Impedance analysis of electrodes made of continuous carbon filaments in a 20 cm2 redox flow cell

The most commonly used electrodes in redox flow batteries are carbon fiber felts, which are assemblies of randomly oriented short fibers. The avenues to improve cell performance are usually pre-treatment to enhance their catalytic activity, modifications of their porosity, and reducing resistivity by increasing compression. Our investigations focus on carbon electrodes made of continuous carbon filaments, where filaments were arranged in specific geometric patterns in order to investigate the effect of the fiber density and spatial distribution across direction of flow on the cell performance. To identify the source of cell voltage losses, electrochemical evaluations were performed with electrochemical impedance spectroscopy (EIS), and compared with commonly used felt electrodes. The electrochemically reversible ferri/ferrocyanide system at 50% state-of-charge (SOC) was used as redox probe for both anolyte and catholyte to avoid varying electrode activities of anode/cathode due to differences in redox species. The findings generate electrode design considerations to reduce voltage losses in redox flow batteries.