Abstract
Aqueous Organic Redox Flow Batteries (ORBAT) are attractive for large-scale energy storage. Quinone based ORBATs have the advantages of being low cost and also environmentally-friendly. However, it has been a challenge to find electrolyte materials that are stable because of the reactivity of the organic molecules with water. For grid-scale energy storage systems to be commercially viable, it is desirable that the levelized cost of energy storage (LCOS) is <5 cents/kWh and the operational life of the system is >10 years.
The flow battery research at the University of Southern California (USC) has previously identified the causes for capacity fade ORBATs operating on quinones. We have demonstrated molecular design strategies to prevent chemical transformations by the Michael reaction with water [1] and acid-catalyzed proto-desulfonation [2]. We have also demonstrated operational approaches to mitigate capacity fade by crossover through the proton exchange membrane separator [2].
Recently, we have developed a highly efficient redox flow battery system based on low-cost water soluble redox materials, which can deliver electricity at the target levelized cost of storage (LCOS) of 5 cents/kWh-cycle. The highly durable molecules at the positive and negative electrode have shown the ability to be discharged at current densities as high as 840 mA/cm2 (Figure 1). We have performed stable charge/discharge cycling experiments for over 700 cycles at 200 mA/cm2 (Figure 2) without any measurable capacity fade.
The system is distinguished by the remarkably low cost of redox materials of $6.5/kWh, the lowest cost for any known type of redox flow battery. Based on our preliminary tests and understanding of the chemical stability of the water-based solutions of the active materials, we can expect the system to realize the targets for long operational life. The ultra-low cost of redox materials, the inherent stability of redox materials, and the high efficiency during charge and discharge, gives this organic flow battery system a competitive edge over other systems.
References
[1] L. Hoober-Burkhardt et al., "A New Michael-Reaction-Resistant Benzoquinone for Aqueous Organic Redox Flow Batteries," J. Electrochem. Soc., vol. 164, no. 4, pp. A600–A607, 2017.
[2] A. Murali et al., "Understanding and Mitigating Capacity Fade in Aqueous Organic Redox Flow Batteries," J. Electrochem. Soc., vol. 165, no. 7, pp. A1193–A1203, 2018.
Figure 1