Synthesis And Supercapacitive Behaviors Of Metal-Organic Frameworks/Graphene Composite Materials

Electrochemical capacitors,also known as supercapacitors,as a important class of energy storage devices with high power density,long cycling life and fast recharge capability,have promising applications as backup or auxiliary power sources in electric vehicles and other electronic devices.The properties of the electrode materials are important factors for the supercapacitors.The electrode materials of ES include carborn,transition metal oxides and conducting polymers.Each of these classes of supercapacitors has strengths and weaknesses:carbonbased materials operate at a very high charge/discharge rate with a long life cycle but have low capacitance,while metal oxide and conducting polymers materials have high capacitance but their redox reactions lead to short cycle life.Recently,Studies on supercapacitors have mainly been focused on perparation of high performance electrode materials with both high power density and energy density.Metal-organic frameworks(MOFs),as a new class of highly porous crystalline hybrid mateials obtained by the assembly of metallic ions or clusters acting as "nodes" and organic ligands acting as linkers,have emerged rapidly in the past years for a wide range of potential applications including gas storage/separation,molecular recognition and ion exchange.Compared with other conventional porous materials,the flexible and various pore structures of MOFs have been the main reason for the particular attention.MOFs not only have high specific surface area but also have unique structures incorporating pseudo-capacitive redox centers that can be directly used as a new type of electrode material.In the present work,a series of MOF materials containing Iron,cobalt,nickel with graphene composites materials were successfully synthesized via solvothermal method,and firstly used as supercapacitor electrode in an aqueous alkaline solution.The relationships between the intrinsic characteristics of the MOF-based composites and its electrochemical properties were investigated in detail.Prussian blue/reduced graphene oxide(PB/rGO)nanohybrids were synthesized via a simple polyvinylpyrrolidone(PVP)-assisted polyol reduction method undermild conditions.It indicates that ratios of starting materials allow a good control on loading and morphology of PB/graphene hybrids,and at ratio of K3Fe(CN)6/GO of 1:2,PB nanocubes get completely embedded into the defect of porous graphene matrices.Graphene nanosheet was decorated by a large amount of cubic PB nanocrystals with the size of 100-200 nm.PB/rGO(1:2)electrode was evaluated with three-electrode configuration at 10 mV/s in 1 M KN03 aqueous electrolyte in the range of-0.2 to 0.8 V,PB/rGO(1:2)exhibits the highest specific capacitance of 251.6 F/g at a scan rate of 10 mV/s and an excellent cycling stability with 92%specific capacitance retained after 1000 cycle tests.The energy density can reach 54.34 Wh/kg at a power density of 1562.5W/kg.The significant enhancement in electrochemical performance over PB/rGO nanocomposites can be attributed to a positive synergetic effect that the PB nanocube interlocked dispersion of rGO sheets superimposes pseudocapacitance from PB on double-layer capacitance from rGO sheets.A series of three-dimensional network structure Co2(OH)2BDC/rGO composites materials have been sucessfully synthesizedby one step solvothermal method and used as an electrode materials for a supercapacitor.It exhibited large specific capacitance,high rate capability and good cycling stability.The reaction temperature,reaction time and GO loading are important factors in determining the phase,morphology and electrochemical property of as-prepared Co2(OH)2BDC/rGO composites.The optimum condition was achieved by the synthesis temperature of 110?,reaction time 24 h and 2 mg/ml of grapheme oxide addition.Co-MOF/rGO composite deliver a large specific capacitance of 225 F/g at a scan rate of 5 mV/s in 1 M KOH electrolyte.Simultaneously,cyclic stability revealed 6.01%drops in specific capacitance after 500 cycles.Nickel-based MOF[Ni3(HCOO)6]DMF/rGO with BET surface area of 163.15 m2/g and pore size of 0.5 nm have been sucessfully synthesized by nickel nitrate as metal precursor,formic acid as organic ligand via solvothermal method.When the synthesis temperature was 80 ?,time 24 h and 8 mg/ml of graphene oxide,the unique structure of[Ni3(HCOO)6]·DMF coated on rGO sheets could be obtained.The highest specific capacitance reached 1068 F/g in 1M KOH electrolyte at the scan rate of 5 mV/s;The capacitance retention was over 96.28%even after 500 cycles,Electrode has lower internal resistance and higher pseudo capacitance.It opens a way for the design of MOFs/rGO composite electrode materials for supercapacitor.