CA2300008C - Stainless steel and titanium for solid polymer electrolyte fuel cell members - Google Patents

Abstract

A low-contact-resistance stainless steel or titanium plate for a solid polymer electrolyte fuel cell member, wherein a noble metal or an alloy of a noble metal is deposited on a portion that comes in contact with other member developing a contact resistance. The stainless steel contains not more than 30% by weight of chromium and, as required, one or more of not more than 10% by weight of molybdenum and not more than 25% by weight of nickel, these components satisfying a relationship, 10 - 0.3 .times. [(Cr%) + 3 (Mo%) + 0.05(Ni%)} .ltoreq. 5, and the remainder being chiefly iron. To mass-produce the solid polymer electrolyte fuel cell in a compact size, further, the invention provides a laminated module for the solid polymer electrolyte fuel cell by using the stainless steel or titanium as the constituent members.

Description

STAINLESS STEEL AND TITANIUM FOR SOLID
POLYMER ELECTROLYTE FUEL CELL MEMBERS
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to a material for solid polymer electrolyte fuel cell members used for automobiles that use electric power as a source of drive and for small-scale generating systems. More specifically, the invention relates to a surface-treated low-contact-resistance material for solid polymer electrolyte fuel cell members.

2. Description of the Related Art In recent years, study of the fuel cells for electric cars has rapidly developed and has been supported by success in the development of a solid polymer electrolyte material.
The solid polymer electrolyte fuel cell is a fuel cell which features the use of an organic membrane of the type selectively transmitting hydrogen ions as an electrolyte, and is different from the conventional alkali fuel cell, phosphorus fuel cell, molten carbonate fuel cell or solid electrolyte fuel cell.
The solid polymer electrolyte fuel cell is a system which uses, as a fuel, pure hydrogen as well as hydrogen gas obtained by reforming alcohols, and produces electric power by electrochemically controlling the reaction thereof with oxygen in the air.
Despite of its small thickness, the solid polymer electrolyte membrane exhibits its function to a sufficient degree by having electrolyte secured in the membrane. Upon controlling the dew point in the cell, therefore, the solid polymer electrolyte membrane works as an electrolyte, and makes it possible to design the cell itself in a compact size and in a simplified manner without using a medium having fluidity, such as an aqueous solution-type electrolyte or a molten salt-type electrolyte.
As the stainless steels for fuel cells, there have heretofore been known a corrosion-resistant stainless steel for molten carbonate fuel cells as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-247852, a highly corrosion-resistant steel plate for molten carbonate fuel cell separators as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-358044, a stainless steel having excellent corrosion resistance against molten salts and a method of producing the same as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-188870, a stainless steel having excellent resistance against molten carbonate as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-165546, a stainless steel having excellent corrosion resistance against molten carbonate as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-225892, and a stainless steel that can be favorably hot worked and exhibits excellent corrosion resistance against molten salts as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-3114620.
Further, as stainless steels for fuel cells that operate in a molten carbonate environment where a high degree of corrosion resistance is required, there have been developed solid electrolyte fuel cell materials used at temperatures of several hundreds of degrees Celsius as represented by metal materials for solid electrolyte fuel cells disclosed in Japanese Unexamined Patent Publications (Kokai) Nos. 6-264193 and 6-293941 and a ferrite-type stainless steel disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-67672.
As a material constituting the solid polymer electrolyte fuel cell that operates in a region of not higher than 100°C, on the other hand, there has been used a carbon-type material since the temperature is not so

- 3 -high and corrosion resistance and durability can be exhibited to a sufficient degree in the environment in which it is used. No study has heretofore been conducted concerning applying a metal-type material such as stainless steel or titanium to the fuel cells of this type.
As for applying a metal material to a separator which is one of the important members of the fuel cell, all that is used as a separator for the phosphoric acid type fuel cell is an amorphous alloy of a nickel group as taught in Japanese Unexamined Patent Publications (Kokai) No. 63-277734, 63-277735, 63-277736 and 63-277737. No study has at all been conducted concerning applying a stainless steel to the separator for solid polymer electrolyte fuel cells nor concerning the concrete shapes thereof.
Use of carbon as a material for constituting the solid polymer electrolyte fuel cell involves problems such as increased cost and increased cell size, hindering the widespread use of the solid polymer electrolyte fuel cells.
The solid polymer electrolyte fuel cell is constituted by a laminate of catalytic electrode units of fine carbon particles and ultrafine noble metal particles on both surfaces of a solid polymer electrolyte membrane that serves as an electrolyte, a current collector of an aggregate of a felt-like carbon fiber (usually called carbon paper) having functions of taking out electric power generated therein as an electric current and of feeding, at the same time, a reaction gas to the catalytic electrode units, and separators for receiving the electric current therefrom and for separating two kinds of reaction gases comprising chiefly oxygen and hydrogen, as well as a cooling medium.
A carbon material has heretofore been used even for the separators. When the fuel cell is to be mounted on an automobile, however, there arise problems such as

- 4 -an increased cost and an increased cell size. Therefore, a study has been conducted concerning applying a stainless steel to the members such as separators.
The present inventors have already disclosed concrete shapes and components for using a stainless steel as members for the solid polymer electrolyte fuel cell, such as separators, in Japanese Unexamined Patent Publications (Kokai) Nos. 11-61146 and 11-62813. When the separators are made of a stainless steel or titanium, however, it has been pointed out that the energy efficiency of the fuel cell is greatly lowered due to a large contact resistance to the carbon paper which serves as a current collector.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a low-contact-resistance material for solid polymer electrolyte fuel cell members for enabling the solid polymer electrolyte fuel cell to exhibit its energy conversion efficiency to a maximum degree. The material was found by studying the contact resistance among the materials that are used.
According to a second aspect of the invention, there is provided a component system of a low cost withstanding the environment of use. The material was found by studying stainless steels to substitute for a carbon material in order to satisfy the demand for decreasing the size and decreasing the cost of the solid polymer electrolyte fuel cell.
According to a third aspect of the invention, there is provided concrete technology for substituting a stainless steel for the carbon material that is used for constituting the members, in order to mass-produce the solid polymer electrolyte fuel cells in compact sizes.
In order to accomplish the above-mentioned objects, the present invention provides the following (1) to (16).
(1) A low-contact-resistance stainless steel plate for solid polymer electrolyte fuel cell members, wherein

- 5 -a noble metal or an alloy of a noble metal is deposited on the surface of a portion that comes in contact with other member developing a contact resistance and from where an oxide film has been removed.
(2) A stainless steel plate as described in (1), wherein the noble metal or the alloy of the noble metal that is deposited has an average thickness of not smaller than 5 nm.
(3) A low-contact-resistance stainless steel plate for solid polymer electrolyte fuel cell members described in (1) or (2), wherein the stainless steel plate contains the following components in following amounts by weight:
Cr: not more than 30~, 10 - 0.3 x ((Cry) + 3 (Mo$) + 0.05(Ni~)} s_ 5, and the remainder being chiefly Fe.
(4) A low-contact-resistance stainless steel plate for solid polymer electrolyte fuel cell members described in (3), further containing one or more of:
Mo: not more than 10$, and Ni: not more than 25~, by weight.
(5) Low-contact-resistance titanium for solid polymer electrolyte fuel cell members, wherein a noble metal or an alloy of a noble metal is deposited on the surface of a portion that comes in contact with other member developing a contact resistance and from where an oxide film has been removed.

(6) Titanium described in (5), wherein the noble metal or the alloy of the noble metal that is deposited has an average thickness of not smaller than 5 nm.

(7) A method of producing a low-contact-resistance stainless steel plate for solid polymer electrolyte fuel cell members by depositing a noble metal or an alloy of a noble metal on the surface of a portion that comes into contact with another member developing a contact resistance and from where an oxide film has been removed , - 6 -by blasting the surface of the plate with particles coated with the noble metal or the alloy of the noble metal.

(8) A method of producing a low-contact-resistance stainless steel plate for solid polymer electrolyte fuel cell members by depositing a noble metal or an alloy of a noble metal on the surface of a portion that comes in contact with another member developing a contact resistance and from where an oxide film has been removed by wet-plating with the noble metal or the alloy of the noble metal while effecting the polishing.

(9) A method of producing a low-contact-resistance titanium for solid polymer electrolyte fuel cell members by depositing a noble metal or an alloy of a noble metal on the surface of a portion that comes in contact with another member developing a contact resistance and from where an oxide film has been removed by blasting the surface of the plate with particles coated with the noble metal or the alloy of the noble metal.

(10) A method of producing a low-contact-resistance titanium for solid polymer electrolyte fuel cell members by depositing a noble metal or an alloy of a noble metal on the surface of a portion that comes into contact with another member developing a contact resistance and from where an oxide film has been removed by wet-plating the noble metal or the alloy of the noble metal while effecting the polishing.

(11) A stainless steel separator for a solid polymer electrolyte fuel cell, wherein the separator has, in the central portion thereof, a corrugated plate structure constituted by a plurality of grooves of which both ends are coupled together by coupling portions, and has, in the peripheral flat portions thereof, holes serving as gas passages for the one reaction gas, holes serving as gas passages for the other reaction gas and holes serving as passages for the coolant, each in a number of two or more.

(12) A stainless steel separator for a solid polymer electrolyte fuel cell described in (11) by using the stainless steel of any one of (1) to (4).

(13) A titanium separator for a solid polymer electrolyte fuel cell, wherein the separator has, in the central portions thereof, a corrugated plate structure constituted by a plurality of grooves of which both ends are coupled together by coupling portions, and has, in the peripheral flat portions thereof, holes serving as gas passages for the one reaction gas, holes serving as gas passages for the other reaction gas and holes serving as passages for the coolant, each in a number of two or more.

(14) A titanium separator for a solid polymer electrolyte fuel cell described in (13) by using titanium of (5) or (6).

(15) A laminated module for a solid polymer electrolyte fuel cell comprising:
A) a stainless steel separator having, in the central portion thereof, a corrugated plate structure constituted by a plurality of grooves of which both ends are coupled together by coupling portions, and having, in the peripheral flat portions thereof, holes serving as gas passages for the one reaction gas, holes serving as gas passages for the other reaction gas and holes serving as passages for the coolant, each in a number of two or more;
B) a spacer having a cut-away portion of a shape corresponding to the central portion of the separator, and having, in the peripheral flat portions thereof, holes serving as gas passages for the one reaction gas, holes serving as gas passages for the other reaction gas and holes serving as passages for the coolant, each in a number of two or more so as to be overlapped on the holes in the separator, the holes serving as passages for the reaction gases and the holes serving as passages for the coolant being communicated . _ 8 _ with the cut-away portion;
C) a solid polymer electrolyte membrane having, in the peripheral flat portions thereof, holes serving as passages for the one reaction gas, holes serving as passages for the other reaction gas and holes serving as passages for the coolant, each in a number of two or more so as to be overlapped on the holes of the separator and on the holes of the spacer; and D) a stainless steel terminating plate having reaction gas feed/drain ports and coolant feed/drain ports at positions corresponding to the reaction gas passages and the coolant passages formed by laminating the separator, the spacer and the solid polymer electrolyte membrane at an end where the separator, the spacer and the solid polymer electrolyte membrane are laminated.

(16) A laminated module for a solid polymer electrolyte fuel cell comprising:
A) a titanium separator having, in the central portion thereof, a corrugated plate structure constituted by a plurality of grooves of which both ends are coupled together by coupling portions, and having, in the peripheral flat portions thereof, holes serving as gas passages for one reaction gas, holes serving as gas passages for the other reaction gas and holes serving as passages for the coolant, each in a number or two or more;
B) a spacer having a cut-away portion of a shape corresponding to the central portion of the separator, and having, in the peripheral flat portions thereof, holes serving as gas passages for the one reaction gas, holes serving as gas passages for the other reaction gas and holes serving as passages for the coolant, each in a number of two or more so as to be overlapped on the holes in the separator, the holes serving as passages for the reaction gases and the holes serving as passages for the coolant being communicated _ g _ with the cut-away portion;
C) a solid polymer electrolyte membrane having, in the peripheral flat portions thereof, holes serving as passages for the one reaction gas, holes serving as passages for the other reaction gas and holes serving as passages for the coolant, each in a number of two or more so as to be overlapped on the holes of the separator and on the holes of the spacer; and D) a titanium terminating plate having reaction gas feed/drain ports and coolant feed/drain ports at positions corresponding to the reaction gas passages and the coolant passages formed by laminating the separator, the spacer and the solid polymer electrolyte membrane at an end where the separator, the spacer and the solid polymer electrolyte membrane are laminated.
The separator used in the solid polymer electrolyte fuel cell has heretofore been obtained by forming grooves in both surfaces of a carbon plate having a thickness of about 5 mm so that the reaction gases and the coolant may flow into predetermined portions in the fuel cell. In this case, however, the carbon material itself is expensive and, besides, laborious work is required by the cutting operation further driving up the cost.
In an attempt to cheaply produce the parts through a mass production system, therefore, the present inventors have conducted a study to supply parts of a predetermined shape by press-molding and punching a thin stainless steel plate, and have succeeded in determining the shapes of the stainless steel parts and of the accompanying constituent parts as well as the laminated module constituted thereby, and have thus completed the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view illustrating a stainless steel separator for a solid polymer electrolyte fuel cell according to the present invention;

Fig. 2 is a schematic view illustrating a spacer A
for the solid polymer electrolyte fuel cell according to the present invention;
Fig. 3 is a schematic view illustrating a spacer B
for the solid polymer electrolyte fuel cell according to the present invention;
Fig. 4 is a schematic view illustrating a~spacer C
for the solid polymer electrolyte fuel cell according to the present invention;
Fig. 5 is a schematic view illustrating a spacer D
for the solid polymer electrolyte fuel cell according to the present invention;
Fig. 6 is a schematic view illustrating a solid polymer electrolyte membrane for the solid polymer electrolyte fuel cell according to the present invention to which a catalytic electrode is imparted;
Fig. 7 is a schematic view illustrating a current collector used in the present invention;
Fig. 8 is a schematic view illustrating a stainless steel terminating plate for the solid polymer electrolyte fuel cell according to the present invention; and Fig. 9 is a schematic view illustrating a laminated module for the solid polymer electrolyte fuel cell according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment according to a first aspect of the invention will now be described.
The contact resistance was measured by arranging jigs having disk-like current-feeding surfaces of a diameter of 30 mm above and below, placing, therebetween, two disk-like metallic test pieces or carbon papers having a diameter of 30 mm and a thickness of 4 mm, placing a weight on the upper part thereof such that the surface pressure of the contacting surfaces was 2.79 kg/cm2, feeding a constant current of a current density of 1.13 A/cm2, and measuring the potential difference across the two disk-like metallic test pieces.

As the materials of the disk-like metallic test pieces, there were selected copper plated with gold to a thickness of 30 Vim, commercially available stainless steels SUS316L, YUS270, YUS260, YUS190L, and class 1 pure titanium for industrial use (YUS stands for a standard specified by Shin-Nihon Seitetsu Co.) As the carbon paper, there was selected a test product of a thickness of 0.6 mm regarded to be most suited for the fuel cells from the standpoint of gas permeability and an electrical conducting property. The carbon paper was cut into a square of a side of 30 mm and was subjected to testing. In order to obtain standard values, first, the test pieces of the stainless steels and titanium were mirror-surface polished on their contact surfaces to measure the contact resistance.
As a result, the contact resistances (unit in mS2~cm2) were gold/gold: 0.02, gold/carbon paper: 5.30, gold/SUS316L: 22.43, SUS316L/SUS316L: 54.90, titanium/carbon paper: 655.83, YUS270/carbon paper:
696.50, YUS260/carbon paper: 679.87, YUS190L/carbon paper: 819.40, SUS316L/carbon paper: 614.52.
The contact resistance of the gold/carbon paper was found by holding the carbon paper by two disk-like test pieces of gold-plated copper, dividing the potential difference between the disk-like test pieces of gold-plated copper by the current density and, then, dividing the value by 2 and, hence, includes the resistance of one-half the thickness of the carbon paper. Further, the contact resistance of the stainless steel or titanium and the carbon paper was found by measuring the potential difference across both ends of a combination of a stainless steel or titanium/carbon paper/gold-plated copper, dividing the potential difference by the current density to obtain the total resistance, and subtracting the contact resistance of gold/carbon paper from the total resistance.

By rearranging the results, it was found that:
O There is almost no resistance on the contact surfaces of gold/gold and gold/carbon paper;
O An oxide film exists on the stainless steel or titanium developing a contact resistance of about several tens of mS2 ~ cm2; and OO A large contact resistance that cannot be expected from the Ohm s law develops at the contact surface between the stainless steel or titanium and the carbon paper.
This phenomenon is deeply related to the fact that the electric conduction of graphite constituting the carbon paper varies depending upon the ~-electrons that are conjugation double-bonded, and a so-called Schottky barrier is formed on the contact interface relative to the stainless steel or titanium having a work function value greatly different from that of graphite, developing a large contact resistance.
Upon considering the contact resistance from the standpoint of semiconductor physics, the data measured this time can be explained consistently. Thus, the contact resistance could be decreased by holding a noble metal (which may be gold, platinum, palladium, silver, copper, tin, lead or the like metal) having a work function value equivalent to that of graphite or by holding an alloy of such a metal between the contact surfaces of the stainless steel or titanium and the carbon paper.
It has been known that a very thin oxide film exists on the stainless steel or titanium, and it has been learned through experiment that this film, too, is a cause of increasing the contact resistance. Concerning this, the noble metal is deposited while removing the film to decrease the contact resistance to the carbon paper.
Therefore, the contact surface of the carbon paper was coated with gold by the ion-plating method to measure the effect of decreasing the contact resistance to a mirror-surface-polished stainless steel, from which it was learned that the contact resistance starts decreasing when gold is deposited in an average thickness of not smaller than 5 nm as calculated from the vaporization rate and vaporization time.
Further, a stainless steel was lightly polished by using a water-resistant silicon carbide paper in a 1N
hydrochloric acid aqueous solution containing 0.5~ of chloroplatinic acid, in order to precipitate a trace amount of platinum on the surface of the metal by the corrosion reaction while mechanically removing the passive film, and the contact resistance to the untreated carbon paper was measured. It was learned that the contact resistance decreases when platinum is deposited in an average thickness of not smaller than 5 nm.
Example 1.
In order to confirm the effect according to the first aspect of the invention, the surfaces of the stainless steel and titanium were treated, and the contact resistances of the contact surfaces of the combinations thereof were measured. The results were as shown in Tables 1 to 8.
A combination number 1 represents a case when both the carbon paper and the stainless steel were not treated (reference 1).
Combination numbers 2 to 28 represent the measured results of contact resistances of when the surfaces of various stainless steels and titanium on which a noble metal was precipitated or deposited while mechanically removing the oxide film, are contacted to the surface of the carbon paper on which gold has been deposited maintaining a thickness of 1000 nm by ionic vaporization.
The results show that the contact resistances of the stainless steels and titanium decrease after the treatment.

In order to precipitate or deposit the noble metal while removing the film, various methods were carried out, such as a method of lightly polishing with the water-resistant silicon carbide paper in a 1N
hydrochloric acid aqueous solution containing 0.5~ of chloroplatinic acid, so that platinum or the like was precipitated in trace amounts by the corrosion reaction, a method of precipitating platinum in larger amounts by the cathodic electrolysis after the above polishing has been effected, and a method of blasting with glass beads which are nonelectrolytically plated with gold or silver.
The effect of decreasing the contact resistance could be observed by any method.
Combination numbers 29 to 45 represent the results of contact resistances on the contact surfaces between stainless steels, between titanium, and between stainless steel and titanium. A decrease in the contact resistance was calculated with the contact resistance of the untreated surfaces of a combination No. 29 as a reference (reference 2). It was proved that the contact resistances could be decreased in all cases.
Combination numbers 46 to 57 represent the measured results of contact resistance between the untreated carbon paper and various stainless steel plates or titanium plate lightly polished with a water-resistant silicon carbide paper #320 in a mixture solution of 0.5$
of potassium chloroplatinic acid and 1N of hydrochloric acid, and leaving them for a predetermined period of time or cathodically treating the surfaces, as a method of depositing a noble metal while removing the film. The effect of greatly decreasing the contact resistance could be observed in all cases.
Combination numbers 58 to 69 represent the measured results of contact resistance between the untreated carbon paper and various stainless steel plates or titanium plate lightly polished with the water-resistant silicon carbide paper #320 in a gold-plating bath, and leaving them for a predetermined period of time or treating the surfaces by cathodic electrolysis, as a method of depositing a noble metal while removing the film. The effect of greatly decreasing the contact resistance could be observed in all cases.
Combination numbers 70 to 81 represent the measured results of contact resistance between the untreated carbon paper and various stainless steel plates or titanium plate lightly polished with the water-resistant silicon carbide paper #320 in a palladium-plating bath, and leaving them for a predetermined period of time or treating the surfaces by cathodic electrolysis, as a method of depositing a noble metal while removing the film. The effect of greatly decreasing the contact resistance could be observed in all cases.
Combination numbers 82 to 93 represent the measured results of contact resistance between the untreated carbon paper and various stainless steel plates or titanium plate lightly polished with the water-resistant silicon carbide paper #320 in a silver-plating bath, and leaving them for a predetermined period of time or treating the surfaces by cathodic electrolysis, as a method of depositing a noble metal while removing the film. The effect of greatly decreasing the contact resistance could be observed in all cases.
Combination numbers 94 to 105 represent the measured results of contact resistance between the untreated carbon paper and various stainless steel plates or titanium plate of which the surfaces are treated by being blasted with glass beads plated with gold, silver or platinum, as a method of depositing a noble metal while removing the film. The effect of greatly decreasing the contact resistance could be observed in all cases.
It is desired that a noble metal or an alloy of the noble metal is deposited on both surfaces that come in contact with each other. The noble metals and the method of depositing the noble metals are not limited to the above-mentioned examples only, but any conventionally employed method or combinations thereof may be used.

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az The baths for plating gold, palladium and silver used for the combination numbers 58 to 93 possessed the compositions as described below.
Composition of a Qold-platincr bath An aqueous solution in which are mixed:
Potassium gold cyanide:KAu(CN)2:4 g/L
Sodium cyanide:NaCN:30 g/L
Soda ash:Na2C03:40 g/L
and maintained at a temperature of 65°C.
Composition of a palladium~lating bath An aqueous solution in which are mixed:
Palladium chloride:PdC13:5 g/L
Hydrochloric acid:HC1:250 ml/L
and maintained at normal temperature.
Composition of a silver-plating bath An aqueous solution in which are mixed:
Silver cyanide:AgCN:5 g/L
Potassium cyanide:KCN:75 g/L
and maintained at normal temperature.
A stainless steel plate was machined into a separator, a paste of a fine carbon powder containing platinum was applied onto a commercially available solid polymer electrolyte membrane and was dried, and a nonwoven fabric of carbon fiber was used as a current collector to obtain a fuel cell.
A pure hydrogen gas or a simulated methanol-cracked gas (25~ of C02, 75$ of Hz) was fed as a fuel gas to the side of the hydrogen electrode, a simulated air gas (20~
of O2, 80~ of NZ) was fed to the side of the oxygen electrode under the atmospheric pressure, the whole cell was held in a high-temperature chamber at a temperature of 90°C, and a short-circuit current flowing into the external unit from the positive electrode to the negative electrode was measured to confirm the performance of the fuel cell.
The electrodes used for the test possessed a size of 100 x 100 mm. By taking the corrosion resistance into _ 27 _ consideration, the separator was formed by press-molding a stainless steel plate YUS270 having a thickness of 0.4 mm so as to possess grooves and holes that sereve as gas passages and coolant passages.
The stainless steel separator possessed a contact surface that was lightly polished by using a silicon carbide paper #320 in a 1N hydrochloric acid containing 0.5$ of potassium chloroplatinic acid by making reference to the results of table 1 and on which surface platinum was precipitated by the cathodic electrolysis conducted for 5 minutes. For the purpose of comparison, further, another stainless steel separator was prepared having a contact surface that was not treated. There were further prepared carbon papers having a contact surface on which gold was deposited maintaining a thickness of about 1000 nm by the ionic vaporization and having a contact surface that was not treated for comparison.
As a result, the solid polymer electrolyte fuel cell that was not treated generated a short-circuit current of only about 15 A, whereas the solid polymer electrolyte fuel cell that was treated to decrease the contact resistance generated a short-circuit current of about 85 A, from which it is obvious that a decrease in the internal contact resistance contributes to greatly increasing the power efficiency.
According to the first aspect of the present invention as described above, it is possible to greatly decrease the contact resistance of the member that so far created a problem in using a stainless steel material, which is cheaper than the traditional carbon materials and makes it possible to decrease the size, as a material of the separator for the solid polymer electrolyte fuel cell that is promising as a generator for cars and as a portable generator. This contributes greatly to putting the solid polymer electrolyte fuel cell into practical use.
Next, described below is a preferred embodiment according to the second aspect of the invention.
In the solid polymer electrolyte fuel cell, the solid polymer electrolyte membrane that selectively transmits hydrogen ions is held by the catalytic electrodes comprising fine particles of carbon and a noble metal, and the electric power is generated by taking out electrons from the oxidation reaction of hydrogen and by taking out electrons from the reducing reaction of oxygen that takes place on the respective electrodes. The electrons are collected by the current collector constituted by a nonwoven fabric having an electrical conducting property, such as carbon fibers, and are guided to the electrically conducting separator.
Single cells having such a basic structure are stacked in series to obtain a cell which as a whole generates a required electromotive power.
The separator requires functions such as one for conducting electricity, as well as a separation function so that a hydrogen gas or a hydrogen-containing gas which is a reaction gas and a gas such as the air containing oxygen will not be mixed together, and, as required, a structural function which permits a coolant such as water to flow inside the cell structure but circulates the coolant and the reaction gases separated from each other.
The carbon material has heretofore been chiefly used as a member for the solid polymer electrolyte fuel cell, such as the separator. However, the carbon material requires an increased production cost for forming grooves and imposes a limitation on decreasing the thickness, making it difficult to decrease the cost of the fuel cell as a whole and to decrease the size. In order to solve this problem, therefore, the present inventors have attempted to use a stainless steel instead of the carbon material and, at the same time, have studied combinations of the additives and their amounts to withstand the environment in which the solid polymer electrolyte fuel cell is used.
The reaction gas, which is a fuel flowing in the solid polymer electrolyte fuel cell, is pure hydrogen, hydrogen containing some impurities, alcohol such as methanol, or a cracked gas of hydrocarbons (representative composition: 25~ of carbon dioxide gas, 75~ of hydrogen and several tens of ppm of carbon monoxide). On the other hand, the reaction gas for controlling the combustion is an oxygen-containing gas and, generally, air from the atmosphere. In order for the solid polymer electrolyte membrane to work as an electrolyte, an amount of water is necessary, and these gases are controlled to have a dew point of about 80°C.
The operation temperature is generally about 90°C.
In this system, the fuel cell starts operating and stops repetitively. It needs not be pointed out, first, that what is most important is that the separator itself does not corrode. In particular, when a cracked gas of methanol is used, the carbon dioxide gas contained therein is absorbed by the condensed water in the fuel cell to form an acidic solution. Besides, the solid polymer electrolyte membrane itself is an acidic solid electrolyte. Therefore, the environment in which the separator is exposed is an acidic aqueous environment at a temperature of from normal temperature up to a boiling point of the coolant such as water (usually, up to about 150°C), and it has been pointed out that the pH drops down to about 2 depending upon the conditions in which it is used. Once the corrosion takes place, the metal ions eluted from the corroded portion contaminates the solid polymer electrolyte membrane even though the corrosion may be of a slight degree, impairing the function for selectively transmitting hydrogen and seriously affecting the cell performance. Thus, the corrosion causes a problem even when it is eluting out trace amounts of ions.
The present inventors have estimated that the elements that impart corrosion resistance in an acidic environment of relatively low temperatures are chiefly Cr, Mo and Ni, have prepared thin plates of stainless steels while changing the amounts of addition and combination of the elements, machined the steel plates into separators, applied a paste of a fine carbon powder containing platinum onto the commercially available solid polymer electrolyte membrane followed by drying, and employed a nonwoven fabric of carbon fiber as a current collector to constitute a fuel cell. A pure hydrogen gas or a simulated methanol-cracked gas (25~ of C02, 75~ of Hz) was fed as a fuel gas to the side of the hydrogen electrode, a simulated air gas (20~ of O2, 80$ of NZ) was fed to the side of the oxygen electrode under the atmospheric pressure, the whole cell was held in a high-temperature chamber maintained at 90°C, and a change in the short-circuit current flowing into the external unit from the positive electrode to the negative electrode was measured with the passage of time to confirm the endurance and reliability of the fuel cell performance (endurance generation testing).
The electrode units used for the testing possessed a size of 100 mm x 100 mm, and the separators were prepared by cutting 4 mm-thick stainless steel plates to form grooves that serve as gas passages. After 100 days have passed from the start of the testing, the external current was measured and a ratio to the current initially generated was found to evaluate the endurance and reliability. Here, it was so judged that the fuel cell could be practically used provided the ratio was not smaller than 0.9. The conditions such as cell size, reaction gas, temperatures for use and the like were selected from the practical point of view, and severe testing was conducted by continuously passing the current for 2400 hours (100 days). Therefore, the stainless steels for practical use could be selected very reliably.
After the endurance generation testing, those having a ratio of the current after the continuous use of 2400 hours/the initial current of not smaller than 0.9 were rearranged for their components, and it was found that a numerical value calculated according to 10 - 0.3 x ([Cr$]
+ 3 x [Mod] + 0.05 x [Nib])~ stands for $ by weight of the elements) served as an effective index provided Cr was contained as an essential component and Mo and Ni were preferably contained. According to the study by the inventors, it was found that when the numerical value calculated according to the above formula is not larger than 5, the separator exhibits suitable properties when pure hydrogen is used as the fuel gas and when the numerical value calculated according to the above formula is not larger than 4, the separator exhibits suitable properties even when a reformed gas of alcohols is used as the fuel gas. That is, the important point of the invention is that the lower limits of the contents of Cr, Mo and Ni, that are basic elements associated with imparting corrosion resistance to the stainless steels, can be expressed by the above-mentioned formula under the environmental conditions in which the separator of the solid polymer electrolyte fuel cell is exposed.
Therefore, the above index makes it possible to specify a stainless steel having a performance sufficient for the solid polymer electrolyte fuel cell and, hence, to provide a material, of low cost, which avoids unnecessary or excess amounts of addition of the elements. As described above, the testing was conducted under the environmental conditions in which the separator was exposed. Since the severest environmental conditions were employed, the invention can be applied to other constituent members made of stainless steel, such as a terminating plate used for the terminating portion of the laminate.
In the present invention, it is important that, in an environment where pure hydrogen is used as the fuel gas, 10 - 0.3 x ([Cr$] + 3 x [Mod] + 0.05 x [Nib]) s 5 is satisfied and in an environment where a reformed gas of alcohols is used as the fuel gas, 10 - 0.3 x ([Cr$] + 3 x [Mod] + 0.05 x [Nib]) s 4 is satisfied. Though the roles of the individual elements have not yet been clarified in detail yet, described below are the elements that are added.
Chromium is a major element that imparts corrosion resistance by establishing a passive state in a corrosive environment to which the invention is related, and exhibits a effect even when it is added alone. The effect is exhibited if the lower limit value of the addition complies with the condition of the above-mentioned formula. The effect saturates when the addition exceeds 30~. Therefore, the upper limit is 30~.
From the standpoint of sufficiently lowering the cost, however, the addition is so adjusted as to satisfy the above formula in a range of not exceeding 23~.
It is desired to add molybdenum since it is considered that molybdenum exhibits the effect of suppressing, particularly, local corrosion in the corrosive environment with which the invention is concerned. The effect is exhibited so far as the lower limit value of addition complies with the condition of the above formula. The effect, however, saturates when the addition exceeds 10$. Therefore, the upper limit is 10~. From the standpoint of sufficiently lowering the cost, however, the addition is so adjusted as to satisfy the above formula in a range of not larger than 7~ and, particularly, not larger than 3~ in a pure hydrogen environment.
It is desired to add nickel since it is considered that nickel further increases the corrosion resistance of the steel material by increasing the austenite phase in the corrosive environment with which the invention deals.
The effect is exhibited so far as the lower limit value of addition complies with the condition of the above formula. The effect, however, saturates when the addition exceeds 25~. Therefore, the upper limit is 25~.
From the standpoint of sufficiently lowering the cost, however, the addition is so adjusted as to satisfy the above formula in a range of not larger than 20~ and, particularly, not larger than 15~ in a pure hydrogen environment.
Though not specified by the above-mentioned formula, copper, which is effective in imparting corrosion resistance, may be suitably added in an amount of not larger than 2.5~ provided it does not sharply drive up the cost, without departing from the scope of the invention. Within a scope investigated by the present inventors, the corrosion resistance in an environment with which the invention is concerned is not affected by the method of production. Therefore, any conventional production method may be employed provided it does not cause any extreme production problem.
Example 2.
The invention will be further described in detail based on the results of the above endurance generation testing over 100 days. Details of the testing conditions are as described above. As a result of putting the stainless steels containing components listed in Table 9 to the test, it was confirmed that the current decreased little after 100 days and the ratio of the current after the testing/the initial current was not smaller than 0.9 when stainless steel materials satisfying a relationship 10 - 0.3 x ([Cry] + 3 x [Mod] + 0.05 x [Nib]) s 5 in the pure hydrogen environment and a relationship 10 - 0.3 x ([Cr$] + 3 x [Mod] + 0.05 x [Nib]) 5 4 in the methanol-reformed gas environment were used. Thus, the separators exhibited their functions to a sufficient degree in the solid polymer electrolyte fuel cell in their respective gaseous environments. When these requirements were not satisfied, the ratios were not larger than 0.9 and the separators failed to exhibit their functions to a sufficient degree as shown in the dotted areas in Table 9.

According to the second aspect of the invention as described above, optimum component ranges are specified for the separator materials for the solid polymer electrolyte fuel cell which is promising as a generator for cars and as a portable generator, making it possible to provide stainless steel materials at a low cost and in a compact size compared with the traditional carbon materials.
Next, described below is a preferred embodiment according to the third aspect of the present invention.
First, Fig. 1 illustrates a stainless steel separator for the solid polymer electrolyte fuel cell.
The separator has, at its central portion 1, a corrugated structure comprising a plurality of grooves, including electrically conducting portions 2 of convex surfaces and passage portions 3 of concave surfaces for the gas or the coolant as shown in a sectional view thereof. The grooves are coupled together at their both ends by coupling portions 8, and the passages are secured permitting the reaction gas or the coolant to flow through the grooves at the central portion. Peripheral portions 7 are provided with holes 4 that serve as passages for the one reaction gas and with holes 5 that serve as passages for the other reaction gas, enabling the reaction gases to be fed or drained. The peripheral portions 7 further have holes 6 serving as passages for the coolant, enabling the coolant to be fed or drained.
The stainless steel plate which is a blank material has a thickness of not larger than 2 mm. Though an optimum thickness is determined from the corrosion resistance and the strength, the stainless steel plate having a small thickness is desired from the standpoint of productivity and cost. From the standpoint of production cost, it is desired that the part is produced by press-molding and punching.
Next, described below are spacers A and B for the solid polymer electrolyte fuel cell. The spacers A and 8 have shapes for securing passages for the one reaction gas and for securing passages for the other reaction gas.
The spacer A is shown in Fig. 2 and the spacer B is shown in Fig. 3. The spacer A has in the peripheral portions 7 thereof holes 10 communicated with the central portion, and the spacer B has in the peripheral portions 7 thereof holes 11 communicated with the central portion, to secure the passages for the one reaction gas and the passages for the other reaction gas, so as to be guided to the groove-coupling portions 8 of the separator. To the central portions are fitted the central portion 1 of the separator. Therefore, the spacers should have a thickness corresponding to the height of concave grooves or convex portions of the separator. Any material may be employed provided the gas does not leak. From the standpoint of cost, however, it is desired to use a resin that is not deformed or does not undergo a chemical change up to 100°C.
Fig. 4 illustrates a spacer C for the solid polymer electrolyte fuel cell. The spacer C secures passages for the coolant and has, in the peripheral portions 7 thereof, holes 12 serving as passages for the coolant, and communicated with the central portion, so that the coolant is guided to the groove-coupling portion of the separator. The spacer C may be sandwiched by two separators, or may be used at an end and may be sandwiched by the separator and the terminating plate, without any difference in the structure except the thickness. Any material may be employed provided the gas does not leak. From the standpoint of cost, however, it is desired to use a resin that is not deformed or does not undergo a chemical change up to 100°C.
Fig. 5 illustrates a spacer D for the solid polymer electrolyte fuel cell. The spacer D is inserted between the solid polymer electrolyte membrane to which a catalytic electrode is imparted as shown in Fig. 6 and the spacer A or B. The spacer D serves as a frame for the catalytic electrode unit 14 formed on the solid polymer electrolyte membrane 13 shown in Fig. 6 and for a current collector 15 shown in Fig. 7, and has a thickness as close to the sum of the thickness of the catalytic electrode unit 14 and the thickness of the current collector 15 as possible. Any material may be employed provided the gas does not leak. From the standpoint of cost, however, it is desired to use a resin that is not deformed or does not undergo a chemical change up to 100°C.
In order to further decrease the number of parts and to further lower the cost, it is desired to employ a spacer E constituted by the spacers A and D as a unitary structure and a spacer F constituted by the spacers B and D as a unitary structure from the standpoint of productivity.
Fig. 8 illustrates a terminating plate for the solid polymer electrolyte fuel cell. The terminating plate has ports 16 and 17 for feeding and draining the reaction gases and ports 18 for feeding and draining the coolant.
The terminating plate works to take out the electric power to the external unit from the solid polymer electrolyte fuel cell that is laminated in series, feeds the reaction gases and the coolant, and applies a suitable pressure to both ends of the laminated module so that no gas leaks or no coolant leaks from the whole laminated structure.
The separators, spacers, terminating plates and the solid polymer electrolyte membranes are laminated and secured, desirably, by using bolts. It is therefore desired that these members have bolt holes 9 formed in the peripheral portions thereof. Thus, the fastening means is contained in the laminate, which is very desirable for decreasing the size. Besides, a reduction in the number of the fastening members makes it possible to lower the cost.
Fig. 9 illustrates a laminated module for the solid polymer electrolyte fuel cell that is constituted in a laminated manner so that separate passages are secured for the one reaction gas, for the other reaction gas and for the coolant. The solid polymer electrolyte membrane 13 to which the catalytic electrode 14 is imparted, current collector 15, spacer A19, spacer B20, spacers C21, 22, spacer D23, stainless steel separator 24, and stainless steel terminating plate 25 are laminated to secure passages 26 for the coolant, passages 27 for the one reaction gas, and passages 28 for the other reaction gas. The reaction gases and the coolant are fed through the reaction gas feed/drain ports 16, 17 and the coolant feed/drain ports 18. Though not diagramed, the spacer E
combines the spacers 19 and 23 integrally together, and the spacer F combines the spacers 20 and 23 integrally together. The shapes of these parts and the structure of the fuel cell are only some examples, and it need not be pointed out that shapes and sizes may be changed provided the same fundamentals apply.
In the solid polymer electrolyte fuel cell, the one reaction gas is usually a fuel gas such as a hydrogen-containing gas or a methanol-reformed gas, and the other reaction gas is usually a combustion-assisting gas such as oxygen-containing gas for controlling the combustion.
As the coolant, water is usually used from the standpoint of cost and safety. That is, the cooling is effected relying on the cooling water and, hence, the solid polymer electrolyte fuel cell is used at temperatures of not higher than the boiling point thereof and, typically, at about 90°C.
The separator and the terminating plate used for the solid polymer electrolyte fuel cell are made of a stainless steel, and are used each in a number of one or more.
Further, the cell may be constituted by combining one or more of the laminated modules.
Example 3.

The members shown in Figs. 1 to 8 and having a square shape of a side of 240 mm were prepared, i.e., stainless steel separators were prepared by using a stainless steel plate having a thickness of 0.5 mm and containing 20~ Cr - 18~ Ni - 6~ Mo - 0.2~ N, and spacers were prepared by using a fluorine-contained resin.
A solid polymer electrolyte membrane available on the market was used, and the catalytic electrode was applied thereon and dried, and was cut into a predetermined shape and was laminated. Further, a nonwoven fabric of carbon fiber was cut into a predetermined shape and was used as the current collector.
A laminated module shown in Fig. 9 was prepared by using the above members as a basic structure. In order to prevent the leakage of gases and coolant, the contacting surfaces of the parts that do not require electric conduction were coated with a thin silicone resin film as a sealing material, and the whole laminate was pressurized by fastening the bolts. The cell consisted of a laminate of 10 stages. After having confirmed that there is no leakage of reaction gases or coolant, pure hydrogen and the air gas were used as the reaction gas, and electric power was generated while so controlling the temperature and the flow rate of the cooling water that the whole cell was maintained at 90°C.
The electromotive force of 5 to 6 V and a short-circuit current of a maximum of 400 A were observed, from which it was proved that the solid polymer electrolyte fuel cell of this system can be constituted to a sufficient degree.
According to the third aspect of the invention, there is provided concrete technical means for using a cheap stainless steel as members of the solid polymer electrolyte fuel cell instead of using the conventional expensive carbon material. It is therefore made possible to enhance the productivity and to greatly decrease the cost, contributing to the widespread use of the solid polymer electrolyte fuel cell.

Claims (18)

1. A low-contact-resistance stainless steel separator plate for use in a solid polymer electrolyte fuel cell, wherein a noble metal or an alloy of a noble metal is deposited on a surface of the plate in contact with carbon materials and from which an oxide film has been removed.

2. The plate according to claim 1, wherein the carbon materials is a current collector.

3. The plate according to claim 1 or 2, wherein the noble metal or the alloy of the noble metal that is deposited has an average thickness of not less than 5 nm.

4. The plate according to any one of claims 1 to 3, wherein the stainless steel plate comprises the following components in following amounts by weight:
Cr: not more than 30%, - 0.3 × {(Cr%) + 3 (Mo%) + 0.05(Ni%) <= 5%, and the remainder Fe.

5. The plate according to any one of claims 1 to 3, wherein the stainless steel plate comprises the following components in following amounts by weight:
Cr: not more than 30%;
10 - 0.3 × {(Cr%) + 3 (Mo%) + 0.05(Ni%)} <= 5%;
and one or more of:
Mo: not more than 10%, and Ni: not more than 25%, by weight;
the remainder Fe.

6. A low-contact-resistance titanium plate for use in a solid polymer electrolyte fuel cell, wherein a noble metal or an alloy of a noble metal is deposited on a surface of the plate in contact with carbon materials and from which an oxide film has been removed.

7. The plate according to claim 6, wherein the carbon materials is a current collector.

8. The plate according to claim 6 or 7, wherein the noble metal or the alloy of the noble metal that is deposited has an average thickness of not less than 5 nm.

9. A method of producing a low-contact-resistance stainless steel plate for use in a solid polymer electrolyte fuel cell by depositing a noble metal or an alloy of a noble metal on a surface of the plate in contact with carbon materials and from which an oxide film has been removed by blasting said surface of the plate with particles coated with the noble metal or the alloy of the noble metal.

10. The method of claim 9 wherein the carbon materials is a current collector.

11. A method of producing a low-contact-resistance stainless steel plate for use in a solid polymer electrolyte fuel cell by depositing a noble metal or an alloy of a noble metal on a surface of the plate in contact with carbon materials and from which an oxide film has been removed by wet-plating said noble metal or said alloy while polishing said surface.

12. The method of claim 11 wherein the carbon materials is a current collector.

13. A method of producing a low-contact-resistance titanium plate for use in a solid polymer electrolyte fuel cell by depositing a noble metal or an alloy of a noble metal on a surface of the plate in contact with carbon materials and from which an oxide film has been removed by blasting said surface of the plate with particles coated with a noble metal or an alloy of said noble metal.

14. The method of claim 13 wherein the carbon materials is a current collector.

15. A method of producing a low-contact-resistance titanium plate for use in a solid polymer electrolyte fuel cell by depositing a noble metal or an alloy of a noble metal on a surface of the plate in contact with carbon materials and from which an oxide film has been removed by wet-plating said noble metal or said alloy while polishing said surface.

16. The method of claim 15 wherein the carbon materials is a current collector.

17. A stainless steel separator for use in a solid polymer electrolyte fuel cell, the separator comprising:
in a central portion thereof, a corrugated plate structure comprising a plurality of stainless steel plates of any one of claims 1 to 5, said plates comprising a plurality of grooves formed therein, and wherein the ends of said plates are coupled together by coupling portions; and in a peripheral portion thereof, holes serving as gas passages for a first reaction gas, holes serving as gas passages for a second reaction gas and holes serving as passages for a coolant, said holes each in a number of two or more.

18. A titanium separator for use in a solid polymer electrolyte fuel cell, the separator comprising:
in a central portion thereof, a corrugated plate structure comprising a plurality of titanium plates of any one of claims 6 to 8, said plates comprising a plurality of grooves formed therein, and wherein the ends of said plates are coupled together by coupling portions;
and in a peripheral portion thereof, holes serving as gas passages for a first reaction gas, holes serving as gas passages for a second reaction gas and holes serving as passages for coolant, said holes each in a number of two or more.