製備二氧化錳/碳奈米纖維複合電極於薄膜電容去離子系統之研究

Electrodeposited MnO2 on Electrospun Carbon Nanofibers as an Electrode for Membrane Capacitive Deionization

黃靖婷(Jing-Ting Huang)

指導教授 ： 侯嘉洪

國立臺灣大學/工學院/環境工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201801829

摘要

因應全球氣候變遷，水資源危機增加以及能源供應不足的挑戰，脫鹽及再生水處理技術日益受到重視且蓬勃發展。薄膜電容去離子技術(Membrane capacitive deionization) 為一具有能源使用效率、環境友善性之脫鹽處理程序。而電極材料的使用為其脫鹽處理效率之關鍵因素，但迫於常見的高孔洞碳材之電吸附容量有所限制，因此，透過快速氧化還原反應增加吸附容量之擬電容(Pseudocapacitance)材料，近年來在電容去離子技術的相關研究中，為研究的發展重點。其中，二氧化錳(Manganese dioxide)因具有高理論電容值(1370 F g−1)、良好的環境友善性和穩定性等優勢，被視為是具有發展潛力的擬電容材料之一。然而，由於二氧化錳的導電性不佳，所以需要選用良好導電性的材料作為基材，藉此發揮其功效。
本研究利用靜電紡絲技術(Electrospinning)加上碳化/活化程序，製備出高導電性碳奈米纖維(Carbon nanofiber, CNF)以及高比表面積碳奈米纖維(Activated carbon nanofiber, ACF)，再利用電沉積程序將二氧化錳附著於碳奈米纖維表面上，製成二氧化錳−碳奈米纖維複合式電極，並進行材料的表面特性及物理特性分析，如:掃描式電子顯微鏡、穿透式電子顯微鏡、X光電子能譜儀、X光繞射光譜儀、接觸角量測、比表面機及孔洞特性分析儀以及熱重分析。之後進行電化學特性分析，使用循環伏安法及交流電阻抗分析，並以批次式的電容去離子實驗，評估其脫鹽效能，找出最有代表性之電極。
由實驗結果可得知，本研究可利用靜電紡絲的技術，配合碳化/活化程序，控制碳奈米纖維的比表面積與導電性。另外，利用電沉積方法，可使無晶型二氧化錳，以奈米片狀構造，成功地披覆於碳奈米纖維上，製備成MnO2/CNF及MnO2/ACF兩種複合電極材料，但是，披覆過程同時也造成材料比表面積的降低，以及導電性的減少。從電化學的結果得知，ACF具有最高的比表面積(769.6 m2 g−1)，在電雙層電容的電荷貯存機制下，其比電容值為59.96 F g−1。若將二氧化錳與高導電性的CNT結合，製備出擬電容的複合材料(MnO2/CNF)，其比表面積為15.7 m2 g−1，比電容值為52.16 F g−1。另外，若將二氧化錳與高表面積的ACF結合，製備出同時具備電雙層電容與擬電容的複合材料(MnO2/ACF)，比表面積仍有443.9 m2 g−1，且其比電容值可顯著增加到70.33 F g−1。
最後將製備的複合電極材料作為陰極置於薄膜電容去離子系統之中，對於10 mM的氯化鈉溶液進行脫鹽。在施加電壓為1.0 V時，ACF的吸鹽量為9.17 mg g−1，同時MnO2/CNF與MnO2/ACF分別可達到11.7及13.2 mg g−1。因此，研究結果顯示，將二氧化錳披覆於碳奈米纖維的表面，利用其法拉第反應的電荷貯存方式，可有效提升材料的吸鹽量。其中，又以同時具有高比表面積與氧化還原活化物質的MnO2/ACF複合材料為最佳，因為其可以同時利用兩種電荷貯存機制。本研究的研究成果，對於以靜電紡絲為基礎的碳電極製備方法，提供策略。

關鍵字

薄膜電容去離子 ； 二氧化錳 ； 靜電紡絲 ； 碳奈米纖維 ； 擬電容

並列摘要

Membrane capacitive deionization (MCDI) is a promising desalination technology with low energy input and high water recovery. Electrode materials play an important role for determination of the desalination performance. Generally, there are two fundamental mechanisms: (1) capacitive electrosorption using highly porous carbons, and (2) pseudocapacitive ion storage using redox materials (i.e., MnO2). Most recently, to achieve high salt adsorption capacity (SAC) of electrode materials, many efforts have been made on the development of pseudocapacitive materials, such as transition metal oxides and conductive polymers, for the applications of CDI and MCDI.
To investigate the dominant species between the electric double layer capacitor, pseudocapacitor and hybrid capacitor, the raw materials of binder-free electrodes (electrospun) were fabricated by electrospinning, and then were electrodeposited with MnO2. Carbonization and activation processes were further conducted to manufacture activated carbon nanofiber (ACF) with high specific surface area and carbon nanofiber (CNF) with high conductivity from electrospun. After manganese dioxide (MnO2) was electrodeposited on both materials, pseudocapacitive (MnO2/CNF) and hybrid (MnO2/ACF) electrodes were obtained. Material characterization (i.e., accelerated surface area and porosimetry system, scanning electron microscope, transmission electron microscope, X-ray photoelectron spectrometer, X-ray diffractometer, thermogravimetric analysis and contact angle meter) confirmed the presence of MnO2 electrodeposited on the substrates. Cyclic voltammetry and electrochemical impedance spectra measurements were conducted to determine the specific capacitance and electrical conductivity of electrodes.
According to the results obtained by cyclic voltammetry, the specific capacitance (at 5 mV s−1) of MnO2/ACF (70.33 F g−1) was higher than ACF (59.96 F g−1). On the other side, the specific capacitance (at 5 mV s−1) of MnO2/CNF (52.16 F g−1) was 10-fold higher than CNF (4.7 F g−1). To investigate the desalination performance, the fabricated electrode materials were tested in a batch-mode MCDI at 1.0 V for 10 mM NaCl. Here, the anode was MnO2/CNF or MnO2/ACF, while the cathode was ACF. As demonstrated, the SAC of MnO2/CNF and MnO2/ACF were 11.7 and 13.2 mg g−1, respectively, which were both higher than that of ACF (9.17 mg g−1). Therefore, the desalination performance can be enhanced by Faradaic ion storage via incorporation of redox-MnO2 with carbon electrode materials. Note that the MnO2/ACF presents the highest SAC among these materials. This reflects that the combination of capacitive electrosorption and pseudocapacitive ion storage is much preferred for MCDI applications. These results can provide a strategy to prepare high-performance capacitive electrodes based on electrospun carbon nanofibers.

並列關鍵字

Membrane capacitive deionization ； electrospinning ； carbon nanofiber ； manganese dioxide ； pseudocapacitance

參考文獻

Avraham, E., Noked, M., Bouhadana, Y., Soffer, A. and Aurbach, D. (2010) Limitations of charge efficiency in capacitive deionization processes III: The behavior of surface oxidized activated carbon electrodes. Electrochim. Acta 56(1), 441-447.

Bao, L., Zang, J. and Li, X. (2011) Flexible Zn2SnO4/MnO2 core/shell nanocable−carbon microfiber hybrid composites for high-performance supercapacitor electrodes. Nano Lett. 11(3), 1215-1220.

Cai, W., Xiong, Z., Hussain, T., Yang, J., Wang, Y. and Liu, J. (2016) Porous MnOX covered electrospun carbon nanofiber for capacitive deionization. J. Electrochem. Soc. 163(13), A2515-A2523.

Chen, B., Wang, Y., Chang, Z., Wang, X., Li, M., Liu, X., Zhang, L. and Wu, Y. (2016) Enhanced capacitive desalination of MnO2 by forming composite with multi-walled carbon nanotubes. RSC Adv. 6(8), 6730-6736.

Chigane, M. and Ishikawa, M. (2000) Manganese oxide thin film preparation by potentiostatic electrolyses and electrochromism. J. Electrochem. Soc. 147(6), 2246-2251.