Nb<sub>2</sub>O<sub>5</sub> Nanostructures as Precursors of Cycling Catalysts for Hydrogen Storage in MgH<sub>2</sub>
Xin Zhang, Xuelian Zhang, Lingchao Zhang, Zhenguo Huang, Limei Yang, Mingxia Gao, Changdong Gu, Wenping Sun, Hongge Pan, Yongfeng Liu
Abstract
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.