Enhancing hydrogen storage performance of MgH₂ with hollow Bi₂Ti₂O₇ catalyst: Synergistic effects of Bi₂Mg₃ alloy phase and Ti polyvalency

The role of catalysts in enhancing the hydrogen storage kinetics of the Mg/MgH₂ system is pivotal. However, the exploration of efficient catalysts and the underlying principles of their design remain both a prominent focus and a significant challenge in current research. In this study, we present a bimetallic oxide of Bi₂Ti₂O₇ hollow sphere as a highly effective catalyst for MgH₂. As a result, the Bi₂Ti₂O₇-catalyzed Mg/MgH₂ system lowers the hydrogen desorption initiation temperature to 194.3 °C, reduces the peak desorption temperature to 245.6 °C, decreases the dehydrogenation activation energy to 82.14 kJ·mol⁻¹, and can absorb 5.4 wt.% of hydrogen within 60 s at 200 °C,demonstrating outstanding hydrogen ab/desorption kinetics, compared to pure MgH₂. Additionally, it can maintain a high hydrogen capacity of 5.2 wt.%, even after 50 dehydrogenation cycles, showing good cycle stability. The characterization results show that the high-valent Bi and Ti in Bi₂Ti₂O₇ are reduced to their low-valent or even zero-valent metallic states during the dehydrogenation and hydrogenation process, thus establishing an in-situ multivalent and multi-element catalytic environment. Density functional theory calculations further reveal that the synergistic effects between Bi and Ti in the Bi-Ti mixed oxide facilitate the cleavage of Mg-H bonds and lower the kinetic barrier for the dissociation of hydrogen molecules, thereby substantially enhancing the kinetics of the Mg/MgH₂ system. This study presents a strategic method for developing efficient catalysts for hydrogen storage materials by harnessing the synergistic effects of metal elements.