A Mechanistic Analysis of Phase Evolution and Hydrogen Storage Behavior in Nanocrystalline Mg(BH<sub>4</sub>)<sub>2</sub> within Reduced Graphene Oxide Sohee Jeong Tae Wook Heo Julia Oktawiec Rongpei Shi ShinYoung Kang James L. White Andreas Schneemann Edmond W. Zaia Liwen F. Wan Keith G. Ray Yi-Sheng Liu Vitalie Stavila Jinghua Guo Jeffrey R. Long Brandon C. Wood Jeffrey J. Urban 10.1021/acsnano.9b07454.s001 https://acs.figshare.com/articles/journal_contribution/A_Mechanistic_Analysis_of_Phase_Evolution_and_Hydrogen_Storage_Behavior_in_Nanocrystalline_Mg_BH_sub_4_sub_sub_2_sub_within_Reduced_Graphene_Oxide/11638212 Magnesium borohydride (Mg­(BH<sub>4</sub>)<sub>2</sub>, abbreviated here MBH) has received tremendous attention as a promising onboard hydrogen storage medium due to its excellent gravimetric and volumetric hydrogen storage capacities. While the polymorphs of MBHalpha (α), beta (β), and gamma (γ)have distinct properties, their synthetic homogeneity can be difficult to control, mainly due to their structural complexity and similar thermodynamic properties. Here, we describe an effective approach for obtaining pure polymorphic phases of MBH nanomaterials within a reduced graphene oxide support (abbreviated MBHg) under mild conditions (60–190 °C under mild vacuum, 2 Torr), starting from two distinct samples initially dried under Ar and vacuum. Specifically, we selectively synthesize the thermodynamically stable α phase and metastable β phase from the γ-phase within the temperature range of 150–180 °C. The relevant underlying phase evolution mechanism is elucidated by theoretical thermodynamics and kinetic nucleation modeling. The resulting MBHg composites exhibit structural stability, resistance to oxidation, and partially reversible formation of diverse [BH<sub>4</sub>]<sup>−</sup> species during de- and rehydrogenation processes, rendering them intriguing candidates for further optimization toward hydrogen storage applications. 2020-01-17 16:37:41 nucleation modeling α phase phase evolution mechanism polymorphic phases γ- phase hydrogen storage capacities onboard hydrogen storage medium Mg Mechanistic Analysis temperature range MBH nanomaterials BH 4 Phase Evolution graphene oxide support Reduced Graphene Oxide Magnesium borohydride metastable β phase rehydrogenation processes MBHg composites exhibit Hydrogen Storage Behavior hydrogen storage applications