Fabrication, Structure Regulation And Supercapacitor Properties Of Ni-based Metal-organic Frameworks

Supercapacitors have attracted much attention because of their high power density and ecxellent cyclability.However,the largest challenge hindering its further development is how to increase the energy density while maintaining its own advantages.The most important component of supercapacitors is electrode materials,whose structure and properties can directly affect the performance of the device.Therefore,to select and design a suitable electrode material is the premise of preparing high energy density supercapacitors.Metal-organic frameworks（MOF）can meet the requirements and have a wide application prospect in the field of supercapacitors due to its high specific surface area and high porosity,but it suffers from its poor conductivity.Under these circumstances,we designed and synthesized the MOF material with layered structure by controlling experimental conditions.Subsequently,the conductivity of MOF material is further improved by compositing conductive materials and regulating structure.As a result,the electrode material with high supercapacitive performance is finally prepared.The following are specific contents:1.We chose nickel nitrate as the metal source and PTA as the organic linker to synthesis Ni-MOF with layered structure by using solvothermal method via controlling time.Furthermore,the effects of reaction time on the structure and properties were investigated.The test results show that 12 h is the best choice to synthesis time.Ni-MOF-12h shows a layered structure with uniform thickness and distribution,stacked together to form amounts of pores and voids,which can increase the contact area with electrolyte ions.The surface functional groups of Ni-MOF-12h can also lead to the activity increase.We use carbon fiber paper（CFP）to prepare electrode,and Ni-MOF-12h/CFP shows the best activity and reversibility in electrochemical tests.At the current density of 2,4,8,16 and 32 Ag^-1,the specific capacitances of Ni-MOF-12h are 483,425,365,291 and 111 Fg^-1,respectively.2.We chose plasma enhanced chemical vapor deposition（PECVD）to prepare CNW on nickel foam,then amorphous Ni（OH）₂ was electrodeposited on CNW as the precursor of Ni-MOF.Finally,vertical Ni-MOF sheet（VNi S）was synthesized on CNW.These strategies about recombination and regulation of structural orientation can further improve the conductivity.CNW prepared by PECVD is directly and vertically grown on the nickel foam substrate,which ensures the purity,the uniform structure and the significantly conductivity.VNi S grows uniformly and vertically on the CNW,which is very different from the morphology of ordinary Ni-MOF materials.VNi S has abundant kinds and quantities of functional groups,which is beneficial to improve the activity.The solid bonding between the CNW and VNi S also improves the stability.So,VNi S shows excellent electrochemical performance and the cyclic voltammetry curves at different sweep speeds remain intact.At the current density of 2,4,8,16 and 32 Ag^-1,the specific capacitances of VNi S are 677,585,465,308 and 175 Fg^-1respectively,which is superior to the Ni-MOF and Ni（OH）₂ electrodes.3.Ni-MOF-12h/CFP and VNi S electrode were integrated with active carbon（AC）to construct asymmetric supercapacitor.The test results are as follows:（1）Ni-MOF-12h/CFP//AC asymmetric supercapacitor can achieve a power density of 1450 W kg^-1 while obtaining a maximum energy density of 13.1 Wh kg^-1.When the supercapacitor reach a maximum power density of 23200W kg^-1,its power density is 4.4 Wh kg^-1.After 5000 charge/discharge cycles,it can keep 78.3%of the initial value.（2）VNi S//AC asymmetric supercapacitor can achieve a power density of 1450 W kg^-1 while obtaining a maximum output energy density of 20.7 Wh kg^-1.When the energy density is 8.8 Wh kg^-1,it can achieve a maximum power density of 23200W kg^-1.After 5000 charge/discharge cycles,the capacitance retention rate can reach 92.3%and the morphology of the supercapacitor remains.These results show that the performance of VNi S exceeds obviously the common Ni-MOF materials.Moreover,it also proves that our control strategy concerning with the conductivity and structural orientation of MOF materials is very effective.