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explained by assuming that there is a first step of the overall reaction that involves an ion exchange process between V3+ ions transported from the bulk of the electrolyte and the hydrogen atoms located at the Ti-N-O-H and Ti-N-H surface bonds. In this way, V3+ ions can be adsorbed easily on the Ti-N and Ti-N-O bonds. In a second step, electron transfer takes place, facilitated by Ti-N and Ti-N-O bonds acting as an electron donor. In the final step, the reduction reaction is completed by an ion-exchange between the V2+ formed on the electrode surface and the protons in the electrolyte. During the discharge process, a similar oxidation reaction in the opposite direction occurs. Unlike our previous article89 based on carbon felt electrodes covered by TiO2, here the outstanding electrical conductivity of TiN provides an explanation for the better battery performance of the electrode due to improved electron transport and transfer. .Furthermore, nitrogen-doped carbon samples have been widely investigated for electrochemical applications203, especially for the oxygen reduction reaction in fuel cells, concluding that the facilitated adsorption of oxygen allowed by the presence of nitrogen improves the electrocatalytic activity of carbon materials204–207. Moreover, the formation of C-OH, C=O, and C-O functionalities has also been shown to enhance cell performance208. The formation of O functional groups increases the standard heterogeneous electron transfer rate for V3+/V2+, from 3.2x 10-7 to 1 x 10-3 cm s-1, one of the highest values ever reported209. Moreover, the introduction of N surface functional groups has also been shown to help decreasing the fraction of the current directed towards H2 evolution89. Among the four main types of nitrogen groups, the quaternary or graphitic-N has been found to be the more stable in the acidic environment208. However, pyrrolic-N has been proposed to be the most electrochemically active nitrogen site for enhancing the catalytic activity in these nitrogen-modified carbon-based electrode materials for VRFB210. 6.1.3.1 Characterization 6.1.3.1.1 Structural characterization The morphological structures of the as-prepared electrode are characterized by FE- SEM. TiO2 nanowire coverage directly growth into the surface of carbon felt with a rod morphology by hydrothermal process (Figure 6.23a). Some changes can be appreciated after the NH3 thermal treatment, which forms local agglomerations of TiO2-nanorods in the case of the CF@TiO2 N500 sample (Figure 6.23b), due to the reductive conditions of the ammonia gas211. In the case of higher annealing temperatures, the rods become more compact and decrease their length (Figure 6.23c and d) with a well-dispersed rods decoration over the carbon felt surface after NH3 treatment. 108PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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