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The ionic resistance of the electrolyte between reference electrode and graphite felt was determined by electrochemical impedance spectroscopy. Figure 4.4 shows the Nyquist plot obtained for the positive (dashed black line) and the negative electrode (solid red line) (in web version). The ionic resistance is given by the Y-interception (imaginary part = 0). A value of 0.05 and 0.04 Ohm were obtained for the positive and negative electrode, respectively. The small values of ionic resistance are due to the large geometrical area of the electrode (25 cm2). We used these ionic resistance values to correct the current intensity in the follow-up experiments. Figure 4.4. –EIS Nyquist plots of the positive (dashed black line) and negative electrode (solid red line) obtained at 50 % of SoC. The frequency range was from 100 KHz - 0.1 Hz. 4.1.2 Single cell evaluation The kinetics of the electrochemical reactions is evaluated at the positive and negative electrode for VRFB “in operando” conditions to illustrate the usefulness of the reference electrode. Figure 4.5a, shows the voltage profiles of the cell and potential profiles the positive and the negative electrode at several current densities in a galvanostatic measurement around 50% SoC. The increasing potential with increased current is due to several factors; ionic resistance of the electrolyte, concentration overpotential and electrode kinetics overpotential. The ionic resistance of the electrolyte is corrected with the values obtained by EIS. It is also used a flow as high as 70 mL min-1 to minimize the concentration gradients (second factor). Thus, the higher potential for higher current density (Figure 4.5b) is significantly affected by the kinetic factor in this case. The potential incremented with increasing current step and it is always larger at the positive electrode. In 64PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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