Development and investigation of ion selective materials for high performance capacitive deionization

Abstract

Capacitive deionization (CDI) is an emerging desalination process that produces fresh water from saline. CDI has been getting attentions due to its energy efficiency compared to the traditional desalination processes such as thermal distillation and reverse osmosis (RO). However, there is an intrinsic limitation in traditional CDI system. It is co-ion expulsion issue. When the electrical double layer formed on the electrode, some of charges are wasted, leading to the deionization performance reduction. To overcome these disadvantages, ion selective materials such as ion exchange membrane and battery materials have been introduced in CDI system. Recently, various materials have been vigorously developed for CDI system. However, the suitable ion selective material has been still required which would be expected to be highly performed, cost effective, and easily processible. Therefore, this study develops new ion selective materials to enhanced CDI performance. In addition, the effect of the ion selective materials on the CDI performances was also investigated. First, Ca-alginate was proposed as an alternative to cation exchange membrane (CEM) in CDI system for hardness control. Ca-alginate, a cost effective and eco-friendly polymer gel, is widely used in pharmaceutical, water treatment, and food-industries. Ca-alginate was successfully coated on the negative electrode using phase separation via sodium-calcium exchange. As a major result, the CDI with Ca-alginate coated electrode (CA-CDI) showed superior deionization performance (approximately 44%) than conventional CDI system. Moreover, MCDI system with Ca-alginate coated electrode had comparable deionization capacity and charge efficiency to conventional MCDI system with commercial cation and anion exchange membranes. Secondly, Ag coating on to the carbon electrode was proposed to enhance the deionization performances. The Ag coated carbon composite electrode was made by coating a small amount of Ag onto a carbon capacitive electrode, exhibiting the characteristics of a battery and a capacitor together. As major results, the CDI deionization capacity (88% more), rate (39% more), and charge efficiency (76%  92%) were dramatically enhanced due to the Ag coating. The significant improvements in deionization performances are explained by the enhanced specific capacity combining the capacitance in the carbon electrode with the Ag mediated charge transfer reaction. In addition, the hybrid CDI with Ag coated carbon composite electrode (73.3 kJ mole-1) is superior to membrane assisted CDI (136.7 kJ mole-1) in terms of energy consumption for deionization due to its low voltage feasible operation. Lastly, it was investigated how the characteristics of ion exchange membrane (IEM) affect the membrane CDI (MCDI) performance. As MCDI performances, deionization capacity, maximum average deionization rate (MADR), and charge efficiency were analyzed. As major results, only MADR was significantly affected by change of IEM characteristics. In addition, MADR showed good positive relationship with transport number divided by total electrical resistance. In conclusion, the research works of this dissertation indicate that the fascinated ion selective materials such as Ca-alginate and Ag coating significantly developed a superior deionization performance and showed great potential for further development in the CDI (MCDI) systems. In addition, the study revealing the relationship between t+/RCEM and MADR would be expected to provide a good insight to develop of highly fast rate-capable MCDI.