JP2005310705A - Fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell, having proper power generation properties, even when the hydrogen concentration in a hydrogen containing gas supplied to an anode is low. <P>SOLUTION: The fuel cell has a separator, of which an anode gas flow path has a width of not less than 1.2 times and not more than 2.0 times that of the cathode gas flow path. The fuel cell has a separator, of which the anode gas flow path has a width of not less than 0.5 times and not more than 0.8 times that of the cathode gas flow path. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fuel cell for generating electricity by electrochemically reacting hydrogen and oxygen.

  The basic structure of a fuel cell is, for example, a MEA (Membrane Electrode Assembly) in which gas diffusion layers having electrode functions are formed on both sides of a solid electrolyte membrane, and a flow path through which gas or the like flows is provided. The structure is sandwiched between the separators.

  Various improvements have been made to the structure of the fuel cell. For example, regarding a flow path formed in a separator, Patent Document 1 discloses that a separator provided with a parallel flow path group including a plurality of parallel grooves is adjacent to a width between the grooves in the parallel flow path group. A fuel cell having a large ridge width between parallel flow path groups is disclosed.

  By the way, pure hydrogen or a hydrogen-containing gas obtained by steam reforming hydrocarbons such as methane is supplied to the anode of the fuel cell, and air is usually supplied to the cathode to generate electric power. A higher hydrogen concentration in the gas supplied to the anode is preferable from the viewpoint of battery performance, and is usually a relatively high concentration of about 70 mol% or more.

In the present specification, mol% indicating a gas composition means mol% of a dry base.
JP 2001-76746 A

  The hydrogen concentration of a hydrogen-containing gas obtained from biomass or a hydrogen-containing gas obtained by partial oxidation reforming of hydrocarbons is relatively low, and may be, for example, about 40 mol% or less. Thus, when power generation is performed with a fuel cell using a gas having a relatively low hydrogen concentration, the reaction efficiency is lowered and it is difficult to obtain good power generation characteristics. Therefore, it is difficult to use such a low hydrogen concentration gas in a fuel cell.

  An object of the present invention is to provide a fuel cell capable of obtaining relatively good power generation characteristics even when the hydrogen concentration of the hydrogen-containing gas supplied to the anode is as low as about 20 mol% to 40 mol%, for example. It is to be.

  In order to achieve the above object, the present inventors have examined each component of the fuel cell, and in particular, by improving the shape of the gas flow path provided in the separator, even when using a fuel gas with a low hydrogen concentration, The inventors have found that it is possible to obtain fuel cell characteristics close to the case where hydrogen concentration fuel gas is used, and have completed the present invention.

  According to the present invention, there is provided a fuel cell having a separator in which the width of the anode gas channel is 1.2 to 2.0 times the width of the cathode gas channel.

  According to the present invention, there is provided a fuel cell having a separator in which the depth of the anode gas channel is 0.5 to 0.8 times the depth of the cathode gas channel.

  In the above fuel cell, it is preferable that the gas diffusion layer is buried in the depth direction of the gas flow path by 10% or more and 50% or less of the initial thickness of the gas diffusion layer on at least one of the anode side and the cathode side.

  In the fuel cell, in at least one of the anode gas channel and the cathode gas channel provided in the separator, the length of the gas channel in the vertically upward direction is 25% or less with respect to the total gas channel length. It is preferable.

  ADVANTAGE OF THE INVENTION According to this invention, even if it is a case where the hydrogen concentration of the hydrogen containing gas supplied to an anode is as low as 20 to 40 mol%, the fuel cell which can acquire a comparatively favorable electric power generation characteristic is provided. .

  In the present invention, the fuel cell may be any type of fuel cell in which hydrogen is the reactant of the electrode reaction at the anode (fuel electrode). For example, a fuel cell of a solid polymer type, a phosphoric acid type, a molten carbonate type, or a solid oxide type can be obtained. Hereinafter, a polymer electrolyte fuel cell (PEFC) will be described as an example.

  The fuel cell has an anode side gas diffusion layer and a cathode side gas diffusion layer each having an electrode function, and a solid polymer electrolyte membrane sandwiched therebetween. A structure in which an electrolyte membrane is integrated between an anode side gas diffusion layer and a cathode side gas diffusion layer is called MEA (Membrane Electrode Assembly). The MEA is sandwiched between separators each provided with an anode gas flow path on one side and a cathode gas flow path on the other side, and one cell is constituted by a combination of these. A fuel cell may have a single cell configuration in which one MEA is sandwiched between two separators. However, in practice, a cell stack having a structure in which a plurality of cells are stacked is generally used. In the separator at both ends of the cell stack or the separator forming the single cell, the gas flow path only needs to be provided on only one surface (the surface in contact with the MEA).

  The separator can also have a flow path for flowing a cooling medium such as cooling water. Alternatively, a member having only a flow path for flowing the cooling medium can be provided inside or around the fuel cell.

  A hydrogen-containing gas is introduced into the anode side of the fuel cell, and an oxygen-containing gas such as air is introduced into the cathode side after being humidified as necessary.

  At this time, a reaction in which hydrogen gas becomes protons and emits electrons proceeds at the anode, and a reaction in which oxygen gas obtains electrons and protons to become water proceeds at the cathode.

  Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings, but the present invention is not limited thereto.

[Flow path width]
By making the width of the anode gas passage formed in the separator 1.2 times or more, preferably 1.4 times or more the width of the cathode gas passage, an anode gas having a relatively low hydrogen concentration, for example, a hydrogen concentration can be obtained. Even when an anode gas of 20 mol% or more and 40 mol% or less is supplied, relatively good battery characteristics can be obtained. It is considered that the relative contact area of the gas to the catalyst layer is increased and the electrode reaction of hydrogen having a low concentration is promoted by the relationship of the channel width.

  On the other hand, the separator also has a function of supporting the MEA and a function of a current collector for collecting current. From the balance with these functions, the width of the anode gas channel is 2.0 times or less than the width of the cathode gas channel. Preferably, it is 1.7 times or less.

  This will be described with reference to FIG. An anode side gas diffusion layer 2 a and a cathode side gas diffusion layer 2 c are provided on both sides of the electrolyte membrane 1 to form the MEA 3. The MEA is sandwiched by the separator 4 having the anode gas channel 5a formed on one surface and the cathode gas channel 5c formed on the other surface. In FIG. 1, both the anode gas and the cathode gas flow along the flow path in a direction perpendicular to the paper surface. The anode gas flow path 5a opens toward the anode side gas diffusion layer 2a, and the cathode gas flow path 5c opens toward the cathode side gas diffusion layer 2c. From the anode gas flow along the flow path, hydrogen diffuses toward the anode-side gas diffusion layer, passes through the electrolyte membrane as protons, and protons and oxygen react with each other in the cathode-side gas diffusion layer. Generate.

  The width Wa of the anode gas channel is set to be 1.2 times or more and 2.0 times or less of the width Wc of the cathode gas channel. Here, the depth of the gas flow path is the same for the anode and the cathode, but this is not always necessary.

  In FIG. 1, the channel cross section is rectangular (including a square) and the channel width is constant, but this is not the only case. In the present invention, the width of the gas flow channel is defined as the opening width of the portion where the flow channel opens toward the MEA.

[Flow path depth]
By making the depth of the anode gas channel 0.8 times or less, preferably 0.7 times or less than the depth of the cathode gas channel, an anode gas having a relatively low hydrogen concentration, for example, a hydrogen concentration of 20 mol% Even when anode gas of 40 mol% or less is supplied, relatively good battery characteristics can be obtained. It is considered that the relative contact area of the gas to the catalyst layer is increased and the electrode reaction of hydrogen having a low concentration is promoted by the relationship of the channel depth.

  On the other hand, from the viewpoint of securing the channel cross-sectional area and suppressing pressure loss, the anode gas channel depth is 0.5 times or more, preferably 0.58 times or more the cathode gas channel depth.

  As shown in FIG. 2, the anode gas flow path depth Da is 0.5 times or more and 0.8 times or less of the cathode gas flow path depth Dc. Here, the width of the gas flow path is the same between the anode and the cathode, but it is not always necessary.

  In FIG. 2, the channel cross section is rectangular (including a square) and the channel depth is constant, but this is not the only case. In the present invention, the depth of the gas channel is defined as the maximum depth of the channel.

  As shown in FIG. 3, the width Wa of the anode gas flow path is set to 1.2 times or more and 2.0 times or less of the width Wc of the cathode gas flow path, and the anode gas flow path depth Da is set to the cathode gas flow path depth. It can also be 0.5 times or more and 0.8 times or less of the thickness Dc.

  The absolute values of the gas channel width and depth are appropriately designed in consideration of the number and length of the channels, the gas amount, and the like. For example, both the anode gas channel and the cathode gas channel have a width of 0. It can be in a range of about 5 mm to 2.5 mm and a depth of about 0.3 mm to 1.2 mm.

  The flow path as described above can be formed, for example, by cutting a plate-like member. Moreover, the separator which has the groove | channel used as a flow path can also be formed by molding. The latter is suitable for mass production.

  In FIGS. 1 to 3, each anode gas channel and each cathode gas channel have a one-to-one correspondence and their centers coincide, but this is not necessarily the case.

[Amount of gas diffusion layer buried]
From the viewpoint of reducing the contact resistance between the gas diffusion layer and the separator and thus improving the battery characteristics, the gas diffusion layer is buried by 10% or more of the initial thickness of the gas diffusion layer in the gas channel depth direction. It is preferable that 15% or more is buried. On the other hand, from the viewpoint of securing the flow path cross-sectional area and suppressing pressure loss, from the viewpoint of ease of manufacture, and from the viewpoint of preventing damage to the gas diffusion layer, the amount of burial is preferably 50% or less, more preferably 30% or less And The gas diffusion layer on the anode side or the cathode side preferably satisfies these conditions, but the gas diffusion layers on both sides preferably satisfy these conditions.

  For this reason, when the MEA and the separator are combined into a single cell or a cell stack, a predetermined ratio of the gas diffusion layer thickness can be compressed and buried by applying a clamping pressure in the cell thickness direction.

This will be described with reference to FIG. Let t ₀ be the initial thickness of the gas diffusion layer (the thickness of the stage before being combined with the separator). FIG. 4 shows a partial cross section of the cathode gas flow path and the gas diffusion layer at the stage where the clamping force is applied by assembling as a single cell or a cell stack. Initial thickness the gas diffusion layer 2c was t _0, a portion in contact with the separator 4 is deformed (compressed) by being clamped, the thickness of this portion is t _2. On the other hand, the part not in contact with the separator of the gas diffusion layer 2c is buried by up to t ₁ within the flow path 5c. The above percentage representing the amount of burial is defined as (t ₀ -t ₂ ) / t ₀ (%).

[Flow path direction]
From the viewpoint of promoting the discharge of condensed water, the length of the gas channel in the vertically upward direction is 25% or less of the total gas channel length in at least one of the anode gas channel and the cathode gas channel. It is preferable. More preferably, the length of the vertically upward gas flow path on both the anode side and the cathode side is 25% or less with respect to the total gas flow path length. In order to do this, it is preferable that the anode gas inlet is positioned higher in the vertical direction than the anode gas outlet. Similarly, the cathode gas inlet is preferably higher in the vertical direction than the cathode gas outlet. More preferably, the gas inlet is at a higher position in the vertical direction than the gas outlet on both the anode side and the cathode side.

  This will be described with reference to FIG. FIG. 5 is a schematic diagram for explaining the arrangement of the gas flow paths, and the gas flow paths are represented by simple lines for simplicity. In FIG. 5, the upper side of the drawing is the vertical upper side. The anode gas is supplied to the anode gas flow path 5a from the anode gas supply port 6a provided in the separator. Here, five gas flow paths are arranged in parallel. The anode gas flow path is first arranged in the horizontal direction from the anode gas supply port side, turning 90 ° and moving downward vertically, turning 90 ° again, turning horizontally, turning 90 ° again, and moving vertically downward In addition, the direction is changed again, and it is arranged horizontally and connected to the anode gas discharge port 7a. In this example, the anode gas flow path never goes vertically upward. Therefore, the ratio of the length of the anode gas passage in the vertically upward direction to the total length of the anode gas passage from the anode gas supply port 6a to the anode gas discharge port 7a is 0%. In the separator shown in FIG. 5, the cathode gas passage having the same layout is formed on the back surface, and the ratio of the cathode gas passage length in the vertically upward direction to the total cathode gas passage length is 0 on the cathode side as well. %. In such a form, in any of the anode gas channel and the cathode gas channel, even if condensed water is generated, the condensed water is easily discharged and the gas flow is not hindered. This contributes to obtaining good battery characteristics.

  What is discussed here is the direction of the gas flow path formed in the separator and the positional relationship between the gas supply port and the gas discharge port when the fuel cell is used.

  The layout of the gas flow path does not have to be bellows, and can be appropriately designed in accordance with the separator shape.

[Separator]
The material of the separator can be appropriately selected from materials known as separators for fuel cells. In the case of PEFC, not only carbon-based materials such as graphite and carbon composite (composite material), but also resin-based materials, metal-based materials, and the like can be used. For example, an expanded graphite sheet subjected to an impermeability treatment, a carbon particle and a thermosetting resin or a thermoplastic resin compression molded or injection molded, a stainless steel sheet subjected to a corrosion resistance treatment and a conductive treatment, etc. are used. be able to.

[Gas diffusion layer]
As the gas diffusion layer, a technique known as a gas diffusion layer of a fuel cell can be appropriately used. In the case of PEFC, for example, a gas diffusion layer in which a catalyst for promoting an electrochemical reaction is appropriately attached to a gas permeable carbon material such as carbon paper or carbon cloth can be used. Examples of the catalyst used for the anode side gas diffusion layer include platinum black, activated carbon-supported Pt (platinum) catalyst or Pt-Ru (ruthenium) alloy catalyst. Examples of the catalyst used for the cathode side gas diffusion layer include May include platinum black and Pt catalyst supported on activated carbon. In general, both anode and cathode catalysts are formed with Teflon, a relatively low molecular weight polymer electrolyte membrane material, activated carbon and the like as necessary to form a porous catalyst layer. A gas diffusion layer can be obtained by providing such a porous catalyst layer on the aforementioned gas-permeable carbon material.

〔Electrolytes〕
As the electrolyte, a known electrolyte membrane or electrolyte plate of a fuel cell can be used as appropriate. In the case of PEFC, Nafion (Nafion, perfluorocarbon sulfonic acid manufactured by DuPont), Gore (Gore, manufactured by Gore), Flemion (Flemion, manufactured by Asahi Glass), Aciplex (produced by Aciplex, Asahi Kasei) A polymer electrolyte membrane known under a trade name such as) is usually used.

[MEA]
The MEA can be manufactured using a known method for manufacturing a fuel cell, in particular, a PEFC MEA. For example, a perfluorocarbon sulfonic acid resin and a platinum-based electrode catalyst supported on carbon are uniformly dispersed in a solvent to prepare an ink, which is applied to a blank such as a Teflon film, and dried and applied repeatedly to obtain a constant thickness. After the film is formed, a blank is stacked on the surface of the solid polymer membrane with the catalyst layer on the inside. After hot pressing, the outer blank is peeled off to obtain a joined body of the formed catalyst layer and the electrolyte membrane. The MEA can be formed by sandwiching the three-layer assembly of the anode catalyst layer + membrane + cathode catalyst layer between a pair of carbon paper or the like.

[Example 1]
First, a polymer electrolyte fuel cell was manufactured by the following method.

  As an anode electrode catalyst, Pt and Ru supported on pyrolytic graphite (Pt and Ru are contained in a total of 54 mass% in the catalyst. The mass ratio of metal is Pt: Ru = 45: 35), as a cathode electrode catalyst A pyrolytic graphite having Pt supported thereon (46% by mass of Pt contained in the catalyst) was used. A predetermined amount of each of the electrode catalysts was measured in a ball mill, an appropriate amount of ultrapure water was added, and the mixture was mixed in the ball mill for 20 minutes. A predetermined amount of a 5% by mass perfluorocarbon sulfonic acid solution (a solvent is a mixture of isopropanol and water) was added to each ball mill and mixed for another 20 minutes to obtain an anode paste and a cathode paste, respectively.

The anode paste is applied to one side of a water-repellent carbon paper (product name: TGP • H • 090, thickness 250 μm) manufactured by Toray Industries, using a metal frame and an applicator to form an anode catalyst layer did. The coating amount was adjusted to be Pt: 0.45 mg / cm ² , Ru: 0.35 mg / cm ² , and perfluorocarbon sulfonic acid: 0.75 mg / cm ² .

The cathode paste was applied to another carbon paper (manufactured by Toray Industries, Inc., trade name: TGP · H · 090, thickness 250 μm) using a metal frame and an applicator to form a cathode catalyst layer. The coating amount was adjusted to be Pt: 0.3 mg / cm ² and perfluorocarbon sulfonic acid: 0.3 mg / cm ² .

  As the proton conductive polymer electrolyte, a perfluorocarbon sulfonic acid having a thickness of 25 μm was used. The carbon paper was joined by hot pressing to both surfaces of the center of the electrolyte membrane so that the applied catalyst layer was in contact with the electrolyte, thereby obtaining an MEA.

  A groove (anode gas flow path) having a width of 1.2 mm is formed on one surface of a plate-like compression molding material of carbon particles and thermoplastic resin by cutting into five bellows, and 0.8 mm on the other surface. As shown in FIG. 5, five grooves having a width (cathode gas flow path) were formed in a bellows shape to form a separator. At this time, the anode gas channel width is 1.5 times the cathode gas channel width.

Five separators and four MEAs were alternately stacked to form a four-layer fuel cell stack (however, the gas flow paths are formed only on the side in contact with the MEA in the separators located at both ends of the fuel cell stack in the stacking direction) In addition, a cooling layer was included in each separator.) At this time, an O-ring was used to seal between the MEA and the separator. A current collector plate, an insulating plate, and an end plate, each plated with gold, were stacked in this order on both ends in the stacking direction of the fuel cell stack, and the whole was tightened with bolts and nuts attached to the end plates on both sides. At this time, each of the four anode side gas diffusion layers (initial thickness: t ₀ a = 250 μm) and the four cathode side gas diffusion layers (initial thickness: t ₀ c = 250 μm) are 30 with respect to the initial thickness. % To adjust. That is, the thickness in the stacking direction of the fuel cell was reduced by 0.30 (t ₀ a + t ₀ c) × 4, that is, 600 μm by tightening.

After aging the fuel cell thus produced, the cell temperature was 80 ° C., the gas humidification temperature was 70 ° C. (both anode gas and cathode gas), hydrogen was 35 mol%, carbon dioxide 25 mol%, nitrogen 40 on the anode side. A low hydrogen concentration gas of mol% was flowed to the cathode side while controlling the gas utilization rate to be 60% and 40%, respectively. The aging conditions were as follows: anode gas: 100 mol% hydrogen, cathode gas: air, cell temperature: 80 ° C., gas humidification temperature: 70 ° C. (both anode gas and cathode gas), current density: 0.5 A / cm ² 8 hours. When the IV characteristic (average value of four cells) of the fuel cell was measured, it was as shown in FIG. In addition, the arrangement | positioning of the fuel cell at the time of evaluation was made for the separator to have the directionality as shown in FIG.

[Comparative Example 1]
A short stack was prepared in the same manner as in Example 1 except that both the anode gas channel width and the cathode gas channel width were set to 0.8 mm, and the IV characteristics thereof were measured. The results are shown in FIG.

As can be seen from the comparison between Example 1 and Comparative Example 1, the gas flow path width on the anode side is 1.5 times that on the cathode side compared to the same gas flow path width on the anode side and cathode side. As for the power generation characteristics obtained from the fuel cell manufactured using the separator, a high voltage was obtained. Specifically, in Example 1, a voltage 12% or more higher than that of Comparative Example 1 was obtained at 0.5 A / cm ² .

[Comparative Example 2]
Using the short stack of Comparative Example 1 and changing the gas supplied to the anode side to a high hydrogen concentration gas with a hydrogen concentration of 75 mol% and a carbon dioxide concentration of 25 mol%, the IV characteristics were the same as in Example 1. When measured, the current density was 0.5 A / cm ² , which was 0.67 V, which was almost the same as the voltage of Example 1.

  As can be seen from a comparison between Example 1 and Comparative Example 2, even when the hydrogen concentration is low, the fuel cell of the present invention can exhibit power generation characteristics close to those when the hydrogen concentration is high.

  The fuel cell of the present invention uses a hydrogen-containing gas having a low hydrogen concentration, such as a hydrogen-containing gas obtained from biomass, or a hydrogen-containing gas produced by partially oxidizing fuel such as methane, liquefied petroleum gas, or kerosene. It can be particularly preferably used when performing the above.

It is a partial cross-sectional schematic diagram for demonstrating the form of a gas flow path about an example of the fuel cell of this invention. It is a partial cross section schematic diagram for demonstrating the form of a gas flow path about another example of the fuel cell of this invention. It is a partial cross section schematic diagram for demonstrating the form of a gas flow path about another example of the fuel cell of this invention. It is a partial section schematic diagram for explaining the amount of burial of a gas diffusion layer. It is a schematic diagram for demonstrating the example of the gas flow path shape of the fuel cell of this invention. It is a graph which shows the IV characteristic of the fuel cell of an Example and a comparative example.

Explanation of symbols

1 Electrolyte 2 Gas diffusion layer (2a: anode side, 2c: cathode side)
3 MEA
4 Separator 5 Gas flow path (5a: anode side, 5c: cathode side)
6 Gas supply port (6a: anode side, 6c: cathode side)
7 Gas outlet (7a: anode side, 7c: cathode side)
W Channel width (Wa: anode side, Wc: cathode side)
D Channel depth (Da: anode side, Dc: cathode side)
t Gas diffusion layer thickness (t ₀ : initial thickness, t ₁ : burial amount, t ₂ : minimum thickness)

Claims (4)

  A fuel cell comprising a separator having a width of an anode gas channel that is 1.2 to 2.0 times a width of a cathode gas channel.   A fuel cell comprising a separator having a depth of an anode gas channel of 0.5 to 0.8 times a depth of a cathode gas channel.   3. The fuel cell according to claim 1, wherein the gas diffusion layer is buried at least 10% to 50% of the initial thickness of the gas diffusion layer in the gas flow path depth direction on at least one of the anode side and the cathode side. .   The length of the vertically upward gas flow path in at least one of the anode gas flow path and the cathode gas flow path provided in the separator is 25% or less with respect to the total gas flow path length. The fuel cell according to any one of the above.

Images