JP2017201608A - Method for manufacturing fuel cell separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To propose a method for manufacturing a fuel cell separator that has both high conductivity and high conductive durability.SOLUTION: A method for manufacturing a fuel cell separator that includes a gas passage includes: a step (301) of applying a carbon black, in parallel with a direction of the gas passage, on a base material that is made of pure titanium or a titanium alloy and has a carbon concentration of 10 atom% or less at a position of depth 10 nm from the outermost surface of the base material; and a step (302) of subjecting a base material on which the carbon black has been applied to heat treatment in a low oxygen partial pressure of 25 Pa or less.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a method for manufacturing a fuel cell separator.

  A fuel cell is a power generation system that directly converts energy released during an oxidation reaction into electrical energy by oxidizing fuel by an electrochemical process, and has a stack structure in which a plurality of single cells are stacked. Yes. A single cell has a structure in which both surfaces of a membrane-electrode assembly are sandwiched between a pair of separators in which grooves serving as flow paths for reactive gases (fuel gas and oxidizing gas) are formed. The membrane-electrode assembly has a structure in which a pair of electrodes made of a porous material is formed on both side surfaces of an electrolyte membrane for selectively transporting hydrogen ions. Since the separator also plays a role of flowing current generated in a single cell to the adjacent single cell, the separator material constituting the separator has a high conductivity (a voltage drop caused by an interface phenomenon between the electrode and the separator). Not only is it required to be low), but also its electrical conductivity is required to be maintained for a long time even in a high-temperature acidic atmosphere inside the fuel cell.

  Patent Document 1 describes a fuel cell separator developed to satisfy such requirements. The separator described in the document includes a metal base material having an oxide film on the surface, a conductive thin film formed on the surface of the metal base oxide film, and a metal base oxide film between the conductive film and the conductive thin film. And an intermediate layer for improving adhesion. By forming an oxide film of the metal substrate itself on the surface of the separator and further forming a conductive thin film on the surface of the oxide film, high conductivity is obtained, and there are pinholes in the conductive thin film. In addition, the dissolution of the metal substrate can be suppressed by the oxide film.

Japanese Patent No. 4147925

  However, since the conductive thin film and the intermediate layer described in the same document are formed by vapor phase growth, the adhesiveness at the interface between the conductive thin film and the intermediate layer may be weak. For this reason, when the separator for fuel cells described in the same document is used, there is a concern that the intermediate layer is peeled off from the conductive thin film, so that high conductivity cannot be maintained.

  This invention is made | formed in view of such a condition, and makes it a subject to propose the method of manufacturing the separator for fuel cells which has high electroconductivity and electroconductive durability.

  In order to solve the above-mentioned problems, a method for producing a separator for a fuel cell according to the present invention includes (i) a substrate made of pure titanium or a titanium alloy, and a carbon concentration at a depth of 10 nm from the outermost surface. A step of applying carbon black to a base material having an atomic ratio of 10 atomic% or less parallel to the direction of the gas flow path; and (ii) a base material coated with carbon black having a low oxygen partial pressure of 25 Pa or less. Heat-treating under oxygen partial pressure.

  By applying carbon black to a base material made of pure titanium or a titanium alloy having a carbon concentration of 10 atomic% or less at a depth of 10 nm from the outermost surface, the titanium atoms of the base material are converted into carbon during heat treatment. Easily diffuses out into black. Then, the base material coated with carbon black is heat-treated under a low oxygen partial pressure of 25 Pa or less so that part or all of the titanium atoms of the base material diffused outward into the carbon black is in an atmosphere. Reacts with a small amount of oxygen in it to become titanium oxide. Thus, a mixed layer in which titanium oxide formed by oxidizing part or all of titanium atoms diffused outward from the substrate and carbon black is formed on the surface of the heat-treated substrate. Carbon black and titanium oxide contained in this mixed layer are stable without being oxidized even in a high-temperature acidic atmosphere (for example, 80 ° C., pH 2) in the fuel cell. Carbon black in the mixed layer forms a conductive path, and titanium oxide exhibits corrosion resistance, so that both high conductivity and conductive durability can be realized.

  Further, since carbon black is fine particles mainly composed of carbon, when carbon black is applied to the surface of a substrate, a plurality of stripes are formed in parallel along the application direction. This streak is a level difference caused by a variation in the thickness of the coating film of carbon black, and the cut surface when the carbon black is cut along a plane perpendicular to the coating direction is uneven. The variation in the thickness of the cut surface when carbon black is cut in a plane parallel to the coating direction is less than the variation in the thickness of the cut surface when carbon black is cut in a plane perpendicular to the coating direction. By applying carbon black parallel to the direction of the gas flow path, variation in the film thickness of the carbon black in the direction parallel to the gas flow path is reduced, and water generated at the cathode by the electromotive reaction of the fuel cell is gas. It can be made easy to flow through the flow path.

  The method for producing a separator for a fuel cell according to the present invention comprises applying carbon black to a base material made of pure titanium or a titanium alloy having a carbon concentration of 10 atomic% or less at a depth of 10 nm from the outermost surface, Since the base material is heat-treated under a low oxygen partial pressure of 25 Pa or less, a fuel cell separator having high conductivity and conductivity durability can be produced. In addition, by applying carbon black to the base material in parallel to the direction of the gas flow path, variation in the thickness of the carbon black film in the direction parallel to the gas flow path is reduced, and is generated at the cathode by the electromotive reaction of the fuel cell. Water can easily flow through the gas flow path.

It is sectional drawing of the single cell in connection with this embodiment. It is a top view of the separator in connection with this embodiment. It is a flowchart which shows the flow of the manufacturing process of the separator in connection with this embodiment. It is a manufacturing-process cross section of the separator in connection with this embodiment. It is a manufacturing-process cross section of the separator in connection with this embodiment. It is explanatory drawing which shows the manufacturing process of the separator in connection with this embodiment. It is a manufacturing-process cross section of the separator in connection with this embodiment. It is a manufacturing-process cross section of the separator in connection with this embodiment. It is a manufacturing-process cross section of the separator in connection with this embodiment.

Hereinafter, embodiments of the present invention will be described with reference to FIGS. Here, the same code | symbol shall show the same element, and the overlapping description is abbreviate | omitted.
As shown in FIG. 1, the single cell 40 has a structure in which both surfaces of the membrane-electrode assembly 10 are sandwiched between a pair of separators 20 and 30. The membrane-electrode assembly 10 includes an electrolyte membrane 11 made of an ion exchange membrane and a pair of electrodes 12 and 13 sandwiching the electrolyte membrane 11. For example, the electrodes 12 and 13 are formed by binding a catalyst layer made of platinum to a diffusion layer made of a porous carbon material. A fuel gas flow path 21 for flowing fuel gas is formed on one surface of the separator 20, and a refrigerant flow path 22 for flowing refrigerant is formed on the other surface. Similarly, an oxidizing gas channel 31 for flowing an oxidizing gas is formed on one surface of the separator 30, and a refrigerant channel 32 for flowing a refrigerant is formed on the other surface. The fuel gas flow channel 21, the oxidant gas flow channel 31, and the refrigerant flow channels 22, 32 are straight flow channels in which repetition of unevenness extends in one direction, or serpentine flow channels having a folded portion in the middle. The electrode 12 serving as the anode faces the fuel gas flow path 21 of the separator 20, and the electrode 13 serving as the cathode faces the oxidizing gas flow path 31 of the separator 30. By supplying fuel gas to the fuel gas channel 21 and supplying oxidizing gas to the oxidizing gas channel 31, an electrochemical reaction occurs in the membrane-electrode assembly 10, and an electromotive force is obtained. In addition, due to this electrochemical reaction, water is generated in the electrode 13 which is a cathode, and the single cell 40 generates heat. And by making a refrigerant | coolant flow into the refrigerant flow paths 22 and 32, heat_generation | fever of the single cell 40 is reduced and the operating temperature of the single cell 40 is maintained in an appropriate range. In FIG. 1, the X direction indicates a direction parallel to the longitudinal direction of the oxidizing gas channel 31, the Y direction indicates a direction parallel to the width direction of the oxidizing gas channel 31, and the Z direction indicates the separator 30. The direction parallel to the thickness direction is shown. The same applies to other drawings.

  As shown in FIG. 2, the separator 30 includes an oxidizing gas inlet 33a, a refrigerant inlet 34a, a fuel gas inlet 35a, a fuel gas outlet 35b, a refrigerant outlet 34b, and an oxidizing gas outlet 33b in addition to the oxidizing gas channel 31 described above. Prepare. The oxidant gas introduced from the oxidant gas inlet 33a into the oxidant gas flow path 31 via the communication part 36a is discharged from the oxidant gas outlet 33b via the communication part 36b. The flow of the refrigerant is the same. The separator 20 is configured similarly to the separator 30.

  FIG. 3 is a flowchart showing the flow of the manufacturing process of the separator 30. As shown in the figure, the manufacturing process of the separator 30 includes a flow path forming process 301, a coating process 303, and a heat treatment process 304, and further includes a carbon concentration reduction processing process 302 as necessary. Hereinafter, each process is explained in full detail.

  As shown in FIG. 4, the flow channel forming step 301 is a step of forming the oxidizing gas flow channel 31 by press-molding the base material 37. As the substrate 37, a substrate made of pure titanium or a titanium alloy is used. As pure titanium, 1-4 types prescribed | regulated to JISH4600 can be mentioned, for example. Examples of the titanium alloy include Ti-Ai, Ti-Nb, Ti-Ta, Ti-6Al-4V, and Ti-Pd. These pure titanium or titanium alloys are light and excellent in corrosion resistance. The base material 37 is preferably a cold plate material of about 0.05 to 1 mm, for example. When the thickness of the base material 37 is set to about 0.05 to 1 mm, there is an advantage that press molding of the oxidizing gas flow path 31 is facilitated while satisfying the requirements for weight reduction and thickness reduction of the separator. The substrate 37 may be in the form of a long band wound in a coil shape, or may be in the form of a sheet cut to a predetermined size. In the flow path forming step 301, the base material 37 made of pure titanium or titanium alloy is rolled and fired, press-molded to form the oxidizing gas flow path 31, and the oil adhering to the surface of the base material 37 is removed by a degreasing process. To do. In this step, the refrigerant channel 32 and various inlets / outlets (oxidizing gas inlet 33a, refrigerant inlet 34a, fuel gas inlet 35a, fuel gas outlet 35b, refrigerant outlet 34b, and oxidizing gas outlet 33b) are formed by press molding of the oxidizing gas channel 31. At the same time, the substrate 37 may be press-molded.

  As shown in FIG. 5, the coating process 303 is performed on the surface of the substrate 37 having a carbon concentration of 10 atomic% or less at a depth of 10 nm from the outermost surface, in parallel with the direction of the oxidizing gas flow path 31. This is a step of applying black 50. In addition, when the carbon concentration at the position of 10 nm depth from the outermost surface of the substrate 37 is 10 atomic% or less, the average carbon concentration between 5 to 50 nm from the outermost surface of the substrate 37 is also 10 atomic% or less. It has been confirmed by experiments of the present inventors. The carbon concentration at a position at a depth of 10 nm from the outermost surface of the substrate 37 can be measured, for example, by analyzing the composition in the depth direction of the substrate 37 using an X-ray photoelectron spectrometer. On the surface of the substrate 37, carbon resulting from adsorption of organic substances or the like present in the atmosphere may be present. In the present invention, a portion excluding the surface layer portion (contamination layer) of the base material 37 resulting from the adsorption of such an organic substance is defined as the “outermost surface”, and the carbon concentration at a position 10 nm deep from the outermost surface is 10 atoms. % Or less. The fact that the carbon concentration at this position exceeds 10 atomic% means that the processing oil or organic substances present in the atmosphere enter the surface layer of the base material 37 during the rolling process for manufacturing the base material 37 and contaminate it. Or they are likely to react with titanium constituting the substrate 37 to form titanium carbide. If the surface layer of the base material 37 is contaminated with processing oil or organic matter present in the atmosphere, or if they react with the titanium constituting the base material 37 to form titanium carbide, When the base material 37 is heat-treated in a heat treatment step 304 described later, it becomes difficult for the carbon black 50 to be bonded to the surface of the base material 37, and it becomes difficult to obtain high conductivity and conductive durability. Therefore, when the carbon concentration at a position 10 nm deep from the outermost surface of the substrate 37 exceeds 10 atomic%, the carbon concentration reduction treatment step is performed after performing the flow path forming step 301 and before performing the coating step 303. Preferably 302 is performed. On the other hand, when the carbon concentration at a position of 10 nm depth from the outermost surface of the substrate 37 is 10 atomic% or less, the application is performed without performing the carbon concentration reduction treatment step 302 after performing the flow path forming step 301. Step 303 can be performed.

  In addition, the carbon concentration at the position of 10 nm depth from the outermost surface of the base material 37 can be kept low by adjusting the rolling process such as performing the rolling reduction per pass of cold rolling at 10% or less. The carbon concentration at that position can be 10 atomic% or less. The carbon concentration at a position 10 nm deep from the outermost surface of the substrate 37 is preferably as low as possible. For example, it is preferably 9 atomic% or less, and more preferably 8 atomic% or less.

  The carbon black 50 is applied to the surface of the substrate 37 by applying an aqueous or oily liquid (dispersion) in which the carbon black 50 powder is dispersed, or by directly applying the carbon black 50 powder. It can be carried out. The aqueous or oily liquid in which the powder of carbon black 50 is dispersed may contain a binder, a surfactant, and the like. However, since the binder, the surfactant, and the like tend to lower the conductivity, A smaller surfactant content is preferred. For example, water can be used as the solvent of the aqueous liquid in which the carbon black 50 powder is dispersed. As the solvent of the oily liquid in which the carbon black 50 powder is dispersed, for example, ethanol, toluene, cyclohexanone, or the like can be used.

The particle size of the carbon black 50 is preferably 20 to 200 nm. Since the carbon black 50 powder tends to form aggregates, it is preferable to adjust the dispersion so that aggregates are not formed. For example, it is preferable to enhance the dispersibility by chemically bonding a functional group such as a carboxyl group to the surface of the carbon black 50 so as to increase the repulsive force between the carbon blacks 50. If the coating amount of the carbon black 50 on the surface of the base material 37 is, for example, 1 μg / cm ² or more, high conductivity and conductive durability can be obtained, and if it is ² μg / cm ² or more, it is more stable. The conductive durability can be obtained. However, when the coating amount of the carbon black 50 exceeds, for example, about 50 μg / cm ^2, the function of improving conductivity and conductivity durability is saturated, and therefore, the upper limit is preferably about 50 μg / cm ² .

  As a method of applying the dispersion liquid in which the carbon black 50 powder is dispersed to the base material 37, for example, a brush coating, a bar coater, a roll coater, a gravure coater, a die coater, a dip coater, a spray coater, or the like can be used. . As a method for applying the carbon black 50 to the substrate 37 in powder form, for example, electrostatic coating can be used.

  FIG. 6 shows a process of applying the carbon black 50 to the substrate 37 using the die coater 80. The application direction of the carbon black 50 is parallel to the direction (X direction) of the oxidizing gas channel 31. When the carbon black 50 is applied to the surface of the substrate 37, a plurality of stripes 51 are formed in parallel along the application direction. The streaks 51 are steps caused by variations in the thickness of the coating film of the carbon black 50. When the coating film of the carbon black 50 is cut along a plane (YZ plane) perpendicular to the coating direction (X direction). The cut surface is undulated in an uneven shape. As shown in FIG. 7, the variation Δh1 in the film thickness of the cut surface when the coating film of the carbon black 50 is cut on a plane (XZ plane) parallel to the coating direction (X direction) is shown in FIG. As shown in the drawing, since the coating film of the carbon black 50 is cut by a plane (YZ plane) perpendicular to the coating direction (X direction), it is smaller than the variation Δh2 in the thickness of the cut surface. By coating the carbon black 50 on the substrate 37 in parallel with the direction of the carbon black 50, variation in the film thickness of the carbon black 50 in the direction parallel to the oxidizing gas flow path 31 is reduced, and the cathode is formed by the electromotive reaction of the fuel cell. The generated water can easily flow through the oxidizing gas channel 31. Here, the film thickness variation of the carbon black 50 means a difference between the maximum film thickness and the minimum film thickness, and the film thickness variation is preferably 50 nm or less. For convenience of explanation, the illustration of the carbon black 50 particles is omitted in FIGS. 7 and 8.

As shown in FIG. 9, the heat treatment step 304 is a step of heat-treating the base material 37 coated with the carbon black 50 under a low oxygen partial pressure where the oxygen partial pressure is 25 Pa or less. On the surface of the heat-treated base material 37, a mixed layer 70 in which titanium oxide 60 in which some or all of the titanium atoms diffused outward from the base material 37 are oxidized and the carbon black 50 is mixed is formed. The mixed layer 70 can impart high conductivity and conductive durability to the separator 30. If the oxygen partial pressure in the heat treatment step 304 exceeds 25 Pa, carbon dioxide may be formed (combusted) by the reaction between carbon and oxygen. That is, the oxidative decomposition of the carbon black 50 occurs, and the titanium on the surface of the substrate 37 is oxidized at the portion where the surface of the substrate 37 is exposed, so that the titanium oxide 60 is excessively generated. Furthermore, since the mixed layer 70 in which the titanium oxide 60 in which part or all of the titanium atoms diffused outward from the substrate 37 are oxidized and the carbon black 50 is not formed is formed, high conductivity and conductivity durability can be obtained. Can not. Therefore, in the heat treatment step 304, the oxygen partial pressure is adjusted to 25 Pa or less under reduced pressure or by using an inert gas (such as Ar gas or nitrogen gas) or a mixed gas of inert gas and oxygen gas. Is preferred. Here, the oxygen partial pressure is preferably in the range of 1.3 × 10 ^{−3 to} 21 Pa, for example, and the temperature of the heat treatment is preferably in the temperature range of 300 to 800 ° C., for example. When the oxygen partial pressure and the heat treatment temperature are within the above ranges, part or all of the titanium atoms diffused outward from the substrate 37 react with a trace amount of oxygen in the atmosphere to form titanium oxide 60, which is oxidized. Formation of the mixed layer 70 in which the titanium 60 and the carbon black 50 are mixed can be promoted.

  On the other hand, when the base material 37 is heat-treated in an atmosphere having a very low oxygen partial pressure, a reaction in which titanium and carbon black 50 are combined to become TiC becomes dominant. Although this TiC has low contact resistance, it is not preferable in a high temperature acidic atmosphere (for example, 80 ° C., pH 2) in the fuel cell because oxidation proceeds and resistance increases. On the other hand, the titanium oxide 60 and the carbon black 50 included in the mixed layer 70 have an advantage that the oxidation does not proceed and is stable even in a high-temperature acidic atmosphere (for example, 80 ° C., pH 2) in the fuel cell. That is, in order to form the mixed layer 70 formed by mixing the titanium oxide 60 and the carbon black 50 on the base material 37, it is necessary that appropriate oxygen exists in the heat treatment atmosphere, and the upper limit of the oxygen partial pressure is For example, it can be 25 Pa, and the lower limit of the oxygen partial pressure can be 0.05 Pa, for example. Moreover, the upper limit of heat processing temperature can be 750 degreeC, for example, and the minimum of heat processing temperature can be 500 degreeC, for example.

  If the carbon concentration at a depth of 10 nm from the outermost surface of the base material 37 exceeds 10 atomic%, the outer side of the titanium atoms from the base material 37 to the carbon black 50 can be obtained even if the heat treatment step 304 is performed. Diffusion is inhibited and the mixed layer 70 is difficult to be formed. In such a case, the carbon concentration reduction treatment step 302 is preferably performed. The carbon concentration reduction treatment step 302 is a step of treating the surface of the substrate 37 before the coating step 303 so that the carbon concentration at a position 10 nm deep from the outermost surface is 10 atomic% or less. In other words, the carbon concentration reduction treatment step 302 is a step of removing a pollutant due to an organic substance or the like on the outermost surface of the substrate 37 and a region where titanium carbide is formed to form a natural oxide film. In the carbon concentration reduction treatment step 302, for example, the substrate 37 is preferably pickled in an acidic aqueous solution containing hydrofluoric acid. As the acidic aqueous solution containing hydrofluoric acid, for example, nitric acid, sulfuric acid, hydrogen peroxide, or the like can be used alone or in combination.

  In the above description, in the process of manufacturing the separator 30, the example in which the carbon black 50 is applied in parallel to the oxidizing gas flow path 31 is shown. However, the carbon black 50 is parallel to the refrigerant flow path 32 on the back side of the oxidizing gas flow path 31. Carbon black 50 may be applied to the surface. Similarly, in the process of manufacturing the separator 20, the carbon black 50 may be applied in parallel to the fuel gas channel 21, and further, the carbon black 50 is parallel to the refrigerant channel 22 on the back side of the fuel gas channel 21. May be applied.

  The embodiments described above are for facilitating the understanding of the present invention, and are not intended to limit the present invention. The present invention can be changed / improved without departing from the spirit thereof, and the present invention includes equivalents thereof. In other words, those in which the person skilled in the art appropriately changes the design of the embodiments are also included in the scope of the present invention as long as they have the features of the present invention. For example, each element included in the embodiment and its arrangement, material, condition, shape, size, and the like are not limited to those illustrated, and can be appropriately changed. Further, the positional relationship such as up, down, left and right is not limited to the illustrated ratio unless otherwise specified. Moreover, each element with which an embodiment is provided can be combined as long as it is technically possible, and the combination thereof is also included in the scope of the present invention as long as it includes the features of the present invention.

DESCRIPTION OF SYMBOLS 10 ... Membrane-electrode assembly 11 ... Electrolyte membrane 12, 13 ... Electrode 20, 30 ... Separator 21 ... Fuel gas flow path 31 ... Oxidation gas flow path 22, 32 ... Refrigerant flow path 37 ... Base material 40 ... Single cell 50 ... Carbon Black 51 ... Stripes 60 ... Titanium oxide 70 ... Mixed layer 80 ... Die coater

Claims (1)

A method for producing a fuel cell separator having a gas flow path,
Carbon black is applied to a base material made of pure titanium or a titanium alloy and having a carbon concentration of 10 atomic% or less at a depth of 10 nm from the outermost surface in parallel to the direction of the gas flow path. Applying step;
Heat-treating the substrate coated with carbon black under a low oxygen partial pressure with an oxygen partial pressure of 25 Pa or less;
The manufacturing method of the separator for fuel cells provided with.

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