Microstructure and electrochemical properties of the Co-BN composites

The Co-BN composites are synthesized by ball-milling metallic Co and boron nitride (BN) powder with a different Co/BN weight ratio. The microstructure, morphology and chemical state of the obtained Co-BN composites are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). After ball milling, Co nanoparticles of 10-30 nin in size are distributed oil the amorphous BN matrix. The electrochemical measurements, including galvanostatic method and cyclic voltammetry (CV), show that the Co-BN composite with the Co/BN weight ratio of 5/1 has a good cycle performance and a high reversible electrochemical capacity in 6M KOH solution. Moreover, the Co-BN composite can be discharged directly Without the first charging process. The higher initial discharge capacity, observed in the first cycle of the pi-c-charged sample, can be explained by an initial reduction of the oxidized species oil the surface of the Co-BN composite and subsequent oxidation. The partial dissolution of Co(OH)(2) in alkaline solution Would further increase the active surface area of the material for the considerable decrease in overpotential after the first cycle. Based oil a structure analysis and electrochemical measurement, the reversible faradic reaction ofthe highly dispersed active Co nanoparticles in the Co-BN composite is dominant. (c) 2007 Elsevier Ltd. All rights reserved.