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

Abstract

Provided is a powder which has high reaction efficiency and which can sustain reaction efficiency even if an oxidation-reduction reaction is repeated. The powder includes 80 mass% or more of iron oxide particles and a covering layer that covers the surface of the iron oxide particles. The covering layer comprises Cr2O3. The average covering thickness of the covering layer is 0.5-10 nm. The average coverage rate of the covering layer is 60% or more.

Description

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

[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.

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

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

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

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

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

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

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

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

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

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

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

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

[0014] 1. A powder comprising 80 mass% or more iron oxide particles and a coating layer covering the surfaces of the iron oxide particles, wherein the coating layer is made of Cr. ２ O ３ wherein the coating layer has an average coating thickness of 0.5 nm or more and 10 nm or less, and the coating layer has an average coverage of 60% or more.

[0015] 2. The iron oxide particles are Fe ２ O ３ and Fe ３ O ４ and wherein the iron oxide particles contain Fe ２ O ３ When the iron oxide particles contain ２ O ３ The crystal and the Cr coating layer ２ O ３ The iron oxide particles have a crystal orientation relationship represented by the following (1) with respect to the Fe３ O ４ When the iron oxide particles contain ３ O ４ The crystal and the Cr coating layer ２ O ３ 2. The powder according to 1 above, having a crystal orientation relationship represented by the following (2) with respect to the (0001) Fe crystal. ２ O ３ / / (0001)Cr ２ O ３ (1) (111)Fe ３ O ４ / / (0001)Cr ２ O ３ (2)

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

[0017] 4. A reactant for a hydrogen generating device using the powder according to 1 or 2 above.

[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.

[0019] 1 is a schematic diagram for explaining a metal-air battery according to one embodiment of the present invention, and FIG. 2 is a schematic diagram for explaining a cell used in Examples.

[0020] The present inventors have conducted extensive research into powders that have high reaction efficiency and can maintain the reaction efficiency even after repeated oxidation-reduction reactions. ２ O ３ The present inventors have found that it is effective to coat the surfaces of iron oxide particles with a coating agent and to set the coating rate and thickness within a predetermined range, and have completed 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 is a powder comprising 80 mass % or more of iron oxide particles and a coating layer covering the surfaces of the iron oxide particles, wherein the coating layer is made of Cr. ２ O ３ The coating layer has an average coating thickness of 0.5 nm or more and 10 nm or less, and the coating layer has an average coverage of 60% or more.

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

[0024] (Type of iron oxide) The type of iron oxide constituting the iron oxide particles is not limited, and may be, for example, Fe ２ O ３ (hematite), Fe ３ O ４ (magnetite), and FeO. However, Fe ２ O ３ and Fe ３ O ４ is stably produced by the oxidation reaction of iron, so from the viewpoint of ease of production, the iron oxide is Fe ２ O ３ and Fe ３ O ４ It is preferable that the iron oxide particles contain one or both of the following: ２ O ３ and Fe ３ O ４ The content ratio of the iron oxide particles is not particularly limited. ２ O ３ The iron oxide may contain only Fe ３ O ４ It may contain only iron oxide.

[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 less than 3 mass% of at least one selected from C, Al, P, S, Cl, and Mn, with the remainder consisting of O, Fe contained as iron oxide, and inevitable impurities.

[0026] (Content) The content of the iron oxide particles is 80 mass% or more relative to the entire powder. If the content of the iron oxide particles relative to the entire powder is less than 80 mass%, the amount of iron oxide contributing to the redox reaction is reduced, resulting in a reduced 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 a hydrogen generator, sufficient reaction efficiency cannot be obtained. Therefore, the content of the iron oxide particles relative to the entire powder is 80 mass% or more. The content of the iron oxide particles relative to the entire 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 entire powder, but it is preferably 97 mass% or less in relation to the content of the coating layer.

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

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

[0030] [Coating Layer] The coating layer covers the surface of the iron oxide particles and functions as a sintering inhibitor. ２ O ３ The average coating thickness is 0.5 nm or more and 10 nm or less, and the average coating rate is 60% or more.

[0031] (Component) The coating layer is made of Cr ２ O ３ When the oxidation-reduction reaction of iron proceeds in a temperature range below 600°C, the standard free energy of formation in the temperature range below 600°C is Fe ２ O ３ and Fe ３ O ４ If the oxide is smaller than 10 ... ２ O ３ It was found that Cr ２ O ３This is because Cr has a density similar to that of iron oxide, so it can be mixed uniformly, and a coating layer is formed over a wide area on the surface of the iron oxide particles. ２ O ３ If an oxide other than Cr is used, sufficient reaction efficiency may not be obtained, and the effect of suppressing sintering is not sufficient, so the reaction efficiency is likely to decrease when the oxidation-reduction reaction is repeated. ２ O ３ It shall consist of the following.

[0032] The coating layer is made of Cr ２ O ３ In addition, it may contain unavoidable impurities.

[0033] The technique described in Patent Document 1 was unable to sufficiently suppress the decrease in reaction efficiency when the oxidation-reduction reaction was repeated. In contrast, the present invention uses Cr as a sintering inhibitor. ２ O ３ By using this material as an anode material for a metal-air battery, high reaction efficiency and excellent charge-discharge cycle characteristics can be achieved, and it can be particularly suitably used as an anode material for a metal-air battery whose operating temperature is less than 600°C.

[0034] In the present invention, it is important that the powder includes a coating layer. ２ O ３ The Cr adheres to the iron oxide particles and functions as a sintering inhibitor. ２ O ３ is the surface of the iron oxide particles. ２ O ３ or iron oxide particles and Cr ２ O ３ In other words, when the powder does not contain a coating layer, the powder is composed of iron oxide particles and Cr particles attached to the particles. ２ O ３The iron-containing powder forms aggregates such as plate-like particles. The more aggregates such as plate-like particles are contained, the lower the density of the compact. In a metal-air battery or a hydrogen generator, 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 and evaluated, the same effect cannot be obtained when used in a metal-air battery or a hydrogen generator. Therefore, in the powder according to the present invention, Cr ２ O ３ The presence of the coating layer covering the surface of the iron oxide particles can prevent a decrease in reaction efficiency when redox reactions are repeated in the actual use environment of a metal-air battery or a hydrogen generating device. ２ O ３ When all of these are present as a coating layer, the decrease in reaction efficiency can be further prevented.

[0035] (Average Coverage) By covering the surfaces of the iron oxide particles with the coating layer at a high coverage, a sintering-inhibiting effect can be obtained, and high reaction efficiency can be maintained. If the average coverage of the coating layer is less than 60%, a sufficient sintering-inhibiting effect cannot be obtained. Therefore, the average coverage of the coating layer is set to 60% or more. The average coverage is preferably 85% or more. On the other hand, the upper limit of the average coverage 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, the reduction reaction of the iron oxide particles and the oxidation reaction of the iron particles produced by the reduction reaction are hindered, and a sufficient oxidation-reduction reaction does not occur in accordance with the amount of iron oxide particles. In other words, when used in a metal-air battery or a hydrogen generation device, the desired reaction efficiency cannot 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, they are measured by the methods shown in the examples.

[0038] (Content) The content of the coating layer relative to the entire powder is not particularly limited, as long as the above-mentioned average coating thickness and average coverage are obtained. However, the higher the content of the coating layer, the thicker the coating layer can be and the higher the coverage of the coating layer can be. Therefore, the content of the coating layer is preferably 3 mass% or more, 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 substances without particular limitations as long as they do not inhibit the oxidation-reduction reaction of the powder. The other substances may be, for example, impurities mixed in when the powder is produced. Examples of the impurities include the materials of the balls and pot of a ball mill, specifically ZrO ２ and Al ２ O ３ 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) The iron oxide particles are Fe ２ O ３ When the iron oxide particles contain ２ O ３ The crystal and the Cr coating layer２ O ３ It is preferable that the iron oxide particles have a crystal orientation relationship represented by the following (1) with the Fe ３ O ４ When the iron oxide particles contain ３ O ４ The crystal and the Cr coating layer ２ O ３ It is preferable that the crystal orientation relationship between the (0001) Fe crystal and the (0001) Fe crystal is expressed by the following formula (2). ２ O ３ / / (0001)Cr ２ O ３ (1) (111)Fe ３ O ４ / / (0001)Cr ２ O ３ (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 relationships (1) and (2) above can be confirmed by STEM and EDS. Specifically, it can be confirmed 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] [Negative electrode active material for metal-air battery] An anode active material for a metal-air battery according to one embodiment of the present invention is an anode active material using the above-described powder. The anode active material may consist of the above-described powder, or may contain components other than the powder. Furthermore, the anode active material may contain powder obtained by reducing iron oxide contained in the above-described powder, or may consist of powder obtained by reducing iron oxide contained in the powder.

[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. An negative electrode 4 is disposed on the opposite surface, and a negative electrode active material 1 is disposed in a sealed space in contact with the negative electrode 4.

[0045] During discharge, the reactions shown by the solid arrows in Figure 1 occur. First, O ２ binds with electrons to form O ２－ Then, an oxidation reaction of iron occurs in the negative electrode active material 1, generating hydrogen. ２－ The electrons flow out of the negative electrode, causing a current to flow from the air electrode 2 to the negative electrode 4 through a wire (not shown).

[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 have flowed into the negative electrode 4. ２－ The resulting O ２－ releases electrons at the air electrode 2 and ２ This causes a current to flow from the negative electrode 4 through a wire (not shown) toward the air electrode 2.

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

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

[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] There is no particular limitation on the manufacturing method of the powder according to the present invention. The powder can be, for example, a mixture of iron oxide powder and Cr ２ O ３ It can be produced by mechanically mixing the powder in a solvent.

[0053] (Raw material powder) In this case, in order to make the content of the iron oxide particles with respect to the entire powder of the present invention 80 mass % or more, the mixing ratio of the iron oxide powder is set to 80 mass % or more. The upper limit of the mixing ratio of the iron oxide powder is not particularly limited, but Cr ２ O ３ In relation to the powder mixing ratio, it is preferable to set the content to 97 mass % or less.

[0054] Also, Cr ２ O ３ Although there are no particular limitations on the upper and lower limits of the powder mixing ratio, it is preferable that the mixing ratio is 3 mass% or more. This allows the content of the coating layer to be 3 mass% or more, thereby improving the average coating rate and the average coating thickness. ２ O ３ The mixing ratio of the powder is set to 20 mass % or less in relation to the mixing ratio of the iron oxide powder, and this also makes it possible to make the coating layer thin by setting the content of the powder to 20 mass % or less.

[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. ２ O ３ In order to facilitate adhesion of the iron oxide powder, 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 Cr ２ O ３ The powder is not particularly limited, and any Cr ２ O ３ Powders can be used, for example, Cr, which is available as a reagent. ２ O ３ Cr powder can be used and is available as an industrial material. ２ O ３ Powder can also be used. ２ O ３ The particle size of the powder is not particularly limited, and may be from several tens of nanometers to several micrometers.

[0058] (Mixing) 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 a ball mill is used, the rotation speed of the ball mill can be slowed down to mix Cr. ２ O ３The 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 is an iron oxide powder containing Cr. ２ O ３ The iron oxide powder can also be produced by vacuum deposition of the above-mentioned iron oxide powder. As the iron oxide powder used for vacuum deposition, the above-mentioned iron oxide powder can be suitably used. In the case of vacuum deposition, the thickness of the coating layer can be increased by increasing the deposition time. In addition, the thickness of the coating layer can be increased by increasing the input power of the vacuum deposition device.

[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] Among samples A to R, for the examples prepared using a ball mill, Fe was used except as described below. ２ O ３ Powder, Fe ３ O ４ Powder and Cr ２ O ３ The powders were blended to form a raw material powder, which was then mixed in a ball mill to obtain the sample. ２ O ３ Powder, Fe ３ O ４ Powder and Cr ２ O ３The powder blending ratio is the same as that of Fe shown in Table 1, except as noted below. ２ O ３ Content, Fe ３ O ４ The content of ZrO was the same as that of the coating layer. ２ The raw material powder and ZrO for mixing were placed in a container made of ２ A ball (10 mm diameter) was placed inside the container, and ethanol was added in a ratio of 50 ml per 10 g of the raw material powder. Then, using a ball mill (Fritsch: Pulverisette 7), mixing was carried out at the rotation speed (number of revolutions) shown in Table 1 for the time shown in Table 1. For sample A, only iron oxide powder was used as the raw material powder. For sample C, iron oxide powder was used in a ratio of 97 mass% and Cr. ２ O ３ 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 3 mass% Cr ２ O ３ The raw material powder used was a mixture of 10 mass % of the powder.

[0065] In addition, as for the example of the manufacturing method using vacuum deposition, Fe ２ O ３ Powder or Fe ３ O ４ Cr is vacuum deposited on the powder ２ O ３ The samples were prepared by coating with the Fe shown in Table 1. ２ O ３ Content and Fe ３ O ４ The content of the coating layer and the content of the coating layer are calculated by dividing the mass of the coating layer by the difference in mass between before and after vacuum deposition. ２ O ３ and Fe ３ O ４ The calculation was carried out using the mass of the powder.

[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 coverage and the average coating thickness were determined by STEM and EDS.

[0067] First, samples A to R were embedded in resin, and cross sections of the powder were cut out to a thickness of 50 nm or less using focused ion beam (FIB) processing. Ten mapping images were obtained at a magnification of approximately 640,000 times using STEM / EDS. The Fe mapping images were used to identify the shape of the iron oxide particles, and the Cr mapping images were used to determine whether a coating layer was present when Cr was present in a layer along the periphery of the iron oxide particle. Next, for samples with a coating layer, the Cr mapping images were used to determine whether a region on the surface of the iron oxide particle where the Cr / Fe mass concentration ratio was 0.2 or more was the Cr. ２ O ３ The coating area was then ２ O ３ The proportion of the coating area covering the surface of one iron oxide particle was calculated as the coating ratio of the coating layer of that particle. The average coating ratio of each sample was calculated as the average of the coating ratios of 10 particles. ２ O ３ The coating thickness for each particle was calculated from the area of ​​the coated region using the following formula, and the average coating thickness was calculated from the average of 10 particles: t = S / C / c × 100, where t is the coating thickness (nm) and S is the Cr ２ O ３ Area of ​​the covered region (nm ２ ), C is the circumference of the iron oxide particle (nm), c is Cr ２ O ３ is the coverage rate (%).

[0068] For samples B, D to N, the mixed Cr was observed by the above-mentioned STEM / EDS observation. ２ O ３ It was confirmed that all of these oxide particles were present as a coating layer. For samples C and O to Q, similar observation revealed oxide particles that were not present as a coating layer, and therefore it was confirmed that the content of the coating layer was less than 3 mass%. It was confirmed that samples A and R did not contain a coating layer.

[0069] (Crystal orientation relationship) For each of the obtained samples, Fe ２ O ３ Crystal or Fe ３ O ４ The crystal and the Cr coating layer ２ O ３ The following method was used to confirm whether the crystal orientation relationship (1) or (2) exists between the Cr crystal and the Cr alloy. ２ O ３ The coated area was identified. ２ O ３ Fe in the coating region and directly below ２ O ３ or Fe ３ O ４ The atomic arrangement of the crystal was confirmed at a magnification of over 2 million times to confirm whether or not there was a 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 Decrease Rate of Reaction Efficiency) First, the samples according to each Example were used in powder form to evaluate the reaction efficiency and the decrease rate of reaction efficiency described below. Figure 2 shows the apparatus used in 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] The cell 6 was maintained at 400°C, and a reduction treatment of iron oxide was carried out by introducing hydrogen gas at 100 ml / min for 60 minutes, and an oxidation treatment of iron by introducing 2.8 vol% water vapor for 180 minutes was repeatedly carried out. The introduction of hydrogen gas and water vapor was carried out while the inside of the cell 6 was kept at 1 atmosphere. As shown in the reaction formula below, the reduction treatment turned iron oxide into metallic iron, and the oxidation treatment oxidized the metallic iron until it was all Fe.３ O ４ 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) The mass change ΔM of sample 7 due to each reduction treatment and oxidation treatment was measured.

[0074] The mass of the powder at the time when the first reduction treatment is completed (M ０ ), one oxidation treatment and one reduction treatment constitute one cycle, and after each oxidation treatment, the mass of the first cycle (M １ ), the mass of the second cycle (M ２ ), ..., mass of the nth cycle (M ｎ At this time, the mass change ΔM ｎ M ｎ -M ０ In addition, the iron oxide contained in the sample according to each example is all converted to metallic iron in the first reduction treatment, and in the next oxidation treatment, the metallic iron is completely oxidized and all the iron contained becomes Fe. ３ O ４ The theoretical mass change in the first cycle is ΔM Ｒ It is defined as:

[0075] First, to evaluate the reaction efficiency, ΔM １ / ΔM Ｒ The evaluation criteria were as follows. If the result was 1 or 2, it could be evaluated as having excellent reaction efficiency. 1: 90% or more 2: 70% or more but less than 90% 3: 50% or more but less than 70% 4: Less than 50%

[0076] Next, to evaluate the rate of decrease in reaction efficiency due to repeated use, ΔM １０ / ΔM １ The evaluation criteria were as follows. If the result was 1 or 2, it could be evaluated as maintaining the reaction efficiency even after repeated redox reactions. 1: 90% or more 2: 70% or more but less than 90% 3: 50% or more but less than 70% 4: Less than 50%

[0077] (Filling rate) Furthermore, assuming that the samples would actually be used as battery materials, etc., molded bodies prepared from the samples according to each example were used to evaluate the filling rate and the rate of decrease in reaction efficiency, which will be described later. 10 g of each sample was prepared and placed in a cylindrical compression molding machine, and a constant load of 0.5 MPa was applied for 1 minute to perform compression molding to produce a molded body. The density of the molded body was calculated from the volume of the obtained molded body, and the density of the molded body divided by the density of the iron oxide particles was used as the filling rate. The evaluation criteria for the filling rate were as follows: A result of 1 can be evaluated as being suitable for use as a battery material and a reactant. 1: 0.2 or more 2: Less than 0.2

[0078] (Decrease in reaction efficiency assuming a real environment) The compact was maintained at 400°C, and a reduction treatment of iron oxide was performed by introducing hydrogen gas at 100 ml / min for 60 minutes, and an oxidation treatment of iron by introducing 2.8 vol% water vapor for 180 minutes was repeatedly performed. The introduction of hydrogen gas and water vapor was performed while the inside of the cell 6 was maintained at 1 atmosphere. The mass change ΔN due to the reduction treatment and oxidation treatment was measured.

[0079] The mass of the powder at the time when the first reduction treatment is completed (N ０ ), one oxidation treatment and one reduction treatment are considered as one cycle, and after each oxidation treatment, the mass of the first cycle (N １ ), the mass of the second cycle (N ２ ), ..., mass of the nth cycle (N ｎ At this time, the mass change ΔN at the nth cycle was measured. ｎ N ｎ -N ０ In each example, the mass of the compact when the filling rate is assumed to be 1.0 is calculated as the product of the volume of the compact and the density of the iron oxide particles. Then, when an amount of powder equivalent to this mass is used and it is assumed that all of the iron contained in the powder is reduced and oxidized in the initial reduction and oxidation treatments, the theoretical mass change in the first cycle is defined as ΔN Ｃ It is defined as:

[0080] In order to evaluate the rate of decrease in reaction efficiency due to repeated use in the case where the battery is filled with the compound and used in an actual environment, ΔN１０ / 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]

[0083]

[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.

[0085] REFERENCE SIGNS LIST 1 negative electrode active material 2 air electrode 3 solid electrolyte 4 negative electrode 5 metal-air battery 6 cell 7 sample

Claims

1. A powder comprising 80 mass% or more iron oxide particles and a coating layer covering the surface of the iron oxide particles, wherein the coating layer is made of Cr. ２ O ３ wherein the coating layer has an average coating thickness of 0.5 nm or more and 10 nm or less, and the coating layer has an average coverage of 60% or more.

2. The iron oxide particles are Fe ２ O ３ and Fe ３ O ４ and wherein the iron oxide particles contain Fe ２ O ３ When the iron oxide particles contain ２ O ３ The crystal and the Cr coating layer ２ O ３ The iron oxide particles have a crystal orientation relationship represented by the following (1) with respect to the Fe ３ O ４ When the iron oxide particles contain ３ O ４ The crystal and the Cr coating layer ２ O ３ The powder according to claim 1, having a crystal orientation relationship between the (0001) Fe crystal and the (0002) Fe crystal, expressed by the following formula (2): 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 using the powder according to claim 1 or 2.

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

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