Study on the relationship between the electrochemical characteristics and desalination performance in capacitive deionization
Date Issued
2016
Date
2016
Author(s)
Chen, Jhih-Jie
Abstract
With climate change and growth of the population, water scarcity is being considered as a serious problem. In response, water desalination technologies can resolve this promising issue. Capacitive deionization (CDI), or referred to electrosorption, has drawn much attention and been developed as a novel electrochemical technology in recent years. Compared to other desalination technologies, CDI has the advantages of lower energy consumption, easy to operate and maintain, and no secondary pollutants. The objective of this study is to have better understanding of CDI performance and to improve the desalination efficiency. Firstly, the electrochemical experiments were carried out to explore the electrical double layer (EDL) capacitance and the electrosorption behaviors at different NaCl concentrations of the activated carbon electrode. Cyclic voltammetry (CV) experiments were performed with a potential window of 0.6 ~-0.4 V with different scan rates (5~1000 mV s-1) and concentrations (0.001 ~ 1 M), in which the EDL capacitance can be calculated from the cyclic voltammograms. In the galvanostatic charge and discharge tests, we used a constant current of 100 mA g-1 to assess the storage capacity of the activated carbon electrode. Electrochemical impedance spectroscopy (EIS) was conducted using the equivalent circuit model to investigate the inner resistance and the capacitive characteristics during the electrosorption process. The second part is to build adsorption-desorption model and to fit the experimental data in a single-pass CDI. The electrosorption performance can be predicted using the CDI model, which can be further utilized to optimize the operational parameters with respect to a higher desalination efficiency. As evidenced by the experimental results, increasing the NaCl concentrations can increase the EDL capacitance. Additionally, the IR drop derived from the galvanostatic charge and discharge tests decreases with an increase in NaCl solutions, suggesting the lower inner resistance for mass transfer. The Nyquest plots obtained by the EIS measurements can further demonstrate that the increase of NaCl concentrations can reduce the inner resistance, which is beneficial for the ion transport through the electrode and the EDL formation within the nanopores. Furthermore, the single-pass CDI experiments indicated that the initial concentration of NaCl solutions has a significant effect on the desalination performance. As the initial concentration increases, the larger electrosorption capacity and the higher charge efficiency can be obtained due to the higher concentration gradient and driving force, in which the electrical double layer is compressed. In a conclusion, through the design process by modeling work, the desalination performance of CDI can be further improved by adjusting the concentrations of salt solutions. This information is very important for the development of CDI technology.
Subjects
Capacitive deionization
cyclic voltammetry
Electrochemical impedance spectroscopy
electrical double layer
electrosorption
Type
thesis
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