Development of Manganese Dioxide/Activated Carbon Composite Electrodes for Enhancing Capacitive Deionization Performance

電容去離子技術(Capacitive deionization, CDI)為近年來備受關注的新穎水處理技術，其原理為藉由電吸附程序去除水中帶電離子，具有操作及維護簡便、無二次汙染、低能量消耗與可逆性等優點。CDI操作原理最早為提供電壓於電極上，利用高比表面積之孔洞碳材作為電極使電極表面形成電雙層(Electrical double-layer, EDL)，以電吸附的方式將離子儲存於電雙層中。然而現階段所製備之孔洞碳材電極受限於低電容值與低電吸附容量等問題而阻礙CDI技術之發展。二氧化錳因具有價格低廉、對環境友善性且擁有相當高的理論比電容值(1370 F g-1)，故常作為應用於超級電容器之電極材料。由於超級電容器與CDI工作原理相近，若能於施加電壓下透過二氧化錳與特定離子完成快速且可逆的法拉第氧化還原反應進行吸附，便能以擬電容(Pusudocapacitance)為基礎提升整體電容進而提升電吸附容量。 本研究目的為製備二氧化錳/活性碳複合電極材料，透過循環伏安之陽極沉積法在特定的電解質濃度以循環伏安法控制沉積圈數將最佳之二氧化錳沉積量覆蓋於活性碳表面。後將製備完成之複合電極材料進行材料表面分析及電化學特性分析。以掃描式電子顯微鏡暨能量散射光譜儀(Scanning electron microscope & Energy dispersive spectrometer)、X光電子光譜儀(X-ray photoelectron spectrometer)、X光繞射光譜儀(X-ray powder diffractometer)等進行材料表面分析﹔而以循環伏安法(Cyclic voltammetry)、定電流充放電(Glavanostatic charge/discharge)、交流電阻抗分析(Electrochemical impedance spectroscopy)進行電化學特性分析。 最後將製備之二氧化錳/活性碳複合電極材料置於CDI系統的陽極，利用於施加電壓下二氧化錳與鈉離子的嵌入反應進行CDI實驗。結果證實於1.0 V之施加電壓、0.01M的氯化鈉水溶液中，此二氧化錳/活性碳複合材料之吸附容量為9.3 mg g-1，較以傳統活性碳材料為電極提升近一倍(5.7 mg g-1)。由此可知，本研究結果證實將活性碳表面修飾，以結合電雙層電容與假性電容兩種方式，大幅提升了CDI實驗的脫鹽效能。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) is a promising water purification technology. Currently, most of CDI devices rely on the utilization of porous carbon electrodes with high specific surface area to electrostatically adsorb ions. In this case, the ions are stored at the electrode/electrolyte interface by electrical double layer (EDL) formation. However, the porous carbon electrodes based on EDL capacitance may suffer from the limitation of electrosorption capacity on targeted ions. Manganese dioxide (MnO2) is often preferred as an active material in supercapacitors for its low cost, non-toxic, and high theoretical capacitance. Most recently, significant efforts have been made on the development of MnO2-porous carbon composite as the electrodes of supercapacitors. Due to the similar working principle of CDI and supercapacitors, the fast and reversible redox-active of MnO2 can ensure the high capacitive behavior to intercalate the specific cations, as well as making the best use of the large pseudocapacitive charge to pursue higher adsorption capacity. Our main object is to develop the MnO2/activated carbon (AC) composite electrodes for enhancing the salt adsorption capacity of CDI cells through the redox-mediated active material of MnO2. Our study uses the anodic electrodeposition method to tune the thickness of MnO2, making the full utilization of MnO2 for the mix-faradaic reactions and retaining the sufficient conductivity and porosity of AC in CDI. The resultant MnO2/AC composite electrodes are characterized by surface morphology, electrochemical properties, and CDI performance. Material characterization (i.e., Scanning electron microscope, X-ray photonelectron spectrosmeter, and X-ray powder diffraction) confirms the presence of MnO2 coated on the AC surface. Cyclic voltammetry, galvanostatic charge/discharge curve, and electrochemical impedance spectra measurements are conducted to determine the specific capacitance and electrical conductivity of MnO2-AC electrode. A pair of batch-mode CDI cells is further performed at an applied voltage of 1.0 V for desalting 0.01M NaCl solution for capacitive desalination. The MnO2/AC is used as a cathode for the EDL formation and Na-intercalation of MnO2. According to the results, the salt adsorption capacity of MnO2/AC electrode is determined to be 9.3 mg g-1, which is about 1.6-fold higher than the AC electrode (5.7 mg g-1). Consequently, by taking the advantage of combination with fast faradaic reactions and double-layer charging, modification of porous AC materials with MnO2 is an emerging approach to achieve high CDI performance