XIA Zhengwei ^{1, 2}, WU Yucheng ¹, ZHANG Haibin ², ZHANG Xinfeng ², LI Canmin ², LIU Dongguang ¹

(1. Department of Materials Science and Technology, Hefei University of Technology, Hefei 230009, Anhui, China;

2. Anhui Chunyuan Coating Technology Co., Ltd., Hefei 230088, Anhui, China)

Extended abstract:[Background and purposes] Proton exchange membrane fuel cells (PEMFCs), which convert hydrogen energy directly into electrical energy and water, have received overwhelming attention, owing to their potential to significantly reduce energy consumption, pollution emissions and reliance on fossil fuels. Bipolar plates are the major part and key component of PEMFCs stack, which provide mechanical strength, collect and conduct current segregate oxidants and reduce agents. They contribute 70–80% weight and 20–30% cost of a whole stack, while significantly affecting the power density. There are three types plates, including metal bipolar plate, graphite bipolar plate and composite bipolar plate. Stainless steel bipolar plates, as one of metal bipolar plate, exhibit promising manufacturability, competitive cost and durability among various metal materials. However, stainless steel would be corroded in the harsh acid (pH 2–5) and humid PEMFCs environment, whereas the leached ions will contaminate the membrane. In addition, the passivated film formed on the surface will increase the interfacial contact resistance (ICR). In order to improve the corrosion resistance and electrical conductivity of steel bipolar plates, surface coatings are essential. Metal nitride coatings, metal carbide coatings, polymer coatings and carbon-based coatings have been introduced in recent years. Carbon-based coatings, mainly including a-C (amorphous Carbon), Ta-C(Tetrahedral amorphous carbon) and DLC (diamond-like carbon), have attracted considerable attention from both academia and industry, owing to their superior performance, such as chemical inertness, mechanical hardness and electrical conductivity. However, Ta-C films as protective coating of PEMFCs have been rarely reported, due to the difficulty in production for industrial application. In this paper, multi-layer Ta-C composite films were produced by using customized industrial-scale vacuum equipment to address those issues.[Methods] Multiple layered Ta-C coatings were prepared by using PIS624 equipment, which assembled filtered cathodic arc evaporation, ion beam and magnetron sputtering into one equipment, while SS304 and silicon specimens were used as substrate for testing and analysis. Adhesion layer and intermediate layer were deposited by using magnetron sputtering at deposition temperature of 150 ℃ and pressure of 3×10⁻¹ Pa, while the sputtering current was set to be 5 A and bias power to be 300 V. The Ta-C layer was coated at arc current of 80–100 A, bias voltage of 1500 V and gas flow of 75 sccm. A scanning electron microscope (CIQTEK SEM3200) was used to characterize surface morphology, coating structure and cross-section profile of the coatings. Raman spectrometer (LabRam HR Evolution, HORIBA JOBIN YVON) was used to identify the bonding valence states. Electrochemical tests were performed by using an electrochemical work station (CHI760, Shanghai Chenhua Instrument Co., Ltd.), with the traditional three electrode system, where saturated Ag/AgCl and platinum mesh were used as the reference electrode and counter electrode, respectively. All samples were mounted in plastic tube and sealed with epoxy resin, with an exposure area of 2.25 cm², serving as the working electrode. Electrochemical measurements were carried out in simulated PEMFCs cathode environment in 0.5 mol·L⁻¹ H₂SO₄+5 ppm F− solution, at operating temperature of 70 ℃. As the cathode environment was harsher than the anode environment, all the samples are stabilized at the open-circuit potential (OCP) for approximately 30 min before the EIS measurements. ICR between bipolar plates and GDL was a key parameter affecting performance of the PEMFCs stack. The test sample sandwiched between 2 pieces of carbon paper (simulate gas diffusion layer, GDL) was placed between 2 gold-plated copper electrodes at a compaction pressure of 1.4 MPa, which was considered to be the conventional compaction pressure in the PEMFCs. Under the same conditions, the resistance of a single carbon paper was measured as well. The ICR was calculated according to the formula ICR= ½(R2−R1)×S, where S was the contact area between GDL and coated stainless steel BPPs. All data of ICR were measured three times for averaging.[Results] The coatings deposited by filtered cathodic arc technology were compact and smooth, which reduced coating porosity and favorable to corrosion resistance. The coating thickness of adhesion and intermediate layers were 180 nm, while the protective Ta-C coating thickness was about 300 nm, forming multiple coating to provide stronger protection for metal bipolar plates.Cr, Ti, Nb and Ta coatings were selected as adhesion layers for comparison. According to electrochemical test, Ta and Nb coatings have higher corrosion resistance. However, Ta and Nb materials would be costly when they are used for mass production. Relatively, Cr and Ti materials were cost effective. Hence, a comprehensive assessment was indispensable to decide the materials to be selected as adhesion layer. Ta-TiN and Ti-TiN combined adhesion and intermediate layer exhibited stronger corrosion resistance, with the corrosion current to be less than 10⁻⁶ A·cm⁻². Ta-C protective coating deposited by using filtered cathodic arc technology indicated displayed higher corrosion resistance, with the average corrosion density to be about 1.26×10⁻⁷ A·cm⁻². Ta-C coating also shown larger contact angle, with the highest hydrophobicity, which was one of the important advantages for Ta-C, in terms of corrosion resistance. According to Raman spectroscopy, the I(D)/I(G)=549.8/1126.7=0.487, with the estimated fraction of sp3 bonding to be in the range of 51 54%. The intermediate layer TiN has higher conductivity than the CrN layer. Considering cost, corrosion performance and ICR result, the Ti-TiN layer combination is recommended for industrial scale application.[Conclusions] Multiple layer coating structure of Ta-C film had stronger corrosion resistance; with more than 50% sp3 content, while it also had larger water contact angle and higher corrosion resistance than DLC film. The filtered arcing deposition technology was able to make the film to be more consistent and stable than normal arcing technology in terms of the preparation of Ta-C. The coating displayed corrosion density of 1.26×10⁻⁷ A·cm⁻² and ICR of less than 5 mΩ·cm², far beyond technical target of 2025 DOE (US Department of Energy). This indicated that the mass-production scale coating technology for PEMFC bipolar plates is highly possible.

Key words: PEMFC; stainless steel bipolar plates; tetrahedral amorphous carbon (Ta-C) films; corrosion resistance; interfacial contact resistance; multiple layers coating