Patent Publication Number: US-2019173100-A1

Title: Method for manufacturing separator for fuel cell

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese patent application JP 2017-233814 filed on Dec. 5, 2017, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a method for manufacturing a separator for fuel cell. 
     Background Art 
     Conventionally inventions about a method for manufacturing a separator for fuel cell have been known, and such a separator has a gas diffusion layer to supply gas to an electrolyte layer having a catalyst layer on the surface (see JP 2006-331670 A, for example). This patent document describes a method for manufacturing a separator for fuel cell that includes a substrate functioning as a separator of the fuel cell and a gas diffusion layer on the surface of the substrate. 
     Such a conventional method for manufacturing a separator for fuel cell includes an applying step and a thermal treatment step (see Claim  1 , for example, in the above document). The applying step applies metal-powder suspended slurry, which can form a metal porous layer after sintering, on the surface of the substrate. The thermal treatment step heats the substrate with the metal-powder suspended slurry applied in the temperature environment that can form the metal porous layer from the metal-power suspended slurry by sintering so as to form the metal porous layer by sintering. 
     SUMMARY 
     A metal porous layer to be a gas diffusion layer of a fuel cell has to have a predetermined thickness. The above patent document specifically describes the method including screen printing to apply metal-powder suspended slurry. In this case, the metal-power suspended slurry applied on the substrate at the applying step may deform and spread over the surface of the substrate due to the self-weight, and it may be difficult to form a metal porous layer of a necessary thickness at the thermal treatment step. 
     One aspect of the present disclosure provides a method for manufacturing a metal separator for fuel cell that can hold the shape of slurry applied on the surface of a metal substrate and so form the structure of a predetermined thickness on the surface of the metal substrate. 
     A method for manufacturing a separator for fuel cell according to one aspect of the present disclosure manufactures a separator for fuel cell including a metal substrate and a conduit-defining part on the surface of the metal substrate. The method includes: partially removing oxide coating covering the surface of the metal substrate to form an application part; applying slurry to the application part after removing the oxide coating; and heating the slurry applied at the application part to form the conduit-defining part. 
     The method for manufacturing a separator for fuel cell according to this aspect includes the step of partially removing an oxide coating covering the surface of the metal substrate to form an application part. This step makes the wettability at the application part to apply slurry better than the wettability at the oxide coating of the metal substrate surrounding the application part. The application part where the oxide coating on the surface of the metal substrate is removed is slightly recessed from the surface of the oxide coating surrounding the application part. 
     With this configuration, at the applying step of slurry to the application part after removing the oxide coating, the slurry applied to the application part does not spread over the surface of the surrounding metal substrate for wetting due to the self-weight. In this way, the slurry can keep the shape and height applied to the application part. Subsequently the slurry applied at the application part is heated to form a conduit-defining part, whereby a conduit-defining part having a predetermined shape and height can be formed. 
     In the method for manufacturing a separator for fuel cell according to the above aspect, the forming of the application part makes a contact angle with pure water at the application part smaller than a contact angle with pure water at the oxide coating, for example. This can make the wettability at the application part to apply slurry better than the wettability at the oxide coating of the metal substrate surrounding the application part. 
     In the method for manufacturing a separator for fuel cell according to the above aspect, the forming of the application part removes the oxide coating with laser light, for example. This allows laser light to be reflected from a newly formed surface of the metal substrate that is exposed at the application part after removing the oxide coating, whereby the oxide coating on the surface of the metal substrate can be removed by the laser light. 
     In the method for manufacturing a separator for fuel cell according to the above aspect, the contact angle with pure water at the application part is 0.75 times or less the contact angle with pure water at the oxide coating, for example. This can prevent the slurry applied to the application part from spreading over the surface of the surrounding metal substrate for wetting due to the self-weight. In this way, the slurry can keep the shape and height applied to the application part reliably. 
     In the method for manufacturing a separator for fuel cell according to the above aspect, the metal substrate is made of pure titanium, and the contact angle with pure water at the application part is less than 20[° ], for example. This can reliably prevent the slurry applied to the application part from spreading over the surface of the surrounding metal substrate for wetting due to the self-weight. In this way, the slurry can keep the shape and height applied to the application part reliably. 
     In the method for manufacturing a separator for fuel cell according to the above aspect, the metal substrate is made of stainless steel, and the contact angle with pure water at the application part is less than 60[°], for example. This can reliably prevent the slurry applied to the application part from spreading over the surface of the surrounding metal substrate for wetting due to the self-weight. In this way, the slurry can keep the shape and height applied to the application part reliably. 
     In the method for manufacturing a separator for fuel cell according to the above aspect, the slurry has viscosity of 1×10 3 [mPa·s] or more and 1×10 4 [mPa·s] or less when shear rate is within the range of 1×10 2 [l/sec] or less, for example. With this configuration, at the applying step of slurry to the application part, the slurry can be applied by screen printing, and the slurry can keep the shape and height applied to the application part. 
     The above aspect can provide a method for manufacturing a separator for fuel cell that can hold the shape of slurry applied on the surface of a metal substrate and so form the structure of a predetermined thickness on the surface of the metal substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a fuel cell; 
         FIG. 2  is a schematic enlarged plan view of the surface of the separator in  FIG. 1 ; 
         FIG. 3  is a flowchart of a method for manufacturing a separator for fuel cell according to one embodiment of the present disclosure; 
         FIG. 4  is a schematic enlarged cross-sectional view of the metal substrate to describe one example of the coating removal step in  FIG. 3 ; 
         FIG. 5  is a schematic enlarged cross-sectional view of the metal substrate when the applying step in  FIG. 3  ends; 
         FIG. 6  is a graph showing the contact angles with pure water at the oxide coating and at the application part of the metal substrates of Example 1; 
         FIG. 7  is a graph showing the contact angles with pure water at the oxide coating and at the application part of the metal substrates of Example 2; 
         FIG. 8  is a graph showing the relationship between the shear rate and the viscosity of the slurry in Examples; and 
         FIG. 9  is a schematic cross-sectional view of the slurry applied to the metal substrates of Comparative Examples. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes one embodiment of a method for manufacturing a separator for fuel cell according to the present disclosure, with reference to the drawings. 
       FIG. 1  is a schematic cross-sectional view of a fuel cell  1 . The fuel cell  1  includes a pair of separators  2 , and a MEGA  3  (Membrane-Electrode-Gas Diffusion Layer Assembly) disposed between these separators  2 , for example. The MEGA  3  includes the lamination of a catalyst layer, a water-repellent layer, and a gas diffusion layer on the surface and the rear face of a polymer electrolyte membrane. 
     Each separator  2  for fuel cell includes a metal substrate  21  and a conduit-defining part  22  on the surface of this metal substrate  21 . The metal substrate  21  is a metal plate-like member made of titanium or titanium alloy or made of a stainless steel, such as SUS316 or SUS447. The metal substrate  21  has a desired shape obtained by press forming or die-cutting, for example. 
       FIG. 2  is a schematic enlarged plan view of the surface of the separator  2  in  FIG. 1 . The conduit-defining part  22  includes a rib on the surface that is opposed to the MEGA  3  of the metal substrate  21 , for example. The conduit-defining part  22  may include a protrusion protruding from the surface of the metal substrate  21  that is opposed to the MEGA  3  toward the MEGA  3 , for example. The conduit-defining part  22  defines a gas conduit  4  between the separator  2  and the MEGA  3 . 
     The fuel cell includes a fuel cell stack including the lamination of a plurality of fuel cells  1 , i.e., single cells, and a housing to store the fuel cell stack, for example. Although not illustrated, the fuel cell  1  has a resin frame around the MEGA  3 , for example, and the outer edges of the pair of separators  2  are joined via this resin frame. The pair of separators  2  and the resin frame have a plurality of manifold holes at the outer edges. 
     The fuel cell generates electricity while receiving reactant gas and coolant supplied via the manifold holes at the individual fuel cells  1  as components of the fuel cell stack. The reactant gas supplied to the fuel cell  1  via the manifold hole for supplying reactant gas is then supplied to the gas conduit  4  between the separator  2  and the MEGA  3  via a groove-like conduit formed at the outer edge of the resin frame, for example. 
     Reactant gas, which was supplied to the gas conduit  4  of the fuel cell  1  and was not used for the reaction at the MEGA  3 , is discharged from the gas conduit  4  to the manifold hole for discharging reactant gas via a groove-like conduit formed at the outer edge of the resin frame, for example. Coolant, which is supplied to the manifold hole for supplying coolant of the fuel cell  1 , is then supplied to a coolant conduit between the separators  2  of the neighboring fuel cells  1  as components of the fuel cell stack. Coolant is then discharged from the manifold hole for discharging coolant. 
       FIG. 3  is a flowchart of a method M 1  for manufacturing a separator for fuel cell according to the present embodiment. As shown in  FIGS. 1 and 2 , the method M 1  for manufacturing a separator for fuel cell of the present embodiment manufactures a separator  2  for fuel cell that includes the metal substrate  21  and the conduit-defining part  22  on the surface of the metal substrate  21 , for example. 
     The method M 1  for manufacturing a separator for fuel cell of the present embodiment, which will be described later in details, includes: a coating removal step S 4  (see  FIG. 4 ) to partially remove an oxide coating  21   a  covering the surface of the metal substrate  21  to form an application part  21   b ; an applying step S 5  (see  FIG. 5 ) to apply slurry  22   s  at the application part  21   b  after removing the oxide coating  21   a ; and a thermal treatment step S 6  to heat the slurry  22   s  applied at the application part  21   b  to form a conduit-defining part  22 . 
     The method M 1  for manufacturing a separator for fuel cell may include, prior to the coating removal step S 4 , a cutting step S 1  to cut the metal substrate  21  from a base material, a forming step S 2  to form the metal substrate  21  cut at the cutting step S 1 , and a washing step S 3  to wash the metal substrate  21 , for example. 
     The cutting step S 1  unwinds a metal thin plate from the roll as a base material, cuts it to be a metal substrate  21  in a desired shape, and then die-cuts such a cut metal substrate  21  to have a plurality of manifold holes, for example. The forming step S 2  presses the metal substrate  21  subjected to the cutting step S to form the metal substrate  21  into a desired shape, for example. 
     The washing step S 3  immerses the metal substrate  21  in acid solution, for example, to wash and remove the oxide adhering to the surface of the metal substrate  21 . The washing step S 3  does not remove the oxide coating  21   a  on the surface of the metal substrate  21  completely, and the metal substrate  21  after the washing step S 3  still has the oxide coating  21   a  on the surface. The washing step S 3  may include water-washing, for example, before or after the washing with oxide to immerse the metal substrate  21  in acid solution. 
       FIG. 4  is a schematic enlarged cross-sectional view of the metal substrate  21  to describe one example of the coating removal step S 4  in  FIG. 3 . As described above, the coating removal step S 4  partially removes the oxide coating  21   a  covering the surface of the metal substrate  21  to form the application part  21   b . In other words, the application part  21   b  is a part of the metal substrate  21  having the oxide coating  21   a  on the surface, from which the oxide film  21   a  on the surface has been selectively removed. 
     At the coating removal step S 4 , the oxide coating  21   a  can be removed by laser cleaning using laser light L, such as YAG laser. More specifically the coating removal step S 4  irradiates the oxide coating  21   a  covering the surface of the metal substrate  21  with laser light L while scanning over the area where the conduit-defining part  22  is to be formed, so as to selectively remove the oxide film  21   a  at the area and form the application part  21   b.    
     In this way, the coating removal step S 4  removes the oxide coating  21   a  on the surface of the metal substrate  21  with laser light L to form the application part  21   b , whereby this step can selectively remove the oxide coating  21   a  only on the surface of the metal substrate  21 . This is because the laser light L is reflected from a newly formed surface of the metal substrate  21  that is exposed at the application part  21   b  after removing the oxide coating  21   a . At the coating removal step S 4 , the oxide coating  21   a  may be removed by sandblasting instead of laser cleaning, for example. 
     The application part  21   b  where the oxide coating  21   a  on the surface of the metal substrate  21  is removed has wettability better than wettability of the oxide coating  21   a  on the surface of the metal substrate  21 . In other words, the application part  21   b  of the metal substrate  21  has a contact angle with pure water that is smaller than that of the oxide coating  21   a . That is, the coating removal step S 4  partially removes the oxide coating  21   a  covering the surface of the metal substrate  21  to form the application part  21   b  having wettability better than that of the oxide coating  21   a . In other words, the method M 1  for manufacturing a separator for fuel cell of the present embodiment includes the coating removal step S 4  to form the application part  21   b , and through this step, the application part  21   b  has a contact angle with pure water that is smaller than the contact angle with pure water at the oxide coating  21   a.    
       FIG. 5  is a schematic cross-sectional view of the metal substrate  21  when the applying step S 5  in  FIG. 3  ends. As described above, the applying step S 5  applies slurry  22   s  to the application part  21   b  after removing the oxide coating  21   a . A method for applying the slurry  22   s  at the applying step S 5  is not limited especially, and various methods, such as screen printing, gravure printing, slot die printing, offset printing, and inkjet printing, may be used. 
     The slurry  22   s  may be prepared, for example, by mixing graphite, acetylene black (carbon black), polyvinyl alcohol (PVOH) and binder with solvent including the mixture of water and ethylene glycol-2-n butylether. The solvents may include ethanol, propylene glycol, ethylene glycol and xylene. 
     As shown in  FIG. 5 , the oxide coating  21   a  on the surface of the metal substrate  21  is removed at the application part  21   b  on the surface of the metal substrate  21 , so that the application part can have improved wettability as compared with the surrounding oxide coating  21   a , and is slightly recessed from the surface of the oxide coating  21   a . With this configuration, the slurry  22   s  applied to the application part  21   b  can be held at the application part  21   b  and does not spread over the surrounding area for wetting. In this way, the slurry can keep a predetermined shape and a necessary height H. From the viewpoint of securing a necessary cross-sectional area for the gas conduit  4  defined by the conduit-defining part  22 , the slurry  22   s  applied at the application part  21   b  has to have a height H of 0.3 [mm] or more from the surface of the metal substrate  21 , for example. 
     As described above, the thermal treatment step S 6  heats the slurry  22   s  applied at the application part  21   b  on the surface of the metal substrate  21  to form the conduit-defining part  22 . This thermal treatment step S 6  vaporizes the solvent included in the slurry  22   s  applied at the application part  21   b  on the surface of the metal substrate  21  to form the conduit-defining part  22  on the surface of the metal substrate  21 . As a result, the separator  2  for fuel cell as shown in  FIGS. 1 and 2  can be manufactured. 
     As stated above, the method M 1  for manufacturing a separator for fuel cell of the present embodiment can hold the shape of the slurry  22   s  applied on the surface of the metal substrate  21  and so form the conduit-defining part  22  of a predetermined thickness on the surface of the metal substrate  21 . 
     That is a detailed description of the embodiments of the present disclosure. The specific configuration of the present disclosure is not limited to the above-stated embodiment, and the design may be modified variously without departing from the spirits of the present disclosure. The present disclosure also covers such modified embodiments. For instance, the oxide coating  21   a  covering the surface of the metal substrate  21  may be a plated layer covering the surface of the metal substrate  21 . 
     More specifically, the method M 1  for manufacturing a separator for fuel cell in this case manufactures a separator for fuel cell having a metal substrate  21  and a conduit-defining part  22  on the surface of the metal substrate  21 . The method may include: partially removing a plated layer covering the surface of the metal substrate  21  to form an application part  21   b ; applying slurry  22   s  at the application part  21   b  after removing the plated layer, and heating the slurry  22   s  applied at the application part  21   b  to form a conduit-defining part  22 . In this case, the plated layer may be a metal layer made of gold (Au), for example. 
     Examples 
     The following describes Examples of the method for manufacturing a separator for fuel cell described in the above embodiment. 
     Three thin plates of pure titanium were prepared, which were metal substrates N 11 , N 12  and N 13  of Example 1. Three thin plates of stainless steel (SUS316) were prepared, which were metal substrates N 21 , N 22  and N 23  of Example 2. Next, the coating removal step was performed to partially remove the oxide coating covering the surface of each of the prepared metal substrates N 11 , N 12 , N 13 , N 21 , N 22  and N 23  and form the application part. 
     At the coating removal step, a laser device was used so that a part of the oxide coating covering the surface of each of the metal substrates N 11 , N 12 , N 13 , N 21 , N 22  and N 23  was irradiated with pulsed YAG laser with the average power of 150 [W] while scanning the laser at the scanning rate of 10 [m/min] with a galvanometer mirror. Next, contact angles [°] with pure water were measured at the oxide coating and at the application part for the metal substrates N 11 , N 12  and N 13  of Example 1 and the metal substrates N 21 , N 22  and N 23  of Example 2. 
       FIG. 6  is a graph showing the contact angles [°] with pure water at the oxide coating and at the application part for the metal substrates N 11 , N 12  and N 13  of Example 1. The metal substrates N 11 , N 12  and N 13  of Example 1 had contact angles with pure water at the oxide coating of 39[°], 32[°] and 38[°], respectively. The metal substrates N 11 , N 12  and N 13  had contact angles with pure water at the application part after removing the oxide coating on the surface of 15[°], 17[°] and 15[°], respectively. 
     That is, while the metal substrates N 11 , N 12  and N 13  made of pure titanium of Example 1 had contact angles with pure water at the oxide coating on the surface of 30[°] or more, the contact angles with pure water at the application part after removing the oxide coating decreased to less than 20[°]. That is, the contact angles with pure water at the application parts was 0.67 time or less the contact angles with pure water at the oxide coating of the metal substrates. Table 1 shows the result. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Metal substrate 
                   
                 Contact angle with pure water [°] 
                   
               
            
           
           
               
               
               
               
            
               
                 (Ex. 1) 
                 Material 
                 Oxide coating 
                 Application part 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 N11 
                 Ti 
                 39 
                 15 
               
               
                 N12 
                 Ti 
                 32 
                 17 
               
               
                 N13 
                 Ti 
                 38 
                 15 
               
               
                   
               
            
           
         
       
     
       FIG. 7  is a graph showing the contact angles [°] with pure water at the oxide coating and at the application part for the metal substrates N 21 , N 22  and N 23  of Example 2. The metal substrates N 21 , N 22  and N 23  of Example 2 had contact angles with pure water at the oxide coating on the surface of 90[° ], 79[] and 100[° ], respectively. The metal substrates N 21 , N 22  and N 23  had contact angles with pure water at the application part after removing the oxide coating on the surface of 40[° ], 55[°] and 50[° ], respectively. 
     That is, while the metal substrates N 21 , N 22  and N 23  made of SUS316 of Example 2 had contact angles with pure water at the oxide coating on the surface of 75[°] or more, the contact angles with pure water at the application part after removing the oxide coating decreased to 55[°] or less. That is, the contact angles with pure water at the application parts was 0.75 time or less the contact angles with pure water at the oxide coating of the metal substrates. Table 2 shows the result. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Metal substrate 
                   
                 Contact angle with pure water [°] 
                   
               
            
           
           
               
               
               
               
            
               
                 (Ex. 2) 
                 Material 
                 Oxide coating 
                 Application part 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 N21 
                 SUS316 
                 90 
                 40 
               
               
                 N22 
                 SUS316 
                 79 
                 55 
               
               
                 N23 
                 SUS316 
                 100 
                 50 
               
               
                   
               
            
           
         
       
     
     Next slurry to be applied at the application part of the metal substrates was prepared. The slurry was prepared by mixing 85 weight parts of graphite, 15 weight parts of acetylene black (carbon black), 5 weight parts of polyvinyl alcohol (PVOH) and 3 weight parts of binder with solvent including the mixture of 49.5 weight parts of water and 5 weight parts of ethylene glycol-2-n butylether. 
       FIG. 8  is a graph showing the relationship between the shear rate [l/sec] and the viscosity [mPa·s] of the prepared slurry. As shown in  FIG. 8 , the slurry was non-Newtonian fluid, and showed the behavior of Bingham fluid. The slurry had the viscosity of 1×10 3 [mPa·s] or more and 1×10 4 [mPa·s] or less when the shear rate was within the range of 1×10 2 [l/sec] or less. 
     Next, the applying step was performed to apply the slurry to the application part of each of the metal substrates N 11 , N 12 , N 13 , N 21 , N 22  and N 23  after removing the oxide coating by screen printing. The screen printing was performed under the conditions of the speed of the squeegee of 30 [mm/sec], the angle of the squeegee of 70[° ], the printing pressure of 0.3 [Mpa], and the rubber hardness of the squeegee of 70 degrees using a durometer (type A). A pre-contact squeegee was not used. As shown in  FIG. 5 , the slurry applied at the application parts of each of the metal substrates N 11 , N 12 , N 13 , N 21 , N 22  and N 23  held a predetermined shape having the height H of 0.3 [mm] or more. 
     Next, the thermal treatment step was performed to heat the slurry applied at the application part of each of the metal substrates N 11 , N 12 , N 13 , N 21 , N 22  and N 23  to form the conduit-defining part. At the thermal treatment step, the slurry was heated for drying under the conditions of 130[° C.] and 30 [sec], and the conduit-defining part  22  was formed as shown in  FIGS. 1 and 2 . 
     The separators of Example 1 and Example 2 obtained through these steps successfully had the conduit-defining part  22  having a predetermined thickness on the surface of the metal substrates N 11 , N 12 , N 13 , N 21 , N 22  and N 23 . The separators for fuel cell of Example 1 and Example 2 therefore had sufficient cross-sectional area of the gas conduit  4  defined with the conduit-defining part  22  so as to improve the power generation efficiency of the fuel cell and have good fuel efficiency. 
     Comparative Examples 
     Thin plates of pure titanium and stainless steel (SUS316) were prepared, which were metal substrates of Comparative Example 1 and Comparative Example 2, respectively. Oxide coating covering the surface of the metal substrates of Comparative Examples 1 and 2 was not removed, and the slurry prepared similarly to Examples was applied by screen printing on the surface of the metal substrates of Comparative Examples 1 and 2. 
       FIG. 9  is a schematic cross-sectional view of the slurry  22 Xs applied to the metal substrate  21 X of Comparative Examples 1 and 2. The slurry  22 Xs applied to the metal substrate  21 X of Comparative Examples 1 and 2 had ooze spreading to the surrounding over the oxide coating  21 Xa on the surface, and the height H was less than 0.3 [mm]. In this way the slurry failed to hold a predetermined shape. 
     Next, the thermal treatment step was performed to heat the slurry  22 Xs applied at the application part of each of the metal substrates of Comparative Examples 1 and 2 to form the conduit-defining part. The thermal treatment step was performed similarly to Examples. The separators of Comparative Examples 1 and 2 obtained through these steps failed to form the conduit-defining part having a predetermined thickness on the surface of the metal substrates. 
     The separators of Comparative Examples 1 and 2 therefore failed to have a sufficient cross-sectional area of the gas conduit defined with the conduit-defining part, which may cause deterioration of the power generation efficiency of the fuel cell and of the fuel efficiency. 
     DESCRIPTION OF SYMBOLS 
     
         
           21  Metal substrate 
           22  Conduit-defining part 
           21   a  Oxide coating 
           21   b  Application part 
           22  Conduit-defining part 
           22   s  Slurry 
         L Laser light 
         M 1  Method for manufacturing separator for fuel cell 
         S 4  Coating removal step (step to form application part) 
         S 5  Applying step (step to apply slurry) 
         S 6  Thermal treatment step (step to form conduit-defining part)