Structure and hydrogen storage properties of 10Htype LPSO in the Gd-Ni-Mg ternary system

In order to discover new Mg-rich phases that could be interesting for hydrogen storage, the Mgrich part of the Gd-Ni-Mg ternary system was explored. Along the Gd/Ni = 4/3 composition line at an atomic percentage of Mg of 80%, the sample with the nominal composition Gd11.4Ni8.6Mg80 gives rise to a single and unreported 10H-type long-period stacking ordered (LPSO) phase. X-ray diffraction and transmission electron microscopy confirmed the 10H-type structure with lattice parameters a = 11.20Å and c = 26.10Å by considering the space group P63/mcm. The hydrogen sorption properties of this phase are investigated. A two-step hydrogen absorption mechanism has been observed: (i) the first step leads to the irreversible decomposition of the compound into a mixture of three hydrides: GdH2, Mg2NiH4 and MgH2 while (ii) the second step consists in the reversible hydrogen absorption of this hydrides mixture, which relies mainly on the reversibility of MgH2 and, to a lesser extent, Mg2NiH4. Hydrogen sorption kinetics were fitted to the Avrami model. The activation energy for H2 absorption obtained is ca.36 kJ·mol-1, which is 3 to 4 times lower than that of pure Mg, highlighting the interest of using this compound to improve hydrogen absorption kinetics. Using the same model, an activation energy of ca.180 kJ·mol-1 is estimated for H2 desorption reaction, a value that remains higher to that reported for the hydrogen desorption reaction in the Mg/MgH2 system.