Improved reversible dehydrogenation properties of Mg(BH4)2 catalyzed by dual-cation transition metal fluorides K2TiF6 and K2NbF7

Two dual-cation transition metal fluorides K2TiF6 and K2NbF7 are introduced into Mg(BH4)2 by ball-milling to catalyze the dehydrogenation of Mg(BH4)2. According to the DSC and TPD results, the onset dehydrogenation temperature of Mg(BH4)2 doped with K2TiF6 and K2NbF7 are remarkably reduced to 105.4 and 118.0 °C, respectively. Meanwhile, both the K2TiF6 and K2NbF7 catalyzed systems can release more than 6.4 wt% H2 under 280 °C, showing an improvement in dehydrogenation kinetics. In addition, the reversible capacity of the Mg(BH4)2–3%K2TiF6 system is 2.7 wt% at 280 °C in 250 min, which is enhanced comparing to that of pristine Mg(BH4)2. X-ray diffraction, Fourier-transformed infrared and 11B nuclear magnetic resonance investigations reveal that the K2TiF6 actually acts as a catalytic precursor to react with Mg(BH4)2, forming active hydrides of KBH4 and TiH2, which further serve as catalyzing agents to facilitate the re-generation of Mg(BH4)2 from intermediates under moderate conditions.