the inventions are related to zinc-air batteries. 
     The expanded view of the zinc-air cell of this invention is presented in  FIG. 1 . The important feature of the cell construction is the added functionality to its basic construction elements. Prismatic frame  1  is a single passive element of the cell that forms the base for assembling other components. The air electrode  2  and the oxygen evolution electrode  3  form the walls of the cell. The oxygen evolution electrode  3  is separated from the electrolyte by a gas tight membrane  4  with high ionic conductivity that prevents the penetration of oxygen inside the cell. Both the air electrodes  2  and  3  are separated from air with waterproof breathable microporous layers  5  and  6  to prevent electrolyte leakage. In addition, two optional hydrophobic gas diffusion layers  7  and  8  can be used to eliminate electrolyte leakage, and to increase the mechanical strength of the cell. Current collectors  9 ,  10  are attached to the air electrodes  2 ,  3  by soldering, welding or any other technique. 
     The upper part of the prismatic frame  1 ,  FIG. 1  is open to accommodate a replaceable zinc anode  11 . The replaceable zinc anode  11 ,  FIG. 2  includes an electroactive zinc composition  12 , ion selective jacket  13 , and current collector  14  in form of spiral ribbon or spiral wire spring and a rubber cap  15 . 
     The replaceable zinc anode  11  is produced by filling the spiral spring  14  with electroactive zinc composition to form a zinc anode. Then the anode is rapped with an ion selective film, followed by sealing the upper and lower openings by ultrasonic welding or an adhesive to form a jacket around said spiral spring frame filled with the electroactive zinc composition. 
     Porous membrane jacket  13  in  FIG. 2  provides an ion conducting layer that prevents zinc ions from penetrating the electrolyte, keeps the zinc anode composition compacted with light pressure of the spiral spring current collector. The jacket  13  forms the outer walls of the replaceable cartridge. The membrane jacket can have several layers, and can comprise of porous plastic, rubber, ceramic or metal stable in alkaline electrolytes. The jacket  13  can be formed of two or more layers of different materials, for example metal mesh and a plastic membrane. 
     The zinc anode  11  is imbedded into the electrolyte that fills the zinc-air cell. An electrolyte is based on 1M-8M solution of KOH, NaOH, LiOH or mixture thereof. The list of optional additives include Ba(OH)2*8H2O, Sr(OH)2*8H2O, carbopol, carboxymetilcellulose, sodium or potassium silicate, titanium dioxide with particle sizes in the range 5 nm-100 nm. 
     The air electrode includes a catalyst, a binder, carbon black and graphite. The catalyst is mixed with the binder, for example polytetrafluorethylene dispersion, or with graphite. Then graphite powder, carbon black, and/or conjugated polymer powder are mixed with said binder dispersion to form a paste. The paste is deposited on the surface of the metal mesh or the metal foam to form the air and oxygen evolution electrodes  2  or  3 . Then the waterproof layers  5  or  6  are applied to the outer sides of the air and oxygen evolution electrodes  2  or  3  while the gas tight membrane  4  to the inner side of any air electrode followed by calendering. The application of the gas tight membrane to the air electrode is optional and is not shown in  FIG. 1 . Then the electrodes are baked at the predetermined temperatures to remove the solvent and the surfactant. After cooling the electrodes are optionally calendered again. 
     The catalysts for oxygen reduction air electrode is selected from manganese dioxide, cobalt oxide, lanthanum strontium manganite, lanthanum strontium cobalt ferrite, platinum phthalocyanine, cobalt phthalocyanine, manganese phthalocyanine, a complex of the manganese salt and polyaniline or mixture thereof. The catalyst is selected in form of microparticles or nanoparticles. 
     Catalysts for oxygen evolution electrode are selected from lanthanum strontium cobalt ferrite, lanthanum strontium manganite, iridium dioxide, ruthenium dioxide, silver, wolfram carbide and other materials. 
     Materials of the current collectors of the air electrodes include nickel, nickel alloys, stainless steel, carbon or graphite in the form of mesh, foam, expanded or perforated sheet. Carbon based materials can be covered by plating, electroless metal deposition, decomposition of a metalloorganic compound to form a metal layer that improves the conductivity of a carbon sheet and its stability to corrosion in alkaline electrolytes. 
     An optional layer of a hydroxide hydrate composition  16  can be placed between the gas tight layer  4 , and the catalyst layer  3 ,  FIG. 3 . The cell components in  FIG. 3  are identical to the cell components in  FIG. 1  with two hydroxide hydrate layers added. A hydroxide hydrate composition layer  17  can be placed between the air electrode  2  and the waterproof layer  5 . This position of the hydroxide hydrate layer is optional also for oxygen evolution electrode  3 . A paste that includes Ba(OH) 2 ×8H 2 O, Sr(OH) 2 ×8H 2 O or (MgCO 3 )×Mg(OH) 2 ×5H 2 O, carbon black powder, graphite powder and a binder is deposited on the surface of the air electrode followed by calendering and baking at predetermined temperature. 
     The metal anode composition  12 ,  FIG. 2 , can be prepared by mixing zinc dust with particle sizes in the interval 900 nm-9000 nm, or metal nanoparticles (30 nm-900 nm) with vermicular graphite, expanded graphite, or carbon black, metal oxides with the alkaline electrolyte. A surface active compound can be added in amount up to 1%. 
     Said metal anode composition can further include a conjugated polymer selected from polyaniline derivatives, polypyrrole derivatives, polyparaphenylene or its derivative, PEDOT salt, polyphenylenevinylene or its derivative or aniline black. 
     Said metal anode composition can include polymers of aniline substitute, polymer of aniline complex, polymer of 4-aminodiphenylamine, polyaniline derivative, polyaniline complex, polyaniline salt, or mixture thereof. 
     We understand conjugated polymer as chemical compound made of multiple monomer units, the number of elementary units starts from 2, i.e. meaning polymer includes oligomer molecules. Polymer is not necessary linear and can be in form of branched or crosslinked chains. The same conjugated polymers can be used as components of the oxygen reduction air electrode or oxygen evolution air electrode. 
     Polymers formed from aniline derivatives include aniline derivative unit, but often have unidentified composition and structure. Many of said polymers of aniline derivative can be produced by adding an oxidant like potassium bichromate, potassium permanganate, potassium persulfate, hydrogen peroxide and many other to aniline derivative solution in water. Many aniline substitutes, aniline complexes and metal salts or oligomers can be polymerized in the same way. 
     Said conjugated polymers can be added in form of powder, or deposited on the surface of the carbon black, graphite, expanded graphite, ptfe powder from solution of a conjugated polymer or during polymerization, and then added to the other components of the electroactive metal composition. Said conjugated polymer is added in amount &gt;1%. The addition of a conjugated polymer decreases hydrogen evolution and increases the shelf life of the metal composition to a period not less than one month. The zinc anode with a conjugated polymer additive can be used in other batteries with alkaline electrolyte, for example nickel-zinc batteries. 
     Ribbon spiral springs or wire spiral springs are used as current collectors  14  to the zinc electrode. The current collectors can be formed of lead, bismuth, indium, and gallium-alloys. The examples of alloys include: In-95%, Bi-5%; In-99%; Bi-1%; In-90%, Zn-10%; In-95%, Zn-5%; In-99%, Zn-1%; In-95%, Sb-5%; Zn-97%, Bi-3%; Zn-99%, Bi-1%; Zn-99%, In-1%; Zn-98%, Ga-2%; Pb-95%, Bi-5%; Pb-99%, Bi-1%; Pb-97%, Ga-3%; Pb-99%, Sb-1%; Pb-95%, In-5%; Pb-99%; In-1%; Pb-98%, Bi-2%. 
     The advantage of a current collector in form of a spiral spring is the simplicity of production that does not require special equipment. Further advantage is the formation of an additional compression force on the electroactive zinc composition after assembling. Additional compression also increases electrical conductivity of the zinc anode. 
     The spring current collectors made of expanded graphite compositions can have an additional layer of a metal alloy deposited on the surface of the spring by electrochemical or electroless deposition, printing, stamping, CVD, electrophoresis, sputtering and other techniques. The layer can comprised of alloys of tin, bismuth, lead, indium or silver which increase the conductivity of said current collectors. 
     Jacket  11  in  FIG. 2  has three functions. The first function is to provide an ion conducting layer, the second function is to form case of the electrode assembly, and the third function is to keep electrode assembly in the lightly compressed state. Ion selective membrane may have several layers, and can comprise of plastic, rubber, ceramic or another material. The replaceable zinc anode can be used repeatedly and recovered by electrochemical reduction inside the cell or outside the cell. 
     The cap  15 ,  FIG. 1  has predetermined shape and fits the opening of the cell. The cap can be formed of rubber, porous rubber or plastic or porous plastic. The rubber or plastic shape can be formed by casting. 
     A current collector of the replaceable zinc anode can be formed of the set of the basic current collectors as demonstrated in  FIG. 4 . In this case all the basic current collectors are joined to form a common current collector. 
     The zinc-air batteries with replaceable zinc anodes can have any shape including concave or convex forms. 
     The replaceable zinc anode of a zinc-air battery has numerous advantages A replaceable zinc anode is safe in operation, does not pose risk of fire or explosion, and does not require heavy containers for transportation. Zinc has high volumetric energy density, and needs limited space for storage; it is safe in operation and can be recharged electrochemically. 
     The replaceable zinc anode has the capability to generate hydrogen. A hydrogen generator is presented in  FIG. 5 . The walls of the hydrogen generator are fixed by an adhesive to the base  18  to form a cathode  19  and an auxiliary electrode  20 . The hydrogen generator has an opening that can accommodate a replaceable zinc anode  11  and the generator can be filled with the alkaline electrolyte. The replaceable zinc anode of the hydrogen generator has the same composition and design as a replaceable zinc anode of the zinc-air cell. 
     The cathode  19  includes a catalyst for hydrogen production. Cathode  19  is made of metallic foil, for example stainless steel, nickel, brass etc. Metal can be covered by another layer with high surface area, for example nickel. The replaceable zinc anode is connected to the cathode through a switch and a variable resistor. When switch is “ON”, and the zinc anode is connected to the cathode, electrons propagate from the zinc anode to the cathode and initiate hydrogen evolution. Said variable resistor controls the rate of hydrogen production. The rate of hydrogen evolution can be controlled by the application of DC voltage too. When positive potential is applied to the zinc anode the production of hydrogen accelerates. 
     The hydrogen generator also includes an oxygen evolution electrode  20  that can be used for electrochemical regeneration of the zinc anode. In the process of hydrogen production the oxygen evolution electrode is disconnected. To regenerate the oxidized zinc anode after discharge, the negative voltage of the DC power supply is applied to the zinc anode while positive voltage to the oxygen evolution electrode. The cathode should be disconnected during regeneration. 
     The oxygen evolution electrode can be made of metal foil or other electrode materials stable in alkaline solutions, and is covered by a catalyst to produce oxygen, for example lanthanum strontium cobalt ferrite, lanthanum strontium manganite, iridium dioxide, ruthenium dioxide, silver, wolfram carbide and other materials. 
     The invention is not limited to the details of the illustrative embodiments, and the present invention can be embodied in other specific forms without departing from essential attributes thereof, and it is desired that the present embodiments will be considered in all respects as illustrative and not restrictive.