Determination of the diffusion coefficient and phase-transfer rate parameter in LaNi5 and MmNi3.6Co0.8Mn0.4Al0.3 using microelectrodes

A potential-step method for determining the diffusion coefficient and phase-transfer parameter in metal hydrides by using microelectrodes was investigated, It was shown that a large potential step is not enough to ensure a completely diffusion-limited mass transfer if a surface-phase transfer reaction takes place at a finite rate. It was shown, using a kinetic expression for the surface phase-transfer reaction, that the slope of the logarithm of the current vs. time curve will be constant both in the case of the mass-transfer limited by diffusion or by diffusion and a surface-phase transfer. The diffusion coefficient and phase-transfer rate parameter were accurately determined for MnNi3.6Co6.8Mn0.4Al0.3 using a fit to the whole transient. The diffusion coefficient was found to be (1.3 +/- 0.3) x 10(-13) m(2)/s. The fit was good and showed that a pure diffusion model was not enough to explain the observed transient. The diffusion coefficient and phase-transfer rate parameter were also estimated for pure LaNi5. A fit of the whole curve showed that neither a pure diffusion model nor a model including phase transfer could explain the whole transient.