Design And Preparation Of Vanadium Nitride Based Electrode Materials And Their Electrochemical Properties

China has an abundant vanadium-bearing resources.However,vanadium pentoxide,vanadium nitride and ferrovanadium are the major vanadium products.In addition,almost 85 percent of vanadium products are used in metallurgy industry.To improve the international competitiveness of China’s vanadium products,it is urgent to develop high value added vanadium-containing products and expand their application field.Supercapacitor is a new type of energy storage and conversion device which can fill the gap between secondary batteries and conventional capacitors.Therefore,supercapacitors have a broad application prospect in the energy storage field.Generally,electrochemical properties of supercapacitors are determined by electrode materials.Recently,vanadium nitride（VN）is considered to be an ideal material for supercapacitor because of its high theoretical specific capacity,excellent electronic conductivity and high vibratory density.However,VN displayed a limited rate capability and a low specific capacitance in practical application as electrode material for supercapacitor.In addition,VN is electrochemically unstable in aqueous electrolyte,which is another problem need to be solved.In this dissertation,the electrochemical properties of VN material was improved by design of the structure of VN material and optimization of the composition of electrolyte.The main innovative progress is summarized as follows:（1）To improve the rate capability of VN,hierarchically nanostructured VN microspheres（HNVNMs）were prepared by a template-free route.The influence of nitridation temperature on the morphology of HNVNMs was systemically investigated.Besides,the formation mechanism of HNVNMs was revealed.It was found that the nitridation temperature has a dramatic effect on the structure and morphology of HNVNMs.When the nitridation temperature was fixed at 650℃,the VN microspheres were assembled with porous nanosheets.The diameter of HNVNMs was in the range of 1～3 μm,and the thickness of the porous nanosheets was typically approximately 30 nm.The electrochemical measurements showed that the HNVNMs exhibited excellent rate capability due to their hierarchical structure,which is propitious to the diffusion of electrolyte.As the scan rate was increased from 2 mV·s-1 to 200 mV·s-1,the capacitance retention was about 59%.Nevertheless,the specific capacitance of HNVNMs was only 120 F·g-1（2 mV·s-1）ascribed to its low surface area and large crystallite size.（2）For the sake of increasing the capacitance of VN,well dispersed VN nanoparticles with small crystallite size and high specific surface area were successfully synthesized by nitridation of Zn₃（OH）₂（V₂O₇）·（H₂O）₂ and followed by alkaline leaching.The effect of NH3 flow,nitridation time and temperature on the crystallite size and specific surface area of VN nanoparticles were examined.In addition,A model for the formation of well dispersed VN nanoparticles was proposed.ZnO obtained by nitridation of Zn₃（OH）2（V207）·（H2O）2 could act as clapboard to prevent VN nanoparticles from growing up and agglomerating.The results demonstrated that decreasing NH3 flow,nitridation time and temperature would increase the content of ZnO in the intermediate products and decrease the crystallite size of VN nanoparticles.Electrochemical measurements displayed that reducing the crystallite size of VN nanoparticles can significantly improve their specific capacitance.VN nanoparticles with the crystallite size of 13 nm exhibited a specific capacitance of 181 F·g-1 at a scan rate of 2 mV·s-1.However,the rate capability of VN nanoparticles was poor.（3）In order to improve the electrochemical properties of VN,such as capacitance,rate capability and cyclic stability,an in situ self-sacrificed template method was developed to prepare vanadium nitride/nitrogen-doped graphene（VN/NGr）nanocomposites by the pyrolysis of a mixture of dicyandiamide,glucose and NH4VO3.The influence of the mass ratio of glucose/NH4VO3 on the composition,structure and morphology were investigated.The characterization results showed that the VN/NGr nanocomposites exhibited an open two-dimensional structure and possessed a high specific surface area and pore volume.Furthermore,VN nanoparticles of the size in the range of 2 to 7 nm were uniformly embedded into the NGr skeleton.In addition,the specific surface area and pore volume of VN/NGr nanocomposites drastically decreased with the de crease of the mass ratio of glucose/NH4VO3,since the two-dimensional structure was destroyed.When the mass ration of glucose/NH4VO3 was 4:2,the VN/NGr nanocomposite showed a high specific capacitance of 255 F·g-1 at a scan rate of 10 mV·s-1,and an excellent cycling stability（94%capacitance retention after 2000 cycles）.Furthermore,as the current density was increased from 1 to 20 A·g-1,about 67%of the specific capacitance was retained.（4）The electrochemical performances of VN can be further enhanced by optimizing the composition of electrolyte.Cyclic voltammetry and electrochemical impedance spectroscopy were applied to investigate the effect of the concentration of KOH solution（CKOH）on the capacitive performance of nanostructured VN.The results revealed that the required overpotential decreased with the increase of CKOH.Additionally,a marked decrease in charge transfer resistance and electrolyte series resistance as well as an obvious increase in double layer capacitance were observed as CKOH increased.Thus,the specific capacitance and the rate capability of VN electrode were improved with the increase of CKOH.Moreover,the highest specific capacitance of 75 F·g-1 was obtained in 6M KOH solution.Dynamics analysis showed that the kinetic process of VN electrode was majorly surface-controlled in diluted KOH solutions at low scan rate,but a mixed process including surface process and diffusion-controlled process existed at high scan rate.However,the kinetic process of VN electrode at high scan rate in concentrated KOH solutions was majorly surface-controlled.Importantly,the concentration of the dissolved oxygen in KOH solution seriously influenced the coulombic efficiency of VN but had no effect on the stability of VN electrode.Decrease of the concentration of the dissolved oxygen in KOH solution could increase the coulombic efficiency of VN electrode.