Reactive hydride system of 2LiH + MgB2/2LiBH4 + MgH2 is of great potential as hydrogen storage material due to its high theoretical hydrogen storage capacity. However, it suffers from sluggish dehydrogenation/hydrogenation kinetics and poor cyclic stability. In the present study, with the incorporation of a 2D MXene phase of Ti3C2 to the 2LiH + MgB2 mixture by ball milling followed by an initial hydrogenation, the system is hydrogenated to 2LiBH4 + MgH2 and shows superior hydrogen storage kinetics and cyclic stability. 9.0 wt % H2 is released at 400 °C within 20 min. The dehydrogenation product can be highly hydrogenated at 350 °C/10 MPa in less than 5 min. The incorporation of 2D Ti3C2 not only leads to an in situ formation of superfine and well dispersive nanosized TiB2 during the ball milling and initial hydrogenation, which serves as heterogeneous nucleation sites for the dehydrogenation product of MgB2, but also helps reduce the particle size of the system during the milling. The synergetic effect of them improves significantly the hydrogen storage kinetics and the reversibility of the system.

Lithium borohydride (LiBH 4 ), with a high hydrogen capacity of 18.5 wt %, is an ideal candidate for hydrogen storage; however, it suffers from high thermal stability, low kinetics, and poor reversibility. Nanoconfinement is an effective strategy to tackle these problems, but a main drawback of nanoconfined systems is the low loading fraction of LiBH 4 , which leads to a low theoretical hydrogen capacity of the systems. It is thus highly desired to design scaffolds with high porosity and a reasonable pore structure for achieving high loading of LiBH 4 . In this work, porous hollow carbon nanospheres (PHCNSs) with uniform size, high specific surface area, large pore volume, and reasonable pore structure are delicately designed and controllably synthesized as the scaffold for confining LiBH 4 . The asprepared PHCNSs can accommodate up to 70 wt % LiBH 4 , while the system still shows a low dehydrogenation temperature of ca. 200 °C and releases rapidly 8.1 wt % H 2 at 350 °C within 25 min. Such a high loading of LiBH 4 and high dehydrogenation capacity at a low temperature have never been reported to date based on our knowledge of carbon-based nanoconfined LiBH 4 systems. Moreover, the system with 60 wt % LiBH 4 shows favorable reversibility and rapid hydrogenation under moderate conditions. The morphology and structure evolutions of the confined systems during cycling are investigated, and the mechanism of the improved hydrogen storage property is proposed. The present work provides further insight into rationally utilizing porous carbon scaffolds with a well-designed structure to improve the hydrogen storage performance of LiBH 4 .

Scheme 1. Illustration of the synthesis process of the TiNb 2 O 7 nanoflakes and the TiNb 2 O 7 -incorporated MgH 2 system.

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