Preparation,Microstructure,and Hydrogen Storage Performance Of Ce-Mg-M Ternary Alloys

Magnesium based hydrogen storage materials have the advantages of high hydrogen storage capacity,high cycling stability,and low price,making them one of the most promising hydrogen storage materials.However,their high reaction temperature and slow hydrogen absorption and desorption kinetics severely limit the application and development of Mg-based hydrogen storage materials.The development of Mg-based hydrogen storage materials with low reaction temperature and high kinetics performance has a significant impact on their commercial application.This article takes Ce Mg₁₂ alloy as the research object and modifies it through methods such as nonstoichiometry,alloying,mechanical ball milling,and adding catalysts.The mechanism of improving the hydrogen storage performance of Mg-based alloys has been explored.Ce Mg₁₂ was alloyed with different metal elements using a vacuum induction melting furnace to prepare the as-cast Ce Mg₁₂M（M=Ca,Mn,Co,Ni）alloys with nonstoichiometric ratios.Research has found that the as-cast alloys before and after hydrogen absorption and desorption have a polycrystalline structure,and the reversible reaction that occurs in Ce Mg₁₂ alloy during hydrogen absorption and desorption is Mg+H₂?Mg H₂ and Ce elements generate stable Ce H₂ phase in-situ during the first hydrogen absorption.After the introduction of alloying elements,the hydrogen storage capacity of the alloy will slightly decrease.The introduced Ca element generates Ca H₂ phase during the first hydrogen absorption and stabilizes in subsequent hydrogen absorption and desorption cycles;The introduced Mn and Co elements are uniformly distributed in the alloy in the form of nanocrystals during the hydrogen absorption and desorption processes;The introduced Ni element forms a Mg₂Ni phase with Mg,adding a reversible reaction of Mg₂Ni+H₂?Mg₂Ni H₄ to the alloy during hydrogen absorption and desorption,which can serve as a hydrogen pump and improve the hydrogen storage kinetics of the alloy.Among the as-cast Ce Mg₁₂ and Ce Mg₁₂M（M=Ca,Mn,Co,Ni）alloys,Ce Mg₁₂Ni alloy has the best comprehensive hydrogen storage performance.Using Ce Mg₁₂Ni alloy as the research object,the effects of different catalyst types and their addition amounts on the hydrogen storage performance of the alloy have been studied.A hydrogen storage alloy with a nanocrystalline and amorphous structure was prepared by adding different amounts（0,3,5,7,10wt.%）of metal-poor oxide Bi₂O₃ to Ce Mg₁₂Ni alloy through ball milling.Bi₂O₃ exists stably in the form of a catalyst during the hydrogen absorption and desorption cycle of the alloy.With the increase of Bi₂O₃ addition,the hydrogen storage capacity of the alloy continues to decrease,and the hydrogen absorption and desorption rate of the alloy first increases and then slows down.When the amount of Bi₂O₃ addition is 5 wt.%,the hydrogen desorption rate of the alloy is the fastest and the dehydrogenation activation energy is the lowest of 63.89 k J/mol.Select 5wt.%as the fixed catalyst dosage and compare the effects of different catalyst types on the hydrogen storage performance of the alloy.The catalytic effects of alkaline earth metal oxide Ba O,transition metal oxide Ti O₂ and metal-poor oxide Bi₂O₃ were compared.It was found that the transition metal oxide Ti O₂showed the best catalytic effect,which was due to the fact that Ti has unsaturated d-layer electrons and can interact with the valence electron of H to weaken the Mg-H bond.Then,the catalytic effects of different transition metal fluorides Ni F₂,Ti F₃,and Zr F₄ were compared.It was found that the added transition metal fluorides generated nanocrystals during ball milling and hydrogen absorption and desorption processes,significantly reducing the grain size of the alloy,resulting in an increase in dislocation density of the alloy,which is beneficial for improving hydrogen storage kinetics performance.However,due to the inability of metal fluoride to reversibly absorb and desorb hydrogen,and the consumption of some Mg to generate stable Mg F₂,the addition of metal fluoride will significantly reduce the hydrogen storage capacity of the alloy.The experiment found that Ti F₃ has the most significant improvement in the hydrogen absorption kinetics of the alloy,resulting in a hydrogen absorption capacity of 3.982 wt.%within 5 min at 100℃,exceeding87%of the saturated hydrogen absorption capacity.Zr F₄ has the most significant improvement in the hydrogen desorption kinetics of the alloy,with the dehydrogenation activation energy reduced to 55.20 k J/mol.Finally,the influence of VB group metal elements（V,Nb,Ta）on the hydrogen storage performance of the alloy was compared.It was found that there are still some large VB group metal particles after ball milling,and VB group elements are prone to generate nanocrystals near Ce H₂ during the hydrogen absorption and desorption processes,limiting the full contact between VB group elements and Mg-based alloys.The addition of VB group metals V,Nb,and Ta leads to the introduction of new reversible reactions of V₂H+H₂?VH₂,Nb+H₂?Nb H and Ta+H₂?Ta₂H in the alloy during the hydrogen absorption and desorption cycles,respectively,which is beneficial for improving the hydrogen storage kinetics of the alloy.The alloy with V addition has the best comprehensive hydrogen storage performance.