Energy Efficiency, Performance Analysis and Multiscale Modelling of Capacitive Deionization

Abstract

Capacitive deionization (CDI) is an emerging technology that utilizes the electrical field between two electrodes to remove the ions from the salty waters. The ions are stored in the electrical double layers (EDLs) forming at the interface between the electrode matrix and the aqueous solution. Until now, numerous effort has been devoted to studying the ion transport mechanisms and to fabricating high-performance electrodes. Yet, there lacks a fair evaluation framework and systematic understanding of the dependences among the operational variables. In this dissertation, we start with the thermodynamically reversible cycle to show the ideal operation of CDI, followed by comparing it with the CDI operation in reality to reveal that the energy efficiency of CDI is relatively low. We move on to understanding the energy losses during the charging and discharge steps of CDI. The quantification of the energetic-kinetic tradeoff for CDI demonstrates that the energetic efficiency cannot be divorced from the kinetic efficiency. We then employ the tradeoff curve to evaluate the two commonly used charging method, i.e. constant voltage and constant current, of CDI, and found out that which is better depends on the separation scenarios. In order to reduce the energy consumption, we decrease the impedance of electrode macropores by filling with conductive polyelectrolytes, with which the energy consumption saves up to half of that in membrane CDI. In the last chapter, we model the continuous operation of flow electrode CDI (FCDI) with an equivalent model that considering the flow electrodes as film electrodes rotating around the electrode chamber. The model successfully capture the phenomelogical characteristics of FCDI including effluent concentration, current density and cell voltages. In summary, this dissertation introduces the framework for CDI performance evaluation, and demonstrates the effective direction to improve the performance.