Patent Publication Number: US-2013244129-A1

Title: Fuel cell separator material, and fuel cell stack using the same

Description:
FIELD OF THE INVENTION 
     The present invention relates to a fuel cell separator material comprising a metal base and an Au plated layer formed on a surface of the metal base, and a fuel cell stack using the same. 
     DESCRIPTION OF THE RELATED ART 
     A polymer electrolyte fuel cell separator has electrical conductivity, electrically connects each single cell of the fuel cell, collects energy (electricity) produced on each single cell, and has flow paths for a fuel gas and air (oxygen) that are provided to each single cell. The separator is also referred to as an interconnector, a bipolar plate or a current collector. 
     Traditionally, as the fuel cell separator, a carbon plate on which gas flow paths are formed has been used. However, it is undesirable in that material and processing costs are high. On the other hand, when a metal plate is used in place of the carbon plate, the metal may corroded in the electric power generation conditions such that the ions eluted therefrom ? are absorbed into a Membrane Electrode Assembly to undesirably decrease the electric power generation performance, or an insulated passive state film is produced on a surface of the metal such that contact resistance between a gas diffusion film and a separator is increased to undesirably decrease the electric power generation performance. To avoid this, there are known technologies that Au plating is coated in a thickness of 0.01 to 0.06 μm on a top of a corrugated separator made of a stainless steel substrate (see Patent Literatures 1 and 2) and a noble metal selected from Au, Ru, Rh, Pd, Os, Ir, Pt or the like is sputter-deposited to form an electrical conductive portion on a stainless steel substrate (see Patent Literature 3). 
     In addition, there are reported technologies that gold is plated in dots or island shapes in a thickness of about 10 nm (0.019 mg/cm 2 ) on a surface of a stainless steel substrate (see Patent Literature 4) and an oxidized film is formed and gold is then plated on a surface of a stainless steel substrate (see Patent Literature 5). 
     PRIOR ART LITERATURE 
     Patent Literature 
     
         
         [Patent Literature 1] Japanese Unexamined Patent Publication (Kokai) Hei 10-228914 
         [Patent Literature 2] Japanese Unexamined Patent Publication (Kokai) Hei 9-22708 
         [Patent Literature 3] Japanese Unexamined Patent Publication (Kokai) 2001-297777 
         [Patent Literature 4] Japanese Unexamined Patent Publication (Kokai) 2004-296381(0007) 
         [Patent Literature 5] Japanese Unexamined Patent Publication (Kokai) 2007-257883 
       
    
     SUMMARY OF THE INVENTION 
     Problems to Be Solved by the Invention 
     However, when the thickness of the gold plating is less than 20 nm in order to decrease costs, coating defects may be easily introduced, and the corrosion resistance of the fuel cell separator cannot be fully provided. Especially, the fuel cell separator is in a severe environment in terms of the corrosion resistance, since it is disposed under acidic atmosphere. 
     In addition, in the technology described in Patent Literature 4, to prevent contact corrosion between dissimilar metals, i.e., between stainless steel and gold, a spontaneous potential of the stainless steel alone is set to 0.48 V to sulfuric acid having a pH of 3 at 90° C. to limit the weight of gold to 1.76 mg/cm 2  or less. Accordingly, the gold plated film is purposely formed not uniform in the island shapes. In general, when the thin metal plate made of stainless steel or the like is exposed in a large area, a large amount of the ions are eluted from the thin plate to undesirably decrease the electric power generation performance. 
     In other words, an object of the present invention is to provide a fuel cell separator material and a fuel cell stack using the same having excellent corrosion resistance, even if an Au plated layer formed on a surface of a thin metal base is thin, and available at lower costs. In a bipolar type separator having two formed separator materials adhered where a fuel gas flows through one material, an oxidizing gas flows through the other material, and cooling water flows through a middle adhered part, corrosion resistance needed at the gas sides and corrosion resistance needed at the cooling water side are different. Another object of the present invention is to provide a fuel cell separator material and a fuel cell stack using the same having a minimal gold film on a surface of a thin metal substrate available at lower costs. 
     Means for Solving the Problems 
     The present invention provides a fuel cell separator material, comprising a thin metal substrate, a first Au plated layer having a thickness of 0.5 to 4 nm formed on one surface of the thin metal substrate, and a uniform second Au plated layer which is thicker than the first Au plated layer formed on the other surface of the thin metal substrate, wherein coverage of the section of the first Au plated layer and coverage of the section of the second Au plated layer observed by a transmission electron microscope are 80% or more. 
     Preferably, the first Au plated layer and the second Au plated layer are electroplated using an Au plating bath having a pH of 1.0 or less and containing sodium bisulfate as a conductive salt. 
     Preferably, the second Au plated layer has a thickness of 7 nm or more. 
     Preferably, the first Au plated layer and the second Au plated layer are wet electroplated on both sides of the thin metal substrate using opposing electrodes by flowing different currents on the both sides. 
     Preferably, the first Au plated layer and the second Au plated layer are formed on parts of the surface of the thin metal substrate. 
     Preferably, the thin metal substrate comprises stainless steel, and the stainless steel is austenitic stainless steel. 
     Preferably, the thin metal substrate has a thickness of 0.05 to 0.3 mm. 
     Preferably, the Au plated layer is seal-treated. 
     Preferably, the sealing treatment is conducted by subjecting to electrolytic treatment of the Au plated layer in a mercapto-based solution. 
     Preferably, the fuel cell separator material of the present invention may be used in a polymer electrolyte fuel cell or a direct methanol fuel cell. 
     A fuel cell separator of the present invention uses the separator material, wherein the second Au plated layer is faced to air electrode and fuel electrode sides. 
     A fuel cell stack of the present invention uses the fuel cell separator material, wherein the second Au plated layer is faced to air electrode and fuel electrode sides. 
     Effect of the Invention 
     According to the present invention, corrosion resistance can be improved and costs can be decreased, even if an Au plated layer formed on a surface of a metal base is thin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a TEM image of a section of a first Au plated layer; 
         FIG. 2  is a schematic diagram of gold coverage (TEM image) on a thin metal substrate; 
         FIG. 3  is a section view of a fuel cell stack (single cell) according to an embodiment of the present invention; 
         FIG. 4  is a section view of a bipolar type separator; 
         FIG. 5  is a section view of a flat type fuel cell stack according to an embodiment of the present invention; and 
         FIG. 6  is a graph showing a time versus an output voltage when an electric power generation test on a single cell using a fuel cell separator material. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the fuel cell separator material according to the embodiments of the present invention will be described below. The symbol “%” herein refers to % by mass, unless otherwise specified. 
     The term “fuel cell separator” herein refers to a fuel cell separator which has electrical conductivity, connects each single cell electrically, collects energy (electricity) produced on each single cell, and has flow paths for a fuel gas (fuel liquid) or air (oxygen) that is provided to each single cell. The separator is also referred to as an interconnector, a bipolar plate and a current collector. 
     Accordingly, the fuel cell separator includes a separator having concave-convex flow paths formed on a surface of a plate-like base, as well as a separator having flow paths with open holes for a gas or methanol formed on a surface of a plate-like base, such as the above-mentioned passive type DMFC separator, which will be described below for detail. 
     &lt;Thin Metal Substrate&gt; 
     The fuel cell separator material requires corrosion resistance and conductivity, and the base (thin metal substrate) requires corrosion resistance. For the thin metal substrate, stainless steel having good corrosion resistance and available at relatively low costs is preferably used. More preferably, austenitic stainless steel is used. 
     Types of the austenitic stainless steel are not especially limited, but include SUS 304, SUS 316L and SUS 301 in compliance with JIS standard. 
     The shape of the thin metal substrate is also not especially limited so long as Au can be plated. However, since the thin metal substrate is press-formed to have the separator shape, the shape is preferably a plate. Specifically, the substrate has preferably a thickness of 0.05 to 0.3 mm. If the thickness of the thin metal substrate is less than 0.05 mm, the separator may have lower stiffness after formed and deform when the fuel cell stack is assembled, assembly man-hour may be increased, and separator losses may be increased. On the other hand, if the thin metal substrate is thicker, the separator has improved stiffness, but if the thickness exceeds 0.3 mm, the stiffness needed for the separator may not be further improved and the weight of the stack may be increased. 
     From the standpoint of forming the Au plated layer smoothly, the surface of the thin metal substrate may be smoothed and cleaned. 
     In order to smooth the surface of the thin metal substrate, finish rolling may be conducted using a roll having, for example, a roll surface roughness Ra of &lt;=0.05 μm. In order to clean, finish annealing may be bright annealing. In addition, although a feed roll in an annealing furnace is generally a carbon roll, the concern is that carbon is adhered to the thin substrate from the carbon roll upon annealing. Therefore a ceramic roll may be used. To avoid oxidation of the surface of the thin substrate, the furnace atmosphere may include hydrogen and nitrogen at a ratio, for example, of 9:1. 
     &lt;First Au Plated Layer&gt; 
     According to the present invention, the Au plated layers are formed on both surfaces of the thin metal substrate. Of these, a first Au plated layer is a uniform layer having a thickness of 0.5 to 4 nm. 
     A plane exposed to the electric power generation conditions (at air electrode and fuel electrode sides) of the fuel cell separator is significantly corroded, and therefore is plated with corrosion resistant gold. However, it has been found that corrosion proceeds gradually at a plane opposite to the plane at the air electrode and fuel electrode sides in the fuel cell separator. A cooling medium (tap water or the like) for reducing, for example, heat of reaction flows through the opposite plane. 
     Through the study to find the thinnest Au plated thickness needed to prevent the gradual corrosion at the cooling medium side by the present inventors, it is found that the uniform layer having a thickness of 0.5 nm or more may work well. The first Au plated layer has a thickness of 0.5 nm or more from the standpoint of corrosion resistance and of not greater than 4 nm from the standpoint of costs. If the thickness of the first Au plated layer exceeds 4 nm, the above-mentioned gradual corrosion may not be further prevented. 
     If the thickness of the first Au plated layer is as thin as 4 nm or less, the Au plated layer may have a dot or island shape such that the thin metal substrate is exposed largely. In general, when the thin metal substrate made of stainless steel is exposed in a large area, a large amount of the ions are eluted from the thin substrate of stainless steel to undesirably decrease the electric power generation performance. Thus, the first Au plated layer should be formed uniformly. 
     The thicknesses of the first Au plated layer and a second Au plated layer described later can be calculated by an electrolytic method, a fluorescent X-rays film thickness meter, and a TEM (transmission electron microscope) image from the sections. As the fluorescent X-rays film thickness meter, SEA5100 manufactured by SII NanoTechnology Inc. can be used, for example.  FIG. 1  shows a TEM image (magnification is 139000) of a section of the first Au plated layer in Example 1. 
     Herein, the fact that the first Au plated layer and the second Au plated layer described later are “uniform” can be determined by observing the section of the plated layers with TEM (transmission electron microscope) (magnification is 100000× or more, generally 139000×). Specifically, when a gold coverage represented by (area of thin metal substrate, which is a base, is not exposed)/(total measurement area) is 80% or more in the TEM image of the section of the plated layers, the Au plated layers are found to be “uniform”. 
     When the gold coverage on the thin metal substrate (TEM image) is schematically shown in  FIG. 2 , the exposed parts of the thin metal substrate are areas B and D, and the unexposed parts of the thin metal substrate are areas A, C and E. Thus, in the TEM image, total lengths of the areas A, C and E in a horizontal direction (A+C+E) are considered as the area of the unexposed parts of the thin metal substrate, and total lengths of the total measurement areas (A+B+C+D+E) are considered as the total measurement areas. The gold coverage can be calculated by {(A+C±E)/(A+B+C+D+E)}×100(%). 
     A method of forming the first Au plated layer uniformly includes electroplating using an Au plating bath having a pH of 1.0 or less and containing sodium bisulfate as a conductive salt. In this case, the composition of the Au plated bath comprises an Au salt, sodium bisulfate and other additives as appropriate. As the Au salt, a gold cyanide salt, a non-cyan-based metal salt (such as gold chloride) and the like can be used. The gold concentration in the Au salt can be about 1 to 100 g/L. The concentration of sodium bisulfate can be about 50 to 100 g/L. 
     When an acidic Au plating bath having a pH of 1.0 or less is used and stainless steel is used as the thin metal substrate, a Cr oxidized film on the surface is easily removed, and adhesion of the Au plated layer can be improved. In addition, a large amount of hydrogen is produced upon plating to activate the stainless steel surface, whereby Au is easily electrodeposited. 
     Also, it is preferred that the acidic Au plating bath be used, and Au be directly plated on the surface of the thin metal substrate such as stainless steel. Conventionally, as to a connector material, the base is Ni underplated and the Au is then plated. Ni is corroded in the electric power generation conditions. So, Au is desirably plated directly on the base without Ni underplating. 
     The Au plating conditions will be described. When the current density is low, a current is concentrated on a convex part of the metal base, so that the plated layer is difficult to be uniform. When the temperature of the plating bath is low, the plated layer may be difficult to be uniform. 
     The concentration of gold in the plating liquid is preferably 1 to 4 g/L, more preferably 1.3 to 1.7 g/L. When the concentration of gold is less than 1 g/L, current efficiency is decreased, so that the plated layer may be difficult to be uniform. 
     From the standpoint of saving gold, it is possible to form the first Au plated layer and the second Au plated layer described layer on only the part requiring electrical conductivity, e.g., the part contacted with the electrodes when a fuel cell separator is formed from the fuel cell separator material. 
     &lt;Second Au Plated Layer&gt; 
     The second Au plated layer is formed on the thin metal substrate at an opposite side of the first Au plated layer. The second Au plated layer is uniform and thicker than the first Au plated layer. 
     In the fuel cell separator, the planes at air electrode and fuel electrode sides are exposed to the electric power generation conditions, and are significantly corroded. Therefore, the thicker Au than the first Au plated layer should be plated. 
     The second Au plated layer should be thicker than the first Au plated layer. For example, the thickness is 5 nm or more, preferably 7 nm or more from the standpoint of corrosion resistance. On the other hand, the second Au plated layer has preferably a thickness of 40 nm or less from the standpoint of costs. If the thickness of the second Au plated layer exceeds 40 nm, the corrosion may not be further prevented. 
     Also, when the second Au plated layer is uniform, the part where the thin metal substrate is exposed by a dot or island shape in the Au plated layer can be decreased. In particular, the amount of the ions eluted from the thin substrate made of stainless steel can be decreased. 
     A method of forming the first Au plated layer and the second Au plated layer on both surfaces of the thin metal film is not especially limited, but wet electroplating for flowing different currents on the both sides (the second Au plated layer has a higher current value than the first Au plated layer) is desirable. 
     &lt;Sealing Treatment&gt; 
     The Au plated layer is preferably seal-treated. If the coating defects are introduced to the Au plated layer, the sealing treatment can fill the defects and maintain the corrosion resistance. A variety of methods of seal-treating the Au plating are known. Preferably, the Au plated layer is subjected to electrolytic treatment in a mercapto-based solution. The mercapto-based solution is obtained by dissolving a compound having a mercapto group in water. The compound having a mercapto group includes a mercapto benzothiazole derivative described in Japanese Unexamined Patent Publication (Kokai) 2004-265695. 
     &lt;Fuel Cell Separator&gt; 
     Then, the fuel cell separator made with the fuel cell separator material according to the present invention will be described below. The fuel cell separator is made by working the above-mentioned fuel cell separator material into the predetermined shape, and comprises reaction gas flow paths or reaction liquid flow paths (channels or openings) for flowing a fuel gas (hydrogen), a fuel liquid (methanol), air (oxygen), cooling water and the like. 
     In the above-mentioned fuel cell separator material, the second Au plated layer is faced to air electrode and fuel electrode sides. 
     &lt;Layered Type (Active Type) Fuel Cell Separator&gt; 
       FIG. 3  shows a section of a single cell of the layered type (active type) fuel cell. In  FIG. 3 , current collector plates  140 A and  140 B are disposed outside of a separator  10  as described later. Generally, when the single cells are layered to form a stack, only a pair of the current collector plates is disposed only on both ends of the stack. 
     The separator  10  has electrical conductivity, contacts with MEA as described later to collect current, and electrically connects respective single cells. In addition, as described later, the separator  10  has channels as flow paths for flowing a fuel gas and air (oxygen). 
     In  FIG. 3 , Membrane Electrode Assembly (MEA)  80  is made by laminating an anode electrode  40  and a cathode electrode  60  on both sides of a polymer electrolyte membrane  20 . On the surfaces of the anode electrode  40  and the cathode electrode  60 , an anode side gas diffusion layer  90 A and a cathode side gas diffusion layer  90 B are laminated, respectively. The Membrane Electrode Assembly herein may be a laminate including the gas diffusion layers  90 A and  90 B. 
     On both sides of the MEA  80 , separators  10  are disposed facing to the gas diffusion layers  90 A and  90   b , and sandwich the MEA  80 . Flow paths  10 L are formed on the surfaces of the separators  10  at the sides of the MEA  80 , and gas can enter and exit into/from an internal spaces  20  surrounded by gaskets  12 , the flow paths  10 L and the gas diffusion layer  90 A (or  90 B) as described later. 
     A fuel gas (hydrogen or the like) flows into the internal spaces  20  at the anode electrode  40 , and an oxidizing gas (oxygen, air or the like) flows into the internal spaces  20  at the cathode electrode  60  to undergo electrochemical reaction. 
     The outside peripherals of the anode electrode  40  and the gas diffusion layer  90 A are surrounded by a frame-like seal member  31  having the almost same thickness as the total thickness of the anode electrode  40  and the gas diffusion layer  90 A. A substantially frame-like gasket  12  is inserted between the seal member  31  and the peripheral of the separator  10  such that the separator is contacted with the gasket  12  and the flow paths  10 L are surrounded by the gasket  12 . The current collector plate  140 A (or  140 B) is laminated on the outer surface (opposite surface of the MEA  80  side) of the separator  10 , and a substantially frame-like seal member  32  is inserted between the current collector plate  140 A (or  140 B) and the peripheral of the separator  10 . 
     The seal member  31  and the gasket  12  form a seal to prevent the fuel gas or the oxidizing gas from leaking outside the cell. When a plurality of the single cells are laminated to form a stack, a gas flows into a space  21  between the outside of the separator  10  and the current collector plate  140 A (or  140 B); the gas being different from that flowing into the space  20 . (When the oxidizing gas flows into the space  20 , hydrogen flows into the space  21 .) Therefore, the seal member  32  is also used as the member for preventing the gas from leaking outside the cell. 
     The fuel cell includes the MEA  80  (and the gas diffusion layers  90 A and  90 B), the separator  10 , the gasket  12  and the current collectors  140 A and  140 B. A plurality of the fuel cells are laminated to form a fuel cell stack. 
     As shown in  FIGS. 3 and 4 , the bipolar type separator has a structure that contact portions of two formed separator materials are adhered by laser welding or the like, a fuel gas flows through one material and an oxidizing gas flows through the other material, and cooling water flows through a middle adhered part. 
     The layered type (active type) fuel cell shown in  FIG. 3  can be applied not only to the above-mentioned fuel cell using hydrogen as the fuel, but also to the DMFC using methanol as the fuel. 
     &lt;Flat Type (Passive Type) Fuel Cell Separator&gt; 
       FIG. 5  shows a section of a single cell of the flat type (passive type) fuel cell. In  FIG. 5 , current collector plates  140  are disposed outside of a separator  100 , respectively. Generally, when the single cells are layered to form a stack, a pair of the current collector plates is disposed only on both ends of the stack. 
     In  FIG. 5 , the structure of the MEA  80  is the same as that in  FIG. 3 , so the same components are designated by the same symbols and the descriptions thereof are omitted. (In  FIG. 5 , the gas diffusion layers  90 A and  90 B are omitted, but there may be the gas diffusion layers  90 A and  90 B.) 
     In  FIG. 5 , the separator  100  has electrical conductivity, collects electricity upon contact with the MEA, and electrically connects each single cell. As described later, holes are formed on the separator  100  for flowing a fuel liquid and air (oxygen). 
     The separator  100  has a stair  100   s  roughly on the center of an elongated tabular base so as to make a section crank shape, and includes an upper piece  100   b  disposed upper via the stair  100   s  and a lower piece  100   a  disposed below via the stair  100   s . The stair  100   s  extends vertically in the longitudinal direction of the separator  100 . 
     A plurality of the separators  100  are arranged in the longitudinal direction, spaces are provided between the lower pieces  100   a  and the upper pieces  100   b  of the abutted separators  100 , and the MEAs  80  are inserted into the spaces. The structure that the MEA  80  is sandwiched between two separators  100  constitutes a single cell  300 . In this way, a stack that a plurality of the MEAs  80  are connected in series via the separators  100  is provided. 
     The flat type (passive type) fuel cell shown in  FIG. 5  can be applied not only to the above-mentioned DMFC using methanol as the fuel, but also to the fuel cell using hydrogen as the fuel. The shape and the number of the openings of the flat type (passive type) fuel cell separator are not limited, the openings may be not only holes but also slits, or the whole separator may be a net. 
     &lt;Fuel Cell Stack&gt; 
     The fuel cell stack of the present invention is obtained by using the fuel cell separator material of the present invention. 
     The fuel cell stack has a plurality of cells connected in series where electrolyte is sandwiched between a pair of electrodes. The fuel cell separator is inserted between the cells to block the fuel gas or air. The electrode contacted with the fuel gas (H 2 ) is a fuel electrode (anode), and the electrode contacted with air (O 2 ) is an air electrode (cathode). 
     Non-limiting examples of the fuel cell stack have been described referring to  FIGS. 3 and 5 . 
     Example 
     &lt;Sample Preparation&gt; 
     For preparation, in order to smooth each thin metal substrate shown in Table 1 and Table 2, finish rolling was conducted using a roll having a roll surface roughness Ra=0.03 μm, and then bright annealing for cleaning was conducted. In addition, a feed roll in an annealing furnace was a ceramic roll, the furnace atmosphere included hydrogen and nitrogen at a ratio of 9:1. 
     Each thin metal substrate thus prepared was electrolytically degreased using a commercially available degreasing liquid Pakuna 105, and then pretreated by acid pickling in a sulfuric acid solution having a pH of 0.5. 
     Then, the following Au plating bath was used to directly plate each acid pickled thin metal substrate with Au. Iridium oxide electrodes were disposed opposingly on both surfaces of the thin metal substrate. By flowing different currents through each iridium oxide electrode, different currents flowed through both surfaces of the thin metal substrate. (The second Au plated layer had a higher current value than the first Au plated layer.) Au plated layers having thicknesses shown in Tables 1 and 2 were electroplated on respective surfaces of the thin metal substrate. Thus, each fuel cell separator material was produced. 
     The Au plating liquid (cyan-based) contained a gold cyanide salt (gold concentration: 1.5 g/L) and sodium bisulfate 70 g/L and had a pH of 0.9. 
     In each fuel cell separator material shown in Table 1, the Au plated layer was formed on an entire surface of each thin metal substrate. On the other hand, in each fuel cell separator material shown in Table 2, the Au plated layer was formed only on a part corresponding to an active area upon power generation of the thin metal substrate. Accordingly, each fuel cell separator material shown in Table 2 was evaluated by cutting out the part on which the Au plated layer was formed. 
     Uniformity and corrosion resistance of the film of the fuel cell separator material thus produced were measured as follows: 
     &lt;Film Uniformity&gt; 
     The section of the sample was observed by TEM (transmission electron microscope) for determination (139000 times). When a gold coverage covering the base is 80% or more, the Au plated layers was “uniform”. Specifically, in the TEM image of the section of the sample, total lengths L 1  of the area of the unexposed parts of the thin metal plate in a horizontal direction were considered as the area of the unexposed parts of the thin metal plate, and total lengths L 2  of the total measurement areas were considered as the total measurement areas. The gold coverage was calculated by (L 1 /L 2 )×100(%). 
     &lt;Corrosion Resistance at the First Au Plated Layer&gt; 
     On the assumption that corrosion environment at the cooling water side of the polymer electrolyte fuel cell upon power generation, the following acceleration test was conducted. 600 ml of a 10 wt ppm chlorine solution having a pH of 5 at 90° C. was used. The pH was adjusted with sulfuric acid, and the chlorine concentration was adjusted with sodium chloride. Each fuel cell separator material was cut out to a size of 40 mm×50 mm, was immersed in the above-described chlorine solution for 168 hours, and was pulled up. Then, Fe, Ni and Cr ions in the solution were quantified by an ICP analysis to measure the metal eluted amounts. 
     &lt;Corrosion Resistance of the Second Au Plated Layer&gt; 
     Based on the corrosion environment of the polymer electrolyte fuel cell upon power generation, the following acceleration test was conducted. Each fuel cell separator material was cut out to a size of 40 mm×50 mm, was immersed in a 600 ml of a sulfuric acid solution having a pH of 1 at 95° C. for 168 hours, and was pulled up. Then, Fe, Ni and Cr ions in the solution were quantified by the ICP analysis to measure the metal eluted amounts. 
     &lt;Contact Resistance&gt; 
     A contact resistance distribution of each Au plated layer was measured using an electric contact simulator (CRS-1 manufactured by Yamazaki Seiki Co., Ltd.) at a voltage range of 200 mV in a measurement mode of under a constant load of 10 gf for a measurement length of 1 mm. A sampling number is  600 , and its average value was used as a contact resistance value. 
     A typical property needed for the fuel cell separators is corrosion resistance under the usage environment (no toxic metal ion elution). Specifically, the corrosion resistance of the first Au plated layer and the second Au plated layer is such that the metal ion eluted amount is desirable 0.05 mg/L or less. If the corrosion resistance of the first Au plated layer exceeds 0.05 mg/L, the eluted metal ions leak to cooling water (the elution of the metal ions increases electrical conductivity of cooling water, whereby a current flows through water), and if the corrosion resistance of the second Au plated layer exceeds 0.05 mg/L, the eluted metal ions are absorbed into the membrane electrode assembly, both of which decrease the electric power generation performance. 
     &lt;Appropriate Thickness of the Thin Metal Substrate&gt; 
     As shown in Table 3, using a stainless steel thin metal substrate having a thickness of 0.03 to 0.3 mm, a separator having a width of 100 mm and a length of 500 mm (a channel shape: a pitch of 2.5 mm, a straight channel having a depth of 0.5 mm) was press-formed. On the assumption of a continuous assembly line for mass-produced fuel cells, a time needed to move 100 separators formed from right to left by hand one by one a distance of 1 m, and deformation (bent, warped) of the separators at the time of the movement were visually determined. The separators were moved carefully in order to prevent the deformation as low as possible. Percentages of unavoidable bent separators were calculated. 
     The results are shown in Tables 1 to 3. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Type of thin metal 
                   
                   
                 First Au 
               
               
                   
                 substrate 
                 Substrate 
                   
                 plated layer 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 Material 
                 thickness 
                 Sealing 
                 Thickness 
                 Form of 
               
               
                   
                 Type 
                 type 
                 (mm) 
                 treatment 
                 (nm) 
                 film 
               
               
                   
               
               
                 Example. 1 
                 Ni base alloy 
                 INCOLOY825 
                 0.10 
                 Mercapto-based 
                 0.5 
                 Uniform 
               
               
                 Example. 2 
                 Ni base alloy 
                 INCONEL625 
                 0.15 
                 Mercapto-based 
                 1.0 
                 Uniform 
               
               
                 Example. 3 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mercapto-based 
                 1.0 
                 Uniform 
               
               
                 Example. 4 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mercapto-based 
                 0.5 
                 Uniform 
               
               
                 Example. 5 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mo-based 
                 2.0 
                 Uniform 
               
               
                 Example. 6 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mercapto-based 
                 4.0 
                 Uniform 
               
               
                 Example. 7 
                 Austenitic 
                 SUS316L 
                 0.20 
                 Mercapto-based 
                 0.5 
                 Uniform 
               
               
                 Example. 8 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mercapto-based 
                 1.0 
                 Uniform 
               
               
                 Example. 9 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mo-based 
                 0.5 
                 Uniform 
               
               
                 Example. 10 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mercapto-based 
                 3.0 
                 Uniform 
               
               
                 Example. 11 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mercapto-based 
                 0.5 
                 Uniform 
               
               
                 Example. 12 
                 Austenitic 
                 SUS316L 
                 0.10 
                 None 
                 0.5 
                 Uniform 
               
               
                 Example. 13 
                 Austenitic 
                 SUS316L 
                 0.30 
                 None 
                 3.0 
                 Uniform 
               
               
                 Example. 14 
                 Austenitic 
                 SUS304 
                 0.10 
                 Mercapto-based 
                 0.5 
                 Uniform 
               
               
                 Example. 15 
                 Austenitic 
                 SUS304 
                 0.10 
                 Mercapto-based 
                 3.0 
                 Uniform 
               
               
                 Example. 16 
                 Austenitic 
                 SUS304 
                 0.10 
                 Mercapto-based 
                 2.0 
                 Uniform 
               
               
                 Example. 17 
                 Austenitic 
                 SUS304 
                 0.10 
                 None 
                 1.0 
                 Uniform 
               
               
                 Example. 18 
                 Austenitic 
                 SUS301 
                 0.07 
                 Mercapto-based 
                 0.5 
                 Uniform 
               
               
                 Example. 19 
                 Austenitic 
                 SUS301 
                 0.10 
                 Mercapto-based 
                 2.0 
                 Uniform 
               
               
                 Example. 20 
                 Austenitic 
                 SUS301 
                 0.10 
                 Mo-based 
                 3.0 
                 Uniform 
               
               
                 Example. 21 
                 Austenitic 
                 SUS301 
                 0.10 
                 None 
                 1.0 
                 Uniform 
               
               
                 Example. 22 
                 Ferritic 
                 SUS430 
                 0.10 
                 Mercapto-based 
                 0.5 
                 Uniform 
               
               
                 Example. 23 
                 Ferritic 
                 SUS430 
                 0.05 
                 Mercapto-based 
                 1.0 
                 Uniform 
               
               
                 Example. 24 
                 Ferritic 
                 SUS430 
                 0.10 
                 None 
                 2.0 
                 Uniform 
               
               
                 Comp. Example. 1 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mercapto-based 
                 0.0 
                 — 
               
               
                 Comp. Example. 2 
                 Austenitic 
                 SUS316L 
                 0.20 
                 Mercapto-based 
                 0.2 
                 Uniform 
               
               
                 Comp. Example. 3 
                 Austenitic 
                 SUS316L 
                 0.10 
                 None 
                 0.0 
                 — 
               
               
                 Comp. Example. 4 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mo-based 
                 0.0 
                 — 
               
               
                 Comp. Example. 5 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mercapto-based 
                 0.5 
                 Ununiform 
               
               
                 Comp. Example. 6 
                 Austenitic 
                 SUS301 
                 0.10 
                 Mercapto-based 
                 0.0 
                 — 
               
               
                 Comp. Example. 7 
                 Austenitic 
                 SUS301 
                 0.07 
                 None 
                 0.3 
                 Uniform 
               
               
                 Comp. Example. 8 
                 Austenitic 
                 SUS301 
                 0.10 
                 Mercapto-based 
                 0.0 
                 — 
               
               
                 Comp. Example. 9 
                 Ferritic 
                 SUS430 
                 0.10 
                 None 
                 1.0 
                 Ununiform 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Metal eluted 
                 Contact resistance 
               
               
                   
                 Second Au 
                 amount (mg/L) 
                 (mΩ) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 plated layer 
                 First 
                 Second 
                 First 
                 Second 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Thickness 
                 Form of 
                 Au plated 
                 Au plated 
                 Au plated 
                 Au plated 
               
               
                   
                   
                 (nm) 
                 film 
                 layer 
                 layer 
                 layer 
                 layer 
               
               
                   
                   
               
               
                   
                 Example. 1 
                 10 
                 Uniform 
                 0.01 
                 0.01 
                 22.0 
                 8.5 
               
               
                   
                 Example. 2 
                 20 
                 Uniform 
                 0.01 
                 0.01 
                 20.5 
                 7.9 
               
               
                   
                 Example. 3 
                 5 
                 Uniform 
                 0.01 
                 0.03 
                 19.0 
                 8.8 
               
               
                   
                 Example. 4 
                 7 
                 Uniform 
                 0.01 
                 0.02 
                 20.5 
                 8.5 
               
               
                   
                 Example. 5 
                 7 
                 Uniform 
                 0.01 
                 0.02 
                 20.3 
                 9.0 
               
               
                   
                 Example. 6 
                 7 
                 Uniform 
                 0.01 
                 0.01 
                 20.5 
                 8.9 
               
               
                   
                 Example. 7 
                 10 
                 Uniform 
                 0.01 
                 0.01 
                 20.1 
                 8.2 
               
               
                   
                 Example. 8 
                 10 
                 Uniform 
                 0.01 
                 0.01 
                 20.2 
                 9.0 
               
               
                   
                 Example. 9 
                 20 
                 Uniform 
                 0.01 
                 0.01 
                 22.5 
                 8.5 
               
               
                   
                 Example. 10 
                 20 
                 Uniform 
                 0.01 
                 0.01 
                 21.2 
                 8.9 
               
               
                   
                 Example. 11 
                 40 
                 Uniform 
                 0.01 
                 0.01 
                 19.0 
                 8.2 
               
               
                   
                 Example. 12 
                 10 
                 Uniform 
                 0.03 
                 0.01 
                 20.2 
                 9.0 
               
               
                   
                 Example. 13 
                 20 
                 Uniform 
                 0.02 
                 0.01 
                 20.2 
                 9.0 
               
               
                   
                 Example. 14 
                 10 
                 Uniform 
                 0.01 
                 0.01 
                 20.2 
                 7.9 
               
               
                   
                 Example. 15 
                 15 
                 Uniform 
                 0.01 
                 0.01 
                 21.2 
                 8.9 
               
               
                   
                 Example. 16 
                 20 
                 Uniform 
                 0.01 
                 0.01 
                 21.5 
                 8.9 
               
               
                   
                 Example. 17 
                 15 
                 Uniform 
                 0.03 
                 0.01 
                 21.5 
                 7.9 
               
               
                   
                 Example. 18 
                 7 
                 Uniform 
                 0.01 
                 0.02 
                 21.5 
                 8.9 
               
               
                   
                 Example. 19 
                 10 
                 Uniform 
                 0.01 
                 0.01 
                 21.2 
                 8.9 
               
               
                   
                 Example. 20 
                 20 
                 Uniform 
                 0.01 
                 0.01 
                 20.2 
                 8.2 
               
               
                   
                 Example. 21 
                 15 
                 Uniform 
                 0.03 
                 0.01 
                 20.3 
                 8.9 
               
               
                   
                 Example. 22 
                 7 
                 Uniform 
                 0.02 
                 0.02 
                 21.2 
                 8.2 
               
               
                   
                 Example. 23 
                 20 
                 Uniform 
                 0.03 
                 0.01 
                 20.3 
                 8.9 
               
               
                   
                 Example. 24 
                 10 
                 Uniform 
                 0.04 
                 0.03 
                 20.3 
                 8.2 
               
               
                   
                 Comp. Example. 1 
                 0 
                 — 
                 1.88 
                 305 
                 20000 or more 
                 9.0 
               
               
                   
                 Comp. Example. 2 
                 7 
                 Uniform 
                 1.22 
                 0.02 
                 80.5 
                 7.9 
               
               
                   
                 Comp. Example. 3 
                 10 
                 Uniform 
                 1.75 
                 0.03 
                 20000 or more 
                 8.2 
               
               
                   
                 Comp. Example. 4 
                 40 
                 Uniform 
                 1.56 
                 0.01 
                 20000 or more 
                 8.9 
               
               
                   
                 Comp. Example. 5 
                 7 
                 Uniform 
                 0.33 
                 0.02 
                 70.8 
                 9.0 
               
               
                   
                 Comp. Example. 6 
                 7 
                 Uniform 
                 1.78 
                 0.02 
                 20000 or more 
                 8.9 
               
               
                   
                 Comp. Example. 7 
                 10 
                 Uniform 
                 0.98 
                 0.02 
                 70.5 
                 7.9 
               
               
                   
                 Comp. Example. 8 
                 20 
                 Uniform 
                 1.86 
                 0.01 
                 20000 or more 
                 7.9 
               
               
                   
                 Comp. Example. 9 
                 15 
                 Uniform 
                 0.12 
                 0.02 
                 75.8 
                 8.9 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Metal eluted 
                 Contact 
               
               
                   
                 amount (mg/L) 
                 resistance (mΩ) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 First Au 
                 Second Au 
                 First 
                 Second 
                 First 
                 Second 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Type of 
                 Substrate 
                   
                 plated layer 
                 plated layer 
                 Au 
                 Au 
                 Au 
                 Au 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 thin metal substrate 
                 thickness 
                 Sealing 
                 Thickness 
                 Form of 
                 Thickness 
                 Form of 
                 plated 
                 plated 
                 plated 
                 plated 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Type 
                 Steel type 
                 (mm) 
                 treatment 
                 (nm) 
                 film 
                 (nm) 
                 film 
                 layer 
                 layer 
                 layer 
                 layer 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example. 25 
                 Austenitic 
                 SUS316L 
                 0.30 
                 None 
                 0.5 
                 Uniform 
                 7 
                 Uniform 
                 0.02 
                 0.2 
                 21.5 
                 8.5 
               
               
                 Example. 26 
                 Austenitic 
                 SUS316L 
                 0.10 
                 Mercapto- 
                 2.0 
                 Uniform 
                 10 
                 Uniform 
                 0.02 
                 0.01 
                 21.5 
                 8.9 
               
               
                   
                   
                   
                   
                 based 
               
               
                 Example. 27 
                 Austenitic 
                 SUS304 
                 0.10 
                 Mercapto- 
                 4.0 
                 Uniform 
                 40 
                 Uniform 
                 0.01 
                 0.01 
                 22.5 
                 9.0 
               
               
                   
                   
                   
                   
                 based 
               
               
                 Example. 28 
                 Austenitic 
                 SUS301 
                 0.10 
                 Mercapto- 
                 3.0 
                 Uniform 
                 10 
                 Uniform 
                 0.01 
                 0.01 
                 20.5 
                 9.0 
               
               
                   
                   
                   
                   
                 based 
               
               
                 Example. 29 
                 Ferritic 
                 SUS430 
                 0.05 
                 Mo-based 
                 1.0 
                 Uniform 
                 20 
                 Uniform 
                 0.03 
                 0.01 
                 21.5 
                 8.9 
               
               
                 Comp. 
                 Austenitic 
                 SUS316L 
                 0.20 
                 Mercapto- 
                 0.2 
                 Uniform 
                 7 
                 Uniform 
                 1.24 
                 0.02 
                 76.2 
                 8.9 
               
               
                 Example. 10 
                   
                   
                   
                 based 
               
               
                 Comp. 
                 Ferritic 
                 SUS430 
                 0.05 
                 Mercapto- 
                 1.0 
                 Ununiform 
                 20 
                 Uniform 
                 1.45 
                 0.01 
                 85.5 
                 8.9 
               
               
                 Example. 11 
                   
                   
                   
                 based 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Type of 
                 Substrate 
                 Processing 
                 Percentage of 
                   
               
               
                 thin metal 
                 thickness 
                 time 
                 bent 
               
               
                 substrate 
                 (mm) 
                 (min) 
                 separators (%) 
                 Remarks 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 SUS316L 
                 0.03 
                 50 
                 30 
                 Comparative 
               
               
                   
                   
                   
                   
                 Example 
               
               
                   
                 0.05 
                 16 
                 0 
                 Example 
               
               
                   
                 0.1 
                 13 
                 0 
                 Example 
               
               
                   
                 0.3 
                 10 
                 0 
                 Example 
               
               
                 SUS301 
                 0.03 
                 45 
                 25 
                 Comparative 
               
               
                   
                   
                   
                   
                 Example 
               
               
                   
                 0.05 
                 15 
                 0 
                 Example 
               
               
                   
                 0.1 
                 13 
                 0 
                 Example 
               
               
                   
                 0.3 
                 10 
                 0 
                 Example 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1 and Table 2, in each Example where the uniform first Au plated layer having a thickness of 0.5 nm or more was formed on one surface (front surface) of the thin metal substrate, and the uniform second Au plated layer thicker than the first Au plated layer is formed on the other surface (back surface), the metal eluted amount was low, and the corrosion resistance was excellent. 
     Also, in each Example, the gold film exists, such that the contact resistance was significantly small (50 mΩ or less). In general, the bipolar separator is assembled by laser welding or the like. The welded parts become electrical paths. In addition, the separator material of each Example had low surface resistance, such that electricity flowed very well. Thus, the performance of the fuel cell was improved. 
     On the other hand, in each Comparative Example 1, 3, 4, 6 or 8 where no first Au plated layer was formed on one surface (front surface) of the thin metal substrate, the metal eluted amount was high, and the corrosion resistance was poor. 
     In each Comparative Example 2, 7 or 10, where the thickness of the first Au plated layer formed on one surface (front surface) of the thin metal substrate was less than 0.5 nm, the metal eluted amount was high, and the corrosion resistance was poor. 
     In each Comparative Example 5, 9 or 11, where the first Au plated layer was formed not uniformly, the metal eluted amount was high, and the corrosion resistance was poor. Comparative Examples 5, 9 and 11 were produced by reference to Examples in Patent Literature 4. 
     As shown in Table 3, in each Example where the thin metal substrate having a thickness of 0.05 mm or more was used, the percentage of the bent separators produced by assembling the fuel cell was 0. So, the work time needed to assemble could be shortened. 
     Then, each separator material of Example 8 and Comparative Example 2 was formed into a separator. Then, a single cell shown in  FIG. 3  was produced. An electric power generation test was conducted on the single cell.  FIG. 6  shows the test conditions, and an output voltage vs. a time. The cell made by the separator material of Example 8 generated power stably for 1000 hrs. In contrast, the cell made by the separator material of Comparative Example 2 had decreasing output voltage (electric power generation performance) as time elapsed. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           10 ,  100  Separator 
           12 ,  12 B Gasket 
           20  polymer electrolyte 
           40  Anode electrode 
           60  Cathode electrode 
           80  Membrane Electrode Assembly (MEA)