Preparation Of MOF-74 Composite Derivatives And Study Of Its Capacitive Properties

To meet the increasing energy demand,energy storage has become one of the most important scientific and technological research topics in today’s society.As a new type of energy storage device,supercapacitors have received great attention from the scientific community.However,the low energy density greatly limits their applications for supercapacitors.Therefore,the design and construction of high-performance electrode materials is an effective means to improve the energy density of supercapacitors.MOFs,as a popular material in recent years,have limited their direct application in supercapacitors due to their low electrical conductivity and poor structural stability.MOF-74,as a class of MOFs,has large specific surface area,rich microporous structure and controllable metal nodes,and its unique structural properties are one of the ideal precursors for building porous materials.While it is used as self-sacrificing templates and precursors by chemical transformation and calcination to prepare to derivative composite electrode materials with excellent electrochemical properties,it can crease the number of exposed active sites and improve stability and conductivity In this paper,MOF-74 is used as a precursor and self-sacrificial template to prepare composite derivative materials,and then electrochemical capacitors are assembled together with another bilayer capacitive materials with expanded potential window and increased energy density.（1）Binary ZnCo-MOF-74 with different Zn/Co molar ratios were prepared by solvothermal method.The mixed oxides Znx Co_1-xO₄ were obtained by high temperature calcination as self-sacrificing templates.The metal oxides obtained at the optimum ratio were investigated.The synthesized materials were prepared as working electrodes for electrochemical performance tests in a three-electrode system,and the Zn_0.5Co_0.5O₄ obtained with Zn/Co=1:1 had the best electrochemical performance.The specific capacitance of the material was 1047.8 F/g at 1 A/g,and it maintained 81.3%after 2000 cycles at 10 A/g.Its specific capacitance and cycling stability were higher than those of the metal oxides obtained at other ratios.The electrochemical performance was tested for the assembled asymmetric supercapacitor device(denoted as Zn_0.5Co_0.5O₄//AC).The results indicated a specific capacitance value of 74.3 F/g at 0.5 A/g.The corresponding energy density was 26.4 Wh/kg at a power density of 536.3 W/kg.This work indicates that the synergistic effect of a suitable bimetallic ratio can effectively promote the conductivity of the electrode material and improve the electrochemical performance.（2）MoS₂,MnNi-MOF-74 and MoS₂/MnNi-MOF-74 were synthesized by solvent thermal synthesis,and spindle-shaped MoS₂/MnNiDH was successfully synthesized by using MoS₂/MnNi-MOF-74 as the precursor,alkalizing at room temperature according to the ion etching/exchange reaction.The four materials were fabricated into working electrodes in a three-electrode system for electrochemical testing,and MoS₂/MnNiDH exhibited excellent electrochemical performance.The specific capacitance of MoS₂/MnNiDH was 2194.4 F/g at1 A/g,and was retained 87.4%of initial value after 5000 cycles at 15 A/g.The prepared asymmetric supercapacitor（MoS₂/MnNiDH//AC）can provide a high energy density of 56.1Wh/kg at a power density of 931 W/kg.The retention was 95%at 15 A/g after 10,000 cycles.The work verifies that the composite material may mitigate volume swelling/shrinking of electrode material and increase its electrochemical cycling stability.（3）The oxide obtained in the first system was used as a precursor to synthesize ZnS/ZnCo₂S₄ by sulfidation reaction.Nano-spherical ZnS/ZnCo₂S₄ was grown on the surface of Ni（OH）2 nanosheets using Ni（OH）₂ nanosheets as supports to improve the electrochemical properties.The nanosphere-interspersed flower-like ZnS/ZnCo₂S₄@Ni（OH）₂heterostructures were successfully prepared by the solvothermal reaction.The three materials were made into working electrodes separately,and the electrochemical performance was tested under the three-electrode system,in which ZnS/ZnCo₂S₄@Ni（OH）₂ showed the most excellent electrochemical performance due to their joint advantages of high conductivity of ZnS/ZnCo₂S₄ and high theoretical specific capacitance of Ni（OH）2.The specific capacitance of ZnS/ZnCo₂S₄@Ni（OH）₂ was up to 2281.4 F/g at 1 A/g.The specific capacitance remained90.4%of the initial after 5000 cycles at 15 A/g.The maximum energy density of the asymmetric solid-state supercapacitor（ZnS/ZnCo₂S₄@Ni（OH）₂//AC/PVA/KOH）can reach61.4 Wh/kg,and its specific capacitance remained 94.5%after 10000 cycles.The work demonstrated that the suitable heterostructures may shorten charge transfer pathway and boost charge storage capability and cycling stability.