Contruction and Application of Carbon-Based Functional Materials by Supercritical CO2 Technology

Author:Fang Ru Yi

Supervisor:zhang wen kui xia yang


Degree Year:2019





Due to their high conductivity,high stability and high adsorption,carbon matrix composites have been widely used in energy and environment fields such as lithium ion batteries and heavy metal adsorption.However,the preparation methods of carbon matrix composites are limited.In these researches,carbon matrix composites are selected as research objects,we designed multi-structure carbon matrix composites based on the characteristics of high dispersion,high permeability and high solubility of supercritical carbon dioxide(SC-CO2).The mechanism of SC-CO2 regulating the structure,interface and morphology of carbon carriers is explored,and the effects of SC-CO2 on the chemical composition and structure of carbon matrix composites are systematically studied.The electrochemical properties and mercury adsorption properties of carbon matrix composites controllable prepared with the assistance of supercritical CO2 were studied.The main research contents are as follows:(1)Microalgae(Nannochloropsis)served as a biological template and carbon source to synthesize Si-O-C microspheres with the assistance of supercritical CO2 fluid.Compared to conventional artificial templates,microalgae is abundant,renewable and available,and can be regarded as a promising biological template.Meanwhile,supercritical CO2 fluid with high penetration,high diffusivity and high dissolving capacity can serve as a superior solvent to guarantee the efficient mass transfer and uniform dispersion of precursors.As anodic materials for Li-ion batteries,Si-O-C microspheres exhibit a high reversible specific capacity of 450 mA h g-1 at a current density of 0.1 A g-1 over 200 cycles,excellent rate cycling stability and high coulombic efficiency(100%).(2)A novel synthetic strategy that utilizes supercritical CO2(SC-CO2)to fabricate C@S composites for Li-S batteries has been successfully developed.Taking the advantages of high infiltrability,excellent diffusivity and superior solvability,SC-CO2not only serves as an intercalator that penetrates into the pores and interlayers of carbon matrices to expand/exfoliate the porous structure and tightly-stacked layered graphite structure,but also plays the role of a marvelous hydrophobic solvent to dissolve sulfur and transfer it into the inner pores and interlayers of carbon matrices.Taking AC@S as an example,it exhibits the highly reversible capacity of 817 mA h g-1 after 100 cycles at0.1 A g-1,and excellent cycling stability with a satisfactory capacity retention of 90.5%.(3)A facile biotemplating method with the assistance of a supercritical CO2(SC-CO2)technique has been developed to construct a unique 3D porous SiOC/Se cathode with a high Se loading for Li-Se batteries with high areal capacity and a long cycling life.An SiOC/Se cathode derived from rice husks achieved an extremely high initial areal capacity of 8.1 mA h cm-2 at 0.1C at an Se loading of 8 mg cm-2,which is the highest Se loading reported thus far.After 200 cycles,the reversible areal capacity remained at 4.1 mA h cm-2 together with a capacity retention of 90%(vs.4.8 mA h cm-2in the 2nd cycle).This excellent performance at a record-breaking Se loading in comparison with earlier Li-Se batteries is attributed to the unique 3D porous conductive network and Si-O-C units set in the porous carbon matrix,which provided continuous electron/ion pathways,enhanced structural stability and strong chemical adsorption for trapping Se and Li2Se,as well as the uniform distribution of Se infiltrated via the SC-CO2 strategy.(4)Using melamine as structural template,carbon and nitrogen source,combined with the high aspect ratio and high conductivity of single-walled carbon nanotubes,selenium and sulphur were evenly infiltrated into the pore channels and layers of the carrier with the assistance of supercritical CO2 fluid.After heat treatment,selenium and sulphur formed a stable Se1-xSx compound,which was tightly integrated into the carrier material of 3D conductive network,NC@SWCNTs@Se1-x-x Sx was successfully synthesized.Composite electrode material.As cathode material,NC@SWCNTs@Se1-x-x Sx-80 composite electrode material maintains 0.2 mAh g-1reversible capacity after 200 cycles at 632 mA g-1 current density.The excellent electrochemical performance can be attributed to the following aspects:Firstly,the 3D conductive network structure of NC@SWCNTs carbon carrier can effectively shorten the ion migration distance,improving the electrolyte penetration and increasing the electron/electron transmission rate.Secondly,some Se substitutes for S to form Se1-xSx compounds,which can effectively improve the conductivity of active substances.Thirdly,with the help of SC-CO2,Se1-xSx is evenly dispersed in the carbon carrier of 3D network structure,which improves the utilization rate of active material.(5)A novel nanoscale zero-valent iron(NZVI)and sulfur bifunctional hierarchical porous macrocellular carbon derived from puffed rice(PRC/NZVI@S)is synthesized by simple one-step carbonization followed by supercritical CO2(SC-CO2)fluid assisted synthetic strategy,which severs as high-efficiency adsorbent for mercury removal from aqueous solutions.The as-synthesized PRC/NZVI@S-10 with moderate NZVI and sulfur double modification offers high selective affinity,removal efficiency,and ultrahigh adsorption capacity of up to 723.46 mg g-1.Such good performance is attributed to the hierarchical porous carbon in the PRC/NZVI@S not only acts as frameworks to stabilize and disperse NZVI,but also provides abundant pores and voids for absorbing Hg(II)from aqueous solutions.Moreover,the absorbed Hg(II)can be reduced to Hg(0)by NZVI and further immobilized by sulfur.Finally,the adsorption mechanism of adsorption-reduction-immobilization is proposed.(6)A novel synthetic strategy has been successfully developed,which utilizes supercritical CO2(SC-CO2)to fabricate sulfur-modified zeolites(zeolites@S)for Hg2+removal from water.Owing to the low viscosity,high diffusivity and excellent dissolving capacity of SC-CO2 as well as the nontoxicity,noninflammability and being a low-cost solvent it exhibits the strong ability to expand the crystal structure of zeolite and the high-efficiency transfer of sulfur in zeolite matrices.Benefiting from this unique microstructure,zeolites@S adsorbents provide abundant pores and voids for absorbing Hg2+,and synchronously offer extra active sites for chemically trapping Hg2+with sulfur.The pseudo-first-order kinetic/pseudo-second-order kinetic models and Langmuir/Freundlich isotherm models have been employed to study the adsorption behaviors and adsorption kinetic parameters.The results demonstrated that zeolites@S-15(sulfur content=15 wt%)exhibited the highest adsorption capacity and the best kinetic parameters for Hg2+relative to other samples.