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6.1.2.2 Single cell performance Consequently, to ensure the potential application of TiO2 nanorods electrodes such as a negative electrode in large-scale VRFB batteries, a flow-type single cell was assembled using a GF-P as a positive electrode, their performance was compared to pristine GF negative electrode using 1M electrolyte concentration. Figure 6.17a, shows a comparison of the voltage profiles developed by all electrodes between 0.8 and 1.8 V at low charge/discharge current densities (ca. 25 mA/cm2). The GF@TiO2:H electrode clearly yields the best performance of all. Comparing the GF-P and TiO2-based electrodes, a significant increase in discharge capacity can be observed in the following order (the discharge capacity value obtained in Ah/L is given in parenthesis): GF-P (8.6) < GF@TiO2 (11) < GF@TiO2 (12.5). These results lead to 93% and 82% of the electrolyte-utilization ratio (theoretical capacity value is ca. 13.4 AhL-1), using GF@TiO2:H and GF@TiO2 electrode, respectively. However, it is obtained near 64% of electrolyte-utilization ratio using a GF-P electrode. Additionally, a dramatically over-potential drop was detected using TiO2 nanorods decorating the GF during the charge and discharge processes at low current. An abrupt IR drop (overvoltage drop when the current was reversed from charge to discharge) was observed in the following order (in parenthesis the value of IR calculated from the Figure 6.17a): GF-P (0.46 V) > GF-TiO2 (0.28 V) ~ GF@TiO2-x:H (0.27 V). These findings are consistent with the efficiency values (Figure 8b). At the same charge/discharge rate (i.e. 25mA/cm2), the VE value increased up to ~ 90% using GF@TiO2 and GF@TiO2-x:H electrodes, in comparison to 61% using a GF-P electrode. The same trends occur with EE values and reach up to ~90% and 58% using GF@TiO2/GF@TiO2:H and GF-P electrode, respectively. Additionally, the CE for GF@TiO2 and GF@TiO2:H electrodes is ~99%, whereas for GF-P electrode the CE is. 95%. The origin of the enhanced performance can be ascribed to the total abatement of the HER carried out by the TiO2-based graphite felt electrodes in comparison with commercial GF-P electrodes. a) 1.8 1.6 1.4 1.2 1.0 Theoretical limit b) GF@TiO 1.8 2 1.6 1.4 1.2 1.0 GF@TiO2:H Theroretical limit GF-P GF-HT GF-TiO2 GF-TiO2:H E/V E/V 25 mA/cm2 125 mA/cm2 0.80 2 4 6 8 10 12 14 Specific Capacity / Ah/L 0.80 2 4 6 8 10 12 14 Specific Capacity / Ah/L Figure 6.17. - a) Galvanostatic full cell charge/discharge profile for GF-P (black line), GF-HT (grey line), GF@TiO2 (blue line) and GF@TiO2:H (red line) at 25 mA/cm2 using 1 M vanadium concentration in 3M H2SO4 as electrolyte. b) Galvanostatic charge/discharge profile for GF@TiO2 (blue line) and GF@TiO2:H (red line) at 125 mA/cm2 using 1 M vanadium concentration in 3M H2SO4 as electrolyte. 99PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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