Electrochemical Studies of Quinonoid Compounds for Aqueous Organic Redox Flow Batteries

Abstract: In the current times of uncertainty in face of climate change, sustainable energy-related technologies are developed with urgency. A candidate of special interest is Aqueous Organic Redox Flow Batteries (AORFB), whose utility depends heavily on the organic molecules used. In this work, the archetypical AORFB molecule, 9,10-anthraquinone-2,7-disulfonic acid (AQDS) was investigated in aqueous solution using a combination of electrochemistry and diffusion NMR in order to show that its battery performance is hampered by self-association. The self-associative species are not reduced in aqueous solution, and consequently, only a fraction of the capacity is redox-accessible in the oxidized state. The self-association was shown to be strong enough that only 27% of the molecules in a typical 1 M AQDS, pH 0 sulfuric acid electrolyte occur in a redox-accessible monomeric form. Furthermore, a class of molecules called naphthalene diimides (NDIs) were for the first time investigated electrochemically in aqueous solution and found to have appealing properties for application in AORFBs. One of the simplest naphthalene diimides, in this work called 2H-NDI, was examined by combining rotating disk electrode voltammetry, 1H-NMR spectroscopy, and diffusion NMR, to show that 2H-NDI also self-associates, and has an equilibrium constant for dimer formation of approximately 150 M-1. The 2H-NDI dimer was shown to have a decreased electrochemical activity, but in contrast to AQDS, the dimerization did not negatively affect bulk electrolytic behavior, likely due to a larger dimer dissociation rate constant. Apart from 2H-NDI, eight other naphthalene diimides, differentiated by the substituent on the naphthalene core, were studied using density functional theory (DFT). The NDIs were disubstituted with Br, F, NH2, NO2, N(CH3)2 (called DMA), OH, and CN, for which reduction potentials and pKa values as well as oxidative and reductive pathways were predicted. These findings were used to give mechanistic insight on experimental work on 2H-NDI, 2Br-NDI and 2DMA-NDI, the last of which has a complex reduction mechanism due to the possibility of being protonated at the amine substituents. Five pH neutral aqueous redox flow batteries using equimolar solutions of 2H-NDI or 2DMA-NDI coupled with 1,1'-bis[3-(trimethylammonio)propyl]ferrocene (BTMAP-Fc) were assembled and studied. In 1 M potassium chloride and 0.5 M potassium phosphate, two batteries with 50 mM 2H-NDI/BTMAP-Fc or 2DMA-NDI/BTMAP-Fc showed appreciable capacity losses while cycling. However, when instead using 1 M ammonium chloride and 0.5 M ammonium phosphate as the supporting electrolyte, a battery with 50 mM 2H-NDI/BTMAP-Fc displayed a coulombic efficiency of 99.92(2)%, an energy efficiency of 83% at 10 mA cm-2, and extraordinarily, no significant decrease in capacity over 320 cycles. At the same current density, 50 mM 2DMA-NDI/BTMAP-Fc showed a coulombic efficiency of 100.00(5)%, an energy efficiency of 90%, and similar to 2H-NDI/BTMAP-Fc, no capacity decrease was detected over 320 cycles, demonstrating two of the most stable redox flow battery chemistries to date.