Structure-controlled Synthesis Of MOFs For Energy Storage Applications

Metal-organic frameworks(MOFs)are ideal materials for supercapacitors due to their large specific surface area,high porosity,versatility and tailability.The composition and structure of MOFs play a crucial role in their electrochemical performance.In this paper,the composition and structure of MOFs were regulated and optimized,and a series of MOFs were prepared by simple and green synthesis method,so as to improve their electrochemical performance.In this paper,Ni²⁺was set to be the metal center,terephthalic acid(PTA)and trimesic acid(BTC)were chosen to be the organic ligand,the mixed ligand method has been adopted,and the ratio of MOFs organic ligand was regulated and optimized and a kind of Ni-MOFs with albizia flower-like spheres@nanosheets structure was prepared.As a regulator,BTC has replaced a part of PTA in Ni-MOFs.The similar molecular size and composition of BTC and PTA can ensure that the introduction of BTC would not destroy the crystal structure of Ni-MOFs,and it has been combined into the Ni-MOFs structure greatly.The albizia flower-like spheres can increase the space between the nanosheets and prevent the stacking of two-dimensional nanosheets,therefore a larger contact area between the electrode and the electrolyte can be provided in the process of charging and discharging.After the introduction of BTC,the adsorption energy of OH^-on the mixed organic ligands can be increased,and the redox reaction kinetics of Ni-MOFs can be improved.When the BTC induction ratio is 20%,the specific capacitance is 920 F?g^-1when the prepared Ni-MOF-2 is 1 A?g^-1 the specific capacitance is 562 F?g^-1when the prepared Ni-MOF-2 is 20 A?g^-1.By adjusting the ratio of Ni²⁺and Co²⁺in the metal center and using BTC as the organic ligand,the hollow spherical Ni/Co-MOFs were successfully prepared by a simple one-step hydrothermal method.The inducer Co²⁺has a significant effect on the structure of MOFs,which can effectively regulate the transformation of the structure of MOFs from yolk-shell to hollow to solid.When the mole ratio of Ni²⁺to Co²⁺is 5:1,Co²⁺can induce the formation of hollow spherical Ni/Co-MOF-1.The space inside the hollow sphere can provide a large enough contact area between the active site and the electrolyte.The regulation of the amount of Co²⁺introduction can reduce the charge transfering resistance,and improve the electrical conductivity,which are more conducive to the adsorption of OH^-on Ni/Co-MOF-1,so that more active sites can participate in the redox reaction.At 1 A?g^-1,the specific capacitance of Ni/Co-MOF-1 electrode can reach 1498 F?g^-1 and still retain 67%of the initial capacitance at 30A?g^-1.The energy density of the Ni/Co-MOF-1//AC asymmetric supercapacitor which is prepared with Ni/Co-MOF-1 as positive electrode and activated carbon(AC)as negative electrode is up to 56 W?h?kg^-1 when the power density is 0.8 k W?kg^-1.Then,we fixed the ratio between the bimetallic center(Ni²⁺/Co²⁺)and used BTC as the organic ligand to regulate the ratio between the metal center and the organic ligand.With the decrease of the proportion of metal centers,the surface of hollow spheres Ni/Co-MOFs becomes rough,and the bulges grow continuously,gradually becoming more and more uniform.The bulging surface can provide more redox active sites,while the hollow spherical structure can make the contact between the electrolyte and the active sites better.As the proportion of metal centers continues to decrease,MOFs generate aggregated spheres,which limit the electrochemical properties of the materials.When the ratio of metal center and organic ligand is1:1,the specific capacitance of the electrode can reach 1656 F?g^-1 at 1 A?g^-1 and maintain 68%of the initial specific capacitance at 20 A?g^-1.Therefore,by adjusting and optimizing the composition and structure,the electrochemical properties can be effectively adjusted to obtain the high-performance pseudocapacitor supercapacitor materials.