以離子交換樹脂塗層/活性碳電極提升薄膜電容去離子技術脫鹽效能之研究

Abstract

電容去離子技術(capacitive deionization, CDI)，其原理係藉由施加外部電場，以電雙層原理將離子電吸附儲存於多孔電極材料表面，達到移除水溶液中離子之目的。若進一步於電極表面放置陰陽離子交換樹脂薄膜(ion-exchange membrane, IEM)，藉由IEM的電性選擇特性，阻擋同離子(co-ion)並允許反離子(counter-ion)穿越薄膜，於電吸附程序中減緩同離子效應的影響，進而提升CDI的充電效率(charge efficiency)與脫鹽能力，則為薄膜電容去離子技術(membrane capacitive deionization, MCDI)。然而，在MCDI技術中，電極與IEM間所存在之空隙和厚度將增加整體系統的阻抗，使得能量損耗增加，故有研究改善之必要性。本研究之目的在於降低電極與IEM間之阻抗，故嘗試將陰陽離子交換樹脂(ion-exchange resin, IER)緊密地直接塗佈於活性碳電極表面，形成離子交換樹脂塗層，以簡單快速的方法合成非均質離子交換膜/活性碳電極(RMCDI)，並以電化學分析、親水性測試與物化表面分析，探討RMCDI之特性。研究結果顯示，離子交換樹脂塗層可降低電極與IEM間之阻抗，並改善增進電極表面的親水性質，且在物化表面分析結果中可知，在薄膜製成工序中不會破壞官能基。且於低掃描速率的循環伏安法實驗中証實，電極的電容特性不會受到塗佈離子交換樹脂層的影響。此外，將RMCDI應用於電吸附程序上，於濃度5 mM NaCl溶液中施加1.2 V之電場進行電容去離子實驗，証實活性碳電極在塗佈離子交換樹脂塗層後，可提升40%之電吸附量(6.21 mg/g-carbon)，且有良好的充電效率(71.4%)及低能源消耗(0.043 kWh/mole)。再者，將RMCDI於連續5次的電吸脫附程序中，其電吸附容量介於6.23至5.91 mg/g-carbon間，顯示能有良好的可逆性與電極再生性。綜上所述，本研究將IER直接塗佈於活性碳電極表面形成離子交換樹脂塗層，可降低電極與IEM間之阻抗，同時保有IEM的性質，提升整體系統脫鹽效能。

Membrane capacitive deionization (MCDI) is an alternative desalination technology, which combined with the ion-exchange membrane (IEM) in front of electrodes. Based on the principle of capacitive deionization (CDI), the ions are removed by applying electric field and ions, which stored in the surface of porous carbon electrodes from aqueous solution. During the process of electrosorption, the IEM plays an important role to exclude the co-ion effect due to the characteristic of permselective. A membrane that completely blocks transport of co-ions while allowing transport of counter-ions simultaneously. However, the IEM is expensive, requires strong physical pressure between membrane and electrodes. In order to solve the problems of contact resistance and thickness of electrodes, the heterogeneous membrane/activated carbon electrode (RMCDI) was developed by adhering the powder of ion-exchange resin (IER) on the surface of electrodes directly. The electrodes were characterized by electrochemical analysis, contact angle and surface structure analysis. The results show IER layer can not only reduce the resistance between electrode and IEM but also improve the hydrophilic properties. In addition, the results also show the functional groups of IER will not be destroyed during product process. For the cyclic voltammetry, the specific capacitance still keeps up after coating IER layer. From the electrosorption experiments of 5 mM NaCl at applied potential of 1.2 V over 30 min period in RMCDI system, the electrosorption capacity (6.21 mg/g-carbon) is 40 % higher than the un-coating electrode and also have good performance of charge efficiency (71.4%) and lower energy consumption (0.0434 kWh/mole). Furthermore, a cyclic test was performance for continuous operation of RMCDI including electrosorption and desorption process for 5 cycles. The electrosorption capacity was measured between 5.91 and 6.23 mg/g-carbon over the repeat operations. This implies that the RMCDI system would have stable performance and good generation of electrodes over the repeated operation