Fuel production method and fuel production device

Abstract

To provide a fuel production method with excellent safety during fuel production, and a fuel production device.SOLUTION: A fuel production method produces CH4 by repeatedly performing Step 1 and Step 2 in this order. Step 1 includes storing hydrogen in a hydrogen storage alloy at a negative electrode by charging an electrochemical cell by a potential control device connected to a positive electrode and the negative electrode and subsequently stopping charging, the electrochemical cell including a positive electrode having Ni2O3H on its surface, the negative electrode including the hydrogen storage alloy, and an aqueous KHCO3 solution. Step 2 includes generating CH4 at the negative electrode by supplying a raw material containing at least one of CO2 and an aqueous KHCO3 solution to the electrochemical cell and subsequently stopping the supply of the raw material. The electrochemical cell is a used nickel-metal hydride secondary battery in the fuel production method and a fuel production device.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a fuel production method and a fuel production apparatus. [Background technology]

[0002] Starting with the Sabatier reaction, which uses nickel as a catalyst to produce methane and water from hydrogen and carbon dioxide, various technologies have been developed to produce fuels such as methane and hydrogen using electrochemical reactions. For example, a method is known in which hydrogen contained in a hydrogen storage alloy used in the negative electrode of an alkaline secondary battery is reacted with carbon dioxide to produce hydrocarbons (Patent Document 1). Also known is a system that generates hydrogen gas from an aqueous solution through an electrochemical reaction using platinum electrodes (Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-008280 [Patent Document 2] Patent Publication No. 2021-001403 Summary of the Invention [Problem to be solved by the invention]

[0004] When producing fuels such as methane through electrochemical reactions, it is desirable to have excellent safety, heat balance, cost, and other factors. For example, when methane and oxygen are generated in the same cell by an electrochemical reaction, there is a risk that they may react with each other, and therefore, a means for recovering the oxygen or the like is required (Patent Document 1). Furthermore, when expensive platinum is used for the electrodes, it may become difficult to reduce the cost of fuel production (Patent Document 2).

[0005] An object of one embodiment of the present disclosure is to provide a fuel production method and a fuel production apparatus that are highly safe when producing fuel. [Means for solving the problem]

[0006] The means for solving the problems include the following aspects. <1> A fuel production method for producing CH4 by repeatedly performing steps 1 and 2, wherein step 1 comprises charging an electrochemical cell including a positive electrode having Ni2O3H on its surface, a negative electrode containing a hydrogen storage alloy, and a KHCO3 aqueous solution, using potential control devices connected to the positive electrode and the negative electrode, respectively, thereby causing hydrogen to be absorbed in the hydrogen storage alloy at the negative electrode, and then stopping the charging; and step 2 comprises supplying a raw material containing at least one of CO2 and a KHCO3 aqueous solution to the electrochemical cell, thereby generating CH4 at the negative electrode, and then stopping the supply of the raw material, wherein the electrochemical cell is a used nickel-metal hydride secondary battery. <2> Prior to step 1, a pretreatment step is performed to form Ni2O3H on the surface of the positive electrode, and the pretreatment step includes repeatedly applying a first potential and a second potential, which are different from each other, to the positive electrode while the positive electrode containing Ni is immersed in an alkaline aqueous solution. <1> The fuel production method according to claim 1. <3> Step 3 is carried out when step 2 is carried out, and step 3 includes recovering the K2CO3 solution produced in step 2, mixing the K2CO3 solution with air to produce a KHCO3 aqueous solution, and supplying the produced KHCO3 aqueous solution as a raw material to an electrochemical cell. <1> or <2> The fuel production method according to claim 1. <4> A fuel production device comprising: a first unit that generates CH4; and a second unit that supplies a CH4 raw material to the first unit; the first unit comprising an electrochemical cell having a positive electrode having Ni2O3H on its surface, a negative electrode that contains a hydrogen storage alloy and generates CH4, and an aqueous KHCO3 solution; and potential control devices connected to the positive electrode and the negative electrode, respectively; and the second unit comprising mixing means that mixes the K2CO3 solution recovered from the first unit with air, and supply means that supplies the aqueous KHCO3 solution generated by the mixing means to the first unit as a raw material. <5> The electrochemical cell is a used nickel-metal hydride secondary battery <4> The fuel production apparatus according to claim 1. [Effects of the Invention]

[0007] According to one embodiment of the present disclosure, a fuel production method and a fuel production apparatus that are excellent in safety when producing fuel are provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is an explanatory diagram illustrating the configuration of the first unit. [Figure 2] FIG. 2 is an explanatory diagram illustrating the configuration of the fuel production device. [Figure 3] FIG. 3 is an explanatory diagram illustrating the configuration of the second unit. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. When embodiments of the present disclosure are described with reference to the drawings, the configuration of the embodiment is not limited to the configuration shown in the drawings. Components indicated by the same reference numerals in the drawings are the same components. In some drawings, only some components may be designated by reference numerals. The dimensional ratios in the drawings do not necessarily represent the actual dimensional ratios. In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In the numerical ranges described in this disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the value shown in the examples. In the present disclosure, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, when there are multiple substances corresponding to each component, the amount of each component means the total amount of the multiple substances unless otherwise specified.

[0010] <Fuel production equipment> The fuel production apparatus of the present disclosure includes a first unit. 1, the first unit 10 includes an electrochemical cell 11 and a potential control device 12. The electrochemical cell 11 includes a positive electrode 13 having Ni2O3H on its surface, a negative electrode 14 containing a hydrogen storage alloy, and a KHCO3 aqueous solution 15 as an electrolyte. The electrochemical cell 11 preferably further includes a recovery port 16 for recovering the generated CH4, K2CO3 solution, etc., and a supply port 17 for supplying raw materials for the generated CH4, etc.

[0011] The electrochemical cell 11 is a used nickel-metal hydride secondary battery. The fuel production device of the present disclosure uses a used nickel-metal hydride secondary battery as a flow reactor. The positive electrode 13 has Ni2O3H on its surface. The positive electrode 13 may be a positive electrode conventionally used in nickel-metal hydride secondary batteries, containing Ni as a positive electrode active material. As described below, the positive electrode 13 containing Ni may be subjected to a pretreatment process to form a positive electrode 13 having Ni2O3H on its surface.

[0012] The negative electrode 14 can be a positive electrode conventionally used in nickel-metal hydride secondary batteries, containing a hydrogen storage alloy as the negative electrode active material. The type of hydrogen storage alloy is not limited, and various alloys can be used as long as they reversibly store and release hydrogen. Specific examples include misch metals such as MmNi5, and expensive, highly active metals including Ni, Co, Mn, and Al. Mm is a mixture of rare earth elements such as Ce and La.

[0013] The electrolyte contained in the electrochemical cell 11 is a KHCO3 aqueous solution 15. In addition to serving as an electrolyte, the KHCO3 aqueous solution 15 also serves as a source of CH4 because it reacts with the hydrogen stored in the hydrogen storage alloy of the negative electrode 14 to produce CH4.

[0014] The potential control device 12 is a device that controls the potential of the positive electrode 13 and the negative electrode 14, and has the same function as a so-called potentiostat.

[0015] Methane synthesis can be carried out by using the first unit 10. The first unit 10 utilizes used nickel-metal hydride secondary batteries, and is therefore an apparatus that can reduce the cost of methane synthesis.

[0016] The fuel production apparatus of the present disclosure can include a second unit in addition to the first unit 10. 2, the fuel production apparatus 100 of the present disclosure includes a first unit 10 and a second unit 20. The first unit 10 produces CH4, and the second unit 20 supplies the first unit 10 with a raw material for CH4.

[0017] 3, the second unit 20 includes a mixing means 21 that mixes the K2CO3 solution recovered from the first unit 10 with air, and a supply means 22 that supplies the KHCO3 aqueous solution produced by the mixing means 21 as a raw material to the first unit 10. The mixing means 21 allows carbon dioxide in the air to be taken in.

[0018] The second unit 20 preferably further includes a supply port 23 for supplying the K2CO3 solution and air recovered from the first unit 10 to the mixing means 21, and a discharge port 24 for supplying the produced KHCO3 aqueous solution as a raw material from the supply means 22 to the first unit 10.

[0019] In the fuel production apparatus 100 of the present disclosure, the electrochemical cell 11 of the first unit 10 uses a used nickel-metal hydride secondary battery as a flow reactor, which not only reduces the cost of the apparatus but also contributes to reducing the cost associated with disposing of the used nickel-metal hydride secondary battery. The second unit 20 also uses air as a raw material to be supplied to the first unit 10, and functions to capture CO2 contained in the air. Therefore, the cost of using isolated CO2 is unnecessary, which contributes to reducing the cost of fuel production. As described above, the fuel production apparatus 100 of the present disclosure is a fuel production apparatus that can reduce the cost of fuel production.

[0020] <Fuel production method> (Step 1 and Step 2) The fuel production method of the present disclosure is a fuel production method in which CH4 is produced by repeatedly performing, in this order, Step 1 and Step 2, which will be described below. In the fuel production method, Step 1 is first performed. (Process 1) Step 1 is a step of charging an electrochemical cell 10 (see FIG. 1) by a potential control device 12.

[0021] During charging, the current density is set to 0-1A / cm 2 It is preferable to keep the voltage at a constant value and charge the battery by constant current charging until the OCV (Open Circuit Voltage) detected by the voltage sensor included in the potential control device 12 reaches 1.4V. The temperature of the electrolyte is preferably not more than 50° C. When the temperature of the electrolyte is to be acquired, the electrochemical cell 10 may be provided with a temperature sensor.

[0022] As described above, it is preferable to charge until the OCV (Open Circuit Voltage) detected by the voltage sensor included in the potential control device 12 reaches 1.4V, and then stop charging when the OCV reaches 1.4V.

[0023] Upon charging, hydrogen is stored in the hydrogen storage alloy at the negative electrode 14. O2 is generated at the positive electrode 13. The generated O2 is preferably released into the atmosphere.

[0024] (Process 2) Step 2 is performed after step 1. Step 2 includes supplying a raw material containing at least one of CO2 and an aqueous KHCO3 solution to the electrochemical cell 10, and generating CH4 at the negative electrode 14. The generated CH4 is recovered and used as fuel, etc.

[0025] A CO2 or KHCO3 aqueous solution is a raw material for CH4 generated at the anode 14. When CO2 is present as a raw material for CH4, a reduction reaction of CO2 proceeds at the anode 14, generating CH4. When a KHCO3 aqueous solution is present as a raw material for CH4, the reaction shown in the following formula (1) proceeds using H absorbed by the anode 14. 2KHCO3+8MH→CH4+K2CO3+3H2O+8M Formula (1)

[0026] A KHCO3 aqueous solution is preferred as the source of CH4 supplied to the electrochemical cell 10 because it does not require the use of CO2.

[0027] In step 2, the potential control device 12 does not apply a voltage to the electrochemical cell 10, and monitors the voltages of the positive electrode 13 and the negative electrode 14. Preferably, the raw material is supplied and CH4 is generated until the voltage reaches 0.6 V while monitoring the voltage. When the voltage reaches 0.6 V, it is preferable to stop the supply of the raw material. This prevents the oxygen storage alloy of the negative electrode 14 from being completely oxidized.

[0028] After step 2, charging is performed again in step 1, and then step 2 is performed. In this way, steps 1 and 2 can be repeatedly performed in this order. The fuel production method of the present disclosure is configured as described above, and therefore does not generate O2 and CH4 simultaneously, making it highly safe. In addition, it uses a thermodynamically favorable reaction, enabling CH4 to be produced with high efficiency.

[0029] (Pretreatment process) A pretreatment step of forming Ni2O3H on the surface of the positive electrode may be carried out prior to step 1. The pretreatment step is preferably carried out when the surface of the positive electrode is not coated with Ni2O3H, or when the surface is coated with Ni2O3H but only slightly.

[0030] The pretreatment step includes repeatedly and alternately applying a first potential and a second potential, which are different from each other, to the positive electrode while the positive electrode is immersed in an alkaline aqueous solution. The first potential is a potential at which Ni becomes tetravalent in a Ni-H2O system, and the second potential is a potential at which Ni becomes divalent in the Ni-H2O system. The potential control device 12 can apply the above-described controlled first and second potentials. In order to form Ni2O3H on the surface of a positive electrode containing Ni by such a method, the method described in JP 2022-45695 A can be adopted.

[0031] The pretreatment step allows sufficient formation of Ni2O3H on the surface of the positive electrode. Ni2O3 has both corrosion resistance and electrical conductivity, so the positive electrode 13 having Ni2O3H on its surface can be used stably for a long period of time.

[0032] (Step 3) Step 3 may be performed during step 2. Step 3 includes recovering the K2CO3 solution produced in step 2, mixing the K2CO3 solution with air to produce an aqueous KHCO3 solution, and supplying the produced aqueous KHCO3 solution to an electrochemical cell as a raw material. Step 3 is carried out in unit 2 (see Figure 2).

[0033] In step 3, first, the K2CO3 solution produced in step 2 is recovered. As shown in the above formula (1), K2CO3 is produced when CH4 is produced. K2CO3 is recovered as a solution.

[0034] The collected K2CO3 solution is mixed with air in the mixing means 21 (see FIG. 3). At this time, the reaction of the following formula (2) proceeds using CO2 in the air. Atmosphere (CO2) + K2CO3 + H2O → 2KHCO3 Equation (2)

[0035] The produced KHCO3 aqueous solution can be used as a raw material for CH4 as shown in formula (1), and is therefore supplied to the electrochemical cell 10.

[0036] The material flow in the disclosed fuel production method (see Figure 2) does not require the use of isolated compounds such as CO2. Therefore, fuel production requires fewer steps. Additionally, the exothermic heat generated in the Sabatier reaction to produce CH4 is offset by the endothermic heat generated by the decomposition of KHCO3, providing an energetic advantage. Furthermore, fuel can be produced at low cost because a used nickel-metal hydride secondary battery can be used as the electrochemical cell 10. Furthermore, O2 and CH4 are not generated simultaneously in the process. Therefore, the fuel production method of the present disclosure is a fuel production method that is highly safe and can reduce the cost of fuel production.

[0037] The present disclosure will be explained in further detail below with reference to experimental examples.

[0038] [Experimental Example 1] It was confirmed as follows that CH4 generation in step 2 was achieved by using a KHCO3 aqueous solution as the electrolyte. An electrochemical cell for testing was used. In this electrochemical cell, a fully charged 4 cm 2 The negative electrode of the MH battery (LaNi5) was used. Pt was used as the counter electrode, and a voltage of 80 V was applied. The current was 0.027 A / cm 2 It was. When the negative electrode was exposed to a saturated potassium carbonate aqueous solution (33.7 g / 100 mL), gas was generated from the negative electrode. The produced gas was collected in a pack and analyzed by gas chromatography, confirming the production of CH4.

[0039] [Experimental Example 2] Gas was generated from the negative electrode in the same manner as in Experimental Example 1, except that a KOH aqueous solution was used as the electrolyte. The gas produced was collected in a pack and analyzed by gas chromatography. It was found that H2 and CO2 were produced, but no fuel gas, including CH4, was produced. [Explanation of symbols]

[0040] 10 Unit 1 11 Electrochemical Cell 12 Potential control device 13 Positive electrode 14 Negative electrode 15 KHCO3 aqueous solution 16 Collection port 17 Supply port 20 Unit 2 21 Mixing means 22 Means of supply 23 Supply port 24 Outlet 100 Fuel production equipment

Claims

1. By repeating steps 1 and 2, CH ４ A fuel production method for producing The step 1 is Ni ２ O ３ A positive electrode having H on its surface, a negative electrode containing a hydrogen storage alloy, and KHCO ３ an electrochemical cell including the positive electrode and the negative electrode, and an aqueous solution, is charged by potential control devices connected to the positive electrode and the negative electrode, respectively, to cause the hydrogen storage alloy to absorb hydrogen at the negative electrode, and then the charging is stopped; In the step 2, CO ２ and KHCO ３ By supplying a raw material containing at least one of the aqueous solutions, CH ４ and then stopping the supply of said raw material; The method for producing fuel, wherein the electrochemical cell is a used nickel-metal hydride secondary battery.

2. Prior to step 1, Ni was applied to the surface of the positive electrode. ２ O ３ A pretreatment step is carried out to form H, 2. The fuel production method according to claim 1, wherein the pretreatment step includes repeatedly applying a first potential and a second potential, which are different from each other, to the positive electrode containing Ni while the positive electrode is immersed in an alkaline aqueous solution.

3. Step 3 is carried out when step 2 is carried out, The step 3 is a step of converting the K generated in the step 2. ２ CO ３ The solution is recovered, and ２ CO ３ By mixing the solution with air, KHCO ３ An aqueous solution of KHCO ３ 10. The method for producing a fuel according to claim 1, further comprising supplying an aqueous solution as a feedstock to the electrochemical cell.

4. CH ４ a first unit for generating CH ４ a second unit that supplies the raw material to the first unit; The first unit is Ni ２ O ３ A positive electrode having H on its surface and a hydrogen storage alloy, ４ and a negative electrode that produces KHCO ３ an electrochemical cell having an aqueous solution, and a potential control device connected to each of the positive electrode and the negative electrode; The second unit is configured to ２ CO ３ A mixing means for mixing the solution with air, and KHCO produced by the mixing means ３ a supply means for supplying an aqueous solution as the raw material to the first unit.

5. 5. The fuel production apparatus according to claim 4, wherein the electrochemical cell is a used nickel-metal hydride secondary battery.

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