Synthesis process and catalytic activity of Nb₂O₅ hollow spheres for reversible hydrogen storage of MgH₂

High operation temperatures and slow reaction kinetics are major obstacles to use MgH₂ as a solid hydrogen store. We report here the synthesis of Nb₂O₅ hollow spheres (o-Nb₂O₅) with wall thickness of approximately 50 nm and mossy surfaces using a facile hydrothermal and calcination process, which showed high activity in catalysis of MgH₂ for hydrogen storage. The dehydrogenation onset temperature of MgH₂ was decreased to 195°C with 7 wt% of o-Nb₂O₅. More than 5.5 wt% H₂ can be desorbed at 300°C within 5 minutes. Hydrogen re-absorption starts even at 25°C and reaches 5.6 wt% within 5 minutes at 200°C. Practical hydrogen capacity stabilizes at 5.8 wt% after 10 cycles of hydrogen uptake/release. The o-Nb₂O₅ was found to be reduced in situ by MgH₂ to low-valence Nb species during the initial dehydrogenation process, which functions as an active catalyst and leads to the enhanced dehydrogenation kinetics.