| With the rapid development of global economy, the energy crisis caused by the consumption of fossil fuels and environmental pollution problems are becoming more and more serious. As a result, there is an urgent need for highly efficient, clean and sustainable sources of energy, as well as new technologies associated with energy conversion and storage. In recent years, as a new form of energy storage, supercapacitors have been widely used in audio-video equipments, hybrid electric vehicles, large industrial equipments, memory back-up devices and other fields due to their higher power density, fast charge and discharge process, large lifespan, low cost, environment friendly and safety, etc. Generally speaking, the perfermance and charge storage of supercapacitors intimately depend on the used electrode materials. Therefore, during the researching process of supercacitors, more and more research focused on developing all kinds of electroactive materials with excellent properties. Up to date, the electrode materials of supercapacitors can be divided into three categories: carbon-based materials, conductive polymers and transition metal compounds. Among them, carbon-based materials are widely used as electrode materials of electric double layer capacitors because of their larger surface area and good electrical conductivity. Although they have larger power density, the smaller capacitance leads to the lower energy density which limits the corresponding application. In contrast, supercapacitors based on conductive polymers and transition metal compounds,can store energy by fast reversible oxidation reactions because these electrode materials have typical Faraday capacitance characteristics. Thus, they can achieve higher specific capacitance and energy density. But for conductive polymers, the corresponding cycle stability is much poorer due to the larger volume contraction and expansion during the process of continuous charging and discharging. For transition metal compounds, although there have some shortcomings(such as: poor conductivity), they can not only provide higher energy density than conventional carbon-based materials, but also possess better electrochemical stability than the polymer materials. Therefore, transition metal compounds are the kind of most-researched electrode materials of supercapacitors.It is well known that the electronegativity of sulfur is lower than that of oxygen. So, metal sulfides have more flexible structure and higher conductivity than metal oxides. At the same time, compared with the single binary sulfides, transition metal ternary sulfides have some other applied advantages, e.g.: richer redox reaction, higher electrochemical activity and lower cost. Hence, transition metal ternary sulfides have become the researching hotspot of electrochemistry. In this dissertation, with the transition metal ternary sulfides applied in supercapacitors as the researching background, CoNi2S4 nanoparticles are synthesized by a facile one-step solvothermal method. Then, CoNi2S4/graphene nanocomposites are prepared by using a simple physical approach and the corresponding electrochemical properties are studied. Secondly, liquid asymmetric supercapacitors are successfully assembled by employing CoNi2S4 nanoparticles as positive electrode and activated carbon as negative electrode, the corresponding electrochemical properties and practical application are studied. Finally, all-solid-state asymmetric supercapacitors are successfully assembled by employing CoNi2S4/graphene nanocomposites as positive electrode and activated carbon as negative electrode. This enhances the CoNi2S4 nanomaterials practical application in the field of portable electronic devices. The studying details are as follows:(1) Synthesis and electrochemical properties of CoNi2S4 nanoparticles and CoNi2S4/graphene nanocomposites.CoNi2S4 nanoparticles were successfully synthesized by a facile one-step solvothermal process. The average size of the obtained nanoparticles is about 8- 15 mm. Then, CoNi2S4/graphene nanocomposites are prepared by a simple physical approach. Electrochemical test results show that CoNi2S4 nanoparticles are a better electrode material of supercapacitors. Meanwhile, when the loaded amount of graphene is 5%(wt%), the pseudocapacitance behaviour of supercapacitor electrodes based on CoNi2S4/graphene nanocomposite is significantly enhanced, the maximum specific capacitance can reach up to 2009.1 F g-1 at a discharge current density of 1 A g-1. Furthermore, these electrodes also show excellent rate capability(1046.4 F g-1 at 20 A g-1) and better electrochemical reversibility. Hence one can see that the CoNi2S4 nanoparticles and the CoNi2S4/graphene nanocomposites have the higher application value of high-performance supercapacitors.(2) Assembly and electrochemical performance of liquid asymmetric supercapacitors based on CoNi2S4 nanoparticles.Liquid asymmetric supercapacitors are successfully assembled by employing CoNi2S4 nanoparticles as positive electrode and activated carbon as negative electrode. Electrochemical results show that the working voltage window of symmetric supercapacitors can be extended to 0- 1.6 V in 3.0 mol L-1 KOH electrolyte. The maximum energy density can be estimated to be 53.1 Wh Kg-1, and the energy density still retains 36.7 Wh Kg-1 even at a higher power density of 7630 W Kg-1. Especially, the liquid asymmetric supercapacitors can still maintain 89% of the initial capacitance after 1000 cycles. Furthermore, two asymmetric supercapacitors linked in series can not only light a red lightemitting diode, but also drive a rotating motor, it indicates that the asymmetric supercapacitors have the good practical application value.(3) Assembly and electrochemical performance of all-solid-state asymmetric supercapacitors based on CoNi2S4/graphene nanocomposites.All-solid-state asymmetric supercapacitors are successfully assembled by employing CoNi2S4/graphene nanocomposites as positive electrode, activated carbon as negative electrode, and polyvinyl alcohol(PVA)/KOH gel are employed as solid-state electrolyte. Electrochemical results show that the as-fabricated all-solid-state asymmetric supercapacitors exhibit an stable operating voltage window of 0- 1.6 V, and the specific capacitance of 34.2 F g-1at the discharge current densities of 3 m A cm-2. Moreover, all-solid-state asymmetric supercapacitors can still maintain 62% of the initial capacitance after 5000 charging-discharging cycles at the discharge current densities of 10 mA cm-2. These remarkable results demonstrate that all-solid-state asymmetric supercapacitors based on CoNi2S4/graphene nanocomposites have the good practical value. |
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