Highly active multivalent multielement catalysts derived from hierarchical porous TiNb₂O₇ nanospheres for the reversible hydrogen storage of MgH₂

Critical limitations in applying MgH₂ as a hydrogen-storage medium include the high H₂ desorption temperature and slow reaction kinetics. In this study, we synthesized hierarchical porous TiNb₂O₇ spheres in micrometer scale built with 20–50 nm nanospheres, which showed stable activity to catalyze hydrogen storage in MgH₂ as precursors. The addition of 7 wt.% TiNb₂O₇ in MgH₂ reduced the dehydrogenation onset temperature from 300 to 177 °C. At 250 °C, approximately 5.5 wt.% H₂ was rapidly released in 10 min. Hydrogen uptake was detected even at room temperature under 50 bar hydrogen; 4.5 wt.% H₂ was absorbed in 3 min at 150 °C, exhibiting a superior low-temperature hydrogenation performance. Moreover, nearly constant capacity was observed from the second cycle onward, demonstrating stable cyclability. During the ball milling and initial de/hydrogenation process, the high-valent Ti and Nb of TiNb₂O₇ were reduced to the lower-valent species or even zero-valent metal, which in situ created multivalent multielement catalytic surroundings. A strong synergistic effect was obtained for hybrid oxides of Nb and Ti by density functional theory (DFT) calculations, which largely weakens the Mg-H bonding and results in a large reduction in kinetic barriers for hydrogen storage reactions of MgH₂. Our findings may guide the further design and development of high-performance complex catalysts for the reversible hydrogen storage of hydrides. [Figure not available: see fulltext.]