Al2O3 Atomic Layer Deposition on Nanostructured ..gamma..-Mg(BH4)2 for H2 Storage

In the context of the growing hydrogen (H2) economy, the demand for H2 storage materials is high, and metal borohydrides are of particular interest. Magnesium borohydride, Mg(BH4)2, has one of the highest hydrogen capacities of all known metal hydrides (14.9 wt % H) but suffers from high operating temperatures, slow kinetics for (de)hydrogenation, and the loss of capacity upon cycling. Strategies to address these challenges include nanoencapsulation and the use of chemical additives. This work is the first to utilize these two strategies simultaneously by using atomic layer deposition (ALD). For this new approach to modify borohydrides, we chose the well-studied Al2O3 ALD process using trimethylaluminum and water. Although there has been limited use of aluminum-based additives for Mg(BH4)2, we demonstrate that the low-temperature H2 capacity was doubled, desorption kinetics were increased by a factor of 3, and 100 cycles of Al2O3 suppressed the release of diborane compared to the uncoated Mg(BH4)2. We identified that the use of trimethylaluminum and water in the ALD process affected the decomposition pathway and that the Al2O3 film growth is dominated by infiltration due to the high porosity of the ..gamma..-phase Mg(BH4)2. From these results, the potential of ALD as a method to functionalize solid-state H2 storage materials is inferred, and recommendations for future ALD processes are presented.