Designing Synthesis And Electrochemical Performances For Supercapacitor Of MOFs Composites

With the consumption of fossil fuels and the deterioration of environmental pollution,supercapacitances,as a new type of green energy storage device,have attracted widespread attention.Metal-organic frameworks?MOFs?is a new kind of porous material with high porosity,low density,large specific surface area,abundant pore structure,adjustable pore size and diversity of topological structure.It has become a very promising electrode material for supercapacitors.Using MOFs as precursors to synthesize advanced functional materials,such as nanoporous carbon materials and metal oxide nanomaterials,which is a new hotspot in the field of electrochemical energy storage.The adjustable pore size of MOFs is beneficial to the rapid migration and diffusion of ions in charge/discharge process.However,the non-conductive nature of MOF itself is one of the greatest obstacles to achieve high capacitance and rate performance.Therefore,the combination of MOF derivatives with nano-carbon materials,metal oxides or conductive polymers can provide an efficient electron transport pathway,enhance the conductivity of MOFs and the electrochemical energy storage performance of composites.Based on the above viewpoints,different kinds of MOF-based composites were prepared and electrochemical energy storage properties were studied in this paper.The main research contents and results are as follows:?1?ZIF-8 regular dodecahedral rhombus was synthesized by simple chemical synthesis with zine acetate and 2-methylimidazole as raw material.Then chitosan carbon aerogels?CAs?was used as substrate,the ZIF-8 polyhedron was assembled on the surface of CAs by simple in-situ deposition method,and the porous ZIF-8 derived carbon/carbon aerogels?ZCCA?composite was prepared by calcining at high temperature.By characterizing the microstructures and morphologies of two samples,it can be concluded that ZCCA composite has large specific surface area and abundant porous structure,which can promote ion migration and electrolyte penetration,and improve the electrochemical energy storage performance.The results of electrochemical tests show that the specific capacitance of ZCCA composite can reach 204.4F/g under the current density of 0.5 A/g,while that of ZIF-8 derived carbon?ZC?is 118.8 F/g.After 2000 cycles,the capacitance retention of ZCCA is 90%.The composite as electrode material of supercapacitor has higher charge/discharge performance and better cycle stability.?2?ZIF-67 polyhedron was synthesized by simple chemical synthesis with cobalt source and 2-methylimidazole as raw material.The ZIF-67 nanocrystals were attracted to CAs nanosheets by in-situ deposition method,and ZIF-67 derived oxide/carbon aerogels?ZIF-67derived Co₃O₄/CAs?composite was obtained through simple heat treatment.The microstructures and morphologies of MOF derived oxides and ZIF-67 derived Co₃O₄/CAs composite were characterized,and the electrochemical energy storage properties of the two kinds of electrode materials were compared.The results show that the discharge capacitance of ZIF-67 derived Co₃O₄/CAs composite electrode can reach 298.8 F/g under the current density of 0.5 A/g,while the specific capacitance of ZIF-67 derived Co₃O₄ is only 111.4 F/g.After 1000cycles,the capacitance loss of composite is only 18%,showing superior capacitance,high charge/discharge efficiency and good cycle stability.These results demonstrate that ZIF-67derived Co₃O₄/CAs is a promising highly performance supercapacitance electrode material.?3?In order to further improve the electrochemical performance of MOF derived oxide/carbon aerogel composite,based on the above,we use cellulose carbon aerogel?CA?instead of CAs as substrate,and introduce NiMoO₄ nanorods with ideal specific capacitance.We synthesized NiMoO₄ nanorods by hydrothermal synthesis and heat treatment,then used them as substrates to make ZIF-67 polyhedron grown into ZIF-67/NiMoO₄/CA in NiMoO₄nanorods,and further to obtain ZIF-67 derived Co₃O₄/NiMoO₄/CA?Co₃O₄/NiMoO₄/CA?ternary composite electrode material.The Co₃O₄/NiMoO₄/CA ternary composite electrode material has good rate performance and excellent cycle stability compared with single material.In addition,Co₃O₄/NiMoO₄/CA ternary composite electrode material and activated carbon?AC?were assembled to form Co₃O₄/NiMoO₄/CA//AC asymmetric capacitors with specific capacitance and specific energy of 125.4 F/g and 34.1 Wh/kg,respectively.When the current density of 0.5 A/g,the capacity retention rate is 84%after 2000 cycles,showing excellent cycle stability and electrochemical energy storage behavior.?4?In order to improve the electrochemical properties of MOF derived carbon/carbon aerogel composites,based on the above research,we have introduced PANI for preparation of MOF derived carbon/PANI porous carbon/cellulose carbon aerogel?ZIF-8-C/PANI-C/CA,ZPCC?ternary composite with good conductivity and thermal stability.By means of simple electrochemical deposition,the PANI was assembled into the surface and voids of CA and carbonized at high temperature to obtain PANI porous carbon/cellulose carbon aerogel?PCC?composite.As a substrate,ZIF-8 polyhedron was grown in PC sheet porous structure,and ZPCC ternary composite was obtained after carbonization at high temperature.The results of electrochemical tests show that the specific capacitance of ZPCC ternary composite reaches391.5 F/g under the current density of 0.5 A/g.When the current density increases to 10.0 A/g,the specific capacity can keep 51.7%of the initial value.And after 2000 cycles under the current density of 5.0 A/g,the specific capacitance loss rate is only 8.8%.Compared with ZC and PCC,the ZPCC ternary composite electrode material has a good rate performance and excellent cycle stability.