CARBON DIOXIDE REDUCTION ELECTRODE CATALYST LAYER, CATHODE, ION EXCHANGE MEMBRANE-ELECTRODE ASSEMBLY AND SOLID ELECTROLYTE ELECTROLYSIS DEVICE

Description

TECHNICAL FIELD

The technology of the present disclosure relates to a carbon dioxide reduction electrode catalyst layer, a cathode, an ion exchange membrane-electrode assembly, and a solid electrolyte type electrolysis device.

BACKGROUND ART

Carbon dioxide is emitted when energy is extracted from fossil fuel and the like. An increase in concentration of carbon dioxide in the atmosphere is said to be one of the causes for global warming. Carbon dioxide is an extremely stable substance, and hence there have hitherto been few ways to utilize carbon dioxide. However, in an era when global warming is becoming more serious, there has been a demand for a new technology for converting carbon dioxide into another substance to recycle the substance as a resource. For example, a carbon dioxide reduction device capable of directly reducing carbon dioxide in a gas phase is being developed.

In general, the carbon dioxide reduction device includes, as a cathode, a gas diffusion layer that takes in a carbon dioxide gas and a catalyst layer that accelerates a carbon dioxide reduction reaction, and is brought into contact with an electrolyte solution via an ion exchange membrane. Due to its structure, the ion exchange membrane has the property of allowing an electrolyte as well as an ion to pass therethrough. As a result, a phenomenon in which the electrolyte supplied to an anode permeates the ion exchange membrane to cause excess moisture in the catalyst layer and a phenomenon in which the electrolyte is precipitated as a salt in the vicinity of the cathode to block a flow passage are often found. Those phenomena may have an adverse influence such as hindering the supply of carbon dioxide to a cathode catalyst to cause a decrease in electrolysis performance, such as a current density or selectivity. In particular, this influence is liable to occur significantly at higher reaction temperature.

Meanwhile, for example, in NPL 1 and NPL 2, the use of pure water without using an electrolyte has been investigated. In addition, in NPL 3, there has been proposed a system in which pure water and a concentrated electrolyte solution are alternately supplied to a cathode.

CITATION LIST

Non Patent Literature

• NPL 1: M. C. O. Monteiro, F. Dattila, B. Hagedoorn, R. Garcia-Muelas, N. Lopez, M. T. M. Koper, Nature catalysis, 2021, 4, 654-662.
• NPL 2: J. A. Rebstock, Q. Zhu, L. R. Baker, Chemical Science, 2022, 13, 7634-7643. NPL 3: B. Endrodi, A. Samu, E. Kecsenovity, T. Halmagyi, D. Sebok, C. Janaky, Nature Energy, 2021, 6, 439-448.

SUMMARY OF INVENTION

Technical Problem

However, in NPL 1 and NPL 2, there has been reported that, when pure water is used without using an electrolyte, the speed and selectivity of the carbon dioxide reduction reaction are significantly reduced.

It has been known that an intermediate in the carbon oxide reduction reaction has relatively low stability, but is sufficiently stabilized by temporarily forming bonds with an alkali metal ion. A method disclosed in NPL 3 seems to resolve the conflicting requirements of suppressing salt precipitation by not using an alkali metal ion and stabilizing an intermediate by using an alkali metal ion, but this method has problems in that a cell operation rate decreases, and stability becomes insufficient owing to the lack of a mechanism for retaining a metal ion in the vicinity of the catalyst.

The technology of the present disclosure has been made in view of the above-mentioned circumstances, and an object of the technology of the present disclosure is to provide a carbon dioxide reduction electrode catalyst layer, a cathode, an ion exchange membrane-electrode assembly, and a solid electrolyte type electrolysis device, each of which has high electrolysis efficiency in a carbon dioxide electrolysis reduction reaction.

Solution to Problem

• • <1> A carbon dioxide reduction electrode catalyst layer, comprising a catalyst, an alkali metal ion, and a polymer material capable of releasing the alkali metal ion. The carbon dioxide reduction electrode catalyst layer according to Item <1>, wherein the catalyst and the alkali metal ion are dispersed and integrated in the polymer material capable of releasing the alkali metal ion.
  • <3> The carbon dioxide reduction electrode catalyst layer according to Item <1>, wherein the carbon dioxide reduction electrode catalyst layer includes a reaction layer containing the catalyst, and a layer containing the alkali metal ion and the polymer material capable of releasing the alkali metal ion.
  • <4> The carbon dioxide reduction electrode catalyst layer according to Item <3>, wherein the reaction layer containing the catalyst, and the layer containing the alkali metal ion and the polymer material capable of releasing the alkali metal ion are integrally formed.
  • <5> The carbon dioxide reduction electrode catalyst layer according to Item <1> or <2>, wherein the polymer material capable of releasing the alkali metal ion includes a cation exchange resin.
  • <6> The carbon dioxide reduction electrode catalyst layer according to Item <1>, <3>, or <4>, wherein the polymer material capable of releasing the alkali metal ion includes a polymer gel.
  • <7> The carbon dioxide reduction electrode catalyst layer according to any one of Items <1> to <6>, wherein the catalyst includes one or more selected from the group consisting of: a catalyst A containing a metal ion selected from the group consisting of: a copper ion; a nickel ion; an iron ion; a cobalt ion; a zinc ion; a manganese ion; a molybdenum ion; and an aluminum ion, a nitrogen-containing compound, and a carbon-containing carrier; and a catalyst B containing an inorganic fine particle selected from the group consisting of: gold; silver; copper; nickel; iron; cobalt; zinc; chromium; palladium; tin; manganese; aluminum; indium; bismuth; molybdenum; and carbon nitride, or a metal complex in which a ligand is coordinated to a metal selected from the group consisting of: copper; nickel; iron; cobalt; zinc; manganese; molybdenum; and aluminum or an ion of the metal, and a carbon-containing carrier.
  • <8> A cathode, comprising the carbon dioxide reduction electrode catalyst layer of any one of Items <1> to <7> and a gas diffusion layer.
  • <9> An ion exchange membrane-electrode assembly, comprising the cathode of Item <8>, a solid electrolyte, and an anode.
  • <10> The ion exchange membrane-electrode assembly according to Item <9>, wherein the solid electrolyte is an anion exchange membrane.
  • <11> A solid electrolyte type electrolysis device, comprising:
    • the cathode of Item <8>;
    • an anode forming a pair of electrodes with the cathode;
    • a solid electrolyte interposed between the cathode and the anode in a contact state; and
    • a voltage application unit configured to apply a voltage between the cathode and the anode.
  • <12> The solid electrolyte type electrolysis device according to Item <11>, further comprising an electrolyte solution in contact with the anode, wherein the electrolyte solution is pure water.
  • <13> The solid electrolyte type electrolysis device according to Item <11> or <12>, wherein the solid electrolyte is an anion exchange membrane.

Advantageous Effects of Invention

According to the technology of the present disclosure, the carbon dioxide reduction electrode catalyst layer, the cathode, the ion exchange membrane-electrode assembly, and the solid electrolyte type electrolysis device, each of which has high electrolysis efficiency in a carbon dioxide electrolysis reduction reaction, can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of an ion exchange membrane-electrode assembly to be suitably used in an embodiment of the present disclosure.

FIG. 2 is a schematic sectional view of a carbon dioxide reduction electrode catalyst layer to be suitably used in this embodiment.

FIG. 3 is a schematic sectional view of a carbon dioxide reduction electrode catalyst layer to be suitably used in this embodiment.

FIG. 4 is a schematic view of a solid electrolyte type electrolysis device to be suitably used in this embodiment.

DESCRIPTION OF EMBODIMENTS

The upper limit values and lower limit values of numerical ranges described herein may be arbitrarily combined. For example, when the range of “from A to B” and the range of “from C to D” are described as numerical ranges, the numerical range of “from A to D” and the numerical range of “from C to B” are also included in the scope of the present disclosure.

In addition, the numerical range of “from a lower limit value to an upper limit value” described herein means that a physical property value is the lower limit value or more and the upper limit value or less unless otherwise stated.

<Carbon Dioxide Reduction Electrode Catalyst Layer>

A carbon dioxide reduction electrode catalyst layer according to an embodiment of the present disclosure includes a catalyst, an alkali metal ion, and a polymer material capable of releasing the alkali metal ion.

The term “carbon dioxide reduction electrode catalyst layer” is hereinafter sometimes simply referred to as “catalyst layer.” In addition, the term “polymer material capable of releasing the alkali metal ion” is hereinafter sometimes simply referred to as “polymer material according to an embodiment of the present disclosure.”

Although the presence of an electrolyte, in particular, an alkali metal ion serving as a cation species, is required for driving a carbon dioxide reduction reaction at a high selectivity, an ion exchange membrane allows the electrolyte to pass therethrough, and hence a phenomenon in which moisture in the catalyst layer becomes excessive and a problem in that the electrolyte is precipitated as a salt in the vicinity of a cathode to block a flow passage may occur as described above. Those phenomena have an adverse influence such as hindering the supply of carbon dioxide to a cathode catalyst, and the electrolysis efficiency is liable to be decreased. In addition, when pure water is used instead of the electrolyte, the stability of an intermediate in the carbon dioxide reduction reaction is decreased, and the electrolysis efficiency is liable to be decreased.

Meanwhile, the carbon dioxide reduction electrode catalyst layer according to the embodiment of the present disclosure has high electrolysis efficiency in the carbon dioxide electrolysis reduction reaction.

Although the reason for the foregoing is not known, it is conceived that, when the catalyst, and the alkali metal ion and the polymer material capable of releasing the alkali metal ion are caused to coexist with each other in the catalyst layer, the alkali metal ion can be stably supplied to the intermediate generated in the carbon dioxide reduction reaction to improve the electrolysis efficiency. In addition, it is conceived that the chemical interaction between the polymer material and the alkali metal ion according to the embodiment of the present disclosure suppresses the detachment of the alkali metal ion from the catalyst layer, and hence the precipitation of the alkali metal ion as a salt in the vicinity of the cathode can be suppressed and thus the electrolysis efficiency can be improved.

As described above, it is conceived that the carbon dioxide reduction electrode catalyst layer according to the embodiment of the present disclosure can control the concentration of the alkali metal ion in the vicinity of the cathode to the extent that the alkali metal ion is not precipitated as a salt while accelerating the reduction reaction, and hence the electrolysis efficiency can be improved.

A carbon dioxide reduction electrode catalyst layer, and a cathode, an ion exchange membrane-electrode assembly, and a solid electrolyte type electrolysis device according to the embodiment of the present disclosure are described in detail below.

[Structure of Catalyst Layer]

FIG. 1 is a schematic sectional view of an ion exchange membrane-electrode assembly to be suitably used in this embodiment.

In FIG. 1, an ion exchange membrane-electrode assembly 50 including a gas diffusion layer 10, a carbon dioxide reduction electrode catalyst layer 20, a solid electrolyte 30, and an anode 40 is illustrated.

The catalyst layer 20 includes a catalyst 23 according to this embodiment, and an alkali metal ion and a polymer material 24 according to this embodiment. A combination of the gas diffusion layer 10 and the catalyst layer 20 forms a cathode according to this embodiment.

As illustrated in FIG. 1, carbon dioxide (CO₂) is supplied to the catalyst layer 20 through the gas diffusion layer 10, and carbon monoxide (CO) is generated by a reduction reaction.

The catalyst layer 20 according to the embodiment of the present disclosure may have a single layer structure as illustrated in FIG. 2, or a laminate structure of two or more layers as illustrated in FIG. 3.

A catalyst layer 20a having a single layer structure illustrated in FIG. 2 includes a catalyst 23a according to this embodiment, and an alkali metal ion and a polymer material 24a according to this embodiment.

A catalyst layer 20b having a laminate structure illustrated in FIG. 3 includes a reaction layer 21 containing a catalyst 23b according to this embodiment, and a layer 22 (also referred to as “cation supply layer 22”) containing an alkali metal ion and a polymer material 24b according to this embodiment.

1. Single Layer Structure

When the catalyst layer 20 according to the embodiment of the present disclosure has a single layer structure, the catalyst layer 20 (catalyst layer 20a) includes the catalyst 23a, and the “alkali metal ion and polymer material capable of releasing the alkali metal ion” 24 in the same layer.

From the viewpoint of stably supplying the alkali metal ion to the intermediate generated in the dioxide reduction reaction in the catalyst layer 20a, it is preferred that the catalyst 23a and the alkali metal ion be dispersed and integrated in the polymer material capable of releasing the alkali metal ion in the catalyst layer 20a.

Although the alkali metal ion is not easily detached from the catalyst layer because of the chemical interaction between the polymer material and the alkali metal ion according to this embodiment, from the viewpoint of further suppressing the detachment of the alkali metal ion from the catalyst layer, it is preferred that part of the polymer material according to this embodiment be substituted with the alkali metal ion. When part of the polymer material according to this embodiment is substituted with the alkali metal ion, the detachment of the alkali metal ion from the catalyst layer can be further suppressed while the alkali metal ion is stably supplied to the intermediate generated in the dioxide reduction reaction. Accordingly, the precipitation of the alkali metal ion as a salt in the vicinity of the cathode can be suppressed and thus the electrolysis efficiency can be improved.

Accordingly, it is preferred that the “alkali metal ion and polymer material capable of releasing the alkali metal ion” 24a included in the catalyst layer 20a be a polymer material which is substituted with the alkali metal ion, and which is capable of releasing the alkali metal ion.

Details of the “alkali metal ion and polymer material capable of releasing the alkali metal ion” 24a included in the catalyst layer 20a are described later.

The content of the catalyst 23a in the catalyst layer 20a is preferably from 50 mass % to 99 mass %, more preferably from 75 mass % to 97 mass %, still more preferably from 90 mass % to 95 mass % from the viewpoint of further improving the electrolysis activity and the reduction reaction speed of CO₂.

The content of the polymer material capable of releasing the alkali metal ion in the catalyst layer 20a (the amount of the polymer material itself, not including the amount of the alkali metal ion) is preferably from 1 mass % to 50 mass %, more preferably from 1 mass % to 30 mass %, still more preferably from 2 mass % to 20 mass % from the viewpoint of further improving the electrolysis activity and the reduction reaction speed of CO₂.

The content of the alkali metal ion in the catalyst layer 20a is preferably from 0.01 mass % to 20 mass %, more preferably from 0.01 mass % to 10 mass %, still more preferably from 0.1 mass % to 5 mass % from the viewpoint of further improving the electrolysis activity and the reduction reaction speed of CO₂.

The content of the alkali metal ion in the catalyst layer 20a may be measured by X-ray photoelectron spectroscopy.

2. Laminate Structure

When the catalyst layer 20 according to the embodiment of the present disclosure has a laminate structure, the catalyst layer 20 may have, for example, a two-layer structure including a layer containing a catalyst, and a layer containing an alkali metal ion and a polymer material capable of releasing the alkali metal ion, or a three-layer structure including a layer containing a catalyst, a layer containing an alkali metal ion, and a layer containing a polymer material capable of releasing the alkali metal ion. Thus, the respective layers may contain different components. In addition, two or more same layers may be laminated as in a four-layer structure obtained by alternately laminating a layer containing a catalyst, and a layer containing an alkali metal ion and a polymer material capable of releasing the alkali metal ion.

In FIG. 3, as an example of the laminate structure, the catalyst layer 20b having a two-layer structure including the reaction layer 21 containing the catalyst 23b, and the cation supply layer 22 containing the “alkali metal ion and polymer material capable of releasing the alkali metal ion” 24b is illustrated.

The reaction layer 21 functions as a layer that accelerates the carbon dioxide reduction reaction in the presence of the catalyst 23b, and the cation supply layer 22 functions as a layer that supplies the alkali metal ion to the reaction layer 21.

It is preferred that the catalyst layer having a laminate structure include the reaction layer 21 containing the catalyst 23b, and the layer 22 (cation supply layer 22) containing the “alkali metal ion and polymer material capable of releasing the alkali metal ion” 24b, as illustrated in FIG. 3, among those layers.

The supply of the alkali metal ion from the cation supply layer 22 to the reaction layer 21 can stabilize the intermediate generated in the dioxide reduction reaction to improve the electrolysis efficiency.

From the viewpoint of supplying the alkali metal ion from the cation supply layer 22 to the reaction layer 21 more efficiently, it is preferred that the reaction layer 21 and the cation supply layer 22 be integrally formed. In other words, it is preferred that the reaction layer 21 and the cation supply layer 22 be adjacent to each other.

In addition, in the catalyst layer 20b, it is preferred that the reaction layer 21 be adjacent to the gas diffusion layer 10, and the cation supply layer 22 be adjacent to the solid electrolyte 30.

The reaction layer 21 contains the catalyst 23b. The same catalyst as the catalyst 23a may be used as the catalyst 23b, and details thereof are described later.

In the reaction layer 21, the catalyst 23b is dispersed in a dispersion medium 25. The dispersion medium 25 is not particularly limited as long as the dispersion medium 25 is a medium capable of dispersing the catalyst 23b, and for example, an ionomer may be used. Details of the ionomer are described later.

The reaction layer 21 may or may not contain the alkali metal ion, but the catalyst layer 20b includes the cation supply layer 22, and hence the content of the alkali metal ion in the reaction layer 21 may be set to 0 mass %.

Details of the “alkali metal ion and polymer material capable of releasing the alkali metal ion” 24b included in the cation supply layer 22 are described later.

The content of the catalyst 23b in the reaction layer 21 is preferably from 50 mass % to 99 mass %, more preferably from 75 mass % to 97 mass %, still more preferably from 90 mass % to 95 mass % from the viewpoint of further improving the electrolysis activity and the reduction reaction speed of CO₂.

The content of the polymer material capable of releasing the alkali metal ion in the cation supply layer 22 is preferably from 5 mass % to 99 mass %, more preferably from 10 mass % to 95 mass %, still more preferably from 20 mass % to 95 mass % from the viewpoint of further improving the electrolysis activity and the reduction reaction speed of CO₂.

The thickness of the cation supply layer 22 is preferably from 0.005 mm to 0.5 mm from the viewpoint of suppressing the precipitation of an alkali metal salt as a salt in the vicinity of the cathode.

The respective components in the catalyst layer are described below. In addition, the following description is given with the reference symbols in FIG. 1 to FIG. 3 being omitted.

[Alkali Metal Ion]

Examples of the alkali metal ion include a lithium ion (Lit), a sodium ion (Na⁺), a potassium ion (K⁺), a rubidium ion (Rb⁺), and a cesium ion (Cs⁺).

The amount of the ion supplied to the intermediate generated in the carbon dioxide reduction reactivity varies depending on the adsorption power of the polymer material capable of releasing the alkali metal ion for the alkali metal ion. Specifically, when a cation exchange resin is used as the polymer material according to this embodiment, and an ion exchange group is a sulfone group, the adsorption power thereof for the alkali metal ion is increased in the order of Li⁺<Na⁺<K⁺<Rb⁺ . . . . It is preferred that the adsorption power be controlled so that the precipitation of a salt is not caused by oversupply while the amount of the alkali metal ion supplied to the intermediate is sufficient.

The adsorption power varies depending on the combination of the alkali metal ion and the polymer material according to this embodiment. The alkali metal ion is preferably one or more selected from the group consisting of: a potassium ion; a sodium ion; a rubidium ion; and a cesium ion, more preferably one or more selected from the group consisting of: a potassium ion; a rubidium ion; and a cesium ion.

The alkali metal ions may be used alone or in combination thereof.

[Polymer Material capable of releasing Alkali Metal Ion]

An ionomer, a polymer gel, or the like may be used as the polymer material capable of releasing the alkali metal ion.

The polymer materials according to this embodiment may be used alone or in combination thereof.

In addition, an ionomer may be used as the dispersion medium in the reaction layer in the catalyst layer having a laminate structure.

(Ionomer)

An ionomer functions as a binder resin in the catalyst layer having a single layer structure or in the reaction layer in the catalyst layer having a laminate structure, is a matrix resin (continuous phase) capable of dispersing and fixing the catalyst according to this embodiment, and also has a function to transfer an ion generated by electrolysis to improve the electrolysis efficiency of CO₂. In addition, the ionomer is preferably conductive, more preferably a polymer electrolyte from the viewpoint of improving the transfer efficiency of the ion generated by electrolysis. It is still more preferred that the polymer electrolyte be an ion exchange resin. The ion exchange resin may be a cation exchange resin or an anion exchange resin.

Examples of the cation exchange resin include a fluororesin having a sulfone group, and a styrene-divinylbenzene copolymer having a sulfone group. In addition, a commercially available product may be used, and examples thereof include Nafion (manufactured by The Chemours Company), Aquivion (manufactured by Solvay Specialty Polymers Japan K.K.), DIAION (manufactured by Mitsubishi Chemical Corporation), and Fumasep (manufactured by FUMATECH BWT GmbH).

An example of the anion exchange resin is a resin having one or more ion exchange groups selected from the group consisting of: a quaternary ammonium group; a primary amino group; a secondary amino group; and a tertiary amino group. A commercially available product may be used, and examples thereof include Sustainion (manufactured by Dioxide Materials), Fumasep (manufactured by FUMATECH BWT GmbH), PENTION (manufactured by Xergy Inc.), DURION (manufactured by Xergy Inc.), NEOSEPTA (manufactured by ASTOM Corporation), and TOYOPEARL (manufactured by Tosoh Corporation).

From the viewpoint of improving conductivity, the anion exchange resin has a basic site density in a dry state of preferably from 2.0 mmol/cm³ to 5.0 mmol/cm³, more preferably 2.5 mmol/cm³ or more and less than 4.5 mmol/cm³, still more preferably 2.9 mmol/cm³ or more and less than 4.5 mmol/cm³.

The basic site density of the anion exchange resin may be obtained from an integral value of a signal obtained by performing ¹H NMR measurement of the anion exchange resin.

In addition, the term “dry state” of the anion exchange resin means a state in which the anion exchange resin does not contain free water. For example, the anion exchange resin may be brought into a dry state by heating in a vacuum.

(Polymer Gel)

It is preferred that a polymer gel having a water retention property and hydrophilicity be used from the viewpoint that the polymer gel encapsulates and releases the alkali metal ion, and ensures ion conductivity. Polyacrylamide, agarose, starch, gelatin, and the like may each be used as the polymer gel having a water retention property and hydrophilicity. Among them, polyacrylamide, gelatin, and agarose are each preferred as the polymer gel, and polyacrylamide and agarose are each more preferred.

When the catalyst layer is used in a single layer structure as illustrated in FIG. 2, the polymer material capable of releasing the alkali metal ion preferably contains an ionomer among those polymer materials, and more preferably contains a cation exchange resin from the viewpoints of easily supplying the alkali metal ion to the intermediate generated in the carbon dioxide reduction reaction, and suppressing the detachment of the alkali metal ion from the catalyst layer.

Further, from the viewpoints of easily supplying the alkali metal ion to the intermediate, and further suppressing the detachment of the alkali metal ion from the catalyst layer, in the catalyst layer having a single layer structure as illustrated in FIG. 2, it is preferred that the “alkali metal ion and polymer material capable of releasing the alkali metal ion” include a cation exchange resin substituted with the alkali metal ion. A specific example thereof is a form in which the sulfone group of the cation exchange resin has an alkali metal ion as a counter ion.

Through use of the cation exchange resin substituted with the alkali metal ion, a cation is supplied from the resin to a catalyst active site during the electrolysis reaction, and the carbon dioxide reduction reaction is easily accelerated.

A method of substituting a cation exchange resin with an alkali metal ion is not particularly limited.

For example, a solution in which a cation exchange resin is dissolved in a solvent such as ethanol is applied onto a base material and dried, and then the base material is immersed in an inorganic salt aqueous solution of, for example, as a carbonate or a bicarbonate containing 0.01 mol/L to 5 mol/L of an alkali metal species. The inorganic salt aqueous solution in which the base material is immersed is placed under a reduced pressure environment so that residual air bubbles are removed, and then left to stand still for 1 hour or more. Thus, metal ion substitution can be performed on the cation exchange resin.

In addition, when the catalyst layer has a laminate structure as illustrated in FIG. 3, the polymer material capable of releasing the alkali metal ion in the cation supply layer preferably includes a polymer gel, and the “alkali metal ion and polymer material capable of releasing the alkali metal ion” preferably includes a polymer gel containing an aqueous solution containing an alkali metal salt. An example of the aqueous solution containing an alkali metal salt is an aqueous solution containing the same components as those of the electrolyte, and specific examples thereof include aqueous solutions of KHCO₃, NaHCO₃, and Cs₂CO₃.

Through use of the polymer gel as the polymer material according to this embodiment in the cation supply layer, the supply of the alkali metal ion from the cation supply layer to the reaction layer can be smoothly performed to improve the electrolysis efficiency.

In the catalyst layer having a laminate structure, the dispersion medium in the reaction layer is preferably an ionomer, and the catalyst is preferably covered with the ionomer. When the catalyst is covered with the ionomer, an ion conductive channel between the covered catalyst and the solid electrolyte is easily formed to facilitate the movement of an ion generated by the reaction, and thus the electrolysis efficiency can be improved.

The dispersion medium in the reaction layer is more preferably a cation exchange resin.

[Catalyst]

The catalyst according to the embodiment of the present disclosure is not particularly limited as long as the catalyst accelerates the carbon dioxide reduction reaction, but from the viewpoint of improving the electrolysis efficiency, it is preferred that the catalyst include one or more selected from the group consisting of: the following catalyst A; and the following catalyst B.

The catalyst A contains a metal ion selected from the group consisting of: a copper ion; a nickel ion; an iron ion; a cobalt ion; a zinc ion; a manganese ion; a molybdenum ion; and aluminum ion, a nitrogen-containing compound, and a carbon-containing carrier.

The catalyst B contains an inorganic fine particle selected from the group consisting of: gold; silver; copper; nickel; iron; cobalt; zinc; chromium; palladium; tin; manganese; aluminum; indium; bismuth; molybdenum; and carbon nitride, or a metal complex in which a ligand is coordinated to a metal selected from the group consisting of: copper; nickel; iron; cobalt; zinc; manganese; molybdenum; and aluminum or an ion of the metal, and a carbon-containing carrier.

In the catalyst A according to the embodiment of the present disclosure, the component that exhibits a catalytic action on the carbon dioxide reduction reaction is the above-mentioned metal ion, but in the technology of the present disclosure, the configuration including the metal ion coordinated to a nitrogen atom on the nitrogen-containing compound and the carbon-containing carrier is referred to as “catalyst”.

In the catalyst B according to the embodiment of the present disclosure, the component that exhibits a catalytic action on the carbon dioxide reduction reaction is the above-mentioned inorganic fine particle or the above-mentioned metal complex, but in the technology of the present disclosure, the above-mentioned inorganic fine particle or the above-mentioned metal complex is referred to as “catalyst source,” and the configuration including the catalyst source and the above-mentioned carbon-containing carrier is referred to as “catalyst”.

[Catalyst Source (Metal Ion) of Catalyst A]

The catalyst A according to this embodiment contains, as a catalyst source, a metal ion selected from the group consisting of: a copper ion; a nickel ion; an iron ion; a cobalt ion; a zinc ion; a manganese ion; a molybdenum ion; and an aluminum ion.

The metal ion in this embodiment has an action of generating at least carbon monoxide by the reduction reaction.

It is preferred that the metal ion be present in a monoatomic state and supported on the carbon-containing carrier according to this embodiment. The activity can be increased by allowing the catalyst source to be present in a monoatomic state.

When the CO₂ reduction reaction is advanced, gold and silver are each widely used as a catalyst source, but gold and silver are rare and expensive. Accordingly, as a catalyst replacing gold and silver, a catalyst preferably has a form in which an ion of a metal except gold and silver is coordinated to a nitrogen atom and thus supported on a carbon-containing simple substance.

The metal of the metal ion in this embodiment is selected from the group consisting of: copper; nickel; iron; cobalt; zinc; manganese; molybdenum; and aluminum.

Among them, nickel, cobalt, iron, copper, zinc, and manganese are each preferred as the metal of the metal ion from the viewpoint of the reaction efficiency of the carbon dioxide reduction reaction, nickel, cobalt, iron, and copper are each more preferred, and nickel, cobalt, and iron are each still more preferred. The catalyst A according to this embodiment may contain only one kind of metal ion, or two or more kinds thereof.

The upper limit of the content of the metal ion in the catalyst A is not particularly limited, but from the viewpoint of further improving the active site density, the upper limit is preferably less than 50 mass %. From the same viewpoint, the content of the metal ion coordinated to the nitrogen atom is more preferably from 0.8 mass % to 15 mass %, still more preferably from 0.9 mass % to 10 mass % in the catalyst A.

In the present disclosure, the term “active site density” means the content (mass %) of the metal ion coordinated to the nitrogen atom of the nitrogen-containing compound in the catalyst A.

The content of the metal ion coordinated to the nitrogen atom may be determined as described below.

The ratio of the content of the metal ion and metal fine particle bonded (coordinated) to the nitrogen atom in the catalyst A may be measured by X-ray absorption fine structure analysis (XAFS).

In addition, the total metal content in the catalyst containing the metal ion and metal fine particle coordinated to the nitrogen atom may be measured by X-ray fluorescence analysis (XRF).

The content of the metal ion coordinated to the nitrogen atom may be calculated from the measurement results of the XAFS and the measurement results of the XRF.

For example, when a Ni ion is used as a catalyst source, the content of the Ni ion coordinated to the nitrogen atom may be calculated by multiplying the total Ni content containing the Ni metal and the Ni ion measured by the XRF by the ratio of the Ni ion in the total Ni determined by the XAFS.

The metal fine particle does not serve as an active site, and is present as a metal aggregate.

[Nitrogen-containing Compound]

The nitrogen-containing compound in the catalyst A is not particularly limited, and examples thereof include pentaethylenehexamine, tetraethylenepentamine, triethylenepentamine, diethylenetriamine, ethylenediamine, and diethylamine. From the viewpoint of improving the electrolysis activity, pentaethylenehexamine, tetraethylenepentamine, and triethylenepentamine are preferred, and pentaethylenehexamine and tetraethylenepentamine are more preferred.

The content of the nitrogen-containing compound in the catalyst A according to this embodiment is preferably from 5 mass % to 75 mass %, more preferably from 10 mass % to 60 mass %, still more preferably from 20 mass % to 50 mass % from the viewpoint of improving the active site density.

[Catalyst Source (Inorganic Fine Particle or Metal Complex) of Catalyst B]

The catalyst B according to this embodiment contains an inorganic fine particle or a metal complex as a catalyst source.

The inorganic fine particle and the metal complex in this embodiment each have an action of generating at least carbon monoxide by the reduction reaction.

(Inorganic Fine Particle)

The inorganic fine particle in this embodiment is an inorganic fine particle of a material selected from the group consisting of: gold; silver; copper; nickel; iron; cobalt; zinc; chromium; palladium; tin; manganese; aluminum; indium; bismuth; molybdenum; and carbon nitride. The materials may be used alone or in combination thereof.

Among them, silver, gold, zinc, tin, copper, and bismuth are each preferred as the material for the inorganic fine particle from the viewpoint of the reaction efficiency of the carbon dioxide reduction reaction, silver, gold, copper, and tin are each more preferred, and silver, gold, and copper are each still more preferred.

The average particle diameter of the inorganic fine particle serving as a catalyst source is preferably 65 nm or less, preferably 60 nm or less, preferably 50 nm or less, preferably 40 nm or less, preferably 30 nm or less from the viewpoint of the reaction speed of the carbon dioxide reduction reaction. In addition, the lower limit value of the average particle diameter is not limited, but is preferably 1 nm or more, more preferably 5 nm or more from the viewpoint of ease of production.

The average particle diameter may be measured by photographic observation or the like with a scanning electron microscope or the like.

(Metal Complex)

The metal complex in this embodiment is a metal complex in which a ligand is coordinated to a metal or an ion of the metal, and the metal used here is selected from the group consisting of: copper; nickel; iron; cobalt; zinc; manganese; molybdenum; and aluminum.

Among them, nickel, cobalt, iron, copper, zinc, and manganese are each preferred as the metal from the viewpoint of the reaction efficiency of the carbon dioxide reduction reaction, nickel, cobalt, iron, and copper are each more preferred, and nickel, cobalt, and iron are each still more preferred. The metal complex may contain only one kind of metal or ion of the metal, or two or more kinds thereof.

The kind of the ligand is not particularly limited, and examples of the metal complex include a phthalocyanine complex, a porphyrin complex, a pyridine complex, a metal-supported covalent triazine framework, and a metal organic framework. Among them, a phthalocyanine complex, a porphyrin complex, a pyridine complex, and a metal-supported covalent triazine framework are preferred, a phthalocyanine complex, a porphyrin complex, and a metal-supported covalent triazine framework are more preferred, and a porphyrin complex and a metal-supported covalent triazine framework are still more preferred. The metal complex may contain only one kind of ligand, or two or more kinds thereof.

[Carbon-containing Carrier]

The catalyst A and the catalyst B each independently contain a carbon-containing carrier.

Carbon generally has conductivity, and hence the carbon-containing carrier according to this embodiment is a conductive carrier.

The carbon-containing carrier according to this embodiment is not limited as long as the carbon-containing carrier is a conductive material that can be used as a gas diffusion layer in an electrode arranged in a device for reducing carbon dioxide, and examples thereof include carbons, such as carbon black (e.g., furnace black, acetylene black, Ketjen black, or medium thermal carbon black), activated carbon, graphite, a carbon nanotube, a carbon nanofiber, a carbon nanohorn, a graphene nanoplatelet, and nanoporous carbon. Among them, carbon black is preferred from the viewpoint of improving the active site density.

From the viewpoints of improving the active site density and improving the current density, the primary particle diameter of the carbon black is preferably from 10 nm to 100 nm, more preferably from 10 nm to 50 nm. The primary particle diameter of the carbon black may be measured with a transmission electron microscope.

From the same viewpoints, the secondary particle diameter (particle diameter of an aggregate) of the carbon black is preferably small, and carbon black with a large amount of functional groups is preferred.

Carbon black may be a commercially available product, and examples thereof include Vulcan (trademark) XC-72 (manufactured by Cabot Corporation), and BLACK PEARLS 2000 (manufactured by Cabot Corporation).

The particle diameter of the catalyst (catalyst A or catalyst B) according to this embodiment is preferably from 10 nm to 50 μm.

When the particle diameter of the catalyst is 50 μm or less, the active site density is increased, and the reduction reaction speed of CO₂ can be improved. In addition, when the particle diameter of the catalyst is 10 nm or more, the precipitation of an electrolyte salt from the solid electrolyte can be suppressed, and workability becomes excellent.

From the viewpoint of further improving the active site density and the viewpoint of facilitating uniform dispersion of the catalyst in the ionomer in the catalyst layer having a single layer structure or the reaction layer in the catalyst layer having a laminate structure, the particle diameter of the catalyst is preferably from 20 nm to 40 μm, more preferably from 30 nm to 20 μm.

The particle diameter of the catalyst may be determined with a laser diffraction/scattering particle diameter distribution measurement device.

[Method of producing Catalyst]

A method of producing the catalyst according to this embodiment is not particularly limited.

An example of a method of producing the catalyst A is a method of producing a catalyst, including: a mixing step of mixing a metal ion, a nitrogen-containing compound, and a carbon-containing carrier to provide a mixture; and a calcination step of calcining the resultant mixture to provide a calcined product. The method of producing the catalyst A may further include a washing step of washing the calcined product, a pulverizing step of pulverizing the calcined product, and the like.

In addition, the catalyst B may be produced by causing an inorganic fine particle and a metal complex each serving as a catalyst source to be supported on the carbon-containing carrier by a known method, such as vapor deposition, precipitation, adsorption, deposition, adhesion, welding, physical mixing, or spraying.

<Cathode>

The cathode according to this embodiment includes the carbon dioxide reduction electrode catalyst layer according to this embodiment described above, and a gas diffusion layer.

The cathode according to this embodiment includes the catalyst layer according to this embodiment, and hence has high electrolysis efficiency.

[Gas Diffusion Layer]

For example, the gas diffusion layer includes carbon paper or a nonwoven fabric, or a metal mesh. Examples thereof include graphite carbon, glassy carbon, titanium, and stainless steel (SUS).

<Ion Exchange Membrane-Electrode Assembly>

The ion exchange membrane-electrode assembly according to this embodiment includes the above-mentioned cathode according to this embodiment, a solid electrolyte, and an anode.

The ion exchange membrane-electrode assembly according to this embodiment includes the cathode including the catalyst layer according to this embodiment, and hence has high electrolysis efficiency.

[Solid Electrolyte]

The ion exchange membrane-electrode assembly according to this embodiment includes a solid electrolyte.

A polymer membrane may be used as the solid electrolyte. Various ionomers may each be used as a polymer. The polymer may be a cation exchange resin or an anion exchange resin, but an anion exchange resin is preferred. That is, the solid electrolyte is preferably an anion exchange membrane.

As the solid electrolyte, a product that is commercially available as a cation exchange membrane or an anion exchange membrane may be used.

In addition, when an anion exchange membrane is used as the solid electrolyte, the anion exchange membrane has a basic site density in a dry state of preferably from 0.5 mmol/cm³ to 5.0 mmol/cm³, more preferably 2.5 mmol/cm³ or more and less than 4.5 mmol/cm³, still more preferably 2.9 mmol/cm³ or more and less than 4.5 mmol/cm³.

As the cation exchange membrane, for example, a strongly acidic cation exchange membrane having a sulfone group introduced into a fluororesin base, Nafion 117, Nafion 115, Nafion 212, or Nafion 350 (manufactured by The Chemrous Company), a strongly acidic cation exchange membrane having a sulfone group introduced into a styrene-divinylbenzene copolymer base, or NEOSEPTA CSE (manufactured by ASTOM Corporation) may be used.

An example of the anion exchange membrane is an anion exchange membrane having one or more ion exchange groups selected from the group consisting of: a quaternary ammonium group; a primary amino group; a secondary amino group; and a tertiary amino group. Specific examples thereof include: NEOSEPTA (trademark) ASE, AHA, ACS, and AFX (all manufactured by ASTOM Corporation); and SELEMION (trademark) AMVN, DSVN, AAV, ASVN, and AHO (all manufactured by AGC Inc.).

The carbon dioxide reduction reaction, that is, the reduction reaction at the cathode according to this embodiment varies depending on the kind of the solid electrolyte. When a cation exchange membrane is used as the solid electrolyte, the reduction reactions of the following reaction formula (1) and reaction formula (2) occur, and when an anion exchange membrane is used as the solid electrolyte, the reduction reactions of the following reaction formula (3) and reaction formula (4) occur.

[Anode]

The oxidation reaction at the anode varies depending on the kind of the solid electrolyte. When a cation exchange membrane is used as the solid electrolyte, the oxidation reaction of the following reaction formula (5) occurs, and when an anion exchange membrane is used as the solid electrolyte, the oxidation reaction of the following reaction formula (6) occurs.

The anode is a gas diffusion electrode including the gas diffusion layer.

For example, the gas diffusion layer includes a metal mesh. Examples of an electrode material for the anode may include Ir, IrO₂, Ru, RuO₂, Co, CoO_x, Cu, CuO_x, Fe, FeO_x, FeOOH, FeMn, Ni, NiO_x, NiOOH, NiCo, NiCe, NiC, NiFe, NiCeCoCe, NiLa, NiMoFe, NiSn, NiZn, SUS, Au, and Pt.

<Solid Electrolyte Type Electrolysis Device>

The solid electrolyte type electrolysis device according to this embodiment includes the above-mentioned cathode according to this embodiment, an anode forming a pair of electrodes with the cathode, a solid electrolyte interposed between the cathode and the anode in a contact state, and a voltage application unit that applies a voltage between the cathode and the anode.

The solid electrolyte type electrolysis device according to this embodiment includes the cathode including the catalyst layer according to this embodiment, and hence has high electrolysis efficiency.

FIG. 4 is a schematic view of a solid electrolyte type electrolysis device that is suitably used in this embodiment.

In FIG. 4, there is illustrated a solid electrolyte type electrolysis device 800 including a cathode 200 according to this embodiment, an anode 400 forming a pair of electrodes with the cathode 200, a solid electrolyte 300 interposed between the cathode 200 and the anode 400 in a contact state, and a voltage application unit 700 that applies a voltage between the cathode 200 and the anode 400.

The solid electrolyte type electrolysis device 800 illustrated in FIG. 4 further includes a cathode collector plate 100, an anode collector plate 500, and an electrolyte solution 600.

The above-mentioned cathode according to this embodiment is used as the cathode 200. In addition, the solid electrolyte 300 is the same as the solid electrolyte 30 in FIG. 1, and the solid electrolyte 300 is preferably an anion exchange membrane. The anode 400 is the same as the anode 40 in FIG. 1.

Details of the cathode 200, the solid electrolyte 300, and the anode 400 are as described above.

The respective elements except the cathode 200, the solid electrolyte 300, and the anode 400 are described below with the reference symbols being omitted.

[Cathode Collector Plate]

Examples of a material for the cathode collector plate include metal materials, such as copper (Cu), nickel (Ni), stainless steel (SUS), nickel-plated steel, and brass. Among them, copper is preferred from the viewpoints of easy workability and cost. When the material for the cathode collector plate is a metal material, the shape thereof is, for example, metal foil, a metal plate, a metal thin film, an expanded metal, a punched metal, or a foam meal.

A gas supply hole for supplying a raw material gas containing carbon dioxide to the cathode and a gas recovery hole for recovering a generated gas containing carbon monoxide may be formed in the cathode collector plate. When the cathode collector plate has the gas supply hole and the gas recovery hole, the raw material gas can be sent to the cathode uniformly and efficiently, and the generated gas (containing an unreacted raw material gas) can be discharged. Only one or two or more gas supply holes and only one or two or more gas recovery holes may be formed independently of each other. In addition, the shape, location, size, and the like of each of the gas supply hole and the gas recovery hole are not limited, and are appropriately set. In addition, when the cathode collector plate has gas permeability, the gas supply hole and the gas recovery hole are not necessarily required.

When the cathode serves to transfer electrons, the cathode collector plate is not necessarily required.

[Anode Collector Plate]

It is preferred that the anode collector plate have electric conductivity in order to receive electrons from the anode, and rigidity for supporting the anode. From such viewpoints, a metal material, such as titanium (Ti), copper (Cu), nickel (Ni), stainless steel (SUS), nickel-plated steel, or brass may be suitably used for the anode collector plate.

A gas flow passage for sending a raw material gas (e.g., H₂O) to the anode may be formed in the anode collector plate. When the anode collector plate has the gas flow passage, a raw material gas can be sent to the anode uniformly and efficiently. The number, shapes, locations, sizes, and the like of the gas flow passages are not limited and are appropriately set.

[Voltage Application Unit]

The voltage application unit serves to apply a voltage between the cathode and the anode through application of a voltage to the cathode collector plate and the anode collector plate. Here, both the collector plates are conductors, and hence receive electrons from the anode while supplying electrons to the cathode. In addition, a control unit (not shown) may be electrically connected to the voltage application unit in order to apply an appropriate voltage.

[Electrolyte solution]

Examples of the electrolyte solution include pure water and an aqueous solution of an electrolyte. Examples of the aqueous solution of an electrolyte include an aqueous solution of a carbonate, an aqueous solution of a bicarbonate (e.g., aqueous solution of KHCO₃), an aqueous solution of a sulfate, an aqueous solution of a borate, an aqueous solution of sodium hydroxide, an aqueous solution of potassium hydroxide, and an aqueous solution of sodium chloride.

The aqueous solution preferably has a pH of 5 or more.

In general, when pure water is used as the electrolyte solution, the alkali metal ion cannot be stably supplied to the intermediate generated in the carbon dioxide reduction reaction, and hence the electrolysis efficiency is decreased. However, the solid electrolyte type electrolysis device according this embodiment includes the catalyst layer according to this embodiment, and hence the alkali metal ion can be stably supplied to the intermediate generated in the carbon dioxide reduction reaction and thus the electrolysis efficiency can be increased, even when pure water is used as the electrolyte solution.

From the viewpoint of suppressing the precipitation of a salt, the concentration of the above-mentioned electrolyte, such as a carbonate or a bicarbonate, in the electrolyte solution is preferably less than 0.1 mol/L.

In addition, from the viewpoint of further suppressing the precipitation of the alkali metal salt as a salt in the vicinity of the cathode, it is preferred that the solid electrolyte type electrolysis device include an electrolyte solution in contact with the anode, and the electrolyte solution be pure water.

(Reaction Gas Supply Unit)

In the solid electrolyte type electrolysis device according to this embodiment, a reaction gas supply unit (not shown) may be arranged on the outside of the solid electrolyte type electrolysis device. That is, it is only required that CO₂ that is a reaction gas be supplied to the catalyst layer included in the cathode, and the reaction gas may be supplied from the reaction gas supply unit to a gas supply hole through, for example, a pipe (not shown), or the reaction gas supply unit may be arranged so that the reaction gas is blown to the surface of the cathode collector plate on an opposite side to a contact surface with the cathode. In addition, a factory emission gas emitted from a factory is suitably used as the reaction gas from an environmental standpoint.

[CO Generation Method]

Next, a CO generation method using the solid electrolyte type electrolysis device according to this embodiment is described.

First, CO₂ that is a reaction gas serving as a raw material is supplied to the solid electrolyte type electrolysis device in a gas phase state by the reaction gas supply unit (not shown). In this case, CO₂ is supplied to the cathode, for example, through the gas supply hole formed in the cathode collector plate.

Next, the CO₂ supplied to the cathode is brought into contact with the catalyst layer of the cathode. As a result, when a cation exchange membrane is used as the solid electrolyte, the reduction reactions of the reaction formula (1) and the reaction formula (2) described above occur, and when an anion exchange membrane is used as the solid electrolyte, the reduction reactions of the reaction formula (3) and the reaction formula (4) described above occur. Thus, a synthesis gas containing at least CO and H₂ is generated.

Next, the generated synthesis gas containing CO and H₂ is sent to a gas recovery device (not shown), for example, through the gas recovery hole formed in the cathode collector plate, and is recovered for each predetermined gas.

EXAMPLES

Next, the technology of the present disclosure is specifically described by way of Examples. However, the technology of the present disclosure is not limited to these examples in any way.

<1. Evaluation of Solid Electrolyte Type Electrolysis Device including Catalyst Layer having Single Layer Structure>

Example 1

[Production of Catalyst]

In a beaker, 0.4 g of carbon black (carbon-containing carrier according to this embodiment) having a primary particle diameter of 30 nm, 1.1 mmol of pentaethylenehexamine, and 0.7 mmol of nickel (II) chloride hexahydrate were mixed into 15 mL of ethanol to provide an ethanol dispersion liquid. After the resultant ethanol dispersion liquid was irradiated with ultrasonic waves for 10 minutes, the ethanol dispersion liquid was dried by heating to evaporate ethanol, to thereby provide a mixture. The resultant mixture was calcined by heating at 900° C. for 10 seconds or more in an inert gas with a calcination furnace. After that, the product was washed with a sulfuric acid aqueous solution, and solid matter was recovered with a suction filter. Further, the solid matter was vacuum-dried at 60° C. overnight to provide Ni complex-supported catalyst powder (intermediate).

The primary particle diameter of the carbon black was determined by laser diffraction particle size distribution measurement.

Further, 0.3 g of the resultant catalyst powder was placed in a pot together with 10 g of zirconia balls each having a diameter of 0.5 mm and 10 mL of water, and pulverized with a planetary ball mill device at a revolution number of 800 rpm for 20 minutes, and a catalyst slurry was recovered. The slurry was washed again with a sulfuric acid aqueous solution, solid matter was recovered with a suction filter, and the solid matter was vacuum-dried at 60° C. overnight to provide final catalyst powder (catalyst A according to this embodiment).

[Production of Cathode] 22 mg of the resultant catalyst powder was dispersed in ethanol, and 3 mg of “Nafion (trademark)” (cation exchange resin) manufactured by The Chemours Company was mixed as a binder into the dispersion liquid. After the mixing, the dispersion liquid was subjected to ultrasonic irradiation for 10 minutes, and the dispersion liquid was left to stand still in a vacuum chamber under a reduced pressure environment of 10 kPa (absolute pressure) for 10 minutes. The dispersion liquid was applied onto carbon paper (gas diffusion layer) with a spray coater so that the supported amount at the time of drying was from 2 mg/cm² to 3 mg/cm². Thus, a cathode precursor was obtained.
(Substitution with Alkali Metal Ion)

The cathode precursor was immersed in a 2 mol/L potassium bicarbonate aqueous solution. Under a state in which the cathode precursor was immersed, the aqueous solution was left to stand still in a vacuum chamber under a reduced pressure environment of 10 kPa (absolute pressure) for 10 minutes so that residual air bubbles in the cathode precursor were removed. After that, the aqueous solution was left to stand still for 12 hours or more, and metal ion substitution was performed on the cation exchange resin to provide a cathode including a catalyst layer having a single layer structure containing the cation exchange resin substituted with an alkali metal ion. The cathode includes the applied film of the dispersion liquid as the catalyst layer, and the carbon paper as the gas diffusion layer.

[Production of Solid Electrolyte Type Electrolysis Device]

An ion exchange membrane using as a base material a fluorine-based resin (basic site density: 2.8 mmol/cm³) having an aromatic ring in a main chain and a quaternary ammonium group bonded as a side chain to the main chain, and an iridium oxide-supported carbon anode (manufactured by Dioxide Materials) were bonded to the above-mentioned cathode to provide an ion exchange membrane-electrode assembly.

The anode had a structure in contact with an electrolyte solution vessel, and pure water was used as the electrolyte solution.

Example 2

A solid electrolyte type electrolysis device of Example 2 was produced in the same manner as in Example 1 except that a 0.01 mol/L KHCO₃ aqueous solution was used as the electrolyte solution in the production of the solid electrolyte Type electrolysis device of Example 1.

Example 3

A solid electrolyte type electrolysis device of Example 3 was produced in the same manner as in Example 1 except that the cathode precursor was immersed in a 2 mol/L rubidium carbonate aqueous solution instead of the potassium bicarbonate aqueous solution in the production of the solid electrolyte type electrolysis device of Example 1.

Example 4

A solid electrolyte type electrolysis device of Example 4 was produced in the same manner as in Example 1 except that a 0.1 mol/L KHCO₃ aqueous solution was used as the electrolyte solution in the production of the solid electrolyte type electrolysis device of Example 1.

Comparative Example 1

A solid electrolyte type electrolysis device of Comparative Example 1 was produced in the same manner as in Example 1 except that the cathode precursor was not subjected to the substitution with the alkali metal ion and the cathode precursor was used as the cathode in the production of the solid electrolyte type electrolysis device of Example 1.

Comparative Example 2

A solid electrolyte type electrolysis device of Comparative Example 2 was produced in the same manner as in Comparative Example 1 except that a 0.01 mol/L KHCO₃ aqueous solution was used as the electrolyte solution in the production of the solid electrolyte type electrolysis device of Comparative Example 1.

Example 5

[Production of Catalyst]

In a beaker, 0.1 g of carbon black (carbon-containing carrier according to this embodiment) having a primary particle diameter of 30 nm was mixed into 100 mL of ethanol, and the resultant ethanol dispersion liquid was irradiated with ultrasonic waves for 10 minutes. The dispersion liquid was then left to stand still in a vacuum chamber under a reduced pressure environment of 10 kPa (absolute pressure) for 10 minutes. After that, 11.7 mL of a 0.1 mol/L AgNO₃ solution and 1 mL of a 2.3 mol/L sodium phosphinate solution were mixed, followed by stirring at 15° C. for 16 hours, to reduce silver nitrate. After the completion of the reaction, the resultant slurry was washed with distilled water, solid matter was recovered with a centrifuge, and the solid matter was vacuum-dried at 60° C. overnight to provide catalyst powder having a Ag particle supported thereon (catalyst B according to this embodiment).

The resultant catalyst is carbon black having the Ag particle supported thereon as a catalyst source, and the mass of the Ag particle to be supported is 40 parts by mass with respect to 100 parts by mass of carbon black having no Ag particle supported thereon.

The primary particle diameter of the carbon black was determined by laser diffraction particle size distribution measurement.

[Production of Cathode]

43 mg of the resultant catalyst powder was dispersed in ethanol, and 6 mg of “Nafion (trademark)” (cation exchange resin) manufactured by The Chemours Company was mixed as a binder into the dispersion liquid. After the mixing, the dispersion liquid was subjected to ultrasonic irradiation for 10 minutes, and was exposed in a vacuum chamber under a reduced pressure environment of 10 kPa (absolute pressure) for 10 minutes. The dispersion liquid was applied onto carbon paper (gas diffusion layer) with a spray coater so that the supported amount at the time of drying was from 2 mg/cm² to 3 mg/cm². Thus, a cathode precursor was obtained.

(Substitution with Alkali Metal Ion)

The cathode precursor was immersed in a 2 mol/L potassium bicarbonate aqueous solution. Under a state in which the cathode precursor was immersed, the aqueous solution was left to stand still in a vacuum chamber under a reduced pressure environment of 10 kPa (absolute pressure) for 10 minutes so that residual air bubbles in the cathode precursor were removed. After that, the aqueous solution was left to stand still for 12 hours or more, and metal ion substitution was performed on the cation exchange resin to provide a cathode including a catalyst layer having a single layer structure containing the cation exchange resin substituted with an alkali metal ion. The cathode includes the applied film of the dispersion liquid as the catalyst layer, and the carbon paper as the gas diffusion layer.

[Production of Solid Electrolyte Type Electrolysis Device]

An ion exchange membrane using as a base material a fluorine-based resin (basic site density: 2.8 mmol/cm³) having an aromatic ring serving in a main chain and a quaternary ammonium group bonded as a side chain to the main chain, and an iridium oxide-supported carbon anode (manufactured by Dioxide Materials) were bonded to the above-mentioned cathode to provide an ion exchange membrane-electrode assembly.

The anode had a structure in contact with an electrolyte solution vessel, and pure water was used as the electrolyte solution.

Comparative Example 3

A solid electrolyte type electrolysis device of Comparative Example 3 was produced in the same manner as in Example 5 except that the cathode precursor was not subjected to the substitution with the alkali metal ion and the cathode precursor was used as the cathode in the production of the solid electrolyte type electrolysis device of Example 5.

<Method of evaluating Solid Electrolyte Type Electrolysis Device>

Through use of each of the solid electrolyte type electrolysis devices of Examples 1 to 5 and Comparative Examples 1 to 3, pure CO₂ was supplied to the cathode, and the CO₂ was subjected to electrolysis with the application potential of the cathode being set to −2.6 V with respect to the anode under the condition that the cell was heated to 90° C., to thereby measure a CO generation current density [mA/cm²] and a CO selectivity [%] at the time of generation of CO. In addition, the presence or absence of the precipitation of a salt in the vicinity of the cathode was visually observed.

The results are shown in Tables 1 to 3.

The results of each of the systems (Example 1, Example 3, and Comparative Example 1) in which Ni was used as the catalyst species and pure water was used as the electrolyte solution are shown in Table 1, the results of each of the systems (Example 2, Example 4, and Comparative Example 2) in which Ni was used as the catalyst species and the potassium bicarbonate aqueous solution was used as the electrolyte solution are shown in Table 2, and the results of each of the systems (Example 5 and Comparative Example 3) in which Ag was used as the catalyst species and pure water was used as the electrolyte solution are shown in Table 3.

	TABLE 1
	Electrolysis evaluation results

				CO
				generation
				current
			Electrolyte	density	CO	Precipitation
	Catalyst	Ionomer	solution	(mA/cm2)	selectivity	of salt

Example 1	Ni	Cation exchange resin	Pure water	190	91%	Absent
		(substitution with K+)
Example 3	Ni	Cation exchange resin	Pure water	11	79%	Absent
		(substitution with Rb+)
Comparative	Ni	Cation exchange resin	Pure water	Less	0%	Absent
Example 1		(no substation with		than 1
		alkali metal)

As is understood from Table 1, when the cation exchange resin was not subjected to alkali metal ion substitution, and the catalyst layer did not contain the alkali metal ion (Comparative Example 1), the CO selectivity was 0%, and the CO generation current density was 1 mA/cm² or less.

Meanwhile, when the cation exchange resin substituted with K⁺ or Rb⁺ was used as the “alkali metal ion and polymer material capable of releasing the alkali metal ion” of the catalyst layer (Examples 1 and 3), the significant CO selectivity was recognized even under the pure water condition. Thus, it is understood that the electrolysis efficiency was high.

	TABLE 2
	Electrolysis evaluation results

				CO
				generation
				current
			Electrolyte	density	CO	Precipitation
	Catalyst	Ionomer	solution	(mA/cm2)	selectivity	of salt

Example 2	Ni	Cation exchange resin	0.01 mol/L	210	90%	Absent
		(substitution with K+)	KHCO3
Example 4	Ni	Cation exchange resin	0.1 mol/L	250	85%	Present
		(substitution with K+)	KHCO3
Comparative	Ni	Cation exchange resin	0.01 mol/L	55	85%	Absent
Example 2		(no substation with	KHCO3
		alkali metal)

As is understood from Table 2, when the potassium bicarbonate aqueous solution was used as the electrolyte solution, the CO selectivity was improved in both of: the system (Comparative Example 2) in which the catalyst layer did not contain the alkali metal ion; and each of the systems (Examples 2 and 4) in which the catalyst layer contained the alkali metal ion, but the CO generation current density was reduced and the electrolysis efficiency was not excellent, in the system (Comparative Example 2) in which the catalyst layer did not contain the alkali metal ion.

	TABLE 3
	Electrolysis evaluation results

				CO
				generation
				current
			Electrolyte	density	CO	Precipitation
	Catalyst	Ionomer	solution	(mA/cm2)	selectivity	of salt

Example 5	Ag	Cation exchange resin	Pure water	4	45%	Absent
		(substitution with K+)
Comparative	Ag	Cation exchange resin	Pure water	1	10%	Absent
Example 3		(no substation with
		alkali metal)

The results shown in Table 3 were similar to the tendency in Table 1. Even in the case where Ag was used as the catalyst species, when the cation exchange resin was not subjected to alkali metal ion substitution, and the catalyst layer did not contain the alkali metal ion (Comparative Example 3), the CO selectivity was 10%, and the CO generation current density was as low as 1 mA/cm².

Meanwhile, when the cation exchange resin substituted with K⁺ was used as the “alkali metal ion and polymer material capable of releasing the alkali metal ion” of the catalyst layer (Example 5), the CO selectivity was higher than that in Comparative Example 3, and the CO generation current density was also higher. Thus, it is understood that the electrolysis efficiency was high.

<2. Evaluation of Solid Electrolyte Type Electrolysis Device including Catalyst Layer having Laminate Structure>

Example 6

[Production of Cathode]

Potassium bicarbonate was mixed into a 0.2 g/L agarose aqueous solution heated to 90° C. so as to have a concentration of 0.5 mol/L, and then the mixture was cooled to provide gel-like solid matter. The resultant solid matter was pulverized in an agate mortar and applied to the surface of the cathode precursor produced in Example 1 on the dispersion liquid applied film side at a supported amount of 50 mg/cm² to form a gel applied film. Thus, a cathode was obtained.

The cathode includes: the dispersion liquid applied film of the cathode precursor serving as a reaction layer in a catalyst layer; the gel applied film serving as a cation supply layer in the catalyst layer; and the carbon paper serving as a gas diffusion layer.

[Production of Solid Electrolyte Type Electrolysis Device]

An ion exchange membrane using as a base material a fluorine-based resin (basic site density: 2.8 mmol/cm³) having an aromatic ring in a main chain and having a quaternary ammonium group bonded as a side chain to the main chain, and an iridium oxide-supported carbon anode (manufactured by Dioxide Materials) were bonded to the above-mentioned cathode to provide an ion exchange membrane-electrode assembly.

The anode had a structure in contact with an electrolyte solution vessel, and pure water was used as the electrolyte solution.

Example 7

A solid electrolyte type electrolysis device of Example 7 was produced in the same manner as in Example 6 except that the potassium bicarbonate was changed to sodium bicarbonate in the gel applied film formation during the production process of the solid electrolyte type electrolysis device of Example 6.

Example 8

A solid electrolyte type electrolysis device of Example 8 was produced in the same manner as in Example 6 except that the potassium bicarbonate was changed to cesium carbonate in the gel applied film formation during the production process of the solid electrolyte type electrolysis device of Example 6.

Comparative Example 4

A solid electrolyte type electrolysis device of Comparative Example 4 was produced in the same manner as in Example 6 except that the potassium bicarbonate used for the gel applied film formation was not mixed in the production of the solid electrolyte type electrolysis device of Example 6.

Example 9

[Production of Cathode]

In a beaker, 0.1 g of carbon black (carbon-containing carrier according to this embodiment) having a primary particle diameter of 30 nm was mixed into 100 mL of ethanol, and the resultant ethanol dispersion liquid was irradiated with ultrasonic waves for 10 minutes. The dispersion liquid was then left to stand still in a vacuum chamber under a reduced pressure environment of 10 kPa (absolute pressure) for 10 minutes. After that, 11.7 mL of a 0.1 mol/L AgNO₃ solution and 1 mL of a 2.3 mol/L sodium phosphinate solution were mixed, followed by stirring at 15° C. for 16 hours, to reduce silver nitrate. After the completion of the reaction, the resultant slurry was washed with distilled water, solid matter was recovered with a centrifuge, and the solid matter was vacuum-dried at 60° C. overnight to provide catalyst powder having a Ag particle supported thereon (catalyst B according to this embodiment).

The resultant catalyst is carbon black having the Ag particle supported thereon as a catalyst source, and the mass of the Ag particle to be supported is 40 parts by mass with respect to 100 parts by mass of carbon black having no Ag particle supported thereon. The primary particle diameter of the carbon black was determined by laser diffraction particle size distribution measurement.

43 mg of the resultant catalyst powder was dispersed in ethanol, and 12 mg of an anion exchange resin (XC-1 (manufactured by Dioxide Materials)) was mixed as a binder into the dispersion liquid. After the mixing, the dispersion liquid was subjected to ultrasonic irradiation for 10 minutes, and was exposed in a vacuum chamber under a reduced pressure environment of 10 kPa (absolute pressure) for 10 minutes. The dispersion liquid was applied onto carbon paper (gas diffusion layer) with a spray coater so that the supported amount at the time of drying was from 2 mg/cm² to 3 mg/cm². Thus, a cathode precursor was obtained.

Cesium carbonate was mixed into a 0.2 g/L agarose aqueous solution heated to 90° C. so as to have a concentration of 0.5 mol/L, followed by cooling, to provide gel-like solid matter. The resultant solid matter was pulverized in an agate mortar and applied to the surface of the produced cathode precursor on the dispersion liquid applied film side at a supported amount of 50 mg/cm² to form a gel applied film. Thus, a cathode was obtained.

The cathode includes: the dispersion liquid applied film of the cathode precursor serving as a reaction layer in a catalyst layer; the gel applied film serving as a cation supply layer in the catalyst layer; and the carbon paper serving as a gas diffusion layer.

[Production of Solid Electrolyte Type Electrolysis Device]

An ion exchange membrane available under the product name “X37-50 grade 60” from Dioxide Materials (ion exchange membrane including polystyrene as a main chain and an imidazolium-based ion exchange group as a side chain), and an iridium oxide-supported carbon anode (manufactured by Dioxide Materials) were bonded to the above-mentioned cathode to provide an ion exchange membrane-electrode assembly.

The anode had a structure in contact with an electrolyte solution vessel, and pure water was used as the electrolyte solution.

Comparative Example 5

A solid electrolyte electrolysis type device of Comparative Example 5 was produced in the same manner as in Example 9 except that the cesium carbonate was not mixed in the gel applied film formation during the production process of the solid electrolyte type electrolysis device of Example 9.

<Method of evaluating Solid Electrolyte Type Electrolysis Device>

Through use of each of the solid electrolyte type electrolysis devices of Examples 6 to 9 and Comparative Examples 4 and 5, pure CO₂ was supplied to the cathode, and the CO₂ was subjected to electrolysis with the application potential of the cathode being set to −2.6 V with respect to the anode under the condition that the cell was heated to 90° C., to thereby measure a CO generation current density [mA/cm²] and a CO selectivity [%] at the time of generation of CO. In addition, the presence or absence of the precipitation of a salt in the vicinity of the cathode was visually observed.

The results are shown in Tables 4 and 5.

The results of each of the systems (Examples 6 to 8 and Comparative Example 4) in which Ni was used as the catalyst species are shown in Table 4, and the results of each of the systems (Example 9 and Comparative Example 5) in which Ag was used as the catalyst species are shown in Table 5.

	TABLE 4
	Electrolysis evaluation results

				CO gen-
		Cation		eration
		supply	Elec-	current	CO	Precip-
	Cat-	layer	trolyte	density	selec-	itation
	alyst	component	solution	(mA/cm2)	tivity	of salt

Example 6	Ni	KHCO3	Pure	20	85%	Absent
			water
Example 7	Ni	NaHCO3	Pure	20	73%	Absent
			water
Example 8	Ni	Cs2CO3	Pure	55	86%	Absent
			water
Compar-	Ni	Absent	Pure	Less	8%	Absent
ative			water	than 1
Example 4

As is understood from Table 4, it was recognized that, even in the case where the reaction layer in the catalyst layer did not contain the alkali metal ion, a high CO selectivity of 70% or more was obtained even when pure water was used as the electrolyte solution, by laminating the cation supply layer containing the alkali metal ion on the reaction layer. In addition, the CO generation current density of each of Examples 6 to 8 was higher than that of Comparative Example 4. Thus, it is understood that the electrolysis efficiency was excellent.

	TABLE 5
	Electrolysis evaluation results

				CO gen-
		Cation		eration
		supply	Elec-	current	CO	Precip-
	Cat-	layer	trolyte	density	selec-	itation
	alyst	component	solution	(mA/cm2)	tivity	of salt

Example 9	Ag	Cs2CO3	Pure	20	81%	Absent
			water
Compar-	Ag	Absent	Pure	Less	19%	Absent
ative			water	than 1
Example 5

As is understood from Table 5, a high CO selectivity was recognized in the same manner even when the Ag catalyst was used. In addition, the effect was recognized also in this experiment using the anion exchange resin as the ionomer that was a dispersion medium of the reaction layer in the catalyst layer, and hence it was recognized that the cation supply was performed directly from the cation supply layer instead of the cation exchange resin.

INDUSTRIAL APPLICABILITY

According to this embodiment, a synthesis gas containing at least CO and H₂ at a desired generation ratio can be generated through use of renewable energy of a solar cell or the like to a voltage application unit, for example, with a CO₂ gas emitted from a factory being used as a raw material with respect to a solid electrolyte type electrolysis device. The synthesis gas generated as described above can generate a fuel base material, a chemical raw material, or the like by a method, such as Fischer-Tropsch synthesis (FT synthesis) or methanation.

REFERENCE SIGNS LIST

• • 10 gas diffusion layer
  • 20 carbon dioxide reduction electrode catalyst layer
  • 21 reaction layer
  • 22 layer containing alkali metal ion and polymer material (cation supply layer)
  • 23 catalyst
  • 24 alkali metal ion and polymer material
  • 30 solid electrolyte (ion exchange membrane)
  • 40 anode
  • 50 ion exchange membrane-electrode assembly
  • 100 cathode collector plate
  • 200 cathode
  • 300 solid electrolyte (ion exchange membrane)
  • 400 anode
  • 500 anode collector plate
  • 600 electrolyte solution
  • 700 voltage application unit
  • 800 solid electrolyte electrolysis device