Effect of Sodium Borohydride on the Hydrogen Diffusion in MmNi<sub>3.6</sub>Mn<sub>0.4</sub>Al<sub>0.3</sub>Co<sub>0.7</sub> Hydride Electrode

Abstract

Anodic polarisation curves of treated and untreated MmNi3.6Mn0.4Al0.3Co0.7 (Mm = mischmetal) electrodes were measured under the conditions of various initial concentrations of absorbed hydrogen (H/M), rates of potential sweep and temperatures. The alloy powders of the electrodes were treated with 6 M NaOH solution containing y M NaBH4 (y = 0.0, 0.005, 0.01, 0.02, 0.03 and 0.05). The initial concentration of absorbed hydrogen in the electrodes varied between 0 and 0.06 H/M, and the working temperature range was 15 °C to 50 °C. Anodic peak current (Ip) at the MmNi3.6Mn0.4Al0.3Co0.7 electrodes increased with an increase in the y value. In addition, the Ip value depended linearly on initial hydrogen concentration and square root of potential rate irrespective of y value. Furthermore, the activation energy for hydrogen diffusion decreased with an increase in y value. From these results, it is considered that the hydrogen diffusion in the MmNi3.6Mn0.4Al0.3Co0.7 alloys and the electrocatalytic activity at their surfaces influence the Ip value and that they are important factors for improving the charge-discharge performance of the negative hydride electrode. Its low temperature (15-30 °C) performances are also improved by the simple powder treatment with the 6 M NaOH solution containing NaBH4 as reducing agent.