Nb 2 O 5 Nanostructures as Precursors of Cycling Catalysts for Hydrogen Storage in MgH 2

High operating temperatures and sluggish kinetics are major obstacles for practical applications of MgH 2 as a solid hydrogen carrier. Introducing nanoscaled high-activity catalysts has been effective in improving the hydrogen cycling of MgH 2 . However, it remains still unclear that between nanoparticle size and morphology, which one is the decisive factor of the catalytic activity of a given catalyst. In this work, we studied this topic by taking nanostructured niobium oxide (Nb 2 O 5 ) as a representative sample. Five types of Nb 2 O 5 catalytic additives with different morphologies and nanosizes were synthesized, and their catalytic activities were compared with commercial microparticles. Our results unambiguously demonstrate that the catalytic activity of Nb 2 O 5 is determined by the primary particle size rather than the morphology and structure because the ultrasmall Nb 2 O 5 nanoparticles that measured ∼5 nm in size enable dehydrogenation of MgH 2 starting at 165 °C after one-cycle activation. The smaller nanoparticle sizes not only enhance the reactivity of Nb 2 O 5 but also lead to more uniform dispersion when ball-milled with MgH 2, which enables in situ formation of more homogeneous and finer Nb-based active species and therefore much higher catalytic activity. This important insight will guide the design and optimization of novel high-activity catalysts for hydrogen cycling of MgH 2 and other hydrogen storage materials.