- 學位論文
以高中孔活性碳電極提升電容去離子技術除鹽性能之研究
Enhancement of Desalination Performance in Capacitive Deionization Using Highly Mesoporous Activated Carbon Electrodes
摘要
電容去離子技術(capacitive deionization, CDI)是應用奈米孔洞材料與電化學原理，並具有發展潛力的電吸附(electrosorption)除鹽技術。技術原理是對系統施加電場，使碳電極孔洞與溶液之間產生電雙層儲存陰、陽離子，達到移除離子之效果。其中活性碳(activated carbon)因具有較高比表面積、低成本與良好的電化學穩定性，故常應用於電容去離子技術。然而，碳電極中的微孔(micropore)結構在電吸附過程中，於孔洞結構中產生電雙層重疊效應(electrical double layer overlapping)，影響電吸附去除效率。本研究的目的為改善微孔結構形成的電雙層重疊影響，利用氫氧化鉀化學活化與二氧化碳物理活化之物理化學活化法製備活性碳，而此技術可藉由操作參數的不同，有效控制碳材孔洞結構特性發展。研究結果顯示，在碳化料與氫氧化鉀浸漬比1:1及二氧化碳氣化時間為兩小時(簡稱CK120)可製備出含有2105.1 m2/g比表面積與孔徑分布於3-5 nm中孔結構，且中孔比例達70.7%之高中孔活性碳(highly mesoporous activated carbon)材料。並進一步將CK120，與高比表面積的微孔碳材(氫氧化鉀浸漬比1:4及二氧化碳氣化時間為零小時，簡稱CK400)和商用活性碳進行電化學分析。結果顯示，於定電流充放電實驗中，CK120有良好的充放電特性，並且觀察到內電阻所導致的電壓降(IR drop)較不明顯。而在循環伏安法實驗分析結果，CK120受到掃瞄速度及濃度變化的影響較不顯著，並有良好的電容值。電吸附應用上，於施加電場為1.0 V，濃度為0.5 mM氯化鈉溶液中進行電容去離子實驗，CK120碳電極(2105.1 m2/g)之電吸附容量為9.72 mg/g-carbon，遠優於CK400碳電極材料(2162.4 m2/g)的4.08 mg/g-carbon。另外，將電吸附數據依擬一階模式結果分析，中孔碳材吸附速率為0.0603 min-1，而微孔碳材只有0.0299 min-1。因此，當增加活性碳材料中孔比例時，對碳材進行電吸附的吸附量、電吸附速率與材料的電容特性皆有提升之效果。
並列摘要
Capacitive deionization (CDI), which is an electrosorption process based on ion separation on highly nanoporous carbon electrodes, offers a promising opportunity to remove inorganic ions from aqueous solutions as a means of water purification and desalination. By applying an electric potential to nanoporous carbon electrodes, ions are electrostatically separated from water and develop an electrical double-layer (EDL) at the electrode-solution interface. Importantly, the utilization of nanoporous carbon materials is a key factor to determine the electrosorption performance. Among carbon materials, activated carbons are commonly used in the electrosorption process because of their high surface area, low cost and high electrochemical stability. In general, most of activated carbons are associated with high microporosity, causing electrical double layer overlapping and leading to poor pore accessibility. With this regard, it is necessary to increase the ratio of mesopores for improving the surface utilization. The objective of this study is to fabricate a highly mesoporous activated carbon electrode that has a high surface area (2105.1 m2/g) and a large proportion of mesopores (70.7%) by physiochemical activation. The prepared activated carbon electrodes are characterized by capacitive measurements, including cyclic voltammetry and galvanostatic charge/discharge curves. The desalting performance of the carbon electrodes is tested by a batch-mode desalination experiment in terms of electrosorption capacity, kinetics, and effective surface area, and compared to those of commercial activated carbons. The results demonstrate the cyclic voltammetry can distinguish between different types of activated carbon materials and their underlying capacitive characteristics. Highly mesoporous activated carbon has less dependence on scan rate and concentration, reflecting better rate capacity for ion electrosorption. From the desalination experiments of 5mM NaCl at 1.0 V, the highly mesoporous activated carbon electrode exhibited a larger ion removal capacity (9.72 mg/g-carbon), higher electrosorption rate (0.0603 min-1) as compared to that of the microporous activated carbon electrode. Overall, mesoporous structure has been proven to have great electrosorption capacity and effective surface utilization in the CDI process.