Phenolic Resin Modified Graphene Materials

Graphene is a new kind of two-dimensional carbon nano-material with only one-atom-thick structure.It possesses high light transmittance,specific surface area and chemical inertness,and has exhibited excellent mechanical strength,high electric conductivity and heat conductivity.Graphene and graphene macroscopic assemblies have shown huge application potentials in energy storage,clean energy,electronic devices and catalyst fields.However,graphene suffers from the drawback of difficulty in fully exfoliation,while its macroscopic assemblies show low strength and poor flexibility with a large amount of defects.In this thesis,we focused on the modification of graphene materials with polymer based amorphous carbon and analysis of the mechanism of properties improvements.Graphene oxide(GO)and phenolic resin were used to fabricate graphene-based carbon fibers with high tensile strength and high elongation at break via utilizing highly effective organic coagulation bath.The effect of preparation parameters such as nozzle orifice diameters,amorphous carbon content and GO solution concentration on the strength,elongation at break and electrical conductivity of graphene-based carbon fibers were systematically researched.Besides,a new kind of hybrid graphene fiber was constructed by applying chemical vapor deposition to in-situ grow N-doped carbon nanotube(N-CNT)arrays on the surface of graphene fibers.The chemical composition,detail morphology and extensive application of this hybrid graphene fiber,which possess high tensile strength,elongation at break and out-standing flexibility,were studied.What's more,phenolic resin was grafted on GO through esterification reaction,and carbonization was conducted to prepare amorphous carbon encapsulated graphene materials.The amorphous carbon coating structure and content were observed and tested.The electrochemical properties of amorphous carbon encapsulated graphene materials were evaluated,and the mechanism of performance improvements were researched.The results showed that GO fibers can be fabricated effectively and continuously in the ethyl acetate/methanol solution of CaCl2 during solution spinning process.The tensile strength of GO fibers increased with the reduction of fiber diameters,which could be achieved by adjusting the spinning solution concentration,extrusion rate and drawing rate.At optimized condition,GO fibers exhibited tensile strength of 215 MPa.Upon the addition phenolic resin into GO spinning solution,graphene-based carbon fibers with high tensile strength and high elongation as break were fabricated.The graphene-based carbon fibers achieved a highest tensile strength of 1.45 GPa(second highest reported value),elongation at break of 1.8%(highest value for graphene fiber)and electrical conductivity of 8.4+104 S m-1.The mechanism that mechanical properties and electrical conductivity could be improved simultaneously was that amorphous carbon could not only enhance densification and reduced structural defects and voids in graphene fibers but also form new C-C bonds between graphene and amorphous carbon during carbonization,which could provide longer slipping distance and interfacial interactions between graphene sheets.Based on the graphene-based carbon fiber,a new kind of carbon hybrid material was constructed,in which in-situ grown N-CNT arrays with confined Ni particles inside covered the surface of graphene fibers.Owing to the stacked graphene sheets and twisted CNTs,the hybrid graphene fibers exhibited tensile strength of 1.09 GPa,elongation at break of 2.0%(highest reported value)and excellent flexibility as it could be twisted to a circle.Attributing to the nitrogen elements on the CNTs,hybrid fibers were suitable to be applied in adsorption of acid gases(C02 adsorption of 0.45 mmol/g at 50 °C and 1 atm),catalysts,polymer composites,fuel cells,etc.Phenolic resin was grafted on GO via esterification reaction,and amorphous carbon encapsulate graphene was prepared through carbonization.The amorphous carbon coating layer could be confirmed by transmission electron microscope observation and the amorphous carbon content was tested to be 12 wt%.The electrochemical performance results showed that the amorphous carbon encapsulate graphene showed high reversible capacity,out-standing cyclic stability and rate performance.After 250 cycles at 50 mA g-1,the reversible capacity reached 376.5 mA h g-1.As the current density increased to 200 mA g-1 and 1A g-1,the capacities remained at 337.8 mA h g-1 and 267.8 mA h g-1.The mechanism analysis indicated that the amorphous carbon layer could ensure the structural integrity of electrode material under high current densities;inhibit the decomposition of electrolyte and solid electrolyte interface film formation;reduce surface film resistance and charge-transfer resistance,thus increasing electrical conductivity of the electrode materials.