Expanding the Scanning Electrochemical Microscopy Toolset for Evaluating Electron Transfer Kinetics of Concentrated and Specialized Redox Electrolytes on Carbon Surfaces

Abstract

Performing quantitative measurements of heterogeneous electron transfer reactions at electrodes (e.g., determining k0 and a) under conditions of high redox active molecule concentration in solution, such as those encountered in electrolytes for redox flow batteries (RFBs), presents significant technical challenges. For example, attempting to use transient voltammetry methodology quickly leads to limitations imposed by iR drop, as currents can be large and resistance, especially in non-aqueous systems, can be high. Additionally, using steady-state techniques based on convective principles, such as the rotating disk electrode (RDE), involve the use of relatively large equipment, with moving parts, and usually with requirements of tens to hundreds of mL of solution, which may be limiting for exploring new compounds at high concentration.To address these issues, here we introduce the use of scanning electrochemical microscopy (SECM) for the determining the rates of electron transfer of redox active species at high concentration in non-aqueous [1] and other unconventional systems, such as deep eutectic solvents. SECM enables the resolution of higher rates of electron transfer compared to transient voltammetry and RDE, and it also enables experiments with spatial resolution, enabling the exploration of reactivity on various sites of a heterogeneous electrode. In this presentation, we will describe theoretical and practical aspects of these measurements, and we will demonstrate their utility using two model systems. The first one consists on the observation of inherent rate asymmetries observed for ferrocene derivatives, ubiquitous redox probes, at the graphite/carbonate solvent interface. These asymmetries are not present on aqueous systems or metallic electrodes, demonstrating the ability of the SECM technique to capture unique behaviors. In the second case, we will discuss how SECM measurements can be used on deep eutectic solvents such as ethaline to address how interactions between redox actives and the electrode or the solvent systems modulate the rate of electron transfer. By systematically modifying the strength of interactions by means of molecule choice, solvent composition, and electrode type and structure, we can gain insight into the processes that determine reactivity at redox flow battery electrodes. This insight is critical for exploring the interfacial aspects such as adsorption, passivation, and double-layer effects on kinetics of electron transfer.[1] Gaddam, R.; Sarbapalli, D.; Howard, J.; Curtiss, L.A.; Assary, R.S.; Rodríguez-López, J. An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces Chem: Asian J., 2023, 18, 2, e202201120. Figure 1