Potassium hydride reduced black TiO2−x for boosting the hydrogenation of magnesium at room temperature

Highlights: • Black TiO₂ is synthesized by milling anatase TiO₂ with KH at room temperature. • The catalyst lowers MgH₂ decomposition temperature to ~194 °C. • 7.1 wt% H₂ is charged within the first 60 s at 230 °C and 30 bars of H₂ pressure. • The catalyzed MgH₂ after dehydrogenation absorbs 6.0 wt% H₂ at room temperature. -- Abstract: MgH₂ has attracted intense interest as one of the most efficient solid-state materials to reversibly store hydrogen owing to its large gravimetric and volumetric hydrogen capacities of about 7.6 wt% and 110 g/L, respectively. However, the high thermodynamic stability of ca. 76 kJ/mol-H₂, and slow kinetics of H₂ de/absorption at temperatures ca. 300 °C, have challenged the broad applications of MgH₂ in the field of energy storage. In an attempt to solve these challenges, this study presents the catalytic effects of adding black TiO_2−x reduced by KH (denoted as K-TiO_2−x) on the hydrogen storage performance of MgH₂. As compared with pristine and ball-milled MgH₂, the addition of 5 wt% of K-TiO_2−x into MgH₂ significantly improves the absorption/desorption behaviors. The doped system respectively absorbs 4.0, and 6.0 wt% H₂ within the first 6 min, and after 180 min of charging at room temperature; the hydrogen can be reversibly released off starting from 194 °C which is ca. 96 °C lower than the additive-free ball-milled MgH₂. The system can also produce ca. 6.5 wt% H₂ upon cycling. Investigations into the improved performance and the structural patterns of MgH₂ +K-TiO_2−x reveal the presence of "K₂Ti₂O₃" pseudo-complex of Ti²⁺ that is liberated in situ during the sorption process of MgH₂; this pseudo-complex is believed to act as a catalyst by providing nucleation sites, oxygen vacancies, and hydrogen diffusion channels at its boundaries with Mg/MgH₂ for the enhanced hydrogen sorption performance observed in the catalyzed MgH₂ composite.