Redox flow batteries (RFBs) are a promising electrochemical storage technology due to their decoupling of energy and power, long service life, and scalability.
1 However, despite moderate commercial success, the widespread deployment of the state-of-the-art system, vanadium RFBs, is stymied by high capital costs. In particular, the high and volatile price of vanadium remains a significant obstacle to adoption, as the cost of the electrolyte alone (ca. 125 $/kWh, with significant fluctuations based on raw materials costs)
2 is close to the United States Department of Energy targets for the total installed system costs (generally cited between 100 and 150 $/kWh).
3,4 This has spurred research into RFB chemistries that utilize abundant and inexpensively-sourced active materials. Arguably the first contemporary RFB system, the iron-chromium (Fe-Cr) chemistry was originally advanced as an energy storage system for deep space missions in the 1970s. This chemistry has recently experienced renewed interest for grid storage as it utilizes relatively low-cost, high-abundance active species, and can remediate crossover losses by utilizing a mixed electrolyte configuration (i.e., the “spectator strategyâ€). However, among other challenges, a critical technical barrier inhibiting widespread Fe-Cr adoption is the high rate of the hydrogen evolution reaction (HER), which reduces system efficiency and causes severe capacity loss. Accordingly, research efforts in literature have primarily focused on catalyst development and electrolyte formulation to reduce HER rates.
5 In this presentation, we develop a protocol for electrochemical purification of the Fe-Cr negative electrolyte by reducing out metal impurities and explore the effect of the protocol on the performance of an Fe-Cr RFB single cell. We show that the purification process results in a notably reduced fade rate in durational galvanostatic cycling of an Fe-Cr RFB cell, and that the effectiveness of the protocol is dependent on the relative amount of electrolyte volume purified to the electrode area. Further, we extract performance metrics (i.e., coulombic, voltaic, and energy efficiencies along with capacity decay rate) of cycled Fe-Cr RFBs reported in the peer-reviewed literature, illuminating a correlation between coulombic efficiency and capacity decay rate. Following this trend, the performance of our cell using purified electrolyte is comparable to the performance of other cells using expensive catalysts and additives, evincing a potential cost reduction pathway for Fe-Cr RFBs.
Acknowledgments
This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. CTW acknowledges additional funding from the National Science Foundation Graduate Research Fellowship Program under Grant No. 1122374. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.
References
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4. ARPA-E, ARPA-E: The First Seven Years, (2016).
5. C. Sun and H. Zhang, ChemSusChem, 15, e202101798 (2022).
