Study On Mechanism Of Supercapacitors Electrode Materials By Combining Experiment And Density Functional Theory

Supercapacitors make a figure in many new energy storage devices,and are widely used in smart grid,new energy vehicles,portable electronic equipment,light railway vehicles and renewable Energy storage due to their fast charge and discharge capability?1-30 s?,high power density(10000 W Kg^-1),long cycle life?10⁶?,wide operating temperature range,safety,etc.However,the commercial application of supercapacitors is hampered because of their lower energy density.Designing and synthesizing electrode materials with reasonable pore size distribution,high specific surface area,high conductivity,high specific capacity and high voltage window is one of the effective methods to achieve high energy density of supercapacitors.In addition,the first-principles calculation based on density functional theory?DFT?has made significant progress in the design,synthesis,simulation calculation and performance evaluation of materials,and has become more and more helpful for the development of new material systems.Utilizing DFT to study the mechanism of the electrode material not only helps to understand the physical and chemical structure of the electrode materials,but also uncovers the relationship between the structure of electrode and the performance of the supercapacitors,and also provides guidance and assistance for the structural design of future electrode materials.The synthesis process and energy storage process of material have a great power on the performance of electrode materials.Therefore,research on growth mechanism and energy storage mechanism of material from the atomic view is beneficial to further explore the electrode materials with better performance.Based on this,this thesis systematically studies the two typical mechanisms of supercapacitor electrode materials through DFT theory calculation.The main research contents and results are as follows.?1?The nucleation mechanism of highly dispersed NiO nanodots anchored on reduced graphene oxide?NiO NDs@rGO?synthesized by solvothermal method is taken as a example.First of all,the NiO NDs@rGO was synthesized using carbonyl nickel and graphene oxide as the source materials.And the structure of the electrode material was studied by experiment and theoretical calculations.The results show that the defects on graphene have a stronger absorption to NiO NDs than those on intact graphene,and NiO NDs are more easily trapped by the defects on graphene through Ni-C and Ni-O-C bonds,which explain the uniform dispersion of NiO NDs on the graphene surface and give a basic understanding of the physicochemical properties of the electrode structure.This unique structure not only improves ion accessibility but also shortens the charge transfer path.The specific capacity of NiO NDs@rGO reaches1020.28 F g^-1 at a current density of 1 A g^-1 in a 2 M KOH aqueous electrolyte,and maintains 92%of initial capacity at 10 A g^-1,showing a high specific capacity and excellent rate performance.?2?As a peseudocapacitance material with high theoretical specific capacity and a large number of pores for electrochemical energy storage,?-MnO₂ is taken as an example to study the storage mechanism of pseudocapacitors.Our group synthesized a self-supporting?-MnO₂/Ni bilayer film composed of an active?-MnO₂ nanosphere layer and a highly conductive but inactive Ni metal layer utilizing a two-step electrochemical deposition method.Experimental results reveal that the self-supporting electrode can undergo highly reversible bending as the electrode is continuously charged and discharged.On this basis,this thesis utilized DFT to prove that the deformation of the MnO₂/Ni film is highly related to the redox pseudocapacitive behavior of MnO₂ layer.The intrinsic redox peseudocapacitance of MnO₂ during charge and discharge causes valence change of Mn element,shortening and prolongation of Mn-O bond,and insertion and extraction of Na⁺ion,which eventually leads to reversible shrinkage and expansion of MnO₂.The results obtained by DFT are completely consistent with the results obtained through experimental observation and testing.Detailed analysis of the DFT at the crystal level is helpful for understanding the intrinsic relationship between redox peseudocapacitance and electrode structure changes.