Chemical state, distribution and role of Ti- and Nb-based additives on the Ca(BH4)2 system

Light metal tetrahydroborates are regarded as promising materials for solid state hydrogen storage. Due to both an high gravimetric hydrogen capacity of 11.5 wt. % and an ideal de-hydrogenation enthalpy of 32 kJ mol-1 H2, Ca(BH4)2 is considered to be one of the most interesting compounds in this class of materials.
In this work, a comprehensive investigation of the effect of different selected additives (TiF4, NbF5, Ti-isopropoxide and CaF2) on the reversible hydrogenation reaction of calcium borohydride is presented combining different investigation techniques. The chemical state of the Nb- and Ti-based additives is studied by X-ray absorption spectroscopy (e.g. XANES). The local structure, size and distribution of the additive/catalyst are presented evaluated by Transmission Electron Microscopy (TEM) coupled with Selected Area Electron Diffraction (SAED) or Energy-dispersive X-ray spectroscopy (EDX). 11B{1H} Solid State Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR) was carried out to detect possible amorphous phases. Formation of TiB2 and NbB2 nanoparticles was observed after milling or upon sorption reactions of the Ti- and Nb-based Ca(BH4)2 doped systems. The formation of transition metal borides nanoparticles is proposed to support the heterogeneous nucleation of CaB6. The {111}CaB6/{1011}NbB2, {111}CaB6/{1010}NbB2 as well as the {111}CaB6/{1011}TiB2 plane pairs have the potential to be the matching planes because the d-value mismatch is well below the d-critical mismatch value (6 %).
Transition-metal borides nanoparticles act as heterogeneous nucleation sites for CaB6, refine the microstructure thus improving the sorption kinetics and, as a consequence, lead to the Ca(BH4)2 reversible formation.