Current at a Rotating Disk Electrode under Electrosorption and Surface Diffusion Limitation

Abstract

Following a model for the current limited by adsorption and surface diffusion at a rotating disk electrode (rde), the case is analyzed when surface diffusion is preceded by adsorption with charge transfer. The electrode surface consists of a distribution of reaction sites embedded in a supporting phase where electrosorption and surface diffusion take place. Electrosorption is described by five parameters, an equilibrium potential (E0ad), a microscopic partial charge transfer (zad), a charge transfer coefficient (βad), and the rate constants for desorption and adsorption (k1 and k2, respectively). Analytical expressions for the current – overpotential (j vs ηad) and the Koutecký – Levich (KL) relation (1/j vs 1/ω1/2) are provided as a function of electrosorption parameters and surface diffusivity (Ds). The KL relation is linear, with the same slope as for a classical rde, whereas the intercept (1/ jK) depends on electrosorption parameters and Ds. Tafel relations for the intercept current (log jK vs. η) are provided for the general and limiting cases. Effects of the radial convective transport in the electrolyte over reaction sites, and of the overlapping of surface diffusion areas are included in the model. The analytical expressions can be used to determine surface diffusion kinetics parameters from rde results.