Probing redoxable organic molecules in the transient near-electrode accumulated regime unveils insidious degradation

Abstract

Organic redox flow batteries (RFBs) are promising for large-scale energy storage due to their eco-friendliness. Recent advances in redox-active organic molecules have improved their resistance to chemical degradation, while (electro)analytical methods for assessing the kinetics of the ‘slowed’ degradation processes are not yet well established. We demonstrated that the substrate generation and tip collection (SG/TC) mode in scanning electrochemical microscopy (SECM), with a long time scale, is powerful for evaluating the ‘slow’ kinetics of chemical degradation. In SG/TC mode, electrogenerated species on a substrate electrode are transiently accumulated in a near-electrode regime by their small flux, which enhances the chemical reaction rates and affects the concentration of redox species detected by a tip electrode. This results in a decrease in tip current, reflecting the degradation of redox species. The detectable rate constants using the SG/TC mode are approximately four orders of magnitude lower than those using the tip generation and substrate collection (TG/SC) mode of SECM. Additionally, the natural convection effect induced by the redox reaction on a large substrate electrode could have less impact on monitoring chemical degradation with a tip electrode because of the enhanced diffusional mass transport by positive feedback, providing an advantage over conventional cyclic voltammetry in a three-electrode cell for a long measurement time scale. The presented analytical method was validated by observing current decay due to slow hydrolysis in a concentrated TEMPO electrolyte. The determined rate constant aligns with numerical calculations corresponding to the coulombic efficiency obtained from charge–discharge testing of RFBs. These findings highlight the potential of SG/TC SECM for rapid operando assessment of redox electrolyte health.