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 a high gravimetric hydrogen capacity of 11.5 wt % and an ideal dehydrogenation enthalpy of 32 kJ mol –1 H 2, Ca(BH 4 ) 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 (TiF 4, NbF 5, Ti-isopropoxide, and CaF 2 ) 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). Transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDX) was used to show the local structure, size, and distribution of the additive/catalyst. 11 B{ 1 H} solid state magic angle spinning-nuclear magnetic resonance (MAS NMR) was carried out to detect possible amorphous phases. The formation of TiB 2 and NbB 2 nanoparticles was observed after milling or upon sorption reactions of the Nb- and Ti-based Ca(BH 4 ) 2 doped systems. The formation of transition-metal boride nanoparticles is proposed to support the heterogeneous nucleation of CaB 6 . The {111}CaB 6 /{1011}NbB 2, {111}CaB 6 /{1010}NbB 2, as well as {111}CaB 6 /{1011}TiB 2 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 boride nanoparticles act as heterogeneous nucleation sites for CaB 6, refine the microstructure thus improving the sorption kinetics, and, as a consequence, lead to the reversible formation of Ca(BH 4 ) 2 .