Thermodynamic reversible cycles of electrochemical desalination with intercalation materials in symmetric and asymmetric configurations

Abstract

Conventional capacitive deionization (CDI) with carbon electrodes is a desalination process based on the formation of electrical double layer. Intercalation capacitive deionization (ICDI), a category of CDI based on intercalation materials as electrodes, achieves desalination by inserting ions into the crystal lattice of the electrode when a voltage is applied. It has been proven numerically that a thermodynamically reversible CDI cycle always consumes electrical work that equals the Gibbs free energy of the separation. We conducted a thermodynamic analysis of a four-stage reversible cycle for both symmetric and asymmetric ICDI using Frumkin isotherm to describe the electrode-solution chemical equilibrium. We provided both analytical and numerical proof showing the electrical work to complete a four-stage ICDI cycle is exactly identical to the Gibbs free energy of separation. Our thermodynamic analysis also shows ICDI is typically more energy efficient than CDI if constant voltage charging and discharge are performed to complete the same separation.