Development of Metallic Bipolar Plate for PEFC By Coating Carbon Thin Film on Stainless Steel
Authors: Ioku Sunao , Inoue Kiyohiro , Sugishita Hiroki , Chigi Yoshitaka , Sugie Yosohiro 
Published: February 2020

Introduction
The polymer electrolyte fuel cell (PEFC) has characteristics that it can operate at a relatively low temperature and can be downsized. Therefore, development is proceeding in a wide field such as a stationary power supply and a power supply for electric vehicles. It is required to reduce the size and weight of the fuel cell as well as to lower the price. For example, it is a method to solve by changing the separator material from a conventional carbon material to a metallic material. Therefore, in this research we proposed the following method aiming at the development of simple and low cost production method. The method is to coat the surface of stainless steel with a resin thin film and then carbonize it at low temperature. We observed the surface and cross section of the obtained carbon film using high resolution SEM. We also examined its applicability as a fuel cell separator by measuring Raman spectroscopy, TG-DTA, XRD, XPS and so on.
Experimental method
The substrate materials used were austenitic SUS304 stainless steel with various thicknesses. Surface pretreatment of the specimen was polished with wet emery paper up to #600. Also, it was used without pretreatment. In the experiment, specimens were used which was rinsed with water after degreasing with acetone before use. The resin coating on the specimen surface was carried out by electrodeposition coating method using various cationic resins. Electrodeposition coating was carried out with varying electrolytic voltage and time at 303K. Carbonization of the resin film was performed in an electric furnace under a nitrogen atmosphere. Sintering temperature was 673K to 1173K for 2 hours. The electrical properties of the carbon-coated film were evaluated by measurement of electrical conductivity. Electrochemical properties were evaluated by measurement of polarization curve. The polarization curves were measured by linear sweep polarization method with sweep rate of 20 mV / min at room temperature in 0.5M H2SO4 solution by a three electrode system using a working(specimen) electrode, a platinum plate as a counter electrode and Hg/Hg2SO4 electrode as a reference electrode. The electric resistance of the carbon film formed on the specimen surface by the carbonization treatment was measured and evaluated by the four-terminal sensing method.
Results and discussion
Figure 1 shows resin coated and its converted carbon film deposited on the specimen surface and cross section SEM photographs. The thickness of resin coated film with about 40 µm was reduced about 2 µm when sintered at 873K. The carbonized film had a smooth surface and no defects were observed. As a result of the tape peeling resistance test, resistance to the adhesion between the carbonized coating film to the substrate had a sufficient ability. This result seems to be due to the formation of the third layer which improves the adhesion at the interface between the substrate metal and the formed carbon film. Figure 2 shows the relationship between the electrical conductivity of the carbon film obtained by heat-treating the resin film formed on the specimen surface of various resin materials at 773 to 1173K and sintering temperature. The electrical resistance of sintered film decreased with increasing heat treatment temperature. And it showed under 10 mΩ·cm at 1073K. In order to improve the electrical conductivity at lower temperature treatment, a coating was prepared by mixing various kinds of carbon powder with resin. As a result, even in the heat treatment at 873K, a value of 10 mΩ·cm or less was obtained. Figure 3 shows polarization curves of specimens that are raw materials and covered by carbon thin film on specimen surface. The polarization curve of the untreated specimen showed an immersion potential of about 0.0 V(vs. SHE) and a passivation current density of 20 µA/cm2. On the other hand, the immersion potential in all carbon-coated specimens was 0.5 to 0.8 V(vs. SHE) and shifted in the noble direction and the anode current became 10 µA/cm2 or less. It was revealed that sufficient corrosion resistance and electric conductivity are exhibited by giving carbon coating treatment. The power generation test was carried out for 180 hours by connecting a load resistance of 0.1 Ω with a carbon film coated separator, as the result the generated voltage was approximately constant value of about 0.7 V during the test period. Therefore, it was clarified that the prepared separator is effective as a fuel cell separator. We intend to make a detailed presentation on the relation between the electrical conductivity and the corrosion resistance. This work was carried out with the assistance of the NEDO.

Figure 1