Secondary battery, catalyst for secondary battery, and method for manufacturing catalyst for secondary battery

Abstract

The problem to be addressed by the present invention is to provide a secondary battery that can be stably charged and discharged, has a prolonged lifespan, and requires less time and effort for maintenance.　A secondary battery 1 has a cell 20 comprising a negative electrode 21, a positive electrode 22, and an electrolyte 23 disposed in contact with the negative electrode 21 and the positive electrode 22 and composed of a gas-impermeable and ion-conductive solid oxide. During charging, carbon dioxide is electrolyzed on the surface of the negative electrode 21, and carbon is deposited on the negative electrode 21 side which is configured as a closed system. A reactor 20 in the closed system comprises: an electrochemical reaction part 20A in which the negative electrode 21 is disposed; and a thermochemical reaction part 20B in which a catalyst 26 for promoting the carbon deposition is disposed. The catalyst 26 disposed in the thermochemical reaction part 20B is a catalyst having a porous substrate composed of one or more types selected from the group consisting of metals and oxides thereof.

Description

Secondary battery, catalyst for the secondary battery, and method for producing the same

[0001] The present invention relates to a secondary battery, and more particularly to a secondary battery (carbon-air secondary battery) that utilizes a combination of electrochemical and thermochemical reactions of carbon. The present invention also relates to a catalyst for such a secondary battery and a method for producing the same.

[0002] Solid oxide fuel cells (SOFCs), which have a laminated structure in which an electrolyte layer (solid electrolyte layer) made of ion-conductive solid oxide (oxide ion conductor) is placed between a positive electrode (air electrode) and a negative electrode (fuel electrode), are expected to be third-generation fuel cells, and their development is currently underway.

[0003] Solid oxide fuel cells use oxide ions (O ２－ The battery uses a solid oxide with high ion conductivity and high permeability to hydrogen (H ２ ), carbon monoxide (CO), methane (CH ４ ) or other reducing agents to generate electrical energy (see, for example, Patent Document 1).

[0004] Furthermore, a solid oxide electrolyzer cell (SOEC) is known as an electrolysis device for water, carbon dioxide, etc. that utilizes the reverse reaction of a solid oxide fuel cell, and a device that can be used as both a solid oxide fuel cell (SOFC) and a solid oxide electrolyzer cell (SOEC) has also been proposed (for example, Patent Document 2).

[0005] To stably supply large amounts of renewable energy-derived electricity, a large-capacity electricity storage technology capable of adjusting the balance between electricity supply and demand is required. As such a technology, the inventors have proposed a carbon / air secondary battery (CASB) (Patent Document 3, Patent Document 4, Non-Patent Document 1).

[0006] Carbon-air secondary batteries are secondary batteries that utilize a combination of electrochemical and thermochemical reactions of carbon, and devices and materials similar to those used in solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) can be used.

[0007] The negative electrode side of a carbon-air secondary battery is a closed system, and carbon and carbon dioxide can be stored inside the closed system. In a carbon-air secondary battery, when charging, carbon dioxide is converted to carbon by an electrochemical reaction at the negative electrode and a thermochemical reaction in the closed system, and carbon is precipitated in the closed system. When discharging, this carbon is used as fuel, converted to carbon dioxide, and stored in the closed system. Therefore, a carbon-air secondary battery can store CO ２ It is emission-free.

[0008] Carbon-air secondary batteries are promising as a compact, large-capacity energy storage technology due to their high theoretical efficiency, theoretical volumetric energy density, and safety. Furthermore, unlike conventional batteries, carbon-air secondary batteries have the advantage that their storage capacity can be increased by increasing the amount of carbon and carbon dioxide stored. This feature makes carbon-air secondary batteries highly promising as a large-capacity energy storage technology.

[0009] Hydrocarbon power generation and carbon dioxide electrolysis are typically operated under conditions that thermodynamically prevent solid carbon deposition. On the other hand, carbon-air secondary batteries deposit solid carbon on the negative electrode during charging and store it as fuel during discharge. Transition metals such as nickel used in solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) are known to have catalytic properties against carbon deposition. Metal catalysts suffer from the problem of degradation due to carbonization.

[0010] Carbon deposition during charging is inevitable in carbon-air secondary batteries. Therefore, how to prevent the deterioration of durability due to carbon deposition is an important issue for the practical application of carbon-air secondary batteries. The development of technology to solve this issue is required.

[0011] Japanese Patent Application Laid-Open No. 9-129256 Japanese Patent Application Laid-Open No. 2010-159458 Japanese Patent No. 7182251 International Publication No. 2024 / 190513

[0012] K. Kameda, S. Manzhos and M. Ihara, J. Power Sources, 516, (2021), 230681

[0013] The present invention has been made in view of the above-mentioned background art, and an object of the present invention is to provide a carbon-air secondary battery that can be stably charged and discharged, has a long life, and requires little maintenance by preventing a decrease in durability due to carbon deposition on the negative electrode side (closed system) of the carbon-air secondary battery.

[0014] The present inventors have conducted extensive research to solve the above problems and have discovered the following facts, which have led to the completion of the present invention.

[0015] In other words, by dividing the negative electrode side (closed system) of the carbon-air secondary battery into an electrochemical reaction section where an electrode (negative electrode) is installed and a thermochemical reaction section where a catalyst that promotes carbon deposition and gasification is installed, carbon deposition becomes easier in the thermochemical reaction section, and carbon deposition on the negative electrode in the electrochemical reaction section can be suppressed, preventing a decrease in the durability of the negative electrode.

[0016] Furthermore, by using a specific material and configuration for the catalyst placed in the thermochemical reaction section, carbon can be more efficiently deposited on the surface of the catalyst placed in the thermochemical reaction section.

[0017] The present invention thus completed is as follows:

[0018] [1] A secondary battery comprising a cell including a negative electrode, a positive electrode, and an electrolyte made of a gas-impermeable and ion-conductive solid oxide placed in contact with the negative electrode and the positive electrode, wherein during charging, carbon dioxide is electrolyzed on the surface of the negative electrode, causing carbon to deposit on the negative electrode side, which forms a closed system, and oxygen is generated at the positive electrode from oxide ions generated at the negative electrode and which have permeated the electrolyte, and during discharging, oxide ions are generated from oxygen at the positive electrode, and carbon and / or carbon monoxide are electrochemically oxidized on the surface of the negative electrode to generate carbon dioxide, wherein a reactor in the closed system has the negative electrode placed inside and is separated from the outside by the electrolyte, and is configured to store carbon deposited during charging, and is composed of an electrochemical reaction section in which the negative electrode is placed, and a thermochemical reaction section in which a catalyst that promotes carbon deposition and gasification is placed, and wherein the catalyst is a catalyst having a porous substrate made of one or more metals and their oxides.

[0019] [2] The secondary battery according to [1], wherein the porous substrate is a porous substrate made of a metal oxide.

[0020] [3] The secondary battery according to [1], wherein the porous substrate is fibrous.

[0021] [4] The secondary battery according to [1], wherein the metal in the porous substrate is one or more metals selected from the group consisting of iron, nickel, cobalt, tungsten, tantalum, palladium, molybdenum, platinum, gold, and copper.

[0022] [5] The secondary battery according to [1], wherein the catalyst is a porous substrate on which a support body composed of a metal and one or more selected from the group consisting of an oxide, carbonate, hydroxide, and chloride thereof is supported.

[0023] [6] The secondary battery according to [5], wherein the metal in the support is one or more metals selected from the group consisting of iron, nickel, cobalt, tungsten, tantalum, palladium, molybdenum, platinum, gold, and copper.

[0024] [7] A catalyst for promoting carbon deposition and gasification for use in the secondary battery according to any one of [1] to [4], characterized in that it has a porous substrate made of one or more materials selected from the group consisting of metals and their oxides.

[0025] [8] A catalyst for promoting carbon deposition for use in the secondary battery according to [5] or [6], characterized in that the catalyst comprises a porous substrate made of at least one selected from the group consisting of metals and their oxides, and a supported body made of at least one selected from the group consisting of metals and their oxides, carbonates, hydroxides, and chlorides.

[0026] [9] A method for producing a catalyst according to [8], characterized in that the porous substrate is impregnated with an aqueous solution containing a metal, and then heat-treated, thereby supporting the supported material on the porous substrate.

[0027] According to the present invention, in a carbon-air secondary battery, carbon deposition on the electrodes (negative electrode) and the like can be prevented, and as a result, a decrease in the durability of the negative electrode side (closed system) is prevented. Therefore, the present invention can provide a carbon-air secondary battery that can be charged and discharged stably, has a long life, and requires little maintenance.

[0028] Furthermore, the catalyst of the present invention can efficiently deposit carbon on its surface and promotes the gasification of carbon, facilitating its consumption. Furthermore, the catalyst of the present invention itself is resistant to deterioration and has a long life. Therefore, the catalyst of the present invention can be used not only in the negative electrode of a carbon-air secondary battery, but also in various technical fields including a process for converting carbon dioxide to carbon (solid carbon) and / or a process for converting carbon (solid carbon) to carbon dioxide.

[0029] 1 is a schematic diagram showing a reaction in a secondary battery of the present invention. (a) During charging (b) During discharging.

[0023] FIG. 1 is a conceptual diagram of a cell, reactor, etc. in a secondary battery of the present invention.

[0024] FIG. 2 is a graph showing the temperature dependency and pressure dependency in an equilibrium state in the Boudouard reaction.

[0025] FIG. 3 is a schematic diagram of a reactor used in Evaluation Examples 1 and 2.

[0026] FIG. 4 is an electron microscope photograph of a catalyst used in Evaluation Example 1. (a) Low magnification (b) High magnification

[0027] FIG. 5 is an electron microscope photograph of a catalyst used in Evaluation Example 2. (a) Low magnification (b) High magnification

[0028] FIG. 6 is a graph showing voltages in the charge step and discharge step of Evaluation Examples 1 and 2.

[0030] The present invention will be described below, but the present invention is not limited to the following embodiments and can be practiced with any modifications.

[0031] The secondary battery of the present invention is a carbon / air secondary battery (CASB). The carbon / air secondary battery was developed by the present inventors, and details thereof are described in Patent Document 3, Patent Document 4, and Non-Patent Document 1.

[0032] A schematic diagram of the vicinity of the electrodes of a secondary battery of the present invention is shown in Figure 1. The secondary battery 1 of the present invention has a cell 10 including a negative electrode 21, a positive electrode 22, and an electrolyte 23 made of a gas-impermeable, ion-conductive solid oxide disposed in contact with the negative electrode 21 and the positive electrode 22.

[0033] The negative electrode 21 side of the secondary battery 1 of the present invention is a closed system 11, and carbon and carbon dioxide are stored in the closed system 11. The positive electrode 22 side of the secondary battery 1 of the present invention is usually an open system 12, and the positive electrode 22 is in contact with oxygen in the air. The oxygen on the positive electrode 22 side is taken into the positive electrode 22 during discharge and participates in the reaction.

[0034] 1(a) is a schematic diagram showing the reaction during charging of the secondary battery 1 of the present invention. During charging, carbon dioxide is electrolyzed on the surface of the negative electrode 21, and carbon is deposited on the negative electrode side, which forms a closed system 11. The specific reaction at the negative electrode 21 during charging is presumed to be as follows.

[0035] CO ２ +2e － → CO+O ２－ (R1) 2CO ←→ CO ２ + C (R2)

[0036] (R1) is a reduction reaction (electrochemical reduction reaction) on the surface of the negative electrode 21. (R2) is a thermochemical reaction between solid carbon, carbon dioxide, and carbon monoxide, known as the Boudouard reaction. From (R1) and (R2), the total negative electrode reaction during charging is as follows:

[0037] CO ２ +4e － → C + 2O ２－ (R3)

[0038] In the case of charging, the overvoltage is generally large, so it is thought to be difficult to electrochemically reduce carbon dioxide directly to carbon (directly causing (R3)). In addition, the overvoltage is also large, so it is thought to be difficult to produce carbon by reducing carbon monoxide produced by (R1) (reaction formula (R4) below). Therefore, the energy efficiency of carbon dioxide electrolysis is low.

[0039] CO + 2e － → C+O ２－ (R4)

[0040] Therefore, in the secondary battery 1 of the present invention, which is a carbon-air secondary battery, carbon monoxide is generated on the surface of the negative electrode 21 by a reduction reaction (R1), and the generated carbon monoxide is converted into carbon dioxide and carbon by a thermochemical reaction (R2), thereby depositing carbon on the side of the negative electrode 21.

[0041] The negative electrode 21 side is a closed system 11, and during charging, carbon (solid carbon) does not necessarily deposit only on the surface of the negative electrode 21, but may also deposit in places other than the surface of the negative electrode 21 (such as the wall surface of the closed system). If carbon (solid carbon) deposits on the surface of the negative electrode 21, it may cause deterioration of the negative electrode 21. Therefore, it is desirable to make it easier for carbon to deposit in places other than the surface of the negative electrode 21 as much as possible. In the present invention, the negative electrode 21 side (closed system 11) is divided into an electrochemical reaction section 20A in which an electrode (negative electrode 21) is installed, and a thermochemical reaction section 20B in which a catalyst 26 that promotes carbon deposition and gasification is installed, and is configured to make it easier for carbon to deposit on the catalyst 26.

[0042] During charging, oxide ions (O ２－ ) to oxygen (O ２ ) is produced (reaction (R5) below).

[0043] O ２－ → 1 / 2O ２ +2e － (R5)

[0044] 1B is a schematic diagram showing the reaction during discharge of the secondary battery 1 of the present invention. During discharge, oxide ions are produced from oxygen at the positive electrode 22 (reaction (R6) below).

[0045] 1 / 2O ２ +2e － → O ２－ (R6)

[0046] The side of the positive electrode 22 is usually an open system 12. The oxygen taken into the positive electrode 22 during discharge may be oxygen in the air or pure oxygen supplied from a gas cylinder or the like. From the viewpoint of cost, etc., it is desirable to use oxygen in the air.

[0047] During discharge, carbon dioxide is produced by electrochemically oxidizing carbon and / or carbon monoxide on the surface of the negative electrode 21. The specific reaction at the negative electrode 21 during discharge is presumed to be as follows.

[0048] C+O ２－→ CO + 2e － (R7) C + 2O ２－ → CO ２ +4e － (R8) CO+O ２－ → CO ２ +2e － (R9) 2CO ←→ CO ２ + C (R2)

[0049] (R7) to (R9) are oxidation reactions (electrochemical oxidation reactions) on the surface of the negative electrode 21. (R2) is the thermochemical reaction between the solid carbon and carbon dioxide and carbon monoxide. The total negative electrode reaction during discharge is as follows:

[0050] C + 2O ２－ → CO ２ +4e － (R10)

[0051] During discharge, carbon attached to the surface of the negative electrode 21 is oxidized by oxide ions to produce carbon monoxide and carbon dioxide (reactions (R7) and (R8)). In addition, carbon monoxide is oxidized by oxide ions on the surface of the negative electrode 21 to produce carbon dioxide (reaction (R9)). Furthermore, during discharge, a thermochemical reaction (R2) occurs between solid carbon and carbon dioxide and carbon monoxide.

[0052] During charging, carbon (solid carbon) does not necessarily deposit only on the surface of the negative electrode 21, but may also deposit in places other than the surface of the negative electrode 21. The carbon (solid carbon) deposited in places other than the surface of the negative electrode 21 is first converted to carbon monoxide by thermochemical reaction (R2) and then contributes to discharge by reaction (R9). As described above, from the viewpoint of preventing deterioration of the negative electrode 21, it is desirable to facilitate carbon deposition in places other than the surface of the negative electrode 21 during charging. That is, it is desirable that the main reaction pathways at the negative electrode during discharge are (R2) and (R9), rather than (R7) or (R8).

[0053] 2 shows a conceptual diagram of the cell 10, reactor 20, etc. in the secondary battery 1 of the present invention. Fig. 2 is a conceptual diagram and does not show the specific shapes of the cell 10, reactor 20, etc.

[0054] The reactor 20 in the closed system 11 of the secondary battery 1 of the present invention has the negative electrode 21 installed inside and is separated from the outside by the electrolyte 23, and is configured to store carbon deposited during charging inside. Note that "the reactor 20 is separated from the outside by the electrolyte 23" means that no exchange of gas occurs between the inside and outside of the reactor 20.

[0055] The electrolyte 23 is made of a gas-impermeable and ion-conductive solid oxide. The electrolyte 23 converts oxide ions (O ２－ ) but does not allow the gases involved in the reaction (oxygen, carbon dioxide, carbon monoxide) to pass through.

[0056] In the secondary battery 1 of the present invention, the negative electrode 21 side is a closed system 11, so that the carbon monoxide partial pressure increases by charging, and carbon is easily deposited. In addition, because the negative electrode 21 side is a closed system 11, carbon dioxide generated on the negative electrode 21 side by discharge is returned to carbon by charging. ２ No emissions.

[0057] The secondary battery 1 of the present invention is configured to store carbon generated by charging and carbon dioxide generated by discharging inside a closed system 11 including a reactor 20. The closed system 11 may have a tank for storing carbon dioxide generated by discharging in addition to the reactor 20. Details of such a tank are described in the specification of Patent Document 4.

[0058] The reactor 20 in the closed system 11 of the secondary battery 1 of the present invention is a reactor consisting of an electrochemical reaction section 20A in which the anode 21 is installed, and a thermochemical reaction section 20B in which a catalyst 26 that promotes carbon deposition is installed. The electrochemical reaction section 20A and the thermochemical reaction section 20B are connected by a pipe 41, and gas flows between them.

[0059] The electrochemical reaction field and the thermochemical reaction field are separated on the negative electrode 21 side of the secondary battery 1 of the present invention. Therefore, in the secondary battery 1 of the present invention, the temperature and pressure of the electrochemical reaction field (electrochemical reaction section 20A) and the thermochemical reaction field (thermochemical reaction section 20B) can be controlled separately.

[0060] The thermochemical reaction section 20B is provided with a catalyst 26 that promotes carbon deposition. In the thermochemical reaction section 20B, the Boudouard reaction (R2) described above occurs.

[0061] FIG. 3 shows the temperature and pressure dependence of the Boudouard reaction (R2) at equilibrium. (R2) leans to the left side at high temperatures and low pressures, and leans to the right side at low temperatures and high pressures. During charging, it is desirable to set conditions that facilitate carbon storage in the thermochemical reaction unit 20B, i.e., to set the thermochemical reaction unit 20B at low temperatures and high pressures. During discharging, it is desirable to set conditions that actively convert the carbon stored in the thermochemical reaction unit 20B to carbon monoxide, i.e., to set the thermochemical reaction unit 20B at high temperatures and low pressures.

[0062] The catalyst 26 that promotes carbon deposition and is installed in the thermochemical reaction section 20B of the secondary battery 1 of the present invention is a catalyst having a porous substrate made of one or more materials selected from the group consisting of metals and their oxides.

[0063] In this specification, the term "porous substrate" refers to any object having pore-like spaces inside. An example of a "porous substrate" is a sponge-like object, but the term "porous substrate" is not limited to such objects. For example, a "porous substrate" also includes an object in which pore-like spaces are formed inside by randomly assembling fibrous objects. For example, FIG. 5 is an electron microscope photograph of the catalyst (a fibrous porous substrate made of iron fibers) used in Evaluation Example 1 described below.

[0064] The porous substrate in the catalyst 26 of the present invention is made of one or more materials selected from the group consisting of metals and their oxides.

[0065] Examples of metals that can be used to form the porous substrate include iron (Fe), nickel (Ni), cobalt (Co), tungsten (W), tantalum (Ta), palladium (Pd), molybdenum (Mo), platinum (Pt), gold (Au), copper (Cu), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), zinc (Zn), aluminum (Al), indium (In), and magnesium (Mg). The porous substrate may be formed from an oxide of any of the above metals. Alternatively, the porous substrate may be formed from a composite oxide containing two or more of the above metals.

[0066] Since carbon deposits on the surface of the porous substrate during charging and gasifies and desorbs from the surface during discharging, the porous substrate should be made of a material that is resistant to deterioration due to carbon deposition, and therefore, the porous substrate is preferably made of a metal oxide.

[0067] The porous substrate may be made of only one of the above-mentioned materials, or may be made of two or more of the above-mentioned materials.

[0068] The porous substrate in the catalyst 26 of the present invention has a porous substrate composed of one or more selected from the group consisting of metals and their oxides, and the porous substrate has the effect of promoting carbon deposition and gasification. The porous substrate has pore-like spaces inside, which gives it a large surface area, thereby increasing the amount of carbon deposition on the surface of the porous substrate. This makes it easier to prevent carbon deposition on other parts of the negative electrode 21, and the secondary battery 1 of the present invention has good durability on the negative electrode 21 side.

[0069] The porosity of the porous substrate (the volume of the voids in the porous substrate divided by the volume occupied by the porous substrate, expressed as a percentage) is preferably 90% or more, more preferably 95% or more, and particularly preferably 97% or more. The porosity is preferably 99.99% or less, more preferably 99.9% or less, and particularly preferably 99.5% or less. When the porosity is within the above range, the amount of carbon deposition on the surface of the porous substrate increases, improving the durability of the negative electrode 21 and facilitating stable charge / discharge characteristics of the secondary battery.

[0070] The catalyst 26 of the present invention has the porous substrate described above. The catalyst 26 may consist of only the porous substrate, or may be a combination of the porous substrate and other elements that do not have catalytic activity. An example of the "other elements" is ceramic that serves as a support for fixing (supporting) the porous substrate in the thermochemical reaction section 20B. An example of such ceramic is alumina (Al ２ O ３ ), zirconia (ZrO ２ ), ceria (CeO ２－δ ), quartz (SiO ２ ), silicon carbide (SiC), ceramics containing these with additives, and composite ceramics of these.

[0071] The catalyst 26 for promoting carbon deposition, which is installed in the thermochemical reaction section 20B of the secondary battery 1 of the present invention, may be a supported material made of one or more selected from the group consisting of metals, and their oxides, carbonates, nitrates, sulfates, hydroxides, and halides (fluorides, chlorides, bromides, and iodides), supported on the porous substrate.

[0072] An example of such a catalyst 26 is shown in Figure 6. Figure 6 is an electron microscope photograph of the catalyst used in Evaluation Example 2, which will be described later, in which iron is supported as a support on a fibrous porous substrate made of iron fibers. In the high-magnification image (Figure 6(b)), particulate iron can be seen adhering to the surface of the fibers.

[0073] Examples of metals constituting the "supported material" supported on the porous substrate include iron, nickel, cobalt, tungsten, tantalum, palladium, molybdenum, platinum, gold, copper, titanium, vanadium, chromium, manganese, zinc, aluminum, indium, and magnesium. The supported material may also be an oxide, carbonate, hydroxide, or chloride of any of the above metals.

[0074] The support may also be made of carbonates, nitrates, sulfates, halides (fluorides, chlorides, bromides, iodides), hydroxides, or oxides of alkali metals or alkaline earth metals. Examples of such alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). Examples of such alkaline earth metals include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba).

[0075] Among carbonates, nitrates, sulfates, halides, hydroxides, and oxides of alkali metals and alkaline earth metals, there are some (molten salts) that have low melting points and are in a molten state (liquid state) in the operating temperature range of the secondary battery of the present invention. Molten salts are in a liquid state when carbon precipitates, and therefore have the advantage of being less susceptible to deterioration.

[0076] Specific examples of the molten salt include lithium carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, lithium nitrate, sodium nitrate, potassium nitrate, rubidium nitrate, strontium nitrate, and barium nitrate. Two or more of these molten salts may be used in combination to form the supported material.

[0077] When the support is made of an oxide, the oxide may be a composite oxide containing two or more of the above-mentioned metals.

[0078] The support may be made of only one of the above-mentioned materials, or may be made of two or more of the above-mentioned materials.

[0079] As described above, the porous substrate in the catalyst 26 of the present invention has the function of promoting carbon deposition and gasification, and the supported material supported on the porous substrate also has the function of promoting carbon deposition and gasification. By supporting the supported material on the porous substrate, carbon is preferentially deposited on the surface of the supported material, making it easier to suppress deterioration of the porous substrate.

[0080] Metals such as iron, nickel, cobalt, tungsten, tantalum, palladium, molybdenum, platinum, gold, copper, titanium, vanadium, chromium, manganese, zinc, aluminum, indium, and magnesium, as well as oxides of these metals, can be used as the material for the porous substrate and the material for the support. In the catalyst 26 of the present invention, the material for the porous substrate and the material for the support may be the same or different.

[0081] An example of a method for supporting a supported material on a porous substrate is a method in which the porous substrate is impregnated with a metal-containing aqueous solution (precursor solution) and then heat-treated. That is, the present invention also relates to a method for producing a catalyst in which a supported material is supported on a porous substrate, characterized in that the porous substrate is impregnated with a metal-containing aqueous solution and then heat-treated, thereby supporting the supported material on the porous substrate.

[0082] The precursor solution may be, for example, an aqueous solution containing a nitrate or sulfate of the metal desired to be supported. The heat treatment is preferably carried out at a temperature in the range of 100° C. to 600° C.

[0083] The catalyst 26 of the present invention is a catalyst that promotes carbon deposition, and can be used as a catalyst for the secondary battery (carbon-air secondary battery) 1 described above.

[0084] Furthermore, the catalyst 26 of the present invention has the effect of efficiently depositing carbon on its surface, making the catalyst itself less susceptible to deterioration, and having a long lifespan. Such an effect can be said to be useful in general technologies that include a process of converting carbon dioxide into carbon (solid carbon). Therefore, the catalyst 26 of the present invention can be applied to applications other than carbon-air secondary batteries.

[0085] The negative electrode 21 of the secondary battery 1 of the present invention includes a negative electrode material, a negative electrode current collector, and the like.

[0086] The negative electrode material is, for example, a composite metal oxide or a cermet. Here, "cermet" refers to a mixture of metal and metal oxide powders that is sintered. The composite metal oxide or cermet is preferably porous.

[0087] As the composite metal oxide, stabilized zirconia (Y) containing yttria is used in order to ensure sufficient output characteristics and durability during discharge. ２ O ３ -ZrO ２ ) (hereinafter, sometimes referred to as "YSZ"); CeO doped with at least one element selected from the group consisting of Gd, La, Y, Sm, Nd, Ca, Mg, Sr, Ba, Dy, Pr, Eu, Er, Ho, Lu, and Yb ２ [Especially Gd-doped CeO ２ (hereinafter, sometimes referred to as "GDC"), Sm-doped CeO ２ ]; Sc ２ O ３ -ZrO ２ (hereinafter sometimes referred to as "ScSZ"); Sm ２ O ３ -CeO ２ (hereinafter sometimes referred to as "SDC"); LaGaO ３ ; etc. are particularly preferred.

[0088] In order to obtain sufficient output characteristics during discharge, the composite metal oxide preferably has a conductivity of 0.01 to 10 S / cm at 1000°C.

[0089] The cermet is preferably a cermet of at least one metal selected from the group consisting of Ni, Pt, Au, Cu, Fe, W, and Ta and a composite metal oxide (particularly the above-mentioned composite metal oxide). From the viewpoint of reliably obtaining sufficient output characteristics during discharge, a cermet of nickel and a composite metal oxide is preferred, and a cermet of nickel and the above-mentioned composite metal oxide is particularly preferred.

[0090] Most preferred, in terms of output characteristics, are a cermet of nickel and YSZ (hereinafter, sometimes referred to as "Ni / YSZ"), a cermet of nickel and GDC (hereinafter, sometimes referred to as "Ni / GDC"), a cermet of nickel and ScSZ (hereinafter, sometimes referred to as "Ni / ScSZ"), or a cermet of nickel and SDC (hereinafter, sometimes referred to as "Ni / SDC").

[0091] The shape and constituent materials of the current collector of the negative electrode 21 are not particularly limited as long as it has electronic conductivity and is chemically and physically stable in the operating temperature range of the secondary battery of the present invention, and the same ones as those provided in known solid oxide fuel cells (SOFCs) can be used.

[0092] The positive electrode 22 of the secondary battery 1 of the present invention includes a positive electrode material, a positive electrode current collector, and the like.

[0093] The composition and shape of the cathode material are not particularly limited, and the same materials as those generally used in cathodes of known solid oxide fuel cells (SOFCs) can be used. For example, (LaSr)MnO ３ (hereinafter sometimes referred to as "LSM") system, (LaSr)CoO ３ (hereinafter, sometimes referred to as "LSC")-based composite metal oxide materials can be preferably used. ０．８５ Sr ０．１５ MnO ３ etc.

[0094] The configuration of the current collector of the positive electrode 22 is the same as that of the current collector of the negative electrode 21 described above, and the constituent material and shape are not particularly limited, and the same materials as those provided in known solid oxide fuel cells (SOFCs) can be used.

[0095] The electrolyte 23 is an ion-conductive solid oxide. The electrolyte 23 contains oxide ions (O ２－) and also functions as a partition wall to prevent direct contact between the reducing agent (the solid carbon described above) and an oxygen-containing gas (e.g., air), and has a gas-impermeable, dense structure. The constituent material of this electrolyte 23 is not particularly limited, and any known material used in solid oxide fuel cells (SOFCs) can be used as appropriate, but it is preferable to form the electrolyte from a material that has high oxide ion conductivity, is chemically stable, and is resistant to thermal shock under conditions ranging from the oxidizing atmosphere on the positive electrode 22 side to the reducing atmosphere on the negative electrode 21 side.

[0096] Preferred examples of materials that satisfy these requirements include stabilized zirconia such as yttria-stabilized zirconia (YSZ) and scandia-stabilized zirconia (ScSZ); lanthanum gallate; and ceria-based solid solutions.

[0097] Furthermore, from the viewpoint of obtaining sufficient output characteristics during power generation, the conductivity of the electrolyte 23 is preferably 0.01 to 10 S / cm at 1000°C.

[0098] The material forming the portion (reactor wall) inside the reactor 20 where the electrolyte 23 is not provided is, for example, mullite, alumina, silicon carbide, quartz, or boron nitride. For example, it is desirable to use the above-mentioned materials because of cost reduction and ease of processing.

[0099] As for more specific shapes of the reactor 20, the cells 10, etc. of the secondary battery 1 of the present invention, shapes described in Patent Document 4 can be appropriately adopted.

[0100] The present invention will be explained in more detail below by way of evaluation examples. The evaluation examples described here are intended to demonstrate that the present invention is theoretically possible to implement and that the effects of the present invention described in this specification can be obtained, but do not necessarily represent modes for implementing the present invention.

[0101] Evaluation Example 1 (Fabrication of Secondary Battery) A carbon-air secondary battery (CASB) was fabricated in which the reactor was separated into an electrochemical reaction section 20A and a thermochemical reaction section 20B, as shown in Fig. 4. The peripheries of the electrochemical reaction section 20A and the thermochemical reaction section 20B could be heated separately by heating means 20C and 20D, respectively.

[0102] Iron fibers (porous substrate made of iron) with a diameter of 20 μm were placed inside the thermochemical reaction section 20B as the catalyst 26. The bulk density of the catalyst (porous substrate) was 0.053 g / cm ３ and the density of iron is 7.87 g / cm ３ Therefore, the porosity of the catalyst (porous substrate) was 99.3%. An electron microscope photograph of this catalyst (porous substrate) is shown in Figure 5 (hereinafter, the catalyst shown in Figure 5 will be referred to as "Catalyst X").

[0103] (Charging step) Gas was introduced from the side of the electrochemical reaction unit 20A, and the gas that passed through the electrochemical reaction unit 20A (negative electrode 21) was sent to the thermochemical reaction unit 20B, and the gas that passed through the thermochemical reaction unit 20B was exhausted to the outside. The temperature of the electrochemical reaction unit 20A was set to 900°C, the temperature of the thermochemical reaction unit 20B was set to 670°C, and charging was performed at 52 mA with a mixed gas of 160 sccm of carbon monoxide and 40 sccm of carbon dioxide supplied to the electrochemical reaction unit 20A. The direction of gas flow during the charging step is shown by the solid line in Figure 4.

[0104] (Discharge step) After the charge step was completed, gas was introduced from the side of the thermochemical reaction unit 20B, and the gas that had passed through the thermochemical reaction unit 20B (catalyst 26) was sent to the electrochemical reaction unit 20A, and the gas that had passed through the electrochemical reaction unit 20A (negative electrode 21) was discharged to the outside. The direction of the gas flow was changed so that the temperature of the electrochemical reaction unit 20A was 900 ° C., the temperature of the thermochemical reaction unit 20B was 670 ° C., 40 sccm of carbon dioxide was supplied to the thermochemical reaction unit 20B, and oxygen was supplied to the positive electrode 22 to maintain a constant oxygen partial pressure. Discharge was performed at 52 mA. The direction of gas flow during the discharge step is shown by the dashed line in FIG. 4.

[0105] Evaluation Example 2 The iron fiber used in Evaluation Example 1 was immersed in an aqueous solution of iron nitrate with a concentration of 0.2 mol / L, and then immersed in 4% H ２ A catalyst was produced in which iron particles were supported on a porous iron substrate by heat treatment at 500°C in an -Ar mixed gas atmosphere. Figure 6 shows an electron microscope photograph of this catalyst (a catalyst in which a support is supported on a porous substrate) (hereinafter, the catalyst shown in Figure 6 will be referred to as "Catalyst Y").

[0106] A carbon-air secondary battery was fabricated in the same manner as in Evaluation Example 1, except that catalyst Y was installed inside the thermochemical reaction section 20B instead of catalyst X, and a charging step and a discharging step were carried out.

[0107] 7 shows the voltages in the charge and discharge steps of Evaluation Examples 1 and 2. In Evaluation Example 2, which used Catalyst Y (a catalyst in which a support body is supported on a porous substrate), almost no decrease in voltage was observed over time during discharge. This suggests that the use of Catalyst Y allows stable carbon deposition and gasification.

[0108] The secondary battery of the present invention is used, for example, as a secondary battery for business use such as in factories and hospitals, for general households, and for transportation equipment such as automobiles, trains, and ships.

[0109] REFERENCE SIGNS LIST 1 secondary battery 10 cell 11 closed system 12 open system 20 reactor 20A electrochemical reaction section 20B thermochemical reaction section 20C heating means 20D heating means 21 negative electrode 22 positive electrode 23 electrolyte 26 catalyst 41 piping

Claims

1. A secondary battery comprising a cell comprising a negative electrode, a positive electrode, and an electrolyte made of a gas-impermeable, ion-conductive solid oxide placed in contact with the negative electrode and the positive electrode; during charging, carbon dioxide is electrolyzed on the surface of the negative electrode, depositing carbon on the negative electrode side, which forms a closed system, and oxygen is generated at the positive electrode from oxide ions generated at the negative electrode and passing through the electrolyte; during discharging, oxide ions are generated from oxygen at the positive electrode, and carbon and / or carbon monoxide are electrochemically oxidized on the surface of the negative electrode to generate carbon dioxide; the reactor in the closed system has the negative electrode placed inside and is separated from the outside by the electrolyte, and is configured to store carbon deposited during charging, and is a reactor consisting of an electrochemical reaction section in which the negative electrode is placed, and a thermochemical reaction section in which a catalyst is placed to promote carbon deposition and gasification; and the catalyst is a catalyst having a porous substrate made of one or more types selected from the group consisting of metals and their oxides.

2. The secondary battery according to claim 1, wherein the porous substrate is a porous substrate made of a metal oxide.

3. The secondary battery according to claim 1, wherein said porous substrate is fibrous.

4. The secondary battery according to claim 1, wherein the metal in said porous substrate is one or more metals selected from the group consisting of iron, nickel, cobalt, tungsten, tantalum, palladium, molybdenum, platinum, gold and copper.

5. The secondary battery according to claim 1, wherein the catalyst is a porous substrate carrying a support composed of at least one metal selected from the group consisting of metal oxides, carbonates, nitrates, sulfates, hydroxides and halides.

6. The secondary battery according to claim 5, wherein the metal in the support is one or more metals selected from the group consisting of iron, nickel, cobalt, tungsten, tantalum, palladium, molybdenum, platinum, gold and copper.

7. A catalyst for promoting carbon deposition and gasification for a secondary battery according to any one of claims 1 to 4, characterized in that it has a porous substrate made of one or more materials selected from the group consisting of metals and their oxides.

8. A catalyst for promoting carbon deposition for use in a secondary battery as described in claim 5 or 6, characterized in that the catalyst comprises a porous substrate made of at least one selected from the group consisting of metals and their oxides, and a supported body made of at least one selected from the group consisting of metals and their oxides, carbonates, nitrates, sulfates, hydroxides and halides.

9. A method for producing a catalyst according to claim 8, characterized in that the porous substrate is impregnated with an aqueous solution containing a metal and then heat-treated, thereby supporting the supported material on the porous substrate.

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