Technical communicationConductive amorphous carbon-coated 316L stainless steel as bipolar plates in polymer electrolyte membrane fuel cells
Introduction
Bipolar plates are a key multifunctional component in polymer electrolyte membrane fuel cells (PEMFC) and account for the large portion of the mass and volume of a typical fuel cell stack [1], [2], [3]. The bipolar plates collect the current of the fuel cells and separate individual fuel cells. They serve as a collector connecting the cathode side of one cell to the anode side of the other one with good conductivity and supply the reactive gases to the anode side (hydrogen gas) and cathode side (oxygen gas) via the flow channels while removing the heat and reaction products (water) [3], [4], [5], [6]. Therefore, high corrosion resistance, low interfacial contact resistance, high gas impermeability, high mechanical strength, and low cost are required for the practical application of the bipolar plate materials [7], [8].
Much effort has been made to develop commercial bipolar plate materials. At present, two types of materials are used in bipolar plates, namely graphite and metal [4], [9]. With respect to corrosion resistance and electrical conductivity, graphite is preferred [7], [10], [11], [12], [13], [14]. However, the fabrication costs of graphite bipolar plates incorporating gas flow channels are high, subsequently limiting its application in bipolar plates. In addition, the graphite bipolar plates are typically several millimeters thick in fuel cell stacks due to its poor mechanical strength and brittle nature and so such PEMFC stacks are heavier and bulkier [15]. The alternative is to use metallic materials in bipolar plates. Metals have advantages such as good mechanical strength, high electrical conductivity, high gas impermeability, low cost, and ease of manufacturing [16], [17], [18], [19]. In addition, the gas flow channels can be easily fabricated in thin metal plates by pressing, but the corrosion resistance and passivation must be considered. Stainless steel is one of the suitable materials in this respect capable of providing satisfactory performance for several thousand hours without obvious power density decline [20]. Due to its self-passivating ability, stainless steel is usually covered by a passive film in the cathode environment and this surface layer prevents the bulk from further corrosion [21]. Although the passive film can decrease the corrosion rate of stainless steel, it will significantly increase the interfacial contact resistance (ICR) between the bipolar plate and back electrode thereby compromising the cell performance due to ohmic loss [17]. On the anode side, metal ions in the stainless steel can leach out into the membrane blocking the sulfonic acid sites and poisoning the catalytic process [2], [22], [23]. The corrosion resistance and electrical conductivity appear to be mutually exclusive in stainless steel, but it is possible to achieve both by coating technology [24], [25], [26].
A promising approach is to deposit a carbon film on the stainless steel substrate. This process combines the advantages of graphite and stainless steel. Show et al. [27], [28] have prepared amorphous carbon film on titanium bipolar plates at various temperature and their results show that the a-C film deposited at 600 °C possesses a low resistivity of 10−3 Ω cm, and the fuel cell assembled from the a-C coated Ti bipolar plate has an output power of 1.4 times higher than that of a bare Ti bipolar plate fuel cell. Chung et al. [29] have coated carbon film on stainless steel 304 by thermal chemical vapor deposition (CVD) at 680 °C using a mixture of C2H2/H2. Both the corrosion tests and PEMFC operation indicate that the carbon film has excellent chemical stability similar to that offered by high purity graphite plate. Fukutsuka et al. [6] have deposited stainless steel 304 with a carbon coating by plasma-assisted chemical vapor deposition. The carbon-coated SS304 exhibits high electrical conductivity and improved corrosion resistance in spite of the absence of a passive film. Fu et al. [9] have prepared a C–Cr composite film on stainless steel by pulsed bias arc ion plating. The results show that the interfacial conductivity and corrosion resistance are improved by the C–Cr film and the C–Cr film coated sample has high surface energy as well.
When the PEMFC is in operation, corrosion of the bipolar plate in the fuel cell system takes place potentiostatically. The anode and cathode environments are at about −0.1 V vs SCE with aerated H2 and 0.6 V vs SCE with air, respectively [30], [31]. Therefore, the bipolar plates in the anode and cathode undergo corrosion at an applied potential which is different from the free potential corrosion. To gauge the corrosion resistance for long-term operation in the simulated PEMFC anode and cathode environments, the potentiostatic test is necessary. However, not much work about carbon coating has been carried out in this respect. In the present study, SS316L is coated with amorphous carbon (a-C) by close field unbalanced magnetron sputter ion plating (CFUBMSIP) [32] and the corresponding effects on the electrical conductivity and corrosion resistance are investigated.
Section snippets
Experimental details
Austenitic stainless steel 316L (SS316L) coupons with size of 15 mm × 15 mm × 5 mm were used in bipolar plates. They were polished with No. 2000 SiC waterproof abrasive paper and No. 5 diamond paste polisher, cleaned with acetone and distilled water in an ultrasonic cleaner, and dried. The a-C film was deposited onto the samples using a close field unbalanced magnetron sputter ion plating (CFUBMSIP) system consisting of two targets of 99.99% pure graphite and two targets of 99.99% Cr. High purity
Surface morphology and structure
Fig. 1 depicts the AFM images showing the surface morphology of the amorphous carbon (a-C) coating on the SS316L sample. The a-C coating prepared by CFUBMSIP is dense and a continuous and compact film is observed, indicating that the a-C coating can block the substrate from direct corrosion. The Raman spectrum acquired from the a-C coated SS316L sample shown in Fig. 2 can be deconvoluted into two Gaussian curves, namely the G-band and D-band corresponding to the graphite band (originating from
Conclusion
316L stainless steel samples were coated with amorphous carbon (a-C) by close field unbalanced magnetron sputter ion plating. The properties of the carbon coating were determined by atomic force microscopy and Raman spectroscopy. The interfacial contact resistance (ICR) and electrochemical behavior were investigated. The results show that the a-C coating is dense, the size of the graphitic crystallites is small, and the number of disordered bands is large. The ICR values determined from the a-C
Acknowledgements
Financial support provided by National Natural Science Foundation of China under contract number 50820125506 and 50601018 and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 112306 are acknowledged. We also thank Huiqin Li in Instrumental Analysis Center of SJTU for the AFM experiments.
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