Combinations of VC and TiC MXenes for Boosting the Hydrogen Storage Performances of MgH.

Two-dimensional vanadium carbide (VC) and titanium carbide (TiC) MXenes were first synthesized by exfoliating VAlC or TiAlC and then introduced jointly into magnesium hydride (MgH) to tailor the hydrogen desorption/absorption performances of MgH. The as-prepared MgH-VC-TiC composites show much better hydrogen storage performances than pure MgH. MgH with addition of 10 wt % of 2VC/TiC initiates hydrogen desorption at around 180 °C; 5.1 wt % of hydrogen was desorbed within 60 min at 225 °C, while 5.8 wt % was desorbed within 2 min at 300 °C. Under 6 MPa H, the dehydrided MgH-2VC/TiC can start to recover hydrogen at room temperature, and 5.1 wt % of H is obtained within 20 s at a constant temperature of 40 °C. The reversible capacity (6.3 wt %) does not decline for up to 10 cycles, which shows excellent cycling stability. The addition of 2VC/TiC can remarkably lower the activation energy for the hydrogen desorption reaction of MgH by 37% and slightly reduce the hydrogen desorption reaction enthalpy by 2 kJ mol H. It was demonstrated that the combination of VC and TiC promotes the hydrogen-releasing process of MgH compared with addition of only VC or TiC, while TiC impacts MgH more significantly than VC in the hydrogen absorption process of MgH at ambient temperatures. A possible mechanism in the hydrogen release and uptake of the MgH-VC-TiC system was proposed as follows: hydrogen atoms or molecules may preferentially transfer through the MgH/VC/TiC triple-grain boundaries during the desorption process and through the Mg/TiC interfaces during the absorption process. Microstructure studies indicated that VC and TiC mainly act as efficient catalysts for MgH. This work provides an insight into the hydrogen storage behaviors and mechanisms of MgH boosted by a combination of two MXenes.