Patent Publication Number: US-2016233523-A1

Title: Fuel cell separator, fuel cell, and manufacturing method of fuel cell separator

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fuel cell separator, a fuel cell, and a manufacturing method of a fuel cell separator. 
     2. Description of Related Art 
     Conventionally, there has been known a technique described in Japanese Patent Application Publication No. 2008-004540 (JP 2008-004540 A), for example, as a technique related to a fuel cell separator. In the technique described in JP 2008-004540 A, in order to improve a corrosion resistance and a conductivity of the separator, a carbon thin film made of minute carbon particles is formed on a base substrate of the separator. 
     If the carbon particles to be formed on a surface of the base substrate are made small, adherence with respect to the base substrate improves. However, a deposition rate is slow, which causes such a problem that production efficiency is low. In the meantime, if a diameter of the carbon particles is increased to improve the production efficiency, such a problem is caused that durability of output of the fuel cell decreases. In addition, for conventional fuel cell separators and fuel cells, downsizing, reduction in cost, resource saving, simplification of manufacture, improvement of usability, and the like are demanded. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention relates to a fuel cell separator. This fuel cell separator includes an electrically-conductive base substrate, and a carbon film formed on the base substrate. The carbon film includes a first layer formed closest to the base material, and a second layer formed farthest from the base substrate. A diameter of carbon particles included in the first layer is 19 nm or less, and is smaller than a diameter of carbon particles included in a layer of the carbon film other than the first layer. A diameter of carbon particles included in the second layer is 40 nm or less. Since the diameter of the carbon particles included in the first layer is 19 nm or less, it is possible to improve adherence between the base substrate and the first layer of the carbon film. Further, since the diameter of the carbon particles included in the second layer is 40 nm or less, it is possible to improve a deposition rate and to improve productive efficiency of the fuel cell separator, in comparison with a case where the carbon film is formed such that a diameter of whole carbon particles thereof is 19 nm or less. Further, it is possible to restrain water including a substance (hereinafter referred to as the corrosive substance) generated by power generation of the fuel cell and corroding the base substrate, from passing through the second layer and penetrating into the base substrate. As a result, it is possible to restrain the base substrate from corroding due to the water including the corrosive substance, thereby making it possible to restrain a decrease in output of a fuel cell. 
     The fuel cell separator according to the above aspect may further includes an intermediate layer containing components of both of the base substrate and the carbon film, the intermediate layer being provided between the base substrate and the carbon film. According to such a configuration, it is possible to further improve adherence between the base substrate and the carbon film by the intermediate layer. 
     A second aspect of the present invention relates to a fuel cell. The fuel cell includes an anode, a cathode, an electrolyte membrane which is sandwiched between the anode and the cathode; the fuel cell separator of the first aspect. According to the second aspect, it is possible to improve the adherence between the base substrate and the first layer of the carbon film, and to restrain the decrease in output of the fuel cell. 
     A third aspect of the present invention relates to a manufacturing method of a fuel cell separator. The manufacturing method includes a step (a) of preparing an electrically-conductive base substrate, and a step (b) of forming a carbon film on the base substrate by plasma CVD. The step (b) may include a step (b 1 ) of forming a first layer of the carbon film as a layer closest to the base substrate, and a step (b 2 ) of forming a second layer of the carbon film as a layer farthest from the base substrate. A flow rate of raw material gas at a time of forming the first layer in the step (b 1 ) may be in a range from ½ to 1/50 of a flow rate of raw material gas at a time of forming the second layer in the step (b 2 ). With such a configuration, it is possible to improve the adherence between the base substrate and the first layer of the carbon film, and to improve the productive efficiency of the fuel cell separator. 
     The present invention is achievable in various aspects other than the above aspects. For example, the present invention is achievable in a manufacturing method of a fuel cell, in a vehicle including a fuel cell, and the like aspects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is an explanatory view to describe a schematic configuration of a fuel cell according to one embodiment of the present invention; 
         FIG. 2  is an enlarged explanatory view illustrating part of a section of a separator; 
         FIG. 3  is a process drawing of a manufacturing method of a separator according to one embodiment of the present invention; 
         FIG. 4  is an explanatory view illustrating, in a graph format, a relationship between a diameter of carbon particles included in a second layer of a carbon film and an increasing amount of a resistance value after a durability test; 
         FIG. 5  is an explanatory view illustrating, in a tabular format, an experimental result of each sample; 
         FIG. 6  is an explanatory view illustrating an SEM picture of a surface of a carbon film of Sample  3 ; 
         FIG. 7  is an explanatory view illustrating an SEM picture of a surface of a carbon film of Sample  9 ; 
         FIG. 8  is an explanatory view illustrating an SEM picture of a surface of a carbon film of Sample  12 ; 
         FIG. 9  is an explanatory view illustrating an SEM picture of a surface of a carbon film of Sample  8 ; 
         FIG. 10  is an explanatory view illustrating an SEM picture of a surface of a carbon film of Sample  11 ; and 
         FIG. 11  is an explanatory view illustrating an SEM picture of a surface of a carbon film of Sample  12 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A mode for carrying out the present invention will be described below based on an embodiment in the following order. 
     A. Embodiment: 
     B. Example: 
     C. Modifications: 
     A. Embodiment 
       FIG. 1  is an explanatory view to describe a schematic configuration of a fuel cell  10  according to one embodiment of the present invention. The fuel cell  10  is a polymer electrolyte fuel cell, and has a stack structure in which a plurality of single cells  14  is laminated. The single cell  14  is a unit module that generates electricity in the fuel cell  10 , and performs power generation by an electrochemical reaction between hydrogen gas and oxygen included in the air. Each of the single cells  14  includes a power generation body  20 , a pair of separators  100  (an anode-side separator  100   an  and a cathode-side separator  100   ca ) sandwiching the power generation body  20  therebetween. 
     The power generation body  20  includes: a membrane electrode assembly (also referred to as MEA)  23  in which catalyst electrode layers  22  (an anode  22   an  and a cathode  22   ca ) are formed on both surfaces of an electrolyte membrane  21 ; and a pair of gas diffusion layers  24  (an anode-side diffusion layer  24   an  and a cathode-side diffusion layer  24   ca ) placed on both sides of the membrane electrode assembly  23 . 
     The electrolyte membrane  21  is a polymer electrolyte membrane made of fluorine-based sulfonic acid polymer as a solid polymeric material, and has a good proton conductivity in a wet condition. In the present embodiment, a Nafion film (NRE212, Nafion is a registered trademark) is used as the electrolyte membrane  21 . However, the electrolyte membrane  21  is not limited to Nafion (registered trademark), and other fluorine-based sulfonic acid membranes such as Aciplex (registered trademark) or Flemion (registered trademark) may be used, for example. Further, as the electrolyte membrane  21 , a fluorine-based phosphoric acid membrane, a fluorine-based carboxylic acid membrane, a fluorine-based hydrocarbon graft membrane, a hydrocarbon-based graft membrane, an aromatic membrane or the like may be used. Furthermore, a composite polymer membrane containing a reinforcing material such as PTFE or polyamide so that a mechanical characteristic thereof is strengthened may be used. 
     The catalyst electrode layers  22  (the anode  22   an  and the cathode  22   ca ) are placed on both sides of the electrolyte membrane  21 , so that when the fuel cell is used, one of them functions as an anode electrode, and the other one of them functions as a cathode electrode. The catalyst electrode layer  22  contains carbon particles (a catalyst carrying carrier) that carry a catalytic metal (platinum, in the present embodiment) that promotes an electrochemical reaction, and a proton-conductive polymer electrolyte (fluorine-based resin, in the present embodiment). A carbon material such as carbon black, carbon nanotube, or carbon nanofiber, or a carbon compound represented by silicon carbide may be used as the electrically-conductive catalyst carrying carrier, instead of the carbon particles. Further, platinum alloy, palladium, rhodium, gold, silver, osmium, iridium, or the like may be used as the catalytic metal, instead of platinum. 
     The gas diffusion layers  24  (the anode-side diffusion layer  24   an  and the cathode-side diffusion layer  24   ca ) are layers for diffusing reactant gas (anode gas and cathode gas) used for an electrode reaction along a surface direction of the electrolyte membrane  21 . In the present embodiment, carbon paper is used as the gas diffusion layers  24 . Note that, as the gas diffusion layers  24 , a carbon porous material such as carbon cloth, or a metal porous material such as metal mesh or foam metal may be used, for example, instead of the carbon paper. 
     The separators  100  (the anode-side separator  100   an  and the cathode-side separator  100   ca ) are made of a member having a gas barrier property and an electronic conductivity. In the present embodiment, the separators  100  are made of titanium. However, the separators  100  may be made of other metallic components, for example, instead of titanium. The separators  100  will be described later in detail. 
     An uneven shape constituting passages where gas and liquid flow is formed on a surface of the separator  100 . More specifically, the anode-side separator  100   an  includes anode gas passages AGC where gas and liquid can flow, between the anode-side separator  100   an  and the anode-side diffusion layer  24   an . The cathode-side separator  100   ca  includes cathode gas passages CGC where gas and liquid can flow, between the cathode-side separator  100   ca  and the cathode-side diffusion layer  24   ca.    
       FIG. 2  is an enlarged explanatory view illustrating part of a section of the separator  100 . The separator  100  includes a metal base substrate  110 , an intermediate layer  112  formed on the metal base substrate  110 , and a carbon film  120  formed on the intermediate layer  112 . Note that the carbon film  120  is formed on that surface of the intermediate layer  112  which makes contact with the gas diffusion layer  24 . 
     The metal base substrate  110  is made of an electrically-conductive metallic component, and in the present embodiment, the separator  100  is made of titanium. However, the metal base substrate  110  may be made of other metal such as stainless steel. 
     The carbon film  120  is formed on the intermediate layer  112 , and improves a conductivity and a corrosion resistance of the separator  100 . The carbon film  120  is formed by depositing carbon particles by plasma CVD. The carbon film  120  includes a first layer  121  formed on a surface of the metal base substrate  110 , and a second layer  122  formed on a surface of the first layer  121 . As will be described later, a diameter of carbon particles included in the first layer  121  is different from a diameter of carbon particles included in the second layer. 
     The intermediate layer  112  contains components of both of the metal base substrate  110  and the carbon film  120 . In the present embodiment, the intermediate layer  112  is made of titanium carbide (TiC). The intermediate layer  112  has good adherence with respect to the metal base substrate  110 , and also has good adherence with respect to the carbon film  120 . In view of this, according to the present embodiment, it is possible to improve adherence between the metal base substrate  110  and the carbon film  120  by the intermediate layer  112 . However, the carbon film  120  may be formed directly on the metal base substrate  110  without forming the intermediate layer  112 . 
     In the present embodiment, the diameter of the carbon particles included in the first layer  121  is smaller than the diameter of the carbon particles included in the second layer  122 , and the diameter of the carbon particles included in the first layer  121  is 19 nm or less. In view of this, according to the present embodiment, the carbon particles included in the first layer  121  is easy to get into minute uneven gaps on the surface of the metal base substrate  110  (the intermediate layer  112  when the intermediate layer  112  is formed). This makes it possible to improve adherence between the first layer  121  of the carbon film  120  and the metal base substrate  110  (the intermediate layer  112  when the intermediate layer  112  is formed). 
     Further, in the present embodiment, the diameter of the carbon particles included in the second layer  122  is 40 nm or less. In view of this, according to the present embodiment, it is possible to improve a deposition rate and to improve productive efficiency of the separator  100 , in comparison with a case where the carbon film  120  is formed such that a diameter of whole carbon particles thereof is 19 nm or less. Further, according to the present embodiment, since gaps between the carbon particles included in the second layer  122  are small, it is possible to restrain water including a corrosive substance (a substance that corrodes the metal base substrate  110  and the intermediate layer  112 ) generated by power generation of the fuel cell, from passing through the second layer  122  and penetrating into the metal base substrate  110  and the intermediate layer  112 . As a result, it is possible to restrain the metal base substrate  110  and the intermediate layer  112  from corroding due to the water including the corrosive substance, thereby making it possible to restrain a decrease in output of the fuel cell. 
     Note that, in the present specification, the “diameter of particles” indicates an average particle diameter, and the average particle diameter is calculated by performing image analysis on an image obtained by FE-SEM (Field Emission-Scanning Electron Microscope). 
       FIG. 3  is a process drawing of a manufacturing method of the separator  100  according to one embodiment of the present invention. In step S 100 , a metal base substrate  110  is prepared. In the present embodiment, a titanium metal base substrate  110  is prepared. 
     In step S 102 , an intermediate layer  112  is formed on the metal base substrate  110 . In the present embodiment, a titanium carbide layer is formed as the intermediate layer  112  on the titanium metal base substrate  110 . 
     In step S 104 , a first layer  121  of a carbon film  120  is formed on the intermediate layer  112 . In the present embodiment, the first layer  121  of the carbon film  120  is formed by plasma CVD using hydrocarbon-based gas. At the time of the plasma CVD, a flow rate of the gas is adjusted so that a diameter of carbon particles included in the first layer  121  of the carbon film  120  becomes 19 nm or less. 
     In step S 106 , a second layer  122  of the carbon film  120  is formed on the first layer  121  of the carbon film  120 . In the present embodiment, the second layer  122  of the carbon film  120  is formed by plasma CVD using hydrocarbon-based gas. At the time of the plasma CVD, a flow rate of the gas is adjusted so that a diameter of carbon particles included in the second layer  122  of the carbon film  120  becomes 40 nm or less. 
     In the present embodiment, the flow rate of raw material gas at the time of forming the first layer  121  in step S 104  is set to be in a range from ½ to 1/50 of the flow rate of raw material gas at the time of forming the second layer  122  in step S 106 . As in the present embodiment, when the flow rate of the raw material gas at the time of forming the first layer  121  is set to be ½ or less of the flow rate of the raw material gas at the time of forming the second layer  122 , it is possible to improve the adherence of the first layer  121  with respect to the metal base substrate  110  (and the intermediate layer  112 ). When the flow rate of the raw material gas at the time of forming the first layer  121  is set to be 1/50 or more of the flow rate of the raw material gas at the time of forming the second layer  122 , it is possible to shorten time required to form the first layer  121 . Thus, when the flow rates of the raw material gases are set as described above, it is possible to improve productive efficiency of the separator  100 . 
     B. Example 
     In this example, a plurality of samples of the fuel cell separator was formed, and a resistance value of each sample was measured. Then, fuel cells were formed by using the samples of the fuel cell separator, and a durability test in which power generation is performed for a predetermined time was performed thereon. After the durability test, a resistance value of each of the samples of the fuel cell separator was measured, so as to measure an increasing amount of the resistance value after the durability test was measured. 
       FIG. 4  is an explanatory view illustrating, in a graph format, a relationship between a diameter of carbon particles included in the second layer  122  of the carbon film  120  and the increasing amount of the resistance value after the durability test. Note that the diameter of the carbon particles of the first layer  121  in each of the samples used in this example is 19 nm or less. 
     According to  FIG. 4 , it can be understood that, as the diameter of the carbon particles included in the second layer  122  becomes smaller, the increasing amount of the resistance value after the durability test is decreased. Further, it can be understood that if the diameter of the carbon particles included in the second layer  122  is 40 nm or less, the resistance value hardly increases, and the increasing amount of the resistance value after the durability test is 5 [mΩ·m 2 ] or less. The reason is as follows: As described above, if the diameter of the carbon particles included in the second layer  122  is 40 nm or less, the gaps between the particles are small, thereby making it possible to restrain water including a corrosive substance generated by power generation of the fuel cell, from passing through the second layer  122  and penetrating into the metal base substrate  110  and the intermediate layer  112 . As a result, it is possible to restrain the metal base substrate  110  and the intermediate layer  112  from corroding due to the water including the corrosive substance. In view of this, it is preferable that the diameter of the carbon particles included in the second layer  122  be 40 nm or less. 
       FIG. 5  is an explanatory view illustrating, in a tabular, format, an experimental result of each of the samples.  FIGS. 6 to 11  are explanatory views each illustrating an SEM picture of a surface of the carbon film  120  of each of the samples. The correspondence between the figures and the samples is as follows. 
       FIG. 6 : Surface of the first layer  121  of Sample  3 
 
 FIG. 7 : Surface of the first layer  121  of Sample  9 
 
 FIG. 8 : Surface of the first layer  121  of Sample  12 
 
 FIG. 9 : Surface of the second layer  122  of Sample  8 
 
 FIG. 10 : Surface of the second layer  122  of Sample  11 
 
 FIG. 11 : Surface of the second layer  122  of Sample  12 
 
     In evaluation of  FIG. 5 , in a case where the increasing amount of the resistance value of a sample after the durability test is more than 5 [mΩ·m 2  (mΩ is milliohm)], it is determined that its durability is low and the sample is evaluated as “B,” and in a case where the increasing amount of the resistance value of a sample after the durability test is not more than 5 [mΩ·m 2 ], it is determined that its durability is high and the sample is evaluated as “A.” 
     According to Sample  1  and Sample  2 , it can be understood that, in a case where the carbon film  120  is not formed in two layers, that is, in a case where the first layer  121  of a small particle diameter is not formed, the increasing amount of the resistance value is large regardless of whether the intermediate layer  112  is provided or not, and the durability is low. 
     According to Sample  3  to Sample  5 , it can be understood that when the diameter of the carbon particles of the first layer  121  is 19 nm or less and the diameter of the carbon particles of the second layer  122  is 40 nm or less, the durability is high. 
     According to Sample  6  to Sample  8 , it can be understood that, even in a case where the diameter of the carbon particles of the first layer  121  is 5 nm or less, when the diameter of the carbon particles of the second layer  122  is more than 40 nm, the durability is low. 
     According to Sample  9  to Sample  13 , it can be understood that, when the diameter of the carbon particles of the first layer  121  is 10 nm or less and the diameter of the carbon particles of the second layer  122  is 30 nm or less, the increasing amount of the resistance value is 2 [mΩ·m 2 ] or less, and thus, the durability is very high. 
     Note that, according to Sample  4  to Sample  13 , it can be understood that when the flow rate of the raw material gas at the time of forming the first layer  121  is in a range from ½ to 1/10 of the flow rate of the raw material gas at the time of forming the second layer  122 , the diameter of the carbon particles of the first layer  121  becomes 19 nm or less. 
     Accordingly, the diameter of the carbon particles of the first layer  121  is preferably 19 nm or less, further preferably 10 nm or less, and particularly preferably 5 nm or less. Further, the diameter of the carbon particles of the second layer  122  is preferably 40 nm or less, and further preferably 30 nm or less. 
     The flow rate of the raw material gas at the time of forming a first layer in which a diameter of carbon particles is 19 nm or less is from 1 sccm to 2000 sccm per 1 m 2  of a processed member e.g., the metal base substrate  110  of the above embodiment. The flow rate of the raw material gas at the time of forming a second layer in which a diameter of carbon particles is 40 nm or less is equal to or smaller than 50000 sccm per 1 m 2  of a processed member e.g., the first layer  121  of the above embodiment and is larger than the flow rate of the raw material gas at the time of forming the first layer. For example, in the sample  11 , the flow rate of the raw material gas at the time of forming the first layer  121  is 500 sccm per 1 m 2  of the metal base substrate  110 . The flow rate of the raw material gas at the time of forming the second layer  122  is 5000 sccm per 1 m 2  of the first layer  121 . 
     C. Modifications 
     Note that the present invention is not limited to the above embodiment and the above example, and is performable in various forms within a range that does not deviate from the gist of the present invention. For example, the following modifications can be employed. 
     Modification 1: In the above embodiment, the carbon film  120  may be constituted by three or more layers. In this case, it is preferable that a diameter of carbon particles included in a layer formed closest to the metal base substrate  110  among the three or more layers constituting the carbon film  120  be smaller than diameters of carbon particles included in the other layers of the carbon film  120 . 
     Further, a diameter of carbon particles included in a layer formed farthest from the metal base substrate  110  among the three or more layers constituting the carbon film  120  is preferably 40 nm or less, and the diameter of the carbon particles included in the layer formed closest to the metal base substrate  110  is preferably 19 nm or less. 
     Modification 2: In the above embodiment, in a case where the metal base substrate  110  is made of titanium, the intermediate layer  112  may be made of TiC 2 , for example. Further, in a case where the metal base substrate  110  is made of stainless steel (SUS), the intermediate layer  112  may be made of Fe 3 C, Cr 23 C 6 , or the like, for example. 
     The present invention is not limited to the above embodiment, example, and modifications, and is achievable in various configurations within a range that does not deviate from the gist of the present invention. For example, those technical features of the embodiment, the example, and the modifications which correspond to the technical features of each aspect described in SUMMARY OF THE INVENTION can be replaced or combined appropriately, in order to resolve some or all of the problems described above or in order to achieve some or all of the above effects. Further, the technical features can be deleted appropriately if the technical features have not been described as essential in the present specification.