Study On The Hydrogen Storage Cycle Performance Of Metal-N-H Composites

The Li-Mg-N-H hydrogen storage composite exhibits relatively high capacity and good reversibility, which is regarded as a promising candidate for practical application. However, a rather high kinetics barrier of hydrogen sorption restricts its extensive application. By adding LiBH4, the kinetic performance of the composite has been improved, but desorption temperature is still relatively high and the cycle performance is relatively poor. Catalysis and composite can effectively improve the performance of kinetics and thermodynamics of the LiMgBNH (1.1MgH2-2LiNH2-0.1LiBH4) hydrogen storage material.In this paper, the catalytic effect of the three different AB5-based alloys (LaNi3.8Al0.75Mn0.45, LaNi4Co and LaNi4.5Mn0.5) on the de-/hydrogenation kinetics performance of the LiMgBNH hydrogen storage material is firstly studied. The three alloys significantly improve the dehydrogenation kinetics, decrease the initial dehydrogenation temperature, and reduce the activation energy. The LaNi4.5Mn0.5 alloy leads to the most significantly improvement on kinetics. There is no new phase formed by the interaction between AB5-based alloys and LiMgBNH composite, the reason for improvement of kinetics of the composites may be that the addition of the alloys results in a weakening of the N-H bond, which is advantageous to dehydrogenation. The dispersive distribution of the AB5-based alloys in the LiMgBNH composite gives full play to the catalytic effect.By comparing the effects of different preparation technologies on dehydrogenation kinetics and cycle performance of composites, it has been found that the 10 wt.% AB5-based alloy samples with high weight ratio of ball to powder exhibits relatively good kinetics performance. The alloys and matrix material particle size is fine, therefore the particles aggregate and grow up more easily in the de-/hydrogenation cycles. The cycle performance is poorer at 150 C and the average decayed rate is about twice that of the LiMgBNH sample. The alloy particle size of 10 wt.% LaNi4.5Mn0.5 sample with low weight ratio of ball to powder increases, and the cycle performance improves, comparing with the LiMgBNH sample. The main reason for capacity fading in cycle process is that the increscent size and the agglomeration of the sample particles hinder the diffusion and mass transfer, then cause the deterioration of kinetics. After decreasing the weight ratio of ball to powder, the LaNi4.5Mn0.5 alloy with large particles plays a role of dispersion and enhanced catalytic, i.e., the alloy particles have been dehiscent and pulverized after cycles, on the one hand, the agglomeration of the sample particles is improved, on the other hand, the dispersion degree of the alloy increases, which can enhance the catalytic effect of the alloy, and then partly offset the deterioration of kinetics caused by particles growth and agglomeration. Finally, the cycle stability is improved by addition of LaNi4.5Mn0.5 alloy.The effect of temperature on the de-/hydrogenation kinetics of the composite was investigated in this paper. When the temperature decreases, the kinetics of the composite deteriorates obviously due to atomic diffusion slowing. Lowering the cycle temperature to 140 C, the cycle performance of the 10 wt.% LaNi4.5Mn0.05 sample with low ball-to-powder weight ratio has not been improved by reason of particles growth slowing down. Insufficient de-/hydrogenation is a main reason for capacity fading.The LaNi4.5Mn0.5 alloy can improve the the particles agglomeration of the composites, but the improvement is limited due to the small addition quantity. The kinetics property of the composite is improved when increase the addition quantity of LaNi4.5Mn0.5 alloy to 30 wt.%, and there is a significant improvement in the particles agglomeration. However, the particles grow up significantly, the cycle performance is worse than the LiMgBNH sample and even 10 wt.% LaNi4.5Mn0.5 sample at 150℃. The reason is that particles of matrix material become finer due to grinding effect of the alloy which grow up more easily in the de-/hydrogenation cycles. To further study the effect of increasing addition quantity of alloy on cycle performance, raising the temperature to 180 C, the cycle performance of the LiMgBNH sample becomes worse, while the cycle performance of the 30 wt.% LaNi4.5Mn0.5 sample has been improved significantly comparing to the sample without adding alloy. The particles grow up more quickly in the cycle process at higher temperature, while the alloy’s the role of dispersion and enhanced catalytic becomes manifest, and then the cycle performance of the sample added alloy is improved.In order to weaken the grinding effect of the alloy, another 30 wt.% LaNi4.5Mn0.5 sample was prepared by three-dimensional swing mixing. The kinetics property of this sample becomes worse comparing to the ball milled sample as the alloy dispersion decreases. Because particle sizes of matrix material have great influence on cycle performance, the cycle performance of the mixed sample improves significantly at 150 C comparing to that of 30 wt.%LaNi4.5Mn0.5 sample, which is better than that of the LiMgBNH sample due to the addition of the alloy.