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5 Single fibre electrode measurements - a versatile strategy for assessing the non-uniform kinetics Experimental Methods Electrochemical experiments were carried out using solutions of K4Fe(CN)6 (BDH AnalaR) and K3Fe(CN)6 (Vickers Laboratories Ltd) dissolved in 1 M KNO3. For single fibre electrode measurements, concentrations of 10 mM K4Fe(CN)6 and 10 mM K3Fe(CN)6 were used. This concentration was chosen to increase the faradaic current measured at these small electrodes. When using carbon felt electrodes, as the total surface area was much larger, 0.1 mM K4Fe(CN)6 and 0.1 mM K3Fe(CN)6 were used to ensure that the total current and thus uncompensated ohmic resistance was minimised. A standard three electrode configuration was employed for all electrochemical measurements, using a Gamry 3000 potentiostat. Dissolved oxygen was purged prior to each experiment by bubbling argon through the electrolyte, and an inert atmosphere was maintained above the electrolyte during all measurements. The temperature of the cell was maintained at 23 ± 1oC. Single carbon fibre electrodes were fabricated using a procedure similar to those reported in literature [179, 180]. Briefly, single carbon fibres from carbon felt (GFE-1, Ceramaterials, USA) were attached to copper wire using electrically conductive carbon glue. This contact was then sealed in epoxy resin to ensure that only the carbon fibre was exposed to the electrolyte. Carbon fibre electrodes were 3-5 mm in length. For the carbon felt electrodes, a section of the same felt, measuring 1 cm by 1 cm with a thickness of 3.5 mm, was held in a 3-D printed holder with only the 1 cm2 front face open to the electrolyte. Electrical contact to the felt was made using a glassy carbon plate attached to a conductive wire insulated from the electrolyte using epoxy resin. The resistance between the working and reference electrodes was quantified using electrochemical impedance spectroscopy (EIS), measured at the open circuit potential with a 10 mV rms sinusoidal potential excitation at frequencies between 1 MHz and 1 Hz. 80% of the resistance measured by EIS was corrected for using positive feedback compensation during cyclic voltammetry at the carbon felt electrodes, with the remaining 20% used to correct the data after the measurements were complete. 53PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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