The Density Functional Theory Study Of Two-Dimensional Electrode Materials:Germanene,Silicene And MXenes

A clean and efficient energy storage/conversion device is urgently needed in our country because of the unbalanced energy distribution,the increasing environmental pollution and the growing scarcity of existing energy sources.Electric double-layer capacitor has attracted much attention.Its competitive advantages are high output power,long lifetime,safety and high efficiency.However,it often shows the low energy density.In order to enhance the energy density,much work on the research and development of high performance electrode materials had been done.In an electric double-layer capacitor,the total interface capacitance(C_T)can be expressed as:1/C_T=1/C_D+1/C_Q.The total interface capacitance is affected by both quantum capacitance(C_Q)and double-layer capacitance(C_D).However,some phenomena have not been scientifically explained because the long neglected the contribution of C_Q.In this thesis,three typical two-dimensional materials with vacancy defects,adsorption,doping and co-doping had been investigated.The regulated electronic structure,the thermodynamic stability and energy storage mechanism had been deeply analyzed to understand the physical mechanism of the enhanced quantum capacitance and energy storage effect.The main work of this thesis was as follows:1.The structural stability and quantum capacitance of pristine germanene adsorbed with metal atoms was systematically studied.All systems exhibit quasi-metallic properties.The adsorption of Cu atom significantly enhanced the quantum capacitance of germanene,with the maximum value of 77.096?F/cm~2.The adsorption of metal atoms played a key role in regulating electronic structure and improving quantum capacitance.3-N doping with single-vacancy germanene showed high quantum capacitance(97.180?F/cm~2).It was found that the interlayer interactions could enhance the quantum capacitance of germanene by modulating the co-doping structure of metal and non-metal atoms.2.The electronic structure and quantum capacitance of Stone Wales topological defect,single-vacancy defect and four kinds of double-vacancy defects were studied based on density functional theory.Among them,topological defects were the most stable with the lowest formation energy of 2.067 eV.For the TMN_x-Si(x=1-3)systems doped with Ag,Au,Cu,Ti,Mn atoms,the structural stability was gradually enhanced with the number of N atom increased.The highest quantum capacitance of the Cu N₂-o-Si system was 145.499?F/cm~2,which could be an ideal anode material for supercapacitors.The increased quantum capacitance was due to the introduction of localized states near the Fermi level.3.The best adsorption site of H,O,F and OH groups on Ti₃C₂ was investigated.Adsorption at the top site of the interlayer Ti atom showed the lower adsorption energy.The adsorption of Al atom on Ti₃C₂ exhibited the best adsorption effect than Li,Na,Mg,K and Ca atoms.Ca atom adsorbed on Ti₃C₂F₂ had the most significant improvement effect of quantum capacitance in the co-doping structures of metal atoms and groups,with the maximum quantum capacitance of 488.153?F/cm~2.It had obvious advantages under positive potential and could be used as an ideal cathode material for supercapacitor.