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.
Highlights
The complex aluminum hydrides (MAlH4 where M = Na, Li, K) are known for their high hydrogen capacities and ability to reversibly cycle hydrogen under moderate temperature and pressure conditions
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
Sodium aluminum hydride (NaAlH4 ) exhibits high gravimetric and volumetric hydrogen capacities, but unlike LiAlH4, NaAlH4 is capable of reversible hydrogen cycling when catalyzed [1]
Summary
Lithium aluminum hydride (LiAlH4 ) exhibits the highest gravimetric and volumetric hydrogen capacities for the first two reactions (theoretical 72 g-H2 /L, 7.9 wt% H); neither Reaction (1) nor Reaction (2) has been proven to be reversible in this material [2]. Sodium aluminum hydride (NaAlH4 ) exhibits high gravimetric and volumetric hydrogen capacities (theoretical 50 g-H2 /L, 5.6 wt%), but unlike LiAlH4 , NaAlH4 is capable of reversible hydrogen cycling when catalyzed (typically with a transition metal catalyst at 1–4 mol%) [1]. [1,5] In addition to the high reversible capacity, this material exhibits moderate desorption enthalpies of 37 kJ/mol H2 (Reaction (1)) and 47 kJ/mol H2 (Reaction (2)) [6], implying 1 bar of H2 pressure is achieved with a (theoretical) desorption temperature of ~35 ◦ C and 110 Multiple studies have demonstrated reversible hydrogen cycling in Ti-catalyzed sodium alanate with over 100 cycles with a measured capacity of ~4 wt% H2 . [1,5] In addition to the high reversible capacity, this material exhibits moderate desorption enthalpies of 37 kJ/mol H2 (Reaction (1)) and 47 kJ/mol H2 (Reaction (2)) [6], implying 1 bar of H2 pressure is achieved with a (theoretical) desorption temperature of ~35 ◦ C and 110
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