Construction Of Supercapacitor Based On MnO₂ And Research On Its Capacitance Performance

With the rapid development of the electronics industry and heavy industry,and the large consumption of non-renewable resources,it is urgent to find an energy storage device with high functional density and long charge and discharge cycle life.In the past few years,supercapacitors have stood out in many energy-saving and environmentally-friendly devices by virtue of their energy saving,high efficiency and high efficiency.In this paper,three kinds of MnO2 based materials were prepared by hydrothermal method,and their properties as electrode materials were studied.The main research contents are as follows:1.Preparation and Capacitance Properties of a-MnO2 Nanowires.Co2+ is used as a dopant to precisely control the crystal phase transformation of MnO2.The tunnel structure is inserted to avoid the collapse of 2×2 tunnel of a-MnO2,and the conversion of 8-MnO2 phase to ?-MnO2 phase is successfully realized.The flexibility of the a-MnO2 nanowires can be precisely controlled by controlling the amount of Co2+ dopant added.When the amount of Cp2+ dopant added is small,curved a-MnO2 nanowires can be prepared;the amount of Co2+ added is increased,and a-MnO2 nanowires are straightened by bending.The electrochemical performance of a-MnO2 nanowires shows that the curved ?-MnO2 nanowires have a certain flexibility.At a current density of 0.1 A g-1,the specific capacitance of MnO2-0.4Co nanowire electrodes is 95 F g-1 which is larger than that of straight MnO2 nanowires.In terms of cycle stability,the MnO2-0.4Co nanowires retain a specific capacitance of 90%.In summary,MnO2-0.4Co nanowires are ideal for supercapacitor electrodes.At the same time,Co-doped ?-MnO2 nanowires also provide a substrate choice for binary composites.2.Preparation and Capacitance Properties of a-MnO2@8-MnO2 Core-Shell Structure.Based on a-MnO2 nanowires,?-MnO2 nanosheets were deposited on the surface of a-MnO2 nanowires by a simple one-step hydrothermal method to form a unique nano-scale structure.This core-shell structure provides good mechanical stability to the material and does not exhibit surface shedding during electrochemical reactions.In order to evaluate the properties of the MnO2-based materials,we conducted a GCD test.At the current density of 0.25 A g-1,the discharge time of a-MnO2@8-MnO2 core-shell nanocomposite electrode is significantly larger than that of a-MnO2 nanowires and 8-MnO2 nanosheets.In a neutral aqueous electrolyte(Na2SO4),the a-MnO2@8-MnO2 electrode can provide a specific capacitance of 206 F g-1,which is superior to the recently reported MnO2-based materials tested under similar conditions.In addition,the high specific capacitance of 206 F g-1 also demonstrates that the electrochemical properties of core-shell nanostructures are generally superior to single-phase materials.The energy density parameter of the ACS device combined with the a-MnO2@8-MnO2 positive electrode and the AC negative electrode was 12.9 W h kg-1 at a power density of 0.23 kW kg-1.At the same time,after 5000 cycles of our device,the loss ratio of the capacitor is about 15%compared with the initial value,which indicates that the device has certain stability during discharge.Our research on MnO2-based materials proves that the core-shell nanostructures are beneficial to improving the electrochemical performance of high-load electrodes.3.Preparation and Capacitance of a-MnO2@rGO Hydrogel.A unique composite supercapacitor material is formed by cross-linking a-MnO2 nanowires in a rGO hydrogel network structure via a simple one-step hydrothermal method.This composite hydrogel material provides good mechanical stability and does not exhibit surface shedding in electrochemical reactions.To evaluate the electrochemical behavior of a-MnO2@rGO hydrogels,we performed a GCD test.At 0.5 A g-1,the discharge time of the ?-MnO2@rGO hydrogel composite electrode is significantly greater than that of the single-phase rGO hydrogel electrode.In a neutral aqueous electrolyte(Na2SO4),the a-MnO2@rGO electrode can provide a specific capacitance of 120 F g-1.The ACS device combined with a-MnO2@rGO positive electrode and AC negative electrode has an energy density of 23.3 W h kg-1 at a power density of 0.23 kW kg-1.In addition,our devices have excellent cycle life and retain>90%of capacitance after 3000 charge/discharge cycles.Our research on a-MnO2@rGO hydrogel shows that the hydrogel composite of MnO2 is beneficial to improving the electrochemical performance of high load electrodes.