The Rational Synthesis Of Cobaltosic Oxide-based Composites Using ZIF-67 As Precursor And Their Electrocatalytic Oxygen Evolution Reaction And Lithium Storage Properties

The development of sustainable energy technologies is important for mitigating increasing environmental pollution and thus achieving sustained economic growth.The Oxygen Evolution Reaction?OER?is a central process in new energy conversion and storage,and rechargeable lithium-ion batteries?LIBs?are considered to be important energy storage devices.Commercial OER electrocatalysts are based on precious metals such as Ir and Ru,but scarcity,high cost and poor stability limit their wide application.In addition,the theoretical low specific capacity(372 mAh g^-1)and low lithium ion transport coefficient(10^-7 to 10^-10 cm² s^-1)of graphite,the anode material for commercial lithium ion batteries,hinder the development of high-performance LIBs.In meeting these challenges,the selection and preparation of effective electrode materials is key to the electrocatalytic OER and LIBs reactions.Cobalt trioxide?Co₃O₄?is considered as a promising electrocatalyst with good stability and catalytic activity,and has a high theoretical specific capacity(890 mAh g^-1)as an anode material for lithium-ion batteries.However,its poor conductivity,volume expansion and nanoparticle agglomeration in the process of lithium inlaying and other deficiencies reduce its electrochemical properties.Constructing nanomorphs,hollows,defects and composites with carbon materials are effective strategies to improve electrocatalytic OER performance and lithium storage performance.In view of the above considerations,in this thesis,three cobalt tetroxide matrix composites were oriented synthesized using ZIF-67 as precursor and their electrocatalytic OER and lithium storage properties were investigated.The research is as follows.?1?Hollow carbon embedded Co₃O₄/reduced graphene oxide nanocomposites and their electrocatalytic evolution propertiesThe combined graphene oxide surface-rich oxygen functional groups are hydrophilic,and deionized water was cleverly selected as a dispersion medium for ZIF-67 and graphene oxide.The ZIF-67/graphene oxide composites were obtained by the precipitation method to achieve uniform particle size ZIF-67 growth in situ on the surface of graphene oxide;the hollow carbon embedded Co₃O₄/reduced graphene oxide nanocomposites were synthesized by controlled cracking and oxidation orientation using the above composites as precursors.Characterized by X-Powder Diffraction?X-Ray Diffraction,XRD?,Scanning Electron Mmicroscope?SEM?,Transmission Electron Microscopy?TEM?,X-ray Photoelectron Spectroscopy?XPS?,it was demonstrated that the prepared Co₃O₄ has a hollow structure,the nanoparticles are homogeneous with graphene oxide in the aqueous solvent and have a smaller nanometer size than those prepared in methanol solvent,XPS proved that the bonding energy of Co is lower than that of Co₃O₄ reported in the literature,indicating that there is a force between Co₃O₄ and reduced graphene oxide.The unique hollow structure and compounding with reduced graphene oxide enhances the OER activity by increasing the active site and enhancing the electrical conductivity.Test results indicate that hollow Co₃O₄@C/rGO-W is at 0.1 M KOH exhibited low overvoltage?382 mV?and tafel slope(62 mV dec^-1).?2?Synthesis of defective hollow nitrogen-doped carbon-covered Co₃O₄composites with defects from ZIF-67 etched in tannic acid as precursor and their electrocatalytic oxygen and lithium storage propertiesXRD demonstrated that the etching of tannic acid did not change the crystal shape of ZIF-67,because the etching agent tannic acid is a large molecular group,which can itself cover the surface of ZIF-67 to prevent the surface from being etched.SEM and TEM showed that HCo₃O₄@NC has a homogeneous hollow structure,and EPR and XPS proved that the surface of HCo₃O₄@NC is rich in defective active sites.As a result,this composite exhibited good OER performance with a low overpotential?360 mV?and low tafel slope(61 mV dec^-1)in 0.1 M KOH.The unique hollow structure can buffer the volume expansion of Co₃O₄ during lithium storage and combine with the high conductivity nitrogen-doped carbon to increase the lithium ion transfer speed during the charge/discharge process,thus improving the cyclic and multiplicity performance of Co₃O₄.The first reversible capacity is 1316.8 mAh g^-1 at a current density of 100 mA g^-1.And after 100 cycles,the specific capacity of the hollow Co₃O₄@NC composite material remains at 1296.5 mAh g^-1,and at a high current density of 1000 mA g^-1,the specific capacity can still be maintained at 408.5mAh g^-1,which is higher than the graphite theory specific capacity.?3?Design synthesis of hollow Co₃O₄@NC/rGO compositesOn the basis of the second work,we introduced the conductively superior reduced graphene oxide into the tannic acid etched ZIF-67 system and obtained HCo₃O₄@NC/rGO composites by simple low-temperature pyrolysis.XRD proved that the graphene composite did not change the crystal shape of Co₃O₄.SEM results showed that Co₃O₄ was uniformly loaded on the graphene surface.BET characterization proved that the etched ZIF-67 was higher than ZIF-67@TA(93.383m²g^-1)after composite with graphene.The specific surface area of ZIF-67@TA/GO is140.8 m²g^-1.The TEM characterization shows that HCo₃O₄@NC presents a homogeneous hollow structure and the nanoparticles are composite with graphene oxide,and the EIS test shows that the semicircle of the HCo₃O₄@NC/rGO electrode is smaller than the semicircle of HCo₃O₄@NC in the high/medium frequency region.As a result,HCo₃O₄@NC/rGO shows high electron conductivity and low charge transfer resistance compared to HCo₃O₄@NC.