Synthesis And Capacitance Performances Of MOFs Loaded On Multi-dimensional Carbon Materials

Effective use of renewable energy,such as solar energy and wind energy,is the best way to alleviate energy crisis and environmental problems.Due to the intermittent supply of renewable energy,many advanced energy storage devices,such as supercapacitors,secondary lithium-ion batteries and superconducting coils,are used to store excess energy.Among them,supercapacitors have attracted people’s attention because of their high charge and discharge rates,good cycling stability,high power density and environmental friendliness.Electrode materials are an important component of supercapacitors,which determine device performances.Metal-Organic Frameworks(MOFs)exhibit high specific capacitance because MOFs have both carbon-material-like double layer capacitance attributed to porous structures and metal-oxide/hydroxide-like pseudocapacitance ascribed to variable-valence transition metal ions with high redox activity,which have been widely used as electrode materials.However,its poor structural stability and conductivity limit its applications in the field of electrochemistry.Therefore,compounding MOFs with carbon materials possessing good conductivity and structural stability may be an effective strategy to solve this problem.In this paper,various MOFs-based composite materials were successfully synthesized by using MWCNT,GO or HA-Na layers as substrates,Ni2+(or Co2+)as central ions,and 4,4’-biphenyldicarboxylic acid(BPDC)as organic ligands.SEM/EDX,TEM,XRD,Raman,FTIR,XPS and other tests were used to characterize structures of composite materials.Cyclic voltammetry(CV),galvanostatic charge and dischargent(GCD),electrochemical impedance spectroscopy(EIS),rate capability,cycling stability and other tests were conducted to analyse electrochemical performances of composite materials.Main research results are elaborated as follows:1)The flower-string-like NiCo-BPDC/MWCNT composites have been successfully synthesized by using MWCNT as a substrate,Ni2+(or Co2+)as central ions and 4,4’-biphenyldicarboxylic acid(BPDC)as organic ligands via a simple solvothermal approach.Compared with the pure hydrangea-like NiCo-BPDC microspheres,these flower-string-like structures are more loosely assembled by NiCo-BPDC nanosheets grows around MWCNT.The MWCNT has a guiding effect on the NiCo-BPDC growth through some bonds or affinity between MWCNT and NiCo-BPDC,and its presence does not affect the formation of the NiCo-BPDC crystals.Many well-dispersed flower-string-like structures can be achieved by adjusting MWCNT dosages.When the amount of MWCNT is 2 mg,the composite has the best capacitance performances.The suitable MWCNT participation can enhance the conductivity and the specific capacitance of the MOFs,and simuneouly reduce their internal resistance and charge transfer resistance.The NiCo-BPDC/MWCNT-2 composites exhibit excellent specific capacitance,good rate capability and excellent cycling stability in a 2 M KOH aqueous electrolyte.It demonstrates a large specific capacitance of 1010 F/g at 0.5 A/g,a good rate capability(760 F/g even at 10 A/g),a good cycling stability and reversibility with the capacitance retention of 828 F/g(100%of the initial capacitance)and coulombic efficiency of approximate 98%at 5 A/g after 3000 cycles.The assembled NiCo-BPDC/MWCNT-2//graphite paper asymmetric supercapacitor has excellent cycling stability and high energy density.The asymmetric supercapacitor holds 100%of the initial value,88.4%of the maximum value at 5 A/g after 3000 GCD cycles.The maximum energy density of asymmetric supercapacitor is 19.7 Wh/kg at a power density of 250 W/kg.2)The Ni(Co)-BPDC/GO composites were synthesized using graphene oxide(GO)as a substrate,Ni2+(or Co2+)as central ions,and 4,4’-biphenyldicarboxylic acid(BPDC)as organic ligands via a hydrothermal approach.In view that rich carboxyl groups on the GO surface can position Ni2+/Co2+ from the surrounding solvent through coordination bonds,strip-shaped MOFs crystals were further grown of strip-shaped MOFs crystals on the GO surface in parallel to form a well-dispersed Ni(Co)-BPDC/GO composite material.In addition,the presence of GO did not affect the growth and crystalline structure of the Ni-BPDC.When the mass of GO is 3 mg,the composite material has the best capacitance performances.Appropriate amount of GO can improve the rate characteristics of the MOFs.Compared with the Ni-BPDC/GO and Co-BPDC/GO composites,the NiCo-BPDC/GO composites exhibit higher specific capacitance,rate capability and cycling stability due to the synergistic coordination of Ni and Co ions.The comparison found that the NiCo-BPDC/GO-3 has the highest specific capacitance,the most excellent rate capability and cycling stability in a 6 M KOH aqueous electrolyte..It exhibits a large specific capacitance of 800 F/g at a current density of 1 A/g,and 500 F/g(capacitance retention is 62.5%)even at 50 A/g.At the current density of 5 A/g,its specific capacitance can remain 450 F/g after 10000 cycles and the retention is 62%.The as-assembled NiCo-BPDC/GO-3//rGO asymmetric supercapacitor also has a large specific capacitance of 134 F/g at 0.5 A/g and a high energy density of 31.6 Wh/kg at a power density of 327.7 W/kg.Its specific capacitance can hold 84%of the initial value at 5 A/g after 5000 cycles.3)The HA-NiCo-BPDC composites with high recombination and high capacitance performance was successfully prepared by hydrothermal method by using sodium humate layer(HA-Na)as substrate,Ni2+(or Co2+)as central ions,and BPDC as ligands.The effects of the content of HA-Na,reaction temperature and Ni2+/Co2+ molar ratio on structures and capacitance performances of HA-NiCo-BPDC composites was discussed in detail.The best preparation process of the composites was determined as follows:the content of humate is 0.941 g HA-Na/g BPDC,(Ni2++Co2+)/BPDC molar ratio is 3:1,Ni2+/Co2+ molar ratio is 1:1,reaction temperature and time are 190℃ and 12 h,respectively.In the HA-NiCo-BPDC composites,the NiCo-BPDC nanosheets are loaded on the surface of the humate layer as a flower-shaped assembly.The HA-NiCo-BPDC(1:1)composites have high specific capacitance(762 F/g at 5 A/g and 410 F/g even at 100 A/g),high rate capability(74%capacity retention from 0.5 A/g to 25 A/g),low internal resistance and charge transfer resistance in a 2 M KOH aqueous electrolyte.The as-fabricated HA-NiCo-BPDC//rGO asymmetric supercapacitor has a large specific capacitance of 101 F/g at 2 A/g,a wide operating potential window of 1.4 V,a good cycling stability(86%capacity retention after 5000 cycles)and a high energy density(high energy density of 31.3 Wh/kg at a power density of 352 W/kg).