Single fibre electrode measurements - a versatile strategy for assessing the non-uniform kinetics at carbon felt electrodes

Abstract

Redox flow batteries (RFBs) are a promising technology for the storage of intermittent renewable energy. Graphite and carbon felts electrodes are commonly used in RFBs as they typically exhibit good conductivity, high liquid permeability, high surface area and are relatively inexpensive. The electrochemical performance of carbon felts is commonly tested by calculating the energy efficiency for charge-discharge cycles of electrochemical cells containing the felt electrodes. This method whilst useful does not provide a direct measure of the kinetics of the redox reactions at the electrode surface. Furthermore, the standard electrochemical methods (e.g. cyclic voltammetry) commonly used to determine the electron transfer kinetics cannot be easily applied to carbon felt electrodes due to the complex mass transport behaviour within the porous felts. In this work we have shown that kinetics at carbon felts can be determined by performing cyclic voltammetry on single carbon fibre electrodes extracted from the carbon felt. This approach avoids the complex mass transport behaviour while also providing a method to determine the distribution of rate constants within a carbon felt electrode. By combining this measured rate constant distribution with the void size distribution of the carbon felt, the cyclic voltammetry of carbon felt electrodes can be successfully simulated over a wide range of sweep rates. Importantly, this approach highlights the necessity of accounting for the rate constant distribution within carbon felt electrodes.