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

Abstract

The present invention provides a powder that has high reaction efficiency and can maintain that reaction efficiency even when redox reactions are repeated. The powder contains 80 mass % or more iron oxide particles and a coating layer that covers the surfaces of the iron oxide particles, the coating layer being made of Cr2O3, the average coating thickness of the coating layer being 0.5 nm or more and 10 nm or less, and the average coverage of the coating layer being 60% or more.

Description

[Technical Field]

[0001] The present invention relates to a powder, a negative electrode active material for a metal-air battery, and a reactant for a hydrogen generation device. [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. 80 mass% or more of iron oxide particles; a coating layer that coats the surface of the iron oxide particles, the coating layer is made of Cr2O3, The average coating thickness of the coating layer is 0.5 nm or more and 10 nm or less, The average coverage of the coating layer is 60% or more. powder.

[0015] 2. The iron oxide particles include one or both of Fe2O3 and Fe3O4; When the iron oxide particles contain Fe2O3, the Fe2O3 crystals of the iron oxide particles and the Cr2O3 crystals of the coating layer have a crystal orientation relationship represented by the following (1): When the iron oxide particles contain Fe3O4, the Fe3O4 crystals of the iron oxide particles and the Cr2O3 crystals of the coating layer have a crystal orientation relationship represented by the following (2): 1. The powder according to 1. (0001)Fe2O3 / / (0001)Cr2O3(1) (111)Fe3O4 / / (0001)Cr2O3(2)

[0016] 3. A negative electrode active material for a metal-air battery, which uses the powder according to 1 or 2 above.

[0017] 4. A reactant for a hydrogen generating device using the powder described in 1 or 2 above. [Effects of the Invention]

[0018] 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]

[0019] [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

[0020] The present inventors have conducted extensive research into powders that have high reaction efficiency and can maintain that efficiency even after repeated oxidation-reduction reactions. As a result, they have discovered that it is effective to coat the surfaces of iron oxide particles with Cr2O3 and set the coating rate and coating thickness within a predetermined range, which has led to the completion of the present invention.

[0021] 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.

[0022] The powder according to the present invention comprises: 80 mass% or more of iron oxide particles, a coating layer that coats the surface of the iron oxide particles, the coating layer is made of Cr2O3, The average coating thickness of the coating layer is 0.5 nm or more and 10 nm or less, The average coverage of the coating layer is 60% or more.

[0023] [Iron oxide particles] The iron oxide particles serve as the base material for the powder.

[0024] (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.

[0025] 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.

[0026] (Content) The content of the iron oxide particles is set to 80 mass% or more relative to the total powder. If the content of the iron oxide particles relative to the total powder is less than 80 mass%, the amount of iron oxide contributing to the redox reaction will be small, resulting in a small amount of hydrogen and charge that can be recovered by the redox reaction using the powder. In other words, when used in a metal-air battery or hydrogen generator, sufficient reaction efficiency will not be obtained. Therefore, the content of the iron oxide particles relative to the total powder is set to 80 mass% or more. The content of the iron oxide particles relative to the total powder is preferably 85 mass% or more, and more preferably 90 mass% or more. There is no particular upper limit to the content of the iron oxide particles relative to the total powder, but in relation to the content of the coating layer, it is preferably 97 mass% or less.

[0027] (particle shape) The shape of the iron oxide particles is not particularly limited, and may be, for example, spherical, polyhedral, or plate-like.

[0028] (Average particle size) The smaller the average particle size (primary particle size) of the iron oxide particles, the larger the specific surface area and the faster the reaction rate. Therefore, the average particle size is preferably 1000 nm or less, and more preferably 700 nm or less. On the other hand, if the average particle size is too small, the coating layer becomes difficult to adhere, which may result in poor reaction efficiency when used in a metal-air battery. Therefore, the average particle size is preferably 10 nm or more, and more preferably 50 nm or more. The particle size of the iron oxide particles is measured as the circle-equivalent diameter of the iron oxide particles. The average particle size of the iron oxide particles is measured using a scanning electron microscope (SEM). More specifically, it is measured using the following method.

[0029] First, the powder is fixed onto 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.

[0030] [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 Cr2O3, has an average coating thickness of 0.5 nm to 10 nm, and has an average coverage of 60% or more.

[0031] (component) The coating layer is made of Cr2O3. When the redox reaction of iron proceeds at temperatures below 600°C, oxides with standard free energies of formation lower than those of Fe2O3 and Fe3O4 at temperatures below 600°C can exist in a thermodynamically stable state. Specific examples of such oxides include chromium oxide, zirconium oxide, niobium oxide, titanium oxide, and vanadium oxide. However, Cr2O3 has been found to be suitable as an oxide that can inexpensively coat the surface of iron oxide particles and suppress sintering. This is because Cr2O3 has a density similar to that of iron oxide, allowing for uniform mixing, and a coating layer is formed over a wide area of ​​the surface of the iron oxide particles. On the other hand, using oxides other than Cr2O3 may not achieve sufficient reaction efficiency, and the effect of suppressing sintering is insufficient, making the reaction efficiency prone to decrease when the redox reaction is repeated. Therefore, the coating layer is made of Cr2O3.

[0032] The coating layer may contain unavoidable impurities in addition to Cr2O3.

[0033] The technology described in Patent Document 1 was unable to sufficiently suppress the decrease in reaction efficiency when redox reactions were repeated. In contrast, the present invention uses Cr2O3 as a sintering inhibitor, which makes it possible to achieve high reaction efficiency and excellent charge-discharge cycle characteristics when used as an anode material for metal-air batteries, and is particularly suitable for use as an anode material for metal-air batteries with operating temperatures below 600°C.

[0034] Furthermore, in the present invention, it is important that the powder contains a coating layer. Cr2O3 functions as a sintering inhibitor by adhering to the iron oxide particles. Here, the iron oxide particles and the Cr2O3 attached thereto take either a form in which the Cr2O3 coats the surface of the iron oxide particles, or a form in which the iron oxide particles and the Cr2O3 particles aggregate to form agglomerates such as plate-like particles. That is, when the powder does not contain a coating layer, the iron oxide particles and the Cr2O3 attached thereto form agglomerates 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 evaluated as is, the same effect will not be obtained when used in metal-air batteries or hydrogen generators. Therefore, in the powder according to the present invention, a coating layer made of Cr2O3 exists on the surface of the iron oxide particles, thereby preventing a decrease in reaction efficiency when redox reactions are repeated in the actual usage environment of a metal-air battery or a hydrogen generation device. Furthermore, preferably, all of the Cr2O3 contained in the powder exists as a coating layer, thereby further preventing a decrease in reaction efficiency.

[0035] (average coverage) By covering the surfaces of the iron oxide particles with the coating layer at a high coverage rate, a sintering-inhibiting effect can be obtained, and high reaction efficiency can be maintained. If the average coverage rate of the coating layer is less than 60%, a sufficient sintering-inhibiting effect cannot be obtained. Therefore, the average coverage rate of the coating layer is set to 60% or more. The average coverage rate is preferably 85% or more. On the other hand, the upper limit of the average coverage rate is not limited and may be 100%.

[0036] (Average coating thickness) Furthermore, if the average coating thickness of the coating layer is less than 0.5 nm, a sufficient sintering suppression effect cannot be obtained. Therefore, the average coating thickness of the coating layer is set to 0.5 nm or more. The average coating thickness is preferably 3 nm or more. On the other hand, if the average coating thickness of the coating layer is greater than 10 nm, it will hinder the reduction reaction of the iron oxide particles and the oxidation reaction of the iron particles produced by the reduction reaction, and a sufficient oxidation-reduction reaction will not occur in accordance with the amount of iron oxide particles. In other words, when used in a metal-air battery or hydrogen generation device, the desired reaction efficiency will not be obtained. Therefore, the average coating thickness of the coating layer is set to 10 nm or less. The average coating thickness is preferably 9 nm or less.

[0037] The average coating 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.

[0038] (Content) The content of the coating layer relative to the total powder is not particularly limited, as long as the average coating thickness and average coverage rate described above are obtained. However, the higher the content of the coating layer, the thicker the coating layer can be and the higher the coverage rate of the coating layer can be. Therefore, the content of the coating layer is preferably 3 mass% or more, and more preferably 5 mass% or more. On the other hand, if the content of the coating layer is too high, the coating layer may be too thick. Furthermore, in relation to the amount of iron oxide, the content of the coating layer is set to 20 mass% or less. The content of the coating layer is preferably 10 mass% or less.

[0039] [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%.

[0040] (Crystal orientation relationship) When the iron oxide particles contain Fe2O3, it is preferable that the Fe2O3 crystals of the iron oxide particles and the Cr2O3 crystals of the coating layer have a crystal orientation relationship represented by the following (1): When the iron oxide particles contain Fe3O4, it is preferable that the Fe3O4 crystals of the iron oxide particles and the Cr2O3 crystals of the coating layer have a crystal orientation relationship represented by the following (2): (0001)Fe2O3 / / (0001)Cr2O3(1) (111)Fe3O4 / / (0001)Cr2O3(2) By satisfying the above conditions, the adhesion between the coating layer and the iron oxide particles is improved, peeling of the coating layer during oxidation-reduction is reduced, and a decrease in the reaction efficiency of the oxidation-reduction reaction due to repeated use can be further suppressed.

[0041] The existence of the crystal orientation relationship of (1) and (2) above can be confirmed by STEM and EDS, specifically by the method described in the Examples.

[0042] The powder according to the present invention can suppress sintering of iron oxide particles during repeated oxidation-reduction reactions and maintain reaction efficiency, and is therefore suitable for use as a reactant in a hydrogen generator that extracts hydrogen generated by oxidation reactions and as a negative electrode active material in a metal-air battery.

[0043] [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.

[0044] 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.

[0045] 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).

[0046] 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).

[0047] As described above, the operating temperature of a metal-air battery using the 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.

[0048] When the negative electrode active material is packed into a cell of a metal-air battery, it is preferable to pack the material at a high packing ratio within a range not exceeding the closest packing ratio in order to increase the battery capacity. The closest packing ratio depends on the shape and size of the particles contained in the powder.

[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 the powder according to the present invention is not particularly limited. For example, the powder can be produced by mechanically mixing iron oxide powder and Cr2O3 powder in a solvent.

[0053] (Raw material powder) In this case, the mixing ratio of the iron oxide powder is set to 80 mass% or more so that the content of the iron oxide particles relative to the total powder of the present invention is 80 mass% or more. Although there is no particular upper limit to the mixing ratio of the iron oxide powder, it is preferably set to 97 mass% or less in relation to the mixing ratio of the Cr2O3 powder.

[0054] Although there are no particular upper or lower limits for the mixing ratio of the Cr2O3 powder, it is preferable to set the mixing ratio to 3 mass% or more. This allows the content of the coating layer to be 3 mass% or more, thereby improving the average coverage and average coating thickness. On the other hand, the mixing ratio of the Cr2O3 powder is set to 20 mass% or less in relation to the mixing ratio of the iron oxide powder. This also allows the content of the coating layer to be 20 mass% or less, thereby making the coating layer thinner.

[0055] 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.

[0056] The average particle size of the iron oxide powder is not particularly limited. However, to facilitate adhesion of Cr2O3 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.

[0057] 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 particle size of the Cr2O3 powder is not particularly limited, and may be approximately several tens of nanometers to several micrometers.

[0058] (mixture) The mixing means is not particularly limited, and a mixer such as a ball mill can be used. The operating conditions of the mixer are not limited. However, for example, when using a ball mill, slowing the rotation speed of the ball mill allows Cr2O3 to adhere to the surface of the iron oxide particles to form a coating layer, prevents the particles from becoming plate-like, and improves the packing rate during compression molding of the resulting powder. 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.

[0059] The solvent used in the mixing is not particularly limited, and an alcohol such as ethanol can be suitably used.

[0060] In addition to the above-mentioned method, the powder according to the present invention can be produced by immersing iron oxide powder in a Cr-containing solution and then drying it. The above-mentioned iron oxide powder can be suitably used as the iron oxide powder to be immersed in the Cr-containing solution.

[0061] The powder according to the present invention can also be produced by vacuum-depositing Cr2O3 onto iron oxide powder. The iron oxide powders described above can be suitably used for vacuum deposition. In the case of vacuum deposition, the thickness of the coating layer can be increased by increasing the deposition time. Furthermore, the thickness of the coating layer can be increased by increasing the input power of the vacuum deposition device. [Example]

[0062] The powder according to the present invention will be described in detail below with reference to examples.

[0063] First, powder samples A to R shown in Table 1 were prepared under the conditions shown in Table 1.

[0064] For samples A to R prepared using a ball mill, Fe2O3 powder, Fe3O4 powder, and Cr2O3 powder were blended to form raw material powders, which were then mixed in a ball mill, except as noted below. 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, except as noted below. For the mixing, the raw material powders 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 raw material powder. Then, using a ball mill (Fritsch: Pulverisette 7), mixing was performed at the rotation speeds (number of revolutions) shown in Table 1 for the times shown in Table 1. For sample A, only iron oxide powder was used as the raw material powder. For sample C, a raw material powder containing 97 mass% iron oxide powder and 3 mass% Cr2O3 powder was used. For samples O to Q, raw material powder containing 97 mass% iron oxide powder and 3 mass% of the oxide powder shown in Table 1 was used, and the samples were prepared under the same conditions as for sample E. For sample R, raw material powder containing 90 mass% iron oxide powder and 10 mass% Cr2O3 powder was used.

[0065] In the examples where the preparation method was vacuum deposition, the samples were prepared by coating Fe2O3 powder or Fe3O4 powder with Cr2O3 by vacuum deposition. The Fe2O3 content, Fe3O4 content, and coating layer content shown in Table 1 were calculated using the mass of the coating layer and the mass of the Fe2O3 and Fe3O4 powders before vacuum deposition, with the difference in mass before and after vacuum deposition being defined as the mass of the coating layer.

[0066] (Measurement of average coverage and average coating thickness of coating layer) For each of the obtained samples, the presence or absence of a coating layer, the average coating rate, and the average coating thickness were determined by STEM and EDS.

[0067] First, samples A–R 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 mass concentration ratio of 0.2 or greater in the Cr2O3 mapping image. The percentage of the surface of an iron oxide particle covered by the Cr2O3-coated region was then calculated as the coating layer coverage of that particle. The average coating layer coverage of each sample was calculated as the average of the coating layer coverages of 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 coating 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 (%).

[0068] For samples B, D to N, the STEM / EDS observation described above confirmed that all of the mixed Cr2O3 was present as a coating layer. For samples C, O to Q, similar observation revealed oxide particles that were not present as a coating layer, confirming that the coating layer content was less than 3 mass%. For samples A and R, it was confirmed that no coating layer was present.

[0069] (Crystal orientation relationship) For each of the obtained samples, whether or not the above-mentioned crystal orientation relationship (1) or (2) exists between the Fe2O3 crystal or Fe3O4 crystal and the Cr2O3 crystal of the coating layer was confirmed by the following method. First, the Cr2O3 coating region was identified by the above-mentioned method. Then, the atomic arrangement of the Cr2O3 coating region and the Fe2O3 or Fe3O4 directly below it was confirmed at a magnification of 2 million times or more to confirm the existence of the crystal orientation relationship.

[0070] Table 1 shows the presence or absence of a coating layer, the average coating rate and average coating thickness, and the presence or absence of a crystal orientation relationship.

[0071] (evaluation) Each of the obtained samples was evaluated for performance as a negative electrode active material.

[0072] (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.

[0073] 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.

[0074] 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:

[0075] 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%

[0076] Next, to evaluate the rate of decrease in reaction efficiency due to repeated use, ΔM 10 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%

[0077] (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

[0078] (Decrease in reaction efficiency in a real environment) The compact was maintained at 400°C, and a reduction treatment of the iron oxide was performed 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 was repeatedly performed. The introduction of hydrogen gas and water vapor was performed while the inside of cell 6 was maintained at 1 atmosphere. The mass change ΔN due to the reduction treatment and oxidation treatment was measured.

[0079] 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 n The 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:

[0080] 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 10 / 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%

[0081] The evaluation results are shown in Table 2.

[0082] [Table 1]

[0083] [Table 2] [Industrial Applicability]

[0084] By using the powder according to the present invention, it is possible to produce a long-life reactant for a hydrogen generation device and a negative electrode active material for an iron-air battery, and therefore the present invention is useful for building a future hydrogen energy society. [Explanation of symbols]

[0085] 1 Negative electrode active material 2 air electrodes 3 Solid electrolyte 4 Negative electrode 5. Metal-air battery 6 cells 7. Sample

Claims

1. 80 mass% or more of iron oxide particles; a coating layer that coats the surface of the iron oxide particles, The coating layer is made of Cr ２ O ３ It consists of The average coating thickness of the coating layer is 0.5 nm or more and 10 nm or less, The average coverage of the coating layer is 60% or more. powder.

2. The iron oxide particles are Fe ２ O ３ and Fe ３ O ４ and The iron oxide particles are Fe ２ O ３ When the iron oxide particles contain ２ O ３ The crystal and the Cr coating layer ２ O ３ The crystal has a crystal orientation relationship represented by the following (1) with the crystal, The iron oxide particles are Fe ３ O ４ When the iron oxide particles contain ３ O ４ The crystal and the Cr coating layer ２ O ３ The crystal has a crystal orientation relationship represented by the following (2) with the crystal. The powder of claim 1. (0001)Fe ２ O ３ / / (0001)Cr ２ O ３ (1) (111)Fe ３ O ４ / / (0001)Cr ２ O ３ (2)

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

4. A reactant for a hydrogen generating device, which uses the powder according to claim 1 or 2.

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