Unraveling charge transport in carbon flow-electrodes: Performance prediction for desalination applications

Carbon based flow-electrodes are a growing research field and find potential application in water treatment processes as well as energy conversion and storage. Flow-electrodes usually consist of a pumpable carbon slurry made of carbon particles suspended in a n electrolyte solution. One application for flow-electrodes is flow-electrode capacitive deionization (FCDI), a membrane-based, electrically-driven desalination method using mostly activated carbon as active material. The use of flow-electrodes in contrast to static electrodes enables a continuous operation and the treatment of high salinity solutions. However, the performance of FCDI processes heavily relies on the activated carbon quality. The process performance results from a wide range of parameters, including the activated carbon sample characteristics, which are usually not sufficiently covered and predicted by standard carbon analyses. With this article, we demonstrate the applicability of electrochemical impedance spectroscopy (EIS) as predictive characterization method for flow-electrode materials. This includes the investigation of influencing system parameters and carbon characteristics, and the development of an equivalent circuit model. The findings display a charge percolation threshold and allow insight into the charge transfer mechanisms in capacitive flow-electrodes. Finally, we demonstrate the possibility to predict and match the desalination performance of flow-electrodes based on different activated carbon types using EIS.