Performance relations in Capacitive Deionization systems

Capacitive Deionization (CDI) is a relatively new deionization technology based on the temporary storage of ions on an electrically charged surface. By directing a flow between two oppositely charged surfaces, negatively charged ions will adsorb onto the positively charged surface, and positively charged ions will adsorb onto the negatively charged surface. To optimize CDI design for various applications, performance relations in CDI systems have to be understood. CDI performance is determined by two factors, adsorption capacity and adsorption rate. The adsorption capacity is important for performance because only a limited amount of ions can be adsorbed onto an electrically charged surface; after the total adsorption capacity is reached the surface has to be discharged. The adsorption rate is important for performance because a higher adsorption rate results in faster removal of ions from a certain stream. The objective of this thesis is to relate the performance of a CDI unit to the specifications of the influent stream and the design aspects of the unit, such as the used electrode materials in the CDI unit. To obtain these relations, the focus in this thesis is on using electrochemical characterization techniques to obtain CDI performance in terms of charge transport, and furthermore linking this charge performance to desalination performance. By using this approach, we found that the total adsorption capacity of a CDI unit is determined by the double-layer area present in the used electrodes, where a higher double-layer area gives a higher adsorption capacity. The adsorption capacity of the double-layer area is in turn dependent on the applied potential and the chemical and physical properties of the treated water. This analysis can be used to screen for activated carbons with a high amount of double-layer area. We found that materials with a high amount of pores with a size around 1.6 nm have a high double-layer area. The adsorption rate of a CDI unit is mainly determined by the absolute conductivity of the influent stream. This relation can be used to optimize spacer and electrode thickness for various influent streams. The charge efficiency, i.e. the amount of ions adsorbed per amount of charge adsorbed, is limited by counter ion expulsion. It could be improved by placing ion-exchange membranes in front of the electrodes. By integrating all obtained relations in a mathematical model, deionization performance could be predicted for CDI systems with and without ion-exchange membranes. This model can be used to predict deionization performance for any operational condition, as well as to identify and resolve bottlenecks in the operation of the CDI system.