Hydrogen generation from the ball milled composites of sodium and lithium borohydride (NaBH4/LiBH4) and magnesium hydroxide (Mg(OH)2) without and with the nanometric nickel (Ni) additive

The composites of (NaBH4+2Mg(OH)2) and (LiBH4+2Mg(OH)2) without and with nanometric Ni (n-Ni) added as a potential catalyst were synthesized by high energy ball milling. The ball milled NaBH4-based composite desorbs hydrogen in one exothermic reaction in contrast to its LiBH4-based counterpart which dehydrogenates in two reactions: an exothermic and endothermic. The NaBH4-based composite starts desorbing hydrogen at 240 °C. Its ball milled LiBH4-based counterpart starts desorbing at 200 °C. The latter initially desorbs hydrogen rapidly but then the rate of desorption suddenly decelerates. The estimated apparent activation energy for the NaBH4-based composite without and with n-Ni is equal to 152 ± 2.2 and 157 ± 0.9 kJ/mol, respectively. In contrast, the apparent activation energy for the initial rapid dehydrogenation for the LiBH4-based composite is very low being equal to 47 ± 2 and 38 ± 9 kJ/mol for the composite without and with the n-Ni additive, respectively. XRD phase studies after volumetric isothermal dehydrogenation tests show the presence of NaBO2 and MgO for the NaBH4-based composite. For the LiBH4-based composite phases such as MgO, Li3BO3, MgB2, MgB6 are the products of the first exothermic reaction which has a theoretical H2 capacity of 8.1 wt.%. However, for reasons which are not quite clear, the first reaction never goes to full completion even at 300 °C desorbing ∼4.5 wt.% H2 at this temperature. The products of the second endothermic reaction for the LiBH4-based composite are MgO, MgB6, B and LiMgBO3 and the reaction has a theoretical H2 capacity of 2.26 wt.%. The effect of the addition of 5 wt.% nanometric Ni on the dehydrogenation behavior of both the NaBH4-and LiBH4-based composites is rather negligible. The n-Ni additive may not be the optimal catalyst for these hydride composite systems although more tests are required since only one n-Ni content was examined.