First-principles Studies of Magnesium Hydride as a Hydrogen Storage Material
Author:Wu Xin Xing
Supervisor:yang jin long
Keyword:BC3 single sheet，Catalyst，charge polarization，first-principles study，hydrogen desorption reaction，hydrogen spillover mechanism，Hydrogen storage material，Kubas interaction，magnesium hydride nanowire，theoretical design
With the traditional energies（coal,oil,natural gas）gradually deplete,the environmental and energy problems become more and more serious.Looking for and utilizing a environmentally friendly and renewable clean energy is a serious challenge facing mankind.Hydrogen energy with a high energy density and burned without pollution,is generally considered as an ideal clean energy.The main technologies of hydrogen energy include:（1）the use of solar energy to produce hydrogen gas,（2）hydrogen storage（hydrogen storage material）,（3）hydrogen fuel cell applications.This paper deals with hydrogen storage materials,which contains five chapters.In chapter one,first,we introduce the first-principles method and DFT.Then,we introduce the quantum chemical software VASP.Last,we introduce the methods of the energy barrier calculations.In chapter two,first,we introduce the basic concepts of the hydrogen economy,hydrogen energy,and the use in reality（the demands decided by America’s department of energy）.Second,we introduce the interactions between the hydrogen storage materials and hydrogen molecule,and introduce the basic concepts of the hydrogen adsorption and desorption reactions（hydrogen capacity,thermodynamics,kinetics,hydrogen desorption enthalpy/energy,energy barrier et al.）.Third,we introduce the experimental research status about the hydrogen storage materials including the traditional metal hydrides,complex metal hydrides,chemical hydrides and porous materials.Four,we list the methods about the adjustments of hydrogen storage properties（decrease the materials’ size and add some catalysts）.Five,we introduce other parameters of hydrogen storage materials（effective hydrogen capacity,overall efficiency,purity of hydrogen gas,number of cycles etc.）.At last we list the theoretical advancements of some hydrogen storage materials（bulk,cluster,nanowire,thin film etc.）.In chapter three,we introduce the experimental advancements of MgH2 nanowires.Based on this,we adopt some structures’ modes consistent with experiments to investigate the hydrogen storage properties of Mg and MgH2 nanowires.The calculated results indicated that the surface structures of Mg and MgH2 nanowires are much different from those of bulk materials,which causes much lower stabilities of nanowires than those of bulk materials.We find that the hydrogen desorption energies and temperatures of MgH2 nanowires are lower than those of MgH2 bulk.While MgH2 nanowires become thinner,the hydrogen desorption energies and temperatures become lower which means easier hydrogen desorption reaction,which is consistent with experiment.The hydrogen desorption temperatures of MgH2 nanowires are between 324 and 450 K,which are much lower than that of bulk MgH2 488 K（the experimental value is 573 K）.Compared with bulk MgH2,the hydrogen desorption temperature of MgH2（96）nanowire with a diameter 2.63 nm decrease 39 K,while that of MgH2（6）nanowire with a diameter 0.63 nm decrease 164 K.While the diameter is thinner than 1.94 nm,the hydrogen desorption energy and temperature decrease rapidly,which means the nanowire with a diameter thinner than 1.94 nm has the best property of hydrogen desorption reaction.Furthermore,through the bader charge analysis,we give out a microscopic mechanism about the change trends of hydrogen desorption energies.When MgH2 nanowires become thinner,the bond interactions between Mg and H atoms become weaker which make the hydrogen desorption energies decrease.For the electronic structures,Mg nanowires are metal,and MgH2 nanowires are semiconductors with indirect gaps.The electronic structures are affected by the quantum size effect and surface effect.As the diameters of MgH2 nanowires decrease,the band gaps increase,showing the quantum size effect.The band gaps of MgH2 nanowires are smaller than that of MgH2 bulk due to the surface effect.Our calculated electronic structures need to be verified by experiment.In chapter four,we firstly introduce the experimental advancements of Mg/MgH2 thin films as hydrogen storage materials,and then introduce the transition metals catalysts.Based on these,the adsorption of a Pd atom on MgH2（110）surface are investigated theoretically.The calculated results indicate that the Pd atom prefers to stay on the subsurface,and then on the surface.We investigate the hydrogen desorption reactions from a Pd atom adsorbed on MgH2（110）surface.The energy barrier of the hydrogen desorption reaction is 1.802 eV for pure MgH2（110）surface.While a Pd atom exists on the subsurface,the energy barrier is 1.649 eV,indicating some content catalytic effect.However,while a Pd atom exists on the surface,the energy barrier is only 1.154 eV（111 kJ/mol）,indicating much better catalytic effect.The hydrogen desorption temperature of pure MgH2 is 573 K.After Pd adsorption,the hydrogen desorption temperature decease to be 367 K,which is much lower than that of pure MgH2,meaning that the reaction rate will increase rapidly.Our calculated energy barriers and hydrogen desorption temperatures are qualitatively consistent with the experimental values.At last,we discuss the microscopic process of the hydrogen desorption reaction through the hydrogen spillover mechanism.In chapter five,we theoretically investigate the possibility of some metal atoms adsorbed on BC3 single sheet as hydrogen storage material.The calculated results indicate that the Li and Ca atom prefers to be a single atom adsorption on BC3.The 3d transition metals（Sc～Zn）prefer to be a large cluster adsorption on BC3.One Li atom could adsorb four hydrogen molecules.One Ca atom could adsorb five hydrogen molecules.One Sc atom could adsorb five hydrogen molecules.The adsorption energy of Sc/H2 is the largest,the value of Li/H2 is the smallest,while the value of Ca/H2 is between them.Through the analysis of charge density difference and density of states,we find that the Li and Ca atom adsorbs hydrogen molecules through charge polarization,while the Sc atom adsorbs hydrogen molecules through charge polarization and Kubas interaction.When the Li atoms are 100%coverage on BC3,the hydrogen capacity is 10 wt%and the adsorption energy is 0.27 eV.When the Ca atoms are 100%coverage on BC3,the hydrogen capacity is 6.5 wt%and the adsorption energy is 0.3 eV.Our theoretically designed hydrogen capacities meet the demands of America’s department of energy.Furthermore,our calculated results need to be verified experimentally.