Abstract
Rare earth elements have excellent catalytic effects on improving hydrogen storage properties of the Mg(2)Ni-based alloys. This study used a small amount of Y to substitute Mg partially in Mg(2)Ni(0.9)Co(0.1) and characterized and discussed the effects of Y on the solidification and de-/hydrogenation behaviors. The Mg(2-x)Y(x)Ni(0.9)Co(0.1) (x = 0, 0.2, 0.3, and 0.4) hydrogen storage alloys were prepared using a metallurgy method. The phase composition of the alloys was studied using X-ray diffraction (XRD). Additionally, their microstructure and chemical composition were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy, respectively. The hydrogen absorption and desorption properties of the alloys were studied using pressure-composition isotherms and differential scanning calorimetric (DSC) measurements. The structure of the as-cast Mg(2)Ni(0.9)Co(0.1) alloy was composed of the peritectic Mg(2)Ni, eutectic Mg-Mg(2)Ni, and a small amount of pre-precipitated Mg-Ni-Co ternary phases, and was converted into the Mg(2)NiH(4), Mg(2)Ni(0.9)Co(0.1)H(4), and MgH(2) phases after hydrogen absorption. Furthermore, the XRD patterns of the alloys showed the MgYNi(4) phase and a trace amount of the Y(2)O(3) phase along with the Mg and Mg(2)Ni phases after the addition of Y. After hydrogen absorption, the phase of the alloys was composed of the Mg(2)NiH(4), MgH(2), MgYNi(4), YH(3), Y(2)O(3), and Mg(2)NiH(0.3) phases. With the increase of Y addition, the area ratios of the peritectic Mg(2)Ni matrix phase in the Mg(2-x)Y(x)Ni(0.9)Co(0.1) (x = 0, 0.2, 0.3, and 0.4) alloys gradually decreased until they disappeared. However, the eutectic structure gradually increased, and the microstructures of the alloys were obviously refined. The addition of Y improves the activation performance of the alloys. The alloy only needed one cycle of de-/hydrogenation to complete the activation for x = 0.4. The DSC curves showed that the initial dehydrogenation temperatures of Mg(2)Ni(0.9)Co(0.1) and Mg(1.8)Y(0.2)Ni(0.9)Co(0.1) were 200 and 156 °C, respectively. The desorption activation energies of the hydrides of the Mg(2)Ni(0.9)Co(0.1) and Mg(1.8)Y(0.2)Ni(0.9)Co(0.1) alloys calculated using the Kissinger method were 94.7 and 56.5 kJ/mol, respectively. Moreover, the addition of Y reduced the initial desorption temperature of the alloys and improved their kinetic properties.