Advanced Microelectrochemical Methods for Capturing Dynamics and Reactive Trends in Redox Flow Battery Electrolyte Systems

Redox flow batteries (RFBs) are emerging devices for energy storage in which charge capacity and power are decoupled. This allows for versatility in the exploration of electrolyte design principles that maximize desirable properties – or combinations of them – such as the concentration and redox potential of the redox active, the conductivity of the electrolyte, or electrode/electrolyte interactions. In this context, we are interested in examining the electrochemical properties of redox active molecules on emerging types of electrolytes which might display exciting new attributes. To understand these properties at a mesoscopic level of interaction between the electrolyte and the electrode, and at concentrations that are relevant for redox-flow battery or on individual particles/ electrode sites relevant to flow batteries, we used techniques based on small electrodes such as scanning electrochemical microscopy (SECM),[1] scanning electrochemical cell microscopy (SECCM), and interdigitated electrode arrays. These techniques allow a broad range of experiments at microscales, including generation-collection and kinetic measurements with minimal convection effects. In this presentation, we will discuss how these capabilities can be used for its application in the evaluation of two emerging electrolyte systems: bicontinuous microemulsions (uEs)[2], and redox-matched flow batteries[3].<br/>As examples, we will describe how confining microemulsion electrolytes between a tip and a substrate leads to stochastic responses that we believe exhibit the impact of charge transfer (i.e., electron and ion transfer processes) across phases. This transport may lead to mechanical deformation and convective effects which are dependent on the nature of the SECM mode used, and other electrolyte properties. The nature and concentration of the electrolyte matter when describing the behavior of the stochastic responses, which are also correlated to macroscopic characteristics (e.g., Coulombic efficiency) of the samples. Another example where electrolyte concentration matters significantly is in the use of polymeric materials for redox-matched flow batteries, in which immobilized redox components are accessed by redox mediators. In these cases, determinining the impacts of ionic strenght and accessibility of the mediator is crucial. To this point, techniques cuh as SECCM help isolate individual components such as particles while microelectrochemical arrays enable the versatile inspection of various reaction conditions using automation [4]. In summary, microelectrode techniques such as SECM, SECCM, and interdigitated arrays hold promise for the elucidation of charge transfer dynamics that are typically lost in bulk electrochemical measurements.<br/><br/>[1] Watkins, T.; Sarbapalli, D.; Counihan, M., et al. J. Mater. Chem. A 2020, 8, 15734.<br/>[2] Imel, B., et al. ACS Appl. Mater. Interfaces 2022, 17, 17, 20179.<br/>[3] Asserghine, A. and Kim, S. et al. ACS Energy Lett. 2024, 9, 2826.<br/>[4] Pence, M.A. et al. ACS Meas. Sci. 2023, 3, 62.