Impedance Analysis of Three-Dimensional Continuous Carbon Filament Embroidered Electrodes in Large 300 cm² Redox Flow Cells

The paper investigates the use of three-dimensional (3D) continuous carbon filament electrodes prepared using tailored fiber placement (i.e. embroidered electrodes) in a 300 cm(2) redox flow cell with 50% state-of-charge (SOC) ferro/ferricyanide redox couple as the probe electrolyte. Electrochemical impedance spectroscopy (EIS) was conducted to identify the different resistance contributions, and thus voltage losses of the electrodes. The findings indicate that: (1) achievingh high frequency resistance values comparable to the felts is possible through side contacting of continuous filament electrodes to the graphite plates, eliminating the need to press the entire electrode structure. (2) The embroidered electrodes can minimize pressure drop, regardless of the electrode thickness, due to the parallel orientation of the carbon filaments to the electrolyte flow, resulting in reduced hydraulic resistance. (3) To reduce charge-transfer resistances, an oxidation treatment is required to improve the wettability of the electrodes, and the duration of the activation treatment must be optimized to avoid filament breakage due to etching. (4) Embroidered electrodes exhibit higher mass transfer coefficients thanfelts, which is attributed to the perpendicular orientation of the carbon filaments to the electrolyte flow. The paper provides avenues for further development of 3D carbon fiber electrodes.