Separator for a fuel cell and method for producing the same

Abstract

Separator for a fuel cell, the separator comprising: an upper metal plate (11) having opposite main sides coated on both sides with a first and a second composite material layer (13, 16), wherein carbon nanotubes (18) are grown on both sides of the upper metal plate (11) where the composite material layers (13, 16) are applied; and a lower metal plate (12) having opposite main sides stacked on the underside of the upper metal plate (11) and a third and a fourth composite material layer (17, 15) applied on both sides, wherein carbon nanotubes (19) are grown on both sides of the lower metal plate (12) where the composite material layers (17, 15) are applied, wherein the second and third composite material layers (16, 17) between the upper metal plate (11) and the lower metal plate (12) are integrally bonded to form a single composite material intermediate layer (14) before solidification, and the carbon nanotubes (18) grown on the upper metal plate (11) on the side of the second composite material layer (16) and the carbon nanotubes (19) grown on the lower metal plates (12) on the side of the third composite material layer (17) are directly bonded to each other.

Description

TECHNICAL FIELDThe present disclosure relates to a separator for a fuel cell and a method for manufacturing the same, and more particularly to the separator for a fuel cell and a method for manufacturing the same, which includes two metal plates and reduces contact resistance between an upper metal plate and a lower metal plate to a minimum.BACKGROUNDA fuel cell normally converts chemical energy into electrical energy by a redox reaction of hydrogen and oxygen. A unit cell of the fuel cell has a low output voltage. Therefore, generally, several to hundreds of cell units are stacked into a fuel cell stack. A separator provides electrical communication between the cell units while the cell units are stacked, and functions as a flow field that separates the reaction gases and flows through the coolant.In a typical separator, the volume and weight of the fuel cell stack can be reduced by reducing the thickness during the manufacturing process, such as by punching, etc., which allows mass production.A metal separator has advantages such as high electric conductivity, excellent mechanical properties, and good workability. However, a typical metal separator easily corrodes in the hot and wet environment of the fuel cell.In the prior art, a technique for coating a polymer substrate having a conductive additive on the surface of the separator has been developed to solve the problem.The separator for a polymer electrolyte fuel cell is formed with a channel through which coolant flows to remove heat generated during operation.As shown in FIG. 8, the conventional separator has two metal plates 1 and 2 as base materials for the coolant passage. The contact resistance is formed on a surface between the two metal plates.A polymer composite material coated on the two metal plates forms a contact surface between a composite material coating 3 and a composite material coating 4 and between each composite material coating and the metal plate, thereby producing a high contact resistance that degrades the fuel cell efficiency.From US 2008 / 0 113 253 A1 a separator for a fuel cell is known, comprising: an upper metal plate with opposite main sides coated on both sides with a first and a second composite material layer; and a lower metal plate with opposite main sides stacked on the underside of the upper metal plate and including a third and a fourth composite material layer applied on both sides, wherein the second and third composite material layers are integrally joined between the upper metal plate and the lower metal plate to form a single composite material intermediate layer before solidification.DE 10 2007 052 832 A1 describes a method comprising providing a bipolar fuel cell plate which contains exposed carbon on an outer surface of the bipolar plate, and reacting a diazonium salt with the exposed carbon, so that a functional group is attached to the exposed carbon, in order to increase the hydrophilicity of the bipolar plate where the functional group is attached.Furthermore, US 2012 / 0 219 881 A1 also discloses a bipolar plate for fuel cells having a flow plate which has a first surface for introducing hydrogen fuel gas and water vapor and a second surface for introducing an oxygen-containing gas, at least some of the first and / or second surface comprising a coating of nanostructured carbon material (NCM) deposited thereon, the coating having a thickness of 1 nm to 5 μm.The above information of this Background section is only for enhancement of understanding of the background of the invention and therefore may contain information that does not form part of the prior art already known to those of ordinary skill in the art in this country.OVERVIEW OF THE DISCLOSUREIt is an object of the present disclosure to provide a separator for a fuel cell and a method for manufacturing the same, wherein carbon nanotubes are grown on both sides of an upper metal plate. Composite material layers are formed by coating a polymer composite material thereon, and the composite material layers between the upper and lower metal plates are integrally bonded to each other before the polymer composite material is dried, so that the contact resistance between the two metal plates is minimized.The object is achieved by a separating element for a fuel cell having the features of claim 1 and a method for producing a separating element for a fuel cell having the features of claim 5.According to an aspect of the present disclosure, a separator for a fuel cell includes an upper metal plate having opposite main sides including first and second composite material layers deposited on both sides, the carbon nanotubes being grown on both sides of the upper metal plate where the composite material layers are deposited, and a lower metal plate having opposite main sides stacked on the lower surface of the upper metal plate, the carbon nanotubes being grown on both sides of the lower metal plate where the composite material layers are deposited and including third and fourth composite material layers deposited on both sides. Here, the second and third composite material layers are integrally joined before solidification to form a single intermediate composite material layer, and the carbon nanotubes grown on the upper metal plate on the second composite material layer side and the carbon nanotubes grown on the lower metal plates on the third composite material layer side are directly joined to each other.According to another aspect of the present disclosure, a method of manufacturing a separator for a fuel cell includes providing an upper metal plate having main opposite sides and a lower metal plate having main opposite sides. Carbon nanotubes are grown on both sides of the upper metal plate and the lower metal plate, respectively, and after the growth, first and second composite material layers are formed on both sides of the upper plate and third and fourth composite material layers are formed on both sides of the lower plate by applying a coating liquid containing a polymer composite material to both sides of the upper and lower metal plates, respectively. A single composite material intermediate layer is formed by stacking the upper metal plate on the lower metal plate, drying the respective composite material layers, and integrally bonding the second composite material layer to the third composite material layer, wherein the carbon nanotubes grown on the upper metal plate on the second composite material layer side and the carbon nanotubes grown on the lower metal plates on the third composite material layer side are directly bonded to each other.BRIEF DESCRIPTION OF THE DRAWINGSThe above and other features of the present invention will be described in detail below, thus not by way of limitation, with reference to specific embodiments which are illustrated in the accompanying drawings only by way of example. FIG. 1 is a schematic view of a cross section of the structure of a separator for a fuel cell according to the present disclosure. FIG. 2 is a partially enlarged view of a separator for a fuel cell according to the present disclosure. FIGS. 3 and 4 are schematic flowcharts of a method for manufacturing a fuel cell according to the present disclosure. FIG. 5 shows a comparative graph of contact resistance between an upper metal plate and a lower metal plate for a separator according to the present disclosure and a separator according to the prior art. FIG. 6 shows the measurement results of contact resistance generated between upper and lower metal plates for various separator examples. FIG. 7 shows the results of an adhesion test with a second and a fourth separator example.. . FIG. 8 is a schematic view of a cross section of the structure of a conventional separator for a fuel cell.It is to be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of the various preferred features illustrative of the principles of the invention. The specific design features of the present invention disclosed herein, including, for example, particular dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and the environment of the field.In the figures, identical reference numerals designate the same or equivalent parts of the present invention throughout the figures of the drawing.DETAILED DESCRIPTIONVarious embodiments of the present invention will be explained in detail below, examples of which are illustrated in the accompanying drawings and described below. Although the invention will be described in conjunction with exemplary embodiments, it is understood that the present description is not intended to limit the invention to these exemplary embodiments. On the contrary, the invention is intended to cover not only the embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined in the appended claims.As shown in FIG. 1, a separator for a fuel cell includes an upper metal plate 11 and a lower metal plate 12. An upper composite material layer 13 is disposed on the upper surface of the upper metal plate 11, a lower composite material layer 15 on the lower surface of the lower metal plate 12, and a composite material intermediate layer 14 between the upper metal plate 11 and the lower metal plate 12.As can be seen from FIG. 4, the first composite material layer 13 and the second composite material layer 16 are respectively disposed on both sides of the upper metal plate 11 where carbon nanotubes are grown.A third composite material layer 17 and a fourth composite material layer 15 are respectively deposited on both sides of the lower metal plate 12 where carbon nanotubes are also grown.The composite material layers 13, 15, 16, and 17 are formed by a coating liquid containing a polymer matrix and a conductive filler, and the upper metal plate 11 is stacked on the lower metal plate 12 before the separator is subjected to a drying process.That is, a coating liquid containing a polymer composite material is applied to the surfaces of the upper and lower metal plates 11 and 12, and then the upper metal plate 11 is stacked on the lower metal plate 12 before drying.The second composite material layer 16 is disposed on the lower surface of the upper metal plate 11 and the third composite material layer 17 is disposed on the upper surface of the lower metal plate 12. Prior to the consolidation of the composite material layers, they are integrally joined into a single composite material layer so as to form a single intermediate composite material layer 14 between the two metal plates.As illustrated in FIG. 2, the carbon nanotubes 18 and 19 grown on the lower surface of the upper metal plate 11 and the upper surface of the lower metal plate 12, respectively, are joined to each other in the composite material intermediate layer 14.Each of the nanotubes grown on the upper and lower metal plates 11 and 12 has a height of 5 to 10 μm, and the composite material layers 13, 15, 16, and 17 on the upper and lower metal plates 11 and 12 have thicknesses of 10 to 20 μm.Thus, the thickness of the upper and lower composite material layers 13 and 15 is 10 to 20 μm, respectively, and the thickness of the intermediate composite material layer 14 is 20 to 40 μm.When the height of the carbon nanotube is below 5 μm, the electrical conductivity decreases. On the other hand, when the height of the carbon nanotube is greater than 10 μm, undesirable defects occur on the coating surface.In addition, the composite material layers 13, 15, 16, and 17 having a thickness below 10 μm separate the composite material layers during assembly of a fuel cell stack, and at over 20 μm, the total resistance increases undesirably.In the separator for the fuel cell described above, a single intermediate composite material layer is interposed between the upper metal plate 11 and the lower metal plate 12, so that the contact surface between the upper and lower metal plates 11 and 12 is omitted, and it becomes possible to eliminate the contact resistance between the two metal plates. In addition, the carbon nanotubes 18 and 19 grown on the upper and lower metal plates 11 and 12 are directly connected to each other, thereby reducing the contact resistance between the upper and lower metal plates 11 and 12 and the composite material intermediate layer 14, and reducing the contact resistance generated between the two metal plates to a minimum.A method for producing a separator element for a fuel cell is schematically illustrated in FIGS. 3 and 4.As shown in FIG. 3, a method of manufacturing a separator for a fuel cell includes the steps of: providing an upper metal plate 11 having opposite main sides and a lower metal plate 12 having opposite main sides; growing nanotubes on both sides of the upper metal plate 11 and the lower metal plate 12, respectively; forming composite material layers 13, 15, 16, and 17 by applying a coating liquid containing a polymer composite material to both sides of the upper metal plate 11 and the lower metal plate 12, respectively, where the carbon nanotubes have been grown; and integrally bonding a second composite material layer 16 of the upper metal plate 11 and a third composite material layer 17 of the lower metal plate 12 by stacking the upper metal plate 11 on the lower metal plate 12, and then subjecting the separator to drying and annealing processes.For the process of growing carbon nanotubes on the surfaces of the upper and lower metal plates 11 and 12, a well-known technique is applied.The first, second, third and fourth composite material layers 13, 15, 16 and 17 are formed by applying a polymer composite material to both sides of the upper and lower metal plates 11 and 12, respectively. The coating liquid applied to both sides of the upper metal plate 11 forms the first and second composite material layers 13 and 16, and the coating liquid applied to both sides of the lower metal plate 12 forms the third and fourth composite material layers 17 and 15.As illustrated in FIG. 4, the lower metal plate 12 is stacked on the lower surface of the upper metal plate 11 before drying the composite material layers 13, 15, 16, and 17 after the composite material layers 13, 15, 16, and 17 are formed on both sides of the upper and lower metal plates 11 and 12.The second composite material layer 16 and the third composite material layer 17 that have not been dried are integrally joined to form the single intermediate composite material layer 14 between the upper metal plate 11 and the lower metal plate 12, so that the contact surface between the upper metal plate 11 and the lower metal plate 12 is omitted.After the intermediate composite material layer 14 is formed between the upper metal plate 11 and the lower metal plate 12, drying and annealing processes are performed to complete the manufacturing process.The polymer composite material contained in the coating liquid includes a polymer matrix and a conductive filler. A conductive polymer as a conductive filler provides adhesion between the upper and lower metal plates 11 and 12 and the respective composite material layers 13, 15, 16, and 17.Polyamideimide (PAI), a thermoplastic polymer having excellent adhesion force with the metal plates as base materials and the carbon nanotubes having excellent moldability, is preferable as a conductive filler.PAI is an aromatic polymer having an amide group and an imide group as repeating units in the polymer chain, and is used where excellent coating stability and processability are required.During the manufacture of the separator, PAI exhibits strong interaction and electron transfer with metal in the annealing process, so that strong adhesion with metal is produced as compared to other polymers.FIG. 5 shows a comparison of contact resistance between an upper metal plate and a lower metal plate for a separator according to the present disclosure and a separator according to the prior art.The separator of the related art is manufactured by joining an upper metal plate and a lower metal plate, and both main sides are coated with a composite material layer, thereby forming a contact surface therebetween.As illustrated in FIG. 5, the contact resistance of the separator according to the present disclosure is significantly lower compared to the separator according to the related art.The method for manufacturing the separator according to the present disclosure will now be described in more detail.Two austenite-based stainless steel (SUS304) substrate layers are provided, and each side of these substrates is then polished with sandpaper and washed with acetone.Each substrate is then acid treated with a 5 percent HF solution for 10 minutes and etched to expose metal catalysts such as Fe, Cr, etc. on the surface of the substrates.The resulting substrates are placed in a heating furnace at 750° C. under a nitrogen atmosphere, into which hydrogen gas and acetylene gas are introduced at a flow rate of 100 sccm and 40 sccm, respectively, to grow carbon nanotubes (CNTs) from the metal catalysts exposed on the surface of the substrate for 1 to 10 minutes. At this time, the growth length of the CNTs can be controlled by controlling the amount of acetylene introduced into the heating furnace.Subsequently, a coating liquid is prepared by adding a polymer composite material containing carbon black and PAI powder to an N-methylpyrrolidone (NMP) solvent, and then applying the coating liquid to the surface of the substrate on which the carbon nanotubes are grown by painting.The two substrate layers are stacked before the coating liquid is dried, so that the coatings of the upper and lower substrates are integrally bonded to form a single coating between the two substrates. Here, the carbon nanotubes grown on the upper and lower substrates are in direct contact with each other.Thereafter, the resulting substrate with the coating is dried at room temperature for 24 hours, followed by annealing at 150°C for two hours.The PAI contained in the coating liquid is a conductive filler and acts as a binder through which the carbon black can bond well to the substrate surface.In addition to the PAI contained in the coating liquid, a conductive polymer dissolved in a solvent may be used as a conductive filler, and a carbon-based material that may be added to the solvent may be selected as a polymer matrix. The materials of the conductive filler material and the polymer matrix may improve the performance of the separator.For comparing the contact resistances of the conductive filler type separators contained in the composite material layer, polyvinyl ester (PVE) was used as the conductive filler of the composite material layer for the first and second separator examples, polyamideimide (PAI) for the third and fourth separator examples, and polypropylene (PP) for the fifth and sixth separator examples.30 Carbon black by weight was used as the polymer matrix of the composite material layer for preparing the first, third and fifth separator examples, and carbon black by weight for preparing the second, fourth and sixth separator examples. Separator Examples were prepared under identical conditions except for the carbon black content.FIG. 6 shows the results of the contact resistance measured between the upper metal plate and the lower metal plate in the separator examples prepared in the above manner.The first, third, and fifth separator examples had contact resistance values of 12, 25, and 30 mΩcm 2 respectively, and the second, fourth, and sixth separator examples had contact resistance values of 10, 21, and 26 mΩcm 2. respectively.That is, when PVE and PAI were used, the contact resistance values of the separators were relatively lower than those of PP, and the lowest contact resistance occurred when PVE was used.FIG. 7 shows the result of the adhesion test (ASTM D 3359) for the composite material layers of the second and fourth separator examples of conductive filler type.The fourth separator example was prepared by applying the PAI-containing coating liquid to the surface of a stainless steel substrate on which carbon nanotubes were grown, and the second separator example was prepared by applying the PVE-containing coating liquid to the surface of a stainless steel substrate on which nanotubes were grown.As shown in Fig. 7, the adhesion of the fourth separator example to the base materials was excellent because the coating on the surface was not peeled from the surface after the application and removal of an adhesive tape. In the second separator, on the other hand, the coating was completely peeled from the base materials together with the carbon nanotubes grown on the surface of the base materials. Thus, the PAI has better adhesion to the base materials than the PVE.Further, the second separator example with PVE had the lowest contact resistance value but very poor adhesion compared to the fourth separator example using PAI.As a result, PVE is not suitable as a conductive filler of the composite material layer. PAI, on the other hand, offers competitive contact resistance compared to other conductive polymers based on its excellent adhesion.As described above, by forming the single composite material intermediate layer between the upper metal plate and the lower metal plate, the contact surface between the upper metal plate and the lower metal plate can be omitted, so that the contact resistance is also omitted.In addition, forming the composite material layers on the carbon nanotubes grown on the upper metal plate and the lower metal plate can decrease the contact resistance between the respective metal plates and the composite material layers, thereby reducing the contact resistance between the upper metal plate and the lower metal plate to a minimum, improving the efficiency of the fuel cell.

Claims

A separator for a fuel cell, the separator comprising: an upper metal plate (11) having opposite main sides coated on both sides with first and second composite material layers (13, 16), carbon nanotubes (18) being grown on both sides of the upper metal plate (11) where the composite material layers (13, 16) are deposited; and a lower metal plate (12) having opposite main sides stacked on the lower surface of the upper metal plate (11), and third and fourth composite material layers (17, 15) deposited on both sides, carbon nanotubes (19) being grown on both sides of the lower metal plate (12) where the composite material layers (17, 15) are deposited, the second and third composite material layers (16, 17) being integrally joined between the upper metal plate (11) and the lower metal plate (12) to form a single intermediate composite material layer (14) before solidification, and the carbon nanotubes (18) grown on the upper metal plate (11) on the side of the second composite material layer (16) and the carbon nanotubes (19) grown on the lower metal plates (12) on the side of the third composite material layer (17) being directly joined to each other.The separator of claim 1, wherein the carbon nanotubes (18) of the top metal plate (11) are grown to a height of 5 to 10 μm.The separator of claim 1, wherein the carbon nanotubes (19) of the lower metal plate (12) are grown to a height of 5 to 10 μm.The separator of claim 1, wherein the first, second, third and fourth composite material layers (13, 16, 17, 15) are coated to a thickness of 10 to 20 μm.A method of manufacturing a separator for a fuel cell, the method comprising the steps of: providing an upper metal plate (11) and a lower metal plate (12), each plate (11, 12) having opposite main sides; growing carbon nanotubes (18, 19) on both sides of the upper metal plate (11) and the lower metal plate (12), respectively; after growing the carbon nanotubes (18, 19), forming first and second composite material layers (13, 16) on both sides of the upper metal plate (11) and third and fourth composite material layers (17, 15) on both sides of the lower metal plate (12) by applying a coating liquid containing a polymer composite material to both sides of the upper and lower metal plates (11, 12), respectively; forming a single composite material intermediate layer (14) by stacking the upper metal plate (11) on the lower metal plate (12) before drying the respective composite material layers (13, 16, 17, 15) and integrally bonding the second composite material layer (16) to the third composite material layer (17) such that the carbon nanotubes (18) grown on the upper metal plate (11) on the second composite material layer (16) side and the carbon nanotubes (19) grown on the lower metal plates (12) on the third composite material layer (17) side are directly bonded to each other.The method of claim 5, wherein the carbon nanotubes (18, 19) are grown to a height of 5 to 10 μm.The method of claim 5, wherein the first, second, third and fourth composite material layers (13, 16, 17, 15) are coated to a thickness of 10 to 20 μm.The method of claim 5, wherein the coating liquid comprises polyamideimide (PAI) as a conductive filler.

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