Superior Kinetic and Cyclic Performance of a 2D Titanium Carbide Incorporated 2LiH + MgB₂ Composite toward Highly Reversible Hydrogen Storage

Abstract

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.

Tuning LiBH4 for Hydrogen Storage: Destabilization, Additive, and Nanoconfinement Approaches

“…The reversibility of pristine LiBH 4 demands harsh conditions (>600 • C and >100 bar H 2 ) [1,41], which makes it impossible for its direct use as a hydrogen storage medium for practical applications. Several investigations have been done heading towards thermodynamic destabilization and kinetic improvement of LiBH 4 [29,30,39,46,49,50,53,[58][59][60][61][62][63][64][65][66][67]69,[72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][89][90][91][92][93][94][95][96][97][98][99].…”

“…Based on this analysis, it is clear that the mutual destabilization effect between LiBH 4 and MgH 2 only occurs upon hydrogenation, but at relatively low temperatures (<413 • C) under equilibrium conditions. For the hydrogenation process carried out under dynamic conditions, the applied temperatures were usually in the range between 300 • C and 400 • C [33][34][35][36][37][38][56][57][58][59][60][61][62][63][64][65][66][67]; hence, the mutual destabilization effect was verified by a one-step curve of hydrogen uptake against time. However, for the dehydrogenation, the thermodynamics limits the behavior of Li-RHC to two main reaction steps, losing the benefit of the destabilization effect.…”

“…The most applied approach to improve the kinetic behavior of the 2LiBH 4 + MgH 2 system was the addition of transition metal (TM) and transition metal compounds (TMC) via mechanical milling [68]. Several works were published about the improvement of the kinetic behavior, and also cycling stability of TM-and TMC-added Li-RHC [56][57][58][59][60][61][62][63][64][65][66][67]. In 2010, Bösenberg et al [57] studied the effects of TMC on the kinetic behavior of Li-RHC and proposed global reaction rate mechanisms for the absorption and mainly desorption of hydrogen.…”

“…Therefore, it was proposed that nanostructured TMB species can act as heterogeneous nucleation sites for MgB 2 , thus improving the dehydrogenation rate of the slowest step, i.e., the decomposition of LiBH 4 (second step), since the decomposition of MgH 2 is quite fast. Based on this concept, either TMBs were added to Li-RHC or TM, and TMC were used as sources to form in situ TMB through the interaction with the Li-RHC [40,[56][57][58][59][60][61][62][63][64][65][66][67]. Puszkiel et al [39,69] proposed another mechanism to explain the effect of a specific TMC on the kinetic behavior of Li-RHC.…”

“…It was found that these nanostructured core-shell Li x TiO y species act as Li + pumps, accelerating both the hydrogenation and dehydrogenation process. Furthermore, a novel kinetic model for the two-step dehydrogenation reaction was developed [39], which can be applied to the Li-RHC independently to the kind of used additive [65,67]. Table 2 provides a summary of some representative additives, describing hydrogenation and dehydrogenation conditions, capacities, and times as well as cycling stability (when it is available).…”