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

To improve the hydrogen storage properties of Mg6Pd and to reduce its cost, Pd has been partly substituted by Ni at the solubility limit of the Mg-6(Pd,Ni) rho-phase. The attained composition is Mg6.2Pd0.25Ni0.65 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 Mg6.2Pd0.25Ni0.65 rho-phase occurs leading to the formation of MgH2, Mg2Ni and Mg6Pd0.7Ni0.3 phases. At high pressure, the Mg6Pd0.7Ni0.3 phase disproportionates into MgH2, Mg2NiH4, MgPd and Mg5Pd2 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. (C) 2012 Elsevier B.V. All rights reserved.