Study On Nanostructure Modulation And Energy Storage Properties Of Potassium Titanate

The storage and conversion of new energy is the key to cope with the imminent energy exhaustion and environmental pollution.Supercapacitor is a new energy storage device with ultra-long cycle life,high power density,fast charge/discharge rate,and environmental friendliness and high safety.However,the low energy density limits its widespread commercial applications.This disadvantage can be overcome by widening the working voltage window or increasing the electrode specific capacitance.To achieve a high voltage window,an organic electrolyte can be used,but it is costly and has poor security.In contrast,aqueous electrolytes have higher ionic conductivity,lower cost and better safety,thus they have more practical value.There are two strategies to increase the energy density of aqueous supercapacitors:(1)Selecting electrode materials with high hydrogen evolution(HER)/oxygen evolution(OER)overpotential to suppress water decomposition to widen the operating voltage;(2)Designing asymmetric supercapacitors(ASCs)by combining the high-capacitance positive/negative materials,so as to widen the operating voltage window and improve energy density of the devices.Ti-based compound materials are non-toxic,low cost,high natural abundance,and have high chemical/electrochemical stability and high hydrogen evolution reaction overpotential,Which could play a key role in in the negative electrode of high-voltage supercapacitors.However,its inherent low electron/ion conductivity still limits its practical application.In this thesis,a self-supporting ultra-thin potassium titanate(KTO)nanobelts array was first obtained on Ti foil by chemical oxidation combined with a simple hydrothermal method,and then modified by using Ar-H₂plasma etching.It was used as the negative electrode of an aqueous asymmetric supercapacitor for the first time.The effects of Ar-H₂plasma etching on the morphology,structure and electrochemical properties of potassium titanate were analyzed by means of SEM,TEM,XRD,XPS,Raman,EPR,UV-vis and UPS combined with DFT calculation.The results show that the layer spacing of potassium titanate increases after treatment,which accelerates the charge transfer and the introduction of oxygen vacancies reduces the band gap so as to improves the conductivity,and the structure rearrangement caused by oxygen vacancies forms TiO₅polyhedron with stronger Ti-O bond force and the introduction of Ti³⁺provides more pseudo-capacitance and inhibits the hydrogen evolution reaction,thus greatly improving the specific capacity and cycle stability.As a result,the capacity increased from 92.8 F g^-1to 341 F g^-1and the Coulombic efficiency increased from 58%to 89%at the current density of 1 A g^-1,and the capacity retention rate increased from 68.4%to 90.1%at a high current density of 10 A g^-1after 10,000 cycles.In addition,a tunnel phase manganese dioxide(K_xMn O₂)nanorod array positive electrode material was prepared by hydrothermal method.It can achieve a wide working potential window of 0-1.2 V,and demonstrated high specific capacitance of 262.6 F g^-1at 1A g^-1,good rate performance(the specific capacitance at 10 A g^-1is 159.8 F g^-1),and excellent cycle stability(83.2%of the initial specific capacitance can be retained after 10000 cycles).The 2.6 V KTO_x//K_xMn O₂aqueous ASCs was assembled and showed high energy density of72.1 Wh kg^-1when the power density is 1300 W kg^-1,which is higher than the previously reported Ti-based supercapacitors,which opens up a new opportunity to promote high-performance Aqueous ASCs.