Research On Electrochemical Properties Of Nanostructured Mn/Co Metal Oxide Supercapacitor Electrode Materials

In recent years,with the rapid development of new energy vehicle and wearable electronics,the requirements for high-performance energy storage devices are becoming unprecedented urgent.New energy storage devices are required to possess not only high energy density,but also high power characteristics along with long service life.Fortunately,supercapacitors?SCs?meet the demands for high power density and long service life of commercial products.However,the commercial applications of SCs are hindered by their intrinsically low energy density.Therefore,how to improve the energy density without sacrificing their advantages is a research hotspot in the field of SCs.According to the energy density equation,enhancing the specific capacitance and broadening the operating potential windows of the devices are two effective strategies for increasing the energy density of SCs.The specific capacitance of the electrode material is possibly increased by adjusting the microstructure and enhancing the conductivity,while an arrangement of fabricating asymmetric SC with two difference types of electrodes has also been shown as an efficient approach to broaden the operating potential window.Thus,the topic of this thesis is focused on the preparation of electrode materials and the assembly of asymmetric SC devices to increase the energy density of SCs.The main contents of this thesis are as follows:?1?Distinctive hierarchical hollow nanostructure composed of MnO₂ nanosheets in-situ growth on hollow carbon nanofibers?MnO₂/HCNFs?was synthesized through co-electrospinning technique and phase separation followed by hydrothermal reaction.The specific surface area of the MnO₂/HCNFs nanocomposites calculated by BET method is estimated to be 151.9 m² g^-1,and the specific capacitance is 293.6 F g^-1 at 0.5A g^-1.An asymmetric SC device was assembled using MnO₂/HCNFs nanocomposites as positive electrode and KOH-activated porous carbon nanofibers?PCNFs?as negative electrode.The specific capacitance of the asymmetric SC device was 63.9 F g^-1.Additionally,the asymmetric SC device could deliver a high energy density of 35.1 Wh kg^-1 and a maximum power density of 8.78 kW kg^-1 with excellent long-term cyclic stability?91.1%capacity retention after 10,000 cycles?.Moreover,two tandem assembled devices could light a red LED for 5 min.?2?Conductive polypyrrole?PPy?was recombined onto MnO₂ nanorods?MNR?to improve the conductivity of composite materials.MNR was first synthesized through hydrothermal method followed by the preparation of uniform PPy cladding on the surface of MNR via vapor polymerization to form MnO₂/polypyrrole core/shell nanorods?PMNR?.The PMNR nanocomposite showed an 89.5%capacitance retention after 5,000 charge/discharge cycles,much higher than that of the MNR electrode?36.2%?.The PMNR electrode showed a higher specific capacitance and electrochemical stability than MNR electrode because of the high conductivity and good stability of PPy coating.NPC and Fe₂O₃@CF have been used as negative electrode materials in seawater-based SCs,while PMNR used as positive electrode material.Thus,PMNR//NPC asymmetric SC,PMNR//Fe₂O₃@CF asymmetric SC,and NPC//NPC symmetric SC were fabricated.Meanwhile,seawater is proposed for the first time as an electrolyte for application in SC devices for the first time.Among the three types of SC,PMNR//Fe₂O₃@CF asymmetric SC achieves the highest specific capacitance,and PMNR//NPC asymmetric SC deliveres the maximum energy density,while the NPC//NPC symmetric SC shows the best cycling stability.?3?The growth of electrochemical active materials by electrodeposition directly on conductive substrates avoids the introduction of binders,resulting in improving the conductivity of the electrodes.Novel nanostructured Fe₂O₃ with a network of 3D mesoporous nanosheets is synthesized on carbon fabric?Fe₂O₃@CF?using a potentially low-cost electrodeposition technique followed by thermal annealing.The Fe₂O₃@CF-10electrode electrodeposited for 10 min exhibited the largest specific capacitance,and can be used as a negative material for asymmetric SCs as an alternative of carbon-based materials.A flexible solid-state asymmetric supercapacitor was assembled using the Fe₂O₃@CF-10 electrode as the negative electrode and MnO₂-covered carbon fabric?MnO₂@CF?as the positive electrode.The assembled SC exhibits a high volumetric capacitance of 2.92 F cm^-3 and a relatively long-term cyclic stability?91.3%initial capacitance retention after 5000 cycles?at a wide operating voltage window of 1.8 V employing a PVA/LiCl gel as electrolyte.The flexible device delivers a maximum energy density of 1.26 mWh cm^-3 and a maximum power density of 462.6 mW cm^-3.In addition,two tandem assembled devices could light 20 red LEDs,and the capacitance retention reaches up to 96%after repetitive bending over 100 bending cycles.?4?The electrodeposition of electrochemical active materials on the conductive CF substrate and the modification of conductive polymers were carried out to improve the electrochemical properties of the electrode materials.MnO₂/PPy core-shell nanowires?NWs?on carbon fabric?MnO₂/PPy@CF?were fabricated through a hydrothermal electrodeposition method followed by vapor-polymerization of PPy.A flexible solid-state asymmetric SC was assembled using MnO₂/PPy@CF-15 electrode as positive electrode with the polymerization time of 15 min and Fe₂O₃@CF-10 electrode as negative electrode.In a CMC-Na₂SO₄ gel electrolyte,the electrochemical window of the assembled flexible device is enlarged 2 V,and the volumetric capacitance is as high as 3.62 F cm^-3.The decrease in volumetric capacitance is only 7.4%after 10000 GCD tests.The maximum energy density and the maximum power density of the flexible device are 1.93 mWh cm^-3 and 159.6 mWcm^-3,respectively.The two flexible devices in series can light 16 parallel red LEDs.After 200 bending experiments,95.8%of the initial capacitance was maintained.?5?The partial replacement of O^2-with N^3-could increase the electronegativity of electrode material by providing additional valence states of metal ions,resulting in the improved the conductivity of the electrode material.The partly nitrogenized Co₃O₄nanosheets?N-Co₃O₄?were prepared by electrodeposition and heat treatment in ammonia atmosphere.A flexible solid-state asymmetric SC employing N-Co₃O₄electrode as a positive electrode and Fe₂O₃@CF-10 electrode as negative electrode shows a maximum specific capacitance of 111.6 F g^-1 in PVA/KOH gel electrolyte with retention of 93.8%initial capacitance after 10000 GCD tests.The maximum energy density and the maximum power density of the flexible device are 39.7 Wh kg^-1 and4.43 kW kg^-1,respectively.These two flexible devices in series can light a red LED for5 min,and the capacitance retention reaches up to 95%after repetitive bending over 100bending cycles.