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7.5 Perspective of Organic Redox Flow Devices Usually, capacity loss in flow batteries can be attributed to redox species crossover through the membrane, oxidation from the outside environment and chemical degradation of redox species. Here major external oxidation is not likely since sealed and purged reservoirs are used. Moreover, a dense and highly selective Nafion 117 membrane was used in all RFB tests displayed and crossover has been shown to be unlikely for the relatively large quinone molecules (p-benzoquinone, o- benzoquinone and sodium 9,10-anthraquinone 2,7-disulfonate). For this reason we attribute the capacity loss to chemical degradation of the organic redox species. According to recent work, o-benzoquinones derivates, as Tiron, transforms to the tri- and tetrahydroxylated versions shortly after the oxidation to the quinone form, since water performs a Michael addition (1,4 to the α,β unsaturated carbonyl compound) as exemplified in Figure 7.16. The multi-hydroxylated quinones are usually found to be electrochemically irreversible and/or insoluble. In the case of increasing the pH, a stronger nucleophile (OH-) is predominant and thus the reduction/oxidation peak ratio is even smaller than in acidic and neutral solution. This is confirmed in a redox flow battery test in Figure 7.42, where an acidic metal free redox flow battery with Tiron losses more than half of its capacity after only 20 cycles. Consequently, quinones with possible leaving groups and unsubstituted sites are likely to be unstable in aqueous solutions. However, the degradation reaction rate depends on the electrolyte used. A further complication is that as both oxidation states are present in the battery during operation and hydroquinones can act as nucleophiles and react with the benzoquinones, there can be unwanted polymerizations at both, low and large pH. Thus, when considering stability the use of only partially substituted BQs and those substituted with possible leaving groups in aqueous RFBs is very limited. As an alternative to single quinone molecules solved in the electrolyte, it could be possible to do a controlled polymerization analogously to Janoschka et al. that has evaluated numerous water-soluble polymers (polymethacrylates and polystyrenes) for application in an aqueous RFB274. A special focus should be placed on their rheological, thermal, and electrochemical properties with a target of < 20mPas to ensure the efficient operation of a pumped RFB. Similarly as they do with TEMPO and viologen as redox active moieties (Figure 7.48)274,275, it can also be done a novel battery type not only to replace common ion-exchange membranes and highly corrosive acidic electrolyte solutions with inexpensive dialysis membranes and pH-neutral sodium chloride solutions, but also avoiding the chemical degradation of single quinones molecules. 206PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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