JP2020105556A - Metal material, separator, fuel battery cell and fuel cell stack - Google Patents

Abstract

To provide a metal material capable of maintaining low contact resistance and maintaining high drainage when used as a separator.SOLUTION: The metal material includes a metal base material and a titanium oxide layer provided on the base material. The titanium oxide layer includes TiOx (1≤x<2) and MOy (one kind or more of M:Nb, Ta and V; 1≤y≤2.5); the Ti and M contents of the titanium oxide layer satisfy 0.005≤[M]/([Ti]+[M])≤0.10 ([Ti]:the Ti content (at%) of the titanium oxide layer, [M]:the M content (at%) of the titanium oxide layer); the C content of the titanium oxide layer is 1-50 mass%; and the titanium oxide layer has at least one kind of titanium carbide and titanium nitride so as to satisfy predetermined requirements in a front surface layer region from the surface to 10 nm in the depth direction.SELECTED DRAWING: Figure 3

Description

The present invention relates to a metal material, a separator including the metal material, a fuel cell including the separator, and a fuel cell stack including a plurality of the fuel cell.

As a material having conductivity, a metal material is used for various purposes. One of such applications is, for example, a fuel cell separator. A fuel cell uses the energy generated during the binding reaction between hydrogen and oxygen to generate electricity. There are various types of fuel cells, such as solid electrolyte type, molten carbonate type, phosphoric acid type, and solid polymer type.

The main functions required for a separator, for example, a polymer electrolyte fuel cell separator are as follows.
(1) Function as a "flow path" for uniformly supplying fuel gas or oxidizing gas to the electrode membrane (anode and cathode) (2) Water generated on the cathode side together with carrier gas such as air and oxygen, Function as a "flow path" that efficiently discharges from the fuel cell to the outside of the system (3) Function as an electrical path by contact with the electrode membrane, and also as an electrical "connector" between two adjacent single cells (4) A function as a "partition wall" between the anode chamber of one cell and the cathode chamber of the adjacent cell between adjacent cells. (5) Cooling is performed in a water-cooled fuel cell including a separator having a flow passage for cooling water. Function as a "partition wall" between the water flow path and the adjacent cell

The material of the separator used in the polymer electrolyte fuel cell needs to be capable of performing such a function. For the separator, a graphite substrate or a carbon-based material using carbon powder hardened with a resin may be used. However, in recent years, a metal-based material is often used for the separator. This is because the metal-based material has an advantage that it is excellent in workability as a property peculiar to metal. That is, it is easy to process a flat metal material into a desired separator shape having the above-mentioned flow path. Further, by using the metal-based material, the thickness of the separator can be reduced, and the weight of the separator can be reduced. Titanium, stainless steel, carbon steel or the like is used as the metal-based material. The separator made of these metallic materials is formed by press working.

A fuel cell separator is required to have high conductivity. When the conductivity of the separator is low, the power generation efficiency of the fuel cell is low. Titanium has high conductivity, but titanium oxide (TiO ₂ ) having low conductivity is formed on the surface thereof. This increases the contact resistance of the titanium material. Even with other metals, the contact resistance increases due to the formation of oxides on the surface.

Therefore, a method of reducing the contact resistance of a metal material having an oxide formed on its surface has been proposed. In Patent Documents 1 and 2, a noble metal is provided on the surface of a metal material. High conductivity is maintained by this noble metal. In Patent Document 3, a titanium oxide having high conductivity is formed by introducing a pentavalent metal into the titanium oxide on the surface. Non-Patent Documents 1 and 2 disclose methods such as physical vapor deposition (PVD) and coating as a method for introducing a pentavalent metal into titanium oxide. In Patent Documents 3 and 4, an oxygen-deficient oxide having high conductivity is formed on the titanium oxide on the surface.

In Patent Document 5, a carbon layer is provided on the surface of a base material made of titanium or a titanium alloy. An intermediate layer containing titanium carbide is formed between the base material and the carbon layer. In this method, the carbon layer reduces the contact resistance. The intermediate layer is formed to improve the adhesion between the base material and the carbon layer.

JP, 2003-105523, A JP 2006-190643 A JP, 2005-224368, A International Publication No. 2017/169712 JP, 2014-22250, A

Meagen A. Gillispie, 4 others, "rf magnetron sputter deposition of transparent conducting Nb-doped TiO2 films on SrTiO3", Journal of Applied Physics 101 (2007) 33125 Jinming Liu, 9 others, “Influence of annealing process on conductive properties of Nb-doped TiO2 diffuse films prepared by sol-gel method”, Applied Surface Science 257 (2011) 10156 Jun Suzuki, 3 others, "Reduction of contact resistance by heat treatment after pickling of titanium alloy containing precious metal element", Titanium, 2006, vol. 54, no. 4, p.258-262

In the fuel cell separator, water is generated by the reaction of oxygen and hydrogen in the flow path of the oxidizing gas. When the drainage of the channel is low, the generated water remains in the channel. As a result, the flow of oxidizing gas is impeded. That is, the flow rate of the oxidizing gas flowing in the channel is reduced. As a result, the amount of reaction between oxygen and hydrogen decreases, and the amount of power generation of the fuel cell decreases.

In Patent Documents 1 to 5, the purpose is to reduce the contact resistance of the separator material, and no attention has been paid to the drainage property of the flow path. For example, in the separator of Patent Document 5, the entire surface is made of a carbon layer such as graphite. However, such a carbon layer has low wettability with water and low drainage.

Therefore, an object of the present invention is to provide a metal material that can maintain low contact resistance and high drainage when used as a separator of a fuel cell. Another object of the present invention is to provide a separator that can maintain low contact resistance and high drainage performance. Still another object of the present invention is to provide a fuel cell and a fuel cell stack capable of maintaining high power generation.

The metal material of the embodiment of the present invention includes a metal base material and a titanium oxide layer provided on the base material,
The titanium oxide layer contains TiO _x (1≦x<2) and MO _y (M: one or more of Nb, Ta and V; 1≦y≦2.5),
The content of Ti and M of the titanium oxide layer satisfies the following formula (1),
C content of the titanium oxide layer is 1 to 50 mass%,
The titanium oxide layer has at least one kind of titanium carbide and titanium nitride in a surface layer region from the surface to a depth direction of 10 nm,
In the surface layer region, the content of C forming the titanium carbide, the content of N forming the titanium nitride, and the content of O forming the TiO _x and the MO _y are represented by the following formula (2). The filling,
The titanium oxide layer has a thickness of 10 to 1000 nm.
0.005≦[M]/([Ti]+[M])≦0.10 (1)
here,
[Ti]: Ti content (at %) of the titanium oxide layer
[M]: M content (at %) of the titanium oxide layer
0.05≦([C]+[N])/[O]≦1.0 (2)
here,
[C]: Content of C constituting titanium carbide in the surface layer region (at %)
[N]: Content of N constituting titanium nitride in the surface layer region (at %)
[O]: Content (at %) of titanium oxide and O constituting MO _y in the surface layer region.

A fuel cell separator according to an embodiment of the present invention includes the metal material.
A fuel cell according to an embodiment of the present invention includes the above separator.
A fuel cell stack according to an embodiment of the present invention includes a plurality of fuel cell units described above.

The metal material of the present invention can maintain low contact resistance and high drainage property when used as a separator. The separator of the present invention can maintain low contact resistance and high drainage property. The fuel cell and fuel cell stack of the present invention can maintain a high power generation amount.

FIG. 1 is a sectional view of a metal material according to an embodiment of the present invention. FIG. 2A is a perspective view of the entire polymer electrolyte fuel cell according to the embodiment of the present invention. FIG. 2B is an exploded perspective view of a fuel cell (single cell) according to an embodiment of the present invention. FIG. 3 is a diagram showing the relationship between the depth from the surface and the concentration of each element in the metal material of one example. FIG. 4 is a diagram showing the configuration of an apparatus for measuring the contact resistance of a metal material.

The present inventors have studied to form the outermost surface layer of the metal material and the separator with a titanium oxide layer having conductivity in order to keep the contact resistance of the metal material and the separator low. Based on such a configuration, the present inventors have studied to increase the wettability (affinity) of water with respect to the metal material and the separator in order to improve the drainage performance of the metal material and the separator.

In the following description, "%" in the chemical composition means "mass%" unless otherwise specified.

<Metal material of the present invention>
FIG. 1 is a sectional view of a metal material according to an embodiment of the present invention. The metal material 11 includes a base material 12 and a titanium oxide layer 13 provided on the base material 12.

<Base material>
The base material is made of metal. Therefore, the base material has conductivity. The electric resistivity of the substrate is preferably 2×10 ⁻⁴ Ω·cm (20° C.) or less, more preferably 1×10 ⁻⁴ Ω·cm (20° C.) or less.

The type of metal element forming the base material is not particularly limited. The substrate may contain substantially only one type of metal element. For example, the base material may be composed of Ti, Pt, or Al except for impurities. Further, the base material may contain a plurality of types of metal elements. For example, the base material may be made of an alloy (including impurities) such as a titanium alloy, stainless steel, or an aluminum alloy.

Pure titanium and titanium alloy have the advantages of excellent corrosion resistance and light weight. Therefore, in applications requiring high corrosion resistance and light weight, for example, applications in fuel cell separators, the base material is preferably made of pure titanium or a titanium alloy. Here, “pure titanium” means a metal material containing 98.8% or more of Ti and the balance being impurities. As pure titanium, for example, pure titanium of JIS type 1 to JIS type 4 can be used. The “titanium alloy” means a metal material containing 70% or more of Ti and the balance consisting of an alloy element and an impurity element. As the titanium alloy, for example, JIS type 11, 13 type, 17 type, or 60 type titanium alloy can be used.

The titanium alloy as the base material preferably contains 6% or less of M (one or more of Nb, Ta, and V). The base material may not contain M. By containing M, the corrosion resistance, conductivity, and mechanical strength of the titanium alloy are increased. In order to sufficiently obtain such effects, the M content of the titanium alloy is preferably 0.1% or more. On the other hand, when the content of M exceeds 6%, the conductivity and bending workability deteriorate. A titanium alloy containing 0.1 to 6% of M is superior to pure titanium in corrosion resistance, conductivity, bending workability, and mechanical strength. In order to obtain these effects more sufficiently, the M content is more preferably 0.5 to 6%.

<Titanium oxide layer>
The titanium oxide layer satisfies the following requirements (i) to (vi).
(I) TiO _x (1≦x<2) and MO _y (M: one or more of Nb, Ta and V; 1≦y≦2.5) are included.
(Ii) The contents of Ti and M satisfy the following formula (1).
0.005≦[M]/([Ti]+[M])≦0.10 (1)
here,
[Ti]: Ti content (at %) of the titanium oxide layer
[M]: M content (at %) of the titanium oxide layer
(Iii) 1 to 50% of C (carbon) is contained.
(Iv) At least one kind of titanium carbide and titanium nitride is contained in the surface layer region from the surface to the depth direction of 10 nm.
(V) In the surface layer region, the content of C constituting titanium carbide, the content of N constituting titanium nitride, and the content of O constituting TiO _x and MO _y satisfy the following formula (2). ..
0.05≦([C]+[N])/[O]≦1.0 (2)
here,
[C]: Content (at %) of C constituting titanium carbide in the surface layer region
[N]: Content of N constituting titanium nitride in the surface layer region (at %)
[O]: Content of titanium oxide and O constituting MO _y (at %) in the surface layer region
(Vi) The thickness is 10 to 1000 nm.

Hereinafter, each of the requirements (i) to (vi) will be described in detail.

<Requirement (i): The titanium oxide layer contains TiO _x (1≦x<2) and MO _y (1≦y≦2.5)>
Titanium oxide represented by the chemical formula with TiO _x is a low-order oxide such as TiO, Ti ₂ O ₃ , and Ti ₄ O ₇ , and has a crystal structure of TiO ₂ and has a part of oxygen (O). It may include a defect. One or more of these titanium oxides are present in the titanium oxide layer. x is the atomic ratio of O to Ti as the average chemical composition of titanium oxide.

The oxide represented by the chemical formula with MO _y (hereinafter referred to as “M oxide”) is M ₂ O ₅ (Nb ₂ O ₅ , Ta ₂ O ₅ , V ₂ O ₅ ) which is the most stable oxide. ) As well as low-order oxides MO ₂ (NbO ₂ , TaO ₂ , VO ₂ ), MO (NbO, TaO, VO), and the like. One or more of these M oxides are present in the titanium oxide layer. y is the atomic ratio of O to M as the average chemical composition of M oxide.

The titanium oxide layer satisfying the requirement (i) has high conductivity. The reason is estimated to be as follows.
(1) Oxygen deficiency in TiO _x , and when y is less than 2.5, the number of electron carriers increases due to oxygen deficiency in MO _y .
The presence of (2) MO _y, oxygen deficiency is stabilized in TiO _x, for example, TiO _x is suppressed to be oxidized to TiO _2.

The titanium oxide layer may further include a composite oxide of Ti and M, for example, Ti _1-z MzO ₂ (0<z≦0.2). The conductivity of such a complex oxide is high.

Whether or not TiO _x (1≦x<2) and MO _y (1≦y≦2.5) are present can be confirmed by XPS (X-ray Photoelectron Spectroscopy). In the XPS spectrum, if the peak of at least one of Ti ³⁺ and Ti ²⁺ exists in the binding energy of Ti and O, it can be determined that 1≦x<2. In this case, a Ti ⁴⁺ peak may be present. Further, regarding the binding energies of M and O, if only the peak of Nb ⁵⁺ is present, it can be determined that y=2.5. If at least one of M ⁴⁺ and M ²⁺ is present, it can be determined that 1≦y<2.5. In this case, an M ⁵⁺ peak may be present.

Whether or not any of the peaks of Ti ³⁺ , Ti ²⁺ , M ⁴⁺ and M ²⁺ is present in the XPS spectrum is determined by, for example, differentiating the waveform of the peak and determining the vicinity of the position where the peak is expected to be present. The determination can be made based on the change in the differential value at. Specifically, it is as follows. The waveform of the XPS spectrum is determined by how the binding energy E and the emission photoelectron intensity I correspond. The emitted photoelectron intensity I can be regarded as a function I(E) of the binding energy E. Here, for example, in the positive direction of E, if the value obtained by differentiating I(E) three times with E changes from a negative value to a positive value near the position where the peak is expected to exist, the peak Can be determined to exist. On the other hand, if there is no such change near the position where the peak is expected to exist, it can be determined that the peak does not exist.

The XPS measurement conditions are as follows.
X-ray: AlK _α (hν = 1486.6 eV (h: Planck's constant, ν: X-ray frequency))
X-ray diameter (diameter): 100 μm
Detector take-in angle: 45°
When the XPS measurement is performed in combination with sputtering, the sputtering conditions are as follows.
Ion type: Ar ⁺
Accelerating voltage: 2-4kV
Raster area: Square area with one side of 1 to 3 mm The measurement under the above conditions can be performed using, for example, a device in which ULVAC-PHI Quantera SXM and PHI 5000 VersaProbe III manufactured by the same are combined. ..

<Requirement (ii): Ti and M contents of the titanium oxide layer satisfy the formula (1)>
The reason why [M]/([Ti]+[M]) is 0.005 or more is to increase the conductivity of the titanium oxide layer. When [M]/([Ti]+[M]) is less than 0.005, the electronic interaction between TiO ₂ and TiO _x and MO _y is weak, and the contribution to conductivity is insufficient. Become. [M]/([Ti]+[M]) is preferably 0.010 or more.

The reason why [M]/([Ti]+[M]) is 0.10 or less is to keep the cost low, improve the workability, and lower the contact resistance. Since M is more expensive than Ti, if [M]/([Ti]+[M]) increases, the raw material cost increases. Further, when [M]/([Ti]+[M]) increases, the ductility of the titanium oxide layer decreases, and thus the workability of the metal material decreases. Furthermore, when [M]/([Ti]+[M]) is large, the contact resistance is high. This is because the amount of M oxide in the titanium oxide layer increases and the M oxide exists in an aggregated state, and along with this, many physical defects occur in the titanium oxide layer. Presumed to be. [M]/([Ti]+[M]) is preferably 0.080 or less.

[Ti] and [M] can be measured by XPS. When the titanium oxide layer is thick (for example, when the thickness of the titanium oxide layer is 100 nm or more), the Ti and M concentration profiles are obtained in the depth direction while sputtering with Ar, and the depths of these concentration profiles are determined. [Ti] and [M] are calculated as the integrated values of the directions, respectively. When a metal material is produced by a method using a sol solution described below, the value of [M]/([Ti]+[M]) is generally almost constant except for the portion near the surface of the titanium oxide layer. Become. Therefore, it is not necessary to perform XPS over the entire thickness of the titanium oxide layer, and [Ti] and [M] at a depth at which typical [M]/([Ti]+[M]) can be obtained. ] May be adopted.

<Requirement (iii): C (carbon) content of the titanium oxide layer is 1 to 50%>
The titanium oxide layer preferably contains a large amount of C that constitutes the carbonaceous material having conductivity. In this case, when the content of C in the titanium oxide layer is 1% or more, the conductivity of the titanium oxide layer is increased and the contact resistance on the surface of the metal material is decreased. Examples of the carbonaceous material having conductivity include graphite, metal carbide, and metal carbonitride.

The wettability of water to carbonaceous materials is generally low. This is disadvantageous in improving drainage when a metal material is used for the separator. However, by dispersing a fine carbonaceous material in the titanium oxide layer, it is possible to prevent the wettability of water with respect to the titanium oxide layer from decreasing. In the production method described below, a sol liquid mixed with an organic polymer is used to form the titanium oxide layer. Graphite generated by carbonization of such an organic polymer is fine and easily dispersed around the crystal of titanium oxide. In this case, it is possible to suppress a decrease in wettability of water with respect to the titanium oxide layer due to the inclusion of the carbonaceous material. For these reasons, the carbonaceous material having conductivity is preferably graphite originating from an organic polymer.

If the C content of the titanium oxide layer is less than 1%, the effect of lowering the contact resistance on the surface of the metal material cannot be sufficiently obtained. The C content of the titanium oxide layer is preferably 3% or more, more preferably 5% or more. Further, when the C content of the titanium oxide layer is 50% or less, the adhesion of the titanium oxide layer to the base material can be increased. When the C content of the titanium oxide layer exceeds 50%, the composition of the titanium oxide layer becomes non-uniform, and the titanium oxide layer easily peels from the substrate. The C content of the titanium oxide layer is preferably 20% or less, and more preferably 15% or less.

The C content of the titanium oxide layer can be measured by the following method. First, the titanium oxide layer is peeled from the base material using an alkaline peeling agent. Then, the mass of the peeled titanium oxide layer is measured. Further, the mass of carbon contained in the peeled titanium oxide layer is measured by the combustion infrared absorption method. Then, the mass of carbon is divided by the mass of the titanium oxide layer to obtain the C content (mass %) of the titanium oxide layer. In the combustion infrared absorption method, specifically, all of the organic carbon and the inorganic carbon contained in the titanium oxide layer are burned at a burning temperature of 1400° C. under an oxygen stream to convert all of these carbon into CO ₂ and CO. The mass of C is quantified by infrared absorption.

<Requirement (iv): The titanium oxide layer has at least one of titanium carbide and titanium nitride in the surface layer region>
The conductivity of titanium carbide and titanium nitride is higher than that of TiO _x . Therefore, when the titanium oxide layer satisfies the requirement (iv), the conductivity of the surface layer region of the titanium oxide layer becomes high, and the contact resistance of the surface of the metal material becomes low. The titanium carbide may be, for example, TiC or Ti ₂ C. The titanium nitride may be, for example, Ti ₃ N ₄ .

In addition, the surface layer region having at least one of titanium carbide and titanium nitride improves drainage of the separator in the fuel cell. This is for the following reason. These compounds have poorer corrosion resistance than TiO _x and are easily corroded or decomposed in a fuel cell environment. Corrosion of these compounds produces at least one of Ti(OH) ₄ and TiO _{2 on} the surface of the separator (titanium oxide layer of the metal material). Ti(OH) ₄ and TiO ₂ have a small contact angle with water. That is, Ti(OH) ₄ and TiO ₂ which are corrosion products have high affinity with water. Therefore, it becomes difficult for water to stay on the surface of the separator in the fuel cell. As a result, drainage performance of the separator is improved.

In the surface layer region, at least one of titanium carbide and titanium nitride is preferably present in a fine and dispersed state. In this case, in a fuel cell environment, fine corrosion products are dispersed and formed on the surface of the titanium oxide layer. The conductivity of the corrosion products is low. However, if fine corrosion products are dispersed and present on the surface of the titanium oxide layer, a conductive path by a substance having higher conductivity, for example, TiO _x and MO _y is secured. Therefore, it is possible to suppress an increase in contact resistance due to the formation of corrosion products.

<Requirement (v): In the surface layer region of the titanium oxide layer, the content of C constituting titanium carbide, the content of N constituting titanium nitride, and the content of O constituting TiO _x and MO _y are Formula (2) is satisfied>
[C] is an index of the amount of titanium carbide in the surface layer region. [N] is an index of the amount of titanium nitride in the surface layer region. When the conductivity of titanium carbide and titanium nitride is higher than that of TiO _x , and ([C]+[N])/[O] increases, the conductivity of the surface layer region of the titanium oxide layer increases. Become. When ([C]+[N])/[O] is less than 0.05, the effect of increasing the conductivity of the surface layer region cannot be sufficiently obtained. Therefore, ([C]+[N])/[O] is set to 0.05 or more. ([C]+[N])/[O] is preferably 0.10 or more, and more preferably 0.15 or more.

On the other hand, when ([C]+[N])/[O] exceeds 1.0, the amount of corrosion (decomposition) of titanium carbide and titanium nitride in the fuel cell environment becomes excessive, and a large amount is present on the surface of the metal material. TiO ₂ is formed. In this case, the contact resistance of the metal material becomes high. Therefore, ([C]+[N])/[O] is set to 1.0 or less. ([C]+[N])/[O] is preferably 0.50 or less.

By satisfying the expression (2), it is possible to maintain high wettability of water with respect to the surface of the metal material in the fuel cell environment while increasing the conductivity of the surface layer region.

The presence of at least one kind of titanium carbide and titanium nitride in the surface layer region and the measurement of [C], [N] and [O] can be carried out by the following method. First, the concentration profiles of C, N, and O are measured by XPS in the surface layer region in the depth direction from the surface of the titanium oxide layer. At that time, C, N and O are intended to constitute titanium carbide, titanium nitride, titanium oxide and MO _y , respectively. Specifically, it is as follows. The concentration of C constituting the titanium carbide is determined from the peak intensity of C1s due to the carbide. The concentration of N constituting titanium nitride is determined from the peak intensity of N1s due to the nitride. The concentration of O constituting titanium oxide and MO _y is obtained from the peak intensity of O1s due to the oxide. In this way, [C], [N], and [O] at one point in the surface layer region excluding the region up to 2 nm from the surface of the titanium oxide layer are obtained.

FIG. 3 is a diagram showing a result of XPS depth direction analysis of a sample of an example (Test No. 1; Example of the present invention) described later, showing a relationship between the depth from the surface and the concentration of each element. Based on the diagram showing the relationship between the depth from the surface and the concentration of each element as shown in FIG. 3, [Ti] and [M] of the formula (1) (requirement (ii)) and the formula (2) (requirement ( v)) [C], “N” and [O] can be determined.

In the titanium oxide layer, when C constituting titanium carbide is detected in the region from the surface to 10 nm, that is, in the surface layer region by the above analysis, it is determined that the titanium oxide layer exists in the surface layer region. When N constituting titanium nitride is detected in the surface layer region, it is determined that titanium nitride is present in the surface layer region. When neither C constituting titanium carbide nor N constituting titanium nitride is detected in the surface layer region, it is determined that neither titanium carbide nor titanium nitride is present in the surface layer region. Whether or not C constituting titanium carbide and N constituting titanium nitride is detected by XPS has a peak of any one of Ti ³⁺ , Ti ²⁺ , M ⁴⁺ and M ^2+. It can be determined by a method similar to the above method for determining whether or not.

<Requirement (vi): The thickness of the titanium oxide layer is 10 to 1000 nm>
When the thickness of the titanium oxide layer is 10 nm or more, the base material can be well protected. When the thickness of the titanium oxide layer is less than 10 nm, the titanium oxide layer forms an electrode (for example, composed of carbon fibers) in a cell of a fuel cell (for example, a polymer electrolyte fuel cell). The substrate is more likely to be exposed when damaged by contact. In this case, an oxide is formed on the exposed portion by corrosion or oxidation. For example, when the substrate is made of pure titanium or titanium alloy, TiO ₂ is formed on the exposed portion. Since the electrical resistance of TiO ₂ is high, this increases the contact resistance of the alloy material (separator). The thickness of the titanium oxide layer is preferably 20 nm or more, more preferably 25 nm or more.

Further, when the thickness of the titanium oxide layer is 1000 nm or less, the electrical resistance in the thickness direction of the titanium oxide layer can be reduced and the adhesion of the titanium oxide layer to the base material can be increased. That is, when the thickness of the titanium oxide layer exceeds 1000 nm, the electric resistance in the thickness direction of the titanium oxide layer becomes so high that it cannot be ignored with respect to the electric resistance of the base material, and the titanium oxide layer is pressed during the pressing process. The layer is easy to peel from the substrate. The thickness of the titanium oxide layer is preferably 500 nm or less, more preferably 300 nm or less.

The thickness of the titanium oxide layer is the average thickness of the titanium oxide layer. The average thickness is the average value of the thicknesses measured at at least three points in the titanium oxide layer. The thickness of the titanium oxide layer at each measurement location is determined by XPS depth direction analysis. Specifically, the O (oxygen) concentration profile is obtained from the surface of the titanium oxide layer in the depth direction while sputtering with Ar. Then, in this O concentration profile, the depth at which the O concentration is halved with respect to the peak near the surface is the thickness of the titanium oxide layer.

In the depth direction analysis by XPS, assuming that the sputtering time and the depth from the surface have a predetermined relationship, the depth is obtained from the surface of the titanium oxide layer by the sputtering time. Therefore, in order to accurately determine the depth from the surface, the relationship between the sputtering time under the conditions for measuring the O concentration profile and the depth actually reached by sputtering is separately calculated. The depth actually reached by sputtering is determined from a SEM (Scanning Electron Microscope) image of a cross section of the titanium oxide layer (metal material) after sputtering including the sputtered portion and perpendicular to the substrate. As a result, the depth can be accurately obtained from the surface of the titanium oxide layer based on the sputtering time.

<Contact angle of water to titanium oxide layer surface>
Hereinafter, the contact angle of water with respect to the titanium oxide layer surface will be simply referred to as “contact angle”. When a metal material is used as a separator of a fuel cell, the contact angle after the fuel cell is operated, for example, after a constant current operation of 0.5 A/cm ² for 500 hours, is preferably 105° or less. As a result, water generated in the flow path of the oxidizing gas flowing through the separator of the fuel cell can be satisfactorily discharged. The contact angle after operation of the fuel cell is more preferably 90° or less, and further preferably 45° or less.

As described above, the corrosion products of titanium carbide and titanium nitride contained in the titanium oxide layer have a high affinity with water. Therefore, the contact angle decreases with the operation time of the fuel cell, so that the water generated in the flow path of the oxidizing gas is satisfactorily discharged over the long-term operation of the fuel cell. The contact angle before starting the operation of the fuel cell may exceed 90°.

The contact angle shall be measured at room temperature according to the sessile drop method of JIS R3257 (1999). In this method, first, water droplets are attached to the surface of the sample. Next, an image of the portion including the end portion of the contact portion between the sample and water as viewed from the direction along the sample surface is obtained. Then, this image is processed by a computer to calculate the angle formed by the surface of the sample and the surface of the water to obtain the contact angle. According to this method, a contact angle that reflects the characteristics of the vicinity of the surface of the sample, for example, the portion from the surface of the sample to a depth of 1 to 2 mm can be obtained.

<Method of manufacturing metal material>
The metal material of the present invention can be manufactured by, for example, a method including a contact step and a heat treatment step described below. In the contacting step, a liquid serving as a raw material for the titanium oxide layer is contacted with the surface of the substrate. In the heat treatment step, the titanium oxide layer is formed by subjecting the base material after the contact step to heat treatment in a non-oxidizing atmosphere.

The liquid used in the contact step is, for example, a sol liquid containing a titanium oxide precursor compound, an M (one or more of Nb, Ta and V) oxide precursor compound, and an organic polymer. You can

In the heat treatment step, titanium oxide and M oxide are formed, and at least part of M is doped in titanium oxide. When a sol liquid is used as the raw material liquid, in the heat treatment step, the titanium oxide is partially reduced by the organic polymer in the sol liquid, and the reduced titanium oxide TiO _x (Ti ₂ O ₃ , Ti ₄ O _{3 (7,} etc.) are formed, and the M oxide is also partially reduced. Further, since the sol liquid contains the organic polymer, a carbonaceous material having conductivity such as graphite is formed in the titanium oxide layer in the heat treatment step.

Further, carbon or its compounds generated by thermal decomposition of the organic polymer reacts with Ti of titanium oxide to form titanium carbide in the titanium oxide layer. Carbon or its compounds may also react with the substrate to form titanium carbide on the substrate. Similarly, when the organic polymer contains nitrogen, nitrogen or a compound thereof generated by thermal decomposition of the organic polymer reacts with Ti of titanium oxide to form titanium nitride in the titanium oxide layer. Nitrogen or its compounds may also react with the substrate to form titanium nitride on the substrate.

Hereinafter, an example of a method for producing a metal material will be described in detail when the above-mentioned sol liquid is used in the contact step.

<Contact process>
The sol liquid may contain the components described below. Examples of titanium oxide precursor compounds include Ti alkoxides (ethoxide, butoxide, propoxide, etc.), Ti chlorides, Ti complex compounds, and titanium oxide fine particles (colloids). One or more of these compounds can be used in the sol liquid.

Examples of the precursor compound of the M oxide include one or more alkoxides of Nb, Ta and V (ethoxide, butoxide, propoxide, etc.), nitrates, chlorides, and oxide fine particles (colloids). .. One or more of these compounds can be used in the sol liquid.

The organic polymer needs to be dissolved in the solvent of the sol liquid. Polyvinylpyrrolidone and polyvinyl acetate are soluble in organic solvents such as various alcohols. Polyvinyl alcohol, polyvinyl pyrrolidone, and various water-based resins are soluble in water. Depending on the type of solvent of the sol liquid, the resin that dissolves in that solvent can be appropriately selected.

Polyvinylpyrrolidone and polyvinyl acetate can partially reduce titanium oxide and M oxide in the heat treatment step, and can increase the conductivity of these oxides. Further, C originating from polyvinylpyrrolidone and polyvinyl acetate remains in the titanium oxide layer as a carbonaceous material having conductivity depending on the heat treatment conditions. With such a carbonaceous material, the conductivity of the titanium oxide layer can be increased. From these points, it is preferable to use at least one of polyvinylpyrrolidone and polyvinyl acetate as the organic polymer.

To prepare a sol liquid by mixing a precursor compound of titanium oxide and a precursor compound of M oxide in a solvent in which they are soluble, compatible or dispersed at a predetermined ratio. You can At that time, if necessary, an acid such as nitric acid or hydrochloric acid may be added to accelerate hydrolysis or polymerization. A stabilizer such as diethanolamine or acetylacetone may be added to the sol liquid.

The organic polymer may be added simultaneously when the titanium oxide precursor compound and the M oxide precursor compound are mixed in a solvent. Further, the organic polymer may be added to this sol solution after preparing a sol solution containing a titanium oxide precursor compound and an M oxide precursor compound.

The molecular weight of the organic polymer is not limited, but when at least one of polyvinylpyrrolidone and polyvinyl acetate is used as the organic polymer, the molecular weight of the organic polymer is preferably 200,000 or less. When the molecular weight exceeds 200,000, the viscosity of the sol solution becomes too high, which may make it difficult to uniformly supply the sol solution to the surface of the base material.

The amount of the organic polymer added to the sol liquid (the ratio of the organic polymer in the sol liquid after the addition; hereinafter referred to as “polymer addition amount”) is not limited. However, if the addition amount is too small, the titanium oxide cannot be reduced sufficiently. Therefore, the amount of polymer added is preferably 1% or more. Further, if the amount of the polymer added is too large, the viscosity of the sol liquid becomes too high, and it becomes difficult to uniformly supply an appropriate amount of the sol liquid to the surface of the base material. If the sol liquid is excessively supplied to the surface of the base material, after the heat treatment step, the carbon content remains unnecessarily and the titanium oxide layer becomes nonuniform. Therefore, the amount of polymer added is preferably 30% or less, and more preferably 15% or less.

The sol solution obtained as described above is attached (contacted) to the surface of the base material by wet coating such as a dip method, a spray method, a bar method, a doctor blade method or the like. The thickness of the sol liquid deposited is adjusted according to the thickness of the titanium oxide layer to be formed.

<Heat treatment process>
In the heat treatment step, the titanium oxide precursor compound and the M oxide precursor compound are thermally decomposed and polymerized to form titanium oxide and M oxide. Furthermore, M is incorporated into the titanium oxide as a dopant. The incorporated M replaces a part of Ti in the titanium oxide layer. Thereby, for example, Ti _1-z M _z O ₂ (0<z≦0.2) is formed. This increases the conductivity of the titanium oxide layer because the titanium oxide in which M is incorporated has high conductivity.

Further, in the heat treatment step, the organic polymer is decomposed to generate C. When the heat treatment process is performed in a non-oxidizing atmosphere, the organic polymer is reductively thermally decomposed to produce a carbonaceous material having conductivity such as graphite. The titanium oxide is reduced by C generated by the decomposition of the organic polymer. At this time, the M oxide may also be reduced. Further, C produced by the decomposition of the organic polymer reacts with Ti to produce titanium carbide. In this way, a titanium oxide layer containing TiO _x , MO _y , a carbonaceous material having conductivity, and titanium carbide is formed.

When the organic polymer contains N, the decomposition of the organic polymer produces N in addition to C. At least one of C and N reduces the titanium oxide. At this time, the M oxide may also be reduced. Further, at least one of C and N reacts with Ti to produce at least one of titanium carbide and titanium nitride. Therefore, in this case, a titanium oxide layer containing at least one of TiO _x , MO _y , a carbonaceous material having conductivity, and titanium carbide and titanium nitride is formed.

From the above, the temperature of the heat treatment needs to be a temperature at which the following reactions (a) to (d) occur.
(A) The titanium oxide precursor compound and the M oxide precursor compound are decomposed to form titanium oxide and M oxide.
(B) The organic polymer is thermally decomposed, and the titanium oxide is reduced by at least one of C and N generated thereby.
(C) At least one of C and N generated by thermal decomposition of the organic polymer reacts with Ti to form at least one of titanium carbide and titanium nitride.
(D) The organic polymer is thermally decomposed to form a carbonaceous material having conductivity, and the carbonaceous material remains in the titanium oxide layer.

Taking the above into consideration, the heat treatment temperature is determined. Specifically, the heat treatment temperature is preferably in the range of 650 to 950°C. The heat treatment time can be appropriately set in consideration of the heat treatment temperature and the like, and can be, for example, 10 seconds to 1 hour.

By setting the heat treatment temperature to 650° C. or higher, titanium oxide and M oxide are reduced by at least one of C and N generated from the organic polymer. Further, by setting the heat treatment temperature to 950° C. or lower, at least one kind of titanium carbide and titanium nitride can be generated in an appropriate amount. Specifically, by setting the heat treatment temperature to 950° C. or lower, the formula (2) is easily satisfied. When the heat treatment temperature exceeds 950° C., at least one of titanium carbide and titanium nitride is excessively generated in the titanium oxide layer, and it is difficult to satisfy the formula (2). Further, if the heat treatment is performed at a temperature higher than 950° C., the energy cost increases and the economical efficiency decreases. Further, when the base material is made of pure titanium or titanium alloy, heat treatment at a temperature higher than 950° C. increases the hardness of the base material and makes it difficult to process it into a separator shape.

The temperature at which the carbonaceous material remains in the titanium oxide layer varies greatly depending on the type of organic polymer. In order to allow the carbonaceous material to remain in the titanium oxide layer, the heat treatment temperature is preferably set as follows. That is, when the organic polymer is polyvinylpyrrolidone, the heat treatment temperature is preferably 650 to 900°C. When the organic polymer is polyvinyl acetate, the heat treatment temperature is preferably 650 to 800°C.

The heat treatment atmosphere needs to be a non-oxidizing atmosphere in order to thermally decompose the organic polymer and cause the reactions (b) to (d). As the non-oxidizing atmosphere, an inert gas atmosphere such as argon or nitrogen, a reducing gas atmosphere such as hydrogen, or a vacuum (reduced pressure atmosphere) can be adopted. The inert gas or the reducing gas may contain oxygen or water vapor as long as it has an oxygen partial pressure of 0.1 Pa or less. When the oxygen partial pressure exceeds 0.1 Pa, the organic polymer is oxidatively decomposed and disappears, and titanium oxide and M oxide are not reduced.

During the heat treatment, various gases are generated due to the decomposition of the titanium oxide precursor compound and the M oxide precursor compound and the thermal decomposition of the organic polymer. Therefore, it is preferable to perform the heat treatment while circulating an inert gas or a reducing gas so that these gases do not stay near the base material and hinder or suppress the desired reaction. When the heat treatment is performed in vacuum, the degree of vacuum is preferably 10 ^-2 Pa or less.

When the base material is made of pure titanium or titanium alloy, before the contacting step, the surface layer of the base material is oxidized to form a preliminary layer containing titanium oxide in advance, and then the contacting step is performed. Good. By oxidizing the surface layer portion of the base material made of pure titanium or titanium alloy, a preliminary layer containing titanium oxide is formed. Since the preliminary layer is formed by direct oxidation of the base material, it has high adhesion to the rest of the base material and has a uniform thickness.

When the contact step using the sol solution is performed on the base material on which the preliminary layer is formed, and then the heat treatment step is performed, at least one of C and N generated by the thermal decomposition of the organic polymer in the sol solution is performed. Not only the titanium oxide originating from the precursor compound but also the titanium oxide contained in the preliminary layer is reduced. Thereby, conductivity is imparted to these titanium oxides. The titanium oxide originating from the preliminary layer shall be part of the titanium oxide layer. By forming the preliminary layer, the thickness of the titanium oxide layer can be easily increased and the adhesion between the titanium oxide layer and the base material can be improved.

The thickness of the preliminary layer is preferably 3 nm or more and less than 1 μm. If the thickness of the preliminary layer is less than 3 nm, the effect of improving the adhesiveness cannot be sufficiently obtained. The thickness of the preliminary layer is more preferably 5 nm or more, further preferably 20 nm or more. On the other hand, when the thickness of the preliminary layer is 1 μm or more, the titanium oxide contained in the preliminary layer is not sufficiently reduced by at least one of C and N generated by thermal decomposition of the organic polymer in the sol solution. As a result, the conductivity of the titanium oxide layer as a whole is lowered. The thickness of the preliminary layer is more preferably 500 nm or less, further preferably 200 nm or less.

The method of forming the preliminary layer, that is, the method of oxidizing the surface layer portion of the base material is not particularly limited. The preliminary layer can be formed by, for example, heat treatment in an oxidizing atmosphere or electrochemical oxidation. When the heat treatment is performed in an oxidizing atmosphere, the oxidizing atmosphere may be an air atmosphere or an atmosphere containing an oxidant. The heat treatment temperature may be, for example, 200 to 900°C. The heating time can be appropriately set so that the preliminary layer has a desired thickness. The electrochemical oxidation is preferably anodic oxidation in a sulfuric acid aqueous solution, an acetic acid aqueous solution, or a phosphoric acid aqueous solution. The oxidizing condition is not limited, but for example, the applied voltage may be several tens of V, and the electrolysis may be performed under a current density of about 0.1 to 10 A/cm ² . The treatment time by anodic oxidation can be appropriately set so that the preliminary layer has a desired thickness.

The metal material of the present invention can be used for a fuel cell separator as described below. Further, the metal material of the present invention can be used for an electrode for electrolysis and the like.

<Fuel cell separator>
The separator of the present invention comprises the above metal material. This separator can be formed into a desired shape by press molding. When the fuel cell is, for example, a polymer electrolyte fuel cell, the separator is provided with grooves serving as flow paths for the fuel gas and the oxidizing gas, as described later. In this case, after a flat base material is press-molded into a desired shape, a titanium oxide layer may be formed on the base material, or a titanium oxide layer may be formed on the flat base material. After the metal material is obtained by pressing, the metal material may be press-molded.

Since the separator of the present invention is provided with the metal material of the present invention, the contact resistance is kept low, and the wettability of water, and hence the drainage property is kept high.

<Fuel cell and fuel cell stack>
The fuel cell of the present invention includes the above separator. A fuel cell stack of the present invention includes a plurality of the fuel cell units. The plurality of fuel cells may be stacked on each other and electrically connected in series.

FIG. 2A is a perspective view of a fuel cell stack according to one embodiment of the present invention, and FIG. 2B is an exploded perspective view of a fuel cell (single cell) that constitutes the fuel cell stack. 2A and 2B show an example in which the fuel cell is a polymer electrolyte fuel cell.

As shown in FIGS. 2A and 2B, the fuel cell stack 1 is an assembly of a plurality of fuel battery cells (single cells). As shown in FIG. 2B, in the single cell, a fuel electrode film (anode) 3 and an oxidant electrode film (cathode) 4 are laminated on one surface and the other surface of the solid polymer electrolyte membrane 2, respectively. Then, the separators 5a and 5b are stacked on both surfaces of this laminated body. The solid polymer electrolyte membrane 2, the fuel electrode membrane 3, and the oxidizer electrode membrane 4 may be an MEA (Membrane Electrode Assembly) that is bonded to each other to form an integral component.

The separators 5a and 5b include the metal material of the present invention. Therefore, the contact resistance of the separators 5a and 5b with respect to the fuel electrode film 3 and the oxidizer electrode film 4 is kept low. The solid polymer electrolyte membrane 2, the fuel electrode membrane 3, and the oxidizer electrode membrane 4 may be made of known materials.

Grooves forming the flow paths 6a and 6b are formed in the separators 5a and 5b, respectively. The channels 6a and 6b form a serpentine channel. In FIG. 2B, the peripheral portions of the separators 5a and 5b are not shown.

A fuel gas (hydrogen or hydrogen-containing gas) G1 is flown through the flow path 6a formed in the separator 5a. As a result, the fuel gas G1 is supplied to the fuel electrode film 3. In the fuel electrode film 3, the fuel gas G1 penetrates the diffusion layer and contacts the catalyst layer. In addition, an oxidizing gas G2 such as air is flowed through the flow path 6b formed in the separator 5b. As a result, the oxidizing gas G2 is supplied to the oxidant electrode film 4. In the oxidant electrode film 4, the oxidizing gas G2 permeates the diffusion layer and contacts the catalyst layer. An electrochemical reaction occurs due to these gases, and a DC voltage is generated between the fuel electrode film 3 and the oxidant electrode film 4. As a result of this reaction, water is generated in the flow path 6b of the separator 5b on the oxidant electrode film 4 side of each fuel cell.

By keeping the drainage of the separator 5b high, the oxidizing gas G2 flowing in the flow path 6b of the separator 5b continues to flow at a high flow rate without being hindered by water. Therefore, the reaction amount between the fuel gas G1 and the oxidizing gas G2 is kept high. The contact resistance of the separators 5a and 5b with respect to the fuel electrode film 3 and the oxidizer electrode film 4 is kept low, so that the fuel cell and the fuel cell stack continue to have low internal resistance. Therefore, a high DC voltage continues to be generated due to the reaction between the fuel gas G1 and the oxidizing gas G2. That is, each fuel cell and the fuel cell stack 1 can maintain a high power generation amount.

In order to confirm the effect of the present invention, a metal material sample was prepared and evaluated by the following method.
1. Sample Preparation Table 1 shows the sample preparation conditions and evaluation results.

(1) Preparation of Base Material As a base material, pure titanium (denoted by “Ti” in Table 1), titanium alloy containing 3% of Ta (denoted by “Ti—Ta” in Table 1), and SUS304. Was used. After the raw material was melted and cast, it was rolled to form a plate having a thickness of 1 mm, and a square piece having a side of 4 cm was cut out from the plate, and this piece was used as a base material. For some substrates, a preliminary layer was formed by preliminary oxidation (oxidation treatment). The pre-oxidation was performed by holding at 500° C. for 5 minutes in an argon atmosphere having a dew point of −70° C.

(2) Contact process First, the sol liquid was prepared in the following procedure. Titanium tetrabutoxide (TBOT), which is a precursor compound of titanium oxide, and a precursor compound of M (Nb, Ta or V) oxide, are dissolved in absolute ethanol (hereinafter referred to as "A liquid"). ) Was prepared. Niobium ethoxide was used as the precursor compound of the Nb oxide. Tantalum ethoxide was used as a precursor compound of Ta oxide. Vanadium ethoxide was used as a V oxide precursor compound. For comparison, in Test No. 16, a sol liquid substantially free of M was prepared.

The amount of each precursor compound in the liquid A was adjusted so that the ratio of Ti and M was the ratio of Ti and M in the titanium oxide layer to be formed. For example, when Ti and Nb form a titanium oxide layer having an atomic ratio of 0.94:0.06, TBOT containing 0.94 equivalent of Ti and niobium ethoxide containing 0.06 equivalent of Nb. Solution A was prepared so as to contain and.

Next, an ethanol solution containing nitric acid and distilled water (hereinafter referred to as “B solution”) was prepared and added to the A solution. The liquid B is for accelerating the hydrolysis of the precursor compound. The addition of the liquid B to the liquid A was performed by adding the liquid B to the liquid A over 2 hours using a dropping funnel while stirring the liquid A. As a result, a transparent sol solution was obtained. Polyvinylpyrrolidone (PVP; molecular weight about 40,000) or polyvinyl acetate (PVAc; molecular weight about 120,000) as an organic polymer was dissolved in this sol solution at a ratio (polymer addition amount) shown in Table 1 to be used in the contact step. A liquid was obtained. For comparison, in Test No. 3, a sol liquid containing substantially no organic polymer was prepared.

Then, the sol liquid was attached to the surface of the substrate by the dip coating method. The coating amount (thickness) of the sol liquid was adjusted by the speed (pulling speed) of pulling up the substrate dipped (immersed) in the sol liquid.

(3) Heat treatment step The base material to which the sol solution was attached as described above was dried at 100° C. for 2 hours, and then heated in an argon stream at the heating temperature shown in Table 1 for 5 minutes to produce a metal material. Got

(4) Characteristics of Metal Material For each metal material, the characteristics relating to the above requirements (i) to (vi) were measured by the method described above. In Table 1, requirement (ii) ([M]/([Ti]+[M])), requirement (iii) (C content), requirement (v) (([C]+[N])/[ O]) and requirement (vi) (titanium oxide layer thickness) are shown. The requirements (i) and (iv) are as follows.

(4-1) Requirement (i)
All the samples other than the test number 3 contained TiO _x (1≦x<2) in the titanium oxide layer. The sample of test number 3 contained TiO ₂ but no TiO _x . In the sample of Test No. 3, TiO ₂ was not reduced because the sol liquid used did not contain an organic polymer. In addition, all the samples other than the test number 16 contained MO _y (1≦y≦2.5) in the titanium oxide layer. In the sample of Test No. 16, MO _y (1≦y≦2.5) was not formed in the titanium oxide layer because the sol liquid used did not substantially contain M.

(4-2) Requirement (iv)
All the samples other than the test number 3 contained titanium carbide in the titanium oxide layer. TiC was detected as the titanium carbide. In these samples, Ti originating from a precursor compound of titanium oxide (titanium tetrabutoxide) and C originating from an organic polymer (polyvinylpyrrolidone or polyvinyl acetate) react with each other to form titanium carbide. It was The samples of test numbers 1, 2, 4-10, 12-14 and 16-18 contained titanium nitride in addition to titanium carbide in the titanium oxide layer. Ti ₃ N ₄ was detected as the titanium nitride. In these samples, Ti originating from the precursor compound of titanium oxide was reacted with N originating from polyvinylpyrrolidone to form titanium nitride. The sample of Test No. 3 contained neither titanium carbide nor titanium nitride in the titanium oxide layer.

(5) Evaluation of Sample (5-1) Measurement of Contact Resistance FIG. 4 is a diagram showing the configuration of an apparatus for measuring the contact resistance of the sample. Using this device, the contact resistance of each sample was measured according to the method described in Non-Patent Document 3. With reference to FIG. 4, first, the produced sample (metal material) S is sandwiched by a pair of carbon papers (TGP-H-90 manufactured by Toray Industries, Inc.) 22 used as a gas diffusion layer for a fuel cell. It was sandwiched between a pair of gold-plated electrodes 23. The area of each carbon paper 22 was 1 cm ² .

Next, a load was applied between the pair of gold-plated electrodes 23 to generate a pressure of 10 kgf/cm ² (9.81×10 ⁵ Pa). In this state, a constant current was passed between the pair of gold-plated electrodes 23, and the voltage drop between the carbon paper 22 and the sample S generated at this time was measured. The resistance value was calculated based on this result. The obtained resistance value is a value obtained by summing the contact resistances of both surfaces of the sample S, so this is divided by 2 to obtain the contact resistance value per one surface of the sample S (initial contact resistance).

Next, using the sample after the initial contact resistance measurement as a separator, a polymer electrolyte fuel cell (single cell) was produced. The reason for using the single cell is that the stacked state greatly affects the evaluation result in the state where the single cells are stacked to form a multi-cell (fuel cell stack). In the fuel cell, as a solid polymer electrolyte membrane, standard MEA for PFEC (using Nafion (registered trademark)-1135) FC50-MEA (membrane electrode assembly (MEA)) manufactured by Toyo Technica was used.

Hydrogen gas having a purity of 99.9999% was passed as the anode-side fuel gas, and air was passed through the fuel cell as the cathode-side gas. The gas pressure of hydrogen gas and air introduced into the fuel cell was 0.04 to 0.20 bar (4000 to 20000 Pa). The entire fuel cell body was kept warm at 70±2°C, and the humidity inside the fuel cell was controlled by setting the dew point at the gas introduction part to 70°C. The pressure inside the fuel cell was 1 atm (1.01×10 ⁵ Pa).

This fuel cell unit was operated at a constant current density of 0.5 A/cm ² . Then, after operating for 500 hours, the separator (metal material) was taken out. With respect to this separator, the contact resistance was measured by the above-mentioned method, and was taken as the contact resistance after power generation. A digital multimeter (KEITHLEY 2001 manufactured by Toyo Technica Co., Ltd.) was used to measure the contact resistance and the current and voltage during operation of the fuel cell.

(5-2) Evaluation of Affinity for Water As an index of drainability of a separator in a fuel cell, a contact angle of water with respect to a metal material surface (hereinafter, simply referred to as “contact angle”) is described in JIS R3257 (1999). ) Was measured according to the sessile drop method. The measurement was performed on the metal material before used in the fuel cell and the metal material (separator) used in the above-mentioned fuel cell operated for 500 hours, and the initial contact angle and the contact angle after power generation were respectively measured.

(6) Evaluation results Table 1 shows the contact resistance and contact angle for each sample at the initial stage and after power generation. The criteria for determining contact resistance are as follows.
Particularly good (⊚): Contact resistance is 10 mΩ·cm ² or less both in the initial stage and after power generation.
Good (◯): The contact resistance after power generation is 30 mΩ·cm ² or less (excluding those satisfying the “especially good” criteria).
Bad (x): The contact resistance after power generation exceeds 30 mΩ·cm ² .
Regarding the contact angle, after power generation, 105° or less was good (◯), and more than 105° was bad (x).

The samples of Examples of the present invention (metal materials satisfying the requirements of the present invention) had low contact resistance after power generation of 30 mΩ·cm ² or less. Further, the contact angles of the samples of the present invention examples after power generation were all as small as 90° or less. By comparing the samples of Test Nos. 12 and 17 with the samples of the examples of the present invention other than these, it can be seen that the above effects can be obtained regardless of whether the base material is pure titanium, titanium alloy or stainless steel. Further, by comparing the samples of Test Nos. 13 and 14 with the samples of the examples of the present invention other than these, it can be seen that the above effects can be obtained regardless of whether M is Nb, Ta or V.

The sample of Test No. 3 was prepared by using the sol liquid that did not substantially contain the organic polymer, and thus the C content of the titanium oxide layer was 0%. This sample had high contact resistance at the initial stage and after power generation. When compared with the samples of test numbers 1-2, 6 and 7, etc., the titanium oxide layer contains a predetermined amount of C, so that the conductivity of the titanium oxide layer is improved and the contact resistance after initial generation and after power generation is increased. It turns out to be low.

Further, in the sample of Test No. 3, the titanium oxide layer contained neither titanium carbide nor titanium nitride, and ([C]+[N])/[O] was 0. The initial contact angle of this sample was 90° or less, but the contact angle after power generation exceeded 90° and increased. In comparison with the samples of Test Nos. 1-2, 6 and 7, it can be seen that a low contact angle can be maintained by the titanium oxide layer containing a predetermined amount of at least one of titanium carbide and titanium nitride. It is speculated that this is because at the time of operation of the fuel cell, at least one corrosion product of titanium carbide and titanium nitride is generated, and the wettability of water to this corrosion product is high.

In the sample of Test No. 11, the titanium oxide layer contained a predetermined amount of C, and the surface layer region contained titanium carbide. However, ([C]+[N])/[O] of the surface layer region of this sample was less than 0.05. Therefore, the effect of improving the wettability of water due to the formation of corrosion products of titanium carbide was not sufficiently obtained, and the contact angle after power generation was large.

In the sample of Test No. 4, the C content of the titanium oxide layer exceeded 50%. In this sample, peeling occurred between the base material and the titanium oxide layer after power generation. For this reason, after power generation, sufficient electrical connection was not obtained between the base material and the titanium oxide layer, and the contact resistance was high.

In the samples of test numbers 5 and 10, ([C]+[N])/[O] exceeded 1.0 in the surface layer region of the titanium oxide layer. In these samples, a large amount of titanium carbide and titanium nitride corrosion products were generated during power generation. The electrical resistance of these corrosion products is high. Therefore, these corrosion products increased the electrical resistance of the titanium oxide layer and increased the contact resistance after power generation.

In the sample of Test No. 16, the sol liquid that does not substantially contain the precursor compound of M oxide was used at the time of production, so that [M]/([Ti]+[M]) of the titanium oxide layer was 0. Met. In Test No. 9, a sol liquid containing a precursor compound of M(Nb) oxide was used at the time of production, but [M]/([Ti]+[M]) was less than 0.005. In these samples, the effect of improving the conductivity of the titanium oxide layer due to the formation of MO _y (1≦y≦2.5) was not obtained at all or sufficiently, and the contact resistance after power generation was high. It was

In the sample of test number 8, [M]/([Ti]+[M]) exceeded 0.10. In this sample, the contact resistance after power generation was high. This is because [M]/([Ti]+[M]) is large, that is, since the amount of M oxide in the titanium oxide layer is large, the M oxide exists in an aggregated state. It is presumed that this is because many physical defects were generated in the titanium oxide layer.

The metal material of the present invention can be used for, for example, a separator of a fuel cell, an electrode for electrolysis (for example, an electrode for electrolysis of water), and the like.

1: Fuel Cell Stack 2: Solid Polymer Electrolyte Membrane 3: Fuel Electrode Membrane 4: Oxidizer Electrode Membrane 5a, 5b: Separators 6a, 6b: Flow Path 11: Metal Material 12: Base Material 13: Titanium Oxide Layer S: Sample (metal material)

Claims (4)

A metal material comprising a metal base material and a titanium oxide layer provided on the base material,
The titanium oxide layer contains TiO _x (1≦x<2) and MO _y (M: one or more of Nb, Ta and V; 1≦y≦2.5),
The content of Ti and M of the titanium oxide layer satisfies the following formula (1),
C content of the titanium oxide layer is 1 to 50 mass%,
The titanium oxide layer has at least one kind of titanium carbide and titanium nitride in a surface layer region from the surface to a depth direction of 10 nm,
In the surface layer region, the content of C forming the titanium carbide, the content of N forming the titanium nitride, and the content of O forming the TiO _x and the MO _y are represented by the following formula (2). The filling,
The metal material, wherein the titanium oxide layer has a thickness of 10 to 1000 nm.
0.005≦[M]/([Ti]+[M])≦0.10 (1)
here,
[Ti]: Ti content (at %) of the titanium oxide layer
[M]: M content (at %) of the titanium oxide layer
0.05≦([C]+[N])/[O]≦1.0 (2)
here,
[C]: Content of C constituting titanium carbide in the surface layer region (at %)
[N]: Content of N constituting titanium nitride in the surface layer region (at %)
[O]: Content (at %) of titanium oxide and O constituting MO _y in the surface layer region. A separator for a fuel cell, comprising the metal material according to claim 1. A fuel cell comprising the separator according to claim 2. A fuel cell stack comprising a plurality of the fuel battery cells according to claim 3.

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