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4.2. Results and Discussion Figure 4.17: Bulk electrolysis of 45 mL 10 mM DHTA in 1 M KOH recorded on a carbon mesh WE. The solution was charged and discharged galvanostatically at ±50 mA with a potential hold at the limit until the current plateaued after each half-cycle. could be caused by an electrochemically active degradation product. On a gold WE, an extra set of peaks appear around 0.4 V vs. SHE (see Figure 4.2 (c), dashed skyblue line). As for DHTA, voltammograms of DHPA were recorded at various scan rates on a GC WE to study if the peaks were faradaic in nature. The voltammograms presented in Figure 4.18 (a) show how the peaks observed at 50 mV s−1 increase in size with increasing scan rate. RDE voltammetry was additionally carried out to investigate the electrochemical profile under hydrodynamic conditions. The recorded linear sweep voltammograms are presented in Figure 4.18 (b) and show activity around the same potentials as observed with CV. Lastly, the stability was studied with bulk electrolysis in two different potential ranges, using a solution of 45mL 5mM DHPA in 1M KOH (12.1mAh theoretical capacity). In the first range, the solution was charged galvanostatically at 25mA until a potential limit of 0.44 V vs. SHE was reached, after which a potentiostatic hold was applied at the limit until the current plateaued. The subsequent discharge was done galvanostatically at −25mA, which resulted in the potential limit of −0.21V vs. SHE being reached immediately. This was followed by a potentiostatic hold at the limit until the current plateaued. These charge-discharge curves are shown as the black lines in Figure 4.19 and show how the initial charge reached the theoretical capacity even before the potential limit was reached, suggesting that side reactions were taking place. The discharge resulted in almost no recovered capacity, which suggests that the oxidised species was either degraded or the potential limit was too high to drive any significant amount of reduction. In the second potential range, the discharge was conducted first, using −25mA and a potential limit of −0.54V vs. SHE, followed by a potentiostatic hold at the limit until the current plateaued. The subsequent charge was conducted to a potential limit of 0.34 V vs. SHE with a current of 25 mA, again followed by a potentiostatic hold. This cycle is shown as the red lines in Figure 4.19. With the lower potential limit, the discharge recovered about 7.0 mA h of the theoretical 12.1 mA h. The subsequent charge recovered 9.2 mA h, which adds to the suspicion that DHPA had degraded to some degree. 61PDF Image | Organic Redox Flow Batteries 2023
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