Hydrogen storage by direct electrochemical hydriding of Mg-based alloys

In this work, pure Mg and as-cast binary Mg–23Ni, Mg–10Ni and ternary Mg–10Ni–5Mm (Mm = mishmetal), Mg–23Ni–0.3P, Mg–10Ni–5Cu, Mg–23Ni–3Al (in wt.%) alloys in form of 0.5 mm thick coupons were subjected to electrochemical hydriding in a 6 mol/L KOH solution at a current density of 40 A/m2 for up to 10 h. Total hydrogen concentrations were measured after hydriding. Hydrogen evolution temperatures were determined by mass spectrometry and differential scanning calorimetry. Structures and phase compositions of both as-cast and hydrided alloys were examined by scanning electron microscopy, energy dispersive analysis and X-ray diffraction. It is found that pure Mg is not suitable for electrochemical hydriding, because the achieved H-concentration is only 0.005 wt.% after hydriding. The best hydriding efficiency is observed for the Mg–23Ni–0.3P (0.55 wt.% H) and Mg–10Ni–5Mm (0.80 wt.% H) alloys. These results suggest that much higher H-concentrations would be obtained when using cathodes with a high specific surface. Hydrogen evolves from the alloys at above 200 °C, as evidenced by DSC and MS. XRD proves that the main portion of absorbed hydrogen is in the form of MgH2 phase. Positive influence of phosphorus and mishmetal on hydriding behavior is discussed in relation to hydriding mechanism.