Theoretical Study Of New Low-Dimensional Catalytic And Gas Storage Materials

With the development of social industrialization,energy shortage and environmental problems such as greenhouse effect are becoming increasingly serious.Therefore,it is urgent to develop new clean energy sources to replace the traditional fossil fuels,as well as energy-related gas storage materials to improve energy utilization efficiency.As a new energy conversion approach,photo(electro)catalysis can accelerate the reaction kinetics with the help of high-performance catalysts,thus greatly improve the energy utilization and conversion efficiency.The progresses of photo(electro)catalytic reactions and energy utilization are closely related to the storage of reactants and product gas molecules,making the materials that can efficiently store gas molecules quite important.Low-dimensional materials are ideal candidates for photo(electro)catalyst and gas storage,owing to their large specific surface area and rich electronic,optical and mechanical properties.In this dissertation,we studied the two-dimensional(2D)Fe3GeX2(X=S,Se and Te),2D metal-organic frameworks(MOF),one-dimensional(1D)carbon nitride nanowires/carbon nanotubes core-shell structure,2D black phosphorene(BP)and phosphorus carbide(?-PC)as photo(electro)catalyst and gas storage materials,by means of first-principles calculations.From the electronic structures of these low dimension materials,we revealed the origins of the catalytic reaction activity,such as sulfur reduction reaction,hydrogen evolution reaction,oxygen evolution reaction and so on.We also proposed the strategies for improving the catalytic activity and light absorption capability of photocatalysis,and integrating the catalytic process and gas storage in one system.These results offer a theoretical guidance for the relevant experimental works.The main research contents and innovative results are as follows:1.Lithium-sulfur battery is a potential energy storage medium with advantages of large capacity,environmental friendliness,and abundant sulfur resources.However,the low conductivity,shuttle effect of lithium polysulfides and slow reaction kinetics hinder the application of lithium-sulfur batteries.Presently,there are two strategies to solve these problems,one is to increase the adsorption strength of lithium polysulfides on the anchoring material to suppress the shuttle effect,the other is to use catalysts to accelerate the reaction kinetics.Based on first-principles calculations,we systematically studied the anchoring effect and catalytic mechanism of 2D magnetic Fe3GeX2(X=S,Se,Te)electrocatalyst in lithium-sulfur batteries.The metallic properties of Fe3GeX2 catalyst ensure the rapid electron transfer during the whole catalytic reaction process.Moderate adsorption of the soluble lithium polysulfides on Fe3GeX2 inhibits the shuttle effect.More interestingly,Fe3GeS2 shows bifunctional electrocatalytic activity for sulfur reduction reaction(SRR)and Li2 S decomposition,which can accelerate the reaction kinetics and improve the sulfur utilization and cycle life of lithium-sulfur batteries.This offers a new approach for the development of highperformance lithium-sulfur batteries.2.Hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)play important roles in the development of renewable energy conversion and storage technologies.Compared with unifunctional catalysts,bifunctional catalysts can not only improve the electrocatalytic performance,but also simplify the device structures and reduce thst.We propose a 2D MOF bifunctional catalyst,monolayer NIIT,for efficient overall water splitting.The active sites of the NIIT monolayer are mainly on the nonmetallic atoms,namely N atoms(HER)and C atoms connecting to S atom(OER).The spatial separation of active sites is conducive to the simultaneous bifunctional catalysis.The overpotentials for HER and OER are only-0.15 V and 0.50 V,respectively,indicating the high electrocatalytic performance.These overpotentials are comparable to those of the commonly used electrocatalysts containing precious metals.Frontier orbital analysis shows that the catalytic activity of the 2D MOF is strongly related to the charge states near Fermi level.This provides a new strategy for the design of non-noble metal catalysts.3.As a promising clean energy source,the production and storage of hydrogen are crucial techniques.Carbon nanotubes(CNTs)have unique characteristics of hollow structure and high mechanical strength.How to use the inner space of CNTs to realize the high-density hydrogen storage has always been research focuses.Here,we proposed an integral strategy for the production and storage of hydrogen in CNT containing a carbon nitride nanowire(CNNW).The first-principles calculations show that,it generates holes and electrons to be distributed respectively on active sites of the CNT and the CNNW by harvesting visible and ultraviolet light.Driven by intrinsic electrostatic field,protons produced by water splitting on the CNT surface penetrate the CNT and react with photo-generated electrons on the CNNW to produce hydrogen molecules in the CNT.The CNNW exhibit excellent HER activity with the overpotentials of-0.17 V,which can be overcome by the potentials of the photogenerated energetic electrons.The produced hydrogen molecules are naturally separated and stably stored inside the CNTs.Owing to the high mechanical strength of CNTs,the high-density hydrogen storage in CNTs can be realized.These results are expected to pave a new feasible way for the production and storage of hydrogen molecules in CNTs.4.In the gas storage process,the adsorption and desorption of gas molecules are usually controlled by changing in temperature or applying high voltage,which requires additional energy consumption.By combined first-principles calculations and molecular dynamics(MD)simulations,we propose that external tensile strain is also capable to regulate the adsorption strength and storage capacity of CO2 on BP and ?-PC.BP has high mechanical flexibility which can withstand tensile strain up to 30%along the armchair direction.?-PC adopts the puckered structure similar to BP,which can resist to critical strain up to 24%.The adsorption of CO2 molecules on the BP or ?-PC layer is all physical in nature,which can facilitate the adsorption and desorption process.The adsorption strength can be effectively controlled by the tensile strain along the armchair direction.In detail,the adsorption energy of CO2 on BP decreases with the uniaxial strain.However,for ?-PC,the adsorption affinity of CO2 on the H1 site can be enhanced and reaches a maximum value under the strain of 11 %.The application of tensile strain can effectively increase the CO2 storage capacity by 16.2% and 7.5%for BP and ?-PC,respectively.These findings provide a new way to tune the CO2adsorption/desorption process and storage capacity by 2D flexible layers.