Fabrication Of UiO-66 And CNTs/Fe₂O₃ For Aqueous Asymmetric Supercapacitor

Supercapacitor is a new type of energy storage device between the secondary battery and the traditional capacitor.Compared with the secondary battery,the supercapacitorexhibitshigherpowerdensity,besidescleanerand environment-friendly.At the same time the supercapacitor demonstrates the advantages of high energy density,high rate pe rformance and long cycle life?up to10w times?distinguished from the traditional capacitor.Currently,the key scientific issues of electrochemical energy storage is to obtain materials or devices with high energy density,large power density and long cycle life.In addition,the cost and environmental protection are often considered in the design process.This article focuses on the existing key scientific problems,besides selects and prepares suitable positive or negative electrode materials to assemble high performance aqueous asymmetric supercapacitors.Firstly,starting from the design and preparation of cathode materials,porous porous UiO-66?HP-UiO-66?was successfully prepared by using one-step hydrothermal and acid etching Zn-MOFs with porous MOFs as raw materials.XRD and XPS results showed that the successful removal of Zn-MOFs in bimetallic MOFs and the single UiO-66 were obtained.N itrogen adsorption and desorption isotherm and pore size distribution results showed that the specific surface area and pore volume of HP-UiO-66 were 874 and 379 m² g^-1,respectively.The mesopores of HP-UiO-66 were mainly distributed in 2.5 to 4.5 nm and 5.5 to 40 nm,the total pore volume and micropore volume were 1.08 and 0.35 cm³ g^-1,respectively.As a supercapacitor electrode material,the specific capacitance of the UiO-66 and HP-UiO-66 electrodes was 101.5 and 849 F g^-1,respectively,at a current density of0.2 A g^-1.The electrochemical performance of HP-UiO-66 was significantly enhanced compared to the bare UiO-66,because the hierarchical porous structure contains mesopores and micropores,and HP-UiO-66 has higher surface area,pore volume and sufficient surface defects than UiO-66,which caused by unstable Zr⁴⁺.In addition,an aqueous asymmetric supercapacitor PC//HP-UiO-66 was assembled using porous carbon PC as negative electrode and HP-UiO-66 as positive electrode,respectively.The highest energy density of PC//HP-UiO-66 ASC was 32 W h kg^-1 power density of240 W kg^-1.When the current density was inc reased from 0.3 A g^-1 to 10 A g^-1,the power density was 7486 W kg^-1 and the energy density was 16 W h kg^-1.Secondly,in order to investigate the universality of hierarchical porous MOFs prepared by one-step hydrothermal and acid etching methods,three hierarchically porous UiO-66 were successfully prepared using one-step hydrothermal and acid etching Fe-MOFs?changed Fe/Zr molar ratios,the difference was marked as HP-UiO-66¹,HP-UiO-66²,and HP-UiO-66³,respectively.?N itrogen adsorption and desorption isotherm and pore size distribution results show that all three materials have a hierarchical porous structure.The specific surface areas of UiO-66,HP-UiO-66¹,HP-UiO-66²,and HP-UiO-66³ were 490,446,229,and 683 m² g^-1.As a supercapacitor electrode material,the specific capacitance of the UiO-66,HP-UiO-66¹,HP-UiO-66²,and HP-UiO-66³ electrodes were 101,732,707,and 773 F g^-1,respectively,at a current density of 0.1 A g^-1.Furthermore,the highest capacitance of HP-UiO-66³ was 7.6 times than 101.5 F g^-1 of bare UiO-66.The electrochemical performance of HP-UiO-66³ was significantly enhanced compared to bare UiO-66,because HP-UiO-66³ has a hierarchical porous structure providing more active sites for supercapacitor energy storage of electrode materials.At present,transition metal oxides still dominate the field of asymmetric supercapacitors,in which Fe₂O₃ and MnO₂ exhibit good electrochemical activity,low cost,environmental protection,and high content in the earth's crust.However,the weak conductivity of Fe₂O₃ and MnO₂ greatly limits their rate performance and cycling stability.We use CNTs and ferrate/permanganate ions to obtain CNTs/Fe₂O₃and CNTs/MnO₂ by direct redox reaction,which is simple and effective,low cost,and clean and environmentally friendly.In the three-electrode electrochemical properties test,both CNTs/Fe₂O₃ and CNTs/MnO₂ exhibited excellent capacitance and rate performance.At a current density of 1.25 mA cm^-2,the specific capacitances of CNTs/Fe₂O₃ and CNTs/MnO₂ were 497.8 mF cm^-22 and 322 mF cm^-2,respectively.The specific capacitance of CNTs/MnO₂ remained unchanged after 2000 CV cycles.97%of the value shows excellent cycle stability.In addition,CNTs/Fe₂O₃ was used as the anode,CNTs/MnO₂ was used as the cathode,and 6 M LiCl was used as the electrolyte.The maximum operating voltage of CNTs/Fe₂O₃//CNTs/MnO₂ ASCs can be stabilized at 1.6 V.When the current density is 7.5 mA cm^-2,the specific capacitance of this capacitor is 207.5 mF cm^-2.According to equations,the energy density of this capacitor is 77.5 mW h cm^-3 when the power density of the capacitor is 6305 mW cm^-3.At a current density of 12.5 mA cm^-2,after 2,000 cycles of charge and discharge,the capacitor retains 50.1%of its initial capacitance,exhibiting better cycle stability.