| As the increasing concerns on the energy crisis and environmental issue,great efforts have been devoted to developing sustainable and environmentally friendly alternative energy sources.Supercapacitors(SCs)are gaining tremendous interest due to their excellent power density,fast charge and discharge,and excellent cycle stability.The practical use of supercapacitor devices is hindered by their low energy density.One strategy is to design nanoscale electrode materials with large surface areas and stable porous nanostructures,as well as a shortened diffusion path of charge carriers to the current collector,which are expected to enlarge the electrode/electrolyte contact surface and improve the electron/ion conductivity in the electrodes,and thus the energy density and power density of supercharge are improved.The main research contents and results are as follows:In this chapter,a cost-effective carbon quantum dots(CQDs)-induced assembly of 2D ultrathin Ni(OH)2 nanosheets into 3D porous hierarchical structure has been developed.The“electrostatic forces”between CQDs and cations drive the“self-assembly”growth of the 3D CQDs/Ni(OH)2 hierarchical structures.The multi-level CQDs/Ni(OH)2 micro-nanostructures endow them high specific surface area and rich porosity.Thanks to their unique structures and the conductivity of the CQDs,the optimized asymmetric supercapacitor displays a maximum specific capacity of 161.3 F g-1,and a high energy density of 57.4 Wh kg-1.To date,zero-dimensional CQDs,as a new carbon nanomaterials,have attracted attention inthe improvement of the electrical conductivity for electrode materials.However,few attempts have been made to the use of CQDs as the structure-directing agent for the assembly of functional micro/nanoelectrodes.Herein,we have developed a general,and simple strategy to fabricate 3D porous hierarchical carbon quantum dots(CQDs)/MxOy(M=Co,Ni)composite nanostructures from thermolysis of corresponding CQDs/M(OH)y,where CQDs play crucial role as a structure-directing agent in tuning the morphologies of the M(OH)y.With the merits of the large electroactive surface area,superior electronic conductivity of CQDs,and fruitful porous structure,the as-fabricated battery-type electrode materials present amazing supercapacitor performance.For example,the as-made porous hierarchical CQDs/Co3O4nanoarchitectures exhibited remarkable electrochemical performance with 1603 F g-1 at 1 A g-1,excellent rate capacity of 70.6%from 1 to 100 A g-1,superior cycling ability(97.0%capacity remained after 2000 cycles).Furthermore,the CQDs/Co3O4//AC hybrid supercapacitor(HSC)exhibits a high specific capacity of 210.4 F g-1 with 74.8 W h kg-1.The practical application of flexible all-solid-state asymmetric supercapacitors(FSASCs)in high energy density devices is limited by their low specific capacity,which can be effectively addressed by designing electrode materials hierarchically on the micro-nanoscale.Herein,well-defined 3D porous hierarchical Cu Co2O4@CQDs and Fe2O3@CQDs architectures are rationally synthesized through a simple CQDs-induced hydrothermal self-assembly technique.Both of the as-prepared Cu Co2O4@CQDs and Fe2O3@CQDs electrodes exhibit improved specific capacity,desirable rate capability and complementary potential range.A FSASCs(Cu Co2O4@CQDs//Fe2O3@CQDs)on graphite paper delivers a high operation voltage of 1.55V,an energy density of 39.5 Wh kg-1 at 1203.7 W kg-1,and excellent cycle stability.The excellent performance is ascribed to the good electronic conductivity with the assistance of CQDs and their unique 3D mesoporous structures with extraordinary specific surface area,which could provide fruitful active sites for electrochemical reactions.The hierarchical Cu Co2O4@CQDs hollow microspheres constructed by one-dimensional(1D)porous nanowires have been successfully prepared through a simple CQDs-induced hydrothermal self-assembly technique.XPS analysis shows the Cu Co2O4@CQDs possesses the Co?-rich surface associated with the oxygen vacancies,which can effectively boost the faradaic reactions and OER activity.The as-synthesized 3D porous Cu Co2O4@CQDs electrode exhibits high activity toward overall electrochemical water splitting,an overpotential of 290 m V for oxygen evolution reaction(OER)and 331 m V for hydrogen evolution reaction(HER)in alkaline media have been achieved at 10 m A cm-2,respectively.Furthermore,an asymmetric supercapacitor(ASC)(Cu Co2O4@CQDs//CNTs)delivers a high energy density of 45.9 Wh kg-1 at 763.4 W kg-1,as well as good cycling ability.The synergy of good electronic conductivity with the assistance of the CQDs and well-defined 3D hollow structures facilitates the subsequent surface electrochemical reactions. |
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