【作者】 黄飞；

【导师】 傅正义； 王为民；

【作者基本信息】 武汉理工大学 ， 材料加工工程， 2009， 博士

【摘要】 作为PEMFC重要组成部分,高性能与低成本的催化剂体系是决定PEMFC实际应用的重要指标之一。目前改善催化剂的方法主要包括：合金Pt催化剂、非Pt催化剂、高效催化剂载体以及优化的催化剂结构。其中,催化剂载体能很好地分散催化剂Pt,减少Pt催化剂的用量,提高催化剂的催化活性。传统的催化剂载体主要是多孔碳黑,这种载体在PEMFC环境中长期运行存在腐蚀问题,从而导致催化剂Pt的流失、迁移和团聚。本课题选取TiB₂导电陶瓷作为载体,从载体的结构和化学性质等方面着手,研究其作为PEMFC催化剂载体的可行性。主要研究内容包括以下三个方面：1)采用行星球磨对TiB₂粉体进行超细化。通过对大粒径的TiB₂粉末进行球磨,研究球磨时间、球磨速度、球料比、球磨气氛以及后续的酸洗和分级工艺的影响,尽量降低TiB₂载体的粒径大小,并对球磨过程中出现的一系列物理化学现象进行解释,尤其是超细化过程中的氧化问题和显微结构演变的问题。通过合理控制一些关键工艺参数,进一步降低TiB₂的氧化；同时采用HRTEM, SEAD等表征手段对球磨中间态进行表征,建立理论同结构演变之间的关系。结果表明TiB₂在高应力作用下择优沿着[0001]和[10-10]方向发生塑形滑移并最终发生裂解。2)特殊形貌结构TiB₂基材料的合成。目前商业化的TiB₂均采用高温固相反应制得,由于其各向异性,容易沿着C轴择优生长形成光滑的六方柱状晶体结构,这种结构不利于Pt的负载和固定,在一定温度场、流场和气体压力下长期运行容易剥落,使Pt发生团聚和迁移,造成成本的浪费和催化效率降低。本文采用球磨工艺及等级分离方法,成功合成出花状双头TiB₂纳米晶须。同时以乙二胺为添加剂,采用HF或过氧化氢溶剂热的方法,成功合成出中空双金字塔结构、凤梨结构、海胆状核壳结构和纳米墙状核壳结构TiB2/Ti02杂化材料。HRTEM、XRD、EDS、XPS结果显示退火后海胆状核壳结构TiB₂/TiO₂杂化材料是一种典型的氮掺杂TiO₂/TiB₂异质结材料。UV-vis光谱表明这种氮掺杂异质结的吸收峰的吸收边发生了明显的蓝移。3)PEMFC化学环境的模拟。通过对氢氟酸环境和过氧化氢环境的模拟,考察TiB₂材料在氢氟酸环境和过氧化氢环境中的运行情况。通过对水热温度、水热时间、浓度、添加剂等参数的控制,详细研究TiB₂材料在这种环境中的形貌、结构和组成的变化,从而进一步验证TiB₂材料在这种环境的稳定性,分析TiB2基材料作为PEMFC催化剂载体的可行性。结果表明TiB₂材料在HF和H₂O₂环境中发生氧化,通过合适工艺的控制,可以得到一定形貌的杂化材料。电化学测试结果表明这种具有特殊结构的杂化材料具有同TiB₂原粉相当的电化学稳定性。更多还原

【Abstract】 As an important component of PEMFC, catalytic system with low cost and excellent catalytic properties is one of decisive targets for application. Currently, the methods of improving the catalytic properties and decreasing the cost of catalyst system include as follows:Pt alloys, non-Pt catalysts, catalyst carrier with high efficiency and the optimum catalytic structure. Traditionally, the Pt catalyst is homogeneously dispersed on porous carbon, which can obviously increase the active areas of Pt catalyst, enhance the catalytic efficiency and reduce the cost. However, the carbon materials would be corroded under such rigorous environment of PEMFC for a long time. Consequently, it would result in loss, transference and conglomeration of Pt catalysts. In this work, TiB2 electric ceramic was used as catalyst carrier and the feasibility was analyzed from the viewpoint of structure evolution and chemical properties of TiB2 materials. The thesis mainly includes three parts as follows:（1） Preparation of ultrafine TiB₂ powders by a planetary ball-milling method. The ultrafine TiB₂ powders could be gained via controlling the milling time, rotation speed, mass ratio of ball to powder, atmosphere, as well as acid-cleaning and hierachical centrifugation. Some physical and chemical phenomena during the milling were analyzed, especially the oxidation of TiB₂ powders and microstructural evolution. The optimum processes for ultrafine TiB2 powders were also discussed. The results show that the twins and cleavage of TiB2-based materials are prone to form along the （0001）/[10-10] slip systems under high stresses.（2） Synthesis of TiB₂-based materials with special morphologies. Currently, commercial TiB2 powders are mostly prepared by high-temperature solid-state reaction. Due to its anisotropy, TiB₂ powders preferentially form slippy two-dimensional hexagonal structure under such high temperature and long holding time, which is deleterious to load Pt nanoparticles. Under a certain temperature field, flow field and stress field in PEMFC, Pt nanoparticles are prone to strip and transfer from the TiB₂ carrier. Consequently, it will increase the cost and decrease the catalytic efficiency. In this work, flowerlike nanowhiskers were successfully prepared by planetary ball-milling and hierachical centrifugation. Moreover, TiO₂/TiB₂ hybrid materials with hollow bipyramidal structure, pineapple structure, urchin structure and nanowall structure have also successfully synthesized by solvothermal method in the liquid of HF and H₂O₂. The results of HRTEM, XRD, EDS and XPS show that urchin TiO₂/TiB₂ hybrid material is actually a kind of typical N-doped TiO₂/TiB₂ heterostructure after annealing. UV-vis spectra indicate that such heterostructure results in obvious blue-shif because of nitrogen dopant.3) Chemical simulation in the environment of PEMFC. The detailed process of TiB₂ in the environment of HF and H₂O₂ was analogized according to the characteristics of PEMFC. The changes of morphology, structure, and composition were investigated via adjusting the hydrothermal temperature, hydrothermal time, concentration, additive, and so on. The results show that TiB₂ will be oxidized in the environment of HF and H₂O₂. However, the oxidization can be minimized through appropriate process. The electrochemical test shows that such hybrid materials with special morphology have equivalent electrochemical stability with that of TiB₂ raw powders.更多还原