Roles of mass transfer and cell architecture in electrochemical desalination performance using polyglycerol activated carbon electrodes

Abstract

Capacitive deionization (CDI) has emerged as a viable alternative for brackish water desalination. Despite the remarkable advances in salt adsorption capacity (SAC) and charge efficiency (QE), improvements in cell architectures regarding the mass transfer aspects are still needed to boost electrode performance. In this work, we report a comprehensive study of flow-by (FBC), flow-through (FTC), and a percolation flow (PFC) cell architectures for brackish water desalination. It was observed that the electrode thickness was a crucial parameter influencing the desalination performance using the FBC, since the concentration gradient decreased towards the substrate interface, due to mass transfer limitations in the interstitial pores, so thin electrodes were recommended. Although the mass transfer and electrode thickness did not impose restrictions for the FTC, the short residence time limited the SAC, which also dramatically decreased after milling the PGAC particles, due to changes in the textural properties of the electrode, despite the fast desalination rates promoted by the mass transfer enhancement. Taking advantage of the beneficial aspects of the FBC and FTC designs, the desalination carried out using the PFC was enhanced by 170 %. With the flow direction perpendicular to the electric field, together with electrolyte percolation, the substantial improvement of the desalination rate provided by the PFC was also observed under more realistic conditions (single-pass and galvanostatic mode). Compared to batch potentiostatic operation, the PFC presented a remarkable desalination rate of 1661 mg g−1 day−1 at 1.0 mA cm−2 and 7.0 mL min−1. These findings enable a better understanding of the mass transfer aspects involved in CDI desalination, revealing the paramount importance of optimization not only of operational parameters, but also of the cell design.