Preparation, Structure Regulation And Hydrogen Storage Properties Of Mg-Ti Composite Films

In the context of the global energy crisis,hydrogen energy has received much attention as a new renewable clean energy source.The transportation and storage of hydrogen energy is an urgent problem that needs to be solved.Many researchers have turned their attention to safe,reliable and low-cost hydrogen storage metals?alloys?.The theoretical hydrogen storage capacity of pure Mg is as high as 7.6 wt%,but the thermodynamic performance of Mg hydride is stable,its high dehydrogenation temperature and slow absorption/desorption kinetics hinder the practical process of Mg as a hydrogen storage material.In order to improve the hydrogen storage performance of Mg-based hydrogen storage metals?alloys?,researchers have used magnetron sputtering to prepare Mg/TM composite films,combining the advantages of nanocrystallization,catalysis and alloying to regulate their thermodynamics and kinetics.After introducing Pd interlayers with high catalytic effect,the hydrogen absorbing kinetics of Mg/Ti multilayer were effectively improved,but the dehydrogenation performance changed little.Therefore,a series of Mg-Ti composite films were prepared by self-developed semi-co-sputtering process,which were optimized by film structure design and process parameter adjustment.The composition,structure and hydrogen storage properties of Mg-Ti composite films were comprehensively studied by XRD,SEM,EDS,TEM,hydrogen storage performance test and other analytical methods.Firstly,Mg/Ti multilayers were prepared.Pd interlayers were introduced to prepare Mg/Ti/Pd.To obtain Mg-Ti/Ti/Pd,the Mg layers were replaced by semi-co-sputtered Mg-Ti layers.The modulation structures were found in the columnar crystals of the Mg-Ti layers,which were consistent with the characteristics of the semi-co-sputtering process,and exhibited a periodic characteristic of alternating deposition of five Mg atomic layers and two Ti atomic layers.Mg-Ti composite films prepared by semi-co-sputtering possessed smaller grains.More importantly,the catalyst element Ti was dispersed in the Mg matrix in the form of atomic layers,which maximized the catalytic effect.Mg-Ti/Ti/Pd performed the best hydrogen absorbing kinetics,which can be completed in 100S,but the absorption is attenuated to 2.0wt%.The desorption of Mg-Ti/Ti/Pd was about 1.3 wt%at a temperature of 473 K,and the desorption of Mg/Ti/Pd and Mg-Ti/Ti/Pd were about 0.4 wt%;Compared with Mg/Ti/Pd?493K?and Mg/Ti?498K?,Mg-Ti/Ti/Pd?458K?also performed lower initial dehydrogenation temperature,indicating that the Mg-Ti semi-co-sputtered layers were able to improve the thermodynamic performance of the composite film.Secondly,to increase the hydrogen storage capacity,the relative content of Mg was increased by structural optimization and composition control.The content of Mg was effectively increased,and the relative atomic ratio is between 67%and 80%.Three groups of sample films were recorded as Mg67Ti33,Mg71Ti29 and Mg80Ti20 according to the relative atomic ratio,and their grain sizes were between 200nm and 400nm.The original films had a preferential growth direction in the?002?,the Mg?002?diffraction peak disappeared and the diffraction peak of MgH₂ appeared after hydrogenation at 423K.The diffraction peak of MgH₂disappeared and the diffraction peak of the Mg?002?reappeared after dehydrogenation at the same temperature,indicating that the Mg-Ti composite film prepared by semi-co-sputtering could absorb and desorb hydrogen at a relatively low temperature and performed good structural stability.Only a small amount of Mg-Ti modulation structures remained in Mg67Ti33,indicating that lower content of Mg was favorable for the formation of the modulation structures.The absorption of Mg67Ti33,Mg71Ti29 and Mg80Ti20 was 2.6wt%,3.4wt%and 3.8wt%at a temperature of 423K and a hydrogen pressure of 6bar,respectively.Mg67Ti33 possessed the fastest hydrogen absorption rate;at the same temperature,the desorption of Mg67Ti33,Mg71Ti29 and Mg80Ti20 were 2.1wt%,1.9wt%and 2.2wt%,respectively.The desorption of Mg67Ti33 was even higher than that of Mg71Ti29,indicating that the Mg-Ti modulation structure could promote the dehydrogenation reaction proceeds.Finally,a stable process of forming Mg-Ti modulation structures was intensively explored.The content of Mg was effectively reduced by fine-tuning of process parameters,and the relative atomic ratio was between 42%and 64%.Four groups of sample films were recorded as Mg42Ti58,Mg55Ti45,Mg60Ti40 and Mg64Ti36.And their grain sizes were 10 nm,20 nm,40nm and 60 nm,which were remarkably refined.The Mg?002?diffraction peak of the original film appears to shift to higher angle,which may be caused by Ti atoms occupying partial positions of Mg lattice in the modulation structures.Mg60Ti40 absorbed and desorbed hydrogen at a temperature of 403K,accompanied by the changes in the relative intensity of the characteristic diffraction peak and peak position shifts,indicating that the hydrogen atoms entered into the Mg lattice mainly in a manner of solid solution.The Mg-Ti modulation structures further improved its thermodynamic performance while ensuring excellent kinetics of the film.The Mg-Ti modulation structures existing in the columnar crystals of the films acted as a"highway"for transporting hydrogen atoms;the thermodynamic stability of Mg was changed because of the occupation of Ti atoms,which lowered the dehydrogenation temperature.