Effect Of Transition Metals Ti,Zr,Nb,Ni,V And Their Carbide As Well As Oxide On Hydrogen Absorption Process Of Mg₁₇Al₁₂:First-principles Study

Hydrogen storage is very important for the large-scale application of hydrogen energy.The theoretical hydrogen storage capacity of Mg₁₇Al₁₂ alloy is4.44 wt.%,and it has the advantages of good reversibility,low weight density and low cost.However,slow kinetics limit its practical application.In this paper,based on density functional theory,the effect of transition metals and their compounds on the hydrogen absorption performance of Mg₁₇Al₁₂(110)surface was studied.Through the adsorption energy of H and H₂ and the energy barrier required for the dissociation of H₂ molecules into two independent H atoms were analyzed for the catalytic mechanism.The results showed that the transition metals and their compounds significantly improved the surface hydrogen absorption performance on Mg₁₇Al₁₂(110).The specific research results were as follows:1.The adsorption and dissociation behaviors of H and H₂ on Mg₁₇Al₁₂(110)surface modified by Ti,Ti C and Ti O₂ were studied by DFT.The results showed that the presence of Ti atoms could effectively improve the adsorption capacity of hydrogen on the surface of Mg₁₇Al₁₂(110).The density of states in the partial wave showed that there was a strong hybridization between the H s orbital and the Ti d orbital.Ti atoms showed better"hydrophilicity"than Mg.In addition,among all the Ti-contained systems,Ti O₂ system showed the best catalytic effect,following by Ti double substitutions system.In Mg₁₅Ti₂Al₁₂(110)and Mg₁₇Al₁₂(110)/Ti O₂systems,the adsorption energies of H were-1.07 e V and-1.49 e V,respectively,and H₂ could be spontaneously dissociated on the surfaces of Mg₁₅Ti₂Al₁₂(110)and Mg₁₇Al₁₂(110)/Ti O₂.2.On the basis of DFT,the catalytic effect of Zr and ZrO₂ on hydrogen absorption properties of Mg₁₇Al₁₂(110)surface was studied.The results were similar to those of the Ti-contained systems.The adsorption energies of all Zr-contained systems for H adsorption were better than that of clean system.The order for the absolute values of adsorption energy of H atom was as follows:Mg₁₇Al₁₂(110)/ZrO₂>Mg₁₅Zr₂Al₁₂(110)>Mg₁₆Zr Al₁₂(110)>Mg₁₇Al₁₂(110)/Zr.The adsorption energy of H atom in ZrO₂ system was-1.42 e V.The electron structure analysis showed that O atom bonds with Mg atom,making more Mg atoms on the surface of(110)were involved in the adsorption process of H.The mechanism of the effect of oxides on the surface hydrogen absorption of Mg₁₇Al₁₂(110)was explained.In addition,the H₂ molecules dissociation on the Mg₁₇Al₁₂(110)/Zr and Mg₁₇Al₁₂(110)/ZrO₂ surfaces could be spontaneous.3.The catalytic effect of transition metal TM(TM=Ti,Ni,V,Nb,Zr)double substitutions on the hydrogen absorption performance of Mg₁₇Al₁₂(110)was investigated by DFT.The results showed that TM dual substitutions significantly improved the adsorption and disintegration properties of(110)surface.Comparing with other TM atom double substitutions systems,the adsorption energy of H and H₂ on the surface containing Nb and Ti was significantly improved.For the dissociation of H₂,the systems containing Nb or V atoms showed good performance,in particular,H₂ dissociated spontaneously on(Ti+V),(Nb+Zr)and(Zr+V)surfaces.In a word,(Ti+V)system and all Nb-containing systems had better hydrogen adsorption and desorption properties.The electron structure analysis showed that there was a strong interaction between H atom and TM atom,indicating the formation of TM-H bond,which explained the mechanism of the effect of double substitutions on the surface hydrogen absorption performance of Mg₁₇Al₁₂(110).