【作者】 吴博；

【导师】 林国强；

【作者基本信息】 大连理工大学 ， 等离子体物理， 2011， 博士

【摘要】 随着经济与社会迅速发展,能源问题逐渐为世界各国所重视。传统的化石能源存在不可再生及严重污染环境等问题,发展无污染可再生能源成为彻底解决能源问题的唯一出路。燃料电池(fuel cell, FC)作为可再生清洁能源的一种重要形式,具有能量转化效率高、无污染等优点。其中,质子交换膜燃料电池(proton exchange membrane fuel cell, PEMFC)在交通、便携式能源以及军事等领域有着广泛的应用前景,特别是有望成为未来主要的车载动力源。双极板(bipolar plates, BPPs)是质子交换膜燃料电池的重要组成部分,起到收集传导电流、分隔氧化剂和还原剂以及支撑电池等作用。双极板占整个电池重量和成本的很大比重,降低双极板成本是降低燃料电池价格的最关键因素之一。但是,传统的人造石墨双极板体积庞大、阻气性差、撞击安全性差、相对成本也较高。金、银等贵金属双极板虽然各方面性能优良,但其价格却极其昂贵,严重阻碍了PEMFC的商业化发展进程。如果能以不锈钢薄板来替代石墨或贵金属板作为双极板,则无疑会大大降低燃料电池的生产成本,但是不锈钢虽然具有很好的导电、导热性,高的机械强度以及易加工等优点,可它在PEMFC的复杂环境中化学稳定性很差,同时板-膜间的接触电阻也很大,致使电池在运行过程中双极板过快腐蚀和功率衰减严重,因此不锈钢薄板作为双极板来直接使用还存在着致命的性能缺陷问题。在不锈钢表面用气相沉积技术制备改性薄膜是解决问题的可行手段,如果沉积的薄膜在电池的氧化、还原环境中具有较强的耐蚀性能,同时又具有很高的导电性,那么与不锈钢基体的其他优良性能相结合,则会达到替代贵金属和石墨双极板的目的,这将大大促进质子交换膜燃料电池的产业化发展进程。本文即针对质子交换膜燃料电池用316L不锈钢双极板进行表面镀膜改性研究,镀膜采用等离子体基PVD技术中的典型工艺——电弧离子镀技术,在普通商用316L不锈钢表面沉积一系列不同成分和组织结构的改性薄膜,目的是开发、寻找既导电又耐蚀,同时又具有一定强化和疏水性能的改性薄膜,使之与不锈钢基体一起构成高性能的双极板,以替代石墨和贵金属双极板,这将在等离子体物理应用技术、新材料开发和燃料电池领域,具有重要的理论和现实意义。具体研究内容及主要研究结果如下：(1)不锈钢双极板表面沉积NiCr金属薄膜研究在不锈钢双极板表面用电弧离子镀技术沉积制备了不同成分的NiCr薄膜以及氮化处理的NiCr薄膜。测试结果表明,镀NiCr薄膜的不锈钢双极板的表面性能有一定改善,其接触电阻较小,薄膜经氮化后更有利于降低接触电阻。在0.85MPa压紧力下,镀膜双极板的接触电阻最小值为20.8 mQ cm2,和原始不锈钢基体相比,降低幅度超过1个数量级。但是在模拟腐蚀环境中,镀膜双极板的耐蚀性能较差,其耐腐蚀性能与不锈钢基体基本相当,说明镀NiCr薄膜无法有效提高双极板的耐蚀性能。分析表明,由于电弧蒸发等离子体中存在固有的中性粒子团簇致使薄膜表面形成大量富Ni的“大颗粒”,是导致镀膜双极板耐蚀性能较差的主要原因。因此在NiCr薄膜材料中摒弃Ni元素有望提高双极板的耐蚀性能。(2)不锈钢双极板表面沉积Cr/CrN多层薄膜研究在不锈钢双极板表面用电弧离子镀技术制备了Cr/CrN双层和Cr/CrN/Cr三明治结构两种Cr-N薄膜。测试结果表明,镀膜双极板的接触电阻和耐腐蚀性能均有所改善,其中镀Cr/CrN/Cr三明治结构的薄膜双极板性能更佳。在0.85MPa压紧力下,镀膜双极板的接触电阻最小值为60.3 mΩcm2,与不锈钢基体相比,降低幅度约1个数量级。在模拟腐蚀环境中,镀膜双极板的腐蚀电位高于基体腐蚀电位。在阴极环境下,镀膜双极板的腐蚀电流密度小于基体腐蚀电流密度；在阳极环境下,仅镀三明治结构薄膜双极板的腐蚀电流密度小于基体腐蚀电流密度。薄膜表征结果表明,直接将CrN薄膜沉积到不锈钢表面时,薄膜会因膜基应力过大而发生破裂；在两者之间增加Cr膜作为过渡层,可以减缓应力而避免薄膜破裂。其中薄膜自身发生破裂是导致镀Cr/CrN薄膜双极板性能较差的主要原因。分析表明,薄膜的多层结构可能导致接触电阻增大,因此改Cr-N薄膜为单层结构并进一步优化薄膜中的成分组合有望进一步提高双极板的性能。(3)不锈钢双极板表面沉积CrNx薄膜研究在不锈钢双极板表面用电弧离子镀技术制备了不同成分的单层CrNx薄膜。测试结果表明,镀膜处理后的双极板性能显著提高。在1.2MPa压紧力下,镀膜双极板的接触电阻最小值为5.8 mΩcm2,和不锈钢基体相比,降低幅度为2个数量级。在模拟腐蚀环境中,与不锈钢基体相比,镀膜双极板的腐蚀电流密度降低幅度也近2个数量级。同时改性双极板的疏水性能也有很多改善,水接触角从不锈钢的73°增大到95°。薄膜表征结果表明,随着镀膜过程中N2流量的增加,薄膜的氮含量也随之增加,薄膜的相组成也从多相Cr+Cr2N变为单相Cr2N,再变为多相Cr2N+CrN,最后变为单相CrN。分析表明,单相组成且有明显择优取向的薄膜接触电阻更小,单相薄膜亦表现出较好的耐蚀性能。该镀膜双极板初步具备替代贵金属双极板的综合性能,但其导电性尚未完全达到贵金属的水平。(4)不锈钢双极板表面沉积CrCx薄膜改性研究在不锈钢双极板表面用电弧离子镀技术制备了一系列不同成分的单层CrCx薄膜。测试结果表明,镀膜改性后的不锈钢双极板性能大幅度提升,特别是双极板的接触电阻大幅度降低,已完全达到贵金属双极板(镀银双极板)的水平。在1.2MPa压紧力作用下,改性双极板的接触电阻最小值达到2.8 mQ cm2,和不锈钢基体相比,降低幅度超过2个数量级。在模拟腐蚀环境中,和不锈钢基体相比,镀膜双极板的腐蚀电流密度降低幅度超过2个数量级。在模拟电池运行环境条件下,改性双极板亦表现出明显优于不锈钢基体的接触电阻和耐腐蚀性能。同时镀膜双极板的疏水性能进一步提高,水接触角提高到105°。薄膜表征结果显示,薄膜主要由非晶碳组成,并包含少量金属Cr晶体相。向薄膜中掺入的Cr元素,可以显著影响碳原子之间的成键方式。分析表明,薄膜中sp2键和sp3键碳原子含量与薄膜接触电阻密切相关,sp2键的比例越高,镀膜双极板的接触电阻越低。(5)镀膜双极板装堆电池运行性能研究鉴于镀CrNx薄膜(Cro50No50)和镀CrCx薄膜(Cro23Co77)双极板的性能突出,分别进行该两种镀膜双极板的小批量处理并组装电池进行运行,考察电池的实际运行性能。测试结果表明,镀CrNx薄膜双极板电池初始性能接近实际应用水平,镀CrCx薄膜双极板电池初始性能达到实际应用水平,并且在200小时强化运行过程中,电池运行稳定且性能未发生明显衰减,已具备批量处理装堆进行中等规模运行试验的条件。更多还原

【Abstract】 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.更多还原