Determination of the Diffusion Coefficient and Phase‐Transfer Rate Parameter in LaNi5 and MmNi3.6Co0.8Mn0.4Al0.3 Using Microelectrodes

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Abstract
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 using a fit to the whole transient. The diffusion coefficient was found to be . 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 . A fit of the whole curve showed that neither a pure diffusion model nor a model including phase transfer could explain the whole transient.

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