JP5660497B2 - Fuel cell separator and method for producing the same - Google Patents

Description

  The present invention relates to a metal separator for a fuel cell that requires corrosion resistance and conductivity, and more particularly to a separator suitable for a polymer electrolyte fuel cell and a method for producing the same.

  The fuel cell is a technology that has been attracting attention in recent years as a device that generates clean energy with little environmental impact. Depending on the type of electrolyte used, the fuel cell can be a polymer electrolyte fuel cell (PEFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), or a solid oxide fuel cell (SOFC). are categorized. Solid polymer fuel cells have lower cell resistance than other types of fuel cells, can be operated at high current density, can be reduced in size and weight, and have low operating temperatures. It is used as a fuel cell.

As shown in FIG. 5, the basic structure of the fuel cell is such that a fuel electrode 102 (functioning as an anode) and an air electrode 103 (functioning as a cathode) are joined to both sides of an electrolyte 101, and hydrogen (H ₂ ) is connected to the fuel electrode 102. ) And air containing oxygen (O ₂ ) is supplied to the air electrode 103. Each electrode is formed with a catalyst layer in which platinum as a reaction catalyst is dispersed between the electrode and the electrolyte 101. On the fuel electrode 102 side, the presence of hydrogen, catalyst, and electrolyte generates protons (H ⁺ ) and electrons (e ⁻ ). The electrons are taken out from the fuel electrode 102 and supplied to the load 104, Moves in the electrolyte 101 and moves toward the air electrode 103 side. On the other hand, on the air electrode 103 side, oxygen, a catalyst, and an electrolyte exist, so that electrons supplied from the load 104 to the air electrode 103, protons that have moved in the electrolyte 101, and oxygen in the air react to water. Will be generated.

  FIG. 6 is an explanatory diagram relating to the basic configuration of a single cell which is the minimum unit of a polymer electrolyte fuel cell. Porous bodies 4 and 5 in which catalyst layers 2 and 3 in which a catalyst is dispersed are formed are bonded to both surfaces of a solid polymer electrolyte membrane 1 formed in layers. Separators 6 and 7 are bonded to the outer surfaces of the porous bodies 4 and 5. A plurality of grooves are formed in the joint portions of the separators 6 and 7 with the porous bodies 4 and 5, and flow paths 8 and 9 through which gas flows are formed by these grooves.

  Then, hydrogen gas is supplied to the flow path 8 from an external supply device (not shown), and air containing oxygen is supplied to the flow path 9 so that the catalyst layer 2 and the porous body 4 function as a fuel electrode. The catalyst layer 3 and the porous body 5 function as an air electrode. Therefore, electrons are taken out from the porous body 4 and supplied to the porous body 5.

  In an actual fuel cell, in order to obtain the necessary power, a stack form in which several tens of such single cells are combined is used. The separator (bipolar plate) described above separates the single cells from each other electrically. Used to connect. In addition, since the separator serves as a flow path for fuel gas and air, the separator has characteristics such as conductivity for connecting single cells, airtightness for preventing mixing of fuel gas and air, and corrosion resistance in a power generation environment. Is required.

  Examples of the material used for the separator mainly include carbon-based materials and metal materials. A separator using a carbon-based material is excellent in terms of corrosion resistance. However, a certain thickness is required to obtain sufficient strength and airtightness, which is a factor that hinders downsizing and thinning. On the other hand, a separator using a metal material can be formed thin because there is no problem in terms of strength and airtightness, but corrosion is likely to occur and there is a problem in terms of corrosion resistance. Therefore, it is necessary to coat the surface of the metal separator with a film having excellent conductivity and corrosion resistance.

  As a surface treatment method for a metal separator, for example, in Patent Document 1, a conductive ceramic film is formed on the surface of a metal member by PVD (Physical Vapor Deposition) such as sputtering and arc ion plating. A separator is described. Patent Document 2 describes a separator in which a dense barrier layer, an intermediate layer, and a conductive thin film layer are formed on a metal substrate layer.

JP 2002-75398 A JP 2010-129303 A

  Patent Document 1 describes that the surface of a metal member is covered with a conductive ceramic film. However, the film formed by PVD has a minute gap called a pinhole, so that sufficient corrosion resistance is obtained. Is difficult. Moreover, in patent document 2, there exists a possibility that an intermediate | middle layer or a dense barrier layer may be oxidized by the acidic water which passed the electroconductive thin film layer, and there exists a possibility that the resistance of a separator may increase and a power generation efficiency may fall.

  Then, an object of this invention is to provide the metal separator for fuel cells excellent in electroconductivity and corrosion resistance, and its manufacturing method.

The fuel cell separator according to the present invention includes a base material made of a metal material , a conductive amorphous film made of TiN formed on the surface of the base material, and a crystallinity formed on the surface of the conductive amorphous film. And a film made of InGaN which is a nitride .

  Another method for manufacturing a fuel cell separator according to the present invention is to form a conductive amorphous film on a substrate surface made of a metal material by physical vapor deposition (PVD), and to form the formed conductive amorphous film. A film made of crystalline nitride is formed on the surface by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).

  In the present invention, since the surface of the base material made of a metal material is coated with a film made of crystalline nitride, the separator is formed on both the fuel electrode side and the air electrode side of the fuel cell. Even when used, airtightness, corrosion resistance and electrical conductivity can be secured and stable power generation performance can be exhibited.

It is a graph which shows the electric potential from the conduction band of a semiconductor to a full band. It is a schematic block diagram of the test apparatus which measures a contact area resistivity. It is a table | surface which shows the evaluation result regarding a corrosion resistance test. It is a table | surface which shows the measurement result regarding a power generation test. It is explanatory drawing regarding the basic structure of a fuel cell. It is explanatory drawing regarding the basic composition of the single cell of a polymer electrolyte fuel cell.

  Hereinafter, embodiments of the present invention will be described. There is no restriction | limiting in particular as a base material of the separator for fuel cells of this invention, The metal material conventionally used for the separator can be used. For example, a metal such as iron, titanium, or aluminum or an alloy containing at least one of these metals can be given. These metal materials are suitable as the base material of the separator in terms of mechanical strength, versatility, ease of processing, and cost. A typical example of the iron alloy is stainless steel, such as austenitic stainless steel such as SUS304 and SUS316, ferritic stainless steel such as SUS430, and martensitic stainless steel such as SUS420. When stainless steel is used as a base material, it can exhibit excellent characteristics in terms of mechanical strength, electrical conductivity, and corrosion resistance compared to other materials.

  The thickness of the substrate is not particularly limited, but can be set from the viewpoints of mechanical strength and ease of processing, and improvement of the energy density of the fuel cell by thinning. For example, when using stainless steel, 0.1 mm to 1 mm is preferable.

  The film made of crystalline nitride is formed by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). For example, in the case of forming by MOCVD method, using a known manufacturing apparatus, an organic metal gas (triethyl gallium, trimethyl gallium, etc.) and a carrier gas (hydrogen or nitrogen) on the substrate surface heated to 500 to 1000 ° C., In addition, a reactive gas (ammonia gas) is blown, and a chemical reaction between the gases occurs on the surface of the substrate, whereby a film made of crystalline nitride can be formed. The film thickness is preferably 0.1 μm to 2 μm because pinholes are generated when the film is formed thin and durability is weakened, and resistance is increased when the film is formed thick.

  Examples of the crystalline nitride constituting the film include semiconductors such as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), indium gallium nitride (InGaN), titanium nitride (TiN), and titanium aluminum nitride. A nitride having the properties of a conductor can be mentioned.

  One or more such crystalline nitrides are formed to form a film on the surface of the substrate. For example, after forming a GaN layer as a buffer layer, an InGaN layer can be formed thereon to form a two-layered film. The film made of crystalline nitride can be improved in characteristics such as conductivity by doping with a different element. For example, the InGaN layer and the GaN layer serving as a buffer are doped with silicon (Si). As a result, the conductivity of the film is improved.

In the power generation environment of the fuel cell, as described above, the electrons generated at the fuel electrode (anode) flow into the fuel electrode side separator, and the flowed electrons flow from the fuel electrode side separator into the air electrode side separator. Electrons flow out to (cathode). When a semiconductor film is formed as a film made of crystalline nitride, the fuel electrode side separator into which electrons flow from the fuel electrode needs to be higher than the potential in the reaction on the fuel electrode side (H ₂ → 2H ⁺ + 2e ⁻ ). .

  FIG. 1 is a graph showing the potential from the conduction band to the full band of a semiconductor (reference: Kiyoteru Yoshida, “Electronic device using GaN”, 1999, Japan Society of Applied Physics, Vol. 68, p. 787; Fujii Masatoshi, “Photochemical reaction and energy conversion function by semiconductor fine particles: focusing on photocatalysis”, 1984, Japan Society of Applied Physics, 53, p.916). As shown in FIG. 1, InGaN has a conduction band potential higher than the oxidation-reduction potential of hydrogen on the fuel electrode side, indicating that it is a material more suitable for a coating used for the fuel electrode side separator than other semiconductors.

  As described above, examples of the method for forming a film made of crystalline nitride include the MOCVD method and the MBE method. In such a film formation method, the crystallinity of the film is easily affected by the surface condition of the substrate. Therefore, until now, it has been limited to film formation on the surface of a base material made of a limited material such as single crystal sapphire. Although it is possible to directly form a film made of crystalline nitride on the surface of a base material made of a metal material by MOCVD or MBE, a large residual stress due to a difference in thermal expansion coefficient between the surface of the base material and the film Phenomena such as generation, peeling of the film, and deterioration of the adhesion of the film are likely to occur.

  In order to avoid such a phenomenon, a conductive amorphous film is formed on the surface of the base material by a PVD method such as a sputtering method or an ion plating method before the film is formed. The influence of the substrate surface can be reduced. In general, when a crystalline film is formed on the surface on which an amorphous film is formed, it is known that the crystalline film becomes amorphous due to the influence of the underlying amorphous film. GaN, InGaN, and the like having an ore structure have strong c-axis orientation and can form a crystalline film even on the surface of an amorphous film (reference: K. Iwata et al., “Gas Source Molecular Beam Epitaxy Growth of GaN on C-, A-, R- and M-Plane Sapphire and Silica Glass Substrates ", 1997, Jpn.J.Appl.Phys.36).

  The conductive amorphous film can be formed by the PVD method as described above. For example, when forming on the base material surface which consists of stainless steel, in order to remove the foreign material on the surface, after the alkali immersion degreasing and the electrolytic degreasing, a cleaning process of washing with pure water and drying is performed. Further, before the film formation, an amorphous film may be formed by a known PVD method by removing the stainless passive film formed on the surface of the substrate by argon plasma. Examples of the amorphous film include conductive metal ceramic films such as TiN, TiC, TiCN, TiAlN, TiAlC, TiAlCN, CrN, CrC, and CrCN. Further, in order to improve the adhesion of the amorphous film, for example, a metal film made of titanium, aluminum, chromium, zirconium, vanadium, niobium, hafnium, tantalum, or an alloy thereof is formed on the substrate surface as an intermediate layer. You may do it. The conductive amorphous film is preferably formed to a thickness of 0.1 μm to 2 μm because cracks are likely to occur as the thickness increases.

<Example 1>
Stainless steel (SUS304) was used as a base material, and a separator having a film made of InGaN as crystalline nitride formed on the base material surface was manufactured. First, the surface of the base material was subjected to alkali dipping degreasing and electrolytic degreasing, and then washed with pure water and dried to perform a cleaning process. InGaN was formed by MOCVD on the surface of the substrate that had been cleaned to remove foreign matter. As a raw material, a mixture of an organometallic gas (triethylgallium and trimethylgallium), a carrier gas (nitrogen), and a reactive gas (ammonia gas) is used, and the substrate temperature is set to 600 ° C. to 800 ° C. and the mixed gas is used. A film having a thickness of 0.5 μm was formed on the substrate surface by blowing on the substrate surface for 60 minutes.

<Example 2>
Using stainless steel (SUS304) as a base material, cleaning the base material surface, forming a TiN film as a conductive amorphous film, and a film made of InGaN as crystalline nitride on the surface of the conductive amorphous film A separator was formed. The conductive amorphous film is formed by a magnetron sputtering method using titanium as a target and sputtering the target with plasma argon, and a TiN film is formed by blowing nitrogen gas and reacting in forming the film. A flow rate ratio of argon and nitrogen was set to 5: 3, and a film was grown for 30 minutes to form a conductive amorphous film having a thickness of 0.6 μm. The film made of crystalline nitride was formed to a thickness of 0.5 μm by blowing a mixed gas onto the surface of the conductive amorphous film in the same manner as in Example 1.

<Example 3>
In Example 2, a separator in which a TiN film as a conductive amorphous film was formed by an arc ion plating method was manufactured. In the arc ion plating method, a titanium target was used as a cathode, a vacuum arc discharge was generated between the anode and the target, the target was evaporated, and a film was formed on the surface of the substrate by ionization. When forming a film, nitrogen gas is blown and reacted with titanium ions to form a TiN film. The flow rate of nitrogen was 640 sccm, and a conductive amorphous film having a thickness of 0.6 μm was formed by growing the film on the surface of the substrate for 30 minutes.

<Comparative example>
As Comparative Example 1, evaluation was performed using stainless steel (SUS304) as it was. Further, as Comparative Example 2, evaluation was performed using stainless steel (SUS316) as it was. As Comparative Example 3, as in Example 2, stainless steel was used as the base material, and a film was formed by forming only a TiN film having a thickness of 0.6 μm as a conductive amorphous film on the base material surface by magnetron sputtering. Went. As Comparative Example 4, as in Example 3, a stainless steel was used as a base material, and a TiN film only having a thickness of 0.6 μm was formed as a conductive amorphous film on the base material surface by an arc ion plating method. And evaluated.

<Evaluation test>
As an evaluation test of the manufactured separator, a contact area resistivity measurement, a corrosion resistance test, and a simple power generation test were performed. The contact area resistivity was measured using the test apparatus shown in FIG. In the test apparatus, the electrode plate 11 is disposed on both surfaces of the electrode plate 10 and the separator 13 via the carbon sheet 12 (TGP-H-060 manufactured by Toray Industries, Inc.), and an insulating plate is added outside the electrode plate 11. Each of the pressure holding members 10 is arranged, and the pressure holding member 10 is pressed so as to sandwich the pressure from both sides, and the respective members are brought into close contact with each other. Then, an ammeter and a voltmeter are connected between the pair of electrode plates 11.

The surface pressure applied to the pressure holding member 10 was 15 kgf / cm ² , the size of the carbon sheet 10 was set to 1 cm × 1 cm, and the contact area of the separator was 1 cm ² . When the current of the pair of electrode plates 11 is made to flow, it is performed in two ways: when flowing from one electrode plate to the other electrode plate and when flowing in the opposite direction, and the resistance value is determined from the current value and voltage value in each case. Asked. The product of the obtained resistance value and the area value of the separator was defined as the contact area resistivity.

  In the corrosion resistance test, the separator was immersed in sulfuric acid adjusted to pH 2.0 and heated to 60 ° C. for 100 hours. Corrosion resistance was evaluated by measuring the contact area resistivity before and after the test and evaluating the change. The evaluation results are shown in FIG.

The contact area resistivity of the separator is required to be 30 mΩ · cm ² or less in order to maintain the performance of the fuel cell. When the contact area resistivity exceeds 30 mΩ · cm ² , the voltage drop between the cells becomes large, the power generation efficiency is lowered, and the performance of the fuel cell is greatly lowered. From the evaluation results, in Examples 1 to 3, the contact area resistivity was 30 mΩ · cm ² or less before and after the corrosion resistance test, and the contact area resistivity and corrosion resistance necessary to maintain the performance of the fuel cell. It can be seen that

For the simple power generation test, carbon carrying platinum on a part of the film-forming surface of the separator in which a film made of the same conductive amorphous film and crystalline nitride as that of Example 3 was formed on one side was added with Nafion. What was apply | coated as a binder was used. On the other side of the separator, a lead wire was connected for use as an electrode, and the entire surface was masked with an insulating material. As a power generation test of the fuel electrode side separator, the separator was placed in an atmosphere filled with hydrogen, and was set in a state where it was immersed in a sulfuric acid aqueous solution as an electrolyte. At that time, except for the film-forming surface of the separator, the aqueous sulfuric acid solution is not touched by masking the insulating material. As a power generation test for the air electrode side separator, the film formation surface was brought into contact with the atmosphere using a separator prepared in the same manner as the fuel electrode side separator, and the lead wires of the fuel electrode side separator and the air electrode side separator were connected via an external load. Connected electrically. As a result, power generation of the fuel cell occurs with the sample in both the anode and cathode power generation environments. Evaluation is performed by measuring the voltage value when the current density at this time is 100 to 300 mA / cm ² . In this test, the smaller the voltage drop with increasing current density, the better the power generation efficiency, and the better the separator performance.

  The measurement results are shown in FIG. As a comparative example, evaluation was performed using stainless steel (SUS304) as it is in the same manner as in comparative example 1. Looking at the measurement results, when the conductive amorphous film and the film were formed as in Example 3, the high voltage was maintained even when the current density was increased, and the power generation performance was excellent. Recognize.

DESCRIPTION OF SYMBOLS 1 ... Electrolyte membrane 2 ... Catalyst layer 3 ... Catalyst layer 4 ... Porous body 5 ... Porous body 6 ... Separator 7 ... Separator 8 ... Channel 9 ..Flow path 101 ... Electrolyte 102 ... Fuel electrode 103 ... Air electrode 104 ... Load

Claims (2)

A base material made of a metal material , a conductive amorphous film made of TiN formed on the surface of the base material, and a film made of InGaN which is a crystalline nitride formed on the surface of the conductive amorphous film A separator for a fuel cell, comprising: A conductive amorphous film is formed on the surface of a substrate made of a metal material by physical vapor deposition (PVD), and metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy is formed on the surface of the formed conductive amorphous film. A method of manufacturing a separator for a fuel cell, wherein a film made of crystalline nitride is formed by (MBE).

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