Thermodynamics and reaction pathways of hydrogen sorption in Mg6(Pd,TM) (TM = Ag, Cu and Ni) pseudo-binary compounds

• Mg6Pd1−xTMx (TM = Ag, Cu and Ni) compounds are synthesized at the TM solubility limit.
• PCI absorption curves exhibit two plateau pressures.
• The high plateau pressure is well above the one of Mg/MgH2.
• The formation of Mg–(Pd,TM) and Mg2NiH4 phases contributes to hydride destabilization.
• A linear correlation between enthalpy and entropy is evidenced.

Mg6(Pd,TM) (TM = Ag, Cu and Ni) pseudo-binary compounds have been synthesized at the TM solubility limit to determine the influence of TM on the thermodynamics and reaction pathways of the Mg6Pd–H system. All compounds exhibit a two-plateau pressure behaviour, being the value of the high plateau pressure well above that of the Mg/MgH2 system. Such destabilization is explained by the formation of different Mg–(Pd,TM) intermetallics and/or Mg2NiH4 hydride phases during the hydrogenation reaction. The formation of these phases not only increases the enthalpy of hydrogenation but also enhances disorder leading to a limited destabilization of the hydrogenated state. This compensation effect is characterized by a linear correlation between enthalpy and entropy terms. In addition, this work also provides the assessment at 623 K of the ternary Mg–Pd–Cu phase diagram in the Mg-rich corner.