In general, a fuel cell has an electrolyte film, a pair of electrodes (an anode and a cathode) each including a catalyst layer and a diffusion layer, and a pair of fuel cell separators (an anode side separator and a cathode side separator) that sandwich the electrodes therebetween. In power generation by the fuel cell, when an anode gas supplied to the anode is a hydrogen gas and a cathode gas supplied to the cathode is an oxygen gas, a reaction of producing hydrogen ions and electrons proceeds on the anode side, and the hydrogen ions reach the cathode side through the electrolyte film, whereas the electrons reach the cathode through an external circuit. Meanwhile, on the cathode side, the hydrogen ions, the electrons, and the oxygen gas react to generate water, thereby emitting energy.
Examples of the fuel cell separator include a separator having a substrate formed of carbon, and a separator having a substrate formed of a metal.
The fuel cell separator having the substrate formed of the metal is superior in mechanical strength and moldability as compared with the fuel cell separator having the substrate formed of carbon. However, as described above, the fuel cell generates moisture during power generation, and hence the fuel cell separator having the substrate formed of the metal is apt to corrode as compared with the fuel cell separator having the substrate formed of carbon. When the fuel cell separator having the substrate formed of the metal corrodes, contact resistance increases, which may result in deterioration in the performance of the fuel cell. It is to be noted that, a simple description “the contact resistance of the fuel cell separator” used herein means both a similar material contact resistance (a contact resistance between similar fuel cell separators) and a diffusion layer contact resistance (a contact resistance between the fuel cell separator and the diffusion layer).
For example, to suppress the corrosion of the metal substrate, there is known a fuel cell separator having a metal substrate subjected to plating of a noble metal such as Au or Pt. However, the noble metals are expensive, and the plating requires use of a large amount of the noble metal, which is not practical. Alternatively, the corrosion of the metal substrate can be suppressed by forming a graphite layer on the metal substrate, but the formation of the graphite metal thereon is technically difficult.
Furthermore, for example, a booklet of International Publication No. 01-006585 discloses a fuel cell separator having a metal substrate coated with diamond-like carbon to suppress the corrosion of the metal substrate.
Furthermore, for example, JP 2003-123781 A discloses a fuel cell separator having a metal substrate coated with diamond-like carbon containing a metal to suppress the corrosion of the metal substrate.
Furthermore, for example, JP 2002-151110 A discloses a fuel cell separator in which an oxide layer is formed on a metal substrate and a conductive layer is further formed on a surface of the oxide layer to suppress the corrosion of the metal substrate and an increase in the contact resistance of the fuel cell separator.
Furthermore, for example, JP 2000-164228 A discloses a fuel cell separator in which a low-electric resistance layer and an anti-corrosion layer are formed on a metal substrate surface to suppress the corrosion of the metal substrate and the increase in the contact resistance of the fuel cell separator.
Furthermore, for example, JP 2001-283872 A discloses a fuel cell separator in which carbon particles dispersed in the manners of islands on a metal substrate are coupled with an upper side of the metal substrate through a chrome carbide layer to suppress the increase in the contact resistance of the fuel cell separator.
However, in the fuel cell separator disclosed in the booklet of International Publication No. 01-006585, the diamond-like carbon corrodes depending on a power generation environment; e.g., an operating temperature of the fuel cell (e.g., 70° C. or above), moisture generated at the time of power generation, or a potential difference involved in a power generation reaction, with the result that the contact resistance of the fuel cell separator inconveniently increases.
Moreover, in the fuel cell separator disclosed in JP 2003-123781 A, the metal corrodes together with the diamond-like carbon depending on a power generation environment of the fuel cell, and consequently the metal is turned to a metal oxide, with the result that the contact resistance of the fuel cell separator increases further.
Additionally, in the fuel cell separators disclosed in JP 2002-151110 A and JP 2000-164228 A, metal materials are used for all of the conductive layer, the low-electric resistance layer, and the anti-corrosion layer, and hence the conductive layer, the low-electric resistance layer, and the anti-corrosion layer corrode depending on a power generation environment of each fuel cell, thereby increasing the contact resistance of each fuel cell separator.
Furthermore, in the fuel cell separator disclosed in JP 2001-283872 A, not the entire metal substrate is covered with the chrome carbide layer and the carbon particles, and hence the metal substrate corrodes depending on a power generation environment of the fuel cell, thereby increasing the contact resistance of the fuel cell separator.