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Chapter 4. Molecular Electrochemistry of Quinones are presented in Figure 4.16. The previously observed peaks all increase in peak current and shift in peak position with increasing scan rate. Figure 4.16: Cyclic voltammograms of 5 mM DHTA in 1 M KOH recorded on a GC WE at various scan rates, initially in the anodic direction. The dashed grey line represents a background recorded in blank solution. Cyclic voltammograms were additionally recorded on a gold WE to study if the electro- chemistry became more well-defined on a different material (see Figure 4.2 (c), dashed salmon line). Compared to the voltammetry on a GC WE, the single peak at −0.8 V vs. SHE became sharper, the set of peaks at −0.25 V vs. SHE became more defined, the peak at 0.3 V vs. SHE shifted down to 0.1 V vs. SHE, and new peaks appeared around 0.55 V and 0.35V vs. SHE on a gold WE. The stability of DHTA in the upper half of the applied potential range was tested via bulk electrolysis, using a solution of 45mL 10mM DHTA dissolved in 1M KOH (24.1mAh theoretical capacity). The solution was charged galvanostatically at 50 mA to a potential limit of 0.64V vs. SHE, after which a potentiostatic hold was applied until the current plateaued at 1.5 mA. This is shown as a plot of potential against capacity in Figure 4.17. The subsequent galvanostatic discharge at −50 mA caused the voltage limit of −0.36 V vs. SHE to be reached immediately, and the following potentiostatic hold recovered only little of the theoretical capacity. This supports the hypothesis that the oxidised species of DHTA is unstable at pH 14. The complicated electrochemistry observed with CV could thus be caused by several decomposition products with different electrochemical profiles. 4.2.3.2 DHPA The tetracarboxy-substituted hydroquinone was synthesised in-house and tested with the goal of achieving a stable structure in alkaline media. The cyclic voltammogram of DHPA on a GC WE presented in Figure 4.2 (c) (solid skyblue line) again shows a complicated electrochemical profile, although with more well-defined peaks than for DHTA. Three sets of peaks are observed around −0.28V, −0.04V, and 0.08V vs. SHE, and a single cathodic peak at −0.74V vs. SHE (this is made clearer in Figure 4.18 (a)). With the 2-electron capability of hydroquinones in mind, two of the sets of peaks can be ascribed to two one-electron processes occurring at different potentials, whereas the last set of peaks 60PDF Image | Organic Redox Flow Batteries 2023
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