ZIF-67 Template Synthesis Of Layered Transition Metals Hydroxide Composites And Their Supercapacitor Properties

The rapid depletion of non-renewable energy sources in the 21st century is causing a serious environmental crisis,which is prompting people to seek clean and sustainable energy sources.At present,the promotion of carbon peak,carbon neutrality has become the world consensus,green and sustainable energy and related energy storage equipment has great market potential.Supercapacitors（SCs）are favored by many researchers because of their high power density,fast charge and discharge ability,long life and strong reliability.However,the energy density of supercapacitors is not ideal,which limits their commercial application.Therefore,it is still a challenging task to explore the fabrication of high performance supercapacitors by designing electrode materials with reasonable structure.Layered bimetallic hydroxide are a promising class of two-dimensional layered materials.Their electrochemical activity is mainly due to the oxidation-reduction of metal atoms and the electron transport during charge-discharge processes,the interaction between OH^-and Layer Double Hydroxides（LDHs）can also significantly improve the electrochemical properties.However,two issues that need to be addressed.First,the aggregation of materials in the electrode manufacturing process and the diffusion of ions in the thicker electrode are insufficient,which are usually hindered by the reduction of ion accessible surface area and the extension of ion transport distance,a slow reaction rate usually results in poor electrochemical performance.The other is the volume expansion during the cycle,the active material comminuted or agglomerated,resulting in poor rate performance and cycling.In order to solve these problems,Ni Co-LDH were fabricated by using ZIF-67as a self-template,and two other electrode materials with different morphologies were prepared by modification.The energy storage mechanism of the electrode material and the cooperation between the materials were studied in detail,and the charge transfer at the interface was accelerated by the morphology control,and the electrical conductivity of the electrode material was gradually increased,the cycle stability and energy density are improved.At the same time,a simple supercapacitor is constructed by assembling the composite material reasonably,and the specific work is as follows:（1）Using ZIF-67 as template and cobalt source,Ni Co-LDH with hollow three-dimensional nano-cage structure were constructed by hydrothermal method and sacrificial template method.The influence of the formation mechanism of microstructure on electrochemical properties was investigated by various testing methods.The results of electrochemical performance test show that the optimum specific capacitance of Ni Co-LDH electrode material with hollow three-dimensional nano-cage structure is 1684 F g^-1at the current density of 1 A g^-1,and the optimum specific capacitance of Ni Co-LDH electrode material is 10 A g^-1at the current density of10 A g^-1,the doubling performance can still be maintained at 60.4%.In addition,the asymmetric supercapacitor assembled by Ni Co-LDH and activated carbon（AC）has an energy density of 23.47 Wh Kg^-1at a power density of 750 W Kg^-1and a capacity retention rate of 70.5%after 5000 cycles of charge and discharge,its excellent performance is confirmed.（2）Based on the advantage of Ni Co-LDH hollow three-dimensional nano-cage structure,the Ni Co-LDH nano-sheets were uniformly grown/wrapped in polypyrrole nanotubes（PPy NTs）,and the composite electrode materials of PPy NTs/Ni Co-LDH were prepared.By adjusting the quality of PPy NTs,it was found that the best electrochemical properties were obtained when the content of PPy NTs was 30 mg.The results of electrochemical test show that the PPy NTs/Ni Co-LDH electrode material has a high specific capacity of 1802 F g^-1at the current density of 1 A g^-1.Assembled PPy NTs/Ni Co-LDH as positive pole,the results show that when the power density is760.2 W Kg^-1,the energy density reaches 35.94 Wh Kg^-1at a stable output voltage of1.5 V,the specific capacitance retention of PPy NTs/Ni Co-LDH was 65.3%after 5000cycles at high current density,which further proved that PPy NTs/Ni Co-LDH had certain potential as positive electrode,on the one hand,it solves the problem that the aggregation of Ni Co-LDH and the higher contact resistance lead to the lower overall electrochemical performance,on the other hand,it can be used as the support of reaction substrate,stabilizes the formed mechanical structure and provides additional pseudocapacitors.（3）In order to effectively alleviate the problem of poor cycling stability of composite materials caused by the volume expansion of PPy NTs during charging/discharging in the previous chapter,the MWCNTs/PPy/Ni Co-LDH composite with high specific capacity and stable structure was successfully constructed.The functionalized carbon nanotubes were used as the reaction substrate to improve the performance of the pseudocapacitor materials,and the polypyrrole nanoparticles could improve the electrical conductivity,the mixed valence of Ni and Co contributes to the formation of electron and ion defects,which improves charge storage properties.MWCNTs/PPy/Ni Co-LDH can reduce the kinetic barrier of redox through synergistic effect,thus resulting in strong interfacial electron transfer.MWCNTs/PPy/Ni Co-LDH in a typical three-electrode system has excellent specific capacity(2114 F g^-1at 1 A g^-1)and rate capability(77.5%capacity retention at 10 A g^-1)in a typical three-electrode system.Asymmetric supercapacitor（ASC）was fabricated by using MWCNTs/PPy/Ni Co-LDH as cathode material and active carbon（AC）as anode material.When the power density is 800.4 W Kg^-1,the energy density can reach 40.6Wh Kg^-1,and after 5000 cycles of charge and discharge at 10 A g^-1,the capacitance retention of ASC is 87.6%,showing excellent cycle stability.