Catalysis of Electrochemical Reactions by Surface-Active Sites: Analyzing the Occurrence and Significance of Volcano Plots by Cyclic Voltammetry

Abstract

Cyclic voltammetry (CV) of heterogeneous electrocatalysts offers a convenient means to critically assess the occurrence of “volcano plots” rendered popular by acid reduction on metal electrodes. The equations relevant to Volmer–Heyrovsky-type reactions shows that the adsorption free energy of the surface-bound intermediate is one of the rate-controlling parameters, which, plotted against the exchange current, could lead to a volcano-looking curve if other rate-controlling factors such as the rate ratio of the two successive electron transfer steps would remain constant upon changing electrocatalyst. This is not necessarily the case in practice, thus blurring the occurrence of volcano plots. Therefore, careful recording and analysis of the CV responses should be a preferred strategy, leading additionally to catalytic Tafel plots for rational electrocatalyst benchmarking. The alternative Volmer–Tafel mechanism gives remarkably rise to S-shaped current–potential responses and to a volcano upon plotting the exchange current against the adsorption standard free energy of the primary intermediate. Again, a wealth of kinetic information results from the characteristics of the current–potential responses.