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Chapter 4. Molecular Electrochemistry of Quinones of 0.093 V vs. SHE. The peaks are again rather broad, which suggests that the electrons are transferred in two successive one-electron processes. No additional studies were performed on this compound due to time constraints. With the near-reversible electrochemical profile in mind, it would be interesting to continue the investigation of PSAQ at a later point. The observed redox potential does however make it less attractive as an electrolyte material, at least at pH 0, as combining it with other typical negolytes or posolytes would produce a low cell voltage. 4.2.4.3 PPAQ(Na) PPAQ was developed for the same reasons as PSAQ, i.e. to achieve a water soluble and chemically stable compound. It was initially synthesised as a sodium salt and later as a pure phosphonic acid, as it proved difficult to properly separate PPAQ(Na) from the NaCl used in the synthesis procedure. The cyclic voltammogram of the sodium salt recorded in 1 M KOH is presented in Figure 4.2 (d) (magenta line). It displays a redox potential of −0.537 V vs. SHE, symmetric peaks and a peak split of 40 mV, but the peak current densities are rather low compared to the other anthraquinones. The most likely explanation is that the purity of the PPAQ(Na) salt is significantly lower than expected. Regardless of this, a Randles-Ševčík analysis was carried out on voltammograms recorded between scan rates of 50 mV s−1 and 500 mV s−1. These are shown in Figure 4.21, with the analysis resulting in diffusion coefficients of 2.77 × 10−8 cm2 s−1 and 2.34 × 10−8 cm2 s−1 for the oxidised and reduced species, respectively. The low calculated diffusion coefficients would also be an effect of a too high estimate of the used concentration, so the diffusion coefficients are likely higher than these estimates. The system otherwise displays reversible behaviour with a near-linear relation between peak currents and square root of scan rate and regression lines intersecting close to (0,0). Figure 4.21: (a) Cyclic voltammograms of 5 mM PPAQ(Na) in 1 M KOH recorded on a GC WE at various scan rates, initially in the cathodic direction. The dashed grey line represents a background recorded in blank solution. (b) Randles-Ševčík plot of the anodic and cathodic peak current densities. The dashed red and blue lines are an extension of the linear regression lines to x = 0. 64PDF Image | Organic Redox Flow Batteries 2023
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