Structural analysis of calcium reactive hydride composite for solid state hydrogen storage

Abstract

Owing to a theoretical hydrogen storage capacity of 10.5 wt% H2, Ca(BH4)2+MgH2, the so‐called calcium reactive hydride composite (Ca‐RHC), has a great potential as a hydrogen storage material. However, its dehydrogenation temperature (∼623 K) is too high for any mobile applications. By addition of 10 mol% of NbF5 into Ca(BH4)2+MgH2, a decrease of the dehydrogenation onset temperature by ∼120 K is observed. In order to understand the reasons behind this desorption temperature decrement two sets of samples [Ca(BH4)2+MgH2 and Ca(BH4)2+MgH2+0.1NbF5] in different hydrogenation states, were prepared. The structural investigation of the above mentioned sets of samples by means of volumetric measurements, anomalous small‐angle X‐ray scattering (ASAXS) and X‐ray absorption spectroscopy (XAS) is reported here. The XAS results show that after the milling procedure NbB2 is formed and remains stable upon further de/rehydrogenation cycling. The results of Nb ASAXS point to nanometric spherical NbB2 particles distributed in the hydride matrix, with a mean diameter of ∼10 nm. Results from Ca ASAXS indicate Ca‐containing nanostructures in the Ca‐RHC+0.1NbF5 samples to be ∼50% finer compared to those without additive. Thus, a higher reaction surface area and shorter diffusion paths for the constituents are concluded to be important contributions to the catalytic effect of an NbF5 additive on the hydrogen sorption kinetics of the Ca(BH4)2+MgH2 composite system.

Influence of milling parameters on the sorption properties of the LiH–MgB2 system doped with TiCl3

“…A tendency towards a plateau in sorption times is visible such that any increase in energy transfer will have no further effect. This too could be attributed to the grain refinement effect of the additive [16,17]. To separate the effect of additive vs. milling conditions, each concentration was kept constant, and comparisons are made with changes in milling conditions.…”

“…Another alternative is to add transition metal halides to increase significantly the sorption kinetics. The impact of transition metal halides has been studied extensively [16][17][18][19]. Their addition to the system reduces particle and crystallite size of the powders, and increases the number of lattice defects and grain boundaries.…”

Tetrahydroborates: Development and Potential as Hydrogen Storage Medium

“…Despite the fact that the TM-boride species assist the formation of CaB 6 , the reversibility of the Ca-RHC system, i.e., Ca(BH 4 ) 2 + MgH 2 as shown in reaction (58), is not complete [137,[198][199][200] …”

Cyclic Dehydrogenation–(Re)Hydrogenation with Hydrogen‐Storage Materials: An Overview

“…155 It has a theoretical GHSC of 11.4 wt % H 2 and is able to reversibly store 8–10 wt % H 2 ; the dehydrogenation enthalpy is lowered by 25 kJ mol −1 H 2 relative to that of pristine LiBH 4 . Alternatively, NaBH 4 and Ca(BH 4 ) 2 have also been proposed to elaborate efficient composites with MgH 2 155–158. Other examples of reactive hydride composites include LiBH 4 –MH 2 (M=Ce, Ca),159 6 LiBH 4 –CaH 2 –3 MgH 2 ,160 LiAlH 4 –2 LiNH 2 ,149, 161 and LiBH 4 –2 LiNH 2 –MgH 2 162.…”