Research On The Composition, Structure And Properties Of TiB₂ Ceramic Particles In The Liquid Of HF And H₂O₂

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.