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3.5 Single cell device set-up The electrolytes are prepared by dissolving 1, 1.6 and/or 2 M vanadium ions (Alfa Aesar) in 3 M H2SO4 (Aldrich, 98%) solutions. The corresponding anolytes and catholytes can be prepared through an electrochemical process. The VRFB single-cell performance was measured using an in-house designed flow cell system, which has been described in Figure 3.8Error! Reference source not found.. The single-cell used was assembled sandwiching the membrane Nafion® 117 (6cm × 6cm) between two pieces of working electrodes with an area of 4 cm2, approximately compressed a 30%. Each electrode in their half- cell is placed in a groove on outer face of the graphite bipolar plates which are etched with 3 serpentines flow fields with diameter of 2 mm. The inner faces of the bipolar plate are deposited a sputtered layer of copper such as current collector. Viton® gasket is used to avoid the electrolyte leakage. Metallic aluminium end-plates are used to close the cell. In order to investigate the effect of several modifications on the carbonaceous felt, different materials have been used as negative and positive electrodes. The single-cell is connected to two glass reservoirs containing 20 mL catholytes and anolytes, respectively. The flow rate at each side is 10 mL min-1 and the negative reservoir was continuously purged with nitrogen to avoid any oxidation of the active species. The flow cell was charged up to 1.8 V and then discharged to 0.8 V, at different current densities, from 12.5 to 350 mAcm-2. Even more, the single cell includes a reference “in-house” electrode, Ag/Ag2SO4. It is included in between the two porous carbon electrodes to follow the reactions on the positive and negative side independently. The performance evaluation method for the VRFB is mainly determined by the following efficiencies: 1) Coulombic efficiency (CE), the ratio of the average discharging capacity to the average charging capacity, 2) Voltage efficiency (VE), the ratio of the average discharging voltage to the average charging voltage, (3) Energy efficiency (EE), VE multiplied by CE/100. According to Faraday’s law, the theoretical capacity in VRFB was calculated, for example, attaining a value of 536 mAh or 13.4 AhL-1 for a 1 M vanadium ions concentration, respectively. The theoretical capacity expressed in Ah L-1 has been calculated taking into account the total volume of the tanks (i.e. 40mL). Additionally, power test were done applying pulsed discharge current during 60 seconds also recording the potential of the cell, as well as, monitoring the potential at Eocv after the mentioned pulse. 51PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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