Structural Construction And Electrochemical Properties Of Metal Organic Frameworks (MOFs) And Their Derived Transition Metal Sulfides

In the face of energy shortage and environmental pollution,it is necessary to develop effective energy storage and conversion devices.Supercapacitors,as a reliable and safe electric energy storage device,have been extensively studied by a large number of scholars due to their ultra-high power density,long-term durability and fast charging and discharging capabilities.The performance of supercapacitors depends largely on electrode materials.Therefore,it is necessary to develop an electrode material with high specific capacitance,excellent rate performance and good cycling stability.Transition metal sulfides(TMS)are considered as a very promising electrode material for supercapacitors due to its high theoretical specific capacity and electrical conductivity.Although TMS has a high specific capacity,its poor rate performance and cycling stability limit its further application.Generally,TMS combined with carbon materials can effectively improve the comprehensive performance of supercapacitors through the synergistic effect of the two.In addition,the overall performance of supercapacitors can be significantly improved by constructing a nano-electrode with large specific surface area and high porosity.In order to develop good electrode materials for their practical applications in supercapacitors,this paper focuses on MOFs(MOF-74,ZIF-L)as precursors,obtaining MOFs derivatives by post-processing and using them as substrates for pseudocapacitive active substances,or using primary MOFs directly as substrates for electrochemical active substances to obtain electrode materials with excellent performance,characterizing their microstructures and testing their electrochemical properties.The main works are as follows:(1)Preparation of Ni Co₂S₄@Fe Ni₂S₄ composites and their electrochemical propertiesOne-dimensional rod-like MOF-74 was used as a precursor to obtain Ni Co₂O₄nanorods by heat treatment under air atmosphere.Subsequently,Ni Co₂O₄ nanorods were used as a substrate to load Ni Fe-LDH nanosheets to synthesize Ni Co₂O₄@LDH composite,and finally Ni Co₂O₄@LDH was subjected to high temperature solvent thermal vulcanization to obtain rod Ni Co₂S₄@Fe Ni₂S₄.Electrochemical test results showed that Ni Co₂S₄@Fe Ni₂S₄ has the most excellent supercapacitance performance compared with Ni Co₂O₄and Ni Co₂O₄@LDH,which may be due to the in situ vulcanization of rod-shaped Ni Co₂O₄ into Ni Co₂S₄ during the vulcanization process,and Ni Co₂S₄ still maintains a one-dimensional structure with a large number of active sites.In addition,the LDH nanosheets were in situ sulfurized into Fe Ni₂S₄ nanoparticles,which may further increase the specific surface area of the material.Among them,the specific capacity of Ni Co₂S₄@Fe Ni₂S₄is 286.5 m Ah g^-1(2063 F g^-1)at a current density of 1 A g^-1.When the current density increases to 10 A g^-1,the specific capacity is 190.5m Ah g^-1 with a capacity retention rate of 66.5%.In addition,the capacity retention rate of Ni Co₂S₄@Fe Ni₂S₄ was 39.9%after 1000 charge/discharge cycles at a high current density of 10 A g^-1.(2)Preparation of C@MCo₂S₄ hierarchical composites and their electrochemical propertiesThe CNT/C hierarchical carbon structure was obtained by heat treatment of the two-dimensional ZIF-L as a precursor under H₂/Ar atmosphere.Subsequently,the CNT/C@LDH composite was obtained by loading Ni Fe-LDH on the CNT/C surface by chemical deposition,and finally the CNT/C@LDH was solvothermal sulfurized to obtain a three-dimensional C@MCo₂S₄ hierarchical composite electrode material,which has excellent electrochemical energy storage performance,which is mainly attributed to the synergistic effect of the carbon substrate with excellent electrical conductivity and MCo₂S₄with high electrochemical activity.The specific capacity of C@MCo₂S₄ is 342.5 m Ah g^-1(2466 F g^-1)at a current density of 1 A g^-1.When the current density further increases to 10 A g^-1,the specific capacity of C@MCo₂S₄ is 235.5m Ah g^-1 and the capacity retention rate is up to 69%.In addition,the hybrid supercapacitors assembled with C@MCo₂S₄ and soybean derived porous carbon as positive and negative electrodes,respectively,showed a maximum energy density of45.9 Wh kg^-1 at a power density of 775 W kg^-1,indicating that C@MCo₂S₄ is promising as the positive material for high-performance supercapacitors.(3)Preparation of nickel-cobalt-sulfur nanosheet arrays supported by MOF and their electrochemical propertiesCo-ZIF/NF was obtained by growing ZIF-L on the surface of nickel foam collector by solution method,followed by growing sulfide nanosheets on the surface of Co-ZIF by electrodeposition method to obtain NCS@Co-ZIF/NF self-supporting electrode material without binder and conductive agent,which has ultrathin nanosheets and unique nanohoneycomb-like structure,thus providing abundant active sites and rich of pores,which facilitates enhanced electrochemical reactions and accelerates the diffusion of electrolyte ions.Due to the above structural advantages and the synergistic effect among the components,NCS@Co-ZIF/NF shows remarkable electrochemical properties with a specific capacity of 144.4 m Ah g^-1(1040 F g^-1)at a current density of 1 m A cm^-2.When the current density increases to 50 m A cm^-2,the specific capacity is 87.5 m Ah g^-1 with a capacity retention rate of 60.5%,which has excellent rate performance.Furthermore,the capacity retention of NCS@Co-ZIF/NF rate was still 76.1%after 1000charge/discharge cycles at a high current density of 50 m A cm^-2,demonstrating excellent cycle stability.In addition,the hybrid capacitor assembled with NCS@Co-ZIF/NF and activated carbon as positive and negative electrodes,respectively,achieved a maximum energy density of 33.9 Wh kg^-1at a power density of 145 W kg^-1.