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Chapter 2. Redox Flow Batteries 2.2.3 DHAQ In 2015, Lin et al. published results for the compound 2,6-dihydroxyanthraquinone (DHAQ) as a negolyte material [29]. The quinone was paired with a posolyte contain- ing the ferri-/ferrocyanide couple, both dissolved in 1M KOH (and additional KOH for DHAQ to account for the released protons), resulting in a theoretical cell voltage of 1.2 V. They achieved power densities of 0.45 W cm−2 at 20 ◦C and 0.7 W cm−2 at 45 ◦C. The in- crease in power density was attributed to a decrease in cell area-specific resistance (ASR) from 0.878Ωcm2 to 0.560Ωcm2. During cell cycling, a coulombic efficiency above 99% was observed for 100 cycles, but also a 0.1 % loss of capacity per cycle. The loss was at this point attributed to leakage from the pumping system. In a later publication by Goulet et al. regarding DHAQ, it was discovered that the capa- city was actually chemical in nature [53]. It was found that the reduced form of DHAQ, dihydroxyanthrahydroquinone (DHAHQ), is less stable than the oxidised form, and that the reduced form undergoes disproportionation to form dihydroxyanthrone (DHA) in al- kaline media [53]. DHA was further found to undergo irreversible dimerisation to products with poor redox properties, leading to irreversible loss of capacity. The group also repor- ted two methods to either prevent or recover some of the lost capacity: Firstly, restricting the upper SOC limit to 88% by reducing the charge voltage limit from 1.6V to 1.25V reduced the capacity fade rate from 5.6 % d−1 to 0.14 % d−1. Secondly, aerating the DHAQ electrolyte in the discharged state was shown to recover approximately 70 % of the lost ca- pacity upon resuming cell cycling. The proposed pathways for capacity loss and recovery are illustrated in Figure 2.5. DHA3– O –O –O– DHAHQ4– O– O– disproportionation O– irreversible dimerisation and further decomposition DHAQ2– –O O– O– O O aeration 2e– Figure 2.5: Illustrations of the reversible redox reaction of DHAQ under normal cell operation in alkaline media (left side) and the decomposition and recovery pathways proposed by Goulet et al. [53]. 12PDF Image | Organic Redox Flow Batteries 2023
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