Electrochemical and Rheological Modeling of Suspension-Based Electrolytes for Redox Flow Batteries

Electrochemical energy storage is poised to play a key role in enabling society-wide decarbonization by facilitating the deployment of variable electricity generators and enhancing existing grid infrastructure. Redox flow batteries (RFBs) possess several technology features which are favorable for cost-effective, long-duration energy storage, including independent scaling of power and energy, long service life, and simplified manufacturing [1]. In particular, the battery configuration enables a diversity of electrolyte compositions and cell materials to be considered, either at the beginning of life or during the installation lifetime (e.g., chemistry swaps). While conventional RFBs are based on redox couples dissolved in electrolyte solutions, more recently, suspension-based electrolytes containing solid charge storage materials have emerged as an alternative formulation. Flowable suspensions enable higher capacities (and thus cell energy densities) and unlock new operating modes (e.g., dissolution and precipitation on suspended particles) [2,3]. However, such electrolytes also frustrate traditional approaches to cell design and operation as their complex electrochemical and rheological behavior presents multifaceted tradeoffs [4,5]. By exploring the essential features of these tradeoffs, we aim to better understand fluid dynamic and electrochemical engineering criteria for high-performance flow cells that operate with suspension-based electrolytes.<br/><br/>In this presentation, we develop and apply a one-dimensional model to derive scaling relationships for suspension-based electrolytes in RFBs. Specifically, we investigate connections between rheological (i.e., non-Newtonian behavior, shear stress) and electrochemical (i.e., species and charge transport) phenomena for electrolyte flow through a planar channel. We then identify key dimensionless groups which describe the relative magnitudes of relevant processes within the flowing suspension-based electrolyte. Through scaling analyses, we assess the importance of each parameter under different dynamic and geometric constraints to enable the identification of favorable materials sets and operating conditions. Ultimately, these dimensionless quantities offer a compact representation of the design considerations for suspension-based electrolytes, allowing more informed materials selection, cell engineering, and system formats. Finally, we hypothesize that these results can be generalized to describe flowable and stationary solid suspensions of utility in multiple electrochemical systems.<br/><br/><b>Acknowledgments</b><br/>This work was funded by the Skoltech – MIT Next Generation Program. B.J.N gratefully acknowledge the NSF Graduate Research Fellowship Program under Grant Number 1122374. 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/><b>References</b><br/>1. M. L. Perry and A. Z. Weber, <i>J. Electrochem. Soc.</i>, <b>163</b>, A5064–A5067 (2016).<br/>2. H. Parant et al., <i>Carbon</i>, <b>119</b>, 10–20 (2017).<br/>3. X. Wang, J. Chai, and J. “Jimmy” Jiang, <i>Nano Materials Science</i>, <b>3</b>, 17–24 (2021).<br/>4. V. E. Brunini, Y.-M. Chiang, and W. C. Carter, <i>Electrochimica Acta</i>, <b>69</b>, 301–307 (2012).<br/>5. N. C. Hoyt, R. F. Savinell, and J. S. Wainright, <i>Chemical Engineering Science</i>, <b>144</b>, 288–297 (2016).