An investigation of the origin of the electrochemical hydrogen storage capacities of the ball-milled Co-Si composites

The Co-Si composites with a molar ratio of 2:1 are synthesized by ball-milling method and their potential as negative electrode materials of Ni-MH batteries is investigated. The microstructure, morphology and chemical state of the ball-milled Co-Si composites are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XRD patterns show that the ball-milled samples for 10 and 20 h contain Co, Si and Co₂Si phases, and the ball-milled samples for 40 and 60 h are mainly amorphous Co₂Si alloys. In contrast to the high initial discharge capacity (1012 mAh/g) obtained for the sample ball-milled for 10 h, the discharge capacities of the samples ball-milled for 40 and 60 h are very low. It indicates that the hydrogen storage capacity of pure Co₂Si alloy is very low. It is found that the formation of active Co nanoparticles and Si oxidation are responsible for the high values of the initial discharge capacities of the ball-milled samples for 10 and 20 h. However, after the first cycle, the discharge capacities of the composites drop below 300 mAh/g. Based on XRD and cyclic voltammetric results, the remaining discharge capacity is mainly contributed by the conversion reaction of Co/Co(OH)₂.