Hydrogen Storage in Magnesium Hydride at Room Temperature Enabled by Graphene-Stabilized Multivalent Niobium Oxides

Reversible hydrogen storage in magnesium hydride (MgH₂) remains hindered by intrinsic, complicated kinetic barriers associated with both hydrogen release and uptake, particularly under mild conditions. In this work, graphene-confined, low-crystallinity niobium oxide nanoparticles are developed to optimize the kinetic barriers across all stages of hydrogen absorption and desorption in MgH₂. This is realized by the synergistic effect of in situ-generated stable multivalent niobium oxide (NbO_x) and the electronically modulating graphene. It is theoretically and experimentally demonstrated that Nb²⁺ enhances H₂ dissociation and diffusion, while Nb⁴⁺ facilitates Mg─H bond cleavage and recombination of H₂. Graphene serves a dual function by modulating the electronic environment at NbO_x interfaces to facilitate charge transfer, while confining nanoparticles to prevent aggregation and hence maintain the catalytic stability of NbO_x. Moreover, graphene suppresses the excessive hydrogen binding tendency of over-reduced Nb⁰, which otherwise traps H and impedes hydrogen diffusion. This integrated structure ensures the stabilization of active Nb species and lowers energy barriers across all key steps of hydrogen storage. As a result, an effective hydrogen absorption even at 0 °C and an onset hydrogen desorption temperature of 155.9 °C is realized. This provides a versatile strategy for engineering multivalent oxides for promoting hydrogen storage of MgH₂.