JP7662504B2 - Fuel cell separator and method for manufacturing fuel cell separator - Google Patents

Description

This disclosure relates to a fuel cell separator that contacts the power generation section of a fuel cell and a method for manufacturing the same.

A fuel cell of a polymer electrolyte fuel cell is made up of a membrane electrode assembly (MEA) in which a solid polymer electrolyte membrane is bonded to an anode electrode and a cathode electrode to form an integrated unit, sandwiched between separators that act as a flow path for hydrogen gas and other gases, and multiple of these are stacked together to form a cell.

Figure 12 is an explanatory diagram showing an exploded view of the cell structure of a fuel cell 60. The XYZ Cartesian coordinate system is shown in Figure 12. The fuel cell 60 shown in the figure is one unit cell, and the fuel cell 60 includes, as its main components, an MEA 5 and fuel cell separators 15 and 16 provided on both sides of the MEA 5.

The MEA 5, which serves as the power generation section, includes as its main components a polymer electrolyte membrane 10, an anode catalyst layer 11, a cathode catalyst layer 12, a gas diffusion layer 13, and a gas diffusion layer 14. The anode catalyst layer 11, which serves as the cathode electrode, is provided on one main surface (the main surface in the -X direction) of the polymer electrolyte membrane 10, and the cathode catalyst layer 12, which serves as the anode electrode, is provided on the other main surface (the main surface in the +X direction) of the polymer electrolyte membrane 10.

When the fuel cell 60 is completed, the polymer electrolyte membrane 10 and the anode catalyst layer 11, and the polymer electrolyte membrane 10 and the cathode catalyst layer 12 are in close contact with each other, and the electrolyte membrane-catalyst layer gaps B1 and B2 shown in FIG. 12 do not exist.

Furthermore, a gas diffusion layer 13 is provided on one main surface of the negative electrode catalyst layer 11, and a gas diffusion layer 14 is provided on the other main surface of the positive electrode catalyst layer 12. The gas diffusion layer is also called a "Gas Diffusion Layer (GDL)."

Then, a fuel cell separator 15 is provided on one main surface of the gas diffusion layer 13. In this case, the fuel cell separator 15 is provided in such a manner that the surface of the convex portion 15t (the portion protruding in the +X direction) contacts one main surface of the gas diffusion layer 13. Similarly, a fuel cell separator 16 is provided on the other main surface of the gas diffusion layer 14. In this case, the fuel cell separator 16 is provided in such a manner that the surface of the convex portion 16t (the portion protruding in the -X direction) contacts the other main surface of the fuel cell separator 16.

The diffusion layer-separator gap B3 indicates the gap between the protruding portion 15t of the fuel cell separator 15 and the gas diffusion layer 13. The diffusion layer-separator gap B4 indicates the gap between the protruding portion 16t of the fuel cell separator 16 and the gas diffusion layer 14. Therefore, when the fuel cell 60 is completed, the diffusion layer-separator gaps B3 and B4 do not exist.

On the other hand, even when the fuel cell 60 is completed, a separator flow path SP15 exists between the recess 15v of the fuel cell separator 15 and the gas diffusion layer 13, and a separator flow path SP16 exists between the recess 16v of the fuel cell separator 16 and the gas diffusion layer 14. The separator flow paths SP15 and SP16 serve as flow paths for hydrogen gas, heated acid solution, etc.

In a fuel cell 60 with such a structure, the gas diffusion layer pairs 13 and 14 provided on both sides of the MEA 5 are layers that provide fuel gas or oxidant gas, respectively, and collect electricity generated by the electrochemical reaction.

Therefore, in order to stabilize the power generation efficiency of the fuel cell 60, it is required to reduce the electrical contact resistance between the gas diffusion layer 13 and fuel cell separator 15 (their convex portions 15t) and between the gas diffusion layer 14 and fuel cell separator 16 (their convex portions 16t), which are in contact with each other, and further to maintain the low contact resistance even after the power generation process using the fuel cell 60 has been completed (first requirement).

In addition, during power generation by the fuel cell 60, the separator pairs 15 and 16 are exposed to an environment in which a voltage is applied in a heated acid solution. For this reason, the separator pairs 15 and 16 are required to have high corrosion resistance against acid (second requirement).

In order to meet these first and second requirements, various improvements have been made to fuel cell separators. For the sake of convenience, in the following explanation, "fuel cell separator" may be abbreviated to "separator" and "gas diffusion layer" may be abbreviated to "GDL" in English.

An example of a conventional fuel cell separator that meets the first and second requirements described above is the fuel cell separator disclosed in Patent Document 1. This fuel cell separator has an antimony-doped tin oxide film (ATO film) formed on the surface of the separator in order to ensure the corrosion resistance of the separator and to achieve low contact resistance with the GDL.

Patent documents 2 to 6 are prior art documents that aim to reduce contact resistance by forming a precious metal on the separator surface.

Patent Document 2 proposes a method of forming a film of an alloy of a precious metal such as Au (gold) or Pt (platinum) with a non-precious metal such as Nb (niobium), Ta (tantalum), Zr (zirconium), or Hf (hafnium) on the surface of a separator substrate using a sputtering method or the like.

The technology disclosed in Patent Document 3 forms a precious metal film on the surface of a separator substrate (thin metal plate). The precious metal film has an average thickness of 10 nm or less, and is provided so that the protrusions are 15 nm or more thick.

Patent document 4 proposes a technique for plating an aluminum substrate for a separator with Ni (nickel) and then plating Au on top of that.

Patent document 5 proposes a technique for depositing dot-shaped Au particles onto the protruding portions of a metal separator (metal substrate) using, for example, an inkjet method, so that the area ratio of the protruding portions is 12% or more.

Patent document 6 discloses a separator structure that includes a titanium oxide layer on a metal substrate made of titanium, and further includes a conductive coating made of conductive particles (size 1 to 100 nm) on top of that.

JP 2019-192437 A JP 2010-182593 A JP 2011-29008 A JP 2013-105629 A JP 2014-139884 A JP 2012-190816 A

However, the conventional fuel cell separators described above are insufficient in terms of satisfying the first and second requirements. This point will be explained below.

The single layer of ATO contained in the conventional separator disclosed in Patent Document 1 was not sufficient to achieve the first requirement of low contact resistance. In other words, it was difficult to reduce the contact resistance with the GDL in the conventional separator with a single layer of tin oxide film coated on the surface.

When realizing the separator disclosed in Patent Document 5, coating the entire surface of the separator substrate with a precious metal would result in high processing costs.

On the other hand, if the precious metal is not formed over the entire surface of the substrate, for example, a partially precious metal formed structure in which the precious metal is scattered on the surface of the substrate can be considered.

Figure 13 is an explanatory diagram showing the problems with the conventional separator disclosed in Patent Document 5. The fuel cell separator 82 shown in Figure 13 (a) employs a precious metal partial formation structure, in which precious metal particles 25 are formed only on a portion of the surface of the metal substrate 20.

The fuel cell separator 82 with a partially precious metal structure shown in FIG. 1(a) can be set to a relatively low initial contact resistance immediately after the device is completed. However, the fuel cell separator 82 is exposed to a harsh environment during power generation in the fuel cell. In this environment, there is a high possibility that metals other than the precious metal will come into contact with the corrosive liquid, which will increase the contact resistance.

This is because, as shown in FIG. 13(b), an oxide layer 28 is formed on the surface of the metal substrate 20 where no precious metal is formed. Over time, the oxide layer 28 reaches the interface between the metal substrate 20 and the precious metal particles 25, and eventually, as shown in FIG. 13(c), the metal substrate 20 and the precious metal particles 25 are separated by the oxide layer 28. In this way, if the degree of corrosion during the use of the fuel cell separator 82 is relatively high, the precious metal particles 25 may fall off from the fuel cell separator 82.

As shown in FIG. 13(c), the contact resistance of the fuel cell separator 82 becomes high after the oxide layer 28 is formed over almost the entire surface of the metal substrate 20. In this way, the conventional fuel cell separator 82 that employs a precious metal partial formation structure cannot stably maintain low contact resistance.

Therefore, if we try to suppress corrosion in a harsh environment where the phenomenon shown in Figure 13 is expected to occur, it would be necessary to provide Au or Pt on the entire surface of the separator substrate, which would be very expensive and would be impractical.

The fuel cell separator disclosed in Patent Document 6 uses titanium oxide as the intermediate layer when titanium is used as the separator substrate, and a non-conductive film layer when stainless steel is used as the separator substrate. A conductive film of Au or Pt is then formed on the intermediate layer by plating or sputtering, realizing a separator structure including the substrate, intermediate layer, and conductive film.

However, the intermediate titanium oxide layer and stainless steel non-conductive film did not have sufficient corrosion resistance in the harsh environments required for fuel cells.

In addition, paragraph [0027] of the specification of Patent Document 6 states that the conductive coating is made of Au, Ag (silver), Cu (copper), Pt, Rh (rhodium), Ir (iridium), etc., but the reality is that if a metal other than Au or Pt is used, corrosion will start from that metal.

Conventional fuel cell separators are constructed as described above, and while they have a relatively inexpensive structure, they have the problem of being unable to stably maintain low contact resistance with the contacting object.

The present disclosure has been made to solve the above problems, and aims to provide a fuel cell separator and a manufacturing method thereof that can stably reduce the contact resistance with the contact object with a relatively inexpensive structure.

The fuel cell separator according to the present disclosure comprises a metal substrate, a conductive oxide film provided on the metal substrate, and a group of precious metal particles provided on the conductive oxide film, the group of precious metal particles including a plurality of precious metal particles, at least a portion of the plurality of precious metal particles being provided discretely, and the constituent material of each of the plurality of precious metal particles being gold or platinum.

The fuel cell separator disclosed herein has a basic three-layer separator structure consisting of a metal substrate, a conductive oxide film, and a group of precious metal particles, and therefore can stably reduce the contact resistance between the group of precious metal particles with the conductive oxide film as the base layer and the object to be contacted.

Furthermore, the fuel cell separator disclosed herein can be produced at a relatively low cost because at least some of the multiple precious metal particles are provided in a discrete manner, thereby reducing the amount of precious metal, such as gold or platinum, used.

FIG. 1 is a cross-sectional view showing a basic structure of a separator which is a basic technology of the present disclosure. 1 is a cross-sectional view showing the structure of a fuel cell separator according to an embodiment of the present invention. 4 is a flowchart showing a process procedure of a manufacturing method of a fuel cell separator according to the present embodiment. FIG. 1 is an explanatory diagram illustrating a schematic configuration of a mist CVD apparatus used in the present embodiment. FIG. 1 is an explanatory diagram showing, in table form, the degree of improvement in contact resistance according to the Au particle area ratio and the average particle size. 1 is a graph showing the relationship between heating temperature and average Au particle size. 1 is a graph showing the relationship between heating temperature and average area ratio of Au. 4 is an explanatory diagram showing, in table form, the contact resistance between the fuel cell separator of the present embodiment and a contact object in various states. FIG. FIG. 9 is an explanatory diagram showing the contact resistance shown in FIG. 8 in the form of a bar graph. FIG. 11 is an explanatory diagram showing, in table form, the contact resistance between a conventional fuel cell separator and a contact object in various states. FIG. 11 is an explanatory diagram showing the contact resistance shown in FIG. 10 in the form of a bar graph. FIG. 2 is an explanatory diagram showing an exploded view of the basic structure of a fuel cell. FIG. 1 is an explanatory diagram showing problems with a conventional separator.

<Embodiment>
(Basic structure)
1 is a cross-sectional view showing a separator basic structure 2R according to the basic technology of the present disclosure. As shown in the figure, the separator basic structure 2R includes a metal substrate 20, a conductive oxide film 21, and a precious metal particle group 22G as main components.

As shown in the figure, a conductive oxide film 21 is provided on the surface of a metal substrate 20. The surface of the conductive oxide film 21 serves as a particle formation surface. A group of precious metal particles 22G is provided on the surface of the conductive oxide film 21.

The precious metal particle group 22G includes a plurality of precious metal particles 22 each in the form of a fine particle, and at least some of the precious metal particles 22 are provided separately from one another. In other words, at least some of the spaces between the precious metal particles 22 are provided with gaps in which no precious metal particles 22 are present. The material constituting each of the precious metal particles is gold (Au) or platinum (Pt).

The precious metal particle group 22G side of the separator basic structure 2R becomes the contact surface with the contact object. In other words, the precious metal particle group 22G with the conductive oxide film 21 as the base layer becomes the contact surface. The contact object may be, for example, the pair of gas diffusion layers 13 and 14 present on both sides of the MEA 5 that is the fuel part in the fuel cell 60 shown in FIG. 12.

The conductive oxide film 21 is a tin oxide film with improved conductivity due to the addition of a third element such as indium or antimony. A tin oxide film with indium added becomes an ITO (Indium Tin Oxide) film, and a tin oxide film with antimony added becomes an ATO (Antimony Tin Oxide) film. In this way, for example, an ATO film (antimony-doped tin oxide film) or an ITO film is used as the conductive oxide film 21.

(Actual structure)
Fig. 2 is a cross-sectional view showing the structure of a fuel cell separator 2 according to the first embodiment of the present disclosure. An XYZ orthogonal coordinate system is depicted in Fig. 2. The fuel cell separator 2 shown in the figure is used, for example, as one of the fuel cell separators 15 and 16 used in the fuel cell 60 shown in Fig. 12.

As shown in the figure, the fuel cell separator 2 shown in FIG. 2 includes a metal substrate 20, a pair of ATO membranes 21A, and a pair of precious metal particle groups 22G as main components.

As shown in the figure, the metal base material 20 has projections and recesses in the X direction and a structure extending in the Z direction. The film thickness of the metal base material 20 is almost uniform. An upper ATO (antimony-doped tin oxide) film 21A is provided on the surface of the metal base material 20 on the +X direction side, and a lower ATO film 21A is provided on the back surface of the metal base material 20 on the -X direction side.

In this way, a pair of ATO films 21A are provided on both sides of the metal substrate 20. A structure including the metal substrate 20 and the pair of ATO films 21A becomes the intermediate structure 30. In the intermediate structure 30, the upper surface of the upper ATO film 21A and the lower surface of the lower ATO film 21A each become particle formation surfaces.

A precious metal particle group 22G is provided on the upper surface of the upper ATO film 21A, and a precious metal particle group 22G is provided on the lower surface of the lower ATO film 21A. Hereinafter, the combination of the upper ATO film 21A and the precious metal particle group 22G provided on the upper surface of the upper ATO film 21A may be referred to as the "upper precious metal formation portion," and the combination of the lower ATO film 21A and the precious metal particle group 22G provided on the lower surface of the lower ATO film 21A may be referred to as the "lower precious metal formation portion."

Each of the pair of precious metal particle groups 22G contains a plurality of precious metal particles 22, and the constituent material of each of the plurality of precious metal particles is gold or platinum.

Therefore, a plurality of particulate (approximately spherical) precious metal particles 22 are provided on the upper surface of the upper ATO film 21A, which is the particle formation surface. At least some of the precious metal particles 22 are attached to the upper surface of the upper ATO film 21A in a discrete state. Therefore, there are gaps at least partially between the precious metal particles 22 in the upper precious metal formation portion.

Similarly, a plurality of particulate precious metal particles 22 are provided on the lower surface, which is the particle formation surface, of the lower ATO film 21A. At least some of the plurality of precious metal particles 22 are attached to the lower surface of the lower ATO film 21A in a discrete state. Therefore, there are at least some gaps between the plurality of precious metal particles 22 in the lower precious metal formation portion.

In addition, the ideal state would be for all of the multiple precious metal particles 22 in each of the upper and lower precious metal formation sections to be dispersed.

In this manner, a pair of precious metal particle groups 22G (a pair of a plurality of precious metal particles 22) is provided on both sides of the intermediate structure 30 in a manner in which a plurality of precious metal particles 22 are attached.

Then, the precious metal particle group 22G with the ATO film 21A as the base layer in the upper precious metal forming portion or the precious metal particle group 22G with the ATO film 21A as the base layer in the lower precious metal forming portion becomes the contact surface with the contact object.

The area ratio of the multiple precious metal particles 22 in the particle formation surface (top surface) of the upper ATO film 21A is set to 30% or less. In other words, the area ratio of the multiple precious metal particles 22 to the area of the top surface of the upper ATO film 21A is set to 30% or less.

Similarly, the area ratio of the multiple precious metal particles 22 in the particle formation surface (lower surface) of the lower ATO film 21A is set to 30% or less. In other words, the area ratio of the multiple precious metal particles 22 to the area of the lower surface of the lower ATO film 21A is set to 30% or less.

In addition, the average diameter (average particle size) of the multiple precious metal particles 22 in the upper precious metal forming portion is set to 10 nm or less. Similarly, the average diameter of the multiple precious metal particles 22 in the lower precious metal forming portion is set to 10 nm or less.

In this way, the fuel cell separator 2 of this embodiment has the separator basic structure 2R shown in Figure 1 on both the front and back surfaces of the metal substrate 20.

In addition, in the fuel cell separator 2 of this embodiment, the multiple precious metal particles 22 in each of the upper precious metal forming portion and the lower precious metal forming portion are subject to an occupation area condition of {occupancy area ratio of 30% or less} and an average diameter condition of {average diameter of 10 nm or less}.

When contact with the contact object is made in the upper precious metal forming part, the multiple precious metal particles 22 in the lower precious metal forming part do not need to satisfy the above-mentioned occupation area condition and the above-mentioned average diameter condition. Similarly, when contact with the contact object is made in the lower precious metal forming part, the multiple precious metal particles 22 in the upper precious metal forming part do not need to satisfy the above-mentioned occupation area condition and the above-mentioned average diameter condition.

(Production method)
The method for manufacturing the fuel cell separator 2 of this embodiment will be described below. Fig. 3 is a flow chart showing the procedure of the method for manufacturing the fuel cell separator 2. The method for manufacturing the fuel cell separator 2 will be described below with reference to this figure.

First, in step S1, an intermediate structure 30 (see FIG. 2) is prepared, which includes a metal substrate 20 and a pair of ATO films 21A provided on the front and back surfaces of the metal substrate 20. In other words, the intermediate structure 30 is an intermediate structure before a pair of precious metal particle groups 22G are formed. The metal substrate 20 may be a SUS304 substrate or a Ti substrate, which will be described later.

The intermediate structure 30 is manufactured by an existing method. For example, a pair of ATO films 21A may be formed on both sides of the metal base material 20 by using a mist CVD method. Specifically, ultrasonic waves are applied to a raw material solution using SnCl ₂ (tin chloride) as a raw material to generate raw material mist, and the raw material mist is sprayed onto the front and back surfaces of the heated metal base material 20, so that the ATO films 21A can be formed on both sides of the metal base material 20.

Figure 4 is an explanatory diagram that shows a schematic configuration of the mist CVD device 7 used in this embodiment to form the fuel cell separator 2. The XYZ Cartesian coordinate system is shown in the figure. Steps S2 and S3 described below are performed using the mist CVD device 7 shown in Figure 4.

The mist CVD device 7 includes a heating chamber 40, a particle formation chamber 50, a transport path 45, and a transport path 46 as its main components.

The heating chamber 40 has a pair of lamp mounting stands 41 on one side (the side on the -X side in the figure) and the other side (the side on the +X side in the figure) of the interior, and each of the pair of lamp mounting stands 41 mounts a plurality of infrared lamps 42. The pair of infrared lamps 42 are arranged so that their infrared radiation directions face each other along the X direction, so the space within the heating chamber 40 sandwiched between the pair of lamp mounting stands 41 becomes the heating space 40s.

Therefore, the heating space 40s in the heating chamber 40 can be heated by infrared irradiation from the multiple infrared lamps 42 provided on each of the pair of lamp mounting tables 41.

A transport path 45 is provided at the upper opening of the heating chamber 40, a transport path 46 is provided at the lower opening of the transport path 45, and a particle formation chamber 50 is provided below the heating chamber 40 via the transport path 46.

Multiple ultrasonic vibrators 38 (two ultrasonic vibrators 38 in FIG. 4) are provided on the bottom surface of the particle formation chamber 50, and the raw material solution 36 is contained below. The raw material solution 36 is a solution containing a precious metal such as gold or platinum. Within the particle formation chamber 50, the space above the raw material solution 36 becomes the mist space 50s.

By performing ultrasonic vibration processing using multiple ultrasonic vibrators 38, ultrasonic waves are applied to the raw material solution 36 in the particle formation chamber 50, atomizing the raw material solution 36 and generating raw material mist MT. The generated raw material mist MT accumulates in the mist space 50s. For example, the raw material mist MT can be reliably accumulated in the mist space 50s by closing the opening at the top of the particle formation chamber 50 or the transport path 46 except when the intermediate structure 30 is being transported into the particle formation chamber 50. Note that an air curtain, for example, can be used as a method for closing the space.

The intermediate structure 30 is held in a suspended state by a holding member 43 connected via a connecting ring 43r. The holding member 43 can be raised and lowered in the ±Z directions.

Step S2 shown in FIG. 3 is performed using the mist CVD device 7 configured in this way. In step S2, the holding member 43 is lowered from above the heating chamber 40 along the transport direction D2 downward, and the intermediate structure 30 is placed in the heating space 40s of the heating chamber 40 via the transport path 45, thereby heating the intermediate structure 30. Therefore, the upper and back surfaces of the intermediate structure 30, i.e., the upper surface of the upper ATO film 21A and the lower surface of the lower ATO film 21A, are each heated by the multiple infrared light lamps 42.

In step S3, the intermediate structure 30 after the heat treatment in step S2 is further lowered by the holding member 43 along the conveying direction D2, and the intermediate structure 30 is placed in the mist space 50s containing the raw material mist MT via the conveying path 46.

The upper surface of the upper ATO film 21A and the lower surface of the lower ATO film 21A are immediately after the heat treatment in step S2. Therefore, when the raw material solution 36 contains gold (Au), a chemical reaction {2AuCl ₃ →2Au+3Cl ₂ } can be caused in the mist CVD method.

Then, in the mist space 50s, the raw material mist MT containing Au atoms that has been misted by the above-mentioned chemical reaction comes into contact with the upper surface of the upper ATO film 21A and the lower surface of the lower ATO film 21A. As a result, multiple Au particles adhere to the upper surface of the upper ATO film 21A and the lower surface of the lower ATO film 21A as multiple precious metal particles 22.

In this way, a precious metal particle group 22G is formed on the upper surface of the upper ATO film 21A and on the lower surface of the lower ATO film 21A. The precious metal particle group 22G includes a plurality of precious metal particles 22 attached to the particle formation surface of the ATO film 21A. Each of the plurality of precious metal particles 22 has a particulate shape, and at least a portion of the plurality of precious metal particles 22 is provided in a discrete manner.

The mist contact time, which is the execution time of step S3, is set to a sufficiently short time of less than 1 second so that the area occupied by the multiple precious metal particles 22 on the particle formation surface of each of the pair of ATO films 21A is 30% or less and the average diameter of the multiple precious metal particles 22 is 10 nm or less.

In this way, by carrying out steps S1 to S3, a fuel cell separator 2 having the structure shown in FIG. 2 can be manufactured. This fuel cell separator 2 satisfies the above-mentioned occupied area conditions and average diameter conditions.

The particle diameter of each of the multiple precious metal particles 22 and the occupied area ratio within the particle formation surface can be adjusted by adjusting the heating temperature of the intermediate structure 30 during the heat treatment in step S2 and the mist contact time, which is the execution time of step S3.

Furthermore, by making the concentration of the raw material solution 36 relatively low and by making the mist contact time relatively short, it is possible to attach all of the multiple precious metal particles 22 to the particle formation surface of the ATO film 21A in a discrete state.

In addition, if it is determined that the occupied area ratio and the average diameter are insufficient in a single execution of steps S2 and S3, the processing of steps S2 and S3 may be repeated multiple times. Specifically, after the first execution of step S3, the holding member 43 is raised in the +Z direction, the unfinished fuel cell separator 2 is placed again in the heating chamber 40, and step S2 is executed a second time. After execution of step S2, step S3 is executed again, thereby executing S2 and S3 a second time.

Similarly, steps S2 and S3 can be performed from the third time onwards. In this way, the intermediate structure 30 or the unfinished fuel cell separator 2 may be placed in the heating chamber 40 and particle formation chamber 50 multiple times, and steps S2 and S3 may be performed multiple times.

(Experimental Results)
5 is an explanatory diagram showing in a table format the degree of improvement in contact resistance according to the average Au area ratio and the average particle size. Note that "average Au area ratio (%)" means the ratio (%) of the area occupied by Au particles, which are the precious metal particles 22, to the area of the particle-formed surface. For example, when the entire upper surface of the upper ATO film 21A becomes the particle-formed surface, the area of the entire upper surface of the upper ATO film 21A becomes the "area of the particle-formed surface."

The term "average particle size (nm)" refers to the average diameter (nm) of the multiple precious metal particles 22 provided on the particle formation surface. The degree of improvement in contact resistance (mΩcm ² ) indicates the degree of resistance reduction (mΩcm ² ) before and after the formation of the Au particles. Specifically, the structure before the formation of the Au particles is the intermediate structure 30 shown in FIG. 2 , and the structure after the formation of the Au particles is the fuel cell separator 2 shown in FIG. 2 , in which Au particles are formed as the precious metal particles 22.

As shown in the figure, by adjusting the mist contact time, which is the execution time of step S3, etc., the Au average area ratio is set to "30%" and the average particle size is set to "5.8 nm" in condition 1, the Au average area ratio is set to "22%" and the average particle size is set to "9.3 nm" in condition 2, and the Au average area ratio is set to "93%" and the average particle size is set to "24.7 nm" in condition 3.

5, the degree of improvement in contact resistance under condition 1 is 6 ( ^mΩcm2 ). That is, under condition 1, the resistance is reduced by 6 mΩ per ^cm2 . Similarly, the degree of improvement in contact resistance under condition 2 is 5.7 ( ^mΩcm2 ), and the degree of improvement in contact resistance under condition 3 is 4 ( ^mΩcm2 ).

From the experimental results shown in Figure 5, conditions 1 and 2 have a significantly smaller average Au area ratio than condition 3. In other words, conditions 1 and 2 have a smaller amount of Au particles attached to the particle formation surface than condition 3, but the degree of improvement in contact resistance is greater than that of condition 3. Therefore, if the emphasis is on achieving a relatively large degree of improvement in contact resistance with the minimum necessary amount of Au particles, it is estimated that the optimal conditions are an average Au area ratio of 30% or less and an average particle size of 10 nm.

Figure 6 is a graph showing the relationship between heating temperature and average Au particle size. In this figure, the horizontal axis is heating temperature (°C) and the vertical axis is Au average particle size (nm). Note that the heating temperature indicates the temperature (°C) in the heating space 40s when the heating process of step S2 is performed, and the Au average particle size indicates the average diameter (nm) of multiple precious metal particles 22 when the precious metal particles 22 are Au particles.

In the figure, particle size change L1 shows the change when the mist contact time, which is the execution time of step S2, is set to 0.1 (sec), and particle size change L2 shows the change when the mist contact time is set to 0.5 (sec).

As shown in particle size change L2 in the figure, when the mist contact time is 0.5 seconds, the average Au particle size exceeds 10 nm when the heating temperature exceeds 430 (°C). On the other hand, as shown in particle size change L1, when the mist contact time is 0.1 seconds, the average Au particle size is kept below 7 nm in the heating temperature range of 350 to 450 (°C).

Figure 7 is a graph showing the relationship between heating temperature and the average area ratio of Au. In this figure, the horizontal axis is heating temperature (°C) and the vertical axis is the average area ratio of Au (%). Note that the average area ratio of Au indicates the ratio (%) of the formation area of multiple precious metal particles 22 to the area of the particle formation surface when the precious metal particles 22 are Au particles.

In the figure, area percentage change L3 shows the change when the mist contact time is set to 0.1 (sec), and area percentage change L4 shows the change when the mist contact time is set to 0.5 (sec).

As shown in area percentage change L4 in the figure, when the mist contact time is 0.5 seconds, the average Au area percentage exceeds 30% when the heating temperature is below 400 (°C). On the other hand, as shown in area percentage change L3, when the mist contact time is 0.1 seconds, the average Au area percentage is kept below 30% in the heating temperature range of 350 to 450 (°C).

Therefore, from the experimental results shown in Figures 6 and 7, it can be inferred that when the heating temperature range in step S2 is {350 to 450°C}, in order to keep the average Au area ratio at 30% or less and the average Au particle size at 10 nm or less within the heating temperature range, the mist contact time must be set to 0.1 seconds or less.

Fig. 8 is an explanatory diagram showing in table form the contact resistance with a contact object in various states of the fuel cell separator 2 of the embodiment. Fig. 9 is an explanatory diagram showing in bar graph form the contact resistance shown in Fig. 8. Specifically, the contact object may be the gas diffusion layer pair 13 and 14 in the fuel cell 60 shown in Fig. 12. Note that the unit of the resistance values shown in Figs. 8 and 9 is all ^mΩcm2 .

8 and 9, the operating specifications of the mist CVD apparatus 7 shown in Fig. 4 are set as follows: the number of ultrasonic vibrators 38 is "2", the ultrasonic frequency of each ultrasonic vibrator 38 is "1.6 MHZ", and the concentration of the _AuCl3 solution serving as the raw material solution 36 is "0.003 mol/L".

8 and 9, the SUS304 substrate and the Ti substrate each show a specific structure of the metal substrate 20. The SUS304 substrate is a typical metal substrate on which a non-conductive film has been formed in advance, and the Ti substrate is a typical metal substrate on which a titanium oxide film (tiOx ₎ has been formed in advance.

In Figures 8 and 9, "before deposition" shows the structure before the precious metal particle group 22G is formed. "after deposition" shows the structure after the precious metal particle group 22G is formed. In other words, "after deposition" shows the overall structure of the fuel cell separator 2.

In the column "Before deposition", "Metal substrate only" shows only the structure of the metal substrate 20 before the conductive oxide film 21 (ATO film 21A) is formed. Furthermore, in the column "Before deposition", "After ATO film formation" shows the structure of the intermediate structure 30 including the metal substrate 20 and the ATO film 21A. The ATO film 21A can be formed on the metal substrate 20 by using an existing mist CVD method in which ultrasonic waves are applied to a raw material solution 36 using _SnCl2 as a raw material to obtain a raw material mist MT.

The fuel cell separator 2 after deposition will be described in detail below. Ultrasonic waves are applied from an ultrasonic vibrator 38 to a raw material solution 36 made from AuCl ₃ to atomize (mist) the raw material solution 36, thereby obtaining a raw material mist MT. Thereafter, by disposing the intermediate structure 30 in a state in which the raw material mist MT is retained in the mist space 50s, a precious metal particle group 22G can be formed on the particle formation surface of the ATO film 21A (step S3 in FIG. 3).

That is, a pair of precious metal particle groups 22G can be formed on the upper surface of the upper ATO film 21A and on the lower surface of the lower ATO film 21A by using the mist CVD method using the mist CVD device 7. The multiple precious metal particles 22 contained in each of the pair of precious metal particle groups 22G become multiple Au particles. In this way, by using the mist CVD method using the mist CVD device 7 for the intermediate structure 30, the fuel cell separator 2 shown in FIG. 2 can be completed.

At this time, the average area ratio of Au in the fuel cell separator 2 was 30%, and the average Au particle size was 5.8 nm.

In Figures 8 and 9, "Before durability test" in the "After deposition" column indicates the state before the fuel cell separator 2 was used, and "After durability test" indicates the state after the fuel cell separator 2 was used in the corrosive and oxidative environment expected for a fuel cell.

As a durability test, a corrosion-oxidation test was conducted in which the fuel cell separator 2 was held for three days in a solution of 800 ml of sulfuric acid (pH 3) containing 3 ppm hydrofluoric acid and held at 80°C while a current of 0.9 V was applied.

As shown in Figures 8 and 9, when the metal substrate 20 is a SUS304 substrate, the contact resistance is "76" for the metal substrate 20 alone, improves to "8.9" for the intermediate structure 30, and improves to "2.9" for the fuel cell separator 2. Furthermore, even after the durability test, the contact resistance of the fuel cell separator 2 is maintained at a low resistance value of "3.2."

As shown in Figures 8 and 9, when the metal substrate 20 is a Ti substrate, the contact resistance is "6.2" for the metal substrate 20 alone, "8.4" for the intermediate structure 30, and improves to "2.1" for the fuel cell separator 2. Furthermore, even after the durability test, the contact resistance of the fuel cell separator 2 is maintained at a relatively low resistance value of "3.8."

As shown in FIG. 9, the fuel cell separator 2 of this embodiment can maintain a low contact resistance value below the target reference resistance value SR (5 ^mΩcm2 ) even after use in the severe corrosive and oxidative environment expected for a fuel cell.

In other words, it can be seen that the low contact resistance of the fuel cell separator 2 of this embodiment can be stably maintained over a long period of time.

Fig. 10 is an explanatory diagram showing in table form the contact resistance between the comparative fuel cell separator and the contact object in various states. Fig. 11 is an explanatory diagram showing in bar graph form the contact resistance shown in Fig. 10. Note that the unit of resistance values shown in Fig. 10 and Fig. 11 is all ^mΩcm2 .

The experimental results shown in Figures 10 and 11, like those shown in Figures 8 and 9, were performed using the operating specifications of the mist CVD device 7 described above.

The comparative fuel cell separator has a structure in which a precious metal particle group 22G (multiple precious metal particles 22) is provided directly on the metal substrate 20.

In Figures 10 and 11, the SUS304 substrate and the Ti substrate each show the specific structure of the metal substrate 20. "Before deposition" in Figures 10 and 11 shows the structure before the precious metal particle group 22G is formed. "After deposition" shows the structure after the precious metal particle group 22G is formed. In other words, "After deposition" shows the overall structure of the comparative fuel cell separator.

By using the mist CVD apparatus 7 shown in FIG. 4 and replacing the intermediate structure 30 with only the metal substrate 20, steps S1 to S3 shown in FIG. 3 are performed to form a precious metal particle group 22G on the particle formation surface of the metal substrate 20.

In the "Before deposition" column, "Metal substrate only" indicates the structure of only the metal substrate 20 before the precious metal particle group 22G is formed.

In the "After deposition" column, "Before durability test" indicates the condition before the comparative fuel cell separator was used, and "After durability test" indicates the condition after the comparative fuel cell separator was used in the corrosive and oxidative environment expected for a fuel cell.

As a durability test, a corrosion test was conducted in which the comparative fuel cell separator was held for three days in a solution of 800 ml of sulfuric acid (pH 3) containing 3 ppm hydrofluoric acid and held at 80°C while a current of 0.9 V was applied. This was similar to the experimental results shown in Figures 8 and 9.

As shown in Figures 10 and 11, when the metal substrate 20 was a SUS304 substrate, the contact resistance was "73" for the metal substrate 20 alone, and improved to "1.73" immediately after the comparative fuel cell separator was completed. However, after the durability test, the contact resistance of the comparative fuel cell separator significantly deteriorated, and the resistance value rose to "21".

As shown in Figures 10 and 11, when the metal substrate 20 was a Ti substrate, the contact resistance was "6.4" for the metal substrate 20 alone, and improved to "2.25" immediately after the comparative fuel cell separator was completed. However, after the durability test, the contact resistance of the comparative fuel cell separator significantly deteriorated, and the resistance value rose to "92.7".

As shown in FIG. 11, the comparative fuel cell separator, after being used in an assumed severe corrosive and oxidative environment, ends up with a high contact resistance value that significantly exceeds the target reference resistance value SR (5 mΩcm ² ).

As described above, the experimental results shown in Figures 10 and 11 reveal that, in a comparative fuel cell separator in which precious metal particle groups 22G are provided on both sides of the metal substrate 20, even if the contact resistance is reduced immediately after completion, the contact resistance value increases significantly when used in an environment expected for a fuel cell, and the low contact resistance cannot be maintained.

In other words, the comparison of Figures 8 and 9 with Figures 10 and 11 reveals the importance of providing a conductive oxide film 21 (ATO film 21A) as an underlayer for the precious metal particle group 22G in order to maintain a low contact resistance of the fuel cell separator even after it has been used in the harsh environment expected for a fuel cell. In other words, it was revealed that the inclusion of a conductive oxide film 21 with high corrosion resistance properties is an essential condition for the separator basic structure 2R.

The experimental results shown in Figures 8 to 11 demonstrate that the fuel cell separator 2 of this embodiment, which has the separator basic structure 2R, can stably maintain low contact resistance for a long period of time.

(effect)
The fuel cell separator 2 of this embodiment has a separator basic structure 2R (see Figure 1) which is a three-layer structure of a metal substrate 20, a conductive oxide film 21, and precious metal particles 22, and therefore can stably reduce the contact resistance between the precious metal particle group 22G and the contact object.

This effect is also evident from the experimental results shown in Figures 8 to 11. The above-mentioned contact objects are fuel cell components, specifically the gas diffusion layer pairs 13 and 14 on both sides of the MEA 5, which serves as the power generation section shown in Figure 12.

In addition, the fuel cell separator 2 of this embodiment can be produced at a relatively low cost by providing at least a portion of the multiple precious metal particles 22 in a discrete manner, thereby reducing the amount of precious metal, such as gold or platinum, used.

Furthermore, by using the ATO film 21A as the conductive oxide film 21 that serves as the underlayer for the precious metal particle group 22G in the fuel cell separator 2, the resistance value of the contact resistance can be stabilized. This effect is also evident from the experimental results shown in Figures 8 to 11.

In addition, in the fuel cell separator 2 of this embodiment, the average diameter of the multiple precious metal particles 22 is set to 10 nm or less. This allows the contact area with the gas diffusion layer pair 13 and 14 to be relatively large, because the gas diffusion layer (GDL) is fibrous and porous.

Therefore, in the fuel cell separator 2 of this embodiment, by setting the average diameter of the multiple precious metal particles 22 to 10 nm or less, it is possible to further reduce the contact resistance with the contact object.

By conducting an experiment such as that shown in FIG. 5 under conditions where a large number of Au average particle sizes below 10 nm are set, the lower limit of the Au average particle size can be recognized in advance as a significant value exceeding "0". For example, the minimum value of the Au average particle size that satisfies the standard resistance value SR or less can be set as the lower limit. Therefore, it is desirable to set the Au average particle size between the previously recognized lower limit and the upper limit of 10 nm.

In addition, in the fuel cell separator 2 of this embodiment, the area occupied by the multiple precious metal particles 22 within the particle formation surface of the conductive oxide film 21 (ATO film 21A) is set to 30% or less.

Therefore, the fuel cell separator 2 of this embodiment can reduce the contact resistance with the contact object while keeping the amount of Au or Pt used as the precious metal particles 22 to a minimum.

By conducting an experiment as shown in FIG. 5 under conditions where a large number of Au average area ratios below 30% are set, the lower limit of the Au average area ratio can be recognized in advance as a significant value exceeding "0". For example, the minimum value of the Au average area ratio that is equal to or less than the reference resistance value SR can be set as the lower limit. Therefore, it is desirable to set the Au average area ratio between the previously recognized lower limit and the upper limit of 30%.

The manufacturing method of the fuel cell separator of this embodiment shown in FIG. 3 can obtain the fuel cell separator 2 of this embodiment including the separator basic structure 2R consisting of the metal substrate 20, the conductive oxide film 21, and the precious metal particles 22 by performing step S3.

The fuel cell separator 2 including the separator basic structure 2R can stably reduce the contact resistance between the precious metal particle group 22G and the contact object.

In addition, by changing the mist contact time, which is the execution time of step S3, it is possible to adjust the average diameter of the multiple precious metal particles 22 contained in the precious metal particle group 22G and the occupied area ratio of the multiple precious metal particles on the particle formation surface.

In the manufacturing method of this embodiment, the mist contact time is set so that the area occupied by the multiple precious metal particles 22 is 30% or less, so that the amount of precious metal (Au, Pt) used as the precious metal particles 22 is kept to a minimum, and a fuel cell separator 2 can be obtained that has reduced contact resistance with the contact object.

Note that within the scope of this disclosure, the embodiments may be modified or omitted as appropriate.

2 fuel cell separator 2R separator basic structure 7 mist CVD device 20 metal substrate 21 conductive oxide film 21A ATO film 22 precious metal particles 22G precious metal particle group 36 raw material solution 38 ultrasonic vibrator 40 heating chamber 50 particle formation chamber

Claims (3)

A metal substrate;
A conductive oxide film provided directly on the surface of the metal substrate;
a group of precious metal particles provided on the conductive oxide film;
The precious metal particle group includes a plurality of precious metal particles,
At least a portion of the plurality of precious metal particles is provided discretely,
The constituent material of each of the plurality of precious metal particles is gold or platinum;
the average diameter of the plurality of precious metal particles is 10 nm or less;
the noble metal particle group is formed on a particle formation surface of the conductive oxide film,
the area ratio of the plurality of precious metal particles to the grain formation surface is 30% or less;
Fuel cell separator. 2. The fuel cell separator according to claim 1,
The conductive oxide film is an antimony-doped tin oxide film.
Fuel cell separator. A method for manufacturing a fuel cell separator, comprising the steps of:
(a) providing an intermediate structure including a metal substrate and a conductive oxide film disposed directly on a surface of the metal substrate;
(b) heating the intermediate structure in a heating chamber;
(c) placing the intermediate structure after the step (b) is performed in a mist space containing raw material mist;
The mist space is provided in a particle formation chamber separate from the heating chamber,
The raw material mist is obtained by atomizing a raw material solution containing a precious metal such as gold or platinum,
After the step (c) is performed, a precious metal particle group including a plurality of precious metal particles is formed on the particle formation surface of the conductive oxide film ,
The mist contact time, which is the execution time of the step (c), is set so that the occupation area ratio of the plurality of precious metal particles in the particle formation surface is 30% or less and the average diameter of the plurality of precious metal particles is 10 nm or less.
A method for manufacturing a fuel cell separator.

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