Abstract:
The present disclosure provides an electrode for an electrolysis device, which allows performance of a catalyst to be efficiently exhibited in an electrochemical reaction of reducing an electrolysis reactant to generate an electrolysis product material. Specifically, a carbon fiber that has a structure in which the carbon fiber contains a part of and/or a whole of a catalyst particle is used as a cathode electrode to greatly improve adherence force of the catalyst particle and enable efficient generation of an electrolysis product material.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a cathode electrode including a region of a carbon fiber having a structure in which the carbon fiber contains a part of and/or a whole of a catalyst particle and to a method for generating an electrolysis product material with the electrode. 
         [0003]    2. Description of the Related Art 
         [0004]    Development of a technique of forming a catalyst into fine particles and making a carrier such as a carbon carry the fine particles on the carrier has been conducted for the purpose of increasing a specific area of a catalyst used for an electrochemical reaction and enhancing reaction efficiency per weight of a catalyst. 
         [0005]    Patent Literature 1 discloses a method for making a surface of a carbon tube membrane carry fine particles containing a carbon dioxide decomposing element by an electrochemical method and using the element for electrochemical reduction of carbon dioxide. 
         [0006]    Patent Literature 2 discloses a method for making a surface of a carbon nanofiber carry a catalyst for a battery by a colloidal method. 
         [0007]    Patent Literature 3 discloses a method for making a carbon material carry a transition metal on a surface of the carbon material by a heat treatment. 
       CITATION LIST 
     Patent Literatures 
       [0008]    PTL 1: Unexamined Japanese Patent Publication No. 2010-255018 
         [0009]    PTL 2: WO 2009/003848 
         [0010]    PTL 3: Unexamined Japanese Patent Publication No. 2002-83604 
         [0011]    PTL 4: Unexamined Japanese Patent Publication No. 2012-209193 
         [0012]    PTL 5: WO 2009/140381 
         [0013]    PTL 6: Unexamined Japanese Patent Publication No. 2010-118269 
         [0014]    PTL 7: U.S. Patent Publication No. 2011/0143253 
         [0015]    The methods disclosed in Patent Literatures 1, 2 and 3 have a problem that the catalyst is carried only on the surface of the carbon material, and therefore adherence of the catalyst with the carbon material is low, resulting in insufficient exhibition of performance of the catalyst particles. 
         [0016]    Particularly, when current efficiency is estimated from a result of electrochemical reduction of carbon dioxide disclosed in Patent Literature 1, the current efficiency of a hydrocarbon as an electrolysis product material derived from the catalyst is 8.5%, which is lower than the current efficiency (40% to 50%) when the catalyst itself is used. 
       SUMMARY 
       [0017]    One non-limiting and exemplary embodiment provides a cathode electrode capable of efficiently generating an electrolysis product material by using a carbon fiber having a structure in which the carbon fiber contains a part of and/or a whole of a catalyst particle (i.e., catalyst-containing structure), to enhance adherence of the catalyst particle with the carbon fiber. 
         [0018]    In one general aspect, the techniques disclosed here feature a method for reducing a reactant electrochemically with an electrolysis device to generate a product material, the method comprising: 
         [0019]    (a) preparing the electrolysis device comprising a cathode chamber, an anode chamber, a cathode electrode, an anode electrode, and a solid electrolyte membrane; 
         [0020]    wherein 
         [0021]    the cathode electrode includes a carbon fiber; 
         [0022]    the carbon fiber includes a first catalyst particle and a second catalyst particle; 
         [0023]    an inside of the carbon fiber is filled with carbon; 
         [0024]    at least a part of the first catalyst particle is located on a surface of the carbon fiber; 
         [0025]    the second catalyst particle is located in the inside of the carbon fiber so as to be surrounded by the carbon with which the inside of the carbon fiber is filled; 
         [0026]    the anode electrode has a region formed of a metal or metal compound; 
         [0027]    a first electrolyte solution is stored in the cathode chamber; 
         [0028]    a second electrolyte solution is stored in the anode chamber; 
         [0029]    the cathode electrode is in contact with the first electrolyte solution; 
         [0030]    the anode electrode is in contact with the second electrolyte solution; 
         [0031]    the first electrolyte solution contains the reactant; and 
         [0032]    the solid electrolyte membrane separates the anode chamber from the cathode chamber; and 
         [0033]    (b) applying a voltage between the anode electrode and the cathode electrode to reduce the reactant on at least one of the first catalyst particle and the second catalyst particle. 
         [0034]    An electrolysis device comprising a cathode electrode according to the present disclosure which includes a carbon fiber having a catalyst-containing structure can efficiently generate an electrolysis product material. 
         [0035]    Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
         [0036]    It should be noted that general or specific embodiments may be implemented as a method, an electrolysis device, a cathode electrode or any selective combination thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  is a view showing an electrolysis device according to a first exemplary embodiment of the present disclosure; 
           [0038]      FIG. 2  is a view showing an electrolysis device according to a second exemplary embodiment of the present disclosure; 
           [0039]      FIG. 3  is a diagram showing pretreatment conditions for a carbon fiber in Examples; 
           [0040]      FIG. 4A  is a scanning electron micrograph of a surface of a carbon fiber; 
           [0041]      FIG. 4B  is a transmission electron micrograph of a surface of a carbon fiber; 
           [0042]      FIG. 5  is a micrograph showing a surface of an electrolyzed carbon fiber in Example 1; 
           [0043]      FIG. 6A  is a scanning electron micrograph before an experiment in Comparative Example 2; 
           [0044]      FIG. 6B  is a scanning electron micrograph after an experiment in Comparative Example 2; 
           [0045]      FIG. 7  is a graph showing a generation amount of a product material per unit time in Examples and Comparative Examples; 
           [0046]      FIG. 8A  is a graph showing a generation amount of carbon monoxide per unit time when gold particles are used; 
           [0047]      FIG. 8B  is a graph showing a generation amount of formic acid per unit time when gallium oxide particles are used; 
           [0048]      FIG. 8C  is a graph showing a generation amount of hydrogen per unit time when platinum particles are used; 
           [0049]      FIG. 9A  is a schematic view of carbon fiber  20 ; and 
           [0050]      FIG. 9B  is a sectional view taken along the line  9 B- 9 B. 
       
    
    
     DETAILED DESCRIPTION 
       [0051]    Hereinafter, the present disclosure is described specifically according to the exemplary embodiments with reference to the drawings. 
       First Exemplary Embodiment 
       [0052]      FIG. 1  is electrolysis device  100  according to a first exemplary embodiment of the present disclosure, which is an electrolysis device that generates an electrolysis product material from an electrolysis reactant, and includes cathode chamber  12  for storing first electrolyte solution  11  containing an electrolysis reactant; electrode  13  for an electrolysis device, as a cathode electrode that is disposed in the cathode chamber so as to be in contact with the first electrolyte solution and includes a region of a carbon fiber having a structure in which the carbon fiber contains a part of and/or a whole of a catalyst particle; solid electrolyte membrane  16  that separates the cathode chamber from anode chamber  15  for storing second electrolyte solution  14 ; anode electrode  17  that is disposed in the anode chamber so as to be in contact with the second electrolyte solution and has a region formed of a metal or metal compound; and external power source  18  for applying a voltage so that the cathode electrode has a negative potential with respect to a potential of the anode electrode. 
         [0053]    According to the first exemplary embodiment, an electrolysis product material can be obtained. 
       Second Exemplary Embodiment 
       [0054]      FIG. 2  is electrolysis device  200  according to a second exemplary embodiment of the present disclosure, which is an electrolysis device that generates an electrolysis product material from an electrolysis reactant, and includes cathode chamber  12  for storing first electrolyte solution  11  containing an electrolysis reactant; electrode  13  for an electrolysis device, as a cathode electrode that is disposed in the cathode chamber so as to be in contact with the first electrolyte solution and that includes a region of a carbon fiber having a structure in which the carbon fiber contains a part of and/or a whole of a catalyst particle; solid electrolyte membrane  16  that separates the cathode chamber from anode chamber  15  for storing second electrolyte solution  14 ; anode electrode  17  that is disposed in the anode chamber so as to be in contact with the second electrolyte solution and has a region formed of a metal or metal compound; external power source  18  for applying a voltage so that the cathode electrode has a negative potential with respect to a potential of the anode electrode; and reference electrode  19  that is disposed in the cathode chamber so as to be in contact with the first electrolyte solution. 
         [0055]    The electrolysis device according to the second exemplary embodiment is suitable because an electrolysis reaction can be controlled by controlling the potential of the cathode electrode, and an influence due to temporal change on the anode electrode side can be eliminated. 
         [0056]    As shown in  FIGS. 1 and 2 , the electrolysis device may have a tube in the cathode chamber. A gaseous electrolysis reactant is supplied to the first electrolyte solution through the tube. Examples of the gaseous electrolyte reactant include oxygen and carbon dioxide. Also as to a liquid/solid electrolysis reactant such as water, an inert gas can be supplied through a sub tube so as to suppress a side reaction. An end of the tube is immersed in the first electrolyte solution. The electrolysis device may also include a voltage measuring device and a current measuring device for monitoring a state of a reduction reaction of an electrolysis reactant. 
         [0057]    The cathode electrode includes a region of a carbon fiber having a structure in which the carbon fiber contains a part of and/or a whole of a catalyst particle (i.e., catalyst-containing structure). The catalyst particle is formed of, for example, a metal, oxide, carbide, nitride, boride, silicide, fluoride, or sulfide. Specific examples of the catalyst particle include platinum, gold, silver, copper and a compound thereof. The catalyst particle is not limited to employment of the substances exemplified above, but the constitution of the catalyst particle is not limited as long as an electrolysis product material can be obtained by catalysis via the catalyst particle. A particle size of the catalyst particle can be freely set in a range not exceeding a fiber diameter of the carbon fiber. In view of a specific surface area of the catalyst, the catalyst particle size is preferably not more than 1 μm, more preferably not more than 100 nm. The fiber diameter of the carbon fiber can also be freely set in a range not being smaller than the particle size of the catalyst particle. In view of the specific surface area described above, the fiber diameter of the carbon fiber is preferably not more than 10 μm, more preferably not more than 1 μm. 
         [0058]    Similarly, a concentration of the catalyst particle in the carbon fiber can also be freely set. A catalytic activity improves as the concentration of the catalyst particle in the carbon fiber increases because a surface area of particles exposed on a surface of the carbon fiber increases. However, when the concentration is too strong, strength of the carbon fiber decreases, causing deterioration in catalytic activity. The concentration of the catalyst particle in the carbon fiber ranges preferably from 5 wt % to 30 wt %, more preferably from 5 wt % to 20 wt %. Such a concentration of the catalyst particle can be, as a direct manner, obtained by heating in oxygen the carbon fiber having the catalyst-containing structure to lose the carbon fiber and comparing weights before and after the heating (thermal analysis measurement). 
         [0059]    Hereinafter, one example for producing a carbon fiber that is included in a cathode electrode and includes a region having a catalyst-containing structure. However, the present disclosure is not limited by the following production example at all. 
         [0060]    The structure can be obtained by carbonizing a fiber formed from a precursor of a carbon fiber, in which catalyst particles are dispersed. Examples of the precursor of a carbon fiber include polypropylene, polyethylene, polystyrene, polyethylene oxide, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, poly-m-phenylene terephthalate, poly-p-phenylene isophthalate, polyvinylidene fluoride, a polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl chloride, a polyvinylidene chloride-acrylate copolymer, polyacrylonitrile, a polyacrylonitrile-methacrylate copolymer, polycarbonate, polyacrylate, polyestercarbonate, a polyamide, an aramid, a polyimide, polycaprolactone, polyamic acid, polylactic acid, polyglycolic acid, collagen, polyhydroxy butyric acid, polyvinyl acetate, a polypeptide, and a high polymer compound such as a copolymer thereof. One selected from the above examples may be used or a plurality of kinds may be mixed. The carbon fiber precursor is not limited to employment of the substances exemplified above. 
         [0061]    The catalyst particles or the carbon fiber precursor may be dispersed in a solvent. Examples of the solvent in which the carbon fiber precursor is dispersed include methanol, ethanol, 1-propanol, 2-propanol, hexafluoroisopropanol, tetraethylene glycol, triethylene glycol, dibenzyl alcohol, 1,3-dioxolane, 1,4-dioxane, methyl ethyl ketone, methyl isobutyl ketone, methyl-n-hexyl ketone, methyl-n-propyl ketone, diisopropyl ketone, diisobutyl ketone, acetone, hexafluoroacetone, phenol, formic acid, methyl formate, ethyl formate, propyl formate, methyl benzoate, ethyl benzoate, propyl benzoate, methyl acetate, ethyl acetate, propyl acetate, dimethyl phthalate, diethyl phthalate, dipropyl phthalate, methyl chloride, ethyl chloride, methylene chloride, chloroform, o-chlorotoluene, p-chlorotoluene, carbon tetrachloride, 1,1-dichloroethane, 1,2-dichloroethane, trichloroethane, dichloropropane, methyl bromide, ethyl bromide, propyl bromide, acetic acid, benzene, toluene, hexane, cyclohexane, cyclohexanone, cyclopentane, o-xylene, p-xylene, m-xylene, acetonitrile, tetrahydrofuran, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, pyridine, and water. One selected from the above examples may be used or a plurality of kinds may be mixed. The solvent is not limited to employment of the substances exemplified above. 
         [0062]    The catalyst particles in a powder state, or in a slurry state in which the catalyst particles are dispersed in the above solvent, are charged into the carbon fiber precursor and dispersed in the mixture. The concentration of the catalyst particles in the carbon fiber can be controlled by adjusting a charging amount of the catalyst particles to be dispersed in the carbon fiber precursor into the carbon fiber precursor. Examples of the method for forming a fiber from the precursor include an electrospinning method and a wet spinning method. Carbonization is carried out in the atmosphere of an inert gas. Preferable examples of the inert gas include nitrogen and argon. Each carbon fiber precursor is preferably subjected to a pretreatment in an atmosphere and temperature conditions unique to the carbon fiber precursor before the carbonization. As to a carbonization temperature, it is suitable to carry out the carbonization at a high temperature in a range not exceeding a melting point of the catalyst particles to improve mechanical strength of the carbon fiber and reduce resistance. In the case of copper, for example, the carbonization is preferably carried out at 600° C. to 1000° C., more preferably 800° C. to 1000° C. 
         [0063]    The cathode electrode may be formed of only the carbon fiber or may have a laminate structure of a base material for holding the carbon fiber and a conductive layer for increasing conductivity of the electrode. The laminate structure includes, for example, a carbon fiber/conductive layer/base material structure, in which the conductive layer and the base material should not be in contact with the electrolyte solution, or the conductive layer and the base material should be formed of a material that is inert as a catalyst because the carbon fiber is structurally impregnated with the electrolyte solution. Examples of the conductive layer that satisfies the conditions include a transparent electrode membrane of ITO or the like, carbon, and various metals. Examples of the base material include glass, an epoxy resin, and a carbon substrate represented by Glassy Carbon (registered trademark). The carbon substrate is preferably used for achieving both the conductivity and catalytic inactivity. The carbon fiber preferably adheres to the conductive layer for increasing electric characteristics of the cathode electrode. Preferable examples of the adhesion method include a method of pressure-adhesion of the carbon fiber with the conductive layer and a method of using a binder such as Nafion (registered trademark) available from E. I. du Pont de Nemours and Company. The configuration of the cathode electrode is not particularly limited as long as of the cathode electrode has a form that provides an effect of reducing carbon dioxide. 
         [0064]    The cathode electrode is in contact with the first electrolyte solution. More accurately, the cathode electrode includes a region of the carbon fiber having the catalyst-containing structure, and the region is in contact with the first electrolyte solution. It is sufficient that if only a part of the cathode electrode is immersed in the first electrolyte solution as long as the region is in contact with the first electrolyte solution. 
         [0065]    The anode electrode includes a conductive substance. Examples of the conductive substance include carbon, platinum, gold, silver, copper, titanium, iridium oxide and an alloy thereof. A material of such a conductive substance is not particularly limited as long as the conductive substance is not decomposed by its own oxidation reaction. 
         [0066]    An oxidation reaction of water on the anode electrode and a reduction reaction of carbon dioxide on the cathode electrode are different independent reaction systems, and the reaction that occurs on the cathode electrode side is never affected by a material of the anode electrode. 
         [0067]    The anode electrode is in contact with the second electrolyte solution. More accurately, the conductive substance included in the anode electrode is in contact with the second electrolyte solution. It is sufficient that if only a part of the anode electrode is immersed in the second electrolyte solution as long as the conductive substance is in contact with the second electrolyte solution. 
         [0068]    The first electrolyte solution is stored in the cathode chamber. The first electrolyte solution is an electrolyte solution having a prescribed concentration, and examples of the first electrolyte solution include an aqueous potassium chloride solution, an aqueous sodium chloride solution, and an aqueous potassium hydrogen carbonate solution. The second electrolyte solution is stored in the anode chamber. The second electrolyte solution is an electrolyte solution having a prescribed concentration, and examples of the second electrolyte solution include an aqueous sodium hydroxide solution and an aqueous potassium hydrogen carbonate solution. 
         [0069]    The solid electrolyte membrane separates a chamber into the cathode chamber for storing the first electrolyte solution and the anode chamber for storing the second electrolyte solution, and is necessary for preventing components of the electrolyte solutions from mixing with each other. The solid electrolyte membrane allows protons to pass therethrough so that the first electrolyte solution on the cathode electrode side is electrically connected to the second electrolyte solution on the anode electrode side. The solid electrolyte membrane is, for example a Nafion (registered trademark) membrane available from E. I. du Pont de Nemours and Company. 
         [0070]    The reference electrode is for measuring the potential of the cathode electrode and is connected to the cathode electrode via a voltage measuring device. As the reference electrode, a silver/silver chloride electrode is used, for example. 
         [0071]    The exemplary embodiments described above are a two-solution system in which the solid electrolyte membrane is used to separate the cathode chamber for storing the first electrolyte solution from the anode chamber for storing the second electrolyte solution. In the exemplary embodiments, when an aqueous sodium chloride solution is used for both the first electrolyte solution and the second electrolyte solution, for example, an electrolysis product material is generated by an electrolysis reaction on the cathode electrode side, while such an electrode should be selected that does not generate a harmful chlorine gas on the anode electrode side. In a case of a one-solution system that includes no solid electrolyte membrane, a reverse reaction on the cathode electrode side occurs causing an electrolysis product material generated to oxidize and return to an electrolysis reactant, therefore such an additional device that immediately removes an electrolysis product material from the reaction system is necessary by, for example, externally constructing a circulating system of the solution. 
         [0072]    In the instant specification, the phrase “structure in which a carbon fiber contains a part of and/or a whole of a catalyst particle (i.e., catalyst-containing structure)” means (I) the cathode electrode includes a carbon fiber, (II) the carbon fiber includes a first catalyst particle and a second catalyst particle, (III) an inside of the carbon fiber is filled with carbon, (IV) at least a part of the first catalyst particle is located on a surface of the carbon fiber, and (V) the second catalyst particle is located in the inside of the carbon fiber so as to be surrounded by the carbon with which the inside of the carbon fiber is filled. 
         [0073]    More specifically, the cathode electrode includes carbon fiber  20  shown in  FIG. 9A . Carbon fiber  20  includes first catalyst particle  21  and second catalyst particle  22 . The inside of carbon fiber  20  is filled with carbon. Needless to say, the surface of carbon fiber  20  is also formed of carbon. 
         [0074]    At least a part of first catalyst particle  21  is located on the surface of carbon fiber  20 . Other part of first catalyst particle  21  may be embedded in carbon fiber  20 . In other words, a part of a region of first catalyst particle  21  is in contact with carbon fiber  20 ; however, the other part of the region of first catalyst particle  21  is exposed so as not to be in contact with carbon fiber  20 . Carbon fiber  20  may include a plurality of first catalyst particles  21 . 
         [0075]    On the other hand, second catalyst particle  22  is located in the inside of carbon fiber  20 . In other words, second catalyst particle  22  is surrounded by the carbon with which the inside of carbon fiber  20  is filled.  FIG. 9B  shows a cross-sectional view taken along the line  9 B- 9 B included in  FIG. 9A . As shown in  FIG. 9B , the whole surface of second catalyst particle  22  is in contact with the carbon with which the inside of carbon fiber  20  is filled. Carbon fiber  20  may include a plurality of second catalyst particles  22 . 
         [0076]    Such carbon fiber  20  improves generation efficiency of the product material. Carbon fiber  20  may be formed by an electrospinning method. 
       Method for Generating Electrolysis Product Material 
       [0077]    A method for generating an electrolysis product material with the above-mentioned electrolysis device is described. 
         [0078]    The electrolysis device may be placed at room temperature and atmospheric pressure, and a high-pressure cell may also be used for accelerating a reaction. 
         [0079]    The external power source applies a voltage to the cathode electrode so that the cathode electrode has a negative potential with respect to a potential of the anode electrode. A value of the voltage applied by the external power source has a threshold necessary for obtaining a reaction for generating an electrolysis product material. This threshold varies depending on, for example, a distance between the cathode electrode and the anode electrode, a type of a material that constitutes the cathode electrode or the anode electrode, and a concentration of the first electrolyte solution. 
         [0080]    A part of the voltage applied to the cathode electrode with respect to the anode electrode is consumed for an oxidation reaction of water on the anode electrode. By using configurations as shown in  FIGS. 1 and 2 , the voltage actually applied to the cathode electrode can be acquired more accurately. The potential of the cathode electrode with respect to a potential of the reference electrode varies depending on a type of a material that constitutes the reference electrode, and, for example, the potential of the cathode electrode is preferably not more than −1.2 V for a reduction reaction of carbon dioxide, not more than −0.8 V for a hydrogen generation reaction, and not more than 0.6 V for an oxygen reduction reaction with respect to a silver/silver chloride electrode. 
         [0081]    As described above, application of an appropriate voltage to the cathode electrode allows reduction of an electrolysis reactant contained in the first electrolyte solution on the cathode electrode. As a result, an electrolysis product material is generated on the surface of the cathode electrode. 
         [0082]    It is suitable to use the solid electrolyte membrane to separate the electrolyte solution on the cathode side from the electrolyte solution on the anode side. 
         [0083]    A reaction current flows through the cathode electrode in response to a reduction reaction of an electrolysis reactant on the surface of the cathode electrode and an oxidation reaction of water on the surface of the anode electrode in the electrolysis device. As shown in  FIGS. 1 and 2 , incorporation of a current measuring device allows monitoring an amount of the reaction current. 
       EXAMPLES 
       [0084]    The present disclosure is described in more detail with reference to Examples below. 
       Example 1 
     Production of Cathode Electrode 
       [0085]    A cathode electrode for an electrolysis device, according to the present disclosure was produced. The cathode electrode includes a carbon fiber having a catalyst-containing structure. 
         [0086]    First, a solution of polyamic acid in N-methylpyrrolidone (manufactured by Ube Industries, Ltd., U-Vanish-A) was used as a precursor solution of a carbon fiber. CuO particles (mean particle size: 20 nm) were used as catalyst particles. As the catalyst particles, used were slurry catalyst particles (manufactured by CIK NanoTek Corporation, CUAP15 Wt %-G180) obtained by dispersing the catalyst particles in an organic solvent mainly consisting of ethanol. The content of CuO in the slurry was 15% by weight ratio. The precursor solution and the catalyst particle solution were mixed at a weight ratio of 9:1 to give a precursor solution of a carbon fiber, in which the catalyst particles were dispersed. 
         [0087]    The precursor solution of a carbon fiber, in which the catalyst particles were dispersed, was subjected to spinning by an electrospinning method to form a fiber of the precursor. The spun fiber was placed on a Glassy Carbon (diameter 25 mm×thickness 0.5 mm) substrate, and was subjected to a pretreatment in an argon (Ar) atmosphere according to a temperature profile shown in  FIG. 3 . Then, the temperature was further raised to 800° C. in the Ar atmosphere and was held for 30 minutes to carry out carbonization of the fiber. 
         [0088]    As a result of observing a surface of the carbon fiber with a scanning electron microscope, several tens of nm particles were confirmed to be present in the carbon fiber having a diameter ranges from several hundred nm to several μm ( FIG. 4A ). As a result of observation with a transmission electron microscope, it was made apparent that the carbon fiber contained a part of and/or a whole of each particle ( FIG. 4B ). This particle was confirmed to be Cu by elemental analysis. It is considered that CuO was reduced in the Ar atmosphere to be Cu. As a result of thermal analysis measurement, a concentration of the Cu particles in the carbon fiber was found out to be about 12 wt %. 
         [0089]    Then, the obtained carbon fiber/Glassy Carbon substrate with the carbon fiber having a catalyst-containing structure was bonded onto a glass plate with a metal sheet (aluminum) interposed between the substrate and the plate. Subsequently, an exposed surface of the metal sheet was covered with an epoxy resin for avoiding contact of the metal sheet with an electrolyte solution, to produce the cathode electrode according to the present disclosure. 
       Assembly of Device 
       [0090]    The electrolysis device shown in  FIG. 2  was produced, the device comprising the cathode electrode described above. The configuration of the electrolysis device according to the present example is as follows. 
         [0091]    Cathode electrode: carbon fiber/Glassy Carbon substrate (surface area: 5 cm 2 ) with the carbon fiber having a catalyst-containing structure 
         [0092]    Anode electrode: platinum 
         [0093]    Distance between electrodes: about 8 cm 
         [0094]    Reference electrode: silver/silver chloride 
         [0095]    Cathode side electrolyte solution: aqueous 0.5 mol/L potassium chloride solution 
         [0096]    Anode side electrolyte solution: aqueous 0.5 mol/L potassium hydrogen carbonate solution 
         [0097]    Solid electrolyte membrane: Nafion membrane (manufactured by E. I. du Pont de Nemours and Company, Nafion 424) 
         [0098]    Supply of carbon dioxide to the cathode side electrolyte solution was carried out by conducting a bubbling treatment of a carbon dioxide gas (flow rate of carbon dioxide: 200 mL/min) through a tube for 30 minutes. 
         [0099]    A chamber was sealed after dissolution of carbon dioxide in the cathode side electrolyte solution. Then, a voltage was applied to the cathode electrode with a potentiostat so that the cathode electrode had a negative potential with respect to a potential of the anode electrode. A value of the applied voltage was controlled by the potentiostat so that the cathode electrode had a potential of −1.8 V with respect to the reference electrode. 
         [0100]    After the application of the voltage for a certain period of time, a type and an amount of a reaction product material generated in the cathode chamber were measured according to gas chromatography and liquid chromatography. As a result, hydrogen (H 2 ), carbon monoxide (CO), formic acid (HCOOH), methane (CH 4 ), ethylene (C 2 H 4 ) aldehydes, and alcohols were detected as reduction product materials of carbon dioxide. That is, a hydrocarbon represented by methane and ethylene was confirmed to be generated by using for the cathode electrode the carbon fiber having the carbon-containing structure. Further, as a result of observing a surface of the carbon fiber with a scanning electron microscope after the electrolysis, catalyst particles were confirmed to be present in the carbon fiber as shown in  FIG. 5 . 
         [0101]    In Example 1, a generation amount of a hydrocarbon per electrolysis time of 1000 seconds was 24.2 μmol. The electrolysis time is equal to a time during which the external power source applied a voltage to the cathode electrode. In addition, generation efficiency (Faradaic efficiency) of a hydrocarbon in Example 1 was not less than 50% and the hydrocarbon was confirmed to be selectively generated. Faradaic efficiency means a ratio of an electric charge used for generating a product material to a whole reaction electric charge, and is calculated by (Faradaic efficiency for generating product material)=(reaction electric charge used for generating product material)/(whole reaction electric charge)×100 [%]. 
       Example 2 
       [0102]    The same experiment as in Example 1 was carried out except for controlling the potentiostat to make the cathode electrode have a potential of −2.0 V with respect to the reference electrode. 
         [0103]    As a result, a hydrocarbon was confirmed to be generated as a reduction product material of carbon dioxide. Further, it was also confirmed that the catalyst particles were present in the carbon fiber after the electrolysis. 
       Example 3 
       [0104]    The same experiment as in Example 1 was carried out except for using an aqueous 3 mol/L potassium chloride solution as the cathode side electrolyte solution. 
         [0105]    As a result, a hydrocarbon was confirmed to be generated as a reduction product material of carbon dioxide. Further, it was also confirmed that the catalyst particles were present in the carbon fiber after the electrolysis. 
       Example 4 
       [0106]    The same experiment as in Example 1 was carried out except for using an aqueous 0.5 mol/L potassium hydrogen carbonate solution as the cathode side electrolyte solution. 
         [0107]    As a result, a hydrocarbon was confirmed to be generated as a reduction product material of carbon dioxide. Further, it was also confirmed that the catalyst particles were present in the carbon fiber after the electrolysis. 
       Example 5 
       [0108]    The same experiment as in Example 1 was carried out except for using particles having a mean particle size of 100 nm as the CuO particles serving as the catalyst particles. 
         [0109]    As a result, a hydrocarbon was confirmed to be generated as a reduction product material of carbon dioxide. Further, it was also confirmed that the catalyst particles were present in the carbon fiber after the electrolysis. 
       Example 6 
       [0110]    The same experiment as in Example 1 was carried out except for making the concentration of the catalyst particles in the carbon fiber 20% by weight ratio. 
         [0111]    As a result, a hydrocarbon was confirmed to be generated as a reduction product material of carbon dioxide. Further, it was also confirmed that the catalyst particles were present in the carbon fiber after the electrolysis. 
       Example 7 
       [0112]    The same experiment as in Example 1 was carried out except for making the concentration of the catalyst particles in the carbon fiber 5% by weight ratio. 
         [0113]    As a result, a hydrocarbon was confirmed to be generated as a reduction product material of carbon dioxide. Further, it was also confirmed that the catalyst particles were present in the carbon fiber after the electrolysis. 
       Comparative Example 1 
       [0114]    The same experiment as in Example 1 was carried out except for using, as the cathode electrode, a carbon fiber/Glassy Carbon electrode with the carbon fiber not including catalyst particles. 
         [0115]    As a result, H 2 , CO, and HCOOH were detected, while a hydrocarbon was not detected. That is, a hydrocarbon was not generated in Comparative Example 1. 
       Comparative Example 2 
       [0116]    The same experiment as in Example 1 was carried out except for using, as the cathode electrode, a carbon fiber/Glassy Carbon electrode with the carbon fiber including no catalyst particle and having Cu deposited only on a surface of the carbon fiber by a solution method.  FIG. 6A  is a scanning electron micrograph before the experiment in Comparative Example 2. 
         [0117]    As a result, the generation efficiency of a hydrocarbon decreased to about a hundredth as compared to the case of Example 1. Further, as a result of observing with a scanning electron microscope the surface of the carbon fiber after the experiment, the catalyst particles were hardly present as shown in  FIG. 6B . It is considered that the catalyst particles on the surface of the carbon fiber were removed during the electrolysis. 
         [0118]    The generation efficiency of a hydrocarbon in Examples 1 to 6 and Comparative Examples 1 and 2 described above is shown in  FIG. 7 . As shown in  FIG. 7 , only in the case in which the carbon fiber/Glassy Carbon substrate with the carbon fiber having a catalyst-containing structure was used as the cathode electrode, a hydrocarbon was selectively generated. This shows that the catalyst particles contained in the carbon fiber selectively reduced carbon dioxide without being removed during the electrolysis. 
       Example 8 
       [0119]    The same experiment as in Example 1 was carried out except for using gold particles (mean particle size: 20 nm) as the catalyst particles. 
         [0120]    As a result, carbon monoxide was confirmed to be generated as a reduction product material of carbon dioxide. It was also confirmed that the catalyst particles were present in the carbon fiber after the electrolysis. 
         [0121]    A generation amount of carbon monoxide per unit time when the gold particles were used is shown in  FIG. 8A . A rate of generation of carbon monoxide was about 67 times as high as the case in which gold particles were not present (Comparative Example 1). This shows that the catalyst particles in the carbon fiber selectively reduced carbon dioxide and generated carbon monoxide. 
       Example 9 
       [0122]    The same experiment as in Example 1 was carried out except for using gallium oxide particles (mean particle size: 50 nm) as the catalyst particles. 
         [0123]    As a result, formic acid was confirmed to be generated as a reduction product material of carbon dioxide. It was also confirmed that the catalyst particles were present in the carbon fiber after the electrolysis. 
         [0124]    A generation amount of formic acid per unit time when the gallium oxide particles were used is shown in  FIG. 8B . A rate of generation of formic acid was about 11 times as high as the case in which gallium oxide particles were not present (Comparative Example 1). This shows that the catalyst particles in the carbon fiber selectively reduced carbon dioxide and generated formic acid. 
       Example 10 
       [0125]    The same experiment as in Example 1 was carried out except for using platinum particles (mean particle size: 20 nm) as the catalyst particles, conducting a bubbling treatment with Ar, and controlling the potentiostat to make the cathode electrode have a potential of −0.8 V with respect to the reference electrode. 
         [0126]    As a result, hydrogen was confirmed to be generated as a reduction product material of water. Further, it was also confirmed that the catalyst particles were present in the carbon fiber after the electrolysis. 
       Comparative Example 3 
       [0127]    The same experiment as in Example 1 was carried out except for using, as the cathode electrode, a carbon fiber/Glassy Carbon electrode with the carbon fiber including no catalyst particle and having Pt deposited only on a surface of the carbon fiber by a solution method, and conducting a bubbling treatment with Ar. 
         [0128]    As a result, a generation amount of hydrogen per unit time decreased to about a thirteenth as compared to the case of Example 10. Further, the catalyst particles were hardly present on the surface of the carbon fiber after the experiment. 
         [0129]    A generation amount of hydrogen per unit time when the platinum particles were used is shown in  FIG. 8C . A rate of generation of hydrogen was about 50 times and 12 times as high as the case in which platinum particles were not present (Comparative Example 1) and the case in which platinum particles were present only on the surface of the carbon fiber (Comparative Example 3), respectively. This shows that the catalyst particles in the carbon fiber selectively reduced water without being removed during the electrolysis and generated hydrogen. 
         [0130]    The present disclosure provides a novel cathode electrode, a novel device, and a novel method, for reducing an electrolysis reactant to generate an electrolysis product material. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100 ,  200  electrolysis device 
           11  first electrolyte solution 
           12  cathode chamber 
           13  cathode electrode 
           14  second electrolyte solution 
           15  anode chamber 
           16  solid electrolyte membrane 
           17  anode electrode 
           18  external power source 
           19  reference electrode 
           1  tube