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(11) | EP 4 488 418 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | ADDITIVE FOR CARBON DIOXIDE REDUCTION CATALYSTS, CATALYST LAYER, CATHODE, ION EXCHANGE MEMBRANE-ELECTRODE ASSEMBLY AND SOLID ELECTROLYTE ELECTROLYSIS DEVICE |
| (57) A catalyst layer that can be suppressed in functional deterioration, an additive
for a carbon dioxide reduction catalyst that is excellent in the electroconductivity
of the electrode catalyst layer and the electrolysis efficiency of the electrolytic
reduction reaction of carbon dioxide, an electrode, an ion exchange membrane-electrode
assembly, and a solid electrolyte electrolysis apparatus are provided. The additive
for a carbon dioxide reduction catalyst includes a carrier having an aryl group on
a surface thereof and containing carbon. |
Technical Field
[0001] The technique of the present disclosure relates to an additive for a carbon dioxide reduction catalyst, a catalyst layer, a cathode, an ion exchange membrane-electrode assembly, and a solid electrolyte electrolysis apparatus.
Background Art
[0002] Carbon dioxide is emitted when energy is extracted from a fossil fuel or the like. The increase of the concentration of carbon dioxide in the atmosphere is said to be one of the causes of the global warming. Carbon dioxide is an extremely stable substance, and therefore there has been substantially no way to use. However, in view of the demand of the times where the global warming becomes more serious, a new technology is needed to convert carbon dioxide into other substances and to recycle as a resource again. For example, a carbon dioxide reduction apparatus capable of directly reducing carbon dioxide in gaseous state is being developed.
[0003] For example, PTL 1 intends to provide a catalyst layer for a carbon dioxide reduction electrode that shows a high partial current density by controlling the wettability and withstands a long-term operation, and describes that the catalyst layer includes a metal catalyst supported on a carbon material, an ion conductive substance, and a hydrophilic polymer, and the ratio (AH2O/AN2) of the BET specific surface area (AN2) obtained through nitrogen adsorption and the BET specific surface area (AH2O) obtained through water vapor adsorption is 0.08 or less.
[0004] PTL 2 intends to enable at least one of the suppression of the generation rate of hydrogen through the side reaction in the reduction reaction of a carbon compound, and the enhancement of the generation rate of the reduction product through the reduction reaction of a carbon compound therein, and describes that the electrode for reduction reaction used in the reduction reaction of a carbon compound is equipped with an electrode body modified with a hydrophobic polymer.
[0005] PTL 3 describes a method capable of solving the technical problem by grafting a molecule having hydrophobicity and/or hydrophilicity on carbon.
[0006] Furthermore, NPL 1 describes a method of preventing an electrode for reducing carbon dioxide from being deteriorated in function, by controlling the wettability of the electrode by adding polytetrafluoroethylene (PTFE) fine particles to the catalyst layer of the electrode.
Citation List
Patent Literatures
Non-patent Literature
Summary of Invention
Technical Problem
[0009] In an electrolysis apparatus including a catalyst layer for performing carbon dioxide reduction reaction and an ion exchange membrane, in the case where the electrolytic solution permeates through the ion exchange membrane and seeps into the catalyst layer, the function of the catalyst layer is degraded.
[0010] PTLs 1 and 2 and NPL 1 try to hydrophobize the catalyst layer by adding a hydrophobic polymer to the catalyst layer, but the electric resistance of the catalyst layer is increased due to the insulating property of the hydrophobic polymer. In addition, PTL 3 enhances the hydrophobicity by supporting a hydrophobic compound on a gas diffusion layer or an intermediate layer between the gas diffusion layer and the catalyst layer, but fails to enhance the hydrophobicity of the catalyst layer itself, and the effect of hydrophobization is limited.
[0011] The present disclosure has been made in view of the circumstances described above, and a problem to be solved by the present disclosure is to provide a catalyst layer that can be suppressed in functional deterioration, an additive for a carbon dioxide reduction catalyst that is excellent in the electroconductivity of the electrode catalyst layer and the electrolysis efficiency of the electrolytic reduction reaction of carbon dioxide, an electrode, an ion exchange membrane-electrode assembly, and a solid electrolyte electrolysis apparatus.
Solution to Problem
[0012]
<1> An additive for a carbon dioxide reduction catalyst, including: a carrier having an aryl group on a surface thereof and containing carbon.
<2> An additive for a carbon dioxide reduction catalyst, including a carrier containing carbon, in which a ratio of a water vapor adsorption amount at 25°C and a water vapor pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa is less than 0.5.
<3> The additive for a carbon dioxide reduction catalyst according to the item <1>, in which the aryl group includes one or more selected from the group consisting of a phenyl group and a condensed ring group having 2 to 6 benzene rings.
<4> The additive for a carbon dioxide reduction catalyst according to the item <3>, in which the condensed ring group having 2 to 6 benzene rings contains one or more selected from the group consisting of a naphthyl group, an anthracenyl group, a phenanthrenyl group, and a pyrenyl group.
<5> The additive for a carbon dioxide reduction catalyst according to any one of the items <1>, <3>, and <4>, in which the aryl group includes one or more of substituents selected from the group consisting of an alkyl group, a fluorinated alkyl group, a phenyl group, a fluorinated phenyl group, and a fluorine atom.
<6> The additive for a carbon dioxide reduction catalyst according to any one of the
items <1> and <3> to <5>, in which the aryl group is any one or more of groups represented
by the formulae (1) to (8):
in the formulae (1) to (8), * represents a bonding site to the surface of the carrier.
<7> A catalyst layer including:
an additive including a carrier having an aryl group on a surface thereof and containing carbon; and
a catalyst including a carrier containing carbon, inorganic fine particles or a metal complex being supported on the carrier.
<8> A catalyst layer including:
an additive including a carrier containing carbon, in which a ratio of a water vapor adsorption amount at 25°C and a water vapor pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa is less than 0.5; and
a catalyst including a carrier containing carbon, inorganic fine particles or a metal complex being supported on the carrier.
<9> The catalyst layer according to the item <7> or <8>, in which the inorganic fine particles are fine particles selected from the group consisting of gold, silver, copper, nickel, iron, cobalt, zinc, chromium, palladium, tin, manganese, aluminum, indium, bismuth, molybdenum, and carbon nitride, and the metal complex is a metal complex containing a metal selected from the group consisting of copper, nickel, iron, cobalt, zinc, manganese, molybdenum, and aluminum, or an ion of the metal, having a ligand coordinated thereto.
<10> The catalyst layer according to the item <7>, in which the aryl group includes one or more selected from the group consisting of a phenyl group and a condensed ring group having 2 to 6 benzene rings.
<11> The catalyst layer according to the item <10>, in which the condensed ring group having 2 to 6 benzene rings contains one or more selected from the group consisting of a naphthyl group, an anthracenyl group, a phenanthrenyl group, and a pyrenyl group.
<12> The catalyst layer according to the item <7>, in which the aryl group includes one or more of substituents selected from the group consisting of an alkyl group, a fluorinated alkyl group, a phenyl group, a fluorinated phenyl group, and a fluorine atom.
<13> The catalyst layer according to the item <7>, in which the aryl group is any
one or more of groups represented by the formulae (1) to (8):
in the formulae (1) to (7), * represents a bonding site to the surface of the carrier.
<14> A cathode including: the catalyst layer according to the item <9>; and a gas diffusion layer.
<15> An ion exchange membrane-electrode assembly including: the cathode according to the item <14>; a solid electrolyte; and an anode.
<16> The ion exchange membrane-electrode assembly according to the item <15>, in which the solid electrolyte is an anion exchange membrane.
<17> A solid electrolyte electrolysis apparatus including:
the cathode according to the item <14>;
an anode constituting a pair of electrodes with the cathode;
a solid electrolyte intervening between the cathode and the anode, in a contact state; and
a voltage application unit applying a voltage between the cathode and the anode.
<18> The solid electrolyte electrolysis apparatus according to the item <17>, in which the solid electrolyte is an anion exchange membrane.
Advantageous Effects of Invention
[0013] The technique of the present disclosure can provide a catalyst layer that can be suppressed in functional deterioration, an additive for a carbon dioxide reduction catalyst that is excellent in the electroconductivity of the electrode catalyst layer and the electrolysis efficiency of the electrolytic reduction reaction of carbon dioxide, a cathode, an ion exchange membrane-electrode assembly, and a solid electrolyte electrolysis apparatus.
Brief Description of Drawings
[0014]
Fig. 1 is a schematic illustration showing an ion exchange membrane-electrode assembly that is favorably used in the present embodiment.
Fig. 2 is a schematic illustration showing a solid electrolyte electrolysis apparatus that is favorably used in the present embodiment.
Fig. 3 is a graph showing the relative water vapor adsorption amounts in Examples and Comparative Example.
Description of Embodiments
[0015] The upper limit values and the lower limit values of the numerical ranges described in the description herein can be optionally combined. For example, in the case where "A to B" and "C to D" are described as numerical ranges, numerical ranges "A to D" and "C to B" are also encompassed in the range of the technique of the present disclosure.
[0016] The numerical range "lower limit value to upper limit value" described in the description herein means the lower limit value or more and the upper limit value or less unless otherwise indicated.
<Additive for Carbon Dioxide Reduction Catalyst>
[0017] The additive for a carbon dioxide reduction catalyst according to the first present embodiment includes a carrier having an aryl group on a surface thereof and containing carbon.
[0018] The additive for a carbon dioxide reduction catalyst according to the second present embodiment includes a carrier containing carbon, in which a ratio of a water vapor adsorption amount at 25°C and a water vapor pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa is less than 0.5.
[0019] The additive for a carbon dioxide reduction catalyst according to the first present embodiment and the additive for a carbon dioxide reduction catalyst according to the second embodiment of the technique of the present disclosure may be generically referred simply to as an "additive for a carbon dioxide reduction catalyst according to the present embodiment".
[0020] A carbon dioxide reduction electrolysis apparatus generally includes a cathode including a gas diffusion layer and a catalyst layer for performing carbon dioxide reduction reaction, an ion exchange membrane, an anode, and an electrolytic solution (electrolyte) supplied to the anode.
[0021] The ion exchange membrane has a nature of permeating not only ions but also an electrolytic solution due to the structure thereof. The following phenomena have been often found: even a slight amount of the electrolytic solution supplied to the anode penetrates through the ion exchange membrane, resulting in excess water inside the catalyst layer; the electrolyte dissolved in the electrolytic solution deposits as a salt in the vicinity of the cathode, and clogs the flow channel of carbon dioxide; and others. The phenomena cause adverse influence, such as inhibition of carbon dioxide supply to the catalyst layer, resulting in deterioration of the electrolytic capabilities, such as the current density and the selectivity. The influence occurs more conspicuously at a higher temperature.
[0022] A method of controlling the water amount of the catalyst layer includes a method of controlling the hydrophobicity of the catalyst layer. PTL 1 described-above controls the hydrophobicity with the addition amount of polyvinyl alcohol, polyvinyl pyrrolidone, or the like, and PTL 2 and NPL 1 control the hydrophobicity by adding polystyrene and PTFE, respectively.
[0023] However, these hydrophobic polymers are insulating materials, and there is a problem that the addition of the hydrophobic polymer increases the electric resistance of the catalyst layer, resulting in heat generation, deterioration in electrolytic efficiency, and the like.
[0024] PTL 3 does not use a hydrophobic polymer, and increases the hydrophobicity by supporting a hydrophobic compound on the gas diffusion layer or the intermediate layer between the gas diffusion layer and the catalyst layer. However, the hydrophobicity of the catalyst layer cannot be enhanced, and thus the effect obtained through the hydrophobization is limited.
[0025] On the other hand, the additive for a carbon dioxide reduction catalyst according to the present embodiment can be used as a constitutional component of the catalyst layer of the electrode equipped with the catalyst layer, and thereby not only the surface of the catalyst layer but also the entire catalyst layer can be hydrophobized without impairing the electroconductivity of the catalyst layer. The mechanism therefor can be estimated as follows.
[0026] The additive for a carbon dioxide reduction catalyst according to the first present embodiment includes a carrier containing carbon excellent in electroconductivity, and simultaneously having an aryl group on the surface thereof, and thereby the additive can impart hydrophobicity to the catalyst layer. The aryl group is fixed to the surface of the carrier through a chemical bond, and thus can securely hydrophobize the catalyst layer, and not only the surface of the catalyst layer but also the interior of the catalyst layer can be hydrophilized.
[0027] The additive for a carbon dioxide reduction catalyst according to the second present embodiment includes a carrier containing carbon excellent in electroconductivity, and simultaneously has a particular ratio of water vapor adsorption amounts of less than 0.5, and thereby the additive is excellent in hydrophobicity while having electroconductivity. Therefore, the addition of the additive to the catalyst layer can hydrophobize not only the surface of the catalyst layer but also the interior of the catalyst layer without impairing the electroconductivity of the catalyst layer.
[0028] As a result, even in the case where the electrolytic solution permeates through the ion exchange membrane, the electrolyte can be suppressed from being attached to the catalyst layer containing the additive for a carbon dioxide reduction catalyst according to the present embodiment, preventing the inhibition of carbon dioxide supply to the catalyst layer. Consequently, the electrolytic capabilities, such as the current density and the selectivity, are not impaired, resulting in an excellent electrolytic efficiency in the electrolytic reduction reaction of carbon dioxide.
[0029] The first embodiment and the second embodiment of the additive for a carbon dioxide reduction catalyst according to the present embodiment will be described sequentially below.
[Additive for Carbon Dioxide Reduction Catalyst according to First Embodiment]
[0030] The additive for a carbon dioxide reduction catalyst according to the first present embodiment includes a carrier having an aryl group on a surface thereof and containing carbon.
[0031] As described above, the additive has an aryl group on the surface of the carrier containing carbon, and thereby the additive has electroconductivity and hydrophobicity, and can impart electroconductivity and hydrophobicity over the entire catalyst layer by adding to the catalyst layer.
(Aryl Group)
[0032] Examples of the aryl group include a phenyl group and a group obtained by removing one hydrogen atom from a condensed ring containing two or more benzene ring (i.e., a condensed ring group).
[0033] Among these, the aryl group preferably includes one or more selected from the group consisting of a phenyl group and a condensed ring group having 2 to 6 benzene rings from the standpoint of suppressing the steric hindrance of the aryl groups on the surface of the carrier, and the standpoint of securing the electroconductivity of the additive. In the case where the number of benzene rings included in the aryl group is 6 or more, the additive less likely inhibits the charge transfer among the carrier, and has excellent electroconductivity.
[0034] Examples of the condensed ring group having 2 to 6 benzene rings include a group obtained by removing one hydrogen atom from a condensed ring, such as naphthalene, anthracene, phenanthrene, pyrene, triphenylene, chrysene, perylene, pentacene, and pentaphene.
[0035] Among the above, the condensed ring group having 2 to 6 benzene rings preferably contains a group obtained by removing one hydrogen atom from at least one of a condensed ring selected from the group consisting of naphthalene, anthracene, phenanthrene, and pyrene. In other words, the condensed ring group having 2 to 6 benzene rings preferably contains one or more selected from the group consisting of a naphthyl group, an anthracenyl group, a phenanthrenyl group, and a pyrenyl group.
[0036] The condensed ring group having 2 to 6 benzene rings is more preferably a pyrenyl group as a substituent.
[0037] The aryl group bonded to the surface of the carrier according to the present embodiment may be unsubstituted, or may further have one kind or two or more kinds of substituents.
[0038] Examples of the substituent include an alkyl group, an alkenyl group, a fluorinated alkyl group, an aryl group, a fluorinated aryl group, and a fluorine atom, and the substituent may further have a substituent.
[0039] Examples of the alkyl group include an alkyl group having 1 to 30 carbon atoms, which may be linear, branched, or cyclic. Specific examples thereof include a methyl group, a benzyl group (phenylmethyl group), a trityl group (triphenylmethyl group), an ethyl group, a n-propyl group, an isopropyl group, a n-butyl group, a tert-butyl group, a n-hexyl group, a cyclohexyl group, a n-octyl group, and a n-dodecyl group. The alkyl group may further have a substituent.
[0040] The upper limit of the number of carbon atoms of the alkyl group, 30, includes the number of carbon atoms of the substituent that can be further substituted. This is also the same as in the alkenyl group, the fluorinated alkyl group, the aryl group, and the fluorinated aryl group described later.
[0041] The number of carbon atoms of the alkyl group is preferably 1 to 25, and more preferably 2 to 20.
[0042] The alkyl group that is linear preferably has 10 to 14 carbon atoms, and the alkyl group that is branched preferably has 1 to 3 carbon atoms and preferably has 1 to 5 unsubstituted phenyl groups.
[0043] Examples of the alkenyl group include an alkenyl group having 2 to 30 carbon atoms, which may be linear, branched, or cyclic. Specific examples thereof include a vinyl group.
[0044] The number of carbon atoms of the alkenyl group is preferably 2 to 25, and more preferably 2 to 20.
[0045] Examples of the fluorinated alkyl group include a fluorinated alkyl group having 1 to 30 carbon atoms, which may be linear, branched, or cyclic. Specific examples thereof include a group obtained by substituting one or more hydrogen atoms in the alkyl group described above with a fluorine atom, examples of which include a fluorinated methyl group and a fluorinated ethyl group.
[0046] The number of carbon atoms of the fluorinated alkyl group is preferably 1 to 25, and more preferably 2 to 20.
[0048] Examples of the aryl group as a substituent include the same groups as in the aryl group bonded to the surface of the carrier according to the present embodiment, and preferably have 6 to 12 carbon atoms. Specific examples thereof include a phenyl group and a naphthyl group.
[0049] Examples of the fluorinated aryl group include a group obtained by substituting one or more hydrogen atoms in the aryl group as a substituent by a fluorine atom, examples of which include a fluorinated phenyl group having 1 to 4 fluorine atoms and a fluorinated naphthyl group having 1 to 7 fluorine atoms.
[0050] In the case where the aryl group bonded to the surface of the carrier according to the present embodiment has a substituent, among the above, the substituent preferably includes one or more selected from the group consisting of an alkyl group, a fluorinated alkyl group, a phenyl group, a fluorinated phenyl group, and a fluorine atom.
[0051] In addition, two or more of one kind of the substituents may exist, or two or more of two or more kinds thereof may exist.
[0052] For example, in the case where the aryl group bonded to the surface of the carrier according to the present embodiment is a phenyl group, the phenyl group may have a structure having two fluorinated methyl groups, or may have a structure in which four of the five hydrogen atoms of the phenyl group are substituted by fluorine atoms, and the remaining one is substituted by a fluorinated phenyl group.
[0053] Furthermore, the aryl group bonded to the surface of the carrier according to the present embodiment is preferably a phenyl group having a substituent, and a condensed ring group having 2 to 6 unsubstituted benzene rings; more preferably a phenyl group having any one of a substituent selected from the group consisting of a triphenylmethyl group, a linear unsubstituted alkyl group having 10 to 14 carbon atoms, and a linear fluorinated alkyl group having 1 to 4 carbon atoms, and a condensed ring group having 4 or 5 unsubstituted benzene rings; and further preferably a phenyl group having any one of a substituent selected from the group consisting of a linear unsubstituted alkyl group having 11 to 13 carbon atoms and a linear fluorinated alkyl group having 2 or 3 carbon atoms, and a condensed ring group having 4 unsubstituted benzene rings.
[0054] More specifically, the aryl group bonded to the surface of the carrier according to the present embodiment is preferably one or more of groups represented by the following formulae (1) to (8). In the formulae (1) to (8), * represents the bonding site to the surface of the carrier according to the present embodiment. The aryl group is more preferably one or more of groups represented by the following formulae (1) to (3) and (5), and further preferably one or more of groups represented by the following formulae (2), (3), and (5).
[0055] The carrier according to the present embodiment may have one kind of the aryl group, or may have two or more kinds thereof.
[0056] The carrier according to the present embodiment may have one aryl group, or may have two or more thereof. The presence of the aryl group that the carrier has can be confirmed and quantitatively determined through infrared spectroscopy.
[Introducing Method of Aryl Group to Surface of Carrier]
[0057] The introducing method of the aryl group to the surface of the carrier according to the present embodiment (i.e., the method of chemical modification) is not particularly limited.
[0058] For example, it is possible that carbon black is used as the carrier according to the present embodiment, and nucleophilic reaction is allowed to occur on the aromatic ring or the like on the surface of the carbon black through diazo reaction with an aromatic compound having one primary amino group as a precursor, so as to form a chemical bond therebetween.
[0059] Examples of the aromatic compound include benzene, and also include a condensed ring compound containing two or more benzene rings, and the condensed ring compound containing two or more benzene rings is preferably a condensed ring compound containing 2 to 6 benzene rings.
[0060] Specific examples thereof include aniline, aminonaphthalene, aminoanthracene, aminophenanthrene, and aminopyrene.
[0061] The aromatic compound having one primary amino group may further have a substituent other than the primary amino group. Examples of the substituent include an alkyl group, an alkenyl group, a fluorinated alkyl group, an aryl group, a fluorinated aryl group, and a fluorine atom, and the substituent may further have a substituent. The details of the substituent that the aromatic compound having one primary amino group can have are the same as the substituent that the aryl group bonded to the surface of the carrier of the additive for a carbon dioxide reduction catalyst according to the first present embodiment can have, and the preferred embodiments thereof are also the same.
[0062] More specifically, the aromatic compound having one primary amino group is preferably one or more of compounds represented by the following formulae (11) to (18).
(Carrier)
[0064] Carbon generally has conductivity, and therefore the carrier according to the present embodiment is a conductive carrier.
[0065] The carbon is not limited, as long as being a conductive material that can be used as a gas diffusion layer in an electrode provided in an apparatus for reducing carbon dioxide, and examples thereof include carbon black (such as furnace black, acetylene black, Ketjen black, and medium thermal carbon black), activated carbon, graphite, carbon nanotubes, carbon nanofibers, carbon nanohorns, graphene nanoplatelets, and nanoporous carbon, in which carbon black is preferred. Furthermore, the structure thereof is preferably a porous structure. Examples of the carbon having a porous structure include a porous carbon material represented by graphene.
[0066] The shape, size, grade, and the like of the carbon black are not limited, and the DBP oil adsorption amount (dibutyl phthalate oil adsorption amount) is preferably 50 to 500 mL/100 g, more preferably 100 to 300 mL/100 g, and further preferably 100 to 200 mL/100 g. The primary particle diameter is preferably 5 to 200 nm, more preferably 10 to 100 nm, and further preferably 10 to 50 nm.
[0067] The DBP oil adsorption amount of the carbon black can be obtained by JIS K6217-4:2001 (determination of oil adsorption amount), and the primary particle diameter can be obtained, for example, by the laser diffraction particle size distribution measurement.
[0068] The carbon black may be a commercially available product, and examples thereof include Vulcan (registered trade name) XC-72 (available from Cabot Corporation), Denka Black HS-100 (available from Denka Co., Ltd.), Ketjen Black EC-600JD (available from Lion Specialty Chemicals Co., Ltd.), and Conductex-7055 Ultra (available from Birla Carbon Corporation).
[0069] The additive for a carbon dioxide reduction catalyst according to the first embodiment preferably includes a carrier having an aryl group on a surface thereof and containing carbon.
[Additive for Carbon Dioxide Reduction Catalyst according to Second Embodiment]
[0070] The additive for a carbon dioxide reduction catalyst according to the second present embodiment includes a carrier containing carbon, in which a ratio of a water vapor adsorption amount at 25°C and a water vapor pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa is less than 0.5.
[0071] In other words, assuming that the water vapor adsorption amount (unit: cm3(STP)/g) at 25°C and a water vapor pressure of 2.2 kPa is represented by a, and the water vapor adsorption amount (unit: cm3(STP)/g) at 25°C and a water vapor pressure of 3.1 kPa is represented by b, the ratio a/b is less than 0.5.
[0072] The water vapor adsorption amount a at 25°C and a water vapor pressure of 2.2 kPa is strongly influenced by the number of molecules adsorbed as a monomolecular layer corresponding to the mutual interaction between the outermost surface of the additive and the adsorbed water molecule, and therefore means the water adsorption capability of the additive. The water vapor adsorption amount b at 25°C and a water vapor pressure of 3.1 kPa is strongly influenced by the number of adsorbed molecules corresponding to the adsorption capacity of the additive, and therefore corresponds to the surface area per unit mass of the additive. Consequently, the ratio (a/b) of the water vapor adsorption amount a with respect to the water vapor adsorption amount b means the surface hydrophilicity of the additive.
[0073] The relationship a/b < 0.5 means that the additive for a carbon dioxide reduction catalyst has high hydrophobicity and high electroconductivity. The ratio a/b is preferably as small as possible, which may be 0, and generally more than 0.01.
[0074] The ratio a/b is preferably 0.5 or less, more preferably 0.4 or less, further preferably 0.35 or less, still further preferably 0.3 or less, still further preferably 0.2 or less, and even further preferably 0.15 or less.
[0075] The measure for allowing the additive for a carbon dioxide reduction catalyst to have a ratio (a/b) of the water vapor adsorption amount a at 25°C and a water vapor pressure of 2.2 kPa with respect to the water vapor adsorption amount b at the same temperature and a water vapor pressure of 3.1 kPa of less than 0.5 is not particularly limited. For example, the relationship a/b < 0.5 can be achieved through chemical modification of the surface of the carrier containing carbon with an aryl group.
[0076] In view of the above, the additive for a carbon dioxide reduction catalyst according to the first present embodiment preferably has a ratio of a water vapor adsorption amount at 25°C and a water vapor pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa of less than 0.5.
[0077] Furthermore, the additive for a carbon dioxide reduction catalyst according to the second present embodiment preferably includes a carrier containing carbon, having an aryl group on a surface of the carrier.
[0078] The additive for a carbon dioxide reduction catalyst with the technique of the present disclosure is preferably coated with an ionomer described later. The ionomer coated on the additive for a carbon dioxide reduction catalyst exerts the hydrophobizing effect also on the catalyst, which similarly exists in the ionomer, and thereby the electrolytic efficiency can be enhanced.
<Catalyst Layer>
[0079] The catalyst layer according to the first present embodiment includes an additive including a carrier having an aryl group on a surface thereof and containing carbon, and a catalyst including a carrier containing carbon, inorganic fine particles or a metal complex being supported on the carrier.
[0080] The catalyst layer according to the second present embodiment includes an additive including a carrier containing carbon, in which a ratio of a water vapor adsorption amount at 25°C and a water vapor pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa is less than 0.5, and a catalyst including a carrier containing carbon, inorganic fine particles or a metal complex being supported on the carrier.
[0081] The catalyst layer according to the first present embodiment and the catalyst layer according to the second present embodiment may be generically referred simply to as a "catalyst layer according to the present embodiment".
[0082] The "additive including a carrier having an aryl group on a surface thereof and containing carbon" contained in the catalyst layer according to the first present embodiment may be referred to as an additive according to the first present embodiment. The "additive having a ratio of a water vapor adsorption amount at 25°C and a water vapor pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa of less than 0.5" contained in the catalyst layer according to the second present embodiment may be referred to as an additive according to the second present embodiment.
[0083] Further, the additive according to the first present embodiment and the additive according to the second present embodiment may be generically referred simply to as an "additive according to the present embodiment".
[0084] In the catalyst layer according to the present embodiment, the catalyst species contained in the catalyst layer is not particularly limited. The catalyst layer according to the present embodiment is preferably used as a catalyst layer containing a carbon dioxide reduction catalyst, but can also be preferably used as various catalyst layers that are demanded to avoid adverse effects due to the water invasion of the catalyst layer, the contact thereof with water vapor, the deposition of salts therein, and the like.
(Additive)
[0085] The additive according to the first present embodiment is the same as the additive for a carbon dioxide reduction catalyst according to the first present embodiment, and the preferred embodiments thereof are also the same.
[0086] The additive including a carrier having an aryl group on a surface thereof and containing carbon, can hydrophobize not only the surface of the catalyst layer but also the interior of the catalyst layer over the entire catalyst layer without impairing the electroconductivity of the catalyst layer containing the additive, and thereby the catalyst contained in the catalyst layer can be prevented from being deteriorated in the functions thereof.
[0087] In the additive according to the second present embodiment, the "ratio of the water vapor adsorption amount at 25°C and a water vapor pressure of 2.2 kPa with respect to the water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa" is the same as the "ratio (a/b) of the water vapor adsorption amount a at 25°C and a water vapor pressure of 2.2 kPa with respect to the water vapor adsorption amount b at the same temperature and a water vapor pressure of 3.1 kPa" in the additive for a carbon dioxide reduction catalyst according to the second present embodiment.
[0088] In the additive according to the second present embodiment, also, a smaller ratio a/b shows higher hydrophobicity and higher electroconductivity. The ratio a/b is preferably as small as possible, which may be 0, and is generally more than 0.01.
[0089] The ratio a/b is preferably 0.5 or less, more preferably 0.4 or less, further preferably 0.35 or less, still further preferably 0.3 or less, still further preferably 0.2 or less, and even further preferably 0.15 or less.
[0090] The additive according to the second present embodiment contained in the catalyst layer can hydrophobize not only the surface of the catalyst layer but also the interior of the catalyst layer over the entire catalyst layer without impairing the electroconductivity of the catalyst layer, and thereby can prevent the catalyst contained in the catalyst layer from being deteriorated in the functions thereof.
[0091] The additive according to the first present embodiment and the additive according to the second present embodiment can hydrophobize not only the surface of the catalyst layer containing the additive, but also the interior thereof over the entire catalyst layer, and also can retain the electroconductivity thereof, and therefore the additives can be preferably used as an additive for various catalyst layers that are demanded to avoid adverse effects due to the water invasion of the catalyst layer, the contact thereof with water vapor, the deposition of salts therein, and the like.
(Catalyst)
[0092] The catalyst layer according to the present embodiment includes a catalyst including a carrier containing carbon, inorganic fine particles or a metal complex being supported on the carrier.
[0093] In the catalyst with the technique of the present disclosure, the component that exhibits the catalytic function is the inorganic fine particles or the metal complex supported on the carrier, and in the technique of the present disclosure, the inorganic fine particles or the metal complex is referred to as a "catalyst source" whereas the carrier having the catalyst source supported thereon is referred to as a "catalyst".
[Carrier]
[0094] The carrier used may be the carrier according to the present embodiment included in the additive for a carbon dioxide reduction catalyst according to the first present embodiment, and preferably contains carbon black.
[0095] The preferred embodiments of the carbon black are the same as the preferred embodiments of the carbon black described for the additive for a carbon dioxide reduction catalyst according to the first present embodiment.
[Inorganic Fine Particles and Metal Complex]
[0096] The carrier according to the present embodiment supports inorganic fine particles or a metal complex thereon as a catalyst source.
[0097] The inorganic fine particles and the metal complex are not particularly limited, as long as being a component exhibiting the catalytic function. In the technique of the present disclosure, the inorganic fine particles mean a metal and an inorganic compound having an average particle diameter of 1 to 100 nm measured through observation of a photograph or the like of a scanning electron microscope or the like.
[0098] For example, in the case where the catalyst source is used in a catalyst layer for a fuel cell, the inorganic fine particles used may be platinum, gold, nickel, ruthenium, rhodium, and the like, and the metal complex used may be a nickel complex, a cobalt complex, an iron complex, a manganese complex, a zinc complex, and the like.
[0099] Furthermore, for example, in the case where the catalyst source is used in a catalyst layer for an electrode of a secondary cell, the inorganic fine particles used may be platinum, gold, nickel, iridium, a metal oxide, and the like, and the metal complex used may be a nickel complex, a cobalt complex, an iron complex, a manganese complex, a zinc complex, and the like.
[0100] In the case where the catalyst layer is used as a catalyst layer for reducing carbon dioxide, the inorganic fine particles and the metal complex is preferably a catalyst source that has at least a function of generating carbon monoxide through reduction reaction.
[0101] Specifically, the inorganic fine particles for reducing carbon dioxide are preferably fine particles selected from the group consisting of gold, silver, copper, nickel, iron, cobalt, zinc, chromium, palladium, tin, manganese, aluminum, indium, bismuth, molybdenum, and carbon nitride. Only one kind of the inorganic fine particles may be used, or two or more kinds thereof may be used in combination.
[0102] Among the above, the material of the inorganic fine particles is preferably silver, gold, zinc, tin, copper, and bismuth, more preferably silver, gold, copper, and tin, and further preferably silver, gold, and copper, from the standpoint of the reaction efficiency of the carbon dioxide reduction reaction.
[0103] The average particle diameter of the inorganic fine particles as the catalyst source for reducing carbon dioxide is preferably 65 nm or less, preferably 60 nm or less, preferably 50 nm or less, preferably 40 nm or less, and preferably 30 nm or less, from the standpoint of the reaction rate of the carbon dioxide reduction reaction. The lower limit of the average particle diameter is not limited, and is preferably 1 nm or more, and more preferably 5 nm or more, from the standpoint of the productivity.
[0104] The average particle diameter can be measured through observation of a photograph or the like of a scanning electron microscope or the like.
[0105] The metal complex as a catalyst for reducing carbon dioxide is a metal complex containing a metal or an ion of the metal, having a ligand coordinated thereto, and the metal ion is preferably selected from the group consisting of copper, nickel, iron, cobalt, zinc, manganese, molybdenum, and aluminum,
[0106] Among the above, the metal is preferably nickel, cobalt, iron, copper, zinc, and manganese, more preferably nickel, cobalt, iron, and copper, and further preferably nickel, cobalt, and iron, from the standpoint of the reaction efficiency of the carbon dioxide reduction reaction. The metal complex may contain one kind of a metal or an ion of the metal, or may contain two or more kinds thereof.
[0107] The kind of the ligand is not particularly limited, and examples thereof include a phthalocyanine complex, a porphyrin complex, a pyridine complex, a metal-modified covalent triazine framework, and a metal organic framework. Among these, a phthalocyanine complex, a porphyrin complex, a pyridine complex, and a metal-modified covalent triazine framework are preferred, a phthalocyanine complex, a porphyrin complex, and a metal-modified covalent triazine framework are more preferred, and a porphyrin complex and a metal-modified covalent triazine framework are further preferred. The metal complex may contain one kind of the ligand, or may contain two or more kinds thereof.
[0108] As described above, the catalyst layer for reducing carbon dioxide according to the present embodiment is preferably a catalyst layer including
the additive according to the present embodiment (i.e., the additive for a carbon dioxide reduction catalyst according to the present embodiment), and
a catalyst including a carrier containing carbon, inorganic fine particles or a metal complex being supported on the carrier, in which
the inorganic fine particles are fine particles selected from the group consisting of gold, silver, copper, nickel, iron, cobalt, zinc, chromium, palladium, tin, manganese, aluminum, indium, bismuth, molybdenum, and carbon nitride, and the metal complex is a metal complex containing a metal selected from the group consisting of copper, nickel, iron, cobalt, zinc, manganese, molybdenum, and aluminum, or an ion of the metal, coordinated thereto.
[0109] The inorganic fine particles and the metal complex are supported on the carrier according to the present embodiment by performing a known method, such as vapor deposition, deposition, adsorption, accumulation, adhesion, welding, physical mixing, and spraying.
[0110] The catalyst with the technique of the present disclosure is preferably coated with an ionomer described later. The ionomer coated on the catalyst facilitates the formation of the ionic conductive channel of the coated catalyst and the solid electrolyte described later, and facilitates the migration of ions formed through the reaction, and thereby the electrolysis efficiency can be enhanced.
[Ionomer]
[0112] The ionomer functions as a binder resin in the catalyst layer to be a matrix resin (continuous phase) capable of dispersing and fixing the additive and the catalyst according to the present embodiment, and also has a function of conducting ions formed through electrolysis and enhancing the electrolysis efficiency of CO2. The ionomer is preferably a polymer electrolyte from the standpoint of enhancing the conductivity. The polymer electrolyte is more preferably an ion exchange resin. The ion exchange resin may be either a cation exchange resin or an anion exchange resin, and is preferably an anion exchange resin.
[0113] With the use of an anion exchange resin, in particular, the anion exchange resin itself has a function of adsorbing carbon dioxide, which can largely enhance the electrolysis efficiency of carbon dioxide, in cooperation with the high ion conduction of the ion exchange resin.
[0114] Examples of the cation exchange resin include a fluorine resin having a sulfone group and a styrene-divinylbenzene copolymer having a sulfone group. A commercially available product may also be used therefor, and examples thereof include Nafion (available from Chemours Company), Aquivion (available from Solvay Specialty Polymers, Inc.), DIAION (available from Mitsubishi Chemical Corporation), and Fumasep (available from Fumatech BWT GmbH).
[0115] Examples of the anion exchange resin include a resin having one or more of an ion exchange group 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 also be used therefor, and examples thereof include Sustainion (available from Dioxide Materials, Inc.), Fumasep (available from Fumatech BWT GmbH), PENTION (available from Xergy, Inc.), DURION (available from Xergy, Inc.), NEOSEPTA (available from Astom Corporation), and TOYOPEARL (available from Tosoh Corporation).
[0116] The anion exchange resin preferably has a base site density in a dry state of 2.0 to 5.0 mmol/cm3, more preferably 2.5 mmol/cm3 or more and less than 4.5 mmol/cm3, and further preferably 2.9 mmol/cm3 or more and less than 4.5 mmol/cm3, from the standpoint of enhancing the conductivity.
[0117] The base site density of the anion exchange resin can be obtained from the integral value of the signal in 1H-NMR measurement of the anion exchange resin.
[0118] The dry state of the anion exchange resin means that the content of free water in the anion exchange resin is 0.01 g or less per 1 g of resin, and for example, the dry state can be obtained by heating the anion exchange resin in vacuum.
[0119] In the case where the cathode according to the present embodiment is used in the ion exchange membrane-electrode assembly described later or the solid electrolyte electrolysis apparatus described later, the ionomer used is preferably the same resin as the solid electrolyte (ion exchange membrane) from the standpoint of enhancing the conductivity.
[0120] The content of the additive according to the present embodiment in the catalyst layer is preferably 1 to 90% by mass, more preferably 5 to 70% by mass, and further preferably 10 to 50% by mass, from the standpoint of enhancing the hydrophobicity of the catalyst layer and suppressing the deterioration in the catalytic function.
[0121] The content of the catalyst according to the present embodiment in the catalyst layer is preferably 5 to 90% by mass, more preferably 10 to 80% by mass, and further preferably 15 to 60% by mass, from the standpoint of further enhancing the production efficiency of a synthetic gas containing CO.
[Cathode]
[0122] The cathode according to the present embodiment includes the catalyst layer for reducing carbon dioxide according to the present embodiment described above, and a gas diffusion layer.
[0123] The cathode according to the present embodiment includes the catalyst layer containing the additive according to the present embodiment, and thereby can stably produce a synthetic gas containing CO without inhibiting the carbon dioxide reduction reaction in the catalyst layer. Accordingly, an excellent electrolytic efficiency of the electrolytic reduction reaction of carbon dioxide can be obtained.
[Gas Diffusion Layer]
[0124] The gas diffusion layer contains, for example, carbon paper or nonwoven fabric, or a metal mesh. Examples thereof include graphite carbon, glassy carbon, titanium, and a stainless steel.
[Ion Exchange Membrane-Electrode Assembly]
[0125] The ion exchange membrane-electrode assembly according to the present embodiment includes the cathode according to the present embodiment described above, a solid electrolyte, and an anode.
[0126] The ion exchange membrane-electrode assembly according to the present embodiment includes the cathode that includes the catalyst layer containing the additive according to the present embodiment, and thereby can stably produce a synthetic gas containing CO without inhibiting the carbon dioxide reduction reaction in the catalyst layer. Accordingly, an excellent electrolytic efficiency of the electrolytic reduction reaction of carbon dioxide can be obtained.
[0127] Fig. 1 is a schematic illustration showing an ion exchange membrane-electrode assembly that is favorably used in the present embodiment. Fig. 1 shows an ion exchange membrane-electrode assembly 50 including a gas diffusion layer 10, a catalyst layer 20, a solid electrolyte layer 30, and an anode 40. The catalyst layer 20 contains an ionomer 22, the multiple catalysts 24 according to the present embodiment, and the multiple additives according to the present embodiment (i.e., the additive for a carbon dioxide reduction catalyst according to the present embodiment) 26. The combination of the gas diffusion layer 10 and the catalyst layer 20 constitutes the cathode according to the present embodiment.
[0128] As shown in Fig. 1, carbon dioxide (CO2) is supplied to the catalyst layer 20 via the gas diffusion layer 10, and carbon monoxide (CO) is formed through reduction reaction.
[Solid Electrolyte]
[0130] The ion exchange membrane-electrode assembly according to the present embodiment includes a solid electrolyte.
[0131] The solid electrolyte used may be a polymer membrane. The polymer used may be various ionomers, and may be a cation exchange resin or an anion exchange resin, and an anion exchange resin is preferred. Accordingly, the solid electrolyte is preferably an anion exchange membrane. The same anion exchange resin as the ionomer used in the catalyst layer described above is more preferably used.
[0132] The solid electrolyte used may be a product that is commercially available as a cation exchange membrane or an anion exchange membrane.
[0133] In the case where an anion exchange membrane is used as the solid electrolyte, the base site density thereof in a dry state is preferably 0.5 to 5.0 mmol/cm3, more preferably 2.5 mmol/cm3 or more and less than 4.5 mmol/cm3, and further preferably 2.9 mmol/cm3 or more and less than 4.5 mmol/cm3.
[0134] Examples of the cation exchange membrane include a strongly acidic cation exchange membrane formed of a fluorine resin as a matrix having a sulfone group introduced thereto, Nafion 117, Nafion 115, Nafion 212, and Nafion 350 (available from Chemours Company), a strongly acidic cation exchange membrane formed of a styrene-divinylbenzene copolymer as a matrix having a sulfone group introduced thereto, and Neosepta CSE (available from Astom Corporation).
[0135] Examples of the anion exchange membrane include an anion exchange membrane having one or more ion exchange group 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 (registered trade name) ASE, AHA, ACS, and AFX (available from Astom Corporation), and Selemion (registered trade name) AMVN, DSVN, AAV, ASVN, and AHO (available from AGC Engineering Co., Ltd.).
[0136] As for reduction reaction of carbon dioxide, the reduction reaction in the cathode according to the present embodiment varies depending on the kind of the solid electrolyte. In the case where a cation exchange membrane is used as the solid electrolyte, the reduction reaction of the reaction formulae (1) and (2) below occurs, and in the case where an anion exchange membrane is used as the solid electrolyte, the reduction reaction of the reaction formulae (3) and (4) below occurs.
CO2+2H-+2e- => CO+H2O (1)
2H++2e- => H2 (2)
H2O+CO2+2e- => CO+2OH- (3)
2H2O+2e- => H2+2OH- (4)
[Anode]
[0137] The oxidation reaction in an anode varies depending on the kind of the solid electrolyte. In the case where a cation exchange membrane is used as the solid electrolyte, the oxidation reaction of the reaction formula (5) below occurs, and in the case where an anion exchange membrane is used as the solid electrolyte, the oxidation reaction of the reaction formula (6) below occurs.
2H2O => O2+4H++4e- (5)
4OH- => O2+2H2O+4e- (6)
[0139] The gas diffusion layer includes, for example, a metal mesh. Examples of the electrode material of the anode include Ir, IrO2, Ru, RuO2, Co, CoOx, Cu, CuOx, Fe, FeOx, FeOOH, FeMn, Ni, NiOx, NiOOH, NiCo, NiCe, NiC, NiFe, NiCeCoCe, NiLa, NiMoFe, NiSn, NiZn, SUS, Au, and Pt.
<Solid Electrolyte Electrolysis Apparatus>
[0140] The solid electrolyte electrolysis apparatus according to the present embodiment includes the cathode according to the present embodiment described above, an anode constituting a pair of electrodes with the cathode, a solid electrolyte intervening between the cathode and the anode, in a contact state, and a voltage application unit applying a voltage between the cathode and the anode.
[0141] The solid electrolyte electrolysis apparatus according to the present embodiment includes the cathode that includes the catalyst layer containing the additive according to the present embodiment, and thereby can stably produce a synthetic gas containing CO without inhibiting the carbon dioxide reduction reaction in the catalyst layer. Accordingly, an excellent electrolytic efficiency of the electrolytic reduction reaction of carbon dioxide can be obtained.
[0142] Fig. 2 is a schematic illustration showing a solid electrolyte electrolysis apparatus that is favorably used in the present embodiment.
[0143] Fig. 2 shows a solid electrolyte electrolysis apparatus 800 including the cathode 200 constituting the electrode according to the present embodiment, an anode 400 constituting a pair of electrodes with the cathode 200, a solid electrolyte 300 intervening between the cathode 200 and the anode 400, in a contact state, and a voltage application unit 700 applying a voltage between the cathode 200 and the anode 400.
[0144] The solid electrolyte electrolysis apparatus 800 shown in Fig. 2 further includes a cathode collector 100, an anode collector 500, and an electrolytic solution 600.
[0145] The electrode according to the present embodiment described above is used as the cathode 200. 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.
[0146] The details of the cathode 200, the solid electrolyte 300, and the anode 400 have been described above.
[0147] The components other than the cathode 200, the solid electrolyte 300, and the anode 400 will be described below while omitting the symbols.
[Cathode Collector]
[0148] Examples of the cathode collector include a metal material, such as copper (Cu), nickel (Ni), a stainless steel (SUS), a nickel-plated steel, and brass, and among these, copper is preferred from the standpoint of the workability and the cost. Examples of the shape of the cathode collector in the case where the material is a metal material include a metal foil, a metal sheet, a thin metal film, an expanded metal, a punching metal, and a metal foam.
[0149] The cathode collector may have provided therein a gas supply hole for supplying the raw material gas containing carbon dioxide to the cathode, and a gas recovery hole for recovering the formed gas containing carbon monoxide. The gas supply hole and the gas recovery hole provided enable the uniform and efficient supply of the raw material gas to the cathode and recovery of the formed gas (including the unreacted raw material gas). Only one or two or more gas supply holes and only one or two or more gas recovery holes may be provided, independently thereon. The shapes, the positions, the sizes, and the like of the gas supply holes and the gas recovery holes are not particularly limited and may be determined appropriately. In the case where the cathode collector has gas permeability, the gas supply holes and the gas recovery holes may not be necessarily provided.
[0150] In the case where the cathode has a function of conducting electrons, the cathode collector may not be necessarily provided.
[Anode Collector]
[0151] The anode collector has electroconductivity for receiving electrons from the anode, and preferably has rigidity supporting the anode. From this standpoint, the anode collector used is preferably a metal material, such as titanium (Ti), copper (Cu), nickel (Ni), a stainless steel (SUS), a nickel-plated steel, and brass.
[0152] The anode collector may have provided therein a gas flow channel for delivering the raw material gas (such as H2O) to the anode. The gas flow channel provided in the anode collector enables the unform and efficient delivery of the raw material gas to the anode. The number, the shape, the position, the size, and the like of the gas flow channel are not particularly limited and may be determined appropriately.
[Voltage Application Unit]
[0153] The voltage application unit has a function of applying a voltage between the cathode and the anode through application of a voltage between the cathode collector and the anode collector. The collectors are conductors, and therefore electrons are supplied to the cathode, whereas electrons are received from the anode. The voltage application unit may have a control unit, which is not shown in the figure, electrically connected thereto for applying an appropriate voltage.
[Electrolytic Solution]
[0155] Examples thereof include a carbonate salt aqueous solution, a hydrogen carbonate salt aqueous solution (such as a KHCO3 aqueous solution), a sulfate salt aqueous solution, a borate salt aqueous solution, sodium hydroxide, a potassium hydroxide aqueous solution, and a sodium chloride aqueous solution.
(Reaction Gas Supply Unit)
[0156] The solid electrolyte electrolysis apparatus according to the present embodiment may have a reaction gas supply unit, which is not shown in the figure, outside the solid electrolyte electrolysis apparatus. Specifically, it suffices that CO2 as the reaction gas is supplied to the catalyst layer of the cathode, in which the reaction gas may be supplied from the reaction gas supply unit to the gas supply hole via a pipe, which is not shown in the figure, or the reaction gas may be sprayed on the surface of the cathode collector opposite to the surface thereof in contact with the cathode. The reaction gas used is preferably a factory emission gas emitted from factories, from the environmental standpoint.
[CO Generating Method]
[0157] A CO generating method using the solid electrolyte electrolysis apparatus according to the present embodiment will be then described.
[0158] First, CO2 in a gas state, which is a reaction gas as a raw material, is supplied to the solid electrolyte electrolysis apparatus with the reaction gas supply unit, which is not shown in the figure. At this time, CO2 is supplied to the cathode, for example, through the gas supply hole provided in the cathode collector.
[0159] Subsequently, CO2 supplied to the cathode is brought into contact with the catalyst layer of the cathode, and thereby the reduction reaction of the reaction formula (1) and the reaction formula (2) described above occurs in the case where a cation exchange membrane is used as the solid electrolyte, or the reduction reaction of the reaction formula (3) and the reaction formula (4) described above occurs in the case where an anion exchange membrane is used as the solid electrolyte, resulting in a synthetic gas containing at least CO and H2 formed.
[0160] Subsequently, for example, the synthetic gas containing at least CO and H2 thus formed is supplied to a gas recovery unit, which is not shown in the figure, through the gas recovery hole provided in the cathode collector, and the prescribed gas species are recovered.
Examples
[0161] The technique of the present disclosure will be then described with reference to examples, but the technique of the present disclosure is not limited to the examples.
<Production of Additive>
[Example 1]
[0162] An ethanol dispersion liquid containing 0.5 g of carbon black having an average particle diameter of 30 nm was irradiated with ultrasonic wave for 10 minutes, and then the dispersion liquid was allowed to stand in a vacuum chamber in a reduced pressure state of 10 kPa (absolute pressure) for 10 minutes. Subsequently, 8.3 mL of a 0.5 mol/L sodium nitrite aqueous solution was added to the dispersion liquid. 4 mmol of 4-tritylaniline (compound represented by the formula (11)) was added to the dispersion liquid, and then 2 mL of hydrochloric acid was added thereto, followed by agitating at 15°C for 5 hours or more. After neutralizing the dispersion liquid by adding a sodium hydroxide solution, the resulting slurry was rinsed with distilled water, and the solid matter was recovered with a centrifugal separator, and then vacuum-dried at 60°C overnight, resulting in an additive of Example 1.
[0163] The primary particle diameter of the carbon black was obtained through laser diffraction particle size distribution measurement.
[Examples 2 to 8]
[0164] Additives of Examples 2 to 8 were produced in the same manner as in the production of the additive of Example 1 except that the basic organic compounds shown in Tables 1 and 2 were used instead of 4-tritylaniline.
[0165] The basic organic compounds used in the production of the additives of Examples 2 to 8 are the compound represented by the formulae (12) to (18) above.
[Comparative Example 1]
[0166] In Comparative Example 1, carbon black having an average particle diameter of 30 nm was used as the additive.
[Comparative Example 2]
[0167] In Comparative Example 2, polytetrafluoroethylene (PTFE) having an aerodynamic particle size of 30 to 50 nm ("Polytetrafluoroethylene Nanopowder" (trade name), available from Nanoshel, Inc.) was used as the additive.
[Comparative Examples 3 and 4]
<Production of Catalyst>
[Examples 1 to 7 and Comparative Examples 1 to 3]
[0169] The same catalyst was used in Examples 1 to 7 and Comparative Examples 1 to 3, and was produced in the following manner.
[0170] In a beaker, 0.1 g of the carbon black carrier (carrier according to the present embodiment) was mixed in 100 mL of ethanol, and the resulting ethanol dispersion liquid was irradiated with ultrasonic wave for 10 minutes. Thereafter, the dispersion liquid was allowed to stand in a vacuum chamber in a reduced pressure state of 10 kPa (absolute pressure) for 10 minutes. Thereafter, 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 therein, and the mixture was agitated at 15°C for 16 hours to reduce silver nitrate. After completing the reaction, the resulting slurry was rinsed with distilled water, and the solid matter was recovered with a centrifugal separator, and then vacuum-dried at 60°C overnight, resulting in catalyst powder of Examples 1 to 7 and Comparative Examples 1 to 3. The catalysts obtained was carbon black having Ag particles supported thereon as a catalyst source, and the mass of the Ag particles supported was 40 parts by mass per 100 parts by mass of the carbon black having except for the Ag particles supported thereon.
[Example 8 and Comparative Example 4]
[0171] The same catalyst was used in Example 8 and Comparative Example 4, and was produced in the following manner.
[0172] In a beaker, 0.4 g of a carbon black carrier (carrier according to the present embodiment), 1.1 mmol of pentaethylenehexamine, and 0.7 mmol of nickel(II) chloride hexahydrate were mixed with 15 mL of ethanol, and the resulting ethanol dispersion liquid was irradiated with ultrasonic wave for 10 minutes. Thereafter, ethanol was evaporated through drying by heating the ethanol dispersion liquid, and the resulting mixture was baked by heating in an inert gas at 900°C for 30 seconds or more with an electric furnace. Thereafter, the product was rinsed with a sulfuric acid aqueous solution, and the solid matter was collected through suction filtration, and vacuum-dried at 60°C overnight, resulting in catalyst powder having a Ni complex supported thereon. The catalyst powder was designated as catalyst powder of Example 8 and Comparative Example 4.
[0173] In the resulting catalyst, the mass of the Ni supported was 1 part by mass per 100 parts by mass of the carbon black except for the Ni complex supported thereon.
<Solid Electrolyte Electrolysis Apparatus>
[Example 1]
[0174] 43 mg of the resulting catalyst powder was again dispersed in ethanol, and the dispersion liquid was mixed with 12 mg of an anion exchange resin as an ionomer having 5 mg of the additive of Example 1 added thereto. The 1H-NMR measurement of the anion exchange resin in a dry state revealed that the base site density was calculated as 2.8 mmol/cm3 from the integral value of the signal. The anion exchange resin is a fluorene based resin as a substrate having an aromatic ring on the main chain and a quaternary ammonium group (quaternary alkylamine group) as a side chain bonded to the main chain.
[0175] After mixing, the dispersion liquid was irradiated with ultrasonic wave for 10 minutes, and then allowed to stand in a vacuum chamber in a reduced pressure state of 10 kPa (absolute pressure) for 10 minutes. The dispersion liquid was coated on carbon paper with a spray coater to provide a cathode. The cathode had the coated film of the dispersion liquid as a catalyst layer, and the carbon paper as a gas diffusion layer.
[0176] The anion exchange membrane having a thickness of approximately 30 µm (base site density: 2.8 mmol/cm3) and a carbon anode having iridium oxide supported thereon (available from Dioxide Materials, Inc.) were adhered to the cathode to provide an ion exchange membrane-electrode assembly.
[0177] The anode had a structure in contact with an electrolytic solution (KHCO3 solution of 0.5 mol/L) tank.
[Examples 2 to 7 and Comparative Example 1 and 2]
[0178] Solid electrolyte electrolysis apparatuses of Examples 2 to 7 and Comparative Example 1 and 2 were produced in the same manner as in the production of the solid electrolyte electrolysis apparatus of Example 1 except that the additive was changed from the additive of Example 1 to any of the additives of Examples 2 to 7 and Comparative Example 1 and 2.
[Example 8]
[0179] A solid electrolyte electrolysis apparatus of Example 8 was produced in the same manner as in the production of the solid electrolyte electrolysis apparatus of Example 1 except that the additive was changed from the additive of Example 1 to the additive of Example 8, and the ionomer was changed from the ionomer of Example 1 to Nafion (available from Chemours Company).
[Comparative Example 3]
[0180] A solid electrolyte electrolysis apparatus of Comparative Example 3 was produced in the same manner as in the production of the solid electrolyte electrolysis apparatus of Example 1 except that no additive was added.
[Comparative Example 4]
[0181] A solid electrolyte electrolysis apparatus of Comparative Example 4 was produced in the same manner as in the production of the solid electrolyte electrolysis apparatus of Example 8 except that no additive was added.
<Evaluation of Solid Electrolyte Electrolysis Apparatus>
[Examples 1 to 7 and Comparative Examples 1 to 3]
[0182] In each of the solid electrolyte electrolysis apparatuses of Examples 1 to 7 and Comparative Examples 1 to 3, CO2 was electrolyzed by supplying pure CO2 to the cathode with an application potential thereof set to -2.6 V with respect to the anode or with a constant current of -1 A/cm2 applied thereto, under condition where the solid electrolyte electrolysis apparatus was heated to 50°C, and the CO formation current density (mA/cm2) and the CO selectivity (%) in forming CO were measured. In the application of the constant current, the voltage (V) was measured instead of the current.
[0183] The results are shown in Table 1.
Table 1
| Basic organic compound | -2.6 V Constant voltage application | -1 A/cm2 Constant current application | |||
| CO Formation current density (mA/cm2) | CO Selectivity | Voltage between 2 Electrodes (V) | CO Selectivity | ||
| Example 1 | 4-tritylaniline | 180 | 98% | 3.3 | 93% |
| Example 2 | 1-aminopyrene | 165 | 98% | 3.4 | 97% |
| Example 3 | 3,5-bis(trifluoromethyl)aniline | 165 | 98% | 3.4 | 97% |
| Example 4 | 4-aminononafluorobiphenyl | 140 | 99% | 3.4 | 97% |
| Example 5 | 4-dodecylaniline | 160 | 99% | 3.4 | 98% |
| Example 6 | 2-aminoanthracene | 140 | 98% | 3.4 | 96% |
| Example 7 | 4-ethylaniline | 155 | 97% | 3.5 | 93% |
| Comparative Example 1 | carbon black | 175 | 95% | 3.7 | 68% |
| Comparative Example 2 | PTFE | 45 | 91% | 3.7 | 20% |
| Comparative Example 3 | none | 130 | 85% | 3.8 | 76% |
<Evaluation of Solid Electrolyte Electrolysis Apparatus>
[Example 8 and Comparative Example 4]
[0184] In each of the solid electrolyte electrolysis apparatuses of Example 8 and Comparative Example 4, CO2 was electrolyzed by supplying pure CO2 to the cathode with an application potential of the cathode set to -1.8 V with respect to the silver/silver chloride reference electrode under condition of room temperature, and the CO formation current density (mA/cm2) in forming CO was measured.
[0185] The results are shown in Table 2.
Table 2
| Basic organic compound | CO Formation current density (mA/cm2) | |
| Example 8 | 2-ethylaniline | 290 |
| Comparative Example 4 | none | 260 |
[0186] It is understood from Tables 1 and 2 that the cases using the additive obtained through chemical modification of the surface of the carrier with an aryl group using various aromatic amine compounds as a raw material (Examples 1 to 7) show relatively low necessary voltages and high CO selectivities in application of a high current of -1 A/cm2. Example 8 shows a relatively high current density.
[0187] On the other hand, Comparative Example 1 using the additive having the surface of the carrier not chemically modified with an aryl group, Comparative Example 2 using the hydrophobic polymer having insulating property as the additive, and Comparative Example 3 using no additive show high necessary voltages and low CO selectivities. Comparative Example 4 using no additive shows a relatively low current density.
<Water Vapor Adsorption Amount of Additive>
[0188] The additives of Examples 1 to 4, 6, and 7 and Comparative Example 1 were measured for the water vapor adsorption amount (a) at 25°C and a water vapor pressure of 2.2 kPa and the water vapor adsorption amount (b) at 25°C and a water vapor pressure of 3.1 kPa with Belsorp-max (available from Microtrac-Bel Japan Co., Ltd.), and the ratios (a/b) thereof are shown in Table 3. In the measurement, after heating 0.2 to 0.3 g of a specimen of the additive to 120°C for 5 hours or more under a vacuum condition for removing the adsorbed gas on the surface thereof in advance, water vapor was introduced under condition of 25°C, and the adsorption amounts at the water vapor pressures were measured.
[0189] A graph of the relative water vapor adsorption amounts plotted against the relative pressure is shown in Fig. 3. In Fig. 3, the relative pressure as the ordinate means a value obtained by dividing the water vapor pressure by the saturated water vapor pressure at 25°C (3.1 kPa). The relative water vapor adsorption amount as the abscissa means a value (a/b) obtained by dividing the water vapor adsorption amount (a) at the relative pressure by the water vapor adsorption amount (b) at the saturated water vapor pressure at 25°C. These are shown by the following expressions.
Relative pressure = (water vapor pressure at each measurement point)/(saturated water
vapor pressure (= 3.1 kPa))
Relative water vapor adsorption amount = (water vapor adsorption amount at each measurement
point)/(water vapor adsorption amount at saturated water vapor pressure)
[0190] In Fig. 3, a relative pressure as the abscissa of 0.7 means a relative pressure of 2.2/3.1, and means that the corresponding relative water vapor adsorption amount as the ordinate is the ratio (a/b).
[0191] The "ratio (a/b)" shown in Table 3 is the relative water vapor adsorption amount as the ordinate at a relative pressure as the abscissa of 0.7.
[0192] In Fig. 3, the details of the curves are as follows.
(Ex. 1): Example 1 (4-tritylaniline)
(Ex.2): Example 2 (1-aminopyrene)
(Ex.3): Example 3 (3,5-bis(trifluoromethyl)aniline)
(Ex.4): Example 4 (4-aminononafluorobiphenyl)
(Ex.6): Example 6 (2-aminoanthracene)
(Ex.7): Example 7 (4-ethylaniline)
(Co-Ex. 1): Comparative Example 1 (non-modified carbon black)
Table 3
| Ratio (a/b) | |
| Example 1 | 0.16 |
| Example 2 | 0.10 |
| Example 3 | 0.35 |
| Example 4 | 0.16 |
| Example 6 | 0.19 |
| Example 7 | 0.46 |
| Comparative Example 1 | 0.52 |
[0193] As understood from Table 3 and Fig. 3, the ratio (a/b) of the water vapor adsorption amount (a) at 25°C and a water vapor pressure of 2.2 kPa with respect to the water vapor adsorption amount (b) at 25°C and a water vapor pressure of 3.1 kPa was less than 0.5. This means that the additives of Examples show high hydrophobicity high electroconductivity.
[0194] As described above, it is estimated that the use of the conductive additive according to the present embodiment exerts the effect of hydrophobization without impairing high electroconductivity.
Industrial Applicability
[0195] In the solid electrolyte electrolysis apparatus according to the present embodiment, for example, CO2 gas emitted from factories is used as a raw material, and renewable energy, such as solar battery, to the voltage application unit, is used, whereby a synthetic gas containing at least CO and H2 at a desired formation ratio can be produced. The synthetic gas produced in this manner can produce fuel substrates, chemical raw materials, and the like through the measures, such as FT synthesis (Fischer-Tropsch synthesis) or methanation.
Reference Sign List
[0196]
10: gas diffusion layer
20: catalyst layer
22: ionomer
24: catalyst
26: additive (additive for carbon dioxide reduction catalyst)
30: solid electrolyte (ion exchange membrane)
40: anode
50: ion exchange membrane-electrode assembly
100: cathode collector
200: cathode
300: solid electrolyte (ion exchange membrane)
400: anode
500: anode collector
600: electrolytic solution
700: voltage application unit
800: solid electrolyte electrolysis apparatus
1. An additive for a carbon dioxide reduction catalyst, comprising a carrier having an
aryl group on a surface thereof and containing carbon.
2. An additive for a carbon dioxide reduction catalyst, comprising a carrier containing
carbon, wherein a ratio of a water vapor adsorption amount at 25°C and a water vapor
pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature
and a water vapor pressure of 3.1 kPa is less than 0.5.
3. The additive for a carbon dioxide reduction catalyst according to claim 1, wherein
the aryl group comprises one or more selected from the group consisting of a phenyl
group and a condensed ring group having 2 to 6 benzene rings.
4. The additive for a carbon dioxide reduction catalyst according to claim 3, wherein
the condensed ring group having 2 to 6 benzene rings contains one or more selected
from the group consisting of a naphthyl group, an anthracenyl group, a phenanthrenyl
group, and a pyrenyl group.
5. The additive for a carbon dioxide reduction catalyst according to any one of claims
1, 3, and 4, wherein the aryl group comprises one or more of substituents selected
from the group consisting of an alkyl group, a fluorinated alkyl group, a phenyl group,
a fluorinated phenyl group, and a fluorine atom.
6. The additive for a carbon dioxide reduction catalyst according to any one of claims
1 and 3 to 5, wherein the aryl group is any one or more of groups represented by the
formulae (1) to (8):
wherein, * represents a bonding site to the surface of the carrier.
wherein, * represents a bonding site to the surface of the carrier.
7. A catalyst layer comprising:
an additive comprising a carrier having an aryl group on a surface thereof and containing carbon; and
a catalyst comprising a carrier containing carbon, inorganic fine particles or a metal complex being supported on the carrier.
8. A catalyst layer comprising:
an additive comprising a carrier containing carbon, in which a ratio of a water vapor adsorption amount at 25°C and a water vapor pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa is less than 0.5, and
a catalyst comprising a carrier containing carbon, inorganic fine particles or a metal complex being supported on the carrier.
9. The catalyst layer according to claim 7 or 8, wherein the inorganic fine particles
are fine particles selected from the group consisting of gold, silver, copper, nickel,
iron, cobalt, zinc, chromium, palladium, tin, manganese, aluminum, indium, bismuth,
molybdenum, and carbon nitride, and the metal complex is a metal complex comprising
a metal selected from the group consisting of copper, nickel, iron, cobalt, zinc,
manganese, molybdenum, and aluminum, or an ion of the metal, having a ligand coordinated
thereto.
10. The catalyst layer according to claim 7, wherein the aryl group comprises one or more
selected from the group consisting of a phenyl group and a condensed ring group having
2 to 6 benzene rings.
11. The catalyst layer according to claim 10, wherein the condensed ring group having
2 to 6 benzene rings contains one or more selected from the group consisting of a
naphthyl group, an anthracenyl group, a phenanthrenyl group, and a pyrenyl group.
12. The catalyst layer according to claim 7, wherein the aryl group comprises one or more
of substituents selected from the group consisting of an alkyl group, a fluorinated
alkyl group, a phenyl group, a fluorinated phenyl group, and a fluorine atom.
13. The catalyst layer according to claim 7, wherein the aryl group is any one or more
of groups represented by the formulae (1) to (8):
wherein, * represents a bonding site to the surface.
wherein, * represents a bonding site to the surface.
14. A cathode comprising: the catalyst layer according to claim 9; and a gas diffusion
layer.
15. An ion exchange membrane-electrode assembly comprising: the cathode according to claim
14; a solid electrolyte; and an anode.
16. The ion exchange membrane-electrode assembly according to claim 15, wherein the solid
electrolyte is an anion exchange membrane.
17. A solid electrolyte electrolysis apparatus comprising:
the cathode according to claim 14;
an anode constituting a pair of electrodes with the cathode;
a solid electrolyte intervening between the cathode and the anode, in a contact state; and
a voltage application unit applying a voltage between the cathode and the anode.
18. The solid electrolyte electrolysis apparatus according to claim 17, wherein the solid
electrolyte is an anion exchange membrane.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.
Patent documents cited in the description
Non-patent literature cited in the description
- Z. XINGL. HUD. S. RIPATTIX. HUX. FENGNature Communications, 2021, vol. 12, 136- [0008]