Understanding The Principles of Charge Transfer to Realize High Capacity Utilization in Redox Targeting Flow Batteries

Redox targeting systems represent a promising path for flow batteries, circumventing the solubility constraints encountered by various organic redox compounds. Yet, the pivotal challenge remains the attainment of high capacity utilization of the target within flow cell systems and a deeper comprehension of the mediator-target interplay. In addressing this gap, we present the development of a high solids accessibility redox targeting flow battery, alongside the formulation of an advanced yet clear thermodynamic model to unravel the intricacies of the mediator-target relationship. Our approach involves a ferrocene bearing insoluble polymer that is synthesized for use as a redox target and matched with different mediator systems: one with a single molecule that has a nearly identical potential and the other with a dual molecule system that has mediators with a voltage offset to encourage charge transfer. These two systems were then tested in a catholyte limited flow cell to determine the capacity accessibility of the mediator-target system. The single molecule system was able to increase the cell’s capacity by 90% of the redox target’s capacity and the dual molecule system was able to increase the capacity by >95% of the redox target’s capacity. To better understand the thermodynamics of each system they are modeled using a Nernst analysis. For the single molecule system, this analysis builds on prior work to suggest that not only is a small voltage offset extremely important, but that the mediator’s accessible state of charge range will determine the redox target's thermodynamic accessibility limits. For the dual molecule system, the model suggests that a medium voltage offset is ideal as it allows for high amount of charge transfer in the preferred direction with enough potential overlap to assist charge transfer in the reverse direction. In summary, our work not only demonstrates the achievable high accessible capacity of redox targeting systems within flow cells but also provides a comprehensive thermodynamic model to elucidate their operational mechanisms and crucial design considerations.