Phase transformation during hydrogen sorption was investigated in ten chosen magnesium-based hydrogen storage (HS) materials. Chemical composition of the materials consisted of Mg, as a principal hydrogen-binding element, additive X and amorphous carbon (CB), as an anti-sticking component. In order to assess the effect of X itself upon the structure, values of concentration of both X and CB were fixed to about 12 wt. %. The influence of X = Mg2Si, Mg2Ge, Mg17Al12, Mg5Ga2, NaCl, LiCl, NaF, LiF and two combinations Ni+Mg17Al12 and Ni+Mg2Si upon the changes in phase composition was tested. Phase content in HS materials was observed (i) after the intensive ball milling (BM), (ii) after the BM followed by hydrogen charging at 623 K and (iii) after the BM and one hydrogen charging/discharging cycle (C/D) at temperature 623 K. The study was carried out by SEM and XRD. It was found that, the C/D is approximately structurally reversible for X = Mg2Ge, Mg17Al12, NaF and LiF. However, additives X = Mg17Al12 and NaF decompose already during the BM. In alloys with combination of Ni with Mg17Al12, new phases NimAln are formed. Phase composition changed during C/D for X = Mg2Si Mg5Ga2 and Ni+Mg2Si due to equilibration of phases composition. Observed structure changes of HS materials with chloride ionic additives NaCl and LiCl are, most likely caused by the relatively strong affinity between Mg and Cl. Hydrogen storage capacity of all studied alloys was 6.0 ± 0.3 wt. % H2.Keywords: Hydrogen storage, Mg alloys, carbon black
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