Super-Nernstian Tafel slopes: An origin in coupled homogeneous kinetics

Simulations are reported relating to the predicted Tafel slopes of voltammograms recorded for a heterogeneous electrochemically reversible electron transfer process coupled to non-linear homogeneous chemical kinetics. Tafel plots are shown to be markedly non-linear with potential, indicating the essential need to be analysed via simulation. In the case of the EC2 reaction, for a simple one-electron reversible electron transfer, the following second order kinetics lead to Tafel slopes with apparent transfer coefficients notably above unity, and so gain an apparent super-Nernstian response. Simulations for a ‘titration’ reaction mechanism, A + e− ⇌ B, B + C → Y are also performed and the effects of electrode radius and analyte concentration on the Tafel slope are investigated and discussed.