The intermittency of wind and solar electricity limits their penetration into our energy mix. Safe, cost-effective, large-scale electrical energy storage could solve this problem. Redox flow batteries (RFBs) are emerging energy storage technologies that are expected to have an increasing market size due to their scalability, safety, and system flexibility. Aqueous organic-based RFBs have gained attention due to their potential low cost and the possibility of tailoring the redox properties by modifying the organic chemical structure. To date, significant progress has been made in improving the stability and consequently the lifetime of organic-based RFBs [1-4]. Increased energy density is also desirable but is constrained by limited organic solubility. In this work, we discuss strategies for enhancing the energy density of organic-based RFBs by utilizing mixtures of redox-active organic molecules. In particular, we demonstrate an anthraquinone-based flow battery in which utilization of multiple tailor-made anthraquinones yields a 40% increase in the volumetric capacity, from 26.8 Ah/L to 37.5 Ah/L, without sacrificing the capacity fade rate of the system. The required principles for designing a mixed redox-active organic-based RFB and the possible trade-offs and opportunities will be discussed. These findings provide a pathway for enhancing the competitiveness of RFBs for practical applications.
[1] Kwabi, David G., Yunlong Ji, and Michael J. Aziz. "Electrolyte lifetime in aqueous organic redox flow batteries: a critical review." Chemical Reviews 120, no. 14 (2020): 6467-6489.
[2] Jin, Shijian, Eric M. Fell, Lucia Vina‐Lopez, Yan Jing, P. Winston Michalak, Roy G. Gordon, and Michael J. Aziz. "Near neutral pH redox flow battery with low permeability and long‐lifetime phosphonated viologen active species." Advanced Energy Materials 10, no. 20 (2020): 2000100.
[3] Wu, Min, Yan Jing, Andrew A. Wong, Eric M. Fell, Shijian Jin, Zhijiang Tang, Roy G. Gordon, and Michael J. Aziz. "Extremely stable anthraquinone negolytes synthesized from common precursors." Chem 6, no. 6 (2020): 1432-1442.
[4] Wu, Min, Meisam Bahari, Yan Jing, Kiana Amini, Eric Fell, Thomas George, Roy Gordon, and Michael Aziz. "Highly Stable, Low Redox Potential Quinone for Aqueous Flow Batteries." Batteries and Supercaps (2022).
