Electrochemical cells and hydrocarbon production equipment

Abstract

To provide an electrochemical cell used for the production of a hydrocarbon by co-electrolysis with carbon dioxide and water vapor.SOLUTION: An electrochemical cell 1 comprises a cathode 2 and an anode 3 arranged on two opposite surfaces 12 and 13 of an oxide ion conductor 11. The cathode 2 has a laminated structure, in which a conductive metallic layer 2A that is configured with a first particle 21 having silver as a principal component and is arranged so as to contact a raw material gas G including carbon dioxide and water vapor, a complex oxide layer 2B that is configured with a second particle 22 having a complex oxide including a transition metal element as a principal component and is arranged so as to contact the oxide ion conductor 11, and an interlayer 2C that is arranged between the conductive metallic layer 2A and the complex oxide layer 2B and is configured with the first particle 21 and the second particle 22 are stacked.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrochemical cell used in the production of hydrocarbons and a hydrocarbon production apparatus using the electrochemical cell. [Background technology]

[0002] In recent years, from the perspective of reducing carbon dioxide emissions and carbon recycling, there has been a growing expectation for the establishment of a technology for co-electrolyzing carbon dioxide and water vapor through an electrochemical process to convert them into hydrocarbons such as methane. Solid oxide electrolysis cells (SOECs), which are generally made of solid oxides, are known as electrochemical cells used in such processes. To promote the desired reactions and increase the reaction efficiency, various studies are being conducted on composite oxides that serve as electrode materials, additive components, electrode structures, and other factors.

[0003] For example, Patent Document 1 discloses an electrode structure in an electrochemical cell used in a fuel cell, in which two regions with different average particle sizes of the electrode material are provided in order to improve the reaction activity of the air electrode active layer that functions as the cathode. Specifically, the average particle size of the first region on the solid electrolyte layer side, which is more likely to affect the reaction activity, is made smaller than that of the second region located on the outer side of the air electrode active layer, thereby increasing the total surface area of ​​the air electrode active layer and increasing the number of sites for adsorption and reaction.

[0004] Patent Document 1 proposes using a perovskite-type composite oxide as an electrode material and adjusting the type and ratio of elements substituting for each region to enhance the reaction activity of the electrode as a whole. It also proposes a method of improving the catalytic activity of an electrode by adding a metal component such as a platinum group metal to the electrode material. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-26339 Summary of the Invention [Problem to be solved by the invention]

[0006] Thus, the selection of electrode materials and improvements to the electrode structure are important for the performance of electrochemical cells. Meanwhile, to generate hydrocarbons using an electrochemical cell, the raw materials, carbon dioxide and water vapor, must be electrolyzed, and then the reaction between the carbon monoxide and hydrogen produced by electrolysis must proceed. However, simply applying a conventional electrochemical cell makes it difficult to advance these reactions, and a single electrochemical cell suitable for generating hydrocarbons has yet to be discovered.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide an electrochemical cell capable of converting carbon dioxide and water vapor into hydrocarbons by an electrochemical process, and a hydrocarbon production apparatus using the electrochemical cell. [Means for solving the problem]

[0008] One aspect of the present invention is An electrochemical cell (1) comprising an oxide ion conductor (11) and a cathode (2) and an anode (3) provided on two opposing surfaces (12, 13) of the oxide ion conductor, The cathode is a conductive metal layer (2A) made of first particles (21) mainly composed of silver and in contact with a source gas (G) containing carbon dioxide and water vapor; a complex oxide layer (2B) that is composed of second particles (22) mainly composed of a complex oxide containing a transition metal element and that is in contact with the oxide ion conductor; an intermediate layer (2C) formed between the conductive metal layer and the complex oxide layer and composed of the first particles and the second particles; And, The oxide ion conductor is composed of at least one kind selected from non-oriented or oriented apatite-type compounds, and the apatite-type compound is a phosphate-based apatite-type compound or an apatite-type composite oxide. , in an electrochemical cell. Another aspect of the present invention is An electrochemical cell (1) comprising an oxide ion conductor (11) and a cathode (2) and an anode (3) provided on two opposing surfaces (12, 13) of the oxide ion conductor, The cathode is a conductive metal layer (2A) made of first particles (21) mainly composed of silver and in contact with a source gas (G) containing carbon dioxide and water vapor; a complex oxide layer (2B) that is composed of second particles (22) mainly composed of a complex oxide containing a transition metal element and that is in contact with the oxide ion conductor; The electrochemical cell has a laminated structure in which an intermediate layer (2C) is provided between the conductive metal layer and the complex oxide layer and is composed of the first particles and the second particles, and the complex oxide containing a transition metal element is a lanthanum-based complex oxide or a ceria-based complex oxide.

[0009] Another aspect of the present invention is a hydrocarbon production apparatus (100) using the electrochemical cell according to the above aspect, a hydrocarbon production section (10) in which the electrochemical cell is disposed so as to close one end of a cylindrical gas flow section (101); the gas flow section has a gas inlet (102) and a gas outlet (103) provided opposite the cathode of the electrochemical cell, the gas inlet section supplies the source gas to the cathode in a gas introduction direction perpendicular to the cathode; The gas discharge section is disposed on the outer periphery of the gas inlet section, and discharges the reaction product at the cathode to the outside of the gas flow section in a direction opposite to the gas inlet direction, in the hydrocarbon production apparatus. [Effects of the Invention]

[0010] When a source gas containing carbon dioxide and water vapor is introduced into the three-layer cathode of the electrochemical cell, hydrogen is first generated by electrolysis of water vapor in the conductive metal layer containing first particles primarily composed of silver, and then diffuses into the intermediate layer. The intermediate layer contains the first particles and second particles primarily composed of a composite oxide containing a transition metal element. As the electrolysis of water vapor progresses, carbon monoxide is generated by electrolysis of carbon dioxide. It is believed that the hydrogen diffusing from the conductive metal layer creates a hydrogen-rich atmosphere in the intermediate layer. Therefore, the generated carbon monoxide quickly reacts with the hydrogen to generate hydrocarbon precursors. The catalytic action of the second particles further electrolytically reduces the precursors to convert them into hydrocarbons. Since hydrocarbon generation has not been observed in either the intermediate layer or the composite oxide layer alone, it is believed that the inclusion of both layers allows precursor generation and its electrolytic reduction to proceed in a stepwise manner, enabling hydrocarbon generation.

[0011] In a hydrocarbon production apparatus using such an electrochemical cell, a small, efficient hydrocarbon production unit can be constructed by providing a gas distribution unit with a gas outlet around the outer periphery of the gas inlet and placing the electrochemical cell on one end of the gas distribution unit. In this gas distribution unit, the gas inlet faces the cathode of the electrochemical cell, and the feed gas is introduced perpendicularly, while the gas flow is in the opposite direction in the gas outlet. This allows the reaction to proceed stepwise, and the precursor-containing gas that reaches the complex oxide layer tends to remain there instead of flowing out. Furthermore, in the intermediate layer, electrolysis, an endothermic reaction, progresses, lowering the temperature, creating conditions favorable for methane production in the adjacent complex oxide layer. This facilitates the electrolytic reduction of the precursor, thereby increasing the methane conversion rate.

[0012] As described above, according to the above-described aspects, it is possible to provide an electrochemical cell capable of converting carbon dioxide and water vapor into hydrocarbons by an electrochemical process, and a hydrocarbon production apparatus using the electrochemical cell. In addition, the symbols in parentheses in the claims and the means for solving the problems indicate the correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is an overall cross-sectional view schematically illustrating the configuration of an electrochemical cell according to a first embodiment. [Figure 2] 3 is a cross-sectional view schematically showing the structure of a cathode, which is a main part of an electrochemical cell in the first embodiment, and is an enlarged cross-sectional view of part A in FIG. 2. FIG. [Figure 3] FIG. 3 is an enlarged cross-sectional view of a main part for explaining a reaction in a cathode of the electrochemical cell in the first embodiment. [Figure 4] FIG. 3 is an enlarged cross-sectional view of a main part showing an example of the configuration of a gas distribution unit of a hydrocarbon production unit using an electrochemical cell in the first embodiment. [Figure 5] 5 is an enlarged cross-sectional view of a main part for explaining the flow of raw material gas supplied to the cathode of the electrochemical cell in the first embodiment, and is an enlarged cross-sectional view of part B in FIG. 4. [Figure 6] FIG. 1 is an overall configuration diagram of a hydrocarbon production apparatus using an electrochemical cell according to a first embodiment. [Figure 7] 7 is an enlarged cross-sectional view of a main part showing an example of the configuration of a gas inlet part of a hydrocarbon production apparatus in the first embodiment, and is an enlarged cross-sectional view of part C in FIG. 6. FIG. [Figure 8] FIG. 5 is an enlarged cross-sectional view of a main part corresponding to the configuration of FIG. 4, illustrating the gas flow in the gas inlet portion of the hydrocarbon production apparatus in the first embodiment. [Figure 9] FIG. 8 is an enlarged cross-sectional view of a main part corresponding to the configuration of FIG. 7, illustrating the gas flow in the gas inlet portion of the hydrocarbon production apparatus in the first embodiment. [Figure 10] FIG. 2 is a histogram showing a comparison of methane conversion rates in a methane production test using a hydrocarbon production apparatus in an example. [Figure 11] FIG. 4 is a graph showing the relationship between methane production rate and pressure in a methane synthesis reaction in a reference example. DETAILED DESCRIPTION OF THE INVENTION

[0014] (Embodiment 1) An embodiment of an electrochemical cell and a hydrocarbon production apparatus will be described with reference to Fig. 1 to Fig. 9. As shown in Fig. 1, the electrochemical cell 1 includes an oxide ion conductor 11, and a cathode 2 and an anode 3 provided on two opposing surfaces 12, 13 of the oxide ion conductor 11. The cathode 2 and the anode 3 face each other with the flat-plate-shaped oxide ion conductor 11 in between, and a raw material gas G containing carbon dioxide (CO2) and water vapor (H2O) is supplied toward the surface 20 of the cathode 2.

[0015] As shown in Figures 2 and 3, the cathode 2 has a structure in which a conductive metal layer 2A composed of first particles 21 containing silver (Ag) as a main component, a complex oxide layer 2B composed of second particles 22 containing a complex oxide containing a transition metal element as a main component, and an intermediate layer 2C composed of the first particles 21 and the second particles 22 are laminated in this order.

[0016] In this case, the conductive metal layer 2A is provided in contact with the space 4 into which the source gas G is introduced, and constitutes the outermost layer of the cathode 2. The surface 20 is shown as an imaginary plane parallel to the oxide ion conductor 11. The complex oxide layer 2B is provided in contact with the surface 12 of the oxide ion conductor 11 on the source gas G side, and constitutes the innermost layer of the cathode 2. The intermediate layer 2C is located between the conductive metal layer 2A and the complex oxide layer 2B and is provided integrally with both of these layers. In the conductive metal layer 2A, the complex oxide layer 2B, and the intermediate layer 2C, the voids formed between the particles communicate with each other, forming diffusion paths 23 through which the source gas G and the generated gas containing the reaction products can diffuse.

[0017] In this way, the cathode 2 has a three-layer structure, and the intermediate layer 2C containing both the first and second particles is formed between the conductive metal layer 2A and the composite oxide layer 2B. This is thought to enable the co-electrolysis of CO2 and HO contained in the source gas G and the reaction of the electrolysis products to proceed stepwise in each layer, making it possible to produce hydrocarbons. When only the conductive metal layer 2A and the composite oxide layer 2B are present and the intermediate layer 2C is not present, or when only the conductive metal layer 2A and the intermediate layer 2C are present and a structure corresponding to the composite oxide layer 2B is not present, hydrocarbon production is not observed. Next, the detailed structure of the cathode 2 and the overall configuration of the electrochemical cell 1 will be described.

[0018] The conductive metal layer 2A of the cathode 2 is a porous conductive metal layer formed by an aggregation of only the first particles 21, and has the function of mainly electrolyzing the water vapor contained in the source gas G. At this time, as shown by [I] in Figure 3, it is believed that a reaction proceeds in which hydrogen (H2) gas is generated by electrolysis of the water vapor. The hydrogen generated in the conductive metal layer 2A diffuses into the intermediate layer 2C together with the source gas G containing unreacted water vapor.

[0019] The first particles 21 are conductive metal particles containing silver as a main metal component, and may contain other catalytic metals to the extent that they do not interfere with the electrode reactions in the conductive metal layer 2 A and the intermediate layer 2 C. Preferably, the first particles 21 can be conductive metal particles containing only silver as a metal component.

[0020] The intermediate layer 2C is a porous mixed material layer formed between the conductive metal layer 2A and the complex oxide layer 2B, and the complex oxide layer 2B is a porous complex oxide layer formed by an aggregation of only second particles 22. Like the conductive metal layer 2A, the intermediate layer 2C contains first particles 21 mainly composed of silver, and like the complex oxide layer 2B, contains second particles 22 mainly composed of a complex oxide containing a transition metal element.

[0021] As shown by [II] in Figure 3, in the intermediate layer 2C, an electrolysis reaction of the source gas G and a reaction of the electrolysis product to generate a methane precursor proceed. That is, as in the conductive metal layer 2A, unreacted water vapor is electrolyzed to generate H, and CO gas contained in the source gas G is electrolyzed to generate carbon monoxide (CO) gas. Furthermore, a methane precursor is generated from the generated CO and H supplied from the conductive metal layer 2A or H generated in the intermediate layer 2C.

[0022] At this time, H2 previously generated in the conductive metal layer diffuses into the intermediate layer 2C, making the intermediate layer 2C hydrogen-rich, and therefore a sufficient amount of H2 exists around the generated CO. As a result, when CO is generated, it quickly reacts with the surrounding H2, and it is thought that methane precursors can be generated from CO molecules and multiple H2 molecules as follows: CO+2H2 → CO(H2)2 CO(H2)2: Methane precursor

[0023] As shown by [III] in Fig. 3, a reaction of generating methane (CH4) gas from a methane precursor proceeds in the complex oxide layer 2B. That is, when the product gas containing the methane precursor generated in the intermediate layer 2C diffuses into the complex oxide layer 2B, the methane precursor is electrolytically reduced by the catalytic action of the second particles 22 and converted to CH4 as follows: CO(H2)2+2e - → CH4+O 2- Oxide ions (O) generated by electrolytic reduction of methane precursor 2- ) passes through the oxide ion conductor 11 in contact with the complex oxide layer 2B and is released to the anode 3 side.

[0024] The second particles 22 are particles of a complex oxide containing at least one element selected from transition metals, and have electrical conductivity to function as an electrolytic electrode in the intermediate layer 2C and as a reduction catalyst for methane precursors in the complex oxide layer 2B. A complex oxide having mixed conductivity of electrons and oxide ions is preferred. Examples of complex oxides having such functionality include perovskite-type complex oxides containing transition metal elements and ceria-based complex oxides containing transition metal elements.

[0025] A perovskite-type composite oxide containing a transition metal element is a composite oxide having a perovskite structure represented by the general formula ABO3 as its basic composition and containing at least one transition metal element. It may also be a composite oxide with a similar structure containing the perovskite structure, such as a composite oxide represented by [(A1)BO3+(A2)O]. In this case, the A1 or A2 element may contain at least one element selected from rare earth elements such as lanthanoid elements and alkaline earth metal elements, and the B element may contain at least one element selected from transition metal elements. Preferably, the A1 or A2 element and the B element are selected from, for example, one or more of the following: A1 or A2: La, Sm, Sr, Ca, Ba B:V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd

[0026] Preferably, a lanthanum-based composite oxide containing lanthanum (La) can be used as a composite oxide of perovskite type or a similar structure. Examples of such lanthanum-based composite oxides include (La,Sr)CoO3, (La,Sr)(Co,Fe)O3, and LaSr(Co,Fe)O4, from the viewpoint of electronic conductivity and ionic conductivity. Specific examples of such composite oxides include LaSrCoO3, LaSrCoO4 ... 0.8 Fe 0.2 O4, etc. However, it is not limited to these.

[0027] Specifically, the ceria-based composite oxide containing a transition metal element has the composition formula Ce 1-x (M1) xIt can be a composite oxide represented by O2 (where 0 < x < 1). The M1 element in the formula is a transition metal element. For example, one or more elements shown below are selected. Preferably, X is a value selected from the range of 0.1 or more and 0.4 or less. M1: V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd Examples of such ceria-based composite oxides include, from the viewpoints of electronic conductivity and ionic conductivity, for example, Ce 1-x (Co, Fe) x O3, etc. are used. Specifically, examples include Ce 0.75 Co 0.2 Fe 0.05 O2, etc. However, it is not limited thereto.

[0028] The ceria-based composite oxide containing a transition metal element imparts electrical conductivity by substituting a part of Ce with a transition metal in cerium oxide (CeO2) having good ionic conductivity, making it metallic in terms of electronic structure. At this time, for example, when Co is selected as the M1 element, more Ce can be substituted, which is advantageous for enhancing electrical conductivity. Also, in addition to electrical conductivity, to impart a catalytic function, it is necessary to adsorb a methane precursor, supply electrons, and reduce it. Therefore, it preferably has a precursor molecule adsorption ability and an electronic structure advantageous for electron supply. When the above-described M1 element is selected, it is considered that such a reaction site is preferably formed in the electrode, and preferably contains a plurality of transition metal elements.

[0029] Preferably, in the intermediate layer 2C, the first particles 21 are formed so as to be connected to the first particles 21 of the conductive metal layer 2A and extend onto the surfaces of the second particles 22, forming an integrated configuration. In this case, the average particle diameter of the first particles 21 is desirably smaller than the average particle diameter of the second particles 22, and the intermediate layer 2C is formed as an aggregate of second particles 22 having the first particles 21 attached to their surfaces. For example, the primary particle diameter of the material of the first particles 21 may be 0.1 μm or less, and the primary particle diameter of the material of the second particles 22 may be 1 μm or less. The first particles 21 on the surfaces of the second particles 22 are electrically connected to each other to form a band-like connected structure, one end of which is electrically connected to the first particles 21 of the conductive metal layer 2A. Preferably, the second particles 22 to which the first particles 21 are attached are formed so that adjacent particles are electrically connected to each other and are connected to the second particles 22 of the complex oxide layer 2B, forming an integrated structure.

[0030] As a result, in the cathode 2, the first particles 21 or the second particles 22 are connected to each other, making it easier to form a three-dimensional network-like conduction path, thereby improving electronic conductivity or ionic conductivity. Furthermore, in the intermediate layer 2C, the area where the first particles 21 and the second particles 22 are in contact with each other expands, making it easier for the electrode reaction to proceed, and since the first particles 21 become smaller, voids between the second particles 22 are secured, improving the gas diffusibility of the diffusion paths 23.

[0031] In this way, by appropriately disposing the intermediate layer 2C between the conductive metal layer 2A and the composite oxide layer 2B in the cathode 2, hydrocarbon production using a single electrochemical cell 1 becomes possible. Preferably, the ratio t1 / t of the thickness t1 of the intermediate layer 2C to the total thickness t (i.e., t = t1 + t2) of the thickness t1 of the intermediate layer 2C and the thickness t2 of the composite oxide layer 2B is configured to be 1% or more, which enables the generation of CH4 by co-electrolysis of the feed gas G. More preferably, by setting the ratio t1 / t to be 5% or more, a practically sufficient methane conversion rate can be obtained. The methane conversion rate can be calculated using the following formula: Methane conversion rate = (methane concentration in produced gas) / [(CO2 concentration in feed gas) - (CO2 concentration in produced gas)]

[0032] On the other hand, even if the ratio t1 / t exceeds 5% and increases to about 50%, the effect of improving the methane conversion rate is limited, and if it increases to about 95%, the methane conversion rate tends to decrease. Furthermore, as the ratio t1 / t increases, the amount of the first particles 21 used increases, so it is preferable to appropriately select the ratio t1 / t so that the desired methane conversion rate is obtained, preferably within a range of 95% or less, more preferably within a range of 50% or less.

[0033] The thickness of the cathode 2 is not particularly limited, but is generally preferably about 20 μm to 30 μm or more to function as an electrode. Preferably, the total thickness t of the intermediate layer 2C and the composite oxide layer 2B is set in the range of 20 μm to 30 μm, and the thickness of the conductive metal layer 2A is appropriately selected from the viewpoint of electronic conductivity. Preferably, the thickness of the conductive metal layer 2A can be set to about 10 μm or more, which is sufficient to obtain electronic conductivity.

[0034] The thickness t1 of the intermediate layer 2C can be controlled to a desired thickness by adjusting the viscosity of the printing paste used to form the conductive metal layer 2A and adjusting the penetration depth when applied to the surface of the complex oxide layer 2B. The thickness of each layer of the cathode 2 can be measured, for example, by observing an image of a cross-section of a sample using an electron microscope. In this case, the region where the second particles 22 having the first particles 21 attached to the surface are present is defined as the intermediate layer 2C, and both sides of it are defined as the conductive metal layer 2A or the complex oxide layer 2B, and the arithmetic average of the thicknesses measured at multiple locations on each layer can be used as the thickness of each layer.

[0035] The oxide ion conductor 11 is a dense solid electrolyte layer disposed between the cathode 2 and the anode 3, and ionizes the oxide ions (O 2- ) is passed through to the anode 3 side. 2- is the electron (e- ) to generate oxygen (O2) gas. The material of the oxide ion conductor 11 is not necessarily limited, but from the viewpoints of voltage resistance, oxide ion conductivity, etc., stabilized zirconia and non-oriented or oriented apatite-type compounds can be preferably used.

[0036] Stabilized zirconia is a zirconia-based oxide obtained by solid-solving a stabilizer, such as an oxide of a rare earth element, in zirconium oxide (ZrO2) having a fluorite-type crystal structure. Examples of stabilizers include yttria (YO3), scandia (Sc2O3), etc., and yttria-stabilized zirconia (YSZ) is preferably used mainly from the viewpoints of ionic conductivity, mechanical and chemical stability, etc.

[0037] The apatite-type compound is a compound having a hexagonal apatite-type crystal structure, and can be a non-oriented apatite-type compound having no orientation axis, or an oriented apatite-type compound including uniaxial and biaxial orientations. In the case of an oriented apatite-type compound, it is preferable that it has a c-axis orientation. Apatite-type compounds generally have higher oxide ion conductivity than stabilized zirconia, which facilitates electrode reactions and allows for the production of more generated gas. Furthermore, oriented apatite-type compounds have higher oxide ion conductivity than non-oriented apatite-type compounds, contributing to improved methane conversion.

[0038] Examples of apatite-type compounds include those having the composition formula (M2) 10 Examples of suitable compounds include those based on the phosphate apatite represented by (PO4)6(X). The M2 site may contain one or more ions selected from the monovalent or divalent metal ions shown below, and the X site may contain one or more anions selected from the anions shown below or HO. M2:Li + , Na + , K. + , Ca 2+ , Mg 2+ , Zn 2+ , Ba 2+ , Mn2+ , Fe 2+ , Cu 2+ X:OH - , F - , Cl - , CO3 2- , H2O Preferably, from the viewpoint of improving oxide ion conductivity, the M2 site is Na + , Ca 2+ and the X site is OH - , CO3 2- Phosphate-based apatite containing Ca is used. 10-x Na 2x / 3 (PO4)5(CO3) x (H2O) x (OH) 2-x / 3 However, the present invention is not limited to these.

[0039] As the apatite-type compound, an apatite-type composite oxide represented by the following formula (1) can also be used: Formula (1):A 9.3+x [T 6.0-y M y ]O 26.0+z (A in the formula is one or more elements selected from the group consisting of La, Ce, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. T in the formula is an element containing Si or Ge, or both. M in the formula is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo.) A composite oxide having an apatite structure, in which x in the formula is a number between -1.4 and 1.5, y in the formula is a number between 0.0 and 3.0, z in the formula is a number between -5.0 and 5.2, and the ratio of the number of moles of A to the number of moles of T (A / T) is between 1.4 and 3.7, can be used as the oxide ion conductor 11.

[0040] In formula (1), element A is a lanthanoid or alkaline earth metal that can form a positively charged ion and form an apatite-type hexagonal crystal structure. Among these, from the viewpoint of further increasing oxide ion conductivity, a combination of element A with one or more elements selected from the group consisting of La, Nd, Ba, Sr, Ca, and Ce is preferred, and a combination of element A with one of La or Nd, or La with one or more elements selected from the group consisting of Nd, Ba, Sr, Ca, and Ce is particularly preferred. Furthermore, T in formula (1) may be an element containing Si or Ge or both.

[0041] The M element in formula (1) is a metal that forms a metastable precursor (A 2+x TO 5+z ), which results in the precursor being transformed into an apatite structure and the crystals being oriented in one direction. From this viewpoint, the M element may be any element that can become gaseous at a temperature of 1000°C or higher at which the precursor assumes the apatite structure and can obtain the necessary vapor pressure. The "necessary vapor pressure" means a vapor pressure that can move in a gaseous state in the atmosphere and diffuse from the surface of the precursor to the interior at grain boundaries or within grains to promote the reaction. From this viewpoint, the element M can be, for example, one or more elements selected from the group consisting of B, Ge, Zn, W, Sn, and Mo. Among these, B, Ge, and Zn are particularly preferred in terms of high degree of orientation and high productivity (orientation speed).

[0042] In formula (1), x is preferably −1.4 or more and 1.5 or less, more preferably 0.00 or more and 0.70 or less, and even more preferably 0.45 or more and 0.65 or less, from the viewpoint of increasing the degree of orientation and oxygen ion conductivity. In order to fill the T element positions in the apatite-type crystal lattice, y in formula (1) is preferably 0.0 or more and 3.0 or less, more preferably 1.0 or more and 2.0 or less, and even more preferably 1.0 or more and 1.6 or less. From the viewpoint of maintaining electrical neutrality within the apatite-type crystal lattice, z in formula (1) is preferably −5.0 or more and 5.2 or less, more preferably −1.5 or more and 1.5 or less, and even more preferably −1.0 or more and 1.0 or less.

[0043] In addition, in formula (1), the ratio of the number of moles of A to the number of moles of T (A / T), in other words, 9.3+x / 6.0-y in formula (1), is preferably 1.4 or more and 3.7 or less, from the viewpoint of maintaining the spatial occupancy rate in the apatite-type crystal lattice. desirable

[0044] As the apatite-type composite oxide represented by the above formula (1), for example, a rare earth silicate-based composite oxide can be used, in which A in the formula is a rare earth element and T in the formula is Si. Examples of rare earth silicate-based composite oxides include La 9.3+x (Si 6.0-y B y )O 26.0+z , La 9.3+x (Si 6.0-y Ge y )O 26.0+z , La 9.3+x (Si 6.0-y Zn y )O 26.0+z However, the present invention is not limited to these.

[0045] When producing a non-oriented apatite-type composite oxide, oxides containing each constituent element are used as raw materials, and the resulting wet mixture is formed into a sheet, which is then heated and fired to obtain a sintered composite oxide. When producing an oriented apatite-type composite oxide, a sintered precursor of the desired apatite-type composite oxide is similarly produced, but excluding a specific element. The precursor is then heated in a gas phase containing the specific element through a gas-solid diffusion process, which causes the specific element to react with the precursor, resulting in an oriented apatite structure having the desired composition.

[0046] In this way, by adopting a manufacturing method including a vapor-solid diffusion process, a composite oxide having an apatite structure in which the crystals are oriented in one direction can be obtained. Furthermore, since the occurrence of cracks in the crystals can be suppressed, it is possible to manufacture an oriented apatite-type oxide ion conductor 11 with a larger area.

[0047] The constituent material of the anode 3 is not necessarily limited, but for example, a perovskite complex oxide containing a transition metal element can be used. Specifically, the perovskite complex oxide containing a transition metal element is a complex oxide containing one or more elements selected from lanthanoid elements, alkaline earth metal elements, rare earth elements, etc. in the A site of the perovskite structure represented by the general formula ABO3, and one or more transition metal elements in the B site. Preferably, (Ba,Sr)CoO3 or the like containing Ba and Sr in the A site and Co in the B site can be used. Examples of such complex oxides include Ba 0.6 Sr 0.4 CoO3, etc. However, the present invention is not limited to these.

[0048] 4, the electrochemical cell 1 configured as above can constitute a hydrocarbon production unit 10 including a gas circulation unit 101. In the hydrocarbon production unit 10, the gas circulation unit 101 includes a gas inlet unit 102 to which a raw material gas G is supplied, and a gas outlet unit 103 from which the produced gas in the electrochemical cell 1 is discharged. The gas circulation unit 101 is configured as a pipe having a gas flow passage therein, and here, the single-tube gas inlet unit 102 and the gas outlet unit 103 are connected at an angle of 90°, and the gas flow passage is bent at an angle of 90° at the intermediate connection.

[0049] The electrochemical cell 1 is arranged such that the cathode 2 forms part of the inner wall of the gas flow section 101 at the curved connection of the gas flow section 101, and the source gas G supplied from the gas inlet 102 to the surface 20 of the cathode 2 is smoothly introduced and the generated gas is smoothly discharged. For example, the angle θ1 between the source gas G flow direction indicated by the white arrow in the figure and the cathode 2 (gas inlet angle) is 45°, and the angle θ2 between the generated gas flow direction and the cathode 2 (gas outlet angle) is 135°. In this case, the source gas G flow direction and the generated gas flow direction are perpendicular to each other. Note that the angle with the cathode 2 is the angle with respect to the cathode surface, for example, the surface 20, which is an imaginary plane, based on the surface 12 of the oxide ion conductor 11 facing the source gas G.

[0050] 5, in the electrochemical cell 1, a raw material gas G is supplied from a gas inlet 102 toward the surface 20 of the cathode 2, and while the raw material gas G diffuses from the conductive metal layer 2A through the intermediate layer 2C to the complex oxide layer 2B, CH is generated by the co-electrolysis of CO and HO in the raw material gas G and the reaction of the electrolysis products, as described above. The generated gas containing CH diffuses laterally through the complex oxide layer 2B toward the gas outlet 103 and is extracted to the outside.

[0051] The hydrocarbon production unit 10 equipped with a gas circulation unit 101 constitutes a main part of a hydrocarbon production apparatus 100, an example of which is shown in Fig. 6. The configuration of the gas circulation unit 101 of the hydrocarbon production unit 10 and the arrangement of the electrochemical cell 1 are not limited to those shown in Figs. 4 and 5 above, and can be modified as appropriate. For example, as shown in Figs. 6 and 7, efficient hydrocarbon production is possible by arranging the gas circulation unit 101 so that the gas introduction direction and gas discharge direction are perpendicular to the electrochemical cell 1 without providing any bending connection parts.

[0052] In FIG. 6, the hydrocarbon production unit 10 includes a double-cylinder gas circulation unit 101. The inner cylinder of the gas circulation unit 101 constitutes a gas inlet 102, and the outer cylinder surrounding the outer periphery of the inner cylinder constitutes a gas outlet 103. The electrochemical cell 1 is arranged to close one end of the gas circulation unit 101. One end of the gas circulation unit 101 extends outward from a housing 10a of the hydrocarbon production unit 10, and the gas inlet 102 and the gas outlet 103 are separated at a branching section 10b. Heaters H are arranged on the outer peripheries of the housing 10a of the hydrocarbon production unit 10 and the separated gas inlet 102 and gas outlet 103, respectively, so that they can be heated or kept warm to a predetermined temperature.

[0053] The gas introduction section 102 includes a CO2 introduction path 102a through which CO2 is introduced and an N2 introduction path 102b through which nitrogen (N2) is introduced as a base gas. A water vapor concentration control tank 102c for storing water (H2O) is disposed in the N2 introduction path 102b upstream of the connection with the CO2 introduction path 102a, and the water vapor concentration is adjusted by passing through the water vapor concentration control tank 102c. A gas sampling pipe 104 is connected to the gas outlet section 103, allowing the generated gas to be analyzed by a gas chromatograph analyzer 105.

[0054] In FIG. 7, gas flow section 101 has gas inlet section 102 facing the center of surface 20 of cathode 2, and gas outlet section 103 facing the outer periphery of surface 20. As indicated by the outline arrows in the figure, source gas G flows in a direction perpendicular to surface 20 of cathode 2, and is introduced toward surface 20 of cathode 2. The flow direction of the generated gas is opposite to the gas inlet direction, and the generated gas is discharged parallel to and opposite the flow of source gas G, away from surface 20 of cathode 2. In this case, the angle θ between the gas flow direction and cathode 2 is 90°, and is the same on the gas inlet side and outlet side (angle θ = gas inlet angle θ1 = gas outlet angle θ2).

[0055] As shown in FIG. 8, in the electrochemical cell 1 shown in FIG. 5, the direction in which the source gas G is introduced is inclined relative to the surface 20. Therefore, after a large amount of electrolysis reaction occurs in region R where a large amount of source gas G is introduced (for example, the region surrounded by a dotted line in FIG. 8), the gas containing the methane precursor tends to flow along with the gas flow. Here, in region R, the temperature drops because a larger amount of endothermic electrolysis reaction occurs, making it a low-temperature region favorable for methane production. However, as the gas containing the methane precursor flows downstream, it may move away from region R, hindering methane production. The produced gas containing CO, H, and CH is directly discharged to the outside of the cathode 2.

[0056] In contrast, as shown in FIG. 9, in the electrochemical cell 1 shown in FIG. 7, the raw material gas G is introduced from a vertical direction. Therefore, after a large amount of electrolytic reaction occurs in the region R where the raw material gas G is introduced (for example, the region surrounded by the dotted line in FIG. 9), the gas containing the methane precursor tends to accumulate in the complex oxide layer 2B and remain near the low-temperature region R. This creates a favorable condition for the electrolytic reduction of the methane precursor, facilitating methane production. This increases the CH4 ratio in the product gas containing CO, H2, and CH4, thereby realizing an efficient hydrocarbon production apparatus 100. Furthermore, the gas flow section 101 has a double-cylinder structure, allowing for a compact structure.

[0057] The electrochemical cell 1 is heated by a heater H arranged around the hydrocarbon production section 10 to a temperature suitable for this series of reactions, for example, in the range of 300°C to 800°C. A lower temperature (for example, 300°C to 600°C) is advantageous for the methanation reaction in terms of equilibrium, while a higher temperature (for example, 500°C to 800°C) is advantageous for catalytic activity on the electrode surface. Preferably, the electrode temperature in the cell is set to a temperature range (for example, 500°C to 600°C) that is advantageous for both of these, and the temperature around the cell is controlled by the heater, taking into account the temperature drop due to electrolysis, which is an endothermic reaction. [Example]

[0058] Examples 1 to 4 An electrochemical cell 1 having the same configuration as in the above-described first embodiment was fabricated, and a methane production test was performed using the hydrocarbon production apparatus 100 shown in Fig. 6 to evaluate the methane conversion rate. The electrochemical cell 1 was fabricated as follows: an electrolyte sheet to serve as the oxide ion conductor 11 was prepared, and a cathode-forming material to serve as the cathode 2 and an anode-forming material to serve as the anode 3 were applied to both sides of the electrolyte sheet by printing, followed by heat treatment.

[0059] <Preparation of electrolyte sheet> Zirconia (ZrO2) containing yttria (YO) at a content of 8 mol% was prepared as the material for the electrolyte sheet. Polyvinyl butyral was added to the obtained yttria-stabilized zirconia powder, and the mixture was mixed in a ball mill with isoamyl acetate, 2-butanol, and ethanol as a mixed solvent to prepare a slurry. As shown in Table 1, yttria-stabilized zirconia containing 8 mol% YO is hereinafter abbreviated as "8YSZ."

[0060] The average particle size of the 8YSZ powder was 0.8 μm. Here, the average particle size is the particle size (diameter) d50 at which the cumulative frequency distribution on a volume basis measured by a laser diffraction / scattering method shows 50%, and the same applies hereinafter.

[0061] The prepared slurry was applied to a resin sheet in the form of a layer using a known doctor blade method, dried, and then peeled off and sintered in air at 1450°C for 2 hours to obtain an electrolyte sheet made of 8YSZ.

[0062] <Preparation of cathode material> The composite oxide containing a transition metal element, which is the material of the composite oxide layer 2B of the cathode 2, is a lanthanum-based composite oxide, such as LaSrCo. 0.8 Fe 0.2 O4 was used (shown as LaSr-based in Table 1). Single oxides of the constituent elements of the lanthanum-based composite oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.; test reagents) were used as raw materials and weighed out to obtain the desired composition. The weighed raw materials were then mixed in a ball mill in ethanol containing 1% by mass of polyvinyl alcohol as a dispersant. Mixing in the ball mill was carried out for 16 hours using zirconia beads with a diameter of 10 mm, after which the mixed raw materials were removed from the ball mill and dried. This dried powder was heat-treated at 1100°C to obtain a powder of a composite oxide containing a transition metal element (average particle size 0.5 μm).

[0063] The obtained composite oxide powder was mixed in terpineol containing 1% by mass of acrylic resin as a dispersant using a ball mill. The ball milling was carried out for 16 hours using zirconia beads with a diameter of 10 mm, and the mixture was then kneaded to adjust the viscosity, thereby preparing a printing paste that would serve as a material for forming the composite oxide layer 2B.

[0064] Silver (Ag) was used as the material for the conductive metal layer 2A of the cathode 2, and printing pastes with different viscosities were prepared. First, a commercially available paste-like silver frit (manufactured by Daiken Chemical Industry Co., Ltd.; "NAG-10") was prepared and added to terpineol to dilute it, or an acrylic resin was added and dissolved to obtain a printing silver paste with the desired viscosity. The silver frit used had an average particle size of 0.02 μm.

[0065] <Preparation of anode material> The composite oxide used as the material for the anode 3 is a perovskite composite oxide, Ba 0.6 Sr 0.4 CoO3 was used. Using the same method as for the composite oxide layer 2B of the cathode 2, the single oxides of the constituent elements used as raw materials were weighed out to achieve the desired composition, dispersed in a vehicle of acrylic resin and terpineol, mixed using a ball mill, and then dried to obtain a dry powder. This dry powder was heat-treated to obtain a composite oxide powder (average particle size 0.5 μm). The obtained composite oxide powder was mixed using a ball mill using the same method, and the viscosity was adjusted by kneading to prepare a printing paste for forming the anode.

[0066] <Preparation of electrochemical cell 1> A printing paste prepared for forming a complex oxide layer 2B was applied by printing to one side of the electrolyte sheet obtained as described above, and then dried. The sheet was then baked by heat treatment at 1000°C for 10 minutes, forming a complex oxide layer 2B. Furthermore, a printing silver paste prepared for forming a conductive metal layer 2A was printed on the surface of the complex oxide layer 2B, followed by heat treatment to form an intermediate layer 2C, and a conductive metal layer 2A was formed on the surface of the intermediate layer 2C, forming a cathode 2.

[0067] At this time, by using silver pastes of different viscosities, the penetration depth into the complex oxide layer 2B was adjusted, and the layer thickness t1 of the intermediate layer 2C was adjusted for each sample. The layer thickness of the conductive metal layer 2A was 10 μm for all samples. When a silver paste with low viscosity was used, a silver paste without viscosity adjustment was printed after printing to adjust the layer thickness to 10 μm. The layer thickness of the complex oxide layer 2B was 25 μm.

[0068] A printing paste prepared for forming the anode 3 was applied by printing to the other side of the electrolyte sheet obtained as described above, and then dried. The sheet was then baked by heat treatment at 1000°C for 10 minutes, forming a composite oxide layer that would become the anode 3. The thickness of the anode 3 was 25 μm.

[0069] Furthermore, platinum (Pt) lead wires for power supply were bonded to each of the cathode 2 and the anode 3 by melting silver frit. The silver frit on the anode 3 was formed by printing a silver frit paste without viscosity adjustment and then heat-treating it at 800°C for 10 minutes.

[0070] In this way, samples of electrochemical cells 1 were obtained in which porous electrodes, the cathode 2 and the anode 3, were laminated on both sides of a dense oxide ion conductor 11 (Examples 1 to 4). As shown in Table 1, the sample of Example 1 was adjusted so that the ratio t1 / t of the layer thickness t1 of the intermediate layer 2C to the total layer thickness t of the composite oxide layer 2B was 1%. The samples of Examples 2 to 4 were electrochemical cells 1 having intermediate layers 2C with ratios t1 / t adjusted to 5%, 50%, and 95%, respectively. The porosity of the oxide ion conductor 11 was 5% or less, and the porosity of the porous electrodes was 50%.

[0071] [Table 1]

[0072] <Evaluation method> A methane production test was carried out using a hydrocarbon production apparatus 100 for the electrochemical cells 1 of Examples 1 to 4. The hydrocarbon production section 10 had a configuration including the gas circulation section 101 shown in FIG. 4 above. A raw material gas G was supplied to the cathode 2 of the electrochemical cell 1 from a gas inlet section 102, and the produced gas was taken out from a gas outlet section 103. The gas flow direction of the gas circulation section 101 was connected so that the gas inlet angle θ1 was 45°, and the gas outlet section 103 was connected so that the gas outlet angle θ2 was 135°. Table 1 shows these gas directions as gas inlet angle θ1 (45°) / gas outlet angle θ2 (135°).

[0073] Feedstock gas G is a gas containing CO2 and water vapor, with the remainder being N2. N2 containing water vapor is generated by bubbling N2 through a water vapor concentration control tank 102c from an N2 inlet conduit 102b. This N2 is mixed with CO2 introduced through a CO2 inlet conduit 102a and supplied to the gas inlet 102. The concentrations of CO2 and water vapor in feedstock gas G are adjusted to a molar ratio suitable for producing CH4. The feedstock gas G is supplied from the gas inlet 102, which is kept at a suitable temperature, to the electrochemical cell 1 located in the hydrocarbon production unit 10, where co-electrolysis is carried out. The methane production conditions are shown below: the gas flow rate of feedstock gas G, the water vapor concentration and CO2 concentration in feedstock gas G, the electrode areas of the cathode 2 and anode 3 of the electrochemical cell 1, and the voltage applied between the two electrodes. Methane production conditions Gas flow rate: 50 ml / min Water vapor concentration: 0.6% (bubbling temperature: 0°C) CO2 concentration: 0.3% Electrode area: 2cm 2 Applied voltage: -1.6V

[0074] The gas flow rate was adjusted so that the ratio of the amount of raw material gas per unit time to the electrode volume (space velocity, SV) was in the range of approximately 1,000 to 10,000 (for example, SV = 3,600). The electrochemical cell 1 was heated as a whole by a heater H arranged around the hydrocarbon production section 10, and the internal temperature of the hydrocarbon production section 10 was controlled to be around 700°C. At this time, the electrolytic reaction (endothermic reaction) of CO2 and HO made the cell temperature lower than the ambient temperature, and it was maintained at approximately 500°C to 600°C, which is advantageous for both the catalytic activity on the surface of the cathode 2 and the methanation reaction.

[0075] The generated gas extracted to the gas outlet 103 was sampled over 10 seconds at 1 ml volume from a gas sampling tube 104 connected to the gas outlet 103, and its components were analyzed using a gas chromatograph (GC490, manufactured by Agilent Technologies, Inc.). The component analysis measured the concentration (mol% / L) of each gas in the generated gas. Based on the measurement results, the methane conversion rate was calculated, and the results are shown in Table 1. Figure 10 shows a bar graph comparing the methane conversion rates. Methane conversion rate = (methane concentration in produced gas) / [(CO2 concentration in feed gas) - (CO2 concentration in produced gas)]

[0076] (Comparative Examples 1 and 2) For comparison, an electrochemical cell 1 was prepared using the same manufacturing process as in Example 1, except that the intermediate layer 2C was not formed on the cathode 2, i.e., the ratio t1 / t was adjusted to 0%, and this was designated Comparative Example 1. In addition, an electrochemical cell 1 was prepared using the same manufacturing process as in Example 1, except that the ratio t1 / t of the intermediate layer 2C was adjusted to 100%, i.e., the composite oxide layer 2B was not formed on the cathode 2, and this was designated Comparative Example 2.

[0077] For the electrochemical cells 1 of Comparative Examples 1 and 2, a methane production test was carried out using the hydrocarbon production apparatus 100 in the same manner as in Example 1, and the produced gas was analyzed by gas chromatography. The results are shown in Table 1 and FIG. 10. As shown in these results, when either the electrochemical cell 1 of Comparative Examples 1 or 2 was used, the production of CO and H by electrolysis was observed, but the production of CH was not confirmed.

[0078] In contrast, CH4 generation was confirmed in all of the electrochemical cells 1 of Examples 1 to 4. Specifically, in Example 1, where the ratio t1 / t was 1%, a methane conversion rate of 20% was obtained, and in Example 2, where the ratio t1 / t was 5%, the methane conversion rate increased significantly to 80%. In Example 3, where the ratio t1 / t was 50%, the methane conversion rate increased further to 85%. In Example 4, where the ratio t1 / t was 95%, the methane conversion rate was 35%, which was lower than in Examples 2 and 3, but higher than in Example 1.

[0079] These results confirmed that the cathode 2 can efficiently generate CH4 in a single electrochemical cell 1 by forming an intermediate layer 2C containing first particles 21 and second particles 22 between the conductive metal layer 2A and the complex oxide layer 2B. The reason for this is not entirely clear, but it is thought that this is due to the fact that water vapor in the source gas G is electrolyzed in the conductive metal layer 2A, and the resulting H2 makes the intermediate layer 2C rich in H2. As a result, when CO2 in the source gas G is further electrolyzed in the intermediate layer 2C to generate CO, these electrolytic products rapidly react to generate a methane precursor, and the electrode reduction reaction of this precursor proceeds selectively in the complex oxide layer 2B, thereby enabling the generation of CH4.

[0080] Such an effect is obtained when the ratio t1 / t is 1% or more at the lower limit, and the methane conversion rate is 20% or higher. As the layer thickness t1 of the intermediate layer 2C increases, the methane conversion rate increases, reaching 80% or higher at 5% to 50%. On the other hand, at the upper limit of the ratio t1 / t, a methane conversion rate of 35% or higher is obtained at 95%, and it is believed that a methane conversion rate according to the ratio t1 / t can be obtained.

[0081] As shown in Figure 11, the methane production rate in the reaction synthesizing methane from H2 and CO2 correlates with pressure. Because this synthesis reaction is a reduced-pressure reaction, it is known that the methane production rate increases under high-pressure conditions; for example, it reaches a near-equilibrium value at a pressure of 3 MPa. Comparing the curves for methane production rates of 30% to 70% at atmospheric pressure, a methane production rate of 50% at atmospheric pressure reaches 95% at a pressure of 3 MPa, and a methane production rate of 70% at atmospheric pressure reaches 95% at a pressure of 1 MPa. The methane production rate of 95% is a value at which the product gas after the reaction can be introduced into a gas pipeline without further purification when adjusting and managing gas, based on the Wobbe index, which indicates the heat input of the gas, and the maximum combustion rate, which indicates the combustion rate.

[0082] Therefore, in practice, a methane conversion rate of 50% or more at normal pressure is sufficient, and a methane conversion rate of 70% or more is more preferable. Therefore, in the electrode configurations of Examples 1 to 4, the ratio t1 / t is about 5% or more, and more preferably about 50% or less.

[0083] (Comparative Example 3) For comparison, an electrochemical cell 1 was fabricated as Comparative Example 3 using the same manufacturing process as in Example 1, except that platinum (Pt) was used instead of Ag as the conductive metal constituting the conductive metal layer 2A and intermediate layer 2C of the cathode 2. In the electrochemical cell 1 of Comparative Example 3, the ratio t1 / t of the intermediate layer 2C was 1%, and the gas flow direction in the gas flow section 101 was gas inlet angle θ1 (45°) / gas outlet angle θ2 (135°).

[0084] For the electrochemical cell 1 of Comparative Example 3, a methane production test was conducted using the hydrocarbon production apparatus 100 in the same manner as in Example 1, and the produced gas was analyzed by gas chromatography. The results are shown in Table 1 and FIG. 10. As shown by these results, when the electrochemical cell 1 of Comparative Example 3 was used, no CH4 generation was confirmed, even though it had a configuration including both the intermediate layer 2C and the composite oxide s2B. The reason for this is not entirely clear, but it is presumed that when Pt is used as the conductive metal, even if a methane precursor is produced, electrolytic reduction does not proceed and methanation does not occur.

[0085] Example 5 In Example 5, the material of the oxide ion conductor 11 was changed from 8YSZ in Example 1 to Ca, which is a non-oriented phosphate apatite. 9.00 Na 0.67 (PO4)5(CO3)(H2O)(OH) 1.67An electrochemical cell 1 was fabricated in which the cathode 2 and anode 3 had the same configuration as in Example 1. For the non-oriented phosphate-based apatite, calcium phosphate [Ca3(PO4)2] and sodium bicarbonate (NaHCO3) were used as raw materials, weighed out to achieve the desired composition, and the raw materials were mixed, dried, and heat-treated using the same manufacturing process as for the composite oxide material in Example 1, to obtain a compound powder with a phosphate-based apatite structure.

[0086] A slurry containing the obtained compound powder was prepared and coated on a resin sheet in the same manner as in the case of 8YSZ in Example 1. Next, the resin sheet was peeled off, and the mixture was fired at 1350°C for 2 hours in an air atmosphere adjusted to contain 5% water vapor, to form an electrolyte sheet that would become the oxide ion conductor 11.

[0087] Materials for the cathode 2 and anode 3 were applied to the obtained electrolyte sheet in the same manner as in Example 1, and the sheet was fired to obtain an electrochemical cell 1. For the electrochemical cell 1 of Example 5, a methane production test was carried out using the hydrocarbon production apparatus 100 in the same manner as in Example 1, and the produced gas was analyzed. The results are shown in Table 1 and FIG. 10.

[0088] Example 6 In Example 6, the material of the oxide ion conductor 11 was changed from 8YSZ in Example 1 to Ca, which is a c-axis oriented phosphate apatite. 9.00 Na 0.67 (PO4)5(CO3)(H2O)(OH) 1.67 Except for the above, an electrochemical cell 1 was produced in the same manner as in Example 5. At that time, first, only calcium phosphate was weighed out as a raw material to a desired amount, and a powder and a slurry for forming a sheet were prepared in the same manner as in Example 5, and a sheet was produced by coating the slurry on a resin sheet.

[0089] The resin sheet was then peeled off from the resulting sheet, which was then embedded in sodium bicarbonate powder. The sheet was then heat-treated at 1300°C for 50 hours in an air atmosphere containing 5% water vapor. This vapor-solid diffusion process allowed calcium phosphate and sodium bicarbonate to react with each other, producing a c-axis-oriented phosphate-based apatite-type compound, forming an electrolyte sheet that would become the oxide ion conductor 11.

[0090] Materials for the cathode 2 and anode 3 were applied to the obtained electrolyte sheet in the same manner as in Example 1, and the sheet was fired to obtain an electrochemical cell 1. For the electrochemical cell 1 of Example 6, a methane production test was carried out using the hydrocarbon production apparatus 100 in the same manner as in Example 1, and the produced gas was analyzed. The results are shown in Table 1 and FIG. 10.

[0091] Example 7 In Example 7, the material of the oxide ion conductor 11 was changed from 8YSZ in Example 1 to La, which is a non-oriented apatite-type composite oxide. 9.3 Si 5.3 B 0.7 O 25 An electrochemical cell 1 was fabricated in the same manner as in Example 1, except that the following was used: First, the single oxides of the constituent elements used as raw materials were weighed to obtain the desired composition, ball milled, and then dried to obtain a dry powder, which was then heat-treated to obtain a composite oxide powder for forming a sheet. Using this composite oxide powder, a slurry was prepared in the same manner as in Example 1 for 8YSZ, and the slurry was applied to a resin sheet. Next, the resin sheet was peeled off, and the resulting slurry was fired in an air atmosphere at 1600°C for 2 hours to form an electrolyte sheet that would become an oxide ion conductor 11.

[0092] Materials for the cathode 2 and anode 3 were applied to the obtained electrolyte sheet in the same manner as in Example 1, and the sheet was fired to obtain an electrochemical cell 1. For the electrochemical cell 1 of Example 7, a methane production test was carried out using the hydrocarbon production apparatus 100 in the same manner as in Example 1, and the produced gas was analyzed. The results are shown in Table 1 and FIG. 10.

[0093] Example 8 In Example 8, the material of the oxide ion conductor 11 was changed from 8YSZ in Example 1 to La, which is a c-axis oriented apatite-type composite oxide. 9.3 Si 5.3 B 0.7 O 25 An electrochemical cell 1 was fabricated in the same manner as in Example 1, except that the following was used: As raw materials, single oxides of the constituent elements excluding B were weighed out to obtain the desired composition, ball milled, and then dried to obtain a dried powder, which was then heat-treated to obtain a precursor powder for forming a sheet. Using this precursor powder, a slurry was prepared in the same manner as in Example 1 for 8YSZ, and the slurry was applied to a resin sheet. Next, the resin sheet was peeled off, and the resulting mixture was fired in an air atmosphere at 1600°C for 2 hours.

[0094] The resin sheet was then peeled off from the resulting sheet, and the sheet was subjected to a heat treatment in a boron oxide (BO) vapor atmosphere at 1550°C for 50 hours. Through this gas-solid diffusion process, the precursor reacted with boron oxide to generate a c-axis oriented apatite-type composite oxide, forming an electrolyte sheet that would become the oxide ion conductor 11.

[0095] Materials for the cathode 2 and anode 3 were applied to the obtained electrolyte sheet in the same manner as in Example 1, and the sheet was fired to obtain an electrochemical cell 1. For the electrochemical cell 1 of Example 6, a methane production test was carried out using the hydrocarbon production apparatus 100 in the same manner as in Example 1, and the produced gas was analyzed. The results are shown in Table 1 and FIG. 10.

[0096] Example 9 In Example 9, the material of the composite oxide layer 2B was changed from the lanthanum composite oxide of Example 1 to a ceria composite oxide, Ce 0.75 Co 0.2 Fe 0.05 Electrochemical cell 1 was produced in the same manner as in Example 1, except that the starting material was Ce(Co,Fe)O (shown as Ce(Co,Fe)O in Table 1). Single oxides of the constituent elements were used as raw materials for the ceria-based composite oxide, and the raw materials were mixed, dried, and heat-treated in the same manufacturing process as for the composite oxide material of Example 1, to obtain a powder of the ceria-based composite oxide.

[0097] Using the obtained composite oxide powder, a printing paste was prepared as a material for forming the composite oxide layer 2B in the same manner as in Example 1. Materials for the cathode 2 and anode 3, including the prepared printing paste, were applied to an electrolyte sheet using 8YSZ similar to that in Example 1, and the sheet was fired to obtain an electrochemical cell 1. For this electrochemical cell 1 of Example 9, a methane production test was carried out using the hydrocarbon production apparatus 100 in the same manner as in Example 1, and the produced gas was analyzed. The results are shown in Table 1 and FIG. 10.

[0098] As shown in the results of Examples 5 to 8 in Table 1 and FIG. 9, by using an apatite-type compound as the material for the oxide ion conductor 11, the methane conversion rate can be improved to 50% or more. In this case, the methane conversion rate of the apatite-type compound having c-axis orientation was higher than that of the non-oriented apatite-type compound, and a methane conversion rate of 70% or more was obtained. Furthermore, a higher methane conversion rate was obtained in Examples 7 and 8, which used an apatite-type composite oxide, than in Examples 5 and 6, which used a phosphate-type apatite-type compound. Furthermore, as shown in the results of Example 9, by using a ceria-based oxide as the material for the second particles 22 constituting the intermediate layer 2C and the composite oxide layer 2B, the methane conversion rate was improved to 90%.

[0099] Example 10 In Example 10, an electrochemical cell 1 fabricated in the same manner as in Example 1 was attached to the hydrocarbon production unit 10 shown in FIG. 7 above, and a methane production test was conducted. The hydrocarbon production unit 10 had a gas flow unit 101 with a double-cylindrical configuration, and was connected so that the gas flow direction of the gas flow unit 101 was perpendicular to the cathode 2. In Table 1, the gas direction (gas introduction angle θ1 / gas discharge angle θ2) is shown as 90° / 90°. The results of a methane production test conducted in the same manner as in Example 1 are shown in Table 1 and FIG. 10.

[0100] As shown in the results of Table 1 and Fig. 10, the methane conversion rate was improved to 52% by changing the gas flow direction in electrochemical cell 1 having the same structure as in Example 1. The reason for this is not entirely clear, but it is presumed that by introducing gas from the vertical direction and discharging the gas in the direction opposite to the gas introduction direction, as shown in Fig. 9 above, the methane precursor is retained in an area that is easily heated, and electrolytic reduction proceeds efficiently.

[0101] Thus, when the ratio t1 / t in the electrochemical cell 1 is 1% or more, by appropriately selecting the materials of the cathode 2 and the oxide ion conductor 11 or by appropriately setting the gas flow direction in the gas flow section 101 of the hydrocarbon production section 10, it is possible to realize a hydrocarbon production apparatus 100 that exhibits a methane conversion rate that is sufficient for practical use.

[0102] In the electrochemical cell 1 configured as described above, the cathode 2 has a three-layer structure having an intermediate layer 2C between the conductive metal layer 2A and the complex oxide layer 2B, which enables hydrocarbon production in a single electrochemical cell 1. The conductive metal layer 2A contains Ag as a main component, the complex oxide layer 2B is mainly composed of a complex oxide containing a transition metal element, and the intermediate layer 2C contains both of these components, thereby achieving high catalytic activity in the cathode 2. Furthermore, the use of such an electrochemical cell 1 makes it possible to realize a highly practical hydrocarbon production device.

[0103] The present invention is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the spirit of the present invention. For example, the electrochemical cell 1 When configuring the hydrocarbon production unit 10 used in the present invention, a plurality of electrochemical cells 1 may be stacked. Furthermore, the configuration of the gas circulation unit 101 of the hydrocarbon production unit 10 and the configuration of the hydrocarbon production apparatus 100 including the hydrocarbon production unit 10 are not limited to the configurations described above and can be changed depending on the application, etc. [Explanation of symbols]

[0104] 1. Electrochemical cell 11 Oxide-ion conductors 12, 13 sides 2 cathodes 3 anode 21 1st particle 22 2nd particle 2A conductive metal layer 2B Composite oxide layer 2C middle layer

Claims

1. An electrochemical cell (1) comprising an oxide ion conductor (11) and a cathode (2) and an anode (3) provided on two opposing surfaces (12, 13) of the oxide ion conductor, The cathode is a conductive metal layer (2A) made of first particles (21) mainly composed of silver and in contact with a raw material gas (G) containing carbon dioxide and water vapor; a complex oxide layer (2B) formed of second particles (22) mainly composed of a complex oxide containing a transition metal element and in contact with the oxide ion conductor; an intermediate layer (2C) provided between the conductive metal layer and the complex oxide layer and constituted by the first particles and the second particles, an electrochemical cell, wherein the oxide ion conductor is composed of at least one kind selected from non-oriented or oriented apatite-type compounds, and the apatite-type compound is a phosphate-based apatite-type compound or an apatite-type composite oxide.

2. An electrochemical cell (1) comprising an oxide ion conductor (11) and a cathode (2) and an anode (3) provided on two opposing surfaces (12, 13) of the oxide ion conductor, The cathode is a conductive metal layer (2A) made of first particles (21) mainly composed of silver and in contact with a raw material gas (G) containing carbon dioxide and water vapor; a complex oxide layer (2B) formed of second particles (22) mainly composed of a complex oxide containing a transition metal element and in contact with the oxide ion conductor; an intermediate layer (2C) formed of the first particles and the second particles and provided between the conductive metal layer and the complex oxide layer, wherein the complex oxide containing a transition metal element is a lanthanum-based complex oxide or a ceria-based complex oxide.

3. An electrochemical cell as described in claim 2, wherein the complex oxide containing the transition metal element is a ceria-based complex oxide containing multiple transition metal elements.

4. 4. The electrochemical cell according to claim 2, wherein the oxide ion conductor is composed of at least one material selected from the group consisting of stabilized zirconia and non-oriented or oriented apatite-type compounds.

5. 5. The electrochemical cell according to claim 4, wherein the apatite-type compound is a phosphate-based apatite-type compound or an apatite-type composite oxide.

6. The apatite-type composite oxide has the following formula: A 9.3+x [T 6.0-y M y ]O 26.0+z (A in the formula is one or more elements selected from the group consisting of La, Ce, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba; T in the formula is an element containing Si or Ge, or both; and M in the formula is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo), In the formula, x is a number between −1.4 and 1.5, In the formula, y is a number of 0.0 or more and 3.0 or less, In the formula, z is a number equal to or greater than −5.0 and equal to or less than 5.2, 6. The electrochemical cell according to claim 1, wherein the ratio of the number of moles of A to the number of moles of T (A / T) in the formula is 1.4 or more and 3.7 or less.

7. 2. The electrochemical cell according to claim 1, wherein the composite oxide containing a transition metal element is a lanthanum-based composite oxide or a ceria-based composite oxide.

8. 8. The electrochemical cell according to claim 7, wherein the composite oxide containing a transition metal element is a ceria-based composite oxide containing a plurality of transition metal elements.

9. A hydrocarbon production apparatus (100) using the electrochemical cell according to any one of claims 1 to 8, a hydrocarbon production section (10) in which the electrochemical cell is arranged so as to close one end side of a cylindrical gas flow section (101); the gas flow section has a gas inlet section (102) and a gas outlet section (103) provided opposite the cathode of the electrochemical cell, the gas inlet section supplies the source gas to the cathode in a gas introduction direction perpendicular to the cathode; The gas outlet portion is disposed on the outer periphery of the gas inlet portion, and discharges the reaction product at the cathode to the outside of the gas flow portion in a gas outlet direction opposite to the gas inlet direction.

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