JP4604302B2 - Polymer electrolyte fuel cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polymer electrolyte fuel cell operating at room temperature and used for portable power supplies, electric vehicle power supplies, domestic cogeneration systems, and the like.
[0002]
[Prior art]
A polymer electrolyte fuel cell is one in which a fuel gas such as hydrogen and an oxidizing gas such as air are electrochemically reacted by a gas diffusion electrode having a catalyst layer such as platinum, and generates electricity and heat simultaneously. Is. A general configuration of such a polymer electrolyte fuel cell is shown in FIG.
[0003]
In FIG. 1, a catalytic reaction layer 2 mainly composed of carbon powder carrying a platinum-based metal catalyst is disposed in close contact with both surfaces of a polymer electrolyte membrane 1 that selectively transports hydrogen ions. Furthermore, on the outer surface of the catalyst layer 2 , a pair of diffusion layers 3 having both gas permeability and conductivity are disposed in close contact therewith. The diffusion layer 3 and the catalytic reaction layer 2 constitute an electrode 4 . The outer electrode 4, the electrode 4 and the polymer electrolyte membrane 1 and the electrode electrolyte membrane assembly formed by (hereinafter, MEA) 5 together with a mechanically secure the connection of the MEA between adjacent electrically in series with each other Further, a conductive separator plate 7 is provided in which a reaction gas is supplied to the electrode and a gas flow path 6 for carrying away water and excess gas generated by the reaction is formed on one surface.
[0004]
Although the gas flow path can be provided separately from the separator plate, a method of providing a gas flow path by providing a groove on the surface of the separator plate is generally used. Further, in the polymer electrolyte fuel cell stack as described above, in order to reduce the electrical contact resistance with the constituent members such as the separator plate and to maintain the sealing property of the fuel gas and the oxidant gas, It is necessary to tighten constantly. For this purpose, it is effective to place an end plate on each end of a large number of unit cells stacked in one direction, fix between both end plates using a fastening member, and apply tightening pressure. is there.
[0005]
In addition, since the fuel cell generates heat during operation, it is necessary to cool the fuel cell with a cooling medium such as cooling water in order to maintain the battery in a good temperature state. Usually, a cooling part for flowing a cooling medium every 1 to 3 cells is inserted between the separators, but in many cases, a cooling medium flow path is provided on the back surface of the separator to form a cooling part. These MEAs, separators, and cooling units are stacked one after the other, stacked in multiple cells, sandwiched between end plates via current collector plates and insulating plates, and fixed from both ends with fastening bolts The structure of the battery.
[0006]
The separator plate of such a polymer electrolyte fuel cell has high conductivity, high gas tightness with respect to the supply gas, and high corrosion resistance and chemical stability with respect to the environment in the fuel cell stack. There is a need. Therefore, the conventional separator plate is made of a carbon material such as glassy carbon or expanded graphite, and the gas flow path is also formed by cutting on the surface thereof, or in the case of expanded graphite, by molding with a mold.
[0007]
In recent years, however, attempts have been made to use metal plates such as stainless steel instead of conventionally used carbon materials.
[0008]
[Problems to be solved by the invention]
When a carbon material is used for a separator plate as in the past, there is a limit to the thickness of the separator plate, which is generally limited to 4 mm, in order to maintain gas tightness and mechanical strength. Is considered to be an obstacle for in-vehicle use.
[0009]
Furthermore, it is difficult for conventional carbon plate cutting methods to reduce the carbon plate material cost as well as the processing cost for cutting the carbon plate, and the method using expanded graphite also has high material cost and is practically used. It is considered as an obstacle for this. In order to reduce the size, cost, and strength of the separator plate, it is very effective to configure the separator plate with a metal material such as stainless steel. However, in the method using a metal material, the metal plate is corroded or dissolved by a long-time operation. When the metal separator corrodes, the electrical resistance and electrical contact resistance of the corroded portion increase, and the battery output decreases.
[0010]
Also, when the metallic separator is dissolved, the dissolved metal ions are mixed into the MEA and trapped at the ion exchange site of the polymer electrolyte, causing a decrease in the ionic conductivity of the polymer electrolyte itself and a decrease in the electrode reaction area. Output decreases. Due to these causes, there has been a problem that the power generation efficiency gradually decreases with respect to operating a polymer electrolyte fuel cell using a metal separator that does not have sufficient corrosion resistance for a long time.
[0011]
[Means for Solving the Problems]
In order to solve such problems, a polymer electrolyte fuel cell according to the present invention includes a hydrogen ion conductive polymer electrolyte membrane and a pair of electrodes arranged at positions sandwiching the hydrogen ion conductive polymer membrane. A polymer electrolyte fuel in which a unit cell is stacked via a conductive separator having a gas flow passage for supplying and discharging a fuel gas containing hydrogen to one of the electrodes and supplying and discharging an oxidant gas containing oxygen to the other In the battery, the conductive separator has a carbon content of not more than 0.03% by weight, a nitrogen content of 0.1% by weight to 0.3% by weight, and a Cr content of 18. 5 wt% or more and 23 wt% or less, Ni content is 12 wt% or more and 20 wt% or less, Mo content is 1.5 wt% or more and 8 wt% or less , and Ti Or Nb, {X + Y ≧ 8 (Z + ), And provided that X, Y, Z, W respectively Ti, Nb, characterized by being configured in addition to austenitic stainless steel such as a carbon, showing the weight percent of nitrogen}.
[0014]
At this time, the stainless steel plate contains Cr of 18.5% or more and 23% or less, Ni of 12% or more and 20% or less, and Mo of 1.5% or more and 8% or less. It is effective.
[0015]
Furthermore, it is effective to add 0.05 wt% or more and 0.2 wt% or less of Cu to the stainless steel plate.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an environment in which a separator plate is exposed in a polymer electrolyte fuel cell, specifically, a reducing atmosphere on the fuel electrode side, an oxidizing atmosphere on the air electrode side, an operating temperature of room temperature to 90 ° C., We have found a separator material that maintains high conductivity and chemical inertness that does not cause corrosion or dissolution against high humidity associated with water generation by adjusting the alloy composition and additive elements of stainless steel. It is in. By using a separator plate made of this stainless steel material, a polymer electrolyte fuel cell stack is provided that is low in cost, excellent in compactness, and does not deteriorate in power generation efficiency even when operated for a long time.
[0020]
Specific alloy compositions and additive elements are shown below. The carbon contained in the stainless steel produces a strong austenite structure and increases the strength of the steel, but causes chromium carbide to precipitate at the grain boundaries, resulting in a chromium deficient layer and intergranular corrosion. The strength of steel as a separator plate for polymer electrolyte fuel cells is sufficient without adding carbon, and the carbon content in stainless steel used for the separator for polymer electrolyte fuel cells is 0.03 wt. It has been found that when it is less than 10%, corrosion such as intergranular corrosion hardly occurs in the battery operating environment.
[0021]
Moreover, in order to avoid the precipitation of chromium carbide, it is also effective to add Ti or Nb that easily binds to carbon. However, in the operating environment of the polymer electrolyte fuel cell, Ti or Nb is used as a separator material. It was found that it was effective to add (Ti + Nb) ≧ 8 (C + N) wt%.
[0022]
In addition, since the polymer electrolyte membrane has a proton free radical such as a sulfonic acid group as a main structure and carbon dioxide is contained in the reformed fuel gas, the vicinity of the electrode of the polymer electrolyte fuel cell is exposed to an acidic atmosphere. Is done. In terms of acid resistance and sulfuric acid resistance of the stainless steel material, it is common and effective to add Mo and Cu. Considering the solid solution limit, saturation of effect, material cost, and acid resistance required as a separator material in the operating environment of polymer electrolyte fuel cell, the amount of Mo added is 1.5 to 8% by weight. Hereinafter, it has been found that it is effective that the amount of Cu added is 0.05 wt% or more and 0.2 wt% or less. Furthermore, it has been found that the corrosion resistance of the polymer electrolyte fuel cell in the operating environment can be further improved by coexisting Mo and Cu.
[0023]
In addition, when nitrogen is added to austenitic stainless steel, it reacts with hydrogen ions in an acidic atmosphere to produce ammonia, thereby reducing the pH drop. That is, the addition of nitrogen is effective for the acid resistance of stainless steel. However, excessive addition of nitrogen causes deterioration of workability due to hardening of the stainless steel. Therefore, as a result of considering the improvement in acid resistance due to the addition of nitrogen and the workability such as rolling and drawing required for the separator material for polymer electrolyte fuel cells, the amount of nitrogen added is 0.1 wt.% For austenitic stainless steel. % To 0.3% by weight was found desirable.
[0025]
Furthermore, as the amount of Cr added to the stainless steel as a separator material increases, the corrosion resistance can be expected to improve. However, as a separator material for a polymer electrolyte fuel cell, it is necessary that the bulk electrical resistance and contact resistance are sufficiently small as well as high corrosion resistance. In the case of stainless steel, the bulk electrical resistance is sufficiently small as a material for a separator, but when the amount of Cr added is increased, the contact resistance increases due to the growth of a passive film layer. Regarding the Cr content in the stainless steel for the separator material under the operating environment of the polymer electrolyte fuel cell, if it is 18% by weight or less, sufficient corrosion resistance cannot be obtained, and if it is 24% by weight or more, the corrosion resistance is Although improved, it has been found that the contact resistance becomes too large as a separator material.
[0026]
【Example】
Embodiments of the present invention will be described below with reference to the drawings.
[0027]
Example 1
First, in order to evaluate the corrosion resistance of the stainless steel substrate in the present invention, an elution test in a sulfuric acid aqueous solution was performed using a sample piece. The chemical composition of the sample sample is shown in Table 1. As comparative samples, SUS304, SUS310S, SUS316, and SUS316L were used (Table 1).
[0028]
[Table 1]
[0029]
Regarding the method and conditions of the dissolution test, first, various stainless steel substrates to be samples were processed into disk-shaped sample pieces each having a diameter of 30 mm and a thickness of 5 mm. The sample piece surface was polished by buffing. These sample pieces were immersed in 100 ml of a sulfuric acid aqueous solution having a concentration of 0.02N, boiled for 100 hours with a reflux apparatus, and the amount of metal ions eluted was quantified and compared by ICP mass spectrometry of the boiled sulfuric acid solution.
[0030]
The result is shown in FIG. Thereby, the carbon content does not exceed 0.03% by weight, Mo is contained in an amount of 1.5 to 8% by weight, Cr is contained in an amount of 18.5 or more, and in some cases, Ti or Nb ( It was found that the austenitic stainless steel added to satisfy (Ti + Nb) ≧ 8 (C + N)% by weight is excellent in corrosion resistance. Further, the carbon content does not exceed 0.03% by weight, the nitrogen content is 0.1 to 0.3% by weight, and in some cases 0.05 to 0.2% by weight of Cu is contained. It was found that the added austenitic stainless steel is excellent in corrosion resistance.
[0031]
(Example 2)
Next, the stainless steel base material used in Example 1 was processed into a sample piece having the same shape and sandwiched between two 400 μm thick carbon papers of the same material as the electrode for the polymer electrolyte fuel cell. The contact resistance was measured. On both sides of the carbon paper, a gold-plated copper plate was installed as a current collector, and the weight dependency of contact resistance was determined by applying an arbitrary weight to the entire sample with an air cylinder.
[0032]
The result is shown in FIG. Thereby, it was found that the contact resistance of SUS310S containing 24 to 26% by weight of Cr, which was excellent in corrosion resistance in Example 1, was too high due to the growth of the passive film. Further, it was found that the sample according to the present invention has a smaller contact resistance than other stainless steels.
[0033]
(Example 3)
Next, each stainless steel substrate used in Example 1 was processed into a separator plate and incorporated into a polymer electrolyte fuel cell stack, and a durability cell operation test was performed. In this example, a fuel cell stack using a carbon separator plate was also subjected to a durability cell operation test and added as a comparative example.
[0034]
First, a method for producing an electrode will be described. A carbon powder having a particle size of several microns or less was immersed in an aqueous solution in which equimolar amounts of chloroplatinic acid and ruthenium chlorate were dissolved, and a platinum-ruthenium catalyst was supported on the carbon powder by reduction treatment. This platinum-supported carbon powder was dispersed in an alcohol solution of a polymer electrolyte to form a slurry.
[0035]
On the other hand, after impregnating an aqueous dispersion of fluororesin with a carbon paper having a thickness of 400 μm serving as an electrode base, this was dried and heat treated at 400 ° C. for 30 minutes to impart water repellency to the carbon paper. Next, the slurry was uniformly applied to one surface of a carbon paper subjected to a water repellent treatment to form a catalyst reaction layer to obtain an electrode.
[0036]
Next, the two electrodes are positioned at the center of the polymer electrolyte so that the surfaces with the catalytic reaction layer face the polymer electrolyte membrane on both sides of the polymer electrolyte membrane that is slightly larger in outer dimensions than the electrodes. After stacking and aligning a gasket made of silicon rubber on the periphery, hot pressing was performed at 100 ° C. for 5 minutes to obtain an electrode electrolyte assembly (MEA). Further, the MEA was cut into a length of 20 cm and a width of 10 cm.
[0037]
The obtained MEA was laminated with 4 cells through a separator plate to obtain a fuel cell stack. The separator plate had a thickness of 3 mm, and a gas channel having a width of 2 mm and a depth of 1 mm was formed by cutting on the surface in contact with the MEA. Moreover, the cooling water channel was arrange | positioned for every 2 cells. Metal end plates made of SUS304 were disposed on the upper and lower portions of the battery stack, and manifolds 11 and 12 were disposed on both sides of the battery stack via insulators 13 and gaskets 10. Fuel gas, air, and cooling water mainly composed of hydrogen were supplied and discharged through the manifolds 11 and 12. The cooling water channel 8 was installed every two cells. FIG. 4 shows a perspective view of the battery stack of this example, and FIG. 5 shows a longitudinal sectional view with a part cut away.
[0038]
The fuel cell stack described above was produced for each of the various stainless steel base separators, and a durability cell operation test was performed on each fuel cell stack while measuring the voltage of each unit cell. The reformed simulated gas (75% by volume of hydrogen, 25% by volume of carbon dioxide, 100ppm of carbon monoxide) is supplied to the anode side, and air is supplied after humidifying with a bubbler to the cathode side, the battery temperature is 75 ° C, the current density It was operated for 2000 hours under the condition of 0.7 A / cm ² . The hydrogen utilization rate was 70%, the oxygen utilization rate was 30%, the anode side bubbler temperature was 75 ° C, and the cathode side bubbler temperature was 65 ° C.
[0039]
The result is shown in FIG. In addition, the battery using the separator by SUS310S was not subjected to the durability test because sufficient initial performance was not obtained because the contact resistance was too large. Thus, it was found that the polymer electrolyte fuel cell separator using the stainless steel according to the present example can maintain high performance for a long time as compared with the separator using the conventional stainless steel.
[0040]
Example 4
In order to clarify the effect of the present invention, an alloy equivalent to a SUS304 stainless steel with a C content changed from 0.01 wt% to 0.08 wt% every 0.01 wt% is prepared, The same corrosion resistance test as in Example 1 was performed. As a result, in the case of an alloy having a C content of 0.03% by weight or less, the total elution amount of Fe, Cr and Ni was 100 mg / L or less, whereas the C content was 0.04% by weight. In the case of the above alloys, the total elution amount of Fe, Cr, and Ni was 200 mg / L or more. Thus, it was found that the separator material for polymer electrolyte fuel cells according to the present invention has high corrosion resistance under the simulated operation environment of the polymer electrolyte fuel cells.
[0041]
(Example 5)
In order to clarify the effect of the present invention, Mo is contained in an alloy having the same composition as the SUS304 stainless steel containing 0.03% by weight of C by changing Mo from 1% to 10% by weight every 1% by weight. The same corrosion resistance test as in Example 1 was performed. As a result, in the case of an alloy having a Mo content of 1% by weight or less, the total elution amount of Fe, Cr and Ni was about 100 mg / L, whereas that of an alloy having a Mo content of 2% by weight or more. In this case, the total elution amount of Fe, Cr, Ni is 80 mg / L or less, and as Mo increases, the total elution amount of Fe, Cr, Ni decreases, and the Mo content is 8% by weight. The total elution amount of Fe, Cr, and Ni decreased to about 50 mg / L. However, when the Mo content was 8% by weight or more, it was found that the total elution amount of Fe, Cr, and Ni did not decrease to about 50 mg / L or less, and the effect of Mo content was saturated.
[0042]
Thus, it was found that the separator material for polymer electrolyte fuel cells according to the present invention has high corrosion resistance under the simulated operation environment of the polymer electrolyte fuel cells.
[0043]
(Example 6)
In order to clarify the effect of the present invention, N is changed from 0% to 0.5% by weight every 0.1% by weight to an alloy having the same composition as SUS304 stainless steel containing 0.03% by weight of C. The same corrosion resistance test as in Example 1 was conducted. As a result, in the case of an alloy having an N content of 0.1% by weight or more, the total elution amount of Fe, Cr and Ni was about 50 mg / L or less, whereas in the case of an alloy not containing N. The total elution amount of Fe, Cr, and Ni was about 100 mg / L.
[0044]
However, it has been found that in the case of an alloy containing 0.4% by weight or more of N, the alloy becomes too hard, and rolling, press forming, etc. become very difficult. As a result, it has been found that the polymer electrolyte fuel cell separator material according to the present invention has high corrosion resistance in a simulated operating environment of the polymer electrolyte fuel cell and provides practical workability.
[0045]
(Example 7)
In order to clarify the effect of the present invention, Cr is added to an alloy containing 0.03% by weight of C, 0.3% by weight of N, and 3% by weight of Mo, with the balance being equivalent to SUS304 stainless steel. Then, the same corrosion resistance test as in Example 1 was performed by changing the Cr content from 18 wt% to 25 wt% by 1 wt%. As a result, in the case of an alloy having a Cr content of 18% by weight, the total elution amount of Fe, Cr and Ni was about 50 mg / L, whereas in the case of an alloy containing 19% by weight or more of Cr. The total elution amount of Fe, Cr and Ni was about 20 mg / L. However, in the case of an alloy containing 24 wt% or more of Cr, it has been found that the contact resistance with the carbon paper that is a constituent member of the polymer electrolyte fuel cell electrode becomes too large. As a result, it has been found that the polymer electrolyte fuel cell separator material according to the present invention has high corrosion resistance in a simulated operating environment of the polymer electrolyte fuel cell and a relatively small contact resistance.
[0046]
(Example 8)
In order to clarify the effect of the present invention, C is 0.03% by weight, N is 0.3% by weight, Cr is 20% by weight, Mo is 3% by weight, Ni is 14% by weight, and the balance substantially. The same corrosion resistance test as that of Example 1 was performed by adding Cu to the Fe alloy in an amount of 0.05% by weight from 0% by weight to 0.3% by weight.
[0047]
As a result, when Cu is not contained, the total elution amount of Fe, Cr and Ni was about 20 mg / L, whereas in the case of an alloy containing 0.05% by weight or more of Cu, The total elution amount of Fe, Cr, and Ni was about 10 mg / L. However, it has been found that the effect of Cu addition is saturated in the case of an alloy containing 0.25% by weight or more of Cu. Thus, it was found that the separator material for polymer electrolyte fuel cells according to the present invention has high corrosion resistance under the simulated operation environment of the polymer electrolyte fuel cells.
[0048]
( Reference Example 1 )
In order to clarify the effect of the present invention, C is contained in an amount of 0.03% by weight, N is 0.3% by weight, Cr is changed from 18% by weight to 25% by weight every 1% by weight, and the balance The same corrosion resistance test as in Example 1 was performed using a substantially Fe ferritic stainless steel. As a result, when the Cr content is 18% by weight, the total elution amount of Fe, Cr and Ni was about 300 mg / L, whereas in the case of an alloy containing 19% by weight or more of Cr, The total elution amount of Fe, Cr, and Ni was 100 mg / L or less. In addition, the total elution amount of Fe, Cr, and Ni decreased as the Cr content increased. However, in the case of an alloy containing Cr by 24% by weight or more, it is a constituent member of a polymer electrolyte fuel cell electrode. It has been found that the contact resistance with a certain carbon paper becomes too large.
[0049]
As a result, it has been found that the polymer electrolyte fuel cell separator material according to the present invention has high corrosion resistance in a simulated operating environment of the polymer electrolyte fuel cell and a relatively small contact resistance. However, since the ferritic stainless steel was poor in workability compared to the austenitic stainless steel, the steel material in Example 9 was a material slightly lacking in workability.
[0050]
( Reference Example 2 )
In order to clarify the effect of the present invention, C is added to 0.015% by weight, N is added to 0.015% by weight, Ti and Nb are added so that (Ti + Nb) ≧ 8 (C + N)% by weight, The same corrosion resistance test as that of Example 1 was performed by using Cr containing a ferritic stainless steel substantially containing Fe, changing from 1 wt% to 25 wt% in increments of 1 wt%. As a result, when the Cr content was 18% by weight, the total elution amount of Fe, Cr and Ni was about 250 mg / L, whereas in the case of an alloy containing 19% by weight or more of Cr, The total elution amount of Fe, Cr, and Ni was 50 mg / L or less. In addition, the total elution amount of Fe, Cr, and Ni decreased as the Cr content increased. However, in the case of an alloy containing Cr by 24% by weight or more, it is a constituent member of a polymer electrolyte fuel cell electrode. It has been found that the contact resistance with a certain carbon paper becomes too large. Further, it has been found that the material of this example has the same level of workability as the austenitic stainless steel.
[0051]
As a result, the polymer electrolyte fuel cell separator material according to the present invention has high corrosion resistance in a simulated operation environment of the polymer electrolyte fuel cell, and has a relatively small contact resistance and excellent workability. It turned out to be.
[0052]
【The invention's effect】
According to the present invention, it is possible to provide a polymer electrolyte fuel cell stack that is low in cost and excellent in compactness, and that does not have a decrease in power generation efficiency even when the battery is operated for a long time.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration in which a part of a general polymer electrolyte fuel cell is cut away. FIG. 2 is a result of an elution test of a stainless steel plate as a component of a first embodiment of the present invention. FIG. 3 is a perspective view of the fuel cell stack manufactured in the second embodiment of the present invention. FIG. 4 is a part of the fuel cell stack manufactured in the second embodiment of the present invention (unit cell). FIG. 5 is a cross-sectional view of a part (unit cell) of a fuel cell stack produced in the third embodiment of the present invention. FIG. 6 is a polymer electrolyte fuel cell of the second embodiment of the present invention. That shows the result of the endurance test
DESCRIPTION OF SYMBOLS 1 Polymer electrolyte membrane 2 Catalytic reaction layer 3 Gas diffusion layer 4 Electrode 5 Electrode electrolyte membrane assembly (MEA)
6 Gas flow path 7 Conductive separator plate 8 Cooling water path 9 Gasket 10 Sealing agent 11 Gas manifold 12 Cooling water manifold 13 Insulator

Claims (2)

A unit cell comprising a hydrogen ion conductive polymer electrolyte membrane and a pair of electrodes arranged at positions sandwiching the hydrogen ion conductive polymer membrane, supplying and discharging a fuel gas containing hydrogen to one of the electrodes, On the other hand, in a polymer electrolyte fuel cell laminated through a conductive separator formed with a gas flow path for supplying and discharging an oxidant gas containing oxygen,
In the conductive separator, the carbon content does not exceed 0.03% by weight, the nitrogen content is 0.1% by weight or more and 0.3% by weight or less , and the Cr content is 18.5% by weight. And 23 wt% or less, the Ni content is 12 wt% or more and 20 wt% or less, the Mo content is 1.5 wt% or more and 8 wt% or less , and
It is composed of an austenitic stainless steel plate to which Ti or Nb is added so as to be {X + Y ≧ 8 (Z + W), where X, Y, Z, and W represent the weight percentages of Ti, Nb, carbon, and nitrogen, respectively}. A polymer electrolyte fuel cell. Stainless steel, a polymer electrolyte fuel cell according to claim 1, characterized in that the addition of a and 0.2 wt% or less of Cu 0.05% by weight or more.