Interfacial Coating of Graphene Nanosheets to Control the Hydrolysis Kinetics of LiAlH₄ at Subzero Temperatures

The hydrolysis of LiAlH4 is a feasible path to supply hydrogen at subzero temperatures due to its high hydrolysis reactivity and hydrogen purity. However, the extremely fast hydrolysis reaction of LiAlH4 hindered its practical application due to its kinetic instability and safety risks caused by the exothermic effect of the reaction. Herein, hydrophobic graphene nanosheets (GNS) have been coated on the surfaces of LiAlH4 particles (LiAlH4@GNS composites) to slow the hydrolysis kinetics of LiAlH4, especially at subzero temperatures. The GNS coating processes have been found to introduce not only GNS coating but also fragmentation of LiAlH4 particles. The coating process was found to hinder the hydrolysis reaction of LiAlH4 with increasing GNS coating to 3.82 wt %, while the hydrolysis of LiAlH4 was found to increase with further GNS coating process due to significant fragmentation effects. The hydrolysis speeds of LiAlH4@GNS (D50 = 100 μm, Dδ = 1.30) with GNS coating (∼3.82 wt %) have been demonstrated to be reduced to 70% of that of pristine LiAlH4, where the conversion rates were found to be still higher than 99.0% at −40 to 0 °C. The GNS coating is considered to serve as a barrier to hinder the free diffusion of H2O molecules toward the reaction interface at the initial stage of the reaction, resulting in longer induction periods of the composites than those of pristine LiAlH4. The hydrolysis of LiAlH4@GNS has been demonstrated to follow the shrinking core model controlled by liquid film diffusion, where the GNS coating plays an important role in extending the diffusion distance and reducing the diffusion efficiency of substances within the liquid film layer.