Nanostructured Oxides Based Supercapacitor:From Electrodes Design To Devices Construction

Supercapacitors are promising energy-storage device with significant superiorities such as high power density,fast charge/discharge rate and long cycling life,which have numerous applications in electric vehicles,modern communications,aerospace and defense and many other fields.However,low energy density limits its wide application.Thus,tremendous research effort has been performed aiming at increasing the energy density of supercapacitors without sacrificing their high-power capability.Based on energy density equation,it is an effective approach to improve energy density through increasing the capacitance and broading the voltage window.This thesis is focused on high-power and high-energy asymmetric supercapacitors.Firstly,we investigate extensively on controllable preparation,structure optimization,energy band design and charge storage mechanism of metal oxide based electrodes materials to enhance their electrochemical performance and capacitance.On this basis,aqueous asymmetric supercapacitors are designed and constructed to broaden the voltage of devices.The details are summatized briefly as follows:1.Optimization of electrical transport properties of cathode materials by doping modification.The continuous flow injection?CFI?equipment was designed and constructed.By accurately controlling the growth liquid concentration,solution acidification induced by hydrolysis of Al³⁺ was suppressed.The ZnO nanowire arrays with high carrier concentration(1.14×10¹⁹ cm^-3)were synthesized.Compared with the common hydrothermal?CH?mothed(4.2×10¹⁶ cm^-3),three orders of magnitude rise of the carrier concentration was achieved.When applying high carrier density Al-doping ZnO NWs as conductive support for NiO-based binder-free supercapacitor electrodes.The capacitance of the electrodes was effectively improved.2.Interface electrical transport performance modulation of cathode materials by band design.ZnO/NiO core-shell structure nanorods array were synthesized on three-dimension carbon clothes as the flexible binder-free electrodes via low temperature hydrothermal method.Au Nanopaticles were further embedded at ZnO/NiO interface through UV reduction approach to study the effect of interfacial electrical transport on the capacitance of electrode material.It resulted in a capacitance of 4.1 Fcm^-2 in ZnO/Au/NiO electrodes at a current density of 5 mA cm^-2,it is an enhancement of 720%compared with ZnO/NiO electrodes(0.5 Fcm^-2).Such an enhanced property is mainly attributed to the charge fast transfer at the electro-electrolyte interfaces provided by Au nanoparticles.Apart from this,electrons can be temporarily trapped and accumulated at the Fermi level because of the localized schottky barrier at Au/NiO interface in charge process until fill the gap between ZnO and NiO,so that additional electrons can be released during discharge.3.Increasing the reactive sites of the anode material by structure designation.Fe₂O₃ hollow nanoshuttles were synthesized without templates and the electrolyte was effectively confined to the nanogap of the hollow nanoshuttles.Besides the increased active materials reaction sites,the shorter interval between a-Fe₂O₃ and electrolyte was achieved enabling a rapid ion transport.The maximum specific capacitance was 249 Fg^-1 at 0.5 Ag^-1.Arrhenius-type equation also was introduced to gain some insights into decline of the capacitance at large current density.After that,the relationship between charge storage and the operating temperature was researched.The results show that as the temperature increasing from the 20 ? to 60 ?,the specific capacitance had no significant change when the working temperature increased,manifesting the electrodes can work stably in this temperature range.4.Construction of asymmetric supercapacitors.An adaptive interface electronic band structure was proposed for ZnO/TiO₂/Ni?OH?₂ core-shell structure nanorods array based cathode electrodes by introducing a TiO₂ embedding layer at the ZnO/Ni?OH?₂ interface.Depending on energy band theory,a stair-like band alignment can help reduce the electron interface transport barrier when it is charging.Consequently,the capacitance was enhanced.On this basis,nanostructured ZnO/Fe₂O₃ anode electrodes were synthesized by hydrothermal method.Based on the charge balance principle,the optimum mass ratio of the positive electrode and negative electrode is 1.00:3.53 and an asymmetric supercapacitor was successfully constructed.It resulted in excellent performances including an extended operating voltage window of 1.6 V,a maximum specific capacitance of 146.8 Fg^-1 at a current density of 1 Ag^-1 and an energy density of 52.2 Whkg^-1 at power density of 1350 Wkg^-1.