The Effect Of Li,Be,Ca,Sr,Ba On The Hydrogenation Properties Of Mg₁₇Al₁₂ Alloy

How to decrease the(de)hydrogenation temperature of the hydrogen storage material is important for hydrogen energy.Mg17Al12 alloy is one kind of Mg-Al alloys that can perform the reversible(de)hydrogenation reactions,and its theoretical hydrogen storage capacity is up to 4.4 wt.%.However,the high(de)hydrogenation temperature hinders the application of Mg17Al12 alloy.In this paper,the effect of various catalysts on hydrogen absorption properties of the Mg1CAl12(110)surface was calculated based on the density functional theory.The catalytic action and mechanism of the catalyst on the Mg17Al12(110)surface were judged from the formation energy of the system,the adsorption energy of H2 and H atoms and the dissociation energy H2 needed to dissociate into two H atoms.Part of the paper was verified by experiments.The results show that the addition of catalysts,as Li and AM have improved the adsorption and dissociation of hydrogen on Mg17Al12(10)surface.The major results are as follows.1.On the basis of density functional theory(DFT),the catalytic effects of Li on hydrogen absorption properties of the Mg,7Al12(110)surfaces are studied.The Li-containing Mg17Al12(10)surfaces were composed of the Li-substituted and Li-adsorbed systems,namely Mg,6LiAl1:(110)and Mg17Al12(110)/Li surfaces,respectively.Their formation energy can confirm that they can exist stably.Calculations indicate that the H(H2)adsorption energy on the Mg17Al12(110)surfaces with a low amount of Li were significantly improved relative to hydrogen on the clean surface.Especially,the H(H2)adsorption energy of Mg17Al12(10)/Li system was-0.68(-0.48)eV,which was three times of the clean system.Moreover,the addition of Li markedly promotes the dissociative properties of H2 on the(110)surface.The barrier energy of H2 dissociation on the Mg17Al12(110)/Li surface was 0.14 eV,which was much lower than that(0.87 eV)on the pure surface.Analysis of electronic structures reveals that the H-s orbital hybridized with the Mg-s evidently in the Li-containing systems,while the hybridization of H-s and Li-s orbitals was also generated in the Li-adsorbed system.Furthermore,the charge density difference of Li-adsorbed system shows that the H2 was close to below Li atom and got some electrons from the latter.The formation of Li-H bond and the interaction between H2 and below Li atom may explain the enhanced hydrogenation properties of Mg17Al12(110)/Li surface.2.Based on density functional theory(DFT),the effects of alkaline-earth metal(AM)on the hydrogenation properties of the Mg-Al alloy are investigated.The related geometry structure,formation energy,H adsorption energy and electronic structure of the AM-doped(AM = Be,Ca,Sr and Ba)bulk Mg17Al12 alloy and Mg17Al12(110)surface are calculated.Results show that the improved hydrogenation properties of the AM-doped Mg17Al12(110)surface are achieved.Moreover,the adsorption energy of H on the Be-doped surface(-0.79 eV)is more excellent than that on the transition metal Ni-doped(-0.44 eV),light metal Li-doped(-0.33 eV)and clean(-0.18 eV)Mg17Al12(110)systems.Analysis of the electronic structures reveals that the Be-H,Ca-H,Sr-H and Ba-H bonds are formed at the same time with the generation of Mg-H bond.Be-H bond is strongest,followed by the Ba-H whose H adsorption energy(-0.31 eV)is also second.Our theoretical predictions may provide a way for hydrogenation of the Mg-Al alloy in experiments.3.Experimental study on the catalytic effect of Ba compounds(BaO,BaCl2,BaF2)doping on Mg17Al12 alloy was carried out and verified by calculation.Given the good overall hydrogenation properties of the Ba-doped system,the hydrogen absorption properties of Mg17Al12 doped with the Ba compounds are investigated experimentally,and the micro-mechanism of hydrogen absorption behavior of BaO-doped system was disscussed.Experimental results denoted that these samples' hydrogen storage capacity was up to 4.0 wt.%,and the hydrogen sorption temperature of Mg17Al12-BaO sample was lowest(187 ?)when the capacity reached to 1.0 wt.%.In the calculation model,(H2)BaO can physically adsorbed on the(BaO-doped)Mg17Al12(110)surface,while the single H atom prefers to adsorb on the bridge site between Mg and Al atoms.The O atom bonded with a neighbor Mg atom after hydrogen adsorption.For improvement,the hydrogen adsorption energy of BaO-doped system(-1.95 eV)was six times that of Ba-doped system(-0.31 eV),and ten times that of clean Mg17Al12(10)surface(-0.18 eV).Moreover,the H2 dissociation barrier energy of BaO-doped system(0.17 eV)was a fifth of that for the clean system(0.87 eV).