Preparation And Electrochemical Properties Of Carbon Nanotube Arrays/Metal Oxide Composites

Usage of a large amount of non-renewable fossil fuel results in severe environmental pollution and energy crisis.Therefore,new renewable energy source and technologies for energy storage have become the main concern of materials science.Supercapacitor,a new energy storage device mainly relying on electrical double layer capacitance and redox pseudocapacitance,has attracted widespread attention due to its high power density,excellent cycle life and fast charge/discharge performance.However,relatively low energy density is still the bottleneck limiting the wide application of supercapacitors.In this thesis,to enhance specific capacitance of supercapacitors,vertically-aligned carbon nanotubes（VACNTs）grown by chemical vapor deposition（CVD）was employed as the 3D conducting scaffold,and high-performance nanocomposite electrode was constructed by decorating pseudocapacitive metal oxide on VACNTs upon supercritical CO₂-assisted impregnation and low-temperature oxidation annealing.Moreover,energy density of supercapacitors was further improved by constructing aqueous asymmetric supercapacitors to broaden working voltage of the devices.Firstly,Co₃O₄/VACNTs composite electrode was designed and prepared.Influence of annealing temperature on morphology,structure and electrochemical performance of the composite was investigated.And Co₃O₄ loading amount was optimized based on electrochemical measurement.Then,Fe₃O₄/VACNTs composite was prepared by the same strategy.Afterwards,an aqueous asymmetric supercapacitor was built by using Fe₃O₄/VACNTs as the negative electrode and Co₃O₄/VACNTs as the positive electrode.It is found that,the Co₃O₄/VACNTs electrode synthesized at 260°C exhibits the best electrochemical performance.The specific capacitance of 833 F/g is achieved at the current density 1 A/g.As the current density increases to 20 A/g,the 89%of the initial capacitance is still remained,suggesting the remarkable rate capacity.The Co₃O₄/VACNTs//Fe₃O₄/VACNT asymmetric supercapacitor can work stably within the potential range of 0-1.8 V and the highest energy density of 36 Wh/kg is reached.After5000 charge/discharge cycles,the capacitance retention of 87%is reached.Then,spinel-type NiFe₂O₄ was anchored on VACNTs by loading two kinds of metal precursors simultaneously and the NiFe₂O₄/VACNTs composite electrode was found to work stably in both the positive and negative potential windows.Therefore,a symmetric supercapacitor was assembled with the bipolar NiFe₂O₄/VACNTs as both the positive and negative electrode in 2 M KOH aqueous electrolyte,and the working potential range was successfully extended to 1.8 V.At the current density of 1 A/g,the supercapacitor exhibit a specific capacitance of 83 F/g.When the power density is 0.89 kW/kg,the full cell reach the maximum energy density of 36.5 Wh/kg.Even after 8000 cycles at 5 A/g,89%of the initial specific capacitance is still maintained.The excellent electrochemical performance of this symmetric supercapacitor is comparable to that of the asymmetric supercapacitor.