In-situ Fabrication Of Multivariate Complex Compound Electrode Materials Based On MOFs And Research On Energy Storage Characteristics

The development of high-quality energy storage devices can effectively reduce the excessive consumption of petrochemical energy and improve the energy storage efficiency of emerging green energy,thus reducing pollution,which is of great significance for the green and sustainable development of the environment.As new energy storage devices,supercapacitors have many advantages,such as environmental friendliness,high electrochemical capacitance,fast charging-discharging,long service life,wide operating temperature range and high-power density,which has attracted wide attention.As the core components,electrode materials are the decisive factors for high-performance supercapacitors.In recent years,as porous nano functional materials,2D sheet metal organic framework compounds(2D-MOF)have attracted extensive attention due to their stable framework structure,controllable nano morphology,adjustable size,good adsorption properties and large specific surface area.At the same time,in order to solve the problem of poor conductivity,2D-MOF is often used as precursor template or sacrificial agent to prepare high-performance electrode materials.Herein,to further explore the application potential of 2D-MOF in the preparation of high-performance electrode materials,carbon fibres(CF)were used as flexible substrate,and two-dimensional nanosheet cobalt based MOFs(2D Co-MOF)or zinc-based MOFs(2D Zn-MOF)nanoarray were constructed as precursor templates or sacrificial agents to prepare high-performance multi metal complex nanoarray electrodes.By studying the structure-activity relationship between the microstructure and properties of these materials,the energy storage behaviours and mechanism of supercapacitors were deeply explored.1.Firstly,2D Co-MOF was grown on carbon fibres to form a uniform and stable template.Then,by adjusting the reaction temperature and time,Ni/Co layered double hydroxides arrays(CF@NiCo-LDH)with hollow structure were fabricated under Ni ion etching,exchange and coprecipitation conditions.Compared with the reported synthesis method,the preparation method used in this experiment was very simple,highly controllable,easy to repeat and the obtained nanostructure was extremely stable.The hollow layered nanoarrays enhanced the flow rate of interlayer ions and further reduced the resistance of ions transport.When the current density was 1.0 A g^-1,the electrochemical capacity reached to134.4 m Ah g^-1 and had an excellent capacitance retention of 78.7%after 5000 cycles under the three-electrode system.However,limited active sites restricted the improvement of electrochemical performance,so further modification was needed to increase the number of effective active sites exposed.The main purpose is to explore the synthesis method,structural characteristics and performance characteristics of 2D-MOF derived polymetallic compounds,and lay the foundation for the follow-up study.2.To make CF@NiCo-LDH more active sites were exposed and higher capacity performance was obtained.NiCo-LDH was transformed into core-shell nanoparticles by controlled reduction and hydrothermal vulcanization(CF@NiCo-A-S).Nickel cobalt alloy as the core effectively enhances the conductivity,and the nickel cobalt nickel coating shell makes more active sites exposed.Based on the synergistic effect,the capacity performance of the granulated nanosheet arrays was significantly improved,reaching 213 m Ah g^-1;the cycle performance was also significantly improved and the capacity retention rate reached 80.2%after 5000 cycles.In addition,carbon nanosheets embedded with iron oxide nanoparticles were prepared by impregnation and calcination(Fe_xO_y@CNS).The energy density and power density of the supercapacitor equipped with the two devices can be as high as 48.2 Wh kg^-1 and 8300 W kg^-1.After 15000charge and discharge,the capacity retention rate can be as high as 83.5%.The as-prepared CF@NiCo-A-S can effectively coordinate the relationship between capacity performance and conductivity,and enhance the energy storage efficiency of electrode materials.It provides a new research idea for the design and development of high-performance electrode materials for supercapacitors.3.Due to the unique lamellar structure of NiCo-LDH provides a good transport channel for electrolyte ions and conductive electrons,it was suggested that more active sites of oxidation reaction could be exposed without changing the original layered structure.Therefore,via a strategy of selective sulfurization,Co₉S₈ nanocrystal particles were embedded within the surface of hollow NiCo-LDH in situ to prepare CF@NiCo-LDH/Co₉S₈ composite nanosheet arrays,which gives full play to the synergistic energy storage advantages of layered double hydroxides and sulfide nanocrystals.The as-prepared CF@NiCo-LDH/Co₉S₈ exhibited an ultrahigh electrochemical capacity of 341.3 m Ah g^-1 at the current density of 5.0 A g^-1 and the maximum energy density of the assembled(active carbon as negative electrode)was38 Wh kg^-1.Moreover,the capacity retention rate is 93.1%after 5000 cycles.Therefore,the method of selective construction of active units on LDH surface was very simple,the nanostructure was extremely stable,and the capacity performance was significantly improved.So,it is an effective way to prepare high-performance supercapacitor materials.4.In view of the ternary metal complex compounds have the advantage of synergistic energy storage of multiple oxidation states,2D Zn-MOF was used as sacrificial agent and etched by Ni and Co ions in aqueous solution containing urea,a three-dimensional complex structure nanoarray of NiCo-Zn-CH and NiCo-CH nanowires was constructed by adjusting the reaction time in one step(CF@NiCo Zn-CH/NiCo-CH).After sulfurization treatment,three-dimensional flexible electrode of ternary metal complex sulfide nanoarray was constructed(CF@NiCo Zn-S/NiCo₂S₄).The capacity is 194 m Ah g^-1.In addition,single-walled carbon nanotubes were doped in the synthesis of 2D Zn-MOF,and then calcined at high temperature to prepare the powder like mixed carbon anode material(CNS-CNT)with a capacity of 215.2 F g^-1.The energy density of the assembled supercapacitor was as high as 49.7 Wh kg^-1and the capacity can still maintain 70.1%after 10000 charge and discharge cycles.5.It was found that in deionized water solution without urea,nickel cobalt zinc ternary metal layered double hydroxide sheet array with more stable structure and better performance can be prepared by adjusting the concentration of Ni and Co ions(CF@NiCo Zn-LDH),which is obviously different from the bimetallic layered double hydroxides constructed in ethanol solution.Compared with bimetallic layered hydroxides,the energy storage characteristics of layered hydroxides are very obvious,and the capacity can be as high as 250 m Ah g^-1.Due to the difference of binding energy of different metals in hydroxides and the solubility products of corresponding metal sulfides,it was found that Co₉S₈ quantum dots(Co₉S₈-QD)were in-situ induced and embedded between NiCo Zn-LDH lamellae,resulting in the intercalation structure(CF@NiCo Zn-LDH/Co₉S₈-QD).The multi-dimensional flexible multi-element metal complex compound array electrode constructed by 0-dimensional quantum dots,1-dimensional carbon fibre,2-dimensional nano sheet and 3-dimensional carbon cloth,the electrochemical capacity was significantly increased to 350.6 m Ah g^-1,and the energy density of supercapacitor was 56.4 Wh kg^-1.After 8000 cycle stability tests,the capacity did not significantly decrease and remained at 95.3%.Therefore,using 2D Zn-MOF as sacrificial agent,the LDH interlayer was modified by quantum dots in situ,which provided a new idea for the preparation of high-performance electrode with multiple complex structures.