Eect of Co content on the kinetic properties of the MlNi4.3ÿXCoXAl0.7 hydride electrodes

By means of linear polarization, Tafel polarization and galvanostatic discharge methods, the kinetic properties of MlNi4.3ÿXCoXAl0.7 (X=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 I0, limiting current density IL, the symmetry factor b, and the apparent di€usion coecient of hydrogen in the a phase Da 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 MlNi4.3ÿXCoXAl0.7 hydride electrodes, with Co content increasing from 0.0 to 1.3, the values of I0, IL, b and Da decreased from 395 to 33 mA/ g, 2263 to 308 mA/g, 0.85 to 0.43, 1.42 10ÿ10 to 2.1 10ÿ11 cm/s, respectively. In order to calculate the electrochemical polarization Ze and concentration polarization Zc based on anodic polarization curve, the experimental anodic polarization curve should be divided into three regions to discuss according to discharge current density Id, namely, Id< IP, IP < Id < If , and Id>If . The calculation of Ze and Zc should use di€erent formula in di€erent regions. When discharge current density Id is smaller than Ip, the traditional equation used to calculate the electrochemical overpotential can not be used, when Ip is between Ip and If , both equations used to calculate the electrochemical overpotential and concentration overpotential can be used, and when Id is larger than If , 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. # 1999 Elsevier Science Ltd. All rights reserved.