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Where ip is the current maximum in amps, n is the number of electrons transferred in the redox event (usually 1), A is the electrode area in [cm2], F is the Faraday constant in [C mol−1], D is the diffusion coefficient in [cm2/s], C is the concentration in [mol/cm3], ν is the scan rate in [V/s], R is the gas constant in [J K−1 mol−1] and T is the temperature in [K]. When a linear behavior is observed between the scan rate and the peak current for the redox couple V3+/V2+, the reaction is diffusion controlled. Prior to use this equation it is worth mention it is applied just as a qualitative approximation because it applies to reversible reactions and planar electrodes while the studied electrodes are porous and quasi-reversible reactions. In order to estimate the mass transfer the Randles-Sevcik equation is used 222. The equation’s plot (Figure 6.29c) gives us a relationship where the slope is directly related to the associated mass transfer, which follows the trend: CF@TiO2 N900 > CF@N900 > CF > CF@TiO2 N500 > CF@TiO2. These values evidence the higher mass transfer on the CF@TiO2 N900 electrode towards the negative reaction, which is directly a consequence of a larger active area of the vanadium ions over the electrode and therefore being less diffusive limited. Moreover, the close value of CF@N900 to CF@TiO2 N900 also evidences a facilitated mass transfer due to larger active area on the electrode’s surface. The Tafel equation was applied in order to confirm the catalytic properties of the different carbon felt modified electrodes (Figure 6.29d), which was mathematically treated and their values disclosed in Table 6.6. As seen, the exchange current density follows the trend (mA): CF@TiO2 N900 (9.58 x 10-1) > CF@N900 (6.87 x 10-1) > CF@TiO2 N500 (3.51 x 10-1) > CF@TiO2 (2.38 x 10-1) > CF (6.6 x 10-2). It is directly related to the equilibrium exchange current on the electrode towards the negative VRFB reaction. Therefore, our synthesized material at high temperature (900oC) shows a superior electron exchange current with the electrolyte towards the redox reaction (V3+/V2+), which implies good catalytic behavior of highly nitrided electrodes. The heterogeneous rate constant was also determined, see Table 6.6, obtaining 1.6 ×10−3 cm s-1 for CF@TiO2 N900 electrode one order of magnitude larger than CF@TiO2 N500 electrode (5.8 x 10-4 cm s-1). CF@TiO2 N900 rate constant value is among the best values in the literature223. 120PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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