Studies of electrochemical properties of TiNi alloy used as an MH electrode - I. Discharge capacity

The electrochemical properties of TiNi alloy used as an MH electrode were investigated. A mathematical model for the electrochemical discharge capacity of TiNi electrode was developed. The model was used to study the effects of various parameters on the discharge capacity, and cycle life. The predicted results from the model fit well with the experimental results. Increasing the discharge current density and charge/discharge cycling current density decreases the discharge capacity, but a high charge/discharge cycling current density increases the cycle life of the hydride electrode. Increasing the exchange current density, or the diffusion coefficient of hydrogen or reducing the particle size increases the discharge capacity. Upon increasing the discharge current density the controlling steps of discharge capacity change from hydrogen diffusion in alpha phase (including oxide film) to the charge-transfer reaction on the hydride electrode surface. However for the TiNi alloy electrode the discharge capacity still depends mainly on the diffusion ability of hydrogen even at high discharge current density (200 mA/g). There exists an optimum dissolved hydrogen content of the alpha phase, C-alpha beta(opt), at which the discharge capacity reaches a maximum value. Also C-alpha beta(opt) increases with increasing discharge current density. (C) 1998 Elsevier Science Ltd. All rights reserved.