Alternating Current Voltammetry: Predicting and Visualizing Harmonics

Alternating current voltammetry (ACV) is gaining popularity for its ability to improve the yield of electrochemical syntheses, and for its ability to improve the sensitivity of electroanalytical measurements. Chief among the analytical advantages of ACV is its ability to generate an alternating current at integer multiples (harmonics) of the applied frequency, effectively gathering several datasets at once. However, interpretation of ACV data is hindered by the lack of a unified theory to predict higher harmonics for arbitrary reaction schemes. To meet the need for quantitative interpretation of ACV data, the present paper outlines a method for predicting an arbitrary number of harmonics for systems with up to two charge transfer events and any number of first-order chemical reaction equilibria. Results are presented up to the third harmonic for a variety of single-step and two-step charge transfer schemes, and results for the fundamental harmonic are presented for the second-order cases of dimerization and disproportionation. Visualizing the alternating current in the complex plane, accounting for both the magnitude and the phase angle of the current, frequently reveals information about an electrochemical scheme that is difficult to discern from plots of the alternating current magnitude alone. In general, irreversible charge transfer causes smaller phase angles, and coupled chemical reactions cause the aspect ratios of complex-plane ACV plots to become more circular. Each scheme has its own fingerprint in ACV data, and these fingerprints give valuable information about the dominant chemical reaction at a given moment during charge transfer.