不同型態之錳氧化物複合電極之製備及其應用於 電容去離子之研究

Abstract

電容去離子技術對半鹹水去除的能源成本較低且回收率高,模組結構簡單且可在常溫常壓下操作,只需將電場移除或倒極即可再生。常用的電容電極材料如錳氧化物,其電容機制主要為擬電容,當其本身具有奈米孔洞時也可能存在電雙層電容機制,使得錳氧化物更具有探討性。 本研究以水熱法合成不同奈米結構(包含針狀及柱狀)之 α-二氧化錳粉末,與碳黑混合製成複合電極,應用於平板式電容中之去離子效能。探討電容去離子系統中不同操作參數(如電壓及電流密度)對 α-二氧化錳複合電極之離子移除效能評估及不同奈米結構 α-二氧化錳之複合電極對不同價數及水合半徑離子的吸附能力。 研究結果顯示,相對於 α-MnO2 柱狀電極,α-MnO2 針狀電極具較大之比表面積、孔洞體積及較高之粗糙度;於高濃度的氯化鈉及氯化鉀之電解液中,α-MnO2 柱狀電極具較高之電流密度,其原因為柱狀的比表面積能使離子接觸的部份較多;使用含量 75%之 α-MnO2 複合電極,在接近 2000 mg/L 的氯化鉀水溶液中,以對稱電極的系統設置,相較於定電流的操作模式,定電壓模式可以獲得較佳之電吸附量,電荷效率較高且可避免副反應之產生,α-MnO2 柱狀複合電極(40 mg/g)的電吸附量高於 α-MnO2 針狀複合電極(33.6 mg/g)。α-MnO2 柱狀複合電極進行不同離子的電吸附效能評估,以水合半徑較小的鉀離子有最高的電吸附量。

Capacitive deionization (CDI) is an emerging water treatment process with low energy consumption, high recovery rate, easy assemble, and can be operated under normal temperature and pressure, that shows its application on desalination of seawater or industrial wastewater. MnO2, a pseudo-capacitive material is one of the promising electrode material for CDI applications because of its high capacity, low cost and environment friendly. In this study, nanorod and nanoneedle α-MnO2 were synthesized by hydrothermal method. Afterward, α-MnO2 was assembled with carbon black (CB) to fabricate α-MnO2 nanorod and α-MnO2 nanoneedle composite electrodes. Their desalination performance and ionic selectivity with different ionic valence states and hydrated radius were be investigated.The optimization of CDI operation by adjustment of applied current, cell voltage of test solution was also conducted.The results show that α-MnO2 nanoneedle has relatively larger surface area, porous volume, and higher roughness than α-MnO2 nanorod. In comparison to α-MnO2 nanoneedle, α-MnO2 nanorod exhibits a higher current density, but the specific capacitance of α-MnO2 nanorod and nanoneedle decrease when the solution concentration nears 2000 mg/L. The results also show that, with different cell voltages and current densities, the maximum electrosorption capacity occurres in constant voltage mode. The α-MnO2 nanorod composite electrodes (40 mg/g as KCl) shows higher electrosorptive capacity than α-MnO2 nanoneedle composite electrodes (33.6 mg/g as KCl) in 30 minutes reaction, indicating its applicable for CDI because of its better accessibility and excellent electrochemical properties for ion transportation in aqueous solution. And, the α-MnO2 nanorod composite electrodes precesses higher electrosorptive capacity in potassium chloride solution, because of its the highest mobility and the smallest hydrated radius comparing with other cations.