Patent Application
20100196768
This invention relates to electrochemical cells. The invention has particular application to zinc/air-based fuel cells and mechanically rechargeable batteries with circulating electrolytes.
Electrochemical zinc/air cells have zinc-based negative electrodes, referred to as anodes in primary cells, and gas-diffusion positive electrodes, referred to as cathodes in primary cells. Such cells electro-catalytically reduce oxygen from air. The electrolyte is typically a concentrated solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) in liquid or gel form.
Zinc/air batteries and fuel cells are commercially appealing for several reasons. Zinc is an attractive anode material because it is abundant, has a low equivalent weight, has a low standard reduction potential in the electrochemical series, and is environmentally favorable compared to alternatives like cadmium. A zinc/air battery or fuel cell can have a relatively small weight and volume because a reactant, oxygen, can be obtained from atmospheric air instead of being stored for use.
Zinc/air fuel cells and mechanically rechargeable batteries can be replenished by adding zinc and by either replacing the electrolyte, which accumulates reaction products during cell operation, or by removing dissolved reaction products from the electrolyte.
In a zinc/air cell, the anodic reaction is commonly written as:
Zn+4OH−→Zn(OH)42−+2e− (1)
In concentrated alkaline electrolytes, the tetrahydroxozincate ion (Zn(OH)42) is highly soluble. It is commonly referred to as the zincate ion. Zinc oxide can precipitate by the following reaction:
Zn(OH)42−→ZnO+H2O+2OH (2)
The cathodic reaction is given by:
½O2+H2O+2e−→2OH− (3)
Anodically dissolved zinc can form supersaturated solutions with concentrations well beyond the equilibrium concentration in alkaline solutions (see e.g., F. R. McLarnon and E. J. Cairns, The Secondary Alkaline Zinc Electrode, Journal of the Electrochemical Society, Vol. 138, Issue 2, p. 645). Electrolyte additives, such as silicate salts, can be used to stabilize the supersaturated solutions and retard zinc oxide precipitation. Details about the differences between supersaturated and undersaturated zincate solutions in alkaline electrolytes are described in C. Debiemme-Chouvy, J. Vedel, M. Bellissent-Funel, and R. Cortes, Supersaturated Zincate Solutions: A Structural Study, Journal of the Electrochemical Society, Vol. 142, No. 5, May 1995, p. 1359.
The high solubility of the zincate ion in alkaline solutions causes longevity and reliability problems in secondary zinc/air batteries. The issues of zinc dendrite formation, which can cause cell shorting, and anode shape change due to preferred locations for the deposition of zinc, are well known in the field. One attempted solution is to use a solid-phase material that can remove tetrahydroxozincate ions from solution by chemical reaction. Calcium hydroxide is often preferred as the material for scavenging zincate ions. Calcium hydroxide can react with the soluble zincate ion to form calcium zincate, a solid phase with low solubility in alkaline electrolytes, by the following reaction:
Ca(OH)2+2Zn(OH)42−+2H2O→Ca(OH)2.2Zn(OH)2.2H2O+4OH− (4)
The solid phase is also referred to as a zincate, and it is common practice to refer to the solid phase by its full name (e.g., ‘calcium zincate’ or ‘magnesium zincate from reaction with magnesium hydroxide’) to avoid confusion with the soluble zincate ion.
Calcium hydroxide powder is often incorporated directly into the negative electrode along with zinc, binders, and other materials. U.S. Pat. No. 4,358,517 discusses using a certain ratio of calcium hydroxide to zinc active material for a nickel/zinc secondary battery for this purpose. U.S. Pat. No. 5,863,676 advocates using calcium zincate, the material formed by the reaction of zincate ions with calcium hydroxide, directly as the active material in a secondary battery. U.S. Pat. Nos. 3,873,367 and 3,516,862 describe using calcium hydroxide for these purposes in sealed, electrically-rechargeable cells. U.S. Pat. Nos. 3,516,862; 2,180,955; 3,497,391; and 3,873,367 discuss integrating calcium hydroxide in sealed zinc batteries. U.S. Pat. No. 3,873,367 discusses the use of magnesium hydroxide in addition to calcium hydroxide. U.S. Pat. No. 4,054,725 discusses using calcium hydroxide within a zinc/air battery to remove carbonate ions introduced as carbon dioxide from unscrubbed air is fed through the air cathode and dissolved into the electrolyte.
Zinc/air fuel cells and mechanically rechargeable batteries have electrolyte-related challenges. If the zinc and air reactants can be supplied continuously to a fuel cell, the only limitation in operating time will be the degradation of electrolyte performance as reaction products accumulate in the electrolyte. The reaction that generates zincate ions from anodically dissolved zinc consumes hydroxide ions, which adversely impacts fuel cell performance by lowering the ionic conductivity of the electrolyte and increasing concentration polarization. If the cell conditions and electrolyte chemistry allow for zinc oxide precipitation, the precipitation reaction will release hydroxide ions but may cause other problems. Precipitated zinc oxide can lower electrical conductivity by coating metallic particles and current collectors, clogging pores in electrodes and separators, and affecting components in systems with circulating electrolytes. The electrolyte will eventually need to be replaced or regenerated because of the accumulation of reaction products. The electrolyte can be regenerated by plating dissolved zinc, but this is not possible or desirable for all systems and applications.
Despite the work that has been done in this field, there remains a need for practical ways to extend the useful electrolyte life and/or improve the performance characteristics of zinc/air fuel cells and mechanically rechargeable batteries.
The present invention has a number of aspect. One aspect of the invention provides zinc/air systems such as primary batteries, fuel cells, and/or mechanically rechargeable batteries that use continuously or intermittently circulating alkaline solutions as an electrolyte. Other aspects of the invention relate to methods for operating and/or methods for maintaining zinc/air primary batteries, fuel cells, and/or mechanically rechargeable batteries.
An example aspect of the invention provides a method for operating a zinc/air system. The system comprises a first zinc-containing electrode; a second gas-diffusion electrode; and an alkaline electrolyte. The method comprises circulating the electrolyte and allowing the circulating electrolyte to contact a zincate-trapping material at a location apart from the first electrode.
Another example aspect of the invention provides a zinc/air electrochemical system. The system comprises a first zinc-containing electrode; a second gas-diffusion electrode; an alkaline electrolyte; and, a zincate-trapping material in contact with the alkaline electrolyte and spaced apart from the first electrode. The system may be, for example, a fuel cell, a primary or secondary battery or the like.
Another example aspect provides an assembly for use in remediating an alkaline electrolyte in a zinc/air electrochemical system. The assembly comprises a zincate-trapping material contained within an electrolyte-permeable enclosure.
Certain embodiments provide methods for the entrapment of dissolved zincate ions into a solid phase. In some embodiments, zincate-trapping material is external to the anode. In some embodiments the zincate-trapping material is outside of the electrochemical cell area. In some embodiments spent zincate-trapping material may be removed and replaced with new trapping zincate-material.
In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following description and claims.
The attached drawings illustrate non-limiting example embodiments of the invention.
Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
Example embodiments of the invention provide ways to remove zincate ions from the electrolyte in zinc/air fuel cells and mechanically rechargeable batteries that use circulating alkaline electrolytes. This description describes example zincate-trapping materials (which may be called ‘zincate scavengers’), example physical forms for the trapping materials, example zinc/air systems and example methods to incorporate zincate-trapping materials in zinc/air systems having circulating electrolytes.
Calcium hydroxide is a suitable material to address electrolyte longevity and performance problems related to electrolyte conductivity, density, concentration polarization of the electrodes, and zinc oxide precipitation in zinc/air fuel cells and mechanically rechargeable batteries. Full or partial removal of zincate ions, which are produced by the anodic dissolution of the zinc anode, can increase the electrolyte conductivity, lower the electrolyte density, and reduce electrode polarization. Further, if desired to operate the fuel cell or battery without zinc oxide precipitation and with or without precipitation-inhibiting electrolyte additives, the removal of zincate ions by the scavenging material can keep the zincate concentration below the threshold for zinc oxide precipitation. Hydroxides and oxides of other alkali earth metals, such as magnesium hydroxide and barium hydroxide, may also be used as zincate-trapping materials. A zincate-trapping material may also be provided in the form of an oxide of calcium or another suitable alkali earth metal. Calcium oxide, for example, undergoes spontaneous hydration in water to form the calcium hydroxide.
The zincate-trapping material comprises calcium in some embodiments. In some embodiments the material comprises one or more of:
Calcium hydroxide is a suitable material for scavenging zincate and has a number of desirable characteristics which may include:
As a demonstration that calcium hydroxide is effective for removing zincate ions from electrolytes used zinc/air fuel cells, calcium hydroxide powder with a mean particle size of approximately 2 microns was added to an exhausted electrolyte from a zinc/air fuel cell and agitated. The electrolyte capacity was approximately 100 Ah/L with an originally 30 wt % KOH electrolyte. Subsequently, solids were collected by filtering the electrolyte after 2 days at room temperature. A sample of the collected material was analyzed by x-ray diffraction. The analysis confirmed that the material was primarily calcium zincate. All diffraction lines greater than 2% relative intensity were indexed to calcium zincate, indicating that the calcium hydroxide conversion to calcium zincate was nearly total. No significant amounts of calcium hydroxide, zinc hydroxide, or zinc oxide were detected in the collected material.
More details about the properties and reactions of calcium hydroxide in zincate-containing alkaline electrolytes are described in the references Y. Wang and G. Wainwright, Formation and Decomposition Kinetic Studies of Calcium Zincate in 20 w/o KOH, Journal of the Electrochemical Society, Vol. 133, No. 9, p. 1869, September 1986, and R. A. Sharma, Physico-Chemical Properties of Calcium Zincate, Journal of the Electrochemical Society, Vol. 133, No. 11, p. 2215, November 1986.
The physical form of the zincate-trapping material can facilitate efficient removal of zincate ions from the electrolyte. Ideally, all of the provided zincate-trapping material (calcium hydroxide for example) is available to be converted to an insoluble zincate-containing reaction product (calcium zincate for example). The availability of zincate-trapping material to trap zincate can be enhanced by providing the zincate-trapping material in a form that provides a relatively high surface area to volume ratio and which discourages the zincate-trapping material from consolidating, packing, or “cementing” in a manner which blocks access by electrolyte to some of the zincate-trapping material.
Where zincate-trapping material is provided in the form of large particles then it is possible that the only that portion of the zincate-trapping material in an outer shell of the particles may be available to trap zincate from an electrolyte. Zincate-trapping material in interior parts of the particles may be shielded from contact with the electrolyte by the surrounding outer shell. Also, it has been reported that calcium hydroxide particles can be passivated by a layer of calcium carbonate, which may be formed by a reaction of calcium hydroxide with carbonate ions. Finally, testing with an unagitated mass of settled particles has shown that the layer of particles in contact with the electrolyte can develop a skinned-over layer of reaction product that prevents good electrolyte circulation and contact with particles underneath the layer of reaction product.
In a flowing electrolyte system the zincate-trapping material may be physically isolated from the zinc electrode and may even be outside of an electrolyte circulation path of the operating zinc/air system.
Approaches for incorporating zincate-trapping material in a system such as a cell or stack having a flowing electrolyte include providing the zincate-trapping material in the form of a loose powder and confining the powder in a desired volume within the system. The loose powder may be agitated to promote electrolyte contact and to prevent cementation. A permeable barrier may be provided to keep a powder or other particles confined to a particular location in a system. The permeable barrier may comprise, for example, a porous polypropylene mesh, an electrolyte-permeable membrane, a sack, an apertured plate, a suitable filter material or the like.
Another approach involves providing a zincate-trapping material in an engineered form in which the zincate-trapping material is fixed.
In the embodiments that follow, calcium hydroxide is described as the zincate-ion trapping material, but any other suitable zincate-trapping material or materials could also be used.
Non-limiting example embodiments which provide zincate-trapping materials in the form of loose particles, such as powders include the following:
Non-limiting example embodiments which involve engineered forms of zincate-trapping material include the following:
An electrolyte 15, such as an aqueous potassium hydroxide solution, is present in space 16 between gas-diffusion electrode 14 and zinc anode 12. Electrolyte 15 is in contact with gas-diffusion electrode 14 and zinc anode 12. Electrolyte 15 is circulated from an electrolyte reservoir 18 through space 16 and back to reservoir 18 by circulation pump 19.
Fuel cell 10 has a potential difference between zinc anode 12 and gas-diffusion electrode 14. The potential difference can drive an electrical current through an external circuit including a load L. As fuel cell 10 operates, zinc metal from zinc anode 12 becomes dissolved in electrolyte 15. The dissolution of zinc into electrolyte 15 causes the composition and properties of electrolyte 15 to change. These changes affect the performance of fuel cell 10.
The zinc loading in the electrolyte can be represented as an electrolyte capacity. The electrolyte capacity may be defined in units of Ah/L. As the electrolyte capacity increases, the voltage produced by the fuel cell decreases when operating at a fixed current. At some point, the performance of the fuel cell will degrade to the point that the electrolyte will need to be replaced. The maximum electrolyte capacity before the electrolyte is considered exhausted depends on the electrolyte composition, fuel cell operating conditions, and the maximum acceptable decrease in performance. As an example, a 45 wt % potassium hydroxide electrolyte may need to be changed at 200 Ah/L for the fuel cell to continue delivering power exceeding the minimum acceptable power.
Zincate ions produced by the anodic dissolution of zinc metal may precipitate out of the solution in the form of zinc oxide. Such precipitation can cause various problems, including the following:
If sufficient zinc is provided at zinc anode 12, the run-time of the fuel cell 10 is limited by the volume of electrolyte 15. The run time may be extended by increasing the volume of electrolyte 15, but this increases the weight and volume of fuel cell 10.
System 20 may comprise a fuel cell or battery arranged in any suitable manner. In some non-limiting example embodiments, the fuel cell or battery has:
Some ways to incorporate a zincate-trapping material such as calcium hydroxide in a zinc/air system include:
Assembly 22A is provided within electrolyte reservoir 18. Assembly 22B is provided in-line in an inlet line 21 to deliver electrolyte 15 to reservoir 18. Assembly 22C is provided in-line in an outlet line 23 that delivers electrolyte 15 from reservoir 18. Assembly 22D is disposed in a cap 24 that closes an opening into electrolyte 18. Assembly 22E is disposed in a loop 25 through which electrolyte is pumped by pump 26. It can be appreciated that, in a range of embodiments of the invention, the assembly 22 that removes zincate from the electrolyte 15 is disposed in a location such that the main flow of electrolyte to and from the assembly in which zinc anode 12 is located is not required to pass through assembly 22.
Embodiments in which the scavenging material is provided in the form of a loose powder or other loose particles may include hardware, such as a mechanical stirrer, to agitate the powder and prevent settling. In some embodiments, a mechanical stirrer or agitator is actuated by a flow of electrolyte. In some embodiments which include a mechanical stirrer or agitator the mechanical stirrer or agitator is driven by a motor, actuator or the like.
In the illustrated embodiment, wall portions 32 are provided by perforated walls (for example, screens, perforated plates, or the like) at each end of assembly 22B. The wall portions constitute electrolyte-permeable barriers and keep pellets 33A inside section 34. Fluid-tight connectors 37 are provided to connect assembly 22B in-line carrying a flow of electrolyte 15.
Electrolyte 15 can flow through section 34 and, in doing so, contacts pellets 33A. Pellets 33A react with zincate from electrolyte 15. Where pellets 33A comprise pellets of calcium hydroxide, over time, pellets 33A become partially or entirely converted to calcium zincate. Assemblies 22 are designed to accommodate any increase in volume as the zincate-trapping material reacts with zincate ions in electrolyte 15.
Assemblies 22 may be field-replaceable. In fuel cell system 20 of
The following example demonstrates the effectiveness of using a zincate-trapping material in a zinc/air fuel cell having a configuration similar that shown in
The principles discussed herein can be applied to make significant reductions in electrolyte volume and mass for a system providing a desired level of performance. For example, assume an electrolyte comprising 45 wt % KOH reached its useful capacity limit at 200 Ah/L (note that this limit is just an example because practical limits are affected by the zinc/air system design and the operating conditions). Based on mass, fully utilized calcium hydroxide is 10.3 times more efficient at trapping an equivalent amount of zincate than 45 wt % KOH at a capacity of 200 Ah/L.
Treatment tank 36 has one or more assemblies 22. In the illustrated embodiment, the assemblies are provided as follows:
Zinc may be recovered from used assemblies 22 in various ways. For example, zincate ions may be allowed to enter a solution from which zincate may be recovered by electroplating. The solution may comprise a potassium hydroxide solution, for example. As the soluble zincate concentration drops below saturation during zinc plating, the calcium zincate in assemblies 22 will release zincate ions and convert back to calcium hydroxide. Alternative options to recover zincate from calcium zincate include concentrating the electrolyte above the calcium zincate stability limit, as described in R. A. Sharma, Physico-Chemical Properties of Calcium Zincate, Journal of the Electrochemical Society, Vol. 133, No. 11, p. 2215, November 1986.
In some embodiments, assemblies 22 may be regenerated in situ. For example, in system 30 as shown in
The chemical reactions that occur during the operation of a fuel cell can result in changes in the concentration of hydroxyl ions in electrolyte 15. For example, while calcium zincate formation tends to concentrate electrolyte 15, zinc dissolution tends to dilute electrolyte 15. If necessary or desired, an active system for managing electrolyte concentration by adding water and/or sodium or potassium hydroxide may be provided.
In some embodiments, calcium hydroxide in assemblies 22 removes both zincate ions and dissolved carbon dioxide in the form of carbonate ions from electrolyte 15. It is usually preferable to remove carbon dioxide from incoming air before it comes in contact with electrolyte.
Some embodiments provide a means for signaling to a user, such as a maintenance person, when the zincate-trapping material is spent. For example, a fuel cell system as described herein may provide the following:
It can be appreciated that embodiments of the invention may provide various advantages over conventional zinc/air fuel cells or mechanically rechargeable batteries, such as the following:
Selected embodiments as discussed herein apply materials that can react with zincate ions in solution to extend the useful life of an electrolyte and improve the electrolyte performance characteristics. In such embodiments removing zincate ions from the electrolyte promotes a high electrolyte conductivity and low concentration of zincate ions.
Where a component (e.g., a pump, reservoir, assembly, device, conductor, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example, the following are possible:
This application claims priority from U.S. patent application No. 60/926,618 filed on 27 Apr. 2007 and entitled ELECTROLYTE REMEDIATION IN ZINC-AIR FUEL CELLS. For purposes of the United States of America, this application claims the benefit under 35 U.S.C. §119 of U.S. patent application No. 60/926,618 filed on 27 Apr. 2007, which is hereby incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US08/05334 | 4/25/2008 | WO | 00 | 3/30/2010 |
| Number | Date | Country | |
|---|---|---|---|
| 60926618 | Apr 2007 | US |
