Waste Polymers Derived Carbon Nanomaterials And Their Applications In Energy Storage And Microwave Absorption

In recent decades,plastics have been widely used in daily life due to its excellent properties.However,plastics usually have a short service life.The large-scale use of plastic products has led to the generation of a large number of waste plastics.These waste plastics are generally difficult to degrade,causing great pressure on the ecological environment.At the same time,the raw materials of plastic products mainly come from non-renewable resources(such as oil and coal).Therefore,the recycling and reuse of waste plastics not only helps to reduce environmental pollution,but also helps save resources,which is in line with the sustainable development strategy.At present,the treatment and recovery methods of waste plastics are mainly as follows:mechanical recovery,landfill,incineration,pyrolysis,etc.However,the above methods usually have some drawbacks.Considering the high carbon content in plastics,carbonizing waste plastics to prepare high value-added carbon materials is a new type of plastic recycling method.Especially in recent years,as nano-carbon materials have exhibited broad applications in the fields of flame retardancy,catalysis,adsorption and separation,and energy storage,polymer carbonization has developed rapidly.For the specific application of carbon materials,its performance heavily depends on the micro-morphology of the carbonized products.Therefore,how to effectively regulate the micro-morphology of the carbonized products while ensuring the high-efficiency carbonization of polymers has become a research hotspot.Based on this,we propose the following carbonization strategies:1.Firstly,we used porous MgO as a template and waste PS as a carbon source to prepare porous carbon nanosheet(CNS).At the same time,in order to further increase the specific surface area of the carbon material and adjust the pore structure,we adopted KOH activation strategy to treat CNS.After that,the carbon nanosheet with a hierarchical pore structure(ACNS-X)was prepared(X stands for activation temperature).The effect of activation temperature on the specific surface area and pore structure of carbon materials was analyzed,and the electrochemical properties of the materials were also tested.Among them,the carbon material treated at 800?(ACNS-800)exhibits the best electrochemical performance.In the three-electrode system,ACNS-800 exhibited a specific capacitance of 323 F g-1 at 0.5 A g-1 in a 6 M KOH electrolyte,good rate capability(222 F g-1 at 20 A g-1).More importantly,when assembled into a two-electrode system,the device displayed an ultra-high energy density of 44.1 Wh kg-1 at a power density of 757.1 W kg-1 in an organic electrolyte.2.In view of the fact that the morphology and structure of carbon materials have important influences on the performance of capacitors.In this part,we continue to regulate and control the morphology of carbon materials.Based on literature reports,carbon materials with three-dimensional(3D)macroporous structure have important applications in the field of energy storage.Therefore,we selected metal oxide nanoparticles as templates and waste PS as carbon source to prepare 3D macroporous carbon material with a honeycomb structure,and chemical activation was adopted to improve the micropores/mesopores content in the carbon material.As a result,3D macroporous carbon with hierarchical pore structure was prepared,and the electrochemical performance in supercapacitors was also studied.From the test results,the obtained 3D hierarchically porous carbon exhibits a high specific capacitance of 284.1 F g-1 at 0.5 A g-1 and good rate performance of 198 F g-1 at 20 A g-1 in a three-electrode system.Moreover,the assembled symmetrical capacitor displays a high energy density of 19.2 W h kg-1 at the power density of 200.7 W kg-1 in aqueous electrolyte(1 M Li2SO4).3.In the first two parts of this thesis,we used the metal oxide as template.In the later purification of the carbon material,a large amount of corrosive acid(such as HCl)was involved,and the template cannot be recycled.In this part,we used soluble salt(K2CO3)as a template,glucose as a carbon source,and urea as a nitrogen source.Then,N-doped carbon material with hierarchical pore structure was prepared via a one-step carbonization method.In this experiment,K2CO3 is used as both template and activator,and can be recycled.The whole synthesis process does not involve corrosive acid,which can be considered a green preparation method.In the three-electrode test,the mass specific capacitance is 361 F g-1 at a current density of 0.5 A g-1.Increasing the current density to 20 A g-1,the specific capacity can be maintained at 240 F g-1,exhibiting good rate performance.When tested in a neutral electrolyte,the device exhibits a high energy density of 17.1 Wh kg-1 at a power density of 207.9 W kg-1.Considering our previous research on the carbonization of waste polymers,this salt template method can be used in the carbonization of waste polyester plastics,such as PET,PC,etc.4.One dimensional rod-like MOF material was prepared from waste PET,and the resultant MOF was carbonized at different temperatures to prepare metal/carbon composites with core-shell structure.The electromagnetic wave absorption properties of pyrolysis products were studied.Among them,the metal/carbon composites prepared at 800? display the best electromagnetic wave absorption performance.The minimum reflection loss(-44.5 dB)can be obtained at the frequency of 17.5 GHz and the matching thickness is 1.5 mm.In addition,the reflection loss is-43.6 dB when the matching thickness is 2.5 mm,also displaying good electromagnetic wave absorption performance.Finally,the influence of pyrolysis temperature on electromagnetic wave absorption performance was discussed by analyzing the dielectric constant and permeability at different temperatures.