High Energy Density Non-Aqueous Organic Redox Flow Batteries

Renewable energy sources such as wind and solar are replacing fossil fuels for electricity generation. However, intermittency of wind and solar limits their wide-spread adoptions. The energy fed into the power grid must be matched with the consumer energy demand to prevent blackouts and destabilization of the grid [1, 2]. Recently, redox flow batteries (RFBs) have gained practical interest among the other energy storage technologies in light of their long lifetime, independent sizing of power and energy, high round-trip efficiency, scalability and design flexibility, fast response, and low environmental impact [3-5]. Organic redox-active materials have recently received attention as they provide competitive electrochemical characteristics, flexible design, and they are abundant in nature [6]. Aqueous designs face commercial difficulty because RFBs have low energy and power density due to the limited cell voltage of 1.23 V. The limited voltage is due to the evolutions of hydrogen and oxygen in the water electrolysis [7]. Solvent substitution is one solution to enable higher energy densities in RFBs, using non-aqueous solution also provides a large design space for enhancement of material solubility, cell potential and the number of electrons stored in the redox species [7-8]. In this study, a new organic redox molecule, tetra amino anthraquinone (Disperse Blue 1: DB-1), is evaluated and compared with other organic systems reported in the literature [5, 7] such as benzoquinone (BQ), naphthoquinone (NQ), anthraquinone (AQ), tetramethyl piperidinyloxyl (Tempo), and phenylenediamine (PD) in non-aqueous solvent by means of cyclic voltammetry. A three-electrode system was utilized to conduct cyclic voltammetry (CV) experiments using glassy carbon working electrodes. The battery performance was evaluated using a flow cell with an electrode area of 2.5 cm2. The electrolytic solution, containing 40 mM DB-1 solution in dimethyl sulfoxide solvent (DMSO) and 1 M Bis (trifluoromethane) sulfonimide lithium salt, was circulated through the cell at a flow rate of 10 cm3 min-1. Graphite felt and Nafion 115 were used as the electrode and membrane, respectively. In addition, density functional theory (DFT) calculations were used to better understand the electrochemical behavior of the active quinone molecules at different oxidation states. Figure 1 shows the molecular orbital energy levels (HOMO and LUMO) of the DB-1 organic dye and other similar organic molecules obtained by DFT calculations in DMSO. A relatively small HOMO-LUMO gap means a lower overpotential required for the oxidation and reduction processes [8]. The DB-1 had narrower bandgaps (<3 eV) than other quinone molecules (> 3.9 eV), suggesting that the selected molecule has better kinetics than other organic molecules. The result describes a systematic evaluation of DB-1 as a redox active material for energy storage applications by means of electronic structures, electrochemical properties, and flow cell studies. Voltammetric studies and DFT calculations (in DMSO solvent) demonstrated that this was capable of forming both anions and cations at electrode potentials ranging from 1.7 to 4.4 V vs. Li, involving up to 6 electron-transfers and exhibiting one of the highest electrode potentials (up to 4.4 V vs. Li) in the literature. Since these quinone derivatives can be extracted from biomass materials, the proposed organic RFB system provides a greener and more sustainable option for grid-scale energy storage applications than metal based redox systems. The proposed chemistry in this work showed high energy efficiency (ca. 68 %) at a current density (20 mA cm-2) with close to 100% capacity retention. These results reveal that the molecular tuning of quinone structures is promising for RFBs applications. Figure 1