Reversible hydrogen storage in the Ni-rich pseudo-binary Mg6Pd0.25Ni0.75 intermetallic compound: Reaction pathway, thermodynamic and kinetic properties

Abstract To improve the hydrogen storage properties of Mg 6 Pd and to reduce its cost, Pd has been partly substituted by Ni at the solubility limit of the Mg 6 (Pd,Ni) ρ-phase. The attained composition is Mg 6.2 Pd 0.25 Ni 0.65 as determined by Energy Dispersive X-Ray (EDX) and X-Ray Diffraction (XRD). Hydrogenation of this compound has been investigated by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM–EDX), Pressure-Composition-Isotherms (PCI) and thermal desorption analysis. On absorption, it decomposes in two steps as evidenced by two distinct plateau pressures. At low pressure, a partial segregation of Mg and Ni out of the pseudo-binary Mg 6.2 Pd 0.25 Ni 0.65 ρ-phase occurs leading to the formation of MgH 2 , Mg 2 Ni and Mg 6 Pd 0.7 Ni 0.3 phases. At high pressure, the Mg 6 Pd 0.7 Ni 0.3 phase disproportionates into MgH 2 , Mg 2 NiH 4 , MgPd and Mg 5 Pd 2 phases. The hydrogenation reaction is reversible providing a hydrogen capacity of 5.6 wt.% H. The reaction enthalpy of the high pressure plateau is less negative than for pure Mg. Furthermore, the activation energy for H-desorption exhibits a dramatic decrease for hydrogen contents above 4 wt.% H, i.e. after the alloy disproportionation.