Investigations On The Hydrogen Storage Performance And Reaction Mechanism Of The Mg(NH₂)₂-2LiH Composite System

Efficient hydrogen storage has been regarded as one of the key technical challenges in the large-scale application of hydrogen energy. The Mg(NH2)2-2LiH composite material system is one of the most promising hydrogen storage materials developed so far. It possesses a reversible hydrogen content of～5.6 wt% and suitable thermodynamic properties that allows it to achieve 1 bar equilibrium hydrogen pressure at～80℃. But the experimentally observed onset temperature for hydrogen desorption was above 140℃even if the sample had been ball-milled intensively indicating the existence of severe kinetic barriers. Besides the poor reacting kinetics, this material also suffers from the degradation in cycling test and the formation of side-product of ammonia. The present research aims to understand the reaction mechanism and alleviate the above three main problems.An efficient non-transition metal catalyst-potassium was discovered for the first time. By doping～3 mol% potassium salts, such as KH, KOH. KF, KNH2, K2CO3, K3PO4 etc., remarkable improvements in hydrogen uptake/release kinetics were obtained. The overall hydrogen desorption of the K-modified system was dramatically reduced by～50℃as compared to the pristine system. Full dehydrogenation and hydrogenation can be achieved at a temperature as low as 107℃. Moreover, side product-ammonia was almost undetectable in the dehydrogenation. However, it was also found that the catalytic performance of potassium was gradually declined upon cycling test at elevated temperatures due to the phase separation of potassium. By re-milling the de-activated sample after cycling. the catalytic activity of potassium can be refreshed. Structural characterizations by applying the synchrotron X-ray absorption fine structure (XAFS) indicated that there was a reversible transformation between K-H components and K-N components during the process of dehydrogenation and hydrogenation. The K-N components are likely the active species for the kinetic enhancement.The cycling durability was improved by introducing the dispersant. It was found that the aggregation/growth of the reactant crystallites is mainly responsible for the poor cyclability of the Mg(NH2)2-2LiH system. Introducing-20 wt% triphenyl phosphate (TPP) in the system efficiently prevented the crystallization of Mg(NH2)2 in the hydrogenation and. in the same time, depressed the growth of crystallite size of Li2Mg(NH)2 in the dehydrogenation, so that a remarkable improvement in the cyclability was achieved. Meanwhile, an enhanced kinetic performance was also obtained due to the pronounced reduction in the crystallite size of reactants by adding TPP. In addition, the thermodynamic properties of the TPP-doped system were also altered because of the persistence of amorphous Mg(NH2)2 during the cycling test, i.e., the decrease in the entropy change resulted in a significant decrease in the equilibrium hydrogen pressure.A series of experiments were designed aiming to disclose the origin of kinetic barriers in the dehydrogenation of the Mg(NH2)2-2LiH system and to understand the mechanism of potassium catalysis. It was found that the thermal stabilities of the transition metal-doped Mg(NH2)2 samples do not correlate with those of the transition metal-doped Mg(NH2)2-2LiH samples revealing that NH3 should not be an indispensable intermediate in the dehydrogenation of Mg(NH2)2-2LiH. Severe kinetic barriers in hydrogen desorption from the Mg(NH2)2-2LiH system are likely from the interface reaction between Mg(NH2)2 and LiH. Potassium can efficiently activate Mg(NH2)2 and promote LiH to participate in the dehydrogenation leading to significant reduction of kinetic barriers in the interface reaction. Three K-containing intermediates, i.c., K2Mg(NH2)4, Li3K(NH2)2 and KII. were found to be involved in the dehydrogenation of the K-modified Mg(NH2)2-2LiH system. The evolution of those intermediates and the transformation among them provide a more energy favorable pathway for the dehydrogenation and thus accelerate the overall reaction rate. According to the above results, an interfacial catalytic mechanism for the heterogeneous solid-state reaction was proposed.