Electrically conductive amorphous carbon coating on metal bipolar plates for PEFC

doi:10.1016/j.surfcoat.2007.07.065

Surface and Coatings Technology

Volume 202, Issues 4–7, 15 December 2007, Pages 1252-1255

https://doi.org/10.1016/j.surfcoat.2007.07.065 Get rights and content

Abstract

Ti bipolar plates coated with amorphous carbon (a-C) film were developed for polymer electrolyte membrane fuel cells (PEMFC). The a-C films were coated at various growth temperatures. A low growth temperature caused the resistivity of the a-C film to be high. An a-C film coating with high resistivity resulted in an increase in the contact resistance between the surface of the bipolar plate and the membrane electrode assembly (MEA) of up to 440 Ωcm² which results in a decrease in the output power of the fuel cell. On the other hand, an a-C film coating with a low resistivity of 10^− 3 Ω cm was formed by an increase in the growth temperature of up to 600 °C. The bipolar plate coated with this a-C film showed low contact resistance (20 mΩ) between its surface and the MEA. Therefore, the fuel cell assembled from Ti bipolar plates coated with the a-C film at 600 °C showed an output power of 1.8 W, giving it an output 1.4 times higher than the output of a bare (not a-C coated) Ti bipolar plate fuel cell.

Introduction

Polymer electrolyte membrane fuel cells (PEMFC) are one of the candidates for practical automobile fuel cells [1], [2]. Some automobile companies have already applied PEMFCs to current cars. The application of fuel cells to automobiles requires miniaturization, high reliability and low cost. The bipolar plates are one of the high cost parts of the PEMFC. Two types of bipolar plates, using graphite and metal in terms of material, are commercially available [2]. Graphite bipolar plates are currently popular for fuel cells. They have advantages which include high chemical stability and high electrical conductivity. The high electrical conductivity reduces the inner resistance of the fuel cell and increases output power. However, graphite is generally fragile to an impact. Moreover, graphite is difficult to process into bipolar plates using mechanical methods. These disadvantages lead to high costs.

The metal bipolar plate is an alternative to graphite bipolar plates. Metal bipolar plates are inexpensive, because they are produced using chemical etching or press processes. Moreover, they generally are thinner than graphite bipolar plates thus resulting in a fuel cell stack that is both light in weight and small in volume. Ti and stainless steel are used as the materials for metal bipolar plates. These metals have a highly resistive oxidized layer on their surface. The oxidized layer increases the inner resistance of the fuel cell. Therefore, fuel cells assembled from metal bipolar plates show a lower efficiency in electric power generation than those made of graphite bipolar plates.

In this study, the oxidized layer is removed by the chemical etching process and then an amorphous carbon (a-C) film is coated onto the surface. The a-C film is electrically conductive, when it is deposited at a high temperature. Therefore, the a-C coated bipolar plate reduces the inner resistance of the fuel cell and increases the output power.

Section snippets

Experiments

A Ti electrode was used as a bipolar plate. The Ti surface was etched by using an HF solution of 1% in order to remove the oxidized layer. The a-C film was deposited on the Ti bipolar plate by using the radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) method. Ethylene was used as the source gas. The RF power, the flow rate of the ethylene gas and the deposition pressure were 150 W, 10 sccm and 0.5 Pa respectively. The growth time was fixed for 3 h. The growth temperature was

Results and discussion

Fig. 1 shows the Raman spectrum for the a-C films deposited at various growth temperatures. The a-C film deposited at R.T. showed broad Raman signals at around 1410 and 1560 cm^- 1. These signals are called the D- and the G-lines, respectively [3], [4], [5], [6]. The D-line is originated from the defects in the graphite crystal. On the other hand, the G-line is assigned to a graphite lattice. The a-C films deposited at 500 and 600 °C show relatively sharp D- and G-lines. The peak positions of the

Summary

The a-C film was coated onto the Ti bipolar plate by the RF-PECVD method. The a-C film was electrically conductive (10^− 3 Ωcm) when it was deposited at a high growth temperature (above 550 °C). The electrically conductive a-C film reduced the contact resistance between its surface and the MEA. Therefore, a fuel cell assembled from Ti bipolar plates coated with electrically conductive a-C film showed low series resistance and high output power. The use of this a-C coating technique for metal

Acknowledgements

The author would like to thank Mr. M. Miki and Mr. T. Nakamura graduate students of Tokai University, for their useful help on the experiments. This work is supported by the study promotion program in Tokai University.

References (7)

F. Alcaide et al.
J. Power Sources
(2006)
V. Mehata et al.
J. Power Sources
(2003)
B.S. Elman et al.
Phys. Rev., B
(1981)

There are more references available in the full text version of this article.

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