Analysis of Influential Factors on Deionization Capacity and Rate in Capacitive Deionization

Abstract

Capacitive deionization (CDI) is an emerging desalination process to produce fresh water from saline water. Compared to conventional desalination processes such as thermal distillation and reverse osmosis (RO), CDI is more simple, energy-efficient, and environment-friendly. These advantages could make CDI as an alternative and/or supplementation to the conventional desalination processes. To make use of the advantages of CDI, a variety of studies have been carried out focusing on improving deionization performance. Among those, carbon electrodes play an important role to determine the performance

thus, many efforts have been made to analyze properties of carbon electrodes, develop a novel carbon material, and modify carbon electrodes. Although there have been considerable advances, systematic studies providing influential factors on deionization performance are insufficient. In this study, deionization performance was investigated especially focusing on capacity and rate to substantially extend previous understandings on CDI. Firstly, to understand deionization capacity, activated carbon materials of various surface areas were fabricated to composite electrodes and their electrochemical and deionization performances were examined. The results showed that a higher capacitance led to a higher capacity, where ~70% of charge capacity converted from capacitance was utilized for deionization. In the further study, a model equation to correlate deionization capacity with salt concentration and applied voltage was used to predict the deionization performance. The equation successfully modeled the experimental results obtained over salt concentration and applied voltage. In particular, when discharge voltage was increased with fixed charge voltage, charge efficiency was enhanced with marginal loss in the capacity, which suggests a possibility of an efficient energy-use by adjusting the operating condition. Secondly, a novel carbon material and analytical method were proposed to investigate deionization rate in CDI. A novel carbon material called metal organic framework (MOF)-derived carbon (MDC) with hierarchic pore structure was synthesized and electrochemical and deionization performances were examined compared to microporous and mesoporous carbons. The result confirmed that hierarchically porous carbon could show outstanding deionization rate mainly due to its unique pore structure. To develop a new method for evaluating rate capability of deionization, potential sweep method was utilized which applies voltage under different scan rates for charging a CDI cell. Deionization capacity was obtained over low to high scan rates and its retention ratio was used as a criterion for the rate capability

high retention ratio represents better rate capability. This method was applicable for various parameters such as electrode thickness, salt concentration, and flow rate. As a result, the thinner electrode, high salt concentration, and high flow rate showed better rate capability. Lastly, a novel concept to evaluate deionization performance was proposed, called the CDI Ragone plot. This plot was developed to show deionization capacity and rate over wide range of current load in constant current operation, thus maximum capacity and rate could be available. Moreover, it allows intuitive acquisition of deionization performance obtained in different parameters. In conclusion, this dissertation could provide an insight to understand deionization capacity and rate. Furthermore, a novel concept was firstly proposed, called the CDI Ragone plot, which could simultaneously demonstrate two important parameters in CDI, which are deionization capacity and rate. Therefore, it would be a comprehensive guide for deionization performance as a future standard in CDI.