Patent Application
20150093657
The present disclosure relates to a direct aluminum fuel cell and an electronic device. More specifically, the present disclosure relates to a direct aluminum fuel cell that is designed to generate electricity by using an anode including an aluminum-containing material as a fuel and using oxygen in the air as a cathode active material, and also relates to various types of electronic devices including such a direct aluminum fuel cell.
An air cell (also called metal-air cell), which uses a high-energy-density metal as an anode active material and uses oxygen in the air as a cathode active material, can operate with a half cell configuration, which makes it possible to reduce the amount of electrode active material to half. Theoretically, such an air cell can attain a high energy density. An aluminum-air cell using an aluminum anode is known as a type of such an air cell and expected to be a large-capacity cell (see, for example, Patent Documents 1 to 3).
To increase the power of conventional aluminum-air cells, investigations have been carried out in which alkaline solutions are used as electrolytes to improve cathode reaction.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-184472
Patent Document 2: Japanese Patent Application Laid-Open No. 2006-147442
Patent Document 3: Japanese Patent Application Laid-Open No. 2012-15025
Patent Document 4: Japanese Patent Application Laid-Open No. 2008-273816
However, conventional aluminum-air cells using alkaline solutions as electrolytes have problems such as alkaline electrolyte-induced severe corrosion of aluminum and cathode degradation which occurs as the alkaline electrolyte is gradually neutralized by absorbing carbon dioxide in the air.
It is therefore an object of the present disclosure to provide a direct aluminum fuel cell in which aluminum as an anode component used as a fuel is prevented from being corroded and the cathode is also prevented from being degraded, and to provide an electronic device having such a direct aluminum fuel cell.
To achieve the object, a first mode of the present disclosure is directed to a direct aluminum fuel cell including: an anode including an aluminum-containing material; a cathode capable of reducing oxygen under neutral or near-neutral conditions; and an electrolytic solution with a pH of 3 to 10, wherein the electrolytic solution contains a buffer substance.
To achieve the object, a second mode of the present disclosure is directed to a direct aluminum fuel cell including: an anode including an aluminum-containing material; and a cathode capable of reducing oxygen under neutral or near-neutral conditions.
To achieve the object, the present disclosure is also directed to an electronic device including at least one piece of the direct aluminum fuel cell according to the first or second mode of the present disclosure.
In the direct aluminum fuel cell according to the first or second mode of the present disclosure or in the direct aluminum fuel cell, according to the first or second mode of the present disclosure, installed in the electronic device of the present disclosure (hereinafter, these direct aluminum fuel cells are also generically referred to as “the direct aluminum fuel cell of the present disclosure and the like”), the buffer substance preferably has a pKa of 4 to 10, which makes it possible to keep the electrolytic solution at a pH of 3 to 10.
Basically, the buffer substance may be of any type having a pKa of 4 to 10, which may be selected as needed. The content of the buffer substance in the electrolytic solution is preferably 0.2 mol or more per liter of the electrolytic solution. The upper limit to the content of the buffer substance in the electrolytic solution may be the maximum solubility of the buffer substance in the electrolytic solution. The content of the buffer substance in the electrolytic solution is more preferably close to the maximum solubility of the buffer substance in the electrolytic solution. In the direct aluminum fuel cell of the present disclosure and the like including those of such a preferred mode, the electrolytic solution may contain halide ions. In this case, the halide ions are preferably chloride ions. In the direct aluminum fuel cell of the present disclosure and the like including those of the preferred mode stated above, the cathode may be designed to be capable of reducing oxygen under conditions with a pH of 3 to 10.The cathode may be an immobilized enzyme electrode including an electrode and an enzyme immobilized thereon.
Alternatively, the cathode may be an electrode including a material capable of reducing oxygen, such as carbon (including charcoal), metal, carbon and metal, or a catalyst (specifically, an oxygen-reducing catalyst or a catalyst capable of reducing oxygen). Alternatively, the cathode may be an electrode containing a catalyst capable of allowing oxygen to undergo four-electron reduction under neutral or near-neutral conditions (for example, the catalyst may be an enzyme). Typically, the direct aluminum fuel cell has a separator between the anode and the cathode. The shape of the anode may be selected as needed. For example, the anode may have a foil shape, a sheet shape, or a plate shape. To increase the contact area between the anode and oxygen, if necessary, at least part of the anode, preferably almost the whole or the whole of the anode may be mesh-shaped or made of a porous material. In a typical example, the anode is made of an aluminum foil. If necessary, the anode may be exchangeably provided. Preferably, the direct aluminum fuel cell is so configured that an insoluble material as a by-product can be removed simultaneously with the exchange of the anode.
Basically, the electronic device may be of any type. The electronic device may be any of portable and stationary devices, examples of which include cellular phones, mobile devices, robots, personal computers, game machines, camera-integrated VTRs (video tape recorders), on-vehicle devices, a variety of home electric appliances, industrial products, and other products.
According to the present disclosure, oxygen is reduced at the cathode under neutral or near-neutral conditions, which makes it possible to prevent self-corrosion of aluminum and to prevent the problem of the degradation of the cathode in contrast to cases where alkaline solutions are used as electrolytes. Therefore, the present disclosure makes it possible to prevent the corrosion of aluminum as an anode component used as a fuel and also to provide a novel direct aluminum fuel cell in which the degradation of the cathode is prevented. The use of this advantageous direct aluminum fuel cell makes it possible to provide high-performance electronic devices and other products.
Hereinafter, the present disclosure will be described based on examples, which however are not intended to limit the present disclosure and in which different values and materials are by way of example only. Examples will be described in the following order.
The aluminum-containing material used to form the anode 11 is preferably, for example, a material including aluminum as a main component, such as elemental aluminum or any of various aluminum alloys. The anode 11 may be in any shape such as a foil, a sheet, or a plate, and may also be in any form such as a bulk, mesh, or porous form.
The cathode 12 is capable of reducing oxygen under neutral or near-neutral conditions, for example, at a pH of 3 to 10, preferably at a pH of 3 to 9, more preferably at a pH of 3 to 8. Specifically, for example, the cathode 12 may be an immobilized enzyme electrode having an immobilized oxygen-reducing enzyme. Examples of the oxygen-reducing enzyme include, but are not limited to, bilirubin oxidase, laccase, and ascorbate oxidase. In addition to the oxygen-reducing enzyme, an electron mediator capable of transferring electrons between the oxygen-reducing enzyme and the cathode 12 is preferably immobilized on the cathode 12. Examples of the electron mediator include, but are not limited to, potassium hexacyanoferrate and potassium octacyanotungstate.
At the cathode 12, oxygen in the air is reduced in the presence of the oxygen-reducing enzyme by electrons from the anode 11 and protons (H+) from the separator 13 filled with the electrolytic solution to produce water. The separator 13 may be, for example, made of a porous membrane or a nonwoven fabric of polyethylene, polypropylene, or other materials. Examples of materials for the nonwoven fabric include, but are not limited to, various types of organic polymer compounds such as polyolefin, polyester, cellulose, and polyacrylamide.
The collector 14 typically includes a metal mesh. The metal mesh may be made of any material capable of withstanding the environment in which the direct aluminum fuel cell is used. Examples of such a material generally include titanium (Ti), nickel (Ni), and stainless steel (e.g., SUS 304). The pore size and other features of the metal mesh are not restricted and may be selected as needed. The collector 15 is designed to be permeable to the electrolytic solution. The collector 15 typically includes a metal mesh like the collector 14.
The electrolytic solution used preferably has a pH of 3 to 10. The electrolytic solution contains a buffer substance. In this example, the buffer substance has a pKa of 4 to 10. Examples of the buffer substance include citric acid, ammonium chloride, phosphoric acid, trishydroxymethylaminomethane, imidazole ring-containing compounds, dihydrogen phosphate ion (H2PO4−), 2-amino-2-hydroxymethyl-1,3-propanediol (abbreviated as Tris), 2-(N-morpholino)ethanesulfonic acid (MES), cacodylic acid, carbonic acid (H2CO3), hydrogen citrate ion, N-(2-acetoamido)iminodiacetic acid (ADA), piperazine-N,N′-bis(2-ethanesulfonic acid) (PIPES), N-(2-acetoamido)-2-aminoethanesulfonic acid (ACES), 3-(N-morpholino)propanesulfonic acid (MOPS), N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES), N-2-hydroxyethylpiperazine-N′-3-propanesulfonic acid (HEPPS), N-[tris(hydroxymethyl)methyl]glycine (abbreviated as Tricine), glycylglycine, and N,N-bis(2-hydroxyethyl)glycine (abbreviated as Bicine). Examples of substances capable of producing dihydrogen phosphate ions (H2PO4) include sodium dihydrogen phosphate (NaH2PO4) and potassium dihydrogen phosphate (KH2PO4). Examples of imidazole ring-containing compounds include imidazole, triazole, pyridine derivatives, bipyridine derivatives, and imidazole derivatives (histidine, 1-methylimidazole, 2-methylimidazole, 4-methylimidazole, 2-ethylimidazole, ethyl imidazole-2-carboxylate, imidazole-2-carboxaldehyde, imidazole-4-carboxylic acid, imidazole-4,5-dicarboxylic acid, imidazol-1-yl-acetic acid, 2-acetylbenzimidazole, 1-acetylimidazole, N-acetylimidazole, 2-aminobenzimidazole, N-(3-aminopropyl)imidazole, 5-amino-2-(trifluoromethyl)benzimidazole, 4-azabenzimidazole, 4-aza-2-mercaptobenzimidazole, benzimidazole, 1-benzylimidazole, and 1-butylimidazole). If necessary, the electrolytic solution may contain, as a neutralizer, at least one acid, for example, selected from the group consisting of hydrochloric acid (HCl), acetic acid (CH3COOH), phosphoric acid (H3PO4), and sulfuric acid (H2SO4) in addition to the buffer substance.
The buffer substance-containing electrolytic solution may also contain, for example, a substance containing a halide ion (such as a chloride ion, a bromide ion, an iodide ion, or a fluoride ion). For example, when a chloride ion-containing substance is added to the buffer substance-containing electrolytic solution, the chloride ion-containing substance may be NaCl, KCl, or the like. The electrolytic solution to be used may also contain an ionic liquid. Any conventionally known ionic liquid may be used, which may be selected as needed.
A gas-liquid separation membrane may be used to form a vessel for containing the electrolytic solution. As a non-limiting example, a polytetrafluoroethylene (PTFE) membrane is preferably used to form the vessel.
The shape of the vessel is selected as needed. The cathode, the anode, the separator, the electrolytic solution, and other components may be housed in a battery case (vessel), which may be in the form of a coin, a flat plate, a cylinder, a laminate, or other forms. The battery case may be of an open-to-atmosphere type having a structure that allows at least the cathode to sufficiently contact with the atmosphere. Alternatively, the battery case may be of a closed type having a gas (air)-introducing duct and an exhaust duct.
The anode 11 is formed by a conventionally known method depending on the material used. Subsequently, the collector 14 is electrically connected to the upper surface of the anode 11. On the other hand, the cathode 12 with an immobilized oxygen-reducing enzyme is formed by dipping the cathode 12 into an enzyme solution containing a dissolved oxygen-reducing enzyme or applying the enzyme solution to the cathode 12. Subsequently, the collector 15 is electrically connected to the upper surface of the cathode 12. The separator 13 is then sandwiched between the anode 11 and the cathode 12, and the anode 11, the cathode 12, and the separator 13 are immersed in the electrolytic solution 17. Thus, the direct aluminum fuel cell shown in
At least one of the anode 11 and the cathode 12 may be integrally formed with the separator 13 so that the manufacturing process can be simplified, the anode 11 or the cathode 12 can have higher mechanical strength, or the movement of protons between the anode 11 and the cathode 12 can be facilitated. The anode 11 and the cathode 12 may be integrally formed with the separator 13. In this case, one of the anode 11 and the cathode 12 may be formed on one surface of the separator 13, and the other of the anode 11 and the cathode 12 may be formed on the other surface of the separator 13.
In this direct aluminum fuel cell, the reactions represented by formulae (1) to (3) below occur at the anode 11 during power generation. Formula (4) is obtained from formulae (2) and (3).
Al→Al3++3e− (1)
Al3++6H2O→[Al (H2O)6]3+ (2)
[Al(H2O)6]3+→[Al(OH)6]3−+6H+ (3)
Al3++6H2O→[Al(OH)6]3−+6H+ (4)
In this process, Al3+ moves from the anode 11 to the cathode 12 through the separator 13 so that electrical energy is generated. At the cathode 12, oxygen in the air is reduced in the presence of an oxygen-decomposing enzyme by electrons from the anode 11 and H+ from the separator 13 filled with the electrolytic solution 17 to produce water.
As is apparent from formula (4), protons are accumulated on the surface of the anode 11, and therefore, unless any measures are taken, the pH at the surface of the anode 11 will decrease so that the aluminum can undergo self corrosion to promote the production of hydrogen gas. In Example 1, however, the electrolytic solution 17 contains a buffer substance with a pKa of 4 to 10, which acts to keep the pH at the surface of the anode 11 neutral or near-neutral (for example, to keep the pH at 3 to 10), so that the aluminum is prevented from undergoing self corrosion and from promoting the production of hydrogen gas. Specifically,
The direct aluminum fuel cell was prepared as described below. Specifically, a square aluminum foil with a dimension of 10 mm×10 mm×12 μm was used as the anode 11. A titanium mesh was electrically connected as the collector 14 to the aluminum foil. An immobilized enzyme electrode including a porous carbon electrode with a dimension of 10 mm×10 mm×2 mm and bilirubin oxidase (BOD) immobilized as an oxygen-reducing enzyme on the carbon electrode was used as the cathode 12. A titanium mesh was electrically connected as the collector 15 to the immobilized enzyme electrode. The separator 13 made of a nonwoven fabric was then sandwiched between the anode 11 and the cathode 12. The resulting laminate of the anode 11/separator 13/cathode 12 was placed in the vessel 16 made of a PTFE membrane, and the vessel 16 was charged with the electrolytic solution 17 and then sealed. The electrolytic solution 17 was a 3 mol/liter NaCl solution containing imidazole as a buffer substance and HCl as a neutralizer and having a pH of 7. The content of the buffer substance was 2 mol per liter of the electrolytic solution.
The output characteristics of the prepared direct aluminum fuel cell were measured at a cell voltage of 0.7 V.
Hereinafter, the advantageous effect of the electrolytic solution 17 including NaCl will be described.
As described above, the direct aluminum fuel cell of Example 1 uses an aluminum-containing material as the anode 11 serving as a fuel and also uses, as the cathode 12, an immobilized enzyme electrode capable of decomposing oxygen under neutral or near-neutral conditions, which makes it possible to prevent the corrosion of the aluminum of the anode 11 used as a fuel and to prevent the degradation of the cathode 12. This direct aluminum fuel cell is also advantageous in that it is safe because the electrolytic solution 17 is neutral or near-neutral.
Example 2 is a modification of Example 1. In Example 2, citric acid was used as the buffer substance in the electrolytic solution 17 instead of imidazole. The content of the buffer substance was 1 mol per liter of the electrolytic solution. The direct aluminum fuel cell was prepared as in Example 1, except for these features. The output characteristics of the direct aluminum fuel cell of Example 2 were measured. The results of the measurement were similar to those of Example 1, but after the power generation was ended, an insoluble by-product aluminum hydroxide did not remain in the electrolytic solution 17.
Example 3 is another modification of Example 1. A direct aluminum fuel cell of Example 3 has a cathode 12 different from that of Example 1. Specifically, for example, the cathode 12 includes an electrode material capable of reducing oxygen, or the cathode 12 includes an electrode including an electrode material and a catalyst capable of reducing oxygen, which is supported on the electrode material. The electrode material may be, for example, a carbon material or a metal material.
In this example, when the cathode 12 includes a carbon material, the carbon material may be in the form of at least one selected from the group consisting of carbon particles, a carbon sheet, and carbon fibers. For example, carbon particles include at least one selected from the group consisting of activated carbon, carbon black, and biocarbon. Alternatively, carbon particles may be other than these. Examples of activated carbon include wood charcoal such as oak charcoal, sawtooth oak charcoal, cedar charcoal, Japanese oak charcoal, or cypress charcoal, rubber charcoal, bamboo charcoal, charcoal from wood briquettes, and coconut shell charcoal.
Examples of carbon black include furnace black, acetylene black, channel black, thermal black, and Ketjen black. In particular, Ketjen black is preferred. Biocarbon is a porous carbon material that is made from a plant-derived raw material with a silicon content of 5% by weight or more, has a specific surface area of 10 m2/g or more as measured by nitrogen BET method, has a silicon content of 1% by weight or less, and has a pore volume of 0.1 cm3/g or more as measured by BJH method and MP method (see Patent Document 4). Specifically, for example, biocarbon is produced as follows. First, ground rice hulls
(Isehikari rice hulls produced in Kagoshima prefecture, Japan) were turned into charcoal by heating at 500° C. for 5 hours in a nitrogen stream, so that a charcoal material was obtained. Subsequently, 10 g of the charcoal material was added to an alumina crucible and then heated to 1,000 at a rate of temperature rise of 5° C./minute in a nitrogen stream (10 liter/minute). Subsequently, the material was turned into a carbonaceous material (porous carbon material precursor) by carbonization at 1,000° C. for 5 hours. The carbonaceous material was then cooled to room temperature. The nitrogen gas continued to flow during the carbonization and the cooling. The porous carbon material precursor was then subjected to an acid treatment by being immersed in a 46% by volume hydrofluoric acid aqueous solution overnight, so that SiO2 was removed, or the porous carbon material precursor was then subjected to an alkali treatment by being immersed in an aqueous sodium hydroxide solution overnight, so that SiO2 was removed. Subsequently, the product was washed with water and ethyl alcohol until a pH of 7 was reached. Finally, the product was dried to give a porous carbon material, namely, biocarbon.
Alternatively, the cathode 12 may include a metal material. In this case, the metal material may be, for example, at least one simple metal selected from the group consisting of cobalt (Co), iridium (Ir), platinum (Pt), silver (Ag), gold (Au), ruthenium (Ru), rhodium (Rh), osmium (Os), niobium (Nb), molybdenum (Mo), indium (In), zinc (Zn), manganese (Mn), iron (Fe), titanium (Ti), vanadium (V), chromium (Cr), palladium (Pd), rhenium (Re), tantalum (Ta), tungsten (W), zirconium (Zr), germanium (Ge), and hafnium (Hf), or an alloy thereof.
The catalyst may be made of, for example, any of a variety of inorganic ceramics such as manganese dioxide (MnO2) (such as electrolytic manganese dioxide (EMD)), tricobalt tetraoxide (Co3O4), nickel oxide (NiO), iron(III) oxide (Fe2O3), ruthenium(IV) oxide (RuO2), copper(II) oxide (CuO), vanadium pentaoxide (V2O5), molybdenum(VI) oxide (MoO3), yttrium(III) oxide (Y2O3), and iridium(IV) oxide (IrO2). Alternatively, the catalyst may be made of, for example, at least one material selected from the group consisting of various precious metals such as gold (Au), platinum (Pt), and palladium (Pd), transition metal oxides, organometallic complexes and polymers thereof (specifically, for example, transition metal porphyrin, phthalocyanine, polymerized porphyrin obtained by polymerization of transition metal porphyrin, and polymerized phthalocyanine obtained by polymerization of phthalocyanine), perovskite, and a product of thermal decomposition of a cobalt salt and polyacrylonitrile. Alternatively, the catalyst may be made of any of other materials including LaBO3 (B: Mn, Co) perovskite oxides, nitrides, or sulfides, and multicomponent perovskite oxides such as La1-xA′xCo1-yFeyO3, wherein A′ is Sr or Ca, and x and y are each from 0.2 to 0.5.
The direct aluminum fuel cell of Example 3 may be the same as the direct aluminum fuel cell of Example 1, except that the cathode 12 has the different features.
The direct aluminum fuel cell of Example 3 was prepared as described below. Specifically, a square aluminum mesh with a dimension of 10 mm×10 mm×170 μm was used as the anode 11. A titanium mesh was electrically connected as the collector 14 to the aluminum mesh. The cathode 12 was prepared by applying biocarbon to the separator 13 made of a nonwoven fabric with a dimension of 10 mm×10 mm×200 μm. A titanium mesh was electrically connected as the collector 15 to the cathode 12. The aluminum mesh anode 11 and the biocarbon cathode 12 were bonded together with the nonwoven fabric separator 13 placed therebetween. The resulting laminate of the anode/separator/cathode was placed in the vessel 16 made of a PTFE membrane, and the vessel 16 was charged with the electrolytic solution 17 and then sealed. The electrolytic solution 17 used was a 4 mol/liter NaCl aqueous solution with a pH of 7. Phosphoric acid was also added at a final concentration of 1.0 mol to the solution, and the pH of the solution was adjusted to 7 with potassium hydroxide.
The output characteristics of the prepared direct aluminum fuel cell of Example 3 were measured at a cell voltage of 0.7 V.
Example 4 is a modification of Examples 1 to 3. The direct aluminum fuel cell of Example 4 differs from the direct aluminum fuel cells of Examples 1 to 3 in that the anode 11 as a fuel is exchangeably provided.
The used fuel cartridge 32 can be replaced with an unused fuel cartridge 32 by the following operation. Specifically, as shown in the schematic cross-sectional view of
The direct aluminum fuel cell of Example 4 has the same features as those of the direct aluminum fuel cells of Examples 1 to 3, except for the features described above.
While the present disclosure has been described based on preferred examples, it will be understood that these examples are not intended to limit the present disclosure and that various modifications of these examples are possible. The values, structures, configurations, shapes, materials, and other features shown in the examples are only by way of example, and if necessary, values, structures, configurations, shapes, materials, and other features different from the above may also be used. It will also be understood that any two or more of Examples 1 to 4 may be combined.
The present disclosure may have the following features.
[2] The direct aluminum fuel cell according to [1], wherein the buffer substance has a pKa of 4 to 10.
[3] The direct aluminum fuel cell according to [1] or [2], wherein the cathode is capable of reducing oxygen under conditions with a pH of 3 to 10.
[4] The direct aluminum fuel cell according to any one of [1] to [3], wherein the electrolytic solution contains halide ions.
[5] The direct aluminum fuel cell according to [4], wherein the halide ions are chloride ions.
[6] The direct aluminum fuel cell according to any one of [1] to [5], further including an oxygen-reducing enzyme immobilized on the cathode.
[7] The direct aluminum fuel cell according to any one of [1] to [6], wherein the cathode includes carbon or metal or contains a catalyst.
[8] The direct aluminum fuel cell according to any one of [1] to [6], wherein the cathode contains a catalyst capable of allowing oxygen to undergo four-electron reduction under neutral or near-neutral conditions.
[9] The direct aluminum fuel cell according to [8], wherein the catalyst is an enzyme.
[10] The direct aluminum fuel cell according to any one of [1] to [9], further including a separator between the cathode and the anode.
[11] The direct aluminum fuel cell according to any one of [1] to [10], which has a foil shape, a sheet shape, or a plate shape.
[12] The direct aluminum fuel cell according to any one of [1] to [11], wherein at least part of the anode is mesh-shaped or includes a porous material.
[13] The direct aluminum fuel cell according to any one of [1] to [12], wherein the anode includes an aluminum foil.
[14] The direct aluminum fuel cell according to any one of [1] to [13], wherein the anode is exchangeably provided.
[15] The direct aluminum fuel cell according to [14], which is so configured that an insoluble material as a by-product can be removed simultaneously with exchange of the anode.
[16] <<Second mode of direct aluminum fuel cell>>A direct aluminum fuel cell including: an anode including an aluminum-containing material; and a cathode capable of reducing oxygen under neutral or near-neutral conditions.
[17] The direct aluminum fuel cell according to [16], wherein the cathode is capable of reducing oxygen under conditions with a pH of 3 to 10.
[18] <<Electronic device>>An electronic device including at least one piece of the direct aluminum fuel cell according to any one of [1] to [17].
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-107338 | May 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/061586 | 4/19/2013 | WO | 00 |
