Mesoscopic Structure Of Carbon Materials Based On The Solid-state Decomposition Of Molecular Templates

The synthesis of carbon materials with well-defined mesostructures is an important way to regulate the properties and functions of carbon materials.The preparation of carbon materials with controlled mesoscopic structure is often based on the template method which generally requires filling carbon-rich precursors inside templates followed by carbonization and removal of the templates.Because the preparation of mesostructured templates and the process of filling carbon-rich precursors are complicated or cumbersome,it is necessary to develop a simple synthetic strategy to prepare carbon materials with controllable mesoscopic structures.Metal-organic frameworks?MOFs?contain a large number of metal ions or metal clusters and variety of organic ligands.MOFs can be used as molecular templates through solid-phase transformation by thermal decomposition.Their metal ions or clusters can form nano-metal elemental or nano-metal oxide particles,and the carbons will evenly coat around them.According to the above strategy,it is possible to prepare carbon materials with mesoscopic structures that are controllable.Based on the carbonization of MOFs,this dissertation mainly investigated the following two aspects.1.At first,cobalt-ion-bridged MOFs?ZIF-67?were selected to synthesize graphitized carbon materials using in situ generated cobalt metal nanoparticles as template agent and catalyst both.The graphite shell networks were obtained by calcination of ZIF-67 at 450°C for 48 h and after acid treatment.The reason was that the in situ formed cobalt nanoparticles could not only provide steric hindrance,but also prompted graphitization of the surrounding carbon materials.After the graphitic shells were connected to each other,the networked structure could be obtained.The graphitic shell networks were the networks of low-porosity carbon materials composed of a few graphitic shells.According to the characterizations of thermogravimetric analysis?TG?,X-ray diffraction analysis?XRD?,scanning electron microscopy?SEM?,high resolution transmission electron microscope?HRTEM?and X-ray photoelectron spectroscopy?XPS?,the formation mechanism of graphitic shell networks was proposed.At the same time,based on the graphitic shell particle model and transmission electron microscopy?TEM?characterization analysis,we clarified the formation mechanism of the capsule structure.Since the graphitic shell networks have the characteristics of high degree of graphitization,network structure,and high electron transfer rate,it was suitable for high-speed and stable storage of sodium ions.The graphitic shell networks were fabricated as the negative electrode of a sodium ion battery by us.The capacity reached 250 mAh g^–1 and 90 mAh g^–1 at current densities of 50 m A g^–1 and 2000 mA g^–1,respectively.At a current density of 1000 mA g^–1,the capacity remained at 120 mAh g^–1 and the Coulombic efficiency approached 99%after1000 cycles.At last,the graphitic shells networks were applied to the anode electrode of sodium ion batteries,which showed a good rate performance and cycle stability.2.We selected alumina-oxygen cluster-bridged MOFs?Al-MIL-53?to synthesize porous carbon materials.Alumina nanoparticle generated by pyrolysis worked as a template agent.Al-MIL-53 powders were prepared by solvothermal method.The morphology of Al-MIL-53 powders were regulated by changing the mixing ratio of solvent.The rod-shaped Al-MIL-53 was selected as the molecular template.SEM and XRD analysis showed that the alumina nanoparticles were formed in situ during the pyrolysis of Al-MIL-53.These nanoparticles were inert steric hindrance templates that could limit the spatial position of carbons.After the alumina was removed by sodium hydroxide solution,the mesoporous carbon materials with a large specific surface area(1585 m² g^–1)and a high porosity(2.56 cm³ g^–1)were prepared.Such large specific surface area and high porosity encouraged us to use this carbon material as positive electrodes for sodium ion battery.Electrochemical tests showed that the capacity reached 70 mAh g^–1 and 35 mAh g^–1 at current densities of 50 mA g^–1 and 2000 mA g^–1,respectively,which demonstrated a good rate performance.At a current density of100 mA g^–1,after a few cycles of stabilization,the capacity remained at 120 mAh g^–1after 500 cycles,which demonstrated the excellent cycling stability.