Modulation on Surface Termination Groups to Optimize the Adsorption Energy and Work Function of Nb2CTx for Enhanced Hydrogen Storage in Magnesium Hydride

Abstract

Exploring high-performance catalysts for hydrogen storage in magnesium hydride (MgH₂) is crucial but still a challenge. Herein, Nb₂CT_x with controllable surface termination groups is developed as an efficient catalyst and the bifunctional modulation (adsorption energy and work function) of different surface termination groups (F, O, OH, or defects) is explored. First, compared to F and O, the introduction of OH on the surface or the direct removal of functional groups both leads to a significant increase in the adsorption of H by Nb₂CT_x. Second, compared to the surface bare, OH-rich Nb₂CT_x has a lower work function, making it easier for hydrogen to enter Mg/MgH₂ from the Nb₂CT_x surface or escape from the Mg/MgH₂ surface into the Nb₂CT_x, thus facilitating the hydrogen ad/desorption properties of MgH₂, i.e., the rate-determining step (RDS) shifts from penetration to diffusion. The Nb₂CT_x-KOH-catalyzed MgH₂ with optimal surface termination groups, therefore exhibits a release of 6.56 wt.% H₂ in 5 min at 250 °C, and 6.46 wt.% H₂ uptake within 5 min at 150 °C. The dehydrogenation and hydrogen uptake activation energies show 49.5% and 60.1% enhancements over pristine MgH₂. In addition, a storage capacity of 5.51 wt.% is maintained after 50 dehydrogenation/hydrogenation cycles.