Methods—Meso-Scale Electrodes for Characterizing Diffusion-Reaction Properties of Redox-Active Organics in Viscous Electrolytes

Abstract

Deep Eutectic Solvents (DESs) have recently gained interest as flow battery electrolytes. Their advantages include a wider electrochemical stability window compared to aqueous electrolytes, higher solubility for redox-active species, and negligible volatility. However, DESs are often highly viscous, and suffer from low ionic conductivities. This can make assessing redox kinetics difficult when attempting to determine their viability for energy storage. In classical voltammetric measurements, low ionic conductivity manifests as high solution resistance, thereby requiring “live” compensation of the electrolyte ohmic drop when performing fast-scan voltammetry. An uncompensated or inadequately-compensated ohmic drop leads to misinterpretation of the voltammetric behavior, e.g., assessing reversibility vs. irreversibility of a redox reaction. Here, we present micro-fabricated electrodes as facile “meso-scale” electrodes, which overcome these issues by nearly eliminating the ohmic drop while retaining uniformity of the current distribution over the electrode surface. Their use in precise transport-kinetics measurements is demonstrated using a redox-active organic, i.e., 4-Hydroxy-TEMPO in an aqueous medium and in ethaline, which is a viscous DES. This study provides a methodical approach to design and to implement voltammetry experiments using meso-scale electrodes leading to reliable measurements of diffusion-reaction properties of 4-Hydroxy-TEMPO.