Synthesis And Electrochemical Performance Of Metal-Organic Framework Materials Constructed By Azocarboxylic Acid Ligands

Metal-organic frameworks(MOFs)built of organic ligands and metal centers exhibits many unique advantages,benefiting from intrinsic structural advantages including high porosity,large specific surface areas,tunable pore sizes,and so on,MOFs have exhibited great potentials in the wide fields of gas storage/separation,catalysis,energy storage and conversion,drug delivery,electrode materials,etc..For electrochemical applications,developing highly efficient and stable multi-functional metal-organic framework electrochemical materials is of prime importance for supercapacitors and electrocatalytic applications.This dissertation uses redox-active metal center or cluster and redox-active organic linker to construct new type of MOFs from the molecular level,which is directly applied to electrochemical applications,such as electrochemical water splitting and supercapacitors.It mainly divides into the following two parts:(1)Cu MOF crystal and Ni MOF crystal are synthesized by selecting redox-active organic linker and metal centers.Then,an in-situ solvothermal method is developed and applied successfully to prepare their nano materials,named as nano-Cu MOF 1-10 and nano-Ni MOF1-10,under a facile and mild condition.In particular,it is found that various morphology and nano sizes for these nano-Cu MOFs and nano-Ni MOFs can be effectively controlled based on solvent effect and surfactant effect.All structures of as-synthesized nano materials are demonstrated by XRD and FT-IR results,which are consistent with the ones of their crystals.More importantly,electrochemical properties can be greatly enhanced from bulk crystals of Cu MOF and Ni MOF to nano-Cu MOF and nano-Ni MOF materials.Their nano materials exhibit excellent supercapacitor performance.Among of them,nano-Ni MOF 3 as supercapacitor electrode shows the best specific capacitance of 1024.44 F g^-1 at a current densities of 1 A g^-1 and maintains very good cycling stability.Its capacitance retention of 49.07%can be observed after 5000 cycles at a current density of 5 A g^-1.In addition,a high-performance asymmetric supercapacitor(ASC)was fabricated by using nano-Ni MOF 3 and activated carbon as two electrodes.A specific capacitance of 38.65 F g^-1 was obtained at a current density of 0.5 A g^-1,and 109%capacity of the initial capacitance at 3 A g^-1 after 5000cycles can be observed.(2)A series of bi-metal organic frameworks(MOF),namely as Fe₂M-MOF(M=Fe,Co,Ni,Zn,Mn)under a simple and mild condition,in which Fe₃ cluster as basic build unit was replaced by the second kind of metal center,at the same time a redox-active organic linker was adopted.The Fe₂M-MOF system as multifunctional catalyst realizes great improvement of electrocatalytic performance for OER and HER.Among of them,Fe₂Co-MOF catalyst exhibits extremely low overpotential of 339 m V at the current density of 10 m A cm^-2 and very small Tafel slope of 36.2 m V dec^-1 in alkaline electrolyte for OER.This result has far exceeded commercial catalyst Ir O₂.Meanwhile,Fe₂Zn-MOF manifests excellent HER activity with small overpotential of 221 m V at 10 m A cm^-2 and low Tafel plots of 174 m V dec^-1.In addition,these catalysts exhibit good long-term stability under working conditions.Systematic investigations are used to explain the enhanced electrocatalytic mechanism.This work provides a facile and effective method for the preparation of multi-functional catalysts for energy conversion applications based on pristine MOF material with redox-active metal centers and organic linkers.