Electrochemical cell

Abstract

This electrochemical cell comprises an anode part and a cathode part that sandwich an electrolyte membrane, the electrochemical cell being such that a first fluid mainly composed of water is supplied to the anode part, and the water is electrolyzed between the anode part and the cathode part to generate protons, wherein the anode part is provided with a first electroconductive plate formed from a composite material containing a first resin and a first carbon material.

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. 2024-051443 filed on March 27, 2024, the entire contents of which are herein incorporated by reference. BACKGROUND ART

[0002] PTL 1 discloses a proton-conduction type electrochemical cell that generates protons from water. This electrochemical cell includes an anode portion and a cathode portion between which an electrolyte membrane lies. The anode portion includes an anode that faces a first surface of the electrolyte membrane and an electrically conductive end plate. The cathode portion includes a cathode that faces a second surface of the electrolyte membrane and an electrically conductive end plate. Each end plate functions as a conductive plate that applies a voltage to the electrochemical cell.

[0003] The anode portion is supplied with a first fluid containing water. As a directcurrent (DC) current is applied between the end plate of the anode portion and the end plate of the cathode portion, protons are generated from water contained in the first fluid. In PTL 1, a substance to be hydrogenated such as toluene is hydrogenated by generated protons so that hydrogen is stored in a stable manner. CITATION LIST PATENT LITERATURE

[0004] PTL 1: Japanese Patent Laying-Open No. 2021-109986 SUMMARY OF INVENTION

[0005] An electrochemical cell in the present disclosure includes an anode portion and a cathode portion between which an electrolyte membrane lies, the electrochemical cell generating protons by supply of a first fluid mainly composed of water to the anode portion and electrolysis of water between the anode portion and the cathode portion. The anode portion includes a first conductive plate formed of a composite material containing a first resin and a first carbon material. 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 an illustrative view of a state of arrangement of a second porous layer in the electrochemical cell described in the embodiment. [Fig. 4] Fig. 4 shows a graph showing relation between pH of an electrolyte solution and a corrosion current density in a test example 2. [Fig. 5] Fig. 5 shows a graph showing relation between an electrical conductivity of an electrolyte solution and a corrosion current density in the test example 2. [Fig. 6] Fig. 6 shows a graph showing relation between operating hours of the electrochemical cell and an amount of thickness decrease of a first conductive plate in a test example 3. [Fig. 7] Fig. 7 is a schematic diagram of a test apparatus in a test example 4. [Fig. 8] Fig. 8 shows a graph showing relation between compression strain and a surface pressure in the test example 4. DETAILED DESCRIPTION

[0007] [Problem to be Solved by the Present Disclosure] The conductive plate which is the end plate of the anode portion in PTL 1 is formed of metal such as titanium. Metal such as titanium is less likely to allow passage therethrough of the first fluid and can prevent leakage of the first fluid from the anode portion, and in addition, it is excellent in mechanical strength. The conductive plate formed of metal such as titanium, however, requires surface treatment to coat the surface with metal such as platinum in order to enhance resistance to oxidation. Surface treatment such as plating is complicated and lowers productivity of the electrochemical cell, also in terms of cost.

[0008] One of objects of the present disclosure is to provide an electrochemical cell including a conductive plate of an anode portion that does not require surface treatment and is excellent in productivity.

[0009] [Advantageous Effect of the Present Disclosure] The electrochemical cell in the present disclosure includes a first conductive plate that does not require surface treatment and is excellent in productivity.

[0010] [Description of Embodiments of the Present Disclosure] Embodiments of the present disclosure will initially be listed and described.

[0011] <1> A proton-conduction type electrochemical cell in the present disclosure includes an anode portion and a cathode portion between which an electrolyte membrane lies, the electrochemical cell generating protons by supply of a first fluid mainly composed of water to the anode portion and electrolysis of water between the anode portion and the cathode portion. The anode portion includes a first conductive plate formed of a composite material containing a first resin and a first carbon material.

[0012] The first conductive plate formed of the composite material is high in electrical conduction owing to the first carbon material. This first conductive plate does not require surface treatment such as plating formed in a conductive plate made of metal, and it is excellent in productivity, also in terms of cost. The first conductive plate formed of the composite material can prevent leakage of the first fluid to the outside of the anode portion. In an example where a plurality of electrochemical cells are stacked, the first resin in the first conductive plate of the anode portion prevents leakage of the first fluid to the cathode portion adjacent to that anode portion. The first resin improves strength of the first conductive plate and reduces oxidation of the first carbon material by the first fluid.

[0013] Since the composite material is formable in a mold, for example, a first conductive plate having a three-dimensional shape provided with grooves for smooth flow of the first fluid is readily obtained.

[0014] <2> In the electrochemical cell described in <1> above, the first carbon material may include graphite and a degree of graphitization in the first conductive plate may be equal to or higher than 0.5.

[0015] Though water which is a main component of the first fluid is generally an electrical insulator, the electrical conductivity of the first fluid may become high due to impurity ions eluted from a constituent member during operations. When a voltage applied to the first conductive plate is high such as when the voltage applied to the first conductive plate is equal to or higher than 1.7 V, the first carbon material in the composite material may be oxidized by the first fluid in which the impurity ions have been introduced, that is, the first conductive plate may be corroded. In contrast, corrosion of the first conductive plate having a degree of graphitization equal to or higher than 0.5 is less likely. Therefore, electrical conduction of the first conductive plate tends to be maintained. Decrease in thickness of the first conductive plate caused by corrosion thereof also due to long-term use also tends to be less. Therefore, the electrochemical cell described in <2> above tends to maintain performance to electrolyze water.

[0016] <3> In the electrochemical cell described in <1> or <2> above, the first fluid may have an electrical conductivity equal to or lower than 200 ^S / cm.

[0017] When the first fluid has the electrical conductivity equal to or lower than 200 uS cm (microsiemens per centimeter), oxidation of the first carbon material in the composite material by the first fluid is less likely.

[0018] <4> In the electrochemical cell described in any one of <1> to <3> above, the first fluid may have a hydrogen ion exponent equal to or larger than four.

[0019] When the first fluid has the hydrogen ion exponent, that is, pH, equal to or larger than four, oxidation of the first carbon material in the composite material by the first fluid is less likely.

[0020] <5> In the electrochemical cell described in any one of <1> to <4> above, a content of the first carbon material in the first conductive plate may be equal to or higher than 70 mass % with a mass of the first conductive plate being defined as 100 mass %.

[0021] If the content of the first carbon material occupied in the first conductive plate is equal to or higher than 70 mass %, the first conductive plate has a high electronic conductivity and has excellent electrical conduction.

[0022] <6> In the electrochemical cell described in any one of <1> to <5> above, a second fluid containing an organic compound having unsaturated bond may be circulated through the cathode portion.

[0023] In the electrochemical cell described in <6> above, as the organic compound having the unsaturated bond is supplied to the cathode portion, a hydride obtained by addition of protons generated at the anode portion to the organic compound can be produced. This electrochemical cell can produce, for example, a hydride such as methylcyclohexane from toluene. Such an electrochemical cell can suitably be used for an apparatus that produces hydrogen carriers.

[0024] <7> In the electrochemical cell described in any one of <1> to <6> above, the cathode portion may include a second conductive plate formed of a composite material containing a second resin and a second carbon material. The second carbon material may include graphite. A content of the second carbon material in the second conductive plate may be equal to or higher than 70 mass % with a mass of the second conductive plate being defined as 100 mass %.

[0025] As the second conductive plate is formed of the composite material and contains at least 70 mass % of the second carbon material including graphite, the second conductive plate is high in electrical conduction owing to the second carbon material and the second resin can lessen leakage of the second fluid to the outside of the cathode portion. In an example where a plurality of electrochemical cells are stacked, the second resin of the second conductive plate of the cathode portion prevents leakage of the second fluid to the anode portion adjacent to that cathode portion.

[0026] Since the composite material is formable in a mold, for example, a second conductive plate having a three-dimensional shape such as a grooved plate can highly accurately be formed. Furthermore, since resin and graphite are resistant to the organic compound above, damage to the second conductive plate by the organic compound contained in the second fluid is less likely.

[0027] In the example where a plurality of electrochemical cells are stacked, the first conductive plate of one electrochemical cell and the second conductive plate of the other electrochemical cell, the one and the other electrochemical cells being adjacent to each other, are shared. In other words, a front side of one conductive plate functions as the first conductive plate and a rear side thereof functions as the second conductive plate, that is, a single conductive plate functions as a bipolar plate.

[0028] <8> In the electrochemical cell described in <7> above, the second resin may be at least one selected from the group consisting of polyester resin, polyamide resin, polyacetal resin, phenol resin, polyvinylidene chloride resin, polyvinylidene fluoride resin, polytetrafluoroethylene resin, epoxy resin, polyoxymethylene resin, and polyphenylene sulfide resin.

[0029] Since the resins listed above are resistant to the organic compound above, damage to the second conductive plate by the second fluid is less likely.

[0030] <9> In the electrochemical cell described in any one of <1> to <8> above, the anode portion may include a first porous layer formed of a conductive material and the first porous layer may have an average thickness not smaller than 0.20 mm and not larger than 0.40 mm.

[0031] When a potential of the first conductive plate becomes sufficiently high to such an extent as oxidizing the first carbon material in the first conductive plate, the first conductive plate may be corroded. If the first porous layer of the anode portion has the thickness equal to or larger than 0.20 mm, a distance for protons generated as a result of a corrosion reaction of the first conductive plate to reach the cathode portion becomes long. With this increase in distance, transfer resistance of protons increases. In order to oxidize the first carbon material in the first conductive plate in the first conductive plate where the transfer resistance has increased, a voltage corresponding to the transfer resistance is further required. Such a voltage is not applied in the electrochemical cell that normally operates.  Therefore, if the first porous layer has an average thickness equal to or larger than 0.20 mm, corrosion of the first conductive plate is less likely. When the thickness is equal to or smaller than 0.40 mm, increase in electrical resistance by the first porous layer is lessened. The first porous layer of the anode portion is expected also to achieve an effect to make the flow of the first fluid in the anode portion uniform.

[0032] <10> In the electrochemical cell described in <9> above, the first porous layer may include a substrate formed of metal and a coating formed at a surface of the substrate. The metal includes at least one selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper, and zinc. A form of the substrate is an aggregate of fibers, a porous sintered body, a foamed formed body, or expanded metal. The coating contains platinum.

[0033] The first porous layer configured as described in <10> above is resistant to the first fluid. Therefore, damage to the first porous layer by the first fluid is less likely.

[0034] <11> In the electrochemical cell described in any one of <1> to <10> above, the cathode portion may include a second porous layer formed of a conductive material and compression strain of the second porous layer in the cathode portion may be not less than 0.3 and not more than 0.8.

[0035] The second porous layer that satisfies the compression strain is excellent in elastic deformability. Even when the thickness of the first conductive plate decreases due to long-term use as described above, a gap between adjacent members included in the electrochemical cell is less likely as the second porous layer elastically deforms. In other words, a state in which the first conductive plate, the first porous layer, the electrolyte membrane, the second porous layer, and the second conductive plate are in surface contact tends to be maintained. Lowering in stress that may be applied to the first conductive plate is expected by relaxation of the stress by the second porous layer when a constituent member of the electrochemical cell receives compressive stress at the time of assembly or receives stress caused by thermal expansion and contraction during operations. In such an electrochemical cell, damage such as fracture of the first conductive plate is less likely.

[0036] The second porous layer of the cathode portion makes the flow of the second fluid in the cathode portion uniform. If the second porous layer of the cathode portion is less likely to elastically deform, a gap is provided between the second porous layer and the electrolyte membrane during operations and the flow of the second fluid becomes non-uniform. In contrast, if the second porous layer is excellent in elastic deformability, the second porous layer is in intimate contact with the electrolyte membrane and a gap is less likely between the second porous layer and the electrolyte membrane. If such an electrochemical cell is used for an apparatus that produces hydrogen carriers, an amount of production of a hydride can be increased.

[0037] <12> In the electrochemical cell described in <11> above, the second porous layer may include a non-woven fabric including carbon fibers.

[0038] The non-woven fabric including carbon fibers has high elastic deformation capability. As the second porous layer has a cushioning property, damage to the first conductive plate such as fracture of the first conductive plate is more readily prevented.

[0039] [Details of Embodiment in the Present Disclosure] A specific example of the electrochemical cell in the present disclosure will be described 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.

[0040] <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. The organic hydride production apparatus 100 in the present example supplies a first fluid 101 stored in a first tank 101T and a second fluid 102 stored in a second tank 102T to the electrochemical cell 1.  The 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 the anode portion 3 and the cathode portion 4.  A positive electrode of the DC power supply is connected to the anode portion 3 and a negative electrode of the DC power supply is connected to the cathode portion 4.

[0041] The first fluid 101 is supplied through a first supply pipe 101A to the anode portion 3. A first pump 101P that delivers the first fluid 101 to the anode portion 3 is arranged at the first supply pipe 101A. A first catalyst layer 30 is arranged in the anode portion 3. As a result of an electrochemical reaction, proton (H+), oxygen (O2), and electron (e-) are generated from water (H2O) contained in the first fluid 101. Proton moves to the cathode portion 4 through the electrolyte membrane 2. Electron generated at the positive electrode flows to the negative electrode through the DC power supply.  The first fluid 101 in the anode portion 3 is discharged to the first tank 101T through a first discharge pipe 101B.

[0042] The first fluid 101 is a fluid mainly composed of water. The first fluid 101 mainly composed of water is pure water or water in which a total concentration of impurities is not higher than 0.1 mM (millimolar).  For example, the first fluid 101 at the time of start-up of the electrochemical cell 1 is pure water.  Commercially available pure water can be employed.  Even though the first fluid 101 at the time of start-up of the electrochemical cell 1 is pure water, with circulation to the electrochemical cell 1, impurity ions derived from a constituent member of the electrochemical cell 1 may be introduced in the first fluid 101.

[0043] The second fluid 102 is supplied through a second supply pipe 102A to the cathode portion 4.  A second pump 102P that delivers the second fluid 102 to the cathode portion 4 is arranged at the second supply pipe 102A. A second catalyst layer 40 is arranged in the cathode portion 4. As a result of an electrochemical reaction, a substance to be hydrogenated contained in the second fluid 102 is hydrogenated by bond between proton that has passed through the electrolyte membrane 2 and electron supplied from the DC power supply in the second catalyst layer 40 to generate a hydride.  The second fluid 102 containing the hydride is discharged from the cathode portion 4 through a second discharge pipe 102B to the second tank 102T.

[0044] The substance to be hydrogenated contained in the second fluid 102 is, for example, an organic compound having unsaturated bond. The organic compound having unsaturated bond is, for example, a monocyclic aromatic compound, a bicyclic aromatic compound, a tricyclic aromatic compound, or an aromatic polymer.  A specific substance name is benzene, toluene, xylene, ethylbenzene, mesitylene, naphthalene, methylnaphthalene, anthracene, or tetralin.  The substance to be hydrogenated exemplified in Fig. 1 is toluene (denoted as TOL in the figure). Hydride produced as a result of hydrogenation of toluene is methylcyclohexane (denoted as MCH below). The second fluid 102 contains toluene and the electrochemical cell 1 in the present example is also referred to as an MCH electrolytic synthesis cell and reactions in the anode portion 3 and the cathode portion 4 are as below. • 3H2O ^ 1.5O2 + 6H+ + 6e- • toluene + 6H+ + 6e- ^ methylcyclohexane

[0045] In performing water electrolysis in the electrochemical cell 1, in the cathode portion 4, hydrogen (H2) is generated from protons. In the electrochemical cell 1 where water electrolysis is performed, a configuration for circulation of the second fluid 102 in the cathode portion 4 is not required. Water that has passed from the anode portion 3 through the electrolyte membrane 2 flows into the cathode portion 4. Therefore, the electrochemical cell 1 where water electrolysis is performed includes a configuration to discharge from the cathode portion 4, hydrogen generated from protons, together with water.

[0046] <<Basic Configuration of Electrochemical Cell>> As shown in Fig. 2, the electrochemical cell 1 including the anode portion 3 and the cathode portion 4 includes the electrolyte membrane 2, a first flow field 5, a second flow field 6, the first catalyst layer 30, and the second catalyst layer 40.

[0047] The electrolyte membrane 2 is formed of a material that allows selective transmission of proton.  The electrolyte membrane 2 separates the first fluid 101 and the second fluid 102 from each other in the electrochemical cell 1. The electrolyte membrane 2 has a thickness, for example, not smaller than 5 um and not larger than 350 um, not smaller than 10 um and not larger than 300 um, or not smaller than 20 um and not larger than 250 um.

[0048] The first flow field 5 is a space in the anode portion 3 where the first fluid 101 flows. An inlet 5A and an outlet 5B of the first flow field 5 are connected to the first supply pipe 101A and the first discharge pipe 101B, respectively, as shown in Fig. 1. In the first flow field 5, the first fluid 101 flows to be in contact with a first surface 21 of the electrolyte membrane 2. The anode portion 3 in the present example includes a first conductive plate 31, a first porous layer 32, and a frame seal portion 33.

[0049] The first conductive plate 31 is a conductive plate material that faces the first surface 21 of the electrolyte membrane 2. The first conductive plate 31 performs a function to apply a voltage to the electrochemical cell 1. The first conductive plate 31 performs also a function to confine the first fluid 101 in the electrochemical cell 1. A not-shown electrode plate is arranged at a surface of the first conductive plate 31 opposite to the first porous layer 32. The not-shown positive electrode of the DC power supply is connected to the electrode plate.

[0050] The first porous layer 32 is a conductive plate material containing a plurality of pores and arranged between the first conductive plate 31 and the first surface 21. The first porous layer 32 performs a function to diffuse the first fluid 101 supplied to the anode portion 3 over the entire the first flow field 5 in the electrochemical cell 1.

[0051] The frame seal portion 33 surrounds an outer periphery of the first porous layer 32. The frame seal portion 33 prevents the first fluid 101 from leaking from the outer periphery of the first porous layer 32 to the outside. Therefore, the first flow field 5 in the present example is mainly defined by a space that is located between the first surface 21 of the electrolyte membrane 2 and the first conductive plate 31 and surrounded by the frame seal portion 33. As the first porous layer 32 is arranged in the space, the pores in the first porous layer 32 can be used as a flow channel for the first fluid 101.

[0052] The first catalyst layer 30 arranged in the first flow field 5 is in contact with or proximate to the first surface 21 of the electrolyte membrane 2. The first catalyst layer 30 promotes electrolysis of water contained in the first fluid 101 to generate proton, oxygen, and electron. The first catalyst layer 30 in the present example is integrally formed on the first surface 21 of the electrolyte membrane 2. The first catalyst layer 30 may integrally be formed in a portion of the first porous layer 32 in contact at least with the electrolyte membrane 2. The first catalyst layer 30 may be a member independent of the electrolyte membrane 2 and the first porous layer 32. In that case, the first catalyst layer 30 is arranged between the electrolyte membrane 2 and the first porous layer 32. The 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. The 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.

[0053] The second flow field 6 is a space in the cathode portion 4 where the second fluid 102 flows.  An inlet 6A and an outlet 6B of the second flow field 6 are connected to the second supply pipe 102A and the second discharge pipe 102B, respectively, as shown in Fig. 1. In the second flow field 6, the second fluid 102 flows to be in contact with a second surface 22 of the electrolyte membrane 2. The second surface 22 is a surface of the electrolyte membrane 2 opposite to the first surface 21.  The cathode portion 4 in the present example includes a second conductive plate 41, a second porous layer 42, and a frame seal portion 43.

[0054] The second conductive plate 41 is a conductive plate material that faces the second surface 22 of the electrolyte membrane 2. The second conductive plate 41 performs a function to apply a voltage to the electrochemical cell 1. The second conductive plate 41 performs also a function to confine the second fluid 102 in the electrochemical cell 1. A not-shown electrode plate is arranged at a surface of the second conductive plate 41 opposite to the second porous layer 42.  The not-shown negative electrode of the DC power supply is connected to the electrode plate.

[0055] The second porous layer 42 is a conductive plate material containing a plurality of pores and arranged between the second conductive plate 41 and the second surface 22. The second porous layer 42 performs a function to diffuse the second fluid 102 supplied to the cathode portion 4 over the entire the second flow field 6.

[0056] The frame seal portion 43 surrounds an outer periphery of the second porous layer 42.  The frame seal portion 43 prevents the second fluid 102 from leaking from the outer periphery of the second porous layer 42 to the outside. Therefore, the second flow field 6 in the present example is mainly defined by a space that is located between the second surface 22 of the electrolyte membrane 2 and the second conductive plate 41 and surrounded by the frame seal portion 43. As the second porous layer 42 is arranged in the space, the pores in the second porous layer 42 can be used as a flow channel for the second fluid 102.

[0057] The second catalyst layer 40 arranged in the second flow field 6 is in contact with or proximate to the second surface 22 of the electrolyte membrane 2. The second catalyst layer 40 promotes a reaction of a substance to be hydrogenated contained in the second fluid 102 to proton from the anode portion 3, which leads to generation of a hydride.  The second catalyst layer 40 in the present example is integrally formed on the second surface 22 of the electrolyte membrane 2. The second catalyst layer 40 may integrally be formed in a portion of the second porous layer 42 in contact at least with the electrolyte membrane 2.  The second catalyst layer 40 may be a member independent of the electrolyte membrane 2 and the second porous layer 42. In that case, the second catalyst layer 40 is arranged between the electrolyte membrane 2 and the second porous layer 42.  The 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. The second catalyst layer 40 may contain an ionomer that binds the catalyst to the support. A known catalyst can be employed.

[0058] The 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, the first conductive plate 31 of one electrochemical cell 1 and the second conductive plate 41 of the remaining electrochemical cell 1 form a single bipolar plate. In the bipolar plate, a first surface of the bipolar plate functions as the first conductive plate 31 and a second surface opposite to the first surface functions as the second conductive plate 41. Therefore, the first porous layer 32 is arranged at the first surface of the bipolar plate and the second porous layer 42 is arranged at the second surface of the bipolar plate.  The first conductive plate 31 and the second conductive plate 41 rather than the bipolar plate are arranged at a first end and a second end of the stacked body, respectively.

[0059] <<Configuration of Each Portion of Electrochemical Cell>> A configuration of each portion of the electrochemical cell 1 will be described below in detail.

[0060] [First Conductive Plate] The first conductive plate 31 is required to be formed of a material that is less likely to react with a substance contained in the first fluid 101. The first conductive plate 31 is, for example, a plate material formed of a composite material of a first carbon material and a first resin.  The first carbon material is, for example, a carbon based material such as graphite or carbon black. The first resin is, for example, at least one selected from the group consisting of polyester resin, polyamide resin, polyacetal resin, phenol resin, polyvinylidene chloride resin, polyvinylidene fluoride resin, polytetrafluoroethylene resin, epoxy resin, polyoxymethylene resin, and polyphenylene sulfide resin.

[0061] The first conductive plate 31 can be made, for example, by thermally pressing a mixture of the first carbon material and the first resin. The first conductive plate 31 may be made by impregnating a porous graphite plate composed of the first carbon material with the first resin.

[0062] In the first conductive plate 31 formed of the composite material, electrical conduction thereof is enhanced owing to the first carbon material and the first resin can prevent leakage of the first fluid 101 to the outside of the anode portion 3. In the example where a plurality of electrochemical cells 1 are stacked, the first conductive plate 31 of the anode portion 3 prevents leakage of the first fluid 101 to the cathode portion 4 adjacent to that anode portion 3. The first resin enhances strength of the first conductive plate 31 and reduces oxidation of the first carbon material by the first fluid 101. As a result of reduction of oxidation, electrical conduction of the first conductive plate 31 owing to the first carbon material is maintained. Furthermore, the first conductive plate 31 formed of the composite material does not require coating with platinum or the like and hence it is inexpensive.  Furthermore, even though the first conductive plate 31 has such a three-dimensional shape as being provided with grooves, it can readily be formed by being formed in a mold.

[0063] In an example where the first carbon material includes graphite, the degree of graphitization in the first conductive plate 31 including graphite may be, for example, equal to or higher than 0.5. Graphite has such a layered structure that a plurality of planar layered structures are bonded by van der Waals force. In each planar layered structure, a plurality of carbon atoms are connected in a hexagonal shape. The degree of graphitization is an indicator that indicates regular layering of the plurality of planar layered structures. The degree of graphitization is obtained by a Franklin's P-value shown in an expression 1 below.  "d(002)" in the expression (1) below represents a value of interplanar spacing d(002) of carbon measured by X-ray diffraction (XRD). Since the first conductive plate 31 is formed of the composite material of graphite and the first resin in the present example, not the degree of graphitization of graphite alone but the degree of graphitization of the first conductive plate 31 including graphite and the first resin is evaluated.

[0064] [Math. 1] P = 1 — 17^1^^2221. . . Expression 1 yj 0.086

[0065] Graphite is obtained by firing carbon in an oxygen-free state. A firing temperature is set, for example, to 1500°C or higher. As the firing temperature is higher, for example, 2000°C or higher or 3000°C or higher, the degree of graphitization of graphite is higher.

[0066] As graphite is higher in degree of graphitization, it is higher in electrical conductivity. In other words, as the first conductive plate 31 containing graphite is higher in degree of graphitization, it is higher in electrical conductivity. It has been clarified by studies conducted by the present inventors that, as the degree of graphitization is higher, oxidation of graphite contained in the first conductive plate 31 by the first fluid 101 is less likely. As oxidation of graphite is less likely, corrosion of the first conductive plate 31 is less likely and electrical conduction of the first conductive plate 31 tends to be maintained.  Therefore, performance of electrolysis of water by the electrochemical cell 1 tends to be maintained.

[0067] The first carbon material may be oxidized when a voltage applied to the first conductive plate 31 is high and the first fluid 101 has a high electrical conductivity. The electrochemical cell 1 can efficiently perform water electrolysis when the potential of the first conductive plate 31 is a high potential such as 1.7 V or higher. Water which is a main component of the first fluid 101, on the other hand, is an insulator. As the electrochemical cell 1 operates, the first fluid 101 may contain impurity ions. These impurity ions increase the electrical conductivity of the first fluid 101. When the electrical conductivity of the first fluid 101 is excessively high, corrosion of the first conductive plate 31 may proceed in a state where the high potential as above is applied. If the electrical conductivity of the first fluid 101 is equal to or lower than 200 ^S / cm, oxidation of the first carbon material in the first conductive plate 31 by the first fluid 101 is less likely. The first fluid 101 may have the electrical conductivity equal to or lower than 100 ^S / cm or equal to or lower than 50 ^S / cm.

[0068] Impurity ions lower pH of the first fluid 101. pH of the first fluid 101 is an index of magnitude of the electrical conductivity of the first fluid 101. Measurement of pH of the first fluid 101 is easier than measurement of the electrical conductivity of the first fluid 101. When pH of the first fluid 101 is excessively low, that is, the electrical conductivity of the first fluid 101 is excessively high, corrosion of the first conductive plate 31 may proceed.  If pH of the first fluid 101 is equal to or higher than four, oxidation of the first carbon material in the first conductive plate 31 by the first fluid 101 is less likely. pH of the first fluid 101 may be equal to or higher than five or equal to or higher than six. In an example where pure water is employed as the first fluid 101 at the time of start-up of the electrochemical cell 1, an upper limit of pH of the first fluid 101 is seven. Since a value of pH may have an error depending on a measurement method, pH of the first fluid 101 may slightly exceed seven.

[0069] The electrical conductivity or pH of the first fluid 101 may be measured by sampling the first fluid 101 in the middle of a flow channel of the first fluid 101. For example, a sampling path that branches from the middle of the first supply pipe 101A in Fig. 1 may be provided and a measurement apparatus may be provided in the sampling path, or a measurement apparatus may be provided in the first tank 101T where the first fluid 101 is stored. According to such a configuration, the electrical conductivity or pH of the first fluid 101 can be measured over time.

[0070] In order to lower the electrical conductivity of the first fluid 101 in contact with the first conductive plate 31 and to bring pH closer to seven, a filter to remove impurity ions may be arranged in a path for circulation of the first fluid 101. The first fluid 101 that contains few impurity ions such as pure water may be kept supplied to the first flow field 5, rather than circulation of the first fluid 101.

[0071] A content of the first carbon material in the first conductive plate 31 with a mass of the first conductive plate 31 being defined as 100 mass % is, for example, equal to or higher than 70 mass %. When the content of the first carbon material occupied in the first conductive plate 31 is equal to or higher than 70 mass %, the first conductive plate 31 has a high electrical conductivity. The content of the first carbon material may be equal to or higher than 75 mass % or equal to or higher than 80 mass %. Since the first conductive plate 31 is formed of the composite material, the first conductive plate 31 contains at least a prescribed amount of first resin. Therefore, an upper limit value of the content of the first carbon material is, for example, 95.5 mass %. A range of the content of the first carbon material is, for example, not lower than 70 mass % and not higher than 95.5 mass %, not lower than 80 mass % and not higher than 95 mass %, or not lower than 85 mass % and not higher than 95 mass %. The content of the first resin is expressed as a value calculated by subtracting the content of the first carbon material from 100 mass %.

[0072] [First Porous Layer] The first porous layer 32 in the present example is a porous plate material formed of a metallic material. The first porous layer 32 includes, for example, a substrate having a three-dimensional network structure and a coating formed at a surface of the substrate. Metal includes at least one selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper, and zinc. Metal includes also an alloy. The first porous layer 32 formed of metal above is resistant to the first fluid 101. Damage to such a first porous layer 32 by the first fluid 101 is less likely. A form of the substrate is an aggregate of fibers such as a nonwoven fabric, a porous sintered body, a foamed formed body, or expanded metal. The coating contains, for example, platinum. The substrate may be a plurality of layered mesh plates including meshes different in size or a plurality of layered mesh plates different in shape of meshes.

[0073] The first porous layer 32 lies between the electrolyte membrane 2 and the first conductive plate 31. As the first porous layer 32 lies between the electrolyte membrane 2 and the first conductive plate 31, it is pressed thereby. Depending on a material for the first porous layer 32, the first porous layer 32 may be compressed by being pressed.  Though the first porous layer 32 made of the metallic material includes a plurality of pores, it is less likely to deform.

[0074] The first porous layer 32 lying between the electrolyte membrane 2 and the first conductive plate 31 in the electrochemical cell 1 has an average thickness, for example, not smaller than 0.20 mm and not larger than 0.40 mm.  In an example where the first porous layer 32 is less likely to deform by being pressed as it is formed, for example, of the metallic material, the average thickness of the first porous layer 32 that is not lying between the electrolyte membrane 2 and the first conductive plate 31 is substantially equal to the average thickness of the first porous layer 32 that is lying between the electrolyte membrane 2 and the first conductive plate 31.  The average thickness of the first porous layer 32 is an average of thicknesses at at least three different locations.

[0075] If the first porous layer 32 has the average thickness equal to or larger than 0.20 mm, a gap between the first conductive plate 31 and the electrolyte membrane 2 is sufficiently large. In other words, since the first flow field 5 having a sufficient size is formed in the anode portion 3, the first fluid 101 more readily diffuses over the entire first flow field 5. If the first porous layer 32 has the thickness equal to or larger than 0.20 mm, a distance for protons generated as a result of corrosion reaction of the first conductive plate 31 to reach the cathode portion 4 becomes longer. With this increase in distance, transfer resistance of protons increases.  In order to oxidize the carbon material in the first conductive plate 31 in the first conductive plate 31 where the transfer resistance has increased, a voltage corresponding to the transfer resistance is further required. The voltage is not applied in the electrochemical cell 1 that normally operates. Therefore, corrosion of the first conductive plate 31 is less likely if the first porous layer 32 has the average thickness equal to or larger than 0.20 mm.

[0076] When the first porous layer 32 has the average thickness equal to or smaller than 0.40 mm, increase in electrical resistance by the first porous layer 32 is lessened. If the first porous layer 32 has the average thickness equal to or smaller than 0.40 mm, the first flow field 5 does not become excessively large and hence the flow of the first fluid 101 in the anode portion 3 can be uniform. Since the first porous layer 32 made of metal tends to be lighter as it is smaller in thickness, ease in assembly of the electrochemical cell 1 improves. The first porous layer 32 small in thickness can achieve reduction in material for the electrochemical cell 1 and a lighter weight of the electrochemical cell 1. The first porous layer 32 may have the average thickness not smaller than 0.22 mm and not larger than 0.40 mm or not smaller than 0.25 mm and not larger than 0.40 mm.

[0077] A porosity of the first porous layer 32 in the electrochemical cell 1 is, for example, not lower than 60% and not higher than 90%. Since the first porous layer 32 formed of the metallic material has low elastic deformation capability, the porosity of the first porous layer 32 removed from the electrochemical cell 1 may be regarded as the porosity of the first porous layer 32 in the electrochemical cell 1. If the porosity is equal to or higher than 60%, the first fluid 101 readily diffuses over the entire first flow field 5. If the porosity is equal to or lower than 90%, strength of the first porous layer 32 tends to be high. The porosity of the first porous layer 32 may be, for example, not lower than 65% and not higher than 85% or not lower than 68% and not higher than 80%.

[0078] [Frame Seal Portion of Anode Portion] A size of the frame seal portion 33 of the anode portion 3 is desirably selected in accordance with a size of the first conductive plate 31 and the first porous layer 32. In the present example, a size of a window portion of the frame seal portion 33 substantially corresponds to the size of the first porous layer 32, and a size of an outer edge of the frame seal portion 33 substantially corresponds to the size of the first conductive plate 31. The frame seal portion 33 is formed of an electrically insulating material resistant to the first fluid 101.  Electrically insulating materials are, for example, epoxy resin, phenol resin, polyamide resin such as PA6 or PA66, polyoxymethylene resin, fluororesin, and polyphenylenesulfide resin.

[0079] [Second Conductive Plate] The second conductive plate 41 in the present example is required to be formed of a material that is less likely to react to the organic compound contained in the second fluid 102. The second conductive plate 41 is, for example, a plate material formed of a composite material of a second carbon material and a second resin. The second conductive plate 41 formed of the composite material may be made by a method the same as that for the first conductive plate 31. The second carbon material is, for example, a carbon-based material such as graphite or carbon black. The second carbon material gives electrical conduction to the second conductive plate 41. The second resin is at least one selected from the group consisting of polyester resin, polyamide resin, polyacetal resin, phenol resin, polyvinylidene chloride resin, polyvinylidene fluoride resin, polytetrafluoroethylene resin, epoxy resin, polyoxymethylene resin, and polyphenylene sulfide resin. Since the listed resins are resistant to the organic compound, damage to the second conductive plate 41 by the second fluid 102 is less likely.

[0080] The second conductive plate 41 contains graphite as the second carbon material. The content of graphite in the second conductive plate 41 may be equal to or higher than 70 mass % with a mass of the second conductive plate 41 being defined as 100 mass %. As the second conductive plate 41 is formed of the composite material and contains at least 70 mass % of the second carbon material containing graphite, electrical conduction thereof increases owing to the second carbon material and the second resin can lessen leakage of the second fluid 102 to the outside of the cathode portion 4. In the example where a plurality of electrochemical cells 1 are stacked, the second resin of the second conductive plate 41 of the cathode portion 4 prevents leakage of the second fluid 102 to the anode portion 3 adjacent to that cathode portion 4. The second conductive plate 41 also is increased in strength by the second resin and can readily be formed by being formed in a mold even when it has such a three dimensional shape as being provided with grooves.

[0081] The second conductive plate 41 may be identical in composition to the first conductive plate 31.  In this case, a single conductive plate can also be used as a bipolar plate. The bipolar plate is a conductive plate that serves as both of the first conductive plate 31 and the second conductive plate 41 in two adjacent electrochemical cells 1.

[0082] <<Configuration of Second Porous Layer>> The second porous layer 42 is a porous plate material formed of a conductive material. The second porous layer 42 is configured to be high in elastic deformation capability. In the electrochemical cell 1, the second porous layer 42 high in elastic deformation capability lies between the electrolyte membrane 2 and the second conductive plate 41 so that the second porous layer 42 is compressed. As a result of this compression, the second porous layer 42 has a smaller average thickness. The second porous layer 42 that lies between the electrolyte membrane 2 and the second conductive plate 41 in the 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 the electrolyte membrane 2 and the second conductive plate 41 in the electrochemical cell 1 is herein regarded as the average thickness of the second porous layer 42. The number of measurements to obtain the average length is at least three, inclusive of measurement at a central position of the electrolyte membrane 2 in a plan view.

[0083] If the second porous layer 42 in a compressed state has the average thickness equal to or larger than 0.15 mm, an amount of elastic deformation of the second porous layer 42 in the electrochemical cell 1 tends to be large.  Consequently, the second porous layer 42 tends to be in intimate contact with the electrolyte membrane 2 and the second conductive plate 41. If the second porous layer 42 has the average thickness equal to or larger than 0.15 mm, a gap between the second conductive plate 41 and the electrolyte membrane 2 is sufficiently large. In other words, since the second flow field 6 having a sufficient size is formed in the cathode portion 4, the second fluid 102 readily diffuses over the entire second flow field 6. If the second porous layer 42 in the compressed state has the average thickness equal to or smaller than 3.0 mm, electrical resistance of the second porous layer 42 and cell resistance of the electrochemical cell 1 are less likely to increase. If the second porous layer 42 has the average thickness equal to or smaller than 3.0 mm, a gap between the second conductive plate 41 and the electrolyte membrane 2 does not become excessively large. In other words, since the second flow field 6 does not become excessively large, the second fluid 102 containing a hydride generated at the second catalyst layer 40 is readily quickly discharged from the second flow field 6. The second porous layer 42 may have the average thickness not smaller than 0.15 mm and smaller than 3.0 mm, 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.

[0084] Fig. 3 is a schematic diagram showing a state of layering of some features in the electrochemical cell 1. When there is a local gap at a boundary 1A between the second porous layer 42 and the electrolyte membrane 2 and a boundary 1B between the second porous layer 42 and the second conductive plate 41, the second fluid 102 is less likely to uniformly spread over the entire surface of the second catalyst layer 40. When an internal pressure in the second flow field 6 changes while the electrochemical cell 1 is operating, a local gap may be provided.  When the compressed second porous layer 42 presses the electrolyte membrane 2 and the second conductive plate 41 at a prescribed surface pressure or higher, the local gap is less likely to be formed at the boundary 1A or 1B. As occurrence of the gap is less likely at the boundary 1A or 1B, the flow of the second fluid 102 tends to be uniform. Consequently, Faraday efficiency of the electrochemical cell 1 tends to improve. Faraday efficiency refers to a ratio of an amount of charges that have contributed to hydrogenation of a substance to be hydrogenated, with a total amount of charges injected into the electrochemical cell being defined as 100%. The second porous layer 42 high in elastic deformation capability exhibits the cushioning property and tends to lessen damage to the first conductive plate 31 and the second conductive plate 41.

[0085] Compression strain of the second porous layer 42 being compressed in the cathode portion 4 of the electrochemical cell 1 is not less than 0.3 and not more than 0.8. The surface pressure applied to the second porous layer 42 compressed in the 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. 8 in a test example 4 which will be described later, as the surface pressure is higher, compression strain is also greater.  Therefore, when compression strain of the second porous layer 42 at the surface pressure of 0.5 MPa is not less than 0.3, compression strain of the second porous layer 42 in the cathode portion 4 to which the surface pressure not lower than 0.5 MPa is applied is also not less than 0.3.  Compression strain in the present example is expressed as a value obtained by dividing an amount of decrease X in thickness of the second porous 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 the second porous layer 42 yet to be compressed. In other words, the amount of decrease X is expressed as t0-t1 where t0 represents the initial thickness of the second porous layer 42 and t1 represents the thickness of the second porous layer 42 compressed at the surface pressure not lower than 0.5 MPa and not higher than 0.8 MPa.  Therefore, compression strain is obtained as (t0-t1) / t0.  For example, when the second porous layer 42 having the initial thickness t0 of 1 mm is compressed at the surface pressure of 0.5 MPa with the thickness t1 of the second porous layer 42 being 0.7 mm, the amount of decrease X is 0.3 mm.  Since compression strain is calculated as compression strain = X / t0, compression strain in this case is 0.3.

[0086] The thickness t0 or t1 of the second porous 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 the second porous 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 the initial thickness t0 is set to 0.7 kPa.  The initial thickness t0 is expressed as a value calculated by averaging thicknesses obtained by measurement at five or more points.  The thickness t1 is obtained similarly to the initial thickness t0, with the surface pressure in measurement of the thickness t1 being varied, for example, to 0.5 MPa.  The thickness t0 or t1 of the second porous layer 42 while it is not assembled to the electrochemical cell 1 is measured.  The initial thickness t0 of the second porous layer 42 is, for example, not smaller than 0.3 mm and not larger than 3.5 mm.

[0087] The second porous layer 42 having compression strain not less than 0.3 and not more than 0.8 is excellent in elastic deformation capability. When such a second porous layer 42 is compressed between the electrolyte membrane 2 and the second conductive plate 41, the second porous layer 42 presses the electrolyte membrane 2 and the second conductive plate 41 between which it lies in the cathode portion 4, in an attempt to return to the thickness before it is compressed.  Therefore, the second porous layer 42 tends to be in surface contact with the electrolyte membrane 2 and the second conductive plate 41.  Even when the thickness of the first conductive plate 31 decreases due to corrosion caused by long-term use, a gap between adjacent members included in the electrochemical cell 1 is less likely owing to elastic deformation of the second porous layer 42. Compression strain may be not less than 0.4. 0.5, 0.6, or 0.7.

[0088] The second porous layer 42 that satisfies the compression strain is, for example, a non-woven fabric containing carbon fibers.  The non-woven fabric is interlaced carbon fibers independent of one another, 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 compression strain is satisfied, the second porous 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 compression strain.  Paper contains a plurality of carbon fibers and a binder that fixes carbon fibers.  Compression 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.

[0089] So long as the compression strain of the second porous layer 42 as a whole is not less than 0.3, the second porous layer may include a member the compression strain of which is less than 0.3. For example, the second porous layer 42 may be configured in such a manner that a plurality of non-woven fabrics different in compression 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.

[0090] 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 um.  When the average diameter is not smaller than 5 pm and not larger than 100 pm and a basis weight of the second porous layer 42 satisfies a range which will be described later, the second porous layer 42 is higher in elastic deformation capability. The carbon fibers may have the average diameter, for example, not smaller than 5 um and not larger than 80 um, not smaller than 5 um and not larger than 50 um, or not smaller than 7 um and not larger than 30 um.

[0091] 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.  For observation of the cross-section, for example, a scanning electron microscope is used. Five or more observation fields of view are taken from one cross-section. Cross-sectional areas of at least three carbon fibers are obtained from each observation field of view.

[0092] The second porous layer 42 being compressed in the electrochemical cell 1 has the porosity, for example, not lower than 40% and not higher than 98%. So long as the porosity of the second porous layer 42 is within the range above, the second porous layer 42 tends to have sufficient conduction and pores while it is excellent in elastic deformation capability.  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 the second porous 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 the porosity of the second porous layer 42 yet to be compressed and an amount of decrease in thickness of the second porous layer 42 in the electrochemical cell 1 are known, the porosity of the second porous layer 42 in the compressed state is obtained by calculation.

[0093] The basis weight of the second porous layer 42 is, for example, not smaller than 50 g / m2 and not larger than 400 g / m2. When the basis weight of the second porous layer 42 is within the range above, the second porous layer 42 tends to have sufficient conduction and pores while it is excellent in elastic deformation capability. 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 the second porous layer 42 per one square meter.  The basis weight is obtained by dividing the mass of the second porous layer 42 by an area while the second porous layer 42 is viewed in a plan view.

[0094] [Frame Seal Portion of Cathode Portion] Specifications such as a constituent material or a size of the frame seal portion 43 of the cathode portion 4 are the same as the specifications such as the constituent material or the size of the frame seal portion 33 of the anode portion 3.

[0095] [Configuration of Flow Channel in First Flow Field] The first flow field 5 may include a flow channel. By including the flow channel, the flow of the first fluid 101 is smooth in the first flow field 5.

[0096] The flow channel is representatively a groove provided in the first conductive plate 31.  A shape, a size, a state of arrangement, and the like of the groove can be selected as appropriate. The groove may have, for example, a linear shape, a wavy shape, or a meandered shape. The form of arrangement of grooves may be, for example, parallel arrangement of a plurality of linear grooves or comb-tooth shaped paths which will be described later.

[0097] The flow channel allows quick diffusion of water contained in the first fluid 101 over the entire first flow field 5. The flow channel allows quick discharge of oxygen generated at the first catalyst layer 30 to the outside of the electrochemical cell 1. The flow channel may be formed in the first porous layer 32.

[0098] In an example where the first conductive plate 31 includes the groove that defines the flow channel, the first conductive plate 31 is larger in area of contact with the first fluid 101 than in an example where the first conductive plate does not include the groove. As the first conductive plate 31 contains graphite high in degree of graphitization, oxidation of graphite is less likely even when the area of contact with the first fluid 101 is large.

[0099] [Configuration of Flow Channel in Second Flow Field] The second flow field 6 may also include a flow channel. The second flow field 6 may include a first flow channel and a second flow channel independent of each other and may include a groove having a structure including an area where a groove that defines the first flow channel and a groove that defines the second flow channel are alternately arranged. For example, the first flow channel includes a plurality of first grooves arranged in parallel and the second flow channel includes a plurality of second grooves arranged in parallel.  In a plan view, at least a part of the plurality of first grooves and at least a part of the plurality of second grooves are alternately arranged in parallel in a direction orthogonal to a direction of extension of the first grooves and the second grooves. The first grooves and the second grooves are, for example, longitudinal grooves that extend from a lower end to an upper end of the second conductive plate 41.

[0100] The first flow channel may further include a lower lateral groove that connects lower ends of the plurality of first grooves and an inlet groove that connects the lower lateral groove and the inlet 6A (Fig. 1). In this case, the first grooves are connected to the inlet 6A. Unlike this example, each of the plurality of first grooves may directly be connected to the inlet 6A. In this case, the lower lateral groove and the inlet groove are not provided in the second conductive plate 41. The first flow channel may be configured such that at least a part of the plurality of first grooves is connected to the lower lateral groove and each of remaining first grooves is directly connected to the inlet 6A.

[0101] The second flow channel may further include an upper lateral groove that connects upper ends of the plurality of second grooves and an outlet groove that connects the upper lateral groove and the outlet 6B (Fig. 1). In this case, the second grooves are connected to the outlet 6B. Unlike this example, each of the plurality of second grooves may directly be connected to the outlet 6B. In this case, the upper lateral groove and the outlet groove are not provided in the second conductive plate 41. The second flow channel may be configured such that at least a part of the plurality of second grooves is connected to the upper lateral groove and each of remaining second grooves is directly connected to the outlet 6B.  The configuration of the groove connected to the inlet 6A and the configuration of the groove connected to the outlet 6B may be the same or different.

[0102] The plurality of first grooves and the plurality of second grooves are arranged to be meshed with each other. Since openings of the first grooves face the second porous layer 42, the second fluid 102 is quickly diffused from the first grooves over the entire second porous layer 42. Consequently, a substance to be hydrogenated contained in the second fluid 102 is efficiently supplied to the second catalyst layer 40. Since openings of the second grooves face the second porous layer 42, the second fluid 102 containing a hydride is quickly collected from near the second catalyst layer 40 to the second grooves.

[0103] <Test Example 1> In a test example 1, the first conductive plates 31 formed from a sample A and a sample B were made and the degrees of graphitization of the first conductive plates 31 was measured.

[0104] [Sample A] The sample A is the first conductive plate 31 made by forming in a mold, a composite material of graphite particles and phenol resin. The content of graphite particles in the sample A was 88 mass % and the content of the phenol resin was 12 mass %.

[0105] A small piece was sampled from the sample A and interplanar spacing d(002) of carbon in the sample A was measured by XRD.  A measurement value was substituted into the expression (1) described above to obtain the degree of graphitization of the sample A. The degree of graphitization of the sample A was 0.644.

[0106] [Sample B] The sample B is the first conductive plate 31 made by forming in a mold, a composite material of graphite particles and phenol resin. The content of graphite particles in the sample B was 93 mass % and the content of the phenol resin was 7 mass %. The degree of graphitization of the sample B was 0.736.

[0107] <Test Example 2> How readily the sample A and the sample B made in the test example 1 were corroded in the electrochemical cell 1 was quantitatively examined. Specifically, a three-electrode measurement cell where the sample A or the sample B was arranged as a working electrode was made, and a density of a current that flowed in the sample A and the sample B under a prescribed condition was measured.

[0108] A plurality of types of electrolyte solutions that simulated the first fluid 101 in which impurity ions had been introduced were initially prepared. These electrolyte solutions were sulfuric acid aqueous solutions obtained by adding sulfuric acid to pure water. The electrical conductivity and pH of the electrolyte solutions vary as a concentration of sulfuric acid in the electrolyte solutions is different. The electrical conductivity and pH are obtained by calculation from an amount of sulfuric acid added to pure water. One-hundred percent oxygen gas was blown into the electrolyte solutions for a prescribed time period to set the electrolyte solutions in a state saturated with oxygen.

[0109] The working electrode formed from the sample A or the sample B, a counter electrode formed from a carbon rod, and a reference electrode formed from a sulfuric acid / mercury sulfate electrode were immersed in the electrolyte solution described above to make a three-electrode measurement cell. The working electrode was made by coating with a heat-shrinkable tube, a part of a plate material obtained by working the sample A or the sample B into a prescribed size. The heat-shrinkable tube is resistant to the electrolyte solution and a portion of the plate material not covered with the heat-shrinkable tube functions as the working electrode in the electrolyte solution. The working electrode had an area of 1 cm x 1 cm.

[0110] A potentiostat / galvanostat (Versa STAT4 manufactured by Princeton Applied research) was connected to the measurement cell to measure polarization characteristics of the sample A or the sample B. Specifically, a voltage was swept at a sweep rate of 1 mV / s (millivolt per second) until a potential of the reference electrode reached 1.7 V or higher from a natural potential to measure the polarization characteristics. During measurement of the polarization characteristics, oxygen gas was kept blown in a manner not impinging on a surface of the working electrode.

[0111] A corrosion current density of each of the sample A and the sample B was obtained from results of the test. The current that flows when the potential is applied to the sample A or B is considered to include also a current based on an oxygen production reaction with decomposition of water together with a corrosion current of graphite. Trial calculation is performed, with the current that flows being totally assumed as the corrosion current of graphite. It can be determined that oxidation of graphite is less likely as the corrosion current is lower. In the present test example, Figs. 4 and 5 show relation between the corrosion current density calculated by dividing the corrosion current by the area of the working electrode and pH or the electrical conductivity of the electrolyte solution. A while circular plot in Figs. 4 and 5 represents a result of the sample A and a black circular plot represents a result of the sample B.

[0112] The abscissa in Fig. 4 represents pH of the electrolyte solution and the ordinate represents the corrosion current density. A Table 1 shows pH of the electrolyte solution used for measurement of the samples A and B. The corrosion current density shown in Figs. 4 and 5 is expressed as a value when the potential of the working electrode is 1.7 V. A unit of the corrosion current density is mA / cm2.

[0113] [Table 1] Sample pH of Electrolyte Solution A 0.6 1.33 2.21 3.14 3.93 5.5 B 0.4 1.27 2.14 3.18 4.11 5.39

[0114] The abscissa in Fig. 5 represents the electrical conductivity of the electrolyte solution and the ordinate represents the corrosion current density. A Table 2 shows the electrical conductivity of the electrolyte solution used for measurement of the samples A and B. A unit of the electrical conductivity is p,S / cm.

[0115] [Table 2] Sample Electrical Conductivity of Electrolyte Solution (pS / cm) A 29 134 520 3400 19200 45000 B 34 167 420 3000 15700 41000

[0116] As shown in Fig. 4, as pH of the electrolyte solution was lower, the corrosion current density of the sample A and the sample B was higher. Therefore, it is estimated that, in the electrochemical cell 1 in the embodiment, as pH of the first fluid 101 is lower, the first conductive plate 31 is more readily corroded. It was found that the sample B higher in degree of graphitization than the sample A was lower in corrosion current density and was less likely to be corroded. As pH of the electrolyte solution is lower, however, a difference in corrosion current density between the sample A and the sample B is smaller. When pH of the electrolyte solution was equal to or higher than four, each of the sample A and the sample B was sufficiently low in corrosion current density.

[0117] As shown in Fig. 5, as the electrical conductivity of the electrolyte solution was higher, the corrosion current density of the sample A and the sample B was higher. Therefore, it is estimated that, in the electrochemical cell 1 in the embodiment, as the electrical conductivity of the first fluid 101 is higher, corrosion of the first conductive plate 31 is more likely. It was found that the sample B higher in degree of graphitization than the sample A was lower in corrosion current density and was less likely to be corroded. As the electrical conductivity of the electrolyte solution is higher, however, the difference in corrosion current density between the sample A and the sample B is smaller. Each of the sample A and the sample B was sufficiently low in corrosion current density when the electrical conductivity of the electrolyte solution was equal to or lower than 200 pS / cm.

[0118] As shown in Figs. 4 and 5, when pH is lower than four or when the electrical conductivity exceeds 200 pS / cm, even the sample A having the degree of graphitization of 0.644 is relatively high in corrosion current density.  Therefore, in an example where the degree of graphitization is lower than 0.5, it is estimated that the corrosion current density higher than in the graph of the sample A may be taken.

[0119] As shown in Figs. 4 and 5, on the other hand, as pH of the electrolyte solution is closer to seven or as the electrical conductivity of the electrolyte solution is lower, the corrosion current density becomes lower. Therefore, it is expected that, in an example where there are fewer impurity ions contained in the first fluid 101, the corrosion current density is sufficiently low if the degree of graphitization of the first conductive plate 31 is equal to or higher than 0.5.

[0120] <Test Example 3> In a test example 3, an amount of thickness decrease of the first conductive plate 31 as the electrochemical cell 1 operated was obtained by simulation. In simulation, the corrosion current density obtained in the test example 2 was used.

[0121] An amount of charges that flow in graphite contained in the sample A and the sample B is obtained from the corrosion current density and an amount of carbon atoms of graphite to be decomposed is obtained by calculation. Since the amount of carbon atoms contained in the samples A and B is obtained by calculation based on formulation at the time when the samples A and B were made, the amount of carbon atoms decomposed with lapse of time is also obtained by calculation. The amount of thickness decrease of the first conductive plate 31 was obtained based on the amount of decomposed carbon atoms. The amount of thickness decrease refers to an amount of decrease in thickness of the first conductive plate 31. Fig. 6 shows results of this simulation.

[0122] The abscissa in Fig. 6 represents operating hours of the electrochemical cell 1 and the ordinate represents the amount of thickness decrease. A unit of operating hours is h (hour). A unit of the amount of thickness decrease is pm.  A black quadrangular plot represents a result of simulation when the first fluid 101 at pH 4 is in contact with the first conductive plate 31 as the sample A and a black circular plot represents a result of simulation when the first fluid 101 at pH 4 is in contact with the first conductive plate 31 as the sample B. A white quadrangular plot represents a result of simulation when the first fluid 101 at pH 5 is in contact with the first conductive plate 31 as the sample A and a white circular plot represents a result of simulation when the first fluid 101 at pH 5 is in contact with the first conductive plate 31 as the sample B.

[0123] As shown in Fig. 6, as the operating hours are longer, the amount of thickness decrease of the first conductive plate 31 increases.  It was found based on comparison between the black quadrangular plot and the white quadrangular plot and comparison between the black circular plot and the white circular plot that, as pH was closer to seven, the amount of thickness decrease of the first conductive plate 31 was smaller. It was found based on comparison between the black quadrangular plot and the black circular plot and comparison between the white quadrangular plot and the white circular plot that the first conductive plate 31 formed from the sample B was significantly smaller in amount of thickness decrease than the first conductive plate 31 formed from the sample A. The sample B is a sample higher in degree of graphitization than the sample A. It was found from these results that the amount of thickness decrease as the electrochemical cell 1 operated could be made smaller by increasing the degree of graphitization of graphite contained in the first conductive plate 31 than by bringing pH of the first fluid 101 closer to seven.

[0124] <Test Example 4> Compression strain of the second porous layer 42 in the electrochemical cell 1 was measured.  The second porous layer 42 is formed from a non-woven fabric containing carbon fibers (a carbon felt below).  A carbon fiber included in the carbon felt had an average diameter of 10 ^m. 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%.  Compression strain is obtained as a value calculated by dividing the amount of decrease X (= t0-t1) in thickness of the compressed second porous layer 42 by the initial thickness t0 of the second porous layer 42 yet to be compressed.  A thickness t1 refers to the thickness of the compressed second porous layer 42.

[0125] The 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. The initial thickness t0 was measured with a surface pressure at 0.7 kPa being applied to the carbon felt for ten seconds.

[0126] Fig. 7 is a schematic diagram of a compression apparatus 9 for measuring compression strain of the second porous layer 42. The compression apparatus 9 includes a lower mount 90 and an upper mount 91. The upper mount 91 is configured as being movable downward.  The compression apparatus 9 applies the surface pressure to a member that lies between the lower mount 90 and the upper mount 91. The compression apparatus 9 can automatically measure the surface pressure applied between the lower mount 90 and the upper mount 91 and an interval between the lower mount 90 and the upper mount 91. The compression apparatus 9 is a commercially available strength evaluation apparatus such as Micro Autograph MST-I type HR manufactured by Shimadzu Corporation.

[0127] In measurement of compression strain of the carbon felt, a lower protective plate 92, the second conductive plate 41, the second porous layer 42, the first conductive plate 31, and an upper protective plate 93 were sequentially layered on the lower mount 90. The lower protective plate 92 and the upper protective plate 93 serve to protect the second conductive plate 41 and the first conductive plate 31 against damage and to apply a uniform surface pressure to the second porous layer 42.  The lower protective plate 92 and the upper protective plate 93 are formed of stainless steel. Among a plurality of members that lie between the lower mount 90 and the upper mount 91, a member the thickness of which changes is only the second porous layer 42. The lower protective plate 92, the second conductive plate 41, the first conductive plate 31, and the upper protective plate 93 are rigid bodies, and thicknesses thereof do not substantially change.

[0128] The compression apparatus 9 is activated to apply the surface pressure to the second porous layer 42.  The surface pressure applied to the second porous layer 42 is gradually increased and a distance of movement of the upper mount 91 corresponding to magnitude of the surface pressure is measured.  The distance of movement can also be expressed as being equal to the amount of decrease X in thickness of the second porous layer 42.  Compression strain of the second porous layer 42 was obtained from the initial thickness t0 and the distance of movement.  Specifically, X / t0 = compression strain.  Compression strain is unitless.  Relation between compression strain and the surface pressure is shown in a graph in Fig. 8.  The abscissa in the graph represents compression strain and the ordinate represents the surface pressure.

[0129] In the test example 4, compression strain also of a carbon cloth and carbon paper was measured.  Compression strain of each of the carbon cloth and the carbon paper was measured with a method the same as the method for the carbon felt. Results thereof are also shown in the graph in Fig. 8.

[0130] As shown in Fig. 8, compression 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 the second porous layer 42 formed from such a carbon felt is compressed between the electrolyte membrane 2 and the second conductive plate 41 as shown in Fig. 3, the second porous layer 42 presses the electrolyte membrane 2 and the second conductive plate 41 at the surface pressure as high as compressive force. Therefore, the second porous layer 42 is in intimate contact with the electrolyte membrane 2 and the second conductive plate 41 and substantially no local gap is produced at the boundary 1A or 1B.  Since there is substantially no local gap at the boundary 1A or 1B, the second fluid 102 readily spreads over the entire second flow field 6. Consequently, improvement in Faraday efficiency of the electrochemical cell 1 is expected.

[0131] Compression 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 the second porous layer 42 formed from such a carbon cloth is arranged between the electrolyte membrane 2 and the second conductive plate 41 as shown in Fig. 3, production of a gap is less likely at the boundary 1A or 1B.

[0132] As described above, the carbon cloth and the carbon felt satisfy compression strain not less than 0.3 even when the surface pressure is relatively as low as approximately 0.1 MPa and satisfies compression strain not less than 0.3 when the surface pressure further increases.  Since the surface pressure in the electrochemical cell 1 is higher than 0.1 MPa, compression strain of the carbon cloth and the carbon felt in the electrochemical cell 1 can be concluded as satisfying 0.3 or more.

[0133] Compression 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 the second porous layer 42 formed from such carbon paper is arranged between the electrolyte membrane 2 and the second conductive plate 41 as shown in Fig. 3, production of a gap is likely at the boundary 1A or 1B.

[0134] In an example where the 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 the electrochemical cell 1 including the second porous layer 42 formed from the carbon paper which is less likely to elastically deform, a gap may be likely at the boundary 1A or 1B in Fig. 3 due to such variation and lowering in Faraday efficiency due to the gap may be likely.

[0135] Even in the presence of the variation above, the second porous layer 42 formed from the carbon felt maintains, by elastic deformation, a state in which it is in intimate contact with the electrolyte membrane 2 and the second conductive plate 41. Consequently, a local gap is less likely at the boundary 1A or 1B in Fig. 3. Therefore, high Faraday efficiency may be maintained for a long time in the electrochemical cell 1 including the second porous layer 42 formed from the carbon felt.

[0136] Furthermore, decrease in thickness of the first conductive plate 31 due to corrosion caused by long-term use may occur. Even in this case, the second porous layer 42 made of the carbon felt elastically deforms so that adjacent members included in the electrochemical cell 1, that is, the first conductive plate 31, the first porous layer 32, the electrolyte membrane 2, the second porous layer 42, and the second conductive plate 41, tend to be maintained in a surface contact state. The surface contact state above further tends to be maintained by increasing the degree of graphitization or bringing pH of the first fluid 101 closer to seven as described above.

[0137] <Test Example 4-2> A plurality of carbon felts X, Y, and Z different from the test example 4 in porosity and basis weight were prepared and compression strain of the carbon felts X, Y, and Z was measured under conditions the same as in the test example 4. A Table 3 shows the porosity, the basis weight, and the average diameter of the carbon fiber of each of the carbon felts X, Y, and Z.  The porosity in the Table 3 is expressed as a value with the carbon felt having the thickness of 1.0 mm being compressed to 0.3 mm.

[0138] [Table 3] Sample Porosity (%) Basis Weight (g / m2) Average Diameter (pm) X 89.5 71 10 Y 84.0 108 10 Z 81.2 127 10

[0139] Compression strain of the 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 average diameter of the carbon fibers was in a range not smaller than 5 um and not larger than 20 um, the carbon felt having compression strain not less than 0.6 and not more than 0.8 could be constructed. REFERENCE SIGNS LIST

[0140] 1 electrochemical cell 1A, 1B boundary 2 electrolyte membrane 21 first surface, 22 second surface 3 anode portion 30 first catalyst layer, 31 first conductive plate, 32 first porous layer, 33 frame seal portion 4 cathode portion 40 second catalyst layer, 41 second conductive plate, 42 second porous layer, 43 frame seal portion 5 first flow field 5A inlet, 5B outlet 6 second flow field 6A inlet, 6B outlet 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 5                    101P first pump, 101T first tank 102 second fluid 102A second supply pipe, 102B second discharge pipe 102P second pump, 102T second tank

Claims

1. An electrochemical cell comprising:an anode portion and a cathode portion between which an electrolyte membrane lies,the electrochemical cell generating protons by supply of a first fluid mainly composed of water to the anode portion and electrolysis of the water between the anode portion and the cathode portion, whereinthe anode portion includes a first conductive plate formed of a composite material containing a first resin and a first carbon material.

2. The electrochemical cell according to claim 1, whereinthe first carbon material includes graphite, anda degree of graphitization in the first conductive plate is equal to or higher than 0.5.

3. The electrochemical cell according to claim 1 or 2, whereinthe first fluid has an electrical conductivity equal to or lower than 200 ^S / cm.

4. The electrochemical cell according to any one of claims 1 to 3, whereinthe first fluid has a hydrogen ion exponent equal to or larger than four.

5. The electrochemical cell according to any one of claims 1 to 4, whereina content of the first carbon material in the first conductive plate is equal to or higher than 70 mass % with a mass of the first conductive plate being defined as 100 mass %.

6. The electrochemical cell according to any one of claims 1 to 5, whereina second fluid containing an organic compound having unsaturated bond is circulated through the cathode portion.

7. The electrochemical cell according to any one of claims 1 to 6, whereinthe cathode portion includes a second conductive plate formed of a composite material containing a second resin and a second carbon material,the second carbon material includes graphite, anda content of the second carbon material in the second conductive plate is equal to or higher than 70 mass % with a mass of the second conductive plate being defined as 100 mass %.

8. The electrochemical cell according to claim 7, whereinthe second resin is at least one selected from the group consisting of polyester resin, polyamide resin, polyacetal resin, phenol resin, polyvinylidene chloride resin, polyvinylidene fluoride resin, polytetrafluoroethylene resin, epoxy resin, polyoxymethylene resin, and polyphenylene sulfide resin.

9. The electrochemical cell according to any one of claims 1 to 8, whereinthe anode portion includes a first porous layer formed of a conductive material, andthe first porous layer has an average thickness not smaller than 0.20 mm and not larger than 0.40 mm.

10. The electrochemical cell according to claim 9, whereinthe first porous layer includes a substrate formed of metal and a coating formedat a surface of the substrate,the metal includes at least one selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper, and zinc,a form of the substrate is an aggregate of fibers, a porous sintered body, a5     foamed formed body, or expanded metal, andthe coating contains platinum.

11. The electrochemical cell according to any one of claims 1 to 10, wherein10            the cathode portion includes a second porous layer formed of a conductivematerial, andcompression strain of the second porous layer in the cathode portion is not less than 0.3 and not more than 0.8.15

12. The electrochemical cell according to claim 11, whereinthe second porous layer includes a non-woven fabric including carbon fibers.

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