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.