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

Abstract

Provided is a powder that has a high reaction efficiency and can maintain said reaction efficiency even after repeated oxidation-reduction reactions. A powder according to the present invention includes iron oxide particles and a coating layer that coats the surface of the iron oxide particles. The coating layer comprises an oxide that includes Cr. The average thickness of the coating layer is no more than 50 nm. The average coverage of the coating layer is at least 50%. The coating layer comprises a Cr diffusion layer that has a gradient composition that has a Cr / Fe concentration ratio that decreases in the thickness direction from the surface side of the coating layer. When d is the thickness of the coating layer, the average gradient in the thickness direction of the Cr / Fe concentration ratio at a thickness of 3d / 4 from the surface of the coating layer is at least 0.01 / nm but no more than 0.20 / nm.

Description

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0014] 1. A powder comprising iron oxide particles and a coating layer coating the surfaces of the iron oxide particles, wherein the coating layer is made of an oxide containing Cr, the average thickness of the coating layer is 50 nm or less, the average coverage of the coating layer is 50% or more, the coating layer comprises a Cr diffusion layer having a gradient composition in which the Cr / Fe concentration ratio decreases in the thickness direction from the surface side of the coating layer, and 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, where d is the thickness of the coating layer, is 0.01 / nm or more and 0.20 / nm or less.

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

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

[0017] 4. The method for producing the powder according to 1 above, comprising: an adhering step of adhering a Cr compound to the iron oxide powder; and a heat treatment step of subjecting the powder after the adhering step to a heat treatment temperature of 600°C or higher.

[0018] 5. The method for producing a powder according to 4 above, wherein the adhesion step comprises 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.

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

[0020] 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.

[0021] 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 surface of iron oxide powder with a coating layer comprising a Cr diffusion layer by subsequently performing a heat treatment, 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 and a coating layer covering the surfaces of the iron oxide particles, wherein the coating layer is made of an oxide containing Cr, the average thickness of the coating layer is 50 nm or less, the average coverage of the coating layer is 50% or more, and the coating layer comprises a Cr diffusion layer having a gradient composition in which the Cr / Fe concentration ratio decreases in the thickness direction from the surface side of the coating layer, and 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, where d is the thickness 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] (Type of iron oxide) The type of iron oxide constituting the iron oxide particles is not limited, and may be, for example, Fe ２ O ３ (hematite), Fe ３ O ４ (magnetite), and FeO. However, Fe ２ O ３ and Fe ３ O ４ is stably produced by the oxidation reaction of iron, so from the viewpoint of ease of production, the iron oxide is Fe ２ O ３ and Fe ３ O ４ It is preferable that the iron oxide particles contain one or both of the following: ２ O ３ and Fe ３ O ４ The content ratio of the iron oxide particles is not particularly limited. ２ O ３ The iron oxide may contain only Fe ３ O ４ It may contain only iron oxide.

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

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

[0029] (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 or a hydrogen generator. 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.

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

[0031] [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] (Components) The coating layer is made of an oxide containing Cr. The oxide containing Cr can stably exist on the surface of the iron particles even under conditions in which the iron oxide particles are reduced. For example, Cr ２ O ３ is an oxide that is more stable than iron oxide in a temperature range below 600°C. Therefore, by using an oxide containing Cr, a high sintering suppression effect can be obtained. 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 at a sufficient density, 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 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 includes 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 be composed 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 also contain unavoidable impurities. The Cr oxide layer may be composed of Cr oxide and unavoidable impurities. That is, the coating layer may be composed of oxides of Fe and Cr and unavoidable impurities, or may be composed 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 oxidation-reduction 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 microscope observation 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 not exceed 0.01 / nm, 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 sintering-inhibiting effect can be achieved by covering the surfaces of the iron oxide particles with the coating layer at 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 oxidation-reduction reaction in accordance with the amount of iron oxide particles. In other words, when used in a metal-air battery or a 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. On the other hand, there is no particular restriction on the lower limit of the average thickness of the coating layer, and it 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, they are measured by the methods shown 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 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 ３The content of the other substances is preferably 17 mass% or less, more preferably 10 mass% or less, and even more preferably 5 mass% or less. The lower limit of the content of the other substances is not limited, and may be 0 mass%.

[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] [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.

[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. 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.

[0046] 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).

[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 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.

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

[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] The method for manufacturing 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] (Adhesion Step) In the adhesion 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, ２ O ３ That is, the adhesion step may be a method of mixing iron oxide powder and Cr powder. ２ O ３ In the mixing step, the mixing ratio of the iron oxide powder is 80 mass % or more, and the Cr powder is mixed with the iron oxide powder. ２ O ３ The mixing ratio of the powder is set to 3 mass% or more. In addition to the above-mentioned method, in order to adhere the Cr compound to the iron oxide powder, ２ O ３ Alternatively, the deposition step may be performed by immersing the iron oxide powder in a Cr-containing solution. ２ O ３ The process may be a vacuum deposition process in which iron oxide powder is vacuum-deposited in a Cr-containing solution or a dipping process in which iron oxide powder is dipped in a Cr-containing solution.

[0056] <Mixing process> Iron oxide powder and Cr ２ O ３When the iron oxide powder is mixed with the Cr powder, 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. ２ O ３ In relation to the powder mixing ratio, the content is set to 97 mass % or less.

[0057] Also, Cr ２ O ３ The mixing ratio of the powder is set to 3 mass% or more. This makes it possible to increase the average coverage. 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.

[0058] 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 average particle size of the powder is not particularly limited, and may be from 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 a ball mill is used, the rotation speed of the ball mill can be slowed down to allow the Cr ２ O ３ This allows the powder to adhere to the surface of the iron oxide particles, 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.

[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> Cr is applied to iron oxide powder ２ O ３ When using a method of vacuum deposition of Cr, the above-mentioned iron oxide powder can be suitably used as the iron oxide powder. In the case of vacuum deposition, the Cr content of the powder after deposition can be controlled by controlling the deposition time and the input power of the vacuum deposition device. ２ O ３ The content of the iron oxide particles and the content of the iron oxide particles can be set within the above-mentioned ranges. In addition, in order to set the average coverage rate within the above-mentioned ranges, it is preferable to perform the deposition 10 times or more. ２ O ３ It is preferable to stir the resulting powder between each deposition so that the powder can be deposited.

[0062] <Immersion Step> When using a method of immersing iron oxide powder in a Cr-containing solution, the above-mentioned iron oxide powder 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 Step) 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 proceeds. 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 proceeds, 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.

[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] Among samples A to S, for the examples in which the deposition method was a ball mill, first, Fe ２ O ３ Powder, Fe ３ O ４ Powder and Cr ２ O ３ The powders were blended to form raw material powder, which was then mixed in a ball mill. ２ O ３ Powder, Fe ３ O ４ Powder and Cr ２ O ３ The powder blending ratio is shown in Table 1. 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 at a ratio of 50 ml per 10 g of the raw material powder. Then, using a ball mill (Fritsch: Pulverisette 7), the mixture was mixed for at least 2 hours at the rotation speed (number of revolutions) shown in Table 1. Then, heat treatment was carried out under the conditions shown in Table 1 to obtain samples. 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 subjected to heat treatment.

[0069] In addition, in the case of vacuum deposition, Fe ２ O ３ The powder was subjected to vacuum deposition the number of times shown in the table. ２ O ３ Between each deposition, the resulting powder was collected and stirred. After that, the sample was heat-treated at 600°C for 2 hours to prepare a sample. In this case, the Fe ２ O ３ Content and Cr ２ O ３ The content is calculated by dividing the mass difference between before and after vacuum deposition by Cr. ２ O ３ As the mass of Cr ２ O ３ and the mass of Fe before vacuum deposition ２ O ３ The powder mass was used to calculate the iron oxide powder content and Cr content in the powder after deposition. ２ O ３ The content of iron oxide particles contained in the obtained sample is as shown in Table 1. The content of iron oxide particles contained in the obtained sample is approximately equal to the content of the iron oxide powder.

[0070] Among the examples, for the examples in which the deposition 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 deposit chromium nitrate, and then the samples were prepared by heat treatment at 600°C for 2 hours. Note that sample O was not subjected to heat treatment.

[0071] (Measurement of Average Coverage and Average Thickness of Coating Layer) The average coverage and average thickness of the coating layer were determined for each of the obtained samples by STEM and EDS.

[0072] First, samples A to S 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 at a magnification of approximately 640,000 were obtained 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 concentration ratio was 0.03 or more was present as Cr. ２ O ３ The coating area was then ２ O ３ The ratio of the coating area to 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 value 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 thickness was calculated from the average of 10 particles: t = S / C / c × 100, where t is the coating thickness (nm), 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 (%).

[0073] (Measurement of Average Gradient) 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 above-mentioned STEM and EDS method. Specifically, for each oxide particle, a line segment was drawn from the surface side of the coating layer of the oxide particle in the thickness direction, and the Cr / Fe concentration ratio (mass concentration ratio) was obtained at 0.2 nm intervals along the line segment. Next, the thickness d of the coating layer was 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 thickness gradient of the Cr / Fe concentration ratio at a thickness of 3d / 4. Similar measurements were performed at 10 locations, and the average of the obtained gradient values ​​was calculated as the average gradient. If the average gradient was 0.01 / nm or greater, the coating layer could be 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 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.

[0077] The cell 6 was maintained at 400°C, and a reduction treatment of iron oxide was carried out by introducing hydrogen gas at 100 ml / min for 60 minutes, and an oxidation treatment of iron by introducing 2.8 vol% water vapor for 180 minutes was repeatedly carried out. The introduction of hydrogen gas and water vapor was carried out while the inside of the cell 6 was kept at 1 atmosphere. As shown in the reaction formula below, the reduction treatment turned iron oxide into metallic iron, and the oxidation treatment oxidized the metallic iron until it was all Fe. ３ O ４The mass of sample 7 changed as the introduction of hydrogen gas and water vapor was repeated: 4H2 + Fe3O4 → 3Fe + 4H2O (reduction treatment) 3Fe + 4H2O → 4H2 + Fe3O4 (oxidation treatment) The mass change ΔM of sample 7 due to each reduction treatment and oxidation treatment was measured.

[0078] The mass of the powder at the time when the first reduction treatment is completed (M ０ ), one oxidation treatment and one reduction treatment are considered as one cycle, and after each oxidation treatment, the mass of the first cycle (M １ ), the mass of the second cycle (M ２ ), ..., mass of the nth cycle (M ｎ At this time, the mass change ΔM ｎ M ｎ -M ０ In addition, the iron oxide contained in the sample according to each example is all converted to metallic iron in the first reduction treatment, and in the next oxidation treatment, the metallic iron is completely oxidized and all the iron contained becomes Fe. ３ O ４ The theoretical mass change in the first cycle is ΔM Ｒ It is defined as:

[0079] 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%

[0080] Next, to evaluate the rate of decrease in reaction efficiency due to repeated use, ΔM ５０ / 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%

[0081] (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

[0082] (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 maintaining a pressure of 1 atmosphere. The mass change ΔN due to the reduction treatment and oxidation treatment was measured.

[0083] 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:

[0084] 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, 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]

[0087]

[0088]

[0089]

[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.

[0091] REFERENCE SIGNS LIST 1 negative electrode active material 2 air electrode 3 solid electrolyte 4 negative electrode 5 metal-air battery 6 cell 7 sample

Claims

1. A powder comprising iron oxide particles and a coating layer covering the surfaces of the iron oxide particles, wherein the coating layer is made of an oxide containing Cr, the average thickness of the coating layer is 50 nm or less, the average coverage of the coating layer is 50% or more, the coating layer comprises a Cr diffusion layer having a gradient composition in which the Cr / Fe concentration ratio decreases in the thickness direction from the surface side of the coating layer, and 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, where d is the thickness of the coating layer, is 0.01 / nm or more and 0.20 / nm or less.

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

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

4. A method for producing the powder described in claim 1, comprising: an adhesion step of adhering a Cr compound to iron oxide powder; and a heat treatment step of subjecting the powder after the adhesion step to a heat treatment temperature of 600°C or higher.

5. The method for producing a powder according to claim 4, wherein the adhesion step comprises 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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