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Table 6.6 – Exchange current density (i0) values and heterogeneous kinetic rate constant obtained from the Tafel plot in 0.05 M V3+ in 1 M H2SO4 at 50% SoC (V2+/V3+) for the electrodes CF, CF@TiO2 and CF@TiO2 N500, CF@TiO2 N900 and CF@N900. Electrode CF@TiO2 CF@TiO2 N900 I0 / mA 0.238 0.958 k0 /cm s-1 3.9 x 10-4 1.6 x 10-3 CF 0.066 1.1 x 10-4 CF@TiO2 N500 0.351 5.8 x 10-4 CF@N900 0.687 1.1 x 10-3 Since the lower contribution of the parasitic reactions must be settled in order to assess a high Coulombic efficiency, as well as long lifespan of the electrodes performance, CV are performed in absence of any vanadium ion to study the HER on the surface of the different studied electrodes (Figure 6.30). The maximum current density values for the as prepared electrodes towards HER increases in the following order (mA cm-2): CF@TiO2 (89) < CF@TiO2 N900 (150) < CF@TiO2 N500 (180) << CF@N900 (248) < CF (258). Compared to CF, it is obvious the large inhibition of hydrogen evolution presented on the felt covered by titanium dioxide as rutile phase, shifting the onset potential of HER, from -0.024 to -0.49 V vs. SHE224. The nitrided samples, CF@TiO2 N500 and CF@TiO2 N900, also inhibited the HER compared to the carbon felt, though not as strongly as before the nitride treatment, -0.15 and -0.30 V vs. SHE onset potential for annealing at 500 and 900oC, respectively. In spite of the inhibition of HER produced on its surface it is not as powerful as in the case of pure titanium dioxide, due to the presence of fewer oxygen C-O centers, as shown by XPS for TiO2 and CF@TiO2 N500 89. Moreover, CF@TiO2 N900 has a more negative onset potential compared to CF@TiO2 N500 as the activation energy towards HER is comparatively larger. Last but not less important, CF@N900 increases the kinetics towards HER, in spite the fact that postpones the onset potential compared to CF. This result related to CF@N900 hydrogen evolution reaction demonstrates the usefulness of TiO2 presence to inhibit H2 formation. Thus, CF@N900 is not studied into a single cell. 121PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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