Exploring the Impact of Temperature on Non-Aqueous Redox Flow Cells

Abstract

Nonaqueous redox flow batteries (NAqRFBs) remain an intriguing storage concept, but further improvements in performance and durability are needed to determine viability. An understudied but potentially-enabling feature of NAqRFBs is the ability to operate over a wide range of temperatures, facilitated by the breadth of solvent, supporting salt, and active species whose physicochemical properties differ from aqueous electrolytes. Elevated temperatures can increase reaction kinetics and transport rates, which are anticipated to improve performance, but may also promote decomposition processes, which are expected to lower durability. Elevated temperatures also increase system complexity, requiring energy input through additional balance-of-plant equipment, consequently necessitating energy balance and heat infrastructure considerations. While there are promising initial reports on molecular stability and electrolyte properties at elevated temperatures,1,2 there remains a need to better understand how temperature impacts performance-defining processes within NAqRFBs.In this presentation, we will discuss how molecular stability, electrolyte properties, and cell performance change as a function of temperature using 10-[2-(2-methoxyethoxy)ethyl]-10H-phenothiazine (MEEPT) as a model redox material.3 With increasing temperature, we observe that, as expected, the kinetic and species transport properties of the electrolyte improve (i.e., electrochemical reversibility, ionic conductivity, viscosity, active species diffusion). We connect these ex-situ findings to cell performance using a symmetric flow cell to quantify resistive losses and cycling stability, observing a marked decrease in cell resistance albeit at the expense of active species stability above a certain temperature threshold. Finally, we assess variable temperature operation of a NAqRFB full cell with MEEPT and (bis(2-(2-methoxyethoxy)ethyl)viologen bis(bis(trifluoromethanesulfonyl)imide) (MEEV-TFSI2) as the positive and negative electrolyte materials, respectively, observing enhancements in power performance but constraints on operating lifetime. These findings highlight that while temperature increases may be used as a means of increasing cell performance, care must be taken not to compromise component durability. While the exact set of conditions that balance this tradeoff are likely to be chemistry-dependent, the methods that will be described in the presentation are suitable for assessing these conditions in a general fashion. Acknowledgements This work was partially supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy. A.H.Q., N.J.M., and B.J.N. gratefully acknowledge the National Science Foundation Graduate Research Fellowship Program under Grant Number 1745302. 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. A.H.Q acknowledges the Alfred P. Sloan Foundation’s Minority Ph.D. (MPHD) Program. The authors thank Dr. Aman Kaur of the Odom Research Group in the Department of Chemistry at the University of Kentucky for insightful discussions and for synthesis of MEEPT-BF4, MEEPT-TFSI, and MEEV-TFSI2 species used in this study. References (1) Odom, S. A.; et al. Meet. Abstr. 2021, MA2021-02 (1), 110.(2) Shkrob, I. A.; et al. Journal of Molecular Liquids 2021, 334, 116533.(3) Kaur, A. P.; et al. Chem. Mater. 2020, 32 (7), 3007–3017. Figure 1