Investigations On Thermodynamics And Kinetics Modification And Mechanisms Of The Li-Mg-N-H Hydrogen Storage System

The Mg(NH2)2-2LiH system possesses a reversible hydrogen capacity of about5.6wt%and favorable thermodynamics, consequently being regarded as one of the most promising hydrogen storage materials so far. However, a rather high kinetic barrier raises the operating temperature, precluding its practical applications. In this paper, starting from the modification of thermodynamics and kinetics, we systematically investigate the modification mechanisms and high-temperature failure mechanisms of alkali metal K-based compounds on the Mg(NH2)2-2LiH system. Meanwhile, the positive role of the same group element Rb-based compounds and the synergetic modification mechanisms of KH and RbH were also studied.The hydrogen storage properties and mechanisms of the Mg(NH2)2-2LiH system with potassium halides were investigated and discussed. The results show that the KF-added sample exhibits superior hydrogen storage properties as～5.0wt%of hydrogen can be reversibly stored in the0.08KF-added sample via a two-stage reaction with an onset dehydrogenation temperature of80℃. However, hydrogen storage behaviors of the samples with KCl, KBr, and KI remain almost unchanged. The fact that KF can readily react with LiH to convert to KH and LiF due to the favorable thermodynamics during ball milling should be the primary reason for its significant effects, as the presence of KH provides a synergetic thermodynamic and kinetic destabilization in the hydrogen storage reaction of the Mg(NH2)2-2LiH system by declining the activation energy of the first-step dehydrogenation as a catalyst and reducing the desorption enthalpy change of the second step as a reactant.KH was directly added to a Mg(NH2)2-2LiH system and the compositional effects on the hydrogen storage properties were investigated systematically. The Mg(NH2)2-2LiH-0.08KH composite displays optimized hydrogen-storage properties, reversibly storing approximately5.2wt%hydrogen through a two-stage reaction and an dehydrogenation onset at70℃. The0.08KH-added sample fully dehydrogenated at130℃begins to absorb hydrogen at50℃, and takes up approximately5.1wt%of hydrogen at140℃. Adding KH significantly enhances the de-/hydrogenation kinetic properties, however, an overly rapid hydrogenation rate enlarges the particle size and raises the dehydrogenation temperature. A cycling evaluation reveals that the KH-added Mg(NH2)2-2LiH system possesses good reversible hydrogen storage abilities, although the operational temperatures for dehydrogenation/hydrogenation increase during cycling. Detailed mechanistic investigations indicate that adding KH catalytically decreases the activation energy of the first dehydrogenation step and reduces the enthalpy of desorption during the second dehydrogenation step as a reactant, significantly improving the hydrogen storage properties of Mg(NH2)2-2LiH.The high-temperature failure mechanisms of K-based additives in the Mg(NH2)2-2LiH system was also investigated and elucidated. The positive effects of K-based additives disappear when the hydrogen release and uptake of the KF-added Mg(NH2)2-2LiH samples were performed at higher temperatures (>200℃). The change in the crystal structure of the dehydrogenation product, the enlargement in the grain and particle sizes of the dehydrogenation/hydrogenation products, and the increase in the inhomogeneous degree of mixing and distribution of K-based additives should be the three most important reasons for the increased operating temperature during the follow-up cycles. In particular, the ability of K-based additives to lower the operating temperature for hydrogen storage in the Mg(NH2)2-2LiH system can be sufficiently recovered after ball milling. Therefore, the failure of K-based additives after high-temperature treatment is only phenomenological instead of being natural. Strictly limiting the dehydrogenation/hydrogenation of the K-added Mg(NH2)2-2LiH system at lower temperatures is critical for maintaining the superior effect of K-based additives.The hydrogen storage properties of RbF-added Mg(NH2)2-2LiH system was investigated systematically. The Mg(NH2)2-2LiH-0.08RbF composite exhibits the optimal hydrogen storage properties as it could reversibly store～4.76wt%of hydrogen with the onset temperatures of80℃for dehydrogenation. At130℃, about70%of hydrogen was rapidly released from the0.08RbF-added sample within180min. In particular, the introduction of RbF significantly enhanced the hydrogenation properties. The fully dehydrogenated Mg(NH2)2-2LiH-0.08RbF sample could absorb about4.8wt%of hydrogen at120℃with the onset temperature of55℃. Further evaluations on thermodynamics and kinetics revealed that both the apparent activation energy and the enthalpy change for hydrogen desorption from the Mg(NH2)2-2LiH-0.08RbF sample were decreased. Structural analyses revealed that a metathesis reaction between RbF and LiH readily took place to convert to RbH and LiF during ball milling. As the actual active species, RbH directly participated in the dehydrogenation reaction in heating process, which provides a more thermodynamically and kinetically favorable reaction pathway. Thus, a concertedly thermodynamic and kinetic destabilization in hydrogen desorption from the RbF-added Mg(NH2)2-2LiH system was achieved.KH and RbH were simultaneously introduced into the Mg(NH2)2-2LiH system to further enhance its hydrogen storage properties, and the corresponding mechanisms were also elucidated. The Mg(NH2)2-2LiH-0.04KH-0.04RbH sample exhibits the optimal hydrogen storage properties as it could reversibly store5.2wt%of hydrogen. The fully dehydrogenated sample at130℃can fully hydrogenated at the temperature as low as120℃. Evaluation on thermodynamics revealed that the enthalpy change for dehydrogenation from the Mg(NH2)2-2LiH-0.04KH-0.04RbH sample was decreased. The synergetic effect of KH and RbH provides a more thermodynamically favorable reaction pathway. Structural characteristics revealed that a solid solution reaction between KH and RbH occurred during ball milling. RbH firstly dissolved from the KH&RbH solid solution and participated in the hydrogen desorption reaction, then the rest of KH participated in the following reaction. The hydrogen storage properties of Mg(NH2)2-2LiH were significantly enhanced by the combined effect of RbH and KH. After50cycles at170℃/2h for dehydrogenation and130℃/2h for hydrogenation, the hydrogen capacity of the Mg(NH2)2-2LiH-0.04KH-0.04RbH sample is still above4.4wt%with the capacity retention of93%. The slightly decreased hydrogen capacity during cycling tests is originated from the unsaturated hydrogenation. The hydrogen capacity can be fully recovered after slightly increased the hydrogenation temperature.