Hydrothermal Preparation And Electrochemical Performances Of ZnWO₄?NiCo₂O₄ Pseudocapacitive Electrode Materials

Due to increasing energy needs and safety concerns arisen in conventional batteries,the development of new energy storage devices have become an urgent demand in portable electronic appliances and energy storing grids.In comparison with conventional capacitors and batteries,supercapacitors are emerging candidates for energy storage having high specific capacitances,energy and power density,prior rate capability,huge current charge and discharge capacity,as well as long cycle stability,which have important applications in the transportation,energy storage and so on.As we know,the electrochemical performances of supercapacitors can depend on the electrode materials and their morphologies and microstrctures which can be tuned by employing various surfactants.The porous layered three-dimensional nanostructures are characteristic of higher specific surface area,abundant active sites and shorter path for the diffusion of electrolyte ions,which can strongly improve the electrochemical performance of supercapacitors.In this dissertation,various surfactants are used to tune the morphologies and microstructures of ZnWO₄ and NiCo₂O₄,and the corresponding electrode materials were tested in three-electrode system to investigate the effect of microstructure on electrochemical performances.The main results and conclusions are as follows:Firstly,ZnWO₄ electrode materials with different morphologies deposited on nickel foam were successfully prepared via a facile hydrothermal route.It is concluded that the stability of the spheroidal structure synthesized by adding a higher concentration of ammonium fluoride?NH₄F?is superior by comparing with the related morphologies and microstructures.The as-prepared nanostructures are multiply characterized and then exposed for electrochemical performances.The experimental results show that ZnWO₄ oblate nanospheres possess unique nanoarchitechtures with specific surface area of 89.47 m²/g.The as-prepared ZnWO₄ oblate nanospheres present excellent electrochemical performances,showing high specific capacity of1198 F/g at 1 A/g,and high retention rate cyclic stability of 96.56%after 1000 cycles at 10 A/g.Subsequently,NiCo₂O₄ electrode materials with diverse morphologies were fabricated by adding different surfactants via a hydrothermal method,where urea can promote the formation of nanosheet-like structures,fluoride ion in NH₄F can increase the active sites on the substrate to promote nucleation and grain growth,and the addition of sodium dodecyl sulfate?SDS?can disrupt the coordination bond and revolutionize the shapes during the nucleation.Meanwhile,many reverse micelles produce as initial nuclei in the supersaturated SDS-urea complex solution and then form amaryllis-like NiCo₂O₄ nanoflower petal structures.NiCo₂O₄ nanowires with high specific surface area?95.23 m²/g?present outstanding stability and electrochemical performances as specific capacitance of 2253 F/g at 2 A/g,power density of 496 W/kg at 45 Wh/kg and high cycling stability of 91.23%after 5000 cycles at 8 A/g.Finally,the addition of SDS in the solution can transform the linear structure into flower-like structure on the basis of successful synthesis rute of NiCo₂O₄ nanowires,resulting in much more excellent and stable microstructural characteristics,that is NiCo₂O₄ nanoflowers show higher specific surface area of 121.52 m²/g than NiCo₂O₄ nanowires.The larger exposed surface can offer plenty of electroactive sites for reduction-oxidation reactions to be occurred efficiently.As a result,NiCo₂O₄ nanoflowers exhibit high conductivity,excellent rate performance and reversibility.The obtained specific capacitance is 2498 F/g at 2 A/g and the power density of 3571 W/kg at 79 Wh/kg.The as-prepared material present remarkable cycling stability of 92.61%retention after 5000 cycles at a constant current density of 8 A/g.Moreover,theoretical calculations on the formation and diffusion energies of Na⁺ion into the surface of NiCo₂O₄ are performed.It confirmed that NiCo₂O₄ material has good conductivity,which can provide evidence for the excellent electrochemical performances.