Enhanced hydrogen storage properties of MgH2 with alkalized Nb2CTx MXene: The role of enlarged interlayer spacing and optimized surface

Abstract

Critical limitations to applying MgH2 as a hydrogen storage medium include high H2 desorption temperature and slow reaction kinetics. In this study, we synthesize a series of Nb2CTx MXene with different enlarged interlayer spacing and surface optimization by alkalization to enhance the hydrogen storage properties of MgH2. Among them, the Nb2CTx MXene (Nb2CTx-5Na) with an alkalization time of 5 h displays the best catalytic activity, which catalyzes MgH2 releasing 5.57 wt% of hydrogen within 12 min at 250 °C. Even at a lower temperature of 200 °C, MgH2 can release about 4 wt% of hydrogen in 3 h under the catalysis of Nb2CTx-5Na. Moreover, an almost constant capacity is observed from the 20th cycle onwards, showing stable cyclicity. The capacity retention of Nb2CTx-5Na/MgH2 is 89.92 % after 40 cycles of hydrogen absorption and desorption. Mechanistic analyses show that the enlarged layer spacing exposes more active sites of intermediate Nb, which effectively accelerates the hydrogen absorption/release kinetics of MgH2. A stable multivalent elemental catalytic environment is formed in situ during the hydrogen absorption and release cycle, which is conducive to the electron transfer between Mg2+ and H– during the hydrogen absorption and desorption process, thus promoting the dissociation and recombination of hydrogen molecules. Additionally, DFT calculations show that Nb2C(OH)x weakens the interaction between Mg and H by prolonging the length of the Mg-H bond, thus reducing the adsorption energy for hydrogen molecules and contributing to the destabilization and diffusion of hydrogen atoms in MgH2.