Design And Optimization Of ZIF-based Asymmetric Supercapacitors

As an efficient and clean energy storage device,supercapacitors have attracted widespread attention in scientific research and industry owing to their high power density,short charging and discharging time,long cycle life,safety and wide operating temperature range.Electrode materials are an important part of supercapacitors,and their composition and structure largely determine the electrochemical performance of supercapacitors.In terms of material selection,porous carbon material is recognized as an excellent electrode material due to its large specific surface area,hierarchical pore structure,good electrical conductivity,good physical and chemical stability and environmental friendliness;Co₃O₄ has more active sites and can undergo a reversible redox reaction.It is a good electrode material for pseudocapacitors.From the structural design point of view,the asymmetric design of the electrodes,that is,the use of different positive and negative electrodes can make full use of the different potential windows of the two electrodes,maximize the working voltage window of the entire supercapacitor,and increase its energy density.Therefore,based on the above ideas,two electrode materials of biomass-derived porous carbon and ZIF-67-derived Co₃O₄were selected for design and assembled into an asymmetric supercapacitor,and the electrochemical performance of the electrode material was further optimized by adjusting the composition of the electrode material in order to improve the energy density of a supercapacitor.In addition,the introduction of the miniaturization design ideas of supercapacitors,while ensuring its energy storage performance,facilitates the application of wearable electronic products.The main research contents and results are as follows:A two-step process of alkaline hydrothermal pretreatment and high-temperature pyrolysis has successfully prepared a highly porous carbon material derived from the seeds of waste biomass Platanus acerifolia.The prepared HK-PSC800 porous carbon material has a very large specific surface area and a microporous/mesoporous structure.In addition,the metal oxide Co₃O₄ was prepared by high-temperature carbonization of ZIF-67 as raw material,and the asymmetric supercapacitor Co₃O₄//HK-PSC800 was assembled with HK-PSC800 as the negative electrode and Co₃O₄ as the positive electrode.Due to the combination of the advantages of carbon materials and metal oxides,the asymmetric supercapacitor has a specific capacitance of 58.8 F g^-1(scan rate of 10 m V s^-1),and a maximum energy density of 27.23 Wh kg^-1.It can be charged and discharged after 3000 cycles.After that,the cycle stability can reach 92.6%.In order to further increase the energy density of the device,another asymmetric supercapacitor was designed.The new Co₃O₄@HK-PSC800 positive material was obtained by pyrolyzing ZIF-67@HK-PSC800 in the air.Then it is assembled with HK-PSC800 negative electrode to form a Co₃O₄@HK-PSC800//HK-PSC800asymmetric supercapacitor.The working potential window of the prepared asymmetric supercapacitor is expanded to 1.5 V,which can provide a high energy density as high as 36.7 Wh kg^-1 at a power density of 1115 W kg^-1.Even if the current density is increased to 5 A g^-1,the energy density can still maintain 18.2 Wh kg^-1 at a power density of 9927 W kg^-1.At a current density of 1 A g^-1,after 3000 charge-discharge cycles,the initial capacitance retention rate is 95.2%.The combination of ZIFs-based metal oxides and prepared porous carbon provides feasibility for the design and optimization of asymmetric supercapacitors for high voltage and high energy density.Using a simple mask-molding method,a paper-based interdigital planar micro-supercapacitor was prepared,and ZIF-8-C800@PANI@G electrode material was prepared by simple low-temperature polymerization,and further by changing the composite material with the quality of ZIF-8-C800,three composite materials were prepared:ZIF-8-C800@PANI@G-1,ZIF-8-C800@PANI@G-2and ZIF-8-C800@PANI@G-3.Finally,it is found that the prepared ZIF-8-C800@PANI@G-2 MSC has the best electrochemical performance,its area capacitance is as high as 164 m F cm^-2,and the energy power density is 19.06μWh cm^-2.This paper-based micro-supercapacitor design strategy has opened up a new path for portable and wearable electronic devices.