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Journal of the American Chemical Society Communication Figure 2. Pausing redox cycling in the reduced state adversely affects battery capacity. Symmetric-cell cycling (see SI) for 8 cycles, followed by a 43 h pause, leads to a large drop in capacity. The first oxidative discharge after the pause shows a transient amount of capacity corresponding to the conversion of accumulated DHA to (DHA)2. concentration of 16% (as determined by NMR analysis). Both quantities correspond well to the observed loss in capacity of 15%. In addition, all of the DHA formed was measurably converted to (DHA)2 (Figures S18−S20). These observations strongly implicate anthrone formation and subsequent dimerization as the primary source of capacity loss. Anthrone and dianthrone species have also been reported to be products of the chemical reduction of anthraquinones and of the electrochemical reduction of anthraquinones in acidic aqueous solutions.44,57−65 The finding that irreversible dimerization is linked to the redox chemistry of anthraquinones may limit the ongoing search for quinones with lower potentials. This is because any decrease in the reduction potential of the quinone/hydro- quinone couple may simultaneously increase the propensity for the hydroquinone to disproportionate into anthrone and quinone. This hypothesis is supported both by calculations (Figure 3; PM7/COSMO on a subset of molecules from ref 41) and the experimental properties of 4,4′-((9,10-anthraqui- none-2,6-diyl)dioxy)dibutyrate (2,6-DBEAQ).34 This negolyte active species has a reduction potential that is 180 mV higher Figure 3. Trade-off between the reduction potential of the quinone- hydroquinone couple and the thermodynamic stability of the hydroquinone form toward anthrone formation (Ntotal = 858 quinones). The mean absolute errors (previously benchmarked in ref 41) of the predicted DHAQ potentials is ∼0.07 V. 8016 than that of DHAQ and is approximately 100 times more stable in terms of RFB lifetime.34 In addition to the associated decrease in battery voltage that this higher reduction potential entails, 2,6-DBEAQ is synthesized from DHAQ, increasing its cost.34 Strategies that can increase the lifetime of anthraqui- nones independently of their redox potential would therefore be valuable. Two simple changes to the operating conditions of the battery may be used to greatly decrease the rate of capacity loss. Because the degradation pathway is initiated by disproportionation of DHAHQ, we hypothesized that avoiding high states of charge would decrease the amount of DHA formed (Figure S21), thereby extending battery life. When a DHAQ/Fe(CN)6 flow battery was cycled at 100 mA/cm2 with a 1.25 V cutoff that accessed 88% of the theoretical capacity, the observed capacity declined at only 0.14%/day (Figures 4A and S22A). By contrast, under more typical conditions with a 1.6 V cutoff that accessed 99.9% of the theoretical capacity, the observed capacity declined at 5.6%/day. Similar results were obtained by using identical potential conditions but with a Coulombic cutoff to restrict the SOC range (Figure S23). Figure 4. (A) Limiting the state of charge reduces the rate of capacity loss in a negolyte-limited DHAQ/Fe(CN)6 full cell. Over the first 1.6 days, the operating state of charge was limited to 88% of the theoretical capacity (i.e., 88% DHAHQ and 12% DHAQ), and the capacity faded at only 0.14%/day. The right-hand segment reflects typical operating conditions (cycling to 99.9% of theoretical capacity), and the capacity faded at 5.6%/day. (B) Symmetric cell cycling in which the capacity-limiting side (5 mL of 0.1 M DHAQ in 1.2 M KOH) demonstrates recovery of 70% of lost capacity after aeration of discharged electrolyte. DOI: 10.1021/jacs.8b13295 J. Am. Chem. Soc. 2019, 141, 8014−8019PDF Image | Extending organic flow batteries via redox state management
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