Effect of Co content on the kinetic properties of the MlNi4.3-xCoxAl0.7 hydride electrodes

By means of linear polarization, Tafel polarization and galvanostatic discharge methods, the kinetic properties of MlNi(4.3-chi)Co(chi)Al(0.7) (chi = 0.0, 0.3, 0.5, 0.7, 0.9, 1.1 and 1.3) hydride electrodes, including high rate dischargeability HRD, exchange current density I-0, limiting current density I-L, the symmetry factor beta, and the apparent diffusion coefficient of hydrogen in the alpha phase D-alpha have been studied systematically. The results show that the HRD of the electrode with high Co content is lower than that with low Co content. For the MlNi(4.3-chi)Co(chi)Al(0.7) hydride electrodes, with Co content increasing from 0.0 to 1.3, the values of I-0, I-L, beta and D-alpha decreased from 395 to 33 mA/g, 2263 to 308 mA/g, 0.85 to 0.43, 1.42 x 10(-10) to 2.1 x 10(-11) cm(2)/s, respectively. In order to calculate the electrochemical polarization eta(e) and concentration polarization eta(c) based on anodic polarization curve, the experimental anodic polarization curve should be divided into three regions to discuss according to discharge current density I-d, namely, I-d < I-p, I-p < I-d < I-f, and I-d > I-f. The calculation of eta(e) and eta(c) should use different formula in different regions. When discharge current density I-d is smaller than I-p, the traditional equation used to calculate the electrochemical overpotential can not be used, when I-p is between I-p and I-f, both equations used to calculate the electrochemical overpotential and concentration overpotential can be used, and when I-d is larger than I-f, because there exists absorption and adsorption of hydrogen, the traditional equation used to calculate concentration overpotential can not be used in the system of hydride electrodes. (C) 1999 Elsevier Science Ltd. All rights reserved.