Fuel cell electrode

Abstract

To prevent elution of precious metals in a high potential state such as idling in an electric vehicle using a fuel cell, while also ensuring high output.SOLUTION: An electrode catalyst for a fuel cell has support particles 11 having a metal oxide and precious metal microparticles 12 supported on the support particles includes both low-loading particles 10, which are support particles 11 having a relatively low loading amount of precious metal microparticles 12, and high-loading particles 20, which are support particles 11 having a relatively high loading amount of precious metal microparticles 12.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an electrode for a fuel cell and a vehicle equipped with the fuel cell. [Background technology]

[0002] Fuel cell catalysts, for example, are made by supporting precious metal particles, such as platinum, ruthenium, or alloys containing these, on the surface of a carbon support. However, these carbon supports are subject to oxidation and corrosion under the high potential conditions that occur during fuel cell startup, causing the supported precious metal particles to fall off and resulting in a decrease in output. Furthermore, the sudden electrical fluctuations that occur during this oxidation and corrosion can cause the precious metal particles to dissolve or aggregate.

[0003] To address this issue, it has been proposed to use ceramics such as tin oxide or tantalum oxide as supports in fuel cell catalysts instead of carbon (see, for example, Patent Document 1). Because ceramic supports do not undergo the oxidation corrosion reaction that occurs with carbon supports, it is possible to prevent the precious metal particles from falling off due to oxidation corrosion.

[0004] Furthermore, ceramic supports made of materials that can become n-type semiconductors, such as tin oxide, are known to form a highly electrically resistive layer called an electron depletion layer near the surface when surrounding oxygen is adsorbed onto the surface. When this electron depletion layer is formed, the resistance of the ceramic support increases, inhibiting electron transfer. It has been proposed that the electron depletion layer can prevent the elution and aggregation of precious metal particles caused by the sudden potential fluctuations that occur when a fuel cell is started by inhibiting electron transfer. This allows the state of the precious metal to be maintained. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-157353 Summary of the Invention [Problem to be solved by the invention]

[0006] However, ceramics generally have lower electrical conductivity than carbon, which means that fuel cell catalysts using ceramic supports have the problem of lower output compared to fuel cell catalysts using carbon supports. To solve this problem, it has been proposed to increase the density of the precious metal particles. By increasing the density, the number of carrier electrons donated from the precious metal particles to the ceramic support, which can become an n-type semiconductor, can be increased, improving electrical conductivity. However, this approach has the drawback of making it difficult for the above-mentioned electron depletion layer to appear due to the increased number of carrier electrons donated to the ceramic support, making it impossible to prevent the elution and aggregation of precious metal particles using the above-mentioned electron depletion layer.

[0007] Therefore, the objective of this invention is to achieve both improved electrical conductivity to increase fuel cell output and prevention of elution and aggregation of precious metal particles to maintain the state of the carrier, which have traditionally been in a trade-off relationship and have not been possible to achieve simultaneously. [Means for solving the problem]

[0008] This invention is An electrode catalyst for a fuel cell, comprising support particles having a metal oxide and noble metal fine particles supported on the support particles, low-loading particles, which are support particles having a relatively low loading amount of the noble metal fine particles; The above-mentioned problems have been solved by providing an electrode catalyst having both high-loading particles, which are support particles having a relatively high loading amount of the noble metal fine particles.

[0009] As one embodiment of the electrode catalyst in the present invention, a form having the low loading particles and the high loading particles in a mixed state can be adopted. [Effects of the Invention]

[0010] In the electrode catalyst according to the present invention, by using a combination of low-loading particles and high-loading particles, an electron depletion layer is generated in the low-loading particles, preventing the elution and aggregation of the precious metal fine particles, while at the same time increasing the donation of carrier electrons to the support particles by the high-loading particles, improving electrical conductivity and thereby improving the output of fuel cells using this electrode catalyst. [Brief explanation of the drawings]

[0011] [Figure 1] Conceptual diagram of a fuel cell using an electrode catalyst according to the present invention. [Figure 2] (a) A conceptual diagram of low-loading particles constituting the electrode catalyst according to the present invention, (b) A conceptual diagram of high-loading particles constituting the electrode catalyst according to the present invention, (c) A conceptual diagram of the vicinity of the surface of a low-loading particle, and (d) A conceptual diagram of the vicinity of the surface of a high-loading particle. [Figure 3] Conceptual diagram of an electrode catalyst that combines low-loading particles and high-loading particles [Figure 4] Schematic diagram of an electric vehicle equipped with a fuel cell having an electrode catalyst according to the present invention. [Figure 5] Graph showing an example of potential fluctuations when an electric vehicle using a fuel cell is operated DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the present invention will be described with reference to the drawings. The present invention relates to an electrode catalyst used in a fuel cell, a fuel cell using the same, and a vehicle equipped with the same. A schematic diagram of a fuel cell 1 is shown in FIG. 1. The fuel cell 1 supplies power to a load 2. Here, a polymer electrolyte fuel cell (PEFC) is shown as an example. An example of the load 2 is a fuel cell-equipped automobile. An anode 3 and a cathode 4 are electrically connected to the load 2. The anode 3 is the fuel electrode, and hydrogen is supplied from the anode-side gas diffusion layer 5, causing a reaction according to the following reaction formula (1). The generated hydrogen ions H + is supplied to the cathode 4, which is the air electrode, through the electrolyte membrane 6. At the cathode 4, oxygen is supplied from the air to the cathode-side gas diffusion layer 7, causing the reaction of the following reaction formula (2).

[0013] Anode: 2H2 → 4H + +4e - ···(1) Cathode: O2+4H + +4e - → 4H2O (2)

[0014] The electrode catalyst according to the present invention causes the above reaction on the surface of the cathode 4 and includes support particles 11 having a metal oxide and precious metal particles 12 supported on the support particles. The electrode catalyst according to the present invention includes low-loading particles 10 and high-loading particles 20, which differ in the amount of precious metal particles 12 supported. A conceptual diagram of the low-loading particles 10 is shown in FIG. 2(a), and a conceptual diagram of the high-loading particles 20 is shown in FIG. The low-loading particles 10 are support particles 11 with a relatively low amount of precious metal particles 12 supported, and the high-loading particles 20 are support particles 11 with a relatively high amount of precious metal particles 12 supported.

[0015] When oxygen molecules are adsorbed onto the surface of the support particles 11, electrons near the surface of the support particles 11 are stolen. Because the number of precious metal particles 12 supported on the surface of the low-loaded particles 10 is small, the supply of carrier electrons from the precious metal particles 12 is insufficient, and an electron depletion layer 13 with a shortage of electrons appears near the surface (FIG. 2(c)). When this electron depletion layer 13 appears, the electrical resistance of the low-loaded particles 10 increases, and the movement of electrons between the support particles 11 is hindered. This hinders the electron movement necessary for the elution of the precious metal particles 12, thereby suppressing the elution of the precious metal particles 12.

[0016] On the other hand, since the high-loading particles 20 have many precious metal particles 12 loaded on their surfaces, many carrier electrons are supplied from the precious metal particles 12, and an electron depletion layer 13 is less likely to appear (FIG. 2(d)). Therefore, electrons move easily in the high-loading particles 20, which contributes to operating the fuel cell 1 itself at high output. Furthermore, when the precious metal particles 12 are in contact with each other, the precious metal particles 12 themselves become an electron conduction path, which reduces the resistance value of the entire catalyst layer of the electrode catalyst and contributes to further improving the output of the fuel cell.

[0017] The metal oxide constituting the carrier particles 11 may be, as a main component, an oxide of a Group 14 element such as Sn, a transition metal element such as Ti, or both. Furthermore, oxides of other metal elements may be contained in addition to the main component. Among these, it is particularly preferable to use an oxide of Sn as the main component. Oxides that can become n-type semiconductors, such as Sn oxide, tend to have excess electrons and readily donate electrons to oxygen molecules, thereby suppressing the ionization of precious metals. The term "main component" refers to a metal element that contains the largest number of atoms compared to the other metal elements.

[0018] The particle size of the carrier particles 11 is preferably 3 nm or more, more preferably 5 nm or more, and even more preferably 10 nm or more. If it is less than 3 nm, it may be too small and may not be able to sufficiently support the precious metal fine particles 12 as a carrier. On the other hand, it is preferably 500 nm or less, more preferably 100 nm or less, and even more preferably 50 nm or less. If it exceeds 500 nm, it is too large and difficult to mix.

[0019] The noble metal constituting the noble metal particles 12 is an element having oxygen reduction activity, and preferably contains platinum in particular. Alternatively, the noble metal may be an alloy containing other transition metals that act as catalytic activity (have oxygen reduction activity) in addition to platinum. Examples of noble metals that can be the other transition metals include palladium, cobalt, and nickel.

[0020] When the noble metal particles 12 are particulate, the particle size is preferably 1 nm or more, and more preferably 3 nm or more. If the particle size is less than 1 nm, they are too likely to dissolve and disappear, and their catalytic activity is likely to be lost. On the other hand, a particle size of 20 nm or less is preferable, and a particle size of 5 nm or less is more preferable. If the particle size exceeds 20 nm, the electrochemically active surface area becomes small, and there is a high risk that the electrode performance will not be fully exhibited. Furthermore, in the case of highly loaded particles, multiple noble metal particles may be strung together like beads to form an aggregate. The particle size of this aggregate of noble metal particles 12 needs to be at least 1 nm or more, and may be 20 nm or more.

[0021] The mass ratio of the precious metal fine particles 12 to the support particles 11 differs between the low-loaded particles 10 and the high-loaded particles 20. The content of the precious metal fine particles 12 relative to the entire low-loaded particles 10 must be 5% by mass or more and 20% by mass or less, which is lower than the content of the high-loaded particles 20. On the other hand, the content of the precious metal fine particles 12 relative to the entire high-loaded particles 20 must be 15% by mass or more and 50% by mass or less, which is higher than the content of the low-loaded particles 10.

[0022] In accordance with the present invention, an electrode catalyst having both low-loading particles 10 and high-loading particles 20 can be used, for example, by mixing these particles. A conceptual diagram of this configuration is shown in FIG. 3. The electrode is composed of a mixed layer 4a containing low-loading particles 10 and high-loading particles 20. The mixture of low-loading particles 10, which can generate an electron depletion layer 13, and high-loading particles 20, which have high electronic conductivity, can balance the suppression of precious metal elution and aggregation with the maintenance of high output. In this case, the mass ratio of the low-loading particles 10 to the high-loading particles 20 is preferably 30:70 to 80:20. If the mass ratio of the low-loading particles 10 is less than 30%, the supply of carrier electrons will be too great, resulting in almost no electron depletion layer being generated, and it will be impossible to prevent the elution and aggregation of precious metal fine particles 12. On the other hand, if the mass ratio of the low-loading particles 10 exceeds 80%, there will be a shortage of carrier electrons, and the electrical conductivity of the support particles 11 made of oxide will not improve, resulting in a significant decrease in output.

[0023] As shown in FIG. 4, a fuel cell 1 equipped with an electrode catalyst according to the present invention can be used in an electric vehicle 30. Electric vehicles include plug-in hybrid vehicles (PHEVs) that can be externally charged or externally powered. A load 2 is connected to the motor of the electric vehicle 30 via a converter, inverter, or the like of the electric vehicle 30, driving the electric vehicle 30. An example of the transition in the electrode potential of the fuel cell when the electric vehicle is driven is shown in FIG. 5. At startup, the potential instantaneously rises to a start-stop potential of 1.0 V or higher. In this high-potential state, platinum elution reactions are likely to occur in an electrode catalyst without an electron depletion layer 13. After startup, the potential temporarily drops when a motor or other load 2 is being driven, but the high-potential state is maintained when the current flowing through the load 2 is small, such as during idling. Even in this situation, precious metal fine particle elution reactions are likely to occur in an electrode catalyst without an electron depletion layer 13. The electrode catalyst of the present invention contains a certain amount of low-loading particles 10, which are likely to cause an electron depletion layer 13 to appear, thereby suppressing the platinum elution reaction under high potential conditions and increasing durability, and also contains a certain amount of high-loading particles 20, which are unlikely to cause an electron depletion layer 13 to appear, thereby preventing a decrease in output and allowing the electrode catalyst to function as a high-output fuel cell and drive electric vehicles with high efficiency. [Explanation of symbols]

[0024] 1 fuel cell 2. Load 3 anode 4 cathodes 5. Anode side gas diffusion layer 6 Electrolyte membrane 7. Cathode side gas diffusion layer 10 Low-loading particles 11 Carrier particles 12 Precious metal fine particles 13 Electron depletion layer 20 High-loading particles 30 Electric Vehicles

Claims

1. An electrode catalyst for a fuel cell, comprising support particles having a metal oxide and noble metal fine particles supported on the support particles, low-loading particles, which are support particles having a relatively low loading amount of the noble metal fine particles; and high-loading particles, which are support particles having a relatively high loading amount of the noble metal fine particles.

2. 10. The electrocatalyst of claim 1 having a mixture of said low loading particles and said high loading particles.

3. A fuel cell using the electrode catalyst according to claim 1 or 2 as a cathode.

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