H₂sorption performance of NaBH₄–MgH₂composites prepared by mechanical activation

Abstract

The current research on solid state hydrogen storage materials for on-board applications is focused on reactive hydrides composites (RHC), i.e. systems based on the improvement of the dehydrogenation thermodynamic of a complex hydride when one (generally the light hydride MgH 2 ) or more hydrides take part to the reaction. The extent of the destabilization, as well as the sorption characteristics of the composites, strongly depends on the structural and nanostructural properties of the constituent hydrides, which are in turn affected by the preparation route. The aim of this work is to evaluate the influence of different mechanical activation conditions on the storage properties of NaBH 4 -MgH 2 composites, up to now scarcely explored in literature. The first results regard composites with 2:1 and 1:2 stoichiometry milled under different atmosphere (Ar or H 2 ). X-ray powders diffraction analysis shows that milling does not lead to the formation of any new phase, but it reduces the average crystallite size of the powders down to nanometric scale. All the mixtures release an H 2 amount close to the theoretical value expected for the full dissociation of both the hydrides and much higher than the target fixed by the US Department of Energy for on-board application. The thermal programmed desorption profiles of the mixtures clearly show two steps, with MgH 2 dissociating first and with higher rate and NaBH 4 gradually dehydrogenating at temperatures close to 400°C. Concerning the 2:1 stoichiometry, when the samples are processed under Ar the two dehydrogenation processes are characterized by a lower starting temperature but also by a lower average rate with respect to the sample milled in H 2 . The 1:2 sample milled under Ar shows the best kinetic performance. Unfortunately, also for this mixture more than 10 h are required to obtain full desorption at a temperature as high as 450°C.

Thermodynamic and Kinetic Investigations on Pure and Doped NaBH4−MgH2 System

“…As evident, two desorption steps are distinguishable in the manometric profiles of both the mixtures. In agreement with our previous findings , and the results by Mao, and in coherence with the pertinent mass losses, the two steps can be attributed to MgH 2 and NaBH 4 desorption, respectively. The starting temperatures of the two processes are 305 and 450 °C in the 2:1 mixture (a) and about 5 °C lower in the 1:2 mixture (b).…”

“…The NaBH 4 −MgH 2 system seems to be a good model for the study of the sorption mechanism in alkaline metal borohydrides−magnesium hydride RHC, due to the fact that only one species, i.e., the intermetallic compound MgB 2 , forms and acts as a destabilizing agent, , while in the Li containing system both MgB 2 and the mixed Li x Mg y alloys could play a role. , Moreover NaBH 4 is more stable upon exposure to air and, hence, easier to handle with respect to the other borohydrides . Finally, the overall system is relatively cheap (<1 Euro/g, i.e., more than 10 times cheaper than the Li containing system).…”

“…In two previous works , we investigated deeply the kinetic sorption properties of binary NaBH 4 −MgH 2 mixtures with 2:1 and 1:2 molar stoichiometry prepared by ball milling. We showed that for both mixtures two desorption steps are present, attributable to the dehydrogenation of MgH 2 and NaBH 4 , respectively.…”

Experimental Evidence of Na2[B12H12] and Na Formation in the Desorption Pathway of the 2NaBH4+ MgH2System

“…We showed in previous papers − that dehydrogenation of the 2NaBH 4 + MgH 2 mixture takes place in two steps, according to the reaction scheme 2 normalNaBH 4 false( normals false) + normalMgH 2 false( normals false) → normalMg false( normals false) + normalH 2 false( normalg false) + 2 normalNaBH 4 false( normals false) 2 normalNaBH 4 false( normals false) + normalMg false( normals false) → 2 normalNa false( normals false) + normalMgB 2 false( normals false) + 4 normalH 2 false( normalg false) with the first step starting at around 330 °C, with release of 1.8 wt % H 2 , and the second one taking place at a minimum temperature of 440 °C, with release of about 7.9 wt % H 2 . Starting from these results, we chose 350 and 450 °C as the temperature values for thermal treatments suitable to study the desorption pathway of the system.…”

“…The NaBH 4 /MgH 2 multicomponent system has a high gravimetric capacity, 9.9 wt %, high volumetric hydrogen density, and a rather low dehydrogenation temperature compared to the single borohydride compound. 15 Moreover, its resistance to moisture and low price (<1h/g) make this system an optimal model for the more attractive-in terms of desorption temperatures and gravimetric capacity-Ca(BH 4 ) 2 /MgH 2 and LiBH 4 / MgH 2 reactive hydride composites.…”