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Chapter 3. Characterisation Methods and acts as the electrochemical counter to the WE. That is, when an oxidation occurs at the WE, a reduction happens at the CE, and vice versa. The CE consists of a conductive and inert material and should have a large surface area compared to the WE so it does not limit the half-reaction occurring at the WE. The RE provides a circuit to the WE over which the potential is applied or measured, and provided the RE has a stable and known potential, the potential at the WE can be measured accurately. A schematic of a three-electrode setup is shown in Figure 3.3. The setup also includes a source of inert gas (typically nitrogen or argon) that is initially used to expel dissolved oxygen from the electrolyte and later used to keep an inert atmosphere above the electrolyte. I Purge gas out Reference electrode Counter electrode Potentiostat E Purge gas in Working electrode Figure 3.3: Schematic of a three-electrode setup with connections to a potentiostat. The purge gas tube is shown in purge mode. One of the parameters that can be determined with CV is the redox potential of a redox couple. As stated previously, it is assumed that the redox potential is equal to the half-wave potential, which is the mid-way point between the anodic and cathodic peak potentials Epa and Epc, as depicted in Figure 3.2 and defined as: E1/2 = Epa + Epc (3.8) 2 Another feature of CV is the ability to visualise the reversibility of an electrochemical system. In a fully reversible system, the current is limited by the mass transfer of material to the surface of the WE, and the oxidation and reduction peak potentials are close to each other (separated by 59/nmV) and independent of the potential scan rate. The current, on the other hand, very much depends on the scan rate. For a fully reversible system, the peak currents (see Figure 3.2 for a graphical representation) increase with ν1/2 as described by the Randles-Ševčík equation, here for a reduction reaction: nF DOx 1/2 Ipc =0.4463nFAc∗Ox RT ν (3.9) 22PDF Image | Organic Redox Flow Batteries 2023
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