Redox flow batteries (RFBs) are a burgeoning technology for use in grid-scale energy storage; however, broad adoption is stymied by large capital costs.^1,2 Housed within the reactor, porous electrodes are critical components to system performance and cost, as they provide surfaces for redox reactions, void space for electrolyte passage, and conductive solids for electron transport.³ Carbon papers, cloths, and felts are commonly used in RFBs, despite their tailored design and function towards the governing physics as fuel cell gas diffusion layers. Consequently, when coupled with geometrically varying flow field structures, these carbon substrates underperform in RFBs with forced liquid advection, resulting in uneven flow and current distributions. These inhomogeneities lead to inefficient operation, motivating the investigation of electrode morphologies and flow conditions that enable more uniform reactions throughout the reactor. One promising approach to assessing electrode performance is the measurement of residence time distribution, a traditional chemical reactor efficacy metric for quantifying flow pathways and hydraulic short-circuiting.⁴ Currently, most residence time estimations in flow cells assess the fluid dynamic spreading of a known solution leaving a reactor, using both experiments and computational fluid dynamics.⁵ However, in the absence of complementary electrochemical performance measurements, hydrodynamic calculations can only serve as a proxy for possible surface accessibility. Additionally, sole measurements of the reactor effluent are susceptible to added variance, which can obscure experimental results⁶ as opposed to spatial quantities within the reactor. Herein, we present an electrochemical method to measure spatially averaged electrochemical residence times in a small-scale flow cell. Using two disparate flow fields each with carbon cloth and paper electrodes, we compare electrochemical residence time responses to conventional steady-state polarization measurements to capture performance scaling relationships. Further, we present complementary modeling approaches, depicting multi-scale residence time calculations to contextualize the experimental values. Together, these approaches have the potential to elucidate the effects of electrode microstructure and flow field selection on cell-level fluid dynamics and electrochemical performance, which, in turn, may be used to advance RFB reactor design. Acknowledgments This work was funded by the Joint Center for Energy Storage Research, an Energy Innovation Hub of the U.S. Department of Energy, Office of Science, Basic Energy (De-AC02-06CH11357). K.M.T. recognizes additional support from the U.S. NSF Graduate Research Fellowship (1122374). References A. Z. Weber et al., J. Appl. Electrochem., 41, 1137 (2011). C. Minke, U. Kunz, and T. Turek, J. Power Sources, 361, 105–114 (2017). K. J. Kim et al., J. Mater. Chem. A, 3, 16913–16933 (2015). A. E. Rodrigues, Chem. Eng. Sci., 230, 116188 (2021). G. Aparicio-Mauricio, F. A. Rodríguez, J. J. H. Pijpers, M. R. Cruz-Díaz, and E. P. Rivero, J. Energy Storage, 29, 101337 (2020). A. Bérard, B. Blais, and G. S. Patience, Can. J. Chem. Eng., 98, 848–867 (2020).