10.1021/acsnano.9b07454.s001 Sohee Jeong Sohee Jeong Tae Wook Heo Tae Wook Heo Julia Oktawiec Julia Oktawiec Rongpei Shi Rongpei Shi ShinYoung Kang ShinYoung Kang James L. White James L. White Andreas Schneemann Andreas Schneemann Edmond W. Zaia Edmond W. Zaia Liwen F. Wan Liwen F. Wan Keith G. Ray Keith G. Ray Yi-Sheng Liu Yi-Sheng Liu Vitalie Stavila Vitalie Stavila Jinghua Guo Jinghua Guo Jeffrey R. Long Jeffrey R. Long Brandon C. Wood Brandon C. Wood Jeffrey J. Urban Jeffrey J. Urban A Mechanistic Analysis of Phase Evolution and Hydrogen Storage Behavior in Nanocrystalline Mg(BH<sub>4</sub>)<sub>2</sub> within Reduced Graphene Oxide American Chemical Society 2020 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 2020-01-17 16:37:41 Journal contribution 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.