Electrochemical Analysis in Charge-Transfer Science: Which Is the Elephant in the Room?
Authors: Vullev Valentine I. , O`Mari Omar 
Published: September 2023

The importance of charge transfer (CT) and charge transport (CTr) for supporting life on Earth and for making our modern ways of living possible cannot be overstated [Phys. Chem. Chem. Phys.
2020, 22, 21583-21629]. Electrochemical analysis is essential for characterizing CT systems and processes [Curr. Opin. Electrochem.
2022, 31, 100862; J. Electrochem. Soc.
2019, 166, H3175-H3187]. While interfaces are inherent for electrochemical cell setups, the effects from them introduce key challenges to the interpretation of measured results. Liquid junctions, formed between different electrolyte solutions, are crucial for separating compartments and ensuring the stability of reference electrodes. The differences in the transport of anions and cations across such liquid-liquid interfaces, give the rise of liquid-junction potentials (E
LJ) that add to the measured voltages. Such liquid-junction potentials between commonly used electrolyte solutions can readily exceed 100 mV. Conversely, the analysis of CT thermodynamics employs the difference between the reduction potentials of electron acceptors and oxidized electron donors. Therefore, using potentials for the donors and the acceptors, measured with the same setup under the same conditions, i.e., with the same E
LJ values added to all of them, can eliminate these liquid-junction potentials [Proc. Natl. Acad. Sci. USA
2021, 118, e2026462118]. As challenging as it can be to characterize E
LJ in everyday electrochemical analysis, it is not the biggest elephant on the room. The electrochemical potentials of donors, acceptors and any redox species, strongly depend on the polarity of the media [J. Phys. Chem. A
2009, 113, 1259-1267]. This dependence becomes especially pronounced as the size of the redox species decreases [J. Phys. Chem. B
2010, 114, 14467-14479]. This dependence on medium polarity also makes the electrochemical potentials susceptible to localized electric fields that molecular dipoles generate, affecting the electrochemical potentials of donors and acceptors and, thus, the Franck-Condon contribution to the CT kinetics [Angew. Chem. Intnl. Ed.
2018, 57, 12365-12369]. Molecular dipoles, therefore, can rectify CT [J. Am. Chem. Soc.
2014, 136, 12966-12973]. As important as the dependence of reduction potentials on medium polarity is, the interfacial nature of electrochemical measurements presents challenges for characterizing this dependence. While the polarity that solvated species experience in the bulk of a electrolyte solution is readily attainable, it is still challenging to determine the polarity of the microenvironment at the electrode surfaces where the redox processes occur. An increase in electrolyte concertation in organic solvents increases their polarity, which is opposite for electrolyte solutions. At the electrode surfaces, however, the increase rigidity of the double layer, especially under applied voltage, decreases the oreitnational polarization, which principally contributed to the dielectric constant of solvents with large dipoles. That is, the polarity that redox species experience at the surface of the working electrode under applied bias is smaller than that in the bulk of the solution. Global-fit analysis on the dependence of measured reduction potentials on the polarity of the solvent and the electrolyte concentrations offers a means not only for extracting liquid-junction potentials, but also for estimating the effective polarity that redox species experience at the surface of the working electrode. Such electrochemical analysis findings improves not only the characterization of CT thermodynamics, but also the quantification of the dipole effects on CT. Understanding the multifaceted effects of dipoles on charge transfer and charge transport has key implications for electronics, photonics and energy science and engineering.