Reversible hydrogen storage in the Ni-rich pseudo-binary Mg_6Pd_(0.25)Ni_(0.75) intermetallic compound: Reaction pathway, thermodynamic and kinetic properties

摘要

To improve the hydrogen storage properties of Mg_6Pd and to reduce its cost, Pd has been partly substituted by Ni at the solubility limit of the Mg_6(Pd,Ni) p-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_2Ni and Mg_6Pd_(0.7)Ni_(0.3) phases. At high pressure, the Mg_6Pd_(0.7)Ni_(0.3) phase disproportionates into MgH_2, Mg_2NiH_4, MgPd and Mg_5Pd_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.