Electrochemical cell

Abstract

An electrochemical cell according to the present invention comprises an anode part, a cathode part, and an electrolyte membrane. The anode part has a first flow field that forms a space through which a first fluid including water flows so as to contact a first surface of the electrolyte membrane. The cathode part has a second flow field that forms a space through which a second fluid including a hydrogenated substance flows so as to contact a second surface of the electrolyte membrane. The second flow field comprises a porous layer formed of a conductive material, and the compressive strain of the porous layer in the cathode part is 0.3-0.8.

Description

TITLE OF INVENTION: Electrochemical Cell TECHNICAL FIELD

[0001] The present disclosure relates to an electrochemical cell. The present application claims priority to Japanese Patent Application No. 2024-051442 filed on March 27, 2024, the entire contents of which are herein incorporated by reference. BACKGROUND ART

[0002] Hydrogen has recently attracted attention as clean energy. For stable storage of hydrogen, addition of hydrogen to a substance to be hydrogenated has been studied. Hydrogen added to the substance to be hydrogenated is extracted by a dehydrogenation reaction at a location where it is demanded. The substance to be hydrogenated is, for example, an organic substance such as toluene.  PTL 1 discloses an electrochemical reduction apparatus that is an organic hydride production apparatus that adds hydrogen to a substance to be hydrogenated.

[0003] The electrochemical reduction apparatus includes an electrolytic cell. The electrolytic cell includes an anode portion and a cathode portion partitioned by an electrolyte membrane. A first fluid containing water is supplied to the anode portion and a second fluid containing a substance to be hydrogenated is supplied to the cathode portion. As a direct-current (DC) current is applied to the electrolytic cell, protons move from the first fluid to the second fluid and hydrogen is added to the substance to be hydrogenated. The electrolytic cell is herein called an electrochemical cell. An electrochemical cell in which a substance to be hydrogenated is hydrogenated by conduction of protons from the anode portion to the cathode portion is a protonconduction type electrochemical cell. CITATION LIST PATENT LITERATURE

[0004] PTL 1: WO2013 / 111586 SUMMARY OF INVENTION

[0005] An electrochemical cell in the present disclosure is an electrochemical cell including an anode portion in which a first fluid containing water flows, a cathode portion in which a second fluid containing a substance to be hydrogenated flows, and an electrolyte membrane arranged between the anode portion and the cathode portion. The anode portion includes a first flow field that defines a space in which the first fluid flows to come in contact with a first surface of the electrolyte membrane and a first catalyst layer arranged in the first flow field. The cathode portion includes a second flow field that defines a space in which the second fluid flows to come in contact with a second surface of the electrolyte membrane and a second catalyst layer arranged in the second flow field. The second flow field includes a porous layer formed of a conductive material. The porous layer in the cathode portion has compressive strain not less than 0.3 and not more than 0.8. BRIEF DESCRIPTION OF DRAWINGS

[0006] [Fig. 1] Fig. 1 is a diagram of operation principles of an electrochemical cell described in an embodiment. [Fig. 2] Fig. 2 is an exploded perspective view of the electrochemical cell described in the embodiment. [Fig. 3] Fig. 3 is a schematic diagram of a first conductive plate provided in the electrochemical cell described in the embodiment. [Fig. 4] Fig. 4 is a schematic diagram of a second conductive plate provided in the electrochemical cell described in the embodiment. [Fig. 5] Fig. 5 is a partial cross-sectional view of the electrochemical cell described in the embodiment. [Fig. 6] Fig. 6 is an illustrative view for illustrating a first ridge area and a second ridge area in the second conductive plate described in the embodiment. [Fig. 7] Fig. 7 is an illustrative view of a position of arrangement of an insulating member in the electrochemical cell described in the embodiment. [Fig. 8] Fig. 8 is an illustrative view of a state of arrangement of a second diffusion layer in the electrochemical cell described in the embodiment. [Fig. 9] Fig. 9 shows a graph showing results in a test example 1. [Fig. 10] Fig. 10 is a graph showing results in a test example 1-2. [Fig. 11] Fig. 11 is a schematic diagram of a test apparatus in a test example 4. [Fig. 12] Fig. 12 shows a graph showing results in test example 4. DETAILED DESCRIPTION

[0007] [Problem to be Solved by the Present Disclosure] In a proton-conduction type electrochemical cell, a side reaction in which protons that have moved to a cathode portion are recombine to generate hydrogen occurs. This side reaction may lower Faraday efficiency of the electrochemical cell. Faraday efficiency herein refers to a ratio of an amount of charges that have contributed to hydrogenation, with the total amount of charges injected into the electrochemical cell being defined as 100%.

[0008] In order to reduce a decrease in Faraday efficiency, the above-described side reaction in the cathode portion should be suppressed.

[0009] One of objects of the present disclosure is to provide an electrochemical cell capable of achieving improvement in Faraday efficiency of a proton-conduction type electrochemical cell.

[0010] [Advantageous Effect of the Present Disclosure] The electrochemical cell in the present disclosure can achieve improvement in Faraday efficiency.

[0011] [Description of Embodiments of the Present Disclosure] Embodiments of the present disclosure will initially be listed and described.

[0012] <1> A proton-conduction type electrochemical cell in the present disclosure is an electrochemical cell including an anode portion in which a first fluid containing water flows, a cathode portion in which a second fluid containing a substance to be hydrogenated flows, and an electrolyte membrane arranged between the anode portion and the cathode portion. The anode portion includes a first flow field that defines a space in which the first fluid flows to come in contact with a first surface of the electrolyte membrane and a first catalyst layer arranged in the first flow field. The cathode portion includes a second flow field that defines a space in which the second fluid flows to come in contact with a second surface of the electrolyte membrane and a second catalyst layer arranged in the second flow field. The second flow field includes a porous layer formed of a conductive material. The porous layer in the cathode portion has compressive strain not less than 0.3 and not more than 0.8.

[0013] The porous layer performs a function to form the second flow field and to diffuse the second fluid over the entire second flow field. Such a porous layer is compressed in the cathode portion in a first direction along a thickness of the cathode portion. The first direction is a direction from the first surface toward the second surface of the electrolyte membrane, which is perpendicular to the first surface of the electrolyte membrane in parallel to the porous layer.

[0014] The compressive strain is compressive strain of the porous layer being compressed in the cathode portion of the electrochemical cell. This compressive strain is expressed as a value calculated by dividing an amount of decrease X in thickness of the compressed porous layer by an initial thickness t0 of the porous layer yet to be compressed.  In other words, compressive strain is expressed as compressive strain = X / t0.  Compressive strain is an indicator of deformability by compression. Great compressive strain of the porous layer having an elastic deformation property by including a plurality of pores means high elastic deformation capability. As compressive strain of the porous layer is greater, limits of amount of decrease X within which elastic deformation capability can be kept, that is, an elastically deformable area of the porous layer, tend to be broader.

[0015] The porous layer having compressive strain not less than 0.3 and not more than 0.8 while a prescribed surface pressure is applied thereto is excellent in elastic deformation capability. In other words, the porous layer having compressive strain not less than 0.3 and not more than 0.8 deforms to a thickness smaller than initial thickness t0 while the prescribed surface pressure is applied thereto. When the surface pressure is lowered or removed, the porous layer with such elastic deformation capability deforms again to a thickness closer to or equivalent to initial thickness t0. When the porous layer with such elastic deformation capability is compressed in the cathode portion, it presses in the cathode portion, a first member and a second member between which the porous layer lies, in an attempt to return to the thickness before compression. The first member is, for example, the electrolyte membrane. The second member is, for example, a conductive plate which will be described later. The porous layer that has expanded within the limits of elastic deformation is in intimate contact with the first member and the second member and a gap is less likely between the porous layer and the first member and between the porous layer and the second member. The gap disturbs the flow of the second fluid in the second flow field and lowers Faraday efficiency of the electrochemical cell. In the electrochemical cell including the porous layer that satisfies the compressive strain defined in <1> above, lowering in Faraday efficiency due to the gap is less likely. Therefore, the electrochemical cell described in <1> above is excellent in Faraday efficiency.

[0016] <2> In the electrochemical cell described in <1> above, the compressive strain may be not less than 0.5.

[0017] The porous layer having compressive strain not less than 0.5 is larger in elastically deformable area than the porous layer having compressive strain not less than 0.3 and less than 0.5. Under the same surface pressure, the porous layer greater in compressive strain is better in flexibility than the porous layer less in compressive strain. Therefore, the porous layer having compressive strain not less than 0.5 is more readily elastically deformed to be in intimate contact with a member adjacent thereto in the electrochemical cell. Consequently, provision of a gap in the electrochemical cell tends to be suppressed and Faraday efficiency of the electrochemical cell is further improved.

[0018] <3> In the electrochemical cell described in <1> or <2> above, the compressive strain may be not less than 0.6.

[0019] The porous layer having compressive strain not less than 0.6 is larger in elastically deformable area than the porous layer having compressive strain not less than 0.3 and less than 0.6. Therefore, the porous layer having compressive strain not less than 0.6 can be compressed to a small thickness while it maintains elastic deformation capability.

[0020] <4> In the electrochemical cell described in any one of <1> to <3> above, the cathode portion may include a conductive plate that faces the electrolyte membrane and a diffusion layer arranged between the conductive plate and the electrolyte membrane. The diffusion layer may be the porous layer.

[0021] With the conductive plate, a voltage is uniformly applied to the cathode portion. With the porous diffusion layer, the second fluid is readily diffused over the entire second flow field. The diffusion layer formed from the porous layer is readily in intimate contact with the electrolyte membrane and the conductive plate.

[0022] <5> In the electrochemical cell described in <4> above, the second flow field may include a plurality of first grooves arranged in parallel and a plurality of second grooves arranged in parallel. At least one of the plurality of first grooves and at least one of the plurality of second grooves may alternately be arranged. The plurality of first grooves and the plurality of second grooves are provided at a surface of the conductive plate that faces the diffusion layer.

[0023] As the grooves structured above are provided in the conductive plate, the second fluid is readily diffused over the entire diffusion layer that faces the conductive plate. The second fluid diffused over the entire diffusion layer is readily in contact with the entire second catalyst layer arranged in the second flow field. Consequently, Faraday efficiency of the electrochemical cell tends to be improved.

[0024] <6> In the electrochemical cell described in any one of <1> or <5> above, the porous layer may have a porosity not lower than 40% and not higher than 98%.

[0025] So long as the porosity of the porous layer is within the range above, a density of the porous layer is not too low or too high and the porous layer is excellent in elastic deformation capability. The porous layer having the porosity not lower than 40% while it is compressed in the electrochemical cell allows the second fluid to easily flow in the cathode portion. Since the porous layer having the porosity not higher than 98% while it is compressed includes a sufficient amount of solid portion, it is excellent in mechanical strength.

[0026] <7> In the electrochemical cell described in any one of <1> to <6> above, the porous layer may be made from a non-woven fabric containing carbon fibers.

[0027] The non-woven fabric containing carbon fibers tends to achieve compressive strain defined in <1>, <2>, or <3> above. The non-woven fabric containing carbon fibers is entangled carbon fibers as a result of interlacing treatment on a plurality of carbon fibers. For example, a needle punch or a water jet is used for the interlacing treatment.

[0028] <8> In the electrochemical cell described in <7> above, the porous layer may have a basis weight not smaller than 50 g / m2 and not larger than 400 g / m2.

[0029] So long as the porous layer has the basis weight not smaller than 50 g / m2 and not larger than 400 g / m2, the density of the porous layer is not too low or too high. The porous layer not too low or too high in density is excellent in elastic deformation capability. When the porous layer having such elastic deformation capability is compressed in the cathode portion between the first member and the second member between which the porous layer lies, it is readily in intimate contact with the first member and the second member without a gap.

[0030] <9> In the electrochemical cell described in <7> or <8> above, the carbon fibers may have an average diameter not smaller than 5 um and not larger than 100 pm.

[0031] The carbon fibers having the average diameter not smaller than 5 pm and not larger than 100 um are not too thin or too thick. The porous layer formed from carbon fibers not too thin or not too thick is excellent in elastic deformation capability. When the porous layer having such elastic deformation capability is compressed in the cathode portion between the first member and the second member between which the porous layer lies, it is readily in intimate contact with the first member and the second member without a gap.

[0032] <10> In the electrochemical cell described in any one of <7> to <9> above, the non-woven fabric may have an average thickness not smaller than 0.15 mm and not larger than 3.0 mm.

[0033] So long as the non-woven fabric which is the porous layer has the average thickness not smaller than 0.15 mm while it is compressed in the electrochemical cell, an amount of elastic deformation of the porous layer in the electrochemical cell tends to be large and consequently the porous layer is readily in intimate contact with a member adjacent thereto. So long as the non-woven fabric in the compressed state has the average thickness not larger than 3.0 mm, increase in electrical resistance of the porous layer and cell resistance of the electrochemical cell is readily lessened.

[0034] <11> In the electrochemical cell described in any one of <1> to <10> above, the first flow field may include a first porous layer formed of a metallic material.

[0035] The first porous layer formed of a metallic material is excellent in rigidity and less likely to deform. When an internal pressure in the cathode portion becomes higher than the internal pressure in the anode portion, the electrolyte membrane may deform toward the anode portion and the electrolyte membrane may be damaged. The first porous layer excellent in rigidity supports the first surface of the electrolyte membrane and lessens excessive deformation of the electrolyte membrane.

[0036] <12> In the electrochemical cell described in <11> above, the first porous layer may include a porous body composed of titanium or a titanium alloy and a precious metal coating formed at a surface of the porous body.

[0037] In the anode portion, oxygen is generated by electrolysis of water. The first porous layer including the porous body composed of titanium or a titanium alloy and the precious metal coating formed at the surface of the porous body is less likely to be oxidized and has long lifetime.

[0038] <13> In the electrochemical cell described in any one of <1> to <12> above, the substance to be hydrogenated may be toluene.

[0039] Toluene is converted to methylcyclohexane (MCH below) by hydrogenation. MCH can store six hydrogen atoms. MCH is liquid at a room temperature and a normal pressure, and a large amount of hydrogen can relatively safely be stored therein.

[0040] <14> In the electrochemical cell described in any one of <1> to <13> above, the first fluid may be composed mainly of water.

[0041] The first fluid mainly composed of water is pure water or an aqueous solution containing pure water as a solvent. The first fluid mainly composed of water is inexpensive and readily available.

[0042] [Details of Embodiment in the Present Disclosure] A specific example of the electrochemical cell in the present disclosure will be described below with reference to the drawings. The same or corresponding elements in the drawings have the same reference characters allotted. A size of a member shown in each drawing is expressed for the purpose of clarification of description and does not necessarily represent an actual dimension. The present invention is not limited to these exemplifications but is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

[0043] <First Embodiment> <<Overview of Organic Hydride Production Apparatus>> Fig. 1 is a diagram of a principle of an organic hydride production apparatus 100 including a proton-conduction type electrochemical cell 1. Organic hydride production apparatus 100 supplies a first fluid 101 stored in a first tank 101T and a second fluid 102 stored in a second tank 102T to electrochemical cell 1. Electrochemical cell 1 includes an anode portion 3 and a cathode portion 4 partitioned by an electrolyte membrane 2. A not-shown DC power supply is connected to anode portion 3 and cathode portion 4. A positive electrode of the DC power supply is connected to anode portion 3 and a negative electrode of the DC power supply is connected to cathode portion 4.

[0044] First fluid 101 is supplied through a first supply pipe 101A to anode portion 3. A first pump 101P that delivers first fluid 101 to anode portion 3 is arranged at first supply pipe 101A. A first catalyst layer 30 is arranged in anode portion 3. As a result of an electrochemical reaction, proton (H+), oxygen (O2), and electron (e-) are generated from water (H2O) contained in first fluid 101. Proton moves to cathode portion 4 through electrolyte membrane 2. Electron generated at the positive electrode flows to the negative electrode through the DC power supply. First fluid 1 in anode portion 3 is discharged to first tank 101T through a first discharge pipe 101B.

[0045] First fluid 101 containing water may be a fluid mainly composed of water. For example, first fluid 101 is pure water or an aqueous solution. Commercially available pure water can be employed. Pure water may contain ions as a result of circulation to electrochemical cell 1. Ions are derived, for example, from at least one of constituent materials for first tank 101T. First fluid 101 may be water vapor.

[0046] Second fluid 102 is supplied through a second supply pipe 102A to cathode portion 4. A second pump 102P that delivers second fluid 102 to cathode portion 4 is arranged at second supply pipe 102A. A second catalyst layer 40 is arranged in cathode portion 4. As a result of an electrochemical reaction, a substance to be hydrogenated contained in second fluid 102 is hydrogenated by bond between proton that has passed through electrolyte membrane 2 and electron supplied from the DC power supply in second catalyst layer 40 to generate a hydrogenated product. Second fluid 102 containing the hydrogenated product is discharged from cathode portion 4 through a second discharge pipe 102B to second tank 102T.

[0047] The substance to be hydrogenated contained in second fluid 102 is, for example, an aromatic hydrocarbon compound containing one or more aromatic rings or a nitrogen-containing heterocyclic aromatic compound. Specific examples of the aromatic compounds above include benzene, naphthalene, anthracene, diphenylethane, pyridine, pyrimidine, pyrazine, quinoline, isoquinoline, N-alkylpyrrole, N-alkylindole, and N-alkyldibenzopyrrole. At least one and at most four hydrogen atom(s) in the aromatic ring of the aromatic hydrocarbon compound or the nitrogen-containing heterocyclic aromatic compound may be substituted with an alkyl group. The "alkyl" above is a linear alkyl group or a branched alkyl group having at least one or at most six carbon atom(s). Alkylbenzene is, for example, toluene or ethylbenzene. Dialkylbenzene is, for example, xylene or diethylbenzene. Trialkylbenzene is, for example, mesitylene. Alkylnaphthalene is, for example, methylnaphthalene. The substance to be hydrogenated in the present example is toluene (denoted as TOL in the figure). A hydrogenated product of toluene is methylcyclohexane (denoted as MCH below). Electrochemical cell 1 in the present example is also referred to as an MCH electrolytic synthesis cell and reactions in anode portion 3 and cathode portion 4 are as below. • 3H2O ^ 1.5O2 + 6H+ + 6e- • toluene + 6H+ + 6e- ^ methylcyclohexane

[0048] <<Basic Configuration of Electrochemical Cell>> As shown in Fig. 2, electrochemical cell 1 includes electrolyte membrane 2, a first flow field 5, a second flow field 6, first catalyst layer 30, and second catalyst layer 40.

[0049] Electrolyte membrane 2 is formed of a material that allows selective transmission of proton. Electrolyte membrane 2 separates first fluid 101 and second fluid 102 from each other in electrochemical cell 1. Electrolyte membrane 2 has a thickness, for example, not smaller than 5 pm and not larger than 300 pm.  Electrolyte membrane 2 having the thickness not smaller than 5 pm readily serves as a partition between first fluid 101 and second fluid 102. Electrolyte membrane 2 having the thickness not larger than 300 pm tends to allow transmission of proton therethrough. Electrolyte membrane 2 may have the thickness not smaller than 10 pm and not larger than 150 pm or not smaller than 20 pm and not larger than 100 pm.

[0050] First flow field 5 is a space in anode portion 3 where first fluid 101 flows. An inlet 5A and an outlet 5B of first flow field 5 are connected to first supply pipe 101A and first discharge pipe 101B, respectively, as shown in Fig. 1. In first flow field 5, first fluid 101 flows to be in contact with a first surface 21 of electrolyte membrane 2. Anode portion 3 in the present example includes a first conductive plate 31, a first diffusion layer 32, and a frame seal portion 33. First conductive plate 31 is a conductive plate material that faces first surface 21 of electrolyte membrane 2. First diffusion layer 32 is a conductive porous layer arranged between first conductive plate 31 and first surface 21. First flow field 5 in the present example is formed in a space that lies between first conductive plate 31 and first surface 21. First flow field 5 includes pores in first diffusion layer 32.

[0051] First conductive plate 31 performs a function to apply a voltage to electrochemical cell 1. First conductive plate 31 performs also a function to confine first fluid 101 in electrochemical cell 1. First conductive plate 31 should be formed of a material less likely to react to a substance contained in first fluid 101. First conductive plate 31 is, for example, a plate formed of a composite material of a conductive material and resin. The conductive material is, for example, a carbonbased material such as graphite or carbon black. The resin is, for example, epoxy resin, phenol resin, polyamide resin such as PA6 or PA66, polyoxymethylene resin, fluororesin such as polytetrafluoroethylene resin, or polyphenylenesulfide resin. The composite material is light in weight and inexpensive, and in addition it can easily be formed by being molded in a die. First conductive plate 31 may be, for example, a metal plate provided with a coating layer. The coating layer is formed of a material having high oxidation resistance, such as platinum. The metal plate is formed, for example, of titanium or a titanium alloy. A not-shown electrode plate is arranged at a surface of first conductive plate 31 opposite to first diffusion layer 32. The not-shown positive electrode of the DC power supply is connected to the electrode plate.

[0052] First diffusion layer 32 performs a function to diffuse first fluid 101 supplied to anode portion 3 over the entire first flow field 5 in electrochemical cell 1. First diffusion layer 32 is a porous layer formed of a metallic material. First diffusion layer 32 includes, for example, a porous body of a three-dimensional network structure made of titanium or a titanium alloy and a precious metal coating formed on a surface of the porous body. Precious metal is, for example, platinum. The porous body includes a porous layer such as a non-woven fabric or a mesh plate. The porous body may be layered mesh plates different in mesh size or layered mesh plates different in mesh shape.

[0053] First diffusion layer 32 lies between electrolyte membrane 2 and first conductive plate 31. First diffusion layer 32 is pressed by electrolyte membrane 2 and first conductive plate 31 by being sandwiched therebetween. Depending on a material for first diffusion layer 32, first diffusion layer 32 may be compressed by the pressing. Though first diffusion layer 32 formed of a metallic material includes a plurality of pores, it is less likely to deform. First diffusion layer 32 as being sandwiched between electrolyte membrane 2 and first conductive plate 31 in electrochemical cell 1 has an average thickness, for example, not smaller than 0.15 mm and not larger than 3.0 mm. In an example where first diffusion layer 32 is less likely to deform by the pressing because of being formed, for example, of a metallic material, the average thickness of first diffusion layer 32 as not being sandwiched between electrolyte membrane 2 and first conductive plate 31 is substantially equal to the average thickness of first diffusion layer 32 as being sandwiched between electrolyte membrane 2 and first conductive plate 31. The average thickness of first diffusion layer 32 is an average of thicknesses at three or more different locations. When first diffusion layer 32 has the average thickness not smaller than 0.15 mm, a gap between first conductive plate 31 and electrolyte membrane 2 is sufficiently large. In other words, since first flow field 5 having a sufficient size is formed in anode portion 3, first fluid 101 is readily diffused over the entire first flow field 5. When first diffusion layer 32 has the average thickness not larger than 3.0 mm, the gap between first conductive plate 31 and electrolyte membrane 2 is not excessively large. In other words, since first flow field 5 is not excessively large, first fluid 101 containing oxygen generated as a result of electrolysis of water is readily quickly discharged from first flow field 5. First diffusion layer 32 may have the average thickness not smaller than 0.2 mm and not larger than 2.5 mm or not smaller than 0.25 mm and not larger than 2.0 mm.

[0054] First diffusion layer 32 has a porosity, for example, not lower than 60% and not higher than 90% in electrochemical cell 1. Since first diffusion layer 32 formed of a metallic material is low in elastic deformation capability, the porosity of first diffusion layer 32 not attached to electrochemical cell 1 should only be regarded as the porosity of first diffusion layer 32 within electrochemical cell 1. When the porosity is not lower than 60%, first fluid 101 is readily diffused over the entire first flow field 5. When the porosity is not higher than 90%, strength of first diffusion layer 32 is readily obtained. First diffusion layer 32 may have the porosity, for example, not lower than 65% and not higher than 85% or not lower than 68% and not higher than 80%.

[0055] Frame seal portion 33 surrounds an outer periphery of first diffusion layer 32. Frame seal portion 33 prevents first fluid 101 from leaking from the outer periphery of first diffusion layer 32 to the outside. Therefore, first flow field 5 in the present example is mainly defined by a space that is located between first surface 21 of electrolyte membrane 2 and first conductive plate 31 and surrounded by frame seal portion 33. As first diffusion layer 32 is arranged in the space, the pores in first diffusion layer 32 can be used as a flow channel for first fluid 101. A size of frame seal portion 33 is desirably selected in accordance with a size of first conductive plate 31 and first diffusion layer 32. In the present example, a window portion of frame seal portion 33 substantially corresponds to the size of first diffusion layer 32, and a size of an outer edge of frame seal portion 33 substantially corresponds to the size of first conductive plate 31. Frame seal portion 33 is formed of an electrically insulating material resistant to first fluid 101. Specific electrically insulating materials are, for example, epoxy resin, phenol resin, polyamide resin such as PA6 or PA66, polyoxymethylene resin, fluororesin, and polyphenylenesulfide resin.

[0056] First catalyst layer 30 arranged in first flow field 5 is in contact with or proximate to first surface 21 of electrolyte membrane 2. First catalyst layer 30 promotes electrolysis of water contained in first fluid 101 to generate proton, oxygen, and electron. First catalyst layer 30 in the present example is integrally formed on first surface 21 of electrolyte membrane 2. First catalyst layer 30 may integrally be formed in a portion of first diffusion layer 32 in contact at least with electrolyte membrane 2. First catalyst layer 30 may be a member independent of electrolyte membrane 2 and first diffusion layer 32. In that case, first catalyst layer 30 is arranged between electrolyte membrane 2 and first diffusion layer 32. First catalyst layer 30 contains a catalyst. The catalyst is, for example, a catalyst based on an oxide of precious metal such as RuO2 or IrO2. This catalyst may be in such a structure that an oxide of precious metal is carried as being dispersed over a substrate made from a metal wire or a metal mesh or such a structure that the substrate is coated with the oxide of precious metal.  First catalyst layer 30 may contain an ionomer that binds the catalyst to the substrate.  Metal contained in the substrate is, for example, one type of metal selected from the group consisting of Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Ta, and W or an alloy mainly composed of the one type of metal. In an example where IrO2 is used for the catalyst, in the structure where the substrate is coated with a thin film composed of IrO2, manufacturing cost is reduced by reduction or the like of an amount of use of expensive precious metal.

[0057] Second flow field 6 is a space in cathode portion 4 where second fluid 102 flows. An inlet 6A and an outlet 6B of second flow field 6 are connected to second supply pipe 102A and second discharge pipe 102B, respectively, as shown in Fig. 1. In second flow field 6, second fluid 102 flows to be in contact with a second surface 22 of electrolyte membrane 2. Second surface 22 is a surface of electrolyte membrane 2 opposite to first surface 21. Cathode portion 4 in the present example includes a second conductive plate 41, a second diffusion layer 42, a frame seal portion 43, and an insulating member 8. Second conductive plate 41 is a conductive plate material that faces second surface 22 of electrolyte membrane 2. Second diffusion layer 42 is a conductive porous layer arranged between second conductive plate 41 and second surface 22. Second flow field 6 in the present example is formed in a space that lies between second conductive plate 41 and second surface 22. Second flow field 6 includes pores in second diffusion layer 42. Insulating member 8 will be described later in a section.

[0058] Second conductive plate 41 performs a function to apply a voltage to electrochemical cell 1. Second conductive plate 41 performs also a function to confine second fluid 102 in electrochemical cell 1. Second conductive plate 41 should be formed of a material less likely to react with a substance contained in second fluid 102. Second conductive plate 41 is, for example, a plate formed of a composite material of a conductive material such as the carbon-based material described above and resin. The composite material is light in weight and inexpensive, and in addition, it can easily be formed by being molded in a die. Second conductive plate 41 may be, for example, a metal plate. The metal plate may be formed, for example, of titanium or a titanium alloy or stainless steel. A not-shown electrode plate is arranged at a surface of second conductive plate 41 opposite to second diffusion layer 42. The not-shown negative electrode of the DC power supply is connected to the electrode plate.

[0059] Second diffusion layer 42 performs a function to diffuse second fluid 102 supplied to cathode portion 4 over the entire second flow field 6. Second diffusion layer 42 is a porous layer formed of a conductive material. Second diffusion layer 42 is, for example, a non-woven fabric containing carbon fibers. The non-woven fabric containing carbon fibers is entangled carbon fibers as a result of interlacing treatment on a plurality of carbon fibers. For example, a needle punch or a water jet is used for the interlacing treatment.

[0060] Second diffusion layer 42 formed from the non-woven fabric has high elastic deformation capability. In electrochemical cell 1, when second diffusion layer 42 having high elastic deformation capability is pressed by electrolyte membrane 2 and second conductive plate 41 by being sandwiched therebetween, second diffusion layer 42 is compressed. As a result of this compression, an average thickness of second diffusion layer 42 becomes smaller. Second diffusion layer 42 sandwiched between electrolyte membrane 2 and second conductive plate 41 in electrochemical cell 1 has the average thickness, for example, not smaller than 0.15 mm and not larger than 3.0 mm. An average length between electrolyte membrane 2 and second conductive plate 41 in electrochemical cell 1 is herein regarded as the average thickness of second diffusion layer 42. The number of times of measurement for obtaining the average length, including measurement at a central position of electrolyte membrane 2 in a plan view, is not smaller than three.

[0061] When second diffusion layer 42 in a compressed state has the average thickness not smaller than 0.15 mm, an amount of elastic deformation of second diffusion layer 42 in electrochemical cell 1 tends to be large. Consequently, second diffusion layer 42 is readily in intimate contact with electrolyte membrane 2 and second conductive plate 41. In addition, when second diffusion layer 42 has the average thickness not smaller than 0.15 mm, a gap between second conductive plate 41 and electrolyte membrane 2 is sufficiently large. In other words, since sufficiently large second flow field 6 is formed in cathode portion 4, second fluid 102 is readily diffused over the entire second flow field 6. When second diffusion layer 42 in the compressed state has the average thickness not larger than 3.0 mm, increase in electrical resistance of second diffusion layer 42 and cell resistance of electrochemical cell 1 is readily lessened. In addition, when second diffusion layer 42 has the average thickness not larger than 3.0 mm, the gap between second conductive plate 41 and electrolyte membrane 2 is not excessively large. In other words, since second flow field 6 is not excessively large, second fluid 102 containing hydrogenated product is generated in second catalyst layer 40 is readily quickly discharged from second flow field 6. Second diffusion layer 42 may have the average thickness not smaller than 0.15 mm and smaller than 3.0 mm, and furthermore not smaller than 0.2 mm and not larger than 2.5 mm or not smaller than 0.25 mm and not larger than 2.0 mm.

[0062] Second diffusion layer 42 has the porosity, for example, not lower than 40% and not higher than 98% in electrochemical cell 1. Since second diffusion layer 42 formed from the non-woven fabric is deformed by compression, the porosity of second diffusion layer 42 in electrochemical cell 1 should only be obtained based on a result of measurement of the porosity of second diffusion layer 42 not attached to electrochemical cell 1. An exemplary method of obtaining the porosity will be described in a section <<Configuration of Second Diffusion Layer>> which will be described later. When the porosity is not lower than 40%, second fluid 102 is readily diffused over the entire second flow field 6. When the porosity is not higher than 98%, strength of second diffusion layer 42 is readily obtained. The porosity of second diffusion layer 42 may be, for example, not lower than 45% and not higher than 97% or not lower than 50% and not higher than 96%.

[0063] Frame seal portion 43 surrounds an outer periphery of second diffusion layer 42. Frame seal portion 43 prevents second fluid 102 from leaking from the outer periphery of second diffusion layer 42 to the outside. Therefore, second flow field 6 in the present example is mainly defined by a space that is located between second surface 22 of electrolyte membrane 2 and second conductive plate 41 and surrounded by frame seal portion 43. As second diffusion layer 42 is arranged in the space, the pores in second diffusion layer 42 can be used as a flow channel for second fluid 102. For specifications such as a constituent material or a size of frame seal portion 43 of cathode portion 4, description about specifications such as the constituent material or the size of frame seal portion 33 of anode portion 3 is desirably referred to.

[0064] Second catalyst layer 40 arranged in second flow field 6 is in contact with or proximate to second surface 22 of electrolyte membrane 2. Second catalyst layer 40 promotes a reaction of a substance to be hydrogenated contained in second fluid 102 to proton and electron, which leads to generation of a hydrogenated product. Second catalyst layer 40 in the present example is integrally formed on second surface 22 of electrolyte membrane 2. Second catalyst layer 40 may integrally be formed in a portion of second diffusion layer 42 in contact at least with electrolyte membrane 2. Second catalyst layer 40 may be a member independent of electrolyte membrane 2 and second diffusion layer 42. In that case, second catalyst layer 40 is arranged between electrolyte membrane 2 and second diffusion layer 42. Second catalyst layer 40 contains a catalyst. The catalyst is, for example, a composition containing first catalyst metal and second catalyst metal. The first catalyst metal contains precious metal which is at least one of Pt and Pd. The second catalyst metal is one type or two or more types of metal selected from the group consisting of Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Sn, W, Re, Pb, and Bi. The catalyst may be such a catalyst that carbon serves as a support and above-mentioned metal or an oxide of the metal is carried on the support. Second catalyst layer 40 may contain an ionomer that binds the catalyst to the support. In addition, a known catalyst can be employed.

[0065] Organic hydride production apparatus 100 in Fig. 1 normally includes a stacked body in which a plurality of electrochemical cells 1 are stacked. In the stacked body, of two adjacent electrochemical cells 1, first conductive plate 31 of one electrochemical cell 1 and second conductive plate 41 of remaining electrochemical cell 1 form a bipolar plate. In the bipolar plate, a first surface of the bipolar plate functions as first conductive plate 31 and a second surface opposite to the first surface functions as second conductive plate 41. Therefore, first diffusion layer 32 is arranged at the first surface of the bipolar plate and second diffusion layer 42 is arranged at the second surface of the bipolar plate. First conductive plate 31 and second conductive plate 41 rather than the bipolar plate are arranged at a first end and a second end of the stacked body, respectively.

[0066] For electrochemical cell 1, highly efficient use of charges injected into electrochemical cell 1 for hydrogenation of the substance to be hydrogenated, that is, improvement in Faraday efficiency, is demanded. A configuration for improving Faraday efficiency of electrochemical cell 1 will be described below.

[0067] <<Configuration of Flow Channel in Second Flow Field>> Second flow field 6 includes a first flow channel 61 and a second flow channel 62. First flow channel 61 and second flow channel 62 in the present example are not directly connected to each other but are independent of each other. Furthermore, cathode portion 4 includes an area where a part of first flow channel 61 and a part of second flow channel 62 are arranged in parallel. Fig. 4 is mainly referred to for description of first flow channel 61 and second flow channel 62. Fig. 4 shows a general flow of second fluid 102 with a bold arrow.

[0068] First flow channel 61 is provided with an inlet end 61A (a first inlet end) directly connected to inlet 6A (Fig. 1) of second flow field 6 and an outlet end 61B (a first outlet end) not directly connected to outlet 6B (Fig. 1) of second flow field 6. Inlet 6A is directly connected to an end of second supply pipe 102A shown in Fig. 1. Therefore, second fluid 102 easily flows into first flow channel 61 through inlet end 61A. First flow channel 61 allows quick diffusion of second fluid 102 over the entire second flow field 6 and increase in opportunity for contact of the substance to be hydrogenated contained in second fluid 102 with second catalyst layer 40. Since outlet end 61B of first flow channel 61 is not directly connected to outlet 6B of second flow field 6, on the other hand, second fluid 102 that flows in first flow channel 61 is not directly discharged to outlet 6B. First flow channel 61 configured as such allows second fluid 102 diffused over the entire second flow field 6 to stay in second flow field 6 to some extent, which promotes generation of a hydrogenated product.

[0069] Second flow channel 62 is provided with an inlet end 62A (a second inlet end) not directly connected to inlet 6A of second flow field 6 and an outlet end 62B (a second outlet end) directly connected to outlet 6B of second flow field 6. Second fluid 102 is taken from the inside of second flow field 6 into second flow channel 62 not directly connected to inlet 6A of second flow field 6, without directly being taken in from inlet 6A of second flow field 6. Therefore, an amount of the substance to be hydrogenated discharged to the outside of electrochemical cell 1 without being in contact with second catalyst layer 40 is reduced and generation of hydrogenated product is promoted. Second flow channel 62, on the other hand, is directly connected to outlet 6B of second flow field 6. Outlet 6B is directly connected to an end of second discharge pipe 102B shown in Fig. 1. Therefore, second fluid 102 is readily discharged from second flow channel 62 through outlet end 62B. Second flow channel 62 configured as such allows quick discharge of second fluid 102 containing hydrogenated product to the outside of electrochemical cell 1.

[0070] In the area where a part of first flow channel 61 and a part of second flow channel 62 are arranged in parallel, second fluid 102 moves from first flow channel 61 to second diffusion layer 42 and thereafter moves through second diffusion layer 42 toward second flow channel 62 while it is in contact with second catalyst layer 40. Second fluid 102 that has moved to second flow channel 62 is quickly discharged from second flow field 6. Therefore, supply of the substance to be hydrogenated to second catalyst layer 40 and discharge of hydrogenated product from second catalyst layer 40 are smooth.

[0071] First flow channel 61 and second flow channel 62 in second flow field 6 improve flowability of second fluid 102 in second flow field 6 and reduce pressure loss in second flow field 6. Therefore, an amount of supply of second fluid 102 to electrochemical cell 1 can be increased. As the amount of supply of second fluid 102 increases, the substance to be hydrogenated is readily supplied to second catalyst layer 40 and hydrogenated product is readily removed from the portion near second catalyst layer 40. In addition, as the amount of supply of second fluid 102 increases, water is readily removed from the portion near second catalyst layer 40. Water passes through electrolyte membrane 2 from first fluid 101 and moves to second fluid 102, and interferes with hydrogenation of the substance to be hydrogenated in second catalyst layer 40. Faraday efficiency in electrochemical cell 1 is enhanced by quick supply of the substance to be hydrogenated and quick discharge of hydrogenated product and water.

[0072] First flow channel 61 and second flow channel 62 in the present example are each a groove provided in second conductive plate 41. An opening of the groove faces second diffusion layer 42. In other words, the opening of the groove is arranged to lead to pores in second diffusion layer 42. Unlike the present example, first flow channel 61 and second flow channel 62 may each be a groove provided in second diffusion layer 42.

[0073] First flow channel 61 defined by the groove provided at second conductive plate 41 allows quick diffusion of second fluid 102 over the entire second conductive plate 41. Second fluid 102 diffused over the entire surface of second conductive plate 41 is readily in contact with second catalyst layer 40. Second fluid 102 containing hydrogenated product is discharged to second flow channel 62 through second diffusion layer 42. Second flow channel 62 defined by the groove provided in second conductive plate 41 allows quick discharge of second fluid 102 containing hydrogenated product to the outside of electrochemical cell 1. Faraday efficiency in electrochemical cell 1 is thus enhanced.

[0074] A shape of the groove that defines first flow channel 61 and second flow channel 62 is not particularly limited. First flow channel 61 and second flow channel 62 in the present example are grooves structured to be independent of each other. Specifically, first flow channel 61 includes a plurality of first grooves 61d and second flow channel 62 includes a plurality of second grooves 62d. In a plan view, at least one of the plurality of first grooves 61d and at least one of the plurality of second grooves 62d are arranged in parallel alternately in a direction orthogonal to a direction of extension of first groove 61d and second groove 62d. First groove 61d and second groove 62d in the present example are each a longitudinal groove that extends in a direction from a lower end toward an upper end of second conductive plate 41. An area where first groove 61d and second groove 62d are alternately arranged corresponds to the area where a part of first flow channel 61 and a part of second flow channel 62 described above are arranged in parallel.

[0075] First flow channel 61 in the present example further includes a first connection groove 61b that connects lower ends of the plurality of first grooves 61d and a supply groove 61f that connects first connection groove 61b and a manifold 4A to each other. The plurality of first grooves 61d extend as being branched from first connection groove 61b. Supply groove 61f extends as being branched from first connection groove 61b in a direction opposite to the direction of extension of first grooves 61d. Supply groove 61f is inlet end 61A of first flow channel 61. Manifold 4A is directly connected to the end of second supply pipe 102A shown in Fig. 1. In other words, supply groove 61f is directly connected by manifold 4A to inlet 6A of second flow field 6 shown in Fig. 1. Second fluid 102 easily flows at inlet end 61A connected to inlet 6A. An end 61e of first groove 61d, on the other hand, is closed. End 61e is outlet end 61B of first flow channel 61. Second fluid 102 less easily flows at outlet end 61B implemented by closed end 61e. Since the opening of first flow channel 61 defined by the groove structured above faces second diffusion layer 42, second fluid 102 is quickly diffused over the entire second diffusion layer 42 from first flow channel 61. Consequently, the substance to be hydrogenated contained in second fluid 102 is efficiently supplied to second catalyst layer 40.

[0076] Unlike the present example, each of the plurality of first grooves 61d may directly be connected to manifold 4A. In this case, first connection groove 61b and supply groove 61f are not provided in second conductive plate 41. Alternatively, first flow channel 61 may be configured such that at least one of the plurality of first grooves 61d is connected to first connection groove 61b and each of remaining first grooves is directly connected to manifold 4A.

[0077] Second flow channel 62 in the present example further includes a second connection groove 62b that connects upper ends of the plurality of second grooves 62d and a discharge groove 62f that connects second connection groove 62b and a manifold 4B to each other. The plurality of second grooves 62d extend as being branched from second connection groove 62b. Discharge groove 62f extends as being branched from second connection groove 62b in a direction opposite to the direction of extension of second grooves 62d. Discharge groove 62f is outlet end 62B of second flow channel 62. Manifold 4B is directly connected to the end of second discharge pipe 102B shown in Fig. 1. In other words, discharge groove 62f is directly connected by manifold 4B to outlet 6B of second flow field 6 shown in Fig. 1. Second fluid 102 easily flows at outlet end 62B connected to outlet 6B. An end 62e of second groove 62d, which is an inlet for second fluid 102 to second flow channel 62, on the other hand, is closed. End 62e is inlet end 62A of second flow channel 62.  Second fluid 102 less easily flows at inlet end 62A implemented by closed end 62e.  Since the opening of second flow channel 62 defined by the groove structured above faces second diffusion layer 42, second fluid 102 containing hydrogenated product is quickly collected in second flow channel 62 from the portion near second catalyst layer 40. Water that has passed through electrolyte membrane 2 is also quickly collected in second flow channel 62 from the portion near second catalyst layer 40.

[0078] Unlike the present example, each of the plurality of second grooves 62d may directly be connected to manifold 4B. In this case, second connection groove 62b and discharge groove 62f are not provided in second conductive plate 41. Alternatively, second flow channel 62 may be configured such that at least one of the plurality of second grooves 62d is connected to second connection groove 62b and each of remaining second grooves is directly connected to manifold 4B. The configuration of the groove connected to manifold 4A and the configuration of the groove connected to manifold 4B may be the same or different.

[0079] Fig. 5 is a partial cross-sectional view of electrochemical cell 1 in Fig. 1 in a direction orthogonal to first groove 61d and second groove 62d in Fig. 4. Fig. 5 shows with a bold arrow, a general flow of second fluid 102 in second diffusion layer 42. Fig. 5 further shows with a bold arrow, a general flow of first fluid 101 in first diffusion layer 32. As shown in Fig. 5, a ridge portion 63 is formed between first groove 61d of first flow channel 61 and second groove 62d of second flow channel 62. The openings of first groove 61d and second groove 62d and ridge portion 63 are covered with second diffusion layer 42. Some of second fluid 102 that flows through first groove 61d passes through second diffusion layer 42 that covers ridge portion 63 and flows into second groove 62d. At that time, the substance to be hydrogenated contained in second fluid 102 is hydrogenated in second catalyst layer 40 to generate hydrogenated product. As the amount of supply of second fluid 102 increases, a flow velocity of second fluid 102 that passes by ridge portion 63 increases, so that the substance to be hydrogenated is quickly supplied to second catalyst layer 40 and hydrogenated product is quickly collected from second catalyst layer 40.

[0080] The flow velocity of second fluid 102 that passes by ridge portion 63 tends to increase, depending on a width of ridge portion 63. As an average width W1 of ridge portion 63 is larger than an average width W2 of first groove 61d and second groove 62d, the flow velocity of second fluid 102 that passes by ridge portion 63 increases. Average width W1 is obtained, for example, as below. Widths at a start end position S1, a terminal end position E1, and an intermediate position M1 over a length of each ridge portion 63 shown in Fig. 4 are measured. An average of measurement values of all widths is average width W1. Average width W2 is obtained, for example, as below. Widths at a start end position S2, a terminal end position E2, and an intermediate position M2 over a length of each of first groove 61d and second groove 62d are measured. An average of measurement values of all widths is average width W2.

[0081] A ratio W1 / W2 between average width W1 and average width W2 is, for example, higher than one and not higher than fifty. There is such tradeoff relation that, as ratio W1 / W2 is higher, the flow velocity of second fluid 102 is higher, whereas pressure loss in second flow field 6 is greater. Specifically, as ratio W1 / W2 is higher, the number of first grooves 61d and second grooves 62d in second flow field 6 decreases. Decrease in number of first grooves 61d and second groove 62d leads to increase in pressure loss in second flow field 6. In other words, the upper limit of the amount of supply of second fluid 102 to electrochemical cell 1 lowers. Increase in average width W1 of ridge portion 63 with decrease in number of first grooves 61d and second grooves 62d, however, increases the flow velocity of second fluid 102 at ridge portion 63. Increase in flow velocity of second fluid 102 at ridge portion 63 tends to lead to enhancement of Faraday efficiency rather than increase in amount of supply of second fluid 102 to electrochemical cell 1. When ratio W1 / W2 is not higher than fifty, the number of first grooves 61d and second grooves 62d does not excessively decrease. When ratio W1 / W2 is not higher than fifty, an effect to lessen increase in flow velocity and increase in pressure loss is satisfactorily obtained. Ratio W1 / W2 may be, for example, not lower than two and not higher than forty-five or not lower than five and not higher than thirty-five.

[0082] Even when a current density while electrochemical cell 1 is operating is high, electrochemical cell 1 desirably achieves high Faraday efficiency not lower than 90% or furthermore not lower than 95%. High Faraday efficiency is obtained by raising ratio W1 / W2 in accordance with magnitude of the current density during operations. For example, in an example where the current density during operations is lower than 1 A / cm2, when ratio W1 / W2 is higher than one, furthermore not lower than three, or furthermore not lower than five, high Faraday efficiency is obtained. For example, in an example where the current density during operations is not lower than 1 A / cm2 that is further higher, when ratio W1 / W2 is higher than five and not higher than forty or furthermore not lower than eight and not higher than thirty-eight or not lower than ten and not higher than thirty-five, high Faraday efficiency is obtained.

[0083] Ridge portion 63 has average width W1, for example, not smaller than 1 mm and not larger than 50 mm. When average width W1 is not smaller than 1 mm, the flow velocity of second fluid 102 at ridge portion 63 tends to sufficiently be high. When average width W1 is not larger than 50 mm, the number of first grooves 61d and second grooves 62d in second flow field 6 is sufficiently large and hence pressure loss in second flow field 6 does not tend to be great. Average width W1 may be, for example, not smaller than 2 mm and not larger than 45 mm or not smaller than 3 mm and not larger than 40 mm.

[0084] First groove 61d and second groove 62d have average width W2, for example, not smaller than 0.5 mm and not larger than 5 mm. When average width W2 is not smaller than 0.5 mm, second fluid 102 is readily diffused over the entire second flow field 6 from first flow channel 61 and diffused second fluid 102 is readily collected in second flow channel 62. When average width W2 is not larger than 5 mm, the amount of second fluid 102 discharged from second flow field 6 without being in contact with second catalyst layer 40 is reduced. Average width W2 may be, for example, not smaller than 0.6 mm and not larger than 3 mm or not smaller than 0.8 mm and not larger than 2 mm.

[0085] First groove 61d and second groove 62d have an average depth d, for example, not smaller than 0.5 mm and not larger than 5 mm. Average depth d is obtained, for example, as below. Depths at a start end position, a terminal end position, and an intermediate position over the length of each of first groove 61d and second groove 62d are measured. An average of measurement values of all depths is average depth d. When average depth d is not smaller than 0.5 mm, pressure loss in second flow field 6 tends to be less. When average depth d is not larger than 5 mm, a thickness of second conductive plate 41 can be made smaller while strength of second conductive plate 41 is maintained. A small thickness of second conductive plate 41 can achieve reduction in size of electrochemical cell 1. Average depth d may be, for example, not smaller than 0.6 mm and not larger than 3 mm or not smaller than 0.8 mm and not larger than 2 mm. The depths of first groove 61d and second groove 62d may be uniform or varied along the lengths of first groove 61d and second groove 62d.

[0086] Unlike the configuration described above, end 61e of first groove 61d of first flow channel 61 and second connection groove 62b of second flow channel 62 in Fig. 4 may be connected to each other by a groove narrower than first groove 61d. In this case again, first flow channel 61 is in such a structure that second fluid 102 less easily flows at outlet end 61B than at inlet end 61A. End 62e of second groove 62d of second flow channel 62 and first connection groove 61b of first flow channel 61 may be connected to each other by a groove narrower than second groove 62d. In this case again, second flow channel 62 is in such a structure that second fluid 102 less easily flows at inlet end 62A than at outlet end 62B.

[0087] <<Configuration of Flow Channel in First Flow Field>> As shown in Fig. 3, first flow field 5 may include a flow channel 50 that smoothens the flow of first fluid 101. Flow channel 50 in the present example is a groove provided in first conductive plate 31. Flow channel 50 in the present example includes a supply groove 51, a first base groove 52, a plurality of branch grooves 53, a second base groove 54, and a discharge groove 55. Supply groove 51 is connected to a manifold 3A that passes through first conductive plate 31. Manifold 3A is directly connected to the end of first supply pipe 101A shown in Fig. 1. In other words, supply groove 51 is directly connected by manifold 3A to inlet 5A of first flow field 5 shown in Fig. 1. First base groove 52 is arranged in a lower portion of first conductive plate 31 and extends in parallel to a surface where electrochemical cell 1 is provided. Second base groove 54 is arranged in an upper portion of first conductive plate 31 and extends in parallel to first base groove 52. Branch groove 53 has a lower end and an upper end connected to first base groove 52 and second base groove 54, respectively. Branch groove 53 extends upward from first base groove 52.

[0088] Discharge groove 55 is connected to a manifold 3B that passes through first conductive plate 31. Manifold 3B is directly connected to the end of first discharge pipe 101B shown in Fig. 1. In other words, discharge groove 55 is directly connected by manifold 3B to outlet 5B of first flow field 5. Flow channel 50 configured above is a straight flow channel through which supply groove 51 to discharge groove 55 are all connected. Flow channel 50 allows quick diffusion of water contained in first fluid 101 over the entire first flow field 5. Flow channel 50 allows quick discharge of oxygen generated in first catalyst layer 30 to the outside of electrochemical cell 1. Unlike the present example, flow channel 50 may be provided in first diffusion layer 32.

[0089] As shown with a bold arrow in Fig. 5, in the straight flow channel, a flow of first fluid 101 from branch groove 53 toward first diffusion layer 32 and a flow of first fluid 101 from first diffusion layer 32 toward branch groove 53 tend to be formed.

[0090] Unlike the present example, flow channel 50 in first flow field 5 may be a groove similar in structure to first flow channel 61 and second flow channel 62 in second flow field 6.

[0091] <<Formation of Non-Current-Conduction Area>> In second flow field 6 of electrochemical cell 1 shown in Fig. 1, a side reaction where proton changes to hydrogen gas may occur. This side reaction lowers Faraday efficiency of electrochemical cell 1. By forming a non-current-conduction area where no current flows along a direction of layering of anode portion 3, electrolyte membrane 2, and cathode portion 4 in electrochemical cell 1, a frequency of occurrence of the side reaction can be lowered. As shown in Fig. 2, the non-current-conduction area is formed by insulating member 8 arranged in at least one of first flow field 5 and second flow field 6. As the frequency of occurrence of the side reaction lowers, Faraday efficiency of electrochemical cell 1 improves.

[0092] Insulating member 8 is superimposed, for example, on at least one of first conductive plate 31 and second conductive plate 41 when electrochemical cell 1 is viewed from a first direction which is perpendicular to first surface 21 and extends from first surface 21 toward second surface 22. A single insulating member 8 or a plurality of insulating members 8 may be provided. In the present example, a single insulating member 8 is layered on second conductive plate 41 in second flow field 6.

[0093] Insulating member 8 in the present example is a sheet in a rectangular frame shape. This insulating member 8 is superimposed on at least a part of a first ridge area 65 shown in Fig. 6. First ridge area 65 is an area shown with chain double-dotted hatching. First ridge area 65 is an area in a band shape along first connection groove 61b of first flow channel 61. First ridge area 65 includes a portion extending from a groove edge of first connection groove 61b of first flow channel 61 to end 62e of second groove 62d of second flow channel 62. In the present example, as shown in Fig. 7, insulating member 8 is arranged such that a lower frame portion thereof covers the entire first ridge area 65. In the present example, insulating member 8 extends to first connection groove 61b. Unlike the present example, insulating member 8 may extend to a part of second groove 62d which is an area close to end 62e. Insulating member 8 may have a portion corresponding to first groove 61d cut away.

[0094] Insulating member 8 is further superimposed on at least a part of a second ridge area 66 shown in Fig. 6. Second ridge area 66 is an area shown with chain doubledotted hatching. Second ridge area 66 is an area in a band shape along second connection groove 62b of second flow channel 62. Second ridge area 66 includes a portion extending from a groove edge of second connection groove 62b of second flow channel 62 to end 61e of first groove 61d of first flow channel 61. In the present example, as shown in Fig. 7, insulating member 8 is arranged such that an upper frame portion thereof covers the entire second ridge area 66. In the present example, insulating member 8 extends to second connection groove 62b. Unlike the present example, insulating member 8 may extend to a part of first groove 61d which is an area close to end 61e. Insulating member 8 may have a portion corresponding to second groove 62d cut away.

[0095] As shown in Fig. 7, a portion of second conductive plate 41 exposed through a window portion of insulating member 8 is a current conduction area 7 where a current flows in the first direction of electrochemical cell 1. In second flow field 6 in the present example, first ridge area 65 and second ridge area 66 are areas where second fluid 102 less easily flows than the area where first groove 61d and second groove 62d are alternately arranged. When the current flows in the first direction in this area where second fluid 102 less easily flows, generation of hydrogen as a result of the side reaction is likely. In the present example, as insulating member 8 in a frame shape covers first ridge area 65 and second ridge area 66, the current does not flow in the first direction in first ridge area 65 and second ridge area 66 and generation of hydrogen due to the side reaction is lessened. Consequently, high Faraday efficiency of electrochemical cell 1 is maintained. Unlike the present example, first ridge area 65 and second ridge area 66 may be covered with respective independent insulating members 8.

[0096] Insulating member 8 is formed of an electrically insulating material resistant to a fluid. Insulating member 8 has a resistivity, for example, not lower than 103 Q^cm. The resistivity may be not lower than 104 Q*cm or not lower than 105 Q*cm. A material satisfying such a resistivity is, for example, epoxy resin, phenol resin, polyamide resin such as PA6 or PA66, polyoxymethylene resin, fluororesin, polyphenylenesulfide resin, glass fibers, insulating rubber, or insulating varnish. Fluororesin is, for example, polytetrafluoroethylene resin or a copolymer of tetrafluoroethylene and perfluoroethylene.

[0097] Insulating member 8 in the form of the sheet is arranged between electrolyte membrane 2 and second diffusion layer 42 or between second diffusion layer 42 and second conductive plate 41. Insulating member 8 arranged at such a position insulates a part of second conductive plate 41. Unlike the present example, in an example where insulating member 8 in the form of the sheet is arranged in first flow field 5, insulating member 8 should only be arranged between electrolyte membrane 2 and first diffusion layer 32 or between first diffusion layer 32 and first conductive plate 31.

[0098] Insulating member 8 may be integrated with second conductive plate 41. For example, insulating member 8 is fixed to second conductive plate 41 with an adhesive or the like. In this case, alignment between insulating member 8 and second conductive plate 41 is not necessary and ease in assembly of electrochemical cell 1 is improved. Insulating member 8 may be a resin material integrated with second conductive plate 41. The resin material is applied, for example, to second conductive plate 41. In this case again, alignment between insulating member 8 and second conductive plate 41 is not necessary and ease in assembly of electrochemical cell 1 is improved.

[0099] Insulating member 8 may be a resin material integrated with second diffusion layer 42. In this case again, alignment between insulating member 8 and second diffusion layer 42 is not necessary and ease in assembly of electrochemical cell 1 is improved. The resin material may be integrated with second diffusion layer 42 by application thereof to second diffusion layer 42 or impregnation of second diffusion layer 42 therewith. In this case, pores in second diffusion layer 42 are closed by the resin material. Therefore, since second fluid 102 does not flow to a portion in second diffusion layer 42 filled with the resin material, pressure loss in cathode portion 4 is reduced. This configuration and insulating member 8 in the form of the sheet may both be used.

[0100] <<Configuration of Second Diffusion Layer>> Second diffusion layer 42 is a porous layer formed of a conductive material. As described above, second diffusion layer 42 is compressed between electrolyte membrane 2 and second conductive plate 41. Fig. 8 is a schematic diagram showing a state of layering of some features in electrochemical cell 1. When there is a local gap at a boundary 1A between second diffusion layer 42 and electrolyte membrane 2 and a boundary 1B between second diffusion layer 42 and second conductive plate 41, second fluid 102 is less likely to uniformly spread over the entire surface of second catalyst layer 40. When an internal pressure in second flow field 6 changes while electrochemical cell 1 is operating, a local gap may be provided. When compressed second diffusion layer 42 presses electrolyte membrane 2 and second conductive plate 41 at a prescribed surface pressure or higher, the local gap is less likely to be formed at boundary 1A or 1B. A thickness of electrolyte membrane 2 may vary by approximately 20% at the maximum due to variation in temperature or content of water. For example, in an example where electrolyte membrane 2 has the thickness of 200 pm, the thickness may vary by approximately 40 pm at the maximum. Since a temperature and a state of flow of water or the like in electrochemical cell 1 are different between when electrochemical cell 1 is operating and when electrochemical cell 1 is not operating, a gap as large as approximately 20% of the thickness of electrolyte membrane 2 at the maximum may be provided due to variation in thickness of electrolyte membrane 2. When second diffusion layer 42 presses electrolyte membrane 2 and second conductive plate 41 at the prescribed surface pressure or higher as described above, variation in thickness of electrolyte membrane 2 can be followed. Therefore, the local gap at boundary 1A or 1B is less likely also in this case.

[0101] Compressive strain of second diffusion layer 42 being compressed in cathode portion 4 of electrochemical cell 1 is not less than 0.3 and not more than 0.8. The surface pressure applied to second diffusion layer 42 compressed in electrochemical cell 1 is, for example, not lower than 0.5 MPa. The upper limit of the surface pressure is, for example, 0.8 MPa. As shown in Fig. 12 in a test example 4 which will be described later, as the surface pressure is higher, compressive strain is also greater. Therefore, when compressive strain of second diffusion layer 42 at the surface pressure of 0.5 MPa is not less than 0.3, compressive strain of second diffusion layer 42 in cathode portion 4 to which the surface pressure not lower than 0.5 MPa is applied is also not less than 0.3. Compressive strain in the present example is expressed as a value obtained, for example, by dividing an amount of decrease X in thickness of second diffusion layer 42 compressed at the surface pressure not lower than 0.5 MPa and not higher than 0.8 MPa by an initial thickness t0 of second diffusion layer 42 yet to be compressed. Alternatively, amount of decrease X is expressed as t0-t1 where t0 represents the initial thickness of second diffusion layer 42 and t1 represents the thickness of second diffusion layer 42 compressed at the surface pressure not lower than 0.5 MPa and not higher than 0.8 MPa. Therefore, compressive strain is obtained as (t0-t1) / t0.  For example, when second diffusion layer 42 having initial thickness t0 of 1 mm is compressed at the surface pressure of 0.5 MPa with thickness t1 of second diffusion layer 42 being 0.7 mm, amount of decrease X is 0.3 mm. Since compressive strain is calculated as compressive strain = X / t0, compressive strain in this case is 0.3.

[0102] Thickness t0 or t1 of second diffusion layer 42 is measured in conformity with a method A under JIS L 1096: 2010. Specifically, a commercially available thickness measurement apparatus is used to measure the thickness of second diffusion layer 42 for a certain time period at a certain surface pressure. The time period is set to ten seconds. The surface pressure in measurement of initial thickness t0 is set to 0.7 kPa. Initial thickness t0 is expressed as a value calculated by averaging thicknesses obtained by measurement at five or more points. Thickness t1 is obtained similarly to initial thickness t0, with the surface pressure in measurement of thickness t1 being varied, for example, to 0.5 MPa. Thickness t0 or t1 of second diffusion layer 42 while it is not assembled to electrochemical cell 1 is measured. Initial thickness t0 of second diffusion layer 42 is, for example, not smaller than 0.3 mm and not larger than 3.5 mm.

[0103] Second diffusion layer 42 having compressive strain not less than 0.3 and not more than 0.8 is excellent in elastic deformation capability. In other words, second diffusion layer 42 having compressive strain not less than 0.3 and not more than 0.8 deforms to a thickness smaller than initial thickness t0 while the prescribed surface pressure is applied thereto. Second diffusion layer 42 with such elastic deformation capability deforms again to a thickness close to or equivalent to initial thickness t0 when the surface pressure lowers or is removed. When such second diffusion layer 42 is compressed between electrolyte membrane 2 and second conductive plate 41, second diffusion layer 42 presses electrolyte membrane 2 and second conductive plate 41 between which it lies in cathode portion 4, in an attempt to return to the thickness before it is compressed. Therefore, second diffusion layer 42 tends to be in surface contact with electrolyte membrane 2 and second conductive plate 41. Therefore, lowering in Faraday efficiency due to the local gap at boundary 1A or 1B is lessened. Compressive strain may be not less than 0.4. 0.5, 0.6, or 0.7.

[0104] Second diffusion layer 42 which is the porous layer may satisfy, for example, elastic deformation characteristics below. • Elastic deformation characteristics: A difference tB-tA in thickness is not smaller than 40 pm where tA represents the thickness of second diffusion layer 42 while a pressure of 0.5 MPa is applied thereto and tB represents the thickness of second diffusion layer 42 under the surface pressure of 0.7 kPa after application of the former pressure.

[0105] Details of a method of measuring the difference in thickness is as below. A load of 0.5 MPa is applied to a specimen of 5 cm x 5 cm. A holding time period is set to one minute. After lapse of one minute, thickness tA is measured in this loaded state. After lapse of one minute since removal of the load, thickness tB is measured under the surface pressure of 0.7 kPa. A method of measuring thickness tB is the same as the method of measuring initial thickness t0 described above.

[0106] Even when the prescribed pressure is applied to second diffusion layer 42 difference in thickness tB-tA of which is not smaller than 40 pm, the second diffusion layer has great capability to return to the thickness before the pressure is applied thereto, that is, it is excellent in elastic deformation capability. As the difference in thickness is larger, elastic deformation capability is higher. The difference in thickness may be not smaller than 50 pm or not smaller than 60 pm, although depending on initial thickness t0 and the surface pressure. The upper limit of the difference in thickness is, for example, initial thickness t0 x 0.9 pm. Second diffusion layer 42 compressive strain of which satisfies 0.6 or more representatively satisfies the elastic deformation characteristics.

[0107] For example, in an example where second catalyst layer 40 is provided at second surface 22 of electrolyte membrane 2, the surface of second catalyst layer 40 may have asperities. When an amount of elastic deformation of second diffusion layer 42 is small, a small gap, for example, of approximately 100 pm or smaller may locally be provided between second catalyst layer 40 and second diffusion layer 42 due to the asperities. This gap may be a through hole for a fluid. Second diffusion layer 42 excellent in elastic deformation capability such as compressive strain not less than 0.3 or furthermore not less than 0.5 or in particular not less than 0.6 could elastically deform to bury such a gap. As second diffusion layer 42 elastically deforms as following small asperities at the surface of second catalyst layer 40, provision of a gap that may serve as the through hole is less likely.

[0108] Second diffusion layer 42 that satisfies the compressive strain is, for example, a non-woven fabric containing carbon fibers. The non-woven fabric containing carbon fibers is interlaced carbon fibers independent of one another as described above, and it does not have a binding portion such as a binder that fixes carbon fibers to one another. The non-woven fabric without the binding portion is high in elastic deformation capability. So long as the compressive strain is satisfied, second diffusion layer 42 may be a woven fabric containing carbon fibers. The woven fabric is a fabric where warp threads and weft threads of carbon fibers are alternately woven. The woven fabric containing carbon fibers is also called a carbon cloth. Paper containing carbon fibers is considered as not satisfying the compressive strain. Paper contains a plurality of carbon fibers and a binder that fixes carbon fibers. For example, an amount of deformation of the thickness of paper is smaller than 20 pm or furthermore approximately 10 pm even when a large surface pressure of 1 MPa is applied, although depending on the thickness of paper yet to be compressed. Compressive strain of such paper to which a surface pressure, for example, of 0.5 MPa is applied is less than 0.2 and the paper hardly deforms.

[0109] So long as the compressive strain of second diffusion layer 42 as a whole is not less than 0.3, the second diffusion layer may include a member the compressive strain of which is less than 0.3. For example, second diffusion layer 42 may be configured in such a manner that a plurality of non-woven fabrics different in compressive strain are layered, a non-woven fabric and a woven fabric are layered, a non-woven fabric and paper are layered, or a non-woven fabric, a woven fabric, and paper are layered.

[0110] The carbon fibers contained in the non-woven fabric have an average diameter, for example, not smaller than 5 pm and not larger than 100 pm. The carbon fibers having the average diameter not smaller than 5 pm and not larger than 100 pm are not too thin or too thick. Second diffusion layer 42 formed from carbon fibers not too thin and not too thick is excellent in elastic deformation capability. Therefore, second diffusion layer 42 compressed between electrolyte membrane 2 and second conductive plate 41 is readily in intimate contact with electrolyte membrane 2 and second conductive plate 41. When the average diameter is not smaller than 5 pm and not larger than 100 pm and a basis weight of second diffusion layer 42 satisfies a range which will be described later, second diffusion layer 42 is higher in elastic deformation capability. The carbon fibers may have the average diameter, for example, not smaller than 5 pm and not larger than 80 pm, not smaller than 5 pm and not larger than 50 pm, or not smaller than 7 pm and not larger than 30 pm.

[0111] The average diameter of carbon fibers is expressed as a value calculated by obtaining a diameter of a circle equal in area to a cross-section of each carbon fiber for each of carbon fibers and averaging diameters. The average diameter of carbon fibers is obtained as below. Second diffusion layer 42 is cut in a direction along the thickness thereof. As a result of this cutting, cross-sections of the plurality of carbon fibers are exposed. The cross-section of second diffusion layer 42 is observed with a microscope and five or more observation fields of view are taken. The microscope is, for example, a scanning electron microscope. A magnification for observation of the cross-section is, for example, not lower than 500x and not higher than 3000x. A diameter of a circle as large as a cross-sectional area of each carbon fiber is obtained for three or more carbon fibers per one observation field of view. The diameters of the circles obtained in all of the five or more observation fields of view are averaged. The calculated average value is the average diameter of carbon fibers.

[0112] Second diffusion layer 42 being compressed in electrochemical cell 1 has the porosity, for example, not lower than 40% and not higher than 98%. So long as the porosity of second diffusion layer 42 is within the range above, a density of second diffusion layer 42 is not too low or too high and hence second diffusion layer 42 is excellent in elastic deformation capability. Pores in second diffusion layer 42 are the flow channel in second fluid 102. Therefore, when the porosity is not lower than 40%, second fluid 102 readily passes through second diffusion layer 42. In addition, the flow velocity of second fluid 102 that passes through second diffusion layer 42 readily increases. When the porosity is not higher than 98%, strength of second diffusion layer 42 tends to be maintained. The porosity may be, for example, not lower than 45% and not higher than 97% or not lower than 50% and not higher than 96%. The porosity is obtained from a cross-sectional photograph of the cross-section of second diffusion layer 42. The cross-sectional photograph is subjected to binarization to obtain an area of a solid portion and a porous portion. An area ratio of the porous portion to the total area of the solid portion and the porous portion is the porosity. When an amount of decrease in thickness of second diffusion layer 42 in electrochemical cell 1 is known, the porosity of second diffusion layer 42 in the compressed state is obtained by calculation.

[0113] The basis weight of second diffusion layer 42 is, for example, not smaller than 50 g / m2 (gram / square meter) and not larger than 400 g / m2.  When the basis weight of second diffusion layer 42 is not smaller than 50 g / m2, the density of second diffusion layer 42 is not excessively low. Such second diffusion layer 42 is excellent in elastic deformation capability. Therefore, second diffusion layer 42 compressed between electrolyte membrane 2 and second conductive plate 41 is readily in intimate contact with electrolyte membrane 2 and second conductive plate 41. When the basis weight is not smaller than 50 g / m2, conductivity of second diffusion layer 42 tends to be high. When the basis weight of second diffusion layer 42 is not larger than 400 g / m2, the density of second diffusion layer 42 is not excessively high. Such second diffusion layer 42 is excellent in elastic deformation capability. Therefore, second diffusion layer 42 compressed between electrolyte membrane 2 and second conductive plate 41 is readily in intimate contact with electrolyte membrane 2 and second conductive plate 41. When the basis weight is not larger than 400 g / m2, pores sufficient for flow tends to be provided in second diffusion layer 42. The basis weight may be, for example, not smaller than 55 g / m2 and not larger than 395 g / m2 or not smaller than 60 g / m2 and not larger than 390 g / m2. The basis weight may be, for example, not smaller than 55 g / m2 and not larger than 200 g / m2 or not smaller than 60 g / m2 and not larger than 150 g / m2. The basis weight is a mass of second diffusion layer 42 per one square meter. The basis weight is obtained by dividing the mass of second diffusion layer 42 by an area while second diffusion layer 42 is viewed in a plan view.

[0114] When second diffusion layer 42 is made from the non-woven fabric containing carbon fibers, the basis weight is not smaller than 50 g / m2 and not larger than 200 g / m2, and the carbon fibers have the average diameter not smaller than 5 pm and not larger than 50 pm, second diffusion layer 42 tends to be high in elastic deformation capability and is in intimate contact with electrolyte membrane 2 and second conductive plate 41. As second diffusion layer 42 is in intimate contact with electrolyte membrane 2 and second conductive plate 41, Faraday efficiency of electrochemical cell 1 may be improved. This effect will specifically be described with reference to test examples which will be described later.

[0115] <<Configuration of First Diffusion Layer>> First diffusion layer 32 formed of a metallic material is smaller in compressive strain than second diffusion layer 42. In other words, first diffusion layer 32 is relatively less likely to deform than second diffusion layer 42. In an example where second diffusion layer 42 has elastic deformation capability, when the internal pressure of cathode portion 4 becomes higher than the internal pressure of anode portion 3, second diffusion layer 42 may strongly press electrolyte membrane 2. As electrolyte membrane 2 pushed by second diffusion layer 42 deforms, electrolyte membrane 2 may be damaged. In addition, a gap may be produced at boundary 1A between electrolyte membrane 2 and second diffusion layer 42 and boundary 1B between second diffusion layer 42 and second conductive plate 41. In the present example, first diffusion layer 32 relatively less likely to deform than second diffusion layer 42 prevents deformation of electrolyte membrane 2. First diffusion layer 32 that is less likely to deform supports first surface 21 of electrolyte membrane 2 and prevents excessive deformation of electrolyte membrane 2.

[0116] <Test Example 1> In a test example 1, influence by first flow channel 61 and second flow channel 62 provided in second flow field 6 of electrochemical cell 1 on Faraday efficiency of electrochemical cell 1 was investigated.

[0117] <<Configuration of Electrochemical Cell>> The configuration of electrolyte membrane 2, anode portion 3, and cathode portion 4 that formed electrochemical cell 1 was as below.

[0118] • Electrolyte membrane 2 Electrolyte membrane 2 was a proton-conductive thin film. Electrolyte membrane 2 had a thickness of 180 Lim (micrometer).

[0119] • Anode portion 3 First catalyst layer 30 is formed on first surface 21 of electrolyte membrane 2. A catalyst contained in first catalyst layer 30 is iridium oxide. An amount of iridium oxide in first catalyst layer 30 is 1.0 mg (milligram) / cm2. First conductive plate 31 is formed of a composite material of a carbon-based material and resin. A surface of first conductive plate 31 that faces first surface 21 is provided with flow channel 50 defined by the groove. Flow channel 50 is the straight flow channel directly connected to inlet 5A and outlet 5B of first flow field 5. Branch groove 53 that defines flow channel 50 had the average width of 1 mm (millimeter) and the average depth of 1.5 mm. A ridge portion formed between adjacent branch grooves 53 had the average width of 1 mm. First diffusion layer 32 is formed of a titanium porous body. A surface of the titanium porous body is coated with platinum. First diffusion layer 32 had a thickness of 0.3 mm.

[0120] • Cathode portion 4 Second catalyst layer 40 is formed on second surface 22 of electrolyte membrane 2. A catalyst contained in second catalyst layer 40 is carbon on which platinum and ruthenium are carried. An amount of catalyst in second catalyst layer 40 is 1.0 mg / cm2. Second conductive plate 41 is formed of a composite material of a carbon-based material and resin. A surface of second conductive plate 41 that faces second surface 22 is provided with first flow channel 61 and second flow channel 62 defined by the grooves. First groove 61d and second groove 62d of first flow channel 61 and second flow channel 62 have average width W2 of 1.0 mm and average depth d of 1.0 mm. Ridge portion 63 formed between first groove 61d and second groove 62d has average width W1 of 25 mm. Ratio W1 / W2 was 25. Second diffusion layer 42 was made from a carbon felt. The carbon felt is formed by disordered entanglement of carbon fibers by the interlacing treatment as described above. Second diffusion layer 42 had initial thickness t0 of 1 mm and second diffusion layer 42 assembled to electrochemical cell 1 had the thickness of 0.3 mm.

[0121] <<Test Conditions>> • Anode portion 3 First fluid 101 was pure water. A heater is arranged at first supply pipe 101A and first fluid 101 is increased in temperature before it is supplied to first flow field 5. The temperature of first fluid 101 at inlet 5A of first flow field 5 was not lower than 50°C.  Increase in temperature by the heater is adjusted such that the temperature of first fluid 101 at outlet 5B of first flow field 5 is not higher than 70°C. A current density at the time of the test was 1.0 A / cm2 (ampere / square centimeter) or 1.5 A / cm2. A flow rate of first fluid 101 under the condition of 1.5 A / cm2 was higher than the flow rate of first fluid 101 under the condition of 1.0 A / cm2.  The current density is expressed as a value calculated by dividing the current fed between first conductive plate 31 and second conductive plate 41 by a current conduction area to which a voltage is applied. The flow rate is an amount of first fluid 101 that flows in one minute through each branch groove 53. Since an amount of supply of first fluid 101 to anode portion 3 has already been known, the flow rate in branch groove 53 is obtained by calculation.

[0122] • Cathode portion 4 Second fluid 102 contains toluene as the substance to be hydrogenated. Toluene is converted to MCH by hydrogenation. A heater is arranged at second supply pipe 102A and second fluid 102 is increased in temperature before it is supplied to second flow field 6. The temperature of second fluid 102 at inlet 6A of second flow field 6 was not lower than 50°C.  Increase in temperature by the heater was adjusted such that the temperature of second fluid 102 at outlet 6B of second flow field 6 was not higher than 70°C. The flow rate of second fluid 102 under the condition of 1.5 A / cm2 was higher than the flow rate of second fluid 102 under the condition of 1.0 A / cm2.

[0123] <<Evaluation Method>> The current was fed to electrochemical cell 1 and second fluid 102 was taken from second tank 102T each time a prescribed time period elapsed. Taken second fluid 102 was analyzed by gas chromatography and a concentration of toluene contained in second fluid 102 was calculated. The concentration of toluene is expressed by percentage of the number of molecules of toluene, with the total number of molecules of toluene and MCH contained in second fluid 102 being defined as 100% (percent).

[0124] Faraday efficiency in cathode portion 4 was obtained. In cathode portion 4, hydrogen is generated by a side reaction where protons not consumed for hydrogenation of toluene are bound to each other. The side reaction in cathode portion 4 is only a hydrogen generation reaction. Therefore, an amount of charges not used for generation of MCH in a total amount of charges injected into electrochemical cell 1 can be obtained by calculation, by measuring a volume of hydrogen generated by the side reaction. A value obtained by subtracting the amount of charges used for generation of hydrogen from the total amount of charges is an amount of charges used for generation of MCH. The amount of charges used for generation of MCH with the total amount of charges being defined as 100% is Faraday efficiency. Faraday efficiency is expressed in a unit %.

[0125] Relation between a toluene concentration and Faraday efficiency each time of lapse of the prescribed time period is shown in a graph in Fig. 9. The abscissa in the graph represents the toluene concentration (mol %) and the ordinate represents Faraday efficiency (%). A black circular plot represents a result of measurement under the condition of the current density of 1.0 A / cm2.  A rhombic plot represents a result of measurement under the condition of the current density of 1.5 A / cm2.  With lapse of time since start of current feed, the toluene concentration in second fluid 102 lowers. Therefore, the plot located toward the left of the graph represents the result of measurement of second fluid 102 taken at the time of lapse of the time period since start of current feed.

[0126] <<Evaluation Result>> In the present test, the flow rate of second fluid 102 in cathode portion 4 is significantly higher than the flow rate of first fluid 101 in anode portion 3. No malfunction, however, due to the internal pressure in electrochemical cell 1 in the present test occurred. The reason why no malfunction occurred in electrochemical cell 1 is estimated as reduction of pressure loss in second flow field 6 by first flow channel 61 and second flow channel 62 arranged in second flow field 6 of cathode portion 4.

[0127] As shown in the graph in Fig. 9, it was clarified that, under the condition of the current density of 1.0 A / cm2, even when the toluene concentration in second fluid 102 is 10%, Faraday efficiency was substantially not lower than 99% and Faraday efficiency was maintained at 95% or higher or furthermore at 98% or higher. It was clarified that, under the condition of the current density of 1.5 A / cm2, even when the toluene concentration in second fluid 102 is 10%, Faraday efficiency was maintained at 90% or higher. Electrochemical cell 1 high in flow rate of second fluid 102 and high in Faraday efficiency can generate a large amount of MCH in a short period of time.

[0128] <Test Example 1-2> Second electrochemical cell 1 and third electrochemical cell 1 different in value of ratio W1 / W2 from the electrochemical cell in test example 1 were prepared and Faraday efficiency of second electrochemical cell 1 and third electrochemical cell 1 was investigated. Ratio W1 / W2 of second electrochemical cell 1 was five and ratio W1 / W2 of third electrochemical cell 1 was forty-five. Test conditions in a test example 1-2 were the same as those in test example 1 where the current density was set to 1.0 A / cm2.

[0129] The result of measurement of electrochemical cell 1 in test example 1 and a result of measurement of second electrochemical cell 1 are shown in a graph in Fig. 10. Fig. 10 is to be viewed similarly to Fig. 9. The black circular plot represents the result of measurement of electrochemical cell 1 in test example 1. A triangular plot represents a result of measurement of second electrochemical cell 1. A quadrangular plot represents a result of measurement of third electrochemical cell 1.

[0130] As shown in Fig. 10, it was clarified that Faraday efficiency of second electrochemical cell 1 ratio W1 / W2 of which was five was substantially 90% even when the toluene concentration was 10% and Faraday efficiency was high. Under a condition where the toluene concentration was the same, Faraday efficiency of third electrochemical cell 1 ratio W1 / W2 of which was forty-five was substantially the same as Faraday efficiency of first electrochemical cell 1 ratio W1 / W2 of which was twenty-five, that is, substantially not lower than 99%, and Faraday efficiency not lower than 98% was maintained. It was found that, at the relatively high current density such as 1 A / cm2, as the value of ratio W1 / W2 is larger, lowering in Faraday efficiency with lowering in toluene concentration was less likely. Under this test condition, pressure loss was extremely great in electrochemical cell 1 ratio W1 / W2 of which exceeded fifty and Faraday efficiency of that electrochemical cell 1 could not be evaluated.

[0131] <Test Example 2> In a test example 2, influence by average width W1 of ridge portion 63 in second flow field 6 on pressure loss and Faraday efficiency in second flow field 6 was investigated.

[0132] In test example 2, a plurality of electrochemical cells 1 different in average width W1 of ridge portion 63 were prepared. Average width W1 was one of three values selected from a range larger than 1 mm and not larger than 30 mm. Features other than average width W1 were the same as in test example 1. First fluid 101 and second fluid 102 were fed to each electrochemical cell 1 and pressure loss in second flow field 6 in each electrochemical cell 1 and Faraday efficiency of each electrochemical cell 1 were measured. The amount of supply of first fluid 101 is the flow rate of first fluid 101 at inlet 5A of first flow field 5. The amount of supply of second fluid 102 was 0.5, 1, 2, or 5 times as large as the amount of supply of first fluid 101. The amount of supply of second fluid 102 is the flow rate of second fluid 102 at inlet 6A of second flow field 6. Table 1 shows results of measurement of pressure loss in second flow field 6 and Table 2 shows results of measurement of Faraday efficiency of electrochemical cell 1.

[0133] "100% TOL*0.4 A / cm2*0.5 h" in Tables 1 and 2 indicates that the concentration of toluene occupied in second fluid 102 before current feed to electrochemical cell 1 was 100% and current feed at the current density of 0.4 A / cm2 was performed for 30 minutes. "10% TOL*0.2 A / cm2*0.5 h" and "5% TOL*0.2 A / cm2*0.5 h" also indicate the toluene concentration before current feed, the current density in current feed, and a time period for current feed. A numerical value shown in the field "amount of supply of second fluid" is a multiplier for the amount of supply of first fluid 101. For example, "1 time" means that the amount of supply of second fluid 102 is the same as the amount of supply of first fluid 101. A numerical value shown in the field "average width W1 of ridge portion" represents a ratio the smallest value of which is "1". In cells in Table 1, a value of pressure loss in second flow field 6 is shown. Pressure loss in second flow field 6 is a difference between the pressure of second fluid 102 at outlet 6B of second flow field 6 and the pressure of second fluid 102 at inlet 6A of second flow field 6. Pressure loss is expressed in a unit kPa (kilopascal). In cells in Table 2, Faraday efficiency of electrochemical cell 1 is shown. Faraday efficiency is expressed in a unit %. A diagonal line is drawn over a cell corresponding to a condition for which measurement was not conducted.

[0134] [Table 1] Amount of Supply of Second Fluid 100% TOL*0.4 A / cm2*0.5 h 10% TOL*0.2 A / cm2*0.5 h 5% TOL*0.2 A / cm2*0.5 h Average Width W1 of Ridge Portion Average Width W1 of Ridge Portion Average Width W1 of Ridge Portion 1 3 5 1 3 5 1 3 5 1 time 8 kPa 13 kPa 25 kPa 8 kPa 13 kPa 29 kPa 31 kPa 60 kPa 2 times 72 kPa 5 times 10 kPa 21 kPa 68 kPa 10 kPa 20 kPa 48 kPa 56 kPa 99 kPa

[0135] [Table 2] Amount of Supply of Second Fluid 100% TOL*0.4 A / cm2*0.5 h 10% TOL*0.2 A / cm2*0.5 h 5% TOL*0.2 A / cm2*0.5 h Average Width W1 of Ridge Portion Average Width W1 of Ridge Portion Average Width W1 of Ridge Portion 1 3 5 1 3 5 1 3 5 1 time 95.9% 99.3% 99.5% 64.5% 99.4% 99.4% 90.3% 93.4% 2 times 98.6% 5 times 98.3% 99.2% 99.4% 82.3% 99.7% 99.7% 94.2% 99.0%

[0136] As shown in Table 1, as average width W1 of ridge portion 63 is larger, pressure loss in second flow field 6 increases. This is because, as average width W1 of ridge portion 63 is larger, the number of first grooves 61d and second grooves 62d decreases and second fluid 102 less easily flows in second flow field 6. In addition, as the amount of supply of second fluid 102 is larger, pressure loss in second flow field 6 increases.

[0137] As shown in Table 2, as average width W1 of ridge portion 63 is larger, Faraday efficiency of electrochemical cell 1 is higher. In addition, as the rate of flow of second fluid 102 to electrochemical cell 1 is higher, Faraday efficiency of electrochemical cell 1 is higher. It was clarified from the results in Tables 1 and 2 that MCH could efficiently be produced by increasing average width W1 of ridge portion 63 and the amount of supply of second fluid 102 within a range where increase in pressure loss can be lessened.

[0138] <<Comparative Example>> As a comparative example, pressure loss of an electrochemical cell in which neither of the first flow field and the second flow field was provided with a groove was considered. In the comparative example, such an electrochemical cell is called a comparative cell. In the comparative cell, the first flow field is defined only by pores in the first diffusion layer in the anode portion and the second flow field is defined only by pores in the second diffusion layer in the cathode portion.

[0139] From the results in Table 1, with the toluene concentration being set to 100% and with the amount of supply of the second fluid being set to "1 time," the pressure of the pump when average width W1 of the ridge portion is "5" is at least three times as high as the pressure of the pump when average width W1 of the ridge portion is "1". With the amount of supply of the second fluid being set to "5 times," the pressure of the pump when average width W1 of the ridge portion is "5" is at least six times as high as the pressure of the pump when average width W1 of the ridge portion is "1". In other words, as the ridge portion is larger, the pressure of the pump is higher.

[0140] It is expected from these results that pressure loss in the second flow field is great in the electrochemical cell provided with the straight flow channel in the first flow field and not provided with the groove in the second flow field. It is expected that pressure loss is extremely great in the comparative cell provided with the groove in neither of the first flow field and the second flow field.

[0141] <Test Example 3> In a test example 3, influence by arrangement of insulating member 8 in electrochemical cell 1 on Faraday efficiency of electrochemical cell 1 was investigated.

[0142] In test example 3, two electrochemical cells 1 where no insulating member 8 was arranged were prepared and Faraday efficiency of each electrochemical cell 1 was investigated. The only difference between two electrochemical cells 1 resides in constituent material for second diffusion layer 42. Second diffusion layer 42 is made from a carbon felt or carbon paper. Average width W1 of ridge portion 63 in second conductive plate 41 is set to a value selected from a range larger than one and not larger than five. Current conduction area 7 of each of these electrochemical cell 1 had an area of 25 cm2.  The toluene concentration of second fluid 102 before current feed was 100% and the current density of current conduction area 7 was 0.4 A / cm2.

[0143] Furthermore, in test example 3, two electrochemical cells 1 where insulating member 8 was arranged were prepared and Faraday efficiency of each electrochemical cell 1 was investigated. Only difference between two electrochemical cells 1 resides in constituent material for second diffusion layer 42. Second diffusion layer 42 is made from a carbon felt or carbon paper. Average width W1 of ridge portion 63 in second conductive plate 41 is set to a value selected from the range larger than one and not larger than five. Insulating member 8 arranged in each electrochemical cell 1 was a sheet in a shape of a rectangular frame formed of polytetrafluoroethylene resin. Insulating member 8 had a resistivity not lower than 1018 Q^cm. The sheet is arranged between electrolyte membrane 2 and second diffusion layer 42. The sheet covers first ridge area 65 and second ridge area 66 shown with hatching in Fig. 6. Current conduction area 7 of each of these electrochemical cells 1 had an area of 17 cm2.  The toluene concentration before current feed was 100% and the current density of current conduction area 7 was 0.4 A / cm2.

[0144] [Table 3] Second Diffusion Layer Carbon felt Carbon paper Insulating Member No Yes No Yes Faraday Efficiency 88.2% 99.4% 85.1% 95.6%

[0145] As shown in Table 3, it was clarified that Faraday efficiency of electrochemical cell 1 was higher by insulating first ridge area 65 and second ridge area 66 with insulating member 8. It was clarified that Faraday efficiency of electrochemical cell 1 was higher by forming second diffusion layer 42 from the carbon felt. In particular, electrochemical cell 1 including insulating member 8 and second diffusion layer 42 formed from the carbon felt achieved Faraday efficiency not lower than 99%.

[0146] <Test Example 4> Compressive strain of second diffusion layer 42 in electrochemical cell 1 in each of test example 1 to test example 3 was measured. Second diffusion layer 42 is formed from the carbon felt. A carbon fiber included in the carbon felt had a diameter of 10 pm.  The carbon felt yet to be compressed had the thickness of 1000 pm, the basis weight of 90 g / cm2, and the porosity of 95.6%. This carbon felt when it was compressed to a thickness of 300 pm had the basis weight of 90 g / cm2 and the porosity of 86.7%. Compressive strain is obtained as a value calculated by dividing amount of decrease % (= t0-t1) in thickness of compressed second diffusion layer 42 by initial thickness t0 of second diffusion layer 42 yet to be compressed.

[0147] Initial thickness t0 of the carbon felt is measured with a commercially available thickness measurement apparatus in conformity with the method A under JIS L 1096: 2010, such as a constant pressured thickness measuring instrument PG-16J (measurement element diameter: O 25.2 mm) manufactured by TECLOCK Co., Ltd. Initial thickness t0 was measured with a surface pressure at 0.7 kPa being applied to the carbon felt for ten seconds.

[0148] Fig. 11 is a schematic diagram of a compression apparatus 9 for measuring compressive strain of second diffusion layer 42. Compression apparatus 9 includes a lower mount 90 and an upper mount 91. Upper mount 91 is configured as being movable downward. Compression apparatus 9 applies the surface pressure to a member that lies between lower mount 90 and upper mount 91. Compression apparatus 9 can automatically measure the surface pressure applied between lower mount 90 and upper mount 91 and an interval between lower mount 90 and upper mount 91. Compression apparatus 9 is a commercially available strength evaluation apparatus such as Micro Autograph MST-I type HR manufactured by Shimadzu Corporation.

[0149] In measurement of compressive strain of the carbon felt, a lower protective plate 92, second conductive plate 41, second diffusion layer 42, first conductive plate 31, and an upper protective plate 93 were sequentially layered on lower mount 90. Lower protective plate 92 and upper protective plate 93 serve to protect second conductive plate 41 and first conductive plate 31 against damage and to apply a uniform surface pressure to second diffusion layer 42. Lower protective plate 92 and upper protective plate 93 are formed of stainless steel. Among a plurality of members that lie between lower mount 90 and upper mount 91, a member the thickness of which changes is only second diffusion layer 42. Lower protective plate 92, second conductive plate 41, first conductive plate 31, and upper protective plate 93 are rigid bodies, and thicknesses thereof do not substantially change.

[0150] Compression apparatus 9 is activated to apply the surface pressure to second diffusion layer 42. The surface pressure applied to second diffusion layer 42 is gradually increased and a distance of movement of upper mount 91 corresponding to magnitude of the surface pressure is measured. The distance of movement can also be expressed as being equal to amount of decrease X in thickness of second diffusion layer 42. Compressive strain of second diffusion layer 42 was obtained from initial thickness t0 and the distance of movement. Specifically, X / t0 = compressive strain. Compressive strain is unitless. Relation between compressive strain and the surface pressure is shown in a graph in Fig. 12. The abscissa in the graph represents compressive strain and the ordinate represents the surface pressure.

[0151] In test example 4, compressive strain also of a carbon cloth and carbon paper was measured. Specifically, compressive strain of each of the carbon cloth and the carbon paper instead of the carbon felt was measured with a method the same as the method for the carbon felt. Results thereof are also shown in the graph in Fig. 12.

[0152] As shown in Fig. 12, compressive strain of the carbon felt at the time of pressurization at the surface pressure of 0.5 MPa was not less than 0.3 and not more than 0.8, or more specifically not less than 0.6 and not more than 0.8. The thickness of the carbon felt is thus greatly changed by compression. The carbon felt is excellent in elastic deformation capability. When second diffusion layer 42 formed from such a carbon felt is compressed between electrolyte membrane 2 and second conductive plate 41 as shown in Fig. 8, second diffusion layer 42 presses electrolyte membrane 2 and second conductive plate 41 at the surface pressure as high as compressive force. Therefore, second diffusion layer 42 is in intimate contact with electrolyte membrane 2 and second conductive plate 41 and substantially no local gap is produced at boundary 1A or 1B. Since there is substantially no local gap at boundary 1A or 1B, second fluid 102 readily spreads over the entire second flow field 6. Consequently, improvement in Faraday efficiency of electrochemical cell 1 is expected.

[0153] Compressive strain of the carbon cloth at the time of pressurization at the surface pressure of 0.5 MPa was not less than 0.3 and more specifically not less than 0.4. In other words, the thickness of the carbon cloth is changed to some extent by pressurization. When second diffusion layer 42 formed from such a carbon cloth is arranged between electrolyte membrane 2 and second conductive plate 41 as shown in Fig. 8, production of a gap is less likely at boundary 1A or 1B.

[0154] As described above, the carbon cloth and the carbon felt satisfy compressive strain not less than 0.3 even when the surface pressure is relatively as low as approximately 0.1 MPa and satisfies compressive strain not less than 0.3 when the surface pressure further increases. Since the surface pressure in electrochemical cell 1 is higher than 0.1 MPa, compressive strain of the carbon cloth and the carbon felt in electrochemical cell 1 can be concluded as satisfying 0.3 or more.

[0155] Compressive strain of the carbon paper at the time of pressurization at the surface pressure of 0.5 MPa was less than 0.2 or more specifically equal to or less than 0.18. In other words, the thickness of the carbon paper is substantially not changed by pressurization. When second diffusion layer 42 formed from such carbon paper is arranged between electrolyte membrane 2 and second conductive plate 41 as shown in Fig. 8, production of a gap is likely at boundary 1A or 1B.

[0156] In an example where electrochemical cell 1 is increased in size or the number of stacked electrochemical cells 1 is increased, a dimension of a member included in the electrochemical cell and fastening pressure for fastening a plurality of electrochemical cells 1 may be varied. Furthermore, variation in dimension due to deformation over time of the member may also be likely. In electrochemical cell 1 including second diffusion layer 42 formed from the carbon paper which is less likely to elastically deform, a gap may be likely at boundary 1A or 1B in Fig. 8 due to such variation and lowering in Faraday efficiency due to the gap may be likely.

[0157] Even in the presence of the variation above, on the other hand, second diffusion layer 42 formed from the carbon felt maintains, by elastic deformation, a state in which it is in intimate contact with electrolyte membrane 2 and second conductive plate 41. Consequently, a local gap is less likely at boundary 1A or 1B in Fig. 8. Therefore, high Faraday efficiency may be maintained for a long time in electrochemical cell 1 including second diffusion layer 42 formed from the carbon felt.

[0158] Electrochemical cell 1 including second diffusion layer 42 formed from the carbon felt is defined as an electrochemical cell A and electrochemical cell 1 including second diffusion layer 42 formed from the carbon paper is defined as an electrochemical cell B, and they are compared with each other. If an attempt to obtain in electrochemical cell B, a flow velocity of toluene as high as that in electrochemical cell A is made with the use of a pump X the same as that for electrochemical cell A, output of pump X becomes equal to or lower than a lower limit value and an appropriate flow rate cannot be maintained. If output of pump X is increased, pressure loss in electrochemical cell B increases before electrolysis. When electrolysis occurs, generation of hydrogen is likely and pressure loss further increases. Therefore, liquid leakage from electrochemical cell B may occur. In view of such aspects, conditions for operations for obtaining high Faraday efficiency are broader for electrochemical cell A including the carbon felt than electrochemical cell B including the carbon paper.

[0159] <Test Example 4-2> A plurality of carbon felts X, Y, and Z different from test example 4 in porosity and basis weight were prepared and compressive strain of carbon felts X, Y, and Z was measured under conditions the same as in test example 4.

[0160] Carbon felt X had the porosity, the basis weight, and a fiber diameter of 89.5%, 71 g / m2, and 10 pm, respectively. Carbon felt Y had the porosity, the basis weight, and the fiber diameter of 84.0%, 108 g / m2, and 10 pm, respectively. Carbon felt Z had the porosity, the basis weight, and the fiber diameter of 81.2%, 127 g / m2, and 10 pm, respectively. The porosity is expressed as a value with the carbon felt having the thickness of 1.0 mm being compressed to 0.3 mm.

[0161] Compressive strain of carbon felts X, Y, and Z was in a range not less than 0.6 and not more than 0.8. This test showed that when the porosity in the compressed state was in a range not lower than 80% and not higher than 90%, the basis weight was in a range not smaller than 70 g / m2 and not larger than 130 g / m2, and the fiber diameter was in a range not smaller than 5 pm and not larger than 20 pm, the carbon felt having compressive strain not less than 0.6 and not more than 0.8 could be constructed. Since these carbon felts X, Y, and Z are excellent in elastic deformation capability, they can contribute to high Faraday efficiency as shown in test examples 1 to 3 when they are used for second diffusion layer 42 of electrochemical cell 1. REFERENCE SIGNS LIST

[0162] 1 electrochemical cell 1A, 1B boundary 2 electrolyte membrane 21 first surface, 22 second surface 3 anode portion 3A, 3B manifold 30 first catalyst layer, 31 first conductive plate, 32 first diffusion layer, 33 frame seal portion 4 cathode portion 4A, 4B manifold 40 second catalyst layer, 41 second conductive plate, 42 second diffusion layer, 43 frame seal portion 5 first flow field 5A inlet, 5B outlet 50 flow channel, 51 supply groove, 52 first base groove, 53 branch groove, 54 second base groove, 55 discharge groove 6 second flow field 6A inlet, 6B outlet 61 first flow channel, 61A inlet end, 61B outlet end 61b first connection groove, 61d first groove, 61e end, 61f supply groove 62 second flow channel, 62A inlet end, 62B outlet end 62b second connection groove, 62d second groove, 62e end, 62f discharge groove 63 ridge portion 65 first ridge area 66 second ridge area 7 current conduction area 8 insulating member 9 compression apparatus 90 lower mount, 91 upper mount, 92 lower protection plate, 93 upper protection plate 100 organic hydride production apparatus 101 first fluid 101A first supply pipe, 101B first discharge pipe 101P first pump, 101T first tank 102 second fluid 102A second supply pipe, 102B second discharge pipe 102P second pump, 102T second tank E1, E2 terminal end position S1, S2 start end position M1, M2 intermediate position W1 average width W2 average width d average depth

Claims

1. An electrochemical cell comprising:an anode portion in which a first fluid containing water flows;a cathode portion in which a second fluid containing a substance to be hydrogenated flows; andan electrolyte membrane arranged between the anode portion and the cathode portion, whereinthe anode portion includesa first flow field that defines a space in which the first fluid flows to come in contact with a first surface of the electrolyte membrane, anda first catalyst layer arranged in the first flow field,the cathode portion includesa second flow field that defines a space in which the second fluid flows to come in contact with a second surface of the electrolyte membrane, anda second catalyst layer arranged in the second flow field,the second flow field includes a porous layer formed of a conductive material, andthe porous layer in the cathode portion has compressive strain not less than 0.3 and not more than 0.8.

2. The electrochemical cell according to claim 1, whereinthe compressive strain is not less than 0.5.

3. The electrochemical cell according to claim 1 or claim 2, whereinthe compressive strain is not less than 0.6.

4. The electrochemical cell according to any one of claims 1 to 3,whereinthe cathode portion includesa conductive plate that faces the electrolyte membrane, anda diffusion layer arranged between the conductive plate and theelectrolyte membrane, andthe diffusion layer is the porous layer.

5. The electrochemical cell according to claim 4, whereinthe second flow field includesa plurality of first grooves arranged in parallel, anda plurality of second grooves arranged in parallel,at least one of the plurality of first grooves and at least one of the plurality of second grooves are alternately arranged, andthe plurality of first grooves and the plurality of second grooves are provided at a surface of the conductive plate that faces the diffusion layer.

6. The electrochemical cell according to any one of claims 1 to 5, whereinthe porous layer has a porosity not lower than 40% and not higher than 98%.

7. The electrochemical cell according to any one of claims 1 to 6, whereinthe porous layer is made from a non-woven fabric containing carbon fibers.

8. The electrochemical cell according to claim 7, whereinthe porous layer has a basis weight not smaller than 50 g / m2 and not larger than 400 g / m2.

9. The electrochemical cell according to claim 7 or 8, whereinthe carbon fibers have an average diameter not smaller than 5 pm and not largerthan 100 ^m.

10. The electrochemical cell according to any one of claims 7 to 9, whereinthe non-woven fabric has an average thickness not smaller than 0.15 mm and not larger than 3.0 mm.

11. The electrochemical cell according to any one of claims 1 to 10, whereinthe first flow field includes a first porous layer formed of a metallic material.

12. The electrochemical cell according to claim 11, whereinthe first porous layer includesa porous body composed of titanium or a titanium alloy, anda precious metal coating formed at a surface of the porous body.

13. The electrochemical cell according to any one of claims 1 to 12, whereinthe substance to be hydrogenated is toluene.

14. The electrochemical cell according to any one of claims 1 to 13, whereinthe first fluid is composed mainly of water.

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