JP2011111381A - Method of forming conductive carbon membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a carbon film excellent in conductivity and corrosion resistance on the surface of stainless steel.
A stainless steel sheet is subjected to a cold rolling treatment of, for example, 9.0% or more, a part of the austenite structure of the surface layer is transformed into a stress-induced martensite structure, and then a pretreatment such as etching is performed on the surface. Thereafter, a carbon film is formed by plasma CVD.
[Selection] Figure 1

Description

The present invention relates to a method for forming a conductive carbon film on the surface of a stainless steel plate used as a separator of a fuel cell, for example.

  In a polymer electrolyte fuel cell, a solid polymer that selectively allows hydrogen ions to pass between a fuel electrode and an air electrode is disposed, and a separator is disposed outside the fuel electrode and the air electrode to form a cell. Stack.

  A cooling water passage is formed outside each separator, hydrogen is supplied between the fuel electrode and the separator, and air (oxygen) is supplied between the air electrode and the separator.

  As the separator, hydrogen and air are separated and a current collecting function is required. Therefore, a separator in which a conductive film is formed on the surface of a stainless steel plate is used.

Patent Document 1 describes that stainless steel is used as a fuel cell separator and a DLC (diamond-like carbon) film is formed on the surface by plasma ion implantation.

In Patent Document 2, as a separator for a fuel cell, a surface of a stainless steel plate is subjected to noble metal plating, and a carbon-containing film is formed by spraying particles of carbon black and a resin-based material on the surface of the noble metal plating by an aerosol deposition method. It is described to form.

Patent Document 3 describes that as a separator for a fuel cell, an uneven surface is formed on the surface of stainless steel, and an amorphous carbon film is formed on the uneven surface by ECR sputtering. Patent Document 4 describes plasma CVD. To form an amorphous carbon film.

In Patent Document 5 and Patent Document 6, in order to improve the surface slidability of the mechanical key for automobiles, the base material made of stainless steel is cold-worked to remove the work-induced martensite generated by the cold work. Later, it is disclosed that a functional film such as a DLC (diamond-like carbon) film or a chromium nitride film is formed on a stainless steel surface.

Non-Patent Document 1 describes that a carbon film is formed on the surface of stainless steel for a separator by plasma CVD.

JP 2009-037977 A JP 2007-31137 A JP 2007-165275 A JP 2008-004540 A JP 2008-031522 A Japanese Patent No. 4291342 Ceramics 43 (2008) No. 6

Since the DLC (diamond-like carbon) film described in Patent Document 1, Patent Document 5 and Patent Document 6 does not have conductivity, it is good for enhancing the slidability of the surface of a mechanical key or the like. As a separator for use, the conductivity is inferior.

  The separators described in Patent Document 2, Patent Document 3, Patent Document 4 and Non-Patent Document 1 are formed with a conductive carbon film on the surface of stainless steel. There are problems with corrosion resistance and coating adhesion.

In order to solve the above-mentioned problem, the method for forming a conductive carbon film according to the present invention is to subject a stainless steel plate to a cold rolling process, transform a part of the austenite structure into a stress-induced martensite structure, and then pretreat the surface. After the application, a carbon film was formed by plasma CVD.

The stress-induced martensitic structure produced by cold rolling has the same crystal structure (bcc) as ferrite and has a low chromium content, and therefore is inferior in corrosion resistance. For this reason, in Patent Documents 5 and 6, a process for removing or reducing stress-induced martensite is performed.

However, according to the knowledge of the present inventor, even if a stress-induced martensitic structure is generated by cold rolling, the martensitic structure is austenite by reverse transformation in the subsequent carbon film formation process (about 700 ° C.) by plasma CVD. As it returned to the structure, it was found that the chromium-deficient region near the grain boundary decreased and the chromium concentration on the stainless steel substrate surface increased.

The chromium and carbon combine to form a carbide (Cr ₂₃ C ₆ ), and this carbide (Cr ₂₃ C ₆ ) serves as a base for the carbon film, contributing to improved adhesion and the formation of a dense film, Even when a defect occurs in a part of the carbon film, it is considered that the corrosion resistance is maintained by the presence of the carbide (Cr ₂₃ C ₆ ).

The transformation into the stress-induced martensite structure appears remarkably when the cold rolling processing rate (actual processing rate) is 9.0% or more.

In order to improve the adhesion of the carbon film, it is preferable to roughen the surface to some extent by a plasma etching process as a pretreatment for film formation.

According to the method for forming a conductive carbon film according to the present invention, a conductive carbon film excellent in corrosion resistance and adhesion can be formed on the surface of a stainless steel plate easily and in a short time.

The perspective view of the cell of the fuel cell which used the stainless steel plate to which the formation method of the conductive carbon film concerning the present invention was applied as a separator. Perspective view of a fuel cell (stack) with multiple cells connected in series (A) is a photomicrograph of the carbon film formed by the method according to the present invention (b) is a photomicrograph of the carbon film formed without cold rolling. Graph showing polarization measurement results AFM photograph of carbon film formed by the method according to the present invention

Embodiments of the present invention will be described below with reference to the accompanying drawings. The method for forming a conductive carbon film according to the present invention can be applied to a separator of a fuel cell.
As shown in FIG. 1, in the fuel cell, catalyst layers 1a and 2a are formed on opposite surfaces of the fuel electrode 1 and the air electrode 2, and a solid polymer film 3 is disposed between the catalyst layers 1a and 2a.
Examples of the solid polymer membrane 3 include a substance that selectively permeates hydrogen ions, such as a fluoropolymer having a sulfonic acid group.

A separator 4 is provided outside the fuel electrode 1, a hydrogen passage 5 is formed between the separator 4 and the fuel electrode 1, and a separator 6 is provided outside the air electrode 2. An air (oxygen) passage 7 is formed between the air electrode 2 and the air electrode 2.

  Hydrogen is supplied between the fuel electrode 1 and the separator 4, and air (oxygen) is supplied between the air electrode 2 and the separator 6. The supplied hydrogen is divided into electrons and hydrogen ions in the catalyst of the fuel electrode 1, and the hydrogen ions pass through the solid polymer 3 to the air electrode 2 and react with oxygen supplied between the air electrode 2 and the separator 6. It becomes water and electrons flow to the air electrode to generate electricity.

  The cells are connected in series to form a stack shown in FIG. In this stack, a carbon film having excellent conductivity is formed on the surface so that the separators 4 and 6 also function as current collector plates.

  In order to form a carbon film on the surfaces of the separators 4 and 6, after cold rolling stainless steel, pretreatment is performed, and then a carbon film is formed by plasma CVD.

SUS304c was used as the stainless steel. The composition ratio (mass%) of SUS304c is as follows: C: 0.05, Si: 0.6, Mn: 0.95, P: 0.028, S: 0.005, Ni: 8.07, Cr: 18. 22, Fe: the balance.

As pretreatment, first, polishing (# 1000) was performed, ultrasonic cleaning was performed in an acetone solution, and the surface was treated by wiping with ethanol and then plasma etching using Ar gas.

The conditions for plasma etching are as follows.
Gas: Ar (25 sccm)
Pressure: 1 torr
Temperature: 750 ° C
Applied power: 90W
Time: 3minAr (25sccm)

Thereafter, a carbon film was formed on the surface of the stainless steel plate that had been pretreated by plasma CVD. The conditions for plasma CVD are as follows.
Reaction gas: C ₂ H ₂ (10 sccm), Ar (25 sccm)
Pressure: 1 torr
Deposition temperature: 750 ° C
Applied power: 90W
Deposition time: 3 hours

As shown in FIGS. 3A and 5, the carbon film prepared by the above method is formed as a dense carbon film having a uniform particle size. On the other hand, it can be seen that the carbon film formed on the surface of the stainless steel plate not subjected to cold rolling has defects as shown in FIG.

The following (Table 1) summarizes the conditions for cold rolling. Reference numeral 1 denotes a sample that was not rolled. In addition, FIG. 1-No. 5 is a graph showing the results of polarization measurement on the sample No. 5, and Table 2 shows the results of contact resistance before and after the formation of the carbon film and after polarization.

The measurement conditions for polarization are as follows.
Counter electrode: Platinum electrode Reference electrode: Mercury-mercuric sulfate electrode Measurement conditions: Room temperature polarization range: -1.0 V to 0.6 V vs. Hg / Hg ₂ SO ₄
Scanning speed: ¹ mVs ^-1
Electrolyte: 0.5M sulfuric acid degassed with nitrogen

From the above FIG. 4, (Table 1) and (Table 2), it can be seen that the carbon coating formed by the method according to the present invention is excellent in conductivity and durability.

DESCRIPTION OF SYMBOLS 1 ... Fuel electrode, 2 ... Air electrode, 1a, 2a ... Catalyst layer, 3 ... Solid polymer film, 4, 6 ... Separator, 5 ... Hydrogen passage, 7 ... Air (oxygen) passage.

Claims (3)

A stainless steel sheet is subjected to a cold rolling process, a part of the austenite structure is transformed into a stress-induced martensite structure, and then a surface is pretreated, and then a carbon film is formed by plasma CVD. Of forming a conductive carbon film. The method for forming a conductive carbon film according to claim 1, wherein the cold rolling treatment is performed at a processing rate of 9.0% or more. The method for forming a conductive carbon film according to claim 1, wherein the pretreatment includes a plasma etching step using Ar gas.

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