Synthesis And Capacitance Properties Of Nickel/cobalt-Based Bimetallic MOFs

Along with the widespread use of energy storage devices,supercapacitors,as a new energy storage component with excellent performances,have become a research hotspot in energy storage in recent years due to the high power density,high energy density,long cycle life and green environmental protection.The difference in the energy storage effect of supercapacitors depends mainly on the structure of electrode materials.Metal-Organic Frameworks(MOFs)are a new type of porous crystalline material with various structures,adjustable pore size,multi-metal sites,ultra-low density,etc.,which have been widely studied as supercapacitor electrode materials.In this paper,4,4’-diphenyl phthalic acid was used as an organic ligand,nickel ions and cobalt ions were used as metal sources,and bimetallic NiCo-MOFs with different morphologies were synthesized by hydrothermal or solvothermal methods.The effects of Ni/Co/BPDC molar ratio,reaction temperature and time on the microstructure and capacitance properties of nickel-cobalt bimetallic MOFs were investigated.The structure-activity relationships of NiCo-MOFs microstructure and its capacitance properties were revealed,and some methods were proposed on the structure adjustment of MOFs.The optimal NiCo-MOFs particles were further heat-treated at different temperatures in a nitrogen atmosphere to further optimize their structure.The structures were characterized by SEM,TEM,XRD,FTIR,Raman,EDS elemental mapping,nitrogen adsorption-desorption analysis,etc.Some electrochemical measurements were performed in an aqueous alkaline electrolyte,such as cyclic voltammetry(CV),galvanostatic charge and dischargent(GCD),electrochemical impedance spectroscopy(EIS),rate capability and cycling stability.The main research contents were as follows:1)A series of NiCo-MOF particles were synthesized by solvothermal method in DMF-EtOH,DMF-DMSO and DMF solvents,respectively.They microscopically presented three-dimensional porous microspheres assembled from one-dimensional nanosheets.The NiCo-MOF-1 synthesized under DMF-EtOH system showed the best morphology and performance.The optimal conditions were 1:1:1,12 h,and 170℃.NiCo-MOF-1 was mainly mesoporous,and the BJH average pore diameter was 15.1 nm,which can be stable in air below 350°C.The specific capacitance of NiCo-MOF-1 reached 936.8 F/g(1.0 A/g).It had a specific capacitance of 449.2 F/g after 1000 charge and discharge at a current density of 1.0 A/g,and the retention rate was 48.8%.The structures were homogenized by low-temperature heat treatments in nitrogen atmosphere,and its specific capacitance was reduced to 656.4 F/g(1.0 A/g),but it had a good rate capability and cycling stability.After 3000 cycles,the specific capacitance was still 486 F/g,and the capacitance retention rate was 81.0%.The NiCo-MOF microspheres assembled in different solvents had different capacitance properties due to the different degree of looseness of the sheet assembly.The specific capacitances of NiCo-MOF prepared in DMF-DMSO and DMF was only 781.1 F/g and 404 F/g at 1.0 A/g,respectively.2)The NiCo-MOF particles were successfully prepared by hydrothermal method,which microscopically demonstrated three-dimensional block structures assembled by one-dimensional micro-nano rods.The formation of one-dimensional nanorods was mainly due to the directional growth of NiCo-MOF on rod-shaped-micelle templates formed by the reaction of BPDC with sodium hydroxide.The optimum conditions were 1:1:2,12 h,170℃.The electrochemical activation of multiple cyclic voltammetry and multiple charge and discharge tests in an alkaline electrolyte revealed that the NiCo-MOF can be rapidly converted to hydroxide and the specific capacitance was increased to 990.7 F/g from the initial value of 156.5 F/g at 1.0 A/g,a high capacity of 1056.6 F/g at 0.5 A/g and a 405.0 F/g capacity at 20 A/g.After 3,000 cycles of charge and discharge at a current density of 1.0 A/g,the specific capacitance was still 553.2 F/g,and the capacitance retention rate was 55.80%.After a low-temperature heat treatment at 400℃ in a nitrogen atmosphere,the specific capacitance was 550.1 F/g at 0.5 A/g,and the capacitance was still 368.0 F/g at 10 A/g.The heat-treated NiCo-MOF had a good rate capability and cycling stability.The capacity retention was 89.6%at 1.0 A/g after 3000 cycles.