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 (NbOx) 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 NbOx interfaces to facilitate charge transfer, while confining nanoparticles to prevent aggregation and hence maintain the catalytic stability of NbOx. 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.