JP2009181918A - Polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell wherein pressure loss of a gas passage is small and thinning and weight reduction are possible and a flooding phenomenon is hardly generated. <P>SOLUTION: The fuel cell includes a membrane-electrode assembly where a solid polymer electrolyte membrane is sandwiched by an anode electrode and a cathode electrode, a gas passage formed outside the membrane-electrode assembly, and a separator for isolating the gas passage from the outside. The gas passage includes a first gas passage sandwiched by the membrane-electrode assembly and the separator at a cathode electrode side and made of a conductive porous body, and a second gas passage arranged between the porous body and the membrane-electrode assembly and made of a groove capable of circulating a gas. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a polymer electrolyte fuel cell in which the pressure loss in an oxidizing gas flow path is small, oxygen is smoothly supplied to a catalyst layer, and an anode side electrode flooding phenomenon hardly occurs.

  Conventionally, as a polymer electrolyte fuel cell, as shown in FIG. 6, a polymer electrolyte membrane 90 is sandwiched between catalyst layers 91a and 91b, and the outside of the catalyst layers 91a and 91b plays a role of current collection and gas diffusion. One having a membrane-electrode assembly 93 (hereinafter referred to as “MEA”) sandwiched between diffusion layers 92a and 92b is known. Both surfaces of the MEA 93 are sandwiched by ribs 95a and 96a of separators 95 and 96 that form gas flow paths 94a and 94b to constitute a unit cell 97. Furthermore, a stack in which a plurality of unit cells are stacked is formed.

In this polymer electrolyte fuel cell, the following electrochemical reaction is performed.
Anode side: H ₂ → 2H ⁺ + 2e ⁻
Cathode side: 1 / 2O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O
Total reaction: H ₂ + 1 / 2O ₂ → H ₂ O
That is, hydrogen is supplied to the gas flow path 94b of the anode-side separator 96, and then supplied to the catalyst layer 91b through the diffusion layer 92b. Then, hydrogen is oxidized by the electrochemical reaction in the catalyst layer 91b to generate protons and electrons. The protons thus generated move through the catalyst layer 91b and the polymer solid electrolyte membrane 90 while drawing water in the form of oxonium ions, and reach the catalyst layer 91a on the cathode side.
On the other hand, the oxygen supplied to the cathode side is reduced to produce water.
Thus, the water generated by the electrochemical reaction inside the polymer electrolyte fuel cell is discharged together with the oxidizing gas flowing through the gas flow path 94a of the separator 95.

  However, in the polymer electrolyte fuel cell, the ribs 95a and 96a are provided in the separators 95 and 96 in order to form the gas flow paths 94a and 94b. There is a possibility that gas is hardly supplied to the portion pressed against 92b, and smooth electrode reaction may be hindered. Further, by providing the ribs 95a and 96a, the thickness of the unit cell 97 is increased, so that there is a problem that the power generation amount per unit volume is reduced.

  On the other hand, a polymer electrolyte fuel cell of a type in which a metal porous body is used for the gas flow path and a rib is not required for the separator is also known (for example, Patent Document 1). Since such a polymer electrolyte fuel cell does not require a rib, gas is smoothly supplied to the membrane electrode assembly over the entire surface of the electrode, and the electrode reaction is not hindered. Further, the thickness can be reduced because the rib is not required.

As a technique related to the present invention, Patent Document 2 describes a fuel cell using a current collector made of a conductive porous material for a fuel cell. However, this is for a solid oxide fuel cell driven at a high temperature and not for a polymer electrolyte fuel cell, and therefore there is no problem of the present invention to prevent flooding phenomenon to be described later.
JP 2007-250487 A JP 2003-86204 A

  In the conventional polymer electrolyte fuel cell in which the gas flow path is formed of a porous metal body and does not require ribs in the separator, the thickness of the porous metal body for forming the gas flow path is reduced. When attempting to reduce the thickness, there is a problem that the pressure loss increases. In addition, since the flow path formed by the metal porous body is narrow, it is difficult to remove water, and there is a possibility that a flooding phenomenon may occur in which water accumulates on the cathode side and gas passage becomes difficult.

  The present invention has been made in view of the above-described conventional situation, and solves the problem of providing a fuel cell in which the pressure loss of the gas flow path is small, the thickness and weight can be reduced, and the flooding phenomenon hardly occurs. It should be a challenge.

  The first aspect of the electrode of the polymer electrolyte fuel cell of the present invention is a membrane electrode assembly in which a solid polymer electrolyte membrane is sandwiched between an anode electrode and a cathode electrode, and is formed outside the membrane electrode assembly. A fuel cell comprising a gas channel and a separator for isolating the gas channel from the outside, wherein the gas channel is a conductive porous material sandwiched between the membrane electrode assembly and a cathode electrode side separator. A first gas flow path composed of a body, and a second gas flow path provided between the porous body and the membrane electrode assembly and having higher gas permeability than the first gas flow path. It is characterized by.

  In the polymer electrolyte fuel cell of the present invention, since the porous body serving as the first gas flow path is sandwiched between the membrane electrode assembly and the separator on the cathode electrode side, the porous body serves as a rib. There is no need to provide a rib on the separator. For this reason, the supply of the oxidizing gas to the membrane electrode assembly is smoothly performed on the entire surface of the electrode, and the electrode reaction is not partially disturbed. Further, the thickness can be reduced because the rib is not required. Furthermore, since the porous body has conductivity, the porous body can serve as a current collector for flowing electricity to the separator. In addition, since the second gas flow path is provided between the porous body and the membrane electrode assembly, even if the first flow path formed by the porous body is clogged with water, The supply of the oxidizing gas is smoothly performed by the two gas flow paths. For this reason, inhibition of the electrode reaction due to the flooding phenomenon can be prevented.

  Such a second gas flow path can be easily formed by providing a groove in the surface of the porous body on the membrane electrode assembly side. Alternatively, a groove may be provided in the cathode side surface of the solid polymer electrolyte membrane so as to serve as the second gas flow path.

  The thickness of the porous body is preferably 1.2 mm or less. When the thickness of the porous body is 1.2 mm or less, the pressure loss due to the oxidizing gas flowing through the porous body increases, and the effect of reducing the pressure loss due to the provision of the second gas channel is great. Further, since the thickness of the porous body is as thin as 1.2 mm or less, the fuel cell can be thinned, and the output per unit volume is increased.

また、ガス流路断面が円以外の場合には、水力直径（すなわち、あるガス流路断面と同じ断面積を有する円管の直径）によって排除圧が決まる。この式に基づく計算によれば、第２のガス流路の水力直径が０．１５ｍｍ以上であれば、排除圧による圧力損失は３ｋＰａ以下となり、ファン等の常圧空気供給源の使用が可能になる。 The hydraulic diameter of the second gas channel is preferably 0.15 mm or more. When the gas channel cross section is a circle, the exclusion pressure ΔP for removing water droplets clogged in the gas channel is determined by the diameter of the circle, and the larger the diameter, the smaller the exclusion pressure (see the following formula (1)) ).

When the gas channel cross section is other than a circle, the exclusion pressure is determined by the hydraulic diameter (that is, the diameter of a circular pipe having the same cross-sectional area as a certain gas channel cross section). According to the calculation based on this formula, if the hydraulic diameter of the second gas flow path is 0.15 mm or more, the pressure loss due to the exclusion pressure is 3 kPa or less, and it is possible to use a normal pressure air supply source such as a fan. Become.

  Embodiments of the membrane electrode assembly and the fuel cell using the same according to the present invention will be described below.

Example 1
In the fuel cell of Example 1, as shown in FIG. 1, both sides of a polymer electrolyte membrane 11 made of Nafion (registered trademark) are covered with a cathode side catalyst layer 12a and an anode side diffusion layer 12b. As the cathode side catalyst layer 12a and the anode side diffusion layer 12b, a water-repellent resin powder and a carbon powder can be mixed and used. Specific examples of the water-repellent resin powder include fluorocarbon polymers (for example, polytetrafluoroethylene (PTFE), tetrafluoroethylene / hexafluoropropylene copolymer (FEP), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA)). ), Polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), ethylene / tetrafluoroethylene copolymer (ETFE), ethylene / chlorotrifluoroethylene copolymer (ECTFE), etc. ). Examples of the carbon powder include carbon black, graphite powder, carbon nanofiber, furnace black, and acetylene black.

  A thin plate-like titanium porous body 13 made of a sintered body of titanium fibers is stacked on the cathode side catalyst layer 12a, and pressed from the outside by a separator 14 made of a stainless steel plate material. Grooves 13a are formed in parallel with each other at a predetermined interval in the direction in which the oxidizing gas flows on the surface of the porous titanium body 13 in contact with the cathode side catalyst layer 12a. The cross-sectional shape of the groove 13a is rectangular. Such a groove 13a can be easily formed by press-molding or cutting a thin plate made of a sintered body of titanium fiber. Air as an oxidizing gas is supplied to the porous titanium body 13 and the groove 13a by an air supply device (not shown). Here, the titanium porous body 13 is a first gas flow path, and the groove 13a is a second gas flow path.

  On the other hand, the anode-side catalyst layer 12b is overlaid with a diffusion layer 15 made of carbon cloth, carbon paper, non-woven fabric made of carbon fiber, and the like, and is pressed from the outside by a separator 16 made of stainless steel. Gas flow paths 16a are formed in the separator 16 at predetermined intervals in parallel with each other in the direction in which hydrogen gas flows. A hydrogen gas is circulated to the gas flow path 16a by a hydrogen circulation device (not shown).

(Comparative Example 1)
In the fuel cell of Comparative Example 1, as shown in FIG. 2, no groove is formed in the titanium porous body 23. Others are the same as those of the fuel cell of Example 1, and the same reference numerals are given to the same components, and the description thereof is omitted.

  FIG. 3 shows the results obtained by calculation for the hydraulic diameter d and the water droplet exclusion pressure in the groove 13a in the fuel cell of Example 1 configured as described above. From this figure, if the hydraulic diameter is 0.15 mm or more, the pressure loss will be less than 3 kPa, and the pressure loss in the gas flow path will be within the upper pressure limit of the fan that is the atmospheric pressure air supply source. I understand.

  In addition, for the fuel cell of Example 1 having a hydraulic diameter d of 0.2 mm and the fuel cell of Comparative Example 1, when humidified air was supplied to the porous titanium body at 70 ° C. so as to have a saturated water vapor pressure. The relationship between the cell temperature and the cathode pressure loss was measured. As a result, in Example 1 in which the groove 13a is provided in the titanium porous body 13 as shown in FIG. It was found to be about 2 kPa lower. From this, it can be seen that the fuel cell of Example 1 is more likely to discharge water than Comparative Example 1, and is less likely to cause flooding.

  Furthermore, the relationship between the current density and the voltage at a cell temperature of 50 ° C. when the air humidified to 70 ° C. to obtain a saturated water vapor pressure was supplied to the porous titanium body was examined. As a result, as shown in FIG. 5, even when the current density was increased in Example 1, the voltage decrease was moderate, whereas in Comparative Example 1, the voltage decreased rapidly as the current density was increased. .

The above results can be interpreted as follows.
In the fuel cell of Embodiment 1, air is supplied to the cathode side titanium porous body 13 and the groove 13a, and hydrogen gas is supplied to the anode side gas flow path 16a. For this reason, hydrogen gas is supplied to the anode side catalyst layer 12b through the diffusion layer 15 on the anode side, where hydrogen is oxidized by an electrochemical reaction to generate protons and electrons. The protons thus generated move in the polymer solid electrolyte membrane 11 while drawing water in the form of oxonium ions, and reach the cathode side catalyst layer 12a. On the other hand, oxygen in the air supplied to the cathode side titanium porous body 13 and the groove 13a is reduced in the cathode side catalyst layer 12a to generate water. Thus, the water accumulated in the cathode catalyst layer 12a is absorbed by the titanium porous body 13 and oozes out into the groove 13a. The water that has oozed into the groove 13a is easily removed by the pressure of the air flowing through the groove 13a. For this reason, in this fuel cell, the titanium porous body 13 is not soaked in water, and the fuel cell is less prone to flooding, and the pressure loss is reduced. Moreover, since it is not necessary to provide the rib for forming a gas flow path in the separator 14, the thickness and weight can be reduced accordingly.

  On the other hand, in the fuel cell of Comparative Example 1, since the groove is not provided in the titanium porous body 23 and it is difficult to discharge water, the water remains in the titanium porous body 23 and the discharge pressure rises. For this reason, it is considered that the flooding phenomenon is likely to occur.

  The present invention is not limited to the description of the embodiments of the invention. Various modifications may be included in the present invention as long as those skilled in the art can easily conceive without departing from the description of the scope of claims.

1 is a partial cross-sectional view of a fuel cell of Example 1. FIG. 4 is a partial cross-sectional view of a fuel cell of Comparative Example 1. FIG. It is a graph which shows the relationship between a hydraulic diameter and a water droplet exclusion pressure. It is a graph which shows the relationship between cell temperature and cathode pressure loss at the time of supplying the air humidified so that it may become saturated water vapor pressure at 70 degreeC to a titanium porous body. It is a graph which shows the relationship between the current density in 50 degreeC of cell temperature, and the voltage at the time of circulating the air humidified so that it may become saturated water vapor pressure at 70 degreeC to a titanium porous body. It is sectional drawing of the conventional polymer electrolyte fuel cell. 6 is a plan view of a diffusion layer according to Example 4. FIG.

Explanation of symbols

11 ... Solid polymer electrolyte membrane 12b ... Cathode side catalyst layer (anode electrode)
12a ... Cathode side catalyst layer (cathode electrode)
DESCRIPTION OF SYMBOLS 11, 12a, 12b ... Membrane electrode assembly 11 ... Gas flow path 13, 13a, 16a ... Gas flow path (13 ... Titanium porous body, 13a ... Groove, 16a ... Gas flow path)
13 ... 1st gas flow path 13a ... 2nd gas flow path

Claims (4)

A membrane electrode assembly in which a solid polymer electrolyte membrane is sandwiched between an anode electrode and a cathode electrode, a gas channel formed outside the membrane electrode assembly, and a separator for isolating the gas channel from the outside A fuel cell comprising:
The gas flow path is provided between a first gas flow path made of a conductive porous body sandwiched between the membrane electrode assembly and a cathode electrode side separator, and the porous body and the membrane electrode assembly. And a second gas passage having a gas permeability higher than that of the first gas passage.   2. The polymer electrolyte fuel cell according to claim 1, wherein the second gas flow path is formed by a groove recessed in the surface of the porous body on the membrane electrode assembly side. 3.   3. The polymer electrolyte fuel cell according to claim 1, wherein the conductive porous body has a thickness of 1.2 mm or less.   4. The polymer electrolyte fuel cell according to claim 1, wherein a hydraulic diameter of the passage is 0.15 mm or more. 5.

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