Diffusion-trapping impedance under restricted linear diffusion conditions

The theoretical expression for the mass-transport impedance is derived considering diffusion-trapping controlled processes studied under restricted linear diffusion conditions (impermeable surface) and assuming uniform distribution of trapping sites in the electrode. The relevant expression applies to hydrogen diffusion-trapping in metals and alloys, as well as ion insertion-trapping in host materials provided the metallic type potential distribution is assumed in electrolyte/host material systems. The impedance diagram shape is discussed based on two dimensionless parameters, i.e. the ratio of the diffusion resistance to the trapping resistance and the ratio of the diffusion time constant to the trapping time constant. A case diagram is plotted to determine the validity conditions for the limiting expressions of the mass-transport impedance derived for diffusion and trapping control. The conditions for characterization of diffusion with trapping from EIS data are discussed depending on the system parameters and the frequency range accessible experimentally. The influence of non-uniform spacial distribution of trapping sites is also discussed. Distribution of trapping sites with different kinetic constants and concentrations is finally envisaged.