Influences on the H2-sorption properties of Mg of Co (with various sizes) and CoO addition by reactive grinding and their thermodynamic stabilities

We attempted to improve the H-2-sorption properties of Mg by mechanical grinding under H-2 (reactive grinding) with Co (with various particle sizes) and with CoO. The thermodynamic stabilities of the added Co and Coo were also investigated. Coo addition has the best influence and addition of smaller particles of Co (0.5-1.5 mum) has a better effect than the addition of larger particles of Co on the H-2-sorption properties of Mg. The activated Mg+10 wt.%CoO sample has about 5.54 wt.% hydrogen-storage capacity at 598 K and the highest hydriding rate, showing an H-a value of 2.39 wt.% after 60 mm at 598 K, 11.2 bar H-2. The order of the hydriding rates after activation is the same as that of the specific surface areas of the samples. The reactive grinding of Mg with Co or CoO and hydriding-dehydriding cycling increase the H-2-sorption rates by facilitating nucleation of magnesium hydride or alpha solid solution of Mg and H (by creating defects on the surface of the Mg particles and by the additive), and by making cracks on the surface of Mg particles and reducing the particle size of Mg, thus shortening the diffusion distances of hydrogen atoms. The cobalt oxide is stable even after 14 hydriding cycles at 598 K under 11.2 bar H-2. Discharge capacities are measured for the samples Mg+10 wt.%COO and Mg+10 wt.%Co (0.5-1.5 mum) with good hydrogen-storage properties.