Synthesis Of Cobalt Selenide-Based Composite Based On Two-Dimensional Metal-Organic Framework And Their Properties For Supercapacitors

Supercapacitors have become the focus of research in the field of energy storage due to their high power density,fast charging and discharging speed,long service life,etc.However,their low energy density limits their application.The improvement of physical and chemical properties of electrode materials is expected to increase their energy density.Cobalt-based selenides have high electrical conductivity and theoretical capacity,but their actual properties are affected by the morphology,structure,surface area,active site and other factors,so it is difficult to be effectively used.Two dimensional(2D)materials have a high aspect ratio and can provide more reactive sites.Its derivatives are expected to improve the conductivity and actual capacity of materials.Based on this,we used 2D MOFs template method,selenitization treatment,multi-level structure control and bimetal-doping methods to construct cobalt-based selenides@selenium-based composite materials and asymmetric supercapacitor devices.The electrochemical properties,influencing factors,and energy storage mechanism were investigated.The main research contents are as follows:(1)The uniform growth of cobalt selenide@selenide(Co Se₂@Se-xh)composites was synthesized by solvothermal method and followed by a selenization annealing process using carbon cloth and carbon paper as current collectors,respectively.The influences of morphology,structure,selenization time,and other parameters on the electrochemical properties of the final product were investigated.The results show that the Co Se₂@Se-2h composite after selenized for two hours has the best electrochemical performance.First-principle calculations demonstrate that the presence of selenium atom can effectively reduce the adsorption energy barrier of hydroxide ions on the surface of the composite.The assembled asymmetric supercapacitor exhibits good cycle stability,high capacitance retention and coulomb efficiency,as well as high power and energy density.(2)A multi-level structure metal selenide@selenium composite was constructed by means of multiple static growth and selenization annealing with Co-ZIF-L as the precursor.The growth time of multilevel structures is regulated.The relationship between the morphology and electrochemical properties of the primary growth(f-Co Se₂@Se)and secondary growth(s-Co Se₂@Se)materials has been studied.The results show that the secondary growth material(s-Co Se₂@Se)shows larger specific capacitance,higher rate performance,and longer cycle stability than those of the primary growth material.An asymmetric capacitor assembled with s-Co Se₂@Se as the positive electrode and activated carbon as the negative electrode shows good electrochemical performance.(3)The bimetallic selenide@selenium((MCo)Se₂@Se,M=Ni,Zn)nanocomposite quadrilateral nanosheets array on carbon cloth was constructed via nickel or zinc doping.The morphology,structure,and electrochemical properties of the nanocomposite quadrilateral nanosheets array were investigated.The energy storage mechanism and theoretical calculation were analyzed and inverstigated and discussed in detail.It is found that the(Ni Co)Se₂@Se electrode shows high specific capacitance and rate performance,which can be attributed to the low electrochemical impedance of the nickel-cobalt selenide and the reduce the adsorption free energy of OH^-.The(Ni Co)Se₂@Se-based asymmetric capacitor device also displays excellent electrochemical energy storage performance.