DETERMINATION OF THE DIFFUSION-COEFFICIENT AND PHASE-TRANSFER RATE PARAMETER IN LANI5 AND MMNI(3.6)CO(0.8)MN(0.4)AL(0.3) USING MICROELECTRODES

A potential-step method for determining the diffusion coefficient and phase-transfer parameter in metal hydrides by using microelectrodes wa s investigated, It was shown that a large potential step is not enough to ensure a completely diffusion-limited mass transfer if a surface-p hase transfer reaction takes place at a finite rate. It was shown, usi ng a kinetic expression for the surface phase-transfer reaction, that the slope of the logarithm of the current vs. time curve will be const ant both in the case of the mass-transfer limited by diffusion or by d iffusion and a surface-phase transfer. The diffusion coefficient and p hase-transfer rate parameter were accurately determined for MnNi3.6Co6 .8Mn0.4Al0.3 using a fit to the whole transient. The diffusion coeffic ient 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 o bserved transient. The diffusion coefficient and phase-transfer rate p arameter 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.