Extreme Monovalent Ion Selectivity Via Capacitive Ion Exchange

Capacitive deionization (CDI) is an emerging technology applied to brackish water desalination and ion selective separations. A typical CDI cell consists of two microporous carbon electrodes, where ions are stored in charged micropore via electrosorption into electric double layers. For typical feed waters containing mixtures of several cations and anions, some of which are polluting, models are needed to guide cell design for a target separation, given the complex electrosorption dynamics of each species. An emerging application for CDI is brackish water treatment for direct agricultural use, for which it is often important to selectively electrosorb monovalent Na⁺ cations over divalent Ca²⁺ and Mg²⁺ cations. Recently, it was demonstrated that utilizing constant-voltage CDI cell charging with sulfonated cathodes and short charging times enabled monovalent-selective separations. Here, we utilize a one-dimensional transient CDI model for a flow-through electrode CDI cell to elucidate the mechanisms enabling such separations. We report the discovery that an asymmetric CDI cell with a chemically functionalized cathode induces electric charges in the pristine anode at 0 V cell voltage, which has important implications for monovalent cation selectivity. Leveraging our mechanistic understanding, with our model we uncover a novel operational regime we term "capacitive ion exchange", where the concentration of one ion species increases while competing species concentration decreases. This regime enables resin-less exchange of monovalent cations for divalent cations, with chemical-free electrical regeneration.