Study of the hydrogenation mechanism of LaCuMg8 ternary phase: The decomposition induces kinetics improvement

The hydrogen sorption properties of LaCuMg8 are investigated. LaCuMg8 crystallizes in the La2Mg17 structure type with the lattice parameters a = 10.1254(2) Å and c = 10.0751(2) Å.An absorption mechanism in 2 steps was reported earlier: (i) the first step, which is irreversible, induces the decomposition of LaCuMg8 into an intimate mixture of 3 phases of sub-micronic size (i.e. LaH3, MgCu2 and MgH2); (ii) the second step consists in the reversible absorption of the “LaCuMg8 activated powder” with kinetics better than for pure magnesium. In order to get a better understanding of the absorption process, it was investigated from thermodynamic, kinetic and morphology points of view. From thermodynamic point of view, the “LaCuMg8 activated powder” behaves like a mixture. Concerning the kinetics, the activation energy was evaluated at about 65 kJ/mol H2, using Avrami–Erofeev model and Arrhenius law. To explain the enhancement of the kinetics, both effects of (i) Mg2Cu/MgCu2 transformation and (ii) morphology induced by the decomposition have been considered.

► LaCuMg8 crystallizes in the La2Mg17 structure type with modified lattice parameters. ► A 2 steps absorption mechanism is reported: decomposition and cycling on MgH2. ► After decomposition, an intimate mixture of 3 sub-micronic phases is obtained.