Study on the relationship between the electrochemical characteristics and desalination performance in capacitive deionization

隨著氣候變遷及世界人口成長，水資源匱乏成為永續發展所面臨的關鍵問題。水淡化技術被視為解決問題的策略之一，藉由去除水體中帶電荷物質，進而得到可利用之淡水資源。電容去離子技術(Capacity deionization, CDI)為新穎的電化學脫鹽程序，相較於其他的除鹽技術所需能源較低；可於低壓力與低電壓下操作；無二次性污染物等優勢，被評估為最具潛力的水淡化技術。本研究分成兩個部分：第一部分運用電化學方法探討活性碳電極的在不同濃度下之電容行為，包含循環伏安法(Cyclic voltammetry, CV)在0.6V至-0.4V的區間內更替不同的掃瞄速度及不同濃度溶液觀測電容變化；充放電實驗(Galvanostatic charge and discharge)利用定電流評估碳電極在不同濃度之電容及電阻；電化學阻抗分析實驗(Electrochemical impedance spectroscopy, EIS)對吸附過程進行等效電路分析。第二個部分經由CDI單元進行連續式電吸附實驗，藉由電吸附模式結合電化學實驗結果探討電吸附行為，評估電容去離子技術之關鍵性因素。由實驗結果可知，NaCl濃度對於活性碳電極之電化學特性包括電阻、電容值等有顯著的影響，進而影響CDI在脫鹽過程中之電吸附能力。由循環伏安法實驗中5 mV s-1掃描速度下，比電容值從1 M的69.4 F g-1隨著濃度下降到0.01 M的14.3 F g-1，由定電流充放電實驗也可觀察到隨著濃度下降充放電時間從1 M的1,641秒下降到0.005 M 的234秒，顯示濃度對充電能力的影響。另外，從脫鹽實驗以及模擬分析結果可知，電吸附之電流效率亦會隨著NaCl溶液濃度增加而提升，從0.002 M之電流效率值54.61 %上升至0.01 M的73.24 %。整體而言，在電吸附過程中，其吸附能力與使用活性碳電極材料之電化學特性極具關聯性。在較高濃度時，雖能源消耗量增加，但因具有較好的電雙層電容行為、較高的電流效率及較低傳輸阻抗，固有較佳之吸附效果。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