Two-dimensional vanadium carbide (V 2 C) and titanium carbide (Ti 3 C 2 ) MXenes were first synthesized by exfoliating V 2 AlC or Ti 3 AlC 2 and then introduced jointly into magnesium hydride (MgH 2 ) to tailor the hydrogen desorption/absorption performances of MgH 2 . The as-prepared MgH 2 −V 2 C−Ti 3 C 2 composites show much better hydrogen storage performances than pure MgH 2 . MgH 2 with addition of 10 wt % of 2V 2 C/Ti 3 C 2 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 2 , the dehydrided MgH 2 −2V 2 C/Ti 3 C 2 can start to recover hydrogen at room temperature, and 5.1 wt % of H 2 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 2V 2 C/Ti 3 C 2 can remarkably lower the activation energy for the hydrogen desorption reaction of MgH 2 by 37% and slightly reduce the hydrogen desorption reaction enthalpy by 2 kJ mol −1 H 2 . It was demonstrated that the combination of V 2 C and Ti 3 C 2 promotes the hydrogen-releasing process of MgH 2 compared with addition of only V 2 C or Ti 3 C 2 , while Ti 3 C 2 impacts MgH 2 more significantly than V 2 C in the hydrogen absorption process of MgH 2 at ambient temperatures. A possible mechanism in the hydrogen release and uptake of the MgH 2 −V 2 C−Ti 3 C 2 system was proposed as follows: hydrogen atoms or molecules may preferentially transfer through the MgH 2 /V 2 C/Ti 3 C 2 triple-grain boundaries during the desorption process and through the Mg/ Ti 3 C 2 interfaces during the absorption process. Microstructure studies indicated that V 2 C and Ti 3 C 2 mainly act as efficient catalysts for MgH 2 . This work provides an insight into the hydrogen storage behaviors and mechanisms of MgH 2 boosted by a combination of two MXenes.

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