Mechanical alloying and electronic simulation of (MgH2+M) systems (M=Al, Ti, Fe, Ni, Cu, Nb) for hydrogen storage

Mg-based alloys are promising candidates for hydrogen storage applications. Here, mechanical alloying (MA) was used to process powder mixtures of MgH2 with 8mol% M (M=Al, Ti, Fe, Ni, Cu and Nb) in order to modify hydrogen storage properties of the Mg hydride. Electronic simulations of the systems were carried out to clarify the mechanisms of the alloy effects. X-ray diffraction (XRD) of the milled samples revealed the formation of new phases: a bcc solid solution phase for the (MgH2+Nb) mixture; TiH2 phase for the (MgH2+Ti); and MgCu2 phase for the (MgH2+Cu). For all the mixtures, a high-pressure phase, γ-MgH2, was also identified after mechanical alloying. Further qualitative and quantitative phase analyses were carried out using the Rietveld method. Scanning electron microscopy (SEM) of the milled powder clearly showed substantial particle size reduction after milling. Dehydrogenation at 300°C under vacuum shows that the (MgH2+Ni) mixture gives the highest level of hydrogen desorption and the most rapid kinetics, followed by MgH2 with Al, Fe, Nb, Ti and Cu. Theoretical predictions show that the (MgH2+Cu) system is the most unstable, followed by (MgH2+Ni), (MgH2+Fe), (MgH2+Al), (MgH2+Nb), (MgH2+Ti). The predicted alloying effects on the stability of MgH2 generally agree with the experimentally observed change in the hydrogen desorption capacity. The differences were discussed in the text.