Powder, negative electrode active material for metal-air batteries, reactant for hydrogen generators, and method for producing powder.

Abstract

The present invention provides a powder that has high reaction efficiency and can maintain that reaction efficiency even after repeated oxidation-reduction reactions. The powder comprises 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 (the ratio of the maximum Ferret diameter Fmax to the minimum Ferret 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.

Description

[Technical Field]

[0001] This invention relates to powder, a negative electrode active material for metal-air batteries, a reactant for hydrogen generators, and a method for producing the powder. [Background technology]

[0002] Because iron is an inexpensive and readily available metal, hydrogen generators and metal-air batteries utilizing the oxidation-reduction reaction of iron, as shown below, are being developed. Here, M is metallic iron and MO is iron oxide. M + H2O → MO + H2 (hydrogen release, discharge) MO + H2 → M + H2O (hydrogen storage, charging)

[0003] To improve the reaction efficiency of the above oxidation-reduction reaction, iron powder is typically used as the metallic iron in hydrogen generators and metal-air batteries.

[0004] Hydrogen generators and metal-air batteries utilize the oxidation-reduction reaction of iron and are therefore used at high operating temperatures. However, when the specific surface area of ​​iron powder is large, the iron particles constituting the powder sinter due to the influence of temperature, causing the iron particles to coarseen. As a result, the specific surface area of ​​the iron powder decreases, and the reaction efficiency of the oxidation-reduction reaction drops significantly with repeated use.

[0005] To address the aforementioned problems, methods are being considered to prevent the sintering of iron particles due to the heat of reaction in oxidation-reduction reactions.

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

[0007] Patent Document 2 discloses an iron oxide particle-containing powder in which a sintering inhibitor is added to iron oxide, and which contains plate-like particles formed by the aggregation of iron oxide particles and the sintering inhibitor. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2015-216109 [Patent Document 2] Japanese Patent Publication No. 2022-15292 [Overview of the project] [Problems that the invention aims to solve]

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

[0010] Firstly, even when using the negative electrode for air batteries disclosed in Patent Document 1, a decrease in the oxidation rate due to iron sintering is likely to occur, and there was a need to improve the performance that maintains reaction efficiency when the oxidation-reduction reaction is repeated.

[0011] Furthermore, when iron-containing powders are used in metal-air batteries or hydrogen generators, they are densely packed into cells, etc. However, the iron oxide particle-containing powder disclosed in Patent Document 2 was unable to sufficiently maintain the reaction efficiency of the oxidation-reduction reaction in such actual usage environments.

[0012] This invention has been made in view of the above circumstances, and aims to provide a powder that has high reaction efficiency and can maintain that reaction efficiency even when oxidation-reduction reactions are repeated. [Means for solving the problem]

[0013] As a result of diligent research, the inventors of this invention discovered that the above objective can be achieved by adopting the following configuration, and thus completed the present invention.

[0014] 1. Iron oxide particles and, A powder containing oxide particles, 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, 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 with respect to the iron oxide particles, powder.

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

[0016] 3. A reactant for a hydrogen generator using the powder according to 1 above.

[0017] 4. An adhesion step of attaching a compound containing at least one of Cr and Si to iron oxide powder, a heat treatment step of subjecting the powder after the adhesion to heat treatment, a reduction-oxidation step of reducing the powder after the heat treatment under the condition of a reduction temperature of 450°C or lower and then oxidizing the reduced powder, comprising: A method for producing the powder according to 1 above.

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

Advantages of the Invention

[0019] According to the present invention, it is possible to provide a powder having high reaction efficiency and capable of maintaining the reaction efficiency even when the redox reaction is repeated.

Brief Description of the Drawings

[0020] [Figure 1]This is a diagram illustrating the morphology of iron oxide particles and oxide particles. [Figure 2] This is a schematic diagram illustrating a metal-air battery according to one embodiment of the present invention. [Figure 3] This is a schematic diagram illustrating the cells used in the example. [Modes for carrying out the invention]

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

[0022] Preferred embodiments of the present invention will be described below. However, the present invention is not limited to the embodiments described below. Furthermore, in this specification, numerical ranges represented by "~" mean a range that includes the numbers written before and after "~" as the lower and upper limits.

[0023] The powder according to the present invention is Iron oxide particles and, A powder containing oxide particles, 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, which is the ratio of the maximum Ferret diameter Fmax to the minimum Ferret diameter Fmin of the oxide particles, is 2.0 or greater. The oxide particles are in contact with the iron oxide particles at an average contact rate of 20% to 80%.

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

[0025] (Types of iron oxide) The type of iron oxide constituting the iron oxide particles is not limited; for example, it may be one or more components selected from the group consisting of Fe2O3 (hematite), Fe3O4 (magnetite), and FeO. However, since Fe2O3 and Fe3O4 are stably produced in the oxidation reaction of iron, from the viewpoint of ease of manufacture, it is preferable that the iron oxide contains one or both of Fe2O3 and Fe3O4. Furthermore, the ratio of Fe2O3 and Fe3O4 content in the iron oxide particles is not particularly limited. That is, the iron oxide particles may contain only Fe2O3 as iron oxide, or only Fe3O4 as iron oxide.

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

[0027] (Content) The content of the iron oxide particles is not particularly limited, but the higher the content of iron oxide particles, the greater the amount of iron oxide that contributes to the oxidation-reduction reaction, which increases the amount of hydrogen and charge that can be recovered by the oxidation-reduction reaction, and further increases the reaction efficiency when used in a metal-air battery or hydrogen generator. Therefore, the content of the iron oxide particles relative to the total 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 total powder is not particularly limited, but in relation to oxide particles, it is preferably 97 mass% or less.

[0028] (particle shape) The shape of the iron oxide particles is not particularly limited and may be spherical, polyhedral, or plate-shaped, for example, but spherical or polyhedral shapes are preferred from the viewpoint of increasing the filling rate into the metal-air battery cell.

[0029] (Average particle size) 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, oxides will not adhere easily, which may conversely reduce the reaction efficiency when used in a metal-air battery or hydrogen generator. Therefore, the average particle diameter is preferably 10 nm or more, and more preferably 50 nm or more. The equivalent circular diameter of the iron oxide particles is used as the particle diameter of the iron oxide particles. The average particle diameter of the iron oxide particles is measured by a scanning electron microscope (SEM). More specifically, it is measured by the method shown below.

[0030] First, the powder is fixed onto a conductive tape, and carbon is deposited to ensure conductivity, creating a sample for measurement. The sample is observed using a scanning electron microscope (SEM), and the equivalent circular 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 diameter of 0.5 nm to 100 nm, and have an average value of the ratio Fmax / Fmin (maximum Ferret diameter Fmax to minimum Ferret diameter Fmin) of 2.0 or more. Furthermore, the oxide particles are in contact with the iron oxide particles at an average contact rate of 20% to 80%.

[0032] (component) 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 oxides having at least one of Cr and Si include Cr2O3, (Fe,Cr)2O3, (Fe,Cr)3O4, Cr2MnO4, SiO2, FeSiO4, and MnSiO3. 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 also include, for example, Fe, Mn, etc. Furthermore, the oxide particles may contain trace amounts of additive elements. 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. There is no upper limit to this proportion, and it may be 100%.

[0033] Preferably, the oxide particles consist of an oxide having at least one of Cr and Si. The oxide particles may also contain unavoidable impurities in addition to the oxide having at least one of Cr and Si, and may contain trace amounts of additives. Examples of additives include C, nitrides, and oxides that do not contain Cr and Si (for example, oxides of at least one element selected from Al, Cl, Mn, and Fe). The total content of additives can be, for example, less than 3% by mass. The oxide particles may contain less than 3% by mass of additives in total, with the remainder consisting of an oxide having at least one of Cr and Si and unavoidable impurities. Furthermore, the total content of C, N, Al, Cl, Mn, and Fe may be less than 3% by mass.

[0034] (Average particle size) The average particle diameter of the oxide particles shall be between 0.5 nm and 100 nm. 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 oxidation-reduction reaction is repeated. Therefore, the average particle diameter shall be 0.5 nm or more. On the other hand, if the average particle diameter is greater than 100 nm, it will hinder the reduction reaction of the iron oxide particles and the oxidation reaction of the iron particles generated by the reduction reaction, and a sufficient oxidation-reduction reaction will not occur in proportion to the amount of iron oxide particles. In other words, the reaction efficiency will decrease when used in a metal-air battery. Therefore, the average particle diameter shall be 100 nm or less. The average particle diameter of the oxide particles shall be the equivalent circle diameter observed by SEM. Specifically, the average particle diameter shall be measured by the method described in the examples.

[0035] (Average value of F-max / F-min) In this invention, it is important to set the average value of Fmax / Fmin, which is the ratio of the maximum Ferret diameter Fmax to the minimum Ferret diameter Fmin of the oxide particles, to 2.0 or higher. 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. In other words, if the adhesion of the oxide particles to the iron oxide particles is low, when the oxidation-reduction reaction is repeated, the contact between the iron oxide particles and the oxide particles will be insufficient, and the reaction efficiency will decrease. Therefore, in order to improve the adhesion, it is necessary to control the shape of the oxide particles, and specifically, it is necessary to set the average value of Fmax / Fmin to 2.0 or higher. Regarding the adhesion between iron oxide particles and oxide particles, the adhesion can also be ensured by agglomerating the iron oxide particles and oxide particles to form aggregates such as plate-shaped particles. However, if oxide particles with an average value of Fmax / Fmin of less than 2.0 are used as primary particles and these aggregate to form aggregates, the density of the molded body will decrease as the amount of 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 oxidation-reduction reaction is repeated. In other words, even if the reaction efficiency is maintained when the powder is used as is for evaluation, the same effect cannot be obtained when it is used in a metal-air battery or hydrogen generator. Therefore, with the powder according to the present invention, by keeping the average value of Fmax / Fmin within the above range, it is possible to prevent a decrease in reaction efficiency when oxidation-reduction reactions are repeated in the actual operating environment of a metal-air battery or hydrogen generator. On the other hand, there is no particular upper limit to the average value of Fmax / Fmin, but it may be, for example, 5.0 or less.

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

[0037] (Average contact rate) It is important that the oxide particles are in contact with the iron oxide particles in order to obtain a sintering suppression effect. Figure 1 shows an example of the morphology of iron oxide particles and oxide particles contained in the powder according to the present invention. The oxide particles shown in Figure 1 are in contact with the iron oxide particles. Here, the ratio of the contact between the iron oxide particles and the oxide particles with respect to the outer circumference of the iron oxide particles is called the contact rate. In the present invention, the average contact rate, which is the average of the contact rates, is set to 20% or more and 80% or less. That is, the oxide particles are in contact with the iron oxide particles with an average contact rate of 20% or more and 80% or less relative to the iron oxide particles. If the average contact rate is less than 20%, a sufficient sintering suppression effect cannot be obtained. Therefore, the average contact rate is set to 20% or more. Furthermore, if the average contact rate is greater than 80%, it will hinder the reduction reaction of the iron oxide particles and the oxidation reaction of the iron particles generated by the reduction reaction, and a sufficient redox reaction will not occur according to the amount of iron oxide particles. That is, the reaction efficiency when used in a metal-air battery will decrease. Therefore, the average contact rate is set to 80% or less.

[0038] The average contact rate is measured by scanning transmission electron microscopy (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 consist of iron oxide particles and oxide particles. In addition to iron oxide particles and oxide particles, the powder may also contain other substances. The other substances are not particularly limited and may be any substances as long as they do not inhibit the oxidation-reduction reaction of the powder. The other substances may be, for example, impurities that are mixed in when the powder is manufactured. Examples of such impurities include the materials of the balls and pots of a ball mill, specifically ZrO2 and Al2O3. The 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 the sintering of iron oxide particles when oxidation-reduction reactions are repeated, thereby maintaining reaction efficiency. Therefore, the powder according to this embodiment can be suitably used as a reactant in a hydrogen generator that extracts hydrogen generated in an oxidation reaction, and as a negative electrode active material in a metal-air battery.

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

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

[0043] During discharge, the reaction shown by the solid arrow in Figure 2 occurs. First, at the air electrode 2, O2 combines with electrons to form O 2- Next, an oxidation reaction of iron occurs in the negative electrode active material 1, and hydrogen is generated. This hydrogen is O 2- Water is generated through the interaction with the other elements, and electrons flow out from the negative electrode side. As a result, a current flows from the air electrode 2 through the wiring (not shown) towards the negative electrode 4.

[0044] During charging, the reverse reaction occurs compared to the discharge reaction, 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 oxygen by electrons flowing into the negative electrode 4. 2- It is broken down into. The generated O 2- At the air electrode 2, electrons are released, becoming O2, which is then released to the outside. As a result, a current flows from the negative electrode 4 towards the air electrode 2 through the wiring (not shown).

[0045] For metal-air batteries using the aforementioned negative electrode active material, it is preferable to set the operating temperature to below 600°C. Furthermore, in order to allow the oxidation-reduction reaction to proceed sufficiently, it is preferable to set the operating temperature to 200°C or higher.

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

[0047] In the hydrogen generator using the reactant according to this embodiment, first, if the reactant contains iron oxide, the iron oxide may be reduced to metallic iron. Then, by bringing the reduced reactant into contact with water vapor, hydrogen can be generated as the metallic iron oxidizes.

[0048] In a hydrogen generator using the aforementioned reagent, it is preferable to keep the temperature of the reagent below 600°C. Furthermore, it is preferable to keep the temperature of the reagent 200°C or higher.

[0049] [Manufacturing method] The powder manufacturing method according to the present invention comprises an adhesion step of attaching a compound containing at least one of Cr and Si to iron oxide powder, a heat treatment step of subjecting the powder after adhesion to heat treatment, and a reduction-oxidation step of subjecting the powder after heat treatment to reduction and the powder after reduction to oxidation.

[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. In addition, iron oxide powder produced from ferrous sulfate or ferrous chloride by a general manufacturing method such as a wet or dry process 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, it is preferable that the average particle size of the iron oxide powder be 50 nm or larger in order to facilitate the adhesion of oxide particles to the surface of the iron oxide particles. On the other hand, if the average particle size of the iron oxide powder is greater than 1000 nm, the specific surface area will decrease, which may reduce the oxidation-reduction rate. Therefore, from the viewpoint of reaction rate, it is preferable that the average particle size of the iron oxide powder be 1000 nm or less.

[0052] (Adhesion process) In the aforementioned deposition step, a compound containing at least one of Cr and Si is deposited onto the iron oxide powder. The method of deposition is not limited. For example, it may be a method of mixing iron oxide powder with an oxide powder containing at least one of Cr and Si. In other words, the deposition step may be a mixing step of mixing iron oxide powder and an oxide powder containing at least one of Cr and Si. In the mixing step, the mixing ratio of iron oxide powder is 80 mass% or more, and the mixing ratio of oxide powder containing at least one of Cr and Si is 3 mass% or more. In addition to the method described above, a method of vacuum deposition of an oxide containing at least one of Cr and Si onto iron oxide powder, or a method of immersing iron oxide powder in a Cr-containing solution or a Si-containing solution can also be employed. In other words, the deposition step may be a vacuum deposition step of vacuum deposition of an oxide containing at least one of Cr and Si onto iron oxide powder, or an immersion step of immersing iron oxide powder in a Cr-containing solution or a Si-containing solution.

[0053] <Mixing process> When mixing iron oxide powder and oxide powder, the average contact rate of the final powder should be 80% or less, and the mixing ratio of the iron oxide powder should be 80 mass% or more to prevent coarsening of the oxide particles. The upper limit of the mixing ratio of the iron oxide powder should be 97 mass% or less, in relation to the mixing ratio of the oxide powder described later.

[0054] Furthermore, the mixing ratio of the oxide powder containing at least one of Cr and Si is set to 3 mass% or more. This ensures that the content of the oxide particles in the final powder is 3 mass% or more, thereby increasing the average contact rate. On the other hand, considering the relationship with 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 for example, powders such as Cr2O3, (Fe,Cr)2O3, (Fe,Cr)3O4, Cr2MnO4, SiO2, FeSiO4, and MnSiO3 can be used. The type of oxide constituting the oxide powder can be the same as that of the oxide particles described above. In addition, 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 in the range of several tens of nanometers to several micrometers.

[0056] The mixing method is not particularly limited, but is preferably mechanical. A mixer such as a ball mill can be used as the mixing method. The operating conditions of the mixer are not limited. However, for example, when using a ball mill, slowing down the rotation speed of the ball mill allows the oxide to adhere to the surface of the iron oxide particles, prevents the particle shape from becoming plate-like, and improves the filling rate when the resulting powder is compressed. From this viewpoint, the rotation speed of the ball mill is preferably 400 rpm or less. In addition, the mixing time is preferably 1.0 hour or more.

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

[0058] <Vacuum deposition process> When using a method of vacuum deposition of an oxide containing at least one of Cr and Si onto 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 those of the oxide particles described above. In the case of vacuum deposition, the content of the oxide and the iron oxide particles in the deposited powder can be kept within the above-mentioned range by controlling the deposition time and the power input to the vacuum deposition apparatus. Furthermore, deposition may be performed multiple times. It is preferable to stir the obtained powder between each deposition.

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

[0060] (Heat treatment process) In the heat treatment process, the powder after the adhesion process is subjected to heat treatment. By applying heat treatment, the adhesion between the oxide particles and the iron oxide particles can be improved.

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

[0062] In the heat treatment step, the holding time is not particularly limited, but by increasing the holding time, the adhesion between the oxide particles and 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 will progress, and sufficient reaction efficiency may not be obtained. For this reason, the holding time for the heat treatment is preferably 30 seconds or more. Furthermore, the holding time for the heat treatment is preferably 24 hours or less. More specifically, if the heat treatment temperature is 600°C, it is preferably 1 hour to 24 hours; if it is 800°C, it is preferably 1 minute to 30 minutes; and if it is 1000°C, it is preferably 30 seconds to 10 minutes.

[0063] (Reduction-oxidation process) In the reduction-oxidation process, the heat-treated powder is reduced at a predetermined reduction temperature, and then the reduced powder is further oxidized. By performing reduction and oxidation in this manner, the oxide particles attached to the iron oxide particles shrink, causing the oxide particles to break while maintaining their attached state, resulting in elongated oxide particles. Therefore, the desired average particle size, average contact rate, and average Fmax / Fmin can be achieved.

[0064] In the reduction-oxidation step, if the reduction temperature is too high, the oxide particles will change from an elongated shape to something closer to a circular shape, making it impossible to achieve the average value of Fmax / Fmin mentioned above. Therefore, the reduction temperature should be 450°C or lower. On the other hand, there is no particular lower limit to the reduction temperature, but increasing the reduction temperature can shorten the time required for reduction. Therefore, a reduction temperature of 300°C or higher is preferable.

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

[0066] By performing the reduction for an appropriate amount of time, the iron oxide powder can be sufficiently reduced. Therefore, it is preferable to perform the reduction for 10 seconds or more.

[0067] In the reduction-oxidation step, the oxidation temperature is not particularly limited, but increasing the oxidation temperature allows for faster oxidation. Therefore, an oxidation temperature of 300°C or higher is preferable. On the other hand, if the oxidation temperature is too high, the iron oxide powder will sinter. Therefore, from the viewpoint of further improving reaction efficiency, an oxidation temperature of 1000°C or lower is preferable.

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

[0069] By performing the oxidation for an appropriate amount of time, the reduced powder can be sufficiently oxidized. Therefore, it is preferable to perform the oxidation for 10 seconds or more. [Examples]

[0070] The powder according to the present invention will be described in detail below based on 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] For samples A to W, in the example where the deposition method was ball milling, first, Fe2O3 powder, Fe3O4 powder, and oxide powder containing at least one of Cr and Si were blended to create the raw material powder. The blending ratios were the same as those shown in Table 1 for the Fe2O3 content, Fe3O4 content, and oxide powder content. The oxides used as at least one of Cr and Si were the types of oxides shown in Table 1. Note that no oxide powder was blended for sample A. The raw material powder was then mixed in a ball mill. For the mixing, the raw material powder and ZrO2 balls (10 mmφ) for mixing were placed in a ZrO2 container, and ethanol was added at a ratio of 50 ml per 10 g of raw material powder. The mixture was then mixed for at least 2 hours using a ball mill (Fritsch: pulverisette 7) at the rotational speed (revolutions per minute) shown in Table 1. For example, sample T was mixed at a rotational speed of 600 rpm for 2 hours.

[0073] Next, samples other than A and T were subjected to heat treatment at 600°C for 2 hours. Samples A and T were not subjected to heat treatment.

[0074] Next, the heat-treated powder was exposed to a hydrogen atmosphere at the reduction temperature shown in Table 1 to reduce the iron oxide to metallic iron, and then exposed to a water vapor-containing atmosphere at the oxidation temperature shown in Table 1 to oxidize the metallic iron and obtain a sample. The content of iron oxide particles and oxide particles in the obtained sample is equivalent to the content of iron oxide powder and oxides containing at least one of Cr and Si in the raw material powder. Samples A and T were not subjected to reduction oxidation.

[0075] Furthermore, in the example where vacuum deposition was used as the deposition method, a sample was prepared by coating Fe2O3 powder with Cr2O3 using vacuum deposition. In this case, the Fe2O3 content and oxide powder content shown in Table 1 were calculated using the difference in mass before and after vacuum deposition as the mass of the oxide powder, and the mass of the Fe2O3 powder before vacuum deposition. Next, sample S was subjected to heat treatment, reduction, and oxidation in the same manner as when using the ball mill described above. The content of iron oxide particles and oxide particles in the obtained sample is equivalent to the Fe2O3 content and oxide powder content shown in Table 1.

[0076] In the examples where the adhesion method was immersion in nitric acid (Cr), iron oxide powder was immersed in a Cr(NO3)3 solution of 35% or more to form a Cr-containing compound on the surface. In the example where the preparation method was immersion in nitric acid (Mn) followed by immersion in nitric acid (Cr), iron oxide powder was immersed in a solution containing a total of 35% or more of Mn(NO3)2 and Cr(NO3)3, and then immersed in a Cr(NO3)3 solution of 35% or more 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 by holding it 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 and obtain the sample.

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

[0080] (Measurement of average particle size of oxide particles, average Fmax / Fmin ratio, and average contact rate) The average particle size of oxide particles and the average Fmax / Fmin values ​​were calculated using SEM measurements.

[0081] First, the obtained sample was observed using SEM at an acceleration voltage of 5kV or less and a magnification of 100,000x or more to obtain an image of the sample. The obtained image was loaded into commercially available image processing software (such as ImageJ), and the equivalent circular diameter and Ferret diameter of the oxide particles were calculated.

[0082] The above-described process was performed on 10 arbitrarily selected oxide particles, and the average of the equivalent circular diameters of the 10 oxide particles was taken as the average particle diameter of the oxide particles. In addition, the maximum value (maximum Ferret diameter) Fmax and the minimum value (minimum Ferret diameter) Fmin of the Ferret diameter were calculated for each oxide particle, and their ratio Fmax / Fmin was calculated for each oxide particle. The average value of Fmax / Fmin was then taken as the average Fmax / Fmin of the oxide particles in the sample.

[0083] Furthermore, the average contact rate was calculated using STEM and EDS.

[0084] The obtained samples were embedded in resin, and powder cross-sections were cut to a thickness of 50 nm or less using focused ion beam (FIB) processing. STEM images were acquired at a magnification of approximately 640,000x. Using the obtained mapping images of either or both Cr and Si, the average contact rate of oxide particles in contact with the iron oxide particle surface was calculated.

[0085] Specifically, among the oxide particles, for oxides containing Cr, regions with a Cr / Fe mass concentration ratio of 0.2 or higher were identified as oxide particles. Then, regions of iron oxide particles were identified on the microscope image, and the ratio of the length of the contact area between the iron oxide particle region and the oxide particle region to the length of the outer circumference of the iron oxide particle region was calculated and defined as the contact rate. The above process was performed for 10 fields of view, and the average value of the obtained contact rates was defined as the average contact rate.

[0086] Furthermore, for oxide particles containing Si, the region with a Si / Fe mass concentration ratio of 0.1 or higher was identified as an oxide particle, and the above processing was performed to calculate the average contact rate.

[0087] (evaluation) The performance of each obtained sample as a negative electrode active material was evaluated.

[0088] (Reaction efficiency and rate of decrease in reaction efficiency) First, the reaction efficiency and the rate of decrease in reaction efficiency, as described later, were evaluated using the samples for each example in powder form. Figure 3 shows the apparatus used for this evaluation. Cell 6 has a capacity of 20 cc and was filled with 7:50 mg of the sample for each example. Cell 6 was equipped 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 equipped with a valve to prevent the ingress of gas from the outside and to maintain the internal pressure at 1 atmosphere. Cell 6 was placed on a thermobalance (not shown) capable of gas introduction and heating.

[0089] Cell 6 was maintained at 400°C, and the reduction treatment of iron oxide was repeatedly performed by introducing hydrogen gas at 100 ml / min for 60 minutes, followed by the oxidation treatment of iron by introducing 2.8 vol% water vapor for 180 minutes. The introduction of hydrogen gas and water vapor was carried out while the inside of cell 6 was maintained at 1 atmosphere. As shown in the reaction equation below, the reduction treatment converted iron oxide to metallic iron, and the oxidation treatment oxidized the metallic iron to Fe3O4. Then, by repeatedly introducing hydrogen gas and water vapor, the mass of sample 7 changed. 4H2 + Fe3O4 → 3Fe + 4H2O (reduction treatment) 3Fe + 4H2O → 4H2 + Fe3O4 (oxidation treatment) Therefore, the mass change ΔM of sample 7 after each reduction and oxidation treatment was measured.

[0090] Starting from the mass of the powder at the completion of the first reduction treatment (M0), one oxidation treatment and one reduction treatment constitute one cycle. After the oxidation treatment of each cycle, the mass of the first cycle (M1), the mass of the second cycle (M2), ..., the mass of the nth cycle (M n ) was measured. At this time, the mass change ΔM in the nth cycle was measured. n to M n-Define it as -M0. Also, assuming that all the iron oxide contained in the sample according to each example becomes metallic iron in the first reduction treatment and all the iron contained after the metallic iron is completely oxidized in the next oxidation treatment becomes Fe3O4, the theoretical mass change in the first cycle is defined as ΔM R Define it as follows.

[0091] First, in order to evaluate the reaction efficiency, ΔM1 / ΔM R was evaluated. The evaluation criteria were as follows. If the result is between 1 and 2, it can be evaluated as having excellent reaction efficiency. 1: 90% or more 2: 70% or more and less than 90% 3: 50% or more and less than 70% 4: Less than 50%

[0092] Subsequently, in order to evaluate the reduction rate of the reaction efficiency due to repeated use, ΔM 20 / ΔM1 was evaluated. The evaluation criteria were as follows. If the result is between 1 and 2, it can be evaluated that the reaction efficiency is maintained even when the redox reaction is repeated. 1: 90% or more 2: 70% or more and less than 90% 3: 50% or more and less than 70% 4: Less than 50%

[0093] (Packing ratio) Furthermore, assuming actual use as a battery material or the like, using the molded body prepared from the sample according to each example, the packing ratio and the reduction rate of the reaction efficiency described below were evaluated. 10 g of each sample was prepared, put into a cylindrical compression molding device, and compressed and molded by applying a constant load of 0.5 MPa for 1 minute. From the volume of the obtained molded body, the density of the molded body was calculated, and the value obtained by dividing the density of the molded body by the density of the iron oxide particles was defined as the packing ratio. The evaluation criteria for the packing ratio were as follows. If the result is 1, it can be evaluated as suitable for use as a battery material and a reactant. 1: 0.2 or more 2: Less than 0.2

[0094] (Reduction rate of reaction efficiency assuming an actual environment) The molded body was maintained at 400°C, and the reduction treatment of iron oxide by introducing hydrogen gas at 100 ml / min for 60 minutes, and the oxidation treatment of iron by introducing 2.8 vol% water vapor for 180 minutes were repeated. The introduction of hydrogen gas and water vapor was carried out while the inside of cell 6 was maintained at 1 atmosphere. The mass change ΔN due to the reduction and oxidation treatments was measured.

[0095] Starting from the mass of the powder at the completion of the first reduction treatment (N0), one oxidation treatment and one reduction treatment constitute one cycle. After the oxidation treatment of each cycle, the mass of the first cycle (N1), the mass of the second cycle (N2), ..., the mass of the nth cycle (N n ) was measured. At this time, the mass change ΔN in the nth cycle was measured. n to N n -N0 is defined as follows. Furthermore, for each example, the mass of the molded body, assuming the packing rate is 1.0, is determined as the product of the volume of the molded body and the density of the iron oxide particles. Then, assuming that an amount of powder equivalent to that mass is used, and that all 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 given by ΔN. C This is how it is defined.

[0096] To evaluate the rate of decrease in reaction efficiency due to repeated use, assuming an environment in which the battery cells are actually used after being filled, ΔN 20 / ΔN C The following criteria were used for evaluation. A result of 1-2 indicates that the reaction efficiency can be maintained even when oxidation-reduction reactions are repeated in actual usage environments. 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] [Table 1]

[0099] [Table 2]

[0100] [Table 3]

[0101] [Table 4] [Industrial applicability]

[0102] Since the powder according to the present invention can be used to manufacture long-life hydrogen generators and negative electrode active materials for iron-air batteries, the present invention is useful for building a future hydrogen energy society. [Explanation of Symbols]

[0103] 1 Negative electrode active material 2. Air electrodes 3 Solid electrolyte 4 Negative electrode 5 Metal-air battery 6 cells 7 samples

Claims

1. Iron oxide particles and, A powder containing oxide particles, The average particle size of the iron oxide particles is 50 nm or more. The oxide particles include an oxide having at least one of Cr and Si, The oxide is a Cr oxide, an Fe and Cr oxide, a Cr and Mn oxide, a Si oxide, an Fe and Si oxide, a Mn and Si oxide, or a Cr and Si 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, which is the ratio of the maximum Ferret diameter Fmax to the minimum Ferret diameter Fmin of the oxide particles, is 2.3 or more. The oxide particles are in contact with the iron oxide particles at an average contact rate of 20% to 80%. powder.

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

3. A reaction agent for a hydrogen generator using the powder described in claim 1.

4. A bonding step involves attaching a compound containing at least one of Cr and Si to iron oxide powder, A heat treatment step in which the powder after adhesion is subjected to heat treatment, The system comprises a reduction-oxidation step in which the heat-treated powder is subjected to reduction at a reduction temperature of 450°C or lower, and the reduced powder is subjected to oxidation. A method for producing the powder according to claim 1.

5. A method for producing the powder according to claim 4, The aforementioned adhesion step is Iron oxide powder: 80 mass% or more, A method for producing powder, comprising a mixing step of mixing an oxide powder containing at least one of Cr and Si: 3 mass% or more.

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