In Situ Energy-Dispersive XAS and XRD Study of the Superior Hydrogen Storage System MgH₂/Nb₂O₅

By in situ energy-dispersive X-ray absorption spectroscopy (EDXAS) and X-ray diffraction (XRD), we analyzed the evolution of niobium in a MgH 2 /Nb 2 O 5 system based on high-energy ball milling during hydrogen cycling. The high time resolution of the EDXAS method allowed us to monitor fast sample changes during this process. Thereby, we demonstrated that the Nb 2 O 5 is already partially reduced during the milling process with the MgH 2 . Further reduction occurs during the heating and cycling processes, in which a lower limit of oxidation state is reached. Hereby, a reaction between the niobium oxide and the Mg/MgH 2 leads to a decrease of crystalline Nb 2 O 5 and the formation of a ternary oxide phase Mg x Nb y O. During the cycling processes a repetitive Nb oxidation−reduction process was observed, which may indicate hydrogen diffusion along the ternary oxide by the formation of metastable niobium hydrides. This points to a mechanism of kinetic sorption improvement by diffusion of hydrogen through pathways of ternary Mg−Nb oxides, which may also reduce the activation energy of the Mg−MgH 2 transition.