Manganese oxide based catalyst and electrode for alkaline electrochemical system and method of its production

Abstract

A catalyst, active layer, and cathode for metal air and other air-assisted cells and methods of producing these are disclosed. The cathode produced comprises a substantially unoxidized carbon support with a manganese or other oxide catalyst. The support maintains its inherent water repellency, conductivity and active sites. The cathode is therefore capable of sustaining significantly high currents for prolonged duration, at higher operating voltages, enabling the extension of metal air technology for higher power devices.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to the fabrication of carbon-based air cathodes loaded with manganese or other oxides for metal air cells, air-assisted alkaline cells and fuel cells, and in particular, relates to the preparation of manganese based oxides for such cathodes.

[0004] Traditional metal-air batteries are usually disk-like in appearance and are therefore referred to commonly as button or coin cells. Other configurations, including cylindrical metal air cells, are known and are applicable to the invention described herein. Metal air cells are disclosed in several patents including U.S. Pat. No. 5,721,065 issued Feb. 24, 1998, assigned to Rayovac Corporation, and entitled “Low Mercury, High Discharge Rate Electrochemical Cell,” and U.S. Pat. No. 6,197,445 issued Mar. 6, 2001, assigned to Rayovac Corporation, and entitled “Air Depolarized Electrochemical Cells,” the disclosures of which are both incorporated by reference herein as if set forth in their entirety.

[0005] Of the potential metal-air battery candidates, zinc-air has received the most attention because zinc is the most electropositive metal, and is relatively stable in aqueous and alkaline electrolytes without significant corrosion. In a zinc-air battery, the anode contains zinc and, during discharge, oxygen from the ambient air and water from the electrolyte are converted at the cathode to hydroxide, the hydroxide oxidizes the zinc at the anode, and water and electrons are released to provide electrical energy.

[0006] In metal-air batteries, a reactive metallic anode is electrochemically coupled to a carbon-based air cathode through a suitable alkaline electrolyte. As is well known in the art, the air cathode is typically a sheet-like member having a surface exposed to the atmosphere (air) and a surface exposed to an aqueous electrolyte of the cell. During operation, oxygen from the air dissociates and is reduced at the cathode, while metal of the anode is oxidized, thereby providing a usable electric current flow through the external circuit between the anode and the cathode. Metal air cells achieve very high energy densities, as the air cathode is very compact yet has essentially unlimited capacity. Because most of the volume is available for the anode active material, a metal-air cell can provide more watt-hours of electromotive force than a typical “two-electrode cell” of similar size, mass and anode composition that contains both anode- and cathode-active materials in approximately equal amounts inside the cell structure.

[0007] A stable gas/liquid/solid interface is important to effective discharge of metal air cells. Conventionally, the air cathode includes a substrate-supported active layer and an air diffusion layer. The active layer comprises a mixture of a carbon support, one or more fine particle catalysts on the support, and a polymeric binder/waterproofing agent such as polytetrafluorethylene (PTFE). The active layer is adhered or laminated to a metallic current-collecting substrate. The substrate is typically a cross-bonded screen having nickel metal strands woven therein, or a fine-mesh expanded metal screen. The air diffusion layer usually includes one or more pure hydrophobic membrane layers laminated to the air side of the active layer. Some metal air cells for high current drain applications employ a dual layer which includes a passive, hydrophobic barrier layer between the active layer and the air diffusion layer. This additional layer makes processing complex and expensive because it has to be fabricated separately then bonded to the active layer.

[0008] It is generally understood that a two-step oxygen reduction process occurs in metal air cells. The process requires diffusion and dissolution of oxygen gas and electrochemical reduction. A two-electron reduction at the carbon support surface produces peroxide ions. The peroxide ions are subsequently reduced in a catalytic step facilitated by the oxide catalyst such.

[0009] The entire composite electrode structure must have high electronic conductivity to ensure effective collection of the current and to reduce ohmic resistance. Without this, an undesirable voltage drop results. In addition to ohmic voltage drop, kinetic (reaction rate dependent) and mass transfer polarization can also reduce the cell voltage. For example, without sufficient available oxygen reduction sites or catalyst, a voltage drop due to kinetic limitations can occur. Insufficient catalyst alone can slow the catalytic step and cause peroxide accumulation, leading to a lower voltage. At high current densities, the cathode is generally under mass transfer control, meaning that mass transport becomes the rate determining step. Hence poor mass transport of reactants (e.g. oxygen) or products, which can be caused by low cathode porosity or by excessive wetting of the electrode, significantly increases polarization and reduces operating voltage. Generally, as the current demand increases during operation, it is believed that the reaction front moves outward toward the air side of the cathode, and more of the cathode surface area participates in the reaction. The liquid electrolyte can film over or flood the cathode surface, thereby blocking air access and reducing the active (available) three-phase interfacial area for reaction. It can eventually break through to the air side of a structurally deficient electrode and puddle between the cathode and the laminated hydrophobic barrier layer. A commonly used term for this condition is “cathode flooding.” The net result of all these phenomena is that the electrode cannot sustain the current density resulting in premature battery failure. Wetting-through of the cathode by electrolyte is further detrimental since the electrolyte is corrosive and leakage can cause damage to expensive components.

[0010] If the carbon support and the entire electrode structure become too hydrophilic for any reason, wetting through of the cathode and performance degradation as noted above cause premature failure.

[0011] In a conventional metal air cathode, one can compensate for a loss in conductivity by adding conducting carbon black. One can also vary, within limits, the amount of the hydrophobic binder in the active layer and the processing conditions to maintain a cathode having a hydrophobic character. However, an increase in the amount of the binder can undesirably reduce the electrode porosity and the number of carbon sites available for reaction. This hinders effective mass transport, which is critical at high current densities. Furthermore, using more binder increases the material cost of the electrodes.

[0012] As new high power, high current density devices raise performance expectations, the requirements for sustaining oxygen reduction over the life of the battery are becoming more demanding.

[0013] It is therefore a goal of air cathode design to increase oxygen reduction and reduce polarization from all sources. For example, attention has already been directed to the catalyst support, the catalyst particles, and on the cathode layer structures employed. The carbon support must have sufficient sites for the oxygen reduction reaction. This depends strongly on the type of carbon, as well as its surface area and surface functionalities. These attributes can depend upon the starting materials used to produce the carbon support and the method of its manufacture.

[0014] Also, inexpensive highly active, fine organic and inorganic catalyst particles should be well distributed throughout the carbon support to ensure rapid and effective consumption of all the peroxide produced to ensure high operating voltage. However, the choice of materials is limited because the catalyst must not only have a high oxygen-reducing activity, but must also withstand the corrosive environment of an electrochemical cell. The availability, cost and environmental or toxicological effects of the materials also have a bearing on the suitable choice for large-scale use of the material in practical systems for consumer applications.

[0015] Manganese oxides are known to be suitable catalysts for carbon-based air cathodes, and various methods are known for producing oxide catalyst on the carbon support. Most methods react the carbon with a strong oxidizing agent such as potassium permanganate (KMnO4) or silver permanganate (AgMnO4). The KMnO4 is reduced to MnO2, while the carbon is oxidized and eventually produces K2CO3. For example, U.S. Pat. No. 4,433,035 and U.S. Pat. No. 5,378,562 both disclose reducing potassium permanganate with either carbon black or activated carbon to produce carbon based air cathodes loaded with manganese oxide. U.S. Pat. No. 3,948,684 teaches using KMnO4 and/or heat to deposit MnO2 catalyst on carbon, and also suggests that MnO2 production is facilitated by using H2O2 with the KMnO4. Both H2O2 and KMnO4 are strong oxidizing agents that can rapidly oxidize the carbon surface. U.S. Pat. No. 3,948,684 taught that the electrodes performed better when Mn(NO3)2 was used. U.S. Pat. No. 4,433,035 describes using KMnO4 as an oxidizing agent on carbon black while adding “uncatalyzed” carbon black, presumably to compensate for the loss in conductivity.

[0016] U.S. Pat. No. 5,378,562 describes using KMnO4 on carbon the room temperature to produce Mn+2. FIGS. 5 & 6 in their patent show increase in impedance with increasing catalyst loading. While this method effectively produces well distributed, fine particle MnO2, the cathodes were developed exclusively for hearing aid battery development which required no more than 10 mA/cm2 at the time. At higher currents, these electrodes are prone to lower conductivity

[0017] Patent publication WO01/37358A2, entitled “Cathodes for Metal Air Electrochemical Cells” discloses an admixture of silver permanganate and carbon black, wherein silver permanganate is reduced in situ by carbon black to form a manganese oxide/silver catalyst mixture supported on carbon, which is used as cathode for oxygen reduction. FIG. 5 in their patent shows that 10% MnO2/C does not perform well compared to 5% MnO2/C, suggesting that high catalyst loadings are detrimental.

[0018] Sol-gel processes have also been employed to produce MnO2 for metal air cathodes. Sol-gel chemistry in aqueous solutions is based on the hydrolysis and condensation of metal ions. Sols are colloidal suspensions of the reaction product that are typically nano-sized. In a “true” sol, by virtue of the charge on the particles, the repulsive forces between adjacent particles can keep them in suspension for long periods of weeks to months. The particle size as well as the agglomeration or “aging” of particles depends on the concentration of reactants and product in the liquid, the type of precursors used, the rate of reaction, pH, etc.

[0019] According to Bach et. al., (J. Solid State Chem., 88,325-333, 1990) to synthesize MnO2 using sol-gel techniques, due to the lack of stable Mn(IV) precursors, one can use redox reactions to obtain MnO2 rather than the typical acid-base type reactions in sol-gel synthesis. Hence soluble inorganic precursors like KMnO4, NaMnO4, LiMnO4, AgMnO4 etc. can be reduced by appropriate organic or inorganic reducing agents to produce a sol, suspension, slurry or gel depending on the material and conditions used for the synthesis. The oxides produced from such low temperature techniques generally produce largely amorphous materials as determined by X ray diffraction analysis. The manganese oxides produced typically have mixed valence states, although careful control of the molar ratio of reactants can ensure a mean oxidation state of +4 for the Mn oxide. The sols can also be treated with acids to promote the disproportionation of Mn3+ to Mn2+ and Mn4+. The Mn2+which is soluble, can be washed away, leaving largely Mn4+.

[0020] French patent 2,659,075, also by Bach et al., entitled “Sol-gel Process for the Preparation of Manganese Oxide” discloses the fabrication of manganese oxide via the reduction of potassium permanganate solution with a carboxylic acid having four carbon atoms. This method produces a manganese (IV) oxide gel using fumaric acid as the reducing agent. It is claimed that the four-carbon nature of the reducing agent yields a gel, in which the manganese oxide particles are suspended. The objective of the patent is to produce crystalline MnO2 for reversible intercalation of Li ions in a rechargeable Li battery. Hence the MnO2 gel is subjected to high temperature heat treatment (calcined) to produce the desired crystal structure and orientation.

[0021] U.S. Pat. No. 6,465,129 entitled ‘Lithium Batteries with New Manganese Oxide Materials as Lithium Intercalation Hosts” describes “sol-gel” technology and the importance of distinguishing between various methods, the different chemical and structural characteristics of the synthesized material, and the end application of inventions. It is incorporated herein by reference. The inventors describe nanoporous, amorphous MnO2 with high lithium intercalating properties, which are not subjected to high temperatures.

[0022] J. Electrochem. Soc., 143(5):1629 (May 1996) (Stadniychuk, et. Al.), incorporated by reference as if set forth herein in its entirety, surveys various methods for producing MnO2. The paper describes the importance of pH and concentration on sol-gel transition when using fumaric acid as reducing agent. It also describes rather complex methods of producing MnO2 nanoparticles for use in thin film alkaline batteries having an electrode predominantly comprising MnO2, where the MnO2, is directly consumed in the reaction. In contrast, the MnO2 of metal air batteries behaves as a catalyst that facilitates a reaction but is not consumed.

[0023] U.S. Pat. No. 6,444,609 entitled “Manganese-based Oxygen Reduction Catalyst, Metal-Air Electrode Including Said Catalyst and Methods for Making the Same Relates to a Sol-gel Process for Making a Catalyst for an Air Electrode.” The inventors disclose combining a manganese alkoxide of valence state +2 with alcohol under suitable conditions to produce a sol, converting the sol to a gel, mixing the gel with carbon to produce a mixture, and then pyrolyzing the mixture at a high temperature to produce the MnO2, which has valence state of +4, on the carbon support.

[0024] An increasing demand for higher power cells has been created by newer devices, such as hearing aids, particularly digital hearing aids. The desired increase in power demands that cells have an ability to operate at higher voltages and at higher currents. Still higher power demands are seen in recent attempts to develop and produce larger batteries in cylindrical or prismatic form, for consumer electronic as well as military, applications. From processing and performance standpoints, it is desirable to preserve the surface chemistry that influences the physico-chemical properties such as wettability and electrical properties of the support carbon materials.

BRIEF SUMMARY OF THE INVENTION

[0025] While, in general, it is known that it is important to maintain the hydrophobicity of the metal air cathode, the prior art has not heretofore appreciated that, at high currents, cathodes can fail as oxidation at the carbon support surface promotes undesired surface micro-hydrophilicity. The inventors have determined the importance of three-dimensional hydrophobic/hydrophilic balance at the micro- and macro levels in the electrode for sustaining high power and high current density discharge. The inventors have further determined that conventional processing methods cause physical or chemical oxidation of the carbon surface and that surface oxygen compounds increase the hydrophilicity of the carbon and make the carbon and the electrode more wettable. It is further believed that oxidation reduces electrochemical activity by consuming active sites that would otherwise participate in the oxygen reduction reaction.

[0026] Accordingly, the present invention is summarized in that a carbon support substantially unoxidized during catalyst loading is advantageously used in an air cathode of a metal air cell for high current drain applications. The carbon support of the invention has micro-hydrophobic properties not seen in the prior art. It is conventional in the art for the carbon support to be provided on its surface with an oxide catalyst that can comprise an oxide of manganese, silver or cobalt, or mixtures thereof, with a manganese oxide, particularly manganese dioxide, being the preferred oxide catalyst. References herein to a manganese compound, such as a permanganate or an oxide, are intended to encompass the other suitable catalysts or catalyst precursors as well.

[0027] In a related aspect, a carbon support having a high level of reactive sites and a low level of oxidation can be selected from available carbon sources for mixing with existing oxide catalyst particles, and can be prepared as described herein such that the carbon is substantially unoxidized after the catalyst has been provided on the support. The carbon support can be activated carbon or conductive carbon black of conventional size. A suitable activated carbon has a surface area of at least 200 m2/g, preferably greater than 700 m2/g. Activated carbons are most commonly obtained by steam or chemical activation of pitch or coal based precursors, to produce extremely high porosity particles with high adsorptive capacity for organic and inorganic compounds. These properties are thought to make the materials appropriate for oxygen reduction reactions, which can be further enhanced by incorporation of catalysts. For such carbons, the molasses number indicates internal porosity, and the iodine number, their surface area in m2/gm. Preferred activated carbons are PWA carbon (Calgon Corporation, Pittsburgh, Pa.)), which is bituminous coal derived, and has a molasses number of 218 and iodine number of 900. Norit SX1G, as well as other grades from Norit Americas Inc, Atlanta, Ga., are suitable peat-based activated carbon with a molasses number of ˜310 and iodine number of 900. A suitable conductive carbon black has a surface area of at least 1200 m2/g. It is preferred that a conductive carbon black have a surface area of at least 1200 m2/g. A preferred carbon black is Black Pearls 2000 or Vulcan XC 72 (Cabot Corporation, Billerica, Mass.). Another suitable carbon black is Ketjen Black (Akzo Nobel Corporation, Chicago).

[0028] The meaning of “substantially unoxidized” in this application refers to further oxidation of the carbon support after the activation process used to produce the support materials. In other words, a goal of this invention is to avoid oxidizing the support while producing a catalyst-loaded support. Preferably, the support is not oxidized during cathode loading in accord with the methods described herein, but oxidation to some extent can be tolerated, for example as much as about 10-20%, or even more, oxidation can be acceptable depending upon the application. The extent of oxidation is best determined operationally by reference to the suitability of a cathode in a high rate application in the manner shown in the accompanying Examples. The statement is not intended to suggest that the starting carbon is free of oxygen-containing surface groups, but rather that the level of such groups is sufficiently low when loaded with catalyst so as not to substantially reduce the function of a cathode under high current conditions as described herein (not more than 10% reduction relative to cathode fabricated similarly without regard to oxidation of the support, e.g., as in Example 1). The level of carbon support oxidation that is acceptable in the invention will vary with the activity of the starting carbon material, and more particularly with its reactivity after cathode formation in the oxygen reduction reaction described above. Accordingly, if the starting material has a large number of reactive sites, the carbon can be partially oxidized without adverse impact upon cathode activity. In contrast, a relatively inactive starting material having the same proportion of oxidization can be rendered unusable in a cathode for high rate applications.

[0029] In another aspect, the invention is further summarized in that an active layer for a cathode for a metal air cell comprises a mixture of a polymeric hydrophobic binder and a carbon support of the invention having supported on its surface the catalyst material. The active layer mixture can be adhered or laminated to a metallic current-collecting substrate, and combined with one or more air diffusion layers in a conventional manner to form a cathode for a metal air cell. The cathode can be incorporated into a metal air cell in a conventional manner. A cathode active layer of the invention typically comprises 70-80% carbon and 2-20% of the oxide catalyst, by weight, with the balance being binder.

[0030] In one aspect, a method for producing an active layer of the invention begins with a method for producing an oxide catalyst suspension that will not oxidize a carbon support when the two are mixed together. In the method, an oxidizing agent, preferably a soluble manganese compound having a valence state higher than +4, is mixed with one or more suitable organic or inorganic reducing agents at a temperature in the range of 10° C. to 100° C. to produce a suspension of oxide(s) containing particles ranging from sub-micron to several microns in size, for example between 100 nanometers and 30 microns in size.

[0031] It should be appreciated that the present invention is intended to include suspensions of various particle sizes, which may be produced by adjusting the starting materials and/or the reaction conditions in a manner known to the art. Substantially all of the primary particles are preferably submicron size, but the primary particles can aggregate to form larger secondary clusters. While sub-micron sized primary oxide particles are desirably employed in the method, the invention is not limited to oxide catalyst having a specific particle size range. For instance, the Examples below demonstrate that oxide catalyst aggregates on the order of 20 microns in size can be used in a cathode having high catalytic activity and hydrophobicity for sustained high current density performance. The oxide particle size distribution can be determined using a Coulter Particle Size Analyzer.

[0032] The oxidizing agent can be selected from a variety of compounds containing manganese of valence greater than +4. Permanganate salts are preferred, for instance, lithium-, sodium-, potassium-, silver-, ammonium- or cobalt-based salts, or mixtures of the salts. A suitable organic reducing agent can have one or more carbon atoms, and can include fumaric acid, citric acid, formic acid, or a salt of these acids, as well as an alcohol, aldehyde, or the like that can be readily oxidized. The reducing agent can also comprise one or more inorganic compounds, such as a nitrate, chloride, sulfate, or perchlorate of various cations, as well as hydrogen peroxide, and the like, which readily react with and reduce the oxidizing agent. The reducing agent can further contain manganese in the +2 valence state (e.g., manganous nitrate, perchlorate,or sulfate) which is oxidized by the permanganate to a higher oxidation state (e.g., +4).

[0033] The mixing of the oxidizing and reducing agents can be accomplished ex situ, under conditions that form particles having the desired size. The catalyst particles are later mixed with a carbon support having the described attributes without exposing the support to an oxidizing agent. The particle suspension produced in an ex situ mixing method is preferably a colloidal sol comprising the oxide particles. The particle suspension is then transferred onto the carbon support (e.g., provided as a slurry, paste, or powder) under agitation to produce a substantially unoxidized carbon support loaded with catalyst for further processing into a metal-air cathode. In a related aspect, the particles and the carbon support can have net opposite charges that enable charge-induced attraction and adsorption of the particles to the carbon surface. It is possible to adjust the deposition of the catalyst particles onto the support by incorporating surfactants or other additives to modify the inherent or imparted charge on the support relative to the charge on the particles.

[0034] Alternatively, the oxide can be deposited in situ on the carbon support by mixing the oxidizing and reducing agents with the carbon support under conditions that favor a redox reaction between the oxidizing and reducing agents over the reaction between the oxidizing agent and the carbon. The rate of reaction between the reducing and oxidizing agents should be at least twice as fast, more preferably five to ten times as fast, as the rate of reaction between the oxidizing agent and the support. Under such conditions, the very fine particles that form in the redox reaction are immediately attracted to and attach to the carbon support, which can also act as a seed or nucleation site. A particle suspension produced in situ with the carbon support as described is more intimately dispersed than would be the case for particle produced ex situ and the carbon will still be substantially unoxidized.

[0035] It is an object of the invention to provide a carbon-supported catalyst for use in a cathode active layer suitable for use in a high performance metal air cell to deliver high power and high current density.

[0036] It is another object of the invention to provide the catalyst at a loading of between about 1% and 20%, preferably between about 5% and 15%, oxide catalyst by weight in the cathode to ensure suitability for use in a high performance metal air cell.

[0037] It is a feature of the invention that a cathode active layer of the invention comprises the carbon-supported catalyst of the invention.

[0038] It is another feature of the invention that the carbon support can be substantially unoxidized during the catalyst loading process.

[0039] It is also an advantage of the invention that the catalyst oxide particles can, if desired, be produced in situ with the carbon support.

[0040] It is an advantage of the invention that the carbon support maintains adequate electrical conductivity and chemical reactivity for high current drain discharge applications.

[0041] It is another advantage that a cathode of the invention maintains catalytic activity and conductivity, and retains a hydrophobic character at both the macro (cathode) and micro (carbon support) levels, and thereby is sufficiently robust to sustain high current densities with minimal flooding.

[0042] It is yet an advantage of the invention that it does not require a high temperature pyrolysis step to produce the Mn+4 oxide.

[0043] Still another advantage of the invention is that the catalyst can function in a single active layer.

[0044] It is yet another advantage of the present invention that readily available, inexpensive compounds are employed in the making of the carbon-supported catalyst of the invention.

[0045] It is a still further advantage of the invention that no gelling step is required for the sols produced, thereby avoiding processing steps and reducing costs.

[0046] A yet further advantage of the invention is that a wider range of oxide catalyst loading is enabled, which is otherwise not possible due to cathode deterioration effected by high catalyst loading in the prior art.

[0047] These and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings which form a part hereof, and which there is shown by way of illustration, and not limitation, preferred embodiments of the invention. Such embodiments do not define the scope of the invention and reference must therefore be made to the claims for this purpose.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Reference is hereby made to the following figures in which like reference numerals correspond to like elements throughout, and in which:

[0049] FIG. 1 is a schematic sectional side elevation view of a zinc-air button cell constructed in accordance with the invention;

[0050] FIG. 2 compares the effects of carbon oxidation on hydrophobicity of cathodes constructed in accordance with the preferred embodiments to that of the prior art;

[0051] FIG. 3 compares the long-term stability and performance of a cathode constructed in accordance with the preferred embodiments to that of the prior art;

[0052] FIG. 4 is a graph illustrating the discharge profile of a zinc air 13 size cell constructed in accordance with the preferred embodiment compared to the prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0053] In this application, high current density is defined as a current density greater than about 50 mA/cm2. A “high performance cell” operates at a high current density and resists flooding and leakage.

[0054] While the preferred embodiments are described with reference to a cathode for a button cell, it should be appreciated by a skilled artisan that the present invention is equally applicable to the fabrication of cathodes for other types of cells, including but not limited to other types of metal-air cells, fuel cells, or any other electrochemical cells that can benefit by having a carbon-supported oxide-based electrode.

[0055] Methods for forming a cathode from a catalyst-coated support and a binder are known and, except as noted, conventional cathode-forming methods can be employed in this invention. Rather, the invention relates to methods for producing a catalyst-coated support having the indicated properties and to the support, coated support, cathode comprising the coated support, and cell comprising the cathode. In accordance with the invention, methods for both ex situ and in situ production are described below, after presenting the structure of a button cell constructed in accordance with the invention.

[0056] Referring to FIG. 1, a metal-air cell, and in particular a button cell 10, is disposed in a battery cavity 12 of an appliance 14. The cavity 12 is generally bounded by a bottom wall 16, a top wall 18, and side walls 20. The negative electrode of the cell 10, commonly referred to as the anode 22, includes an anode can 24 that contains anode active material 26. The anode can 24 has a top wall 28 and an annular downwardly-depending side wall 30. Top wall 28 and side wall 30 have, in combination, an inner surface 32 and outer surface 34. Side wall 30 terminates in an annular can foot 36, and defines a cavity 38 within the anode can, which contains the anode material 26.

[0057] The positive electrode, commonly referred to as the cathode 40, includes a cathode assembly 42 contained within a cathode can 44. Cathode can 44 has a bottom 46 and an annular upstanding side wall 47. Bottom 46 has a generally flat inner surface 48, a generally flat outer surface 50, and an outer perimeter 52 defined on the flat outer surface 50. A plurality of air ports 54 extend through the bottom 46 of the cathode can to provide avenues for air to flow into the cathode. An air reservoir 55 spaces the cathode assembly 42 from the bottom 46 and the corresponding air ports 54. A porous diffusion layer 57 fills the air reservoir 55, and presents an outer reaction surface 90 for the oxygen. Side wall 47 of the cathode can has an inner surface 56 and an outer surface 58. It should be appreciated that an air mover (not shown) could be installed to assist in air circulation.

[0058] The cathode assembly 42 includes an active layer 72 that is interposed between a barrier layer 74 and air diffusion layer 57. Active layer 72 facilitates the reaction between the hydroxyl in the electrolyte and the cathodic oxygen of the air. Barrier layer 74 is a micro-porous plastic membrane, typically polypropylene, having the primary function of preventing anodic zinc particles from coming into physical contact with the remaining elements of the cathode assembly 42. Barrier layer 74 however, does permit passage of hydroxyl ions and water therethrough to the cathode assembly.

[0059] The anode 22 is electrically insulated from the cathode 40 via a seal, that includes an annular side wall 62 disposed between the upstanding side wall 47 of the cathode can and the downwardly-depending side wall 30 of the anode can. A seal foot 64 is disposed generally between the can foot 36 of the anode can and the cathode assembly 42. A seal top 66 is positioned at the locus where the side wall 62 of seal 60 extends from between the side walls 30 and 47 adjacent the top of the cell.

[0060] The outer surface 68 of the cell 10 is thus defined by portions of the outer surface 34 of the top of the anode can, outer surface 58 of the side wall 47 of the cathode can, outer surface 50 of the bottom of the cathode can, and the top 66 of seal 60.

[0061] As is detailed below, the cathode 16 is loaded with manganese oxide using any of several methods to obtain an oxide-coated carbon support in which the carbon is substantially unoxidized. Cathodes made with the coated support of the invention achieve higher operating voltages than prior art cathodes, and furthermore to improve long-term performance during operation.

[0062] In an ex situ method for fabricating a carbon-based air cathode with manganese dioxide catalyst particles, a solution of an oxidizing agent (potassium permanganate) and a reducing agent (sodium formate) can be combined at room temperature and at a generally neutral pH (range from about 6 to 8) to produce a manganese oxide sol according to the following reaction. The manganese oxide suspensions can also be prepared from the reduction of potassium permanganate solution by sodium formate in acidic or alkaline solutions.

4KMnO4+6HCOONa→4MnO2+3CO2+3H2O+2K2CO3+3Na2CO3  (1)

[0063] Next, the particles in suspension are mixed with a carbon slurry that comprises the carbon support and the mixture is stirred to disperse the manganese oxide particles into the carbon matrix. If desired, the suspension of carbon slurry and manganese oxide can be heated. A suspension of manganese oxide on carbon is thus produced in a slurry form.

[0064] Alternatively, in a method for depositing the catalyst on the carbon substrate in situ, the carbon substrate is highly agitated. Separate streams or sprays of the oxidizing agent and the reducing agent can be mixed above the agitated carbon substrate and react with one another to form small oxide particles, preferably colloidal particles, before contacting the surface of the carbon support. When the particles contact the carbon support, they can be immediately adsorbed to and evenly dispersed on the support which can further act as a seed or nucleating surface. Because the conditions favor a redox reaction between the oxidizing and reducing agents over the reaction between the oxidizing agent and the carbon, the available oxidizing agent is substantially consumed before it has an opportunity to contact the carbon substrate such that the substrate is not oxidized in the process. Because the particles and the carbon support can have net opposite charges, the particles can be attracted to and adsorbed on the carbon surface.

[0065] Without regard to whether the catalyst oxide-coated carbon support was prepared by in situ or ex situ method, the processing continues in a standard manner to produce an air cathode. Briefly, the binder/waterproofing agent is added to the suspension, and the resulting mixture is stirred prior to filtering and washing. In a preferred embodiment, 25 grams of Teflon T-30 PTFE suspension are added to the suspension, though other waterproofing agents could be used, such as polyethylene.

[0066] The mixture is filtered and rinsed with H2O to remove any soluble impurities before being dried at step 118. In particular, the mixture is dried at 90° C. for approximately 14 hours in accordance with the preferred embodiment. Finally, it is rolled to provide an active catalyst layer for the resulting air cathode. The catalyst layer is then laminated to a nickel screen current collector at its inner surface and a PTFE layer at its outer surface to provide an air diffusion layer. In order to prevent electrical contact between the air cathode and anode, a separator is applied on the inner surface of the nickel screen. The separator can comprise a traditional non-woven fabric, or could alternatively comprise a conformal separator, as is understood by one having ordinary skill in the art.

[0067] The fabrication process is completed to produce a carbon-based air cathode loaded with substantially evenly distributed manganese dioxide particles that provide a catalyst to the oxygen reduction reaction that occurs during discharge of the cell. The cathode may then be installed into a metal-air cell in a conventional manner.

EXAMPLE 1

[0068] For comparison, a conventional method for preparing an air cathode was undertaken. A carbon slurry was prepared by placing 1700 mL of distilled water in a mixing vessel and adding 490 grams of PWA activated carbon (Calgon) and stirring the mixture for 30 minutes. A 0.35M solution of KMnO4 was prepared and 773 grams of that solution was slowly poured into the mixing vessel containing the carbon slurry. This mixture was stirred for an additional 30 minutes at room temperature. 10 g of Black Pearls 2000 conducting carbon black was added and mixed for an additional 10 minutes. A waterproofing agent, primarily T-30 suspension (DuPont), was added to the aforementioned mixture in the amount of 125 grams. This final mixture was stirred for an additional 15 minutes.

[0069] The resultant cathode mixture was filtered through a Buchner funnel, rinsed with distilled water and filtered again. The resultant mix remaining in the filter was then dried at approximately 90° C. in air for 8-24 hours. Once dried, the mix was pulverized in a high intensity mixer for approximately 10 minutes. It was then rolled to provide the active catalytic layer and subsequently laminated onto the current collector to yield the final electrode. The final dry composition contains 3.8% MnO2. All of the carbon was activated by the permanganate.

EXAMPLE 2

[0070] The procedure of Example 1 was repeated, except that the concentration of KMnO4 solution was increased to yield 8% MnO2 in the final product. All of the carbon was activated by the permanganate.

EXAMPLE 3

[0071] The procedure of Example 1 was repeated, except that only one half of the total amount of carbon was mixed with the KMnO4 solution and allowed to react. Then, the remaining half of the carbon was added to the mixture and processing continued as in Example 1. This resulted in a cathode in which the carbon was 50% oxidized (activated).

EXAMPLE 4

[0072] To produce a substantially unoxidized support by an ex situ method, the following steps were performed. A sodium formate solution was prepared by first placing 180 grams of distilled water in a tank, and adding 20 grams of sodium formate powder. The mixture was stirred for approximately five minutes. Next, 350 ml of potassium permanganate solution (1.73N) was added to the sodium formate solution, and the resulting mixture was stirred for approximately an additional 10 minutes at a temperature between 25° and 100° C. to produce a manganese oxide suspension according to Reaction (1) above. The amorphous manganese oxide particles produced ranged in size from about 100 nanometers to about 30 microns, and had an average particle aggregate size in the range of 20 to 26 microns.

[0073] A carbon slurry was prepared by placing 500 grams of distilled water in a tank, adding 7 grams of Black Pearls 2000, and 93 grams of Norit SX1G (having a BET surface area of approximately 1500 m2/g and 900 m2/g, respectively), and stirring the mixture for 15 minutes. The manganese oxide suspension was poured into a tank containing the carbon slurry, and the suspension was stirred for approximately one hour. A waterproofing agent, and in particular 25 grams of Teflon T-30, was added to the suspension and the resulting mixture was stirred for approximately 10 minutes.

[0074] The resulting cathode mixture was then treated to provide a cathode. In particular, the mixture was filtered on Buchner funnel, and rinsed with H2O before being dried at 90° C. for approximately 14 hours. Finally, it was rolled to provide the catalyst active layer. The catalyst layer was then laminated and treated in the manner described above to produce a cathode.

EXAMPLE 5

[0075] To produce a substantially unoxidized support by an in situ method, the method of Example 4 was repeated to produce a cathode using the same components, except the carbon slurry was agitated for 10 minutes, and then the potassium permanganate solution and the sodium formate solution were simultaneously added slowly at room temperature so as to enter the vigorously stirred slurry as a single stream.

EXAMPLE 6

[0076] To illustrate the effect of carbon oxidation on the hydrophobicity and high current capability, polarization curves of the cathodes of Examples 1-5 were obtained as is shown in FIG. 2. Polarization measurements were carried out in a single compartment cell with three-electrode configuration using Solartron 1286 with CorrWare for Windows. The electrode potential was measured and referred to zinc wire reference, while the counter electrode was made of Platinum gauze. Attention is directed to the capability of the cathodes at greater than 50 mA/cm2 (high current density). When all of the carbon support is oxidized (Example 1), the voltage drops off quite significantly, even when the amount of MnO2 catalyst was increased from 3.8% to 8% (Example 2). When only half the carbon is oxidized but the amount of MnO2 catalyst is maintained at 3.8% (Example 3), the high current capability is significantly improved. Notably, however, superior performance was observed where the carbon was prepared in accordance with the invention (Examples 4 and 5).

EXAMPLE 7

[0077] Long-term performance tests of carbon based air cathodes were conducted at constant current density over a period of time and recording the corresponding changes in electrode potential. This test determines the sustained robustness of the electrodes, particularly from flooding in the mass transport controlled region of the polarization curves of FIG. 3. A rapid drop off in voltage implies that the three phase interface in the structure and the hydrophobicity of the finished electrode are inadequate to sustain high currents. The electrodes were the same as those in FIG. 2, except the electrode of Example. 5 is not shown.

[0078] A very high current density of 200 mA/cm2 was applied to stress the electrodes. The results show a substantial performance improvement for the present invention compared to all others, with minimal voltage drop over the duration of the test.

EXAMPLE 8

[0079] It has further been determined that the present cathode produces a metal-air cell having an increased operating voltage, when compared to prior art metal-air cells. In particular, referring to FIG. 4, the discharge profile of zinc-air 13 size cells having cathodes constructed in accordance with the present invention are compared to those of the prior art. As illustrated, the present cell achieves an operating voltage of almost 30 mV greater than prior art cells throughout the usable life of the cell. While the present cell becomes fully depleted a few hours sooner than the conventional cell, a skilled artisan would appreciate that the voltage of the conventional cell is substantially low so as to render the cell useless for its intended purpose during this time.

EXAMPLE 9

[0080] The cathodes of the invention were shown to exhibit the following performance:

Current Density (I) Voltage (V)
mA/cm2              (V)
100                 >1. 1
150                 >1.05
200                 >0.9

[0081] The data presented demonstrate the importance of reducing or eliminating carbon oxidation when preparing a high performance air cathode.

[0082] The invention has been described in connection with what are presently considered to be the most pratical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangement included within the spirit and scope of the invention, as set forth by the appended claims.

Claims

1. A carbon support comprising at least one of conductive carbon black and activated carbon and having an oxide catalyst loaded thereupon, the carbon support being substantially unoxidized.

2. A carbon support as claimed in claim 1, the conductive carbon black having a surface area of at least 1200 m2/g.

3. A carbon support as claimed in claim 1, the activated carbon having a surface area of at least 700 m2/g.

4. A carbon support as claimed in claim 1, the oxide catalyst comprising manganese.

5. A carbon support as claimed in claim 1, the oxide catalyst comprising manganese dioxide.

6. A carbon support as claimed in claim 1, the oxide catalyst comprising primary particles of submicron size.

7. A carbon support as claimed in claim 1, the oxide catalyst comprising secondary particles of between about 100 nanometers and 30 microns in size.

8. An active layer for an air cathode, the active layer comprising a hydrophobic binder and a carbon support comprising at least one of conductive carbon black and activated carbon and having an oxide catalyst loaded thereupon, the carbon support being substantially unoxidized.

9. A cathode active layer as claimed in claim 8 comprising, by weight, 70-80% carbon and 2-20% oxide catalyst.

10. An air cathode for an electrochemical cell, the air cathode comprising a metallic substrate, an active layer on a first side of the substrate, a hydrophobic diffusion layer on a second side of the substrate, the active layer comprising a hydrophobic binder and a carbon support comprising at least one of conductive carbon black and activated carbon and having an oxide catalyst loaded thereupon, the carbon support being substantially unoxidized.

11. An air cathode as claimed in claim 10 wherein the diffusion layer comprises PTFE.

12. An air cathode as claimed in claim 10 comprising, by weight, 70-80% carbon and 2-20% oxide catalyst.

13. An electrochemical cell comprising: an anode; an air cathode comprising a metallic substrate, an active layer on a first side of the substrate, a hydrophobic diffusion layer on a second side of the substrate, the active layer comprising a hydrophobic binder and a carbon support comprising at least one of conductive carbon black and activated carbon and having an oxide catalyst loaded thereupon, the carbon support being substantially unoxidized; a separator between the anode and the cathode; and an electrolyte in contact with the anode and the cathode.

14. A method for making an air cathode for an electrochemical cell, the method comprising the steps of: combining a carbon slurry comprising at least one of conductive carbon black and activated carbon, with a suspension of oxide catalyst particles having particles ranging in size from about 100 nanometers to about 30 microns and a waterproofing agent to form a mixture, the carbon in the mixture being substantially unoxidized; forming the mixture into an active layer for an air cathode, the carbon in the cathode being substantially unoxidized; and incorporating the active layer into an air cathode.

15. A method for making an air cathode as claimed in claim 14 wherein the suspension of oxide catalyst particles is formed by a method comprising the step of mixing an oxidizing agent that comprises manganese with a reducing agent to yield a suspension of oxide catalyst particles.

16. A method for making an air cathode as claimed in claim 14 wherein the oxidizing agent comprises a soluble manganese compound having a valence state higher than +4.

17. A method for making an air cathode as claimed in claim 14 wherein the oxidizing agent comprises a permanganate salt selected from the group consisting of a lithium salt, a sodium salt, a potassium salt, a silver salt, an ammonium salt, a cobalt salt, and a mixture thereof.

18. A method for making an air cathode as claimed in claim 14 wherein the reducing agent is selected from the group consisting of an organic reducing agent and an inorganic reducing agent.

19. A method for making an air cathode as claimed in claim 14 wherein the reducing agent is selected from the group consisting of a nitrate, a chloride, a sulfate, and a perchlorate of a manganese compound having a valence of +2 and hydrogen peroxide.

20. A method for making an air cathode as claimed in claim 14 wherein the reducing agent is selected from the group consisting of fumaric acid, citric acid, formic acid, a salt of any of the foregoing acids, an alcohol that can be readily oxidized, and an aldehyde that can be readily oxidized.

21. A method for making an air cathode for an electrochemical cell, the method comprising the steps of: mixing an oxidizing agent comprising manganese with a reducing agent and with a carbon support under conditions that favor a redox reaction between the oxidizing agent and the reducing agent over a reaction between the oxidizing agent and the carbon to form oxide catalyst particles and to load the particles in situ onto the carbon support; mixing a waterproofing agent with the oxide-catalyst-loaded carbon support to form a mixture; forming the mixture into an active layer for an air cathode, the carbon in the cathode being substantially unoxidized; and incorporating the active layer into an air cathode.

22. A method for making an air cathode as claimed in claim 21 wherein the oxidizing agent comprises a soluble manganese compound having a valence state higher than +4.

23. A method for making an air cathode as claimed in claim 21 wherein the oxidizing agent comprises a permanganate salt selected from the group consisting of a lithium salt, a sodium salt, a potassium salt, a silver salt , an ammonium salt, a cobalt salt, and a mixture thereof.

24. A method for making an air cathode as claimed in claim 21 wherein the reducing agent is selected from an organic reducing agent and an inorganic reducing agent.

25. A method for making an air cathode as claimed in claim 21 wherein the reducing agent is selected from the group consisting of a nitrate, a chloride, a sulfate, a perchlorate and hydrogen peroxide.

26. A method for making an air cathode as claimed in claim 21 wherein the reducing agent is selected from the group consisting of fumaric acid, citric acid, formic acid, a salt of any of the foregoing acids, an alcohol that can be readily oxidized, and an aldehyde that can be readily oxidized.