溶盐电沉积石墨与碳化物涂层研究

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	溶盐电沉积石墨与碳化物涂层研究
	吕旺燕
学位类型	博士
导师	曾潮流
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	腐蚀科学与防护
关键词	熔融碳酸盐电沉积石墨碳化铬碳化钛 Molten Carbonates Electrodeposition Graphite Chromium Carbides Titanium Carbides
摘要	"质子交换膜燃料电池(PEMFC)是一种将氢和氧中的化学能转化成电能的装置。它具有排放低、能量密度高和操作温度低等优点而受到广泛关注。尽管PEMFC发展已取得了很大进展，但材料成本仍是PEMFC实用化面临的一个挑战。双极板是PEMFC的核心多功能部件，占据电池组重量和成本的绝大部分。目前PEMFC双极板材料主要有石墨、金属及相关复合材料。与石墨类材料相比，金属材料在强度、抗气体渗透、规模化生产及加工成薄板以提高电池比功率等方面显示明显优势，但其面临的主要问题是腐蚀问题，这导致电池性能下降。为此，必须提高金属双极板的抗腐蚀性能，并降低接触电阻。本文在含碳酸盐的熔盐体系中研究了碳酸根离子的直接电化学还原，以期在不锈钢表面沉积石墨与碳化物涂层，并采用X-射线衍射、扫描电镜、透射电镜及拉曼光谱等表征了电沉积产物的微观结构与形貌。论文取得的主要结果如下：计算了Li2CO3、Na2CO3和K2CO3在不同温度的标准电化学还原电位。Li2CO3还原成碳的电位明显比Na2CO3 和K2CO3还原成碳及碱金属离子还原成碱金属的电位正。在熔盐中加入Li2CO3有利于碳的沉积。实验结果表明，CO32-离子在不同金属表面的还原析碳电位相近，且随温度升高而正移，其还原过程遵循一步还原机制。在750 0C熔融(Li, K)2CO3中，通过CO32-的直接还原成功地制备了纳米碳纤维。制备的纳米碳纤维分别呈现平直状和螺旋状这二种形式，且具有多壁结构。螺旋状碳纤维分别具有单螺旋和双螺旋生长方式。无论是平直的还是螺旋状的纳米碳纤维都是中空的，且都裹有金属铁颗粒。这些铁颗粒对纳米碳纤维的生长起催化作用。电沉积电流密度越大，得到的纳米碳纤维的管径越大。在沉积电流密度为192 mA/cm2条件下，通过在900 0C熔融NaCl-Na2CO3中的恒流电沉积可以在多弧离子镀铬和钛涂层表面获得致密、粘附性良好的高结晶度石墨层，而在304不锈钢表面沉积的碳层则疏松、粘附性差，石墨化程度低。同时，在石墨层下形成了碳化物。多弧离子镀铬和钛涂层的细小、近球形表面形貌有助于石墨晶的形核和生长，而基体成分的变化对此无明显影响。过大或过小沉积电流不利于产生高结晶度石墨层。提高沉积温度有助于石墨层的生长。以(Na, K)2CO3为碳源，能够在钛表面电合成碳化钛；以(Na, K)2CO3为碳源，以K2TiF6为钛源，成功地在316不锈钢表面沉积了均匀、致密的碳化钛涂层。制备的碳化钛涂层在1 M H2SO4溶液中具有很宽的钝化稳定区，且能显著降低基体与石墨的接触电阻；在900 oC ，NaCl-KCl-NaF共晶熔盐中，以K2TiF6作为钛源，以石墨阳极为碳源，采用脉冲电镀方法成功地在304不锈钢表面合成了均匀、致密的碳化钛涂层。"
其他摘要	" Proton exchange membrane fuel cell (PEMFC) is a power generation device that converts electrochemically the chemical energy of hydrogen and oxygen (or air) into electricity with water as the only byproduct. PEMFC has received wide attention due to its reduced emissions, high power density and low operating temperatures. Although great progresses have been achieved in PEMFCs, materials cost is still a great challenge to their commercialization. As a key multifunctional component of PEMFCs, bipolar plates account for a significant part of total weight and cost of the PEMFC stack. The materials investigated so far for bipolar plates include graphite, metals and composites. Metallic materials with respect to graphite exhibit great competitiveness due to their higher mechanical strength, no gas permeability, much superior manufacturability, and easily being machined to be thin plates to achieve the higher power density. The main challenge of metals, however, is the corrosion in PEMFC environments, which causes considerable power degradation. Therefore, it is necessary to enhance the corrosion resistance of metallic bipolar plates, with acceptable contact resistance. In this thesis, the direct electroreduction of carbonate ions in molten salts containing carbonates has been investigated in an attempt to deposit graphite and carbides coatings on stainless steels, and the structure and morphology of the deposits were examined by X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy. The main achievements are as follows: The standard potentials for the electrochemical reduction of Li2CO3, Na2CO3 and K2CO3 at different temperatures have been calculated. It is shown that the potentials for the reduction of Li2CO3 into carbon are much more positive than those for Na2CO3 and K2CO3, and than those for the reduction of alkali ions into metals. Therefore, the presence of Li2CO3 in molten salts is helpful to the deposition of carbon. It is shown by cathodic polarization and cyclic voltammetry that the potentials for the reduction of CO32- into carbon on various metals are similar, and shift positively with increasing the temperature. The electrochemical reduction of CO32- follows a one-step mechanism. Carbon nanofibers have been deposited by the direct electroreduction of CO32- in molten (Li, K)2CO3 at 750 0C. The obtained carbon nanofibers exhibit straight and helical forms, respectively, with a multi-walled structure. Both straight and helical carbon nanofibres are hollow, and iron particles were fixed inside the fibres. These iron particles have acted as a catalyst for the generation of carbon nanofibers. A higher deposition current density gave rise to the production of thick carbon fibres. A compact, adhesive and well-crystallized graphite film could be obtained on 304 stainless steel coated with multi-arc ion plated Cr and Ti coatings,respectively, by the galvanostatic deposition in molten NaCl-Na2CO3 at the current density of 192 mA/cm2 at 9000C, while the carbonaceous deposits on the bare steel were loose, and exhibited a poor adhesion to the substrate, with a small amount of graphitic crystallites. In addition, carbides were formed beneath the carbonaceous deposits. The small, coarse and spherical appearance of the ion plated Cr and Ti coatings probably helps to the nucleation and growth of graphitic crystallites. However, the growth of crystallized graphite has little relation to the chemical composition of the substrate. A higher or lower deposition current density is unfavorable to the generation of a well-crystallized graphite film. Increasing the deposition temperature helps to the growth of graphite. Titanium carbide coatings have been prepared by the potentiostatic electroreduction of CO32- on titanium in molten CaCl2-NaCl -(Na, K)2CO3 at 900 0C. A compact and uniform titanium carbide coating has been electrodeposited potentiostatically on 316 stainless steel in molten CaCl2-NaCl-(Na, K)2CO3-K2TiF6，among which (Na, K)2CO3 and K2TiF6 act as the carbon and titanium sources, respectively. It was shown by potentiodynamic polarization measurements in 1 M H2SO4 solution at room temperature that the steel covered with titanium carbide coatings exhibited a significantly wider passive region than the bare steel. Meanwhile, the coatings decrease obviously the contact resistance between the substrate and graphite. When K2TiF6 was used as the titanium source, and graphite anode as the carbon source, a dense and uniform titanium carbide coating could also be obtained on 304 stainless steel by pulse-plating technique in molten NaCl-KCl-NaF-K2TiF6 at 900 0C."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64496
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	吕旺燕. 溶盐电沉积石墨与碳化物涂层研究[D]. 北京. 中国科学院金属研究所,2012.