Electrochemical Analysis in Charge-Transfer Science: Polarity at Electrode Surfaces Affect Electrochemical Analyses for Charge-Transfer Systems
Authors: O`Mari Jaime O. 
Published: February 2024

Electrochemical analysis is a vital component of the science of charge transfer (CT), and its importance cannot be overstated. Many fields, including energy science, photoredox and electrocatalysis, biomedical sensor development, environmental engineering, and electronics, rely heavily on CT and require accurate electrochemistry. [Phys. Chem. Chem. Phys.
2020, 22, 21583-21629] As a result, it is critical to pay close attention to the details of collecting, implementing, and interpreting electrochemical results to advance this broad field of science and engineering. [Current Opinion in Electrochemistry
2022, 31, 100862]

The interfaces of an electrochemical cell present specific challenges for the interpretation and applicability of the analysis. The polarity of the medium on the electrode surface is not the same as the polarity of the bulk medium, and this has a significant influence on the behavior of charged and dipole species. Solvation in a condensed medium controls the dynamics of processes involving CT, including intramolecular redistribution of electron density. Polarity affects several properties of solvated species, including solubility, reactivity, electrochemical potential, and spectral transitions. [J. Phys. Chem. B
2023, 127, 6, 1443–1458]

To understand changes in electrochemical potential during oxidation and reduction, it is important to analyze the relationship between the bulk electrolyte concentration and the Born microenvironmental polarity experienced by the species analyzed in the diffuse Helmholtz layer and at the surface of the working electrode during the heterogeneous steps of CT. This analysis can be used to assess the polarity experienced by redox species on the surface of polarizing working electrodes.

Another critical issue in electrochemical analysis is the liquid junction that connects two different solutions, typically through a porous medium. Differences in the transport properties and activity of ions on both sides of the junction create a potential, known as the liquid potential (ELJ), which contributes to the measured voltage. To minimize ELJs, a miscible solvent solution and an electrolyte composed of ions with the same mobility throughout the transition can be used. ELJs can also be eliminated using a cell configuration with a pseudo-reference electrode that does not contain a liquid interface. However, the potential of pseudo-reference electrodes depends on the composition of the sample solution in which they are immersed, so care must be taken when interpreting the measured values.

Overall, electrochemical analysis is a complex and critical part of CT science, and its accurate implementation is essential in advancing this broad field of science and engineering. Understanding the influence of the polarity of the medium, the relationship between the bulk electrolyte concentration and the Born microenvironmental polarity, and the liquid junction is crucial for interpreting electrochemical results and evaluating CT thermodynamics.