| Energy issues are gaining increasing attention throughout the world with the development of the economy and society. The traditional fossil energy is non-renewable energy sources and causes serious environmental pollution. The only way out is to develop non-polluted and renewable power sources. As an important renewable clean energy source, fuel cell (FC) exhibits many advantages such as high energy conversion efficiency and near-zero emission. Among various fuel cells, proton exchange membrane fuel cells (PEMFCs) are the most promising candidate to be widely used in the field of automotive application, portable power source and military application, especially as the primary automotive power sources in the future.As one of the important components in PEMFCs, bipolar plates (BPPs) play a role of collecting currents, separating oxidants from fuels and supporting the cell stack. Bipolar plates are the most heavy and expensive components in PEMFCs. So reducing the cost of bipolar plate is one of the key factors to lower the price of PEMFC. However, the traditional artificial graphite bipolar plates are bulky and with high gas permeability, low impact safety and relatively high cost. The noble metal bipolar plates, such as gold or silver, have good performance in all respects. However, they are so expensive that the high cost greatly prevents PEMFCs from commercial application. If stainless steel sheets can be used as bipolar plates to replace the graphite or noble metal plates, the cost of fuel cells would be greatly reduced. The stainless steels have many advantages such as good electrical conductivity, good thermal conductivity, high mechanical strength, and easy processing. However, the chemical stability of the stainless steel in the complicated environment is bad. Meanwhile, the interfacial contact resistance (ICR) of the stainless steel plate is so high which could cause the rapid corrosion and power degradation of the cells during the operation. Thus, stainless steel sheets couldn’t be used as bipolar plates directly. Preparing modified films on the stainless steel surface by physical vapor deposition (PVD) is an effective way to solve these problems. If the deposited films show higher corrosion resistance in both oxidizing and reducing environment and better electrical conductivity, we would achieve the goal of replacing noble metal and graphite bipolar plates by combining the good performance of the films with those of the stainless steel substrate, which would greatly promote the development in PEMFC industry. In this paper, film deposition on 316L stainless steel (SS316L) bipolar plates for PEMFCs is carried out. Arc ion plating (AIP), which is a typical PVD method, is applied to prepare a series of modified films with different chemical compositions and structures on commercial SS316L plates. Our aim is to obtain some films with high electrical conductivity, high corrosion resistance and appropriate hydrophobic property. The stainless steel bipolar plates coated with the modified films exhibit superb performance, which could replace the graphite and noble metal bipolar plate. Our research has an important theoretical and practical contribution to the fields of plasma physics application, new material development and fuel cell. The details of the research and results are as follows:(1) Research on the coated NiCr films on stainless steel bipolar platesA series of NiCr films with different chemical composition and nitrided NiCr film are synthesized on stainless steel bipolar plates by AIP. The test results show that the performance of the NiCr films coated bipolar plates is improved. The ICRs of the coated bipolar plates decrease. Nitridation treatment is benefit for reducing the ICR further. At a compaction force of 0.85 MPa, the lowest ICR of the coated bipolar plates is 20.8 mΩcm2. Compared with that of the stainless steel substrate, the ICR of the coated bipolar plates reduces by more than one order of magnitude. However, the corrosion resistance of the coated bipolar plate is worse. In simulated corrosive environment, the coated bipolar plate only shows similar corrosion resistance to the uncoated stainless steel substrate, which means NiCr films couldn’t effectively enhance the corrosion resistance of the bipolar plates. Our analysis indicates that the Ni-rich droplets on the film surface induced by neutral particles produced from the plasma by AIP lead to the worse corrosion resistance. So it can be concluded that removing Ni from the NiCr film would improve the corrosion resistance of the bipolar plate.(2) Research on the multiple-layer Cr/CrN films on stainless steel bipolar platesA double-layer Cr/CrN film and triple-layer Cr/CrN/Cr film with a sandwich structure are prepared on stainless steel bipolar plates by AIP. The test results show that the ICR and corrosion resistance of the coated bipolar plates are both improved. The Cr/CrN/Cr film coated bipolar plate shows the better performance. The ICR reaches 60.3 mΩcm2 at a compaction force of 0.85 MPa, which is about one order of magnitude lower than that of the stainless steel substrate. The corrosion potentials of coated bipolar plates are higher than that of the stainless steel substrate in simulated corrosive environment. In the cathodic environment, the corrosive current density of coated bipolar plate is lower than that of the stainless steel substrate. In the anodic environment, only Cr/CrN/Cr film coated bipolar plate displays lower corrosive current density than the stainless steel substrate. The film characterization results show that depositing a CrN film on the stainless steel substrate directly would result in the film cracking owing to the large internal stress in the film. Adding a Cr film as the intermediate layer between the CrN film and stainless steel substrate would weaken the internal stress and prevent the film from cracking, which is the primary reason for the worse performance of the Cr/CrN film coated bipolar plates. The analysis shows that multiple layer structure would raise the ICR of the coated bipolar plate. So changing multiple layer structure to single layer structure and optimizing the film composition could improve the performance of the coated bipolar plates further.(3) Research on the coated CrNx films on stainless steel bipolar plates.A series of single-layer CrNx films with different chemical composition are formed on stainless steel bipolar plates by AIP. The test results indicate that performance of the coated bipolar plates is greatly improved. The lowest ICR of the coated bipolar plate is 5.8 mΩcm2. The ICRs of the coated bipolar plates reduce by nearly two orders of magnitude comparing with that of the stainless steel substrate. The corrosive current density of the coated bipolar plates in the simulated environment also reduces by nearly two orders of magnitude. The hydrophobic property of the coated bipolar plate is improved. The contact angle increases from 73°for the stainless steel substrate to 95°for the coated bipolar plates. The film characterization results indicate that N content of the CrNx films increases as the nitrogen flow rate increases, and the phase composition accordingly changes from the mixtures of Cr and Cr2N phases, pure Cr2N phase through Cr2N and CrN phases, to pure CrN phase. The analysis indicates that lower ICRs are generally obtained by the single-phase CrNx films with a preferred orientation. The CrNx films with single phase structure also exhibit a better corrosion resistance. The CrNx film coated bipolar plates have shown good enough performance to replace the noble metal bipolar plates.(4) Research on the coated CrCx films on stainless steel bipolar platesA series of single-layer CrCx films with different chemical composition are deposited on stainless steel bipolar plates by AIP. It can be seen from the test results that the performance of the coated bipolar plates is greatly improved, especially the great decrease in ICRs of the coated bipolar plates, which cause the coated bipolar plates have reached the level of noble metal bipolar plates such as Ag-plated bipolar plates. A lowest ICR of 2.8 mΩcm2 is obtained at a compaction force of 1.2 MPa. Comparing with the stainless steel substrate, ICRs of the coated bipolar plates reduce by more than two orders of magnitude. In simulated corrosive environment, the corrosive current density of the coated bipolar plates also reduces by more than two orders of magnitude. The hydrophobic properties of the coated bipolar plates are improved, and the contact angle reaches 105°. The film characterization results indicate that the CrCx films are mainly composed of amorphous carbon matrix mixed with a few Cr crystalline phase. Doping Cr in the carbon-based films would obviously influence the carbon atoms bondings. It’s found that ICRs of the coated bipolar plates have a close relation with the sp3 and sp2 carbon atoms content in the CrCx film. The higher sp2 carbon atoms content, the lower ICR of the coated bipolar plate.(5) Research on the operation performance of PEMFCs with the coated bipolar platesThe CrNx film (Cr0.50N0.50) and CrCx film (Cr0.23C0.77) coated bipolar plates are produced for their excellent performance. The coated stainless steel plates are used as bipolar plates to assemble PEMFCs, and the operation performance of the stack is investigated. The results show that the PEMFCs with CrNx film coated bipolar plates exhibit an initial performance close to the practical application level. Meanwhile, the initial performance of the PEMFCs with CrCx film coated bipolar plates has reached the application level. During the 200-hour testing, the cells work stably and no remarkable degradation in cell performance is detected, which suggests that the CrCx film coated bipolar plates have an opportunity to be used in the further testing. |
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