開發高效能電容去離子之電極於提升脫鹽之研究

Development of high-performance capacitive deionization electrodes for enhancing desalination capacity

劉乃綾(Nei-Ling Liu)

指導教授 ： 侯嘉洪

國立臺灣大學/工學院/環境工程學研究所/博士(2020年)

https://doi.org/10.6342/NTU202004191

摘要

面對人口增加以及工業與農業之快速發展嚴重影響水循環以及各物質流的平衡下，發展一個對環境有善、低能源消耗和高效率之永續水資源處理技術，為全球水處理技術發展的一項目標。在許多脫鹽的技術中，電容去離子技術(Capacitive Deionization, CDI)為一項新興的水處理技術，其原理為施加一電場於兩電極板，使得水體中的帶電荷物質在電場的驅使下儲存於電極表面，進而移除水中的離子污染物。而在電容去離子的實驗中，為提升電極的平均脫鹽容量(Mean deionization capacity, MDC)，電極材料為影響脫鹽容量一項關鍵因子，如何有效提升脫鹽容量，為近年研究的主要項目。本研究為提升電容去離子技術的脫鹽容量，從商用的活性碳電極出發，藉由製備活性碳電極應用於CDI程序，並從過程中分析活性碳的表面及電化學特性，以了解電極材料對於脫鹽容量的影響，研究中選用AC1及AC2兩種活性碳進行比較，AC1具有較高的比表面及良好的孔洞分布，在連續式的去離子實驗中，脫鹽容量為5.08 mg/g較AC2的2.39 mg/g。在選擇商用活性碳進行的試驗中，我們可以得知比表面積越高且中孔洞及微孔洞分布平均的材料可有效的提升脫鹽容量。然而，再以活性碳進行電容去離子之應用後得知，常用的粉末狀碳材受到黏著劑(polymer binder)及材料表面的不規則結構，使得碳電極導電性下降，電荷傳輸不易以及離子無法有效快速的於奈米孔洞中進行傳輸等限制。以靜電紡絲技術配合碳化及活化程序，製備出片狀且具有階層孔洞性及高比表面積及良好導電性的活性碳纖維電極。其中以二氧化碳進行活化程序，並且在溫度900°C活化時間為90分鐘的ACF-900-90活性碳纖維具有最高比表面積1300 m2/g，並且中孔洞的比例佔總孔體積的39% (0.26 cm3/g)。將ACF-900-90活性碳纖維電極應用於電容去離子得程序中，使用電壓為1.6 V之薄膜電容去離子(Membrane capacitive deionization, MCDI)技術中進行實驗，脫鹽容量高達10.53 mg/g，相較於活性炭高出近一倍，並且在電化學穩定性的測試實驗中有良好的穩定性，靜電紡絲技術可調控纖維的粗細及纖維的分布，碳化及活化的程序可控制活性碳纖維表面奈米孔徑的分布。藉由靜電紡絲技術與碳化活化程序的結合所製備出之活性碳纖維電極，有效提升電容去離子之平均脫鹽容量。再者，為更近一步提升脫鹽容量，製備出擬電容材料(Pseudocapacitive material)─二氧化錳與碳纖維之複合電極，以碳纖維做為基材，將二氧化錳以化學沉積的方式，披覆於碳纖維表面。另一方面，藉由改變碳纖維的的導電性，同時探討不同導電程度之碳纖維電極在製備成複合式電極後對於二氧化錳擬電容之影響。實驗結果中顯示，以碳化溫度1000度之碳纖維做為基材所製備出之MnO2/CF-1000複合式電極在於MCDI系統進行實驗後，平均脫鹽容量進一步提升至28.4 mg/g，並且相較於其他基材之碳纖維，在能源使用上顯著較低，並且電流效率均高於85%以上。在本研究的成果中，成功藉由不同方式進而改善且有效提升電容去離子之脫鹽容量。

關鍵字

電容去離子 ； 階層孔洞材料 ； 擬電容材料 ； 脫鹽

並列摘要

The sustainability of freshwater supply is a pressing issue that affects the overall global community. Among various water desalination technologies, capacitive deionization (CDI) is an electric-field assisted desalination process that removes salt ions or ionic contaminants from aqueous solutions. To optimize the performance of CDI, selecting an appropriate material for CDI electrodes is crucial to fulfil a given series of requirements, particularly mean deionization capacity (MCD). The objective of this thesis is to relate the performance of CDI system to the type of electrode materials used. The research starts with the preparation of activated carbon electrodes, followed by the application of the designed electrodes into the CDI cell, and furthermore linking the surface and electrochemical characteristics to the desalination performance. Two types of commercial activated carbon, AC1 and AC2 were selected for comparison, which AC1 has higher specific surface and well balance of mesoporous and microporous structure. In the single-pass experiment, AC1 exhibited a higher salt adsorption capacity of 5.08 mg/g than that of AC2 (2.39 mg/g). We found that materials with higher specific surface area as well as evenly distributed mesopores and micropores give a higher salt adsorption capacity. Yet, the electrosorption capacity limitation is a bottleneck in CDI due to irregular pore structures and the use of polymer binder in porous carbon electrodes. By developing an integrated solution for both limitations, electrospun ACFs were designed with desired pore structures and without the use of polymer additives. The electrospun activated carbon fiber (e-ACF) prepared from polyacrylonitrile by electrospinning and subsequent oxidation, carbonization, and activation processes with N2/CO2 at 900°C for 90 min, had a high specific surface area of 1300 m2/g and a 39% increase in mesopores relative to total pore volume. As a result of batch-mode CDI experiments at 1.6 V, the electrospun ACFs had a promising electrosorption capacity of 10.53 mg/g and good cycling performance. Consequently, without the use of additives, the electrospun ACF electrodes possess good conductivity and favorable pore structures (e.g., high specific surface area, large pore volume, and enhanced mesoporosity) for electrosorption of ions. After clarifying the use of porous materials as CDI electrodes, next would be to further improve MDC by incorperating pseudocapacitive MnO2 to electrospun carbon fibers. First, the carbon fibers were prepared by sintering at different temperatures to obtain various conductivities. MnO2 was then coated to the carbon fibers by chemical deposition. The CF/MnO2 composites were tested to determine the impact of substrate conductivity to the pseudocapacitance of MnO2. The MDC of MnO2/CF-1000//e-ACF was found to be at 28.4 mg/g. With the modifications for CDI electrodes, the desalination capacity was successfully improved.

並列關鍵字

capacitive deionization ； hierarchical porous carbon material ； pseudocapacitive material ； desalination

參考文獻

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