JP2005267902A - Polymer electrolyte fuel cell - Google Patents

Abstract

An object of the present invention is to provide a polymer electrolyte fuel cell having good startability even when it is humidified or not humidified.
A polymer electrolyte fuel cell includes a porous fuel gas diffusion layer having a large number of pores, a fuel catalyst layer, an electrolyte membrane, an oxidant gas catalyst layer, and a large number of pores. And a porous gas diffusion layer for an oxidant having layers. The gas diffusion layer for the oxidant has a pore distribution with a pore size on the horizontal axis and a pore volume on the vertical axis, and the high frequency peak of the pore volume is set within the range of the pore diameter of 10 to 30 μm. In addition, when the total pore volume is 100% by volume, the total pore volume of pores exceeding 30 μm is set to 20% by volume or less.
[Selection] Figure 2

Description

  The present invention relates to a polymer electrolyte fuel cell suitable for startup with low or no humidification.

  A polymer electrolyte fuel cell supplies fuel such as hydrogen to a gas diffusion layer on the fuel side, and supplies an oxidant gas such as air to a gas diffusion layer on the oxidant side, and performs electrochemical reaction on a catalyst such as platinum. It reacts automatically and generates electricity and heat simultaneously by a power generation reaction. Here, water is generated on the oxidant electrode side by the power generation reaction.

  By the way, the polymer electrolyte used in the polymer electrolyte fuel cell has the required ionic conductivity when it is wet with water. On the other hand, the electrode reaction as a battery is a water generation reaction that occurs at the three-phase interface of the catalyst, the polymer electrolyte, and the reaction gas. Accordingly, if water vapor in the supplied gas and water produced by the electrode reaction are not quickly discharged from the gas diffusion layer for the oxidant, water will remain in the electrode and the gas diffusion layer for the oxidant. In this case, the gas flow path in the oxidant gas diffusion layer is blocked with generated water (flooding phenomenon), the gas diffusibility is deteriorated, and the characteristics of the fuel cell are deteriorated.

  From such a viewpoint, measures are taken in the gas diffusion layer used in the polymer electrolyte fuel cell to promote moisture retention of the polymer electrolyte and discharge of water. That is, the gas diffusion layer is generally made of carbon nonwoven fabric such as carbon paper made of carbon fiber, carbon cloth, or the like. These gas diffusion layers are preliminarily treated with a water repellent treatment using a dispersion of polytetrafluoroethylene material, etc., so that the generated water generated by the electrode reaction can be discharged quickly, and a polymer electrolyte membrane or In general, the polymer electrolyte in the electrode is in an appropriate wet state.

  As another method, a measure is taken to mix water-repellent-treated carbon particles in the electrode catalyst layer to discharge excess generated water in the electrode catalyst layer.

  By the way, the low humidification operation condition with a small amount of humidification is preferable because the humidifier can be downsized. However, since the amount of water contained in the oxidant gas supplied to the oxidant electrode side is small, the initial wet state of the electrolyte membrane cannot be created. For this reason, there is a problem that it is difficult to obtain power generation performance at the time of startup. Furthermore, even if generated water is generated by the power generation reaction, the generated water is easily taken away to the gas downstream side of the gas diffusion layer. Moreover, since there is little pressure loss upstream of the gas, the gas flow rate is fast, and the electrolyte membrane tends to be dry. Accordingly, a sufficient cell reaction has not occurred, and the power generation output of the fuel cell has not sufficiently increased.

Further, in Patent Document 2 (Japanese Patent Laid-Open No. 8-124583) or Patent Document 2 (Japanese Patent Laid-Open No. 6-262562), the current collector is composed of carbon cloth, and the mesh of the cloth is the gas inlet. An example in which the gas diffusibility is improved by gradually roughing in the direction from the outlet to the outlet is disclosed.
JP-A-8-124583 JP-A-6-262562

  According to the above-described conventional technology, the startability is not good during the low humidification operation in which the humidification amount of the oxidant gas (generally air) supplied to the oxidant electrode is low. This is presumably because of the large amount of water brought back in the gas diffusion layer for the oxidant, particularly the gas upstream region of the gas diffusion layer for the oxidant.

  The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a polymer electrolyte fuel cell with good startability even in low humidification operation or non-humidification operation. And

  The inventor has been eagerly developing solid polymer fuel cells. And, in the gas diffusion layer for oxidant, particularly in the gas upstream region of the gas diffusion layer for oxidant, if the take-back of water such as generated water is suppressed, the startability is improved even in low humidification operation. This was discovered and confirmed by a test, and the present invention was completed.

That is, the polymer electrolyte fuel cell according to the present invention is a porous gas fuel having a large number of pores and supplied with fuel, a fuel catalyst layer containing a catalyst, an ionic conductivity An electrolyte membrane having a catalyst, a catalyst layer for an oxidant gas containing a catalyst, and a gas diffusion layer for an oxidant that is porous and has a large number of pores and is supplied with an oxidant gas. In molecular fuel cells,
The gas diffusion layer for the oxidant has a pore distribution with a pore size on the horizontal axis and a pore volume on the vertical axis, and the high frequency peak of the pore volume is set within the range of the pore diameter of 10 to 30 μm. and,
When the total pore volume is 100% by volume, the total pore volume of pores exceeding 30 μm is set to 20% by volume or less.

  According to the present invention, in the gas diffusion layer for oxidant, most of the pores are 10 to 30 μm and the number of pores exceeding 30 μm is reduced. As a result, the carry-back of water in the oxidant gas diffusion layer can be reduced, and in particular, the take-up of water in the gas upstream region of the oxidant gas diffusion layer can be reduced. As a result, even when the humidity is low or non-humidified, the wet state of the electrolyte membrane is quickly created, so that the reaction activity is enhanced and the battery performance is exhibited.

  In addition, if the pores have the above-described sizes, in the gas diffusion layer for the oxidant, particularly in the gas downstream region of the gas diffusion layer for the oxidant, the generated water generated by the power generation reaction is discharged to the gas downstream side. Can contribute to the reduction of submergence of the catalyst and gas clogging (flooding phenomenon) due to water. Therefore, even when the humidity is low or non-humidified, the startability is good. Here, low humidification means that the relative humidity of the oxidant gas is 50% or less. Non-humidified refers to the state of natural humidity in the atmosphere.

  According to the polymer electrolyte fuel cell according to the present invention, the high frequency peak of the pore volume is set within the range of the pore diameter of 10 to 30 μm, and when the entire pore volume is 100% by volume, The total pore volume of pores exceeding 30 μm is set to 20% by volume or less.

  As a result, it is possible to suppress the take-back of water in the gas upstream region and to ensure the water discharge performance in the gas downstream region. Therefore, the balance between the water retention performance and the drainage performance in the cell can be well maintained even under the low humidification operation condition or the non-humidification operation condition.

  As a result, the startability and output performance of the fuel cell can be improved. Further, by enabling low-humidification operation or non-humidification operation, auxiliary devices such as a humidifier and a water tank can be simplified or eliminated, the size of the fuel cell power generation system can be reduced, and the cost can be reduced. If the humidification operation is performed, the humidifier can be eliminated.

  According to the present invention, the oxidant gas diffusion layer has a pore distribution in which the horizontal axis is the pore diameter and the vertical axis is the pore volume, and the high frequency peak of the pore volume is within the range of the pore diameter of 10 to 30 μm. In addition, when the total pore volume is 100% by volume, the total pore volume of pores exceeding 30 μm is set to 20% by volume or less.

  Here, the high frequency peak of the pore volume can be set within the range of the pore diameter of 10 to 30 μm and within the range of 20 to 30 μm. Further, the total pore volume of pores exceeding 30 μm can be set to 19% by volume or less, 18% by volume or less, and 15% by volume or less. The high frequency peak is preferably a single peak, but may be a double peak in some cases.

  Further, if the cumulative pore volume percentage of pores exceeding 30 μm is K2, and the cumulative pore volume percentage of 20 to 30 μm is K1, K1 / K2 = 1.1 to 5.0, 1.2 to 4.0. Or 1.4 to 3.0 or 1.6 to 2.5. As a result, pores exceeding 30 μm can be decreased while pores having a diameter of 20 to 30 μm can be increased.

  As the oxidant gas diffusion layer, a porous substrate having gas permeability and conductivity can be used. Then, the porous substrate can be formed by impregnating a water repellent material such as a fluororesin and applying a conductive ink containing a conductive substance. As the porous substrate, for example, a carbon sheet formed of an aggregate of carbon fibers can be used. Examples of the carbon sheet include carbon paper and carbon cloth. As thickness with porous base materials, such as a carbon sheet, 100-500 micrometers, 150-450 micrometers, and 300-400 micrometers can be illustrated. Generally, when the carbon sheet is thin, the pore size tends to be large.

  Examples of the present invention will be described below.

  First, 15 g of carbon black (particulate conductive material, manufactured by Cabot Corporation: VULCAN XC72R) is mixed in 80 g of water (dispersion medium), stirred for 10 minutes using a stirrer, and then a dispersion stock solution (trade name: 17 g of POLYFLON D-1, manufactured by Daikin Industries, Ltd.) was added and stirred for another 10 minutes to produce a carbon paste (dispersion) (dispersion production process). The above dispersion stock solution has a content concentration of tetrafluoroethylene (hereinafter referred to as PTFE) of 60% by weight.

  The above-mentioned carbon paste was applied to both surfaces of carbon paper (porous substrate, Torayca TGP-H-120 manufactured by Toray Industries, Inc., size 200 mm × 250 mm, thickness 350 μm) with a screen printer (application process) ).

  Next, the carbon paper coated with the carbon paste was placed in a drying furnace (atmospheric pressure) maintained at a temperature of 80 ° C. and held for 30 minutes to evaporate water (evaporation step).

  Thereafter, the carbon paper was held at a sintering temperature of 380 ° C. for 60 minutes to sinter PTFE to produce a gas diffusion layer (PTFE sintering step).

The pore distribution in the gas diffusion layer was measured by mercury porosimetry. As a measurement condition of the mercury intrusion method, an auto pore apparatus IV (manufactured by Micrometrics) was used, and about 1 gram of a gas diffusion layer was put into a measurement cell, and mercury intrusion was performed (injection pressure was 0.1 to 33000 psia).
Then, the polymer electrolyte fuel cell (single cell) shown in FIG. 1 was assembled and a power generation test was conducted. In this case, pure hydrogen gas (utilization rate: 70%, gauge pressure: 40 kPa) was used as the fuel, and air (utilization rate: 35%, gauge pressure: 50 kPa) was used as the oxidant gas. The temperature of the cooling water discharged from the fuel cell was 70 ° C. As the oxidant gas, the air was not humidified. The humidification of the fuel was low and the relative humidity was 30%.

  FIG. 1 is a conceptual diagram of a polymer electrolyte fuel cell (single cell). As shown in FIG. 1, a polymer electrolyte fuel cell (single cell) includes a fuel gas distribution plate 10 having a fuel passage 10a to which fuel is supplied, a fuel gas diffusion layer 11, a proton conductor, and a catalyst. The catalyst layer 12 for fuel having a conductive material (carbon black), the electrolyte membrane 13 formed of a polymer material having proton conductivity, and the oxidant having a proton conductor, catalyst, and conductive material (carbon black) The catalyst layer 14, the oxidant gas diffusion layer 15, and the oxidant gas distribution plate 16 having the oxidant gas passage 16a to which the oxidant gas is supplied are sequentially stacked in the thickness direction. The fuel gas flow plate 10 and the oxidant gas flow plate 16 have gas permeability and conductivity. The fuel gas diffusion layer 11 is basically formed of the same type as the oxidant gas diffusion layer 15.

(Comparative Example 1)
Except for the carbon paper being TGP-H-060 (thickness 180 μm), gas diffusion layers for oxidant gas and fuel were produced in the same manner as in Example 1. Furthermore, the pore distribution of the gas diffusion layer and the power generation characteristics of the fuel cell were measured in the same manner as in Example 1.

(Comparative Example 2)
A gas diffusion layer was produced in the same manner as in Example 1 except that the carbon paper was TGP-H-090 (thickness: 270 μm). Furthermore, the pore distribution of the gas diffusion layer and the power generation characteristics of the fuel cell were measured in the same manner as in Example 1.

(Pore distribution)
2 to 4 show the pore distribution of the examples. As can be seen from FIG. 2, according to the example, the frequency of pores having a pore diameter of 10 to 30 μm (especially a pore diameter of 20 to 30 μm) is high, and a pore diameter of 24.2 μm (about 23 to 26 μm). ) Having the highest frequency (single peak).

  Further, as can be understood from FIGS. 3 and 4, when the total pore volume is 100% by volume, the total pore volume of pores exceeding 30 μm was set to about 17% by volume (20% by volume or less). . In other words, the total pore volume of pores of 30 μm or less was set to about 83 volume% (80 volume% or more).

  Further, according to the example, as can be understood from FIG. 4, the cumulative pore volume percentage up to a pore diameter of 10 μm is about 33%, and the cumulative pore volume percentage where the pore diameter exceeds 30 μm is about 17%. In view of this, the cumulative pore volume percentage with a pore diameter of 10 to 30 μm was about 50% (100% −33% −17% = 50%).

  In addition, according to the examples, as can be understood from FIG. 4, the cumulative pore volume percentage up to 20 μm is about 45%, and the cumulative pore volume percentage exceeding 30 μm is about 17%. In view of this, the cumulative pore volume percentage with a pore diameter of 20 to 30 μm was about 38% (100% −45% −17% = 38%).

  With such a pore size and pore distribution, it is presumed that the take-back of water in the gas upstream region that tends to dry can be suppressed. In addition, it is assumed that water discharge is ensured in the gas downstream region where flooding is likely to occur. Therefore, considering FIG. 4, the cumulative pore volume percentage with a pore size of 20-30 μm is about 30-70%, especially 30-65%, 33-65%, and further 35-55%. it can.

  In addition, according to the example, considering FIG. 4, the cumulative pore volume percentage up to a pore diameter of 20 μm is about 45%, and the cumulative pore volume percentage where the pore diameter exceeds 28 μm is about 20%. In consideration of the fact, the cumulative pore volume percentage with a pore diameter of 20 to 28 μm was about 35% (100% −45% −20% = 35%). Thus, the pore diameter can be concentrated to about 30 to 40% by volume within the range of 20 to 28 μm.

  Since the fuel gas diffusion layer is the same type as the oxidant gas diffusion layer, as described above, in the pore distribution in which the horizontal axis is the pore diameter and the vertical axis is the pore volume, When the entire volume is 100% by volume, the high frequency peak of the pore volume is set within the range of the pore diameter of 10 to 30 μm (about 23 to 26 μm), and the entire pore volume is 100% by volume. In this case, the total pore volume of pores exceeding 30 μm is set to 20% by volume or less (about 18% by volume). Therefore, according to FIG. 4, if the cumulative pore volume percentage of pores exceeding 30 μm is K2 (18 volume%) and the cumulative pore volume percentage of 20-30 μm is K1 (38 volume%), K1 / K2 = 38% by volume / 18% by volume≈2.1. In consideration of K1 / K2 in Examples and Comparative Examples, K1 / K2 = 1.2 to 4.0, or 1.4 to 3.0, or 1.6 to 2.5 can be set.

  5 to 7 show the pore distribution of Comparative Example 1. FIG. As can be seen from FIG. 5, according to Comparative Example 1, the frequency of pores having a pore diameter of 32.9 μm was the highest (single peak). Further, as can be understood from FIGS. 6 and 7, when the total pore volume is 100% by volume, the total pore volume of pores exceeding 30 μm was set to about 40% by volume. In other words, the total pore volume of pores of 30 μm or less was set to about 60% by volume.

  Further, according to Comparative Example 1, as can be understood from FIG. 7, the cumulative pore volume percentage up to 20 μm is about 40%, and the cumulative pore volume percentage over 30 μm is about 40%. In consideration of the fact, the cumulative pore volume percentage with a pore diameter of 20 to 30 μm was about 20% (100% −40% −40% = 20%).

  Further, according to Comparative Example 1, as can be understood from FIG. 7, the cumulative pore volume percentage up to a pore diameter of 10 μm is about 35%, and the cumulative pore volume percentage where the pore diameter exceeds 30 μm is about 40%. In consideration of the fact, the cumulative pore volume percentage with a pore diameter of 10 to 30 μm was about 25% (100% −40% −35% = 25%). According to FIG. 7, K1 / K2 = 20% by volume / 40% by volume = 0.5.

  8 to 10 show the pore distribution of Comparative Example 2. As can be understood from FIG. 8, according to Comparative Example 2, a double peak of a pore diameter of 24.2 μm and a pore diameter of 33.0 μm was obtained. Moreover, the peak of the pore diameter of 33.0 μm was higher than the peak of the pore diameter of 24.2 μm. Further, as can be understood from FIGS. 9 and 10, when the total pore volume is 100% by volume, the total pore volume of pores exceeding 30 μm was set to about 25% by volume. In other words, the total pore volume of pores of 30 μm or less was set to about 75% by volume.

  Further, according to Comparative Example 2, as can be understood from FIG. 10, the cumulative pore volume percentage up to 20 μm is about 50%, and the cumulative pore volume percentage exceeding 30 μm is about 25%. In consideration of the fact, the cumulative pore volume percentage with a pore diameter of 20 to 30 μm was about 25% (100% −50% −25% = 25%).

  In addition, according to Comparative Example 2, as can be understood from FIG. 10, the cumulative pore volume percentage up to 10 μm is about 40%, and the cumulative pore volume percentage exceeding 30 μm is about 25%. Taking this into consideration, the cumulative pore volume percentage with a pore diameter of 10 to 30 μm was about 35% (100% −40% −25% = 35%). Note that, according to FIG. 10, K1 / K2 = 25% by volume / 25% by volume = 1.

  FIG. 11 shows the results of a power generation test under the above power generation conditions. As shown in FIG. 11, the fuel cell according to the example had a higher cell voltage than the fuel cell according to Comparative Example 1 and the fuel cell according to Comparative Example 2. On the other hand, since Comparative Example 1 and Comparative Example 2 have many large pores exceeding 30 μm, it is presumed that there is a large amount of water brought back in the gas upstream region of the oxidant gas diffusion layer. For this reason, according to Comparative Examples 1 and 2, the cell voltage is unlikely to increase during the low humidification operation in which the humidification amount of the oxidant gas (air) supplied to the oxidant electrode of the fuel cell is small.

  On the other hand, in the embodiment, since the pore diameter is generally small in the oxidant gas diffusion layer, there are few large pores exceeding 30 μm, and therefore, the water is not brought back in the oxidant gas diffusion layer. Therefore, in the embodiment, even when the humidification amount of the oxidant gas (air) supplied to the oxidant electrode of the fuel cell is small, the polymer electrolyte membrane is less dried and the balance between water retention and drainage is reduced. This is presumably due to the fact that the

(Other)
In the above embodiment, carbon black (VULCAN XC72R) is used as a conductive material attached to the gas diffusion layer, but other carbon blacks are also possible. For example, acetylene black, boron-added acetylene black (BMAB), or Ketchenen black may be used. Furthermore, carbon nanofibers, carbon nanotubes, vapor grown carbon fibers, carbon fillers, carbon fibers, graphite fibers, and the like can be used as conductive materials. Also, in the examples, a mixed paste of PTFE and carbon black is used, but if it is a component that decomposes below the sintering temperature of PTFE, it can be a pore forming agent, such as polypinyl alcohol, dietary fiber, etc. It is also possible to make holes positively.

  Moreover, although carbon paper was used as the porous substrate in the examples, for example, paper such as carbon material, ceramic material, glass material, polymer material, cloth, felt, mesh, and the like can be used.

  In the above-described embodiments, the humidification of the air as the oxidant gas is set to low humidification. In addition, the present invention is not limited to the embodiments described above and shown in the drawings, and can be implemented with appropriate modifications within a range not departing from the gist.

  The present invention can be used in, for example, fuel cell power generation systems for vehicles, stationary devices, electric devices, and electronic devices.

It is a conceptual diagram of a fuel cell. It is a graph which shows the relationship between a pore diameter and a pore volume concerning an Example. It is a graph which concerns on an Example and shows the relationship between a pore diameter (0-1000 micrometers) and a cumulative pore volume percentage. It is a graph which concerns on an Example and shows the relationship between a pore diameter (10-50 micrometers) and a cumulative pore volume percentage. 6 is a graph showing a relationship between a pore diameter and a pore volume according to Comparative Example 1. It is a graph which concerns on the comparative example 1 and shows the relationship between a pore diameter (0-1000 micrometers) and a cumulative pore volume percentage. It is a graph which concerns on the comparative example 1 and shows the relationship between a pore diameter (10-50 micrometers) and a cumulative pore volume percentage. 4 is a graph showing a relationship between a pore diameter and a pore volume according to Comparative Example 2. It is a graph which concerns on the comparative example 2 and shows the relationship between a pore diameter (0-1000 micrometers) and a cumulative pore volume percentage. It is a graph which concerns on the comparative example 2 and shows the relationship between a pore diameter (10-50 micrometers) and a cumulative pore volume percentage. It is a graph which shows the result of a power generation test.

Explanation of symbols

  In the figure, 10 is a gas distribution plate for fuel, 11 is a gas diffusion layer for fuel, 12 is a catalyst layer for fuel, 13 is an electrolyte membrane, 14 is a catalyst layer for oxidant, 15 is a gas diffusion layer for oxidant, 16 The gas distribution board for oxidants is shown.

Claims (3)

A fuel gas diffusion layer having a large number of pores and supplied with fuel, a fuel catalyst layer containing a catalyst, an electrolyte membrane having ion conductivity, and an oxidant gas containing a catalyst In a polymer electrolyte fuel cell in which a catalyst layer and a porous gas having a large number of pores and an oxidant gas diffusion layer to which an oxidant gas is supplied are stacked in this order,
The gas diffusion layer for oxidizing agent is
In the pore distribution with the horizontal axis as the pore diameter and the vertical axis as the pore volume, the high frequency peak of the pore volume is set within the range of the pore diameter of 10 to 30 μm, and
A solid polymer fuel cell, wherein the total pore volume of pores exceeding 30 μm is set to 20% by volume or less when the total pore volume is 100% by volume.   2. The polymer electrolyte fuel cell according to claim 1, wherein the oxidant gas diffusion layer is formed of an aggregate of carbon fibers. 3. The gas diffusion layer for an oxidant according to claim 1 or 2, wherein the pores having a pore diameter of 20 to 30 [mu] m are 30 to 70% by volume when the entire pore volume is 100% by volume. A solid polymer fuel cell.

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