Reliable Information about Electrochemical Potentials from Cyclic Voltammograms That Look “Messy”
Authors: Vullev Valentine I. , O`Mari Jaime O. 
Published: February 2024

The importance of voltammetry for characterizing electronic properties of molecular species and (nano)materials cannot be overstated. Among the most broadly used electroanalytical techniques, cyclic voltammetry (CV) offers an incomparably facile means for estimating standard electrochemical potentials, E
(0), which are essential for analysis of charge transfer, energy conversion and photocatalysis [Phys. Chem. Chem. Phys.
2020, 22, 21583-21629]. Redox irreversibility, liquid-junction potentials, and uncertainties about the microenvironment on the surfaces of polarized working electrodes present key challenges for the validity of CV analysis [Curr. Opin. Electrochem.
2022, 31, 100862]. As an average of anodic and cathodic peak potentials extracted from cyclic voltammograms, half-wave potentials, E
(1/2), serve as a good approximation for E
(0). This approximation is acceptable for results from voltammograms exhibiting even partial chemical reversibility [J. Electrochem. Soc.
2019, 166, H3175-H3187]. Conversely, cyclic voltammograms lacking an anodic or cathodic peak die to irreversible reduction or oxidation, respectively, render estimating E
(1/2) impossible. In fact, irreversibility of electrochemical oxidation and reduction is more or less the rule rather than the exception. Statistical analysis of a series of voltammograms allows us to demonstrate that the potentials at the inflection points at the rise of the cathodic and anodic waves represent a good approximation of E
(1/2) for reduction and oxidation, respectively [J. Electrochem. Soc.
2019, 166, H3175-H3187]. The use of inflection potentials significantly broadens the applicability of CV for extracting meaningful results from voltammograms showing irreversible behavior. It allows us to investigate in parallel a series of electron-rich and electron-deficient polycyclic aromatic species not only with similar structures, but also with similar spin-density distributions of their radical ions, regardless their stability and reversible redox behavior in different solution media. Reformulating the Rehm-Weller equation for CT thermodynamic analysis, offers a means for eliminating the effects of liquid-junction potentials while studying electron donor-acceptor pairs [Proc. Natl. Acad. Sci. USA
2021, 118, e2026462118]. Employing this approach to donors and acceptors with similar structures reveals patterns correlated with the microenvironment at the surfaces of polarized working electrodes. Global-fit analysis of the dependence of CV-extracted reduction potentials on the concentration of the supporting electrolyte for different solvents yields information about the liquid-junction potentials in the electrochemical cell and about the polarity of the microenvironment of the redox species at the electrode surface [J. Phys. Chem. B
2023, 127, 1443-1458]. These outcomes offer a key foundation not only for improved voltammetric estimates of standard electrochemical potentials of redox species, but also for the polarity dependence of these potentials, permitting their broad applicability to systems in widely different media.