Quantum Capacitance Of Typical Two-Dimensional Electrode Materials:A First Principles Study

In electric double-layer supercapacitors,the total interface capacitance?C_T?can be expressed as,1/C_T=1/C_Q+1/C_D,where C_D and C_Q are the double-layer capacitance and quantum capacitance.Although the total interfacial capacitance is restricted by both C_Q and C_D,researchers have ignored the contribution of C_Q for a long time,which caused some experimental phenomena to be unexplained correctly,and lacked a way to further increase the total interfacial capacitance and energy density.Aiming at the problem of low energy storage of electric double-layer supercapacitors,we have optimized the structure of some typical two-dimensional materials such as graphene-based electric double-layer electrode materials,tried to understand the mechanism of adjusting quantum capacitance,and obtained a way to enhance quantum capacitance,to provide new ideas for developing high-performance two-dimensional electrode materials for electric double layer capacitors.Based on the density functional theory calculation,the structure of the two-dimensional electrode material has been optimized,by introducing defects,doping,and adsorption of atoms and clusters,and the quantum capacitance has been improved,and the novel electrochemical properties such as capacitance of some typical two-dimensional materials have been obtained.Meanwhile,the mechanism of the change in quantum capacitance has been revealed.The main results we obtaind are as follows:1.Two-dimensional materials such as graphene and silicene are affected by quantum confinement effect and small density of states,and the quantum capacitance is close to zero near Fermi level.Based on the first-principles calculation method of density functional theory,two-dimensional electrode materials such as graphene are effectively modulated in electronic structure through doping/co-doping,adsorption of metal atoms and clusters,etc.Promoting the formation of local states near Dirac points of the electrode materials and/or the shift of Fermi level,will result in the enhancement of quantum capacitance.Excellent nitrogen-doped graphene-based electrode material is selected,and its quantum capacitance is as high as 118?F/cm².We find that 3-N and 3-S doped single vacancy graphene has relatively stable crystal structure and excellent quantum capacitance,and is suitable for being used as an ideal electrode material for symmetric supercapacitors.N/S and N/P co-doped single vacancy graphene is suitable for asymmetric supercapacitors.Through the quantitative correlation between quantum capacitance and electric double layer capacitance,we fit the total interfacial capacitance of 3-N doped single vacancy graphene,confirm the contribution of quantum capacitance to energy storage,and design graphene-based electrode materials with high specific capacity and stable structure.2.There are still challenges and unknowns in the synthesis and thermodynamic stability of silicene and germanene existing independently.We have systematically studied the thermodynamic stability of silicene under different supercell sizes,doping types,doping concentrations and different temperatures by using the first principle molecular dynamics simulation method under NVT ensemble,and determined its instability conditions and microscopic mechanism in the atomic scale range.It is found that the above silicene doped system has thermodynamic stability at 300K and700K.At 1200K,with the change of dynamic simulation time,we observed the increase and fluctuation of Si-Si average bond length.After the temperature rises to1500K,it is observed that the average bond lengths of Si-Si and Si-N increase sharply,and the doped silicene system begins to collapse around 1500 K.3.Based on the first principles of density functional theory,we studied the stability and quantum capacitance of transition metal atoms adsorbed on pristine and single vacany silicene/germanene.All of these systems doped with metal atoms such as silicene and germanene exhibit quasi-metallic characteristics,accompanied by obvious electron transfer and formation of defect states near Fermi level.Adsorption of single vacancy germanene by metal atoms plays a key role in adjusting the electronic structure of silicene and germanene and improving quantum capacitance.In particular,Ti atom adsorbs single vacancy silicene/germanene,which significantly improves quantum capacitance by 80.1?F/cm² and 76.5?F/cm²?doping concentration 3.1%?.In the aspect of energy storage research,no one has yet studied the quantum capacitance of germanene.We have studied systematically the thermodynamic stability and quantum capacitance of germanene with single vacancy defect,Stone Wales topological defect and five kinds of double vacancy defects.The formation energy of Stone Wales topology defect is the lowest?1.65eV?,and the increase of quantum capacitance is attributed to the local state formed near Fermi level.Compared with Ti?Au,Ag,Cu,Al?,3-B?N,P,S?doped single vacancy graphene?silicene,germanene?,3-N doped single vacancy graphene,germanene and Ti?Cu?doped single vacancy silicene have higher quantum capacitance.The above theoretical calculations provide some references for the practical application of two-dimensional electrode materials in supercapacitors and field effect transistors.4.In recent years,layered MoS₂ has become a promising energy storage electrode material due to its excellent electrical and electrochemical properties,good mechanical properties,various electronic states?semiconductor,metal and charge density wave states?and good environmental characteristics.We have studied the thermodynamic stability and quantum capacitance of metal and nonmetal atom doped pristine and single vacancy V_S single layer MoS₂.The doped atoms form strong bonds with MoS₂,and the optimized structure system shows metallic property.It is also confirmed that Al replaces S atom in single vacancy V_S single-layer MoS₂ and B replaces S atom in pristine single-layer MoS₂,both of which have higher quantum capacitance of 80.2?F/cm² and 200.9?F/cm² respectively.As the doping concentration of Al and B atoms increases,the quantum capacitance monotonically increases.This will be helpful for further understanding the properties of chemically modified MoS₂-based electrode materials.5.The effects of the type,size and adsorption mode for Pt clusters on the stability of single-vacancy graphene have been studied.It is found that the close-packed structure is more stable than Sutton-Chen structure for the adsorption of Pt_n cluster on single-vacancy graphene,except the magic number n=13.As the cluster size increases,the adsorption Pt cluster with a close-packed structure on a single vacancy graphene gradually changes from a single point contact to triangular and rhombus contact,so that the area of contact the adsorption stability could be enhanced.We have also studied the interaction between Li clusters and graphene.Considering the thermal effect,we have analyzed the Gibbs free energy of the Li clusters with under different configurations and different thermodynamic temperatures on graphene.The nucleation possibility of alkali metal clusters on graphene has been estimatd,and the capacitance and cycle stability have been probed.In summary,we have used the density functional theory calculation to study the structural stability and capacitance for some typical two-dimensional materials such as graphene,silicene,germanene,etc.Through above theoretical simulation,we have deeply understood the electronic structure and charge storage mechanism for these electrode materials,and the possibility of these two-dimensional materials as energy storage electrode materials has been evaluated theoretically.In general,one can improve the capacitance for the double-layer capacitor by increasing the specific surface area of electrode materials,but in fact this improvement is generally limited.However,in this dissertation,we have tried to improve the capacitance by increasing the quantum capacitance,and to clarify the mechanism of introducing defects,doping,surface adsorption and etc.to increase the quantum capacitance for two-dimensional materials.This investigation may provide a new idea for optimizing the electrochemical performance with a high current density and high cycle stability for the 2D electrode material and strengthening double-layer supercapacitor in the future.