Preparations,microstructures And Hydrogen Storage Properties Of Mg/Nbo_x(x=0~2.5)composite Films

Magnesium is considered as the most promising solid-state hydrogen storage material for its high energy density,environmental benefits,low cost,and abundant resource.The theoretical hydrogen storage capacity of pure Mg is up to 7.6 wt%,but the sluggish de/hydriding kinetics and excessive thermodynamic stability of Mg/MgH₂ hindered the practical applications.Thermodynamic and kinetic tuning of magnesium-based systems are mainly via alloying with other elements,nanostructuring of Mg,catalyzing,forming composite structures,changing reaction pathways,and so on.The Mg/TMOx composite films using for hydrogen storage can coordinately tune the kinetics and thermodynamics of Mg?MgH₂ reaction,which simultaneously takes the advantage of nanostructuring,catalyzing,and alloying.However,it seldom be seen the study of exploring the effects of original growth morphologies and microstructures of Mg/TMOx composite films on their hydrogen storage properties,and there is still lack of the microstructural evolutions to directly illuminate the catalytic mechanism of transition metals and their oxides affecting on Mg?MgH₂ reaction.A class of effective catalysts for Mg?MgH₂ reaction are transition metal Nb and its oxide Nb₂O₅.Therefore,in this work,we prepared a series of Mg/NbO_x composite films by magnetron sputtering to exactly tailor the sizes,compositions,interfaces,and crystallinities of constituent phases at nanoscale,and synthetically investigated the relationship of their morphologies,microstructures and properties via the testing methods of XRD,SEM,TEM,XPS,and Sieverts-type Gas Reaction Controller,employing intensive analysis to the catalytic mechanisms of transition metal Nb and multi-valence NbOx on Mg?MgH₂ reaction.The results of our study are shown as follows:Firstly,the sputtering parameters for the preparation of Mg/Nb composite films were optimized.We studied the effects of orignal growth morphologies and microstructures on their dehydrogenation properties via changing the sizes,heating the substrates and adding bias voltages during sputtering to regulate the strcutures of Mg/Nb 8-layer films.Results show that the Nb interlayers with thicknesses of 1nm and 2nm exhibited the superior catalytic effects,which can lower the dehydrogenation temperatures to 110 °C and 122 °C,respectively.Besides,it is found that substrate temperatures and bias voltages can affect the dehydrogenation performances via altering the growth morphologies and microstructures of Mg/Nb 8-layer films.The substrate temperature of 125 °C and bias voltage of 300 V during sputtering presents the vastest enhancement to the dehydrogenation performances of Mg?100 nm?/Nb?1 nm?8-layer film and Mg?100 nm?/Nb?10 nm?8-layer film,and the dehydrogenation temperatures can decrease to 100 °C and 114 °C,respectively.Secondly,the hydrogen storage properties and microstructural evolutions during de/hydrogenation processes of Mg/Nb,Mg/Nb₂O₅,and Mg/?Nb₂O₅+Nb?composite films were compared.We employed in-depth microstructural characterizations of Mg?100 nm?/NbOx?10 nm?32-layer films,and provide illuminating insights into the catalytic mechanism of multivalence NbOx on Mg?MgH₂ reaction.For the Mg/Nb,Mg/Nb₂O₅,and Mg/?Nb₂O₅+Nb?32-layer films,the third exhibits the best de/hydrogenation properties.The Mg/?Nb₂O₅+Nb?32-layer film can absorb 4.9 wt% H₂ within 3 min at 225 °C and 4.2 wt% H₂ within 15 min at 200 °C under a hydrogen pressure of 3 MPa,respectively.Meanwhile it can release 2.6 wt% H₂ within 50 min at 225 °C.For the Mg/?Nb₂O₅+Nb?32-layer film,the hydrogen evolution starts at 132 °C and the peak temperature of hydrogen desorption is 180 °C,and the activation energy of dehydrogenation is approximately-42 kJ/mol-1.The superior catalyzing effects on the de/hydrogenation reactions extremely depended on the phase transitions occurred among binary NbOx?Nb,NbO2,Nb₄O₅,and Nb₂O₅?and Mg-Nb-O ternary compounds?MgNb2O3.67 and MgNb0.5O2.25?during the sputtering,hydrogenation,and dehydrogenation processes,respectively.The multiple-valence Nb ions can act as an intermediate for the electron transferring between Mg/Mg²⁺ and H/H-.Finally,we fabricated Mg-Nb alloy films by co-sputtering to compare the differences of properties and catalytic effects between co-sputtered and multilayer films,and studied the effect of Nb content on the morphologies,microstructures,and hydrogen storage properties of MgNb alloy films.It is found that the co-sputtered Mg_xNb1-x alloy films exhibite different phase structures with the Nb content increasing,which respectively form Mg?Nb?solid solution,bcc-?Mg,Nb?,and Nb?Mg?solid solution.The hydrogen absorption capacity of Mg_xNb1-x alloy films generally decrease with the Nb content increasing,and they all exhibit the fast hydrogenation kinetics at 200 °C under a hydrogen pressure of 3 MPa,which just take several minutes to achieve the saturated hydrogen absorption capacity.The Mg0.90Nb0.10 alloy film shows the maximum capacity,which can absorb 5.7 wt% H₂ within 8 min.Aslo,the dehydrogenation speeds of all Mg_xNb1-x alloy films at 250 °C are no big differences,and the dehydrogenation capacity generally decrease with the Nb content increasing.Besides,the Mg0.90Nb0.10 alloy film aslo shows the maximum capacity,which can release 4.46 wt% H₂ within 60 min.The hydrogenation kinetics of the Mg0.90Nb0.10 alloy film is obviously superior to the Mg?100 nm?/Nb?10 nm?32-layer film at 200 °C under a hydrogen pressure of 3 MPa,which must be attributed to the differences of Nb existing forms and distributions in both.