Exfoliation of compact layered Ti2VAlC2 MAX to open layered Ti2VC2 MXene towards enhancing the hydrogen storage properties of MgH2

Magnesium hydride (MgH2) possesses a high hydrogen storage capacity of 7.6 wt%, but suffers from severely high operating temperature and slow kinetics. In this work, a bimetallic open layered Ti2VC2 MXene was successfully synthesized by exfoliation of the compact layered Ti2VAlC2 MAX. Both Ti2VC2 MXene and Ti2VAlC2 MAX show good enhancing impact on the hydrogen storage properties of MgH2, with Ti2VC2 being better than Ti2VAlC2. The MgH2 + 10 wt% Ti2VC2 and the MgH2 + 10 wt% Ti2VAlC2 start dehydrogenation at 170 and 204 degrees C, respectively and both can start rehydrogenation at room temperature. The MgH2 + 10 wt% Ti2VC2 and the MgH2 + 10 wt% Ti2VAlC2 can respectively absorb 5.5 and 4.5 wt% of hydrogen within 10 min at 150 celcius, with dehydrogenation activation energies of 71 and 78 kJ mol-1 H2, respectively. The reversible capacity of the MgH2 + 10 wt% Ti2VC2 only slightly decreases from 6.2 to 6.1 wt% after 100 cycles. Both Ti2VC2 MXene and Ti2VAlC2 MAX are layered structure containing active Ti and V elements and can provide a large number of channels for hydrogen and electron transfer during the hydrogen sorption process of MgH2, which contributes to the excellent hydrogen storage performances of MgH2-Ti2VC2/Ti2VAlC2. However, the delaminated Ti2VC2 MXene possesses wider layer spacing due to the removal of the Al layers, which leads to more attachment sites for MgH2 than the Ti2VAlC2 MAX does and refining of MgH2. Thus, the MgH2-Ti2VC2 have better hydrogen storage properties than that of MgH2-Ti2VAlC2. This work provides new insights to understand the role of MAX and MXene in tailoring the hydrogen storage of Mg-based hydrogen storage materials.