Preparation And Electrochemical Performance Of Transition Metal Hydroxide Composites Based On MOFs

Nowadays,the rapid development of human beings leads to a straight upward trend in the demand for energy,and there is a growing demand for electrochemical energy storage.However,the most widely used secondary battery is often complained that it cannot meet the needs of human development due to its low power density.At the moment,there are also growing calls for a new type of energy storage device to replace secondary batteries.Supercapacitors happen to have a high energy density,but they also have the advantages of long cycle life,high specific capacitance and so on.As a result,more and more researches are being carried out on supercapacitors.For now,the challenge for supercapacitors is to improve their energy density.Among them,scientists are already focusing on improving the electrochemical properties of electrode materials to produce supercapacitors with even greater advantages.For the electrode materials of supercapacitors,previous conclusions have been drawn that the composite of multiple materials is the only effective method to prepare electrode materials for supercapacitors with excellent performance.At the same time,the great potential of new materials,metal organic frameworks(MOFs),has been discovered by more and more scientists.Therefore,we are developing to give full play to the advantages of MOFs into electrode materials.In summary,this paper is carried out according to the following works:(1)Based on the precursor of Ni Co-MOF-74,ternary Ni(OH)₂/Co(OH)₂/Mg(OH)₂with the rod-like structure was designed and synthesized via the seed method and the solvothermal method.By adjusting the amount of load,it can be found that when the mass ratio of load and template is1:1,NCM-2(Ni(OH)₂/Co(OH)₂/Mg(OH)₂-2)is successfully prepared and exhibits the the best electrochemical characteristic.For NCM-2 electrode,the 1481.78 F g^-1specific capacitance at 1A g^-1and the 92.68%cyclic stability after 5000 cycles prove the superiority of NCM-2.In addition,when we used NCM-2 as the positive and AC(activated carbon)as the negative to make a supercapacitor,the specific capacitance is up to 156 F g^-1and the energy density can reach to 55.69Wh kg^-1as the current density is 1 A g^-1.(2)The combination of carbon materials and active metal materials can prepare newfashioned and hierarchical porous materials,and the synergistic effect of the two materials can effectively optimize the performance of electrode materials.The electrode material(GO/Zn-Co-Ni LDH)of this article is prepared via combining a trimetallic hydroxide(Zn-Co-Ni LDH)derived from ZIF-8 and a carbon material(graphene oxide)used to enhance the conductivity.The emerging trimetallic hydroxide with a high theoretical specific capacitance is easy to reunite frequently.Based on the characteristics of ZIF-8,we can change the structure of Zn-Co-Ni LDH,which can solve the agglomeration phenomenon.And in the end the cotton-like fluffy structure of GO/Zn-Co-Ni LDH(abbreviated as GO/ZCN)let itself exhibit a 1873.33 F g^-1high specific capacitance when the current density shows as 1 A g^-1.We can also utilize compressed GO/Zn-Co-Ni LDH to acquire striking cycling stability(97.71%capacitance retention at 10 A g^-1after 5000 charge-discharge circulation)which is improved greatly compared to the uncompressed GO/Zn-Co-Ni LDH.The device of GO/ZCN//AC ASC displays a high energy density of 64.91 Wh kg^-1at a power density of 800 W kg^-1and excellent cycling stability performance(a specific capacitance retention of 98.96%is obtained after 10000 charge-discharge cycles).(3)Through a series of treatments of cotton,biomass carbon material can be obtained,and named as CCO.Composite material CCO/Co-Ni-Mn LDH combining the biomass carbon(CCO)and the MOF material(ZIF-67)is successfully prepared depending on seed method,solvent ther mal method and calcining.Simultaneously,the impact of load upon the sacrifice template CCO/ZIF-67 and electrochemical performance of CCO/Co-Ni-Mn LDH is further studied,the optimized CCO/Co-Ni-Mn LDH-2(CCO/CNM-2)composite electrode exhibits better electrochemical properties as well.In the case of a current density of 1 A g^-1,the high specific capacitance of 2995.56 F g^-1can be calculated in CCO/CNM-2,in addition,as the current density is set to 10 A g^-1,the capacitance value can still maintain 91.8%of initial capacitance after experiencing 5000 cycles of charging and discharging.The voltage window of the assembled CCO/CNM-2//activated carbon(AC)asymmetric supercapacitor can reach 1.6 V,and the maximum energy density can reach 104.98Wh kg^-1.