Sub- and super-Nernstian Tafel slopes can result from reversible electron transfer coupled to either preceding or following chemical reaction

Abstract

A general scheme for an electrochemically reversible one-electron electrode reaction containing both preceding and following coupled chemical kinetics is analysed in the light of simulation in the context of linear sweep voltammetry at a macro-electrode. In particular, we explore the influence of the coupled chemistry on the observed Tafel slope at potentials where it is influenced minimally by mass transport effect. In the absence of coupled chemistry, an apparent transfer coefficient of unity is observed, as expected for a simple electrochemically reversible one-electron process. However, the presence of a preceding chemical reaction serves to lower the apparent transfer coefficient markedly below unity; the coupled kinetics create the illusion of electrochemical irreversibility. In contrast, a following chemical reaction can give an apparent transfer coefficient in excess of unity creating a ‘super-Nernstian’ response. Thus, a full range of Tafel slopes, both below and above unity, characterize an intrinsically reversible reaction in the presence of coupled kinetics. • Simulation of electrochemically reversible processes with coupled chemistry. • Effects of following and preceding chemistry on Tafel slope is analysed. • Preceding chemical reaction decreases the apparent transfer coefficient. • Following chemical reaction increases the apparent transfer coefficient.