JP2009238376A - Fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell capable of homogenizing a state of wetness on a cathode side in a polymer electrolyte fuel cell, and capable of operation at a higher load. <P>SOLUTION: A polymer electrolyte membrane 11 is pinched by reaction layers 12a, 12b, the reaction layers 12a, 12b are pinched by a cathode side diffusion layer 13 and an anode side diffusion layer 14, and furthermore, the cathode side diffusion layer 13 and the anode side diffusion layer 14 are pinched by separators 17, 18. In the cathode side diffusion layer 13, micro porous layers 13b, 13c, 13d are formed on both faces of a diffusion layer base material 13a, and in the micro porous layers 13c, 13d on a gas flow passage side, gas is easier to be permeated on the downstream side than on the upstream side. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a fuel cell and can be suitably used for a polymer electrolyte fuel cell system having no humidifier.

  A polymer electrolyte fuel cell includes a membrane-electrode assembly in which a membrane made of a polymer electrolyte is sandwiched between reaction layers, and the outside of the reaction layer is sandwiched between diffusion layers that play a role of current collection and gas diffusion. ing. In the reaction layer, carbon particles supporting a catalyst such as platinum and a solid polymer electrolyte such as Nafion (registered trademark, Nafion (manufactured by Dupont)) are mixed. The both surfaces of the membrane-electrode assembly are sandwiched by separators having air or hydrogen gas flow paths to form unit cells, and 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 of the separator on the anode side, and supplied to the reaction layer through the diffusion layer. Then, hydrogen is oxidized by the electrochemical reaction in the reaction layer to generate protons and electrons. The protons thus generated move in the reaction layer and the polymer solid electrolyte while drawing water in the form of oxonium ions, and reach the cathode side. On the other hand, oxygen supplied to the cathode side combines with oxonium ions to generate water.

  As described above, in the polymer electrolyte fuel cell, water is generated on the cathode side, and further, water moves from the anode side to the cathode side, so that the diffusion layer on the cathode side is flooded. A phenomenon may occur, and gas permeation through the diffusion layer may be difficult.

  On the other hand, particularly in an air-cooled solid polymer fuel cell, the membrane ratio is increased by increasing the stoichiometric ratio (that is, the ratio of the amount of air supplied to the amount of air necessary for reaction with the supplied hydrogen gas). Since the electrode assembly is cooled, the amount of water carried away by the air supplied to the cathode side increases, and the membrane-electrode assembly tends to be in a dry state.

Therefore, a microporous layer that is denser and more water-repellent than the diffusion layer is formed on both surfaces of the diffusion layer on the cathode side, and prevents the inside of the diffusion layer from being immersed in water due to the water repellency of the microporous layer. Prevention of drying of the electrolyte membrane is performed (Patent Document 1 and Patent Document 2).
Japanese Patent Laid-Open No. 9-245800 Japanese Patent Laid-Open No. 2005-174607

  However, the degree of wetness on the cathode side in polymer electrolyte fuel cells varies depending on the location. For example, a uniform microporous layer that is denser and more water-repellent than the diffusion layer is formed on both sides of the diffusion layer on the cathode side. However, it was difficult to make all the places properly wet. This is because, even on the same cathode side, an oxidizing gas with a low humidity flows on the upstream side of the gas flow path, so that it is easy to dry, and the downstream side contains the moisture present on the cathode side and an oxidizing gas in a high humidity state. This is because the moisture is less likely to evaporate than the upstream side, and it tends to be in a wet state.

  The present invention has been made in view of the above-described conventional circumstances, and can make the degree of wetness on the cathode side of a polymer electrolyte fuel cell more uniform, and can be operated at a higher load. And providing a fuel cell system is a problem to be solved.

In the fuel cell according to the first aspect of the present invention, the solid polymer electrolyte membrane is sandwiched from both sides by a reaction layer containing a catalyst, sandwiched by a diffusion layer that allows gas permeation from the outside of the reaction layer, and further the diffusion layer In the fuel cell sandwiched by the separator that forms the gas flow path from the outside of the
The diffusion layer on the cathode side has microporous layers that are denser and more water-repellent than the diffusion layer substrate on both sides of the diffusion layer substrate, and the microporous layer on the gas flow path side is downstream of the upstream side. This is characterized in that gas permeation is easier.

In the fuel cell according to the first aspect of the present invention, the microporous layers that are denser and more water-repellent than the diffusion layer substrate are formed on both sides of the diffusion layer substrate of the diffusion layer on the cathode side. The water repellency of the layer can prevent the inside of the diffusion layer from being immersed in water, and can also prevent the polymer electrolyte membrane from drying.
In addition, since the microporous layer on the gas flow path side is more easily permeated with the gas on the downstream side where it is likely to be wet than the upstream side where it is likely to be dry, the wet state can be made more uniform. For this reason, even if the flow rate of the oxidizing gas is increased in a high load operation, it is difficult for the upstream side to be overdried or the downstream side to be overwet. For this reason, operation with a higher load becomes possible.

  Examples of a method for forming a microporous layer having a gas permeability different depending on the location on the surface of the diffusion layer base include changing the pore diameter of carbon depending on the location or changing the content of carbon or water repellent. And so on.

  In the fuel cell of the second aspect of the present invention, the microporous layer on the gas flow path side formed on the surface of the diffusion layer base material on the cathode side has carbon whose pore diameter is controlled by a gap generated between the primary particles. The water-repellent resin is contained, and the pore diameter of the downstream carbon is made larger than the pore diameter of the upstream carbon.

一方、反応ガスは極めて小さな径の通路であっても水よりも容易に通過することができる。本発明の第２の局面の燃料電池では、カソード側の拡散層の表面に形成されているガス流路側のマイクロポーラス層にはカーボンが含有されており、下流側のカーボンの細孔径は上流側のカーボンの細孔径よりも大きくされている。このため、上流側のマイクロポーラス層は水が透過し難くなり、反応層及び固体高分子電解質層の乾燥を防ぐことができる。また、下流側のマイクロポーラス層は、ガスも液体も通しやすくなるため、酸化ガスによる水の持ち去り量が増えるため、湿潤過多となりやすい下流側拡散層が水浸しの状態になることを防ぐことができる。 The gap between the primary particles varies depending on the production method of carbon, and the pore diameter can be controlled by this. The gas permeability can be easily controlled by changing the pore diameter of carbon. Further, it is known that the water pressure ΔP required when water passes through the communicating pores is represented by the following equation (1).

On the other hand, the reaction gas can pass through even a very small diameter passage more easily than water. In the fuel cell of the second aspect of the present invention, the microporous layer on the gas flow path side formed on the surface of the diffusion layer on the cathode side contains carbon, and the pore diameter of the carbon on the downstream side is upstream. The pore diameter of the carbon is larger. For this reason, it is difficult for water to permeate through the upstream microporous layer, and drying of the reaction layer and the solid polymer electrolyte layer can be prevented. Also, the downstream microporous layer facilitates the passage of gas and liquid, and the amount of water taken away by the oxidizing gas increases, thereby preventing the downstream diffusion layer, which tends to be excessively wet, from becoming immersed. it can.

  As for the selection of the pore diameter, the optimal pore diameter varies depending on the conditions such as the stoichiometric ratio of the air flowing to the cathode side, the operating temperature of the fuel cell, the humidity of the air, etc. Normally, when operating with an air-cooled fuel cell, the upstream pore diameter is 0.005 to 0.03 μm and the downstream pore diameter is 0.05 to 0.3 μm at a stoichiometric ratio of 10 to 200. It is preferable to do.

  Further, in the fuel cell according to the second aspect of the present invention, since the water-repellent resin is added to the microporous layer, the water wettability of the microporous layer is reduced, and the water permeation is prevented without hindering the gas permeability. Can be prevented. For this reason, drying of the membrane-electrode assembly can be prevented without hindering the supply of oxygen gas to the reaction layer.

Hereinafter, embodiments embodying a fuel cell according to the present invention will be described.
(Embodiment)
The fuel cell according to the embodiment is configured by stacking a plurality of fuel cell single-layer cells 10 shown in FIG. The fuel cell single layer cell 10 includes a polymer electrolyte membrane 11 made of Nafion (registered trademark) sandwiched from both sides by reaction layers 12a and 12b containing a platinum catalyst, and a cathode side diffusion layer from the outside of the reaction layers 12a and 12b. 13 and the anode side diffusion layer 14, and further sandwiched by separators 17 and 18 that form gas flow paths 15 and 16 from the outside of the cathode side diffusion layer 13 and the anode side diffusion layer 14.

  The cathode side diffusion layer 13 is a porous microporous material containing carbon powder and a water repellent resin on the reaction layer 12a side of a diffusion layer base material 13a made of carbon woven fabric, carbon paper or the like capable of gas diffusion. Layer 13b is formed. A small pore diameter microporous layer 13c containing carbon having a small pore diameter is formed on the upstream surface of the diffusion layer base material 13a on the gas flow path 15 side, and the downstream surface on the gas flow path 15 side. A large pore diameter microporous layer 13d containing carbon having a large pore diameter is formed. Carbon particles are usually formed by agglomeration of fine particles to form secondary particles, and these secondary particles have pores communicating with the outside (that is, gaps formed in the carbon particles). The smaller the pore diameter is, the more difficult it is for water to pass through the microporous layer. For this reason, the pore diameter of the carbon used as the raw material for the microporous layer substantially matches the pore diameter of the microporous layer. Therefore, the pore diameter of the microporous layer can be controlled by setting the pore diameter in the gap generated in the carbon particles.

  The anode side diffusion layer 14 has porous microporous layers 14b and 14c containing carbon powder and a water repellent resin formed on both sides of the diffusion layer base material 14a.

  As the diffusion layer base materials 13a and 14a, for example, carbon cloth, carbon paper, a nonwoven fabric made of carbon fiber, or the like can be used. Further, as the microporous layers 12b and 12c, 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.

  In the fuel cell single-layer cell 10 of Embodiment 1 configured as described above, the microporous material is denser and more water-repellent than the diffusion layer base material 13a on both sides of the diffusion layer base material 13a in the diffusion layer 13 on the cathode side. Since the layers 13b, 13c, and 13d are formed, the inside of the diffusion layer base material 13a is prevented from being immersed in water due to water repellency, and the movement of water from the reaction layer 12b to the diffusion layer 13 side is prevented. Drying of the electrolyte membrane 11 can be prevented.

  Further, the microporous layers 13c and 13d of the diffusion layer base material 13a in the diffusion layer 13 on the cathode side are formed with a small pore diameter microporous layer 13c containing carbon having a small pore diameter on the upstream side that is easy to dry. A large-pore-diameter microporous layer 13d containing carbon having a large pore diameter is formed on the downstream side of the gas flow path 15 that tends to become wet. For this reason, the wet state can be made more uniform, and even if the flow rate of the oxidizing gas is increased in a high load operation, it becomes difficult for the upstream side to become dry and the downstream side to become excessively wet. For this reason, operation with a higher load becomes possible.

Hereinafter, examples in which embodiments are described more specifically will be described in detail.
Example 1
<Preparation of microporous layer>
A fine powder of polytetrafluoroethylene was mixed with the carbon black powder (primary particle diameter of 16 nm) so as to be 35 parts by weight to prepare a microporous layer having a small pore diameter that was self-supported by welding.
In the same manner, a self-supporting large pore diameter microporous layer was prepared using carbon black having a primary particle diameter of 35 nm.

  FIG. 2 shows the result of measuring the pore distribution of the small pore diameter microporous layer and the large pore diameter microporous layer thus obtained with a porosimeter. From this figure, it was found that the small pore diameter microporous layer has a central peak at a pore diameter of 0.015 μm, and the large pore diameter microporous layer has a central peak at 0.15 μm.

<Adjustment of diffusion membrane with reaction layer>
Prepare a plain weave carbon cloth cut into a rectangle of the specified size, stack a small-pore microporous layer on one side, and stack a small-pore microporous layer on the other half, and on the other half A large pore diameter microporous layer is stacked. Then, they are integrated by thermocompression bonding at 345 ° C.

Next, 2 g to 200 g of Nafion (registered trademark) solution (5 wt% water / isopropanol solution) is added to 1 g of commercially available platinum-supported carbon (platinum content 60 wt%), and stirred with a hybrid mixer to obtain a catalyst paste. The surface of the film integrated by the method was printed so that the amount of Pt supported was 0.01 to ² mg / cm ² and dried to obtain a diffusion layer with a reaction layer.
Instead of printing on the diffusion layer to form the reaction layer, the catalyst paste is printed on a polytetrafluoroethylene sheet, dried, and peeled to produce a self-supporting reaction layer film. The reaction layer may be formed by thermocompression bonding with a diffusion layer.

  A cathode-side reaction layer-attached diffusion layer prepared as described above, and an anode-side reaction layer-attached diffusion layer in which a small-pore-diameter microporous layer is pressure-bonded only on the reaction layer side are prepared. The membrane was sandwiched between reaction layers from both sides of the molecular electrolyte membrane and pressed by hot pressing. Further, oxygen and hydrogen gas supply passages were formed on the outside thereof to produce a fuel cell single layer cell.

(Comparative Example 1)
In Example 2, in the production of the diffusion layer with the anode-side reaction layer, the large pore size microporous layer was not used, and the small pore size microporous layer was bonded to both surfaces of the diffusion layer. Others are the same as those of the fuel cell single-layer cell of Example 1, and the description is omitted.

<Evaluation>
About the fuel cell single layer cell of Example and Comparative Example 1 produced in this way,
While introducing 0.1 MPa of dry hydrogen gas into the gas flow path of the anode-side separator, while controlling with a control device so that the stoichiometric ratio of supply air becomes a predetermined value in the gas flow path of the cathode-side separator The relationship between current and cell voltage was measured. The cell temperature was adjusted to 50 ° C. with a temperature controller.

  As a result, as shown in FIG. 3 to FIG. 6, as the stoichiometric ratio increases, it can be seen that the fuel cell single layer cell of the example maintains a higher cell voltage up to a higher current than the comparative example, It was found that power generation at high load is possible. The reason for this is explained as follows.

That is, when the stoichiometric ratio is less than 10, since the amount of air supplied to the cathode reaction layer is small, no large pressure is generated in the reaction layer. For this reason, regardless of the pore size of the microporous layer, a flooding phenomenon occurs and the cell voltage is greatly reduced.
On the other hand, when the stoichiometric ratio is 10 or more, the amount of air supplied to the cathode side reaction layer increases and the pressure in the reaction layer increases. In this case, in the fuel cell unit cell of the comparative example, since the pore diameter of the microporous layer on the gas flow path side joined to the cathode side diffusion layer is as small as 0.015 μm, it is difficult for water to be discharged even when the pressure increases. The flooding phenomenon generally reduces the cell voltage. On the other hand, in the fuel cell unit cell of the example, since the pore diameter of the microporous layer on the gas flow path side is as large as 0.15 μm.

  In the measurement of the current-cell voltage, the humidity at the air inlet and outlet was measured at the same time, and the relationship between the current and the moisture content in the electrode was determined by calculation from the results shown in FIGS. As a result, as shown in FIGS. 7 to 10, it was found that the fuel cell single-layer cell of the example had less water content than the comparative example as the stoichiometric ratio increased. Also from this result, it can be seen that water is easily discharged on the cathode side of the fuel cell single-layer cell of the example, and the flooding phenomenon hardly occurs even when the current is increased.

<Fuel cell system>
As described above, since the fuel cell of the present invention is unlikely to be dried or flooded on the cathode side even if the stoichiometric ratio is increased, air-cooled fuel cell in which the fuel cell is cooled by flowing air to the cathode side. It can utilize suitably for a system.
The following system can be considered as such an air-cooled fuel cell system. That is,
(1) A solid polymer electrolyte membrane is sandwiched from both sides by a reaction layer containing a catalyst, is sandwiched by a diffusion layer that allows gas permeation from the outside of the reaction layer, and further forms a gas flow path from the outside of the diffusion layer. A fuel cell sandwiched between separators, a fuel gas supply hand supply for supplying a fuel gas to a gas flow path on the anode electrode side of the fuel cell, and an oxidizing gas and a refrigerant gas on a gas flow path on the cathode electrode side of the fuel cell As a fuel cell system comprising air supply means for supplying air and control means for controlling the temperature of the fuel cell by controlling the air supply amount,
The diffusion layer on the cathode side has microporous layers that are denser and more water-repellent than the diffusion layer substrate on both sides of the diffusion layer substrate, and the microporous layer on the gas flow path side is downstream of the upstream side. The fuel cell system is characterized in that the gas permeation is made easier, and the control means controls the stoichiometric ratio of the supply air to be in the range of 10 to 200.

  As is clear from the above embodiments, this fuel cell system can exhibit a high voltage characteristic at a high load especially at a high stoichiometric ratio of 10 or more. However, when the stoichiometric ratio exceeds 200, the membrane / electrode assembly is dried by drying, which may cause a voltage drop. Various air blowers such as a sirocco fan and a turbo fan can be used as the air supply means. Furthermore, the air-cooled solid polymer fuel cell can be driven without a methanol reformer.

  An embodiment of such a fuel cell system is shown in FIG. This fuel cell system is air-cooled, and a fuel cell stack 40, a hydrogen gas supply system 50 for supplying hydrogen gas as a fuel gas to the fuel cell stack 40, and air as an oxidizing gas to the fuel cell stack 40. The air supply system 60 for supplying and the control apparatus 70 for controlling the air blower 61 provided in the air supply system 60 are provided.

  The fuel cell stack 40 has a structure in which the single-layer cells for fuel cells of the above-described embodiments are stacked, and in order to introduce air into the air flow path of the fuel cell stack 40 upstream of an air flow path (not shown). The air manifold 41 is provided, and an air duct 42 for exhausting air is provided downstream of the air flow path. The air duct 42 is provided with a temperature sensor 43 for measuring the temperature of the exhaust gas. The temperature sensor 43 is connected to the control device 70, receives a signal related to the temperature from the temperature sensor 43, and controls the blower 61. It can be controlled.

  The air supply system 60 is provided with a blower 61 for taking in outside air, and is connected to the air manifold 41 through a supply pipe 62. The blower 61 has a capacity of a total pressure of 30 kPa. Note that the air supply system 30 is not provided with a humidifier.

  In the hydrogen supply system 50, a hydrogen cylinder 51 serving as a hydrogen source is connected to a hydrogen pump 58 via adjustment valves 52 to 57, and further connected to a hydrogen gas flow path (not shown) of the fuel cell stack 40 from the hydrogen pump 58. Has been.

When driving the fuel cell system configured as described above, the adjustment valves 52 to 57 of the hydrogen supply system 50 are adjusted to a predetermined pressure, supplied to the hydrogen pump 58, and further driven by driving the hydrogen pump 58. Then, hydrogen gas is supplied to the hydrogen gas flow path of the fuel cell stack 40 at a predetermined flow rate.
Further, the blower 61 of the air supply system 60 is driven to supply air from the air manifold 41 to the air flow path of the fuel cell stack 40. Thereby, an electrochemical oxidation reaction of hydrogen gas and an electrochemical reduction reaction of oxygen are performed in the fuel cell stack 40, and a current flows to the load 80.
Excess air supplied to the air flow path is exhausted from the air duct 42, and the temperature sensor 43 senses the temperature of the exhausted air and transmits a temperature-related signal to the control device 70. Further, the control device 70 compares the temperature sensed by the temperature sensor 43 with the set temperature. If the sensed temperature is higher than the set temperature, the output of the blower 61 is increased to send more air, and the fuel cell stack 40 Air-cool. Further, when the sensed temperature is lower than the set temperature, the output of the blower 61 is lowered to lower the air flow rate. As a result, the temperature of the fuel cell stack 40 increases. Thus, the stoichiometric ratio can be adjusted by the control device 70.

  In the fuel cell system of the example, the microporous layer having a finer water repellency than the diffusion layer substrate is formed on both sides of the diffusion layer substrate of the diffusion layer on the cathode side. As a result, the diffusion layer can be prevented from being immersed in water, and the polymer electrolyte membrane can be prevented from drying. Further, since the microporous layer on the gas flow path side is more easily permeated with gas on the downstream side where it is likely to be wet than the upstream side where it is likely to be dry, the wet state can be made more uniform. For this reason, even if the flow rate of the oxidizing gas is increased in a high load operation, it is difficult for the upstream side to be overdried or the downstream side to be overwet. For this reason, operation with a higher load becomes possible. For this reason, although the air-cooled fuel cell is driven with a large stoichiometric ratio, a decrease in the output of the fuel cell system is prevented.

  The present invention is not limited to the description of the embodiments and examples of the invention described above. Various modifications are also included in the present invention as long as those skilled in the art can easily conceive without departing from the scope of the claims.

It is a schematic cross section of the single layer cell for fuel cells of an embodiment. It is a graph which shows the measurement result of the pore distribution of the microporous layer prepared in the Example. It is a graph which shows the relationship between the electric current and cell voltage in the stoichiometric ratio 2 of the fuel cell single layer cell of an Example. It is a graph which shows the relationship between the electric current in the stoichiometric ratio 3 of the fuel cell single layer cell of an Example, and a cell voltage. It is a graph which shows the relationship between the electric current and cell voltage in the stoichiometric ratio 5 of the fuel cell single layer cell of an Example. It is a graph which shows the relationship between the electric current in the stoichiometric ratio 10 of the fuel cell single layer cell of an Example, and a cell voltage. It is a graph which shows the relationship between the electric current in the stoichiometric ratio 2 of the fuel cell single layer cell of an Example, and the moisture content in an electrode. It is a graph which shows the relationship between the electric current in the stoichiometric ratio 3 of the fuel cell single layer cell of an Example, and the moisture content in an electrode. It is a graph which shows the relationship between the electric current in the stoichiometric ratio 5 of the fuel cell single layer cell of an Example, and the moisture content in an electrode. It is a graph which shows the relationship between the electric current in the stoichiometric ratio of 10 of the fuel cell single layer cell of an Example, and the moisture content in an electrode. It is a systematic diagram of a fuel cell system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Solid polymer electrolyte membrane 12a, 12b ... Reaction layer 13, 14 ... Diffusion layer 13a, 14a ... Diffusion layer base material 13b, 13c, 13d, 14b, 14c ... Microporous layer 13b, 13c, 14b, 14c ... Small Pore diameter microporous layer 13d ... Large pore diameter microporous layer

Claims (2)

A solid polymer electrolyte membrane is sandwiched from both sides by a reaction layer containing a catalyst, sandwiched by a diffusion layer that allows gas permeation from the outside of the reaction layer, and further sandwiched by a separator that forms a gas flow path from the outside of the diffusion layer Fuel cell,
The diffusion layer on the cathode side has microporous layers that are denser and more water-repellent than the diffusion layer substrate on both sides of the diffusion layer substrate, and the microporous layer on the gas flow path side is downstream of the upstream side. A fuel cell characterized in that is more easily gas permeable.   The microporous layer on the gas flow path side formed on the surface of the diffusion layer base material on the cathode side contains carbon and water-repellent resin whose pore diameters are controlled by gaps formed between the primary particles. 2. The fuel cell according to claim 1, wherein the pore diameter of the carbon is larger than the pore diameter of the upstream carbon.

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