Diffusion layer approximation under transient conditions

First, a theoretical expression of diffusion layer thickness is derived for simple (one-step) electrochemical reactions with reversible electron-transfer processes investigated by LSV under semi-infinite linear diffusion conditions in the electrolytic solution. The above expression is compared to previous results in electrochemical literature. In particular, it is shown that Padé-approximants used in previous works fail to predict the potential dependence of diffusion layer thickness over the whole potential domain. More generally, the change of diffusion layer thickness with respect to time is derived in this article in terms of reaction rate and mass-transport conditions, irrespective of the interfacial reaction mechanism and the electron-transfer kinetics. The formulation applies to one-dimensional diffusion processes eventually coupled with homogeneous chemical reactions, using electrochemical techniques such as potential and current step methods, linear sweep voltammetry (LSV), etc. Finally, experimental investigation of FeIII/FeII redox couple by LSV is presented as an illustration of the theoretical derivation in this work.