Electrodes Made of Continuous Carbon Filament Rovings: Effect of the Filament Arrangement on Cell Voltage Losses 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 the use of embroidered carbon fiber electrodes, where continuous filaments in the form of rovings can be arranged in specific geometric patterns. The advantage is that in addition to the usual options of pretreatment, porosity and compression, modifications on the orientation of the carbon fibers and fiber spatial distribution can also be performed in order to investigate the effect of the fiber arrangement on the cell performance. We report on such investigations in this paper, where the embroidered electrodes were modified according to number of carbon filaments, their orientation to electrolyte flow, and their spatial distribution across direction of flow. In order 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% SOC was used as redox probe for both anolyte and catholyte to avoid interference from varying electrode activities in response to differences between the catholyte and anolyte redox systems. The findings generate electrode design considerations to achieve more uniform current distributions and reduce voltage losses in redox flow batteries.