In recent years, as fuel cells for automobiles, polymer electrolyte fuel cells have started to progress rapidly. The polymer electrolyte fuel cell is a fuel cell that uses hydrogen and oxygen, and also uses an organic film (composites with inorganic materials are also being developed) of a hydrogen-ion-selective-transmission type as electrolyte. Examples of hydrogen used as a fuel include pure hydrogen and a hydrogen gas obtained by modifying alcohols.
However, current fuel cell systems use components and members having high unit costs, and the cost for components and members needs to be lowered largely for consumer use. Further, for use in automobiles, the current fuel cell systems need not only to lower the cost, but also to downsize a stack, which serves as the center of the fuel cell. A polymer electrolyte fuel cell has a structure in which a membrane electrode assembly (hereinafter also referred to as MEA) including a solid polymer film, electrodes, and a gas diffusion layer, is sandwiched between separators, and a large number of MEAs are laminated to form a stack.
Examples of characteristics required for the separator include electron conductivity, a property of isolating an oxygen gas and a hydrogen gas at the respective electrodes, low contact resistance with the MEA, favorable durability in the environment inside a fuel cell, and the like. Here, the gas diffusion layer (GDL) in the MEA is generally formed with a carbon paper consisting of integrated carbon fibers, and accordingly, the separator is required to have favorable contact conductivity with carbon. Since a stainless steel or a titanium material used as materials for a separator generally has low contact conductivity with carbon without any treatment, many techniques have been proposed to increase the contact conductivity with carbon. A passivation film having low conductivity can serve as an obstacle to higher contact conductivity with carbon. Although this problem could be solved at the expense of the durability, extremely high durability is still required for a separator in the environment inside the fuel cell, which is a highly corrosive environment. For this reason, currently, it is quite difficult to develop a satisfactory metal material. Carbon separators have been the mainstream so far; however, if meal separators become available, the fuel cell itself can be downsized, and further, a break does not occur in the manufacturing process of the fuel cell. Accordingly, metal separators are said to be necessary to enable mass production and diffusion.
Under such circumstances, for example, Patent Document 1 discloses a technique that makes it possible to lower contact resistance of a stainless steel effectively, in terms of thinning, reducing weight, and the like, by use of a special stainless steel obtained by precipitating a conductive compound in a steel material. Highly durable titanium is also being studied to be used for a separator. In the same manner as a stainless steel, titanium has high contact resistance with the MEA by the presence of a passivation film on the outermost surface of titanium, and accordingly, for example, Patent Document 2 discloses an invention that enables a TiB-based precipitate to be diffused in titanium and the contact resistance with the MEA to be lowered. Patent Document 3 discloses a titanium alloy for a separator. The titanium alloy contains, by mass %, 0.5 to 15% Ta and a limited amount of Fe and O as necessary. Further, in the titanium alloy, a range from the outermost surface to 0.5 μm in depth has an average nitrogen concentration of 6 atomic % or more, and contains tantalum nitride and titanium nitride. It is also disclosed that, in a manufacturing method therefor, it is preferable to heat the titanium alloy at temperatures of 600 to 1000° C. for three seconds or more under a nitrogen atmosphere. Patent Documents 4, 5, and 6 disclose a technique to thrust a conductive material into the superficial layer by a blasting method or a roll processing method in a manufacturing process of a titanium or stainless steel metal separator. In this case, a surface microstructure in which the conductive material is disposed to penetrate a passivation film formed on the metal surface secures both contact conductivity with carbon and durability.
Patent Document 7 discloses a manufacturing method for a fuel cell separator, including converting impurities including titanium carbide or titanium nitride formed on the surface of titanium into oxide by anode oxidizing treatment, and then performing plating treatment. It is known that titanium carbide or titanium nitride formed on the surface of titanium is dissolved while being exposed to a corrosive environment during use and is re-precipitated as oxide to lower the contact conductivity. This method can be said to suppress oxidation of these impurities during generation of electricity (during use) and to increase durability. However, to secure conductivity and durability, an expensive plated film is necessary. Patent Document 8 discloses a technique to form an oxide film as a corrosion-resistant film by coating the surface of a titanium-based alloy with BN powder and by performing heat treatment thereon, the titanium-based alloy being used as a base material and being obtained by alloying Group 3 elements in the periodic table. This is a technique to increase conductivity by doping, with impurity atoms, a position of a titanium atom in a crystal lattice of the oxide film serving as a passivation film of the titanium alloy. Patent Documents 9 and 10 disclose a technique to form, in rolling processing of a fuel cell separator made of titanium, an altered layer containing titanium carbide on the superficial layer by use of carbon-containing rolling oil, and to form a high-density carbon film thereon to secure conductivity and durability. Thus, conductivity with a carbon paper is increased. However, since durability is secured by the carbon film, making a fine carbon film leads to satisfaction of required performance. The interface between simple carbon and titanium has high contact resistance, and accordingly, titanium carbide that increases conductivity is disposed therebetween. This surface structure can generate a corrosion product that inhibits contact conductivity because the altered layer and the base material cannot be prevented from being corroded in a case in which the carbon film has a defect.
Patent Documents 11, 12, 13, 14, and 15 disclose titanium and a titanium alloy fuel cell separator that is similar to the structure disclosed in Patent Document 9 and has a structure mainly including a carbon layer, a titanium carbide intermediate layer, and a titanium base material laminated in this order. Although a manufacturing process is different from that in Patent Document 9 in that the titanium carbide intermediate layer is formed after the carbon layer is formed in advance, a mechanism of increasing durability by the carbon layer is similar. Patent Document 16 discloses a manufacturing method of applying graphite powder and rolling and annealing the graphite powder for mass production. It can be said that the function of a conventional carbon separator is realized by adding the carbon layer and the titanium carbide intermediate layer to the surface of an unbreakable titanium base material. However, since the titanium carbide intermediate layer has low durability, there remains a concern that this surface structure can generate a corrosion product that inhibits contact conductivity because the intermediate layer and the base material cannot be prevented from being corroded in a case in which the carbon film has a defect. Under such circumstances, Patent Document 17 discloses a technique to dispose titanium carbide and titanium nitride, which are conductive materials, on the surface of titanium, and to cover not only titanium but also the conductive materials with titanium oxide having a passivation function. This invention secures contact conductivity, and in addition, increases durability. However, it is necessary to further increase environmental deterioration resistance of the titanium oxide film covering the conductive materials in order to further lengthen the fuel cell life. Patent Document 18 discloses a titanium material for a polymer electrolyte fuel cell separator in which a coating film made of titanium compound particles and titanium oxide is formed on the surface of a titanium substrate. Patent Document 19 discloses a polymer electrolyte fuel cell separator in which an oxide layer is formed on the surface of pure titanium or a titanium alloy and conductive compound particles are adhered thereto. Patent Document 20 discloses a titanium material for a polymer electrolyte fuel cell separator having a superficial layer structure in which Ti compounds containing C or N are dispersed and the Ti compounds are covered with titanium oxide.