Hydrogen generating alloy, experimental teaching materials, negative electrode material for magnesium batteries, and hydrogen generating agent for power generation.

Abstract

To provide a hydrogen generating alloy which has high material stability and can generate a large amount of hydrogen through hydrolysis, requiring less energy for the production process, an experimental material, a negative electrode material for a magnesium battery, and a hydrogen generating agent for power generation.SOLUTION: This hydrogen generating alloy has a chemical composition comprising 5-30 mass% of Ca, with the balance being Mg and impurities, and has a metal structure containing a lamella structure comprising a Mg phase mainly composed of Mg and a Mg2Ca phase mainly composed of Mg2Ca.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a hydrogen generation alloy, an experimental teaching material, a negative electrode material for a magnesium battery, and a hydrogen generation agent for power generation.

Background Art

[0002] In recent years, the main energy sources that support human activities are fossil fuels such as petroleum and natural gas, and carbon dioxide generated incidentally when extracting energy from them is regarded as a cause of various environmental problems. Therefore, efforts have been made mainly in developed countries to achieve a carbon-free society that does not use energy accompanied by carbon dioxide emissions. As another problem, these fossil fuels are natural resources with limited reserves and generated over a very long period, so there is concern that they will run out. For these reasons, it is required to develop a sustainable energy source that can supply energy stably.

[0003] As an energy resource to solve these problems, hydrogen has attracted attention. The reasons include that carbon dioxide is not generated during combustion, it can be directly converted into electrical energy by a fuel cell, the energy efficiency is high, and it is abundantly present on the earth as a compound such as water. Therefore, establishing a technology that can supply hydrogen stably is expected to lead to the solution of the above problems we are currently facing.

[0004] However, as described above, while hydrogen itself has a high potential as a sustainable energy source, methods such as electrolysis of water and steam reforming of fossil fuels currently used in industrial hydrogen production have problems such as high power consumption, carbon dioxide emissions, and consumption of fossil fuels. Therefore, it is urgent to establish a method for producing hydrogen that consumes as little power as possible and does not involve carbon dioxide emissions.

[0005] As a method for producing hydrogen, hydrolysis using metals has been studied. This method involves immersing a metal in an aqueous solution to react the metal with water and generate hydrogen, and it has three main advantages, as described below. First, the hydrogen generation mechanism is simple; second, it does not involve the consumption of electricity or the emission of carbon dioxide; and finally, hydrogen can be stored in a state that is easy to store and transport, as it is metal and water rather than a gas.

[0006] In particular, magnesium (Mg) is an active metal that hydrolyzes to produce hydrogen, as shown in equation (1) below, with 1 mole of hydrogen being produced for every 1 mole of Mg.

[0007] Mg+2H2O→Mg(OH)2+H2…(1) formula

[0008] Since the atomic weight of Mg is relatively small at 24.3, it can be expected that a large amount of hydrogen can be generated per unit mass of Mg. In fact, calculations show that if 1 kg of Mg were completely hydrolyzed according to the reaction in equation (1) above, 921 L of hydrogen would be obtained under standard conditions.

[0009] Furthermore, Mg is present in neutral aqueous solution. 2+ Because it exists as such, the hydrolysis reaction proceeds even in a neutral aqueous solution. In addition, chloride ions (Cl) are present in the aqueous solution used for hydrolysis. - It is widely known that the presence of ) improves the hydrolysis rate of Mg alloys.

[0010] However, in the hydrolysis reaction of Mg, there is a problem in that the pH of the Mg surface increases as the reaction progresses, and a passivation film of Mg(OH)2 forms on the Mg surface, hindering the hydrolysis reaction. Therefore, to address this problem, measures such as accelerating the hydrolysis reaction by machining Mg or adding alloying elements have been investigated.

[0011] For example, Non-Patent Document 1 discloses a technique for suppressing the reduction in the reaction rate of hydrolysis reactions by ball milling Mg or its hydride to produce a fine powder. According to the technique described in Non-Patent Document 1, the reactivity is improved by increasing the specific surface area of ​​the material or by applying strain to the material, and the reaction is greatly accelerated.

[0012] Furthermore, Non-Patent Documents 2 and 3 disclose a technique for generating a second phase in the Mg matrix by adding a metal element to Mg, and for accelerating the reaction by causing galvanic corrosion between the Mg matrix and the second phase. Specifically, a technique is disclosed in which a bulk Mg-X binary eutectic alloy (X=Ni, Cu, Sn) is fabricated, and hydrogen is generated by hydrolyzing the eutectic alloy in a 3.5 wt.% NaCl aqueous solution. [Prior art documents] [Non-patent literature]

[0013] [Non-Patent Document 1] M.-H. Grosjean, M. Zidoune, L. Roue, J.-Y. Huot. Int. J. Hydrogen Energy 2006; 31: 109-119. [Non-Patent Document 2] Song-Lin. Li, Hung-Mao Lin, Jun-Yen Uan. Int. J. Hydrogen Energy 2013; 38: 13520-13528. [Non-Patent Document 3] Song-Lin Li, Jenn-Ming Song, Jun-Yen Uan, Journal of Alloys and Compounds 2019; 772: 489-498. [Overview of the project] [Problems that the invention aims to solve]

[0014] However, the technology described in Non-Patent Document 1 has drawbacks, such as requiring a large amount of energy during material processing with a ball mill, and making it difficult to store the material due to its high reactivity.

[0015] Furthermore, in the technologies described in Non-Patent Documents 2 and 3, a galvanic cell is formed in which the electrochemically less noble Mg phase acts as the anode and the electrochemically noble Mg2X phase acts as the cathode. However, while the Mg phase hydrolyzes to generate hydrogen, the Mg2X phase acting as the cathode does not hydrolyze, resulting in a problem where the amount of hydrogen generated per unit mass of the alloy is small.

[0016] Thus, in conventional methods for generating hydrogen using the hydrolysis reaction of Mg, it has been difficult to achieve high material stability and a large amount of hydrogen generation while suppressing the energy required for production.

[0017] This invention has been made in view of the above circumstances, and aims to provide a hydrogen generating alloy, experimental teaching material, a negative electrode material for magnesium batteries, and a hydrogen generating agent for power generation, which have high material stability and can generate a large amount of hydrogen by hydrolysis reaction while suppressing the energy required for manufacturing. [Means for solving the problem]

[0018] The inventors focused on Mg-Ca alloys from three perspectives. Firstly, Mg-Ca alloys form a eutectic structure consisting of two phases, Mg and Mg2Ca, when the Ca content is between 1.34 mass% and 45.2 mass%. Here, the eutectic composition is Mg-16.2 mass%Ca. Therefore, if a Mg-Ca alloy has a eutectic structure, it can be expected that the Mg-Ca alloy will exhibit reaction behavior similar to the Mg-X (X=Ni, Cu, Sn) eutectic alloy described above, and that all of the anode phase in the alloy will react.

[0019] Second, the saturated calomel electrode potential of the Mg2Ca phase is -1.85 V (vs SCE), which is lower than that of pure Mg at -1.64 V (vs SCE). Therefore, in the galvanic corrosion occurring between the Mg phase and the Mg2Ca phase, the Mg2Ca phase is considered to act as an anode and be preferentially corroded as shown in the following formula (2).

[0020] Mg2Ca + 3H2O → 2Mg(OH)2 + Ca(OH)2 + 3H2…(2) formula

[0021] Third, since the Mg phase acting as a cathode in the above galvanic corrosion is also chemically active, after the reaction of the Mg2Ca phase proceeds to a certain extent, the reaction of the above formula (1) is started, and it is considered that the Mg phase contributes to hydrogen generation. In this case, since all the phases constituting the metal react with water, it can be expected that the amount of hydrogen generated per mass of the alloy will be more than that of the above-mentioned Mg-X eutectic alloy. For example, the theoretically hydrogen generation amount per 1 kg of the Mg-Ni eutectic alloy under standard conditions is 526 L, the theoretically hydrogen generation amount per 1 kg of the Mg-Cu eutectic alloy is 422 L, and the theoretically hydrogen generation amount per 1 kg of the Mg-Sn eutectic alloy is 442 L. The hydrogen generation amounts of these eutectic alloys are values calculated assuming that the cathode does not hydrolyze. On the other hand, the theoretically hydrogen generation amount per 1 kg of the Mg-Ca eutectic alloy in which the cathode also dissolves is 863 L, and it can be expected that the Mg-Ca alloy will generate a large amount of hydrogen by hydrolysis. And based on the above, the present inventors have studied in detail the hydrolysis reaction of the Mg-Ca eutectic alloy and have arrived at the present invention.

[0022] The gist of the present invention is as follows: [1] The hydrogen generation alloy according to one aspect of the present invention has a chemical composition of, in mass%, Ca: 5 to 30 mass%, and the balance: Mg and impurities, and the metal structure includes a lamellar structure composed of a Mg phase mainly composed of Mg and a Mg2Ca phase mainly composed of Mg2Ca. [2] In the hydrogen generation alloy according to [1] above, the chemical composition may contain Ca: 16.2 to 20 mass%. [3] The hydrogen generation alloy described in [1] or [2] above may contain a total of 0.01 to 1% by mass of one or more selected from the group consisting of Ni, Cu, and Sn in place of a part of the Mg. [4] In the hydrogen generation alloy described in any one of [1] to [3] above, the thicknesses of the Mg phase and the Mg2Ca phase in the lamellar structure may both be 50 nm or more and 5000 nm or less. [5] In the hydrogen generation alloy described in any one of [1] to [4] above, the metal structure may consist only of the lamellar structure.

[0023] [6] The experimental teaching material according to another aspect of the present invention uses the hydrogen generation alloy described in any one of [1] to [5] above.

[0024] [7] The negative electrode material for a magnesium battery according to still another aspect of the present invention uses the hydrogen generation alloy described in any one of [1] to [5] above.

[0025] [8] The hydrogen generation agent for power generation according to still another aspect of the present invention uses the hydrogen generation alloy described in any one of [1] to [5] above.

Advantages of the Invention

[0026] According to the present invention, it is possible to provide a hydrogen generation alloy, an experimental teaching material, a negative electrode material for a magnesium battery, and a hydrogen generation agent for power generation, which can suppress the energy required for production, have high material stability, and generate a large amount of hydrogen by a hydrolysis reaction.

Brief Description of the Drawings

[0027] <​​​​​​​This graph shows the change in hydrogen generation over time when Mg-16.2Ca, an example of a hydrogen-generating alloy according to the same embodiment, is immersed in an etching solution with a changed temperature. [Figure 4] This graph shows the change in the amount of hydrogen generated over time when Mg-10Ca, an example of a hydrogen-generating alloy according to the same embodiment, is immersed in an etching solution with a changed temperature. [Figure 5] This graph shows the change in the amount of hydrogen generated over time when Mg-15Ca, an example of a hydrogen-generating alloy according to the same embodiment, is immersed in an etching solution with a changed temperature. [Figure 6] This graph shows the change in the amount of hydrogen generated over time when Mg-20Ca, an example of a hydrogen-generating alloy according to the same embodiment, is immersed in an etching solution with a changed temperature. [Figure 7] These are the X-ray diffraction patterns of samples obtained by hydrolyzing Mg-16.2Ca, an example of a hydrogen-generating alloy according to the same embodiment, in an etchant at 20°C, 40°C, and 60°C for 2 hours. [Figure 8] This is a schematic diagram illustrating the hydrolysis behavior. [Figure 9] This is a schematic diagram of the apparatus used in the hydrolysis test in the example. [Figure 10] These are the XRD diffraction patterns of each sample prepared in the examples. [Figure 11] This graph shows the thickness of the Mg phase and Mg2Ca phase in the lamellar structure of each sample prepared in the examples. [Figure 12] This is an SEM image showing the cross-section of Mg-15Ca after corrosion, prepared in the example. [Figure 13] These are TEM transmission images and EDS maps showing the elemental distribution of the Mg-15Ca matrix alloy before casting in the examples. [Figure 14] This figure shows the change over time when pure Mg in the example is immersed in an etching solution. [Figure 15] This figure shows the change over time when Mg-10Ca in the example is immersed in an etching solution. [Figure 16]This figure shows the change over time when Mg-15Ca in the example is immersed in an etching solution. [Figure 17] This figure shows the change over time when Mg-16.2Ca in the example is immersed in an etching solution. [Figure 18] This figure shows the change over time when Mg-20Ca in the example is immersed in an etching solution. [Modes for carrying out the invention]

[0028] Below, a hydrogen generation alloy, experimental teaching material, negative electrode material for magnesium batteries, and hydrogen generation agent for power generation according to embodiments of the present invention will be described with reference to the attached drawings. However, the present invention is not limited to the following embodiments.

[0029] <Hydrogen-generating alloy> First, a hydrogen-generating alloy according to one embodiment of the present invention will be described. The hydrogen-generating alloy according to this embodiment has a chemical composition of 5-30% by mass of Ca, with the remainder being Mg and impurities, and its microstructure includes a lamellar structure consisting of a Mg phase mainly composed of Mg and a Mg2Ca phase mainly composed of Mg2Ca. A detailed description follows below.

[0030] (chemical composition) Ca: 5~30% by mass Calcium (Ca) forms the intermetallic compound Mg2Ca with magnesium (Mg). If the Ca content is 5% by mass or more, the hydrolysis reaction at room temperature is promoted, starting from a lamellar structure consisting of a Mg phase mainly composed of Mg and a Mg2Ca phase mainly composed of Mg2Ca. The Ca content is preferably 10% by mass or more. Furthermore, if the Ca content is 16.2% by mass or more, the rate of the hydrolysis reaction increases, and a large amount of hydrogen can be generated in a short time. Therefore, the Ca content is more preferably 16.2% by mass or more. On the other hand, from the viewpoint of promoting the hydrolysis reaction, a higher Ca content is acceptable, but if it is too high, the activity of the Ca-Mg alloy increases, making it difficult to handle. Therefore, the Ca content should be 30% by mass or less. The Ca content is preferably 20% by mass or less.

[0031] Remainder: Mg and impurities The remainder of the hydrogen-generating alloy according to this embodiment consists of Mg and impurities. Examples of impurities include Li, Na, Al, Si, Cl, Ca, Mn, Fe, Co, Ni, Cu, Zn, Sr, Ba, and Pb. The impurity content is preferably as low as possible and may be 0%. From the viewpoint of maintaining the amount of hydrogen generated, the impurity content is preferably 1% by mass or less. More preferably, the impurity content is 0.1% by mass or less.

[0032] One or more elements selected from the group consisting of Ni, Cu, and Sn: 0.01 to 1% by mass Nickel (Ni), copper (Cu), and tin (Sn) are elements that form intermetallic compounds with magnesium (Mg). The presence of these intermetallic compounds promotes the hydrolysis reaction of magnesium. As a result, the amount of hydrogen generated per unit time increases. Therefore, when the hydrogen-generating alloy according to this embodiment is used in situations where a large amount of hydrogen is required in a short time, it is preferable that the hydrogen-generating alloy contains one or more elements selected from the group consisting of Ni, Cu, and Sn in place of a portion of the magnesium. From the viewpoint of increasing the amount of hydrogen generated per unit time, the content of one or more elements selected from the group consisting of Ni, Cu, and Sn is preferably 0.01% by mass or more, and more preferably 0.25% by mass or more. On the other hand, as mentioned above, intermetallic compounds formed between Ni, Cu, or Sn and magnesium are less susceptible to hydrolysis. Therefore, if the content of one or more elements selected from the group consisting of Ni, Cu, and Sn is too high, the amount of hydrogen generated per unit mass may decrease. Therefore, the content of one or more elements selected from the group consisting of Ni, Cu, and Sn is preferably 1% by mass or less, and more preferably 0.5% by mass or less.

[0033] The chemical composition of the Mg-Ca alloy is measured using EDX (Energy Dispersive X-ray Spectroscopy). Specifically, five measurement points are arbitrarily selected at a magnification of 100x, and the composition of these measurement points is analyzed using EDX. The average value of the compositions at these five points is then taken as the chemical composition of the Mg-Ca alloy.

[0034] (Composition dependence of Mg-Ca alloy on hydrolysis reaction) The inventors used Mg-Ca alloys with different Ca content as hydrogen-generating alloys and investigated the composition dependence of the Mg-Ca alloys on the hydrolysis reaction by changing the temperature of the etchant used in the hydrolysis reaction. A 3.5 mass% NaCl solution was used as the etchant. The initial reaction rate of the hydrolysis reaction at each temperature (10 to 40°C) for each alloy was calculated, and Arrhenius plots were created from these. Figure 1 shows the created Arrhenius plots. Based on the created Arrhenius plots, the activation energy of each alloy was calculated from the Arrhenius equation shown in equation (3) below.

[0035]

number

[0036] In equation (3) above, r is the reaction rate in the initial stages of the hydrolysis reaction, and E a is the activation energy (J / mol), R is the gas constant (J / (K·mol)), and T is the reaction temperature (K).

[0037] Table 1 shows the calculated activation energies for each alloy. Note that Mg-10Ca represents an Mg-Ca alloy with a Ca content of 10 mass%, Mg-15Ca represents an Mg-Ca alloy with a Ca content of 15 mass%, Mg-16.2Ca represents an Mg-Ca alloy with a Ca content of 16.2 mass%, and Mg-20Ca represents an Mg-Ca alloy with a Ca content of 20 mass%. Mg-10Ca and Mg-15Ca are hypoeutectic alloys, Mg-16.2Ca is a eutectic alloy, and Mg-20Ca is a hypereutectic alloy. Furthermore, the activation energies of Mg2Ca and pure Mg, as shown in Table 1, are conventionally known values.

[0038] [Table 1]

[0039] As shown in Table 1, the activation energies of Mg-16.2Ca and Mg-20Ca are 20 kJ·mol. -1 This was approximately the same as the activation energy of Mg2Ca. On the other hand, the activation energies of Mg-10Ca and Mg-15Ca were 44 kJ·mol. -1 The activation energies were around the same value as those of pure Mg. From this, it can be inferred that hydrolysis of the Mg2Ca phase is dominant in the eutectic alloy Mg-16.2Ca and the hypereutectic alloy Mg-20Ca. Furthermore, since the hypoeutectic alloys Mg-10Ca and Mg-15Ca contain primary Mg crystals (dendritic structure), it is thought that hydrolysis of Mg in the primary Mg crystals is also proceeding simultaneously. As shown in Table 1, a Ca content of 16.2% by mass or more is more preferable, based on the activation energy.

[0040] (Metal structure) The hydrogen-generating alloy according to this embodiment has a lamellar structure consisting of a Mg phase mainly composed of Mg and a Mg2Ca phase mainly composed of Mg2Ca. Figure 2 shows an example of the metallic structure of the hydrogen-generating alloy according to this embodiment. Figure 2 shows backscattered electron images of Mg-Ca alloys with different Ca content captured by SEM (Scanning Electron Microscope). Figure 2(a) is a backscattered electron image of an Mg-Ca alloy with a Ca content of 10 mass% (Mg-10Ca), (b) is a backscattered electron image of an Mg-Ca alloy with a Ca content of 15 mass% (Mg-15Ca), (c) is a backscattered electron image of an Mg-Ca alloy with a Ca content of 16.2 mass% (Mg-16.2Ca), and (d) is a backscattered electron image of an Mg-Ca alloy with a Ca content of 20 mass% (Mg-20Ca).

[0041] The hydrogen-generating alloys according to this embodiment have a lamellar structure, as shown in Figure 2. Mg-10Ca and Mg-15Ca are hypoeutectic alloys and, as shown in Figures 2(a) and (b), have a dendritic structure consisting mainly of Mg phase and a lamellar structure consisting of Mg phase and Mg2Ca phase. Mg-16.2Ca is a eutectic alloy and, as shown in Figure 2(c), has a lamellar structure consisting of Mg phase and Mg2Ca phase. Mg-20Ca is a hypereutectic alloy and, as shown in Figure 2(d), has a dendritic structure consisting of Mg2Ca phase and a lamellar structure consisting of Mg phase and Mg2Ca phase. In Figures 2(a) to (d), the darkened areas are the Mg phase and the brightened areas are the Mg2Ca phase.

[0042] [Lamellar tissue] The lamellar structure is a layered structure consisting of an Mg phase and an Mg2Ca phase. Galvanic corrosion occurs at the interface between dissimilar metals, but in the hydrogen-generating alloy according to this embodiment, the thickness of the Mg phase and Mg2Ca phase in the lamellar structure is small, and the interface area between the Mg phase and Mg2Ca phase per unit amount is large. Therefore, hydrolysis reactions are likely to occur in the lamellar structure. As a result, the hydrolysis reaction rate of the hydrogen-generating alloy is high, and the amount of hydrogen generated per unit time can be increased. Furthermore, generally, in the hydrolysis reaction of Mg, the pH near the Mg surface increases as the reaction progresses, and a passivation of Mg(OH)2 is formed on the Mg surface, hindering the hydrolysis reaction. However, in the hydrogen-generating alloy according to this embodiment, because the lamellar structure is very fine, the Mg(OH)2 produced in the hydrolysis reaction is released to the outside of the lamellar structure (outside the reaction system) by hydrogen bubbles (hydrogen bubbles) that are similarly produced. Furthermore, the passivated Ca(OH)2 produced by the hydrolysis reaction can also be released to the outside of the lamellar structure by hydrogen bubbles, similar to Mg(OH)2. Therefore, the hydrolysis reaction can continue without being affected by Mg(OH)2, and most of the Mg constituting the Mg-Ca alloy can contribute to the hydrolysis reaction, generating a large amount of hydrogen.

[0043] The thickness of the Mg phase and Mg2Ca phase in the lamellar structure can be, for example, 25 nm to 50 μm for both. Since the interface between the Mg phase and the Mg2Ca phase acts as a starting point for galvanic corrosion and facilitates hydrolysis reactions, a finer lamellar structure is preferable. Therefore, the thickness of the Mg phase and Mg2Ca phase in the lamellar structure is preferably 5000 nm or less, more preferably 1000 nm or less, and even more preferably 250 nm or less. On the other hand, if the thickness of the Mg phase and Mg2Ca phase in the lamellar structure is too thin, Mg(OH)2 or Ca(OH)2 formed during hydrolysis may not be discharged and may remain in the lamellar structure. However, if the thickness of the Mg phase and Mg2Ca phase in the lamellar structure is 50 nm or more, the above problem is less likely to occur. Therefore, the thickness of the Mg phase and Mg2Ca phase in the lamellar structure is preferably 50 nm or more, and more preferably 100 nm or more.

[0044] The thickness of the Mg phase and Mg2Ca phase in the lamellar structure is determined by obtaining a backscattered electron image at 10,000x magnification using a scanning electron microscope (SEM), measuring the thickness of the Mg phase and Mg2Ca phase at 10 arbitrary locations in the backscattered electron image, and taking the average values ​​of these measurements as the thickness of the Mg phase and Mg2Ca phase.

[0045] The thickness of the Mg and Mg2Ca phases in the lamellar structure can be controlled by the cooling rate during hydrogen alloy production. By decreasing the cooling rate, the thickness of the Mg and Mg2Ca phases in the lamellar structure can be increased, while by increasing the cooling rate, their thickness can be decreased.

[0046] As mentioned above, since lamellar structures have many fine interfaces, the larger the area ratio of the lamellar structure, the greater the amount of hydrogen generated per unit time in the initial stages of the hydrolysis reaction. Therefore, the lamellar structure ratio relative to the entire metal structure can be 100%.

[0047] [Dendrite Organization] A dendritic structure is a dendritic structure composed of either a Mg phase or a Mg2Ca phase. As shown in Figure 2, the dendritic structure is composed of either a Mg phase or a Mg2Ca phase depending on the Ca content. In galvanic corrosion that occurs between the Mg phase and the Mg2Ca phase, the Mg2Ca phase acts as an anode and corrodes preferentially. Therefore, when a metal structure contains a dendritic structure composed of the Mg2Ca phase, the reaction rate in the initial stages of the hydrolysis reaction is greater than when it contains a dendritic structure composed of the Mg phase, resulting in a higher hydrogen generation per unit time. Consequently, when a metal structure contains a dendritic structure, it is preferable that the phase constituting the dendritic structure is the Mg2Ca phase. In other words, a hypereutectic alloy is preferable to a hypoeutectic alloy.

[0048] When the Mg-Ca alloy is a hypereutectic alloy, if the dendritic structure ratio to the entire metal structure is 5% or more, the reaction rate in the initial stages of the hydrolysis reaction increases for the reasons mentioned above. Therefore, when the Mg-Ca alloy is a hypereutectic alloy, the dendritic structure ratio to the entire metal structure is preferably 5% or more, and more preferably 15% or more. On the other hand, when the Mg-Ca alloy is a hypereutectic alloy, if the dendritic structure ratio to the entire metal structure is 17% or less, the lamellar structure ratio increases, and the reaction rate in the initial stages of the hydrolysis reaction increases. Therefore, when the Mg-Ca alloy is a hypereutectic alloy, the dendritic structure ratio to the entire metal structure may be 17% or less, or even 0%.

[0049] When the Mg-Ca alloy is a hypoeutectic alloy, the dendrite structure is composed of the Mg phase, resulting in a lower initial reaction rate for hydrolysis compared to a hypereutectic alloy. Therefore, from the viewpoint of the initial reaction rate for hydrolysis, it is preferable for the dendritic structure ratio to the total metal structure to be small when the Mg-Ca alloy is a hypoeutectic alloy. When the Mg-Ca alloy is a hypoeutectic alloy, the dendritic structure ratio to the total metal structure is preferably 70% or less, more preferably 50% or less, and even more preferably 0%.

[0050] Lamellar and dendritic structure ratios are calculated by acquiring five fields of view of backscattered electron images at 100x magnification using a scanning electron microscope (SEM), measuring the lamellar and dendritic structure ratios for each image using image analysis software, and then calculating the average value. For example, in the image analysis software imageJ, the coarse structure formed as the primary crystal is determined to be a dendritic structure, while the other eutectic microstructure is determined to be a lamellar structure.

[0051] Up to this point, the hydrogen-generating alloy according to this embodiment has been described. The hydrogen-generating alloy according to this embodiment can be manufactured, for example, by the following method. For example, raw materials whose composition has been adjusted to achieve a desired chemical composition are charged into a carbon crucible and cast under an Ar (argon) atmosphere. For the mold, for example, a stainless steel mold can be used. For the cooling method, for example, air cooling can be used. To control the thickness of the Mg phase and Mg2Ca phase in the lamellar structure, the cooling rate can be adjusted. For example, the weight ratio of the mold to the amount of casting can be adjusted, or a mold with a different thermal conductivity, such as one made of copper, can be used. Furthermore, the solidification rate can be slowed by preheating the mold, or accelerated by water cooling. To adjust the cooling rate more precisely, the above parameters can be optimized to further adjust the cooling rate. The manufacturing method described above is merely one example, and the manufacturing method of the hydrogen-generating alloy according to this embodiment is not limited to the method described above.

[0052] <Method for generating hydrogen> In this embodiment, the hydrogen-generating alloy undergoes a hydrolysis reaction upon contact with a corrosive solution, generating hydrogen. The corrosive solution is not particularly limited and may be an acidic solution, a neutral solution, or an alkaline solution.

[0053] From the viewpoint of increasing the reaction rate of the hydrolysis reaction and the amount of hydrogen generated per unit time, the corrosive solution is preferably an acidic or alkaline solution. From the above viewpoint, an acidic solution preferred for the corrosive solution is a solution with a pH of 7 or less, and an alkaline solution preferred for the corrosive solution is a solution with a pH of 7 or more and 11 or less. Examples of the above-mentioned acidic solutions include carbonated beverages, vinegar, citric acid, fruit juice, and acidic detergents. Examples of the alkaline solutions mentioned above include NaCl aqueous solution, body fluids, baking soda, and alkaline detergents.

[0054] From the standpoint of ease of availability and safety, the corrosive solution is preferably tap water, commercially available water, or seawater.

[0055] The temperature of the etchant is not particularly limited, but it is preferably between 20°C and 40°C. When the temperature of the etchant is within the above temperature range, the amount of hydrogen generated per unit time increases. The temperature of the etchant is more preferably 30°C or higher. The inventors speculate on the reason for this as follows. Figure 3 shows a graph of the change in the amount of hydrogen generated over time when Mg-16.2Ca is immersed in an etchant with a changed temperature. Figure 3(b) is an enlarged graph of Figure 3(a) from the start of immersion (0 min) to 20 min. Figure 4 shows a graph of the change in the amount of hydrogen generated over time when Mg-10Ca is immersed in an etchant with a changed temperature. Figure 4(b) is an enlarged graph of Figure 4(a) from the start of immersion (0 min) to 20 min. Figure 5 shows a graph of the change in the amount of hydrogen generated over time when Mg-15Ca is immersed in an etchant with a changed temperature. Figure 5(b) is a magnified graph of Figure 5(a) from the start of immersion (0 min) to 20 min. Figure 6 shows a graph of the change in hydrogen generation over time when Mg-20Ca is immersed in etching solutions at different temperatures. Figure 6(b) is a magnified graph of Figure 6(a) from the start of immersion (0 min) to 20 min. A 3.5 mass% NaCl solution was used as the etching solution. Figure 7 shows the X-ray diffraction patterns after immersion of Mg-16.2Ca in etching solutions at 20°C, 40°C, and 60°C for 2 hours. Figure 8 shows a model of hydrolysis behavior hypothesized by the inventors.

[0056] As shown in Figure 3, in the range of etching solution temperatures from 10°C to 40°C, the amount of hydrogen generated at 2 hours from the start of immersion increased as the etching solution temperature increased. However, at etching solution temperatures of 50°C and 60°C, the amount of hydrogen generated at 2 hours from the start of immersion decreased sharply compared to the case at 40°C. As shown in Figure 4, when examining the change in hydrogen generation during the first 20 minutes of immersion with Mg-16.2Ca, at etching solution temperatures of 50°C and 60°C, although the amount of hydrogen generated was higher for the first few minutes compared to the case below 40°C, the amount of hydrogen generated per unit time thereafter decreased. Similar trends were observed with other compositions, as shown in Figures 4-6.

[0057] As shown in Figure 7, a peak for Mg(OH)2 was observed regardless of the etching solution used at any temperature. Comparing the magnitude of the diffraction peaks of Mg(OH)2 after hydrolysis of Mg-16.2Ca using the etching solutions at each temperature, a larger peak was observed at 60°C compared to other temperatures. From this result, it can be inferred that the volume fraction of Mg(OH)2 in the Mg-Ca alloy is larger when hydrolysis is performed using the etching solution at 60°C compared to other temperatures.

[0058] As mentioned above, the higher the temperature of the etchant, the greater the initial hydrolysis rate and the greater the amount of hydrogen generated per unit time. However, as shown in Figure 8, as the reaction progresses, the pH increases, and passive Mg(OH)2 and Ca(OH)2 are generated and aggregate, making contact between the Mg2Ca phase and the etchant difficult, which is thought to slow down the reaction rate. On the other hand, when the temperature of the etchant is 40°C or lower, the initial reaction rate of the hydrolysis reaction is not large, and the generation rate of Mg(OH)2 and Ca(OH)2 is small, so it is thought that the generated Mg(OH)2 and Ca(OH)2 are released outside the lamellar structure by hydrogen bubbles. Therefore, when the temperature of the etchant is 40°C or lower, it is thought that the amount of hydrogen generated per unit time increases compared to when it is 50°C or higher.

[0059] Furthermore, while Ca(OH)2 is also produced during the hydrolysis of Mg-Ca alloys, it is known that Ca(OH)2 reacts with CO2 as shown in equation (4) below to form CaCO3. Considering that Ca(OH)2 can dissolve through the reaction shown in equation (4) below, it is thought that when the amount of Ca(OH)2 produced is small, Mg(OH)2, rather than Ca(OH)2, primarily inhibits the hydrolysis reaction.

[0060] Ca(OH)2+CO2→CaCO3+H2O…(4) formula

[0061] The method for generating hydrogen using the hydrogen-generating alloy according to this embodiment is to bring the hydrogen-generating alloy into contact with an etching solution. For example, the hydrogen-generating alloy may be immersed in the etching solution, or the etching solution may be sprayed or poured onto the hydrogen-generating alloy.

[0062] As described above, the hydrogen-generating alloy according to this embodiment is manufactured by casting a specific amount of metal to achieve a desired chemical composition, does not require processing to increase reactivity, and can generate a large amount of hydrogen by contacting it with an etchant. Furthermore, since the hydrogen-generating alloy according to this embodiment is a stable solid in air, it is easy to transport. Also, the condensation point of hydrogen is -252.6°C, and to handle hydrogen as a liquid, it is necessary to lower the temperature below this point. However, since the hydrogen-generating alloy according to this embodiment is a stable solid at room temperature, energy loss is small. Moreover, the volume of the hydrogen-generating alloy according to this embodiment is significantly smaller than the volume of gaseous hydrogen it can generate. Therefore, the hydrogen-generating alloy according to this embodiment can be said to be a hydrogen source that allows for the easy transport of large amounts of hydrogen.

[0063] Furthermore, readily available and easy-to-handle liquids such as tap water, seawater, or saltwater can be used as the corrosive solution. Therefore, the hydrogen-generating alloy described above is suitable for experimental use.

[0064] Furthermore, the hydrogen-generating alloy described above can also be used as a negative electrode material for magnesium batteries. Specifically, a magnesium-air battery can be constructed using oxygen from the air as the positive electrode active material and the hydrogen-generating alloy as the negative electrode active material. More specifically, electricity can be generated by arranging a negative electrode material made of the hydrogen-generating alloy and a positive electrode material that supplies electrons to air (oxygen) as the positive electrode material via a separator, and bringing them into contact with an electrolyte. As described above, the hydrogen-generating alloy according to this disclosure generates a large amount of hydrogen per unit mass, and since it is a solid hydrogen source, it also generates a large amount of hydrogen per unit volume. In other words, it generates a large amount of electricity per unit mass and per unit volume. Therefore, by using the hydrogen-generating alloy according to this disclosure, a battery with a large discharge capacity can be made. Furthermore, by controlling the metal structure of the hydrogen-generating alloy, the rate of the hydrolysis reaction can be controlled, and the power density of the battery can be adjusted.

[0065] Furthermore, the hydrogen-generating alloy described above can also be used as a hydrogen-generating agent for hydrogen power generation, which generates electricity by burning hydrogen. As stated above, the hydrogen-generating alloy according to this disclosure generates a large amount of hydrogen per unit mass, and since it is a solid hydrogen source, it also generates a large amount of hydrogen per unit volume. Therefore, by using the hydrogen-generating alloy according to this disclosure in hydrogen power generation, a relatively large amount of energy can be obtained even with a small volume of hydrogen-generating alloy.

[0066] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and can be modified in various ways. [Examples]

[0067] The following are examples of the present invention. The conditions in these examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0068] Mg-x mass%Ca (x=10, 15, 16.2, 20) was fabricated using a high-frequency induction furnace. The base materials used in the fabrication are shown below. ·Mg-10Ca: Mg-10Ca master alloy ·Mg-15Ca: Mg-15Ca master alloy ·Mg-16.2Ca: Mg-20Ca master alloy, pure Mg (99.9 mass% Mg) grains ·Mg-20Ca: Mg-20Ca master alloy The above-mentioned base material was melted in a carbon crucible under an Ar atmosphere at a temperature of 800°C, and the molten metal was poured into a stainless steel mold and air-cooled to obtain the sample. Each sample was processed with a cutter and used for the following analysis.

[0069] (XRD analysis) The phase of each sample was identified by XRD. Each sample was polished in the following order with SiC abrasive paper of #320, #500, #800, #1200, #2000, and #4000 grit, and then mirror-polished with diamond paste of particle sizes of 3 μm, 1 μm, and 0.25 μm.

[0070] (SEM-EDX analysis) The metallographic structure of each prepared sample was observed using SEM, and the chemical composition of each sample was measured using EDX. Specifically, the surface of each sample was polished using the same method as the sample used for XRD analysis, and the resulting surface was subjected to SEM-EDX analysis. Five measurement points were arbitrarily selected at a magnification of 100x, and the composition of each measurement point was analyzed by surface analysis using EDX. The average value of the compositions at the five points was taken as the chemical composition of the Mg-Ca alloy. Furthermore, the dendritic structure ratio of each sample relative to the metal structure was determined using the image analysis software imageJ. Specifically, the area ratio of the dendritic structure relative to the total area of ​​the field of view was calculated for each backscattered electron image of five fields acquired at 100x magnification, and this average value was defined as the dendritic structure ratio. Furthermore, the area ratio of the Mg2Ca phase in the lamellar structure was determined using the image analysis software imageJ. Specifically, the area ratio of the Mg2Ca phase relative to the total area of ​​the lamellar structure observed in each of the five backscattered electron images acquired at 3000x magnification was calculated, and the average value of this was taken as the area ratio of the Mg2Ca phase. Furthermore, the thicknesses of the Mg phase and Mg2Ca phase were measured at three arbitrary locations in the backscattered electron images of each obtained sample at a magnification of 10,000, and the average values ​​of these measurements were taken as the thicknesses of the Mg phase and Mg2Ca phase.

[0071] (Short-term corrosion surface observation) For observation of the corroded surface, observations were performed using a focused ion beam (FIB), scanning electron microscope (SEM), and transmission electron microscope (TEM). In this observation, the cross-section of the sample was observed after immersion of the sample surface for 10 seconds. The preparation of the sample and the method of observing the cross-section for each method are described below.

[0072] [Cross-sectional observation using FIB and SEM] Mg-15Ca was used for cross-sectional observation using FIB. Mg-15Ca obtained by casting was cut into 2mm x 5mm x 1mm pieces with a cutter, polished in that order with SiC abrasive paper of #320, #500, #800, #1200, #2000, and #4000 grit, and then mirror-polished in that order with diamond paste of particle sizes of 3μm, 1μm, and 0.25μm. The mirror-polished surface was immersed for 10 seconds in a 3.5% by mass NaCl aqueous solution prepared with tap water and salt, then rinsed in ethanol until no more bubbles were generated, and then dried. After that, carbon was deposited on the surface to protect it from the ion beam. On the surface obtained by the method described above, a rectangular parallelepiped-shaped hole perpendicular to the surface was created by irradiating it with a Ga ion beam in a FIB-SEM combined device. Subsequently, the plane along the direction of the Ga ion beam irradiation in the rectangular parallelepiped hole was observed using the SEM of the combined device.

[0073] [TEM observation] The Mg-15Ca matrix alloy before casting was used as the observation sample. Two 5mm x 5mm x 2mm samples were cut from the matrix alloy, and their surfaces were polished in the following order using SiC abrasive paper: #320, #500, #800, #1200, #2000, and #4000. After polishing, the surfaces were mirror-polished in the following order using diamond paste with particle sizes of 3μm, 1μm, and 0.25μm. The mirror-polished surfaces were immersed for 10 seconds in a 3.5% by mass NaCl aqueous solution prepared with tap water and salt, then rinsed in ethanol until no more bubbles were produced, and then dried. The two dried samples were bonded together using epoxy resin, with the mirror surfaces facing each other, and then clamped in a fixing jig and left for one day to bond. The bonded samples were cut to a thickness of 1 mm using a precision cutting machine, and both sides were polished in the following order with SiC abrasive paper: #800, #1200, #2000, #2400, #3000, #4000, and #5000, until the thickness was 100 μm or less. After polishing, the samples were milled in an ion milling apparatus until the center of the sample was sufficiently thin. The samples prepared in this way were observed using a TEM, and EDS maps showing the distribution of Mg and Ca were obtained using EDS.

[0074] (Observation of hydrolysis behavior) The hydrolysis behavior of pure Mg, Mg-10Ca, Mg-15Ca, Mg-16.2Ca, and Mg-20Ca was observed. The hydrolysis test of the above samples was performed by the water displacement method using the apparatus shown in Figure 9. In detail, the apparatus consisted of a burette filled with water, a water tank containing water, an Erlenmeyer flask for hydrolyzing the sample, and an Al bead bath for adjusting the temperature of the etchant in the Erlenmeyer flask. The Erlenmeyer flask contained a 3.5% by mass NaCl aqueous solution prepared by mixing tap water and sodium chloride (99% by mass NaCl) as the etchant, and the sample was immersed in the etchant. The opening of the Erlenmeyer flask was provided with a vent and a tube leading to the inside of the burette, and the part of the opening other than the vent and tube was covered with a rubber stopper. Each sample was immersed in the etchant for a maximum of 31 days. The sample dimensions and etchant temperature were as follows. • Sample dimensions: 10mm x 10mm x 5mm (Errors were kept within ±5% for each length.) • Etching solution temperature: 10°C, 20°C, 30°C, 40°C, 50°C, 60°C (The temperature of the etching solution was maintained within ±1°C of each of the above temperatures during immersion in an Al bead bath.)

[0075] (Measuring hydrolysis reaction rate) The hydrolysis reaction rate was determined by measuring the amount of hydrogen generated during the hydrolysis test performed on each sample using the same method as described above. However, the immersion time for each sample was set to 2 hours.

[0076] The results of each analysis are explained below.

[0077] (XRD analysis results) Figure 10 shows the XRD diffraction patterns of each sample. Since the diffraction peaks observed in each sample coincided with the Mg peak and the Mg2Ca peak, it was found that all of the prepared samples contained both an Mg phase and a Mg2Ca phase.

[0078] (SEM-EDX analysis) Figure 2 shows backscattered electron images obtained by SEM observation. In Figure 2, an image at a magnification of 10,000 is interpolated onto an image at a magnification of 100x. Table 2 shows the chemical composition of each sample calculated by EDX analysis.

[0079] [Table 2]

[0080] As shown in Table 2, the Ca content of each sample was found to be within ±1 mass% of the target composition. Furthermore, as shown in Figures 2(a) and (b), lamellar structures and dendritic structures due to primary Mg crystals were observed in the microstructures of the hypoeutectic alloys Mg-10Ca and Mg-15Ca. In addition, as shown in Figure 2(c), it was found that the eutectic alloy Mg-16.2Ca had almost no dendritic structures. In addition, as shown in Figure 2(d), lamellar structures and dendritic structures due to primary Mg2Ca crystals were observed in the hypereutectic alloy Mg-20Ca.

[0081] Table 3 and Figure 11 show the thicknesses of the Mg and Mg2Ca phases in the lamellar structure of each sample.

[0082] [Table 3]

[0083] The thickness of the Mg and Mg2Ca phases in the lamellar structure depends on the degree of supercooling during metal solidification. In this example, the thicknesses of the Mg and Mg2Ca phases in the lamellar structure were as shown in Table 3, depending on the chemical composition of each sample.

[0084] Table 4 shows the dendritic structure ratio relative to the metallic structure of each sample, and the area ratio of the Mg2Ca phase in the lamellar structure.

[0085] [Table 4]

[0086] As shown in Table 4, the area percentage of the dendritic structure was found to increase as the composition deviated from the eutectic composition of 16.2 mass%Ca. Furthermore, the area percentage of the Mg2Ca phase in the lamellar structure ranged from 34.8% to 39.4%.

[0087] (Results of observation of corroded surface) [Cross-sectional observation using FIB and SEM] Figure 12 shows the results of observing the corrosion cross-section using a FIB-SEM combined device. As shown in Figure 12, primary Mg crystals, fine Mg phases, and vacancies between the fine Mg phases were observed in the corrosion cross-section. The presence of residual primary Mg crystals suggests that the Mg2Ca phase constituting the lamellar structure was selectively hydrolyzed, forming a structure with fine vacancies as shown in Figure 12.

[0088] [TEM observation] Figure 13 shows the transmission image and EDS map obtained by TEM. Figure 13(a) is the transmission image, Figure 13(b) is the EDS map of Mg in the field of view of (a), and Figure 13(c) is the EDS map of Ca in the field of view of (a). Figure 13 shows the lamellar structure. From Figures 13(a) and (b), it can be seen that a large amount of Mg remains. This is thought to be because the Mg2Ca phase of the lamellar structure selectively reacted with the etching solution, leaving the Mg phase behind. Furthermore, from Figure 13(c), it can be seen that Ca is present on the surface of the Mg phase.

[0089] (Observation of hydrolysis behavior) Figures 14-18 show the changes over time when pure Mg, Mg-10Ca, Mg-15Ca, Mg-16.2Ca, and Mg-20Ca were immersed in an etching solution. As shown in Figures 14-18, each sample released hydrogen bubbles after 1 minute of immersion in the aqueous solution. On the other hand, the reactions of Mg-10Ca, Mg-15Ca, Mg-16.2Ca, and Mg-20Ca continued without stopping, albeit at different rates. During the reaction, the shape of the samples was not maintained for Mg-10Ca, Mg-16.2Ca, and Mg-20Ca. Mg-16.2Ca became a white powder on the 9th day, as shown in Figure 17 (d-5), and Mg-20Ca became a white powder on the 14th day, as shown in Figure 18 (e-6). When each sample was in the white powder state, no hydrogen bubbles were generated. As shown in Figure 15 (b-6), even after 31 days from the start of the reaction, Mg-10Ca did not become a completely white powder. Instead, it was a mixed powder consisting of fine white powder and a slightly whitish, coarser gray powder. Trace amounts of hydrogen bubbles were generated from the mixed powder form of Mg-10Ca. As shown in Figure 16 (c-6), the shape of the Mg-15Ca sample was maintained, and in addition, white powder had locally aggregated on the alloy surface. In this state, hydrogen bubble generation continued mainly from the parts where the shape had collapsed. For pure Mg, hydrogen bubbles were also generated after 31 days, but the amount was significantly less than that of Mg-15Ca.

[0090] (Measuring hydrolysis reaction rate) Figures 3-6 show the hydrolysis results for each sample at different erosion temperatures. As shown in Figures 3-6, a large amount of hydrogen was generated for all samples. In particular, Mg-16.2Ca and Mg-20Ca showed a large amount of hydrogen generation in the range of 20-40°C.

Claims

1. The chemical composition is, in mass%, Ca: 5-30 mass%, with the remainder being Mg and impurities. The metallic structure consists of an Mg phase, which is mainly Mg, and Mg ２ Mg, mainly Ca ２ A hydrogen-generating alloy containing a lamellar structure consisting of a Ca phase.

2. The hydrogen-generating alloy according to claim 1, wherein the chemical composition contains Ca: 16.2 to 20% by mass.

3. The hydrogen-generating alloy according to claim 1 or 2, wherein, in place of a portion of the Mg, it contains a total of 0.01 to 1% by mass of one or more elements selected from the group consisting of Ni, Cu, and Sn.

4. The Mg phase and the Mg in the lamellar structure ２ A hydrogen-generating alloy according to any one of claims 1 to 3, wherein the thickness of the Ca phase is 50 nm or more and 5000 nm or less.

5. The hydrogen-generating alloy according to any one of claims 1 to 4, wherein the metallic structure consists solely of the lamellar structure.

6. An experimental teaching material using the hydrogen-generating alloy described in any one of claims 1 to 5.

7. A negative electrode material for a magnesium battery using the hydrogen-generating alloy described in any one of claims 1 to 5.

8. A hydrogen generating agent for power generation using the hydrogen generating alloy described in any one of claims 1 to 5.

Images