Theoritical Study On The Thermodynamic Stability Of Reversible Hydrogen Ab/desorption About Ca₇Ge-type Mg Based Hydrides

The theoretical hydrogen storage capacity of Mg₇TiH₁₆ with a crystal structure of Ca₇Ge type is 6.9wt.%,which makes it be a potential hydrogen storage material.However,the dehydrogenation temperature is 605K that is a obstacle for its application;besides,there is no binary Mg-Ti alloy,the hydride cannot absorb hydrogen repeatedly in the form of Mg₇Ti that means the hydrogen ab/desorption process is irreversible.It was found that a fluorite-like Mg₇TiH₁₆ phase appeared in Mg-Ti-H multilayer,which can reversibly ab/desorb hydrogen,but it is not clear about mechnism of keeping the stability of Mg₇TiH₁₆ before and after dehydrogeation in the multilayer structure.Therefore,this paper first proposed that alkali metals doping Mg₇TiH₁₆ and studied the effect of alkali metals on dehydrogenation thermodynamics and stability of Mg₇TiH₁₆ after hydrogen release;secondly,the stability and dehydrogenation performance of the MgH₂/TiH₂,TiH₂/Mg₇TiH₁₆ and MgH₂/Mg₇TiH₁₆ interface were investigated by the methods of First Principle Theory,ab Molecular Dynamics（AIMD）,Climbing Image Nudged Elastic Band（CINEB）,the mechanism of Mg-Ti-H multilayer stabilizing Mg₇TiH₁₆ was explored;based on the above results,the influence of the interface on the reversible hydrogen absorption and desorption of Mg₇TiH₁₆ doped with alkali metal is further discussed;in addition,taking full advantage of the Ca₇Ge crystal structure of Mg₇TiH₁₆,the Mg₇TiX2（X=C,N,O,F,S and Cl）alloys with the same structure were designed,and their feasibility as a hydrogen storage material were explored.The effects of alkali/alkali earth metal（Li,Na,K and Ca）doping on the dehydrogenation thermodynamics of Mg₇TiH₁₆ were studied.Among the four kinds of doping metals,Li doping can not only ensure the stability of the doping system,but also reduce the hydrogen release energy.Li substitution doping weakens the interaction between Mg-H and Ti-H,and thus reduces the release energy of H around the doped atom.Although the hydride structure of Mg_7-xLi_xTiH₁₆（x=3.125at.%～25at.%）with different Li doping concentration is thermodynamically stable,Mg_7-xLixTi would collapse and can not absorb and release hydrogen reversibly.When all Mgl atoms are replaced by Li atoms to form Mg₆LiTiH₁₆,the hydrogen storage capacity increases from 6.9wt.%of Mg₇TiH₁₆ to 7.44wt.%.There are only two kinds of tetrahedral interstice in the Mg₆LiTiH₁₆,so the interaction between metal-H is relatively uniform,and the crystal structure of the hydride is symmetrical.The first principle molecular dynamics study shows that Mg₆LiTiH₁₆ structure may start to be unstable and H atom will dissociate at 500K.In order to study the interaction of hydrides in Mg-Ti-H multilayers,MgH₂/TiH₂,TiH₂/Mg₇TiH₁₆ and MgH₂/Mg₇TiH₁₆ inerfaces were built.The MgH₂/Mg₇TiH₁₆ interface with the smallest interfacial adhension work will be formed first when Mg/Ti multilayer is prepared under the atmosphere of H2.In TiH₂/Mg₇TiH₁₆ and MgH₂/TiH₂ interfaces,the TiH₂ slab is more likely to play the role of catalyst to accelerate hydrogen release.And the construction of the interface has a positive effect on the improvement of dehydrogenation thermodynamics of hydrides,the most ideal interface model among the three interfaces is MgH₂/Mg₇TiH₁₆.Although there is no Mg₇Ti alloy,Mg₇Ti in Mg/Mg₇Ti interface can be stable by Mg atom losing charge and Ti atom obtaining charge.The hydrogen prefer to first absorb in the tetrahedral interstice around the Ti atom in the interface of Mg/Mg₇Ti,and the strong bond between Ti and H has a positive effect on the hydrogen absorption process.The establishment of the interface is helpful to maintain the stability of Mg₇TiH₁₆ after dehydrogenation and to realize the reversibility of hydrogen absorption and desorption.The stability of MgH₂/Mg₆LiTiH₁₆ interface before and after hydrogen release was discussed,and the dehydrogenation thermodynamics of MgH₂/Mg₆LiTiH₁₆ interface was investigated.The appearance of the interface reduces the hydrogen release energy of Mg₆LiTiH₁₆ slab,and the interface of Mg/Mg6LiTi formed after hydrogen release is stable.The study of electronic structure shows that uniform electron gas and a small amount of charge accumulate in the interface area.The metal atoms on both sides of the interface keep the stability by metal bonds and a small amount of ion bonds.In the Mg₆LiTiH₁₆ slab,Ti atoms obtain the charge,Mg and Li atoms lose the charge.However,compared with the Mg₆LiTiH₁₆ bulk,the number of charge Ti atoms obtaining is significantly reduced,and the number of charge Mg atoms losing is also reduced,while the number of charge contributed by Li is slightly increased.The change of the number of gain and loss electrons reflects the change of interaction of Mg-Li-Ti,which is also the main reason that Mg₆LiTiH₁₆ can keep stability in the interface structure after hydrogen release.Seven kinds of Mg₇TiX2（X=C,N,O,F,P,S and Cl）alloys with Ca₇Ge structure were designed.Among them,three kinds of alloys,Mg₇TiF₂,Mg₇TiO₂ and Mg₇TiS₂ are stable and can be synthesized.After fully hydrogenation,the formation energies of the three hydrides are negative,and the hydrogen storage capacity of Mg₇TiF₂H₁₄,Mg₇TiO₂H₁₄ and Mg₇TiS₂H₁₄ are 5.23wt.%,5.35wt.%,4.76wt.%,respectively.Mg₇TiO₂ and Mg₇TiS₂ become unstable under the temperature of 300K,while the formation energy of Mg₇TiF₂ is still negative at 300K or even higher temperature（400K）,which thermodynamically stable.The phonon calculation of Mg₇TiF₂ shows that there is no virtual frequency in the phonon spectrum and the structure is stable,which is expected to become a new hydrogen storage material.