Preparation Of ZIF-derived Cobalt-based Sulfur/selenides And Their Supercapacitor Properties

Supercapacitors play an important role in many renewable energy sources as the efficient electrochemical energy storage devices.Metal cobalt-based compound have been extensively studied as the electrode material of supercapacitors,but the sluggish ion/electron diffusion and poor electron conductivity cause low electrochemical performance and limite their developement in the field of energy storage.Therefore,the research of new metal cobalt-based compound electrode materials is the key to achieving the supercapacitors with excellent energy storage performance.Cobalt-based sulfides exhibit superior electrochemical properties than their corresponding oxides in terms of electrical conductivity,redox reactions.Metal cobalt-based selenides offer better electrical conductivity,higher electrochemical activity and better application prospects than the corresponding sulfides.This paper briefly describes the energy storage mechanisms and the research progress of various types of electrode materials for supercapacitors,proposes the use of composite highly conductive materials to prepare cobalt-based sulfides and cobalt-based selenides and their composites.The morphology,elemental composition,pore size distribution and other structural characterisation of the prepared electrode materials are studied.The pattern between their electrochemical properties and structure,and the effect on the energy storage of supercapacitors are measuried.The work is as follows:1.The hierarchical porous Co₉S₈ nanowire arrays situ-grown on nickel foam using zeolite imidazole framework(ZIF-67)as the sacrificial template are successfully prepared by multi-step experiments.The introduction of ZIF-67 template is favourable to the formation of porous structure,provides large surface areas and short diffusion channels,reduce volume expansion as well as ensuring structural stability.ZIF-67-derived Co₉S₈ shows a large specific capacitance due to its abundant redox sites and high electrical conductivity.The electrochemical results show the Co₉S₈/NF electrode has the specific capacitance of 2952.4 F g^-1 at the current density of 1 A g^-1 and retention rate of 75.5%at 20 A g^-1,and 75%after 5000cycles.The assembled Co₉S₈/NF//AC ASC shows a remarkable specific energy of 48.2 Wh kg^-1 at the high specific power of 799.8 W kg^-1 with 77%retention after 5000 cycles.2.To improve the rate performance and cycling properties of the cobalt-based compounds,CoSe₂ nanosheets are successfully prepared on nickel foam using Se powder as the selenium source with ZIF-67 as the sacrificial template.The effect of hydrothermal temperature on the structure and electrochemical properties of the prepared CoSe₂ electrode materials is investigated.The results show that the CoSe₂-180 electrode material exhibits more excellent electrochemical performance,owing to its unique nanosheet structure provides a high active surface and fast ion transport channels.The sheets are cross-linked to form a network structure providing fast electron transfer channels.The ZIF-67 backbone is not only a fast electron transfer pathway,but also adapts to the volume expansion of selenide during the redox process.Thanks to its unique porous structure,the CoSe₂-180 electrode exhibits a high specific capacitance(1939.6 F g^-1),outstanding rate performance(84%)and excellent cycling performance(84%).The CoSe₂-180//AC ASC device achieves a high energy density of 45.6Wh kg^-1 at a specific power of 800.8 W kg^-1 with outstanding cycling performance(80%retention after 5000 cycles).3.Co₃S₄/CoSe₂ nanosheets are successfully prepared on nickel foam by the hydrothermal method using both S and Se powders as a sulfide and selenium sources.The effects of different amounts of sulfide and selenium sources on the morphology and electrochemical properties of the prepared cobalt-based selenium-sulfide are further investigated.The synthesised nanosheet structure provides a large surface area that serves both as a storage site for electrolyte ions and facilitates ion transport and diffusion within the electrolyte and material,enhancing electrochemical behaviour.The influence of the synergistic effect of S^2-and Se^2-on improving the electrochemical properties of the material was analysed.The electrochemical results showed that the Co₃S₄/CoSe₂(1:1.5)electrode has a specific capacitance of 2952.4 F g^-1 at 1 A g^-1 and a retention rate of 72.7%at 20 A g^-1.The specific capacitance was able to be maintained 72.5%of the initial vlaue after 5000 cycles.In addition,Co₃S₄/CoSe₂(1:1.5)//AC ASC assembled with Co₃S₄/CoSe₂(1:1.5)as the positive electrode showed a maximum energy density of 29.3 Wh kg^-1 at a specific power of 799.1 W kg^-1 with75%retention after 5000 cycles.