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

• Main Authors: Jason Graetz, John J. Vajo
• Format: Article
• Language: English
• Published: MDPI AG 2020-11-01
• Series: Energies
• Subjects: Hydrogen Storage 1; hydrogenation 2; dehydrogenation 3; alanates 4; electrolyte 5
• Online Access: https://www.mdpi.com/1996-1073/13/22/5868

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.