Preparation Of Transition Metal-based Compouds (Fe,Co,Ni) And Its Application In Supercapacitor

As a new green energy storage device,supercapacitor have been widely employed in the areas of electronic products,electric cars,hybrid electric vehicles,wireless communication facilities,signal monitoring,solar energy as well as wind power generation due to its advantages of high power density,high charge–discharge efficiency and long cycle life,showing great application prospect.Development of supercapacitors with high energy density,long cycle life and low cost is one of the most important aspects of future research in this field.To achieve this goal,the transition metal-based compounds?Fe,Co,Ni?with high theoretical capacity and low price are the ideal electrode materials.Therefore,the object of this dissertation is transition metal–based compounds?Fe,Co,Ni?and by controlling their composition and structures,we have studied their energy storage performance and mechanism.The dissertation provides experimental basis for the exploration of electrode materials with high performance and lost cost.1.Synthesis of Ni,Co-?A compounds and study on its energy storage performance.NiCoO₂ and corresponding sulfides and selenides are prepared by a facile hydrothermal method followed by ion-exchange and they are investigated as positive electrode materials for supercapacitors.The influence of different non–metallic element on the energy storage performance of these compounds are studied.The results show that Ni-Co selenides exhibits higher areal capacitance(1.1 F cm^-2)at the scan rate of 5mV s^-1,but it drops quickly as the scan rate increases,indicating its poor rate capability compared to NiCoO₂.Cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?results indicate that as the atomic number of?A group increases,the conductivity of nickel and cobalt-based compounds increases,and diffusion–controlled energy storage process dominant,which is in favor of the diffusion of electrolyte ions and electrons.Therefore,Ni-Co selenides exhibits higher areal capacitance.Meanwhile,NiCoO₂ shows better rate capability since non–diffusion–controlled capacitive effect occupies a large proportion to the total capacity in NiCoO₂ compared to sulfides and selenides.2.Contrallable synthesis of Ni-Co sulfides nanowire arrays and study on its energy storage performance.Ni-Co sulfides nanowire arrays are grown on nickel foam by a facile hydrothermal method followed by ion-exchange and they are investigated as positive electrode materials for supercapacitors.The nanowire morphology of precursor is reserved.The results show that Ni-Co sulfides exhibit higher specific capacitance than that of Ni-Co oxides at the same discharge current density.BET results show that the specific surface area of Ni-Co sulfides decreases due to the subsequent elimination of certain porosity after ion-exchange reaction.The results demonstrate that the change in composition and electronic structure of electrode material has greater impact on its energy storage performance.3.Controllable synthesis of NiCo₂S₄ nanosheets and study on its energy storage performance.NiCo₂S₄ nanosheets are synthesized by a facile hydrothermal method followed by ion-exchange and they are investigated as positive electrode materials for supercapacitors.The energy storage tests show that by controlling the morphology of Ni-Co sulfides,NiCo₂S₄ nanosheets exhibits considerable specific capacitance,better rate capability and good cycling performance compared to 1D nanowires arrays.It can be ascribed to the efficient ion and electron transport in 2D nanosheet,as well as its accommodation of volume change during charge and discharge process.4.Controllable preparation of hierarchical nickel and cobalt hybrid inorganic–organic nanosheet supported nanowires and study on its energy storage performance.Hierarchical nickel and cobalt inorganic–organic nanostructures on carbon fiber paper have been fabricated by a one-step hydrothermal approach.The results show that HMT molecules and NO₃^-ions have been intercalated into the Ni–Co hydroxides and the formation of hierarchical structure is not only controlled by the precursor concentration but also the molar ratio of HMT to Ni/Co.When used as positive electrode materials for supercapacitors,it exhibits considerable specific capacitance(1307 F g^-1,1.7 F cm^-2)and excellent long term cycling performance.The interconnected nanosheet-nanowire structure affords rich open spaces and efficient passways for ion diffusion and electron transport.Besides,the intercalation of anions and HMT molecule may have enhanced the cycling stability of the electrode.5.Controllable synthesis of Fe₂O₃ negative materials and study on its energy storage performance.Fe₂O₃ with nanoparticles,nanosheets and nanorods morphologies are synthesized by hydrothermal method,respectively,and they are investigated as negative electrode materials for supercapacitors.The impact of morphologies and electrolyte solutions on the supercapacitive behavior for Fe₂O₃ and the charge storage mechanism are studied.Results show that energy storage performance of Fe₂O₃ is related to morphology,voltage range and electrolyte.Fe₂O₃ nanosheet exhibits higher specific capacitance,however,it shows poor rate capability.For Fe₂O₃ tested under-1.2–0 V,the redox reaction is a diffusion–controlled process,therefore,it exhibit higher specific capacitance than that of-0.8-0V.Additionally,tests show that under same voltage range,Fe₂O₃ nanosheet exhibit higher capacity but poor rate capability in LiCl/LiOH compared to NaCl/NaOH,vice versa.6.Assembly and test of prototype supercapacitor devices.Different prototype supercapacitor devices are assembled by different nickel and cobalt–based compounds employed as positive electrode,and active carbon and FeOOH nanorods as negative electrode,respectively,with KOH aqueous solution as electrolyte,to evaluate the practical value of as–prepared electrode materials,including Ni–Co nanowires//active carbon,Ni–Co based nanosheet-nanowire composite//active carbon and NiCo₂S₄nanosheet//FeOOH nanorod.The results show that NiCo₂S₄ nanosheet//FeOOH nanorod device exhibits higher energy density(45.9 Wh kg^-1)and power density(8.6 kW kg^-1),which can be ascribed to the higher specific capacitance of negative electrode based on pseudocapacitance storage mechanism as well as better capacitance match under different current densities between NiCo₂S₄ nanosheet//FeOOH nanorod device.The test of prototype supercapacitor devices demonstrates that the capacitance match of positive and negative electrode is in favor of improving the power density and employing pseudocapacitance as negative electrode can help to improve the capacitance and energy density of the device.