Systematic study of first-row transition metals chlorides as reaction accelerators for Mg/ MgH2 for hydrogen storage application

Abstract

Mg has been proposed as a possible hydrogen storage material. However, Mg necessarily requires the addition of a suitable material with catalytic activity to improve hydriding and dehydriding reactions. Some chlorides of transition metals have been tested as improvers of hydriding/dehydriding reactions in Mg/MgH2. However, experimental preparation and tested conditions are dissimilar among different published reports. A systematic study of first-row transition metal chlorides as additives is presented here to understand the various factors affecting the hydriding/dehydriding reactions in Mg/MgH2 prepared and tested under the same conditions. The added transition metal chlorides (TMClx, x = oxidation state) include metals in oxidation states +4, +3, and +2 whenever commercially available. Additional chlorides, such as ZrCl4 and AlCl3, were included for comparison. The mixtures of Mg-TMClx were produced employing cryogenic ball milled and characterized by i) temperature-programmed hydriding and dehydriding, ii) X-ray photoelectron spectroscopy at Mg, transition metals, and chlorine 2p edges, iii) X-ray diffraction, and iv) scanning electron microscope. The best additives are VCl3>TiCl4>NiCl2>AlCl3. In contrast, the worst additives among the first-row transition metal chlorides were CrCl2, ZnCl2, and CuCl2. They do not improve at all the hydrogen uptake kinetics of pure Mg. Chemical state, side reactions, crystal structure, and morphology were discussed as factors affecting the behavior of the hydriding and dehydriding reactions. A complex interrelationship among these factors is observed. However, the electronegativity of the transition metal and an interaction of the type Mg-TM-Cl, are proposed as the main factors for the observed improvement of hydriding and dehydriding reactions.