JP7770852B2 - Electrochemical cells and hydrocarbon production equipment - Google Patents

Description

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

In recent years, from the perspective of reducing carbon dioxide emissions and recycling carbon, there has been a growing desire to establish 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 from solid oxides, are known as electrochemical cells used in such processes, and research is currently underway into composite oxides that serve as electrode materials, additive components, electrode structures, and other aspects to promote the desired reactions and increase reaction efficiency.

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

Patent Document 1 proposes using a perovskite-type composite oxide as the electrode material and adjusting the type and ratio of elements that substitute for each region to increase the reaction activity of the electrode as a whole. It also proposes a method of improving the catalytic activity of the electrode by adding a metal component, such as a platinum group metal, to the electrode material.

Japanese Patent Application Laid-Open No. 2018-26339

As such, 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 drive these reactions, and a single electrochemical cell suitable for generating hydrocarbons is yet to be discovered.

The present invention was made in consideration of these issues, and aims to provide an electrochemical cell that can convert carbon dioxide and water vapor into hydrocarbons through an electrochemical process, and a hydrocarbon production device that uses the electrochemical cell.

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 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 ,
The electrochemical cell is configured such that 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 .
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 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;
The electrochemical cell has a laminated structure in which the conductive metal layer and the composite oxide layer are provided with an intermediate layer (2C) composed of the first particles and the second particles, and the composite oxide containing a transition metal element is a lanthanum-based composite oxide or a ceria-based composite oxide.

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 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 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.

When a feed 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, converting them to hydrocarbons. Because hydrocarbon generation has not been observed in the intermediate layer or composite oxide layer alone, it is believed that including both layers allows precursor generation and its electrolytic reduction to proceed in a stepwise manner, enabling hydrocarbon generation.

A hydrocarbon production device using such an electrochemical cell can be configured as a compact, efficient hydrocarbon production unit by providing a gas distribution section with a gas outlet section around the periphery of the gas inlet section and placing the electrochemical cell on one end of the gas distribution section. In this gas distribution section, the gas inlet section 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 section. This allows the reaction to proceed stepwise, and 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, increasing the methane conversion rate.

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.

1 is an overall cross-sectional view schematically illustrating a configuration of an electrochemical cell according to a first embodiment. 3 is a cross-sectional view schematically showing the structure of a cathode, which is a main part of the electrochemical cell according to the first embodiment, and is an enlarged cross-sectional view of part A in FIG. 2. FIG. FIG. 3 is an enlarged cross-sectional view of a main part for explaining a reaction at a cathode of the electrochemical cell according to the first embodiment. FIG. 2 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. FIG. 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 . 1 is an overall configuration diagram of a hydrocarbon production apparatus using an electrochemical cell according to a first embodiment. FIG. 7 is an enlarged cross-sectional view of a main part showing a configuration example of a gas inlet part of a hydrocarbon production apparatus according to the first embodiment, and is an enlarged cross-sectional view of part C in FIG. 6 . 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. 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. 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. FIG. 4 is a graph showing the relationship between methane production rate and pressure in a methane synthesis reaction in a reference example.

(Embodiment 1)
1 to 9, embodiments of an electrochemical cell and a hydrocarbon production apparatus will be described. 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 feed gas G containing carbon dioxide ( _CO2 ) and water vapor ( _H2O ) is supplied toward the surface 20 of the cathode 2.

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 primarily composed of silver (Ag), a complex oxide layer 2B composed of second particles 22 primarily composed of a complex oxide containing a transition metal element, and an intermediate layer 2C composed of the first particles 21 and the second particles 22 are layered in this order.

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 facing the source gas G, 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 layers. The voids formed between the particles in the conductive metal layer 2A, the complex oxide layer 2B, and the intermediate layer 2C communicate with each other, forming diffusion paths 23 through which the source gas G and the generated gas containing reaction products can diffuse.

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 complex oxide layer 2B. This is thought to enable stepwise co-electrolysis of _CO2 and _H2O contained in the source gas G and reaction of the electrolysis products in each layer, making it possible to produce hydrocarbons. When only the conductive metal layer 2A and the complex 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 complex 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.

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 water vapor contained in the source gas G. At this time, as shown by [I] in Fig. 3, it is believed that a reaction proceeds in which hydrogen ( _H2 ) gas is produced by electrolysis of water vapor. The hydrogen produced in the conductive metal layer 2A diffuses into the intermediate layer 2C together with the source gas G containing unreacted water vapor.

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

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 composed only of an aggregate of second particles 22. Like the conductive metal layer 2A, the intermediate layer 2C contains first particles 21 whose main component is silver, and like the complex oxide layer 2B, it contains second particles 22 whose main component is a complex oxide containing a transition metal element.

3, in the intermediate layer 2C, an electrolytic 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 .

At this time, _H2 previously generated in the conductive metal layer diffuses into the intermediate layer 2C, making the intermediate layer 2C in a hydrogen-rich state, 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 a methane precursor can be generated from a CO molecule and multiple _H2 molecules as follows.
CO+2H ₂ → CO(H ₂ ) ₂
CO( _H2 ) ₂ : methane precursor

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(H ₂ ) ₂ +2e ^- → CH ₄ +O ^2-
Oxide ions (O ²⁻ ) generated by electrolytic reduction of the methane precursor permeate through the oxide ion conductor 11 in contact with the complex oxide layer 2B and are released to the anode 3 side.

The second particles 22 are particles of a complex oxide containing at least one element selected from transition metals. They are electrically conductive and 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 with mixed conductivity of electrons and oxide ions is preferred. Examples of complex oxides with such functionality include perovskite-type complex oxides containing transition metal elements and ceria-based complex oxides containing transition metal elements.

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 having 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

Preferably, a lanthanum-based composite oxide containing lanthanum (La) can be used as a composite oxide of perovskite type or a similar structure. From the viewpoint of electronic conductivity and ionic conductivity, examples of such lanthanum-based composite oxides include (La,Sr) _CoO3 , (La,Sr)(Co,Fe) _O3 , and LaSr(Co,Fe _{) O4 .} Specific examples of such composite oxides include _{LaSrCo0.8Fe0.2O4} , but are not limited thereto.

Specifically, the ceria-based composite oxide containing a transition metal element can be a composite oxide having a composition formula represented by Ce1 _-x (M1) _xO2 (where 0<x<1). The M1 element in the formula is _a transition metal element, and may be, for example, one or more of the following elements: Preferably, X is a value selected from the range of 0.1 to 0.4.
M1: V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd
Examples of such ceria-based composite oxides include Ce1 _-x (Co,Fe) _{xO3 and} the like, from the viewpoint of electronic conductivity and _{ionic conductivity} , and _specifically include _{Ce0.75Co0.2Fe0.05O2} , but are not limited thereto.

Ceria-based composite oxides containing transition metal elements have a metallic electronic structure and electrical conductivity due to the substitution of some of the Ce atoms in cerium oxide (CeO ₂ ), which has good ionic conductivity, with a transition metal. In this case, for example, selecting Co as the M1 element allows for the substitution of more Ce atoms, which is advantageous for increasing electrical conductivity. Furthermore, to impart catalytic functionality along with electrical conductivity, it is necessary to adsorb methane precursors, supply electrons, and reduce them. Therefore, it is preferable for the oxide to have the ability to adsorb precursor molecules and an electronic structure that is advantageous for electron supply. When the above-mentioned M1 element is selected, it is believed that such a reaction site is advantageously formed within the electrode, and it is therefore desirable for the oxide to contain multiple transition metal elements.

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 preferably 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 with 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 multiple first particles 21 on the surface of the second particles 22 are electrically connected to each other to form a band-like string, one end of which is electrically connected to the first particles 21 of the conductive metal layer 2A. Preferably, the second particles 22 with the first particles 21 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 configuration.

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 three-dimensional mesh-like conduction paths, 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 expands, making it easier for the electrode reaction to proceed, and as the first particles 21 become smaller, gaps between the second particles 22 are secured, improving the gas diffusion properties of the diffusion path 23.

In this way, by appropriately disposing the intermediate layer 2C between the conductive metal layer 2A and the complex 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 of the thickness t1 of the intermediate layer 2C and the thickness t2 of the complex oxide layer 2B (i.e., t = t1 + t2) 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 methane conversion rate sufficient for practical use 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)]

On the other hand, even if the ratio t1/t exceeds 5% and increases to around 50%, the effect of improving the methane conversion rate is limited, and if it is increased to around 95%, the methane conversion rate tends to decrease. Furthermore, as the ratio t1/t increases, the amount of 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, and more preferably within a range of 50% or less.

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

The thickness t1 of the intermediate layer 2C can be controlled to the desired thickness by adjusting the viscosity of the printing paste used to form the conductive metal layer 2A and 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 the cross section of a sample using an electron microscope. In this case, the region where the second particles 22 with 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 complex oxide layer 2B, and the arithmetic average of the thicknesses measured at multiple locations on each can be used as the thickness of each layer.

The oxide ion conductor 11 is a dense solid electrolyte layer disposed between the cathode 2 and the anode 3, and allows oxide ions (O ²⁻ ) generated on the cathode 2 side to pass through to the anode 3 side. The O ²⁻ that has passed through to the anode 3 releases electrons (e ⁻ ), generating oxygen (O ₂ ) 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.

Stabilized zirconia is a zirconia-based oxide obtained by dissolving 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 ( _Y2O3 ) and scandia _{( Sc2O3} ). Preferably, yttria-stabilized zirconia (YSZ) is used primarily from the viewpoints of ionic conductivity, mechanical and chemical stability, etc.

Apatite-type compounds are compounds with a hexagonal apatite-type crystal structure, and can be non-oriented (i.e., no orientation axis) or oriented (including uniaxial and biaxial orientations). In the case of oriented apatite-type compounds, they preferably have 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 larger amounts of generated gas. Furthermore, oriented compounds have higher oxide ion conductivity than non-oriented compounds, contributing to improved methane conversion.

Examples of apatite-type compounds include compounds based on phosphate apatite, whose basic composition is represented by the formula (M2) ₁₀ (PO ₄ ) ₆ (X) _2. The M2 site may contain, for example, one or more ions selected from the monovalent or divalent metal ions shown below, and the X site may contain, for example, one or more ions selected from the anions shown below or H ₂ O.
M2: Li ⁺ , Na ⁺ , K ⁺ , Ca ²⁺ , Mg ²⁺ , Zn ²⁺ , Ba ²⁺ , Mn ²⁺ , Fe ²⁺ , Cu ²⁺
X: OH ⁻ , F ⁻ , Cl ⁻ , CO ₃ ²⁻ , H ₂ O
From the viewpoint of improving oxide ion conductivity, it is preferable to use a phosphate-based apatite in which the M2 site contains Na ⁺ and Ca ²⁺ and the X site contains OH ^- and CO ₃ ^2- . Examples of such apatite-type compounds include, but are not limited to, Ca _10-x Na _2x/3 (PO ₄ ) ₅ (CO ₃ ) _x (H ₂ O) _x (OH) _2-x/3 .

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 of -1.4 or more and 1.5 or less, y in the formula is a number of 0.0 or more and 3.0 or less, z in the formula is a number of -5.0 or more and 5.2 or less, and the ratio of the number of moles of A to the number of moles of T (A/T) is 1.4 or more and 3.7 or less, can be used as the oxide ion conductor 11.

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 the 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.

When an oriented apatite structure is to be formed, the M element in formula (1) is introduced by reaction with a metastable precursor (A _2+x TO _5+z ) in the gas phase, thereby converting the precursor into an apatite structure and orienting the crystals in one direction.
From this viewpoint, the M element may be any element that can attain the necessary vapor pressure by becoming gaseous at a temperature of 1000° C. or higher at which the precursor assumes the apatite structure. The term "necessary vapor pressure" means a vapor pressure that can move in a gaseous state in the atmosphere and progress the reaction by diffusing from the surface of the precursor to the interior at grain boundaries or within grains.
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 them, B, Ge, and Zn are particularly preferred in terms of a high degree of orientation and high productivity (orientation speed).

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 being able to increase the degree of orientation and oxygen ion conductivity.
In terms of filling 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.

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

The apatite-type composite oxide represented by the above formula (1) can be, for example, a rare earth silicate-based composite oxide 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 La9.3 _+x ( _{Si6.0-yBy )} O26.0 _+z , _La9.3+x ( _{Si6.0- yGey} ) _O26.0+z , _La9.3+x (Si6.0 _{- yZny} ) _O26.0+z , etc. However, they are not limited to these.

When producing a non-oriented apatite-type complex oxide, oxides containing each constituent element are used as raw materials in a conventional manner, wet-mixed, formed into a sheet, and heated and fired to obtain a sintered complex oxide. When producing an oriented apatite-type complex oxide, a sintered body of a precursor to the desired apatite-type complex oxide is similarly produced, but excluding a specific element. Then, a vapor-solid diffusion process is performed in which the precursor is heated in a vapor phase containing the specific element, causing the specific element to react with the precursor to form an oriented apatite structure with the desired composition.

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

The constituent material of the anode 3 is not necessarily limited, but may be, for example, a perovskite complex oxide containing a transition metal element. 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, and rare earth elements in the A site of a 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, but are not limited _{to , Ba0.6Sr0.4CoO3} .

As shown in FIG. 4, an electrochemical cell 1 configured in this manner can form a hydrocarbon production section 10 equipped with a gas distribution section 101. In the hydrocarbon production section 10, the gas distribution section 101 includes a gas inlet section 102 to which raw material gas G is supplied, and a gas outlet section 103 from which the produced gas in the electrochemical cell 1 is discharged. The gas distribution section 101 is configured as a pipe with an internal gas flow channel, and here, the single-tube gas inlet section 102 and gas outlet section 103 are connected at a 90° angle, with the gas flow channel bent 90° at the intermediate connection.

The electrochemical cell 1 is arranged so 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 so that the raw material gas G supplied from the gas inlet 102 to the surface 20 of the cathode 2 can be smoothly introduced and the generated gas can be smoothly discharged. For example, the angle θ1 (gas inlet angle) between the raw material gas G flow direction indicated by the white arrow in the figure and the cathode 2 can be inclined at 45°, and the angle θ2 (gas outlet angle) between the generated gas flow direction and the cathode 2 can be inclined at 135°. In this case, the raw material 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 raw material gas G.

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 co-electrolysis of _CO and _H O 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.

The hydrocarbon production unit 10, which includes a gas distribution unit 101, constitutes the main part of a hydrocarbon production apparatus 100, an example of which is shown in Figure 6. The configuration of the gas distribution unit 101 of the hydrocarbon production unit 10 and the arrangement of the electrochemical cell 1 are not limited to those shown in Figures 4 and 5 above, and can be modified as appropriate. For example, as shown in Figures 6 and 7, efficient hydrocarbon production is possible by arranging the gas distribution unit 101 so that it does not have any bent connectors and the gas introduction direction and gas discharge direction are perpendicular to the electrochemical cell 1.

In FIG. 6, the hydrocarbon production unit 10 includes a double-cylinder gas distribution unit 101. The inner cylinder of the gas distribution unit 101 constitutes the gas inlet 102, and the outer cylinder surrounding the outer periphery of the inner cylinder constitutes the gas outlet 103. The electrochemical cell 1 is arranged to close one end of the gas distribution unit 101. One end of the gas distribution unit 101 extends outward from the housing 10a of the hydrocarbon production unit 10, and the gas inlet 102 and gas outlet 103 are separated at the branching section 10b. Heaters H are arranged on the outer periphery of the housing 10a of the hydrocarbon production unit 10 and the separated gas inlet 102 and gas outlet 103, respectively, to heat or maintain a predetermined temperature.

The gas inlet section 102 includes a _CO2 inlet passage 102a through which _CO2 is introduced and an _N2 inlet passage 102b through which nitrogen ( _N2 ) serving as a base gas is introduced. A water vapor concentration control tank 102c in which water ( _H2O ) is stored is disposed in the _N2 inlet passage 102b upstream of the connection with the _CO2 inlet passage 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.

In Figure 7, the gas flow section 101 has the gas inlet section 102 facing the center of the surface 20 of the cathode 2, and the gas outlet section 103 facing the outer periphery of the surface 20. As indicated by the white arrows in the figure, the raw material gas G flows perpendicular to the surface 20 of the cathode 2 and is introduced toward the surface 20 of the 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 the raw material gas G, away from the surface 20 of the cathode 2. In this case, the angle θ between the gas flow direction and the 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).

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 the region R where a large amount of source gas G is introduced (for example, the region surrounded by the 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 electrolysis reaction, which is an endothermic reaction, occurs, making it a low-temperature region favorable for methane production. However, since the gas containing the methane precursor flows downstream, it may move away from region R, making it difficult for methane production to proceed. The produced gas containing CO, _H2 , and _CH4 is directly discharged to the outside of the cathode 2.

In contrast, as shown in Figure 9, in the electrochemical cell 1 shown in Figure 7, the feed gas G is introduced from a vertical direction. Therefore, after a large amount of electrolytic reaction occurs in region R where a large amount of feed gas G is introduced (for example, the region surrounded by a dotted line in Figure 9), the gas containing the methane precursor tends to remain in the complex oxide layer 2B and remain near region R, which is a low-temperature region. 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.

The electrochemical cell 1 is heated by a heater H located 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 (e.g., 300°C to 600°C) is advantageous for the methanation reaction, while a higher temperature (e.g., 500°C to 800°C) is advantageous for catalytic activity on the electrode surface. It is preferable to keep the electrode temperature within the cell within a temperature range favorable for both reactions (e.g., 500°C to 600°C), and to control the temperature around the cell with a heater, taking into account the temperature drop due to electrolysis, which is an endothermic reaction.

Examples 1 to 4
An electrochemical cell 1 having the same configuration as in the first embodiment described above 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 material to serve as the cathode 2 and an anode material to serve as the anode 3 were applied to both sides of the electrolyte sheet by printing, followed by heat treatment.

<Preparation of electrolyte sheet>
Zirconia ( _ZrO2 ) containing yttria ( _Y2O3 ) was prepared as the material for the electrolyte sheet, with the content being 8 mol%. 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% _Y2O3 will be abbreviated as "8YSZ" below.

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 volume-based cumulative frequency distribution measured by the laser diffraction/scattering method shows 50%, and this will be used hereinafter as well.

The prepared slurry was applied in layers onto a resin sheet using a known doctor blade method and then dried. The resin sheet was then peeled off and sintered in air at 1,450°C for 2 hours to obtain an electrolyte sheet made from 8YSZ.

<Preparation of cathode material>
The composite oxide containing a transition metal element _used for the composite oxide layer 2B of the cathode 2 was _a lanthanum-based composite oxide, _{LaSrCo0.8Fe0.2O4} (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 to achieve 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. The mixed raw materials were then removed to a tray and dried. The 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).

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

Silver (Ag) was used as the material for the conductive metal layer 2A of the cathode 2, and printing pastes of different viscosities were prepared. First, 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 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.

<Preparation of anode material>
The composite oxide used for the anode 3 was _{Ba0.6Sr0.4CoO3 ,} a _perovskite composite oxide. Using the same method as for the composite oxide layer 2B of the cathode 2, single oxides of the constituent elements used as raw materials were weighed to the desired composition, dispersed in a vehicle of acrylic resin and terpineol, mixed in a ball mill, and then dried to obtain a dry powder. This dried powder was heat-treated to obtain a composite oxide powder (average particle size 0.5 μm). The obtained composite oxide powder was ball-milled in the same manner and kneaded to adjust the viscosity, thereby preparing a printing paste for forming the anode.

<Preparation of Electrochemical Cell 1>
A printing paste prepared for forming the 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 the complex oxide layer 2B. Furthermore, a printing silver paste prepared for forming the 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 the conductive metal layer 2A was formed on the surface of the intermediate layer 2C, forming the cathode 2.

By using silver pastes with 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 set to 10 μm for all samples. When a low-viscosity silver paste 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 set to 25 μm.

The printing paste prepared for forming the anode 3 was then 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.

Furthermore, platinum (Pt) lead wires for power supply were attached to each of the cathode 2 and anode 3 by melting silver frit. The silver frit on the anode 3 was printed using silver frit paste without viscosity adjustment, and then heat-treated at 800°C for 10 minutes.

In this way, electrochemical cell 1 samples were obtained in which porous electrodes, a cathode 2 and an 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 had the ratio t1/t of the thickness t1 of the intermediate layer 2C to the total thickness t of the composite oxide layer 2B adjusted to 1%. Furthermore, the samples of Examples 2 to 4 had 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%.

<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 flow 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 flow 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°).

The feed gas G contained _CO2 and water vapor, with the remainder being _{N2. N2} containing water vapor was produced by bubbling _N2 from the _N2 inlet passage 102b through the water vapor concentration control tank 102c, and this was mixed with _CO2 introduced from the _CO2 inlet passage 102a and supplied to the gas inlet 102. The concentrations of _CO2 and water vapor in the feed gas G were adjusted to a molar ratio suitable for producing _CH4 . The feed gas G was supplied from the gas inlet 102, which was kept at a suitable temperature, to the electrochemical cell 1 arranged in the hydrocarbon production section 10 for co-electrolysis. The methane production conditions are shown below: the gas flow rate of the feed gas G, the water vapor concentration and _CO2 concentration in the feed gas G, the electrode areas of the cathode 2 and anode 3 of the electrochemical cell 1, and the voltage applied between the 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: −1.6 V

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 _H2O made the cell temperature lower than the ambient temperature, and the cell temperature 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.

The generated gas extracted to the gas outlet 103 was sampled over 10 seconds in an amount of 1 ml 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. A bar graph comparing the methane conversion rates is shown in FIG. 10.
Methane conversion rate = (methane concentration in produced gas) / [( _CO2 concentration in feed gas) - ( _CO2 concentration in produced gas)]

(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 complex oxide layer 2B was not formed on the cathode 2, and this was designated Comparative Example 2.

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 _H2 by electrolysis was observed, but the production of _CH4 was not confirmed.

In contrast, the generation of _CH4 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 was higher than in Example 1.

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 necessarily 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 electrolysis products rapidly react to generate a methane precursor, and the electrode reduction reaction of this methane precursor proceeds selectively in the complex oxide layer 2B, making it possible to generate _CH4 .

This effect is achieved when the ratio t1/t is at the lower limit of 1% or more, resulting in a methane conversion rate of 20% or more. As the layer thickness t1 of the intermediate layer 2C increases, the methane conversion rate increases, reaching 80% or more at a ratio of 5% to 50%. On the other hand, at the upper limit of the ratio t1/t, a methane conversion rate of 35% or more is achieved at a ratio of 95%, and it is believed that a methane conversion rate consistent with the ratio t1/t can be achieved.

As shown in FIG. 11 as a reference example, the methane production rate in the reaction synthesizing methane from _H and _CO correlates with pressure. Because this synthesis reaction is a reduced-pressure reaction, it is known that synthesis under high-pressure conditions increases the methane production rate; for example, it reaches a nearly equilibrium value at a pressure of 3 MPa. Comparing the curves for methane production rates of 30% to 70% at atmospheric pressure, if the methane production rate is 50% at atmospheric pressure, it reaches 95% at a pressure of 3 MPa, and if the methane production rate is 70% at atmospheric pressure, it reaches 95% at a pressure of 1 MPa. A methane production rate of 95% is a value at which the post-reaction product gas can be introduced into a gas pipeline or the like without undergoing a purification process when adjusting and managing gas based on the Wobbe index, which is an index indicating the amount of heat emitted and the maximum combustion rate, which is an index indicating the combustion rate, both of which are indicators of gas flammability.

Therefore, in practice, a methane conversion rate of 50% or more at atmospheric pressure is sufficient, with a methane conversion rate of 70% or more being more preferable. Therefore, for the electrode configurations of Examples 1 to 4, the ratio t1/t is preferably selected to be around 5% or more, and more preferably around 50% or less.

(Comparative Example 3)
For comparison, an electrochemical cell 1 was fabricated as Comparative Example 3 by 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 such that the gas inlet angle θ1 (45°)/gas outlet angle θ2 (135°).

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.

Example 5
In Example 5, an electrochemical cell 1 _was fabricated in which the material of the oxide ion conductor 11 was changed from 8YSZ in Example 1 to non-oriented phosphate apatite, _Ca9.00Na0.67 ( _PO4 ) ₅ (CO3)( _H2O )(OH) _1.67 , and the configurations of _the cathode 2 and anode 3 were the same as those in Example 1. The non-oriented phosphate apatite was prepared by weighing calcium phosphate [ _Ca3 ( _PO4 ) ₂ ] and sodium bicarbonate ( _NaHCO3 ) to obtain the desired composition as raw materials, and mixing, drying, and heat treatment of the raw materials were carried out in the same manufacturing process as for the composite oxide material in Example 1 to obtain a compound powder with a phosphate apatite structure.

A slurry containing the obtained compound powder was prepared in the same manner as for 8YSZ in Example 1 and applied to a resin sheet. The resin sheet was then peeled off, and the resulting mixture was fired at 1,350°C for 2 hours in an air atmosphere adjusted to contain 5% water vapor, forming an electrolyte sheet that would become the oxide ion conductor 11.

The 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 conducted 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 Figure 10.

Example 6
In Example 6, an electrochemical cell 1 was fabricated in the _same manner as in Example 5, except that the material for the oxide ion conductor 11 was changed from 8YSZ in Example 1 to c-axis oriented phosphate apatite, _Ca9.00Na0.67 ( _PO4 ) ₅ ( _CO3 )( _H2O )(OH) _1.67 . First, a desired amount of calcium phosphate alone was weighed as the raw material, and a powder and a slurry for forming a sheet were prepared in the same manner as in Example 5, and a sheet was fabricated by coating the powder and a slurry on a resin sheet.

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 1,300°C for 50 hours in an air atmosphere adjusted to contain 5% water vapor. This gas-solid diffusion process caused 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.

The 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 conducted 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 Figure 10.

Example 7
In Example 7, an electrochemical cell 1 was fabricated in _the same _manner as in Example 1, except that the material for the oxide ion conductor 11 was replaced with a non-oriented apatite-type composite oxide, _{La9.3Si5.3B0.7O25} , instead of _8YSZ in Example 1. 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 dried powder, which was then heat-treated to obtain a composite oxide powder for sheet formation. 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. The resin sheet was then peeled off, and the resulting mixture was fired in an air atmosphere at 1600°C for 2 hours to form an electrolyte sheet for the oxide ion conductor 11.

The 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 conducted using the hydrocarbon production apparatus 100, as in Example 1, and the produced gas was analyzed. The results are shown in Table 1 and Figure 10.

(Example 8)
In Example 8, an electrochemical cell 1 was fabricated in _the same _manner as in Example 1, except that the material for the oxide ion conductor 11 was a c-axis-oriented apatite-type composite oxide, _{La9.3Si5.3B0.7O25} , instead of _8YSZ in Example 1. In this case, 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 sheet formation. 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. The resin sheet was then peeled off, and the resulting mixture was fired in air at 1600°C for 2 hours.

The resin sheet was then peeled off from the resulting sheet, and the sheet was subjected to a heat treatment in a boron oxide (B ₂ O ₃ ) 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, and an electrolyte sheet serving as the oxide ion conductor 11 was formed.

The 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 conducted 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 Figure 10.

Example 9
In Example 9, an electrochemical cell 1 was produced in the _same manner as in Example 1, except that the material of the complex oxide layer 2B was changed to a ceria-based complex oxide, _{Ce0.75Co0.2Fe0.05O2} (shown as Ce( _Co ,Fe) _O in Table 1), instead of the lanthanum-based complex oxide of Example 1. The ceria-based complex oxide was produced using single oxides of the constituent elements as raw materials, and the raw materials were mixed, dried, and heat-treated in the same production process as for the complex oxide material of Example 1, to obtain a ceria-based complex oxide powder.

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 the same 8YSZ as 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 conducted using the hydrocarbon production apparatus 100, as in Example 1, and the produced gas was analyzed. The results are shown in Table 1 and Figure 10.

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

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 also shown in Table 1 and FIG. 10.

As shown in the results in Table 1 and Figure 10, by changing the gas flow direction for an electrochemical cell 1 with the same structure as in Example 1, the methane conversion rate improved to 52%. The reason for this is not entirely clear, but it is presumed that by introducing gas vertically and discharging it in the direction opposite to the gas introduction direction, as shown in Figure 9 above, the methane precursor remains in an area that is easily heated, allowing electrolytic reduction to proceed efficiently.

In this way, when the ratio t1/t in the electrochemical cell 1 is 1% or higher, by appropriately selecting the materials for 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 sufficient for practical use.

In the electrochemical cell 1 configured as described above, the cathode 2 has a three-layer structure with an intermediate layer 2C between the conductive metal layer 2A and the complex oxide layer 2B, making it possible to produce hydrocarbons using a single electrochemical cell 1. The conductive metal layer 2A contains Ag as its primary component, the complex oxide layer 2B is primarily composed of a complex oxide containing a transition metal element, and the intermediate layer 2C contains both of these components, resulting in 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.

The present invention is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the gist of the present invention. For example, when configuring a hydrocarbon production unit 10 using an electrochemical cell 1, a plurality of electrochemical cells 1 may be stacked. Furthermore, the configuration of the gas distribution 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.

REFERENCE SIGNS LIST 1 electrochemical cell 11 oxide ion conductor 12, 13 surface 2 cathode 3 anode 21 first particle 22 second particle 2A conductive metal layer 2B composite oxide layer 2C intermediate layer

Claims (9)

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 . 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. The electrochemical cell according to claim 2 , wherein the composite oxide containing a transition metal element is a ceria-based composite oxide containing a plurality of transition metal elements . 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. The electrochemical cell described in claim 4, wherein the apatite-type compound is a phosphate-based apatite-type compound or an apatite-type composite oxide. 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. 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. The electrochemical cell described in claim 7, wherein the composite oxide containing a transition metal element is a ceria-based composite oxide containing multiple transition metal elements. 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 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 gas discharge direction opposite to the gas inlet direction.