The development of next-generation redox flow batteries (RFBs) heavily relies on the advancement of new component materials (e.g., redox electrolytes, membranes/separators) that enable low-cost, high-performance, and long-duration energy storage systems.¹ The efficient evaluation of candidate materials thus requires simple and robust methods for investigating the evolution of system components during operation—in particular, near real-time monitoring of the battery state-of-health would enable timely interventions that extend lifetime or prevent safety incidents. Prior work has primarily comprised the measurement of electrolyte state-of-charge,^2–4 and while this can qualitatively describe the state of the redox electrolyte, knowledge of the individual active species concentrations is needed to quantitatively assess the causes of performance loss. Accordingly, we have established the use of microelectrodes for studying the ex situ decomposition of RFB active materials, showing that this technique can simultaneously describe contributions from self-discharge reactions and molecular decomposition.⁵ However, considering the flow disturbances (e.g., pump oscillations, irregular flow, non-uniform mixing, etc.) associated with operating RFBs, it is especially challenging to integrate microelectrodes directly into flow cells, as the steady-state current can be highly variable due to undesired advection. Here, we seek to expand the versatility of microelectrodes by leveraging and directing this flow to study redox electrolytes within an operating cell. In this presentation, we describe the integration of microelectrodes into an in-line, flow-through electrochemical cell for use as an operando diagnostic tool to measure redox species concentrations in RFBs (see Figure). The cell consists of a three-electrode assembly, with a pseudo-reference electrode, to accurately quantify species concentrations via the steady-state current obtained from the microelectrode. First, we validate the proposed working principle using multi-physics simulations, and assess the measurement protocol ex situ under quiescent and flow conditions. Using a model electroactive compound (N-(2-(2-methoxyethoxy)ethyl)phenothiazine, MEEPT) prepared at varying states of charge, voltammetry reveals that the steady-state current can be reliably measured with a platinum wire pseudo-reference electrode, which can be used in place of standard fritted reference electrodes to reduce contamination and drift. Subsequent e x situ current measurements under steady flow yield a stable current response that increases with flow rate, in general agreement with simulation results. Importantly, as long as the flow conditions remain consistent throughout testing, the current remains linear with concentration, allowing the concentration to be determined from the steady-state current using an empirical mass transfer coefficient. As a proof-of-concept, we demonstrate the use of this sensor in a symmetric, non-aqueous redox flow cell using MEEPT. Finally, and perhaps most notably, this diagnostic tool is comprised of commercial off-the-shelf components, making it a readily accessible to any laboratory studying RFBs. Acknowledgments This work was funded by the National Science Foundation (NSF) under Award Number 1805566. B.J.N. and K.M.T. gratefully acknowledge the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. References B. R. Chalamala, T. Soundappan, G. R. Fisher, M. R. Anstey, V. V. Viswanathan, and M. L. Perry, Proceedings of the IEEE, 102, 976–999 (2014). C. Stolze, J. P. Meurer, M. D. Hager, and U. S. Schubert, Chem. Mater., 31, 5363–5369 (2019). M. Skyllas-Kazacos and M. Kazacos, Journal of Power Sources, 196, 8822–8827 (2011). C. Stolze, M. D. Hager, and U. S. Schubert, Journal of Power Sources, 423, 60–67 (2019). J. A. Kowalski, A. M. Fenton, B. J. Neyhouse, and F. R. Brushett, J. Electrochem. Soc., 167, 160513 (2020). Figure 1