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4.2. Results and Discussion Figure 4.5: RDE experiment of 5mM K3[Fe(CN)6] in 1M KCl adjusted to pH 12 with KOH recorded on a GC WE. (a) Linear sweep voltammograms recorded at 5 mV s−1 in the cathodic direction at rotation rates from 100–2500 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 kinetic parameters of [Fe(CN)6]3– and [Fe(CN)6]4– at pH 12 were determined from the intercept and slope of the Tafel plots in Figures 4.5 (d) and 4.6 (d), which are also shown together in Figure 4.7. The linear regressions were carried out from an absolute overpotential of 75mV and upward, and in both cases a discernible deviation from lin- earity is observed at lower overpotentials. Standard rate constants of 3.73 × 10−3 cm s−1 and 3.23 × 10−3 cm s−1 and transfer coefficients of 0.429 and 0.412 were calculated for [Fe(CN)6]3 – and [Fe(CN)6]4 – , respectively. Values of 5.4 × 10−3 cm s−1 and 4.7 × 10−3 cm s−1 were previously reported at pH 7.50 [81], whereas another study reported a rate constant of 0.320 cm s−1 for [Fe(CN)6]3 – at pH 10.0 [49]. The calculated transfer coefficients sug- gest that the two half-reactions are relatively symmetric, although the coefficients do not add up to 1 as would be expected from the Butler-Volmer equation (see Equation 3.7). This could be caused by the coefficients being measured during two separate experiments, which will inherently differ slightly in measuring conditions (state of the working elec- trode surface, impurities in the electrolyte, temperature, etc.). The experiment could with advantage be repeated with both redox species present in the same electrolyte and recording linear potential sweeps in both the anodic and cathodic directions. 51PDF Image | Organic Redox Flow Batteries 2023
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