Influence of Crossover on Capacity Fade of Symmetric Redox Flow Cells

Organic redox reactants offer tunable structures through chemical synthesis, thereby unlocking a vast space of physical and chemical properties for the design of high-performance flow-based electrochemical systems. Evaluating the stability of these redox reactants under conditions of electrochemical cycling between oxidized and reduced states is of paramount importance for choosing materials with which to build durable electrochemical devices, including long-lifetime redox flow batteries. Organic redox reactants in flow batteries are subject to myriad state-of-charge (SOC) dependent chemical reactions that can result in structural changes and the loss of redox activity under cycling conditions, and these reactions may be superimposed on top of crossover though the membrane, complicating the matter of understanding the cause of capacity fade.<br/><br/>The volumetrically unbalanced compositionally symmetric cell (hereafter referred to as symmetric cell) method was devised to isolate the contribution of chemical decomposition to capacity fade.¹ In principle the symmetric cell minimizes concentration gradients across the membrane, which would otherwise drive crossover. However, under conditions when the time-averaged SOC of capacity limiting side (CLS) and non-capacity limiting side (NCLS) deviate from 50%, net crossover may occur and influence the measured capacity.²<br/><br/>We tested symmetric cells of anthraquinone disulfonic acid (AQDS) with Nafion membranes of varied thickness and manufacture (NR211, NR212, N115, and N117, ranging 25–183 μm). Membranes were tested both as-received and pretreated with a common procedure of soaking in water at elevated temperature and then dilute hydrogen peroxide.³ We found no significant difference in capacity fade rates of symmetric cells with any of the membranes as-received, indicating a negligible influence of crossover. However, we observed increased capacity fade with increased crossover flux through pre-treated membranes. Supported by zero-dimensional modeling and <i>operando</i> UV-vis spectrophotometry, we propose a mechanism for net crossover in AQDS symmetric cells based on a higher time-averaged concentration of quinhydrone dimers in the NCLS than in the CLS, driving net crossover of AQDS reactants out of the CLS. Further, we illustrate other hypothetical scenarios of net crossover using the zero-dimensional model. Overall, many membrane-electrolyte systems used in symmetric cell studies have sufficiently low crossover flux as to avoid the influence of crossover on capacity fade, but under conditions of higher crossover flux, complex interactions of crossover and chemical reactions may result in diverse capacity fade trajectories, the mechanisms of which may be untangled with appropriate measurements and modeling.<br/><br/><br/>[1] Goulet, M.-A.; Aziz, M. J. Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods. <i>Journal of The Electrochemical Society</i> <b>2018</b>, <i>165,</i> A1466–A1477<br/><br/>[2] Nolte, O.; Volodin, I. A.; Stolze, C.; Hager, M. D.; Schubert, U. S. Trust is good, control is better: A review on monitoring and characterization techniques for flow battery electrolytes. <i>Materials Horizons</i> <b>2021</b>, <i>8(7)</i>, 1866-1925<br/><br/>[3] Lin, K.; Chen, Q.; Gerhardt, M. R.; Tong, L.; Kim, S. B.; Eisenach, L.; Valle, A. W.; Hardee, D.; Gordon, R. G.; Aziz, M. J.; Marshak, M. P. Alkaline Quinone Flow Battery. <i>Science</i> <b>2015</b>, <i>349,</i> 1529–1532