Kinetic improvement in the Mg(NH₂)₂-LiH storage system by product seeding

Mg(NH₂)₂-2LiH is considered a promising hydrogen storage composition because of its reversibility, moderate operating conditions, and favorable thermodynamics. Two barriers which remain for this system are reducing (eliminating) ammonia release which occurs in parallel with hydrogen desorption and further reducing the desorption temperature toward its thermodynamically predicted value (∼90 °C). Here, we evaluate the Mg-(NH₂)₂-2LiH composition in the presence of various amounts of its decomposition product, Li₂Mg(NH)₂. From this screening study, we demonstrate that seeding the composition with the addition of 10 wt % excess Li₂Mg(NH)₂ is indeed effective for decreasing the peak hydrogen desorption temperature by 40 °C (to 180 °C) relative to the unseeded composition. This kinetic enhancement is quantified by the corresponding reduction in activation energy (E_a) from 88.0 to 76.2 kJ/mol for the unseeded and seeded forms, respectively. Similarly, temperature-programmed desorption mass spectrometry (TPD - MS) data reveals that the improved hydrogen desorption temperature also translates to a significant reduction in the amount of ammonia liberated. The effect of cycling a fresh sample containing product seed is also determined and reveals that after 13 charge/ discharge cycles, a 25% decrease in performance is observed. Finally, isothermal desorption kinetics for the seeded sample show rapid hydrogen desorption within minutes at 220 °C (whereas the unseeded form requires 2 h) and full desorption at 180 °C within 2 h (whereas the unseeded form is only 50% complete in the same time frame).