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 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). 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. 11B{1H} solid state magic angle spinning-nuclear magnetic resonance (MAS NMR) was carried out to detect possible amorphous phases. The formation of TiB2 and NbB2 nanoparticles was observed after milling or upon sorption reactions of the Nb- and Ti-based Ca(BH4)2 doped systems. The formation of transition-metal boride nanoparticles is proposed to support the heterogeneous nucleation of CaB6. The {111}CaB6/{1011}NbB2, {111}CaB6/{1010}NbB2, as well as {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 boride nanoparticles act as heterogeneous nucleation sites for CaB6, refine the microstructure thus improving the sorption kinetics, and, as a consequence, lead to the reversible formation of Ca(BH4)2.