Study On Preparation Of Titanium-based Compound Nanomaterials And Their Electrochemical Properties

New materials for electrochemical energy device application have attracted rapidly increasing attention due to its ever increasing significance in human’s life. With respect to the well-established materials for electrochemical energy devices there exist obvious limitations at present. Therefore, it is of great importance to develop novel materials with superior properties to replace the traditional ones for electrochemical energy devices. Due to the abundance and excellent chemical stability, titanium（Ti）- based compound materials are highly promising for electrochemical energy applications such as lithium ion batteries, supercapacitors, and fuel cells. However, up to now the research on Ti- based compound materials is not enough yet. In order to promote the application of Ti-based compound materials, it is necessary to investigate their preparation, structure, and properties from the viewpoint of electrochemical energy application. In this dissertation, novel Ti- based compound nanomaterials including Ti O_2-x（x≥0） nanoparticles, Ti N nanowire spheres, and Ti C /C composite nanofibers were prepared and their electrochemical performances were investigated. The main research work and results are as follows:Non- stoichiometric Ti O_2-x nanoparticles were synthesized by a facile solvothermal method using zinc powders as the reducing reagents. The amount of Ti3+ can be easily controlled by changing the Zn:Ti molar ratio while the morphology, structure, and specific surface area of the Ti O 2-x nanoparticles keep stable during the reducing and synthesizing process. The concentrations of Ti3+ for the samples prepared at the Zn:Ti molar ratios of 0%, 2%, 4%, and 6% were measured to be 1.3%, 4.0%, 4.0%, and 7.2%, respectively. The adding of Ti3+ could increase the conductivity of the Ti O_2-x nanoparticles, leading to improvement of the electrochemical p erformance. The main research focuses on the electrochemical performance for lithium- ion battery application. It is found that both the platform specific capacity and the pseudocapacitance on the material interface were increased by the adding of Ti3 +. It is also found that the adding of Ti3+ facilitates the intercalation process. The electrochemical performance of the Ti O_2-x nanoparticles, such as reversible capacity, rate performance, and stability, is dependent on the Zn:Ti molar ratio. The optimal one is obtained at the Zn:Ti molar ratio of 4%, which is much better than the pure Ti O2. The reversible capacity of 202.1 m Ah/g at the current density of 100 m A/g and 79.1 m Ah/g at 3000 m A/g were achieved for the optimal sample, showing good rate performance. Meanwhile, this sample shows high capacity retention of 96% after 50 cycles at the current densities of 1000 m A/g.Flower- like Ti N nanowire spheres were prepared by hydrothermal process and subsequent nitridation treatment in N H3. Ethylene glycol（EG） was used to control the morphology of the samples and the well-structured nanowire spheres were prepared at the EG:H2O volume ratio of 3:1. The diameter of the flower- like Ti N spheres is about 3 μ m in average and the diameter of the constitut ing nanowires is 15-20 nm. The effects of nitridation temperature on the morphology, crystallinity, and surface valence were systematically studied, which indicates that the nanowire spheres nitrided at 900 oC possess excellent morphology, high phase purity, and good crystallinity. The ORR catalytic activity of the Ti N nanowire spheres nitrided at different temperatures were tested in 0.1 mol/L KO H solut ion, showing that the samples nitrided at 900 oC possess excellent electrocatalyt ic performance due to the excellent nanowire structure and high nitridation degree. The onset potential, peak potential, and peak current density for the samples nitrided at 900 oC are-0.129 V,-0.274 V, and 0.64 m A/cm2, respective ly. For the optimal sample the O RR proceeds following a nearly four electron transfer route with high stability.Continuous Ti C /C composite nanofibers（TCCNFs） were prepared by electrospinning and subsequent calcining in protective atmosphere. By changing the atmosphere of stabilization process the TCCNFs with different Ti C contents were successfully synthesized with uniform morphology and average diameter about 100 nm. The diameter of Ti C nanoparticles is 15-20 nm. The TCCNFs stabilized in Ar contain much more Ti C than those stabilized in air. By comparing the O RR catalyt ic activit ies of the different samples, it is found that the TCCNFs stabilized in air possess the best electrocatalyt ic activity and good catalytic stability. The onset potential, peak potential, and peak current density for the sample are-0.07 V,-0.18 V, and 0.712 m A/cm2, respectively. The specific capacitance s of the TCCNFs stabilized in Ar and air were measured to be 77.8 F /g and 130.0 F/g at the current density of 0.1 A/g, respectively, which are much higher than pure Ti C nanoparticles and the reported carbon nanofibers, leading to a synergistic enhancing effect between Ti C and carbon. Reversible valance change of Ti atoms was observed during charge/discharge process, indicative of the occurrence of pseudoreaction. The capacitance retention reaches 98.9% and 93.0% for the TCCNFs stabilized in Ar and air after 25 000 cycles, respectively, showing excellent stability.