Effect of diffusion in underpotential deposition: Simulated and experimental results

Diffusion limitation can play a role in shaping the CVs during upd formation, in spite of the fact that the amount of metal deposited is very small, of the order of 1-2 nmol/cm2. In this paper, the shapes of cyclic voltammograms are evaluated quantitatively by digital simulation. The Frumkin adsorption isotherm and reversibility are assumed. A dimensionless parameter P is defined, which is proportional to the ratio between the diffusion-limited and the surface-controlled peak current densities. A domain in which diffusion can be neglected is presented by a series of lines of constant P in plots of logcbulk vs. log|v|. This will allow one to choose the combinations of concentrations and sweep rates where experiments can be conducted without significant interference by diffusion limitation. The results of simulation were compared for the case of upd formation of lead on a polycrystalline silver substrate. Very narrow CV peaks are observed for this system, and the value of the interaction parameter in the Frumkin isotherm is found to be f = −2.5 ± 0.1. Good agreement between experiment and the results of simulation was found when this value was employed.