Construction Of Cobalt-based Oxides Nanoarrays Materals And Their Supercapacitive Properties

The rapid development of renewable energy storage and related application industries has tremendously promoted the research in the field of high performance electrochemical energy storage devices and electrodes materials.Among a variety of energy storage device,supercapacitors as known as a high power device,have been applied widely in energy transition and storge field due to numerous outstanding properties such as high power density,fast charge/discharge time and excellent cycling stability.Unfortunately,because the energy density is too low,the development of supercapacitor is greatly restricted.One of the determining factors that affect their energy storage performance is electrode materials.Therefore,designing and fabricating high performance electrode materials is a great importance in the field of supercapacitors.Cobalt-based oxides,characterized as high theory value,multiple valence states and rich resource storage,are considered valuable to improve energy densities at large extent.However,it should be pointed out that there are several problem remain unresolved,for example,poor electroconductibility results in much lower specific capacitance than the theoretical values and unsatisfactory cycle life fails to meet the requests of suercapacitors.In this paper,nanoarray structure with abundant morphology was fabricated by hydrothermal method in hope of solving the deficiency of cobalt-based materials.Besides,the energy storage performance of cobalt-based materials can be enhanced by the introduction of graphene or Mo-based materials.The main research and results are as followed:(1)The Co₃O₄ nanowires/nanoflakes composite array structure was constructed on a three dimensional nickel foam substracte,through hydrothermal mathed.In this case,the combination of nanowaires and nanoflakes arrays is found to have increased,the conductivity,specific surface area and active sites of the Co₃O₄ materials,thus improving the electrochemical performance of the as-prepared electrode materials.Their specific capacitance can reach 2053.1 F g^-1 at a current density of 1 A g^-1.The hybrid supercapacitors devices that are made of Co₃O₄-based heterogeneous nanoarry structure and N-rGO can obtain high specific energy of 22.2 Wh kg^-1 while still maintaining 93.3%of their initial capacitance even after 10000 cycles,shows an excellent cycling stability.(2)The ultra-thin Ni_xCo_3-xO₄ nanowires arrays were grown on the nickel foam-supported graphene framework using a two-step hydrothermal process.The increased specific surface area of electrode materials is attributed to a unique intersected nanowire network array structure,which can obtain rich active sites and facilitate the electrolyte accessibility to surface of the electrode.As a result,a rapid electroic transmission and ion diffusion coule be realized in this way,thus improving the electrochemical energy storage performance of electrode materials.Thanks to the synergistic effect of between the crossed nanowires structure and graphene,the specific capacitance value for Ni_xCo_3-xO₄ nanowire structure material were obtained to be 2619.3 F g^-1,far higher than those either nanosheet material fabricated by one-step hydrothermal process(1632.4 F g^-1)or pure nanowire structure by two-step hytrdothermal process(1950.7 F g^-1).Among all the hybrid supercapacitors devices that were made of Ni_xCo_3-xO₄/graphene nanowire materials,the maximum specific energy have been obtain as 24.8 Wh kg^-1.After 8000 cycles,its specific capacitance retention still maintains 90.3%.This remarkable performance makes Ni_xCo_3-xO₄ nanowire a promising material in practical application in the field of supercapacitors.(3)The Ni-Co thermometal oxides precursor was firstly prepared by hydrothermal method.And then ultrathin nanosheet structure of Ni-Co-Mo trimatllic oxide was in-situ synthesized by only using Na₂MoO₄ solution through a second hydorthermal process.The two-step hydrothermal methods has effectively reduce the thickness of nanosheet and improved conductivity of electrode materials by introducing Mo element.This can,in a large extent,enhance the energy storage and electrochemical properties of Ni-Co oxide nanosheets as electrode materials.The results show that Ni-Co-Mo mixed trimetallic oxide material exhibits a specific capacitance of 1837.3 F g^-1.After6000 cycles,it keeps 81.2%of the intial specific capacitance retention.The result of experimentation indicates that Ni-Co-Mo trimetallic oxide ultrathin nanosheet mixed material has high specific capacitance and cycling stability.