Fuel cell system

Abstract

To suppress a decrease in power generation performance in a dry environment in a fuel cell system equipped with an open cathode polymer electrolyte fuel cell.SOLUTION: A fuel cell system includes an open-cathode polymer electrolyte fuel cell including an anode catalyst layer and a cathode catalyst layer, and a hydrogen gas supply device for supplying hydrogen gas to the anode catalyst layer. The cathode catalyst layer includes tin oxide-based particles as a cathode catalyst support. The fuel cell system may further include a humidifier for humidifying the hydrogen gas and / or a fan for introducing outside air to the cathode catalyst layer, wherein the humidifier and / or the fan are provided between the hydrogen gas supply device and the anode catalyst layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fuel cell system, and more particularly to a fuel cell system equipped with an open cathode polymer electrolyte fuel cell. [Background technology]

[0002] A polymer electrolyte fuel cell (PEFC) comprises a membrane electrode assembly (MEA) in which catalyst layers are bonded to both sides of an electrolyte membrane. A gas diffusion layer is usually disposed on the outside of the catalyst layer. Furthermore, a current collector (separator) with gas flow channels is disposed on the outside of the gas diffusion layer. A PEFC typically comprises a structure (fuel cell stack) in which multiple unit cells each consisting of such an MEA, gas diffusion layer, and current collector are stacked.

[0003] Fuel cells are broadly classified into closed cathode fuel cells and open cathode fuel cells depending on the method of supplying oxidant gas to the cathode. A "closed cathode fuel cell" is a type of fuel cell that supplies outside air, the pressure and humidity of which have been adjusted using auxiliary devices such as a compressor and a humidifier, to the cathode. An "open cathode fuel cell" refers to a type of fuel cell in which outside air is supplied directly to the cathode.

[0004] Open cathode fuel cells are further broadly classified into forced convection type and free breathing type. "Forced convection" refers to a type of fuel cell that uses a fan to supply outside air to the cathode. Forced convection fuel cells are being considered for use in small mobility vehicles. "Naturally aspirated" refers to a type of fuel cell in which outside air is supplied to the cathode by diffusion alone, without the use of a fan. Naturally aspirated fuel cells are being considered for application in small portable devices.

[0005] Because open cathode fuel cells directly use outside air as a cooling medium and oxidant gas for the cells, they do not require auxiliary equipment such as a compressor to supply outside air or a humidifier to humidify the outside air. As a result, fuel cell systems equipped with open cathode fuel cells are generally more compact than fuel cell systems equipped with closed cathode fuel cells, and the overall system cost is also lower (see Non-Patent Document 1).

[0006] Various proposals have been made regarding such open cathode fuel cells. For example, Patent Document 1 discloses a fuel cell that is not a polymer electrolyte fuel cell, but a solid oxide fuel cell having an open cathode structure and divided into a plurality of cell modules; an internal manifold for connecting the cathode flow paths of a plurality of cell modules in series; a cooling gas supply pipe for supplying cooling air to the internal manifold; A fuel cell stack comprising:

[0007] The same document states: (A) When the cathode flow paths of the cell modules are connected in series by an internal manifold and cooling air is supplied to the internal manifold, the high-temperature air discharged from the outlet of the cathode flow path of the upstream cell module is mixed with the cooling air, causing a decrease in temperature, and the cooled air is supplied to the cathode flow path of the next cell module; and (B) This reduces the temperature distribution inside the cell stack. is stated.

[0008] In an open-cathode polymer electrolyte fuel cell, outside air is introduced directly into the cell without passing through a humidifier. Therefore, depending on the temperature and humidity of the outside air, the environment inside the cell can become dry. For example, if the operating temperature of the cell is 40°C, and outside air at a temperature of 25°C and a relative humidity of 100% is introduced into the cell, the relative humidity inside the cell will be 43%. Similarly, if outside air at a temperature of 25°C and a relative humidity of 50% is introduced into the cell, the relative humidity inside the cell will be 22%.

[0009] Carbon is used as the cathode catalyst support in conventional open cathode polymer electrolyte fuel cells (see Non-Patent Documents 2 and 3). However, because carbon is water-repellent, moisture in the catalyst layer is likely to decrease in a dry environment. As a result, the proton conductivity of the ionomer in the catalyst layer decreases, leading to a decline in power generation performance. Furthermore, the cathode catalyst support of a polymer electrolyte fuel cell is exposed to harsh conditions, and depending on the operating conditions, the cathode catalyst support may deteriorate. However, there has not been a fuel cell system proposed to date that includes an open-cathode polymer electrolyte fuel cell, has high power generation performance in a dry environment, and is also highly durable. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent Publication No. 2021-044164 [Non-patent literature]

[0011] [Non-Patent Document 1] Jundika G. Kurnia et al., Appl. Energy, 2021, 283, 116359 [Non-patent document 2] Zi'ang Xiong et al., Int. J. Hydrogen Energy, 2016, 41, 9191 [Non-patent document 3] Robert W. Atkinson III et al., ECS Trans., 2017, 80, 461 Summary of the Invention [Problem to be solved by the invention]

[0012] The problem to be solved by the present invention is to suppress a decrease in power generation performance in a dry environment in a fuel cell system equipped with an open cathode polymer electrolyte fuel cell. Another problem to be solved by the present invention is to suppress a decrease in durability in a fuel cell system equipped with an open cathode polymer electrolyte fuel cell. [Means for solving the problem]

[0013] In order to solve the above problems, the fuel cell system according to the present invention comprises: an open cathode polymer electrolyte fuel cell including an anode catalyst layer and a cathode catalyst layer; a hydrogen gas supply device for supplying hydrogen gas to the anode catalyst layer; Equipped with The cathode catalyst layer contains tin oxide particles as a cathode catalyst support. [Effects of the Invention]

[0014] In a fuel cell system equipped with an open cathode polymer electrolyte fuel cell, the use of tin oxide particles as a cathode catalyst support provides the following effects. (a) Because tin oxide particles are hydrophilic, moisture is easily retained within the cathode catalyst layer, preventing a decrease in power generation performance in a dry environment. (b) When the tin oxide-based particles are porous and have mesopores, and the cathode catalyst particles are supported within the mesopores, the deterioration of power generation performance due to catalyst poisoning is suppressed. (c) Carbon supports are oxidized and deteriorated when exposed to a high potential, whereas tin oxide particles are not oxidized and deteriorated even when exposed to a high potential. (d) In an open cathode polymer electrolyte fuel cell, the potential of the cathode catalyst layer does not drop to near the hydrogen potential, so the destabilization of tin oxide particles at low potential does not become a problem. [Brief explanation of the drawings]

[0015] [Figure 1]FIG. 1 is a schematic diagram of a fuel cell system (with anode humidification) according to a first embodiment of the present invention. [Figure 2] FIG. 10 is a schematic diagram of a fuel cell system (without anode humidification) according to a second embodiment of the present invention.

[0016] [Figure 3] This is the IV curve of an open cathode fuel cell system (with anode humidification) at an outside air temperature of 30°C and an outside air humidity of 50%. [Figure 4] FIG. 10 is a graph showing the dependence of ORR mass activity at 0.84 V on ambient humidity (ambient temperature 30° C.) in an open cathode fuel cell system (with anode humidification). [Figure 5] FIG. 10 is a graph showing the dependence of current density at 0.6 V on ambient humidity (ambient temperature 30° C.) in an open cathode fuel cell system (with anode humidification).

[0017] [Figure 6] This is the IV curve of an open cathode fuel cell system (with anode humidification) at an outside air temperature of 25°C and an outside air humidity of 50%. [Figure 7] FIG. 10 is a graph showing the dependence of ORR mass activity at 0.84 V on ambient humidity (ambient temperature 25° C.) in an open cathode fuel cell system (with anode humidification). [Figure 8] FIG. 10 is a diagram showing the dependence of current density at 0.6 V on ambient humidity (ambient temperature 25° C.) in an open cathode fuel cell system (with anode humidification).

[0018] [Figure 9] This is the IV curve of an open cathode fuel cell system (without anode humidification) at an outside air temperature of 30°C and an outside air humidity of 50%. [Figure 10] This is the IV curve of an open cathode fuel cell system (without anode humidification) at an outside air temperature of 25°C and an outside air humidity of 50%. [Figure 11] ORR mass activity at 0.84 V (ambient air humidity 50%) for an open cathode fuel cell system (without anode humidification). [Figure 12] Current density at 0.6 V (outside air humidity 50%) of an open cathode fuel cell system (without anode humidification). DETAILED DESCRIPTION OF THE INVENTION

[0019] [Configuration 1] an open cathode polymer electrolyte fuel cell including an anode catalyst layer and a cathode catalyst layer; a hydrogen gas supply device for supplying hydrogen gas to the anode catalyst layer; Equipped with The cathode catalyst layer contains tin oxide particles as a cathode catalyst support. Fuel cell system.

[0020] [Configuration 2] 2. The fuel cell system according to configuration 1, further comprising a humidifier provided between the hydrogen gas supply device and the anode catalyst layer for humidifying the hydrogen gas.

[0021] [Configuration 3] 3. The fuel cell system according to configuration 1 or 2, further comprising a fan for introducing outside air into the cathode catalyst layer.

[0022] [Configuration 4] The cathode catalyst layer is an electrode catalyst in which Pt-based particles are supported on the surfaces of the tin oxide-based particles; Catalyst layer ionomer and 4. The fuel cell system according to any one of configurations 1 to 3, comprising:

[0023] [Configuration 5] 5. The fuel cell system according to any one of the above aspects 1 to 4, wherein the tin oxide based particles are made of SnO2 containing at least one element selected from the group consisting of Sb, Nb, Ta, and W.

[0024] [Configuration 6] The tin oxide particles are It has a structure in which porous primary particles are fused together in a beaded pattern (a beaded structure), Specific surface area is 30m 2 / g or more 6. The fuel cell system according to any one of configurations 1 to 5.

[0025] [Configuration 7] 7. The fuel cell system according to any one of configurations 1 to 6, wherein the tin oxide particles contain pores with a pore diameter of 4 nm or more and 15 nm or less.

[0026] [Configuration 8] The tin oxide particles are (a) Sb-SnO2 particles made of SnO2 doped with Sb, the doping amount of Sb being 2.5 at% or more and 15.0 at% or less, and / or (b) Nb-SnO2 particles made of Nb-doped SnO2, the doping amount of Nb being 2.5 at% or more and 15.0 at% or less. 8. The fuel cell system according to any one of configurations 1 to 7, comprising:

[0027] [Configuration 9] A fuel cell system according to any one of configurations 4 to 8, which satisfies the following formulas (1) and (2): MA1(A / g Pt )≧700 …(1) MA2(A / g Pt )≧-2.3RH amb +800 …(2)

[0028] however, "MA1(A / g Pt )" means, Cell temperature T cell :40℃, outside temperature T amb :25℃, Outside air relative humidity RH amb :50% or more and 100% or less, Hydrogen gas humidity: 100% of the outside air temperature, cell voltage: 0.84V The mass activity of the oxygen reduction reaction when power generation is performed under the conditions MA2 (A / gPt )" means, Cell temperature T cell :40℃, outside temperature T amb :30℃, Outside air relative humidity RH amb :50% or more and 100% or less, Hydrogen gas humidity: 100% of the outside air temperature, cell voltage: 0.84V Mass activity of the oxygen reduction reaction when power generation is performed under the conditions.

[0029] An embodiment of the present invention will be described in detail below. [1. Fuel Cell System] 1.1. First embodiment Fig. 1 shows a schematic diagram of a fuel cell system (with anode humidification) according to a first embodiment of the present invention. In Fig. 1, a fuel cell system 10a includes: an open cathode polymer electrolyte fuel cell 12 including an anode catalyst layer and a cathode catalyst layer; a hydrogen gas supply device 14 for supplying hydrogen gas to the anode catalyst layer; a humidifier 16 for humidifying hydrogen gas, which is provided between the hydrogen gas supply device 14 and the anode catalyst layer; a fan 18 for introducing outside air into the cathode catalyst layer; It is equipped with:

[0030] [1.1.1. Open cathode polymer electrolyte fuel cell] An open cathode polymer electrolyte fuel cell (hereinafter simply referred to as "fuel cell") 12 includes a membrane electrode assembly (MEA) in which an anode catalyst layer is bonded to one side of an electrolyte membrane and a cathode catalyst layer is bonded to the other side. Typically, gas diffusion layers are disposed on the outside of each of the anode catalyst layer and the cathode catalyst layer. Furthermore, separators with gas flow channels are disposed on the outside of each of the gas diffusion layers. The fuel cell 12 includes a unit cell composed of such an MEA, gas diffusion layer, and separator. In the present invention, the term "open cathode fuel cell" refers to a case in which the fuel cell is composed of only a unit cell, or a case in which the fuel cell is provided with a structure (stack structure) in which multiple unit cells are stacked.

[0031] In general, the cathode catalyst layer and the cathode catalyst layer are each (a) an electrode catalyst in which catalyst particles are supported on the surface of a catalyst support; (b) Catalyst layer ionomer and It is equipped with:

[0032] In the present invention, the cathode catalyst layer contains tin oxide particles as a cathode catalyst support, which is different from conventional open cathode polymer electrolyte fuel cells. Furthermore, in the present invention, the cathode catalyst layer an electrode catalyst in which Pt-based particles are supported on the surfaces of tin oxide-based particles; Catalyst layer ionomer and The cathode electrode catalyst will be described in detail later.

[0033] Other materials constituting the fuel cell 12 are not particularly limited, and the most suitable material can be selected depending on the purpose. Examples of materials for the anode catalyst support include carbon particles and conductive oxide particles.

[0034] Examples of materials for the anode catalyst particles include: (a) Precious metals (Pt, Au, Ag, Pd, Rh, Ir, Ru, Os), (b) an alloy containing two or more precious metal elements; (c) Alloys containing one or more precious metal elements and one or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.); etc.

[0035] Examples of materials for the electrolyte membrane and catalyst layer ionomer include: (a) perfluorocarbon sulfonic acid polymers such as Nafion®, Flemion®, Aquivion®, and Aciplex®; (b) Highly oxygen-permeable ionomers consisting of polymeric compounds containing acid groups and cyclic structures in their molecular structure (see, for example, References 1 to 4). etc.

[0036] [Reference 1] Japanese Patent Application Laid-Open No. 2003-036856 [Reference 2] International Publication No. 2012 / 088166 [Reference 3] JP 2013-216811 A [Reference 4] JP 2006-152249 A

[0037] Examples of materials for the gas diffusion layer include carbon paper, carbon cloth, etc. A water-repellent layer made of a mixture of water-repellent polymers and conductive particles is usually formed on the surface of the gas diffusion layer. The separator may be made of, for example, carbon or stainless steel.

[0038] [1.1.2. Hydrogen gas supply device] The hydrogen gas supply device 14 is for supplying hydrogen gas to the anode catalyst layer of the fuel cell 12. In the present invention, the structure of the hydrogen gas supply device 14 is not particularly limited as long as it is capable of supplying a required amount of hydrogen gas to the anode catalyst layer. The hydrogen gas supply device 14 may be, for example, a hydrogen gas cylinder, a liquid hydrogen storage device, or a tank containing a hydrogen storage alloy.

[0039] [1.1.3. Humidifier] The humidifier 16 is used to humidify the hydrogen gas. The humidifier 16 is provided between the hydrogen gas supply device 14 and the anode catalyst layer of the fuel cell 12. The humidifier 16 is not necessarily required. However, if the hydrogen gas is humidified using the humidifier 16, the electrolyte membrane and catalyst layer are appropriately humidified even when the humidity of the outside air is low. As a result, a decrease in the proton conductivity of the electrolyte membrane or the catalyst layer ionomer can be suppressed. In the present invention, the structure of the humidifier 16 is not particularly limited as long as it is capable of humidifying hydrogen gas to a desired humidity level.

[0040] Fans The fan 18 is used to introduce outside air into the cathode catalyst layer of the fuel cell 12. The fan 18 is not necessarily required. However, when the fan 18 is used to forcibly supply outside air to the cathode catalyst layer, a higher output can be obtained compared to when the outside air is supplied by diffusion alone (natural intake type). Furthermore, when the fan 18 is used to forcibly supply outside air to the cathode catalyst layer, the fuel cell 12 is cooled, and an excessive temperature rise in the fuel cell 12 can be suppressed. In the present invention, the structure of the fan 18 is not particularly limited as long as it is capable of forcibly supplying a required amount of outside air to the cathode catalyst layer.

[0041] [1.2. Second embodiment] 2 is a schematic diagram of a fuel cell system (without anode humidification) according to a second embodiment of the present invention. In FIG. 2, a fuel cell system 10b includes: an open cathode polymer electrolyte fuel cell 12 including an anode catalyst layer and a cathode catalyst layer; a hydrogen gas supply device 14 for supplying hydrogen gas to the anode catalyst layer; a fan 18 for introducing outside air into the cathode catalyst layer; It is equipped with: The fuel cell system 10b according to this embodiment does not include a humidifier for humidifying hydrogen gas. This is what makes it different from the first embodiment. The rest of the configuration is the same as the first embodiment, so a description thereof will be omitted.

[0042] [1.3. Characteristics] In the fuel cell systems 10a and 10b according to the present invention, the mass activity can be improved by optimizing the configuration of each component (particularly the composition of the cathode catalyst support). It is particularly preferable that the fuel cell systems 10a and 10b satisfy the following formulas (1) and (2). MA1(A / g Pt )≧700 …(1) MA2(A / g Pt )≧-2.3RH amb +800 …(2)

[0043] however, "MA1(A / g Pt )" means, Cell temperature T cell :40℃, outside temperature T amb :25℃, Outside air relative humidity RH amb :50% or more and 100% or less, Hydrogen gas humidity: 100% of the outside air temperature, cell voltage: 0.84V The mass activity of the oxygen reduction reaction when power generation is performed under the conditions MA2(A / g Pt )" means, Cell temperature T cell :40℃, outside temperature T amb :30℃, Outside air relative humidity RH amb :50% or more and 100% or less, Hydrogen gas humidity: 100% of the outside air temperature, cell voltage: 0.84V Mass activity of the oxygen reduction reaction when power generation is performed under the conditions. "Mass activity" refers to the value obtained by dividing the current density under certain conditions by the mass of Pt contained in the cathode catalyst particles per unit area of ​​the cathode.

[0044] [1.3.1. Formula (1)] Equation (1) is expressed as follows: the cathode catalyst particles are Pt-based particles, and the ambient temperature T amb is 25℃, outside air relative humidity RH amb When the mass activity (MA1) measured under the above conditions is 50% or more and 100% or less, RH amb It means that the temperature is equal to or higher than a predetermined threshold, regardless of the outside air temperature T amb is 25℃, outside air relative humidity RH amb The condition of "relative humidity of 50% or more and 100% or less" corresponds to a condition where the relative humidity inside the cell is 22 to 43%.

[0045] When the configuration of each part of the fuel cell systems 10a and 10b is optimized, MA1 is 700A / g Pt By further optimizing the configuration of each part, MA1 can achieve 800A / g Pt More than 900A / g Pt or more, or 1000A / g Pt That's all.

[0046] [1.3.2. Formula (2)] Equation (2) is expressed as follows: the cathode catalyst particles are Pt-based particles, and the ambient temperature T amb is 30℃, ambient relative humidity RH amb When the mass activity (MA2) measured under the above conditions is 50% or more and 100% or less, RH amb The condition of equation (2) corresponds to the condition where the relative humidity inside the cell is slightly higher than that of equation (1).

[0047] When the configuration of each part of the fuel cell systems 10a and 10b is optimized, MA2 is -2.3RH amb +800(A / g Pt ) or more. By further optimizing the configuration of each part, the MA2 can achieve a maximum of -2.3RH amb +850(A / g Pt ) or more, -2.3RH amb +900(A / g Pt ) or more, -2.3RH amb +950(A / g Pt) or above, or -2.3RH amb +1050(A / g Pt ) or more.

[0048] [2. Cathode Electrocatalyst] In the present invention, the electrode catalyst for the cathode is Tin oxide particles, Cathode catalyst particles supported on the surface of tin oxide particles It is equipped with:

[0049] [2.1. Tin oxide particles] [A. Composition] In the present invention, tin oxide particles are used as the cathode catalyst support. "Tin oxide particles" refers to particles made of SnO2 or particles made of SnO2 containing a dopant. In the present invention, the type of dopant is not particularly limited. Examples of dopants include Nb, Sb, W, Ta, and Al. SnO2 may contain any one of these dopants, or may contain two or more of them.

[0050] Among these, the tin oxide-based particles are preferably SnO2 doped with Sb, Nb, Ta, and / or W. In particular, the tin oxide-based particles are (a) Particles made of SnO2 doped with Sb (Sb-SnO2 particles), and / or (b) Particles made of Nb-doped SnO2 (Nb-SnO2 particles) is preferred.

[0051] Sb-SnO2 particles have higher electrical conductivity than SnO2 particles containing other dopants, and are therefore suitable as a support for supporting cathode catalyst particles. Furthermore, although Nb-SnO2 particles have lower electrical conductivity than Sb-SnO2 particles, they have the advantage that the dopant (Nb) is less likely to be eluted than Sb-SnO2 particles.

[0052] In Sb-SnO2 particles, the greater the amount of Sb doped, the higher the electrical conductivity. To achieve this effect, the amount of Sb doped is preferably 2.5 at% or more. The amount of Sb doped is more preferably 5.0 at% or more. On the other hand, if the doping amount of Sb is excessive, the carrier concentration becomes excessive and the conductivity may decrease. Therefore, the doping amount of Sb is preferably 15.0 at% or less. The doping amount of Sb is more preferably 10.0 at% or less.

[0053] Similarly, in Nb-SnO2 particles, the greater the Nb doping amount, the higher the conductivity. To achieve this effect, the Nb doping amount is preferably 2.5 at% or more. The Nb doping amount is more preferably 5.0 at% or more. On the other hand, if the doping amount of Nb is excessive, the carrier concentration becomes excessive and the conductivity may decrease. Therefore, the doping amount of Nb is preferably 15.0 at% or less. The doping amount of Nb is more preferably 10.0 at% or less.

[0054] [B. Specific surface area] Generally, the larger the specific surface area of ​​the tin oxide particles, the more highly dispersed the cathode catalyst particles can be supported, improving the ORR mass activity. Therefore, the larger the specific surface area of ​​the tin oxide particles, the better. To obtain high ORR mass activity, the specific surface area of ​​the tin oxide particles should be 30 m 2 / g or more. The specific surface area is preferably 50 m 2 / g or more, more preferably 60m 2 / g or more, more preferably 90m 2 / g or more, more preferably 100m 2 / g or more.

[0055] [C. Pore diameter] The tin oxide particles may be solid particles, but are preferably porous particles having pores with a pore diameter of 50 nm or less (hereinafter also referred to as "mesopores") inside. Here, "mesopores" generally refer to pores with a diameter of 2 nm or more and 50 nm or less. However, in the present invention, unless otherwise specified, the term "mesopores" includes pores with a diameter of less than 2 nm (so-called "micropores") in addition to pores with a diameter of 2 nm or more and 50 nm or less. "Pore diameter" refers to the average diameter of mesopores. The pore diameter is obtained by analyzing the adsorption side data of the nitrogen adsorption isotherm of tin oxide particles using the BJH method and determining the pore diameter at which the pore volume is maximum (the most frequent peak value or mode pore diameter).

[0056] When tin oxide-based particles have mesopores, if cathode catalyst particles are supported on the tin oxide-based particles, a high proportion of the cathode catalyst particles will be present in the mesopores. Therefore, when an electrode catalyst is formed by supporting cathode catalyst particles in the mesopores of tin oxide-based particles and a catalyst layer is produced using such an electrode catalyst and an ionomer, it is possible to suppress poisoning of the cathode catalyst particles by the ionomer and the resulting performance degradation.

[0057] When tin oxide-based particles have mesopores, the pore diameter (size of the mesopores) affects the performance of the electrode catalyst. Generally, if the pore diameter is too small, it becomes difficult to support cathode catalyst particles in the mesopores. As a result, when a catalyst layer is produced using the electrode catalyst according to the present invention and an ionomer, the cathode catalyst particles may be poisoned by the ionomer. Therefore, the pore diameter is preferably 1 nm or more. The pore diameter is more preferably 2 nm or more, 3 nm or more, 4 nm or more, or 5 nm or more.

[0058] On the other hand, if the pore diameter is too large, the ionomer may penetrate into the mesopores and poison the cathode catalyst particles supported in the mesopores. Therefore, the pore diameter is preferably 20 nm or less. The pore diameter is more preferably 15 nm or less, 10 nm or less, or 8 nm or less. In particular, when the pore size of the tin oxide particles is set to 4 nm or more and 15 nm or less, high catalytic activity can be obtained when a catalyst layer is prepared using the particles as a carrier.

[0059] [D. Shape] In the present invention, the shape of the tin oxide-based particles is not particularly limited as long as the above-mentioned conditions are satisfied. The tin oxide-based particles may be isolated particles or may be particles having a beaded structure in which porous primary particles are fused together. Here, the term "beaded structure" refers to a structure in which primary particles are fused together in a beaded shape.

[0060] Using the method described below, tin oxide-based particles with a beaded structure in which porous primary particles are fused together can be obtained. In particles with a beaded structure (i.e., secondary particles), the primary particles are loosely connected to each other, resulting in relatively large voids between the primary particles. Therefore, when an electrode catalyst is produced using tin oxide-based particles with a beaded structure and a catalyst layer is produced using this and an ionomer, appropriate voids are formed within the catalyst layer. As a result, the gas diffusion resistance of the catalyst layer is reduced. Furthermore, because the primary particles are composed of an aggregate of minute crystallites, there are relatively small voids (mesopores) inside the primary particles, and therefore, when these are used as catalyst supports, it is possible to prevent the cathode catalyst particles from being poisoned by the ionomer.

[0061] The shape of the primary particles is not particularly limited. When tin oxide-based particles are produced using the method described below, the primary particles are usually not perfectly spherical but have an irregular shape with an aspect ratio of about 1.1 to 3.

[0062] [E. Average diameter of primary particles] The term "average particle size of primary particles" refers to the average value of the maximum dimension of primary particles measured by observation with a scanning electron microscope (SEM). When the tin oxide-based particles are particles having a beaded structure in which porous primary particles are fused together, the average particle size of the primary particles is not particularly limited, and an optimum value can be selected depending on the purpose.

[0063] Generally, if the average particle size of the primary particles is too small, it becomes difficult to support the cathode catalyst particles. Therefore, the average particle size of the primary particles is preferably 0.05 μm or more. The average particle size is more preferably 0.06 μm or more, and even more preferably 0.07 μm or more. On the other hand, if the average particle size of the primary particles is too large, the thickness of the catalyst layer increases, and the ionic resistance and electronic resistance in the catalyst layer increase. Therefore, the average particle size of the primary particles is preferably 2 μm or less. The average particle size is more preferably 1 μm or less, and even more preferably 0.5 μm or less.

[0064] [F. Electrical Conductivity of Compacted Powder] "Conductivity of a powder compact" means (a) Tin oxide particles are molded using two stainless steel disks and a plastic jig with a cylindrical hole. (b) The value obtained by applying a pressure of 2.4 MPa to the obtained green compact and measuring the voltage while passing a constant current through it.

[0065] The conductivity of the compact (i.e., tin oxide-based particles) mainly depends on the type and amount of dopant. When the composition of the tin oxide-based particles is optimized, the conductivity of the compact can be increased to 1×10 -3 When the manufacturing conditions are optimized, the conductivity is 1×10 -2 S / cm or more. By using the method described below, it is possible to synthesize tin oxide particles even if the conductivity of the compact is about 10 S / cm.

[0066] [G. Pore Volume] The "pore volume" refers to the volume of mesopores contained in the primary particles, and does not include the volume of voids between the primary particles. The pore volume can be obtained by analyzing the adsorption data of the nitrogen adsorption isotherm of the tin oxide particles by the BJH method and calculating the value of P / P0 = 0.03 to 0.99.

[0067] When the tin oxide-based particles according to the present invention are used as a cathode catalyst support, if the pore volume is too small, the proportion of cathode catalyst particles supported in the pores will be small. Therefore, the pore volume is preferably 0.1 mL / g or more. The pore volume is preferably 0.15 mL / g or more, and more preferably 0.2 mL / g or more. On the other hand, if the pore volume is too large, the proportion of the pore walls of the tin oxide particles decreases, resulting in low electronic conductivity. Furthermore, the amount of ionomer penetration increases, which may result in catalyst poisoning and reduced activity. Therefore, the pore volume is preferably 1 mL / g or less. The pore volume is preferably 0.7 mL / g or less, and more preferably 0.5 mL / g or less.

[0068] [H. Tap Density] "Tap density" refers to a value measured in accordance with JIS Z 2512. When the tin oxide-based particles according to the present invention are used as a cathode catalyst support, if the tap density of the tin oxide-based particles is too small, the thickness of the cathode catalyst layer becomes too large, resulting in a decrease in proton conductivity. 3 The tap density is preferably 0.01 g / cm or more. 3 More preferably, 0.05 g / cm 3 That's all. On the other hand, if the tap density is too high, when a cathode catalyst layer is fabricated using this, it becomes difficult to ensure voids in the cathode catalyst layer that are capable of suppressing flooding. Therefore, the tap density is set to 1.0 g / cm. 3 Preferably, the tap density is 0.75 g / cm or less. 3 The following is the result.

[0069] [I. Method for producing tin oxide particles] Tin oxide-based particles can be produced by various methods. For example, solid tin oxide-based particles can be produced by co-precipitation (Reference 5) or flame deposition (Reference 6). [Reference 5] F. Takasaki et al., J. Electrochem. Soc., 2011, 158, B1270 [Reference 6] K. Kakinuma et al., ACS Appl. Mater. Interfaces, 2019, 11, 34957

[0070] On the other hand, tin oxide-based particles having a structure in which porous primary particles are fused together in a beaded shape (a beaded structure) can be produced by a method (a template method) in which tin oxide is precipitated in the pores of a template and the template is then removed. Specifically, tin oxide particles having a beaded structure and porous primary particles can be produced using mesoporous carbon as a template. Mesoporous carbon is also produced using mesoporous silica as a template. Mesoporous silica is typically synthesized by condensation polymerization of a silica source in a reaction solution containing a silica source, a surfactant, and a catalyst.

[0071] When synthesizing mesoporous silica, if the concentrations of the surfactant and the silica source in the reaction solution are each limited to a specific range, mesoporous silica having a beaded structure and having a specific range of specific surface area, pore size, etc. can be obtained. This mesoporous silica with a beaded structure is used as a first template, carbon is deposited in the first template, and the first template is removed to obtain mesoporous carbon with a beaded structure. Furthermore, when mesoporous carbon with a beaded structure is used as a second template, tin oxide is deposited in the second template, and the second template is removed to obtain tin oxide particles with a beaded structure. Details of the template method are described below.

[0072] [2.2. Cathode catalyst particles] [2.2.1. Composition] In the present invention, the composition of the cathode catalyst particles is not particularly limited as long as it has activity in the oxygen reduction reaction (ORR).The cathode catalyst particles are particularly preferably Pt-based particles. Here, "Pt-based particles" refers to particles made of Pt or a Pt alloy. The Pt-based particles are supported on the surface of the tin oxide-based particles (i.e., the outer surface or the inner surface of the mesopores of the tin oxide-based particles).

[0073] When the Pt-based particles are made of a Pt alloy, the composition of the Pt alloy (i.e., the type and content of alloying elements) is not particularly limited, and an optimum composition can be selected depending on the purpose. (a) Alloys containing Pt and one or more precious metal elements other than Pt (e.g., Pt-Pd alloys, Pt-Ru alloys, Pt-Ir alloys, etc.); (b) Alloys containing Pt and one or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.) (e.g., Pt-Fe alloys, Pt-Co alloys, Pt-Ni alloys, Pt-Cr alloys, Pt-V alloys, Pt-Ti alloys, etc.); etc.

[0074] When Pt-based particles contain metal elements other than Pt, if the Pt content is too low, catalytic activity may decrease. Therefore, the amount of Pt contained in the Pt-based particles is preferably 30 at% or more. The amount of Pt is more preferably 40 at% or more, 50 at% or more, or 60 at% or more.

[0075] [2.2.2. Average particle size] The term "average particle size of Pt-based particles" refers to the average value of the maximum dimension of Pt-based particles measured by observation with a scanning electron microscope (SEM). The average particle size of Pt-based particles affects mass activity. Generally, if the average particle size of Pt-based particles becomes too large, the mass activity of the Pt-based particles decreases. Therefore, the average particle size of Pt-based particles is preferably 5 nm or less. The average particle size is more preferably 4 nm or less. On the other hand, if the average particle size of the Pt-based particles is too small, the components that make up the particles, such as Pt, tend to dissolve. Therefore, the average particle size of the Pt-based particles is preferably 1 nm or more. The average particle size is more preferably 2 nm or more.

[0076] 2.2.3. Loading amount The amount of Pt-based particles supported is not particularly limited, and an optimal amount can be selected depending on the purpose. Generally, if the amount of Pt-based particles supported is too small, the thickness of the cathode catalyst layer required to obtain a predetermined basis weight increases, and the electron resistance, proton transfer resistance, and / or gas diffusion resistance of the cathode catalyst layer increase. Therefore, the amount of Pt-based particles supported is preferably 5 mass% or more. The amount is more preferably 10 mass% or more, and even more preferably 15 mass% or more. On the other hand, if the amount of Pt-based particles loaded is excessive, the Pt-based particles will aggregate on the support surface, which will actually reduce the activity of the electrode catalyst. Therefore, the amount of Pt-based particles loaded is preferably 60 mass% or less. The amount is more preferably 50 mass% or less, and even more preferably 40 mass% or less.

[0077] [3. Manufacturing method of mesoporous silica (first template)] In order to produce tin oxide particles having a structure in which porous primary particles are fused together in a beaded shape (a beaded structure), it is first necessary to produce mesoporous silica (a first template) having a beaded structure. (a) preparing precursor particles by condensation polymerization of the silica source in a reaction solution containing the silica source, a surfactant, and a catalyst; (b) separating the precursor particles from the reaction solution and drying them; (c) If necessary, the dried precursor particles are subjected to a diameter expansion treatment; (d) calcining the precursor particles This is obtained by:

[0078] [3.1. Polycondensation process] First, in a reaction solution containing a silica source, a surfactant, and a catalyst, the silica source is polycondensed to obtain precursor particles (polycondensation step).

[0079] 3.1.1. Silica Source In the present invention, the type of silica source is not particularly limited. Examples of the silica source include: (a) tetraalkoxysilanes such as tetramethoxysilane, tetraethoxysilane, tetraisopropoxysilane, tetrabutoxysilane, dimethoxydiethoxysilane, and tetraethyleneglycoxysilane; (b) trialkoxysilanes such as 3-mercaptopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, and 3-(2-aminoethyl)aminopropyltrimethoxysilane; As the silica source, any one of these may be used alone, or two or more of them may be used in combination.

[0080] 3.1.2. Surfactants When a silica source is polycondensed in a reaction solution, adding a surfactant to the reaction solution causes the surfactant to form micelles in the reaction solution. Because hydrophilic groups are clustered around the micelles, the silica source is adsorbed to the surface of the micelles. Furthermore, the micelles with the adsorbed silica source self-assemble in the reaction solution, causing the silica source to polycondense. As a result, mesopores originating from the micelles are formed inside the primary particles. The size of the mesopores can be controlled (1 to 50 nm) primarily by the molecular length of the surfactant.

[0081] In the present invention, an alkyl quaternary ammonium salt is used as the surfactant. The alkyl quaternary ammonium salt refers to a compound represented by the following formula (a): CH3-(CH2) n -N + (R1)(R2)(R3)X - (a)

[0082] In formula (a), R1, R2, and R3 each represent an alkyl group having 1 to 3 carbon atoms. R1, R2, and R3 may be the same or different. To facilitate aggregation of alkyl quaternary ammonium salts (micelle formation), it is preferable that R1, R2, and R3 are all the same. Furthermore, it is preferable that at least one of R1, R2, and R3 is a methyl group, and it is preferable that all of them are methyl groups. In formula (a), X represents a halogen atom. The type of halogen atom is not particularly limited, but X is preferably Cl or Br in view of availability.

[0083] In formula (a), n represents an integer of 7 to 21. Generally, as n decreases, a spherical mesoporous material with a smaller central pore diameter is obtained. On the other hand, as n increases, the central pore diameter increases, but if n is too large, the hydrophobic interaction of the alkyl quaternary ammonium salt becomes excessive. As a result, a layered compound is produced, and a mesoporous material cannot be obtained. n is preferably 9 to 17, and more preferably 13 to 17.

[0084] Among those represented by formula (a), alkyltrimethylammonium halides are preferred, such as hexadecyltrimethylammonium halide, octadecyltrimethylammonium halide, nonyltrimethylammonium halide, decyltrimethylammonium halide, undecyltrimethylammonium halide, and dodecyltrimethylammonium halide. Among these, alkyltrimethylammonium bromide or alkyltrimethylammonium chloride is particularly preferred.

[0085] When synthesizing mesoporous silica, one type of alkyl quaternary ammonium salt may be used, or two or more types may be used. However, since the alkyl quaternary ammonium salt serves as a template for forming mesopores within the primary particles, the type of alkyl quaternary ammonium salt significantly affects the shape of the mesopores. To synthesize silica particles with more uniform mesopores, it is preferable to use one type of alkyl quaternary ammonium salt.

[0086] 3.1.3. Catalyst When polycondensing a silica source, a catalyst is usually added to the reaction solution. When synthesizing particulate mesoporous silica, the catalyst may be an alkali such as sodium hydroxide or aqueous ammonia, or an acid such as hydrochloric acid.

[0087] 3.1.4. Solvent The solvent used may be water, an organic solvent such as alcohol, or a mixed solvent of water and an organic solvent. Alcohol is (1) Monohydric alcohols such as methanol, ethanol, and propanol, (2) Dihydric alcohols such as ethylene glycol, (3) Trihydric alcohols such as glycerin, Either is fine. When a mixed solvent of water and an organic solvent is used, the content of the organic solvent in the mixed solvent can be selected arbitrarily depending on the purpose. Generally, adding an appropriate amount of organic solvent to the solvent makes it easier to control the particle size and particle size distribution.

[0088] 3.1.5. Composition of reaction solution The composition of the reaction solution affects the external shape and pore structure of the synthesized mesoporous silica. In particular, the concentrations of the surfactant and silica source in the reaction solution have a significant effect on the average primary particle size, pore size, pore volume, and tap density of the mesoporous silica particles.

[0089] [A. Surfactant concentration] If the surfactant concentration is too low, the particle precipitation rate will be slow and a structure in which primary particles are linked together will not be obtained. Therefore, the surfactant concentration must be 0.03 mol / L or more. The surfactant concentration is preferably 0.035 mol / L or more, more preferably 0.04 mol / L or more.

[0090] On the other hand, if the surfactant concentration is too high, the particle precipitation rate becomes too fast, and the primary particle size easily exceeds 300 nm. Therefore, the surfactant concentration must be 1.0 mol / L or less. The surfactant concentration is preferably 0.95 mol / L or less, and more preferably 0.90 mol / L or less.

[0091] B. Silica Source Concentration If the concentration of the silica source is too low, the particle precipitation rate will be slow, and a structure in which primary particles are connected will not be obtained. Alternatively, the surfactant will be excessive, and uniform mesopores may not be obtained. Therefore, the concentration of the silica source must be 0.05 mol / L or more. The concentration of the silica source is preferably 0.06 mol / L or more, and more preferably 0.07 mol / L or more.

[0092] On the other hand, if the silica source concentration is too high, the particle precipitation rate becomes too fast, and the primary particle diameter easily exceeds 300 nm. Alternatively, sheet-like particles may be obtained instead of spherical particles. Therefore, the silica source concentration must be 1.0 mol / L or less. The silica source concentration is preferably 0.95 mol / L or less, and more preferably 0.9 mol / L or less.

[0093] C. Catalyst Concentration In the present invention, the catalyst concentration is not particularly limited. Generally, if the catalyst concentration is too low, the particle precipitation rate will be slow. On the other hand, if the catalyst concentration is too high, the particle precipitation rate will be fast. It is preferable to select the optimum catalyst concentration depending on the type of silica source, the type of surfactant, the target physical property values, etc.

[0094] 3.1.6 Reaction conditions A silica source is added to a solvent containing a predetermined amount of surfactant, and hydrolysis and polycondensation are carried out, whereby precursor particles containing silica and surfactant are obtained, with the surfactant acting as a template. The optimum reaction conditions are selected depending on the type of silica source, the particle size of the precursor particles, etc. In general, the reaction temperature is preferably −20 to 100° C. The reaction temperature is more preferably 0 to 90° C., and even more preferably 10 to 80° C.

[0095] [3.2. Drying process] Next, the precursor particles are separated from the reaction solution and dried (drying step). Drying is carried out to remove the solvent remaining in the precursor particles. The drying conditions are not particularly limited as long as the solvent can be removed.

[0096] [3.3. Diameter expansion process] Next, if necessary, the dried precursor particles may be subjected to a diameter expansion treatment (diameter expansion step). The "diameter expansion treatment" refers to a treatment for expanding the diameter of mesopores in the primary particles. Specifically, the diameter-enlarging treatment is carried out by subjecting the synthesized precursor particles (from which the surfactant has not been removed) to a hydrothermal treatment in a solution containing a diameter-enlarging agent, which can enlarge the pore size of the precursor particles.

[0097] Examples of the diameter expanding agent include: (a) Hydrocarbons such as trimethylbenzene, triethylbenzene, benzene, cyclohexane, triisopropylbenzene, naphthalene, hexane, heptane, octane, nonane, decane, undecane, and dodecane; (b) Acids such as hydrochloric acid, sulfuric acid, and nitric acid; etc.

[0098] The reason why the pore size increases upon hydrothermal treatment in the presence of hydrocarbons is thought to be that rearrangement of silica occurs when the diameter-expanding agent is introduced from the solvent into the pores of the more hydrophobic precursor particles. Furthermore, the pore size increases when hydrothermal treatment is performed in the presence of an acid such as hydrochloric acid. This is thought to be due to the dissolution and reprecipitation of silica inside the primary particles. When the manufacturing conditions are optimized, radial pores are formed inside the silica. When this is subjected to hydrothermal treatment in the presence of an acid, silica dissolution and reprecipitation occurs, converting the radial pores into interconnected pores.

[0099] The conditions for the diameter-enlarging treatment are not particularly limited as long as the desired pore diameter is obtained. Usually, it is preferable to add about 0.05 mol / L to 10 mol / L of a diameter-enlarging agent to the reaction solution and perform hydrothermal treatment at 60 to 150°C.

[0100] [3.4. Firing process] Next, after carrying out a diameter expansion treatment as necessary, the precursor particles are calcined (calcination step), thereby obtaining mesoporous silica particles having a beaded structure. The calcination is carried out to dehydrate and crystallize the precursor particles with residual OH groups and to thermally decompose the surfactant remaining in the mesopores. The calcination conditions are not particularly limited as long as they allow for dehydration, crystallization, and thermal decomposition of the surfactant. Calcination is usually carried out by heating in the atmosphere at 400°C to 700°C for 1 to 10 hours.

[0101] [4. Manufacturing method of mesoporous carbon (second mold)] Next, mesoporous silica having a beaded structure is used as a template to produce mesoporous carbon having a beaded structure (second template). (a) preparing mesoporous silica as a first template; (b) depositing carbon in the mesopores of the mesoporous silica to prepare a silica / carbon composite; (c) removing silica from the composite. This is obtained by: In order to promote graphitization of the obtained mesoporous carbon, the mesoporous carbon may be heat-treated at a temperature higher than 1500° C. after removing the silica.

[0102] [4.1. First mold preparation process] First, mesoporous silica to be used as the first template is prepared (first template preparation step). Details of the method for producing mesoporous silica are as described above, and therefore will not be described again.

[0103] [4.2. Carbon deposition process] Next, carbon is deposited in the mesopores of the mesoporous silica to produce a silica / carbon composite (carbon deposition step). Specifically, carbon deposition in mesopores is as follows: (a) introducing a carbon precursor into the mesopores; (b) Polymerizing and carbonizing the carbon precursor within the mesopores This is done by:

[0104] 4.2.1. Introduction of carbon precursors The term "carbon precursor" refers to a material capable of producing carbon by thermal decomposition. Specific examples of such carbon precursors include: (1) A polymer precursor that is liquid at room temperature and thermally polymerizable (e.g., furfuryl alcohol, aniline, etc.), (2) A mixture of an aqueous solution of carbohydrates and an acid (for example, a mixture of monosaccharides such as sucrose, xylose, glucose, or a mixture of disaccharides or polysaccharides with an acid such as sulfuric acid, hydrochloric acid, nitric acid, or phosphoric acid), (3) A mixture of two-component curing polymer precursors (e.g., phenol and formalin), etc. Among these, polymer precursors can be impregnated into mesopores without dilution with a solvent, allowing a relatively large amount of carbon to be produced in the mesopores with a relatively small number of impregnation cycles. Furthermore, they have the advantage of not requiring a polymerization initiator and being easy to handle.

[0105] When a liquid or solution carbon precursor is used, the amount of liquid or solution adsorbed per one time is preferably as large as possible, and is preferably such an amount that the entire mesopores are filled with the liquid or solution. When a mixture of an aqueous solution of a carbohydrate and an acid is used as the carbon precursor, the amount of acid is preferably the minimum amount that can polymerize the organic material. Furthermore, when a mixture of two-component curing polymer precursors is used as the carbon precursor, the optimum ratio is selected depending on the type of polymer precursor.

[0106] 4.2.2. Polymerization and carbonization of carbon precursors The polymerized carbon precursor is then carbonized within the mesopores. Carbonization of the carbon precursor is carried out by heating mesoporous silica containing the carbon precursor to a predetermined temperature in a non-oxidizing atmosphere (e.g., in an inert atmosphere, vacuum, etc.). Specifically, the heating temperature is preferably 500°C or higher and 1200°C or lower. If the heating temperature is lower than 500°C, the carbon precursor will not be sufficiently carbonized. On the other hand, if the heating temperature exceeds 1200°C, silica and carbon will react, which is not preferable. The optimal heating time is selected depending on the heating temperature.

[0107] The amount of carbon generated in the mesopores should be at least the amount that allows the carbon particles to maintain their shape when the mesoporous silica is removed. Therefore, if the amount of carbon generated in one filling, polymerization, and carbonization process is relatively small, it is preferable to repeat these processes multiple times. In this case, the conditions for each repeated process may be the same or different. Furthermore, when each of the steps of filling, polymerization, and carbonization is repeated multiple times, each carbonization step may be performed at a relatively low temperature, and after the final carbonization step is completed, another carbonization step may be performed at a higher temperature. If the final carbonization step is performed at a higher temperature than the previous carbonization steps, the carbon introduced into the pores in multiple steps is more likely to be integrated.

[0108] [4.3. First mold removal step] Next, the mesoporous silica serving as the first template is removed from the composite (first template removal step), thereby obtaining mesoporous carbon (second template) having a beaded structure. Specific methods for removing mesoporous silica include: (1) A method of heating the complex in an alkaline aqueous solution such as sodium hydroxide, (2) Etching the composite with an aqueous hydrofluoric acid solution; etc.

[0109] [4.4. Graphitization process] Next, if necessary, the mesoporous carbon is heat-treated at a temperature higher than 1500°C (graphitization step). When carbonizing a carbon source within the mesopores of mesoporous silica, the heat treatment temperature must be low to suppress the reaction between silica and carbon. As a result, the degree of graphitization of the carbon after carbonization is low. To achieve a high degree of graphitization, it is preferable to heat-treat the mesoporous carbon at a high temperature after removing the first template.

[0110] If the heat treatment temperature is too low, graphitization will be insufficient. Therefore, the heat treatment temperature is preferably higher than 1500° C. The heat treatment temperature is preferably 1700° C. or higher, and more preferably 1800° C. or higher. On the other hand, if the heat treatment temperature is made higher than necessary, there is no difference in the effect and it is of no practical benefit. Therefore, the heat treatment temperature is preferably 2300°C or less. The heat treatment temperature is preferably 2200°C or less.

[0111] [5. Manufacturing method of tin oxide particles] A method for producing tin oxide particles having a beaded structure includes the steps of: A first step of preparing mesoporous carbon having a beaded structure; a second step of precipitating tin oxide or tin oxide containing a dopant (hereinafter, these are also collectively referred to as "Sn-containing oxide") in the mesopores of the mesoporous carbon to obtain a Sn-containing oxide / carbon composite; a third step of removing carbon from the Sn-containing oxide / carbon composite; It is equipped with:

[0112] [5.1. 1st step] First, mesoporous carbon having a beaded structure is prepared (Step 1). Details of the method for producing mesoporous carbon are as described above, and therefore will not be described here.

[0113] [5.2. 2nd process] Next, a Sn-containing oxide is precipitated in the mesopores of the mesoporous carbon (second step), thereby obtaining a Sn-containing oxide / carbon composite. Specifically, the deposition of the Sn-containing oxide in the mesopores is carried out by introducing a precursor of the Sn-containing oxide into the mesopores and converting the precursor into the Sn-containing oxide.

[0114] 5.2.1. Precursors Specific examples of precursors for forming Sn-containing oxides in mesopores include: (1) A compound containing a metal element constituting a Sn-containing oxide, which is soluble in a solvent, and which can be oxidized by dissolved oxygen in the solvent and precipitated; (2) A compound containing a metal element constituting an Sn-containing oxide and capable of forming a metal oxide by thermal decomposition or hydrolysis; etc.

[0115] Compounds that can be oxidized and precipitated by dissolved oxygen include: (1) Salts containing divalent Sn, such as SnCl2, (2) Salts containing Nb, Sb, W, Ta, or Al, such as NbCl5, SbCl3, WCl6, TaCl5, and AlCl3; etc.

[0116] Compounds capable of forming metal oxides by thermal decomposition or hydrolysis include: (1) Chlorides such as SnCl4, SnCl2, NbCl5, SbCl3, WCl6, TaCl5, and AlCl3, (2) Alkoxides such as tungsten ethoxide (W(OC2H5)6), Sn(OC2H5)2, Sn(OC(CH3)3)4, Nb(OC2H5)5, Ta(OC2H5)5, Sb(OC2H5)3, and Al(OC2H5)3; (3) Acetylacetonate salts such as tin acetylacetonate (Sn(CH3COCHCOCH3)2) and Al(CH3COCHCOCH3)3; (4) Acetates such as Sn(CH3COO)2 and Sb(CH3COO)3, etc.

[0117] 5.2.2. Introduction of precursors into pores When the precursor is a liquid, it may be adsorbed directly into the pores of the mesoporous carbon. Alternatively, the precursor may be dissolved in an appropriate solvent, and the solution may be adsorbed into the pores of the mesoporous carbon. When the precursor is dissolved in a solvent, the type of solvent and the concentration of the precursor are not particularly limited, and an optimum solvent and precursor concentration may be selected depending on the purpose.

[0118] 5.2.3. Conversion of precursors to oxides After the precursor is adsorbed, the precursor is converted into an Sn-containing oxide. The conversion method is not particularly limited, and an optimal method is selected depending on the type of precursor. For example, when a chloride is used as a precursor, mesoporous carbon is dispersed in a solution containing the chloride and stirred in air. With continued stirring, the chloride is eventually adsorbed into the mesopores of the mesoporous carbon, and the chloride in the mesopores gradually becomes a Sn-containing oxide due to the dissolved oxygen.

[0119] For example, when an alkoxide is used as a precursor, the alkoxide or a solution thereof is added to mesoporous carbon, and the alkoxide or its solution is impregnated into the mesopores. When this is heated to a predetermined temperature, polycondensation of the alkoxide occurs, producing a Sn-containing oxide in the mesopores. If a sufficient amount of Sn-containing oxide cannot be formed in the mesopores by one cycle of adsorption of the precursor and conversion to the Sn-containing oxide, the adsorption and conversion may be repeated multiple times.

[0120] [5.3. Third step] Next, carbon is removed from the Sn-containing oxide / carbon composite (third step), thereby obtaining the tin oxide-based particles according to the present invention. The method for removing carbon is not particularly limited, and various methods can be used. Examples of the carbon removal method include: (1) A method of heating a Sn-containing oxide / carbon composite in an oxidizing atmosphere, (2) Oxygen plasma etching of Sn-containing oxide / carbon composites; etc. The removal conditions such as the heating temperature and heating time are not particularly limited as long as they are conditions that completely remove carbon without causing the crystallites of the Sn-containing oxide to become coarse.

[0121] [6. Effect] [6.1. Suppression of degradation of power generation performance in dry environments] In open cathode fuel cells, outside air is introduced directly into the cell without passing through a humidifier, which can create a dry environment inside the cell depending on the temperature and humidity of the outside air. On the other hand, carbon supports are water-repellent and have poor water retention. Therefore, in the case of a cathode catalyst layer containing a carbon support, the moisture content within the cathode catalyst layer decreases in a dry environment, reducing the proton conductivity of the ionomer within the cathode catalyst layer. As a result, the power generation performance of the fuel cell declines.

[0122] In contrast, tin oxide particles are hydrophilic and have high water retention. Therefore, when used as a cathode catalyst support, moisture is easily retained in the cathode catalyst layer even in a dry environment. As a result, the deterioration of power generation performance in a dry environment can be suppressed.

[0123] 6.2. Suppression of catalyst poisoning by ionomers Generally, the lower the humidity in the catalyst layer, the more easily the sulfonic acid groups of the ionomer are adsorbed onto the surface of the catalyst particles, and as a result, the lower the humidity in the catalyst layer, the more easily the catalyst is poisoned by the sulfonic acid groups of the ionomer.

[0124] In contrast, when the tin oxide-based particles are porous and have mesopores, and the cathode catalyst particles are supported within the mesopores, contact between the cathode catalyst particles and the ionomer is prevented, thereby suppressing catalyst poisoning. Therefore, when the tin oxide-based particles are used as a cathode catalyst support for an open-cathode polymer electrolyte fuel cell, it is possible to suppress the deterioration of power generation performance due to catalyst poisoning.

[0125] 6.3. Oxidative Deterioration of Support In a polymer electrolyte fuel cell, the cathode is sometimes exposed to a high potential during operation, and the carbon support is easily oxidized and deteriorated when exposed to a high potential. In contrast, tin oxide-based particles do not undergo oxidative degradation even when exposed to high potentials, and therefore, when used as a cathode catalyst support for open-cathode polymer electrolyte fuel cells, it is possible to suppress a decrease in durability due to oxidative degradation.

[0126] 6.4. Suppression of destabilization of tin oxide particles in low-potential environments In a closed cathode fuel cell, if the cathode is sealed with the air purged (in other words, the oxygen remaining in the cathode flow channel is completely consumed) during shutdown, hydrogen gas on the anode side may permeate the electrolyte membrane and move to the cathode side, causing the potential of the cathode catalyst layer to drop to near the hydrogen potential. Carbon supports are relatively stable in a strongly acidic, low-potential environment near the hydrogen potential. However, tin oxide-based particles may become unstable (i.e., dopants and SnO2 may be eluted) in a strongly acidic, low-potential environment near the hydrogen potential.

[0127] In contrast, in an open cathode polymer electrolyte fuel cell, because of its structure, air is always present at the cathode, the potential of the cathode catalyst layer does not drop to near the hydrogen potential, and therefore, even if tin oxide-based particles are used as a cathode catalyst support in an open cathode polymer electrolyte fuel cell, the instability of tin oxide-based particles at low potential does not become a problem. [Example]

[0128] (Examples 1 and 2, Comparative Example 1) 1. Sample Preparation 1.1. Pt / Sb-SnO2 (Example 1) 1.1.1. Preparation of beaded starburst silica To a mixed solvent of 4.6 g of methanol (MeOH) and 4.6 g of ethylene glycol (EG), 56.3 g of 30 mass% aqueous cetyltrimethylammonium chloride solution was added and stirred at room temperature. 8.8 g of 1 M NaOH was added and heated to 50°C. Hereinafter, this solution will be referred to as "first solution." Next, 12.3 g of tetraethoxysilane (TEOS) was dissolved in a mixed solvent of 6.5 g of MeOH and 6.5 g of EG, hereinafter referred to as the "second solution."

[0129] The second solution was added to the first solution, which had been heated to 50°C. After the mixture became cloudy, heating was stopped and the mixture was stirred for an additional 4 hours or more. Filtration and redispersion in purified water were repeated twice, and the mixture was then dried at 45°C. The dried powder was then calcined in air at 550°C for 6 hours to obtain connected starburst mesoporous silica with radial pores (hereinafter referred to as "connected starburst silica (CSS)").

[0130] 1.1.2. Preparation of beaded starburst carbon 0.5 g of CSS was placed in a PFA container, and furfuryl alcohol (FA) was added in an amount equal to the pore volume of the CSS, allowing it to penetrate into the pores of the CSS. This was then heat-treated at 150°C for 24 hours to polymerize the FA. This was then heat-treated in a nitrogen atmosphere for 6 hours at 500°C to further carbonize the FA. This process was repeated twice, and then further heat-treated in a nitrogen atmosphere for 6 hours at 900°C to obtain a CSS / carbon composite.

[0131] This composite was immersed in a 12% HF solution for 4 hours to dissolve the silica component. After dissolution, it was repeatedly filtered and washed, and then dried at 45°C to obtain a connected starburst carbon (hereinafter referred to as "connected starburst carbon (CSC)") with radial pores. The obtained CSC had a BET specific surface area of ​​2122 m. 2 / g, pore volume: 1.3 mL / g, pore diameter: 2.2 nm.

[0132] 1.1.3. Preparation of bead-like mesoporous Sb-SnO 0.03 g of SbCl3 (99.9 mass%, Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in 4 mL of concentrated hydrochloric acid (35 mass%, Fujifilm Wako Pure Chemical Industries, Ltd.), and after dilution with 36 mL of purified water, 5.0 g of SnCl2 (99.9 mass%, Fujifilm Wako Pure Chemical Industries, Ltd.) was further added and dissolved. 0.1 g of CSC was added and dispersed in this solution. This dispersion was stirred in air at room temperature for 2 hours, after which 200 mL of purified water was added and stirred in air for an additional 4 hours. Subsequently, filtration and redispersion in purified water were repeated twice, and then the mixture was dried at 45 °C to obtain a beaded Sb-SnO2 / carbon composite.

[0133] This beaded Sb-SnO2 / carbon composite was treated in an air atmosphere at 320°C for 24 hours, and then further treated in an air atmosphere at 450°C for 3 hours to obtain pale blue beaded mesoporous Sb-SnO2. The Sb doping content of the resulting beaded mesoporous Sb-SnO2 was 2.5 at%. The mode pore diameter (mode) determined from N2 adsorption measurements was 7.3 nm.

[0134] [1.1.4. Pt loading (colloidal method)] Pt nanoparticles were supported on beaded mesoporous Sb-SnO2 (pore diameter: 7.3 nm) using a colloidal method. First, 6 mL of 0.4 M NaOH / EG solution was mixed with 6 mL of 0.04 mM HPtCl (Fujifilm Wako Pure Chemical Industries, Ltd.) / EG solution. This mixture was heated at 160 °C for 3 min while stirring in a microwave synthesizer (Monowabe 400, Anton Paar) to obtain a Pt nanoparticle colloidal solution.

[0135] Next, 126 mg of Sb-SnO2 powder was added to 8 mL of Pt nanoparticle colloidal solution and stirred overnight at room temperature. 0.2 mL of 1 M HNO3 was then added, and the mixture was stirred at room temperature for 1 hour. This process was repeated twice. 0.5 mL of 1 M HNO3 was then added, and the mixture was stirred at room temperature for 1 hour. This process was then repeated twice, with filtration and redispersion in purified water. Finally, the solid was dried in a vacuum at 70°C to obtain Pt / Sb-SnO2 (Pt loading: 20 mass%).

[0136] 1.2. Pt / Nb-SnO2 (Example 2) 1.2.1. Preparation of bead-like mesoporous Nb-SnO A mixture was obtained by mixing 250 mL of purified water, 4 mL of concentrated hydrochloric acid (35 mass%, Fujifilm Wako Pure Chemical Industries, Ltd.), 5.0 g of SnCl2 (Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.074 g of NbCl5 (Fujifilm Wako Pure Chemical Industries, Ltd.). 0.1 g of CSC was added to this mixture and dispersed. After stirring this dispersion in air at room temperature for 4 hours, it was filtered and redispersed in purified water twice. The mixture was then dried at 45°C to obtain a beaded mesoporous Nb-SnO2 / carbon composite.

[0137] This beaded Nb-SnO2 / carbon composite was treated in an air atmosphere at 300°C for 24 hours, and then further treated in an air atmosphere at 700°C for 3 hours to obtain beaded mesoporous Nb-SnO2. The Nb doping content of the obtained beaded mesoporous Nb-SnO2 was 4.8 at%. The mode pore diameter (mode) determined from N2 adsorption measurements was 12 nm.

[0138] [1.2.2. Pt Support (ALD)] Next, Pt particles were supported on the surface of a support made of beaded mesoporous Nb-SnO2 using atomic layer deposition (ALD). The Pt precursor used was MeCpPtMe3 (methylcyclopentadienyltrimethylplatinum, manufactured by Tri Chemical Laboratory Co., Ltd.). A test tube containing 100 mg of the support was heated to 150°C, and the Pt precursor container was heated to 60°C using a mantle heater. The following ALD cycle (1-4) was repeated 27 times to obtain Pt / Nb-SnO2. The Pt support ratio (the ratio of the mass of Pt to the total mass of the electrode catalyst) was 26.1 mass%.

[0139] 1.Pt precursor supply: MeCpPtMe3 / Ar, 50ccm, 20min 2. Purge: Ar, 200ccm, 5min 3. Pt precursor reduction: H2, 100 ccm, 5 min 4. Purge: Ar, 200ccm, 5min

[0140] [1.3. Pt / C (Comparative Example 1)] A commercially available Pt / C catalyst (TEC10V30E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., Pt loading ratio 30 mass%) was used as it was as an electrode catalyst for the cathode.

[0141] [1.4. Preparation of cathode catalyst layer sheet] A cathode catalyst layer sheet was produced using the above-mentioned Pt / Sb—SnO 2 , Pt / Nb—SnO 2 , or Pt / C.

[0142] Predetermined amounts of the electrode catalyst, purified water, ethanol, propylene glycol, and ionomer dispersion (21.2 mass%, D2020) were weighed and mixed. The amount of ionomer was adjusted so that the ratio (I / S) of the ionomer mass (I) to the carrier mass (S) was 0.26. This mixture was subjected to alternating shaking and ultrasonic dispersion three times to obtain a catalyst ink. The resulting catalyst ink was applied to a polytetrafluoroethylene (PTFE) sheet using an applicator (gap height: 4 mil) and dried to obtain a cathode catalyst layer sheet. The platinum content was 0.11 mg. Pt / cm 2 It was decided.

[0143] [1.5. Preparation of anode catalyst layer sheet] An anode catalyst layer sheet was prepared in the same manner as the cathode catalyst sheet, except that 30 mass% Pt / Ketjen (registered trademark) was used as the electrode catalyst. The platinum content was 0.05 mg. Pt / cm 2 The I / C (ionomer / carbon mass ratio) was set to 1.0.

[0144] 1.6. Preparation of MEA The cathode catalyst layer sheet and the anode catalyst layer sheet were each cut into 1 cm squares, and the cut catalyst layers were transferred to a Nafion (registered trademark) membrane (NR211) by hot pressing to prepare an MEA. The hot pressing conditions were 120°C and 0.89 kN / cm. 2 , and 5 minutes.

[0145] 2. Test Method [2.1. Fuel cell system 10a (with anode humidification)] Assuming use in the fuel cell system 10a shown in Figure 1, a single cell test was conducted simulating temperature and humidity. 2 The cell temperature was maintained at 40°C. amb ) is assumed to be 30℃ or 25℃, and the respective T amb Relative humidity (RH ambAir gas (500 ccm) containing 50 to 100% water vapor was introduced into the cathode. amb H2 gas (500 ccm) containing water vapor equivalent to a relative humidity of 100% at 100°C was introduced into the anode. The power generation performance under each condition was evaluated by sweeping the voltage from 0.1 V to the OCV (open circuit voltage) at a sweep rate of 20 mV / s.

[0146] 2.2. Fuel Cell System 10b (without anode humidification) Assuming use in the fuel cell system 10b shown in Figure 2, a single cell test was conducted simulating temperature and humidity. 2 The cell temperature was maintained at 40°C. amb ) is assumed to be 30℃ or 25℃, and the respective T amb Relative humidity (RH amb Air gas (500 ccm) containing 50% water vapor was introduced into the cathode. Unhumidified H2 gas was introduced into the anode. amb Assuming a temperature of 30°C, the H2 gas flow rate was set to 50 ccm. amb When the temperature was assumed to be 25°C, the H2 gas flow rate was set to 10 ccm. The power generation performance under each condition was evaluated by sweeping the voltage from 0.1 V to the OCV (open circuit voltage) at a sweep rate of 20 mV / s.

[0147] [3. Results] 3.1. Fuel cell system 10a (with anode humidification) [3.1.1. Performance at an outside temperature of 30°C] Figure 3 shows the IV curves of a fuel cell system 10a using Pt / Sb-SnO2, Pt / Nb-SnO2, or Pt / C as a cathode at an ambient temperature of 30°C and an ambient humidity of 50%. Figure 4 shows the dependence of ORR mass activity at 0.84V (ambient temperature 30°C) on ambient humidity for the fuel cell system 10a. Figure 5 shows the dependence of current density at 0.6V (ambient temperature 30°C) on ambient humidity for the fuel cell system 10a.

[0148] At all humidity levels, the ORR mass activity increased in the following order: Example 2 (Pt / Nb-SnO2) > Example 1 (Pt / Sb-SnO2) > Comparative Example 1 (Pt / C). Examples 1 and 2, which used a porous SnO2 support, showed higher ORR mass activity than Comparative Example 1, which used a solid carbon support. This is thought to be because in Examples 1 and 2, the Pt fine particles supported inside the mesopores were not directly coated with the ionomer, thereby reducing catalyst poisoning by the sulfonic acid groups of the ionomer. Moreover, the ORR mass activity of Example 1 was lower than that of Example 2. This is thought to be because Sb doped in SnO2 is more easily eluted than Nb, and the eluted Sb ions poisoned the catalyst.

[0149] On the other hand, the current density at 0.6 V increased in the following order: Comparative Example 1 (Pt / C) > Example 1 (Pt / Sb-SnO2) > Example 2 (Pt / Nb-SnO2). This is consistent with the order of the conductivity of the supports, and is thought to indicate that the lower the conductivity of the support, the greater the IR loss.

[0150] [3.1.2. Performance at an outside temperature of 25°C] Figure 6 shows the IV curves of a fuel cell system 10a using Pt / Sb-SnO2, Pt / Nb-SnO2, or Pt / C as a cathode at an ambient temperature of 25°C and an ambient humidity of 50%. Figure 7 shows the ambient humidity dependence of ORR mass activity at 0.84V (ambient temperature 25°C) for the fuel cell system 10a. Figure 8 shows the ambient humidity dependence of current density at 0.60V (ambient temperature 25°C) for the fuel cell system 10a.

[0151] As with the ambient temperature of 30° C., the ORR mass activity increased in the following order at all humidities: Example 2 (Pt / Nb-SnO2) > Example 1 (Pt / Sb-SnO2) > Comparative Example 1 (Pt / C). Also, the current density at 0.6 V increased in the following order at all humidities: Example 1 (Pt / Sb-SnO2) > Comparative Example 1 (Pt / C) > Example 2 (Pt / Nb-SnO2).

[0152] In tests assuming an ambient temperature of 25°C, the water vapor contained in the H2 gas supplied to the anode was less than that at an ambient temperature of 30°C, creating a drier environment inside the cell than at an ambient temperature of 30°C. Under such an environment, the water content in the catalyst layer, which uses a water-repellent carbon support, decreases. As a result, the proton conductivity of the ionomer in the catalyst layer is thought to have decreased, resulting in increased IR loss. In fact, in Comparative Example 1, the current density at 0.6 V decreased as the ambient humidity decreased (Figure 8). In contrast, when hydrophilic SnO2 is used as the support, moisture is easily retained in the catalyst layer even in a dry environment. This is thought to be why Example 1 achieved a higher current density at 0.6 V than Comparative Example 1.

[0153] 3.2. Fuel Cell System 10b (without anode humidification) Figure 9 shows the IV curves of a fuel cell system 10b using Pt / Sb-SnO2 or Pt / C as the cathode at an ambient temperature of 30°C and ambient humidity of 50%. Figure 10 shows the IV curves of the fuel cell system 10b at an ambient temperature of 25°C and ambient humidity of 50%. Figure 11 shows the ORR mass activity of the fuel cell system 10b at 0.84V (ambient humidity of 50%). Figure 12 shows the current density of the fuel cell system 10b at 0.60V (ambient humidity of 50%).

[0154] In the fuel cell system 10b, unhumidified H gas is introduced to the anode, creating a drier environment within the cell than in the fuel cell system 10a. Under such an environment, Example 1, which used a porous SnO support, was found to be superior to Comparative Example 1, which used a solid carbon support, in both ORR mass activity and current density at 0.6 V.

[0155] From the above, it was shown that in an environment simulating a fuel cell system equipped with an open cathode polymer electrolyte fuel cell, using a porous SnO2 support for the cathode provides higher catalytic activity than using a carbon support. Furthermore, under conditions where the cell interior is dry, it was shown that the performance in the power range (current density at a cell voltage of 0.6 V) is also higher when using a porous SnO2 support than when using a carbon support.

[0156] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0157] The fuel cell system according to the present invention can be used in small mobility vehicles, small portable devices, and the like. [Explanation of symbols]

[0158] 10a, 10b Fuel cell system 12 Closed cathode polymer electrolyte fuel cell 14 Hydrogen gas supply device 16 Humidifier 18 Fans

Claims

1. an open cathode polymer electrolyte fuel cell including an anode catalyst layer and a cathode catalyst layer; a hydrogen gas supply device for supplying hydrogen gas to the anode catalyst layer; Equipped with The cathode catalyst layer contains tin oxide particles as a cathode catalyst support. Fuel cell system.

2. 2. The fuel cell system according to claim 1, further comprising a humidifier for humidifying the hydrogen gas, the humidifier being provided between the hydrogen gas supply device and the anode catalyst layer.

3. 2. The fuel cell system according to claim 1, further comprising a fan for introducing outside air into the cathode catalyst layer.

4. The cathode catalyst layer is an electrode catalyst in which Pt-based particles are supported on the surfaces of the tin oxide-based particles; Catalyst layer ionomer and 10. The fuel cell system of claim 1, comprising:

5. The tin oxide particles are SnO containing at least one element selected from the group consisting of Sb, Nb, Ta and W. 2 2. The fuel cell system according to claim 1, comprising:

6. The tin oxide particles are It has a structure in which porous primary particles are fused together in a beaded shape (a beaded structure), Specific surface area is 30m 2 / g or more The fuel cell system according to claim 1 .

7. 2. The fuel cell system according to claim 1, wherein the tin oxide particles contain pores with a pore diameter of 4 nm to 15 nm.

8. The tin oxide particles are (a) Sb-doped SnO 2 and the doping amount of Sb is 2.5 at % or more and 15.0 at % or less. 2 particles, and / or (b) Nb-doped SnO 2 and the Nb-doping amount is 2.5 at % or more and 15.0 at % or less. 2 particle 10. The fuel cell system of claim 1, comprising:

9. 5. The fuel cell system according to claim 4, wherein the following formulas (1) and (2) are satisfied: MA 1 (A / g Pt )≧700 …(1) Mạ 2 (A / ') Pt )≧-2.3RH amb +8000 …(2) however, "MA 1 (A / g Pt )" means セルTemperatureT cell 40℃, ambient temperature T amb 25℃ Outside air relative humidity RH amb :50% or more and 100% or less, Hydrogen gas humidity: 100% of the outside air temperature, cell voltage: 0.84V The mass activity of the oxygen reduction reaction when power generation is performed under the conditions "MA 2 (A / g Pt )" means セルTemperatureT cell 40℃, ambient temperature T amb 30℃ Outside air relative humidity RH amb :50% or more and 100% or less, Hydrogen gas humidity: 100% of the outside air temperature, cell voltage: 0.84V Mass activity of the oxygen reduction reaction when power generation is performed under the conditions.

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