Experimental and Computational Studies of Disperse Blue in Organic Non-Aqueous Redox Flow Batteries
Authors: Pahlevaninezhad Maedeh , Leung Puiki , Pahlevani Majid , Ponce de Leon Carlos , Roberts Edward P.L. 
Published: May 2020

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]. Converting this intermittent power into a base-load power is a challenge in the energy sector. Energy storage systems (ESSs) can store and supply large scale energy as required to address this challenge. 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]. Redox-active materials is an important constituent of RFBs since battery cycling performance is highly dependent on redox-active properties such as solubility, redox potential, chemical stability, and cost. 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: DB), 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 by using a flow cell design with an electrode area of 2.5 cm2. The electrolytic solution: 40 mM DB 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 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 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 results of the CV and charge-discharge experiments will be presented demonstrating that this organic molecule can improve RFBs energy density.
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Figure 1