Theoretical Study Of Two-dimensional New Energy Materials

With the economic development and the improvement of living standards,the human demand for energy has continuously risen.The search for new energy and new materials has become a topic of great relevance.Graphene,a single-atom-thick flat two-dimensional(2D)carbon material has attracted much interest due to its excellent electrical conductivity and mechanical properties.In addition,graphene possesses high theoretical specific surface area which provides a rich platform for surface chemistry.Because of its extraordinary physical and chemical properties,graphene has many potential applications,such as nanoelectronics,transparent electrodes,optical devices,and energy storage.Motivated by the great successes of graphene,more and more 2D materials have been studied in recent years.In this thesis,we focus on exploration of several graphene-like nanomaterials for new-energy-related applications,including gas storage,gas separation and rechargeable battery.All the calculations are performed by means of first-principles calculations based on density-functional theory(DFT).We reveal the internal mechanisms of the influence of geometrical structures and electronic properties on molecular,atomic adsorption and diffusion properties and provid the theoretical basis for the relevant practical applications.The main resultsin this dissertation are summarized as follows:1.As the most promising energy storage system(ESS),rechargeable alkali-metal(AM)ion batteries have been attracting great attention.A suitable anode material is quite crucial for successful development of AM ion batteries.Using first-principles calculations,we propose that a two-dimensional(2D)group-IV monochalcogenide,germanium sulfide nanosheet(GSNS),can serve as high-performance anodes for the AM(AM=Li,Na,and K)ion batteries.The interaction between AM atoms and GSNS is strong enough to prevent the clustering of the intercalated AM atoms that may occur in other 2D materials.The low energy barriers for the diffusion of AM atoms on GSNS,0.236(Li),0.090(Na)and 0.050 eV(K),suggest the high charge/discharge rate of the GSNS anodes.Low average electrode potentials and high AM storage capacity up to 512 mA h g-1(for Na)can be achieved in the AM/GSNS systems.In view of the higher abundances of Na and K than that of Li,our work offers a promising anode material for the development of low-cost Na and K ion batteries with high performance.2.Phosphorus compounds have been intensively investigated as potential negative electrode materials of alkali-metal(AM)ion batteries.However,the practical application is greatly hindered by the low conductivity due to the semiconducting nature of phosphorus and rapid structural degradation during cycling.Based on first-principles calculations,we proposed a new two-dimensional phosphorus carbide compound ?0-PC monolayer(PCM)as a promising anode material for AM(AM=Li,Na,and K)ion batteries.The PCM exhibits excellent electric conductivity and high charge/discharge rate due to the low energy barriers for the diffusion of AM atoms,66(Li),39(Na)and 36 meV(K).The storage capacities of AM ions are Li,Na and K atoms on PCM can be as high as 1247.1 mAh g-1(Li)623.5 mA h g-1(Na),and 623.5 mA h g-1(K),respectively.More importantly,the PCM possesses ultrahigh stiffness(Cx=186.08 J/m2(zigzag),Cy=123.16 J/m2(armchair))which can effectively avoid structure degradation during discharging/charging cycles.These interesting properties open avenue to overcome the drawbacks of phosphorus compounds in the AM ion batteries.3.Despite the high theoretical capacity of lithium-sulfur(Li-S)batteries,their commercialization is severely hindered by low cycle stability and low efficiency,stemming from the dissolution and diffusion of lithium polysulfides(LiPSs)in the electrolyte.In this study,we propose a novel two-dimensional(2D)conductive metal-organic framework(MOF),namely Cu-BHT,as a promising sulfur host material for high-performance Li-S battery.The excellent conductivity of the Cu-BHT eliminates the insulating nature of S-based electrodes.The dissolution of LiPSs into the electrolyte is largely prevented by the strong interaction between Cu-BHT and LiPSs.In addition,orientated deposition of Li2S on Cu-BHT facilitates the kinetics of LiPSs redox reaction.Therefore,the use of Cu-BHT for Li-S battery cathodes is expected to suppress the LiPSs shuttle effect and to improve overall performance,which is ideal for practical application of Li-S batteries.4.An efficient membrane for helium separation from natural gas is quite crucial for cryogenic industries.However,most experimentally available membranes fail in separating helium from small molecules in natural gas,such as H2,as well as in 3He/4He isotopes separation.Using first-principles calculations,we theoretically demonstrated that the already-synthesized graphitic carbon nitride(g-C3N4)has high efficiency in helium separation from the gas molecules(H2,N2,CO and CH4)in natural gas and the noble gas molecules(Ne and Ar).The selectivity of He over Hz molecule at room temperature is calculated to be as high as 107.More interestingly,the g-3N4 membrane can also serve as a quantum sieving membrane for 3He/4He separation with a predicted transmission ratio of 18 at 49 K,thus offers a combined means of both He and 3He isotope separation.5.We report our first-principles calculations on the possibility of Ca-decorated silicene sheet and zigzag silicene nanoribbons(ZSiNRs)as hydrogen storage medium.We predict that Ca atoms prefer to disperse on the silicene or at the edges of ZSiNRs without clustering,due to the strong binding between Ca and Si atoms.By adsorbing Ca atoms on the both sides of silicene,the hydrogen storage capacity can reach to 6.17 wt%(gravimetric density)with an average adsorption energy of 0.265 eV/H2,which are quite optimial for reversible hydrogen adsorption and desorption at ambient conditions.The hydrogen storage capacity can be further improved to 8.43 wt%with the average adsorption energy in the range of 0.182-0.269 eV/H2 in the Ca-decorated ZSiNRs.These findings indicate that the Ca-decorated silicene and ZSiNRs have potential application in hydrogen storage.