Controllable Synthesis Of Flower-like Nickel Metal-organic Phosphate And Their Derivatives And Performance Research Of Energy Storage

Electrochemical energy storage devices are considered as promising systems to solve current environmental problems such as fossil fuels and carbon dioxide emissions,as well as to meet the needs of emerging markets such as electronic and electric vehicles in the future.As an electrochemical energy storage device,supercapacitors(SCs)have attracted much attention because its rate performance and cycle stability are better than those of batteries.Considering that the electrochemical performance of SC s strongly depends on the active material of the electrode and the corresponding charge storage mechanism.Therefore,new materials with high capacity,fast charge storage kinetics and electrochemical stability are needed to overcome the limited energy density.In recent years,metal-organic framework(MOFs)materials and their derivatives have attracted great attention as advanced electrode materials because of their various topologies,adjustable porosity,clear crystal structure and rich redox active centers.In this paper,nickel metal-organic phosphate framework(Ni-MOPh)(Ni-MOF),Ni-MOPh derivatives and bimetal-organic phosphate framework(NiCo-MOPh,NiMn-MOPh)were synthesized successively by hydrothermal reaction and direct calcination method,using nickel,cobalt and manganese as metal sources and phenylphosphonic acid as organic ligands.Using the above materials as electrode materials,the electrochemical energy storage behavior of SCs was studied.1.Nickel acetate was reacted with phenylphosphonic acid by solvothermal method for 2 h.Ni-MOPh(B1)material with 3D flower-like structure self-assembled from two-dimensional(2D)nanosheets can be prepared.This material is used as the electrode material of SCs.In the three-electrode system of 3 M KOH,it has a high specific capacitance of 403.1 F g-1(0.5 A g-1).At a higher current density(5.0 A g-1),it experienced 2000 cycles of continuous charge and discharge,and its capacity retention was more than 62.9%.In addition,the aqueous devices made of B1 and activated carbon(AC)can also show the best electrochemical activity in the same series of electrode materials.2.Using Ni-MOPh material as precursor,a series of Ni-MOPh derivatives were obtained by direct calcination.The electrochemical properties of three-electrode SCs and two-electrode aqueous devices for A-(Ni-MOPh)-D(600℃)were studied.After electrochemical tests,it is found that it has higher capacity and better electrochemical stability than the precursor B1.This is because it retains the unique flower-like superstructure of the precursor,allowing full diffusion of electrolytes and rapid electron transfer,while the new pore structure exposes more electrochemical active centers in the electrolyte.In addition,in the amorphous state,the lattice energy of the material is lower,which is beneficial to chemical intercalation/de-intercalation or redox reaction.Therefore,the reasonable design of derived materials is an effective way to improve the electrical conductivity of MOF.3.Based on the experimental method of synthesizing 3D flower-shaped Ni-MOPh materials,NiCo-MOPh and NiMn-MOPh were synthesized by simple one-step solvothermal synthesis.By adjusting the molar ratio between metals,the best microstructure and optimal charge storage capacity can be obtained.The results show that Ni:Co=5:1(NiCo-MOPh-3)and Ni:Mn=10:1(NiMn-MOPh-2)are the most suitable molar ratio.The samples synthesized under these conditions are used as electrode materials.In the three-electrode system,the specific capacitances of NiCo-MOPh-3 and NiMn-MOPh-2 can reach 694.4 and 569.1 F g-1,respectively,when the current density is 0.5 A g-1.In the water system devices,after 2000 cycles,the capacity retention of NiCo-MOPh-3//AC and NiMn-MOPh-2//AC is as high as 95.7%and 91.4%,respectively.The reason why bimetallic MOPh has excellent performance is that more Ni2+vacancies can be generated when Co2+and Mn2+ with small ion radius partially replace Ni2+in Ni-MOPh.This leads to more free holes in NiCo-MOPh-3 and NiMn-MOPh-2,which helps to improve the electrical conductivity of the material.