Recent years have seen development of fuel cells that have good power generation efficiency and do not emit carbon dioxide from the viewpoint of preserving the global environment. This fuel cell generates power by causing hydrogen and oxygen to react with each other. A basic structure of a fuel cell resembles a sandwich and is constituted by an electrolyte membrane (i.e., ion exchange membrane), two electrodes (i.e., a fuel electrode and an air electrode), a diffusion layer for diffusing hydrogen and oxygen (air), and two separators. Phosphoric-acid fuel cells, molten carbonate fuel cells, solid-oxide fuel cells, alkaline fuel cells, proton-exchange membrane fuel cells, and the like have been developed in accordance with the type of electrolyte used.
Of these fuel cells, proton-exchange membrane fuel cells in particular have the following advantages over molten carbonate fuel cells, phosphoric-acid fuel cells, and the like:
(a) Operating temperature is significantly low, i.e., about 80° C.
(b) Weight- and size-reduction of the fuel cell main body is possible.
(c) The time taken for start-up is short and fuel efficiency and output density are high.
Accordingly, proton-exchange membrane fuel cells are one of the most prospective fuel cells today, for onboard power supplies for electric vehicles and portable and compact dispersed power systems for household use (stationary type compact electric generator).
A proton-exchange membrane fuel cell is based on the principle of extracting power from hydrogen and oxygen through a polymer membrane and has a structure shown in FIG. 1, in which a membrane-electrode assembly 1 is sandwiched by gas diffusion layers 2 and 3 such as carbon cloths and these form a single constitutional element (also known as a single cell). Electromotive force is generated between the separators 4 and 5.
The membrane-electrode assembly 1 is also known as MEA (Membrane-Electrode Assembly) and is made by integrating a polymer membrane and an electrode material such as carbon black carrying a platinum catalyst, the electrode material being provided on front and back surfaces of the polymer membrane. The thickness of the membrane-electrode assembly 1 is several ten to several hundred micrometers. The gas diffusion layers 2 and 3 are frequently integrated with the membrane-electrode assembly 1.
When proton-exchange membrane fuel cells are applied to the usages described above, several ten to several hundred single cells described above are connected in series to form a fuel cell stack, and the fuel cell stack is used.
The separators 4 and 5 are required to have
(A) a function of a separator that separates between single cells each other, as well as
(B) a function of an electric conductor that carries electrons generated;
(C) a function of a channel for oxygen (air) and hydrogen (air channels 6 and hydrogen channels 7 in FIG. 1); and
(D) a function of a discharge channel for discharging water and gas generated (air channels 6 and hydrogen channels 7 also serve as this discharge channel).
In order to use a proton-exchange membrane fuel cell in practical application, separators having good durability and conductivity must be used.
The durability expected is about 5000 hours for fuel cells for electric vehicles and about 40000 hours for stationary type electric generators used as compact dispersed power systems for household use and the like.
Proton-exchange membrane fuel cells that have been put to practice hitherto use carbon materials as separators. However, since the separators using carbon materials are susceptible to fracture upon impact, they have the drawbacks that not only the size-reduction is difficult but also the process cost for forming channels is high. In particular, the cost problem has been the largest impediment for spread of fuel cells.
In response, attempts have been made to use a metal material, in particular, stainless steel, instead of carbon materials as the material for separators.
The operating environment the separators are exposed to are characteristic in that the environment is acidic and has a high temperature of 70° C. or higher and the expected potential range is as wide as from about 0 V vs SHE to 1.0 V vs SHE or higher (hereinafter all potentials are versus SHE and simply denoted as V). In order to use stainless steel, the corrosion resistance in the expected potential range needs to be improved. In particular, at and near 1.0 V, transpassive dissolution of Cr, which is the main element of the stainless steel, occurs and thus it is difficult to maintain corrosion resistance solely by Cr on one hand. On the other hand, Cr is primarily responsible for maintaining the corrosion resistance at 0.6 V or less. Thus, according to the conventional art, the corrosion resistance could not be maintained in a wide potential range from a low potential to a high potential.
For example, patent document 1 discloses a stainless steel for a separator in which the corrosion resistance is improved from the composition aspect by increasing the Cr and Mo contents.
Patent document 2 discloses a method for producing a separator for a low-temperature-type fuel cell characterized in that a stainless steel sheet containing 0.5 mass % or more of Cu is subjected to alternation electrolytic etching of alternately performing anodic electrolyzation at a potential of +0.5 V or more and cathodic electrolyzation at a potential between −0.2 V and −0.8 V in an aqueous solution of ferric chloride.
Patent document 3 discloses a stainless steel conductive part and method of producing the same that has excellent conductivity and low contact electrical resistance formed by modifying a passive film on a stainless steel surface by injecting fluorine in the passive film.