Design And Synthesis Of Multi-scale Carbon-based Composites For Hybrid Supercapacitors

Supercapacitors involving the surface electrochemical reactions over electrode materials with a high specific capacitance,long lifespan and good safety in use,hold a great promise in the future electrical energy storage fields.However,how to further enhance the reaction kinetics,boost the rate capability and energy density of the electrode materials while to maintain the high power density,still remains a major challenge.In this thesis,we have tried to develop methodologies and stratages for the design and synthesis of multi-scale carbon-based composites with a focus on the optimization of the porous structure and physicochemical properties,which have been demonstrated to be effective to enhance the transport of electrolyte ions and electrons,and to improve the reaction kinetics,as well as finally to deliver much higher energy density.The main results are summarized below.Hierarchical nanocomposites composed of carbon nanotubes(CNT)coupled with NiCoAl-layered double hydroxide(NiCoAl-LDH)nanosheets are fabricated by a simple urea precipitation method.The incorporated CNT functions as the conducting scaffold,which helps to improve the electrical conductivity of the composite electrodes that can deliver a specific capacitance of 1035 F g-1,better than the pristine LDH with a specific capacitance of 950 F g-1.In addition,Zero-dimensional carbon black(CB),one-dimensional CNT,two-dimensional reduced graphene oxides(RGO),and three-dimensional CNT/RGO composites were further tested as the building blocks to couple with the LDH nanosheets,with an aim of figuring out the impact of different nano structured carbon on the pore structure and the electrical conductivity of the nanocomposites.Of the electrodes fabricated,the nanocomposite consisting of CNT/RGO and LDH exhibits the highest rate capability of 72%due to high pore volume and low charge transfer resistance.3D architecture composites LDH-NPs/CH-NWs made of NiCo-carbonate hydroxide(NiCo-CH)nanowires coupled with NiCoAl-LDH nanosheets have been successfully grown on flexible graphite paper by one-step hydrothermal method.The NiCo-CH nanowires in the nanosheets matrix function as a scaffold for the dispersion of the NiCoAl-LDH nanosheets,resulting in a relatively loose and open structure,which helps to create abundant electroactive centers and large interfacial area for fast electrolyte ions transport.The monolithic composite electrodes show a high specific capacitance of 1297 F g-1 and excellent rate capability of 59%.The hybrid supercapacitors fabricated using the LDH-NPs/CH-NWs composites as the positive electrode and AC as negative electrode show a high energy density of 58.9 Wh kg-1.Ultrafast assembly of the monolithic composite G-CH films with alternate connections of the graphene and monodispersed NiCo-CH nanowires is achieved by a simple hydrothermal technique followed by vacuum filtration,which can be finished quickly in 25 s,yielding the composite films with a high volume density of 2.1 g cm-3.The GO-induced monodispersed NiCo-CH nanowires function as a scaffold and/or support,leading to relatively loose and open channels that favor the fast electrolyte ions transport.Because of this,the G-CH films as electrodes can deliver a superior volumetric capacitance of 2936 F cm-3.It is also found that combining well-interconnected conductive CNT networks into the monolithic composite films can further improve the rate capability of the electrodes.A simple strategy has been developed for the fabrication of edge site-enriched NiCo-sulfide(Ni-Co-S)decorated on graphene frameworks via in situ chemically converted method from NiCo-CH and graphene precursors,yielding two-dimensional composite architectures Ni-Co-S/G.In this novel process,the Kirkendall effect involving the anion exchange reaction,e.g.the etching-like effect of the S2-ions,plays a crucial role for the formation of the edge site-enriched nanostructure.The experimental results and the density functional theory calculations have revealed that the Ni-Co-S edge sites have a high electrochemical activity and strong affinity for OH-in the electrolyte,which can exhibit the excellent specific capacitance of 1492 F g-1 and superb rate capability of 96%.The hybrid supercapacitors with both high power density of 22.1 kW kg-1 and high energy density of 28.4 Wh kg-1 have been successfully fabricated using Ni-Co-S/G composite as the positive electrode and two-dimensional porous carbon nanosheets(PCNS)as the negative electrode.Ultrathin NiCo-oxide(NiCo2O4)nanosheets have been successfully grown on graphene surface in a vertical orientation via homogeneous precipitation followed by thermal annealing treatment in air,in which polyaniline(PANI)acts as the structure coupling bridge between the two components.The experimental data and the density functional theory calculations have shown that PANI can effectively tune the surface electronic structural states of NiCo2O4 nanosheets,and result in strong coupling effects between the conponents.The as-made composites NiCo2O4-P-G are rich in electrochemical active sites,and feature 3D open frameworks with hierarchical pore structure,also show the excellent rate capability of 84%and good electrochemical stability as supercapacitor electrodes.