JP2002367621A - Solid polymer fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid polymer fuel cell having a metal separator 5 restrained from corrosion, maintaining high power generating efficiency for long time. SOLUTION: The solid polymer fuel cell has a structure of interposing an oxidation electrode 2 and a fuel electrode 3, arranged on both surfaces of the solid polymer film 1, between metal separators 5. Products of erosion are preliminarily removed from the solid polymer film 1, the oxidation electrode 2, and the fuel electrode 3 by the cleaning using boiling water, or by the electrolytic reaction of water generated by humid atmosphere or electricity conduction in the water solution, and a stainless steel, on the surface of which, a Ni layer is formed, is used as a metal separator 5. As the products of erosion are preliminarily removed from a thin film electrode assembly, pH is not lowered when the operation of the fuel cell is started, and as the essential excellent electron conductive property of Ni is maintained for long time, a solid polymer fuel cell with small loss of Joule heat is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polymer electrolyte fuel cell which has attracted attention as a clean power source.

[0002]

2. Description of the Related Art Among fuel cells, a polymer electrolyte fuel cell can operate at a temperature of 100 ° C. or less and has an advantage that it can be started in a short time. Also, since each member is made of solid, the structure is simple and maintenance is easy,
It can be applied to applications exposed to vibration and shock. Furthermore, it has advantages such as high power density, suitable for miniaturization, high fuel efficiency, and low noise. From these advantages, applications for electric vehicles are being studied. If a fuel cell capable of providing the same mileage as a gasoline-powered vehicle can be mounted on the vehicle, NO _x and SO _x will hardly be generated and CO
It becomes a very clean power source for the environment, such as halving the occurrence of ₂ .

[0003] The solid polymer fuel cell utilizes the fact that a solid polymer resin membrane having a proton exchange group in the molecule functions as a proton conductive electrolyte. The structure is such that a fuel gas such as hydrogen flows on one side of the polymer film and an oxidizing gas such as air flows on the other side. More specifically, as shown in FIG. 1, a solid polymer membrane 1 having a platinum-based catalyst coated on both surfaces supplies a gas to the supported catalyst and extracts a generated current from a gas diffusion electrode (oxidizing electrode 2 and The fuel electrodes 3) are joined, and the separators 5 face each other via the gaskets 4. An air supply port 6 and an air discharge port 7 are formed in the separator 5 on the oxidation electrode 2 side, and a hydrogen supply port 8 and a hydrogen discharge port 9 are formed in the separator 5 on the fuel electrode 3 side.

[0004] The separator 5 is formed with a plurality of grooves 10 extending in the flow direction of the hydrogen g and the oxygen or air o for the conduction and uniform distribution of the hydrogen g and the oxygen or air o. Heat generated during power generation is removed by a water cooling mechanism that circulates the cooling water w sent from the water supply port 11 through the inside of the separator 5 and then discharges the cooling water w from the drain port 12. Hydrogen supply port 8
The hydrogen g that has been fed into the gap between the fuel electrode 3 and the separator 5 through the solid polymer membrane 1 as protons that have emitted electrons, passes through the solid polymer membrane 1, receives electrons on the oxidation electrode 2 side, and Combustion by oxygen or air o passing through the gap. Therefore, when a current is applied to each of the separators 5 and 5 that are in contact with the oxidizing electrode 2 and the fuel electrode 3 and a load is connected, power can be taken out.

Since the amount of power generation per cell of the fuel cell is extremely small, a plurality of cells are stacked with one unit of a solid polymer film sandwiched between separators 5 and 5 as shown in FIG. This increases the amount of power that can be extracted. In a structure in which a large number of cells are stacked, the contact resistance between the oxidizing electrode 2 and the fuel electrode 3 and each of the separators 5 has a great effect on the power generation efficiency. In order to improve the power generation efficiency, a separator having good conductivity and low contact resistance with the oxidizing electrode 2 and the fuel electrode 3 is required, and a graphite separator is conventionally used similarly to the phosphoric acid type fuel cell. .

[0006]

SUMMARY OF THE INVENTION Graphite separators are:
The material cost and processing cost are high, which raises the price of the fuel cell as a whole and lowers the productivity. In addition, a graphite separator that is brittle in material is likely to be damaged when subjected to vibration or impact. Therefore, it has been proposed in Japanese Patent Application Laid-Open No. Hei 8-180883 to produce a separator from a metal plate by pressing or punching. However, the oxidation electrode 2 side through which oxygen or air o passes is in an acidic atmosphere having an acidity of pH 2 to 3. A metal material that withstands such a strongly acidic atmosphere and satisfies the characteristics required for the separator has not been put to practical use so far.

[0007] For example, an acid-resistant material such as stainless steel is considered as a metal material that can withstand a strong acid. These materials exhibit acid resistance due to a strong passivation film formed on the surface, but the passivation film increases surface resistance and contact resistance. When the contact resistance increases, a large amount of Joule heat is generated at the contact portion, resulting in a large heat loss, which lowers the power generation efficiency of the fuel cell. As a metal material which does not form an oxide film or a passive film on the surface, noble metals such as Au and Pt are known. Au is a material that can withstand acidic atmosphere,
Since it is very expensive, it is not practical as a fuel cell separator material. Pt is also a material that does not easily form an oxide film and withstands an acidic atmosphere, but cannot be said to be a practical material because it is very expensive like Au. On the other hand,
Ni is a very inexpensive and excellent electronic conductor as compared with Au and Pt, but does not have sufficient corrosion resistance in an acidic atmosphere of pH 2-3.

[0008]

SUMMARY OF THE INVENTION The present invention has been devised to solve such a problem, and utilizes the characteristics of Ni as an excellent electronic conductor by reducing the corrosiveness of the fuel cell atmosphere. It is an object of the present invention to provide a polymer electrolyte fuel cell with high power generation efficiency at a low cost by suppressing corrosion of a metal separator by making it possible.

In order to achieve the object, the polymer electrolyte fuel cell of the present invention has a structure in which an oxidizing electrode and a fuel electrode disposed on both sides of a solid polymer membrane are sandwiched by metal separators. The membrane, the oxidizing electrode, and the fuel electrode are those from which corrosion products have been removed in advance, and the metal separator
It is a stainless steel plate on which a layer is formed. As a method for removing corrosion products in advance, washing in boiling water, electrolysis of water generated by energization in a humid atmosphere or an aqueous solution, or the like is employed. As a metal separator, a surface roughness Ra: 0.5 to 30 g / m ^{2 is} applied to the surface of a stainless steel plate roughened to 0.4 to 1.5 μm.
It is preferable that a Ni layer is formed with an adhesion amount of.

[0010]

During operation of a polymer electrolyte fuel cell, sulfate ions are eluted from a thin film electrode assembly which is a constituent material of the cell. Sulfate ions are mainly derived from the resin used when fixing the Pt-based catalyst to the catalyst layer in the thin-film electrode assembly.
Lower H. Therefore, the present applicant energizes the electrode constituent members such as the solid polymer membrane 1, the gas-permeable oxidizing electrode 2 and the fuel electrode 3 in contact with the aqueous solution, so that the impurities can be set before the fuel cell is assembled. Was proposed (Japanese Patent Application No. 2000-093707).

In the subsequent research process by the present inventors, by removing impurities in advance, the corrosiveness of the fuel cell atmosphere is reduced, and a stainless steel separator having a Ni layer can be used. It has been found that the inherent properties of an excellent electronic conductor can be utilized. As a result, even if a large number of cells are stacked, the contact resistance with the carbon electrode is kept low for a long period of time, the Joule heat generated due to the increase in the contact resistance is suppressed, and the fuel cell is assembled with high power generation efficiency.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As a base material of a metal separator, there is no particular restriction on the type of steel as long as it exhibits the corrosion resistance required in a fuel cell atmosphere. Steel plates are used. In order to secure the corrosion resistance required for the fuel cell, a stainless steel sheet containing 12% by mass or more of Cr is preferable. In consideration of assembly and weight reduction of the fuel cell, a stainless steel plate having a plate thickness of 0.1 to 0.4 mm is preferable.

The surface of a stainless steel plate is electroplated,
A Ni layer is formed by an electroless plating method or the like. In order to secure the adhesion between the underlying stainless steel and the Ni layer, it is preferable to form the intermediate layer by Ni strike plating with the deposition efficiency adjusted to 15% or less, and then perform Ni main plating. When the Ni layer is formed by the electroplating method, a plating bath such as a Watts bath, a total chloride bath, a total sulfate bath or the like is used. The Ni layer
In order to lower the contact resistance by covering the entire surface of the stainless steel plate, the coating is preferably formed with an adhesion amount of 0.5 g / m ² or more. The effect of the Ni layer to lower the contact resistance increases according to the amount of deposition, but it saturates at 80 g / m ² and it is not economical to form a thicker layer.

If the surface of the stainless steel sheet is roughened before the formation of the Ni layer, the Ni
The adhesion of the layer is improved. Roughening is also effective in reducing the contact resistance of a stainless steel plate. The reduction of contact resistance due to surface roughening is proposed by the present inventors in Japanese Patent Application No. 2000-276893. For example, when a Ni layer is formed to such an extent that irregularities remain on the surface of a roughened stainless steel sheet, Due to the surface irregularities, a good adhesion state is obtained with the carbon electrode, and high electron conductivity of Ni acts synergistically to further reduce the contact resistance of the stainless steel plate. In order to maintain the concavo-convex shape even after the Ni layer is formed, the surface roughness is R
a: It is preferable to roughen the stainless steel plate so as to have a thickness of 0.4 to 1.5 μm.

For the surface roughening of the stainless steel sheet, for example, alternating electrolytic etching in an aqueous ferric chloride solution is employed, and the surface roughness is preferably adjusted to Ra: 0.4 to 1.5 μm. Fine pits are formed on the surface of the alternating electrolytically etched stainless steel plate, and the stainless steel plate is roughened. Ra: The surface roughness of 0.4 to 1.5 μm is a condition obtained from the results of investigation and research by the present inventors, and a stainless steel sheet having a Ni layer formed after the surface roughening treatment is good for a carbon electrode. Contact in an intimate contact state, effectively reducing contact resistance. Further, by roughening the surface to Ra: 0.4 μm or more, the adhesion of the Ni layer is also improved. However, if the surface is excessively roughened with Ra: more than 1.5 μm, pits formed on the surface of the stainless steel plate become unevenly distributed, and the adhesion of the Ni layer is rather lowered.

A Ni layer is formed on the surface of the roughened stainless steel plate by the method described above.
It is preferable to form with an adhesion amount of 0 g / m ² . When the adhesion amount of the Ni layer is 0.5 g / m ² or more, the entire unevenness on the surface of the stainless steel plate is covered with the Ni layer, and the effect of lowering the contact resistance due to the inherent high electron conductivity of Ni becomes remarkable. However, when the Ni layer is formed with an adhesion amount exceeding 30 g / m ² , the unevenness on the surface of the roughened stainless steel plate becomes Ni.
The effect of the concavo-convex shape filled with the layer and brought into contact with the carbon electrode in a good contact state of the stainless steel plate is impaired, and the effect of the surface roughening treatment for reducing the contact resistance is reduced.

The stainless steel sheet on which the Ni layer has been formed is obtained by removing corrosion products in advance by washing in boiling water,
Alternatively, after corrosion products are removed in advance by electrolysis of water generated by energization in a humid atmosphere or an aqueous solution, they are incorporated into a thin-film electrode assembly. When impurities are removed by washing using boiling water, the solid polymer membrane 1, the oxidizing electrode 2 and the fuel electrode 3 are immersed in boiling pure water for about 5 minutes. Since impurities dissolve in boiling water, repeated immersion in boiling water while appropriately replacing pure water reduces the impurity concentration in the thin-film electrode assembly. Preferably, boiling water immersion is repeated three times or more.

When impurities are removed by the electrolysis reaction of water, a thin film electrode assembly is assembled by sandwiching the oxidizing electrode 2 and the fuel electrode 3 disposed on both sides of the solid polymer membrane 1 with a non-corrosive separator material such as carbon. Thereafter, a voltage is externally applied to the thin-film electrode assembly while the inside of the cell is in contact with water. The environment is such that impurities easily dissolve out of the electrode components due to the electrolysis reaction of water generated by the application of voltage. It is preferable that the electrolysis reaction be continued until the concentration of the impurities eluted from the electrode component drops to 10 ppm.

In the fuel cell assembled with the electrode constituent member having the reduced impurity concentration and the stainless steel separator 5 having the Ni layer formed thereon, the elution of impurities from the thin-film electrode assembly during power generation is suppressed. , N
It does not enter the low pH range that promotes corrosion of the i-layer. As a result, excellent electronic conductivity inherent to Ni is obtained, and the contact resistance with the carbon electrode is maintained low, resulting in a polymer electrolyte fuel cell with high power generation efficiency.

[0020]

EXAMPLE A stainless steel sheet A having the composition shown in Table 1 and a BA-finished stainless steel sheet A and a stainless steel sheet B whose surface was roughened by electrolytic etching and having a Ni layer was used as a base material of the metal separator 5. A steel plate separator 5 was prepared.

[0021]

In the electrolytic etching, Fe ³⁺ : 55 g /
1, using an aqueous ferric chloride solution having a liquid temperature of 57.5 ° C., an anode current density of 3.0 kA / m ² and a cathode current density of 0.
5kA / m ² , treatment time 60 seconds, alternating electrolysis cycle 5H
Alternating electrolysis was performed under the conditions of z. Observation of the surface of the etched stainless steel plate revealed that hemispherical pits having an average diameter of 2 μm and an average depth of 1 μm were uniformly formed over the entire surface, and the surface roughness was Ra: 0.5 μm.

For the formation of the Ni layer, a method was used in which Ni strike plating was performed in advance and then Ni main plating was performed. Ni strike _{plating, NiSO 4 · 6H 2 O:} 400g /
l, Na ₂ SO ₄ : 100 g / l containing solution temperature: 55 ° C, p
Using a plating bath of H1.5, the current density at the time of electroplating was set to 5 A / dm ^2, and electricity was supplied for 12 seconds. In this _{plating, NiCl 2 · 6H 2 O:} 300g / l, H 3 BO 3:
A plating bath having a solution temperature of 55 ° C. and a pH of 2.0 containing 30 g / l was used, and the current density during electroplating was set at 10 A / dm ² . Under these conditions, for the stainless steel sheet A, the total adhesion amount was 44.5 g / m2 by setting the energization time to 290 seconds.
The ^second Ni layer to form a Ni layer of coating weight 9.0 g / m ² by the the energization time stainless steel B 58 seconds.

[Removal of impurities by boiling water immersion] After immersing the solid polymer membrane 1, the oxide electrode 2 and the fuel electrode 3 in boiling pure water for 5 minutes, replacing the pure water and repeating the boiling pure water immersion three times. To remove impurities. Next, the oxidation electrode 2 and the fuel electrode 3 were superimposed on the solid polymer membrane 1 to form a thin-film electrode assembly, which was combined with stainless steel separators A and B on which a Ni layer was formed to constitute a fuel cell. Humidified hydrogen and oxygen were supplied to the fuel cell, and the fuel cell was started. PH, F of water discharged from fuel cell
By measuring the e ion concentration and the Ni ion concentration, the acidity inside the cell and the corrosion of the separator 5 were evaluated. For comparison, a thin-film electrode assembly from which impurities were not removed was also provided with a fuel cell including a separator 5 made of a stainless steel plate having a Ni layer formed thereon, and the pH, Fe ion concentration and Ni ion The concentration was measured.

As can be seen from the measurement results shown in FIG. 2, in the fuel cell according to the present invention, a decrease in pH of water discharged from the fuel cell at the time of start-up is greatly suppressed on both the oxidizing electrode 2 side and the fuel electrode 3 side. It had been. On the other hand, in the fuel cell in which the stainless steel plate separator 5 was incorporated without removing impurities beforehand, the pH dropped significantly at the time of startup. Further, by maintaining the current density at a constant value of 0.5 A / m ² ,
After the operation for 0 hour, the stainless steel plate separator 5 was taken out from the thin film electrode assembly and the state of corrosion was investigated. As a result, no corrosion was detected in the stainless steel plate separator 5 in contact with the thin film electrode assembly from which impurities were removed, as compared with the stainless steel plate separator 5 in contact with the thin film electrode assembly in which impurities were not removed in advance. . This corrosion suppression is presumed to be due to the suppression of the pH drop at the time of starting the fuel cell.

Since corrosion of the Ni layer is suppressed, N
The original characteristics as an excellent electron conductor were maintained, and the contact resistance on the surface of the separator 5 did not increase as shown in FIG. On the other hand, in the separator 5 made of stainless steel plate that comes into contact with the thin-film electrode assembly from which impurities were not removed in advance, the contact resistance after 100 hours of operation was significantly increased. Among them, the stainless steel plate surface is roughened and Ni
The stainless steel sheet B on which the layer was formed exhibited lower contact resistance both before and after the test as compared with the stainless steel sheet A finished with BA.

Regardless of which of the stainless steel plates A and B was used as the separator substrate, Fe ions and Ni ions were not detected in the water discharged from the fuel cell. On the other hand,
Water discharged from a fuel cell constituted by bringing a stainless steel plate into contact with a thin-film electrode assembly from which impurities have not been removed in advance contains 0.2 ppm of Fe ions and 0.2 p
pm Ni ions were detected.

[Removal of Impurities by Electrolysis Reaction of Water] A fuel cell was constructed by combining a thin-film electrode assembly in which an oxidation electrode 2 and a fuel electrode 3 were superimposed on a solid polymer membrane 1 with a carbon separator. After maintaining the inside of the fuel cell in a humid atmosphere, water was electrolyzed by applying a DC voltage between both electrodes and applying a current at a current density of 0.1 A / dm ² for 10 hours. Further, current was supplied under the same conditions with the applied voltage reversed. After the impurities were removed by electrolysis of water by energization, the carbon separator was removed from the thin-film electrode assembly and replaced with a stainless steel separator 5 on which a Ni layer was formed. The fuel cell was started by supplying humidified hydrogen and oxygen to the assembled fuel cell, and the pH, Fe ion concentration and Ni ion concentration of water discharged from the fuel cell were measured in the same manner. As can be seen from the measurement results in FIG. 4, a decrease in the pH of water discharged from the fuel cell at the time of startup was significantly suppressed on both the oxidizing electrode 2 side and the fuel electrode 3 side.

Further, after operating the fuel cell for 100 hours while maintaining the current density at a constant value of 0.5 A / dm ² , the stainless steel separator 5 taken out of the thin film electrode assembly was observed. Corrosion was not detected at this time, and in this case also, it can be seen that corrosion was suppressed because there was no decrease in pH when the fuel cell was started. N
Since the corrosion of the i-layer was suppressed, the characteristics of Ni as an excellent electron conductor were maintained, and the contact resistance on the surface of the separator 5 did not increase as shown in FIG. Above all, the stainless steel sheet B in which the surface of the stainless steel sheet was roughened to form a Ni layer showed lower contact resistance before and after the test than the stainless steel sheet A finished with BA. In addition, when any of the stainless steel plates A and B was used as the separator substrate, Fe ions and Ni ions were not detected in the water discharged from the fuel cell.

From the above results, the solid polymer membrane 1, oxide electrode 2, fuel electrode 3 from which impurities were previously removed by washing with boiling water or electrolysis of water, and a stainless steel plate separator 5 having a Ni layer formed thereon It has been confirmed that by combining the above, the polymer electrolyte fuel cell having the low contact resistance and the high power generation efficiency without corrosion of the separator 5 is formed.

[0031]

As described above, the polymer electrolyte fuel cell according to the present invention is assembled by bringing the stainless steel plate on which the Ni layer is formed into contact with the thin film electrode assembly from which impurities have been removed in advance. Therefore, during operation of the fuel cell, the pH of the fuel cell
A polymer electrolyte fuel cell that suppresses the decrease, maintains the original excellent electronic conductivity of Ni for a long period of time, generates little Joule heat due to contact resistance even in a state where a plurality of fuel cells are stacked, and has high power generation efficiency Becomes

[Brief description of the drawings]

FIG. 1 is a sectional view (a) and an exploded perspective view (b) showing an internal structure of a fuel cell using a solid polymer membrane as an electrolyte.

FIG. 2 is a graph showing a change in pH of water discharged from an oxidation electrode side (a) and a fuel electrode side (b) when a fuel cell having a thin-film electrode assembly from which impurities have been removed by boiling water immersion is started.

FIG. 3 is a graph comparing the contact resistance of the separator surface after operation of the fuel cell for 100 hours with that before operation.

FIG. 4 shows a change in pH of water discharged from an oxidation electrode side (a) and a fuel electrode side (b) when a fuel cell having a thin-film electrode assembly from which impurities have been removed in advance by an electrolysis reaction of water is started. Graph

FIG. 5 is a graph comparing the contact resistance of the separator surface after operation of the fuel cell for 100 hours with that before operation.

[Explanation of symbols]

1: solid polymer membrane 2: oxidized electrode 3: fuel electrode
5: Separator

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Tsuyoshi Shimizu 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd. (72) Inventor Keiji Izumi 5 Ishizu Nishimachi 5 Sakai City, Osaka Prefecture Nissin Steel Co., Ltd. In Technical Research Institute (72) Inventor Yoshikazu Morita 5, Nisshin Steel Corporation, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd. (72) Inventor Shinichi Kamoshida 5, Ishizu Nishimachi, Sakai City, Osaka Nisshin Steel Corporation Technical Research Center ( 72) Inventor Toshiki Kanetsugu 5 Ishizu Nishicho, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd.Technical Research Institute (72) Inventor Go Takahashi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Yagami Yuichi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F-term (reference) 5H026 AA06 BB00 CC03 CX05 EE02 EE08 E E18 HH00 HH05

Claims (3)

[Claims] 1. A structure in which an oxidizing electrode and a fuel electrode disposed on both sides of a solid polymer film are sandwiched between metal separators, and the solid polymer film, the oxidizing electrode and the fuel electrode have previously been removed from corrosive substances. Wherein the metal separator is a stainless steel plate having a Ni layer formed on the surface thereof. 2. A solid polymer membrane from which a corrosive substance has been removed in advance by an electrolysis reaction of water generated by washing with boiling water or energization in a humid atmosphere or an aqueous solution, an oxidized electrode,
2. The polymer electrolyte fuel cell according to claim 1, wherein a fuel electrode is used. 3. Surface roughness Ra: 0.5 to 30 g / m 2 on the surface of a stainless steel sheet roughened to 0.4 to 1.5 μm.
^3. The polymer electrolyte fuel cell according to claim 1, wherein a metal separator having a Ni layer formed with an adhesion amount of ² is used as a metal separator.