Distinguishing Heterogeneous and Homogeneous CE Mechanisms: Theoretical Insights into Square-Wave Voltammetry

A theoretical study of an electrode mechanism where the electrode reaction (E) is preceded by a chemical reaction that takes place solely at the electrode surface (C-het) is presented under conditions of square-wave voltammetry (SWV). Rigorous mathematical solutions in the form of integral equations derived by means of Laplace transforms are presented for the surface concentration of all species involved in the electrode mechanism, yielding explicit recurrent formulas for the simulation of the voltammetric response. The theory approximately predicts that the chemical reaction starts at the beginning of the voltammetric experiment, disregarding its occurrence in the short time period between inserting the electrode in the solution until starting the voltammetric experiment. It is demonstrated that SWV can differentiate between the common CE mechanism, where C is a homogeneous chemical reaction taking place in the vicinity of the electrode, and the current ChetE mechanism, which is relevant for plethora of electrocatalytic processes at electrodes modified with catalytically active enzymes and/or noble-metal nanoparticles, as well as for electrocatalytic processes of fundamental importance such as CO2, N-2, O-2, and H+ reductions.