3D Flower-like Hierarchical Microspheres Of Cobalt-based Materials:Design,Synthesis And Applications In Supercapacitors

The hierarchical flower-like Co₃O₄/Co?OH?₂ spheres with porous structure exhibit excellent electrochemical performance and attract extensive interest due to the enlarged surface area,a range of short ion-diffusion channels and interconnected pore network for rapid ion-transport.In spite of the unique properties,the flower-like Co₃O₄ shows limit capacitance probably due to poor electron transport and insufficient surface area.Two optimal strategies are usually employed to enhance the good electrochemical performance of the supercapacitors:?I?enhancing the large surface areas of the electrode materials,and?II?improving the electron transport between active materials by incorporation of highly conductive carbon materials?such as CNT and Graphene?into the electrode materials.In this dissertation,a series of Co-based materials of DBS-Co?OH?₂,DBS-Co₃O₄,Co?OH?₂-CNT>N-PEGm,Co₃O₄-CNT>N-PEGm and Co₃O₄-G>N-PEGm with ultrathin nanosheets have been prepared as pseudocapacitor materials by employing a solvothermal method.The 3D rGO and 3D rGO-CNT>N-PEGm aerogels have been prepared as electrical double-layer capacitors materials.Moreover,the asymmetric supercapacitors have been fabricated with the as-prepared materials to achieve high energy density and power density for energy storage application,with excellent long cycle life.The main contents are as follows:?1?A new 3D flower-like?-Co?OH?₂ hierarchical microspheres with dodecyl benzene sulfate?DBS?-intercalated structure has been fabricated for the first time by employing a solvothermal method.The DBS-?-Co?OH?₂ structure can be synergistically determined by various characterization methods?EDS,TGA,FTIR and XPS?.Moreover,the DBS-?-Co?OH?₂ microspheres possess ultrathin Co?OH?₂nanosheets only 2.4-4.0 nm?4-6 single atomically thick layers?and a specific surface as 210.54 m² g^-1.Based on the morphological evolution of the precursor at different reaction temperatures,a new growth mechanism of the flower-like microspheres has been rationally proposed with a gradual“OH^--incursion”in an outside-inside strategy.On the basis of various characterization methods,we have deeply investigated the transition state of the morphological evolution to further verify the mechanism.?2?An advanced flower-like DBS-Co₃O₄ hierarchical microsphere composed of ultrathin nanosheets with thickness of⁴ nm has been synthesized from DBS-Co?OH?₂ flower sphere in the sintering process.The DBS-Co₃O₄ exhibits specific capacitance value of 1104.4 F g^-1 at a current density of 0.5 A g^-1 in 6 M KOH solution.Meanwhile,the 3D rGO aerogels are prepared by a facile method as negative electrode materials,and the specific capacitance of 213.3 F g^-1 can be achieved at a current density of 2 A g^-1.Hence,the asymmetric supercapacitors of the double“ion-buffering reservoirs”are fabricated with the DBS-Co₃O₄ as positive electrode of promising Faradaic pseudocapacitors materials and the 3D rGO aerogels as the negative electrode of electrical double-layer capacitors materials,exhibiting a high energy density of 25.5 Wh kg^-1 at the power density of 400 kW kg^-1 with excellent cycling performance.?3?We have synthesized a novel dodecyl benzene sulfate?DBS?-intercalated 3D flower-like?-Co?OH?₂-CNT>N-PEGm hierarchical microsphere composites,in which the incorporating CNT>N-PEGm were modified by Nitrene Chemistry with the methoxypolyethylene glycol?mPEG?.The Co?OH?₂-CNT>N-PEGm composites as Faradaic pseudocapacitors materials show high specific capacitance value of 436.1 F g^-1 at a current density of 0.5 A g^-1 in 6 M KOH solution.Furthermore,the asymmetric supercapacitors show an energy density of 12.6 Wh kg^-1 at the power density of 400 kW kg^-1,which are fabricated with Co?OH?₂-CNT>N-PEGm as positive electrode material and active carbon as negative electrode material.?4?A novel 3D flower-like Co₃O₄-CNT>N-PEGm hierarchical microsphere has been synthesized by incorporating carbon nanotubes?CNTs?modified with methoxypolyethylene glycol?mPEG?via Nitrene Chemistry into the Co₃O₄microsphere uniformly.In this composite,the coordination bonds have formed between the lone pair of electrons on ether-oxygen groups of the polymer chains on the CNTs surface and cobalt ions,yielding close function of CNT>N-PEGm with Co₃O₄ nanostructures.Moreover,the Co₃O₄-CNT>N-PEGm composites can provide a range of short ion-diffusion channels at the exterior of Co₃O₄ microspheres,meanwhile the CNT>N-PEGms provide electron transport channels to improve the electron transport in the interior of Co₃O₄ microspheres.Hence,the high specific capacitance value of 1513.2 F g^-1 at a current density of 0.5 A g^-1 has been achieved for Co₃O₄-CNT>N-PEGm in 6 M KOH solution.Furthermore,the asymmetric supercapacitors fabricated with Co₃O₄-CNT>N-PEGm as positive electrode material and active carbon as negative electrode material show an excellent capacitance of119.9 F g^-1 at a current density of 0.5 A g^-1 and an energy density of 42.6 Wh kg^-1 at the power density of 400 kW kg^-1.?5?The novel 3D flower-like Co₃O₄-G>N-PEGm composites have been synthesized by employing a solvothermal method,in which the incorporated graphene nanosheets are modified with methoxypolyethylene glycol?mPEG?via Nitrene Chemistry to form the 2D macromolecular brushes.In the as-prepared composites,the coordination bonds have been formed between the lone pair of electrons on ether-oxygen groups of the PEG polymer chains on the surface of the G>N-PEGm macromolecular brushes and cobalt ions.The as-prepared composites can provide a range of short ion-diffusion channels in the interior of Co₃O₄ microspheres and the G>N-PEGm nanosheets can function as electron transport channels to improve the electron transport at the exterior of the microspheres.A high specific capacitance value of 1625.6 F g^-1 at a current density of 0.5 A g^-1 can be achieved in KOH solution.Furthermore,the 3D rGO-CNT>N-PEGm aerogels as negative electrode materials exhibit high reversible specific capacitance of 313.8 F g^-1 at a current density of 2 A g^-1.Based on the high electrochemical performance of the both electrode materials,the double ion-buffering reservoirs of asymmetric supercapacitors configured with the Co₃O₄-G>N-PEGm as positive electrode and 3D rGO-CNT>N-PEGm as negative electrode can deliver a high energy energy density of 34.4 Wh kg^-1 at the power density of 400 kW kg^-1.