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Chapter 4. Molecular Electrochemistry of Quinones Levich and Tafel linear regressions. The diffusion coefficient is higher than the literature value of 3.66 × 10−6 cm2 s−1, whereas the standard rate constant is about 3.4 times smaller than the previously reported 2.12 × 10−3 cm s−1 [32]. The latter is not surprising, as it was already observed that the cyclic voltammogram recorded in this work (see Figure 4.2 (b), lime line) appeared less reversible than observed in the literature [32]. Figure 4.14: RDE experiment of 5 mM DHBQ in 1 M KOH recorded on a GC WE. (a) Linear sweep voltammograms recorded at 5 mV s−1 in the cathodic direction at rotation rates from 300–2500 rpm. The areas shaded in blue and red show where the Levich and Koutecký-Levich currents were determined. (b) Levich plot. (c) Koutecký–Levich plot. (d) Tafel plot. The dashed lines in subplots (b)–(d) are extensions of the linear regression lines to x = 0. The chemical stability was tested via RRDE measurements, which are presented in Figure 4.15. The collection efficiencies are close to the theoretical value for all rotation rates, which would lead to the conclusion that DHBQ is chemically stable in 1 M KOH. However, in the work conducted by Yang et al. [32], prolonged cycling in a full cell configuration against K4[Fe(CN)6] revealed a fade rate of 0.24 % cycle−1 or 12.5 % d−1 in a Nafion 115 based cell (the calendar fade rate was not given directly in the reference and was instead estimated from the stated experimental parameters). Crossover of DHBQ was found to account for only 5 % of the capacity fade; the remaining capacity fade was attributed to degradation of DHBQ, which was confirmed by the detection of new species with nuclear magnetic resonance (NMR) spectroscopy and liquid chromatography experiments [32]. 58PDF Image | Organic Redox Flow Batteries 2023
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