The electrochemical charge and discharge behavior of Co powders has been investigated by using X-ray diffraction (XRD), charge and discharge testing, and electrochemical impedance spectroscopy (EIS) at room temperature. Mechanical milling (MM) has been used to treat the Co powders for a comparative experiment. Mechanical milling induces a phase transition of fcc phase to hcp phase, an increase in particle size, and a decrease in grain size. The results of the XRD indicate a reversible reaction between Co and Co(OH)(2). The non-milled Co has a higher discharge capacity at a current density of 60 mA g(-1) as compared to the milled one. However, the milled Co presents a better HRD, in spite of the discrepancy in particle size. The results of EIS show that the electrochemical reaction process of Co powders consists of three steps, that is the charge-transfer of Co/Co(OH)(2) or Co/CoHx, the mass-transfer of HCoO2-, and hydrogen diffusion within Co, depending on the depth of discharge. (c) 2009 Elsevier Ltd. All rights reserved.