Magnesium secondary battery

Abstract

We provide a magnesium secondary battery that functions as a fuel cell with high efficiency and high performance. [Solution] The magnesium secondary battery 10 has a fuel electrode 30, an air electrode 50, and a reaction plate 60 made of an amphoteric metal containing at least magnesium, and an electrochemical reaction occurs between the fuel electrode 30 and the air electrode 50 by adding an alkaline aqueous solution within a reaction chamber 13 surrounded by a ceramic filter 20.

Description

[Technical Field]

[0001] The present invention relates to a magnesium secondary battery. [Background technology]

[0002] Magnesium batteries have been known for some time. For example, Patent Document 1 discloses a magnesium battery including a negative electrode (cathode) body, a positive electrode (anode) body containing magnesium, and a separator interposed therebetween. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-182435 Summary of the Invention [Problem to be solved by the invention]

[0004] The magnesium battery disclosed in Patent Document 1 uses an electrolyte such as saline solution to induce an electrochemical reaction and generate a required electromotive force. However, the electromotive force is only about 1.5 to 2.0 V, and the battery is unable to be used in automobile batteries due to insufficient power. Furthermore, a magnesium oxide coating formed on the surface of the cathode body by polyvalent carboxylate ions inhibits the electrochemical reaction, making it impossible to reuse the cathode body without replacing it; it can only be used for about 4 or 5 cycles, and it is impossible to use it for more than 100 cycles.

[0005] Considering future energy consumption, it has been predicted that fossil fuels such as coal, gasoline, and diesel will run out within the 21st century. Even more serious, the burning of fossil fuels is exacerbating global environmental problems such as global warming and acid rain. Therefore, attention is being focused on clean energy obtained by burning hydrogen, an environmentally friendly clean energy source that does not produce harmful substances such as carbon dioxide and nitrogen oxides when burned. Furthermore, according to estimates by the National Institute of Advanced Industrial Science and Technology (NEDO), running a car on a polymer electrolyte fuel cell requires 32 grams of catalyst per vehicle for a small car (80 kW output) and 60 grams for a medium-sized car, raising concerns about the depletion of platinum as a resource.

[0006] Various types of fuel cells have been developed, but polymer electrolyte fuel cells (PEFCs), which burn hydrogen and cause a chemical reaction between hydrogen and oxygen through a process that is the reverse of the electrolysis of water, to generate electrical energy, are beginning to become popular for use in household power sources and automobiles.While several methods for generating the hydrogen needed for fuel cells have been proposed, attempts are currently being made to generate hydrogen by improving natural gas, coal gas, gasoline, ethanol, etc., and then using that hydrogen as fuel to generate electricity.

[0007] However, many manufacturers in the automotive industry are beginning to identify problems with EVs. For example, 1) fuel cells are expensive, accounting for up to 50% of the EV's price; 2) even minor damage from accidents is often impossible to repair or assess, leading to high insurance premiums; and 3) the amount of fuel cells that end up as waste is increasing. As a result, the original aims of "promoting a circular economy" and "contributing to environmental issues through the spread of sustainable EVs" are becoming less relevant, and there is an urgent need to develop and manufacture low-cost fuel cells that are easier to repair.

[0008] Therefore, there was a need for a magnesium battery that could be used as a secondary battery with high efficiency and high performance (such as the ability to operate at room temperature) as a fuel cell, which could continuously generate electrochemical energy by continuously supplying hydrogen fuel and oxidant from an external source, without using resources that are at risk of depletion.

[0009] An object of the present invention is to provide an improvement over conventional magnesium batteries, which is a magnesium secondary battery that functions as a fuel cell with high efficiency and high performance. [Means for solving the problem]

[0010] In order to solve the above technical problems, the present invention relates to a magnesium secondary battery which has a fuel electrode, an air electrode, and a reaction plate made of an amphoteric metal containing at least magnesium, and in which an electrochemical reaction occurs between the fuel electrode and the air electrode when an alkaline aqueous solution is added in a reaction chamber surrounded by a ceramic filter. [Effects of the Invention]

[0011] The magnesium secondary battery according to the present invention can be used for at least 100 cycles or more, and can therefore be used as a fuel cell that provides a stable power source for a long period of time. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a perspective view of a magnesium secondary battery according to an embodiment of the present invention; [Figure 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Figure 3] 1 is a schematic diagram showing a power generation system using a magnesium secondary battery according to an embodiment of the present invention; [Figure 4] FIG. 2 is an enlarged view of the constituent materials of the fuel electrode according to the present embodiment. [Figure 5] A perspective view of a car that uses magnesium secondary batteries. DETAILED DESCRIPTION OF THE INVENTION

[0013] The embodiments described below are merely examples of devices and methods for embodying the technical idea of ​​the invention, and the technical idea of ​​the present invention is not limited to those described below. The technical idea of ​​the present invention can be modified in various ways within the scope of the claims. In particular, it should be noted that the drawings are schematic and may differ from the actual product. Descriptions of parts that are already known technology have been omitted.

[0014] The magnesium secondary battery 10 includes a lid 11, a main body 12, and a ceramic filter 20, a fuel electrode (anode) 30, a diffusion sheet 40, an air electrode (cathode) 50, and a reaction plate 60 made of an amphoteric metal, all of which are housed in a reaction chamber 13 within the main body 12.

[0015] In this specification, a battery cell 100 refers to an element that is housed in a reaction chamber 13 inside a main body 12 and that is configured to include a ceramic filter 20, a fuel electrode (anode) 30, a diffusion sheet 40, and an air electrode (cathode) 50. In the illustrated example, only one battery cell 100 is shown, but it is preferable that the magnesium secondary battery 10 has a configuration in which current is collected from a plurality of battery cells 100.

[0016] The fuel electrode 30 contacts the ceramic filter 20 and the diffusion sheet 40, decomposes the hydrogen generated in the reaction chamber 13 into hydrogen ions and electrons, and sends the electrons to the air electrode 50 via a conductor. The diffusion sheet 40 is made of a carbon cloth (or carbon paper) composited with a solid polymer electrolyte, and is in contact with the fuel electrode (anode) 30 and the air electrode (cathode) 50 on both sides.

[0017] The diffusion sheet 40 serves to diffuse the hydrogen ions generated at the anode 30 and deliver them evenly to the cathode 50. At the cathode 50, the hydrogen ions sent through the diffusion sheet 40 react with oxygen in the air that has passed through the cathode 50 to produce water. During this reaction, the electrons generated at the anode 30 are consumed, generating electricity.

[0018] In the magnesium secondary battery 10 according to the present invention, hydrogen molecules generated inside are diffused through the ceramic filter 20, decomposed into hydrogen ions and electrons at the fuel electrode 30, the hydrogen ions are diffused through the diffusion sheet 40, and oxidized at the air electrode 50. Hydrogen generation takes place in the reaction chamber 13 surrounded by the cylindrical ceramic filter 20.

[0019] In the reaction chamber 13, hydrogen is generated by adding an alkaline aqueous solution to a reaction plate 60 made of an amphoteric metal. An amphoteric metal is a metal that reacts with both acid and alkali to generate hydrogen, and examples of amphoteric metals include aluminum (AI), zinc (Zn), tin (Sn), lead (Pb), and magnesium (Mg).

[0020] The reaction plate 60 according to the present invention contains magnesium carbon nanotubes in a composition ratio of 40 to 60% and a small composition ratio, for example, about 0.02 to 1.0%, of a thermoplastic synthetic resin. Known materials such as polyethylene terephthalate (PET), polypropylene (PP), and polyethylene (PE) can be used as the thermoplastic synthetic resin, but polypropylene (PP) is preferably used.

[0021] Examples of alkalis that can react with amphoteric metals to generate hydrogen include carbonates such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, magnesium hydroxide, sodium bicarbonate, and potassium carbonate. In particular, sodium hydroxide and potassium hydroxide are suitable for use in the present invention. Also suitable is alkaline electrolyzed water produced by electrolyzing water.

[0022] These alkalis are added dropwise to the amphoteric metals as aqueous solutions. The pH of the acid to be added should preferably be between 9 and 14. This is because, although the reaction to generate hydrogen occurs even at a pH below 9, the product coats the metal, hindering the reaction from proceeding. On the other hand, as the alkali concentration increases, the viscosity of the alkaline water increases, making it difficult to add the alkali. In particular, at pH levels above 14, the reaction occurs rapidly, making it difficult to control.

[0023] The cylindrical ceramic filter 20 that forms the side wall of the reaction chamber 13 can be made of any porous material, regardless of its composition. This porous ceramic filter transports hydrogen generated inside the reaction chamber 13 to the outside of the ceramic filter 20 by utilizing the increase in gas pressure inside the reaction chamber. The cross-sectional shape of the cylinder is circular or polygonal. In the case of a polygon, the corners should be obtuse, and a hexagonal shape is suitable, as it minimizes space loss due to the accumulation of multiple cells. The cylindrical ceramic filter 20 is either formed and fired in-situ, or cut from a long ceramic tube.

[0024] The fuel electrode 30 is formed on the outside of the cylindrical ceramic filter 20. A carbon-based electrode is used for the fuel electrode 30. Specifically, it is manufactured by coating a mixture of powders of activated carbon, carbon black, ketjen, graphite, carbon nanotubes, carbon nanohorns, etc. with a solid polymer electrolyte.

[0025] Known solid polymer electrolytes include fluorine-based polymers with sulfonic groups, such as Nafion (DuPont) and Flemion membrane (Asahi Glass Co., Ltd.), and both are suitable for use in the present invention. To apply carbon-based powder to the ceramic filter 20, these solid polymer electrolytes are dissolved in an organic solvent, and the carbon-based powder is decomposed and applied. As the organic solvent for dissolving the solid polymer electrolyte, a liquid with a boiling point of 70°C to 200°C is used. Specifically, butyl acetate is suitable.

[0026] A conductor (not shown) is connected to the fuel electrode 30. There are no restrictions on the material of this conductor as long as it has good conductivity, but a conductor with low electrical resistance is suitable. The conductor is coated and insulated except for the connection with the fuel electrode 30. There are no particular restrictions on the connection between the conductor and the fuel electrode 30 as long as it has a structure that allows the conductor to be connected to the fuel electrode 30 and conduct electricity, but if the end of the conductor is connected to a metal flake, the flake portion of the conductor can be applied together when applying carbon-based powder to the ceramic filter 20. The thickness of the flake is usually about 0.1 to 0.5 mm.

[0027] A diffusion sheet 40 is attached to the outer periphery of the fuel electrode 30. The diffusion sheet 40 is a sheet for uniformly diffusing hydrogen ions, and is made by combining a solid polymer electrolyte 42 with carbon cloth or carbon paper 41. When the fuel electrode 30 is attached to the ceramic filter 20, the diffusion sheet 40 coated with an organic solution of the solid electrolyte should be attached before the surface dries.

[0028] The air electrode 50 is attached in close contact with the outside of the diffusion sheet 40. The air electrode 50 is a porous tubular member made of a mixture of activated carbon and a carbon-based material. Known examples of activated carbon include carbon black and ketjen black, but in the present invention, the carbon-based material is carbon fiber such as PAN carbon fiber made from carbonized acrylic fiber or PITCH carbon fiber made from carbonized pitch.

[0029] To manufacture the cylindrical component that will become the air electrode (cathode) 50, activated carbon and carbon-based material are mixed and molded with a glue, and then fired in a reducing atmosphere. A mixing ratio of 3:7 by weight of activated carbon to carbon-based material is suitable. If the amount of activated carbon is too small, the performance as an electrode decreases. Also, if the amount of carbon-based material is too small, the physical strength will be insufficient. The glue added during mixing is a glue that carbonizes during reduction firing, and industrially available polyvinyl alcohol is suitable. A weight ratio of approximately 1:10 is suitable for the amount of glue added and the mixture of activated carbon and carbon-based material.

[0030] The kneaded mixture of activated carbon and carbonaceous material is fired in a reducing atmosphere. The formation must correspond to the shape of the ceramic filter 20, and the inner diameter of the tubular member that will become the air electrode 50 is preferably 3 to 10 mm larger than the outer diameter of the ceramic filter 20. The thickness of the tubular member that will become the air electrode 50 of the ceramic filter 20 is approximately 10 mm. The formed tubular member is fired at 300 to 800°C in a reducing atmosphere that is blocked off from oxygen.

[0031] To attach the air electrode 50 to the diffusion sheet 40 in close contact with it, the diffusion sheet 40, which is in close contact with the outer periphery of the fuel electrode 30, is coated with a solution of the solid polymer electrolyte in an organic solvent, and then inserted into the cylindrical air electrode 50. In this case, the organic solvent solution of the solid polymer electrolyte is coated on the inside of the cylindrical member that will become the air electrode 50.

[0032] A conductor (not shown) is connected to the air electrode 50. This conductor can be made of any material as long as it has good conductivity, but a conductor with low electrical resistance is preferred. The conductor is insulated except for the connection with the air electrode 50. The connection between the conductor and the air electrode 50 is not particularly limited as long as it has a structure that allows the conductor to be connected to the anode 30 and conduct electricity. However, when using a conductor with a thin metal flake attached to the end, a ceramic filter 20 with a diffusion sheet 40 attached to it is inserted into the air electrode 50, and then the thin metal flake connected to the conductor is sandwiched between the diffusion sheet 40 and the air electrode 50. The thickness of the thin metal flake is typically approximately 0.1 to 0.3 mm. The connection between the conductor and the air electrode 50 is a hole or contact surface formed on the top, bottom, or side of the air electrode 50 for inserting a conductor.

[0033] The set consisting of the ceramic filter 20, fuel electrode 30, diffusion sheet 40, air electrode 50, and lead wires is dried to remove the organic solvent. A dripping device 19 is located on the lid 11, and an outlet 15 is located on the bottom.

[0034] There are no restrictions on the materials for the main body 11 and the lid 12, as long as they are impermeable to electrons and hydrogen gas and resistant to acids and alkalis, but synthetic resins are suitable because they are easy to process. A set consisting of a ceramic filter 20, a fuel electrode 30, a diffusion sheet 40, an air electrode 50, and an attached lead wire is attached vertically. A backing 16 is provided between the top and bottom of the main body 12. A silicone backing is suitable as the backing 16.

[0035] A reaction plate 60 made of an amphoteric metal that reacts with an alkaline aqueous solution to generate hydrogen is placed in the reaction chamber 13. The reaction plate 60 is in the form of a plate with a thickness of 0.1 to 10 mm, and the size of the plate is set to match the shape and size of the reaction chamber 13.

[0036] The size of the reaction plate 60 is formed to be 0.5 to 20 mm smaller than the inner diameter of the ceramic filter 20, and is a size that allows easy insertion and removal into the reaction chamber 13. The reaction plates 60 are stacked in layers one above the other while maintaining a certain interval between them, and the appropriate interval between the stacked reaction plates 60 is in the range of 0.5 to 50 mm, with 5 to 20 mm being more appropriate.

[0037] The reaction plates 60 made of amphoteric metal have a mesh structure or 1 to 30 through-holes per plate. For example, when an alkaline aqueous solution is dropped onto the top of multiple reaction plates 60 stacked vertically in layers at an angle, the solution reacts with the amphoteric metal. This aqueous solution falls onto the lower reaction plate 60 through the through-holes and slides down while repeating the reaction on the new reaction plate 60.

[0038] Then, when it reaches the lowest position of the reaction plate 60, it falls to the bottom of the reaction chamber 13. The mesh size of the mesh-structured reaction plate 60 is preferably 0.1 to 10 mm. If the mesh size is small, the water droplets that pass through may be concentrated in one place on the lower surface of the reaction plate 60, and conversely, if the mesh size is large, the water droplets may fall before they have had a chance to fully react with the amphoteric metal.

[0039] The reaction plate 60 having through holes is formed by cutting out a punch plate with through holes punched to match the shape of the reaction chamber 13, or by punching a cut-out amphoteric metal plate with a needle having a sharp tip.

[0040] A support is required to support the amphoteric metal reaction plates 60 so that they can be stacked one above the other while maintaining a certain distance between them. There are no restrictions on the structure of the support as long as it allows the amphoteric metal reaction plates to be stacked one above the other while maintaining a certain distance between them, but the support must not be one that can prevent water droplets from dropping from the upper reaction plate 60.

[0041] Furthermore, in order to support the amphoteric metal reaction plate so that it is not submerged in the waste liquid until the reaction with the amphoteric metal is completed and the waste liquid containing dissolved reaction by-products accumulates at the bottom, it is preferable to select a material for the support that provides extra height at the bottom that does not react with alkaline water with a pH of 9 to 14. Stainless steel is particularly suitable because of its ease of processing.

[0042] The multiple amphoteric metal reaction plates 60 mounted on the support are set at an allowable angle of 3 to 45 degrees relative to the horizontal, with an angle of 5 to 30 degrees being most suitable. If the inclination angle is less than 3 degrees, the alkaline aqueous solution hardly moves on the reaction plate 60, causing the reaction to occur only in a portion of the reaction plate 60, potentially reducing the overall reaction efficiency. Furthermore, an increase in the amount of amphoteric metal that ends up unreacted is uneconomical. Conversely, if the inclination angle exceeds 45 degrees, the alkaline aqueous solution falls to the bottom before it has a chance to react with the amphoteric metal, potentially preventing the reaction from proceeding fully. This wasteful consumption of the alkaline aqueous solution is uneconomical.

[0043] Because the reaction between an amphoteric metal and an alkaline aqueous solution is an exothermic reaction, the magnesium secondary battery 10 of the present invention can operate at low temperatures, for example, -40°C, as long as the acidic or alkaline aqueous solution that is dripped does not freeze. Also, it can operate at high temperatures as long as the plastics used for the body 12 and lid 11 do not deform.

[0044] The dripping device 19 used in the present invention is structured to include a connection port for connecting a hose or pipe through which the alkaline aqueous solution is delivered, and a nozzle for dripping the alkaline aqueous solution onto the amphoteric metal reaction plate 60. The dripping device 19 is attached to the lid 11 at the highest position of the inclined reaction plate 60 so that the alkaline aqueous solution can be dripped. The dripping device 19 is attached so as to prevent hydrogen generated in the reaction chamber 13 from leaking from the fuel cell 100.

[0045] In the battery cell 100, as the reaction between the alkali and amphoteric metal progresses, the pressure in the reaction chamber 13 where hydrogen is generated increases, and the drip rate slows. When the pressure in the reaction chamber 13 increases, the amount of hydrogen that reaches the anode 30 through the ceramic filter 20 increases, and the amount of power generated increases. When the hydrogen pressure in the reaction chamber 13 decreases, the drip rate of the alkaline aqueous solution increases, promoting the generation of hydrogen and allowing the power generation to be maintained constant.

[0046] Since the fuel cell 100 of the present invention obtains hydrogen through the reaction between an amphoteric metal and an acidic or alkaline aqueous solution, after a certain amount of power generation, the amphoteric metal is consumed and hydrogen is no longer generated, making power generation impossible. In such a case, the pump is stopped, the dripping device 19 is removed, the lid 11 is removed, the support body in the reaction chamber 13 together with the remaining reaction plate 60 is taken out, the blind plug 14 of the exhaust port 15 is opened, the reaction by-products remaining in the reaction chamber 13 are removed, and the reaction chamber 13 is cleaned.

[0047] Thereafter, the discharge port 15 is closed with the blanking plug 14, and the stored support is set anew so that the reaction plates 60 are stacked one on top of the other in layers while maintaining a certain distance from each other, and at an angle of 3 to 45 degrees with respect to the horizontal plane.

[0048] The support is made of a material that does not react with alkaline water of pH 9 to 14, so if a reaction plate 60 made of an amphoteric metal is attached, it can be operated repeatedly.

[0049] After power generation, the reaction by-products recovered from the reaction chamber 13 can be recycled by conventional techniques and returned to the original metal. For example, in a magnesium secondary battery 10 in which sodium hydroxide is dropped onto an aluminum reaction plate 60, aluminum hydroxide and sodium aluminate are recovered. From this, aluminum oxide (alumina) is obtained, which can then be electrolyzed (molten salt electrolysis) to obtain metallic aluminum.

[0050] The magnesium secondary battery 10 having the above configuration can be used for at least 100 cycles, preferably 300 to 500 cycles, whereas magnesium batteries used as ordinary primary batteries can only be used for about 1 to 4 cycles. Furthermore, since it does not generate harmful substances like lithium batteries, it is environmentally friendly and can be safely disposed of. Furthermore, since it does not self-discharge before use, it has excellent long-term storage properties.

[0051] 3, a power generation system for a magnesium secondary battery according to the present invention will be described. An amphoteric metal reaction plate 60 is set on one or more fuel cell cells 100, a hose or pipe for supplying water from a tank 300 storing an alkaline aqueous solution is opened, and a pump controller 400 that controls the transport pump 200 issues an operation command to the pump 200. This causes the pump 200 to operate using the power of a pump battery 500, and the alkaline aqueous solution is sent to each fuel cell 100 and dripped onto the surface of the amphoteric metal reaction plate 60 via a dripping device 19, generating hydrogen.

[0052] Each battery cell 100 generates electricity using hydrogen generated in the reaction chamber 13 as fuel. The generated DC electricity is supplied to an inverter 600 and converted into AC output for use. By collecting electricity from multiple battery cells 100, a magnesium secondary battery 10 that generates a large amount of power can be obtained.

[0053] Figure 4 shows a magnified view of a portion of the surface of the anode 30 observed with an electron microscope. Referring to Figure 4, the constituent material of the anode 30 includes first particles 71 made of magnesium, second particles 72 made of carbon nanotubes, and third particles 73 made of a thermoplastic synthetic resin, preferably polypropylene. Because the first to third particles 71 to 73 are spherical and have no corners, the gaps between the particles are smaller than if they were rectangular and had corners, allowing for a higher density.

[0054] Referring to Fig. 5, automobile 800 is an electric automobile that uses an electric motor as a power source for running. Alternatively, it is a hybrid automobile that can select and use an electric motor or an engine as a power source for running. Automobile 200 has a battery pack 810, which has a secondary battery module to which multiple magnesium secondary batteries 10 are connected. It is preferable that automobile 200 further has a charge control device electrically connected to the secondary battery module.

[0055] The present invention can include at least the following embodiments, which can be adopted separately or in combination with each other. (1) The alkaline aqueous solution has a pH of 9 to 14. (2) The reaction plate contains magnesium, carbon nanotubes, and a thermoplastic synthetic resin. (3) The thermoplastic synthetic resin is polypropylene. [Industrial Applicability]

[0056] The magnesium secondary battery according to the present invention can be widely applied in various fields as a power source for automobiles, airplanes, trains, personal digital assistants, artificial satellites, etc. [Explanation of symbols]

[0057] 10: Magnesium secondary battery 13: Reaction chamber 20: Ceramic filter 30: Fuel electrode (anode) 50: Air electrode (cathode) 60: Reaction plate

Claims

1. The fuel cell has a fuel electrode, an air electrode, and a reaction plate made of an amphoteric metal containing at least magnesium, A magnesium secondary battery in which an electrochemical reaction occurs between the fuel electrode and the air electrode by adding an alkaline aqueous solution in a reaction chamber surrounded by a ceramic filter.

2. 2. The magnesium secondary battery according to claim 1, wherein the alkaline aqueous solution has a pH of 9 to 14.

3. 3. The magnesium secondary battery according to claim 1, wherein the reaction plate contains magnesium, carbon nanotubes, and a thermoplastic synthetic resin.

4. 3. The magnesium secondary battery according to claim 1, wherein the thermoplastic synthetic resin is polypropylene.

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