Modified Design Of Titanium-based Nanotube Arrays For The Application In Supercapacitors

Supercapacitors are one type excellent electrochemical energy storage devices,owing to their high energy density,superior power density,lower cost,outstanding cycling stability and safety in comparison with traditional capacitors,batteries and fuel cells.Ti O₂nanotube arrays（NTAs）are a potential remarkable electrochemical energy storage material due to the characteristics of cheap,non-toxic,stable chemical properties and high specific surface area.However,Ti O₂is an n-type semiconductor material with a wide band gap（³.2 e V for anatase and³.0 e V for rutile）,which led to low electrochemical activity and poor charge transfer efficiency(～1×10^-12S cm^-1to 1×10^-7S cm^-1).It seriously affects the improvement of electrochemical performance,and further limits its application in the field of supercapacitors.Therefore,it is urgent to develop a simple and effective modification strategy to optimize its electronic structure,thereafter improving the intrinsic electrochemical activity and conductivity of Ti O₂,and then expand the application in the field of supercapacitors.Hence,the thesis is committed to optimizing the electronic structure of Ti-based nanomaterials through metal doping,nonmetal doping and defect engineering construction,thus improving the conductivity and electrochemical activity,thereby promoting the intrinsic electrochemical performance of Ti-based electrode materials.The main research contents and results are as follows:（1）In this work,we successfully prepared black Ti-Nb-O nanotube arrays with co-doping of Nb and Ti³⁺/V_oby in-situ anodization of Ti-Nb alloy and metal blackening.Metal blackening is one type typical non-contact reduction.We systematically investigated the effects of bulk-phase Nb doping and metal blackening on the morphology,structure,phase composition,element composition,element valence and electrochemical performance of electrode materials by combining with microstructure characterization and electrochemical performance test.Meanwhile,we optimized the blackening temperature and blackening time to prepare the best performance of electrode materials.It could be found that the bulk-phase Nb doping synergetic metal blackening significantly improved the conductivity and electrochemical activity,and then increased the areal capacitance by 76.28 times.Black Ti-Nb-O exhibited the areal capacitance of 15.90 m F cm^-2(0.1 m A cm^-2).Besides,it owned the outstanding rate capability(81.70%capacitance retention when the scan rate increased from 10 m V s^-1to 100 m V s^-1)and remarkable cycling stability（75.86%capacitance retention after 1000 cycles）.（2）In this work,we successfully prepared R-Ti-Nb-O nanotube arrays with co-doping ofNb and Ti³⁺/V_oby in-situ anodization of Ti-Nb alloy and cathodic reduction,which owned the features of simple operation,low cost and high efficiency.We systematically studied the effects of Nb-doped synergistic with cathodic reduction on the microstructure and electrochemical properties of electrode materials.Besides,we optimized crystallization temperature,reduction voltage and reduction time.DFT calculation further revealed the enhanced mechanism of Nb doping and cathodic reduction.It could be found that Nb doping and Ti³⁺/V_oco-doping indeedly improve the conductivity,electrochemical activity,carrier density and hydrophilicity of the electrode material with the narrowed band gap,the upshift of the Fermi level and low-energy VB bottom,thus enhanced the electrochemical performance of the electrode material.Meanwhile,the energy storage mechanism of electrode materials was also revealed.As a result,R-Ti-Nb-O displayed the areal capacitance of 19.56 m F cm^-2at the current density of 0.1 m A cm^-2,which exhibited the impressive rate capability(81.70%capacitance retention when the scan rate increased from 10 m V s^-1to 100 m V s^-1)and outstanding cycle stability（76.76%capacitance retention after 1000 cycles）.This work would provide a feasible modification strategies and theoretical basis for improving the electrochemical performance of Ti-based nanotube arrays.（3）In this work,based on functional element doping synergetic defect engineering design,we successfully prepared black Ti-Nb-P-O nanotube arrays with bulk-phase Nb/P co-doping synergetic V_oself-doping by in-situ anodization of Ti-Nb alloy and one-step phosphorization.This project skillfully utilized the generated strong reducing atmosphere during the phosphorization process to simultaneously realize bulk-phase P-doped and V_oself-doping.We systematically studied the effects of Nb/P co-doping and V_oself-doping on the microstructure and electrochemical properties,and the effects of phosphorization temperature and phosphorization time on the electrochemical properties were also explored to prepare the best performance of electrode materials.It could be observed that Nb/P co-doping and V_oself-doping could reduce the grain size and inhibit the transformation of anatase to rutile phase,which was beneficial for the improvement of carrier density and conductivity.Meanwhile,Ti³⁺/V_oand Nb⁴⁺/V_ocould be introduced during the phosphorization process,which was beneficial to improve the conductivity of the electrode material.DFT calculation further revealed the enhanced mechanism of the electrochemical properties by Nb doping and one-step phosphorization.It could be found that bulk-phase Nb/P co-doping and the V_oself-doping could promote the surface reaction kinetics,hydrophilicity,conductivity and electrochemical activity of the electrode materials with the reduced work function,the narrowed band gap,the upshift of the Fermi level,broadened and produced the new valence band level bottom.At the same time,the larger electronegativities of Nb and P were beneficial to enhance the structural stability of the electrode materials.Black Ti-Nb-P-O depicted the areal capacitance of 36.60 m F cm^-2at the current density of 0.1 m A cm^-2,which presented high rate performance(80.75%capacitance retention when the scan rate increased from 10m V s^-1to 100 m V s^-1)and impressive cycle stability（84.00%areal capacitance retention after5000 cycles）.Besides,an aqueous symmetrical supercapacitor was assembled,which owned a wide voltage window of 2 V and exhibited the high energy density(3.51 m Wh cm^-3)at the high power density(1869.16 m W cm^-3).This work may pave a promising pathway to develop a new-type and high-performance Ti-based oxide nanostructures for enhanced electrochemical energy storage.