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3.4.3 Cycling Protocols Cell cycling can be carried out in a few different ways instrumentally, with the major difference being whether the voltage or the current is being controlled. The most common method is galvanostatic cycling, i.e. drawing a constant current until predefined voltage limits are reached. These limits are typically in the region of 300–500mV above and below the open circuit voltage (OCV). An issue with this method is that at high current densities, or if the system inherently has a high internal resistance, the voltage limits are reached before all the capacity is utilised. A slow increase in internal resistance with time, which could be caused by membrane ageing, will also decrease the achieved capacity with time. It can therefore be necessary to end a galvanostatic period with a potentiostatic hold at the voltage limit to access all (or at least more) of the available capacity [38, 65]. Less commonly, batteries are cycled potentiostatically, i.e. at a constant voltage with predefined current limits. For small-scale laboratory experiments, the current limits are typically set to a few mAcm−2 or as a percentage of the initial current [24, 25, 38, 64, 65]. Cycling potentiostatically makes it possible to run the battery under exhaustive bulk electrolysis conditions, i.e. accessing all the available capacity, without reaching voltage limits that could damage the battery. 3.5 Polarisation Curves Another important metric for RFBs is the power density, which is the power per geometric electrode area the battery can deliver at a certain voltage and current density. It is commonly determined by recording polarisation curves of the battery, i.e. recording the cell voltage as a function of the current density, or vice versa, at a fixed SOC. The power density is calculated as the product of current density and cell voltage: Pcell = iEcell (3.23) An example of a polarisation curve and the corresponding power density curve is presented in Figure 3.8. The peak power density is typically reported in literature for a given RFB system. Polarisation curves can also be used to give a qualitative assessment of the processes causing overvoltages in the system, which can be divided into ohmic, kinetic, and mass transport overvoltages. The typical appearances of these overvoltage contributions are shown as the shaded areas above the polarisation curve in 3.8. The kinetic overvoltage is caused by the resistance associated with the electron transfer between electrode and redox- active material. The ohmic overvoltage is caused by the internal resistance of the cell and is linear over the entire range, as it follows Ohm’s law. This overvoltage can further be resolved into contributions from electrode electronic, contact electronic, electrolyte ionic, and membrane ionic processes. This was recently done in a quinone-bromide flow cell through a combination of full cell, half cell, and voltage-probe experiments [66]. Lastly, the mass transport overvoltage is caused by insufficient transport of active material to the electrode surface. 3.5. Polarisation Curves 29PDF Image | Organic Redox Flow Batteries 2023
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