Synthesis process and catalytic activity of Nb ₂ O ₅ hollow spheres for reversible hydrogen storage of MgH ₂

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.