Preparation,microstructure And Hydrogen Desorption Properties Of Mg-ni Hydrogen Storage Films

Mg-based hydrogen storage materials are considered to be one of the most promising hydrogen storage materials due to their light weight,low cost,abundant source,large hydrogen storage capacity,and adapt hydrogen absorption/desorption platform.However,the stable thermodynamic properties and slow kinetic properties of the Mg/Mg H₂ system limit its practical application.Alloying and microstructure control can effectively improve its hydrogen storage properties,but there are also defects such as reduced capacity and so on;Thin films have the advantages of both,and the modification of the material on the nano-scale can avoid a substantial reduction in hydrogen storage capacity.In addition,thin film materials are more conducive to studying the microscopic phase transition before and after the reaction and revealing the reaction mechanism of hydrogen absorption and desorption.Magnetron sputtering is a common process method for preparing film materials,which can quickly and easily carry out structure control and composition design of them in the nanometer scale.In this paper,the two-component Mg-Ni composite hydrogen storage films were prepared by the method of magnetron sputtering and the distribution of H in the films was directly characterized by TOF-SIMS.Combined with first-principles theoretical calculations,the structure transition behavior and micro-mechanism of the Mg-Ni-(H)film before and after the reaction were studied;the effects of anisotropic electron migration caused by different Mg/Ni atomic ratios on the thermodynamics and kinetics of hydrogen adsorption and desorption reactions were explored;the stability of the Mg/Ni interface structure in the hydrogen absorption and desorption cycle was investigated.The main research results of this paper are as follows:First of all,the Mg_1-xNi_x(x=0.5,0.3,0.2,0.1)composite hydrogen storage films with different Mg/Ni atomic ratios were prepared by a rotating mask combined with a semi-co-sputtering process.The dehydrogenation temperature and dehydrogenation kinetics of the films were tested;the microstructure and the element distribution of the films were characterized before and after the hydrogen absorption and desorption reaction.The results show that the Ni content can significantly affect the dehydrogenation performance of the Mg-based films.The higher the proportion of Ni atoms,the lower the dehydrogenation temperature,but the lower the effective hydrogen storage mass;The semi-co-sputtering technology of rotating mask can make the mixture of Mg and Ni more uniform,and effectively promote the improvement of Ni on the hydrogen storage performance of Mg-based films.The phase of the as-deposited films varied with Mg/Ni atoms.It changed from amorphous and Ni nanocrystalline structure to all amorphous structure,and then to Mg nanocrystalline with(002)preferred orientation and amorphous structure with the increase of Mg/Ni atomic ratio;after hydrogenation,the films were mainly composed of Mg₂Ni H_0.3,Mg₂Ni H₄ and Mg H₂.Secondly,Mg-Ni-X(X=3,5,7,9)multilayer films with the same composition but different Mg/Ni interface numbers were prepared by alternately sputtering Mg and Ni and by controlling the sputtering time.The hydrogen desorption properties of all multilayer films were tested and compared with those of pure Mg films,it was found that the Mg/Ni interface could significantly promote the dehydrogenation behavior of the films,and the more Mg/Ni interfacial layers,the lower dehydrogenation temperature and the faster dehydrogenation kinetics.Among them,the hydrogen desorption capacity of Mg-Ni-9 film was as high as5.29wt.%;H element in the hydrogenated film was mainly distributed at the interface,indicating that the hydrogen absorption of it starts at the interface.By characterizing the microstructure of the films after the hydrogen absorption and desorption cycle,it can be seen that Mg(002)is bonded with Ni(111)at the Mg/Ni interface in the as-deposited multilayer films.After hydrogen absorption,the interface is disturbed due to the formation of hydrides Mg₂Ni H₄ and Mg H₂,whereas it can return to the combined state of Mg(002)and Ni(111)after dehydrogenation,which maintains well structural stability.Finally,the adsorption,diffusion and electronic properties of H at the Mg(0001)/Ni(111)interface were investigated by first-principles calculation method based on density functional theory.The results show that the adsorption of H atoms in the interface layer is more stable;and the diffusion energy barrier of H atoms in the interface layer is 0.257 e V,which is lower than 0.564 e V of Ni layer and 0.505 e V of Mg layer,showing a faster diffusion rate,which indicates that the interface can provide more diffusion channels of hydrogen atoms,thus promoting the adsorption and desorption of hydrogen.