Abstract
Nonaqueous redox flow batteries (NAqRFBs) are a promising, but nascent, concept for cost-effective grid-scale energy storage. While most studies report new active molecules and proof-of-concept prototypes, few discuss cell design. The direct translation of aqueous RFB design principles to nonaqueous systems is hampered by a lack of materials-specific knowledge, especially concerning the increased viscosities and decreased conductivities associated with nonaqueous electrolytes. To guide NAqRFB reactor design, recent techno-economic analyses have established an area specific resistance (ASR) target of <5 Ω cm2. Here, we employ a state-of-the-art vanadium flow cell architecture, modified for compatibility with nonaqueous electrolytes, and a model ferrocene-based redox couple to investigate the feasibility of achieving this target ASR. We identify and minimize sources of resistive loss for various active species concentrations, electrolyte compositions, flow rates, separators, and electrode thicknesses via polarization and impedance spectroscopy, culminating in the demonstration of a cell ASR of ca. 1.7 Ω cm2. Further, we validate performance scalability using dynamically similar cells with a ten-fold difference in active areas. This work demonstrates that, with appropriate cell engineering, low resistance nonaqueous reactors can be realized, providing promise for the cost-competitiveness of future NAqRFBs.
Highlights
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Most nonaqueous redox flow batteries (NAqRFBs) studies have focused on molecular discovery and electrolyte characterization at dilute active species concentrations,[13,14,15,16,17,18,19] with only a few published attempts to engineer higher performance systems with concentrated electrolytes.[20,21,22,23,24,25,26]
At present, reported NAqRFBs exhibit inadequate performance and durability,[8] but identifying performance limiting factors can be difficult because shortcomings could be attributed to active species degradation, insufficient separator selectivity, or poor cell design.[27]
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