Evaluation of Extremely Stable Redox Active Materials and Porous Electrodes for Redox Flow Batteries Using Static Cells and Physics-Based Modeling
Authors: Sosa Jordan D. , Aziz Michael J. 
Published: December 2024

Aqueous Organic Redox Flow Batteries (AORFBs) have shown growing potential as a stationary energy storage technology used to alleviate the issue of intermittencyi for renewable energy sources. For systems with charge/discharge duration of 10 or more hours, the capital cost of a flow battery asymptotically approaches the cost of the electroactive material.ii While the field has made many new materialsiii and developed proper protocols for evaluating their decompositioniv, uncertainty in the fade rates of extremely stableiii materials remains comparable to the fade rates themselvesv, frustrating the evaluation of prospects of the material for commercialization. Additionally, evaluation of new materials in flow cells requires a significant amount of material synthesis, whereas evaluation methods requiring less material could accelerate the development of AORFBs.vi

To reduce noise in measurements by removing pumping, splashing, and bubbling of solutions that occurs in flow cells, and to reduce the amount of required active material, we use static cells composed solely of porous electrodes sandwiched about a membrane, gaskets, and graphitic current collectors with no flow. With reduced material consumption, increased cycling rate, and less noise in capacity measurements, we more rapidly and more accurately determine the capacity fade rates for extremely stable materials for AORFBs (Figure 1) and test the limits for cycle-denominated fade rates. In addition, we discuss relevant reporting statistics for making fair comparisons between extremely stable materials.

To evaluate overpotentials in the system due to kinetics, mass transfer, and Ohmic resistance, we model the system using Newman’s porous electrode modelvii to ensure the use of proper cycling protocols to access all of the capacity of the system.

Figure 1: (left) An example of potentiostatic cycling of 0.1 M 2,6-DPPEAQ at pH 14 in a volumetrically unbalanced compositionally symmetric static cell with an applied voltage square wave of amplitude 0.20 V across the cell, held until the current drops to 1 mA/cm2. After cycling for a day while the electrolyte equilibrates, a capacity fade rate is evaluated from the slope of the natural log of discharge capacity vs time. (right) A whisker and box plot for capacity fade measurements from several potentiostatic symmetric cell cycling experiments in a flow cell from Ref [5], static cells with no attention to temperature fluctuations, and static cells where temperature fluctuations are kept within 1 degree Celsius.

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[ii] F. R. Brushett, M. J. Aziz, and K. E. Rodby. ACS Energy Lett. 2020, 5, 879−884

[iii] David G. Kwabi, Yunlong Ji, and Michael J. Aziz, “Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review.” Chemical Reviews 2020 120 (14), 6467-6489

[iv]M.A. Goulet and M. J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”. J. Electrochem. Soc, 165 (7) A1466-A1477 (2018)

[v] Eric M. Fell and Michael J. Aziz, “High-Throughput Electrochemical Characterization of Aqueous Organic Redox Flow Battery Active Material.” 2023 J. Electrochem. Soc.
170 100507

[vi] Cao, Y., Aspuru-Guzik, A. “Accelerating discovery in organic redox flow batteries.” Nat Comput Sci
4, 89–91 (2024)

[vii] J. Newman and C. W. Tobias, J. Electrochem. Soc., 1962, 109, 1183

Figure 1