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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Aluminum hydride (AlH 3) is a promising candidate for hydrogen storage due to its high hydrogen density of 10 wt%. Several polymorphs of AlH 3 (e.g., α, β, and γ) have been successfully synthesized by wet chemical reaction of LiAlH 4 and AlCl 3 in ether solution followed by desolvation. However, the synthesis process of α'-AlH 3 from wet chemicals still remains unclear. In the present work, α'-AlH 3 was synthesized first by the formation of the etherate AlH 3 through a reaction of LiAlH 4 and AlCl 3 in ether solution. Then, the etherate AlH 3 was heated at 60 • C under an ether gas atmosphere and in the presence of excess LiAlH 4 to remove the ether ligand. Finally, α'-AlH 3 was obtained by ether washing to remove the excess LiAlH 4. It is suggested that the desolvation of the etherate AlH 3 under an ether gas atmosphere is essential for the formation of α'-AlH 3 from the etherate AlH 3. The as-synthesized α'-AlH 3 takes the form of rod-like particles and can release 7.7 wt% hydrogen in the temperature range 120-200 • C.

MgH2 has a high hydrogen content of 7.6 wt% and possesses good reversibility under normal conditions. However, pristine MgH2 requires a high temperature above 300 °C to release hydrogen, with very slow kinetics. In this work, we utilized Ti3CN MXene to reduce the operating temperature and enhance the kinetics of MgH2. The initial temperature of MgH2 decomposition can be lowered from 322 °C for pristine MgH2 to 214 °C through the employment of Ti3CN. The desorbed MgH2 + 7.5 wt% Ti3CN can start absorption at room temperature, while the desorbed pristine MgH2 can only start absorption at 120 °C. The employment of Ti3CN can significantly improve the hydrogen release kinetics of MgH2, with the desorption activation energy decreasing from 121 to 80 kJ mol−1. Regarding thermodynamics, the desorption enthalpy changes of MgH2 and MgH2 + 7.5 wt% Ti3CN were 79.3 and 78.8 kJ mol−1, respectively. This indicates that the employment of Ti3CN does not alter the thermal stability of MgH2. Phase evolution studies through the use of X-ray diffraction and electron diffraction both confirm that Ti3CN remains stable during the hydrogen release and uptake process of the composite. This work will help understand the impact of a transition metal carbonitride on the hydrogen storage of MgH2.