阅读说明：本技术 二氧化碳电解-碳燃料电池一体型装置 (Carbon dioxide electrolysis-carbon fuel cell body type device ) 是由 伊藤靖彦 锦织德二郎 渡部浩行 于 2017-08-24 设计创作，主要内容包括：提供利用使用了熔融盐的经一体化的电化学反应体系,能够将电能与化学能(电析碳)相互转换的基于二氧化碳电解的碳析出-碳燃料电池一体型装置等。本发明提供的二氧化碳电解-碳燃料电池一体型装置具备：容纳包含氧化物离子的熔融盐的电解浴槽；浸渍于熔融盐中的碳析出/燃烧电极；浸渍于熔融盐中而与碳析出/燃烧电极电连接的氧气产生电极；浸渍于熔融盐中的氧气还原电极；用于向熔融盐中供给含二氧化碳的气体从而生成碳酸根离子的二氧化碳气体供给部；用于向碳析出/燃烧电极与氧气产生电极之间施加在碳析出/燃烧电极上碳酸根离子被还原并析出碳的电压的电源；和用于向氧气还原电极供给含氧气体从而在熔融盐中生成氧化物离子的氧气供给部。(A carbon dioxide electrolysis-carbon fuel cell -type device or the like is provided, which is capable of converting electric energy and chemical energy (electrowinning carbon) to each other by using an -based electrochemical reaction system using molten salt, wherein the carbon dioxide electrolysis-carbon fuel cell -type device is provided with an electrolytic bath containing molten salt containing oxide ions, a carbon precipitation/combustion electrode immersed in the molten salt, an oxygen generation electrode immersed in the molten salt and electrically connected to the carbon precipitation/combustion electrode, an oxygen reduction electrode immersed in the molten salt, a carbon dioxide gas supply unit for supplying a carbon dioxide-containing gas into the molten salt to generate carbon ions, a power supply for applying a voltage between the carbon precipitation/combustion electrode and the oxygen generation electrode, the voltage being applied between the carbon precipitation/combustion electrode and the oxygen generation electrode, the voltage causing the carbon precipitation/combustion electrode to reduce carbonate ions and precipitate carbon, and an oxygen supply unit for supplying an oxygen-containing gas to the oxygen reduction electrode to generate oxide ions in the molten salt.)

1, A carbon dioxide electrolysis-carbon fuel cell type device, comprising:

an electrolytic bath that contains a molten salt containing oxide ions;

at least carbon precipitation/combustion electrodes partially immersed in the molten salt;

an oxygen generation electrode at least partially immersed in the molten salt and electrically connected to the carbon deposition/combustion electrode;

an oxygen reduction electrode at least partially immersed in the molten salt;

a carbon dioxide gas supply unit for supplying a carbon dioxide-containing gas into the molten salt to generate carbonate ions;

a power supply for applying a voltage between the carbon evolution/combustion electrode and the oxygen generation electrode, the voltage causing the carbonate ions to be reduced and carbon to be evolved on the carbon evolution/combustion electrode; and

an oxygen gas supply unit for supplying an oxygen-containing gas to the oxygen gas reduction electrode to generate oxide ions in the molten salt.

2. The carbon dioxide electrolysis-carbon fuel cell type device according to claim l, wherein the device includes an oxygen collecting section for collecting oxygen generated at the oxygen generating electrode.

3. The carbon dioxide electrolysis-carbon fuel cell body-type device according to claim 1 or claim 2, which comprises a carbon dioxide gas collecting unit for collecting carbon dioxide gas generated at the carbon deposition/combustion electrode.

4. The carbon dioxide electrolysis-carbon fuel cell -type device of any of claims 1-3, wherein the carbon evolution/combustion electrode constitutes at least portions of an inner wall of the electrolytic bath.

5. The carbon dioxide electrolysis-carbon fuel cell body-type device of any of claims 1-4, wherein the molten salt comprises at least 1 of an alkali metal halide and an alkaline earth metal halide.

6. The carbon dioxide electrolysis-carbon fuel cell -type device of any of claims 1-4 wherein the molten salt comprises at least 1 of an alkali metal carbonate and an alkaline earth metal carbonate.

7, systems comprising the carbon dioxide electrolysis-carbon fuel cell -type device of any of claims 1 to 6, and

a carbon dioxide gas storage portion for storing carbon dioxide gas generated at the carbon deposition/combustion electrode.

8, systems comprising the carbon dioxide electrolysis-carbon fuel cell -type device of any of claims 1 to 6, and

a carbon storage portion for storing carbon generated on the carbon deposition/combustion electrode.

9. The method of using the carbon dioxide electrolysis-carbon fuel cell -type device of any of claims 1 to 6, comprising the steps of:

a carbon electroprecipitation step of supplying a carbon dioxide-containing gas from the carbon dioxide gas supply unit into the molten salt, applying a voltage between the carbon deposition/combustion electrode and the oxygen generation electrode by the power supply, and reducing the carbonate ions on the carbon deposition/combustion electrode to deposit carbon; and

and a power generation step of supplying an oxygen-containing gas from the oxygen gas supply unit to the oxygen reduction electrode to generate oxide ions in the molten salt and generate carbon dioxide gas on the carbon deposited on the carbon deposition/combustion electrode.

Technical Field

The present invention relates to a carbon dioxide electrolysis-carbon fuel cell -type device using an electrolytic bath containing a molten salt and a method for using the same, a system in which the -type device is combined with a carbon dioxide gas storage unit, and a system in which the -type device is combined with a carbon storage unit, and particularly to a carbon dioxide electrolysis-carbon fuel cell -type device capable of utilizing and making compatible an electrodeposition reaction of carbon derived from carbon dioxide using an electrolytic bath containing a molten salt and a combustion reaction of carbon in a Direct Carbon Fuel Cell (DCFC) as a reaction completely opposite to the electrodeposition reaction and the combustion reaction of carbon .

Background

In japan approved "paris agreement" as a framework of international global warming countermeasures in 2020 and onward, the goal is to reduce the emission of greenhouse gases in the whole world in 2050 by half as compared with the current emission.

However, there is a contradiction that the amount of Carbon dioxide emission increases when the proportion of thermal power generation is increased for load following, and there is a contradiction that the amount of Carbon dioxide emission increases also in , being an effective measure against direct reduction of Carbon dioxide is a CCS (Carbon dioxide Capture and Storage)), and for example, a technique of storing high-concentration Carbon dioxide separated and recovered from exhaust gas of a thermal power plant underground or the like has been studied.

On the other hand, although studies are being conducted to actively and effectively utilize recovered carbon dioxide rather than storing it underground, for example, EOR (enhanced oil recovery) causes an increase in the amount of petroleum used, and when it is intended to use it as a synthetic raw material such as methanol, dimethyl ether, various polymers, etc., a hydrocarbon raw material is required in addition to carbon, and hydrogen is also required as a reducing agent, and as a result, carbon dioxide emission is often increased, and no promising technology has been found.

In another aspect , the applicant found that the use of a "molten salt" as a functional liquid enables the "electrolytic deposition of carbon dioxide" from carbon dioxide as a raw material^2-) When carbon dioxide is supplied to the molten salt of (2), carbonate ions (CO) are generated according to the formula (1)₃ ^2-) And (3) carbon dioxide absorption reaction.

In molten salt: CO 2₂+O^2-→CO₃ ^2-…(1)

When the carbonate ions are reduced at the cathode, carbon is deposited on the cathode according to the formula (2), but various carbon deposits ranging from dense to porous can be obtained by controlling the electrolysis conditions at this time.

And (3) cathode reaction: CO 2₃ ^2-+4e^-→C+3O^2-…(2)

By using an insoluble anode for part of the oxide ions generated, oxidation occurs at the anode and oxygen is generated according to the formula (3).

And (3) anode reaction: 2O^2-→O₂+4e^-…(3)

The oxide ions remaining in the molten salt without being oxidized at the anode can be utilized in the carbon dioxide absorption reaction of formula (1), and when formulae (1) to (3) are combined, the decomposition reaction of carbon dioxide into carbon and oxygen is performed as follows.

And (3) total reaction: CO 2₂→C+O₂…(4)

On the other hand, , the deposited carbon is generated by inputting electric power, and if electric power can be obtained by using the carbon as a fuel, a new energy storage system can be constructed similarly to the case of using hydrogen produced by electrolyzing water.

In the DCFC, for example, according to the following expressions (5) and (6), when the reduction of oxygen is performed at the positive electrode and the reaction of generating carbon dioxide from carbon and an oxide ion is performed at the negative electrode, and these reactions are combined, the reaction of generating carbon dioxide from carbon and oxygen is performed according to the expression (7).

And (3) positive pole reaction: o is₂+4e^-→2O^2-…(5)

And (3) cathode reaction: c +2O^2-→CO₂+4e^-…(6)

And (3) total reaction: c + O₂→CO₂…(7)

As for the solid "carbon fuel" currently used in DCFC, coke fine powder has been used in many cases so far, and there are cases where carbon black is used as in the lorentzier research institute, and carbon generated by decomposition of hydrocarbons as in the tokyo industrial university, but it is expected that carbon obtained from carbon dioxide by molten salt electrolysis as described above can also be used as such carbon fuel.

As described above, the carbon electrodeposition reaction from carbon dioxide and the carbon combustion reaction in the Direct Carbon Fuel Cell (DCFC) are completely opposite reactions, and a device capable of converting electrical energy and chemical energy (electrolysis product) into each other can be constructed by using the same electrochemical system as a water electrolysis fuel cell -type device in which hydrogen obtained by electrolysis of water is used as a fuel for a fuel cell.

However, unlike the electrolysis of water (hydrogen evolution reaction), the carbon from carbon dioxide is not easily separated and recovered as a gas (hydrogen) but as a solid (carbon), since the electrolysis product is deposited and remains on the electrode, no attempt has been made to use the gas (carbon) as a fuel for a fuel cell.

Disclosure of Invention

Problems to be solved by the invention

Accordingly, an object of the present invention is to provide a carbon dioxide electrolysis-based carbon deposition-carbon fuel cell -type device and a method for using the same, which can convert electrical energy and chemical energy (electrodeposition carbon) into each other by using an -made electrochemical reaction system using an electrolytic bath containing a molten salt, a system in which the -type device is combined with a carbon dioxide gas storage unit, and a system in which the -type device is combined with a carbon storage unit.

Means for solving the problems

The present inventors have conducted extensive studies on the structure of a carbon deposition and carbon fuel cell using a carbon dioxide electrolysis apparatus and a reaction system thereof, and as a result, have found that focusing on the reaction system represented by the above expressions (1) to (4) for immobilizing and storing carbon dioxide as carbon by molten salt electrolysis and the reaction system represented by the above expressions (5) to (7) for obtaining electric power using carbon as a fuel are completely opposite reactions, the use of an electrochemical reaction system formed by using an electrolytic bath containing a molten salt is most effective in constructing a carbon dioxide electrolysis-carbon fuel cell -type apparatus, and have completed the present invention.

That is, the present invention provides carbon dioxide electrolysis-carbon fuel cell type devices, each of which includes an electrolytic bath containing a molten salt containing oxide ions, a carbon deposition/combustion electrode at least partially immersed in the molten salt, an oxygen generation electrode at least partially immersed in the molten salt and electrically connected to the carbon deposition/combustion electrode, an oxygen reduction electrode at least partially immersed in the molten salt, a carbon dioxide gas supply unit for supplying a carbon dioxide-containing gas into the molten salt to generate carbonate ions, a power supply unit for applying a voltage between the carbon deposition/combustion electrode and the oxygen generation electrode, the voltage being applied to the carbon deposition/combustion electrode, the carbonate ions being reduced and the carbon being deposited, and an oxygen supply unit for supplying an oxygen-containing gas to the oxygen reduction electrode to generate oxide ions in the molten salt.

When the carbon dioxide electrolysis-carbon fuel cell -type device of the present invention functions as a carbon dioxide electrolysis device for immobilizing and storing carbon dioxide in the form of carbon by molten salt electrolysis, carbon dioxide is supplied to the presence of oxygen ions (oxide ions, O ions) in an electrolytic bath by using a carbon dioxide gas supply unit^2-) To produce carbonate ions (CO) according to the formula (1)₃ ^2-) And (3) carbon dioxide absorption reaction.

In molten salt: CO 2₂+O^2-→CO₃ ^2-…(1)

When carbon is deposited on the carbon deposition/combustion electrode according to the formula (2) when the carbonate ions are reduced on the carbon deposition/combustion electrode at least partially immersed in the molten salt, various carbon electrodeposits ranging from dense to porous can be obtained by controlling electrolysis conditions by a power source applied between a cathode and an anode, which will be described later.

And (3) cathode reaction: CO 2₃ ^2-+4e^-→C+3O^2-…(2)

Part of the oxide ions generated is oxidized at the oxygen generation electrode electrically connected to the carbon deposition/combustion electrode, and oxygen is generated according to the formula (3).

And (3) anode reaction: 2O^2-→O₂+4e^-…(3)

The oxide ions remaining in the molten salt without being oxidized at the anode can be utilized in the carbon dioxide absorption reaction of formula (1), and when formulae (1) to (3) are combined, the decomposition reaction from carbon dioxide into carbon and oxygen is performed as follows.

And (3) total reaction: CO 2₂→C+O₂…(4)

Therefore, in order to function as a carbon dioxide electrolysis device for immobilizing and storing carbon dioxide as carbon, the carbon dioxide electrolysis-carbon fuel cell -type device of the present invention needs to include an electrolytic bath containing a molten salt containing oxide ions, a carbon dioxide gas supply unit for supplying a carbon dioxide-containing gas into the molten salt to generate carbonate ions, a carbon deposition/combustion electrode at least part of which is immersed in the molten salt, an oxygen generation electrode at least part of which is immersed in the molten salt and electrically connected to the carbon deposition/combustion electrode, and a power supply for applying a voltage between the carbon deposition/combustion electrode and the oxygen generation electrode, the voltage causing the carbon deposition/combustion electrode to deposit carbon by reducing the carbonate ions.

In the carbon dioxide electrolysis-carbon fuel cell -type device of the present invention, since oxygen is generated at the oxygen generating electrode as the cathode (see formula (2)), it is preferable to provide an oxygen collecting unit for efficiently collecting the oxygen generated at the electrode, and in this case, there is an advantage that the recovered oxygen can be reused for the reaction shown in formula (5) or the like, for example.

In another aspect , when the carbon dioxide electrolysis-carbon fuel cell -type device of the present invention functions as a carbon fuel cell that obtains electric power using carbon as a fuel, oxygen is reduced at the positive electrode in accordance with the expression (5) by supplying an oxygen-containing gas to the oxygen reduction electrode as the positive electrode in the electrolytic bath by the oxygen supply unit.

And (3) positive pole reaction: o is₂+4e^-→2O^2-…(5)

Further, on the carbon deposition/combustion electrode electrically connected to the oxygen reduction electrode and serving as a negative electrode, carbon dioxide is generated from carbon of the carbon deposition/combustion electrode and the oxide ions in the molten salt according to the formula (6).

And (3) cathode reaction: c +2O^2-→CO₂+4e^-…(6)

When the formulas (5) and (6) are combined, a reaction of generating carbon dioxide from carbon and oxygen is performed as shown in the formula (7).

And (3) total reaction: c + O₂→CO₂…(7)

Therefore, in order to function as a carbon fuel cell for obtaining electric power from carbon as a fuel, the carbon dioxide electrolysis-carbon fuel cell -type device of the present invention needs to include an electrolytic bath containing a molten salt containing oxide ions, an oxygen reduction electrode at least part of which is immersed in the molten salt, a carbon deposition/combustion electrode at least part of which is immersed in the molten salt and electrically connected to the oxygen reduction electrode, and an oxygen supply unit for supplying an oxygen-containing gas to the oxygen reduction electrode to generate oxide ions in the molten salt.

In the carbon dioxide electrolysis-carbon fuel cell of the present invention, since carbon dioxide gas is generated at the carbon deposition/combustion electrode as the negative electrode (see formula (6)), it is preferable to provide a carbon dioxide gas collecting unit for efficiently collecting carbon dioxide gas generated at the electrode.

Thus, the method of using the carbon dioxide electrolysis-carbon fuel cell of the present invention is characterized by comprising 2 steps involving completely opposite reactions, namely a carbon deposition step of supplying a carbon dioxide-containing gas from a carbon dioxide gas supply unit into a molten salt, applying a voltage between a carbon deposition/combustion electrode and an oxygen generation electrode by a power supply, and reducing carbonate ions on the carbon deposition/combustion electrode to deposit carbon, and an electric power generation step of supplying an oxygen-containing gas from an oxygen supply unit into an oxygen reduction electrode to generate oxide ions in the molten salt and generate carbon dioxide gas on the carbon deposited on the carbon deposition/combustion electrode.

The carbon deposition/combustion electrode used in the present invention may be formed as a single body with the electrolytic bath, such as an electrode rod immersed in the molten salt in the electrolytic bath, or may be formed as a body with the electrolytic bath so as to constitute at least parts of the inner wall of the electrolytic bath.

In the present invention, carbonate ion (CO) is used as a source of carbon ions in an electrolytic bath₃ ^2-) And oxide ion (O)^2-) The molten salt present stably preferably contains at least of alkali metal halides and alkaline earth metal halides.

In the present invention, if a system is configured to include the carbon dioxide gas storage unit for storing the carbon dioxide gas generated at the carbon deposition/combustion electrode in addition to the carbon dioxide electrolysis-carbon fuel cell -type device, the high-concentration carbon dioxide gas (see formula (7)) generated when the system is used as a carbon fuel cell can be stored underground.

In the present invention, if the system is configured to include the carbon storage unit for storing carbon generated at the carbon deposition/combustion electrode in addition to the carbon dioxide electrolysis-carbon fuel cell -type device, carbon (see formula (4)) generated when the system is used as a carbon dioxide electrolysis device can be used as a solid fuel for a carbon fuel cell (see formula (7)) or a highly functional carbon member.

Effects of the invention

According to the present invention, a carbon dioxide electrolysis apparatus for immobilizing and storing carbon dioxide in the form of carbon, which is originally different in structure, and a carbon fuel cell for obtaining electric power using carbon as a fuel can be integrated into a carbon dioxide electrolysis-carbon fuel cell -type apparatus capable of converting electric energy and chemical energy (electrodeposited carbon) into each other, and a system in which the -type apparatus is combined with a carbon dioxide gas storage and/or a carbon storage, by using an electrochemical reaction system formed into using an electrolytic bath containing a molten salt.

Further, according to the present invention, 2 steps involving the completely opposite reaction can be performed by using an -made electrochemical reaction system using an electrolytic bath containing a molten salt, a carbon electrolysis step of supplying a carbon dioxide-containing gas from a carbon dioxide gas supply unit into the molten salt, applying a voltage between a carbon deposition/combustion electrode and an oxygen generation electrode by a power supply, reducing carbonate ions on the carbon deposition/combustion electrode to deposit carbon, and a power generation step of supplying an oxygen-containing gas from an oxygen supply unit into the oxygen reduction electrode to generate oxide ions in the molten salt and generate carbon dioxide gas on the carbon deposited on the carbon deposition/combustion electrode.

Drawings

Fig. 1 is a schematic diagram schematically showing a carbon dioxide electrolysis-carbon fuel cell -type device according to embodiments of the present invention.

Fig. 2 is an explanatory view for explaining a state in which the carbon dioxide electrolysis-carbon fuel cell -type device shown in fig. 1 is used as a carbon dioxide electrolysis device (power storage).

Fig. 3 is an explanatory view for explaining a state where the carbon dioxide electrolysis-carbon fuel cell -type device shown in fig. 1 is used as a carbon fuel cell (power generation).

Fig. 4 is an energy/mass flow diagram for the case of using a water electrolysis-hydrogen fuel cell -type device using hydrogen as an energy carrier and a carbon dioxide electrolysis-carbon fuel cell -type device shown in fig. 1 using carbon as an energy carrier.

Fig. 5 is a schematic diagram schematically showing a carbon utility energy system using the carbon dioxide electrolysis-carbon fuel cell -type device shown in fig. 1.

Detailed Description

Hereinafter, a carbon dioxide electrolysis-carbon fuel cell -type device and a method of using the same, a system in which the -type device and a carbon dioxide gas storage unit are combined, and a system in which the -type device and a carbon storage unit are combined according to embodiments of the present invention will be described in detail with reference to the drawings.

Fig. 1 is a schematic diagram schematically showing a carbon dioxide electrolysis-carbon fuel cell -type device 1 according to embodiments of the present invention.

As can be understood with reference to fig. 1, the carbon dioxide electrolysis-carbon fuel cell type device 1 according to the present embodiment includes an electrolytic bath 2 that contains a molten salt 20 containing oxide ions, in the electrolytic bath 2, a carbon deposition/combustion electrode 3, an oxygen generation electrode 4, an oxygen supply unit 5, and a carbon dioxide gas supply unit 6 are disposed so that portions are immersed in the molten salt 20, respectively, the oxygen supply unit 5 supplies an oxygen-containing gas to the oxygen reduction electrode 4 to generate oxide ions in the molten salt 20, and the carbon dioxide gas supply unit 6 supplies a carbon dioxide-containing gas to the molten salt 20 to generate carbonate ions, and further, a pair of electrodes of the carbon deposition/combustion electrode 3 and the oxygen generation electrode 4 and a pair of electrodes of the carbon deposition/combustion electrode 3 and the oxygen generation electrode 4 are electrically connected, respectively, and a power supply for applying a voltage between the carbon deposition/combustion electrode 3 and the oxygen generation electrode 4 is connected to the pair of the carbon deposition/combustion electrode 3 and the oxygen generation electrode 4, and the voltage is applied to reduce the carbonate ions on the carbon deposition/combustion electrode 3 to deposit carbon.

A. Carbon dioxide electrolysis-carbon fuel cell body type device

1. Electrolytic bath

In the present embodiment, carbonate ions (CO) are generated in the electrolytic bath 20₃ ^2-) And oxide ion (O)^2-) The main molten salt is an alkali metal halide, an alkaline earth metal halide, a mixture thereof, or the like. When used as a carbon dioxide electrolysis apparatus (power storage), O is added first^2-A source, further blown with carbon dioxide, thereby passing through the reaction CO of formula (1)₃ ^2-When the product is supplied to an electrolytic bath, the reactions of the formulae (2) and (3) can be smoothly carried out, and when the product is used as a carbon fuel cell (power generation), O is similarly added in advance to ^2-The source of the oxygen-containing gas is such that the cell reactions of the formulae (5) and (6) can be smoothly performed, that is, it is preferable to add O in advance to the carbon dioxide electrolysis-carbon fuel cell type device^2-A source. Oxide ion (O) consumed by carbon dioxide electrolysis of formula (3) or carbon fuel cell of formula (6)^2-) The same amount of O is supplied as the reactions of the formulae (2) and (5) proceed, respectively, and therefore, in principle, O in the electrolytic bath is supplied as a whole of the reactions^2-Is kept constant.

Here, as the alkali metal halide, an alkali metal halide can be usedCompounds such as LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, LiBr, NaBr, KBr, RbBr, CsBr, LiI, NaI, KI, RbI, CsI, etc., can be used as the alkaline earth metal halide₂、CaF₂、SrF₂、BaF₂、MgCl₂、CaCl₂、SrCl₂、BaCl₂、MgBr₂、CaBr₂、SrBr₂、BaBr₂、MgI₂、CaI₂、SrI₂、BaI₂And (c) a compound such as a quaternary ammonium compound.

As oxide ion (O)^2-) Sources thereof include alkali metal oxides and alkaline earth metal oxides, and Li may be used as the alkali metal oxide₂O、Na₂O、K₂Oxides such as O, and oxides such as MgO, CaO, BaO, and the like can be used as the alkaline earth metal oxides.

In the molten salt, O may be substituted for the molten salt^2-Source of added CO₃ ^2-Or carbonate ion (CO) as a main molten salt₃ ^2-) The carbonate of (1). In this case, O is generated by the reverse reaction of the formula (1) depending on the bath composition and bath temperature^2-Oxygen generation reaction of formula (3) for smoothly performing carbon dioxide electrolysis, and formula (6) for carbon fuel cell, O^2-Since a high concentration is advantageous, it is preferable to sufficiently supply O to the electrolytic bath in advance by the reaction of the formula (2)^2-. In particular O in the electrolytic bath^2-In a low concentration state, the oxygen gas of the formula (3) may be generated as an anode reaction, and a chlorine generation reaction of the following formula may be performed.

And (3) anode reaction: 2Cl^-→Cl₂+2e^-…(8)

When the carbon dioxide electrolysis-carbon fuel cell -type device is operated, the generation of chlorine gas may cause adverse effects such as deterioration in the mass balance of ions in the bath and deterioration of the structural material, and therefore, attention is required.

As containing carbonate ions (CO)₃ ^2-) The molten salt of (2) includes alkali metal carbonates, alkaline earth metal carbonates, and the like. As alkali metalsCarbonate, Li may be used₂CO₃、Na₂CO₃、K₂CO₃Etc. as the alkaline earth metal carbonate, MgCO may be used₃、CaCO₃、BaCO₃And the like.

The above compounds may be used alone, or 2 or more of them may be used in combination. The combination of these compounds, the number of compounds to be combined, the mixing ratio, and the like are not limited, and can be appropriately selected according to a preferred operating temperature range.

In addition, the electrolytic bath (molten salt 20) is designed to rapidly supply carbonate ions (CO) in the vicinity of specific electrodes for the purpose of promoting the respective electrode reactions₃ ^2-) Oxide ion (O)^2-) Or conversely, the electrolytic bath may be circulated in the direction by an appropriate method such as blowing an inert gas or stirring, and the like, between the cathode and the anode, or between the cathode and the anode.

The temperature of the electrolytic bath containing the molten salt 20 (bath temperature) is not particularly limited, and is generally advantageous in terms of supplying a substance and promoting a reaction as the bath temperature of the electrolytic bath is higher, but is also advantageous in that evaporation of the molten salt becomes significant in a high temperature region exceeding 900 ℃, and the material of the electrolytic bath 2 usable at a high temperature is limited and treatment becomes difficult, and therefore a treatment temperature of about 250 ℃ to 800 ℃ is preferable as an actual bath temperature, and a treatment temperature of about 350 ℃ to 700 ℃ is particularly more preferable.

B. As carbon dioxide electrolysis for immobilizing/storing carbon dioxide in the form of carbon by molten salt electrolysis Situation in which the device is functioning

Fig. 2 is an explanatory view illustrating a state in which the carbon dioxide electrolysis-carbon fuel cell -type device 1 shown in fig. 1 is used as a carbon dioxide electrolysis device (power storage).

1. Cathode (carbon deposition)Combustion electrode 3)

When the carbon dioxide electrolysis-carbon fuel cell -type device 1 of the present embodiment functions as a carbon dioxide electrolysis device for immobilizing and storing carbon dioxide in the form of carbon by molten salt electrolysis, oxide ions (O) are present in the electrolytic bath 2^2-) The molten salt 20 of (2) is supplied with a carbon dioxide-containing gas by using the carbon dioxide gas supply part 6 to generate carbonate ions (CO) according to the formula (1)₃ ^2-) And (3) carbon dioxide absorption reaction.

In molten salt: CO 2₂+O^2-→CO₃ ^2-…(1)

When the carbonate ions are reduced at by the carbon deposition/combustion electrode 3 partially immersed in the molten salt 20, carbon is deposited on the carbon deposition/combustion electrode 3 according to the formula (2), and at this time, various carbon electrodeposits ranging from dense to porous can be obtained by controlling electrolysis conditions by a power source applied between a cathode and an anode, which will be described later.

And (3) cathode reaction: CO 2₃ ^2-+4e^-→C+3O^2-…(2)

The reactions of the formulae (1) and (2) (blowing of carbon dioxide and electrolytic deposition of carbon) may be performed separately as completely independent stepwise reactions, or may be performed all at once. Particularly, when these reactions are performed simultaneously, it is preferable to circulate the bath solution by an appropriate method as described above.

The cathode material (carbon deposition/combustion electrode 3) is not limited to metal, and any of various materials may be used as long as it is a cathode material that stably exists in a solid phase or a liquid phase at the treatment temperature in the present embodiment and has conductivity, and the carbon deposition/combustion electrode 3 used in the present embodiment may be formed into a body with the electrolytic bath 2 so as to constitute at least portions of the inner wall of the electrolytic bath 2, although not shown.

2. Anode (oxygen generating electrode 4)

Part of the generated oxide ions is oxidized at the oxygen generation electrode 4 electrically connected to the carbon deposition/combustion electrode 3 and oxygen is generated according to the formula (3).

And (3) anode reaction: 2O^2-→O₂+4e^-…(3)

As the insoluble anode (oxygen generating electrode 4), an electrode using a noble metal such as platinum or gold, and Ni may be used_XFe_3-XO₄A nickel ferrite represented by (X ═ 0.1 to 2.0), or a nickel ferrite comprising the following formula: ni_XCo_1-XO (X ═ 0.1 to 0.5) or the formula: ni_XCo_3-XO₄And a nickel-cobalt oxide conductive ceramic electrode or a conductive diamond electrode represented by (X ═ 0.3 to 1.5).

Oxide ions (O) remaining in the molten salt without being oxidized at the anode^2-) The decomposition reaction of the following formula (1) into carbon and oxygen can be utilized in the carbon dioxide absorption reaction of the formula (1), and when the formulas (1) to (3) are combined, the decomposition reaction of carbon dioxide into carbon and oxygen is performed as follows.

And (3) total reaction: CO 2₂→C+O₂…(4)

3. Conditions of electrolysis

As for the electrode potential at the time of electrolysis, a power supply 7 is used to supply carbonate ions (CO)₃ ^2-) The electrode potential or the electrolytic current is controlled in the form of a reduced potential region. For example, when molten LiCl-KCl having a bath temperature of about 500 ℃ is used in the electrolytic bath, CO is preferably generated at a specific ratio₃ ^2-About 1.2V (Li) of the reduction reaction of (1)⁺/Li basis) and a potential at which Li metal does not precipitate (a potential higher than about 0V).

4. Blowing conditions of a gas containing carbon dioxide

As the carbon dioxide-containing gas to be blown into the carbon dioxide gas supply unit 6, an exhaust gas from a thermal power plant or the like is conceivable. The gas component other than carbon dioxide containing an inert gas such as argon or nitrogen is not problematic, but contains moisture or NO_x、SO_xIt is preferably removed in advance because it may dissolve in various ion forms and decrease the efficiency of carbon electrodeposition at the cathode. With respect to oxygen, if adoptedThere is no problem in the inclusion of the blowing gas if the blowing gas is not directly contacted with the cathode.

In addition, the catalyst is based on carbon dioxide and oxide ions (O)^2-) Carbonate ion (CO)₃ ^2-) Since the formation reaction of (a) proceeds as a gas-liquid reaction, the smaller the bubble size (bubble diameter) of the carbon dioxide-containing gas, the larger the specific surface area per unit volume, and the higher the reactivity. Therefore, the bubble diameter of the carbon dioxide-containing gas depends on the temperature of the molten salt and O^2-The concentration, the number of bubbles of the carbon dioxide-containing gas, and the like, and the preferred bubble diameter of the carbon dioxide-containing gas can be appropriately determined depending on the scale of the carbonate production unit, the magnitude of the electrolytic current, and the like.

The diameter of the bubbles of the carbon dioxide-containing gas is not critical, but is preferably about 100nm to 10mm, more preferably about 1 μm to 1 mm. The bubble diameter referred to herein means a diameter of a bubble which is supplied with the carbon dioxide-containing gas to the molten salt 20 or immediately after the supply, and is accompanied by oxide ions (O)^2-) The reaction of (3) is reduced. The number of carbon dioxide-containing gas bubbles per unit volume is also determined by the temperature of the molten salt and O^2-The concentration, the bubble diameter of the carbon dioxide-containing gas, and the like.

For example, when micron-sized bubbles are to be generated, the object is achieved by passing the bubbles through a porous member made of Pyrex (registered trademark), quartz, silicon nitride, silicon carbide, boron nitride, or alumina, and when submicron-sized fine bubbles are to be generated, the bubbles of the carbon dioxide-containing gas may be further refined by ultrasonic wave application or the like.

In the molten salt 20 as the electrolytic bath, the temperature of the carbon dioxide-containing gas blown into the molten salt 20 by the carbon dioxide gas supply unit 6 is not particularly limited, and it is preferable to heat the molten salt 20 in advance until the temperature of the carbon dioxide-containing gas approaches the temperature of the molten salt 20 in order to suppress the temperature fluctuation of the electrolytic bath. The heating of the carbon dioxide-containing gas may be performed by providing a heater or the like in addition to the carbon dioxide gas flow path, or may be performed by providing a carbon dioxide-containing gas flow path in the electrolytic bath and using the heat of the molten salt 20 in the electrolytic bath.

To promote the reaction of carbon dioxide and oxide ion (O)^2-) Carbonate ion (CO)₃ ^2-) For the purpose of the formation reaction of (2), the molten salt 20 is preferably stirred. As a means for such stirring, bubbling with a carbon dioxide-containing gas or an inert gas may be used, or an agitator (agitator) having a driving part such as an impeller may be used.

In the carbon dioxide electrolysis-carbon fuel cell -type device 1 according to the present embodiment, since oxygen is generated in the oxygen generation electrode 4 as the anode (see formula (2)), the electrode 4 is provided with the oxygen collection unit 30 for efficiently collecting the generated oxygen, and in this case, the recovered oxygen can be reused for the reaction shown in formula (5), for example.

C. The case where the fuel cell functions as a carbon fuel cell for obtaining electric power using carbon as fuel

1. Positive electrode (oxygen reduction electrode 5)

Fig. 3 is an explanatory view for explaining a state in which the carbon dioxide electrolysis-carbon fuel cell -type device 1 shown in fig. 1 is used as a carbon fuel cell (power generation).

When the carbon dioxide electrolysis-carbon fuel cell -type device 1 of the present embodiment functions as a carbon fuel cell that obtains electric power using carbon as a fuel, oxygen is reduced at the positive electrode in accordance with the expression (5) by supplying an oxygen-containing gas to the oxygen reduction electrode 5 as the positive electrode in the electrolytic bath 2 by the oxygen supply unit 8.

And (3) positive pole reaction: o is₂+4e^-→2O^2-…(5)

It is not problematic to contain an inert gas such as argon or nitrogen as a component other than oxygen contained in the oxygen-containing gas blown into the oxygen gas supply portion 8. With respect to carbon dioxide, it does not interfere with the bathHowever, the carbon mass balance is lost, and therefore, it is preferable to remove the carbon in advance. Moisture also becomes oxide ions (O)^2-) Hydroxide ion (OH)^-) Since there is a possibility that an undesired reaction may occur in the electrode, the removal is preferable.

2. Negative electrode (carbon deposition/combustion electrode 3)

Further, with respect to the carbon deposition/combustion electrode 3 which is electrically connected to the oxygen reduction electrode 5 and serves as a negative electrode, carbon dioxide is generated from the carbon of the carbon deposition/combustion electrode 3 and the oxide ions in the molten salt 20 according to the formula (6).

And (3) cathode reaction: c +2O^2-→CO₂+4e^-…(6)

When the formulas (5) and (6) are combined, a reaction of generating carbon dioxide from carbon and oxygen is performed as in the formula (7).

And (3) total reaction: c + O₂→CO₂…(7)

In the carbon dioxide electrolysis-carbon fuel cell -type device 1 according to the present embodiment, the carbon dioxide gas collecting unit 30 is provided to efficiently collect the carbon dioxide gas generated in the carbon deposition/combustion electrode 3 as the negative electrode (see formula (6)), and it is noted that the carbon dioxide generated in the negative electrode and the oxide ions (O) in the bath^2-) Carbonate ion (CO) is generated upon contact₃ ^2-) Therefore, it must be quickly discharged to a position not in contact with O^2-In the contacted gas phase. In addition, regarding O in the bath^2-The ion concentration is preferably kept low to the extent that it does not come into contact with carbon dioxide in the vicinity of the negative electrode. For the CO generated₃ ^2-When the carbon dioxide electrolysis-carbon fuel cell -type device 1 functions as a carbon dioxide electrolysis device, it can be used for the carbon electrodeposition reaction of formula (2), but it inhibits the carbon dioxide absorption reaction of formula (1), and therefore CO is generated at this stage₃ ^2-And is not preferred.

3. Power generation efficiency of carbon fuel cell

In the carbon dioxide electrolysis-carbon fuel cell -type device 1 according to the present embodiment, the power generation efficiency when used as a carbon fuel cell can be calculated by multiplying the ratio (Δ G/Δ H) of the gibbs energy change (Δ G) of the combustion reaction converted into electric power to the enthalpy change (Δ H) by the fuel utilization rate and the voltage efficiency, and when solid carbon is used as the fuel, Δ G/Δ H is likely to approach 1 even at high temperature, and the fuel utilization rate is likely to approach 1 compared to that of gaseous hydrogen, so the carbon dioxide electrolysis-carbon fuel cell -type device 1 according to the present embodiment can be expected to obtain a higher power generation efficiency than hydrogen.

Table 1 shows (Δ G/Δ H) of the electrochemical combustion reaction of hydrogen and carbon, expected fuel utilization rate, and power generation efficiency in the carbon dioxide electrolysis-carbon fuel cell -type device 1 according to the present embodiment, it should be noted that the voltage efficiency is the same as , which is 0.8, and in the case where solid carbon is used as the fuel, Δ G/Δ H is likely to approach 1 even at high temperature, and the fuel utilization rate is likely to approach 1 even compared to gaseous hydrogen, and therefore, it is expected that a very high power generation efficiency of , which is 0.80, is obtained compared to the case of hydrogen (0.54).

[ Table 1]

Combustion reaction	ΔG/ΔH(923K)	Fuel utilization rate	Efficiency of voltage	Efficiency of power generation
					H3+1/2O2(g)→H2O(g)	0.80	0.85	0.80	0.54
C+O2(g)→CO2(g)	1.00	1.00	0.80	0.80

Reference documents: yi Yuan academic, Hydrogen energy System, 36(2), 17(2011)

FIG. 4 shows an energy/substance flow diagram for the case of using a water electrolysis-hydrogen fuel cell -type device using hydrogen as an energy carrier and a carbon dioxide electrolysis-carbon fuel cell -type device 1 using carbon as an energy carrier, the energy efficiency (87%, electrolysis voltage 1.7V) of the water electrolysis device is determined using the highest value of the current industrial electrolysis, and the values (68%, electrolysis voltage 1.5V) of the carbon dioxide electrolysis device are set based on the actual values of the carbon electroanalysis experiment of the applicant₂When the ratio of the electric energy input for electrolysis to the electric energy obtained from the carbon fuel cell (overall efficiency) was 54%, the carbon dioxide electrolysis-carbon fuel cell type device was judged to be significantly larger than the water electrolysis-fuel cell type device by 47%.

As described above, the method of using the carbon dioxide electrolysis-carbon fuel cell -type device 1 according to the present embodiment is characterized by including 2 steps involving completely opposite reactions, namely, a carbon electrolysis step of supplying a carbon dioxide-containing gas from the carbon dioxide gas supply unit 6 into the molten salt 20, applying a voltage between the carbon deposition/combustion electrode 3 and the oxygen generation electrode 4 by the power supply 7, and reducing carbonate ions on the carbon deposition/combustion electrode 3 to deposit carbon, and a power generation step of supplying an oxygen-containing gas from the oxygen supply unit 8 into the oxygen reduction electrode 5 to generate oxide ions in the molten salt 20 and generate carbon dioxide gas on the carbon deposited on the carbon deposition/combustion electrode 3.

D. Utilizes carbon dioxide electrolysis-carbon combustionCarbon-based active energy system of material battery type device

Fig. 5 is a schematic diagram schematically showing a carbon utility energy system using the carbon dioxide electrolysis-carbon fuel cell -type device 1 shown in fig. 1.

As will be understood from fig. 5, in the present embodiment, the carbon dioxide gas storage unit 9 for storing the carbon dioxide gas generated in the carbon deposition/combustion electrode 3 is added to the carbon dioxide electrolysis-carbon fuel cell -type apparatus 1, whereby the carbon-use energy system a (1a) can be configured, and it is assumed that the carbon dioxide gas having a high concentration generated when the carbon-use energy system a (1a) is used as a carbon fuel cell (see formula (7)) is stored underground.

In the present embodiment, the carbon storage unit 10 for storing carbon generated in the carbon deposition/combustion electrode 3 is added to the carbon dioxide electrolysis-carbon fuel cell -type device 1, whereby the carbon-use energy system B (1B) can be configured as a carbon-use energy system B (1B) which can use carbon (see formula (4)) generated when it is used as a carbon dioxide electrolysis device as a solid fuel for a carbon fuel cell (see formula (7)) or a high-performance carbon member.

Fig. 5 also shows cases of the flow direction of carbon and carbon dioxide, and electrical energy in a "carbon-utility energy system" that was conceived in year 2050.

In order to achieve the target of carbon dioxide emission reduction (80%) in 2050 years, it is estimated that it is necessary to achieve a renewable energy ratio of 50% to the total generated power (10 million MWh), and Utilization and storage of recovered carbon dioxide (CCUS: carbon dioxide Capture, Utilization&Storage) carbon dioxide handling 2 hundred million t-CO₂And (4) a year. If 2 million t-CO₂Carbon dioxide per year is converted to carbon by the "carbon-utilizing energy system", and 5400 million t-C per year of carbon is produced.

As for carbon obtained by electrolysis in the "carbon-based energy system", 3 functions of "application to energy conversion equipment and the like as a high-performance carbon material", "application to solid fuel for fuel cells", and "storage in a state of solid carbon" are expected. Among them, carbon used as a solid fuel for a fuel cell is discharged as high-concentration carbon dioxide at the time of power generation, and the carbon dioxide can be recycled as a carbon raw material and can be stored underground as high-concentration carbon dioxide.

Next, assuming that carbon dioxide electrolysis is performed in half-time (12 hours) of 1 day on average and electricity is generated in half-time by using carbon obtained by using 100MW class carbon dioxide electrolysis-carbon fuel cell -type apparatus 1 of the present invention, and 50% of the amount of electricity generated by renewable energy is temporarily stored, the system is introduced into about 600 seats, and the amount of carbon used for electricity generation in the apparatus is 3500 ten thousand t-C/year, and the carbon becomes high-concentration carbon dioxide at the time of electricity generation, and is stored underground, thereby directly contributing to reduction of carbon dioxide (1 a).

Finally, the amount of solid carbon stored in the state of 1600 ten thousand t-C/year. In order to convert carbon dioxide into solid carbon and store the carbon dioxide, a simple calculation requires more energy than underground storage, but underground storage requires injection of carbon dioxide into an underground aquifer or the like at a depth of 1000m or more from the earth's surface, and there is a limit to a formation suitable for stable storage of carbon dioxide for a long period of time. In contrast, carbon has a density about 3 times higher than that of carbon dioxide under high pressure (underground), and since the weight of carbon contained in carbon dioxide is 12/44, the volume thereof can be reduced to 1/10 or less of the volume of carbon dioxide under high pressure (underground). Further, in consideration of the fact that it can be stored in any place without requiring high pressure and can be used as fuel or various raw materials in some cases, the storage of carbon dioxide into solid carbon can be a sufficient choice as a realistic option for reducing the amount of carbon dioxide in the atmosphere (1 b).

As described above, the carbon dioxide electrolysis-carbon fuel cell type device 1 of the present invention is a core technology, and the carbon utility energy system a (1a) having the carbon dioxide gas storage unit 9 and the carbon utility energy system B (1B) having the carbon storage unit 10 are configured, and further, the carbon utility energy systems a, B (1a, 1B) of the present invention are combined with thermal power generation and power generation based on natural energy such as sunlight and wind power via a smart grid or the like, so that the obtained carbon is used as fuel (energy carrier) of a power generation facility directly using solid carbon fuel, whereby the power system can be stabilized, and utilization and storage (CCUS) of recovered carbon dioxide can be promoted.

Description of the reference numerals

1. carbon dioxide electrolysis-carbon fuel cell body type device

11. carbon deposition apparatus based on carbon dioxide electrolysis

12. carbon fuel cell

1 a. carbon active energy system A

1B. carbon active energy system B

2. electrolytic bath

20. molten salt

3. carbon deposition/combustion electrode

30. carbon dioxide gas collecting section

4. oxygen generating electrode

40. oxygen gas collecting part

5. oxygen reduction electrode

6. carbon dioxide gas supply section

7. power supply

8. oxygen gas supply part

9. carbon dioxide gas storage section

10. carbon reservoir