STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
BACKGROUND OF THE INVENTION
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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+.
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.
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.
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.
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.
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
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.
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.
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).
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.
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.
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.
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.
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).
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.
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.
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.
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.
It is a feature of the invention that a cathode active layer of the invention comprises the carbon-supported catalyst of the invention.
It is another feature of the invention that the carbon support can be substantially unoxidized during the catalyst loading process.
It is also an advantage of the invention that the catalyst oxide particles can, if desired, be produced in situ with the carbon support.
It is an advantage of the invention that the carbon support maintains adequate electrical conductivity and chemical reactivity for high current drain discharge applications.
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.
It is yet an advantage of the invention that it does not require a high temperature pyrolysis step to produce the Mn+4 oxide.
Still another advantage of the invention is that the catalyst can function in a single active layer.
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.
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.
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.
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.