Preparation Of Carbon Quantum Dots @ MnOx Composites And Its Supercapacitor Performance

In the context of the energy crisis,improving energy storage efficiency is one of focuses to solve the problem.The combination of high power density and high energy density of supercapacitors makes it an alternative to lithium-ion batteries for energy storage applications in all areas.Manganese oxide material?MnOx?has attracted attention due to its large theoretical specific capacitance.At the same time,it has become a research hotspot because of its abundant resources,low price,easy preparation and environmental friendliness.However,MnOx as a transition metal oxide has a large defect of low electron conductivity and slow ion diffusion.This thesis tried to solve this problem by studying the appropriate reaction conditions for a one-step reaction method to synthesize carbon material@MnOx composite materials and using these materials for supercapacitor electrode materials.In addition,with the development of flexible and wearable electronic devices,the design and preparation of flexible supercapacitor devices has become a new trend.Based on carbon material@MnOx composites and their synthesis methods,2D flexible supercapacitor devices and linear supercapacitor devices were designed and fabricated.The specific research contents are as follows:?1?Investigate and regulate the optimal reaction conditions for the synthesis of MnOx@carbon quantum dots composites.Potassium permanganate?KMnO₄?and nitrogen-doped carbon quantum dots?N-CDs?are used as reaction materials with a single variable of the hydrothermal reaction,like the reaction temperature,the reaction time,the mass ratio of the two reactants for the synthesis of carbon quantum dots@Mn₃O₄ composites?MCDs1-2?and carbon quantum dots@MnO₂composites?MCDs1-8,MCDs-Me1-2?.MCDs1-2 is a composite product of hausmannite Mn₃O₄ and carbon quantum dots.The microscopic morphology is a regular octahedron with a side length of 50nm to 200nm.The carbon quantum dots are wrapped uniformly in the nanosheet.MCDs1-8 composites and MCDs-Me1-2 composites are obtained by different synthetic methods,but they have the same morphology and composition.They are the composite products of birnessite?-MnO₂ and carbon quantum dots.MnO₂ grew on the carbon quantum dots and formed the nanosheets.?2?Research on the performance of supercapacitors for three composite materials.A suitable potential window was selected for each of the three composite materials under the two electrodes system and the electrolyte of 1M Na₂SO₄ aqueous solution.The specific capacitance and cycle performance data were obtained under the potential window.Comprehensive comparison of relevant data shows that MCDs1-8 has the most excellent electrochemical performance among the three materials.At a potential window of-0.8 V to 0.8 V,the MCDs1-8 material exhibits a specific capacitance of 204 F·g^-1 at a sweep rate of 50 mV·s^-1.?3?Design and fabrication of 2D flexible supercapacitors.The MCDs1-8 was used as the electrode material and prepare a hydrogel electrolyte with sodium polyacrylate as the polymer substrate and 1M sodium sulfate was added.A 2D flexible supercapacitor device with"sandwich"type was designed and fabricated.The electrochemical performance test of the device at various bending angles shows that any bending does not affect its performance.Composite nanosheets formed by the birnessite?-MnO₂ and graphene quantum dots was grown on the surface of the carbon fiber by a one-step hydrothermal reaction to build MCF-GQD.It was used as a one-dimensional linear supercapacitor electrode,and the length specific capacitance was increased by 45.9%compared with the material of carbon fiber grown only with MnO₂?MCF?.The conductivity exhibited on the impedance spectrum is also much better than the MCF.