In Situ Dynamic Monitoring Of The Formation Mechanism And Electrochemical Performance Of ZIFs-Derived Carbon-Based Materials

As a kind of porous carbon material,ZIFs derived carbon processes the advantages of abundant nitrogen doping and ultra-high specific surface area,which is a promising electrode material for supercapacitors（SCs）.However,the shortcomings such as easy adhesion agglomeration,small pore diameter and large resistance restrict their practical applications in SCs.Therefore,exploring the formation mechanism of ZIFs derived carbon materials is a key factor to improve the electrochemical performance of SCs.In this paper,using ZIFs as the starting point,a variety of derived carbon materials was fabricated by adjusting the carbonization temperature and compounding other compounds with excellent electrical conductivity.The formation mechanism and regulation mode were investigated and realize the controllable preparation of the derived carbon.The relationship between structure,composition and electrochemical performance was characterized and analyzed.1.Dynamic observing the formation mechanism of ZIF-8（Zn）derived porous carbon by in-situ TEM and their double layer capacitance characteristics.In virtue of the large surface area,high porosity and low boiling point of Zn,ZIF-8（Zn）are promised to be ideal self-templates for the fabrication of hierarchical porous carbon architectures.However,the Zn volatilization and microstructural resolution of Zn-ZIFs derived porous carbon are still not clearly defined.Herein,porous carbon derived from ZIF-8 was prepared by pyrolysis route as electrode material for supercapacitors.The pyrolysis mechanisms of ZIF-8,especially Zn volatilization and microstructural resolution processes,were dynamically studied by in-situ heating TEM and ex-situ detecting techniques.The porous carbon skeleton maintains the pristine dodecahedral shape of ZIF-8 with slight shrinks and collapses,which possess large specific surface area and high microporosity.By increasing the carbonization temperature,Zn was evaporated and N was decomposed out of the porous carbon skeleton.Interestingly,a quantity of Zn element can still be detected in the carbon skeleton even if at 1100^oC.This result is quite different from the previous literatures,for they reported that metal-free porous carbon can be prepared when the temperature was higher than the boiling point of Zn（907^oC）.Cs-corrected HAADF-STEM image and synchrotron radiation spectra indicate that the residual Zn element is uniformly anchored in the carbon skeleton as N/C-coordinated single Zn atom.Electrochemical performance indicates that the specific capacitance of ZIF-8 derived N-doped porous carbon is proportional to the specific surface area.The porous carbon synthesized at1100^oC,which shows the highest specific surface area of 1269 m² g^-1,exhibits the highest specific capacitance of 210.0 F g^-1 at 1.0 A g^-1.A symmetric supercapacitor（SSCs）assembled by the optimal ZIF-8 derived porous carbon delivers an energy density of 16.0Wh kg^-1 at a powder density of 1063.0 W kg^-1,which also possesses excellent cycling stability of about 90%after 10000 cycles.This work is of great significance to deeply understand the microstructural resolution and properties of Zn-ZIFs derived porous carbon,guiding their practical applications.2.Fabrication of bimetallic CoZn-ZIFs derived N-doped porous Co/C electrode materials and their double layer capacitance characteristics.The bimetallic CoZn-ZIFs prepared by hydrothermal method were used as the precursors.N-doped porous Co/C electrode materials were prepared by Ar-annealing treatment of the precursors at 900^oC.The influence of molar ratio of Zn and Coon the nanostructures and capacitance characteristics are investigated systematically.The results show that the Conanoparticles are conducive to the formation of CNTs in the Co/C nanocomposites.As the size of Conanoparticles grows up,both the diameter and length of CNTs are increased first and then decreased.Conanoparticles with small size are beneficial to catalyze the formation of CNTs.The electrochemical performance can be boosted by increasing the amount of CNTs in the N-doped porous Co/C electrode materials:the optimal specific capacitance can reach201 F g^-1 at 1 A g^-1,and the rate capability is 80%at 10 A g^-1.These are improved in comparison with the ZIF-8 derived N-doped porous carbon that reported in the above chapter;the power density and energy density of the assembled SSC are 1896.9 W kg^-1 and20.8 Wh kg^-1 at 1 A g^-1;The capacitance retention rate maintains 90%after 10000 charge-discharge cycles.The remarkable electrochemical properties are mainly attributed to the formation of CNTs,which can improve the electron conductivity and enhance the transmission speed of carriers.3.Package of Mg_xFe_3-xO₄//N-doped porous Co/C asymmetric supercapacitors and the capacitance characteristics.To utilize the advantages of remarkable cyclic stability of ZIFs derived carbon and overcome its low energy density,we assembled asymmetric supercapacitors（ASCs）using nitrogen-doped porous Co/C prepared in the previous chapter as the negative electrode and Mg-doped Fe₃O₄ nanoparticles as the positive electrode.The capacitance characteristics were investigated,and the potential application of ZIFs derived carbon in SCs was discussed.As a transition metal oxide,Mg doped Fe₃O₄nanoparticles exhibit pseudocapacitance feature,which shows a specific capacitance of681 F g^-1 at 1 A g^-1.The power density and energy density of the assembled ASCs are improved to be 1896.9 W kg^-1 and 29.2 Wh kg^-1 at 1 A g^-1,respectively.However,the cycle stability decreases:the specific capacitance remains 76%after 5000 cycles of charge and discharge.These preliminary results indicate that the energy density of assembed asymmetric supercapacitors can be increased,but the cycle stability is decreased.