Understanding the Role of Electrode Thickness on Redox Flow Cell Performance

The electrode thickness is a critical design parameter to engineer high-performing flow cells by impacting the available surface area for reactions, current and potential distributions, and required pumping power. To date, redox flow cell assemblies employ repurposed off-the-shelf fibrous electrodes which feature a broad range of thicknesses. However, comprehensive guidelines to select the optimal electrode thickness for a given reactor architecture remain elusive. Here, we investigate the effect of the electrode thickness in the range of 200 - 1100 µm on the cell performance by stacking electrode layers in four different flow cell configurations – Freudenberg paper and ELAT cloth electrodes combined with flow-through and interdigitated flow fields. We employ a suite of polarization, electrochemical impedance spectroscopy and pressure drop measurements together with pore network modeling simulations to correlate the electrode thickness for various reactor designs to the electrochemical and hydraulic performance. We find that thicker electrodes (420 µm paper electrodes and 812 µm cloth electrodes) are beneficial in combination with flow-through flow fields, whereas when using interdigitated flow fields, thinner electrodes (210 µm paper electrodes and 406 µm cloth electrodes) result in a better current density and pressure drop trade-off. We hope our findings will aid researchers and technology practitioners in designing their electrochemical flow cells under convective operation.