Potassium hydride reduced black TiO2−x for boosting the hydrogenation of magnesium 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 TiO2−x reduced by KH (denoted as K-TiO2−x) on the hydrogen storage performance of MgH2. As compared with pristine and ball-milled MgH2, the addition of 5 wt% of K-TiO2−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-TiO2−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.