Robust architecture of 2D nano Mg-based borohydride on graphene with superior reversible hydrogen storage performance

Efficient technical strategies to synthesize hydrides with high capacity and favorable reversibility are significant for the development of novel energy materials. Herein, nano Mg-based borohydride, Mg(BH4)2, with robust architecture was designed and prepared by confining on graphene through a solution self-confinement method. The Mg(BH4)2 confined on graphene displays a wrinkled 2D nano layer morphology within 8.8 nm thickness. Such 2D nano Mg(BH4)2 can start dehydrogenation at 67.9 °C with a high capacity of 12.0 wt.%, which is 190.5 °C lower than pristine Mg(BH4)2. The isothermal dehydrogenation tests and kinetics fitting results indicate the 2D nano Mg(BH4)2 possesses much-enhanced dehydrogenation kinetics of 31.3 kJ/mol activation energy, which is only half of pristine Mg(BH4)2. The thermodynamics of the 2D nano Mg(BH4)2 is also verified by PCT tests, of which Gibbs free energy value for the confined 2D nano Mg(BH4)2 is estimated to be -18.01 kJ/mol H2, lower than -16.36 kJ/mol H2 of pristine Mg(BH4)2. Importantly, the reversibility of the confined 2D nano Mg(BH4)2 is significantly enhanced to over 90% capacity retention with relatively kinetics stability during 10 cycles. The mechanism analyses manifest that Mg(BH4)2 exhibits stable 2D nano morphology during 10 cyclic tests, resulting in the greatly reduced H diffusion path and the improved de/rehydrogenation kinetics of the 2D nano Mg(BH4)2. Based on theoretical calculations of Mg(BH4)2 and the intermediate MgB12H12 confined on graphene, the charge transfer status of both samples is modified to facilitate de/rehydrogenation, thus leading to the significant thermodynamic improvements of the reversible hydrogen storage performances for 2D nano Mg(BH4)2. Such investigation of the Mg-based borohydride will illuminate prospective technical research of energy storage materials.