Utilizing Microelectrodes to Understand Indirect Redox Targeting Reactions for High Energy Density Flow Batteries

Abstract

One of the major problems with widespread implementation of renewable energy is the lack of cost-effective long-duration grid-scale energy storage filling in for the intermittent nature of solar and wind energy. Redox flow batteries (RFBs) are considered a promising solution to this problem due to their decoupled energy and power capabilities, allowing them to be scaled up for long-duration more efficiently and cost-effectively than other electrochemical energy storage devices [1]. The energy capacity of RFBs is primarily determined by the quantity of active species in the electrolyte, which is a product of the volume and the concentration. Thus, the limiting factor for energy density in RFBs is typically the solubility of the active species [2]. One probable path to enhancing the energy density without increasing the concentration is by utilizing the concept of indirect redox targeting reactions [3]. These redox-targeting flow batteries (RTFB) use solid charge storage materials in the electrolyte tanks to boost capacity, allowing the active species to act as mediators which transfer their charge by reversibly reducing or oxidizing the solid material [4].In this study, cobalt hexacyanoferrate (CoHCF) was used as a solid capacity booster in an electrolyte containing vanadium(iv/v)bis-hydroxyiminodiacetate (VBH) as mediators in acetonitrile. Constant current cycling was performed in a symmetric cell configuration with vanadium ions shuttling between vanadium(iv) and vanadium(v) oxidation states. To monitor the concentration of redox active species accurately and in real-time, a carbon fiber ultramicroelectrode (UME) was installed on the capacity limiting half-cell. This in-line UME set-up provides a better understanding of how CoHCF interacts with mediators and insights on active species crossover between half-cells. In addition, this study aims to examine the kinetics of the indirect redox-targeting reactions. As a rarely studied charge transfer process, very few studies have been reported on the kinetics of indirect redox targeting reactions [5]. Scanning electrochemical microscopy is used to understand how counter cations affect the kinetics of the reaction between the solid booster material and the mediator. Various counter cations (e.g., Ca2+, Na+, Li+) are used with the previously mentioned VBH-based mediators in order to determine the effective rate constants (keff) as well as the reversibility of the charge transfer reactions.