NaH doped TiO2 as a high-performance catalyst for Mg/MgH2 cycling stability and room temperature absorption

This paper presents the catalytic effect of NaH doped nanocrystalline TiO2 (designated as NaTiOxH) in the improvement of MgH2 hydrogen storage properties. The catalyst preparation involves ball milling NaH with TiO2 for 3 hr. The addition of 5 wt% NaTiOxH powder into MgH2 reduces its operating temperature to ∼185 °C, which is ∼110 °C lower than the additive-free as-milled MgH2. The composite remarkably desorbs ∼7.2 wt% H2 within 15 min at ∼290 °C and reabsorbs ∼4.5 wt% H2 in 45 min at room temperature under 50 bar H2. MgH2 dehydrogenation is activated at 57 kJ/mol by the catalyst. More importantly, the addition of 2.5 wt% NaTiOxH catalyst aids MgH2 to reversibly produce ∼6.1 wt% H2 upon 100 cycles within 475 hr at 300 °C. Microstructural investigation into the catalyzed MgH2 composite reveals a firm contact existing between NaTiOxH and MgH2 particles. Meanwhile, the NaTiOxH catalyst consists of catalytically active Ti3O5, and "rod-like" Na2Ti3O7 species liberated in-situ during preparation; these active species could provide multiple hydrogen diffusion pathways for an improved MgH2 sorption process. Furthermore, the elemental characterization identifies the reduced valence states of titanium (Ti<4+) which show some sort of reversibility consistent with H2 insertion and removal. This phenomenon is believed to enhance the mobility of Mg/MgH2 electrons by the creation and elimination of oxygen vacancies in the defective (TiO2-x) catalyst. Our findings have therefore moved MgH2 closer to practical applications.