JP2016136481A - Fuel battery cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel battery cell in which the gas diffusion performance of a gas diffusion layer in the neighborhood of a rib of a separator can be kept high under discharge, thereby suppressing degradation of the performance.SOLUTION: In a fuel battery cell, at least spherical carbon, polytetrafluoroethylene and dispersant are mixed to prepare fluid dispersion having a viscosity of 2,000 to 3,000 cp. The fluid dispersion is coated on one surface of carbon paper, and after the fluid dispersion is coated, the carbon paper is burned at 330°C to form a water repellant layer on one surface, thereby forming a gas diffusion layer having the water repellant layer at least at the cathode electrode side. The water repellant layer of the gas diffusion layer having the water repellant layer and a cathode electrode are in contact with each other.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a fuel cell that can maintain high gas diffusibility of a gas diffusion layer in the vicinity of a rib of a separator during discharge, and as a result, suppress performance degradation.

  The gas diffusion layer on the cathode electrode side of the fuel cell is usually provided with a water repellent layer for the purpose of improving the discharge of water produced by the discharge reaction. Patent Document 1 is a gas diffusion layer for a fuel cell including a gas diffusion layer base material and a conductive water repellent layer, wherein the conductive water repellent layer contains a conductive material and a water repellent material, There is disclosed a fuel cell gas diffusion layer which is a porous layer having pores with a pore diameter of 0.2 μm to 2 μm inside and the inner surface of the pores made of a water repellent material.

JP 2009-059524 A

  As described in Patent Document 1, the water repellent layer is usually provided on one side of the gas diffusion layer. The water repellent layer is formed, for example, by applying a water repellent paste to one side of a substrate such as carbon paper.

  Incidentally, Patent Document 1 has a description that a serpentine type flow path is formed on the MEA side surface in the thickness direction of the separator. As described above, in so-called flat fuel cells, it is usual to use a sandwiched body in which a membrane / electrode assembly is sandwiched between a pair of gas diffusion layers and further sandwiched by a pair of separators having a groove structure. is there. The groove structure has a function of supplying gas to the gas diffusion layer and discharging excess gas and moisture. In a flat fuel cell, a predetermined pressure is usually applied in a direction perpendicular to the plane. When using a separator having a groove structure, a portion other than the groove structure in the separator (the portion may be referred to as “rib” hereinafter) is pressed more strongly against the gas diffusion layer than a portion corresponding to the groove structure. As a result of the partial compression of the gas diffusion layer in the vicinity of the rib, the gas diffusibility of the portion is weakened. As described above, when a separator having a groove structure is used, the gas diffusibility of the gas diffusion layer in the vicinity of the rib is required. In particular, in addition to the gas diffusivity in the direction perpendicular to the plane of the fuel cell, it is parallel to the plane. Directional gas diffusivity is important.

As a result of the study by the present inventors, in the fuel cell using the gas diffusion layer disclosed in Patent Document 1, when the water repellent paste penetrates into the gas diffusion layer, the gas is sufficient to the vicinity of the rib of the separator. It was revealed that the performance of the fuel cell deteriorated without being supplied.
The present invention has been accomplished in view of the above-described circumstances, and provides a fuel cell that maintains the gas diffusibility of the gas diffusion layer in the vicinity of the rib of the separator at the time of discharge, and as a result can suppress performance degradation. Objective.

  The fuel cell of the present invention comprises a membrane / electrode assembly comprising an anode electrode on one side of the electrolyte membrane and a cathode electrode on the other side, a pair of gas diffusion layers sandwiching the membrane / electrode assembly, And a pair of separators further sandwiching the membrane-electrode assembly and the pair of gas diffusion layers and having a groove structure on the side facing the gas diffusion layer, at least on the cathode electrode side The gas diffusion layer mixes at least spherical carbon, polytetrafluoroethylene, and a dispersion medium to prepare a dispersion having a viscosity of 2,000 cp to 3,000 cp, and the dispersion is applied to one side of the carbon paper. A gas diffusion layer with a water-repellent layer, wherein a water-repellent layer is formed on one side by firing the carbon paper after coating and applying the dispersion at 330 ° C., Characterized in that said cathode electrode and water-repellent layer in the aqueous layer with gas diffusion layer is in contact.

  According to the present invention, the gas diffusibility of the gas diffusion layer in the vicinity of the rib of the separator is obtained by using a gas diffusion layer with a water repellent layer that has been baked under a specific temperature using a dispersion having a specific range of viscosity. Can be kept high during discharge, and as a result, performance degradation of the fuel cell can be suppressed.

It is the cross-sectional schematic diagram which expanded and showed the laminated | stacked part of the separator and the gas diffusion layer in each fuel cell of Examples 1-2 and Comparative Examples 1-3. It is a graph which shows the relationship between the in-plane gas diffusion resistance of the fuel battery cell of Examples 1-2 and Comparative Examples 1-3, and the viscosity of the dispersion liquid for water-repellent layers used for manufacture of these fuel battery cells. 5 is a graph in which IV curves of fuel cells of Example 1 and Comparative Example 3 are superimposed. 5 is a graph in which IV curves of fuel cells of Example 1 and Comparative Example 3 are superimposed.

  The fuel cell of the present invention comprises a membrane / electrode assembly comprising an anode electrode on one side of the electrolyte membrane and a cathode electrode on the other side, a pair of gas diffusion layers sandwiching the membrane / electrode assembly, And a pair of separators further sandwiching the membrane-electrode assembly and the pair of gas diffusion layers and having a groove structure on the side facing the gas diffusion layer, at least on the cathode electrode side The gas diffusion layer mixes at least spherical carbon, polytetrafluoroethylene, and a dispersion medium to prepare a dispersion having a viscosity of 2,000 cp to 3,000 cp, and the dispersion is applied to one side of the carbon paper. A gas diffusion layer with a water-repellent layer, wherein a water-repellent layer is formed on one side by firing the carbon paper after coating and applying the dispersion at 330 ° C., Characterized in that said cathode electrode and water-repellent layer in the aqueous layer with gas diffusion layer is in contact.

  As the electrolyte membrane, the anode electrode, and the cathode electrode in the membrane-electrode assembly used in the present invention, those usually used for the membrane-electrode assembly constituting the fuel cell can be used.

  Of the pair of gas diffusion layers sandwiching the membrane / electrode assembly, at least the gas diffusion layer on the cathode electrode side will be described as follows: (1) dispersion preparation step, (2) coating step, and (3) firing. A gas diffusion layer with a water repellent layer produced through the process is used.

(1) Dispersion preparation step This step is a step of preparing a dispersion having a viscosity of 2,000 cp to 3,000 cp by mixing at least spherical carbon, polytetrafluoroethylene, and a dispersion medium.
For example, ketjen black can be used as the spherical carbon.
As the dispersion medium, for example, alcohol such as ethanol, water, and a mixed solution thereof can be used.
As a method for adjusting the viscosity to 2,000 cp to 3,000 cp, for example, when ketjen black is used as spherical carbon and ethanol is used as a dispersion medium, spherical carbon: polytetrafluoroethylene: water = (70 to 90 Mass%): (10 to 25 mass%): (2 to 10 mass%).
Moreover, as a method of adjusting a viscosity, the method of mixing a thickener in addition to spherical carbon, polytetrafluoroethylene, and a dispersion medium is mentioned, for example. As the timing of adding the thickener to the mixture of the three types of raw materials (spherical carbon, polytetrafluoroethylene, and dispersion medium) is later, the viscosity tends to be higher. The timing of adding the thickener to the mixture is preferably just before the resulting dispersion is subjected to the coating step. For example, polyethylene oxide can be used as the thickener.
As a method for measuring the viscosity of the obtained dispersion liquid, a method using a general cone plate type (E type) rotational viscometer can be exemplified.

(2) Coating step This step is a step of coating the dispersion obtained in the dispersion preparation step on one side of the carbon paper.
The coating amount of the dispersion liquid is, for example, 4 to 50 mg / cm ² although it depends on the composition of the dispersion liquid.
The application method is not particularly limited. Examples of the coating method include die coating and spray coating. At the time of application, the penetration of the dispersion liquid into the carbon paper as the base material may be suppressed by a physical method. For example, when applying the dispersion, wind may be blown from the side opposite to the application surface through the carbon paper to suppress the penetration of the dispersion into the carbon paper by using wind power. Alternatively, the carbon paper may be arranged in parallel with the ground, and the dispersion liquid may be applied to the carbon paper from below in the vertical direction, so that the penetration of the dispersion liquid into the carbon paper may be suppressed using gravity.

  As in the case of using a combination of a so-called flat separator and a porous material, such a permeation of the dispersion does not need to be considered in a configuration in which the gas diffuses uniformly in the planar direction of the fuel cell. However, when a separator having a groove structure is used as in the present invention, the gas diffusibility mainly in the plane direction varies greatly depending on the amount of the dispersion soaked, as shown in the examples described later.

(3) Firing step This step is a step of forming a water-repellent layer on one side of the carbon paper by firing the carbon paper after applying the dispersion at 330 ° C or higher (preferably 330 ° C).
The upper limit of the firing temperature is not particularly limited as long as it is a temperature usually used for firing at the time of forming the water repellent layer. The upper limit of the firing temperature may be 500 ° C., for example.
Thus, the gas diffusion layer with a water repellent layer obtained through the firing step is disposed so that the surface on which the water repellent layer is formed is in contact with the cathode electrode.

  The gas diffusion layer on the anode electrode side may be a gas diffusion layer with a water repellent layer or a gas diffusion layer (carbon paper or the like) that has not been subjected to a water repellent treatment.

The pair of separators used in the present invention further sandwich a sandwich body including a membrane / electrode assembly and a gas diffusion layer. Each separator has a groove structure on the side facing the gas diffusion layer.
As the shape of the groove structure, a groove structure of a separator usually used for a fuel cell is adopted.

  Thus, by using a dispersion liquid having a viscosity of 2,000 cp or more and 3,000 cp or less and using a gas diffusion layer obtained through firing at a firing temperature of 330 ° C. or more, the dispersion of the carbon paper is infiltrated. In addition, the gas diffusivity in the vicinity of the rib in the gas diffusion layer can be kept high during discharge, and as a result, the performance degradation of the fuel cell can be suppressed.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited only to these examples.

1. Production of fuel cell [Example 1]
First, a water repellent layer dispersion was prepared. 14% by mass of spherical carbon (acetylene black), 6% by mass of polytetrafluoroethylene, and 79.8% by mass of a dispersion medium (water) were mixed. After mixing these three types of raw materials, just before applying to carbon paper (gas diffusion layer substrate), 0.2% by mass of polyethylene oxide (see JP2013-191537A) was further mixed as a thickener. .
The viscosity of the prepared dispersion for water repellent layer was measured with a cone plate type (E type) rotational viscometer at a temperature of 25 ° C. and a rotational speed of 25 rpm for 15 seconds.
The water repellent layer dispersion was die coated on one side of carbon paper at a coating amount of 20 mg / cm ² . Thereafter, the carbon paper was fired at 330 ° C. to obtain a gas diffusion layer with a water repellent layer.

Next, a membrane / electrode assembly having an anode electrode on one side of the electrolyte membrane and a cathode electrode on the other side was prepared. Subsequently, the membrane / electrode assembly was sandwiched between a pair of gas diffusion layers. At this time, the gas diffusion layer in contact with the cathode electrode is the gas diffusion layer with the water repellent layer, and the water repellent layer (the surface coated with the water repellent layer dispersion) in the gas diffusion layer with the water repellent layer, and the cathode electrode And touched. The gas diffusion layer on the anode electrode side was carbon paper.
The sandwiched body was further sandwiched between a pair of separators to obtain a fuel cell of Example 1. At this time, a separator having a groove structure on one side was used, and the separator was sandwiched so that the surface and the gas diffusion layer were in contact with each other.

[Example 2]
The types and mixing ratio of the spherical carbon, PTFE, and dispersion medium are the same as those in Example 1. After mixing these three types of raw materials, 0.2% by mass of polyethylene oxide was further mixed as a thickener at a time earlier than Example 1.
The viscosity of the prepared dispersion for water repellent layer was measured by the same method as in Example 1 and found to be 2,000 cp.
Next, a gas diffusion layer with a water repellent layer was prepared in the same manner as in Example 1. Along with the obtained gas diffusion layer with water repellent layer, the same membrane / electrode assembly as in Example 1, a gas diffusion layer (carbon paper) on the anode electrode side, and a pair of separators were used. A fuel cell was produced.

[Comparative Example 1]
The types and mixing ratio of the spherical carbon, PTFE, and dispersion medium are the same as those in Example 1. After mixing these three types of raw materials, 0.2 mass% of polyethylene oxide was further mixed as a thickener immediately before being applied to carbon paper (gas diffusion layer substrate).
The viscosity of the prepared dispersion for water-repellent layer was measured by the same method as in Example 1. As a result, it was 3,000 cp.
Next, the water repellent layer dispersion was die coated on one side of the carbon paper at a coating amount of 20 mg / cm ² . Thereafter, the carbon paper was fired at 250 ° C. to obtain a gas diffusion layer with a water repellent layer.
Along with the obtained gas diffusion layer with water repellent layer, the same membrane / electrode assembly as in Example 1, a gas diffusion layer (carbon paper) on the anode electrode side, and a pair of separators were used. A fuel cell was produced.

[Comparative Example 2]
The types and mixing ratio of the spherical carbon, PTFE, and dispersion medium are the same as those in Example 1. After mixing these three kinds of raw materials, 0.2% by mass of polyethylene oxide was further mixed as a thickener at a time earlier than Example 1 and later than Example 2.
The viscosity of the prepared dispersion for water-repellent layer was measured by the same method as in Example 1. As a result, it was 2,500 cp.
Next, the water repellent layer dispersion was die coated on one side of the carbon paper at a coating amount of 20 mg / cm ² . Thereafter, the carbon paper was fired at 250 ° C. to obtain a gas diffusion layer with a water repellent layer.
In combination with the obtained gas diffusion layer with water repellent layer, the same membrane / electrode assembly as in Example 1, a gas diffusion layer (carbon paper) on the anode electrode side, and a pair of separators were used. A fuel cell was produced.

[Comparative Example 3]
The types and mixing ratio of the spherical carbon, PTFE, and dispersion medium are the same as those in Example 1. These three types of raw materials and 0.2% by mass of polyethylene oxide as a thickener were mixed at a time.
The viscosity of the prepared dispersion for water-repellent layer was measured by the same method as in Example 1, and was 1,500 cp.
Next, the water repellent layer dispersion was die coated on one side of the carbon paper at a coating amount of 20 mg / cm ² . Thereafter, the carbon paper was fired at 250 ° C. to obtain a gas diffusion layer with a water repellent layer.
In combination with the obtained gas diffusion layer with water repellent layer, the same membrane / electrode assembly as in Example 1, a gas diffusion layer (carbon paper) on the anode electrode side, and a pair of separators were used. A fuel cell was produced.

2. Gas diffusivity evaluation About the fuel cell of Examples 1-2 and Comparative Examples 1-3, the limit current value (maximum current value) was measured on condition of the following.
Hydrogen gas flow rate: 1 NL / min
Air flow rate: 1NL / min
Back pressure: 150kPa
Cell temperature: 60 ° C
Dew point: 55 ° C

FIG. 1 is an enlarged schematic cross-sectional view showing a laminated portion of a separator and a gas diffusion layer in each of the fuel cells of Examples 1-2 and Comparative Examples 1-3. The numerical value in FIG. 1 represents length (mm).
In FIG. 1, a gas flow path g is provided between the separator c and the gas diffusion layer d. The gas flow path g is a part of a groove structure provided in the separator c. As shown in FIG. 1, the thickness of the gas diffusion layer d is 0.2 mm (= 200 μm). Moreover, as shown in FIG. 1, the space | interval between the gas flow paths g is 9.2 mm, the width of the gas flow path g is 1.2 mm, and the depth of the gas flow path g is 1.5 mm. A bent arrow extending from the gas flow path g to the vicinity of the center of the gas diffusion layer d indicates a supply gas diffusion path. The gas diffusion distance L is 0.275 mm. In addition, the depth b of the laminated part which concerns on FIG. 1 is 10 (mm).
A window a was provided with a width of 7 mm at the center of the space between the gas flow paths g. In the case of the measurement limit current I _lim (A), the diffusion resistance R _{mol @ L} corresponding to the gas diffusion distance _L is obtained by the following equation (1).

(In the above formula (1), R _{mol @ L} is the diffusion resistance corresponding to the diffusion distance L (m), F is the Faraday constant (96485 (sAmol ⁻¹ )), and R is the gas constant (8.31 (JK ^{− 1} mol ⁻¹ )), T is the temperature (K), P _{O2_ff} is the oxygen partial pressure (Pa) in the flow path, I _lim is the measurement limit current (A), and d is the thickness of the gas diffusion layer (m the), b is the gas diffusion layer depth of the _{(m), R other} diffusion resistance than molecular diffusion resistance (e.g. knudsen diffusion resistance) (s / m), a is a window (m), L is the diffusion length (M) is shown respectively.)
The diffusion resistance R _{mol @ L} corresponding to the gas diffusion distance L obtained from the above equation (1) was defined as the in-plane gas diffusion resistance in the fuel cell.

FIG. 2 shows the relationship between the in-plane gas diffusion resistance of the fuel cells of Examples 1 and 2 and Comparative Examples 1 to 3 and the viscosity of the water repellent layer dispersion used in the production of these fuel cells. It is a graph. FIG. 2 is a graph in which the vertical axis represents the in-plane gas diffusion resistance (s / m) and the horizontal axis represents the viscosity (cp) of the water repellent layer dispersion. The square plots in FIG. 2 show the experimental results using a gas diffusion layer with a water repellent layer obtained by firing at 330 ° C. The rhombus plots in FIG. 2 show the experimental results using a gas diffusion layer with a water repellent layer obtained by firing at 250 ° C.
First, from the result of Comparative Example 3 in FIG. 2, the gas diffusion layer with a water repellent layer obtained using the water repellent layer dispersion having a viscosity of 1,500 cp has an in-plane gas diffusion resistance of the fuel cell of 25 s. It can be seen that / m. This is presumably because the water-repellent layer dispersion having a low viscosity soaked into the inside of the carbon paper, which deteriorated the gas diffusibility of the water-repellent layer-attached gas diffusion layer.
Further, from the results of Comparative Examples 1 to 3 in FIG. 2, it is understood that the gas diffusion layer with a water repellent layer obtained by baking at 250 ° C. has an in-plane gas diffusion resistance of the fuel battery cell of 22 s / m or more. . This is because the gas diffusibility becomes low as a result of the water-repellent layer dispersion soaking into the carbon paper due to the firing temperature being too low.
On the other hand, from the results of Examples 1 and 2 in FIG. 2, the gas diffusion layer with a water repellent layer obtained by using a dispersion for a water repellent layer having a viscosity of 2,000 to 3,000 cp and baking at 330 ° C. It can be seen that the in-plane gas diffusion resistance of the fuel battery cell is 21 s / m or less. This is because the water-repellent layer dispersion can be kept high on the surface of the carbon paper by keeping the viscosity of the water-repellent layer dispersion in a specific range and increasing the firing temperature, so that the gas diffusibility can be kept high. is there.
In this way, by setting the viscosity of the water repellent layer dispersion to 2,000 to 3,000 cp and the firing temperature to 330 ° C. or higher, the amount of the water repellent layer dispersion permeated can be suppressed, and the water repellent obtained The gas diffusibility of the layered gas diffusion layer can be maintained high.

3. IV Test First, with respect to the fuel cells of Example 1 and Comparative Example 3, for the purpose of evaluating gas diffusivity in a direction perpendicular to the plane of the cell, an IV test was performed under the following conditions, and an IV curve was obtained. Obtained (FIG. 3).
・ Fuel gas: Hydrogen gas, dew point 55 ℃
Oxidant gas: 1% O ₂ (99% N ₂ ), dew point 55 ° C.
-Cell temperature: 60 ° C
It is. The gas diffusion resistance is calculated by the following formula (2).

(In the above formula (2), R _mol represents gas diffusion resistance, and F, R, T, P _{O2 — ff} , I _lim , and R _other represent the same as in the above formula (1).)

Next, for the fuel cells of Example 1 and Comparative Example 3, for the purpose of evaluating gas diffusivity in a direction parallel to the plane of the cell, an IV test was performed under the following conditions to obtain an IV curve. (FIG. 4).
・ Fuel gas: Hydrogen gas, dew point 55 ℃
Oxidant gas: air, dew point 55 ° C
-Cell temperature: 60 ° C

3 and 4 are graphs in which the IV curves of the fuel cells of Example 1 and Comparative Example 3 are overlaid. From the above test method, the gas diffusivity in the direction perpendicular to the plane of the fuel cell can be evaluated from FIG. 3, and the gas diffusivity in the direction parallel to the plane of the fuel cell can be evaluated from FIG.
In FIG. 3, there is no significant difference in performance between the fuel cells of Example 1 and Comparative Example 3. From these results, it can be seen that these fuel cells have no difference in gas diffusivity in a direction perpendicular to the plane of the fuel cells.
On the other hand, in FIG. 4, the fuel cell of Example 1 has a higher current density over almost the entire potential region of 0.05 to 0.9 V compared to the fuel cell of Comparative Example 3. From this result, it can be seen that Example 1 has significantly higher gas diffusivity in the direction parallel to the plane of the fuel cell than Comparative Example 3.
Based on the consideration of FIG. 2 above, since the water repellent layer dispersion used in Comparative Example 3 has a lower viscosity than the water repellent layer dispersion used in Example 1, the amount of soaking into the carbon paper As a result, the gas diffusibility deteriorates. Here, when the gas diffusibility direction is decomposed into a direction parallel to the plane of the fuel cell and a direction perpendicular to the plane, the effect of reducing the thickness of the gas diffusion layer itself and the water repellent layer are shown in FIG. As a result of the mutual cancellation of the effects of the soaking liquid dispersion, the gas diffusivity in the direction perpendicular to the plane of the fuel cell is almost the same between Example 1 and Comparative Example 3.
Therefore, when the above consideration and the result of FIG. 4 are taken into consideration, it can be confirmed that the influence of the viscosity of the water repellent layer dispersion affects the gas diffusivity in the direction parallel to the plane of the fuel cell.

a Window c Separator d Gas diffusion layer g Gas flow path

Claims (1)

A membrane / electrode assembly comprising an anode electrode on one side of the electrolyte membrane and a cathode electrode on the other side;
A pair of gas diffusion layers sandwiching the membrane-electrode assembly;
A pair of separators further sandwiching the membrane-electrode assembly and the pair of gas diffusion layers and having a groove structure on the side facing the gas diffusion layer;
A fuel cell comprising:
At least the gas diffusion layer on the cathode electrode side,
By mixing at least spherical carbon, polytetrafluoroethylene, and a dispersion medium, a dispersion having a viscosity of 2,000 cp to 3,000 cp is prepared,
Apply the dispersion to one side of carbon paper,
It is a gas diffusion layer with a water repellent layer in which a water repellent layer is formed on one side by firing the carbon paper after applying the dispersion at 330 ° C.,
The fuel cell, wherein the water repellent layer in the gas diffusion layer with the water repellent layer is in contact with the cathode electrode.