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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on KBH4 by Materials Project

KBH4 crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. K1+ is bonded in a 11-coordinate geometry to eleven H+0.50+ atoms. There are a spread of K–H bond distances ranging from 2.57–2.76 Å. B3- is bonded in a distorted water-like geometry to two H+0.50+ atoms. There is one shorter (1.28 Å) and one longer (1.29 Å) B–H bond length. There are three inequivalent H+0.50+ sites. In the first H+0.50+ site, H+0.50+ is bonded in a distorted single-bond geometry to two equivalent K1+ and one H+0.50+ atom. The H–H bond length is 0.75 Å. In the second H+0.50+ site, H+0.50+ is bonded in a distorted single-bond geometry to four equivalent K1+ and one B3- atom. In the third H+0.50+ site, H+0.50+ is bonded in a distorted single-bond geometry to three equivalent K1+ and one B3- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on KBH4 by Materials Project

KBH4 crystallizes in the tetragonal P4_2/nmc space group. The structure is three-dimensional. K1+ is bonded in a 12-coordinate geometry to twelve equivalent H+0.50+ atoms. There are four shorter (2.81 Å) and eight longer (2.82 Å) K–H bond lengths. B3- is bonded in a tetrahedral geometry to four equivalent H+0.50+ atoms. All B–H bond lengths are 1.23 Å. H+0.50+ is bonded in a single-bond geometry to three equivalent K1+ and one B3- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electrolyte-Assisted Hydrogen Cycling in Lithium and Sodium Alanates at Low Pressures and Temperatures

An investigation of electrolyte-assisted hydrogen storage reactions in complex aluminum hydrides (LiAlH4 and NaAlH4) reveals significantly reduced reaction times for hydrogen desorption and uptake in the presence of an electrolyte. LiAlH4 evolves ~7.8 wt% H2 over ~3 h in the presence of a Li-KBH4 eutectic at 130 °C compared to ~25 h for the same material without the electrolyte. Similarly, NaAlH4 exhibits 4.8 wt% H2 evolution over ~4 h in the presence of a diglyme electrolyte at 150 °C compared to 4.4 wt% in ~15 h for the same material without the electrolyte. These reduced reaction times are composed of two effects, an increase in reaction rates and a change in the reaction kinetics. While typical solid state dehydrogenation reactions exhibit kinetics with rates that continuously decrease with the extent of reaction, we find that the addition of an electrolyte results in rates that are relatively constant over the full desorption window. Fitting the kinetics to an Avrami-Erofe’ev model supports these observations. The desorption rate coefficients increase in the presence of an electrolyte, suggesting an increase in the velocities of the reactant-product interfaces. In addition, including an electrolyte increases the growth parameters, primarily for the second desorption steps, resulting in the observed relatively constant reaction rates. Similar effects occur upon hydrogen uptake in NaH/Al where the presence of an electrolyte enables hydrogenation under more practical low temperature (75 °C) and pressure (50 bar H2) conditions.

25 ENERGY STORAGE↗