Improved hydrogen storage dynamics of as-milled Mg22(LaY)2Ni10 alloy catalyzed by MoS2

Abstract

The composites composed of Mg22(LaY)2Ni10 as-quenched alloy and x wt% MoS2 (x = 0–10) catalyst were prepared by high-energy ball milling. The phase composition, microstructure and hydrogen storage dynamics were characterized by X-ray powder diffraction, scanning electron microscopy, high resolution transmission electron microscopy, differential scanning calorimetry and Sievert's measure method, respectively. Experimental results show that there are Mg2Ni, LaMg3, La2Mg17 and Mg24Y5 phases in the composites, the addition of MoS2 leads to the formation of MgS and its intensity strengthens with the increase of MoS2. The sequence in which the average grain size decreases from large to small is 0 wt% MoS2, 10 wt% MoS2, 1 wt% MoS2, 5 wt% MoS2 and 3 wt% MoS2. For granularity, the suitable amount of MoS2 is 3 wt%, which gives a useful influence on decreasing particle size and enhancing grinding efficiency in this experiment. For the 3 wt% and 10 wt% MoS2 composites, there are a large number of amorphous embedded by a small amount of nano scale grains. The misorientation angle between two adjacent Mg2Ni grains belongs to the large angle grain boundary. Compared with the other composites, the amorphous phase of the 3 wt% MoS2 composite is relatively large, and the phase boundaries of atoms are relatively disordered. These composites have good activation characteristics. The hydrogen absorption capacity can reach their respective maximum after 2–4 cycles. The hydrogen absorption rate is greater whether the composites contain MoS2 at higher temperature (350 °C), and can attain saturation point within 18 min. By contrast, the 1 wt% and 3 wt% MoS2 composites can saturate within 5.4 and 8.5 min, respectively. Their maximum saturated hydrogen absorption capacity reduces from 3.811 wt% to 2.748 wt% when MoS2 catalyst rises from 0 to 10 wt%. The time required for 90% of the saturated hydrogen absorption capacity (T90%Cmax) both first shorten and then extend with the increase of MoS2. Although the saturated hydrogen absorption capacity of 3 wt% MoS2 composite is lower than 0 wt% and 1 wt% composites, its T90%Cmax are superior to them. At higher temperature (350 °C), the hydrogen absorption saturation ratio (R1min) rises from 57.49% to 86.29% with MoS2 amount increases from 0 to 5 wt%, and then lessens to 67.54% when the MoS2 amount further increase up to 10 wt%. The amount of saturated hydrogen desorption capacity (C′max) of the composite decreases with the increase of MoS2. Compared with 0 wt% MoS2 and 10 wt% MoS2 composites, the activation energy of hydrogenation and dehydrogenation for the 3 wt% MoS2 composite is relatively low. Comprehensive comparison find that the 3 wt% MoS2 composite has the best hydrogen desorption kinetics, a small or excessive amount of MoS2 both decreases dehydrogenating rate. Comprehensive analysis shows that the 3 wt% MoS2 composite contains amorphous and nanocrystalline crystals, displaying a good hydrogen absorption/desorption kinetics, even at 100 °C. Excessive or too little MoS2 can improve the hydrogen absorption and desorption performance of the as-milled composites in our experiment.