Theory of water treatment by capacitive deionization with redox active porous electrodes

Abstract

Capacitive deionization (CDI) for water treatment, which relies on the capture of charged species to sustain the electrical double layers (EDLs) established within porous electrodes under an applied electrical potential, can be enhanced by the chemical attachment of fixed charged groups to the porous electrode electrodes (ECDI). It has recently been demonstrated that further improvements in capacity and energy storage can be gained by functionalization of the electrode surfaces with redox polymers in which the charge on the electrodes can be modulated through Faradaic reactions under different cell voltages in a capacitive process that can be called “Faradaic CDI” (FaCDI). Here, we extend recent mathematical models developed for the characterization of CDI and ECDI systems to incorporate the redox mediated contributions by allowing for the variable chemical charges generated by reactions in FaCDI. The lumped model developed here assumes the spacer channel is well-mixed with uniform electrosorption in each electrode. We demonstrate that the salt adsorption performance characterization of the fixed chemical charge ECDI and variable chemical charge FaCDI materials can be unified within a common theoretical framework based on the point of zero charge (PZC) of the electrode material. In the latter case the PZC is determined by the equilibrium potentials of the redox couples immobilized on the porous electrodes. The new model is able to predict the experimentally observed enhanced and inverted performance of CDI cells, and illuminates the benefit of choosing redox active materials for water treatment applications. The deionization performance of FaCDI cells is shown to be superior to that of CDI and ECDI systems with equilibrium adsorption capacities 50–100% higher than attained with CDI systems, and at smaller cell voltages, depending on the redox potentials of the Faradaic moieties.