Elucidating Design Principles for Redox-Mediated Flow Batteries Through Physics-Based Modeling

Redox-mediation, or redox-targeting, is an emerging concept within the flow battery community that offers a pathway to dramatic increases in energy density while retaining independent scaling of power and energy. In these systems, soluble redox-active mediators act as charge shuttles between the electrodes within the electrochemical cell and the “off-electrode” materials in the external tanks.^1,2 Literature reports indicate that by incorporating solid active materials, redox-mediated flow batteries (RMFBs) can extend the charge-storage capacity of the flow battery,^2-3 accommodate previously inaccessible sets of active materials,⁴ and unlock new modes of operation.^5-6 However, the presence of reactive solid active materials in the tanks significantly alters the performance characteristics of the flow battery which may frustrate assessments of technical and economic potential. Accordingly, we have developed a qualitatively-validated mathematical model to aid in predicting the behavior of RMFBs.⁷ The framework employs physics-based constitutive equations to represent the transient mass and charge dynamics in the flow cell and tank, for both the dissolved mediators and solid active species. Notably, we use mixed potential theory as a thermodynamically-consistent method of describing the driving force and rates between the mediator and solid.<br/><br/>In this presentation, we will discuss how this model can offer useful insights into the RMFB design-space. We employ dimensional analysis, relationships for pressure drop in porous media, and principles of cycling stability to explore how material properties, operating protocol, and system architecture connect to trade-offs in capacity utilization, power output, and parasitic pumping losses. By outlining general performance trends and incorporating constrained optimization, we demonstrate how this model framework can serve as a tool for guiding the operation of present-day RMFBs and establishing material property targets for next-generation RMFBs. Ultimately, this work aims to provide actionable design strategies that accelerate the development of redox-mediated systems.<br/><br/><i>Acknowledgements</i><br/>N.J.M gratefully acknowledges the NSF Graduate Research Fellowship Program under Grant Number 2141064. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.<br/><br/><i>References</i><br/>1. Qi & Koenig Jr., J. Vac. Sci. Technol. B, 2017, 35, 040801<br/>2. Yan and Wang, <i>Adv. Mat.</i> 2018, 30 (47).<br/>3. Chen et al., <i>Joule, </i>2019, 3, 2255–2267<br/>4. Jiao et al., <i>ACS Nano</i>, 2022, 16, 14262−14273<br/>5. Fan et al., <i>ACS Energy Letters, </i>2016, 2, 615−621<br/>6. Fenton et al., <i>ACS Omega, </i>2022, 7, 44, 40540–40547<br/>7. Matteucci et al., <i>ECS Meeting Abstracts</i> 2023, MA2023-01 (3), 765.