MnO₂ Hierarchical Nanostructures: Synthesis,Characterization And Evaluation Of Electrochemical Performance For The Excellent Aqueous Asymmetrical Supercapacitor

Topics of this thesis covered the fabrication of supercapacitors mainly via the modification of one-pot hydrothermal method,the morphology control and ionic hybridization of MnO₂ that possesses the excellent theoretical specific capacitance(1370 F g^?1),and also the low cost,low toxicity and abundance in the earth.These topics were set up on the comprehensive summary of literatures concerning with the methodological merits and disadvantages of supercapacitor materials and the fabrication processes,and also for improvement,the modifications,which constituted the main content of first chapter.In the second chapter,aimed at improving the electric efficacy of supercapacitor,a novel binder and conductive agent-free electrode material was designed,namely integrating aluminum into ultrathin nanosheets of Ni Co₂-OH deposited on the nickel foam via the one-pot hydrothermal process.Variable amounts of Al were introduced.It was observed that Al remarkably affected the morphology and electrical performance of Ni Co₂Al_x-OH.At first,as Al was introduced into Ni Co₂-OH,the transfiguration occurred fromβ-phase of Ni Co₂-OH toα-phase of Ni Co₂Al_x-OH.Meanwhile,at any constant current density,the specific capacitance of Ni Co₂Al_x-OH electrode increased with the increase of Al amount,except for Ni Co₂Al_0.5-OH electrodes,which gave an utmost specific capacitance of 1,570 F g^?1 at the current density of 2 A g^?1 but at the other current densities,the specific capacitances of Ni Co₂Al_0.5-OH electrodes were lower than those of Ni Co₂Al_0.6-OH electrodes.Cycling performances of Ni Co₂Al_x-OH electrodes at the current density of 20 A g^-1for 6,000 cycles exhibited that the introduction of Al was harmless in a proper range of Al amounts.The specific capacitances generally kept constant in 6,000 cycles,except for Ni Co₂Al_0.6-OH electrode which the specific capacitance decreased from～840 to～600 F g^?1.This was the reason that Ni Co₂Al_0.5-OH was selected as the model for the further studies.For example,the specific capacitance retention of Ni Co₂Al_0.5-OH was 98%at the current density of 20 A g^?1 after 6000 cycles.Using Ni Co₂Al_0.5-OH as the positive electrode(Ni Co₂Al_0.5-OH//AC),a high energy density of 56.8 Wh kg^?1 was obtained at the power density of 618 W kg^?1.This result indicated that Ni Co₂Al_0.5-OH was a potential electrode material for the fabrication of aqueous asymmetric supercapacitor with the excellent performance.In the third chapter,a simple and environmentally benign one-pot hydrothermal strategy was developed for the synthesis of hierarchical grass-flower nanostructure of MnO₂ without the use of either templates or acid.SEM and TEM were used to confirm the morphology of MnO₂.Because of synergistic effects of nano grass and nano flower,the nano grass-flower hierarchical structure exhibited specific capacitance of 230 F g^?1 at the current density of 1.0A g^?1 and acquires outstanding cycling stability after 10000 cycles with specific capacitance retention of 81%.For the evaluation of the electrochemical performance,cyclic voltammetry（CV）and galvanostatic charge-discharge（GCD）cycling tests were conducted.Further,an aqueous asymmetrical supercapacitor device（nano grass-flower MnO₂//AC and nano grass-MnO₂//AC）was constructed.Nano grass-flower MnO₂//AC illustrated an energy density of30 Wh kg^?1 with a maximum power density of 18000 W kg^?1.This work will provide a new strategy to synthesize worthwhile nanostructures that reveal potential applications in energy storage electrodes.In the fourth chapter,an alternative approach was made to self-assemble low crystalline ultra-thin MnO₂ nanosheets into MnO₂ nanospheres by a cost-effective,one-pot and instant ultrasonic synthesis without the assistance of acid or template within an hour for high performance and high-rate supercapacitors.The MnO₂ nano-spheres are demonstrated to be excellent working electrodes in a three-electrode cell revealing a remarkable specific capacitance of 309 F g^?1 at the current density of 1.0 A g^?1 and good cyclic stability with capacitance retention of 90%after 5000 cycles in 1 M Na₂SO₄ electrolyte aqueous solution.Further,an aqueous asymmetrical supercapacitor（MnO₂//AC）device was constructed whose maximum workable potential window for the MnO₂//AC device reached 0-2.0 V.MnO₂//AC illustrated an energy density of 40 Wh kg^?1 with a maximum power density of 19000 W kg^?1.Besides,this device sustained a long cycle life with 85%capacitance retention after 5000 cycles.In the fifth chapter,core-shell heterostructure of birnessite type Fe-doped MnO₂ binder and conductive agent-free electrode was prepared on carbon nanofiber cloth via a hydrothermal method,which improved the conductivity and capacitance for the high-performance aqueous asymmetrical supercapacitor.The Fe-MnO₂ electrode had a specific capacitance of about 350F g^?1 at the current density of 1 A g^?1.We further drafted a high potency aqueous asymmetric supercapacitor（Fe-MnO₂//AC）with Fe-MnO₂ as the positive electrode at the extendable working potential window of 0-2.0 V which manifested an outstanding energy density of 40Wh kg^?1 with a maximum power density of 3956 W kg^?1 and an excellent cycling performance with capacitance retention of 88.8%at the current density of 10 A g^?1 after 6000 cycles.In the sixth chapter,an electrode of uniform and ultrathin nanosheets of silver-doped manganese oxide（Ag₂-MnO₂）was constructed on carbon cloth through a hydrothermal synthesis with an outstanding specific capacitance for the high-performance aqueous asymmetrical supercapacitor.The Ag₂-MnO₂electrode bore the highest specific capacitance of350 F g^?1 at 1 A g^?1 and 280 F g^?1 at a current density of 40 A g^?1.Besides,an aqueous asymmetric supercapacitor（Ag₂-MnO₂//AC）assembled with Ag₂-MnO₂ as a positive electrode exhibited a wide potential window of 0-2.0 V and a remarkable energy density of 54 Wh kg^?1at a power density of 2000 W kg^?1.26 Wh kg^?1energy density was retained even at a higher power density of 19500 W kg^?1.This device offered excellent cycling stability with capacitance retention of 80%after 10,000 GCD cycles.In the seventh and last chapter,based on the redox couple reaction of Mn³⁺/Mn⁴⁺,the theoretical capacitance of manganese dioxide（MnO₂）has a very high value of 1370 F g^?1 but the specific capacitance of MnO₂ in the reported literature is still under 300 F g^?1.The potential window of MnO₂ in the previously published studies is approximately 1 V which considering E=?CV²,limits the power and energy densities far below the theoretical value.In this work,we developed an ultra-thin K⁺dopedδ-MnO₂ nanosheets array electrode the potential window of which extended to 0-1.2 V with a highly reversible capacitance of 366 F g^?1.When the potential window reaches 1.2 V,the fast-redox reaction of MnO₂ and the K⁺intercalation/deintercalation process restrains water decomposition in kinetics.A high potency aqueous asymmetric supercapacitor（K-MnO₂//AC）was drafted with K-MnO₂ as a positive electrode which exhibited a stable working potential window of 0-2.2 V in 1 M Na₂SO₄aqueous electrolyte.This device has demonstrated an excellent energy density of 56 Wh kg^?1at a power density of 550 W kg^?1 and an ultralong cycle performance with capacitance retention of 98%over 10000 cycles at the current density of 10 A g^?1.This approach leads to new prospects for emerging high workable potential window aqueous energy storage devices.