Powder, negative-electrode active material for metal-air battery, reactant for hydrogen generator, and method for producing powder

Abstract

Provided is a powder which has a high reaction efficiency and which, even when repeatedly undergoing oxidation / reduction reactions, can retain the reaction efficiency. The powder comprises iron oxide particles and oxide particles, wherein the oxide particles include an oxide of Cr and / or Si and the oxide particles have an average particle diameter of 0.5-100 nm. The oxide particles have a ratio of the maximum Feret diameter Fmax to the minimum Feret diameter Fmin, Fmax / Fmin, of 2.0 or greater on average. The oxide particles are in contact with the iron oxide particles at an average degree of contact with the iron oxide particles of 20-80%.

Description

Powder, negative electrode active material for metal-air battery, reactant for hydrogen generation device, and method for producing powder

[0001] The present invention relates to a powder, a negative electrode active material for a metal-air battery, a reactant for a hydrogen generation device, and a method for producing the powder.

[0002] Iron is a metal that is inexpensive and can be supplied stably, so hydrogen generators and metal-air batteries are being developed that utilize the iron oxidation-reduction reaction shown below. Here, M is metallic iron and MO is iron oxide. M + H2O → MO + H2 (hydrogen release, discharge) MO + H2 → M + H2O (hydrogen storage, charge)

[0003] In order to increase the reaction efficiency of the above oxidation-reduction reaction, iron powder is usually used as the metallic iron in hydrogen generators and metal-air batteries.

[0004] Hydrogen generators and metal-air batteries utilize the iron oxidation-reduction reaction and are therefore operated at high temperatures. However, when the specific surface area of ​​iron powder is large, the iron particles that make up the iron powder sinter together under the influence of temperature, causing the iron particles to become coarse. As a result, the specific surface area of ​​the iron powder decreases, and the reaction efficiency of the oxidation-reduction reaction decreases significantly with repeated use.

[0005] In order to solve the above-mentioned problems, methods for preventing sintering of iron particles due to the reaction heat of the oxidation-reduction reaction have been investigated.

[0006] For example, Patent Document 1 discloses an anode for an air battery in which particulate iron is coated with a compound that is chemically stable at the operating temperature of the air battery, such as an oxide of aluminum or zirconium.

[0007] Patent Document 2 discloses a powder in which a sintering inhibitor is added to iron oxide, and the powder contains iron oxide particles that contain plate-like particles formed by agglomeration of iron oxide particles and the sintering inhibitor.

[0008] JP 2015-216109 A JP 2022-15292 A

[0009] According to Patent Documents 1 and 2, it is possible to provide an air battery that can maintain reaction efficiency even when redox reactions are repeated (excellent charge-discharge cycle characteristics). However, Patent Documents 1 and 2 have the following problems.

[0010] First, even when the air battery negative electrode disclosed in Patent Document 1 is used, the oxidation rate is likely to decrease due to sintering of iron, and there has been a demand for an improvement in the performance of maintaining reaction efficiency when redox reactions are repeated.

[0011] Furthermore, when iron-containing powder is used in a metal-air battery or a hydrogen generation device, it is densely packed into a cell or the like, but the iron oxide particle-containing powder disclosed in Patent Document 2 was unable to maintain sufficient reaction efficiency of the redox reaction in such an actual usage environment.

[0012] The present invention has been made in view of the above circumstances, and has as its object to provide a powder that has high reaction efficiency and can maintain the reaction efficiency even when oxidation-reduction reactions are repeated.

[0013] As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.

[0014] 1. A powder comprising iron oxide particles and oxide particles, wherein the oxide particles contain at least one of Cr and Si as an oxide, the oxide particles have an average particle size of 0.5 nm or more and 100 nm or less, the average value of the ratio Fmax / Fmin of the maximum Feret diameter Fmax to the minimum Feret diameter Fmin of the oxide particles is 2.0 or more, and the oxide particles are in contact with the iron oxide particles at an average contact rate of 20% or more and 80% or less.

[0015] 2. A negative electrode active material for a metal-air battery, using the powder described in 1 above.

[0016] 3. A reactant for a hydrogen generating device using the powder described in 1 above.

[0017] 4. A method for producing the powder according to 1 above, comprising: an adhering step of adhering a compound containing at least one of Cr and Si to iron oxide powder; a heat treatment step of heat treating the powder after the adhering step; and a reduction-oxidation step of reducing the heat-treated powder at a reduction temperature of 450°C or less, thereby oxidizing the reduced powder.

[0018] 5. The method for producing a powder according to 4 above, wherein the adhering step is a mixing step of mixing 80 mass% or more of iron oxide powder and 3 mass% or more of a powder of an oxide containing at least one of Cr and Si.

[0019] According to the present invention, it is possible to provide a powder that has high reaction efficiency and can maintain the reaction efficiency even when oxidation-reduction reactions are repeated.

[0020] The present invention relates to a metal-air battery, a method for manufacturing a metal-air battery, and a method for manufacturing the same, and a method for manufacturing the same.

[0021] The present inventors have conducted extensive research into powders that have high reaction efficiency and can maintain that reaction efficiency even after repeated oxidation-reduction reactions. As a result, they have discovered that it is effective to bring fine oxide particles containing at least one of Cr and Si as an oxide into contact with part of the surface of iron oxide particles, and have completed the present invention.

[0022] Preferred embodiments of the present invention will be described below. Note that the present invention is not limited to the following embodiments. Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0023] The powder according to the present invention is a powder comprising iron oxide particles and oxide particles, wherein the oxide particles contain at least one of Cr and Si as an oxide, the average particle diameter of the oxide particles is 0.5 nm or more and 100 nm or less, the average value of the ratio Fmax / Fmin of the maximum Feret diameter Fmax to the minimum Feret diameter Fmin of the oxide particles is 2.0 or more, and the oxide particles are present in contact with the iron oxide particles at an average contact rate of 20% or more and 80% or less.

[0024] [Iron Oxide Particles] The iron oxide particles serve as the base material for the powder.

[0025] (Type of iron oxide) The type of iron oxide constituting the iron oxide particles is not limited, and may be, for example, Fe ２ O ３ (hematite), Fe ３ O ４ (magnetite), and FeO. However, Fe ２ O ３ and Fe ３ O ４ is stably produced by the oxidation reaction of iron, so from the viewpoint of ease of production, the iron oxide is Fe ２ O ３ and Fe ３ O ４ It is preferable that the iron oxide particles contain one or both of the following: ２ O ３ and Fe ３ O ４ The content ratio of the iron oxide particles is not particularly limited. ２ O ３ The iron oxide may contain only Fe ３ O ４ It may contain only iron oxide.

[0026] The iron oxide particles may consist of iron oxide, but may also contain inevitable impurities in addition to the iron oxide. Furthermore, the iron oxide particles may contain trace amounts of additional elements. Examples of additional elements include C, Al, P, S, Cl, and Mn. The total content of the additional elements may be less than 3 mass%. The iron oxide particles may contain less than 3 mass% of at least one selected from C, Al, P, S, Cl, and Mn, with the remainder consisting of O, Fe contained as iron oxide, and inevitable impurities.

[0027] (Content) The content of the iron oxide particles is not particularly limited, but the higher the content of the iron oxide particles, the greater the amount of iron oxide that contributes to the redox reaction, and therefore the greater the amount of hydrogen and charge that can be recovered by the redox reaction, resulting in higher reaction efficiency when used in a metal-air battery or a hydrogen generator. Therefore, the content of the iron oxide particles relative to the entire powder is preferably 80 mass% or more, more preferably 85 mass% or more, and even more preferably 90 mass% or more. The upper limit of the content of the iron oxide particles relative to the entire powder is not particularly limited, but is preferably 97 mass% or less in terms of the relationship with the oxide particles.

[0028] (Particle Shape) The shape of the iron oxide particles is not particularly limited and may be, for example, spherical, polyhedral, or plate-like. From the viewpoint of increasing the filling rate in the cell of the metal-air battery, however, spherical or polyhedral shapes are preferred.

[0029] (Average Particle Diameter) The smaller the average particle diameter (primary particle diameter) of the iron oxide particles, the larger the specific surface area and the higher the reaction rate. Therefore, the average particle diameter is preferably 1000 nm or less, and more preferably 700 nm or less. On the other hand, if the average particle diameter is too small, the oxide is less likely to adhere, which may result in poor reaction efficiency when used in a metal-air battery or a hydrogen generator. Therefore, the average particle diameter is preferably 10 nm or more, and more preferably 50 nm or more. The particle diameter of the iron oxide particles is measured as the circle-equivalent diameter of the iron oxide particles. The average particle diameter of the iron oxide particles is measured using a scanning electron microscope (SEM). More specifically, it is measured using the following method.

[0030] First, the powder is fixed on a conductive tape, and carbon is evaporated to ensure conductivity, to prepare a measurement sample. The measurement sample is observed with a SEM, and the circle-equivalent diameter of the iron oxide particles is calculated by image processing.

[0031] [Oxide Particles] The oxide particles function as a sintering inhibitor for the iron oxide particles. The oxide particles contain at least one of Cr and Si as an oxide, have an average particle size of 0.5 nm to 100 nm, and have an average ratio Fmax / Fmin of the maximum Feret diameter Fmax to the minimum Feret diameter Fmin of 2.0 or more. The oxide particles are in contact with the iron oxide particles at an average contact rate of 20% to 80%.

[0032] (Components) The oxide particles contain at least one of Cr and Si as an oxide. That is, the oxide particles contain an oxide having at least one of Cr and Si. Examples of the oxide having at least one of Cr and Si include Cr ２ O ３ , (Fe, Cr) ２ O ３ , (Fe, Cr) ３ O ４ , Cr ２ MnO ４ , SiO ２ , FeSiO ４ , MnSiO ３ The oxide particles may contain multiple types of oxides. The metal elements contained as oxides in the oxide particles are not limited to Cr and Si, and may include, for example, Fe, Mn, etc. Furthermore, the oxide particles may contain a trace amount of an additive element. However, in order to more effectively suppress sintering, the total proportion of Cr and Si in the metal elements contained as oxides in the oxide particles is preferably 25% or more in atomic ratio. The upper limit of this proportion is not limited and may be 100%.

[0033] The oxide particles are preferably made of an oxide containing at least one of Cr and Si. The oxide particles may contain unavoidable impurities in addition to the oxide containing at least one of Cr and Si, and may also contain trace amounts of additional components. Examples of the additional components include C, nitrides, and oxides that do not contain Cr or Si (e.g., an oxide of at least one element selected from Al, Cl, Mn, and Fe). The total content of the additional components may be less than 3 mass%. The oxide particles may contain a total content of the additional components of less than 3 mass%, with the remainder consisting of an oxide containing at least one of Cr and Si and unavoidable impurities. The total content of C, N, Al, Cl, Mn, and Fe may also be less than 3 mass%.

[0034] (Average Particle Diameter) The average particle diameter of the oxide particles is 0.5 nm or more and 100 nm or less. If the average particle diameter is less than 0.5 nm, it is difficult to obtain a sufficient sintering suppression effect, and the reaction efficiency decreases when the redox reaction is repeated. Therefore, the average particle diameter is 0.5 nm or more. On the other hand, if the average particle diameter is greater than 100 nm, the reduction reaction of the iron oxide particles and the oxidation reaction of the iron particles produced by the reduction reaction are hindered, and a sufficient redox reaction does not occur according to the amount of iron oxide particles. In other words, the reaction efficiency decreases when used in a metal-air battery. Therefore, the average particle diameter is 100 nm or less. The average particle diameter of the oxide particles is the circle-equivalent diameter observed with a SEM. Specifically, the average particle diameter is measured by the method described in the Examples.

[0035] (Average Value of Fmax / Fmin) In the present invention, it is important that the average value of Fmax / Fmin, which is the ratio of the maximum Feret diameter Fmax to the minimum Feret diameter Fmin of the oxide particles, is 2.0 or more. If the average value of Fmax / Fmin is less than 2.0, the reaction efficiency will be poor when the oxidation-reduction reaction is repeated. That is, if the adhesion of the oxide particles to the iron oxide particles is low, the contact between the iron oxide particles and the oxide particles will be insufficient when the oxidation-reduction reaction is repeated, resulting in a decrease in reaction efficiency. Therefore, in order to improve the adhesion, it is necessary to control the shape of the oxide particles, specifically, to make the average value of Fmax / Fmin 2.0 or more. Note that the adhesion between the iron oxide particles and the oxide particles can also be ensured by aggregating the iron oxide particles and the oxide particles to form aggregates such as plate-like particles. However, if oxide particles with an average value of Fmax / Fmin less than 2.0 are used as primary particles and these aggregate to form aggregates, the density of the molded product will decrease as the amount of the aggregates increases. In metal-air batteries or hydrogen generators, if the iron-containing powder is not packed at a sufficient density, the reaction efficiency will decrease when the redox reaction is repeated. In other words, even if the reaction efficiency is maintained when the powder is used as is, the same effect will not be obtained when used in metal-air batteries or hydrogen generators. Therefore, by setting the average value of Fmax / Fmin within the above range, the powder according to the present invention can prevent a decrease in reaction efficiency when the redox reaction is repeated in the actual usage environment of the metal-air battery or hydrogen generator. Meanwhile, the upper limit of the average value of Fmax / Fmin is not particularly limited, but may be, for example, 5.0 or less.

[0036] The maximum Feret diameter Fmax, the minimum Feret diameter Fmin, and the average value of Fmax / Fmin of the oxide particles are measured by SEM, specifically by the method described in the examples.

[0037] (Average Contact Ratio) It is important that the oxide particles be in contact with the iron oxide particles to obtain the sintering-inhibiting effect. FIG. 1 shows an example of the morphology of iron oxide particles contained in the powder according to the present invention. The oxide particles shown in FIG. 1 are in contact with the iron oxide particles. Here, the contact ratio refers to the percentage of the iron oxide particles in contact with the periphery of the iron oxide particles. In the present invention, the average contact ratio, which is the average of the contact ratios, is set to 20% or more and 80% or less. That is, the oxide particles are in contact with the iron oxide particles at an average contact ratio of 20% or more and 80% or less. If the average contact ratio is less than 20%, a sufficient sintering-inhibiting effect cannot be obtained. Therefore, the average contact ratio is set to 20% or more. On the other hand, if the average contact ratio is greater than 80%, the reduction reaction of the iron oxide particles and the oxidation reaction of the iron particles produced by the reduction reaction are hindered, and a sufficient redox reaction corresponding to the amount of iron oxide particles does not occur. In other words, the reaction efficiency decreases when used in a metal-air battery. Therefore, the average contact ratio is set to 80% or less.

[0038] The average contact ratio is measured by a scanning transmission electron microscope (STEM) and energy dispersive X-ray spectroscopy (EDS). Specifically, it is measured by the method described in the examples.

[0039] [Powder] The powder according to the present invention may be composed of iron oxide particles and oxide particles. The powder may also contain other substances in addition to the iron oxide particles and oxide particles. The other substances may be any substances without particular limitations as long as they do not inhibit the oxidation-reduction reaction of the powder. The other substances may be, for example, impurities mixed in when the powder is produced. Examples of the impurities include the materials of the balls and pots of a ball mill, specifically ZrO ２ and Al ２ O ３The content of the other substances is preferably 17 mass% or less, more preferably 10 mass% or less, and even more preferably 5 mass% or less. The lower limit of the content of the other substances is not limited, and may be 0 mass%.

[0040] The powder according to this embodiment can suppress sintering of iron oxide particles during repeated oxidation-reduction reactions and maintain reaction efficiency. Therefore, the powder according to this embodiment can be suitably used as a reactant in a hydrogen generator that extracts hydrogen generated by oxidation reactions and as an anode active material in a metal-air battery.

[0041] [Negative electrode active material for metal-air battery] An anode active material for a metal-air battery according to one embodiment of the present invention is an anode active material using the above-described powder. The anode active material may consist of the above-described powder, or may contain components other than the powder. Furthermore, the anode active material may contain powder obtained by reducing iron oxide contained in the above-described powder, or may consist of powder obtained by reducing iron oxide contained in the powder.

[0042] A metal-air battery (iron-air battery) using the negative electrode active material according to this embodiment will be described with reference to Fig. 2. As shown in Fig. 2, in a metal-air battery 5, an air electrode 2 is disposed on one surface of a solid electrolyte 3, and the air electrode 2 is in contact with air. An negative electrode 4 is disposed on the opposite surface, and a negative electrode active material 1 is disposed in a sealed space in contact with the negative electrode 4.

[0043] During discharge, the reactions shown by the solid arrows in Figure 2 occur. First, O ２ binds with electrons to form O ２－ Then, an oxidation reaction of iron occurs in the negative electrode active material 1, generating hydrogen. ２－ The electrons flow out of the negative electrode, causing a current to flow from the air electrode 2 to the negative electrode 4 through a wire (not shown).

[0044] During charging, the reverse reaction to that during discharging occurs, as shown by the dashed arrow in Figure 2. First, a reduction reaction of iron occurs in the negative electrode active material 1, producing water. The produced water is then converted into hydrogen and O by the electrons that flow into the negative electrode 4. ２－ The resulting O ２－ releases electrons at the air electrode 2 and ２ This causes a current to flow from the negative electrode 4 through a wire (not shown) toward the air electrode 2.

[0045] The operating temperature of a metal-air battery using the above-described negative electrode active material is preferably less than 600° C. In addition, in order to allow the oxidation-reduction reaction to proceed sufficiently, the operating temperature is preferably 200° C. or higher.

[0046] [Reactant for Hydrogen Generator] A reactant for a hydrogen generator according to one embodiment of the present invention is a reactant using the above powder. The reactant may contain powder obtained by reducing iron oxide contained in the powder, or may consist of the powder. The reactant may contain components other than the powder.

[0047] In the hydrogen generation device using the reactant according to this embodiment, when the reactant contains iron oxide, the iron oxide may be reduced to metallic iron. Then, by bringing water vapor into contact with the reduced reactant, hydrogen can be generated as the metallic iron is oxidized.

[0048] In the hydrogen generating device using the reactant, the temperature of the reactant is preferably set to less than 600° C. Also, the temperature of the reactant is preferably set to 200° C. or higher.

[0049] [Manufacturing Method] The manufacturing method of the powder according to the present invention includes an adhering step of adhering a compound containing at least one of Cr and Si to iron oxide powder, a heat treatment step of heat treating the powder after the adhering step, and a reduction-oxidation step of reducing the heat-treated powder and oxidizing the reduced powder.

[0050] (Iron Oxide Powder) The iron oxide powder is not particularly limited, and any iron oxide powder can be used. For example, iron oxide powder available as an industrial material such as a pigment or ferrite raw material can be used. Iron oxide powder produced from ferrous sulfate or ferrous chloride by a general production method such as a wet method or a dry method can also be used. The type of iron oxide constituting the iron oxide powder is the same as that of the iron oxide particles.

[0051] The average particle size of the iron oxide powder is not particularly limited. However, in order to facilitate adhesion of the oxide particles to the iron oxide particle surface, it is preferable that the average particle size of the iron oxide powder is 50 nm or more. On the other hand, if the average particle size of the iron oxide powder is larger than 1000 nm, the specific surface area may become small and the oxidation-reduction rate may decrease. Therefore, from the viewpoint of reaction rate, it is preferable that the average particle size of the iron oxide powder is 1000 nm or less.

[0052] (Adhesion Step) In the adhesion step, a compound containing at least one of Cr and Si is adhered to the iron oxide powder. The adhesion method is not limited. For example, it may be a method of mixing the iron oxide powder with a powder of an oxide containing at least one of Cr and Si. That is, the adhesion step may be a mixing step of mixing the iron oxide powder and a powder of an oxide containing at least one of Cr and Si. In the mixing step, the mixing ratio of the iron oxide powder is 80 mass% or more, and the mixing ratio of the powder of the oxide containing at least one of Cr and Si is 3 mass% or more. In addition to the above-mentioned methods, it is also possible to employ a method of vacuum-depositing an oxide containing at least one of Cr and Si onto the iron oxide powder, or a method of immersing the iron oxide powder in a Cr-containing solution or a Si-containing solution. In other words, the adhesion step may be a vacuum deposition step of vacuum-depositing an oxide containing at least one of Cr and Si onto the iron oxide powder, or an immersion step of immersing the iron oxide powder in a Cr-containing solution or a Si-containing solution.

[0053] <Mixing step> When mixing iron oxide powder and oxide powder, the average contact rate of the final powder is set to 80% or less, and the mixing ratio of the iron oxide powder is set to 80 mass% or more in order to prevent the oxide particles from becoming coarse. The upper limit of the mixing ratio of the iron oxide powder is set to 97 mass% or less in relation to the mixing ratio of the oxide powder described below.

[0054] The mixing ratio of the oxide powder containing at least one of Cr and Si is set to 3 mass% or more, thereby making it possible to increase the content of the oxide particles in the final powder to 3 mass% or more and to increase the average contact ratio. On the other hand, in relation to the mixing ratio of the iron oxide powder, the mixing ratio is set to 20 mass% or less.

[0055] The oxide powder is not particularly limited, and examples thereof include Cr ２ O ３ , (Fe, Cr) ２ O ３ , (Fe, Cr) ３ O ４ , Cr ２ MnO ４ , SiO ２ , FeSiO ４ , MnSiO ３ Powders such as the above can be used. The type of oxide constituting the oxide powder can be the same as the oxide particles described above. Furthermore, for example, oxide powders available as reagents can be used, and oxide powders available as industrial materials can also be used. The average particle size of the oxide powder is not particularly limited, and may be on the order of several tens of nanometers to several micrometers.

[0056] The mixing means is not particularly limited, and mechanical mixing is preferred. A mixer such as a ball mill can be used as the mixing means. The operating conditions of the mixer are not limited. However, for example, when a ball mill is used, slowing down the rotation speed of the ball mill allows the oxide to adhere to the surface of the iron oxide particles, prevents the particles from becoming plate-like, and improves the packing rate of the resulting powder during compression molding. From this perspective, the rotation speed of the ball mill is preferably 400 rpm or less. Furthermore, the mixing time is preferably 1.0 hour or more.

[0057] The mixing is preferably carried out in a solvent, which is not particularly limited, and alcohol such as ethanol can be suitably used.

[0058] <Vacuum Vapor Deposition Process> When using a method of vacuum-depositing an oxide containing at least one of Cr and Si onto an iron oxide powder, the iron oxide powder described above can be suitably used as the iron oxide powder. Furthermore, the components of the oxide containing at least one of Cr and Si can be the same as the components of the oxide particles described above. In the case of vacuum vapor deposition, the oxide content and iron oxide particle content of the powder after deposition can be within the above-mentioned ranges by controlling the vapor deposition time and the input power of the vacuum vapor deposition device. Furthermore, vapor deposition may be performed multiple times. It is preferable to stir the resulting powder between each vapor deposition.

[0059] <Immersion Step> When using a method of immersing an iron oxide powder in a Cr-containing solution or a Si-containing solution, the above-mentioned iron oxide powder can be suitably used as the iron oxide powder. The Cr-containing solution or the Si-containing solution is not particularly limited, and a solution such as Cr nitrate can be used.

[0060] (Heat Treatment Step) In the heat treatment step, the powder after the adhesion step is subjected to heat treatment. By performing the heat treatment, it is possible to increase the adhesion between the oxide particles and the iron oxide particles.

[0061] In the heat treatment step, the heat treatment temperature is not particularly limited, but by increasing the heat treatment temperature, the adhesion between the oxide particles and the iron oxide particles in the final powder can be improved. Therefore, the heat treatment temperature is preferably 600°C or higher. On the other hand, if the heat treatment temperature is too high, sintering of the iron oxide powder progresses. Therefore, from the viewpoint of further increasing the reaction efficiency, the heat treatment temperature is preferably 1000°C or lower.

[0062] In the heat treatment step, the holding time of the heat treatment is not particularly limited, but by extending the holding time, the adhesion between the oxide particles and the iron oxide particles in the final powder can be improved. On the other hand, if the holding time is too long, the sintering of the iron oxide powder progresses, and sufficient reaction efficiency may not be obtained. Therefore, the holding time of the heat treatment is preferably 30 seconds or more. Furthermore, the holding time of the heat treatment is preferably 24 hours or less. More specifically, if the heat treatment temperature is 600°C, the holding time is preferably 1 hour to 24 hours, if it is 800°C, the holding time is preferably 1 minute to 30 minutes, and if it is 1000°C, the holding time is preferably 30 seconds to 10 minutes.

[0063] (Reduction-oxidation step) In the reduction-oxidation step, the heat-treated powder is reduced at a predetermined reduction temperature, and the reduced powder is then oxidized. By performing the reduction and oxidation in this manner, the oxide particles attached to the iron oxide particles shrink, and the oxide particles are cracked while maintaining their attached state, resulting in elongated oxide particles. As a result, the desired average particle size, average contact ratio, and average value of Fmax / Fmin can be achieved.

[0064] In the reduction-oxidation step, if the reduction temperature is too high, the oxide particles will change from elongated to circular, and the average value of Fmax / Fmin described above will not be achieved. Therefore, the reduction temperature is set to 450°C or less. On the other hand, although there is no particular lower limit for the reduction temperature, by increasing the reduction temperature, the time required for reduction can be shortened. Therefore, the reduction temperature is preferably 300°C or more.

[0065] In the reduction, hydrogen gas can be suitably used as a reducing agent.

[0066] The iron oxide powder can be sufficiently reduced by carrying out the reduction for an appropriate period of time, and therefore, the reduction is preferably carried out for 10 seconds or more.

[0067] In the reduction-oxidation step, the oxidation temperature during oxidation is not particularly limited, but by increasing the oxidation temperature, oxidation can be completed in a short time. Therefore, the oxidation temperature is preferably 300°C or higher. On the other hand, if the oxidation temperature is too high, sintering of the iron oxide powder progresses. Therefore, from the viewpoint of further increasing the reaction efficiency, the oxidation temperature is preferably 1000°C or lower.

[0068] In the oxidation, water vapor can be suitably used as an oxidizing agent.

[0069] By carrying out the oxidation for an appropriate period of time, the reduced powder can be sufficiently oxidized, and therefore, the oxidation is preferably carried out for 10 seconds or more.

[0070] The powder according to the present invention will be described in detail below with reference to examples.

[0071] First, powder samples A to W shown in Tables 1 and 2 were prepared under the conditions shown in Tables 1 and 2.

[0072] Among samples A to W, for the examples in which the deposition method was a ball mill, first, Fe ２ O ３ Powder, Fe ３ O ４ The powder of Fe and the powder of an oxide containing at least one of Cr and Si were mixed to prepare a raw material powder. ２ O ３ Content, Fe ３ O ４ The content and the oxide powder content were the same ratio. As the oxide containing at least one of Cr and Si, the oxides shown in Table 1 were used. Note that no oxide powder was blended with Sample A. The raw material powders were mixed in a ball mill. During the mixing, ZrO ２ The raw material powder and ZrO for mixing were placed in a container made of ２ A ball (10 mm diameter) was placed in the container, and ethanol was added in a ratio of 50 ml per 10 g of the raw material powder. Then, using a ball mill (Fritsch: Pulverisette 7), mixing was carried out for 2 hours or more at the rotation speed (number of revolutions) shown in Table 1. For example, sample T was mixed for 2 hours at a rotation speed of 600 rpm.

[0073] Next, the mixed powders were subjected to a heat treatment at 600° C. for 2 hours except for samples A and T. Samples A and T were not subjected to a heat treatment.

[0074] The heat-treated powder was then exposed to a hydrogen atmosphere at the reduction temperature shown in Table 1 to reduce the iron oxide to metallic iron, and then the metallic iron was oxidized by exposure to a water vapor-containing atmosphere at the oxidation temperature shown in Table 1 to obtain a sample. The contents of iron oxide particles and oxide particles contained in the obtained sample were equivalent to the contents of iron oxide powder and oxide containing at least one of Cr and Si in the raw material powder. Note that samples A and T were not subjected to reduction oxidation.

[0075] In addition, in the case of vacuum deposition, Fe ２ O ３ Cr is vacuum deposited on the powder ２ O ３ In this case, the Fe shown in Table 1 was used to prepare the sample. ２ O ３ The content of Fe and oxide powder is calculated by dividing the mass of oxide powder by the difference between the mass before and after vacuum deposition. ２ O ３ The mass of the powder was calculated using the mass of the powder. Next, sample S was subjected to heat treatment, reduction, and oxidation in the same manner as in the case of using the ball mill described above. The content of iron oxide particles and oxide particles contained in the obtained sample was calculated using the Fe ２ O ３ The content is equal to that of the oxide powder.

[0076] In the examples in which the adhesion method was immersion in Cr nitrate, the iron oxide powder was mixed with 35% or more of Cr (NO ３ ) ３ The iron oxide powder was immersed in a solution to form a Cr-containing compound on the surface. ３ ) ２ and Cr(NO ３ ) ３ After immersion in a solution containing a total of 35% or more of Cr(NO ３ ) ３ The substrate was immersed in the solution to form a Cr-containing compound on the surface.

[0077] Next, the immersed powder was subjected to a drying treatment at 200°C, a salt decomposition treatment at 400°C, and then a heat treatment at 600°C for 2 hours.

[0078] Next, the heat-treated powder was exposed to a hydrogen atmosphere at the reduction temperature shown in Table 2 to reduce the iron oxide to metallic iron, and then exposed to a water vapor-containing atmosphere at the oxidation temperature shown in Table 2 to oxidize the metallic iron, thereby obtaining a sample.

[0079] The oxide particles contained in the obtained powder contained the elements shown in Tables 1 and 2 as oxides.

[0080] (Measurement of Average Particle Diameter, Average Value of Fmax / Fmin, and Average Contact Ratio of Oxide Particles) The average particle diameter and average value of Fmax / Fmin of oxide particles were calculated by measurement using a SEM.

[0081] First, the obtained sample was observed by SEM at an acceleration voltage of 5 kV or less and a magnification of 100,000 times or more to obtain an image of the sample. The obtained image was read into commercially available image processing software (e.g., ImageJ), and the circle-equivalent diameter and Feret's diameter of the oxide particles were calculated.

[0082] The above-mentioned process was carried out on 10 arbitrarily selected oxide particles, and the average value of the circle-equivalent diameters of the 10 oxide particles was taken as the average particle diameter of the oxide particles.Furthermore, the maximum value (maximum Feret diameter) Fmax and the minimum value (minimum Feret diameter) Fmin of the Feret diameter for each oxide particle were calculated, and the ratio Fmax / Fmin was calculated for each oxide particle.The average value of Fmax / Fmin was then taken as the average value of Fmax / Fmin of the oxide particles of the sample.

[0083] The average contact rate was calculated by STEM and EDS.

[0084] The obtained sample was embedded in resin, and a cross section of the powder was cut out to a thickness of 50 nm or less using focused ion beam (FIB) processing, and an STEM image was taken at a magnification of approximately 640,000. Using the obtained mapping images of one or both of Cr and Si, the average contact ratio of oxide particles in contact with the surface of the iron oxide particles was calculated.

[0085] Specifically, for oxide particles containing Cr, regions with a Cr / Fe mass concentration ratio of 0.2 or more were identified as oxide particles. Then, iron oxide particle regions were identified on the microscope image, and the ratio of the length of the contact portion between the iron oxide particle region and the oxide particle region to the periphery of the iron oxide particle region was calculated and used as the contact ratio. The above process was performed on 10 fields of view, and the average of the obtained contact ratios was used as the average contact ratio.

[0086] Furthermore, for oxides containing Si among the oxide particles, regions where the Si / Fe mass concentration ratio was 0.1 or more were identified as oxide particles, and the above-mentioned processing was carried out to calculate the average contact ratio.

[0087] (Evaluation) Each of the obtained samples was evaluated for performance as a negative electrode active material.

[0088] (Reaction Efficiency and Decrease Rate of Reaction Efficiency) First, the samples according to each Example were used in powder form to evaluate the reaction efficiency and the decrease rate of reaction efficiency described below. Figure 3 shows the apparatus used for this evaluation. Cell 6 had a capacity of 20 cc, and 50 mg of sample 7 according to each Example was placed inside. Cell 6 was provided with a gas inlet and a gas outlet to allow gas flow, and was sealed except for the gas inlet and gas outlet. The gas outlet was provided with a valve to prevent the intrusion of gas from outside and to maintain the internal pressure at 1 atmosphere. Cell 6 was placed in a thermobalance (not shown) that was capable of gas introduction and heating.

[0089] The cell 6 was maintained at 400°C, and a reduction treatment of iron oxide was carried out by introducing hydrogen gas at 100 ml / min for 60 minutes, and an oxidation treatment of iron by introducing 2.8 vol% water vapor for 180 minutes was repeatedly carried out. The introduction of hydrogen gas and water vapor was carried out while the inside of the cell 6 was kept at 1 atmosphere. As shown in the reaction formula below, the reduction treatment turned iron oxide into metallic iron, and the oxidation treatment oxidized the metallic iron until it was all Fe. ３ O ４The mass of sample 7 changed as the introduction of hydrogen gas and water vapor was repeated: 4H2 + Fe3O4 → 3Fe + 4H2O (reduction treatment) 3Fe + 4H2O → 4H2 + Fe3O4 (oxidation treatment) The mass change ΔM of sample 7 due to each reduction treatment and oxidation treatment was measured.

[0090] The mass of the powder at the time when the first reduction treatment is completed (M ０ ), one oxidation treatment and one reduction treatment are considered as one cycle, and after each oxidation treatment, the mass of the first cycle (M １ ), the mass of the second cycle (M ２ ), ..., mass of the nth cycle (M ｎ At this time, the mass change ΔM ｎ M ｎ -M ０ In addition, the iron oxide contained in the sample according to each example is all converted to metallic iron in the first reduction treatment, and in the next oxidation treatment, the metallic iron is completely oxidized and all the iron contained becomes Fe. ３ O ４ The theoretical mass change in the first cycle is ΔM Ｒ It is defined as:

[0091] First, to evaluate the reaction efficiency, ΔM １ / ΔM Ｒ The evaluation criteria were as follows. If the result was 1 or 2, it could be evaluated as having excellent reaction efficiency. 1: 90% or more 2: 70% or more but less than 90% 3: 50% or more but less than 70% 4: Less than 50%

[0092] Next, to evaluate the rate of decrease in reaction efficiency due to repeated use, ΔM ２０ / ΔM １ The evaluation criteria were as follows. If the result was 1 or 2, it could be evaluated as maintaining the reaction efficiency even after repeated redox reactions. 1: 90% or more 2: 70% or more but less than 90% 3: 50% or more but less than 70% 4: Less than 50%

[0093] (Filling rate) Furthermore, assuming that the samples would actually be used as battery materials, etc., molded bodies prepared from the samples according to each example were used to evaluate the filling rate and the rate of decrease in reaction efficiency, which will be described later. 10 g of each sample was prepared and placed in a cylindrical compression molding machine, and a constant load of 0.5 MPa was applied for 1 minute to perform compression molding to produce a molded body. The density of the molded body was calculated from the volume of the obtained molded body, and the density of the molded body divided by the density of the iron oxide particles was used as the filling rate. The evaluation criteria for the filling rate were as follows: A result of 1 can be evaluated as being suitable for use as a battery material and a reactant. 1: 0.2 or more 2: Less than 0.2

[0094] (Decrease in reaction efficiency assuming a real environment) The compact was maintained at 400°C, and a reduction treatment of iron oxide was performed by introducing hydrogen gas at 100 ml / min for 60 minutes, and an oxidation treatment of iron by introducing 2.8 vol% water vapor for 180 minutes was repeatedly performed. The introduction of hydrogen gas and water vapor was performed while the inside of the cell 6 was maintained at 1 atmosphere. The mass change ΔN due to the reduction treatment and oxidation treatment was measured.

[0095] The mass of the powder at the time when the first reduction treatment is completed (N ０ ), one oxidation treatment and one reduction treatment are considered as one cycle, and after each oxidation treatment, the mass of the first cycle (N １ ), the mass of the second cycle (N ２ ), ..., mass of the nth cycle (N ｎ At this time, the mass change ΔN at the nth cycle was measured. ｎ N ｎ -N ０ In each example, the mass of the compact when the filling rate is assumed to be 1.0 is calculated as the product of the volume of the compact and the density of the iron oxide particles. Then, when an amount of powder equivalent to this mass is used and it is assumed that all of the iron contained in the powder is reduced and oxidized in the initial reduction and oxidation treatments, the theoretical mass change in the first cycle is defined as ΔN Ｃ It is defined as:

[0096] In order to evaluate the rate of decrease in reaction efficiency due to repeated use in the case where the battery is filled with the compound and used in an actual environment, ΔN２０ / ΔN Ｃ The evaluation criteria were as follows. If the result was 1 or 2, it could be evaluated as maintaining reaction efficiency even when redox reactions were repeated in an actual usage environment. 1: 30% or more 2: 20% or more but less than 30% 3: 10% or more but less than 20% 4: Less than 10%

[0097] The evaluation results are shown in Tables 3 and 4.

[0098]

[0099]

[0100]

[0101]

[0102] By using the powder according to the present invention, it is possible to produce a long-life hydrogen generation device and a negative electrode active material for an iron-air battery, and therefore the present invention is useful for building a hydrogen energy society in the future.

[0103] REFERENCE SIGNS LIST 1 negative electrode active material 2 air electrode 3 solid electrolyte 4 negative electrode 5 metal-air battery 6 cell 7 sample

Claims

1. A powder comprising iron oxide particles and oxide particles, wherein the oxide particles contain at least one of Cr and Si as an oxide, the average particle size of the oxide particles is 0.5 nm or more and 100 nm or less, the average value of the ratio Fmax / Fmin of the maximum Feret diameter Fmax to the minimum Feret diameter Fmin of the oxide particles is 2.0 or more, and the oxide particles are in contact with the iron oxide particles at an average contact rate of 20% or more and 80% or less.

2. A negative electrode active material for a metal-air battery using the powder according to claim 1.

3. A reactant for a hydrogen generating device using the powder according to claim 1.

4. A method for producing a powder according to claim 1, comprising: an adhesion step of adhering a compound containing at least one of Cr and Si to iron oxide powder; a heat treatment step of heat treating the powder after the adhesion step; and a reduction-oxidation step of reducing the heat-treated powder at a reduction temperature of 450°C or less, thereby oxidizing the reduced powder.

5. A method for producing a powder according to claim 4, wherein the adhering step is a mixing step of mixing 80 mass% or more of iron oxide powder and 3 mass% or more of powder of an oxide containing at least one of Cr and Si.

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