Ni9S8/Fe5Ni4S8-mediated reversible catalysis: Enhancing the hydrogen storage performance of MgH2 under mild conditions

• A reversible catalyst Ni 9 S 8 /Fe 5 Ni 4 S 8 was synthesized via a one-step hydrothermal method. • The MgH 2 -5 wt.% Ni 9 S 8 /Fe 5 Ni 4 S 8 composite releases 5 wt.% H 2 at 225 °C and absorbs 4 wt.% H 2 at 45 °C. • The reversible catalytic role of Ni 9 S 8 /Fe 5 Ni 4 S 8 in enhancing MgH 2 hydrogen storage was revealed. MgH 2 is favored for its high hydrogen storage density (7.6 wt.%), however, its practical application is hindered by stable thermodynamics and sluggish kinetics. Although the introduction of highly active catalysts effectively improves the kinetic performance of MgH 2 , the catalytic mechanism of complex multiphase catalysts remains poorly understood, posing formidable challenges to the rational design of truly reversible catalysts. In this study, a highly-effective reversible catalyst Ni 9 S 8 /Fe 5 Ni 4 S 8 was constructed to extremely enhance both the hydrogen desorption and regeneration capability of MgH 2 . Remarkably, at a mild temperature of 225 °C, MgH 2 Ni 9 S 8 /Fe 5 Ni 4 S 8 rapidly desorbs 4.13 wt.% H 2 in 3000 s. The composite also efficiently absorbs hydrogen at near-ambient temperatures, absorbing 4.0 wt.% H 2 in 7200 s at 45 °C. In contrast, ball‑milled MgH 2 alone hardly releases any hydrogen at 225 °C and shows almost no hydrogen uptake at 45 °C. Detailed studies revealed that the improved hydrogen ab/desorption behavior of Mg/MgH 2 is attributed to the in-situ generation of catalytically active sites (Fe/Mg 2 Ni heterostructures and “hydrogen pump” of Mg 2 Ni/Mg 2 NiH 4 ) during hydrogen cycling. Density functional theory (DFT) calculations revealed that significant electron transfer at the Fe/Mg 2 Ni heterostructure reduces hydrogen absorption capacity, thereby promoting hydrogen ab/desorption processes in Mg/MgH 2 . Concurrently, the Mg 2 Ni/Mg 2 NiH 4 “hydrogen pump” facilitates H transfer. These findings yield deeper insights into the reversibly catalytic function of multiphase catalysts for enhancing metal hydride hydrogen storage. The Ni 9 S 8 /Fe 5 Ni 4 S 8 catalyst dynamically generates MgS, Fe, and Mg 2 Ni/Mg 2 NiH 4 phases in-situ, creating abundant interfaces that facilitate hydrogen diffusion, thus achieving fast de/re-hydrogenation at low temperatures.