APPARATUS FOR PRODUCING ORGANIC HYDRIDE

Abstract

An apparatus for producing an organic hydride includes: an anode electrode that generates protons by oxidizing water; a cathode electrode that generates an organic hydride by hydrogenating a substance to be hydrogenated with the protons; an electrolyte membrane that has an EW of less than 980 and is arranged between the anode electrode and the cathode electrode so as to transfer the protons from the anode electrode side to the cathode electrode side; and a low water content layer that is arranged between the electrolyte membrane and the cathode electrode and that has a lower water content than that of the electrolyte membrane.

Claims

1. An apparatus for producing an organic hydride comprising: an anode electrode that generates protons by oxidizing water; a cathode electrode that generates an organic hydride by hydrogenating a substance to be hydrogenated with the protons; an electrolyte membrane that has an equivalent weight (EW) of less than 980 and is arranged between the anode electrode and the cathode electrode so as to transfer the protons from the anode electrode side to the cathode electrode side; and a low water content layer that is arranged between the electrolyte membrane and the cathode electrode and that has a lower water content than that of the electrolyte membrane.

2. The apparatus for producing an organic hydride according to claim 1, comprising: a high water content layer that is arranged between the electrolyte membrane and the anode electrode and that has a higher water content than that of the electrolyte membrane.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0010] Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:

[0011] FIG. 1 is a schematic diagram of an organic hydride production system according to an embodiment;

[0012] FIG. 2 is a cross-sectional view of an apparatus for producing an organic hydride; and

[0013] FIG. 3 is a diagram showing the characteristics of respective electrolytic cells according to exemplary embodiments and comparative examples, the improvement rate of the Faradaic efficiency, and the amount of change in voltage.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, the present invention will be described based on preferred embodiments with reference to the drawings. The embodiments do not limit the technical scope of the present invention and are shown for illustrative purposes, and not all the features described in the embodiments and combinations thereof are necessarily essential to the invention. Therefore, regarding the details of the embodiments, many design modifications such as change, addition, deletion, etc., of the constituent elements may be made without departing from the spirit of the invention defined in the claims. New embodiments resulting from added design change will provide the advantages of the embodiments and variations that are combined.

[0015] In the embodiments, the details for which such design change is possible are emphasized with the notations according to the embodiment, in the embodiment, etc. However, design change is also allowed for those without such notations. Optional combinations of the constituting elements described in the embodiments are also valid as embodiments of the present invention. The same or equivalent constituting elements, members, and processes illustrated in each drawing shall be denoted by the same reference numerals, and duplicative explanations will be omitted appropriately. The scales and shapes of parts shown in each figure are set for the sake of convenience in order to facilitate the explanation and shall not be interpreted in a limited manner unless otherwise mentioned. Terms like first, second, etc., used in the specification and claims do not indicate an order or importance by any means and are used to distinguish a certain feature from the others. Some of the components in each figure may be omitted if they are not important for explanation.

[0016] FIG. 1 is a schematic diagram of an organic hydride production system 1 according to an embodiment. The organic hydride production system 1 as an example includes an apparatus 2 for producing an organic hydride, an anolyte supply device 4, and a catholyte supply device 6. In FIG. 1, a part of the structure of the apparatus 2 for producing an organic hydride is omitted. Although only one apparatus 2 for producing an organic hydride is shown in FIG. 1, the organic hydride production system 1 may include a plurality of apparatuses 2 for producing an organic hydride. In this case, the respective apparatuses 2 for producing an organic hydride are stacked in the same direction such that the anode electrode 10 and the cathode electrode 12 are arranged in the same direction, and electrically connected in series. Note that the apparatuses 2 for producing an organic hydride may be connected in parallel, or may be a combination of series connection and parallel connection.

[0017] The apparatus 2 for producing an organic hydride is an electrolysis cell for generating an organic hydride by hydrogenating a substance to be hydrogenated, which is a dehydrogenated product of the organic hydride, by an electrochemical reduction reaction. The apparatus 2 for producing an organic hydride includes a membrane electrode assembly 8, a pair of plate members 16a and 16b, and a pair of gaskets 18a and 18b. The membrane electrode assembly 8 includes an anode electrode 10 (anode), a cathode electrode 12 (cathode), and an electrolyte membrane 14.

[0018] The anode electrode 10 generates protons by oxidizing water. The anode electrode 10 has, for example, a metal such as iridium (Ir), ruthenium (Ru), or platinum (Pt), or a metal oxide thereof as an anode catalyst that oxidizes water. The anode catalyst may be dispersedly supported or coated on a base material having electron conductivity. The base material includes a material containing, for example, a metal such as titanium (Ti) or stainless steel (SUS) as a main component. Examples of the form of the base material include a woven fabric sheet or a nonwoven fabric sheet, a mesh, a porous sintered body, a foamed molded body (foam), and an expanded metal.

[0019] The cathode electrode 12 generates an organic hydride by hydrogenating a substance to be hydrogenated with protons. The cathode electrode 12 contains, for example, platinum or ruthenium as a cathode catalyst for hydrogenating the substance to be hydrogenated. It is preferable that the cathode electrode 12 also contains a porous catalyst support that supports the cathode catalyst. The catalyst support includes an electron-conductive material such as porous carbon, a porous metal, or a porous metal oxide.

[0020] Furthermore, the cathode catalyst is coated with an ionomer (cation exchange ionomer). For example, the catalyst support, which is in the state of supporting the cathode catalyst, is coated with an ionomer. Examples of the ionomer include a perfluorosulfonic acid polymer such as Nafion (registered trademark), Flemion (registered trademark), Fumion (registered trademark), or Aciplex (registered trademark). It is preferable that the cathode catalyst is partially coated with the ionomer. As a result, it is possible to efficiently supply three elements (the substance to be hydrogenated, a proton, and an electron) necessary for an electrochemical reaction in the cathode electrode 12 to the reaction field.

[0021] The cathode electrode 12 according to the present embodiment has a catalyst layer 12a and a diffusion layer 12b. The catalyst layer 12a is disposed closer to the electrolyte membrane 14 than the diffusion layer 12b. The catalyst layer 12a contains the cathode catalyst, the catalyst support, and the ionomer described above. The diffusion layer 12b is in contact with a main surface of the catalyst layer 12a on a side opposite to the electrolyte membrane 14. The diffusion layer 12b uniformly diffuses the substance to be hydrogenated supplied from the outside into the catalyst layer 12a. The organic hydride generated in the catalyst layer 12a is discharged to the outside of the cathode electrode 12 through the diffusion layer 12b. The diffusion layer 12b includes a conductive material such as carbon or a metal. In addition, the diffusion layer 12b is a porous body such as a sintered body of fibers or particles or a foamed molded body. Examples of the material included in the diffusion layer 12b include a carbon woven fabric (carbon cloth), a carbon nonwoven fabric, and carbon paper. Note that the diffusion layer 12b may be omitted.

[0022] The electrolyte membrane 14 is disposed between the anode electrode 10 and the cathode electrode 12. The electrolyte membrane 14 moves protons from the anode electrode 10 side to the cathode electrode 12 side. The electrolyte membrane 14 as an example is composed of a solid polymer electrolyte membrane having protonic conductivity.

[0023] The plate member 16a and the plate member 16b are made of a metal such as stainless steel or titanium, for example. The plate member 16a is stacked on the membrane electrode assembly 8 from the side of the anode electrode 10. The plate member 16b is stacked on the membrane electrode assembly 8 from the side of the cathode electrode 12. Accordingly, the membrane electrode assembly 8 is sandwiched between the pair of plate members 16a and 16b. A gap between the plate member 16a and the membrane electrode assembly 8 is sealed with the gasket 18a. A gap between the plate member 16b and the membrane electrode assembly 8 is sealed with the gasket 18b. When the organic hydride production system 1 includes only one apparatus 2 for producing an organic hydride, the pair of plate members 16a and 16b can correspond to so-called end plates. When the organic hydride production system 1 includes a plurality of apparatuses 2 for producing an organic hydride, and another apparatus 2 for producing an organic hydride is arranged next to the plate member 16a or the plate member 16b, the plate member can correspond to a so-called separator.

[0024] The anode flow path 20 is connected to the anode electrode 10. The anode flow path 20 feeds and discharges the anolyte LA to and from the anode electrode 10. A groove may be provided on a main surface facing the anode electrode 10 side in the plate member 16a, and this grove may constitute the anode flow path 20.

[0025] The cathode flow path 22 is connected to the cathode electrode 12. The cathode flow path 22 feeds and discharges the catholyte LC to and from the cathode electrode 12. A groove may be provided on a main surface facing the cathode electrode 12 side in the plate member 16b, and this grove may constitute the cathode flow path 22.

[0026] The anolyte LA is supplied to the anode electrode 10 by the anolyte supply device 4. The anolyte supply device 4 includes an anolyte tank 24, a first anode pipe 26, a second anode pipe 28, and an anode pump 30. The anolyte LA is stored in the anolyte tank 24. The anolyte LA contains water to be supplied to the anode electrode 10. Examples of the anolyte LA include an aqueous sulfuric acid solution, an aqueous nitric acid solution, an aqueous hydrochloric acid solution, pure water, and ion-exchanged water.

[0027] The anolyte tank 24 is connected to the anode electrode 10 by the first anode pipe 26. One end of the first anode pipe 26 is connected to the anolyte tank 24, and the other end of the first anode pipe 26 is connected to the anode flow path 20. The anode pump 30 is provided in the middle of the first anode pipe 26. The anode pump 30 can be constituted by a known pump such as a gear pump or a cylinder pump, for example. Note that the anolyte supply device 4 may circulate the anolyte LA using a liquid feeding device other than the pump. The anolyte tank 24 is connected to the anode electrode 10 by the second anode pipe 28. One end of the second anode pipe 28 is connected to the anode flow path 20, and the other end of the second anode pipe 28 is connected to the anolyte tank 24.

[0028] The anolyte LA in the anolyte tank 24 flows into the anode electrode 10 through the first anode pipe 26 by driving of the anode pump 30. The anolyte LA flowing into the anode electrode 10 is subjected to an electrode reaction in the anode electrode 10. The anolyte LA in the anode electrode 10 is returned to the anolyte tank 24 through the second anode pipe 28. As an example, the anolyte tank 24 also functions as a gas-liquid separator. In the anode electrode 10, oxygen gas is generated by the electrode reaction. Therefore, the oxygen gas is mixed into the anolyte LA discharged from the anode electrode 10. The anolyte tank 24 separates the oxygen gas in the anolyte LA from the anolyte LA and discharges the oxygen gas to the outside of the system.

[0029] In the anolyte supply device 4 according to the present embodiment, the anolyte LA is circulated between the anode electrode 10 and the anolyte tank 24. However, the present invention is not limited to this configuration, and the anolyte LA may be sent from the anode electrode 10 to the outside of the system without being returned to the anolyte tank 24.

[0030] The catholyte LC is supplied to the cathode electrode 12 by the catholyte supply device 6. The catholyte supply device 6 includes a catholyte tank 32, a first cathode pipe 34, a second cathode pipe 36, a third cathode pipe 38, a cathode pump 40, and a separator 42. The catholyte LC is stored in the catholyte tank 32. The catholyte LC contains an organic hydride raw material (substance to be hydrogenated) to be supplied to the cathode electrode 12. As an example, the catholyte LC does not contain an organic hydride before the start of the operation of the organic hydride production system 1, and after the start of the operation, the organic hydride generated by electrolysis is mixed in, whereby the catholyte becomes the liquid mixture of the substance to be hydrogenated and the organic hydride. The substance to be hydrogenated and the organic hydride are preferably a liquid at 20 C. and 1 atm.

[0031] The substance to be hydrogenated and the organic hydride are not particularly limited as long as they are organic compounds capable of adding/desorbing hydrogen by reversibly causing a hydrogenation reaction/dehydrogenation reaction. As the substance to be hydrogenated and the organic hydride used in the present embodiment, an acetone-isopropanol type, a benzoquinone-hydroquinone type, an aromatic hydrocarbon type, and the like can be widely used. Among these, the aromatic hydrocarbon type is preferable from the viewpoint of transportability during energy transport or the like. In general, aromatic hydrocarbon-based substances to be hydrogenated and organic hydrides are hydrophobic.

[0032] An aromatic hydrocarbon compound used as the substance to be hydrogenated is a compound containing at least one aromatic ring. Examples of the aromatic hydrocarbon compound include benzene, alkylbenzene, naphthalene, alkylnaphthalene, anthracene, and diphenylethane. The alkylbenzene contains a compound in which 1 to 4 hydrogen atoms in the aromatic ring are substituted with a linear alkyl group or a branched alkyl group having 1 to 6 carbons. Examples of such a compound include toluene, xylene, mesitylene, ethylbenzene, and diethylbenzene. The alkylnaphthalene contains a compound in which 1 to 4 hydrogen atoms in the aromatic ring are substituted with a linear alkyl group or a branched alkyl group having 1 to 6 carbons. Examples of such a compound include methylnaphthalene. These compounds may be used alone or in combination.

[0033] The substance to be hydrogenated is preferably at least one of toluene and benzene. It is also possible to use a nitrogen-containing heterocyclic aromatic compound such as quinoline, isoquinoline, N-alkylpyrrole, N-alkylindole, or N-alkyldibenzopyrrole as the substance to be hydrogenated. The organic hydride is obtained by hydrogenating the above-described substance to be hydrogenated, and examples thereof include cyclohexane, methylcyclohexane, dimethylcyclohexane, and decahydroquinoline.

[0034] The catholyte tank 32 is connected to the cathode electrode 12 by the first cathode pipe 34. One end of the first cathode pipe 34 is connected to the catholyte tank 32, and the other end of the first cathode pipe 34 is connected to the cathode flow path 22. The cathode pump 40 is provided in the middle of the first cathode pipe 34. The cathode pump 40 can by constituted by a known pump such as a gear pump or a cylinder pump, for example. Note that the catholyte supply device 6 may circulate the catholyte LC using a liquid feeding device other than the pump.

[0035] The separator 42 is connected to the cathode electrode 12 by the second cathode pipe 36. One end of the second cathode pipe 36 is connected to the cathode flow path 22, and the other end of the second cathode pipe 36 is connected to the separator 42. The separator 42 has a known gas-liquid separator and a known oil-water separator. The separator 42 is connected to the catholyte tank 32 by the third cathode pipe 38. One end of the third cathode pipe 38 is connected to the separator 42, and the other end of the third cathode pipe 38 is connected to the catholyte tank 32.

[0036] The catholyte LC in the catholyte tank 32 flows into the cathode electrode 12 through the first cathode pipe 34 by driving of the cathode pump 40. The catholyte LC flowing into the cathode electrode 12 is subjected to an electrode reaction in the cathode electrode 12. The catholyte LC in the cathode electrode 12 flows into the separator 42 through the second cathode pipe 36. The hydrogen gas may be generated by the side reaction in the cathode electrode 12. Therefore, the hydrogen gas may be mixed in the catholyte LC discharged from the cathode electrode 12. The separator 42 separates the hydrogen gas in the catholyte LC from the catholyte LC and discharges the hydrogen gas to the outside of the system. In addition, water moves from the anode electrode 10 to the cathode electrode 12 together with protons. Therefore, the water may be mixed in the catholyte LC discharged from the cathode electrode 12. The separator 42 separates the water in the catholyte LC from the catholyte LC and discharges the water to the outside of the system. The catholyte LC from which the hydrogen gas and the water have been separated is returned to the catholyte tank 32 through the third cathode pipe 38.

[0037] In the catholyte supply device 6 according to the present embodiment, the catholyte LC is circulated between the cathode electrode 12 and the catholyte tank 32. However, the present invention is not limited to this configuration, and the catholyte LC may be sent to the outside of the system from the cathode electrode 12 without being returned to the catholyte tank 32.

[0038] A reaction that occurs in a case where toluene (TL) is used as an example of the substance to be hydrogenated in the apparatus 2 for producing an organic hydride is as follows. The organic hydride obtained in a case where toluene is used as the substance to be hydrogenated is methylcyclohexane (MCH).

<Electrode Reaction in Anode Electrode>

##STR00001##

<Electrode Reaction in Cathode Electrode>

##STR00002##

[0039] That is, the electrode reaction in the anode electrode 10 and the electrode reaction in the cathode electrode 12 proceed in parallel. The protons generated by electrolysis of water at the anode electrode 10 pass through the electrolyte membrane 14 together with water molecules and move to the cathode electrode 12. Electrons generated by electrolysis of water are supplied to the cathode electrode 12 via an external circuit. The protons and electrons supplied to the cathode electrode 12 are used for the hydrogenation of toluene in the cathode electrode 12. As a result, methylcyclohexane is generated.

[0040] Therefore, according to the organic hydride production system 1 according to the present embodiment, the electrolysis of water and the hydrogenation reaction of the substance to be hydrogenated can be performed in one step. For this reason, organic hydride production efficiency can be increased as compared with a conventional technique in which the organic hydride is produced by a two-step process which includes a process of producing hydrogen by water electrolysis or the like and a process of chemically hydrogenating the substance to be hydrogenated in a reactor such as a plant. Furthermore, since the reactor for performing the chemical hydrogenation and a high-pressure vessel for storing the hydrogen produced by the water electrolysis or the like are not required, a significant reduction in facility cost can be achieved.

[0041] In the cathode electrode 12, the following hydrogen gas generation reaction may occur as a side reaction together with the hydrogenation reaction of the substance to be hydrogenated which is the main reaction. As the supply amount of the substance to be hydrogenated to the cathode electrode 12 becomes insufficient, this side reaction is likely to occur.

<Side Reaction That Can Occur in Cathode Electrode>

##STR00003##

[0042] The apparatus 2 for producing an organic hydride is supplied with power from an external power supply (not shown). When power is supplied from the power supply to the apparatus 2 for producing an organic hydride, a predetermined cell voltage is applied between the anode electrode 10 and the cathode electrode 12 of the apparatus 2 for producing an organic hydride, and an electrolytic current flows. The power supply sends power supplied from a power supply device to the apparatus 2 for producing an organic hydride. The power supply device can be constituted by a power generation device that generates power using renewable energy, for example, a wind power generation device, a solar power generation device, or the like. Note that the power supply device is not limited to the power generation device using renewable energy, and may be a system power supply, a storage device storing power from the power generation device using renewable energy or the system power supply, or the like. A combination of two or more of these devices may be used. Further, the configuration of the organic hydride production system 1 is not limited to those described above, and the configuration of each part can be appropriately changed.

[0043] Next, a structure of the apparatus 2 for producing an organic hydride will be described in detail. FIG. 2 is a cross-sectional view of the apparatus 2 for producing an organic hydride. The apparatus 2 for producing an organic hydride according to the present embodiment includes a low water content layer 44 and a high-water content layer 46 in addition to the above-described configuration.

[0044] The electrolyte membrane 14 has an equivalent weight (EW) of less than 980. EW is the dry mass of the electrolyte per mol of sulfonic acid groups in the electrolyte membrane 14. The lower the EW, the more hydrophilic sulfonic acid groups the electrolyte membrane 14 has, and therefore the higher the water content. The EW of the electrolyte membrane 14 is preferably 950 or less, more preferably 900 or less, and even more preferably 870 or less. For example, the electrolyte membrane 14 is formed of a polymer having an EW of less than 980. Examples of the polymer that can be used for the electrolyte membrane 14 include a perfluorosulfonic acid polymer and the like. By lowering the EW of the electrolyte membrane 14 to less than 980, the water content of the electrolyte membrane 14 can be increased compared to a case where the EW is 980 or more. Thereby, the ion transfer resistance of the electrolyte membrane 14 can be reduced. Therefore, the cell voltage in the organic hydride production can be reduced.

[0045] Therefore, by increasing the water content of the electrolyte membrane 14, the resistance of the electrolyte membrane 14 is reduced, and the current density can be increased while suppressing the increase in the cell voltage. On the other hand, when the water content of the electrolyte membrane 14 is increased, the affinity of the electrolyte membrane 14 for the substance to be hydrogenated decreases. For this reason, the hydrophobic substance to be hydrogenated is less likely to be supplied to the reaction field. As a result, side reactions are likely to occur due to insufficient substance to be hydrogenated in the reaction field. Therefore, the efficiency of the electrode reaction at the cathode electrode 12, that is, the Faradaic efficiency, can be reduced.

[0046] On the other hand, in the present embodiment, the low water content layer 44 is arranged between the electrolyte membrane 14 and the cathode electrode 12. One main surface of the low water content layer 44 is in contact with the electrolyte membrane 14, and the other main surface of the low water content layer 44 is in contact with the catalyst layer 12a. The low water content layer 44 has a lower water content (higher EW) than that of the electrolyte membrane 14 when the low water content layer 44 has ion exchange ability and is therefore more hydrophobic than the electrolyte membrane 14. For example, the low water content layer 44 is formed of a polymer (for example, ionomer) having a lower water content than that of the polymer constituting the electrolyte membrane 14. The water content (%) in the present embodiment is defined by the following equation (1). The expression polymer in hydrous state in equation (1) means, for example, a polymer after immersion in pure water for one hour.

[00001]$\begin{array}{cc}\mathrm{water}\mathrm{content}=\left(\mathrm{weight}\mathrm{of}\mathrm{water}\mathrm{contained}\mathrm{in}\mathrm{polymer}/\mathrm{weight}\mathrm{of}\mathrm{polymer}\mathrm{in}\mathrm{hydrous}\mathrm{state}\right)100& \left(1\right)\end{array}$

[0047] The method for forming the low water content layer 44 is not particularly limited, and a known method can be employed. For example, a method may be employed where a polymer constituting the low water content layer 44 is applied to the surface of the electrolyte membrane 14 or the surface of the cathode electrode 12 or where a thin film of the polymer is pressed on the surface of the electrolyte membrane 14 or the surface of the cathode electrode 12.

[0048] Examples of the polymer that can be used for the low water content layer 44 include Nafion (registered trademark), Fumion (registered trademark), and the like. The low water content layer 44 may or may not function as an ion exchange membrane. By making the water content of the low water content layer 44 lower than the water content of the electrolyte membrane 14, it is possible to make it easier for the substance to be hydrogenated to reach the reaction field. Thereby, the shortage of the substance to be hydrogenated can be avoided, and the occurrence of side reactions can be suppressed.

[0049] That is, the combination of setting the EW of the electrolyte membrane 14 to less than 980 and installing the low water content layer 44 between the electrolyte membrane 14 and the cathode electrode 12 allows for both the suppression of the increase in the cell voltage in the organic hydride production and the suppression of the decrease in Faradaic efficiency. Further, since the current density can be easily increased, the production efficiency per unit time of the organic hydride can be improved. Further, the miniaturization and the like of the apparatus 2 for producing an organic hydride can be realized, and the cost of members for the apparatus 2 for producing an organic hydride can be thereby reduced. Further, by suppressing the increase in the cell voltage, the cost of measures against heat generation required for the apparatus 2 for producing an organic hydride can be reduced.

[0050] In the present embodiment, the high-water content layer 46 is arranged between the electrolyte membrane 14 and the anode electrode 10. One main surface of the high-water content layer 46 is in contact with the electrolyte membrane 14, and the other main surface of the high-water content layer 46 is in contact with the anode electrode 10. The high-water content layer 46 has a higher water content (lower EW) than that of the electrolyte membrane 14 and is therefore more hydrophilic than the electrolyte membrane 14. For example, the high-water content layer 46 is formed of a polymer having a higher water content than that of the polymer constituting the electrolyte membrane 14. The method for forming the high-water content layer 46 is not particularly limited, and a known method can be employed. For example, a method may be employed where a polymer constituting the high-water content layer 46 is applied to the surface of the electrolyte membrane 14 or the surface of the anode electrode 10 or where a thin film of the polymer is pressed on the surface of the electrolyte membrane 14 or the surface of the anode electrode 10.

[0051] Examples of the polymer that can be used for the high-water content layer 46 include Aquivion (registered trademark), Fumion (registered trademark), and the like, which have a higher water content than that of the electrolyte membrane 14. By interposing the high-water content layer 46 between the electrolyte membrane 14 and the anode electrode 10, the access of water in the anode catalyst is improved, and the increase in the cell voltage and the decrease in the Faradaic efficiency can be further suppressed. Note that the high-water content layer 46 may be omitted. In this case, the electrolyte membrane 14 and the anode electrode 10 are in contact with each other.

(Exemplary Variations)

[0052] The apparatus 2 for producing an organic hydride according to the above-described embodiment can include the following exemplary variations. That is, the cathode electrode 12 may contain an ionomer having a lower water content than that of the electrolyte membrane 14. Examples of such a low water content ionomer include Nafion (registered trademark) and the like. By adding a low water content ionomer to the cathode electrode 12, the substance to be hydrogenated can easily reach the reaction field in the same manner as in the case when the low water content layer 44 is installed, and the occurrence of side reactions can be thus suppressed. Therefore, the same effect as that of the low water content layer 44 can be achieved.

[0053] The addition of a low water content ionomer to the cathode electrode 12 may be performed instead of the installation of the low water content layer 44, or may be performed along with the installation of the low water content layer 44. That is, the apparatus 2 for producing an organic hydride only needs to include at least one of the low water content layer 44 and the low water content ionomer contained in the cathode electrode 12. When the apparatus 2 for producing an organic hydride does not include the low water content layer 44, the electrolyte membrane 14 and the cathode electrode 12 are in contact with each other.

[0054] Further, a water-repellent layer (hydrophobic layer) may be provided between the low water content layer 44 and the cathode catalyst layer 12a. Examples of such a water-repellent layer include a layer made of a material in which a water-repellent fluororesin such as a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP) is added to ketjen black. The water-repellent layer can be formed by a known method, for example, by applying a dispersion liquid of the material to the surface of the catalyst layer 12a. This allows the substance to be hydrogenated to reach the reaction field more easily, and the Faradaic efficiency can be improved.

[0055] The embodiments may be defined by the items described in the following.

[Item 1]

[0056] An apparatus (2) for producing an organic hydride including: [0057] an anode electrode (10) that generates protons by oxidizing water; [0058] a cathode electrode (12) that generates an organic hydride by hydrogenating a substance to be hydrogenated with the protons; [0059] an electrolyte membrane (14) that has an equivalent weight (EW) of less than 980 and is arranged between the anode electrode (10) and the cathode electrode (12) so as to transfer the protons from the anode electrode (10) side to the cathode electrode (12) side; and [0060] a low water content layer (44) that is arranged between the electrolyte membrane (14) and the cathode electrode (12) and that has a lower water content than that of the electrolyte membrane (14).

[Item 2]

[0061] The apparatus (2) for producing an organic hydride according to Item 1, including: [0062] a high-water content layer (46) that is arranged between the electrolyte membrane (14) and the anode electrode (10) and that has a higher water content than that of the electrolyte membrane (14).

EXEMPLARY EMBODIMENTS

[0063] Hereinafter, exemplary embodiments of the present invention will be explained. However, these exemplary embodiments are merely examples for suitably explaining the present invention and do not limit the present invention in any way.

First Exemplary Embodiment

[0064] As the electrolyte membrane, a polyfluorosulfonic acid cation exchange membrane (Aquivion (registered trademark) E87-05S, manufactured by Solvay) was prepared. The EW of this electrolyte membrane is 870, and the film thickness is 50 m.

[0065] The water content of the electrolyte membrane was measured by the following procedure. That is, the electrolyte membrane cut out into 2 cm squares was dried in a dryer for 24 hours. The weight of the electrolyte membrane was measured after the drying. Subsequently, the dried electrolyte membrane was immersed in pure water for one hour. Then, the water attached to the surface of the electrolyte membrane was wiped off, and the weight of the electrolyte membrane containing water was measured. The weight of the water contained in the electrolyte membrane was obtained from the difference between the weight of the electrolyte membrane after the drying and the weight of the electrolyte membrane containing the water. Then, the water content (%) of the electrolyte membrane was calculated based on the following equation (2).

[00002]$\begin{array}{cc}\mathrm{water}\mathrm{content}=\left(\mathrm{weight}\mathrm{of}\mathrm{water}\mathrm{contained}\mathrm{in}\mathrm{electrolyte}\mathrm{membrane}/\mathrm{weight}\mathrm{of}\mathrm{electrolyte}\mathrm{membrane}\mathrm{in}\mathrm{hydrous}\mathrm{state}\right)100& \left(2\right)\end{array}$

[0066] A polyfluorosulfonic acid cation-exchange ionomer (Nafion (registered trademark) D2020CS, EW: 1100, manufactured by DuPont) was applied to one surface of the electrolyte membrane so as to form a low water content layer. The thickness of the low water content layer was 10 m.

[0067] The water content of the low water content layer was measured by the following procedure. That is, the weight of a 12 cm square aluminum foil was measured, and the weight per 2 cm square of aluminum foil was calculated. An ionomer was spray-applied to this aluminum foil such that the aluminum foil was laminated with a low water content layer having a thickness of 10 m. An aluminum foil with a low water content layer cut out into 2 cm squares (hereinafter referred to as laminated aluminum foil) was dried in a dryer for 24 hours. The weight of the laminated aluminum foil was measured after the drying. Subsequently, the dried laminated aluminum foil was immersed in pure water for one hour. Then, the water attached to the surface of the laminated aluminum foil was wiped off, and the weight of the laminated aluminum foil containing water in the low moisture content layer was measured. The weight of the water contained in the low water content layer was obtained from the difference between the weight of the laminated aluminum foil after the drying and the weight of the laminated aluminum foil containing the water. Further, the weight of the low water content layer in a hydrous state was obtained from the difference between the weight of the laminated aluminum foil containing water and the weight per 2 cm square of the aluminum foil. Then, the water content (%) of the low water content layer was calculated based on the following equation (3).

[00003]$\mathrm{water}\mathrm{content}=\left(\mathrm{weight}\mathrm{of}\mathrm{water}\mathrm{contained}\mathrm{in}\mathrm{low}\mathrm{water}\mathrm{content}\mathrm{layer}/\mathrm{weight}\mathrm{of}\mathrm{low}\mathrm{water}\mathrm{content}\mathrm{layer}\mathrm{in}\mathrm{hydrous}\mathrm{state}\right)100$

[0068] A cathode catalyst ink was prepared by mixing a PtRu/C catalyst (TEC61E54, manufactured by TANAKA PRECIOUS METAL TECHNOLOGIES Co., Ltd.), a polyfluorosulfonic acid cation exchange ionomer (Nafion (registered trademark) D2020CS, EW: 1100, manufactured by DuPont), pure water, and 1-propanol (manufactured by Wako). The catalyst loading density of the catalyst ink was 1 mg/cm.sup.2, and the ionomer/carbon ratio (I/C) was 0.5. The prepared cathode catalyst ink was applied to the surface of the low water content layer so as to form a cathode catalyst layer.

[0069] As the anode electrode, a dimensionally stable electrode (DSE) (manufactured by De Nora Permelec Ltd.) coated with IrO.sub.2 on a Ti substrate was prepared. Then, this DSE electrode was stacked on the other surface of the electrolyte membrane. Thereby, the electrolytic cell (apparatus for producing an organic hydride) according to the first exemplary embodiment was obtained.

[0070] Toluene serving as a cathode liquid was circulated to the cathode of the obtained apparatus for producing an organic hydride at a flow rate of 20 mL/min. Further, a 1 mol/L sulfuric acid aqueous solution serving as an anode liquid was circulated on the anode side at a flow rate of 60 mL/min. Then, constant-current electrolysis was performed at a temperature of 60 C. and a current density of 1 A/cm.sup.2. In addition, the voltage at the time of the constant-current electrolysis was measured. Then, the amount of change in voltage with respect to the voltage in the second comparative example described later was calculated. Further, the Faradaic efficiency was calculated from the amount of electricity consumed by the constant-current electrolysis and the amount of organic hydride generated. Then, the improvement rate (%) of the Faradaic efficiency with respect to the Faradaic efficiency in the second comparative example described later was calculated based on the following equation (4).

[00004]$\mathrm{improvement}\mathrm{rate}=\left[1-\left(100-\mathrm{Faradaic}\mathrm{efficiency}\mathrm{in}\mathrm{exemplary}\mathrm{embodiment}\right)/\left(100-\mathrm{Faradaic}\mathrm{efficiency}\mathrm{in}\mathrm{second}\mathrm{comparative}\mathrm{example}\right)\right]100$

Second Exemplary Embodiment

[0071] An electrolysis cell was prepared, constant current electrolysis was performed, and the voltage change amount and the improvement rate were calculated in the same manner as in the first exemplary embodiment, except that a low water content layer was formed by pressing a thin film of ionomer onto the electrolyte membrane instead of applying an ionomer.

Third Exemplary Embodiment

[0072] An electrolysis cell was prepared, constant current electrolysis was performed, and the voltage change amount and the improvement rate were calculated in the same manner as in the first exemplary embodiment, except that Fumion (registered trademark) FSLA-1020 (EW: 960-1000, manufactured by FUMATECH) was used as the polyfluorosulfonic acid-based cation exchange ionomer instead of Nafion (registered trademark) D2020CS.

Fourth Exemplary Embodiment

[0073] An electrolysis cell was prepared, constant current electrolysis was performed, and the voltage change amount and the improvement rate were calculated in the same manner as in the first exemplary embodiment, except that a high-water content layer was provided between the electrolyte membrane and the anode electrode. The high-water content layer was formed by applying a polyfluorosulfonic acid cation-exchange ionomer (Fumion (registered trademark) FSLA-710, EW: 710-740, manufactured by FUMATECH) to the other surface of the electrolyte membrane. The thickness of the high-water content layer was 10 m. Further, the water content of the high-water content layer was measured in the same procedure as that in the case of the low water content layer.

Fifth Exemplary Embodiment

[0074] An electrolysis cell was prepared, constant current electrolysis was performed, and the voltage change amount and the improvement rate were calculated in the same manner as in the first exemplary embodiment, except that a water-repellent layer was provided between the low water content layer and the cathode catalyst layer. The water-repellent layer was formed by applying a Nafion (registered trademark) dispersion liquid to which Ketjen Black (EC600JD, manufactured by Lion Corporation) containing FEP (120-JRB, manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd.) was added to the surface of the low water content layer facing opposite to the electrolyte membrane. The thickness of the water-repellent layer was 10 m, and the I/C was 0.5.

First Comparative Example

[0075] An electrolysis cell was prepared, constant current electrolysis was performed, and the improvement rate was calculated in the same manner as in the first exemplary embodiment, except that Aquivion (registered trademark) E98-05S was used as the electrolyte membrane instead of Aquivion (registered trademark) E87-05S and that no low water content layer was provided. The EW of this electrolyte membrane is 980.

Second Comparative Example

[0076] An electrolysis cell was prepared, constant current electrolysis was performed, and the voltage and the Faradaic efficiency were calculated in the same manner as in the first exemplary embodiment, except that no low water content layer was provided.

[0077] FIG. 3 is a diagram showing the characteristics of respective electrolytic cells according to exemplary embodiments and comparative examples, the improvement rate of the Faradaic efficiency, and the amount of change in voltage. As shown in FIG. 3, compared to the second comparative example where the electrolyte membrane had an EW of less than 980 but did not have a low water content layer, the Faradaic efficiency was improved by 20% or more in the first to fifth exemplary embodiments where the electrolyte membrane had an EW of less than 980 and had a low water content layer. Further, the improvement rate was higher than that in the first comparative example where the EW of the electrolyte membrane was 980. Further, the Faradaic efficiency was further improved in the fifth exemplary embodiment with a water-repellent layer compared to the first to fourth exemplary embodiments without a water-repellent layer. Further, the increase in voltage with respect to that in the second comparative example was able to be suppressed to 25 mV or less in the first to fifth exemplary embodiments. The voltage was able to be further reduced in the fourth exemplary embodiment with a high-water content layer.

[0078] From the above, it has been confirmed that by using an electrolyte membrane having an EW of less than 980 and providing a low water content layer between the electrolyte membrane and the cathode electrode, it is possible to suppress both the increase in cell voltage and the decrease in the Faradaic efficiency in the production of organic hydrides. Further, it has been confirmed that the cell voltage can be further reduced by providing a high-water content layer between the electrolyte membrane and the anode electrode. Also, it has been confirmed that the Faradaic efficiency can be further improved by providing a water-repellent layer between the low water content layer and the cathode electrode.