Preparation And Hydrogen Storage Properties Of Mg-M(M=Ga, Ba) Based Hydrogen Storage Alloys

Mg-based hydrogen storage alloys are considered as one of the most promising hydrogenstorage alloys due to their high hydrogen storage capacity, abundance resources andenvironment-friendly. However, MgH₂are limited in practical application for its demerits ofhigh de/hydriding temperature and sluggish kinetics. Based on the review of research anddevelopment of Mg-based hydrogen storage alloys. The hydrogen storage properties andphase transition de/hydrogenation of Mg-M （M=Ga、Ba） binary alloys are studied in thispaper. Mg₅Ga₂,Mg₁₇Ba₂alloys were prepared by induction melting, while Mg/Mg₅Ga₂,MgH₂/Mg₅Ga₂composites were prepared by mechanically milling. The microstructure andphase transformation de/hydriding samples were identified by using X-ray diffraction （XRD）and Scanning Electron Microscope （SEM）. Their hydrogen storage properties were measuredby Gas Reaction Controller （AMC） and Differential Scanning Calorimetry （DSC）. Thede/hydriding mechanism and hydrogen storage properties of Mg-M（M-Ga、Ba） binary alloyswere revealed.For Mg-Ga system, The de/hydriding mechanism of Mg₅Ga₂alloy was analysised. Theresults showed that the de/hydriding reaction of Mg₅Ga₂alloy is reversible, and the equationcan be depicted as: Mg₅Ga₂+H₂2Mg₂Ga+MgH₂. Mg₅Ga₂phase can reduce thedehydring temperature of MgH₂. The dehydring process of MgH₂/Mg₅Ga₂composite includestwo steps: one is Mg₂Ga reacted with MgH₂to form Mg₅Ga₂and H₂, and the enthalpy andentropy of this reaction are68.7kJ mol^-1H₂and123.7J K^-1mol^-1, respectively. On the otherside MgH₂decomposed to Mg and H₂, with the enthalpy and entropy of this reaction being77.1kJ mol^-1H₂and135.9J K^-1mol^-1, respectively. The dehydriding enthalpy and entropy ofMg/Mg₅Ga₂composite are76.9kJ mol^-1H₂and139.4J K^-1mol^-1, respectively. The activationenergy of dehydriding of Mg/Mg₅Ga₂composite is153kJ/mol, which is lower than pureMgH₂. Thermal decomposition analysis indicates the dehydriding temperature of Mg/Mg₅Ga₂composite is also lower than pure MgH₂。For Mg-Ba system, the phase transition during de/hydriding process and hydrogenstorage properties of Mg₁₇Ba₂alloy were investigated. XRD patterns showed that theMg₁₇Ba₂alloy reacted with hydrogen and transformed into MgH₂and Ba₆Mg₇H₂₆; while for the dehydriding process, MgH₂and Ba6Mg7H₂6transformed to Mg₁₇Ba₂、Ba₂MgH₆and Mg.The reversible hydrogen storage capacity of Mg₁₇Ba₂alloy is4.2wt.%H₂. Mg₁₇Ba₂alloy couldhydrogenated at543K, and thermal decomposition analysis indicated the dehydridingtemperature of the alloy is633K, which is about50K lower than pure MgH₂. Meanwhile thedehydriding enthalpy and the de/hydriding activation energy were calculated, and comparedwith that of MgH₂.