Role Of Surface And Interface On Hydrogenation/Dehydrogenation Of Mg-H Systems: A Theoretical Study

The hydrogen storage content of mangesium hydride is relativelyhigh with a gravimetricdensity of7.6wt%, a volumetric density of110gL-1,and an energy density of9MJkg-1. Thenature has a wide abundance of low cost magnesium, leading Mg a promising candidate forthecommercial hydrogen storage materials.In the Introduction,this dissertationintroduces the world energy crisis and the energydilemma that China is facing, and then comes up with a solution of application of cleanhydrogen energy. Afterwards, a detailed review of hydrogen storage materials, whichemphasizes the potential and application obstacles of Mg-based hydrogen storage materials, isprovided.The review continues to focus on solutions for hydrogen storage improvements, andsome other existing problems. It is further indicated that the present Mg-based hydrogen storagematerials still need more fundamental researches on hydrogenation properties of Mg surfaces.Using the first-principles approaches, we systematically studythe stabilities andrelaxations of various Mg surfaces with different Miller indexes in Chapter Two, researches thestrain effect on the Mg(0001) hydrogen storage properties in Chapter Three, investigates thetheorectical modelling methods and the stability-determining factors of Mg/MgH2interfaces,and discovers the atoimic adsorption and diffusion mechanism of hydrogen on Mg(1013) slab.In Chapter Two, we systematically discussthe stabilities and relaxations of various Mgsurfaces with different Miller indexes. Usually the stabilities of high index surfaces arelowerthan that of low index ones, but this prediction is not always true for Mg surfaces. Thecomputational results show that Mg(1010) is the most unstable surface in the series of Mg(101n)(n=1-9). Due to the fact that two kinds of bonds, i.e. the basal bonds and non-basal bonds, withvarying bond energies exist in hcp Mg, the number of breaking these two kinds of bonds willdirectly impact surface energies.Therefore the low-index Mg surfaces are not necessarily stable,as they may possess of higher density of broken bonds. Later, a surface-energy predicting modelis built to quantitatively explain the relative Mg surface stabilities. The last part of Chapter Twofinds that the Friedel oscillation is the mechanism of low index Mg surface relaxations. But for the high index ones, the mechanism is a combination of charge smoothing effect and dramaticcharge depletion, rather than the Friedel oscillations.In Chapter Three, we further investigate biaxial strain effects on hydrogenation propertiesof Mg(0001) slab. The phase diagram of structural stabilities shows biaxial strains significantlyinfluence the hydrogenation properties of the system. When the coverage is less than2ML, thedominating adsorption structure is H-Mg-H trilayer. The lattice constants of the trilayer arefound to be less than those of Mg(0001), thus, from a thermodynamic perspective, negativestrains (i.e., compress) are helpful for the formation of trilayers in initial stage of hydrogenstorage. As the H coverage increases, the MgH2(110) bulk-like structure starts to prevailenergetically. Sinceits lattice constants are larger than those of Mg, the tensilestrains arebeneficial for the MgH2(110) bulk-like structure. As the H coverage increases further, theenergy advantage between the H-Mg-H trilayer and MgH2(110) bulk-like structure is reversed,thus leading to a phase transition from Mg to MgH2. In Chapter Three, it is concluded that thestrains is capable of tuning the Mg hydrogenation/dehygrogenation properties.In Chapter Four, we systematically discussthe theorectical modelling methods and thestability-determining factors of Mg/MgH2interfaces, and the defect formations in interfaceregions. The considered interface stalibity-determining factors include the surface energies oftwo phases, the interface mutual constants, and the relative position of two phases. The conceptof the mutual constants is proposed to study the elastic effects on interface stabilities by tuningthe lattice constants and two phase relative position of interface models. The results manifestthat the surface energies of Mg and MgH2have the biggest influence on the Mg/MgH2interfacial energies. The mutual constants and two phase relative position also induce thechange in interfacial energies. It is suggested that these three factors should be fully consideredduring the modelling process. Mg-H bonds are the key to interface stabilities. Lastly in ChapterFour, the interface and strain effects on defect formation are studied and both found to bebeneficial to defect formations.In Chapter Five, the atoimic adsorption and diffusion mechanism of hydrogen onMg(1013) slab are discovered and made comparisons with the Mg(0001) cases. Firstly, the Mg(1013) slab is successfully prepared by experimental groups, and the excellent H soroptionproperites of the slab, i.e. H desorption temperature of392K, are verified, compared to592Kof Mg(00001) slab. Then, the theoretical results indicate that the more numbers of danglingbonds on Mg(1013) lead to easier hydrogen adsorption at low coverage. Interestingly, the largenumber of non-closed-packed tunnels on Mg(1013) highly facilitates the hydrogen diffusionand makes a big difference with the Mg(0001) cases. Verification and explanation of H expresspenetration on Mg(1013) is proved by the theoretical calculation results.