The influence of Cu substitution on the hydrogen sorption properties of magnesium rich Mg-Ni films

This paper describes the hydrogen storage properties of Magnesium rich ternary Mg-Ni-Cu films of 1.5 μm thickness using binary Mg-Ni and Mg-Cu as baselines, and aims to elucidate the precise influences of alloying element Cu on the hydrogen sorption kinetics, thermodynamics and cycleability. Mg-rich Mg-Ni-(Cu) alloys show two stages during absorption. The first stage due to the absorption of Mg not alloyed in the form of Mg₂Ni and Mg₂Cu, hereafter denoted as free-Mg, is very quick, but the second one due to the absorption of intermetallic Mg₂Ni and/or Mg₂Cu is significantly slower. This sequence is confirmed by XRD characterizations at different absorption stages. The rapid first stage absorption is mainly catalyzed by the intermetallic phase, Mg₂Ni. Cu substitution improves the desorption kinetics, but severely decreases the kinetics of the second absorption stage. Failure to completely absorb Mg₂Cu to MgH₂ and MgCu₂ in consecutive absorption cycles leads to complete loss of desorption-ability in binary Mg-15 at.%Cu. XRD combined with TEM shows that segregation of Mg₂Cu towards the grain boundaries is responsible for this. Pressure-Composition Isotherms are used to examine the thermodynamic properties of the alloys. The thermodynamic properties of the Low-Temperature (LT-) Mg₂NiH₄ are determined for the first time experimentally, and are found to be ΔH = −78.6 kJ/mol H₂ and ΔS = −147.83 J/K-mol H₂. It is found that the Cu substitution has no influence on the plateau pressure of MgH₂ from free-Mg phase, but slightly increases the plateau pressure of LT-Mg₂NiH₄.