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6.29b) of all electrodes studied resembled a semi-circle at high frequency region and a sloped line at the low frequency region ascribed to charge transfer process and the diffusion process, respectively. The data were fitted using Randles equivalent circuit (Figure 6.29b onset). The following elements can be ascribed to the fitting: i) the Rs element represents the ohmic resistant, combining the electrolyte resistance, connection resistance, and the electrode resistance; ii) CPE-1 has a Faradic component (Y1), which is a double layer capacitance (Cdl) of the interface between the electrode and the electrolyte, and a no-Faradic component (n1), iii) charge transfer resistance (Rct), and iv) CPE-2 has a Faradic component (Y2), which represents the diffusion capacitance in pores of the electrodes, and a no-Faradic component (n2). The data parameters obtained are listed in Table 6.5 with an error less than 10%. Table 6.5 – Parameters obtained from the Nyquits fitting plots with the equivalent circuit model of Figure 6.29b. Sample CF@TiO2 CF@TiO2 N500 CF@TiO2 N900 CF@N900 CF Rs / Ohm 10.4 11.7 12.3 13.9 11.4 CPE-1 CPE-2 Rct / Ohm Y1 (Cdl) n1 Y2 n2 0.004451 0.92136 80.5 0.020095 0.89663 34.4 0.084218 0.76553 1.25 0.034198 0.61148 4.79 0.000322 0.97239 122 0.77467 0.81453 1.589 0.91352 2.781 0.6693 1.83 0.75279 0.96348 0.58562 Consequently, CF@TiO2 N900 sample has an electrochemical charge transfer resistance undeniable low compare to other electrodes, 1.25 Ohm, which is more than three times better compared to CF@N900 electrode. There is a great variation in the charge transfer resistance of the CF@TiO2 sample compared to the nitrided electrodes. This process highly improves the charge transfers between the negative electrode and the vanadium ions, especially for CF@TiO2 N900, when TiN phase is formed, as well as nitrogen groups functionalization over the carbon felt, as XPS previously shown (Figure S4). It makes CF@TiO2 N900 as the best electrode synthesized in this study, reducing the charge transfer resistance almost two orders of magnitude compared to CF@TiO2221. It is also worth considering CF@N900, which performed the second best, helped by the N and O-groups over the carbonaceous surface, showing an Rct of 4.79 Ohm. Moreover, Cdl values are found to follow the same trend as the Rct, obtaining the larger value for CF@TiO2 N900 (8.4 x 10-2 F) electrode due to higher amount of the active sites available for the reaction, which also agrees well with the CV analysis. Besides, in cyclic voltammetry, the peak current, ip, depends not only on the concentration and diffusion properties of the electroactive species but also on the scan rate, as described by the Randles-Sevcik equation: 119PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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