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

Abstract

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.

Electrolytes in Multiple-Phase Hydrogen Storage Reactions

“…The unusual kinetic behavior observed in the 2nd desorption step of NaAlH 4 was also seen with an electrolyte in MgH 2 /Sn [1] and LiAlH 4 2nd step [2]. In multiphase hydrogen storage reactions (and more generally, solid-state reactions) without an electrolyte, the majority of the reaction can be treated as the first order, where the rate continuously decreases or the temperature must be continuously increased to maintain a constant rate.…”

“…The materials and methods including the Sieverts apparatus, the 1000 bar hydrogenation system, and the 11 B NMR setup (Bruker, Billerica, MA, USA) used for the MgH 2 /Sn, MgB 2 , LiAlH 4 , and NaAlH 4 experiments have been described completely in Refs. [1,2]. For the additional MgB 2 hydrogenation experiments, a custom high-pressure manifold was constructed using a 25 mL Series 4740 pressure vessel from Parr Instruments (Moline, IL, USA) with valves and fittings from High-Pressure Equipment (Erie, PA, USA).…”

“…The influence of electrolytes was also explored for the dehydrogenation of LiAlH 4 and NaAlH 4 [2]. These hydrides dehydrogenate in two steps beginning from a single phase.…”

“…For NaAlH 4 catalyzed with 3 mol% TiCl 3 , dehydrogenation was compared without an electrolyte and with 50 wt% diglyme [2]. No additional electrolyte salt was included with the diglyme.…”

“…In our previous work, an investigation of electrolyte-assisted hydrogen storage reactions in destabilized hydrides (MgH 2 /Sn) and complex hydrides (LiAlH 4 , NaAlH 4 , Mg(BH 4 ) 2 ) revealed significantly reduced reaction times for hydrogen desorption and uptake in the presence of an electrolyte [1,2]. In this work, we provide motivation and background for the use of electrolytes by describing the kinetic limitations in terms of subjective kinetic temperatures and excess free energies.…”