A Mathematical Framework for Modeling Energy Dense Redox-Mediated Flow Batteries

Abstract

Electrochemical processes can accelerate global decarbonization efforts by enabling the broad adoption of renewable electricity, facilitating the deployment of electric vehicles, and affording new opportunities in sustainable chemical manufacturing. However, this transition requires new electrochemical technologies with performance, cost, and scalability metrics that align with emerging demands. This need has inspired exploration into innovative systems that leverage unconventional materials and/or reactor designs. One such approach is the use of redox-mediated electrode reactions, which may expand the portfolio of feasible reactants, device configurations, and operating modes. In brief, a soluble and stable redox-couple (the mediator) is paired with an off-electrode material of interest (typically in the solid phase); this combination results in a two-step reaction system, where the mediator reacts on the electrode surface within an electrochemical cell before driving a separate redox reaction with the spatially removed material.1-2 Prior literature has shown how this approach can enhance the charge-storage capacity of redox flow batteries, facilitate difficult-to-perform electrochemical transformations, and unlock spatial and temporal flexibility in electrochemical processes.3-6 While promising, the performance of these systems is governed by coupled reactive-transport processes which present complex (and transient) interdependencies that frustrate navigation through the multi-dimensional design space. As such, technology development is slowed by the material- and time-intensive nature of experimental campaigns. To this end, modeling and simulation can offer an inexpensive and accelerated path to disaggregate interrelated physical processes, to quantify performance descriptors, and to inform experimental studies.7-8 In this presentation, we will describe a mathematical modeling framework that uses traditional chemical engineering principles to represent a redox-mediated electrochemical system consisting of a flow cell, a tank containing solid reactants, and a recirculating stream containing a soluble mediator. This domain aligns with common redox-mediated flow battery configurations, which enables both validation of the simulated electrochemical and fluid dynamic performance as well as insight into experimental observations reported in the literature. We will highlight the utility of low-dimensional models derived from this framework to provide deeper quantitative understanding into these phenomena. Through dimensional analyses, relevant dimensionless groups can be established that allow for generalization of model findings (e.g., scaling relationships, property tradeoffs) and articulation of design criteria for conceptual systems relevant to electrochemical energy storage and conversion. Acknowledgements This work was funded by the Skoltech – MIT Next Generation Program. N.J.M and B.J.N gratefully acknowledge 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. CTM gratefully acknowledges support under and awarded by the Department of Defense, Office of Naval Research, through the National Defense Science and Engineering Graduate Fellowship.