Preparation And Electrochemical Properties Of MOF@ Graphene-based Composites

Due to the energy crisis and environmental pollution caused by the excessive use of fossil fuels,the research on new high-efficiency electrochemical energy storage systems such as lithium-ion batteries?LIBs?and supercapacitors has become one of the most important research fields in the world.In this paper,a series of MOF@graphene-based composites were prepared by in-situ self-assembly method.As the precursor,nitrogen-doped carbon materials with different structures were obtained by high-temperature carbonization,and used as supercapacitor electrodes and anode materials for LIBs.The main research contents are as follows:?1?ZIF-8@GQDs?GQDs:graphene quantum dots?composite was prepared as a precursor by in-situ self-assembly method.A new porous carbon material NC@GQDs was prepared by high temperature calcination and etching,this material has a multi-stage pore structure and a high specific surface area(668 m² g^-1).As an electrode of a supercapacitor,it has excellent electric double layer capacity and long cycle stability.After 9999 cycles at 2 A g^-1,the current density maintains a specific capacitance of 130 F g^-1 with a coulomb efficiency approaching 100%.The composite can also be used as an anode material for LIBs.At a current density of 100 mA g^-1,the discharge capacity of the electrode is maintained at 493 mA h g^-1 after 200 cycles,with high reversible capacity and good cycling stability.The excellent electrochemical performance of NC@GQDs can be attributed to the synergistic effect of MOFs-based porous carbon and graphene quantum dots.?2?ZIF-67@GQDs composites were successfully synthesized by a simple in-situ self-assembly method.As a precursor,a new porous carbon material Co/NC@GQDs was prepared by high temperature calcination and etching.The material has a specific surface area of 454.17 m² g^-1 and is mainly mesoporous.The XPS data indicates that the Co/NC@GQDs composite has a high nitrogen content and graphite nitrogen content of30.4%,thereby exhibiting better electrochemical performance than Co/NC.When used as an anode material for LIBs,Co/NC@GQDs electrode exhibits high reversible capacity and good rate performance and cycle stability.At the current density of 150 mA g^-1 and 500 mA g^-1,the discharge specific capacity after 150 cycles reached 477.4 and 353.4 mA h g^-1.?3?Graphene oxide?GO?was used as a two-dimensional nanosheet substrate,and a bimetallic zeolite imidazole skeleton?Co/Zn-ZIFs?was used as a MOF template.Co/NC@GO has been synthesized through carbonization of an in situ grown Co/Zn-ZIFs on graphene oxide?GO?.The composite is composed of graphite-carbon coated Co nanoparticles,N-doped carbon skeletons and graphene skeletons,and has a layered micropore-macropore structure.When Co/NC@GO was used as the anode material of LIBs,at the current density of 200,500 and 1000 mA g^-1,the discharge specific capacity reaches689.8,533.2 and 456.1 mA h g^-1 after 400 cycle respectively,and the discharge specific capacity is stable.When the current density changes from 50 to 100,200,500 and 1000 mA g^-1,the discharge capacity of Co/NC@GO electrode is 800.1,703.4,613.5,548.6 and 463.2mA h g^-1,respectively.It exhibits excellent cycle stability and rate performance.