Studies on Aqueous Redox Flow Batteries Based on Water-Soluble Quinone Redox Couples

Abstract

Organic Redox Flow Batteries (ORBAT) based on aqueous electrolyte present a promising solution for grid-scale and distributed electrical energy storage direly needed for integrating the intermittent renewable energy sources because the proposed materials are potentially low cost, eco-friendly, and sustainable [1]. In ORBAT, water-soluble organic redox couples such as benzoquinone and anthraquinone derivatives can be used as both the positive and negative materials. These materials are exhibit fast kinetics of electron transfer and high charge capacity. The use of water as the solvent lowers the cost and makes the system safe and eco-friendly. In addition, ORBAT does not use any heavy metals. Researchers at Harvard have also been working on similar systems using bromine as the positive electrode material [2]. Thus, the overall cost can be low enough to achieve the DOE-specified target of $100/kWh for the entire battery system. In our previous study [1], we have constructed redox flow cells based on quinone compounds, such as 4,5-Dihydroxy-1,3-benzenedisulfonic acid (DSQ) and Anthraquinone-2,6-disulfonic acid (AQDS) and measured the rechargeability and current-voltage characteristics. Since then we have conducted further cycling of these cells. No significant loss of capacity was observed over at least a 100 cycles (Figure 1 a). However, the charge-discharge efficiency is low initially (Figure 1 b), and this is now understood to arise from the conversion of the oxidation product of DSQ by the Michael Addition reaction to the 2,4,5- trihydroxybenzene-1,3-disulfonic acid [3]. We will describe the polarization characteristics and the mechanism of this reactant transformation as studied by electrochemical methods, NMR, and UV spectroscopy. These studies offer insights into the requirements for the design of new molecules. Acknowledgement The work presented here was funded by ARPA-E (Open FOA DE-AR0000353) and the Loker Hydrocarbon Institute of University of Southern California. Figure 1. Cycling experiment of 1M DSQ, 1M AQDS, 1M sulfuric acid. Charge and discharge at 80mA/cm2. a) Charge-discharge capacity. b) Charge-discharge efficiency.