Understanding the Impact of Compression on the Active Area of Carbon Felt Electrodes for Redox Flow Batteries

The active material in the electrolyte of redox flow batteries needs to be transported without hindrance to the electrode surface to undergo reaction. This is usually ensured by utilizing high surface area carbon electrodes to create a surplus of available sites for the electrochemical reactions. Typically, the geometrical area or the specific surface area of the electrodes is used as a physical measure to relate performance data in experimental studies. Herein, we report on the differences between the physical surface area and the electrochemically active area, and further elucidate transport pathways to the active sites in real three-dimensional structures of carbon felt electrodes for aqueous redox flow batteries with organic active materials. We apply a combination of local X-ray tomography, electrochemical analysis, and computational fluid dynamic simulations. We reveal that the real accessible area for the conversion of the active material in carbon felt electrodes is determined by hot-spots (being isolated regions/zones where the highest flow velocity and, thus, enhanced mass transport and electrochemical conversion can be expected) caused by nonhomogeneous arrangement and locally compressed carbon fibers. By implication, the actually utilizable area of the electrode is always a unique measure and needs to be monitored during battery lifetime to compare the performance data for various cell setups and different active materials. In the end, a better comparison of the performance data will lead to an increase in efficiency and cycling stability for future redox flow batteries and their successful application in large-scale energy storage.