Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells

https://doi.org/10.1016/j.ijhydene.2018.12.184Get rights and content

Highlights

  • Chromium-molybdenum carbide coatings are deposited on SS 316 L bipolar plates.

  • A very low corrosion current density of 0.23 μA cm−2 is obtained.

  • A low interfacial contact resistance of 6.5 mΩ cm2 is achieved.

Abstract

Superior corrosion resistance and high electrical conductivity are crucial to the metallic bipolar plates towards a wider application in proton exchange membrane fuel cells. In this work, molybdenum carbide coatings are deposited in different thicknesses onto the surface of 316 L stainless steel by magnetron sputtering, and their feasibility as bipolar plates is investigated. The microstructure characterization confirms a homogenous, compact and defectless surface for the coatings. The anti-corrosion performance improves with the increase of the coating thickness by careful analysis of the potentiodynamic and potentiostatic data. With the adoption of a thin chromium transition layer and coating of a ∼1052 nm thick molybdenum carbide, an excellent corrosion current density of 0.23 μA cm−2 is achieved, being approximately 3 orders of magnitude lower than that of the bare stainless steel. The coated samples also show a low interfacial contact resistance down to 6.5 mΩ cm2 in contrast to 60 mΩ cm2 for the uncoated ones. Additionally, the hydrophobic property of the coatings’ surface is beneficial for the removal of liquid water during fuel cell operation. The results suggest that the molybdenum carbide coated stainless steel is a promising candidate for the bipolar plates.

Introduction

The proton exchange membrane fuel cell (PEMFC) has great promise as a new power generation technology for the stationary and transportation applications due to its high power density, high efficiency, zero carbon emission and fast response [1], [2]. The major components of PEMFCs include the membrane electrode assembly (MEA), the gas diffusion layer (GDL) and the bipolar plate (BP). The bipolar plate is particularly important because it accounts for ∼80% of the total stack weight and more than 30% of the stack cost. The BP plays multi-roles in a PEMFC stack, such as providing the mechanical support for the single cells, separating and guiding the reactant gas flows, collecting the electrical current, providing channels for removing the water product and facilitating water management [3]. Therefore, the BPs must have excellent electrical conductivity, high thermal conductivity, low gas permeability and certain mechanical strength [4]. On these properties, the Department of Energy (DOE) of the United States proposed some criteria to be achieved in 2020 towards the commercialization of BPs [5]. The criteria are summarized in Table 1.

Conventionally, graphite is used for bipolar plates due to its excellent electrical conductivity, thermal conductivity and chemical inertness, and good power output has been achieved with PEMFC stacks using the graphite BPs [6]. However, graphite is brittle and needs complex technique to be shaped into BPs, resulting in dramatic cost increase hindering the commercialization of PEMFCs [7]. In recent years, more and more studies were focused on the application of metallic materials (especially stainless steels, SS) in bipolar plates. Metallic BPs are very attractive due to the excellent mechanical strength, good electrical conductivity, low gas permeability, high thermal conductivity, ease of shaping into sheets and low cost production at large scales. However, the major drawbacks lie in that metallic BPs are prone to corrosion in the perfluorosulfonic acid and electrochemical environment of PEMFCs [8], [9]. The dissolved metal cations can migrate into MEAs and reduce the performance. Moreover, a non-conductive oxide layer may be formed on the surface of BPs due to corrosion, causing significant increase of contact resistance between BPs and GDLs thereby decreasing the stack performance [10], [11]. To solve these problems, protective and conductive surface coatings are demanded for metallic bipolar plates.

Up to now a variety of protective coating materials, such as graphite diamond-like carbon, conductive polymers, noble metal and metal nitrides, have been investigated for improving the corrosion resistance of the metallic BPs. For example, Sisan et al. prepared carbon film for the SS 316 L BPs using Physical Vapor Deposition (PVD) technique, and a film of only 200 nm thickness exhibited sufficient corrosion tolerance for use in PEMFCs [12]. Also, dense carbon films have been obtained through Chemical Vapor Deposition (CVD) in C2H2/H2 for protection of the SS 304 BPs [13], [14]. The carbon films, which consisted of highly crystallized graphite and amorphous carbon, exhibited decent electrical conduction and corrosion resistance comparable to that of graphite BPs. Feng et al. reported SS 316 BPs with a carbon coating of several micrometers’ thickness [15], showing not only a good corrosion resistance but also hydrophobicity that can benefit the water management. Additionally, Cr doped carbon was reported as another effective anti-corrosion coating for SS 316 BPs [16]. It was also found that the addition of Cr prompted the sp3 of carbon to change into sp2, resulting in a decrease of the contact resistance. Although carbon coating is advantageous considering the simple chemical composition and cost-effective manufacturing, its bonding to the SS substrate is usually insufficient and may fall off the substrate [12]. Further, carbon can be oxidized during PEMFC stack start-up where there is a high voltage instantaneously [17]. Some conductive polymers have also been investigated as coating materials of metallic BPs. For example, a polyaniline (PANI) coated SS 304 bipolar plate has demonstrated high corrosion resistance [18], although the contact resistance between the BPs and GDL was still too high for PEMFC applications [19].

Noble metals (e.g. Au or Ag) having excellent chemical stability and electrical conductivity could be good candidates for coatings of the metallic BPs [20]. However, the high cost limits their application in PEMFCs. Similar to the noble metals, some low-cost transition metal carbides and nitrides also exhibit good corrosion tolerance and electrical conductivity, therefore attracting a lot of interest to serve as anti-corrosion coatings in PEMFCs. For instance, a ∼2 μm thick TiN conductive coating [21] can successfully reduce the corrosion current density of the SS 316 BPs down to 1 μA cm−2. Also, densified TiC coatings have been applied onto the SS 304 BPs [22]. During the deposition process, any defects such as macro-particles or pinholes in the coating layer should be avoided since they are harmful to the lifetime of PEMFCs [23]. Barranco et al. employed cathodic arc evaporation technique for the deposition of CrN on Al-5083 [24], and studied the effect of coating thickness on the coating structure as well as the corrosion resistance. It was found that the number of defects can be significantly reduced with increase of the coating thickness [24]. Composite or multi-layered coatings were also developed to achieve defectless surface and improve the corrosion resistance of the BPs. Nam et al. prepared multi-layered coatings on the SS 316 L via RF sputtering [25], and a low corrosion current density ∼0.7 μA cm−2 was obtained with a 0.1 μm thick CrN inner layer and a 0.9 μm thick TiN outer layer.

Molybdenum is one important alloying element to stainless steel 316 for improvement of the anti-pitting corrosion resistance to acids. Molybdenum carbide has been well known as a low cost wear-resistive material [26] and an effective catalyst [27], [28]. It has a unique interstitial crystal structure with the smaller carbon atoms filling half of the total octahedral interstices in the lattice of the bigger metal atoms. Molybdenum carbide exhibits simultaneously some extent of metal-like (e.g. electrical conduction) and carbon-like (e.g. chemical inertness in acid) properties, which are demanded by the BPs of PEMFCs [29], [30], [31], [32]. To the best of our knowledge, there is little study so far on the molybdenum carbide coatings for metallic BPs of PEMFCs in literature. In this study, we deposited molybdenum carbide on SS 316 L by magnetron sputtering and investigated its feasibility as BPs. Instead of using Mo target together with carbon-source gases or carbon target [33], the magnetron sputtering deposition of molybdenum carbide was conducted directly with a single Mo2C target in this work, which is simple, easy to handle and cost-effective. The effect of coating thickness and adoption of an additional transition layer on the corrosion resistance as well as the contact resistance were investigated.

Section snippets

Sample preparation

Commercial SS 316 L plates were used as the substrate in this study. The SS 316 L plates were cut into small pieces in the dimension of 15 mm × 15 mm x 2 mm by wire electrical discharge machining. To remove the oxide layer and clean the surface, the SS 316 L samples were polished in turns with No. 600, 800, 1000, 1200 and 1500 SiC waterproof abrasive papers. The samples were then rinsed with acetone and distilled water, and finally dried with nitrogen purge gas. A magnetron sputtering equipment

Surface morphology and composition

SEM images in Fig. 2 show the surface morphology of the SS 316 L blank sample without coating and the one coated with molybdenum carbide. According to Fig. 2a, there were some micro-scratches on the surface of the pre-treated SS 316 L without coating before the electrochemical tests. After coating, the SS 316 L exhibited a compact, flat and homogeneous surface (Fig. 2b) without showing any scratches. This indicates that the coating can effectively cover the SS316L surface and eliminate the

Conclusions

Molybdenum carbide coatings were applied onto SS 316 L as protective coatings and successfully tested as bipolar plates of PEMFCs. The molybdenum carbide was deposited in different thicknesses by DC magnetron sputtering, and a smooth and dense surface was obtained without showing any major defect. The anti-corrosion property was investigated by potentiodynamic and potentiostatic polarization tests in the simulated PEMFC environments. The corrosion current of the SS 316 L was found to decrease

Acknowledgements

Thanks for Huhang Ma from the Materials Characterization and Preparation Center, Huqiang Yi and Xiaoming Zhang from the Department of Material Science and engineering, Southern University of Science and Technology, for guidance of the sputtering experiment and help on single cell testing. Thanks for the financial support by the Shenzhen Peacock Plan (KQTD2016022620054656), the National Key Research and Development Program of China (No. 2017YFB0102701), the Development and Reform Commission of

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