Tuning the Cation/Anion Adsorption Balance with a Multi-Electrode Capacitive-Deionization Process

Capacitive deionization (CDI) is an emerging technique for purifying water by removing ions. Recent experimental studies have reported that the anion/cation adsorption can be naturally imbalanced, even for a solution with just sodium and chloride, and suggested a link between imbalance and Faradaic leakages. However, these effects have been missing from conventional models. In this work, we developed a new circuit model to better understand the connection between Faradaic leakages and adsorption imbalance. The theory demonstrates that the effect emerges in a model that includes leakages, considers leakages on both electrodes separately, and considers different leakage resistance on the two electrodes. Having the model, it is possible to analyze and quantify the influence of the leakage resistance and other material properties on the adsorption imbalance. Leveraging these results, we further present a multi-electrode (ME) device design. The setup adds a third electrode to the spacer channel and can tune or eliminate the adsorption imbalance based on appropriately distributing the voltage across the electrodes. In conclusion, we describe a charge leakage mechanism responsible for the imbalance of ion adsorption and a flexible device design to tune the anion/cation removal.(c) 2023 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/ by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.

Automated summary

Capacitive deionization (CDI) is a promising technique for purifying water by eliminating ions. Recent experiments have shown a natural imbalance in anion/cation adsorption even for solutions containing only sodium and chloride, and suggested a connection with Faradaic leakages. This study presents a new circuit model that considers leakages, separately accounts for leakages on both electrodes, and incorporates different leakage resistances. The model allows analysis and quantification of the influence of leakage resistance and other material properties on adsorption imbalance. Based on these findings, a multi-electrode (ME) device design is proposed to tune or eliminate the adsorption imbalance by distributing voltage across the electrodes appropriately.