The Preparation And Electrochemical Performance Of The Multi-level Structure Nickel-cobalt Hydroxide Composite Electrode

With the advantages of high power density,good rate performance and cycle stability,and environmentally friendly,supercapacitors have become one of the most promising assistant power source for electric vehicles.As the most important component of the supercapacitor,the properties of the electrode materials play a decisive role in the overall performance of the supercapacitor.Therefore,an effective way to improve the performance of supercapacitors is to prepare high performance electrode materials.Of all kinds of electrode materials,Faraday electrode materials which can be reversible electrochemically reduced or oxidized are gradually used in supercapacitors because of their high theory specific capacities.This paper briefly introduces the development,structure,principle and characteristics of supercapacitors as well as the research progress of electrode materials.The research bottleneck and trends of Faraday electrode materials are analyzed in detail.In this paper,composite electrodes with hierarchical structure coated with graphene oxide and porous graphene oxide were prepared respectively by step-wise hydrothermal method and conductive layer coating method.The morphology,structure and electrochemical properties of the electrode materials were systematically studied.The main research contents and results have been summarized as follows:（1）Firstly,the Ni-Co hydroxide was selected as the active materials,the three-dimensional nickel foam as the substrate,and composite electrode with a vertical nano-array structure was obtained by a simple hydrothermal method.Then,composite electrodes with hierarchical structures were prepared by multi-step hydrothermal treatment and coating of several graphene oxide conductive layers.The number of active materials and conductive layers of the composite was optimized according to the electrochemical measurements,so as to obtain an electrode with good electrochemical performance under high mass loading conditions.The specific mass and areal capacity of the hierarchical electrode could reach 874.9 C·g^-11 and 2.41 C·cm^-22 at 1 mA·cm^-2.Even at a high current density of 50 mA·cm^-2,47%of the capacity was obtained.Hybrid supercapacitor was assembled with the optimized hierarchical electrode as the positive electrode and commercial activated carbon as the negative electrode,achieving a high energy density of 33.5 Wh·kg^-11 at a power density of 369.3 W·kg^-1.And,the areal energy density can reach 0.39 mWh·cm^-2.These results indicated that it was efficient to improve the areal energy density of the device by hierarchical structural design of the electrode.（2）To investigate the influence of different conductive interlayers in the hierarchical electrodes,porous graphene was used as conductive layer instead of graphene for preparing composite electrode.Morphological characterization showed that the porous graphene was a porous network structure,which could be embedded in the interspace of the Ni-Co hydroxide nanoarrays,so as to enhance the contact area between the active materials and conductive agent.The electrochemical performance results of the electrode showed that the porous graphene as the conductive layer displayed superior rate performance than the graphene.