JP2004192932A - Polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polymer electrolyte fuel cell which protects metallic separators 20R, 20L from corrosion and does not increase contact resistance resulting from the corrosion by injecting fuel and an oxidizer from the inside of the separators 20R, 20L toward gas diffusion electrodes 14, 15. <P>SOLUTION: The fuel cell has a structure, in which a polymer ion exchange film 11 provided with a fuel electrode 12 and an oxidation electrode 13 on the both sides, respectively, is held between the diffusion electrodes 14, 15, and hollow metallic separators 20R, 20L with a plurality of protruded portions 21 are formed with a prescribed spacing thereon are opposed to the diffusion electrodes 14, 15. The fuel and the oxidizer are injected inside the separators 20R, 20L and blown out from gas exhaust holes 22 disposed on the top face of the protruded portions 21 toward the diffusion electrodes 14, 15. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
[Industrial applications]
The present invention relates to a polymer electrolyte fuel cell using a metal separator such as stainless steel.
[0002]
[Prior art]
The polymer electrolyte fuel cell has little effect on the environment and is expected to be used as an electric energy supply source in various fields, such as a power source for automobiles, since it can be started and generated even at a low temperature of about room temperature. The polymer electrolyte fuel cell includes a membrane-electrode assembly 10 in which catalyst electrode layers 12 and 13 are formed on both surfaces of a polymer ion exchange membrane 11 and sandwiched between gas diffusion electrodes 14 and 15 made of porous carbon. (FIG. 1).
When the H ₂ -containing fuel is fed to the catalyst electrode layer 12 (fuel electrode) side, H ₂ becomes protons H ⁺ on the fuel electrode 12. The protons H ⁺ permeate through the polymer ion exchange membrane 11 in the presence of water, move to the catalyst electrode layer 13 (oxidation electrode), and receive O ₂ in the oxidant sent to the oxidation electrode 13 side and an external circuit. It reacts with electrons e ⁻ flowing from 16 and is discharged out of the system as water (reaction product). The flow of electrons e ⁻ along the external circuit 16 is extracted as electric energy, but the amount of electricity extracted from the single membrane-electrode assembly 10 is extremely small. Thus, by stacking a large number of membrane-electrode assemblies 10, electric power that can be used practically is obtained.
[0003]
When many membrane-electrode assemblies 10 are stacked, a separator is interposed between individual membrane-electrode assemblies 10. Conventionally, a separator obtained by cutting graphite having a good electrical conductivity into a predetermined shape has been used. However, a separator made of graphite is disadvantageous in that its productivity is poor and its impact resistance is poor. Therefore, instead of graphite separators, metal separators such as stainless steel having excellent corrosion resistance have been researched and developed. The metal separator is easier to process than the graphite separator, and is effective in reducing the stack thickness. The present inventors have also introduced a stainless steel sheet used for a fuel cell separator (JP-A-2000-277133).
[0004]
[Problems to be solved by the invention]
In a polymer electrolyte fuel cell in which a large number of cells are stacked, the contact resistance is integrated for each cell, and the generated power is consumed as Joule heat. Therefore, in order to improve the power generation efficiency of the fuel cell, it is required to reduce the contact resistance as much as possible. However, the metal separator is corroded by being exposed to the severe environment in the cell, and the contact resistance to the gas diffusion electrode increases. Corrosion of the metal separator is caused by the fact that sulfone groups dropped off from the polymer ion exchange membrane 11 during operation of the fuel cell become acidic substances such as sulfate ions, and the acidic substances flow out into the gas to form an acidic atmosphere in the cell. Is believed to be the main cause.
[0005]
Corrosion which causes an increase in contact resistance is suppressed by using stainless steel having good corrosion resistance for the separator substrate. In addition, the conductivity of the separator substrate is ensured by applying graphite, carbon black, a conductive metal or the like to the stainless steel surface.
Metal separators that have been proposed up to now are premised on a corrosive atmosphere inside the fuel cell unit, and the mainstream of material development is to endure the corrosive atmosphere and maintain a low contact resistance. However, when exposed to a severe atmosphere for a long period of time, corrosion of the separator surface is inevitable, and the life of the metal separator is limited due to an increase in contact resistance due to corrosion.
[0006]
[Means for Solving the Problems]
The present invention has been devised to solve such a problem, and suppresses an increase in contact resistance due to corrosion by mitigating a corrosive environment to which a metal separator is exposed. An object is to provide a polymer electrolyte fuel cell that can maintain high power generation efficiency.
[0007]
In order to achieve the object, the polymer electrolyte fuel cell of the present invention sandwiches a polymer ion exchange membrane having a fuel electrode and an oxidation electrode on both sides by gas diffusion electrodes, and a plurality of raised portions are formed at predetermined intervals. The hollow metal separator has a structure facing the gas diffusion electrode, and the gas ejection holes for ejecting the fuel and oxidant fed into the metal separator toward the gas diffusion electrode have a top surface of the raised portion. It is characterized by being formed in.
[0008]
Embodiment and operation
In the polymer electrolyte fuel cell according to the present invention, hollow metal separators 20R and 20L having a plurality of raised portions 21 formed at predetermined intervals are opposed to the gas diffusion electrodes 14 and 15 (FIG. 2). . A gas outlet 22 is formed substantially at the center of the top surface of the raised portion 21. A fuel such as hydrogen is fed to the metal separator 20R on the fuel electrode 12 side, and an oxidant such as oxygen is fed to the metal separator 20L on the oxidation electrode 13 side.
[0009]
The fuel and oxidant sent into the metal separators 20 </ b> R and 20 </ b> L are ejected from the gas ejection holes 22 toward the gas diffusion electrodes 14 and 15. A part of the fuel and the oxidant pass through the gas diffusion electrodes 14 and 15 and are consumed in the cell reaction, and then flow into the groove 23 between the adjacent raised portions 21. As the fuel and oxidant sent to the metal separators 20R and 20L, a part of the fuel and oxidant sent to the fuel electrode 12 and the oxidation electrode 13 can be used, or the entire amount of the fuel and oxidant can be used as the metal separator 20R. , 20L to the fuel electrode 12 side and the oxidation electrode 13 side.
[0010]
The fuel and the oxidizing agent are sent into the metal separators 20R and 20L and are ejected from the gas ejection holes 22 so that the inside of the metal separators 20R and 20L discharges between the supply-side gas flow path Gin and the adjacent raised portion 21. Means the side gas flow path Gout. The top surface of the raised portion 21 in contact with the gas diffusion electrodes 14 and 15 is located upstream of the gas diffusion electrodes 14 and 15 with respect to the flow of fuel and oxidant.
[0011]
When the fuel and the oxidant sent to the gas diffusion electrodes 14 and 15 cause a cell reaction, an acidic substance is eluted from the polymer ion exchange membrane 11, but the acidic substance rides on the gas flow and the discharge-side gas flow path Gout. Will be sent to In the vicinity of the top surface of the raised portion 21, the acidic substance is diluted by the fuel and the oxidizing agent jetted from the gas jetting holes 22 to decrease the concentration. As a result, despite the fact that the top surface of the raised portion 21 is in contact with the gas diffusion electrodes 14 and 15, there is no adhesion of an acidic substance, and the top surface is protected from a corrosion reaction. Therefore, an increase in contact resistance due to corrosion is prevented, and the power generation efficiency of the fuel cell is maintained at a high level for a long time.
[0012]
In addition, opening the gas ejection holes 22 on the uneven surfaces of the metal separators 20R and 20L (or the current collectors) facing the gas diffusion electrodes 14 and 15 is a technique known per se (Japanese Patent Application Laid-Open No. 2002-63915). However, since the gas supply port is opened irrespective of the uneven shape, it is not possible to expect a discharge of the acidic substance along the flow of the gas or a decrease in the concentration due to the dilution of the acidic substance near the top surface of the raised portion 21. . On the other hand, when the gas ejection holes 22 are opened at the top surface of the raised portion 21, the top surface of the raised portion 21 is located upstream of the gas diffusion electrodes 14 and 15 with respect to the flow of the fuel and oxidant, and the acidic substance is discharged. In addition, the corrosion of the metal separators 20R and 20L is suppressed.
[0013]
【Example】
A SUS304 stainless steel plate having a thickness of 0.3 mm was stretched to form raised portions 21 having a diameter of 3 mm and a height of 0.5 mm at intervals of 5 mm. A hole having a diameter of 0.3 mm was formed in the center of the top surface of the raised portion 21 to form a gas ejection hole 22. By attaching a back plate 24 to the surface opposite to the gas ejection holes 22, hollow metal separators 20R and 20L were produced.
Platinum-supported carbon black was applied to the polymer ion exchange membrane 11 (nafion 115), and the fuel electrode 12 was provided on one surface of the polymer ion exchange membrane 11 and the oxidation electrode 13 was provided on the other surface. The coating amount of the platinum-supported carbon black was adjusted so that the platinum adhesion amount was 0.5 mg / cm ² . Next, the membrane-electrode assembly 10 was produced by sandwiching the polymer ion exchange membrane 11 between carbon fiber nonwoven fabrics (gas diffusion electrodes 14 and 15) having a thickness of 0.5 mm.
[0014]
The polymer electrolyte fuel cell was assembled by sandwiching the membrane-electrode assembly 10 between the metal separators 20R and 20L.
While supplying hydrogen gas (fuel) and oxygen gas (oxidizing agent) to the obtained polymer electrolyte fuel cell, under conditions of a cell temperature of 80 ° C., a gas humidification temperature of 80 ° C., and an output current density of 0.3 A / cm ² . Ran continuously. During continuous operation, the cell voltage was measured as needed, and the change over time in the cell voltage was investigated. As a result of the investigation, the initial voltage of 0.6 V was maintained even after 500 hours from the start of operation, and no increase in contact resistance due to corrosion was detected. According to the result of disassembling the fuel cell and observing the surfaces of the metal separators 20R and 20L, the corrosion area ratio occupying the top surface of the raised portion 21 was extremely small.
[0015]
For comparison, a stainless steel plate of the same material having no gas ejection holes 22 in the ridges 21 was used as a separator, and hydrogen gas and oxygen gas were fed between the ridges 21 and the fuel cell was continuously operated under similar conditions. Upon operation, the cell voltage dropped immediately after the operation. When 100 hours had elapsed after the start of operation, the cell voltage dropped to 0 V, and the function as a fuel cell was lost. When the surface of the separator was observed after disassembly of the fuel cell, the entire surface of the raised portion 21 was corroded.
As is clear from this comparison, the fuel and the oxidizing agent are fed into the hollow metal separators 20R and 20L, and are sent out from the gas ejection holes 22 toward the gas diffusion electrodes 14 and 15, whereby the gas diffusion electrodes 14 and 15 It was confirmed that the corrosion of the protruding portion 21 in contact was suppressed, and the power generation efficiency was maintained at a high level for a long period of time.
[0016]
【The invention's effect】
As described above, the polymer electrolyte fuel cell of the present invention uses a hollow metal separator as a separator when stacking fuel cells, and contacts the gas diffusion electrode of the membrane-electrode assembly. The structure is such that fuel and oxidant are ejected from the gas ejection holes provided in the protruding portion toward the gas diffusion electrode. The ejection of the fuel and the oxidizing agent suppresses the acidic substance generated by the battery reaction from adhering to or approaching the surface of the raised portion, and reduces the corrosive effect of the environment where the raised portion in contact with the gas diffusion electrode is exposed. As a result, there is no increase in contact resistance due to corrosion, and the polymer electrolyte fuel cell has stable power generation efficiency at a high level for a long period of time.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating the operation of a polymer electrolyte fuel cell. FIG. 2 is a schematic diagram illustrating a main part of a polymer electrolyte fuel cell according to the present invention.
10: Membrane-electrode assembly 11: Polymer ion exchange membrane 12: Fuel electrode 13: Oxidation electrode 14, 15: Gas diffusion electrode 16: External circuit 20R, 20L: Metal separator 21: Raised portion 22: Gas ejection hole 23 : Groove 24: Back plate Gin: Supply gas flow path Gout: Discharge gas flow path

Claims (1)

A structure in which a polymer ion exchange membrane having a fuel electrode and an oxidation electrode on both sides is sandwiched between gas diffusion electrodes, and a hollow metal separator having a plurality of raised portions formed at predetermined intervals faces the gas diffusion electrode. A polymer electrolyte fuel cell characterized in that a gas ejection hole for ejecting a fuel and an oxidant fed into a metal separator toward a gas diffusion electrode is formed on a top surface of a raised portion.

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