Controllable Preparation And Application Of Bimetal Organic Framework(BMOF) As Electrode Material In Supercapacitors

The high power density of supercapacitors(SCs)are attractive in applications such as regenerative braking systems,uninterruptible power supplies,and power tools.However,as an energy storage device,its low energy density is difficult to adapt to the current market application.But,electrodes are cores of the SCs,so improving the energy density of SCs may start with designing and improving the performance of electrode materials.The metal organic frameworks(MOFs)materials has attracted wide attention due to their higher surface area,ordered pore channels and tunability.Despite the above advantages,its low specific capacitance,poor chemical stability,low conductivity and large impedance all require to be improved.The introduction of a second metal ion into MOFs to prepare bimetal organic frameworks(BMOFs)may help us to solve the above problems.The introduced metal ions can promote the rearrangement of atoms in the MOFs lattice,to increase dislocation density and microscopic stress,thus enhancing the conductivity and stability of the material.Two metals in the BMOFs enhance their specific capacitance synergistically.To improve the electrochemical performance of MOF,bimetallic Co/Ni-MOF and Co/Zn-MOF with different morphology and structures are designed to explore the influence of morphology,structure and components on the capacitance properties,and to reveal the energy storage mechanism of BMOFs.The specific results are as follows:(1)Co/Ni-MOFs with different morphologies and crystal structures designed by adjusting the ratio of cobalt and nickel ions,and then explore how these changes to improve its electrochemistry performance.It is shown that regulating the proportion of metals can increase the dislocation density of the BMOFs,to boost the conductivity of the material,perhaps due to the newly introduced metal ions promoting the rearrangement of atoms in the BMOFs and increase the active sites.Regulating the ratio of cobalt and nickel ions can also increase the microscopic stress of the material and improve its stability.Meanwhile,the introduced nickel ions in the BMOFs play a synergistic effect with cobalt ions,which is beneficial to increase the specific capacitance.The Co/Ni-MOF-2 prepared at the ratio of cobalt and nickel ions is 1:2 has the best electrochemical performance.It is because that the thinned wall hollow spherical structure can increase the contact area of electrode and electrolyte,greatly shorten the ions transmission path.From the structural point of view,this is because:the dislocation density and microscopic stress of the Co/Ni-MOF-2 are the largest,so it has the best conductivity and stability.The excellent electrochemical performance of Co/Ni-MOF-2 is:the maximum specific capacitance(2608.8 F g-1);the highest retention(59%);the smallest impedance(Rct is 0.57 Ω,Rs is 0.42 Ω)and the best conductivity(τ0 is 16.7 s).The electrode reaction dynamics reveals that the charge storage of the Co/Ni-MOF-2 is mainly controlled by the diffusion process.The self-assembled asymmetric supercapacitor Co/Ni-MOF-2//AC(the cathode material is Co/Ni-MOF-2,and the anode material is corn silk-derived biochar(AC))has a high voltage window of 1.7 V.When the energy density is 48.4 Wh kg-1,the power density is 850.02 W kg-1,and the capacitance retention rate is 85%after 5000 charge-discharge cycles.(2)A series of Co/Zn-MOFs with different morphologies and crystal structures were controllably synthesized,the influence of the introduction of zinc ions with a larger radius on the morphology and structure of the prepared BMOFs was researched,and the reasons for the improvement of its electrochemical performance were explored.The results show that the introduction of zinc ions increases the interlayer spacing of the BMOFs,which facilitates the insertion of electrolyte ions.The increased dislocation density is conducive to improve the conductivity and the reinforced microscopic stress increase the stability of the Co/Zn-MOFs.The cobalt and zinc ions in the BMOFs play a synergistic effect to boost the specific capacitance of the material.Meanwhile,the surface linear Co/Zn-MOF-2 prepared at the cobalt-zinc ratio of 1:1 has the most excellent metal synergistic effect,the largest dislocation density and microscopic stress,so it has the best electrochemical performance.The super performance of Co/Zn-MOF-2 is the largest specific capacity(1605.4 F g-1 under 1 A g-1),the smallest impedance(Rct is 0.42 Ω,Rs is 0.64 Ω),and the highest conductivity(τ0 is 19.8 s).The electrode reaction kinetics of Co/Zn-MOF-2 reveals that charge storage is mainly controlled by the diffusion process.The asymmetric supercapacitor Co/Zn-MOF-2//AC has a high voltage window of 1.7 V.When the energy density is 37.2 Wh kg-1,the power density is 851.1 W kg-1.The above results further indicate that Co/Zn-MOF-2 has good application potential as an electrode material.