Electrode catalyst for fuel cell and fuel cell

Abstract

To provide an electrode catalyst and a fuel cell capable of suppressing a decrease in catalytic activity.SOLUTION: A fuel cell electrode catalyst 1 is used in an electrode of a fuel cell. The electrode catalyst 1 has a conductive carbon fiber 2 containing molybdenum sulfide, and a catalytic metal 3 that has the effect of promoting an electrode reaction in a fuel cell and is supported on the conductive carbon fiber 2. The concentration of sulfur atoms on the surface of the conductive carbon fiber 2, which is obtained by analyzing the surface of the electrode catalyst 1 using X-ray photoelectron spectroscopy, may be 0.3 atomic % or more and 2 atomic % or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electrode catalyst for a fuel cell and a fuel cell. [Background technology]

[0002] A fuel cell such as a solid polymer electrolyte fuel cell has a membrane electrode assembly including an electrolyte membrane, an anode provided on one side of the electrolyte membrane, and a cathode provided on the other side of the electrolyte membrane. The fuel cell generates an electromotive force between the anode and the cathode by supplying a fuel to the anode of the membrane electrode assembly and an oxidant to the cathode, thereby generating electricity.

[0003] The electrodes of fuel cells contain an electrode catalyst for promoting electrode reactions. For example, Patent Document 1 describes a supported catalyst for fuel cells having a layered structure in which a carbon-based support, metal oxide particles, and catalytic metal particles are layered in this order. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-27918 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, conventional electrode catalysts, including the supported catalyst of Patent Document 1, have a problem in that their catalytic activity decreases due to catalyst poisoning when power is generated continuously. Therefore, there is a demand for electrode catalysts that can maintain high catalytic activity for a longer period of time.

[0006] The present invention has been made in view of the above problems, and aims to provide an electrode catalyst and a fuel cell capable of suppressing a decrease in catalytic activity. [Means for solving the problem]

[0007] One aspect of the present invention is a fuel cell electrode catalyst used in an electrode of a fuel cell, comprising: A conductive carbon fiber containing molybdenum sulfide; The present invention also relates to an electrode catalyst for a fuel cell, comprising: a catalytic metal supported on the conductive carbon fiber, the catalytic metal having a function of promoting an electrode reaction in the fuel cell. Effect of the Invention

[0008] The catalytic metal in the fuel cell electrode catalyst (hereinafter referred to as "electrode catalyst") is supported on the surface of conductive carbon fiber containing molybdenum sulfide. By supporting the catalytic metal on the surface of conductive carbon fiber containing molybdenum sulfide in this manner, it is possible to suppress a decrease in catalytic activity due to poisoning of the catalytic metal.

[0009] As described above, according to the above-described aspects, it is possible to provide an electrode catalyst and a fuel cell capable of suppressing a decrease in catalytic activity. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of an electrode catalyst according to the first embodiment. [Diagram 2] FIG. 2 is a partial cross-sectional view showing a main part of a fuel cell including an electrode catalyst according to a second embodiment. [Diagram 3] FIG. 3 is an explanatory diagram showing the current-potential curves of each test agent in the experimental examples. [Figure 4] FIG. 4 is an explanatory diagram showing the time-current curves of each test agent in the experimental example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] (Embodiment 1) An embodiment of the electrode catalyst will be described with reference to Fig. 1. The electrode catalyst 1 of this embodiment is configured to be used in an electrode of a fuel cell. As shown in Fig. 1, the electrode catalyst 1 has conductive carbon fiber 2 containing molybdenum sulfide, and catalytic metal 3 that has the effect of promoting an electrode reaction in a fuel cell and is supported on the carbon fiber. The configuration of the electrode catalyst 1 will be described in more detail below.

[0012] For example, polyacrylonitrile-based carbon fibers containing molybdenum sulfide and pitch-based carbon fibers containing molybdenum sulfide can be used as the conductive carbon fibers 2. Molybdenum sulfide is uniformly dispersed in the conductive carbon fibers 2, and a part of it is exposed on the surface of the conductive carbon fibers 2.

[0013] The concentration of sulfur atoms on the surface of the conductive carbon fiber 2, which is obtained by analyzing the surface of the electrode catalyst 1 using X-ray photoelectron spectroscopy (so-called XPS), is preferably 0.3 atomic % or more and 2 atomic % or less. By setting the concentration of sulfur atoms on the surface of the conductive carbon fiber 2 within the above-mentioned specific range, a sufficient amount of molybdenum sulfide can be exposed on the surface of the conductive carbon fiber 2. As a result, poisoning of the catalytic metal 3 can be more effectively suppressed, and a decrease in catalytic activity can be suppressed for a longer period of time.

[0014] The concentration of sulfur atoms on the surface of the conductive carbon fiber 2 is more preferably 0.4 atomic % or more and 1.5 atomic % or less, further preferably 0.5 atomic % or more and 1.0 atomic % or less, and particularly preferably 0.6 atomic % or more and 0.9 atomic % or less. In this case, the effect of suppressing the poisoning of the catalytic metal 3 is further improved, and the catalytic activity is further increased, thereby making it possible to further improve the power generation output of the fuel cell.

[0015] The concentration of sulfur atoms on the surface of the conductive carbon fiber 2 can be adjusted, for example, by the amount of molybdenum sulfide added when producing the conductive carbon fiber 2. The conductive carbon fiber 2 may also contain an additive such as conductive carbon particles.

[0016] The average diameter of the conductive carbon fibers 2 is preferably 250 nm or more and 500 nm or less. In this case, the specific surface area of ​​the electrode catalyst 1 becomes larger, so that the contact frequency between the fuel or oxidizing agent and the electrode catalyst 1 can be increased. As a result, the effect of promoting the electrode reaction can be further increased.

[0017] The method for calculating the average diameter of the conductive carbon fibers 2 is, for example, as follows. First, an enlarged photograph of the electrode catalyst 1 is obtained by observing the electrode catalyst 1 using a scanning secondary electron microscope (SEM). A plurality of measurement positions are set randomly on the conductive carbon fibers 2 in the enlarged photograph, and the diameter of the conductive carbon fibers 2 at each of these measurement positions is measured. The arithmetic mean value of the diameters of the conductive carbon fibers 2 at the plurality of measurement positions thus obtained is taken as the average diameter of the conductive carbon fibers 2. The number of measurement positions set on the enlarged photograph is not particularly limited, but the more the number of measurement positions is, the more accurate the average diameter can be obtained. From this viewpoint, the number of measurement positions set on the enlarged photograph is preferably 10 or more.

[0018] The conductive carbon fiber 2 used in the electrode catalyst 1 is obtained by carbonizing an organic fiber made of an organic material, which contains, for example, molybdenum sulfide. For example, polyacrylonitrile or coal pitch can be used as the organic material constituting the organic fiber.

[0019] The catalytic metal 3 is supported on the surface of the conductive carbon fiber 2. As the catalytic metal 3, a metal having a property of functioning as a catalyst for the electrode reaction of the fuel cell can be used. More specifically, as the catalytic metal 3, for example, a precious metal such as platinum, palladium, and ruthenium, and an alloy containing these precious metals can be used. From the viewpoint of more reliably obtaining the effect of suppressing the poisoning of the catalytic metal 3, it is preferable that the catalytic metal 3 is palladium. In addition, since palladium has excellent catalytic activity for the oxidation reaction of formic acid, the electrode catalyst 1 having palladium as the catalytic metal 3 is particularly suitable for a fuel cell that uses formic acid as fuel.

[0020] The amount of the catalytic metal 3 supported is preferably 20 mass % or more and 40 mass % or less with respect to the mass of the electrode catalyst 1. In this case, the effect of suppressing poisoning of the catalytic metal 3 can be more reliably obtained.

[0021] The electrode catalyst 1 can be produced, for example, by the following method. First, an organic material such as polyacrylonitrile or coal pitch is mixed with molybdenum sulfide, and then the mixture is spun to produce organic fibers containing molybdenum sulfide. The spinning method for producing the organic fibers is not particularly limited, and an appropriate method may be selected from known methods depending on the desired diameter of the conductive carbon fiber 2 and the properties of the organic material used.

[0022] Next, the organic fibers are heat-treated to carbonize the organic matter, producing conductive carbon fibers 2 containing molybdenum sulfide. Thereafter, a catalytic metal 3 is supported on the surface of the conductive carbon fibers 2 by a known method. In this manner, an electrode catalyst 1 can be obtained.

[0023] The electrode catalyst 1 of this embodiment has conductive carbon fiber 2 containing molybdenum sulfide and catalytic metal 3 supported on the surface of the conductive carbon fiber 2. In this way, by supporting the catalytic metal 3 on the surface of the conductive carbon fiber 2 containing molybdenum sulfide, it is possible to suppress a decrease in catalytic activity due to poisoning of the catalytic metal 3. The reason why such an effect is obtained is not necessarily clear at this point, but it is thought that, for example, the electronic state of the catalytic metal 3 changes due to an interaction between the molybdenum sulfide exposed on the surface of the conductive carbon fiber 2 and the catalytic metal.

[0024] (Embodiment 2) In this embodiment, an example of a fuel cell including the electrode catalyst 1 of embodiment 1 will be described with reference to Fig. 2. Among the symbols used in the following embodiments, the same symbols as those used in the previous embodiments indicate the same components as those in the previous embodiments unless otherwise specified.

[0025] The electrode catalyst 1 can be applied to various types of fuel cells, such as fuel cells using hydrogen as fuel, direct methanol fuel cells using methanol as fuel, and direct formic acid fuel cells using formic acid as fuel. Among these fuel cells, fuel cells using liquid fuels containing carbon atoms, such as formic acid and methanol, have a problem that the activity of the catalytic metal is easily reduced because carbon monoxide, which causes poisoning of the catalytic metal, may be generated in the electrode reaction of the anode. In contrast, the electrode catalyst 1 can suppress the poisoning of the catalytic metal 3, and therefore can suppress the reduction in catalytic activity for a long period of time even in fuel cells using liquid fuels containing carbon atoms. Therefore, the electrode catalyst 1 is useful for fuel cells using liquid fuels containing carbon atoms, particularly direct formic acid fuel cells using formic acid as fuel.

[0026] FIG. 2 shows an example of a fuel cell 4. The fuel cell 4 has a membrane electrode assembly (hereinafter referred to as "MEA") 41 including an electrolyte membrane 411 and an electrode catalyst 1 provided on both sides of the electrolyte membrane 411, a first separator 42 provided on one side of the MEA 41, and a second separator 43 provided on the other side of the MEA 41. The first separator 42 is configured to be able to supply fuel to the MEA 41, and the second separator 43 is configured to be able to supply an oxidant to the MEA 41. That is, the fuel cell 4 is configured such that the electrode catalyst 1 on the first separator 42 side of the MEA 41 serves as the anode 1a, and the electrode catalyst 1 on the second separator 43 side serves as the cathode 1c. The fuel cell 4 supplies fuel from the first separator 42 to the anode 1a and supplies an oxidant from the second separator 43 to the cathode 1c, thereby causing an electrode reaction on the anode 1a and the cathode 1c to generate electricity. In FIG. 2, the structure of the electrode catalyst 1 is simplified for the sake of convenience.

[0027] The fuel cell 4 may have one or more unit cells 40 including an MEA 41, a first separator 42, and a second separator 43. For example, when the fuel cell 4 is composed of one unit cell 40 as shown in FIG. 2, the electrode catalyst 1 serving as the anode 1a and the electrode catalyst 1 serving as the cathode 1c may be electrically connected to an external load, and a current may be drawn from each electrode. Although not shown in the figure, when the fuel cell 4 has a plurality of unit cells 40, these unit cells 40 may be stacked together to form a cell stack. In this case, each unit cell 40 may be electrically connected in series or in parallel with the other unit cells 40.

[0028] The electrolyte membrane 411 in the MEA 41 has electrical insulation properties and is capable of transporting hydrogen ions (H + The cation exchange resin is made of a cation exchange resin that is configured to selectively allow permeation of cations such as cations of 1 to 100 carbon atoms. Examples of such cation exchange resins include perfluoroalkylsulfonic acid polymers (e.g., Nafion (registered trademark) manufactured by DuPont).

[0029] The first separator 42 may be made of an electric conductor such as a metal material, such as gold-plated stainless steel, a conductive nonmetallic material, such as conductive carbon, or a conductive composite material. The first separator 42, which is electrically conductive, functions as a current collector that collects electrons generated by the electrode reaction in the MEA 41, and can conduct the electric power generated in the MEA 41 to the outside.

[0030] The first separator 42 is provided with a fuel flow channel 421 configured to be able to supply fuel to the anode 1a. The shape and arrangement of the fuel flow channel 421 may take various forms. For example, the fuel flow channel 421 may be a groove provided in the first separator 42, having a cross-sectional shape such as a rectangular, triangular, or semicircular shape. The fuel flow channel 421 may have, for example, a plurality of straight portions arranged in parallel to each other and a folded portion connecting an end of each straight portion to an end of a straight portion adjacent to the straight portion, and may be arranged in a meandering manner in a plan view seen from the contact surface side, but other arrangements may also be used.

[0031] Between the MEA 41 and the first separator 42, a fuel diffusion layer 44 for diffusing the fuel supplied from the first separator 42 may be provided as necessary. As the fuel diffusion layer 44, for example, carbon paper, which is a composite material of conductive carbon fibers and carbon, or carbon cloth, which is a woven material of conductive carbon fibers, can be used. When liquid fuel is used as the fuel, the fuel diffusion layer 44 is preferably carbon cloth. Since the carbon cloth has appropriate gaps between the conductive carbon fibers constituting the carbon cloth, the liquid fuel can quickly penetrate into the gaps between the conductive carbon fibers due to surface tension. Therefore, by using the carbon cloth as the fuel diffusion layer 44, the liquid fuel can be more easily diffused in the fuel diffusion layer 44.

[0032] Furthermore, a sealant 46 made of an elastomer may be provided between the outer periphery of the first separator 42 and the electrolyte membrane 411 of the MEA 41. The sealant 46 is disposed so as to be in close contact with each of the first separator 42 and the MEA 41. By providing the sealant 46 between the first separator 42 and the MEA 41, unintended leakage of fuel from the internal space of the fuel cell 4 to the outside can be suppressed. As the elastomer used for the sealant 46, for example, a polymer material having rubber elasticity, such as ethylene-propylene-diene rubber (i.e., EPDM), can be used.

[0033] The second separator 43 may be made of an electric conductor such as a metal material, such as gold-plated stainless steel, a conductive nonmetallic material, such as conductive carbon, or a conductive composite material, similar to the first separator 42. The second separator 43, which is electrically conductive, functions as a current collector that supplies electrons generated in the anode 1a and passed through an external load to the cathode 1c, and can conduct the electric power generated in the MEA 41 to the outside.

[0034] The second separator 43 is provided with an oxidant flow path 431 configured to be able to supply an oxidant to the cathode 1c. Note that air, oxygen gas, or the like can be used as the oxidant. The shape and arrangement of the oxidant flow path 431 can take various forms, similar to the fuel flow path 421. For example, the oxidant flow path 431 may be a groove having a cross-sectional shape such as a rectangular shape, a triangular shape, or a semicircular shape, and provided in the second separator 43. In addition, the oxidant flow path 431 may have, for example, a plurality of straight portions arranged in parallel to each other and a folded portion connecting an end of each straight portion to an end of a straight portion adjacent to the straight portion, and may be arranged in a meandering manner in a plan view seen from the contact surface side, but other arrangements are also possible.

[0035] If necessary, an oxidant diffusion layer 45 for diffusing the oxidant supplied from the second separator 43 may be provided between the MEA 41 and the second separator 43. As the oxidant diffusion layer 45, for example, carbon paper which is a composite material of conductive carbon fiber and carbon, or carbon cloth which is a woven fabric of conductive carbon fiber, can be used.

[0036] Furthermore, a sealant 47 made of an elastomer may be provided between the outer periphery of the second separator 43 and the electrolyte membrane 411 of the MEA 41. The sealant 47 is disposed so as to be in close contact with each of the second separator 43 and the MEA 41. By providing the sealant 47 between the second separator 43 and the MEA 41, it is possible to suppress unintended leakage of the oxidant from the internal space of the fuel cell 4 to the outside. As the elastomer used for the sealant 47, for example, a polymer material having rubber elasticity, such as ethylene-propylene-diene rubber (i.e., EPDM), can be used.

[0037] The anode 1a and cathode 1c of the fuel cell 4 of this embodiment are composed of the electrode catalyst 1. Therefore, it is possible to suppress a decrease in power generation output due to poisoning of the catalytic metal 3 during power generation.

[0038] (Experimental Example) In this example, electrode catalysts were prepared by varying the blending amount of molybdenum sulfide, and the catalytic activity and the maintenance performance of the catalytic activity of the obtained electrode catalysts were evaluated. The preparation method and evaluation method of the electrode catalyst in this example will be described below.

[0039] [Method of producing electrode catalyst] First, conductive carbon particles ("Vulcan XC-72R" manufactured by Fuel Cell Earth) and molybdenum sulfide were dissolved in dimethylformamide. Polyacrylonitrile was further added to this solution, and the mixture was stirred at a temperature of 80°C for 5 hours, and then stirred at room temperature for 1 day to obtain a spinning ink. "Vulcan" is a registered trademark of Cabot Corporation.

[0040] Next, the spinning ink was spun by electrostatic spinning to obtain organic fibers containing molybdenum sulfide. The obtained organic fibers were heated in air at a temperature of 250°C for 10 hours for stabilization treatment, and then heated in nitrogen at a temperature of 1000°C for 1 hour to carbonize the polyacrylonitrile. The carbon fibers thus obtained were heated in a water vapor atmosphere with a dew point of 70°C at a temperature of 850°C for 1 hour for activation treatment. The above operations were carried out by changing the amount of molybdenum sulfide added to the spinning ink to obtain three types of conductive carbon fibers with different molybdenum sulfide contents.

[0041] Next, a catalytic metal was supported on the surface of the conductive carbon fiber by the following method. First, PdCl2 and ethylene glycol were mixed, and the pH of the resulting mixture was adjusted to within the range of 9 to 10 using a 2 mol / L aqueous sodium hydroxide solution. The mixture after the pH adjustment was refluxed at a temperature of 160°C for 3 hours to obtain a catalytic solution.

[0042] A mixture of conductive carbon fiber and ethylene glycol was added to this catalyst solution, and then the pH of the catalyst solution was adjusted to within the range of 1 to 2 using a 0.5 mol / L aqueous nitric acid solution. This catalyst solution was stirred overnight to precipitate palladium as a catalytic metal on the surface of the conductive carbon fiber. Thereafter, the conductive carbon fiber was filtered out of the catalyst solution, and then washed and vacuum dried to obtain electrode catalysts (test agents S1 to S3). When the catalytic metal was supported under these conditions, the amount of the catalytic metal supported in the test agents S1 to S3 was 20 mass% with respect to the mass of each test agent (i.e., the sum of the mass of the conductive carbon fiber and the mass of the catalytic metal).

[0043] Table 1 shows the composition of the surface of the electrode catalyst measured by X-ray photoelectron spectroscopy and the average diameter of the conductive carbon fibers calculated based on the SEM image. Note that test agent R1 in Table 1 is an electrode catalyst for comparison with test agents S1 to S3. Test agent R1 has the same configuration as test agents S1 to S3, except that it does not contain molybdenum sulfide in the conductive carbon fiber. Also, the method for preparing test agent R1 is the same as the method for preparing test agents S1 to S3, except that molybdenum sulfide is not added to the spinning ink.

[0044] [Catalytic activity] The catalytic activity was evaluated based on the current-potential curve obtained by cyclic voltammetry. Specifically, 5 mg of the test agent was dispersed in 320 μL of ethanol to prepare a catalyst dispersion. 25 μL of a 5% by mass perfluoroalkylsulfonic acid polymer solution was added to the catalyst dispersion to prepare a catalyst ink. 2.5 μL of the obtained catalyst ink was applied to the surface of a glassy carbon electrode and then dried to prepare a working electrode.

[0045] Next, the working electrode, a platinum wire electrode as a counter electrode, and an Hg / HgSO4 electrode as a reference electrode were attached to a three-electrode electrochemical cell, and the electrochemical cell was then filled with a test solution containing 0.5 mol / L sulfuric acid and 3 mol / L formic acid, with nitrogen dissolved therein until saturated.

[0046] The electrochemical cell prepared as described above was connected to a potentio / galvanostat ("Automatic Polarization System HSV-110" manufactured by Hokuto Denko Corporation). The potential of the working electrode was then swept to obtain a current-potential curve. The potential sweep range was from 0 V to +1.2 V with respect to the standard hydrogen electrode, and the sweep rate was 10 mV / s. The current-potential curve was obtained in a room temperature environment.

[0047] Figure 3 shows the current-potential curves obtained by cyclic voltammetry. The vertical axis of Figure 3 is the current density ratio obtained by dividing the current density per mass of palladium supported on the electrode catalyst by the current density at the start of measurement for test agent R1, and the horizontal axis is the potential of the working electrode relative to the standard hydrogen electrode (unit: V vs. NHE). Table 2 shows the maximum current density ratio in the current-potential curve for each test agent.

[0048] [Maintaining catalytic activity] The catalytic activity maintenance performance was evaluated based on the time-current curve obtained by the chronoamperometry method. Specifically, a three-electrode electrochemical cell was prepared in the same manner as in the evaluation of catalytic activity, in which a working electrode, a platinum wire electrode as a counter electrode, and an Hg / HgSO4 electrode as a reference electrode were attached. After filling the electrochemical cell with the same test solution as the test solution used in the cyclic voltammetry method, the electrochemical cell was connected to a potentio / galvanostat. Then, a potential of +0.60 V against a standard hydrogen electrode was applied to the working electrode for 3600 seconds, and the current flowing through the working electrode during this period was measured. The chronoamperometry method was performed at room temperature.

[0049] FIG. 4 shows a time-current curve obtained by the chronoamperometry method. The vertical axis of FIG. 4 is the current density ratio obtained by dividing the current density per mass of palladium supported on the electrode catalyst by the current density at the start of measurement for test agent R1, and the horizontal axis is the elapsed time (unit: seconds) from the start of measurement. Table 2 shows the current density ratio at the start of measurement for each test agent, the current density ratio at the end of measurement, and the maintenance rate of catalytic activity (unit: %). The maintenance rate of catalytic activity is a value expressed as a percentage of the ratio of the current density at the end of measurement to the current density at the start of measurement.

[0050] [Table 1]

[0051] [Table 2]

[0052] As shown in Table 1, the catalytic metal in the test agents S1 to S3 is supported on the surface of the conductive carbon fiber containing molybdenum sulfide, so that the poisoning of the catalytic metal can be suppressed compared to the test agent R1 that does not contain molybdenum sulfide. Therefore, as shown in FIG. 4 and Table 2, the test agents S1 to S3 showed a higher catalytic activity retention rate than the test agent R1.

[0053] Among these test agents S1 to S3, test agent S2, in which the concentration of sulfur atoms on the surface of the electrode catalyst is 0.6 atomic % or more and 0.9 atomic % or less, showed a higher catalytic activity retention rate than test agents S1 and S3. Therefore, from the comparison of test agents S1 to S3, it can be understood that the electrode catalyst, in which the concentration of sulfur atoms on the surface is 0.6 atomic % or more and 0.9 atomic % or less, is more excellent in the effect of suppressing catalyst poisoning. In addition, as shown in FIG. 3 and Table 2, it can be understood that test agent S2 has a higher maximum current density and higher catalytic activity than test agents S1 and S3.

[0054] Although aspects of the electrode catalyst and fuel cell have been described above based on the embodiments and experimental examples, the specific aspects of the electrode catalyst and fuel cell according to the present invention are not limited to the aspects of the above-mentioned embodiments and experimental examples, and can be applied to various embodiments without departing from the gist thereof. [Explanation of symbols]

[0055] 1 Electrode catalyst for fuel cells 2. Conductive carbon fiber 3. Catalytic metals

Claims

1. A fuel cell electrode catalyst used in an electrode of a fuel cell, A conductive carbon fiber containing molybdenum sulfide; a catalytic metal supported on the conductive carbon fiber, the catalytic metal having an effect of promoting an electrode reaction in the fuel cell;

2. 2. The fuel cell electrode catalyst according to claim 1, wherein the concentration of sulfur atoms on the surface of the conductive carbon fiber, as obtained by analyzing the surface of the electrode catalyst using X-ray photoelectron spectroscopy, is 0.3 atomic % or more and 2 atomic % or less.

3. 3. The electrode catalyst for a fuel cell according to claim 1, wherein the amount of the catalytic metal supported is 20% by mass or more and 40% by mass or less with respect to the mass of the electrode catalyst.

4. 3. The fuel cell electrode catalyst according to claim 1, wherein the catalytic metal is palladium.

5. 3. The fuel cell electrode catalyst according to claim 1, wherein the conductive carbon fibers have an average diameter of 250 nm or more and 500 nm or less.

6. A fuel cell comprising the fuel cell electrode catalyst according to claim 1 or 2.

7. 7. The fuel cell of claim 6 configured to use formic acid as a fuel.

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