Preparation Of 3D Mn-based Nanocomposites And Their Application In Supercapacitors

Supercapacitors attract much attention as an energy storage device with high energy density,high current charge and discharge characteristics,and friendly to the environment.Electrochemical performance of capacitors strongly depends on the electrode materials.Therefore,how to improve the electrochemical performance from the perspective of electrode materials is a hotspot in this field.Mn-based oxide is a supercapacitor electrode material because of low cost,environmental safety,and high theoretical capacities.However,the poor conductivity of Mn-based oxide limits its practical application in supercapacitors.In order to improve the conductivity and specific capacitance of the Mn-based oxide electrode materials,we combine it with the high conductivity material and design novel 3D structure.The performance of the supercapacitor was systematically studied and analyzed.The major research contents and results were shown as followings.?1?Network-like interconnection MnO₂/C porous microsphere precursor were synthesized by a simple process of hydrothermal treatment and the other two kinds of Mn₂O₃/C and Mn₃O₄/C porous microspheres were obtained by calcinating at different temperatures.The following reasons may account for the formation of mesoporous MnO₂/C composites.Whisker-like MnO₂ forms on the carbon surface spheres owing to the "oriented attachment" and "self-assembly" processes.During the reaction,the resulting ions?CO32-and HCO3-?,which had played an important role in the formation of the pores or pore channels.We performed cyclic voltammetry?CV?,Electrochemical impedance?EIS?and galvanostatic charge-discharge of the ECs using a three-electrode system to evaluate the electrochemical properties of the as-prepared four samples.Porous Mn2O3/C microsphere shows the largest specific capacitance of 237 F g^-1 at a current density of 0.4 A g^-1,a above 90%capacitance retention after 5000 cycles.A high-voltage Solid-state electrochemical capacitor?EC?was assembled with Mn₂O₃/C spheres as the electrode in a gel electrolyte?Na₂SO₄/PVA?.This gel electrolyte-based Solid-state EC is reversibly cycled within a wide voltage region of 0-2.0 V.Moreover,a high specific capacitance of 57.5 F g^-1 at a current density of 0.12 A g^-1,a superior long-term cyclic stability and a nearly 89%capacitance retention after 2000 cycles are observed.?2?Three-dimensional polyhedron-core/double-shellCuO@C@MnO₂ array on nickel foam was fabricated by a facile,stepwise hydrothermal approach.This smart electrode design offers several advantages as follows:the selected Ni foam substrate with 3D network structure increasing surface area can provide ideal electron pathway and load more active materials per unit electrode area.Polyhedron CuO with well-defined single-crystalline nanostructure serve as both the backbone and conductive connection for MnO₂.The C layer can increase the conductivity and protect thte inner structure of CuO as result of improving the durability.Ultrathin nanosheets-like MnO₂ can increase the contact area with electrolyte,enable fast redox reaction.When used for supercapacitors,CuO@C@MnO₂ shows excellent electrochemical performances such as high specific capacitances of 609 F g^-1 at a current density of 1 A g^-1 and long-term cycling stability.A high-voltage Solid-state electrochemical capacitor?EC?is assembled with CuO@C@MnO₂ as the electrode in an gel electrolyte?Na₂SO₄/PVA?.This gel electrolyte-based Solid-state EC is reversibly cycled within a wide voltage region of-0.2-1.8 V.Moreover,a high specific capacitance of 146 F g^-1 at a current density of 1.67 A g^-1,a superior long-term cyclic stability and a nearly 100%capacitance retention after 5000 cycles are observed.