Study On Hydrogen Absorption And Desorption Characteristics And Mechanism Of R-Mg-Ni Compounds(R=Nd,Gd,Er)

R-Mg-Ni layer intermetallic superlattice compounds are stacked repeatedly by[RMg Ni₄]units with different ratios of[RNi₅]units along the c-axis direction.They have been widely studied in electrochemical performance as the negative electrode of nickel-hydrogen battery.But the mechanism of these alloys in gaseous hydrogen sorage should be studied furtherly.The difference of rare earth R and the alter of[RNi₅]/[RMg Ni₄]ratio can greatly change its hydrogen storage mechanism,which directly affects its hydrogen storage performance.In this paper,Nd,Gd and Er with atomic radii of 1.82,1.80 and 1.76?are selected as the research objects.Laves phase RMg Ni₄ and layered R₂Mg Ni₉,R₃Mg Ni₁₄ and R₄Mg Ni₁₉superlattice compounds were prepared by fixing R/Mg=1 in[RMg Ni₄]unit and sintering again after high-frequency melting.The phase structure was accurately determined by X-ray full spectrum fitting,and the phase and composition were further determined by back-scattering electron diffraction（EBSD）,transmission electron microscope（TEM）and energy spectrum（EDS）.Their hydrogen ab/de-sorption properties and cycle stability were measured by sieverts device.The effect of rare earth atomic radius on the hydrogen storage performance of Laves type compound RMg Ni₄ was studied,and the correlation between layered compounds R₂Mg Ni₉,R₃Mg Ni₁₄ and R₄Mg Ni₁₉ and[RMg Ni₄]unit hydrogen storage performance was further studied.The obtained alloys RMg Ni₄ and R₂Mg Ni₉ are single-phase and have Mg Cu₄Sn and Pu Ni₃ structures respectively.R₃Mg Ni₁₄ and R₄Mg Ni₁₉ superlattice compounds are mainly composed of 2H and 3R isomers and a small amount of other phases.With the decrease of the radius of rare earth atoms,the lattice constant and cell volume decrease.In layered compounds,Mg atoms only replace the rare earth position in the[rmgni4]structural unit,but do not enter the[RNi₅]unit.The structural units of 2H and 3R phases are consistent,but the stacking repetition periods are different.Their structural unit parameters（a,c/N）are close.At the same time,the reduction of rare earth atom radius promotes the formation of3R phase in both R₃Mg Ni₁₄ and R₄Mg Ni₁₉compounds.RMg Ni₄ and layered R₂Mg Ni₉,R₃Mg Ni₁₄ and R₄Mg Ni₁₉ superlattice compounds can absorb and release hydrogen reversibly and have good structural stability.After 50 hydrogen absorption and desorption cycles,there is no phase decomposition and hydrogen induced amorphous phenomenon.When the fixed[rmgni4]unit R/Mg=1,R_（Nd,Mg）/R_Ni=1.38,so the criterion of hydrogen induced amorphous can be improved,which needs to be further studied.The size of rare earth atoms is the main factor affecting the hydrogen storage capacity.With the decrease of the radius of rare earth atoms,the gap occupied by hydrogen atoms becomes smaller,the hydrogen storage capacity decreases,and their hydrogen absorption and desorption platform pressure increases.R₃Mg Ni₁₄ and R₄Mg Ni₁₉ superlattice compounds have polytypic structures.The hydrogen absorption and desorption properties of 2H and 3R phases are close,and there is only one platform pressure in the whole hydrogen absorption and desorption process.With the decrease of the radius of rare earth atoms,the enthalpy of hydride formation decreases,and the absolute value of enthalpy of compounds formed by rare earth Er is quite different from that of compounds formed by Nd and Gd.Lattice strain is the main reason for the attenuation of hydrogen storage capacity.With the decrease of the radius of rare earth atoms,the gap radius that can be occupied by hydrogen atoms decreases,and the retention of hydrogen atoms in the lattice leads to the increase of lattice defects.At the same time,the interaction between hydrogen atoms and lattice defects further induces dislocations,and the lattice strain increases with the decrease of the radius of rare earth atoms.In layered superlattice compounds,[RMg Ni₄]shrinks and[RNi₅]expands to adapt to the same size of a-axis.With the increase of the proportion of[RNi₅]units,the structural unit parameters（a,c/N）of RMg Ni₄,R₂Mg Ni₉,R₃Mg Ni₁₄ and R₄Mg Ni₁₉ series compounds decrease.With the increase of[RNi₅]unit layer,the lattice holes that can be occupied by hydrogen atoms increases,and the hydrogen storage capacity increases.As the enthalpy of hydrogen absorption decreases,the equilibrium pressure increases and the hysteresis decreases significantly in the process of hydrogen absorption and desorption.The greater the binding effect of[RNi₅]unit on[RMg Ni₄],the more uniform strain is shown,and the more stable the cycle process of hydrogen absorption and desorption.The hydrogen storage performance and cycle stability of layered R₂Mg Ni₉,R₃Mg Ni₁₄ and R₄Mg Ni₁₉ compounds are closely related to[RMg Ni₄]unit.Among the three rare earths,Nd and Gd are more suitable to be replaced as A-side atoms,and Er also has good stability,but its hydrogen storage capacity is relatively low.They can be replaced with rare earth atoms with larger atomic radius to improve their properties.