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Chapter 3. Characterisation Methods calculated as the ratio between the measured ring and disk currents: Nex = −Ir (3.19) Id If there is a significant difference between the experimental and maximum collection effi- ciencies, it means that material was lost between the disk and ring, e.g. to an irreversible chemical reaction, as illustrated in Figure 3.6. One indication that a chemical reaction is taking place is if the measured collection efficiency decreases with decreasing electrode rotation rate, as the lower rotation rate allows more time for the chemical conversion to take place while the species travel from disk to ring. 3.3 Bulk Electrolysis The voltammetry techniques of stationary and rotating electrodes described in Section 3.2 are carried out at a low ratio of WE area to electrolyte volume. The resulting low currents ensure that the bulk concentration of active species does not change significantly with time. If the goal on the other hand is to considerably change the concentration of active species, this can be achieved by bulk electrolysis. The technique is also carried out in a three-electrode configuration, but the planar disk WE is exchanged with a high surface area WE, such as a porous carbon mesh. The CE is placed in a chamber filled with blank supporting electrolyte and separated from the main compartment by an ion- exchange membrane or a sintered glass or ceramic junction. The RE is placed in the same compartment as the WE, and should in the same way as for the voltammetry techniques exhibit a stable potential for the WE potential to be measured against. Furthermore, to ensure that the electrolysis of active species is not limited by mass transport, the solution is continuously stirred using e.g. a magnetic stirring bar. The exhaustive electrolysis conditions are achieved by applying a constant potential across the system while measuring the current, or vice versa. Bulk electrolysis is useful for several types of studies, such as probing the number of electrons available from a redox-active species, assessing the stability in solution, or carrying out electrochemical syntheses [62]. 3.4 Cell Cycling The prevalent method to test the performance of any type of battery is through cycling, i.e. alternately charging and discharging the battery several hundreds or thousands of times. Several important parameters are obtained through this method, including the charge and discharge capacities, the coulombic efficiency, the capacity fade rate, the voltage efficiency, and the electrical efficiency. Each of these terms is introduced in the following, together with the different cell cycling methods that may be employed. 26PDF Image | Organic Redox Flow Batteries 2023
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