JP7494354B2 - Electrochemical cell and method for producing the same, electrochemical module, and electrochemical cartridge - Google Patents

Description

This disclosure relates to electrochemical cells and manufacturing methods thereof, electrochemical modules, and electrochemical cartridges.

A solid oxide fuel cell (SOFC) is equipped with a single element (cell) consisting of a fuel electrode, a solid electrolyte membrane, and an air electrode, and an interconnector that electrically connects adjacent cells. Although the generated voltage per cell is small, by connecting multiple cells in series to form a cell stack, it is possible to increase the voltage and obtain a practical output.

In a cell stack, when fuel gas is supplied to the fuel electrode and an oxidizing gas such as oxygen is supplied to the air electrode, the oxygen in the oxidizing gas supplied to the air electrode is ionized and passes through the solid electrolyte membrane to reach the fuel electrode. An electrochemical reaction occurs between the oxygen ions that have reached the fuel electrode and the fuel gas, generating a potential difference between the fuel electrode and the air electrode, and electricity is generated by extracting this potential difference to the outside.

The greatest feature of SOFCs is their high power generation efficiency. Therefore, there is a demand for SOFCs to achieve higher power density while maintaining resistance to cell degradation during power generation. To improve power generation performance and degradation behavior, it is important to understand the behavior of the surfaces and interfaces of the electrodes and electrolytes. (See Non-Patent Document 1)

Junichiro Otomo, "Material development of solid oxide fuel cells and high performance through structural control", Surface Chemistry, Vol. 32, No. 2, pp. 93-98, 2011

In a cell, a large interfacial resistance exists at the interface between the air electrode and the solid electrolyte membrane. As described in Non-Patent Document 1, one method for reducing the resistance at the interface between the air electrode and the solid electrolyte membrane is to introduce a dense intermediate layer of ceria-based oxide between the air electrode and the solid electrolyte membrane.

However, when an electronically conductive air electrode is laminated on an intermediate layer of a ceria-based oxide, the interfacial resistance between the air electrode and the intermediate layer may become high.

At the interface between the solid electrolyte membrane and the air electrode, an electrode reaction occurs at the three-phase interface, accompanied by the adsorption, dissociation, and movement of oxygen. A three-phase interface is an interface where three phases coexist: ion conduction (electrolyte), oxygen supply (gas diffusion), and electron supply (electronic conductivity).

Figure 10 shows a schematic diagram of the cathode side of a conventional cell. An cathode 113' is formed on a solid electrolyte membrane 111. The cathode 113' includes an cathode intermediate layer 113a' and an cathode conductive layer 113b'. The cathode intermediate layer 113a' is ionically conductive in an oxidizing atmosphere and therefore functions as an electrolyte. Pores P included in the cathode intermediate layer 113a' and the cathode conductive layer 113b' can serve as oxygen gas diffusion paths. The cathode conductive layer 113b' has electronic conductivity. In Figure 10, the interface between the cathode intermediate layer 113a' and the cathode conductive layer 113b' can be a three-phase interface field.

However, the ceria-based oxide, which is the main component of the air electrode intermediate layer 113a', does not have electronic conductivity. Therefore, electrons cannot pass through the air electrode intermediate layer 113a', and the three-phase interface field on the air electrode 113' side is only the interface between the air electrode intermediate layer 113a' and the air electrode conductive layer 113b', resulting in high interface resistance of the electrode.

This disclosure has been made in light of these circumstances, and aims to provide an electrochemical cell that reduces the resistance at the interface between the oxygen electrode intermediate layer and the oxygen electrode conductive layer compared to conventional electrochemical cells, a manufacturing method thereof, an electrochemical module, and an electrochemical cartridge.

To solve the above problems, the electrochemical cell and its manufacturing method, electrochemical module, and electrochemical cartridge disclosed herein employ the following measures.

The present disclosure provides an electrochemical cell in which a hydrogen electrode, a solid electrolyte membrane, and an oxygen electrode are laminated in this order, the oxygen electrode including, from the solid electrolyte membrane side, an oxygen electrode intermediate layer and an oxygen electrode conductive layer, the oxygen electrode conductive layer being a sintered body of a material including a perovskite oxide represented by a general formula _ABO3 , in which an A site is La and Sr and/or Ca, and a B site is Mn, the oxygen electrode intermediate layer being a sintered body of a material including Sm-doped ceria, and having pores, a first texture, and a second texture, the first texture including 50 atomic % or more of Ce and not including Ca, Sr, or Mn, the second texture including an element selected from the group consisting of Ca, Sr, and Mn, and the area ratio of the second texture in a cross section of the oxygen electrode intermediate layer being 20% or more and 60% or less of the total area of the first texture and the second texture.

The present disclosure provides an electrochemical module including the electrochemical cell described above.

The present disclosure provides an electrochemical cartridge having the electrochemical module described in the above disclosure.

The present disclosure provides a method for manufacturing an electrochemical cell in which a hydrogen electrode, a solid electrolyte membrane, and an oxygen electrode are laminated in this order, the oxygen electrode including, from the solid electrolyte membrane side, an oxygen electrode intermediate layer and an oxygen electrode conductive layer, the oxygen electrode conductive layer being a sintered body of a material containing a perovskite oxide represented by a general formula _ABO3 , in which the A site is La and Sr and/or Ca, and the B site is Mn, and the oxygen electrode intermediate layer is a sintered body of a material containing Sm-doped ceria, the method comprising obtaining conditions under which a structure containing an element selected from the group consisting of Ca, Sr, and Mn is formed in 20% or more and 60% or less of a total area excluding pores in a cross section of the oxygen electrode intermediate layer, and co-sintering the oxygen electrode intermediate layer and the oxygen electrode conductive layer in accordance with the obtained conditions to form the oxygen electrode intermediate layer.

The oxygen electrode conductive layer, which is formed by sintering a material containing the perovskite oxide, has electronic conductivity. The oxygen electrode intermediate layer, which is formed by sintering a material containing Sm-doped ceria, has ionic conductivity. By forming the oxygen electrode into a two-layer structure consisting of an oxygen electrode conductive layer and an oxygen electrode intermediate layer, the interface resistance between the solid electrolyte membrane and the oxygen electrode can be reduced.

The first structure, which is mainly composed of elements derived from Sm-doped ceria, performs the function of ion conductivity. The second structure, which contains an element selected from the group consisting of Ca, Sr, and Mn, performs the function of electronic conductivity. When the total area of the cross section of the oxygen electrode intermediate layer is constant, as the second structure increases, the proportion of the first structure decreases, and the ionic conductivity of the oxygen electrode intermediate layer decreases, and as the first structure increases, the proportion of the second structure decreases, and the electronic conductivity of the oxygen electrode intermediate layer decreases. By setting the area ratio of the second structure in the cross section of the oxygen electrode intermediate layer to 20% to 60% of the total area of the first structure and the second structure in the cross section of the oxygen electrode intermediate layer, it is possible to form an electronic path while securing an ion path in the oxygen electrode intermediate layer.

By including pores that serve as gas paths, a first ionically conductive structure, and a second electronically conductive structure, the electrode reaction field (three-phase interface) expands three-dimensionally inside the oxygen electrode intermediate layer. This reduces the resistance at the interface between the oxygen electrode conductive layer and the oxygen electrode intermediate layer.

FIG. 2 is a diagram showing an embodiment of a cell stack according to the present disclosure. FIG. 2 illustrates an embodiment of a SOFC module according to an embodiment of the present disclosure. FIG. 2 illustrates one embodiment of a cross-section of a SOFC cartridge according to an embodiment of the present disclosure. 1 is a scanning electron microscope photograph of a cross section of an air electrode intermediate layer of a test specimen. 1 is a diagram showing semi-quantitative analysis results of energy dispersive X-ray analysis (EDX). 1 is a graph illustrating the effects of Mn, Ca, and Sr on the electronic conductivity of a test specimen. 7 is a chart summarizing the values read from FIG. 6. 1 is a graph showing the relationship between the proportion of the second texture in the air electrode intermediate layer and electronic conductivity. 1 is a schematic diagram of a cross section (air electrode side) of a fuel cell according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram of a cross section (air electrode side) of a conventional fuel cell.

Below, an embodiment of the electrochemical cell and its manufacturing method, electrochemical module, and electrochemical cartridge according to the present disclosure will be described with reference to the drawings. Note that the following description of the fuel cell should be read as appropriate substituting the term electrochemical cell.

In the following, for ease of explanation, the positional relationship of each component is described using the expressions "above" and "below" with respect to the paper surface, indicating the vertically upper side and vertically lower side, respectively. Also, in this embodiment, for those that can obtain the same effect in the vertical direction and the horizontal direction, the vertical direction on the paper surface is not necessarily limited to the vertically upper and lower directions, but may correspond to, for example, a horizontal direction perpendicular to the vertical direction.

In the following, a cylindrical (tubular) cell stack for a solid oxide fuel cell (SOFC) is described as an example, but this is not necessarily the case and a flat cell stack may be used, for example. The fuel cell is formed on a substrate, but instead of a substrate, an electrode (fuel electrode or air electrode) may be formed thick and serve as the substrate as well.

(Cylindrical cell stack structure)
First, referring to FIG. 1, a cylindrical cell stack using a substrate tube will be described as an example according to this embodiment. When a substrate tube is not used, for example, the fuel electrode may be formed thick and used as the substrate tube, and the use of the substrate tube is not limited. In addition, the substrate tube in this embodiment will be described as having a cylindrical shape, but the substrate tube may be cylindrical and is not necessarily limited to a circular cross section, and may be, for example, elliptical. A cell stack such as a flat tubular cylinder in which the peripheral side surface of a cylinder is crushed vertically may also be used. Here, FIG. 1 shows one aspect of a cell stack according to this embodiment.

The cell stack 101 includes, for example, a cylindrical base tube 103, a plurality of fuel cells 105 formed on the outer circumferential surface of the base tube 103, and an interconnector 107 formed between adjacent fuel cells 105. The fuel cell 105 is formed by stacking a fuel electrode 109, a solid electrolyte membrane 111, and an air electrode 113. The cell stack 101 also includes a lead film 115 electrically connected via the interconnector 107 to the air electrode 113 of the fuel cell 105 formed at the end of the axial direction of the base tube 103 among the plurality of fuel cells 105 formed on the outer circumferential surface of the base tube 103, and a lead film 115 electrically connected to the fuel electrode 109 of the fuel cell 105 formed at the other end of the outer circumferential surface.

(Description of materials and functions of each component of the cell stack)
The base tube 103 is made of a porous material and is mainly composed of, for example, CaO-stabilized _ZrO2 (CSZ), a mixture of CSZ and nickel oxide (NiO) (CSZ+NiO), _{Y2O3 -} stabilized _ZrO2 (YSZ), or _MgAl2O4 . This base tube 103 supports the fuel cell 105, the interconnector 107, and the _lead film 115, and diffuses the fuel gas supplied to the inner peripheral surface of the base tube 103 through the pores of the base tube 103 to the fuel electrode 109 formed on the outer peripheral surface of the base tube 103.

The fuel electrode 109 is made of a composite oxide of Ni and a zirconia-based electrolyte material, and Ni/YSZ is used, for example. The thickness of the fuel electrode 109 is 50 μm to 250 μm, and the fuel electrode 109 may be formed by screen printing a slurry. In this case, the Ni component of the fuel electrode 109 has a catalytic effect on the fuel gas. This catalytic effect is to react with the fuel gas supplied through the substrate tube 103, for example, a mixed gas of methane (CH ₄ ) and water vapor, and reform it into hydrogen (H ₂ ) and carbon monoxide (CO). The fuel electrode 109 also electrochemically reacts the hydrogen (H ₂ ) and carbon monoxide (CO) obtained by reforming with oxygen ions (O ^2- ) supplied through the solid electrolyte membrane 111 near the interface with the solid electrolyte membrane 111 to generate water (H ₂ O) and carbon dioxide (CO ₂ ). At this time, the fuel cell 105 generates electricity from the electrons released from the oxygen ions.
Fuel gases that can be supplied to the fuel electrode 109 of a solid oxide fuel cell and used include hydrocarbon gases such as hydrogen ( _H2 ), carbon monoxide (CO), and methane ( _CH4 ), city gas, and natural gas, as well as gasified gases produced by gasification equipment from carbon-containing raw materials such as petroleum, methanol, and coal.

YSZ, which has airtightness that makes it difficult for gas to pass through and high oxygen ion conductivity at high temperatures, is mainly used for the solid electrolyte membrane 111. This solid electrolyte membrane 111 moves oxygen ions (O ²⁻ ) generated at the air electrode 113 to the fuel electrode 109. The thickness of the solid electrolyte membrane 111 located on the surface of the fuel electrode 109 is 10 μm to 100 μm, and the solid electrolyte membrane 111 may be formed by screen printing a slurry.

The air electrode 113 dissociates oxygen in an oxidizing gas such as air supplied thereto near the interface with the solid electrolyte membrane 111 to generate oxygen ions (O ²⁻ ). The air electrode 113 is coated with a slurry by screen printing or using a dispenser.

An oxidizing gas is a gas that contains approximately 15% to 30% oxygen. Air is a typical example, but other gases such as a mixture of combustion exhaust gas and air, or a mixture of oxygen and air can also be used.

The air electrode 113 includes an air electrode intermediate layer 113a and an air electrode conductive layer 113b. The air electrode intermediate layer 113a is disposed in contact with the solid electrolyte membrane 111 and the interconnector 107. The air electrode conductive layer 113b is disposed on and in contact with the air electrode intermediate layer 113a.

The cathode conductive layer 113b has electronic conductivity. The thickness of the cathode conductive layer 113b is 20 μm or more and 1500 μm or less. When applied to a cylindrical cell stack, the thickness of the cathode conductive layer 113b may be 500 μm or more and 1500 μm or less.

The material of the cathode conductive layer 113b is a perovskite oxide represented by the general formula _ABO3 . In the general formula _ABO3 , the A site is selected from the group consisting of La, Sr, and Ca. The B site is Mn. The perovskite oxide represented by the general formula _ABO3 is, for example, _LaMnO3 doped with Sr and/or Ca. _LaMnO3 doped with Sr and Ca can be produced by a general solid-phase mixing method or liquid-phase mixing method. The contents of La, Mn, Sr, and Ca can be adjusted when the raw materials are mixed.

The perovskite oxide represented by the general formula _ABO3 may have an A-site deficient composition. In the A-site deficient composition, the A/B molar ratio is preferably 0.90 to 0.98. For example, by setting A=0.95 mol, B=1.00 mol, and the A/B molar ratio to 0.95, the A-site is deficient and the B-site is excessive.

The perovskite oxide represented by the general formula _ABO3 may have an A-site excess composition. In the A-site excess composition, the A/B ratio is preferably 1.02 to 1.10. For example, when A=1.05 mol, B=1.00 mol, and the A/B ratio is 1.05, the B site is deficient and the A site is excessive.

The air electrode intermediate layer 113a has ionic conductivity and electronic conductivity. The thickness of the air electrode intermediate layer 113a is 10 μm or more and 20 μm or less.

The air electrode intermediate layer 113a has pores, a first texture, and a second texture. The area ratio of the second texture in the air electrode intermediate layer 113a is 20% to 60%, preferably 30% to 40%, of the total area of the first texture and the second texture in the cross section. In the cross section of the air electrode intermediate layer 113a, the area ratio of the pores may be 5% to 50% of the total area of the pores, the first texture, and the second texture in the cross section.

The cathode intermediate layer 113a is made of a material having high ionic conductivity and excellent catalytic activity. The material of the cathode intermediate layer 113a is Sm-doped ceria. The Sm-doped ceria may be Sm _1-x Ce _x O ₂ (0.8≦x≦0.9).

The material of the cathode intermediate layer 113a may be doped with elements in the A and B sites of the material of the cathode conductive layer 113b. The amount of the added elements is 1.0 or less in terms of the molar ratio of the elements in the A and B sites.

The first structure is composed mainly of elements derived from Sm-doped ceria. The Ce content of the first structure is 50 atomic % or more. The first structure does not contain Mn, Ca, or Sr. Here, "does not contain" indicates that the content is 0.4 atomic % or less.

The second structure contains an element derived from Sm-doped ceria and an element (electronically conductive element) selected from the group consisting of Ca, Sr, and Mn. The Ce content of the second structure is less than 50 atomic %. The electronically conductive element content of the second structure can be 0.5 atomic % or more and 50 atomic % or less.

The interconnector 107 is made of a conductive _perovskite oxide represented by M1 _{- xLxTiO3} (M is an alkaline earth metal element, L is a lanthanoid element) such as _SrTiO3 , and the slurry is screen-printed. The interconnector 107 is a dense film so that the fuel gas and the oxidizing gas do not mix. The interconnector 107 also has stable durability and electrical conductivity under both oxidizing and reducing atmospheres. The interconnector 107 electrically connects the air electrode 113 of one fuel cell 105 to the fuel electrode 109 of the other fuel cell 105 in adjacent fuel cells 105, thereby connecting the adjacent fuel cells 105 in series.

The lead film 115 is made of a composite material of Ni and a zirconia-based electrolyte material, such as Ni/YSZ, or M1- _xLxTiO3 (M is an alkaline earth metal element, L is a lanthanoid element) such as _SrTiO3 , since it is required to have electronic conductivity and a thermal expansion coefficient close to that of the other materials constituting the cell stack 101. This lead film 115 conducts the DC power generated by the multiple fuel cell cells 105 connected in series by the interconnector 107 to the vicinity of the end of the cell stack 101.

(Manufacturing method of cylindrical cell stack)
A slurry for each component is prepared by mixing the material powders of the fuel electrode 109, solid electrolyte membrane 111, interconnector 107, lead membrane 115, air electrode intermediate layer 113a, and air electrode conductive layer 113b with an aqueous vehicle.

The base tube 103 on which the fuel electrode 109, solid electrolyte membrane 111, and interconnector 107 slurry film are formed is co-sintered in air. The sintering temperature is specifically set to 1350°C to 1450°C.

Next, the base tube 103 on which the slurry films of the air electrode intermediate layer 113a and the air electrode conductive layer 113b are successively formed on the co-sintered base tube 103 is sintered in the atmosphere. The sintering temperature is specifically set to 1100°C to 1250°C. The sintering temperature here is set to a lower temperature than the co-sintering temperature after the base tube 103 to the interconnector 107 are formed.

The above process results in a cylindrical cell stack 101 in which fuel cells 105 are formed on a base tube 103.

Regarding the material composition, sintering temperature, and sintering procedure of the air electrode intermediate layer 113a and the air electrode conductive layer 113b, the conditions under which a structure (second structure) containing an element selected from the group consisting of Ca, Sr, and Mn is formed in the air electrode intermediate layer 113a in an amount of 20% to 60% of the total area of the air electrode intermediate layer excluding pores are obtained through preliminary tests, etc. Air electrode intermediate layer 113a is formed according to the above obtained.

For example, by co-sintering the cathode intermediate layer 113a and the cathode conductive layer 113b, the elements contained in the cathode conductive layer 113b are diffused into the cathode intermediate layer 113a. This forms a second structure in the cathode intermediate layer 113a.

When the material of the cathode conductive layer 113b has an A-site deficient composition, Mn, which is in excess of the stoichiometric composition of the perovskite structure, diffuses in large amounts into the cathode intermediate layer 113a.

When the material of the cathode conductive layer 113b has an A-site excess composition, Ca and Sr, which are in excess of the stoichiometric composition of the perovskite structure, diffuse in large amounts into the cathode intermediate layer 113a.

By increasing the sintering temperature and atomizing the material of the cathode conductive layer 113b, the diffusion of elements contained in the material of the cathode conductive layer 113b into the cathode intermediate layer 113a can be promoted. When the sintering temperature is increased, the shrinkage behavior of the cathode conductive layer material is measured, the temperature is changed by 50°C from the starting point of sintering shrinkage, a prototype cell is fabricated, and the change in the area ratio of the second structure is obtained from the results of cross-sectional structure observation, making it possible to control the rate of change of the second structure. In addition, the same control is possible by extending the sintering time even at the same sintering temperature. Regarding the particle size, the rate of change of the second structure can be controlled by fabricating a prototype cell using materials with different particle sizes and obtaining the change in the area ratio of the second structure from the results of cross-sectional structure observation after the particle size change.

(Explanation of SOFC module structure and function of each element)
Next, the SOFC module and the SOFC cartridge according to the present embodiment will be described with reference to Fig. 2 and Fig. 3. Fig. 2 shows one aspect of the SOFC module according to the present embodiment. Fig. 3 shows a cross-sectional view of one aspect of the SOFC cartridge according to the present embodiment.

As shown in FIG. 2, the SOFC module (fuel cell module) 201 includes, for example, a plurality of SOFC cartridges (fuel cell cartridges) 203 and a pressure vessel 205 that houses the plurality of SOFC cartridges 203. Although FIG. 2 illustrates a cylindrical SOFC cell stack 101, this is not necessarily the case, and a flat cell stack may be used, for example. The SOFC module 201 also includes a fuel gas supply pipe 207, a plurality of fuel gas supply branch pipes 207a, a fuel gas exhaust pipe 209, and a plurality of fuel gas exhaust branch pipes 209a. The SOFC module 201 also includes an oxidizing gas supply pipe (not shown), an oxidizing gas supply branch pipe (not shown), an oxidizing gas exhaust pipe (not shown), and a plurality of oxidizing gas exhaust branch pipes (not shown).

The fuel gas supply pipe 207 is provided outside the pressure vessel 205 and is connected to a fuel gas supply unit that supplies fuel gas with a predetermined gas composition and a predetermined flow rate corresponding to the power generation amount of the SOFC module 201, and is also connected to multiple fuel gas supply branch pipes 207a. This fuel gas supply pipe 207 branches and guides the fuel gas at a predetermined flow rate supplied from the fuel gas supply unit to multiple fuel gas supply branch pipes 207a. In addition, the fuel gas supply branch pipe 207a is connected to the fuel gas supply pipe 207 and is connected to multiple SOFC cartridges 203. This fuel gas supply branch pipe 207a guides the fuel gas supplied from the fuel gas supply pipe 207 to the multiple SOFC cartridges 203 at an approximately equal flow rate, and approximately uniforms the power generation performance of the multiple SOFC cartridges 203.

The fuel gas discharge branch pipe 209a is connected to the multiple SOFC cartridges 203 and is also connected to the fuel gas discharge pipe 209. This fuel gas discharge branch pipe 209a guides the exhaust fuel gas discharged from the SOFC cartridges 203 to the fuel gas discharge pipe 209. In addition, the fuel gas discharge pipe 209 is connected to the multiple fuel gas discharge branch pipes 209a, and a portion of the fuel gas discharge pipe 209 is disposed outside the pressure vessel 205. This fuel gas discharge pipe 209 guides the exhaust fuel gas discharged from the fuel gas discharge branch pipe 209a at a substantially uniform flow rate to the outside of the pressure vessel 205.

The pressure vessel 205 is operated with an internal pressure of 0.1 MPa to approximately 3 MPa and an internal temperature of ambient temperature to approximately 550°C, so a material that has strength and corrosion resistance to oxidizing agents such as oxygen contained in oxidizing gases is used. For example, stainless steel materials such as SUS304 are suitable.

In this embodiment, a configuration in which multiple SOFC cartridges 203 are grouped together and stored in the pressure vessel 205 is described, but this is not limited thereto, and for example, the SOFC cartridges 203 may be stored in the pressure vessel 205 without being grouped together.

(Explanation of the structure of the SOFC cartridge and the function of each element)
As shown in Fig. 3, the SOFC cartridge 203 includes a plurality of cell stacks 101, a power generation chamber 215, a fuel gas supply header 217, a fuel gas discharge header 219, an oxidizing gas (air) supply header 221, and an oxidizing gas discharge header 223. The SOFC cartridge 203 also includes an upper tube plate 225a, a lower tube plate 225b, an upper heat insulator 227a, and a lower heat insulator 227b. In this embodiment, the SOFC cartridge 203 has a structure in which the fuel gas supply header 217, the fuel gas discharge header 219, the oxidizing gas supply header 221, and the oxidizing gas discharge header 223 are arranged as shown in Fig. 3, so that the fuel gas and the oxidizing gas flow in opposing directions between the inside and outside of the cell stack 101. However, this is not necessarily required, and for example, the inside and outside of the cell stack 101 may flow in parallel, or the oxidizing gas may flow in a direction perpendicular to the longitudinal direction of the cell stack 101.

The power generation chamber 215 is an area formed between the upper insulator 227a and the lower insulator 227b. This power generation chamber 215 is an area in which the fuel cells 105 of the cell stack 101 are arranged, and generates electricity by electrochemically reacting fuel gas with oxidizing gas. The temperature near the center of the cell stack 101 in the longitudinal direction of the power generation chamber 215 is monitored by a temperature measurement unit (such as a temperature sensor or thermocouple), and during steady-state operation of the SOFC module 201, a high-temperature atmosphere of approximately 700°C to 1000°C is created.

The fuel gas supply header 217 is an area surrounded by the upper casing 229a and upper tube plate 225a of the SOFC cartridge 203, and is connected to the fuel gas supply branch pipe 207a by the fuel gas supply hole 231a provided at the top of the upper casing 229a. The multiple cell stacks 101 are joined to the upper tube plate 225a by a seal member 237a, and the fuel gas supply header 217 guides the fuel gas supplied from the fuel gas supply branch pipe 207a through the fuel gas supply hole 231a into the inside of the base tube 103 of the multiple cell stacks 101 at a substantially uniform flow rate, thereby substantially equalizing the power generation performance of the multiple cell stacks 101.

The fuel gas discharge header 219 is an area surrounded by the lower casing 229b and the lower tube plate 225b of the SOFC cartridge 203, and is connected to the fuel gas discharge branch pipe 209a (not shown) by the fuel gas discharge hole 231b provided in the lower casing 229b. In addition, the multiple cell stacks 101 are joined to the lower tube plate 225b by a seal member 237b, and the fuel gas discharge header 219 collects the exhaust fuel gas that passes through the inside of the base tube 103 of the multiple cell stacks 101 and is supplied to the fuel gas discharge header 219, and directs it to the fuel gas discharge branch pipe 209a via the fuel gas discharge hole 231b.

The oxidizing gas of a predetermined gas composition and a predetermined flow rate corresponding to the power generation amount of the SOFC module 201 is branched to the oxidizing gas supply branch pipe and supplied to the multiple SOFC cartridges 203. The oxidizing gas supply header 221 is an area surrounded by the lower casing 229b, the lower tube plate 225b, and the lower heat insulator 227b of the SOFC cartridge 203, and is connected to an oxidizing gas supply branch pipe (not shown) by an oxidizing gas supply hole 233a provided on the side of the lower casing 229b. This oxidizing gas supply header 221 guides a predetermined flow rate of oxidizing gas supplied from the oxidizing gas supply branch pipe (not shown) through the oxidizing gas supply hole 233a to the power generation chamber 215 through the oxidizing gas supply gap 235a described later.

The oxidizing gas discharge header 223 is an area surrounded by the upper casing 229a, upper tube plate 225a, and upper insulation 227a of the SOFC cartridge 203, and is connected to an oxidizing gas discharge branch pipe (not shown) via an oxidizing gas discharge hole 233b provided on the side of the upper casing 229a. This oxidizing gas discharge header 223 guides the exhaust oxidizing gas supplied to the oxidizing gas discharge header 223 from the power generation chamber 215 through an oxidizing gas discharge gap 235b (described later) to the oxidizing gas discharge branch pipe (not shown) via the oxidizing gas discharge hole 233b.

The upper tube plate 225a is fixed to the side plate of the upper casing 229a between the top plate of the upper casing 229a and the upper insulator 227a so that the upper tube plate 225a, the top plate of the upper casing 229a, and the upper insulator 227a are approximately parallel. The upper tube plate 225a also has a number of holes corresponding to the number of cell stacks 101 provided in the SOFC cartridge 203, into which the cell stacks 101 are inserted. The upper tube plate 225a airtightly supports one end of the multiple cell stacks 101 via either or both of the sealing member 237a and the adhesive member, and isolates the fuel gas supply header 217 from the oxidizing gas discharge header 223.

The upper insulation 227a is disposed at the lower end of the upper casing 229a so that the upper insulation 227a, the top plate of the upper casing 229a, and the upper tube plate 225a are approximately parallel to each other, and is fixed to the side plate of the upper casing 229a. The upper insulation 227a also has a number of holes corresponding to the number of cell stacks 101 provided in the SOFC cartridge 203. The diameter of the holes is set to be larger than the outer diameter of the cell stacks 101. The upper insulation 227a has an oxidizing gas exhaust gap 235b formed between the inner surface of the hole and the outer surface of the cell stack 101 inserted into the upper insulation 227a.

The upper insulation 227a separates the power generation chamber 215 from the oxidizing gas discharge header 223, and prevents the atmosphere around the upper tube sheet 225a from becoming hot, which would result in a decrease in strength and an increase in corrosion caused by the oxidizing agent contained in the oxidizing gas. The upper tube sheet 225a, etc. is made of a metal material that is resistant to high temperatures, such as Inconel, and prevents the upper tube sheet 225a, etc. from being thermally deformed by being exposed to the high temperatures in the power generation chamber 215 and the large temperature difference within the upper tube sheet 225a, etc. The upper insulation 227a also guides the exhaust oxidizing gas that has passed through the power generation chamber 215 and been exposed to high temperatures through the oxidizing gas discharge gap 235b to the oxidizing gas discharge header 223.

According to this embodiment, the structure of the SOFC cartridge 203 described above allows the fuel gas and the oxidizing gas to flow in opposite directions inside and outside the cell stack 101. As a result, the exhaust oxidizing gas exchanges heat with the fuel gas supplied to the power generation chamber 215 through the inside of the base tube 103, and is cooled to a temperature at which the upper tube plate 225a, etc., made of a metal material, does not deform, such as buckling, and is supplied to the oxidizing gas discharge header 223. In addition, the fuel gas is heated by heat exchange with the exhaust oxidizing gas discharged from the power generation chamber 215 and is supplied to the power generation chamber 215. As a result, fuel gas preheated to a temperature suitable for power generation can be supplied to the power generation chamber 215 without using a heater or the like.

The lower tube plate 225b is fixed to the side plate of the lower casing 229b between the bottom plate of the lower casing 229b and the lower insulator 227b so that the lower tube plate 225b, the bottom plate of the lower casing 229b, and the lower insulator 227b are approximately parallel to each other. The lower tube plate 225b also has a number of holes corresponding to the number of cell stacks 101 provided in the SOFC cartridge 203, into which the cell stacks 101 are inserted. The lower tube plate 225b airtightly supports the other ends of the multiple cell stacks 101 via either or both of the sealing member 237b and the adhesive member, and isolates the fuel gas discharge header 219 from the oxidizing gas supply header 221.

The lower insulator 227b is disposed at the upper end of the lower casing 229b so that the lower insulator 227b, the bottom plate of the lower casing 229b, and the lower tube plate 225b are approximately parallel to each other, and is fixed to the side plate of the lower casing 229b. The lower insulator 227b also has a number of holes corresponding to the number of cell stacks 101 provided in the SOFC cartridge 203. The diameter of the holes is set to be larger than the outer diameter of the cell stacks 101. The lower insulator 227b has an oxidizing gas supply gap 235a formed between the inner surface of the hole and the outer surface of the cell stack 101 inserted into the lower insulator 227b.

The lower insulator 227b separates the power generation chamber 215 from the oxidizing gas supply header 221, and prevents the atmosphere around the lower tube sheet 225b from becoming hot, which would result in a decrease in strength and an increase in corrosion caused by the oxidizing agent contained in the oxidizing gas. The lower tube sheet 225b is made of a metal material that is resistant to high temperatures, such as Inconel, and prevents the lower tube sheet 225b from being exposed to high temperatures and being thermally deformed due to a large temperature difference within the lower tube sheet 225b. The lower insulator 227b also guides the oxidizing gas supplied to the oxidizing gas supply header 221 through the oxidizing gas supply gap 235a to the power generation chamber 215.

According to this embodiment, the structure of the SOFC cartridge 203 described above allows the fuel gas and the oxidizing gas to flow in opposite directions inside and outside the cell stack 101. As a result, the exhaust fuel gas that passes through the inside of the base tube 103 and the power generation chamber 215 exchanges heat with the oxidizing gas supplied to the power generation chamber 215, and is cooled to a temperature at which the lower tube plate 225b made of a metal material does not deform, such as by buckling, and is supplied to the fuel gas discharge header 219. In addition, the oxidizing gas is heated by heat exchange with the exhaust fuel gas and is supplied to the power generation chamber 215. As a result, it is possible to supply the oxidizing gas heated to a temperature required for power generation to the power generation chamber 215 without using a heater or the like.

The DC power generated in the power generation chamber 215 is conducted to the vicinity of the end of the cell stack 101 by the lead film 115 made of Ni/YSZ or the like provided on the multiple fuel cell cells 105, and then collected by the current collector rod (not shown) of the SOFC cartridge 203 via a current collector plate (not shown) and taken out to the outside of each SOFC cartridge 203. The DC power conducted out to the outside of the SOFC cartridge 203 by the current collector rod is connected to the generated power of each SOFC cartridge 203 in a predetermined number of series and parallel connections, conducted out to the outside of the SOFC module 201, and converted to a predetermined AC power by a power conversion device (such as an inverter) such as a power conditioner (not shown) and supplied to the power supply destination (for example, a load facility or a power system).

Next, the operation and effects of the fuel cell according to the above embodiment will be described.

(Test specimen preparation)
A cross section of a fuel cell was cut out from the cell stack produced according to the above embodiment to prepare a test specimen.

The materials of each component of the cell stack are as follows:
Base tube: calcium stabilized zirconia (Ca content 15 mol%)
Anode: Ni:YSZ (Y content 8 mol%) = 50:50 (mass ratio), film thickness 120 μm
Solid electrolyte membrane: YSZ (Y content 8 mol%), thickness 80 μm
_{Interconnector : Sr0.9La0.1TiO3}
Air electrode intermediate layer: _{Sm0.2Ce0.8O2 ,} film thickness ₁₅ μm
Air electrode conductive layer: _{La0.5Sr0.25Ca0.25MnO3 , film} thickness 1000 μm, A _/ B ratio=0.95

The sintering conditions for the base tube and interconnector were 1400°C for 4 hours, and the sintering conditions for the air electrode intermediate layer and air electrode conductive layer were 1200°C for 2 hours.

(Composition analysis of the air electrode intermediate layer)
The composition of the air electrode intermediate layer was analyzed using a scanning electron microscope (SEM) "SU6600" (manufactured by Hitachi High-Tech Corporation) equipped with an elemental analysis device. The elemental analysis device was an EDX (Energy Dispersive X-ray spectrometer).
The test specimen was subjected to energy dispersive X-ray analysis (EDX). The analysis results are shown in Figures 4 and 5. Figure 4 is an SEM photograph (field of view size (magnification): 5000 times) of the air electrode intermediate layer portion of the test specimen surface. Figure 5 shows the semi-quantitative analysis result of the air electrode intermediate layer. Since carbon vapor deposition was performed, the semi-quantitative value of C is excluded in Figure 5, and the sum of Ce, Sm, O, Mn, Ca and Sr is set to 100.

In the air electrode intermediate layer in Figure 4, black areas (threshold 0-60.frn), gray areas (threshold 61-195.frn), and white areas (threshold 196-255.frn) were observed. When the total area of the black, gray, and white areas is taken as 100%, the area ratios of the black, gray, and white areas (average values obtained by analyzing three points each) were 17%, 26%, and 55%, respectively. When the total area of the gray and white areas is taken as 100% (no pores), the area ratios of the gray and white areas were 32% and 68%, respectively. The black areas correspond to pores.

According to Figure 5, Ce, Sm, and O were detected in the white area, but Mn, Ca, and Sr were not detected. The white area was a structure (first structure) that mainly contained elements derived from the material of the air electrode intermediate layer. The white area contained 50 atomic % or more of Ce.

In the grey area, Mn, Ca and Sr were detected in addition to elements (Ce, Sm and O) derived from the material of the air electrode intermediate layer. Because Ca, Mn and Sr are not contained in the material of the air electrode intermediate layer, it is believed that the elements contained in the material of the air electrode conductive layer diffused into the air electrode intermediate layer during co-sintering. The grey area contained elements derived from the material of the air electrode conductive layer, and was a structure (second structure) with a different composition from the first structure. The amount of Ce contained in the grey area was less than that in the white area, less than 50 atomic %. According to Figure 5, the grey area contained 12-14 atomic % Mn, 1-2.1 atomic % Ca and 1-2.1 atomic % Sr.

Although not shown in the figure, surface analysis of the test specimen revealed that Mn, Ca, and Sr had each diffused from the cathode conductive layer to the cathode intermediate layer.

(Electronic Conductivity)
Figure 6 shows the effects of Mn, Ca, and Sr on the electronic conductivity of the test specimen (Source: Matsui et al., J. Mater. Chem. A, 2020, 8, 11867-11873). In the figure, the upper horizontal axis is temperature (°C), the lower horizontal axis is temperature (1000T ^-1 /K ^-1 ), and the vertical axis is electronic conductivity (log [σ / Scm ^-1 ]). Figure 7 is a chart summarizing the values read from Figure 6.

The electronic conductivity of _SmMnO3 was 0.3 Scm ^-1 at 600 °C. This value is higher than that of _SmCeO2 without Mn. This suggests that the inclusion of Mn can increase the electronic conductivity of the test specimen.

According to Figure 6, the inclusion of Ca or Sr increased the electronic conductivity. The higher the Ca content, the higher the electronic conductivity. When the Ca content was low, the higher the temperature, the higher the electronic conductivity tended to be. It is presumed that Sr shows a similar tendency to Ca. When the amount of Ca or Sr added was the same as that of Sm, the electronic conductivity was approximately constant regardless of temperature.

The Sm-doped ceria, which is the material of the air electrode intermediate layer, has high ionic conductivity, but low electronic conductivity in the oxidizing atmosphere on the air electrode side. According to Figures 4 to 7, the second structure (gray area) that may have been formed by the diffusion of Mn, Ca and/or Sr from the air electrode conductive layer to the air electrode intermediate layer is thought to have electronic conductivity.

The electronic paths of composites that contain electronically conductive materials within electronically insulating materials can be considered using percolation theory. In percolation theory, it is believed that electronic conductors aggregate above a certain concentration (threshold) and form clusters that connect the entire system, resulting in electronic conductivity. This threshold is called the percolation threshold. Percolation is a theory that examines how the target substances are connected within a system and how these characteristics are reflected in the properties of the diameter. A group formed by connecting certain substances is called a cluster.

Figure 8 shows the relationship between the proportion of the second texture in the air electrode intermediate layer according to the embodiment and the electronic conductivity. In Figure 8, the horizontal axis represents the proportion of the second texture (vol%), and the horizontal axis represents the electronic conductivity (S/cm). The proportion of the second texture was calculated assuming that the total volume of the first texture and the second texture is 100%.

FIG. 8 was estimated from Bruggeman's equation (1).
σ _c is the electronic conductivity of the cathode intermediate layer, V _f is the volume fraction of the second texture, σ _f is the electronic conductivity of the second texture, and σ _m is the electronic conductivity of the first texture. The electronic conductivity of the first texture was assumed to be 0.001 Scm ^-1 . For the electronic conductivity of the second texture, an approximation formula was created for each test piece data in the graph of Figure 6, and an interpolation formula was used based on the correlation between the slope of the approximation formula and the Ca content of the EXP coefficient. The electronic conductivity of Sr was assumed to be equivalent to Ca.

According to Figure 8, as the volume fraction of the second tissue increased, the electronic conductivity also increased. When the volume fraction of the second tissue was 20% or more, electronic conductivity greater than the linear effect was observed. The slope of the graph was greatest when the volume fraction of the second tissue was between 30% and 40%. According to Figure 8, a volume fraction of the second tissue of about 33% is the percolation threshold. The volume fraction can be thought of as the area fraction. The results in Figure 8 are consistent with the results in Figures 4 to 7.

In the air electrode intermediate layer, there is a trade-off between electronic conductivity and ionic conductivity. As the electronically conductive second structure increases, the proportion of the ionically conductive first structure decreases, and the ionic conductivity of the air electrode intermediate layer decreases. Since ionic conductivity is necessary for the air electrode intermediate layer, it is preferable that the proportion of the second structure be 60% or less.

Figure 9 is a schematic cross-sectional view of the air electrode side of a fuel cell according to the above embodiment in which the air electrode intermediate layer includes the second structure.

The air electrode intermediate layer 113a of the fuel cell according to the embodiment includes pores P, a first texture _S1 , and a second texture _S2 . The pores P can serve as oxygen gas diffusion paths. The air electrode intermediate layer 113a is ionically conductive in an oxidizing atmosphere, and therefore functions as an electrolyte. The air electrode intermediate layer 113a including the second texture _S2 has electronic conductivity in addition to ion conductivity. Thus, a three-phase interface is formed inside the air electrode intermediate layer 113a including the second texture _S2 . By imparting electronic conductivity to the air electrode intermediate layer 113a, the three-phase interface (electrode reaction field) can be expanded three-dimensionally, and the reaction resistance can be reduced more than ever before.

The above embodiment can also be applied to a high temperature steam electrolysis (SOEC) cell that produces hydrogen by applying electric power in the same configuration as the fuel cell 105. In that case, the fuel cell 105 in the above embodiment can be replaced with a hydrogen generating unit that generates hydrogen without generating electricity.

<Additional Notes>
The electrochemical cell and the manufacturing method thereof, the electrochemical module, and the electrochemical cartridge described in the above-described embodiments can be understood, for example, as follows.

The electrochemical cell (105) according to the present disclosure has a hydrogen electrode (109), a solid electrolyte membrane (111), and an oxygen electrode (113) laminated in this order, the oxygen electrode including, in order from the solid electrolyte membrane side, an oxygen electrode intermediate layer (113a) and an oxygen electrode conductive layer (113b), the oxygen electrode conductive layer being a sintered body of a material including a perovskite oxide represented by the general formula _ABO3 , in which the A site is La and Sr and/or Ca, and the B site is Mn, the oxygen electrode intermediate layer being a sintered body of a material including Sm-doped ceria, and having pores, a first texture, and a second texture, the first texture including 50 atomic % or more of Ce and not including Ca, Sr, or Mn, the second texture including an element selected from the group consisting of Ca, Sr, and Mn, and the area ratio of the second texture in a cross section of the oxygen electrode intermediate layer being 20% or more and 60% or less of the total area of the first texture and the second texture.

The oxygen electrode conductive layer, which is formed by sintering a material containing the perovskite oxide, has electronic conductivity. The oxygen electrode intermediate layer, which is formed by sintering a material containing Sm-doped ceria, has ionic conductivity. By forming the oxygen electrode into a two-layer structure consisting of an oxygen electrode conductive layer and an oxygen electrode intermediate layer, the interface resistance between the solid electrolyte membrane and the oxygen electrode can be reduced.

The first structure, which is mainly composed of elements derived from Sm-doped ceria, has an ionic conductivity function. The second structure, which contains an element selected from the group consisting of Ca, Sr, and Mn, has an electronic conductivity function. When the total area of the cross section of the oxygen electrode intermediate layer is constant, as the second structure increases, the proportion of the first structure decreases, and the ionic conductivity of the oxygen electrode intermediate layer decreases, and as the first structure increases, the proportion of the second structure decreases, and the electronic conductivity of the oxygen electrode intermediate layer decreases. By setting the area ratio of the second structure in the cross section of the oxygen electrode intermediate layer to 20% to 60% and preferably 30% to 40% of the total area of the first structure and the second structure in the cross section of the oxygen electrode intermediate layer, an electronic path can be formed while securing an ionic path in the oxygen electrode intermediate layer.

By including pores that serve as gas paths, a first ionically conductive structure, and a second electronically conductive structure, the electrode reaction field (three-phase interface) expands three-dimensionally inside the oxygen electrode intermediate layer. This reduces the reaction resistance at the interface between the oxygen electrode conductive layer and the oxygen electrode intermediate layer.

In one embodiment of the disclosure above, the area ratio of the pores in the cross section of the oxygen electrode intermediate layer may be 20% or more of the total area of the pores, the first structure, and the second structure.

By keeping the pore area ratio within the above range, a three-phase interface can be more reliably formed within the oxygen electrode intermediate layer.

An electrochemical module equipped with the electrochemical cell disclosed above, and an electrochemical cartridge equipped with the electrochemical module, have improved power generation efficiency compared to conventional methods.

The method for manufacturing an electrochemical cell according to the present disclosure includes a hydrogen electrode (109), a solid electrolyte membrane (111), and an oxygen electrode (113) laminated in this order, the oxygen electrode including, from the solid electrolyte membrane side, an oxygen electrode intermediate layer (113a) and an oxygen electrode conductive layer (113b), the oxygen electrode conductive layer being a sintered body of a material containing a perovskite oxide represented by the general formula _ABO3 , the A site being La and Sr and/or Ca, and the B site being Mn, and the oxygen electrode intermediate layer being a sintered body of a material containing Sm-doped ceria, and the method includes obtaining conditions under which a structure containing an element selected from the group consisting of Ca, Sr, and Mn is formed in 20% to 60% of a total area of a cross section of the oxygen electrode intermediate layer excluding pores, and co-sintering the oxygen electrode intermediate layer and the oxygen electrode conductive layer in accordance with the obtained conditions to form the oxygen electrode intermediate layer.

In one embodiment of the above disclosure, the conditions may be any one of the material composition of the oxygen electrode conductive layer, the material composition of the oxygen electrode intermediate layer, the sintering temperature of the oxygen electrode intermediate layer and the oxygen electrode conductive layer, and the sintering procedure.

101 Cell stack (fuel cell)
103 Base tube (base)
105 Fuel cell (power generation element) (electrochemical cell)
107 interconnector 109 fuel electrode (hydrogen electrode)
111 Solid electrolyte membrane 113 Air electrode (oxygen electrode)
113a: Air electrode intermediate layer (oxygen electrode intermediate layer)
113b Air electrode conductive layer (oxygen electrode conductive layer)
115 Lead film 201 SOFC (fuel cell) module (electrochemical module)
203 SOFC (Fuel Cell) Cartridge (Electrochemical Cartridge)
205 Pressure vessel 207 Fuel gas supply pipe 207a Fuel gas supply branch pipe 209 Fuel gas exhaust pipe 209a Fuel gas exhaust branch pipe 215 Power generation chamber 217 Fuel gas supply header 219 Fuel gas exhaust header 221 Oxidizing gas supply header 223 Oxidizing gas exhaust header 225a Upper tube plate 225b Lower tube plate 227a Upper heat insulator 227b Lower heat insulator 229a Upper casing 229b Lower casing 231a Fuel gas supply hole 231b Fuel gas exhaust hole 233a Oxidizing gas supply hole 233b Oxidizing gas exhaust hole 235a Oxidizing gas supply gap 235b Oxidizing gas exhaust gap 237a, 237b Seal member

Claims (7)

A hydrogen electrode, a solid electrolyte membrane, and an oxygen electrode are laminated in this order.
the oxygen electrode includes an oxygen electrode intermediate layer and an oxygen electrode conductive layer in this order from the solid electrolyte membrane side,
the oxygen electrode conductive layer is a sintered body of a material containing a perovskite oxide represented by the general formula _ABO3 , in which the A site is La, Sr and/or Ca, and the B site is Mn;
the oxygen electrode intermediate layer is a sintered body of a material containing Sm-doped ceria, and has pores, a first structure and a second structure;
The first structure contains 50 atomic % or more of Ce and does not contain Ca, Sr, or Mn,
the second structure contains an element selected from the group consisting of Ca, Sr, and Mn;
An electrochemical cell comprising: a cross-section of the oxygen electrode intermediate layer in which an area ratio of the second texture is 20% or more and 60% or less of a total area of the first texture and the second texture. The electrochemical cell according to claim 1, wherein the area ratio of the second structure in the cross section of the oxygen electrode intermediate layer is 30% to 40% of the total area of the first structure and the second structure. The electrochemical cell according to claim 2, wherein the area ratio of the pores in the cross section of the oxygen electrode intermediate layer is 5% or more of the total area of the pores, the first structure, and the second structure. An electrochemical module comprising an electrochemical cell according to any one of claims 1 to 3. An electrochemical cartridge comprising the electrochemical module according to claim 4. A hydrogen electrode, a solid electrolyte membrane, and an oxygen electrode are laminated in this order.
the oxygen electrode includes an oxygen electrode intermediate layer and an oxygen electrode conductive layer in this order from the solid electrolyte membrane side,
the oxygen electrode conductive layer is a sintered body of a material containing a perovskite oxide represented by a general formula _ABO3 , in which the A site is La, Sr and/or Ca, and the B site is Mn;
The oxygen electrode intermediate layer is a sintered body of a material containing Sm-doped ceria,
obtaining a condition in which a structure containing an element selected from the group consisting of Ca, Sr, and Mn is formed in an amount of 20% to 60% of a total area of the cross section excluding pores in the cross section of the oxygen electrode intermediate layer;
the oxygen electrode intermediate layer and the oxygen electrode conductive layer are co-sintered according to the obtained conditions to form the oxygen electrode intermediate layer. The method for manufacturing an electrochemical cell according to claim 6, wherein the conditions are any one of the material composition of the oxygen electrode conductive layer, the material composition of the oxygen electrode intermediate layer, the sintering temperature of the oxygen electrode intermediate layer and the oxygen electrode conductive layer, and the sintering procedure.