Preparation And Electrochemical Properties Of Metal Oxide Nano-heterostructures Based On Metal Organic Frameworks

Metal oxides were widely used as electrode materials for supercapacitors due to their pseudo-capacitive properties.The composite structure of multicomponent metal oxide,due to the synergistic effect of various metal elements,is expected to improve the electrochemical performance of electrode materials.Especially,binary oxide nanoheterostructure is the focus of current research in supercapacitor electrode material.Meta-Organic Frameworks?MOFs?is made of metal ions and organic ligands through coordination bond self-assembly with periodic porous structure.MOFs exhibit high surface area,uniform pore distribution and high chemical stability and have been used in a widely application.Currently,MOFs have been demonstrated as promising templates to create novel porous metal oxides by pyrolysis to remove organic coordination polymer chain.This method has many advantages,such as:metal oxide derivatives can inherit porous structure of MOFs precursors;the synthesis process can be easy controlled,et al.In this work,we developed a facile method to fabricate a metal oxides nanoheterostructure via a solid–solid conversion process using core–shell MOFs@MOFs as a template.The electrochemical energy storage properties of materials have been investigated.Herein,we set out to prepare core–shell structured ZIF-8@ZIF-67,which consists of ZIF-8 crystals as the core and ZIF-67 crystals as the shell,through a seed-mediated growth method.By thermal treatment of ZIF-8@ZIF-67 crystals,the Co₃O₄/Zn O nanoheterostructure is successfully prepared.The prepared metal organic framework precursor and Co₃O₄/Zn O nanoheterostructures were characterized by using the means of X-ray diffraction?XRD?,transmission electron microscopy?TEM?,electron spectroscopy?EDS?,thermal gravimetric analysis?TG?.The electrochemical properties of the nanoheterostructure were investigated and the application of the material as an electrode for supercapacitors is demonstrated.Specific work as follows:1.Preparation and characterization of ZIF-8@ZIF-67 core-shell structure.Core-shell ZIF-8@ZIF-67 was synthesized by a solvothermal method under100? using 2-methylimidazole as ligand and Co/Zn as metal elements.The structure and morphology of the crystals have been investigated.The results show that the core-shell structure of ZIF-8@ZIF-67 was synthesized successfully.2.Preparation and characterization of Co₃O₄/ZnO nanoheterostructure based on ZIF-8@ZIF-67 precursor.?1?The heterogeneous structure of Co₃O₄/ZnO was prepared through a two-steps pyrolysis method using ZIF-8@ZIF-67 as the template.Firstly,ZIF-8@ZIF-67 was calcined under a N2 at 500 o C.After that,the obtained samples were then calcined under O2 flow at 350 o C.The properties of sample were investigated deeply.?2?The sample of Co₃O₄/ZnO nanoheterostructures was prepared as working electrode.The electrochemical performance of materials was evaluated in a three-electrode configuration and asymmetric supercapacitor.Cyclic voltammetry?CV?and galvanostatic charge discharge test?GDC?were used on the electrochemical test.The experimental results show that,in the alkaline aqueous electrolyte?6M,KOH?,the specific capacitance of electrode material is up to 415 F/g when the current density is 0.5 A/g.By testing at high current density and high charge discharge cycles,the results show that the material has better rate performance and cycle stability.An asymmetric supercapacitor was fabricated by utilizing the as-synthesized nanoheterostructures as the positive electrode and activated carbon as the negative electrode in 6 M KOH.The symmetric supercapacitors achieve a high energy density up to 43.2 Wh/kg at a power density of 1401 W/kg.?3?As a comparative study,Co₃O₄ and ZnO powder was prepared through a solid–solid conversion process using ZIF-67 and ZIF-8 as template.Co₃O₄-Zn O mixture was prepared by grinding and sintering of Co₃O₄ and Zn O powder.We compared the electrochemical properties of the mixture of Co₃O₄-Zn O and Co₃O₄/Zn O nanoheterostructures.The results show that the electrochemical properties of nanoheterostructures Co₃O₄/Zn O are superior to those of the mixture of Co₃O₄-Zn O.The reason is due to its special nanoheterostructure.3.Preparation and characterization of Co₃O₄//Zn O heterostructures based on thick shell ZIF-8@ZIF-67 precursor.By changing the quality ratio of metal ions and organic ligands,the shell thickness of the core-shell ZIF-8@ZIF-67 is changed.We used the same preparation method to obtain Co₃O₄//Zn O heterostructure and investigated its electrochemical performance.We compared electrochemical properties of Co₃O₄//Zn O heterostructure based on the ZIF-8@ZIF-67 precursors with different shell thickness.