Effect Of Rare Earth Substitution On Hydrogen Storage Properties Of Ca-Mg-Ni Compounds

On the basis of the review of the latest research progress of R-Mg-Ni?R=rare earth elements?hydrogen storage compounds,the Ca?197 pm?element in Ca-Mg-Ni compounds were partly substituted by Nd?182 pm?,Gd?180 pm?and Er?176 pm?with decreasing atomic radii.Characterization and analysis were measured by X-ray diffraction/Rietveld full-spectrum fitting,high-resolution electron diffraction,gaseous P-C-T hydrogen storage test and electrochemical properties tests.The effects of the substitution of rare earth elements on phase structure,gaseous hydrogen storage properties and electrochemical performance of AB₂,AB₃ and A₂B₇-type Ca-Mg-Ni compounds were systematically studied.The brief contents and results of this research are as follows:?1?After the substitution,CaMgNi₄ was converted from a C15 type structure?space group:Fd-3m?to a C15b type structure of R_0.5Ca_0.5MgNi₄?space group:F-43m?.The more ordered atomic sites in the lattice after the substitution resulted in a larger lattice parameter for Nd_0.5Ca_0.5MgNi₄?a=7.0591????than that of CaMgNi₄?a=7.0173????.Although the hydrogen absorption capacities of R_0.5Ca_0.5MgNi₄ alloys were reduced compared to that of CaMgNi₄?1.33 wt.%?,the equilibrium pressures for hydrogen absorption-desorption were significantly increased and flatter.The hydriding enthalpy of R_0.5Ca_0.5MgNi₄ changed from-46.3 to-26.8 kJ/mol H₂ with the decrease of cell volume from Nd_0.5Ca_0.5MgNi₄ to Er_0.5Ca_0.5MgNi₄.In addition,the R_0.5Ca_0.5MgNi₄ alloy electrodes showed better cycle stabilities and high-rate discharge performance than CaMgNi₄ because of their better corrosion resistance and faster hydrogen diffusion rates.?2?Both Ca₂MgNi₉ and RCaMgNi₉ exhibited the PuNi₃-type single-phase structures.The hydrogen storage capacity of Ca₂MgNi₉ can reach 1.73 wt.%,but its hydride was too stable to release hydrogen completely.Compared with Ca₂MgNi₉,the thermodynamics of hydrogen absorption-desorption of RCaMgNi₉ compounds were significantly improved.The hydriding enthalpy changed from-30.4 to-19.5 kJ/mol H₂ with the decrease of cell volume from 510.7???³?NdCaMgNi₉?to 498.1???³?ErCaMgNi₉?,showing favorable application prospects.In addition,the RCaMgNi₉ alloy electrodes exhibited better cycle stabilities and high-rate discharge performance than Ca₂MgNi₉,especially in the case of Nd substitution.The main reason for the poor cycle stability of Ca₂MgNi₉ was the severe phase decomposition during charge and discharge.?3?A₂B₇ alloys contained the 3R-type rhombohedral structure and the 2H-type hexagonal structure,and the content of the 3R phase increased as the radii of the substituted atoms decreased.The hydrogen storage capacities of the A₂B₇ alloys were further increased,but the equilibrium pressures for hydrogen absorption-desorption were more inclined.In terms of electrochemical performance,the corrosion resistance of the A₂B₇-type alloys in the alkaline solution was enhanced,but the tendency of phase decomposition was increased.?4?In contrast,for each structure,the Ca-Mg-Ni alloy has the largest hydrogen storage capacity,and the hydrogen storage capacities of the Nd,Gd,and Er substituted alloys decrease in turn;The equilibrium pressures for hydrogen absorption of the alloys satisfy a linear relationship with the average subunit volume,and the linear slope decreases with the increase of the AB₅ unit;For all alloys,the hydrogenation enthalpies and the average subunit volumes basically satisfy a linear relationship,that is,the absolute values of hydrogen absorbing enthalpies decrease as the unit cell volumes reduce,and their hydrides change toward the unstable state,which is more favorable to the process of hydrogen absorption and desorption.In general,the substitution of rare earth elements lead to more excellent hydrogen storage properties and electrochemical performances due to the more ordered atomic sites occupation.The Nd-substituted alloys have the best improvement effect on the Ca-Mg-Ni compounds,especially in the cyclic stabilities and the high-rate discharge performances.