Electrochemical and Hydrogen Transport Kinetic Performance of MlNi[sub 3.75]Co[sub 0.65]Mn[sub 0.4]Al[sub 0.2] Metal Hydride Electrodes at Various Temperatures

Abstract

Temperature dependence of the electrochemical and hydrogen transport kinetic performance of MlNi3.75Co0.65Mn0.4Al0.2 metal hydride electrode have been systematically investigated in the temperature range -20 to 85°C. The results show that temperature has pronounced effects on a series of characteristics of the electrode. With the increase of temperature, the total resistance Rtotal (LP) and Rtotal (EIS), the polarization resistance Rp, and the ohmic resistance Ro decrease, while the exchange current density i0, the symmetry factor β, the limiting current density iL, and the hydrogen diffusion coefficient D increase. Under anodic polarization, the activation overpotential ηa and the concentration overpotential ηc increase with the decrease of temperature and with the increase of discharge current density. At lower discharge current densities, charge transfer is the rate-determining step of the electrode reaction, whereas at higher discharge current densities, the diffusion of hydrogen in the bulk of the electrode is the rate-controlling step. At medium discharge current densities, the electrode reaction is mainly determined by hydrogen diffusion in the electrode at lower temperatures, while it is mainly controlled by charge transfer at higher temperatures. At intermediate temperatures, the electrode reaction is under mixed control of charge transfer and hydrogen diffusion. The activation energy for charge transfer and for hydrogen diffusion in the electrode were calculated to be 28.1 and 19.9 kJ mol-1, respectively.