# Fine-tuning the indirect electrochemical reaction in redox-mediated flow batteries

## TL;DR

This paper studies the indirect electrochemical reaction in redox-mediated flow batteries, showing how it can be controlled and optimized for better battery performance.

## Contribution

The study introduces a direct method to analyze and tune the indirect electrochemical reaction using spectroscopy and ion concentration.

## Key findings

- The state-of-charge of the solid active material can be measured directly using spectroscopic methods.
- Supporting ion type and concentration significantly affect the redox reaction between the redox mediator and solid active material.
- Periodic-DFT analysis of the solid active material provides a basis for a thermodynamic framework to optimize the reaction.

## Abstract

Redox-mediated flow batteries (RMFBs) are a promising, emerging energy storage technology and have the potential to drastically increase the capacity of conventional redox flow batteries (RFBs) while maintaining their architectural flexibility. In these systems, a solution-phase active material is pumped between the RFB cell stack and storage tanks and is responsible for direct charge/discharge of the battery system. This material acts as a redox mediator (RM) between the electrochemical apparatus and a solid active material (SAM), which remains in the storage tanks and comprises the capacity of the system. Characteristics of the indirect electrochemical reaction between RM and SAM, which occur in the storage tank, external to the RFB stack, have so far been inferred from conventional RFB performance metrics. Herein, we report a study of this heterogeneous process that is based on spectroscopic measurements, carried out on the active materials, rather than interpretation of distal electrode processes. This provides independent information on the SAM's state-of-charge, a critical property of RMFB performance that is typically not measured directly. Further, we demonstrate that the redox reaction between the RM and the SAM, which is required for efficient operation, may be tuned by hundreds of mV, or even completely inhibited, by altering the type and concentration of supporting ions in the electrolyte. Finally, we report a periodic-DFT investigation of the vibrational spectroscopy of the SAM, which lays the groundwork for a thermodynamic framework that will be used to characterize and optimize the indirect electrochemical reaction.

An indirect electrochemical reaction is made viable through the oxidation of (RM) VBH2−/1− and reduction of (SAM) CoHCF (CoIIFeIII) accompanied by selective potassium-ion intercalation into the CoHCF lattice.