Synthesis and hydrogen desorption properties of nanocomposite magnesium hydride with sodium borohydride (MgH2+NaBH4)

Abstract

The effects of nanocompositing with sodium borohydride (NaBH4) additive on the synthesis and dehydriding properties of nanocrystalline magnesium hydride (β-MgH2) were studied. The nanocomposite hydrides with catalytic NaBH4 were processed by controlled reactive mechanical milling (CRMM) of Mg + X wt.% NaBH4 mixtures (X = 2, 10 and 20) in the magneto-mill Uni-Ball-Mill 5 under reactive hydrogen atmosphere. The X-ray diffraction (XRD) patterns of the as-milled nanocomposite powders reveal the presence of nanocrystalline β-MgH2 and NaBH4 having nanograins within the 12–14 and ∼55 nm range, respectively. Scanning electron microscopy shows pronounced particle size (expressed as equivalent circle diameter, ECD) reduction from ECD ∼9.5 μm for the as-received Mg powder to ECD ∼1 μm for the β-MgH2 + NaBH4 nanocomposite powders after 100 h of reactive milling. Both the β-MgH2 + 2 and 10 wt.% NaBH4 nanocomposites show a single differential scanning calorimetry (DSC) endothermic decomposition peak with the maximum at temperature ∼438 and ∼338 °C, respectively. A single peak at ∼338 °C clearly indicates that both hydrides are simultaneously decomposed acting catalytically on one another. The DSC curve for the MgH2 + 20 wt.% NaBH4 nanocomposite shows an endothermic peak triplet with the first small peak at ∼305 °C and the second large peak at ∼330 °C most likely from the decomposition of β-MgH2, and the third smaller peak at ∼405 °C possibly from the decomposition of NaBH4. Apparently, the presence of 10 and 20 wt.% NaBH4 in the nanocomposite mixture with β-MgH2 lowers the decomposition temperature by as large as 100 °C. XRD patterns show that the principal decomposition product after DSC test up to 500 °C is the elemental nanocrystalline Mg (nanograin size 30–69 nm) and additionally MgB2 compound in the MgH2 + 10 and 20 wt.% NaBH4 nanocomposites. After DSC decomposition of MgH2 + 20 wt.% NaBH4 nanocomposite the elemental Mg product exhibits unit cell volume expansion which suggests a formation of substitutional solid solution of Na in Mg.