Thermodynamics and reaction pathways of hydrogen sorption in Mg_6(Pd,TM) (TM = Ag, Cu and Ni) pseudo-binary compounds

摘要

Mg_6(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 Mg_6Pd-H system. All compounds exhibit a two-plateau pressure behaviour, being the value of the high plateau pressure well above that of the Mg/MgH_2 system. Such destabilization is explained by the formation of different Mg-(Pd,TM) intermetallics and/or Mg_2NiH_4 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.