Influence of Co and Pd on the formation of nanostructured LaMg2Ni and its hydrogen reactivity

Abstract

Abstract The influence of elemental additives 1–10 at.% Co or 5 at.% Pd on the phase formation of the orthorhombic LaMg 2 Ni phase and its hydrogen reactivity in La 25 Mg 50 Ni 25 alloys was studied. The induction melting, the melt-spinning and the intensive ball milling routes, respectively, were used. By controlled heat treatment of glassy melt-spun Co- or Pd-substituted La 25 Mg 50 Ni 25 ribbons the formation of a La 1− x Mg 2 Ni 1− y Co x + y ( x + y = 0.2) phase in combination with LaMg 3 proceeds or LaMg 2 Ni 0.8 Pd 0.2 as single phase, respectively. Co is dissolved in the LaMg 2 Ni alloy up to 5 at.% and occupies the La positions in the unit cell predominantly. Pd occupies the Ni position in LaMg 2 Ni only. The activation energies for crystallisation E AC of La 1− x Mg 2 Ni 1− y Co x + y and LaMg 2 Ni 0.8 Pd 0.2 were estimated. The most effective method for achieving the substituted hydride LaMg 2 NiH 7 as single phase was the intensive ball milling in a planetary RETSCH mill under a 0.5 MPa hydrogen atmosphere. The elemental substitution of La 25 Mg 50 Ni 25 by 5 at.% Co or 5 at.% Pd reduced the reaction time for the hydride formation drastically from 7.5 h to 3 h for Pd and to 1.5 h for Co. The hydrogen desorption from these substituted hydride powders proceeds in two steps: the decomposition into LaH 2−3 and a solid solution of Co or Pd with Mg 2 Ni with E AT ≈ (75 ± 5) kJ/mol and E AT ≈ (160 ± 5) kJ/mol. The activation energy for decomposition is much higher in both cases as for the pure LaMg 2 NiH 7 . Particularly, the Pd-containing hydride shows reversible interstitial hydrogen absorption/desorption behaviour in the lower temperature region before decomposition and could be of interest for further studies regarding electrochemical applications. The recombination to the starting phases >540 °C or 530 °C, respectively, is combined with the simultaneous evaporation of Mg in both cases of substitution.