Structural Regulation And Electrochemical Properties Of Manganese Oxide Nanoelectrode Materials

As pseudocapacitor electrode material,manganese oxides?MnO₂?have been given wide research attention.Although manganese oxides have many advantages,such as high theoretical specific capacitance,wide electrochemical potential window,low cost,natural abundance,and environmental benignity,its actual specific capacitance is far below its theoretical value.Because manganese oxides have poor electronic conductivity,the charge transfer in the process of charge/discharge is bad and makes their effective utilization low.Research results indicate that two approaches can effectively improve the electrochemical performance of MnO₂ electrode.One is the preparation of nano-size manganese oxide electrodes with high specific surface area and holey structure.Another is the preparation of MnO₂-based nanocomposites based on the synergistic effects from their individual constituents.Therefore,the design and controllable preparation of nano-size manganese oxide electrodes with high specific surface area or MnO₂-based nanocomposites is a meaningful research topic.Via different strategies,methods and effective technical routes,the nano-size manganese oxide electrodes with high specific surface area or MnO₂-based nanocomposites are expected to be prepraed and their electrochemical performance are systematically investigated.In the present thesis,a series of nano-size manganese oxides with high specific surface area or MnO₂-based nanocomposites based on the synergistic effects have been prepared by self-sacrifical template method,acid assisted heat treatment technology,and a facile in-situ redox strategy.The structure,morphologies,and electrochemical properties of the as-synthesised materials have been studied.The main research contents and results are as follows:MnO₂/RGO nanocomposite has been prepared by a simple redox reaction between RGO and KMnO₄,in which RGO as a self-sacrifical template.The amount of MnO₂ in the MnO₂/RGO composite and electrochemical properties can be controlled by changing the concentration of KMn04.MnO₂?1.45?/RGO composite electrode exhibits the highest specific capacitance of 274 F g-1 at a scan rate of 10 mV s-1 in 1 mol L-1 Na2S04,it is better than pure RGO electrode.An asymmetric electrochemical capacitor is assembled based on MnO₂?1.45?/RGO as positive electrode and RGO as negative electrode.The capacitor can be worked reversibly in a voltage of 0-1.8 V,which gives a specific capacitance of 53 F g-1 at a current density of 0.25 A g-1 and energy density of 23.9 Wh kg-1.Moreover,the capacitor exhibits excellent cycle stability,and its capacitance retention is 96%after repeating the galvanostatic charge-discharge 1000 cycles at a current density of 2 A g-1.MnO₂?1.45?/RGO composite is a potential electrode material for the capacitor with high energy and power densities.This in-situ growth method not only prepares the MnO₂-based composites,but also provides a new preparation method for other transition metal oxide nanocomposites.H-Birnessite is firstly prepared by hydrothermal synthesis and ion-exchange method.Then by heating treatment a mixture of H2SO₄ and H-Birnessite,porous MnO₂ nanomaterials are finially prepared.The pore structure,specific surface area and the electrochemical properties of porous MnO₂ nanomaterials electrodes are connected with the molar ratio of H2SO₄/MnO₂.The as-synthesised MnO₂ electrode with a molar ratio of 14.4 gives a specific capacitance of 253 F g-1 at a current density of 0.5 A g-1,which is higher than H-Birnessite electrode?154 F g-1?.Moreover,the capacitance retention increases to 62.1%from 45.5%.An asymmetric electrochemical capacitor is assembled based on MnO₂?14.4?as positive electrode and RGO as negative electrode.The capacitor can be worked reversibly in a voltage of 0-1.8 V,which gives a specific capacitance of 61.5 F g-1 at a current density of 0.5 A g-1 and energy density of 27.7 Wh kg-1.And its capacitance retention is 96.8%after repeating the galvanostatic charge-discharge 5000 cycles at a scan rate of 50 mV s-1.The sulfuric acid assisted heat treatment technology is an effective approach to prepare MnO₂ electrode material with high specific surface area and porous structure.By heating the delaminated MnO₂ nanosheet slurry with NH4HSO₄,porous-assembled MnO₂ with different specific surface area can be controllable prepared by adjusting the molar ratio of NH4HSO₄/MnO₂.The as-synthesised MnO₂ electrode with a molar ratio of 2.1 gives a large specific surface area of 456 m2 g-1 and a specific capacitance of 281 F g-1 at a current density of 0.25 A g-1,which is higher than MnO₂ electrode without NH4HSO₄ addition?124 F g-1?.The MnO₂?2.1?electrode shows a good cycling stability of about 1.9%loss after 2000 cycles at a current density of 2 A g-1.An asymmetric electrochemical capacitor is assembled based on Mn02?2.1?as positive electrode and GR as negative electrode.The capacitor can be worked reversibly in a voltage of 0-1.8 V,which gives a specific capacitance of 64.3 F g-1 at a current density of 0.5 A g-1 and energy density of 28.9 Wh kg-1.And its capacitance retention is 92%after repeating the galvanostatic charge-discharge 2000 cycles at a current density of 2 A g-1.The porous-assembled MnO₂ with large specific surface area shows a potential application,and the acid assisted heat treatment technology provides a new route for preparing other transition metal oxide nanostructures with large specific surface area.A novel holey technique for MnO₂ nanosheets has been developed by in-situ redox reaction between the delaminated Mn02 nanosheets and the adsorbed Fe²⁺ ions at room temperature,in which Fe²⁺ ions come from a redox reaction between Cu wire and Fe3+ions in the reaction system.The holey ratio of MnO₂ nanosheets can be controlled by changing the in-situ redox times,and nanopores with an average diameter of 4.3 nm can be uniformly introduced in the basal plane of MnO₂ nanosheets and the assembled materials from these holey MnO₂ nanosheets show a large specific surface area of 288 m2 g-1.The porous MnO₂ prepared by treating 6 h?MnO₂-6?shows a specific capacitance of 303 F g-1 at a current density of 0.25 A g-1,it is higher than that of the delaminated MnO₂ electrode?148 F g-1?.Moreover,the electrode exhibits excellent capacitance retention and cycle stability,and its capacitance retention is 88.7%after 4500 cycles at a scan rate of 20 mV s-1.An asymmetric electrochemical capacitor is assembled based on MnO₂-6 as positive electrode and RGO as negative electrode.The capacitor can be worked reversibly in a voltage of 0-1.8 V,which gives a specific capacitance of 58.8 F g-1 at a current density of 0.5 A g-1 and energy density of 26.5 Wh kg-1 at a power density of 457 W kg-1.The result shows that the in-situ redox etching method is an effective approach to introduce nanoporous into 2D MnO₂ nanosheets.It improves the specific capacitance and capacitance retention of MnO₂ electrodes,and provides a good idea for the preparation of other porous transition metal oxide.