Synthesis,Growth Mechanism And Supercapacitor Performance Of Carbon Incorporated On Pseudocapacitor Nano-structured Arrays

With the increasing demand of electronic vehicles and high-power density energy-storage device,supercapacitors,with longer lifespan,better safety and faster charge-discharge capability,are being considered as the most promising power devices.To realize the application of the supercapacitor,energy density as well as power density is the typical issue to be addressed.As the packaging technology of the supercapacitor device is relatively simple,improving the capacitive behaviours of electrode material is the key point.Ideal electrode materials are required to have the following features:(1)high specific surface area,which guarantees the specific capacitance and energy density;(2)Porosity,which dominates the specific capacitance and the rate capability;(3)high electronic conductivity,also affects the rate capability and power density;(4)high thermal?chemical?structural stability,which determines cyclic performance.As we know,pseudosupercapacitors have higher theoretical capacitance value than electrical double-layer capacitor,however,low conductivity restricts capacitance behaviors of metal oxides-based electrodes.In this thesis,we construct one or two dimensional nanostructures which provide direct pathway for the electrochemical reaction.And these structures can provide higher surface area as the skeleton of the electrode material,with the carbon incorporated with various electrode materials(CoO,NiCo2O4,NiCo2S4),the structure,surfaces of the electrode,electronic conductivity are changed.A lot of interesting phenomena are found and investigated by using kinds of characterization methods.Meanwhile,we also investigate the activation process of pseudocapacitors in the alkaline electrolyte.The main research work and innovations are concluded as followings:1.A low temperature in situ CVD method to fabricate CoO@C nanostructure arrays(CCNAs)based on 3D nickel foam is developed to improve the electrochemical behaviors of CoO.The growth mechanism has been proposed and confirmed by Raman,SEM,TEM,XPS.In this structure,CoO not only is a capacitor material,but also selected as a catalyst to catalyze the decomposition of acetylene in a reduction and catalytic process during the CVD process.In this study,apart from partial amorphous carbon,the crystalline conductive carbon shell is observed,which is different with current works.Deposition time plays an important role in the morphology,structure,electrochemical behaviors of CCCNAs.As a consequence,the optimized CCNAs exhibit a high specific capacitance of 3277.4 F/g(6.76 F/cm2)at the current density of 1 mA/cm2.Even at the high current density of 50 mA/cm2,the specific capacitance of the CCNAs remains 1481.6 F/g and the CCNAs show a small capacitance loss after 10,000 cycles.The assembled asymmetric supercapacitor using the optimized CCNAs as the positive electrode delivers a maximum energy density and power density of 58.89 W h/kg(5 mA).2.Ternary NiCo2O4 has evoked tremendous interest for its better electrical conductivity and higher redox activity compared to nickel oxides and cobalt oxides.Herein,a facile and practical strategy has been developed to construct carbon-CoO-NiO-NiCo2O4 nanosheet hetero-structured arrays with high electrochemical performance for supercapacitors.Such method provides new solutions for the synthesis of multiple composition of pseudocapacitor materials.The growth mechanism of the novel electrode is investigated by XRD,TEM,XPS analysis and minimum ion rearrangement.The CVD reaction temperature for the carbon deposition is as low as 350?,which is the same as the annealing temperature of synthesizing NiCo2O4 from NiCox(OH)y nanosheet hetero-structured arrays,suggesting that the method is cost-effective and practical.The optimal carbon-CoO-NiO-NiCo2O4 nanosheet hetero-structured arrays exhibit a large areal apacitance of 5.23 F/cm2(2602.0 F/g)in the 6 M KOH aqueous electrolyte,good rate capability and excellent cycling performance even with a rising trend after 7000 cycles.3.Carbon-NiCo2S4 nanosheet hetero-structured arrays are firstly synthesized and investigated as supercapacitors.Compared to NiCo2O4,NiCo2S4 exhibited an electric conductivity?100 times as high as that of NiCo2O4,and could offer richer redox reactions than the corresponding single sulphides.Here,in the framework of novel carbon-Coo-NiO-NiCo2O4 nanosheet hybrid hetero-structured arrays on the nickel foam,we further developed and firstly synthesized carbon-NiCo2S4 nanosheet hetero-structured arrays using a simple hydrothermal method.The NiCox(OH)y or NiCo2O4 could be converted to NiCo2S4 in anion-exchange reaction of different solubility product constant.Such unique carbon-NiCo2S4 structure exhibits high areal capacitance of 8.33 F/cm2 and high specific capacitance of 1893.2 F/g in a potential window 0.8 V and higher mass loading of 4.4 mg/cm2.4.In the research of cycle experiment of pseudocapacitor material in the alkaline electrolyte,activation process is observed.Such activation process usually leads to a capacitance boost.A preliminary study is done with the EIS,CV,SEM,XRD method to study the activation process of the NiCo2O4 nanosheet arrays on nickel foam in the alkaline electrolyte.