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4. EIS at Carbon Fiber Cylindrical Microelectrodes Abstract The theoretical mass-transport impedance at cylindrical microelectrodes has been known for some time, but an experimental verification is given here for the first time, using carbon fiber electrodes taken from a commercial carbon felt. Nonlinear least-squares fitting of the impedance spectrum of a redox couple at the reversible potential enables accurate evaluation of a composite parameter containing the standard rate constant and mean diffusivity, without measuring the fiber diameter, length or area. Introduction The use of fiber-based materials in electrochemistry has seen a resurgence recently, with the use of carbon felts as electrodes in redox flow batteries and electrospun polymers in emerging energy technologies. It is important to be able to characterize individual fibers and relate this information to the properties of the assembled material. At the time when microelectrodes were being developed, there was an interest in cylindrical microelectrodes [139-144], but they were harder to manipulate and there were indications that convection effects disrupted the desired radial diffusion [145, 146]. There are reports of fits of cyclic voltammetry and chronoamperometry data to numerical simulations involving cylindrical diffusion [140]. The mass transport impedance has an analytical solution, but has apparently not been applied to the case of long cylindrical microelectrodes, for which the radius is less than the diffusion length. We here study the Fe(CN) 3− / Fe(CN) 4− redox couple at individual carbon fibers, and show 66 that electrochemical impedance spectroscopy (EIS) enables accurate extraction of the standard rate constant. 41PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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