Preparation Of Dense Activated Carbon Materials And Their Electrochemical Energy Storage Performance

Activated carbons are widely used as electrode materials for supercapacitors due to their good electrical conductivity,large specific surface area,high chemical stability and environmental friendliness.But based on their electrical double-layer energy storage mechanism,increasing specific surface area and enriching pore structure will reduce conductivity and compaction density while ensuring high gravimetric capacitances,the obtained deep and branched porous structure will destroy the internal long-distance electron transport channel and increase the transmission resistance of electrolyte ion,resulting in the low surface and space utilization,low specific capacitance（especially volumetric capacitance）and poor rate performance.From this point of view,on the basis of ensuring the overall high conductivity,using graphene quantum dots and potassium fulvic which own conjugated structures as precursors to prepare a series of dense activated carbons through three aspects of structural design:assembly mode,activation mode of activation reagent and optimization of intrinsic structure of the precursor.The effect of the preparation technology and precursors on the density and microstructure of the materials were detailedly characterized by scanning electron microscopy,transmission electron microscopy,X-ray diffraction,and N2absorption/desorption tests,respectively.And the relationship between the structure of materials and electrochemical performance was deeply explored by cyclic voltammetry,galvanostatic charge-discharge,and impedance tests,respectively.On this basis,the corresponding synthesis and energy storage mechanism were proposed.The main contents are as follows:To graphene quantum dots with conjugated structure and chitosan as raw materials,the positively charged chitosan and negatively charged graphene quantum dots（GQDs）are assembled to dense precursor in aqueous solution benefiting from the strong intermolecular electrostatic attraction.Through subsequent carbonization and activation,large-surface-area and high density activated carbon was prepared by electrostatic densification method.The introduction of graphene quantum dots builds the whole continuous conductive network.Based on densification from within the precursor and appropriate activation process,the dense and porous structure achieves the integration of high specific surface area(3359 m² g^-1)and high compaction density(0.75 g cm^-3),solving the problem that the conductivity and compaction density decrease with the increase of specific surface area,promoting the efficient charge storage and ion transport dynamics,leading to excellent gravimetric capacitance(1 A g^-1,341F g^-1)and rate performance(100 A g^-1,238 A g^-1)as well as high volumetric capacitance(1 A g^-1,256 F cm^-3).To graphene quantum dots with conjugated structure as raw material,small amount of KOH was employed for the ion exchange mainly with-COOH to form the immobilized K⁺,uniformly distributed K groups serve as the activating agents in the subsequent pyrolytic etching process,a dense activated carbon with ordered and interconnected microporous structure was prepared by homogeneous activation.Graphene quantum dots with highly crystallized nature provide a complete conductive network.The ordered and interconnected porous structure constructed by homogeneous activation not only achieves high compaction density(0.60 g cm^-3)at large specific surface area(1884 m² g^-1),balancing the pore structure and density of material,but also effectively solves the problem that deep and branched porous structure obtained from traditional activation blocks the electrons and ions transport channels,improving surface and space utilization,accelerating the electrons and ions transmission dynamics,leading to well gravimetric capacitance(1 A g^-1,315 F g^-1),rate performance(80 A g^-1,177 F g^-1),and volumetric capacitance(1 A g^-1,189 F cm^-3).To inexpensive biomass potassium fulvic acid with conjugated structure as raw material,highly doped and high density layered stacked porous activated carbon were prepared through self-pyrolysis and subsequent homogeneous activation.The conjugated structure of potassium fulvic acid builds continuous conductive pathway for electron transfer.Meanwhile,the highly doped characteristics and layered stacked porous structure provide convenient infiltration channels and sufficient diffusion space for ion transport,which balances the contradictory relationship between material structure and space.At the optimal carbonization temperature and alkali-carbon ratio,the obtained material shows high gravimetric/volumetric capacitance(1 A g^-1,324 F g^-1/204 F cm^-3)and rate performance(50 A g^-1,capacitance retention of 73%).Importantly,even at high mass loading of 20 mg cm^-2,it shows remarkably high gravimetric capacitance(10 A g^-1,164 F g^-1).Therefore,this simple and efficient preparation method of dense activated carbon provides a new idea for the structural design of dense energy storage electrode materials.