Preparation of Carbon and Carbon-based Composites and Study of the Capacitive Performance

Author:Li Yi Ju

Supervisor:wang gui ling


Degree Year:2018





Supercapacitor,as a novel electrochemical energy storage device,possesses the advantages of high power density,excellent cycling stability,superior charge/discharge rate performance and non-pollution.Therefore,supercapacitors can be applied in many applications.Electrode materials,as one of the key components in supercapacitors,have a decisiveeffectontheoutputperformance.Generally,constructingappropriate micro-nanostructures of electrode materials contributes to enhancing the electrochemical performance.Differentwiththebulkmaterials,theelectrodematerialswith micro-nanostructures have unique physical and chemical properties,such as high specific surface area and abundant nanopores,which have been regarded as the important research interests in the field of energy materials.In this thesis,the reasonable construction and assembly of various electrode materials with micro-nanostructures were achieved using the facile and universal chemical synthesis methods.Moreover,the electrochemical performance as the electrode materials of supercapacitors have been systematically tested.The main contents are as followed:The three-dimensional(3D)manganese dioxide/reduced graphene oxide(MnO2/RGO)composites were prepared using layer-by-layer spray coating self-assembly and electrodeposition methods,respectively.(a)For the layer-by-layer spray coating method,the graphene oxide dispersion,potassium permanganate and manganese chloride solutions were sprayed in turns on the reduced graphene oxide wrapped nickel foam in a high-temperature incubator.Next,the obtained samples were calcinated in tube furnace under high temperature to form the 3D multilayered MnO2/RGO electrode materials.The MnO2 nanoparticles are uniformly embedded in the interconnected RGO layers,which can form 3D porous conductive network for electron transport and electrolyte permeation.The obtained MnO2/RGO electrode materials have a relatively high discharge specific capacitance of 267 F g-11 at a current density of 0.25 A g-1.(b)For the electrodeposition method,the graphene oxide was first electrodeposited and reduced on the nickel foam.After freeze drying,the 3D crosslinked RGO arrays were self-assembled on the nickel foam.Subsequently,the MnO2nanosheets were in-situ conformally grown on the RGO nanosheets using the facile hydrothermal method to form the 3D MnO2/RGO composite electrode materials.The electrode materials with 3D open and porous microstructures contributes to building duel pathways for ion and electron transport.The obtained electrode has a high discharge specific capacitance of 462 F g-1at a current density of 0.5 A g-1.Moreover,even at a high current density of 10 A g-1,the capacitance retention of the electrode keeps 93.1%after cycling 5,000cycles,indicating excellent cycling stability.The interconnected graphene-like carbon nanosheets derived from the precursors of willow catkin cellulose with hollow tube structure were successfully prepared using the one-step carbonization and activation method.The unique crosslinked carbon nanosheet networks can facilitate the electrolyte permeation and electron transfer.Owing to the multilayer and hollow structure of the willow catkin cellulose,the nanostructures of the carbon can be facilely controlled by adjusting the ratio of activating agent to the precursor.After that,the optimized graphene-like carbon nanosheets were doped with nitrogen(N)and sulfur(S)heteroatoms,which can bring in pseudocapacitance and further enhance the capacitive performance.The N,S co-doped carbon nanosheets have a high discharge specific capacitance of 298 F g-11 at a current density of 0.5 A g-1.Even at an ultrahigh discharge rate of 50 A g-1,the specific capacitance still has 233 F g-1,which demonstrates superior rate performance.Additionally,the electrode has a high specific capacitance of 98%after 10,000 cycles at a current density of5 A g-1,displaying an excellent cycling durability.The assembled symmetric supercapacitor based on the N,S co-doped carbon nanosheets has a high energy density of 21.0 Wh kg-11 at a power density of 180 W kg-1,which is far superior than the commercial supercapacitors.The sandwich-like hierarchically porous carbon materials were prepared using the multi-templates method.First,the polyvinyl alcohol(PVA),nickel nanopowders and zinc chloride were added in graphene oxide solution to form the uniform mixture.The PVA is the carbon precursor,the zinc chloride is regarded as high-temperature molten salt template and the graphene oxide acts as the soft template for supporting the porous carbon.The nickel nanopowders serve as the hard template,which can bring in abundant macropores after acid etch.The target carbon product was obtained using the facile pyrolysis and acid wash.The obtained carbon materials with open pore structure and reasonable pore size distribution have a high specific surface area of 1235 m2 g-11 and a pore volume of 1.3 cm3 g-1.The discharge specific capacitance of the electrode can reach to 274.8 F g-1at a current density of 0.2 A g-1.The discharge specific capacitance still has 206 F g-11 at a high current density of 20 A g-1,displaying favorable rate performance.The assembled symmetric supercapacitor based on the sandwich-like porous carbon has a high energy density of 18.47 Wh kg-11 at a power density of179.9 W kg-1.The MnO2 nanosheets were then in-situ grown on the hierarchically porous carbon materials to act as the positive electrode.The assembled asymmetric supercapacitor based on the porous carbon and porous carbon/MnO2 composite has a high energy density of25.93 Wh kg-1at a power density of 199.9 W kg-1.In addition,the asymmetric cell has a high capacitance retention of 112.2%after 10,000 cycles at a current density of 2 A g-1,exhibiting an excellent cycling stability.The flexible electrode was prepared using the facile chemical reduction and electrodeposition methods.The non-woven cotton microfibers were first wrapped by reduced graphene oxide nanosheets to form the flexible conductive substrate.Then,the nickel sulfide was deposited on the RGO-coated cotton microfibers by one-step constant voltage electrodeposition method to form the flexible and wearable electrode.The 3D interlaced RGO-wrapped cotton microfibers build a conductive network for fast electron transport.The open macropores between the cotton microfibers contribute to electrolyte permeation.The obtained flexible electrode based on the non-woven cotton textile has a high specific discharge capacitance of 775 F g-11 at a current density of 0.5 A g-1.The specific capacitance retention keeps 88.1%after 1000 cycles at a current density of 2 A g-1.The facile,low-cost and high-performance flexible electrode materials offer a strategy for the flexible and wearable electronics.