Exploring High Power Nonaqueous Flow Cell Design
Authors: Milshtein Jarrod David , Barton John Leonard , Darling Robert M. , Brushett Fikile R. 
Published: February 2020

Nonaqueous redox flow batteries (NAqRFB) utilizing redox active organic molecules offer an attractive alternative to metal-based aqueous redox flow batteries for achieving cost-effective, grid-scale energy storage. In particular, nonaqueous electrolytes provide large electrochemical windows, enabling 3 – 4 V cells, and organic active materials span a broad materials design space.1 While several studies report on the discovery of new redox active organic molecules2,3 and even demonstrate organic-based NAqRFB prototypes4, few efforts discuss advances in NAqRFB reactor design. A lack of materials specific knowledge of new nonaqueous electrolytes makes translating aqueous flow cell design principles to nonaqueous systems difficult, especially considering the increased viscosities and decreased conductivities associated with nonaqueous systems. Additionally, prototyping laboratory-scale flow cells employing newly discovered active materials leads to difficulty in determining whether cell failure arises from active material degradation or inadequate flow cell design. Thus, advancing NAqRFB cell design requires development of chemistry-agnostic design guidelines and systematic investigation of device configurations using well-characterized electrolytes with near practical active species concentrations.
This work combines experimental and modeling efforts to quantify performance-limiting factors in nonaqueous flow cell design. First, numerical modeling of electrochemical processes in high performance interdigitated flow fields predicts cell polarization as a function of various characteristic dimensionless groups relating to electrolyte properties, electrode morphology, and cell geometry. To capture the wide array of possible materials and cell design configurations, we perform a parametric sweep of the numerical model across the feasible design space, computing dimensionless polarization curves for the characteristic dimensionless groups. From the dimensionless polarization curves, cell designs that minimize area specific resistance (ASR) for a particular set of input materials properties can be identified (Figure 1), guiding flow cell design.
Second, we prove 4-acetylamino-2,2,6,6-tetramethyl-1-oxo-piperidine-1-oxyl (AcNH-TEMPO) as a redox pair with the required traits to serve as a platform chemistry for systematic cell-level performance analysis. Both AcNH-TEMPO and its oxidized cation salt are commercially available, stable over many cycles, and soluble in the electrolyte of interest (≥ 0.5 M in 1 M lithium tetrafluoroborate (LiBF4) / propylene carbonate (PC)); additionally the AcNH-TEMPO couple exhibits fast redox kinetics. Using this model redox pair, we investigate critical electrolyte properties including viscosity, conductivity, and UV-vis absorbance spectra as a function of state-of-charge (SOC). Across all SOCs of the model electrolyte (0.5 M AcNH-TEMPO / 1 M LiBF4 / PC), conductivity varies by only 13 %, but viscosity swings drastically by 45 %. Well-defined absorbance peaks at visible wavelengths suggest that electrolyte SOC could even be monitored in-situ with UV-vis. Furthermore, cyclic voltammetry helps to determine the redox potential and reversibility of both the reduced and oxidized forms of AcNH-TEMPO. The redox potential of AcNH-TEMPO conveniently lies well within the electrochemical window of many common nonaqueous electrolytes, allowing for controlled flow cell studies with no electrolyte decomposition.
Third, after validating the model compound, a single-electrolyte flow cell study5 exploits the AcNH-TEMPO couple to explore experimental pathways towards achieving an aggressive area specific resistance (ASR) target of ≤ 5 Ω cm2 and validating the numerically computed design rules. A state-of-the-art vanadium flow cell5 (Figure 2a) serves as the initial architecture with various nonaqueous compatible separators and membranes. For each flow cell configuration, impedance (Figure 2a) and polarization (Figure 2b) measurements are recorded at several flow conditions, quantifying the total ASR, as well as the ohmic, kinetic, and mass transfer impedance contributions. Specifically, we demonstrate the effects of separator conductivity, electrode thickness, and flow rate on ASR. We also investigate, for the first time, the performance enhancements afforded by operating NAqRFBs at elevated temperatures. This work culminates in a demonstration of a NAqRFB reactor, which achieves our ASR target, showing promise for economically viable NAqRFBs with high performance reactors.

Acknowledgments

We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research and the National Science Foundation Graduate Research Fellowship Program.

References

1. R. M. Darling, K. G. Gallagher, J. A. Kowalski, S. Ha, and F. R. Brushett, Energy Environ. Sci., 7, 3459–3477 (2014).
2. J. Huang et al., Adv. Energy Mater., 5, 1401782 (2015).
3. C. S. Sevov et al., J. Am. Chem. Soc., 137, 14465–14472 (2015).
4. W. Duan et al., J Mater Chem A, 4, 5448–5456 (2016).
5. R. M. Darling and M. L. Perry, J. Electrochem. Soc., 161, A1381–A1387 (2014).

Figure 1