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1500 3 M MeSO3H 1000 500 0 -500 -1000 -1500 0.0 0.05 M 1,4-Dihydroxybenzene 0.05 M 1,2-Dihydroxybenzene 0.5 1.0 1.5 E vs. NHE / V i / mA/g Figure 7.8. - Cyclic voltammetry comparative between the para- and orto- forms of the simplest benzoquinones (dihydroxybenzene). Therefore, further screening over a variety of quinone derivate molecules is done to evaluate their solubility among various types of solvents (Table 7.2) in order to improve the solubility of p-benzoquinone with similar reversibility and intensity, as well as select an optimum redox molecule for the anolyte, more specifically, 9,10- anthraquinone-2,7-disulphonic acid (AQDS), extremely rapid and reversible two- electron two-proton reduction. This quinone/hydroquinone is implemented in an aqueous flow battery with inexpensive carbon electrodes. They are also natural products which have been extracted for millennia from common sources such as rhubarb. Moreover, it could be a renewable source for future anthraquinone-based electrolyte solution. Knowing the principal basis of an efficient rechargeable redox flow battery is the rapid kinetics of charge-transfer at the positive and negative electrodes. Many organic redox couples, especially from the quinone family, undergo rapid proton- coupled electron transfer without the need for dissociating high energy bonds. Consequently, these redox couples have relatively high rate constants for the charge-transfer process (10-3 to 10-4 cm s-1), which is 2 to 3 orders of magnitude higher than commercial vanadium redox system266,267. Typically organic redox couples generate a battery voltage close to 1 V. The values of diffusion coefficients are about an order of magnitude smaller in aqueous solution than in non-aqueous solvents such as acetonitrile268. In aqueous solutions, the observed extent of decrease in the values of diffusion coefficients with increase in molecular mass is about 6 x 10-9 cm2 s-1 per unit of molecular mass. This coefficient is an order of magnitude lower than that observed in acetonitrile. Thus, besides the effect of molecular mass, the molecular diameter resulting from the salvation and the interaction of ionic groups with water through 161PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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