Sorption properties and reversibility of Ti(IV) and Nb(V)-fluoride doped-Ca(BH4)2–MgH2 system

Abstract

In the last decade, alkaline and alkaline earth metal tetrahydroborates have been the focuses of the research due to their high gravimetric and volumetric hydrogen densities. Among them, Ca(BH4)2 and the Ca(BH4)2 + MgH2 reactive hydride composites (RHC), were calculated to have the ideal thermodynamic properties which fall within the optimal range for mobile applications. In this study, the addition of NbF5 or TiF4 to the Ca(BH4)2 + MgH2 reactive hydride composite system was attempted aiming to obtain a full reversible system with the simultaneous suppression of CaB12H12. Structural characterization of the specimens was performed by means of in-situ Synchrotron Radiation Powder X-ray diffraction (SR-PXD) and 11B{1H} Solid State Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR). The evolution of the chemical state of the Nb- and Ti-based additives was monitored by X-ray Absorption Near Edge Structure (XANES). The addition of NbF5 or TiF4 to the Ca(BH4)2 + MgH2 system have not suppressed completely the formation of CaB12H12 and only a slight improvement concerning the reversible reaction was displayed just in the case of Nb-doped composite material.