Research On The Pseudocapacitance Properties Of Nickel-based Materials Electrode For High-performance Supercapacitors

Environmental pollution and energy shortage are the world's two crises because of the increased population of the world and rapid consumption of fossil fuels.As a result,exploring renewable energies,such as solar energy,wind energy and tidal energy,is of urgent need.Great efforts have been devoted to develop energy storage devices,such as lithum battery,fuel cell and supercapacitor,in order to couple these thchnologies with renewable energy sources for powering the electrical grid.In particularly,supercapacitors have received more and more attentions because of their high power density,fast charge-discharge process,and long lifetime.In general,supercapacitors can be divided into two types according to the charge storage mechanism:electrical double-layer capacitors(EDLCs)and pseudocapacitors.As for EDLCs,electrical energy is stored by the diffusion and accumulation of electrostatic charges at theinterface between electrode and electrolyte.Carbon-based materials are usually used as electrode materials for EDLCs.In contrast,pseudocapacitors are principally dominated by Faradaic redox reactions,which possess much higher specific capacitance than that of EDLCs.With the increased demand in energy storage materials,investigating high performance electrode materials of pseudocapacitors have became the key task.In this paper,aiming to improve the specific capacitance of electrode materials,we have prepared different types of nickel-based materials by differnet methods.The surface morphology,phase structure,and electrochemical properties of these nickel-based materials have been studied systematically.Main contents of this study are as follows:(1)A novel bimetallic Ni/Co-based metal-organic framework(Ni/Co-MOF)was successfully synthesized via a simple solvothermal method,and used as electrode material for high performance supercapacitors.After doping of Co element,the Ni/Co-MOF materials retain the original crystalline topology structure of Ni3(BTC)2·12H2O.The as-obtained Ni/Co-MOF demonstrates an excellent specific capacitance of 1067 and 780 F/g at current densities of 1 and 10 A/g,respectively,and can also retain 68.4%of the original capacitance after 2500 cycles.The synthetic procedure can be applied to synthesize other bimetallic MOFs to enhance their conductive property.(2)A facile one-step electrodeposition method is developed to prepare 3D Ni3S2 interconnected nanosheet arrays on Ni foam as electrodes for suprecapacitors,resulting in excellent pseudocapacitance performance.The composition,microstructure and morphology of the prepared Ni3S2 materials were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),Raman spectroscopy and X-ray photoelectron spectroscopy(XPS),The electrochemical capacitance properties were tested by cyclic voltammetry(CV)and galvanostatic charge-discharge(GCD)measurements in a three-electrode system.Taking advantage of the highly conductive 3D architectures,the Ni3S2/NF-10 electrode exhibits a superior specific capacitance of 2850 F/g even at a current density of 1 A/g.Remarkably,a specific capacitance of 1972 F/g could be achieved at a high current density of 10 A/g,indicating its excellent rate capability.With the increase of the electrodeposition time,the 3D architectures of Ni3S2 begin to disappear,resulting in a reduced specific capacitance.The appropriate electrodeposition conditions are the key to the preparation of high performance electrode materials,Our results show that this unique 3D Ni3S2 is promising for electrochemical energy applications.(3)Ni3S2 coated on well-designed hierarchical porous Ni@Ni foam(Ni3S2-Ni@NF)was successfully fabricated by a novel method,consisting of hydrogen-bubble template electrodeposition followed by in situ cyclic voltammetry electrodeposition.The physical phase and morphology properties of the as-prepared samples were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and Raman spectroscopy.The electrochemical capacitance properties were tested by an electrochemical workstation.The Ni3S2-Ni@NF electrode demonstrates a much higher specific capacitance of 4.56 F/cm2 at a current density of 2 mAJcm2.The Ni3S2-Ni@NF also exhibits excellent rate capability(4.06 F/cm2 at a current density of 20 mA/cm2)and cycle performance(72.96%after 1000 cycles).(4)A facile one-step hydrothermal method was developed to synthesize self-supported Ni3S2 on a Ni foam substrate with a loose and porous structure,and the pore size can be adjusted by controlling the sulfuration time.Combining the advantages of both porous structure and self-supported structures,electrochemical tests show that Ni3S2@NF-12 demonstrate a high specific capacitance(1.87 F/cm2 at 2 mA/cm2).To further improve the electrochemical performance of Ni3S2,we use etched Ni foam as current collector to synthesize self-supported Ni3S2 by one-step hydrothermal method(labeled as Ni3S2@MNF).The etching process created a 3D net structure on the Ni foam surface.Different from the results of Ni3S2@NF,Ni3S2@MNF developed a hierarchical structure consisting of nanowire subunits.Because of this unique structure,when the sample was applied as an electrode for supercapacitors,it demonstrated an ultrahigh areal capacitance of 3.32 F/cm2 at a current rate of 2 mA/cm2,which is much higher than that of Ni3S2@NF.