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For a reversible reaction: For an irreversible reaction: ip 1⁄4 2:69 105n3=2ACD1=2v1=2: 1 ip 1⁄4 2:99 105n3=2a1=2ACD1=2v1=2, 2 ð3:2Þ ð3:3Þ 0.0035 0.0030 0.0025 0.0020 0.0015 0.0010 0.0005 0 –0.0005 –0.0010 0.00893 mol l–1 0.0161 mol l–1 0.0196 mol l–1 0.0232 mol l–1 0.0286 mol l–1 6 1 2 3 4 5 5 3 4 1 2 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 potential/V versus SCE Figure 4. LSV curves of 2.8 M H2SO4 solution with different concentrations of Fe(III) at a scan rate of 0.001 V s21. iron ions underwent oxidation reaction and then formed an oxidation peak at 0.4–0.6 V. Figure 4 shows the LSV curves of 2.8 M H2SO4 solution with different concentrations of Fe(III) at a scan rate of 0.001 V s21. It can be observed that the current density was increased with the concentration of Fe(III) at 1.0 V, which is the redox potential of V(IV)/V(V), so it illustrated that iron ions perhaps are competitive with vanadium ions for the adsorption on the electrode surface and redox reaction. The side reaction of the Fe(III)/Fe(II) may have a bad influence on the performance of VRFB. To further investigate the effect of Fe(III) on the kinetics of electrode reaction, a series of CV curves for test electrolyte, containing different concentrations of Fe(III) on the graphite electrode at various scan rates, are shown in figure 5. It presents the typical characteristics of a quasi-reversible one-electron process for the anodic and cathodic peak potentials that change gradually with the scanning rates. A plot of redox peak currents as a linear function of the square root of scan rates with different concentrations of Fe(III) further verified the quasi-reversible process for the V(V)/V(IV) redox reaction, as shown in figure 6. Theoretically, the value of diffusion coefficient for a quasi-reversible reaction (D) is between that for a reversible (D1) one and for an irreversible (D2) one. For a reversible and irreversible one-step and one- electron reaction, the peak current ip is given in equation (3.2) and equation (3.3), respectively [25–28]: where n is the number of electrons transferred in the reaction, A is the surface area of the working electrode (1 cm2), C is the bulk concentration of the primary reactant, D1 and D2 are the diffusion coefficients for a reversible reaction and an irreversible reaction, respectively, and y is the scanning rate. The corresponding diffusion coefficients of the electrolyte with different concentrations of Fe(III) obtained from equation (3.2) and equation (3.3) are listed in table 3. It is clearly shown that the diffusion coefficient of V(IV) species increased from (2.06–3.33) 1026 cm2 s21 to (2.44–3.95) 1026 cm2 s21 with 0.0196 mol l21 Fe(III), while it decreased from (2.06–3.33) 1026 cm2 s21 to (1.78– 2.88) 1026 cm2 s21 with 0.0286 mol l21 Fe(III), indicating an improvement in the diffusion of V(IV) ions when the Fe(III) concentration was below 0.0196 mol l21. 3.3. Electrochemical impedance spectroscopy Figure 7 shows the Nyquist plots of the electrolytes with different concentrations of Fe(III), in order to analyse the electrode reaction–diffusion kinetics of V(IV) species and the processes of mass transfer and charge transfer for the V(V)/V(IV) redox couple. Each plot consisted of a semicircle in the high- frequency region and a sloped line in the low-frequency region. The semicircle in the high-frequency royalsocietypublishing.org/journal/rsos R. Soc. open sci. 6: 181309 Downloaded from https://royalsocietypublishing.org/ on 11 January 2023 current density (A cm–2)PDF Image | Effect of Fe3 positive electrolyte vanadium redox flow
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