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(6.1). As it can be seen in Figure 6.13.a, there is a relevant modification in the HER caused by the presence of the TiO2 coating layer onto the graphite felt, while the GF-P electrode shows a maximum value of current-to-weight ratio up to 5.5 Ag- 1, GF@TiO2 and GF@TiO2:H electrodes present 0.6 and 0.2 Ag-1, respectively. The lower onset potential value showed in Figure 6.13.b also clearly evidences the poor HER that takes place on the TiO2 nanorods. In case of GF-P electrode, the onset potential was -0.1 V vs. NHE, while this potential was shifted to more negative values, up to -0.6 V vs. NHE for TiO2-based electrode. The most striking feature is that the HER contribution is very important using GF-P as negative electrode in VRFB at the V3+/V2+ redox potential, (ca. -0.26 V vs. NHE), indicating the competition between both reaction. a) 0.0 -1.5 -3.0 -4.5 -6.0 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 b) 0.5 0.0 -0.5 -1.0 -0.4 -0.3 -0.2 -0.1 E vs. NHE / V GF@TiO2 GF@TiO2:H 2H+ + 2e- H2 GF@TiO2 GF@TiO2:H E0= -0.26 V V3+ + e- V2+ E vs. NHE / V Figure 6.13. -. a) Linear sweep voltammetry (LSV) of pristine graphite felt, GF@TiO2 and GF@TiO2:H electrodes using a 1M sulphuric media with potential window of 0 to - 1V; b) Hydrogen-evolution reaction at E0= -0.26 V obtained from figure a) at the standard reduction potential value for negative reaction. Scan rate: 2 mV/s. Nevertheless, the hydrogen-evolution reaction is totally absent using GF@TiO2 or GF@TiO2:H electrodes at this potential, guarantying a superior performance. These results can be thermodynamically explained by a largely negative Gibbs free- energy of the intermediate hydride state (ΔGH*), indicating a highly energetic adsorption of the atomic hydrogen on TiO2-site. From volcano-type relationship (Figure 6.14a), the hydrogen-binding energy (HBE) of the intermediate hydride state is obtained, ca. 70 kcal mol-1. This HBE value is large, leading to a slow desorption in successive steps and in consequence, reducing the hydrogen-evolution rate (jo). The optimal value of ΔGH* should be zero (i.e. ΔGPtH*~0.09 eV), corresponding to the highest hydrogen- evolution rate; for instance, in the case of well-known highly efficient platinum catalyst (see the schematic representation of free-energy diagram for TiO2 and Pt electrocatalyst in Figure 6.14b). 96 i / A/g i / A/g GF-P GF-PPDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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