Powder, negative electrode active material for metal-air battery, reactant for hydrogen generation device, 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 includes iron oxide particles and a coating layer that coats the surfaces of the iron oxide particles, the coating layer being made of an oxide containing Cr, the average thickness of the coating layer being 50 nm or less, the average coverage of the coating layer being 50% or more, the coating layer having a Cr diffusion layer with a gradient composition in which the Cr / Fe concentration ratio decreases from the surface side of the coating layer in the thickness direction, and the average gradient of the Cr / Fe concentration ratio in the thickness direction at a position 3d / 4 thickness from the surface of the coating layer, where d is the thickness of the coating layer, is 0.01 / nm or more and 0.20 / nm or less.

Description

[Technical Field]

[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. [Background technology]

[0002] Iron is a metal that is inexpensive and available in stable supply, so hydrogen generators and metal-air batteries are being developed that utilize the iron redox reaction shown below, where M is metallic iron and MO is iron oxide. M + H2O → MO + H2 (hydrogen release, discharge) MO + H2 → M + H2O (hydrogen storage, charging)

[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 a negative electrode 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 aggregation of iron oxide particles and the sintering inhibitor. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-216109 [Patent Document 2] Japanese Patent Publication No. 2022-15292 Summary of the Invention [Problem to be solved by the invention]

[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 improved performance in maintaining reaction efficiency when redox reactions are repeated.

[0011] Furthermore, when iron-containing powders are used in metal-air batteries or hydrogen generators, they are densely packed into cells, etc., 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. [Means for solving the problem]

[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. Iron oxide particles; a coating layer that coats the surface of the iron oxide particles, the coating layer is made of an oxide containing Cr, The coating layer has an average thickness of 50 nm or less, The average coverage of the coating layer is 50% or more, the coating layer includes a Cr diffusion layer having a gradient composition in which the Cr / Fe concentration ratio decreases from the surface side of the coating layer in a thickness direction, where d is the thickness of the coating layer, the average gradient of the Cr / Fe concentration ratio in the thickness direction at a position 3d / 4 thickness from the surface of the coating layer is 0.01 / nm or more and 0.20 / nm or less. powder.

[0015] 2. A negative electrode active material for a metal-air battery, which uses 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 deposition step of depositing a Cr compound onto the iron oxide powder; 2. The method for producing the powder according to 1 above, further comprising a heat treatment step of subjecting the powder after adhesion to a heat treatment temperature of 600° C. or higher.

[0018] 5. The method for producing the powder according to 4 above, The method for producing a powder, wherein the adhering step is a mixing step of mixing 80 mass % or more of iron oxide powder and 3 mass % or more of Cr2O3 powder. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram for explaining a metal-air battery according to one embodiment of the present invention. [Figure 2]FIG. 1 is a schematic diagram for explaining a cell used in an example. DETAILED DESCRIPTION OF THE INVENTION

[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 discovered that it is effective to coat the surface of iron oxide powder with Cr2O3 and then perform heat treatment to coat the surface of the iron oxide particles with a coating layer that includes a Cr diffusion layer, 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 comprises: Iron oxide particles; a coating layer that coats the surface of the iron oxide particles, the coating layer is made of an oxide containing Cr, The coating layer has an average thickness of 50 nm or less, The average coverage of the coating layer is 50% or more, the coating layer includes a Cr diffusion layer having a gradient composition in which the Cr / Fe concentration ratio decreases from the surface side of the coating layer in a thickness direction, When the thickness of the coating layer is d, the average gradient of the Cr / Fe concentration ratio in the thickness direction at a position 3d / 4 thickness from the surface of the coating layer is 0.01 / nm or more and 0.20 / nm or less.

[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, and may be, for example, one or more components selected from the group consisting of Fe2O3 (hematite), Fe3O4 (magnetite), and FeO. However, because Fe2O3 and Fe3O4 are stably produced in the oxidation reaction of iron, from the viewpoint of ease of production, it is preferable that the iron oxide contain one or both of Fe2O3 and Fe3O4. Furthermore, the content ratio of Fe2O3 and Fe3O4 contained in the iron oxide particles is not particularly limited. That is, the iron oxide particles may contain only Fe2O3 as the iron oxide, or may contain only Fe3O4 as the 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 at least one element selected from C, Al, P, S, Cl, and Mn in a total content of less than 3 mass%, 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 through the redox reaction, resulting in higher reaction efficiency when used in a metal-air battery or 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 it is preferably 97 mass% or less in relation to the coating layer.

[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 packing rate in the cell of the metal-air battery, spherical or polyhedral shapes are preferred.

[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 faster 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 coating layer becomes difficult 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 by the following method.

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

[0031] [Coating layer] The coating layer covers the surface of the iron oxide particles and functions as a sintering inhibitor. The coating layer is made of an oxide containing Cr, has an average thickness of 50 nm or less, and an average coverage of 50% or more. The coating layer also includes a Cr diffusion layer having a gradient composition of Cr / Fe concentration ratio.

[0032] (component) The coating layer is made of an oxide containing Cr. The oxide containing Cr can remain stable on the surface of the iron particles even under conditions in which the iron oxide particles are reduced. For example, Cr2O3 is an oxide that is more stable than iron oxide in a temperature range below 600°C. Therefore, the use of an oxide containing Cr can achieve a high sintering suppression effect. The coating layer may be made of an oxide of Fe and Cr, or may be made of a Cr oxide and an oxide of Fe and Cr.

[0033] In the present invention, it is important that the powder contains a coating layer made of a Cr-containing oxide. The Cr-containing oxide functions as a sintering inhibitor by adhering to the iron oxide particles. Here, the iron oxide particles and the Cr-containing oxide attached to the particles take either a form in which the oxide coats the surface of the iron oxide particles, or a form in which the iron oxide particles and the oxide particles aggregate to form aggregates such as plate-like particles. That is, when the powder does not contain a coating layer, the iron oxide particles and the oxide attached to the particles form aggregates such as plate-like particles. The more agglomerates such as plate-like particles are contained, the lower the density of the compact. In metal-air batteries or hydrogen generators, if the iron-containing powder is not packed densely, the reaction efficiency during repeated oxidation-reduction reactions will decrease. 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, in the powder of the present invention, a coating layer made of a Cr-containing oxide exists on the surface of the iron oxide particles, thereby preventing a decrease in reaction efficiency when used in metal-air batteries or hydrogen generators.

[0034] (Cr diffusion layer) The coating layer may include a Cr diffusion layer. The coating layer may be made of a Cr diffusion layer.

[0035] The Cr diffusion layer has a gradient composition in which the Cr / Fe concentration ratio decreases from the surface side of the coating layer in the thickness direction, and the Cr concentration increases toward the surface side of the coating layer.

[0036] The Cr diffusion layer may contain unavoidable impurities in addition to the oxides of Cr and Fe.

[0037] (Cr oxide layer) The coating layer may include a Cr oxide layer, or may consist of a Cr diffusion layer and a Cr oxide layer disposed on the surface side of the Cr diffusion layer. The Cr oxide layer may contain Cr oxide and, in addition to Cr oxide, may contain unavoidable impurities. The Cr oxide layer may consist of Cr oxide and unavoidable impurities. That is, the coating layer may consist of oxides of Fe and Cr and unavoidable impurities, or may consist of Cr oxide, oxides of Fe and Cr, and unavoidable impurities.

[0038] (Average gradient of Cr / Fe concentration ratio) From the viewpoint of obtaining sufficient adhesion at the interface between the iron oxide particles and the coating layer, it is important that the gradient of the Cr / Fe concentration ratio near the interface between the coating layer and the iron oxide particles is small. Therefore, the average gradient of the Cr / Fe concentration ratio in the thickness direction at a position 3d / 4 thickness from the surface of the coating layer, where d is the thickness of the coating layer, is set to 0.20 / nm or less. This allows for the production of powder with high reaction efficiency and the ability to maintain reaction efficiency even after repeated redox reactions. On the other hand, even when a Cr diffusion layer is not present, i.e., when the gradient composition is not observed in the coating layer, the average gradient may slightly exceed 0. This is due to measurement accuracy reasons (e.g., the spread of the electron beam within the observation sample during transmission electron microscopy and overlapping of information in the depth direction of the observation sample). However, when a Cr diffusion layer is not present, the average gradient will never be 0.01 / nm or more, even when measurement accuracy is taken into consideration. Therefore, the average gradient is set to 0.01 / nm or more. The average gradient may be 0.010 / nm or more. Here, the thickness d of the coating layer is defined as the distance from the surface of the coating layer to the point where the Cr / Fe concentration ratio first reaches 0.03 when the Cr / Fe concentration ratio is plotted in the thickness direction of the coating layer from the surface side of the coating layer. A specific method for determining the average gradient is as described in the Examples.

[0039] (average coverage) The coating layer can exert a sintering-inhibiting effect by covering the surfaces of the iron oxide particles with a high coverage. If the average coverage of the coating layer is less than 50%, a sufficient sintering-inhibiting effect cannot be obtained. Therefore, the average coverage of the coating layer is set to 50% or more. The average coverage is preferably 70% or more, and more preferably 90% or more. On the other hand, the upper limit of the average coverage is not limited and may be 100%.

[0040] (average thickness) If the average thickness of the coating layer is greater than 50 nm, it will interfere with the reduction reaction of the iron oxide particles and the oxidation reaction of the iron particles produced by the reduction reaction, preventing a sufficient redox reaction in accordance with the amount of iron oxide particles. In other words, when used in a metal-air battery or hydrogen generator, the desired reaction efficiency will not be achieved. Therefore, the average thickness of the coating layer is set to 50 nm or less. The average thickness of the coating layer is preferably 30 nm or less. Meanwhile, the lower limit of the average thickness of the coating layer is not particularly limited and may be greater than 0 nm. However, if the average thickness of the coating layer is less than 0.5 nm, a sufficient sintering suppression effect may not be achieved. Therefore, the average thickness of the coating layer is preferably 0.5 nm or more.

[0041] The average thickness and average coverage of the coating layer are measured by a scanning transmission electron microscope (STEM) and energy dispersive X-ray spectroscopy (EDS), specifically by the methods described in the examples.

[0042] [Powder] The powder according to the present invention may be composed of iron oxide particles and a coating layer. The powder may also contain other substances in addition to the iron oxide particles and coating layer. The other substances may be any substance without particular limitation as long as they do not inhibit the redox reaction of the powder. The other substances may be, for example, impurities mixed in during the production of the powder. Examples of such impurities include the materials of 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%.

[0043] 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.

[0044] [Anode active material for metal-air batteries] 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 powder. The anode active material may consist of the 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 powder, or may consist of powder obtained by reducing iron oxide contained in the powder.

[0045] A metal-air battery (iron-air battery) using the negative electrode active material according to this embodiment will be described with reference to Fig. 1. As shown in Fig. 1, 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. A 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.

[0046] During discharge, the reactions shown by the solid arrows in Figure 1 occur. First, O2 combines with electrons at the air electrode 2 to form O 2- Then, an oxidation reaction of iron occurs in the negative electrode active material 1, generating hydrogen. 2- This causes the air electrode 2 to bond with the cathode to generate water, and electrons flow out from the anode side, causing a current to flow from the air electrode 2 to the anode 4 through a wire (not shown).

[0047] During charging, the reverse reaction to that during discharging occurs, as shown by the dashed arrow in Figure 1. 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 electrons that flow into the negative electrode 4. 2- The resulting O 2- releases electrons at the air electrode 2 to become O2, which is then released to the outside. As a result, a current flows from the negative electrode 4 to the air electrode 2 through a wire (not shown).

[0048] The operating temperature of a metal-air battery using the above-mentioned negative electrode active material is preferably less than 600° C. Furthermore, in order to allow the oxidation-reduction reaction to proceed sufficiently, the operating temperature is preferably 200° C. or higher.

[0049] [Reactant for hydrogen generation device] 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 a powder obtained by reducing iron oxide contained in the powder, or may consist of the powder. The reactant may also contain components other than the powder.

[0050] 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.

[0051] 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.

[0052] [Manufacturing method] The method for producing powder according to the present invention comprises an adhering step of adhering a Cr compound to iron oxide powder, and a heat treatment step of heat treating the powder after the adhering step.

[0053] (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.

[0054] The average particle size of the iron oxide powder is not particularly limited. However, in order to facilitate the formation of the above-described coating layer and Cr diffusion layer on the surface of the iron oxide particles, it is preferable that the average particle size of the iron oxide powder be 10 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 be 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 be 1000 nm or less.

[0055] (Attachment process) In the adhering step, a Cr compound is adhered to the iron oxide powder. The method for adhering the Cr compound to the iron oxide powder is not limited. For example, a method of mixing the iron oxide powder with Cr2O3 powder may be used. That is, the adhering step may be a mixing step of mixing the iron oxide powder with Cr2O3 powder. In the mixing step, the mixing ratio of the iron oxide powder is set to 80 mass% or more, and the mixing ratio of the Cr2O3 powder is set to 3 mass% or more. In addition to the above-mentioned methods, other methods for adhering the Cr compound to the iron oxide powder include a method of vacuum-depositing Cr2O3 onto the iron oxide powder or a method of immersing the iron oxide powder in a Cr-containing solution. In other words, the adhering step may be a vacuum deposition step of vacuum-depositing Cr2O3 onto the iron oxide powder or an immersion step of immersing the iron oxide powder in a Cr-containing solution.

[0056] <Mixing process> When iron oxide powder and Cr2O3 powder are mixed, the mixing ratio of the iron oxide powder is set to 80 mass% or more in order to reduce the average gradient and the average thickness, and 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 Cr2O3 powder.

[0057] The mixing ratio of Cr2O3 powder is set to 3 mass% or more, which increases the average coverage. On the other hand, the mixing ratio of Cr2O3 powder is set to 20 mass% or less in relation to the mixing ratio of iron oxide powder.

[0058] The Cr2O3 powder is not particularly limited, and any Cr2O3 powder can be used. For example, Cr2O3 powder available as a reagent can be used, or Cr2O3 powder available as an industrial material can also be used. The average particle size of the Cr2O3 powder is not particularly limited, and may be approximately several tens of nanometers to several micrometers.

[0059] 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 using a ball mill, slowing down the rotation speed of the ball mill allows Cr2O3 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, by extending the mixing time, the coating layer can be made thicker. From this perspective, the mixing time is preferably 1.0 hour or more.

[0060] The mixing is preferably carried out in a solvent, which is not particularly limited, and alcohol such as ethanol can be suitably used.

[0061] <Vacuum deposition process> When using a method of vacuum-depositing Cr2O3 onto iron oxide powder, the iron oxide powder described above can be suitably used. In the case of vacuum deposition, the Cr2O3 content and iron oxide particle content of the powder after deposition can be adjusted to the above-mentioned ranges by controlling the deposition time and the input power of the vacuum deposition device. Furthermore, to achieve an average coverage within the above-mentioned ranges, it is preferable to perform deposition 10 or more times. It is also preferable to stir the resulting powder between each deposition so that Cr2O3 can be deposited on almost the entire surface of the iron oxide particles.

[0062] <Soaking process> When using a method of immersing iron oxide powder in a Cr-containing solution, the above-mentioned iron oxide powders can be suitably used as the iron oxide powder. The Cr-containing solution is not particularly limited, and a solution such as Cr nitrate can be used. The number of immersions is preferably 29 times or less to prevent the coating layer from becoming too thick.

[0063] (Heat treatment process) In the heat treatment step, the powder after the adhering step is subjected to heat treatment, thereby forming a Cr diffusion layer.

[0064] In the heat treatment step, if the heat treatment temperature is low, the average gradient cannot be reduced. Therefore, the heat treatment temperature is set to 600°C or higher. On the other hand, although there is no particular upper limit to the heat treatment temperature, if the heat treatment temperature is too high, sintering of the iron oxide powder will proceed. Therefore, from the viewpoint of further increasing the reaction efficiency, the heat treatment temperature is preferably set to 1000°C or lower.

[0065] In the heat treatment step, the holding time of the heat treatment is not particularly limited, but by extending the holding time, the average gradient can be reduced. On the other hand, if the holding time is too long, 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. [Example]

[0066] The powder according to the present invention will be described in detail below with reference to examples.

[0067] First, powder samples A to S shown in Tables 1 and 2 were prepared under the conditions shown in Tables 1 and 2.

[0068] For samples A to S in which the deposition method was a ball mill, Fe2O3 powder, Fe3O4 powder, and Cr2O3 powder were first blended to form a raw material powder, which was then mixed in a ball mill. The blending ratios of the Fe2O3 powder, Fe3O4 powder, and Cr2O3 powder were the same as the Fe2O3 content, Fe3O4 content, and coating layer content shown in Table 1. For the mixing, the raw material powder and ZrO2 balls (10 mm diameter) were placed in a ZrO2 container, and ethanol was added at a ratio of 50 ml per 10 g of the raw material powder. Then, using a ball mill (Fritsch: Pulverisette 7), mixing was performed for at least two hours at the rotation speed (number of rotations) shown in Table 1. The samples were then heat-treated under the conditions shown in Table 1 to obtain them. The content of iron oxide particles contained in the obtained samples was approximately equal to the content of iron oxide powder in the raw material powder. Samples A, D, and N were not heat-treated.

[0069] In addition, for examples in which vacuum deposition was used as the deposition method, Fe2O3 powder was vacuum-deposited the number of times shown in the table to coat it with Cr2O3. Between each deposition, the resulting powder was collected and stirred. Further, samples were prepared by heat-treating the powder at 600°C for 2 hours. In this case, the Fe2O3 content and Cr2O3 content shown in Table 1 were calculated using the mass of Cr2O3 and the mass of the Fe2O3 powder before vacuum deposition, with the difference in mass before and after vacuum deposition being defined as the mass of Cr2O3. That is, the iron oxide powder content and Cr2O3 content in the powder after deposition are as shown in Table 1. Furthermore, the content of iron oxide particles contained in the obtained sample was approximately equal to the content of the iron oxide powder.

[0070] Among the examples, for the examples in which the adhesion method was immersion in chromium nitrate, the iron oxide powder was immersed in a chromium nitrate solution the number of times shown in Table 2 to adhere the chromium nitrate, and then the sample was produced by heat treating at 600°C for 2 hours. Note that sample O was not heat treated.

[0071] (Measurement of average coverage and average thickness of coating layer) For each of the obtained samples, the average coverage and average thickness of the coating layer were determined by STEM and EDS.

[0072] First, samples A to S were embedded in resin, and cross-sections of the powder were cut out to thicknesses of 50 nm or less using focused ion beam (FIB) processing. Ten mapping images were acquired using STEM / EDS at approximately 640,000x magnification. The Fe mapping image was used to identify the shape of the iron oxide particles, and the Cr mapping image was used to determine whether a coating layer was present if Cr was present in a layer along the periphery of the iron oxide particle. Next, for samples with a coating layer, the Cr2O3-coated region was defined as the area of ​​the iron oxide particle surface with a Cr / Fe concentration ratio of 0.03 or greater in the Cr2O3 mapping image. The coverage rate of the coating layer on a particle was then calculated as the percentage of the Cr2O3-coated region covering a single iron oxide particle. The average coverage rate of the coating layer for each sample was calculated as the average coverage rate of the coating layer for 10 particles. The coating thickness for each particle was calculated from the area of ​​the Cr2O3-coated region using the following formula, and the average thickness was calculated from the average of the 10 particles. t=S / C / c×100 where t is the coating thickness (nm), S is the area of ​​the Cr2O3 coating region (nm 2 ), C is the circumference of the iron oxide particle (nm), and c is the coverage of Cr2O3 (%).

[0073] (Mean gradient measurement) Next, for each of the obtained samples, the average gradient in the thickness direction of the Cr / Fe concentration ratio at a position 3d / 4 thickness from the surface of the coating layer was determined.

[0074] First, EDS line analysis was performed at a magnification of approximately 5,000,000 times using the STEM and EDS method described above. Specifically, for each oxide particle, a line segment was drawn from the surface of the coating layer of the oxide particle in the thickness direction, and the Cr / Fe concentration ratio (mass concentration ratio) was measured at 0.2 nm intervals along the line segment. The thickness d of the coating layer was then calculated as the distance from the surface of the coating layer to the point where the concentration ratio first reached 0.03. The average gradient of the concentration ratio in the region 3d / 4±d / 8 nm deep from the surface of the coating layer was then used as the gradient of the Cr / Fe concentration ratio in the thickness direction at a depth of 3d / 4 nm. Similar measurements were performed at 10 locations, and the average of the gradient values ​​was calculated as the average gradient. If the average gradient was 0.01 nm or greater, the coating layer was evaluated as having the Cr diffusion layer. For samples A, B, and N, the average gradient could not be measured because no coating layer was observed.

[0075] (evaluation) Each of the obtained samples was evaluated for performance as a negative electrode active material.

[0076] (Reaction efficiency and rate of decrease in reaction efficiency) First, the samples according to each Example were used in powder form to evaluate the reaction efficiency and the rate of decrease in reaction efficiency, which will be described later. Figure 2 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.

[0077] Cell 6 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, followed by an oxidation treatment of iron by introducing 2.8 vol% water vapor for 180 minutes. The introduction of hydrogen gas and water vapor was performed while the pressure inside cell 6 was maintained at 1 atmosphere. As shown in the reaction formula below, the reduction treatment converted iron oxide to metallic iron, and the oxidation treatment oxidized the metallic iron to Fe3O4. The 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) Therefore, the mass change ΔM of sample 7 due to each reduction treatment and oxidation treatment was measured.

[0078] Starting from the mass of the powder at the completion of the first reduction treatment (M0), one cycle of oxidation treatment and reduction treatment is counted as one cycle, and after each oxidation treatment, 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 n M n In addition, assuming that the iron oxide contained in the sample according to each example is all converted to metallic iron in the first reduction treatment, and that the metallic iron is completely oxidized in the next oxidation treatment so that all the contained iron becomes Fe3O4, the theoretical mass change in the first cycle is defined as ΔM R It is defined as:

[0079] First, to evaluate the reaction efficiency, ΔM1 / 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%

[0080] Next, to evaluate the rate of decrease in reaction efficiency due to repeated use, ΔM50 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 oxidation-reduction 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%

[0081] (Filling rate) Furthermore, assuming actual use as a battery material, etc., molded bodies prepared from the samples according to each example were evaluated for their packing ratio and the rate of decrease in reaction efficiency, which will be described later. Ten grams 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 packing ratio was determined by dividing the density of the molded body by the density of the iron oxide particles. The packing ratio was evaluated based on the following criteria: A result of 1 indicates that the sample is suitable for use as a battery material or reactant. 1:0.2 or more 2: Less than 0.2

[0082] (Decrease in reaction efficiency in a real environment) The compact was maintained at 400°C and repeatedly subjected to a reduction treatment of the iron oxide by introducing hydrogen gas at 100 ml / min for 60 minutes and an oxidation treatment of the iron by introducing 2.8 vol% water vapor for 180 minutes. The introduction of hydrogen gas and water vapor was carried out while maintaining a pressure of 1 atmosphere. The mass change ΔN due to the reduction and oxidation treatments was measured.

[0083] Starting from the mass of the powder at the time when the first reduction treatment is completed (N0), one oxidation treatment and reduction treatment is considered as one cycle, and after each oxidation treatment, 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 n N nThe mass of the compact in each example, assuming the filling rate to be 1.0, is calculated as the product of the volume of the compact and the density of the iron oxide particles. The theoretical mass change in the first cycle, assuming that an amount of powder equivalent to this mass is used and that all of the iron contained in the powder is reduced and oxidized in the initial reduction and oxidation treatments, is defined as ΔN C It is defined as:

[0084] In order to evaluate the rate of decrease in reaction efficiency due to repeated use in the case where the material is filled into a battery cell or the like and used in an actual environment, ΔN 50 / 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%

[0085] The evaluation results are shown in Tables 3 and 4.

[0086] [Table 1]

[0087] [Table 2]

[0088] [Table 3]

[0089] [Table 4] [Industrial Applicability]

[0090] 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. [Explanation of symbols]

[0091] 1 Negative electrode active material 2 air electrodes 3 Solid electrolyte 4 Negative electrode 5. Metal-air battery 6 cells 7. Sample

Claims

1. Iron oxide particles; a coating layer that coats the surface of the iron oxide particles, the coating layer is made of an oxide containing Cr, The coating layer has an average thickness of 50 nm or less, The average coverage of the coating layer is 50% or more, the coating layer includes a Cr diffusion layer having a gradient composition in which the Cr / Fe concentration ratio decreases from the surface side of the coating layer in a thickness direction, where d is the thickness of the coating layer, an average gradient of the Cr / Fe concentration ratio in the thickness direction at a position of 3d / 4 thickness from the surface of the coating layer is 0.01 / nm or more and 0.20 / nm or less. powder.

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

3. A reactant for a hydrogen generating device, which uses the powder according to claim 1.

4. an adhering step of adhering a Cr compound to the iron oxide powder; The method for producing a powder according to claim 1, further comprising a heat treatment step of subjecting the powder after the deposition to a heat treatment temperature of 600°C or higher.

5. 5. The method for producing the powder according to claim 4, The adhesion step is performed by mixing 80 mass % or more of iron oxide powder and 3 mass % or more of Cr. ２ O ３ The method for producing a powder includes a mixing step of mixing the powder with the raw material.

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