Preparation And Hydrogen Production Characteristics Of Novel Magnesium Matrix Composites

Light metal Mg has the advantages of wide sources,mild reaction,simple process and high hydrogen production density.It is considered as a hydrolytic hydrogen production material with good application prospects and has become a research hotspot in recent years.However,Mg generates dense Mg（OH）₂ during the hydrolysis process,which hinders the continuous hydrolysis reaction,thus making the hydrolysis kinetics and the reduction of hydrogen conversion difficult to be applied in practice.In this thesis,Mg-Bi₂O₃,Mg-Bi₂MoO₆/CNTs（Carbon nanotube）and Mg-BiOCl/CNTs were prepared by ball milling and discharge plasma sintering to improve the hydrogen production performance and oxidation resistance of mg-based hydrogen production materials.The mechanism of hydrogen production was studied by XRD,SEM-EDS,TEM,XPS and DFT.Firstly,this paper prepared transition metal oxide magnesium matrix composites by ball milling method,and studied the effect of transition metal oxides（Bi₂O₃,Sb₂O₃,WO₃,MoO₃,MoO₂,CrO₃,V₂O₅,TiO₂）on the performance of hydrogen production by Mg hydrolysis.Among them,Bi₂O₃ and MoO₃ have better improvement effect on Mg hydrolysis reaction.Based on this,bismuth oxygenated salt is further selected as dopant to study its effect on Mg hydrogen production performance,and Mg-10 wt%Bi₂MoO₆ has better hydrogen production performance.The yield and maximum rate of hydrogen production were 858.4 mL·g^-1 and 1062.6 mL·g^-1·min^-1,respectively.The theoretical calculation results show that the Bi atom doping can change the local charge distribution of Mg,enhance the adsorption energy of Mg for H₂O,and reduce the adsorption energy of H atom after H₂O dissociation,promoting the hydrolysis reaction.Then,Mg-Bi₂MoO₆-C composites were prepared by ball milling on the basis of Mg-Bi₂MoO₆.The effects of carbon materials（GR,RGO and CNTs）on hydrogen production performance of Mg-Bi₂MoO₆-C composites were studied.The results showed that:The introduction of CNTs by hydrothermal preparation of Bi₂MoO₆/CNTs can more effectively improve the hydrogen production performance of Mg-water reaction compared with the direct addition of carbon materials in the ball milling stage.At 298.15 K,the yield,rate and maximum rate of hydrogen production of Mg-Bi₂MOO₆/CNTs reached 860.9 mL·g^-1,99.2% and 2172.4 mL·g^-1·min^-1,respectively.The mechanism of hydrogen production was further studied.The results showed that CNTs covered on the surface of Mg powder prevented the agglomeration of Mg,resulting in the formation of smaller Mg in the composite after ball milling.In addition,during the ball milling process,MoO₃ and elemental Bi were generated in situ,which promoted the hydrolysis reaction.In addition,BiOF,BiOBr and BiOCl were synthesized by solution method,doped with Mg powder by ball milling method,and a series of Mg-BiOX（X=F,Cl,Br）composites were prepared by discharge plasma sintering method.The results show that BiOCl can effectively improve the hydrogen production performance of Mg.At 333.15K,the hydrogen production yield of Mg-BiOCl is 832.3 mL·g^-1,the hydrogen production rate is 99.1%,and the maximum hydrogen production rate is 593.8 mL·g^-1·min^-1.Antioxidation experiments showed that the hydrogen yield of the bulk material was 76.3% after 21 days of exposure to air.The mechanism shows that the elemental Bi and Mg produced by the solid phase reaction of BiOCl/CNTs during SPS sintering form a corrosive cell structure,thus accelerating the reaction.