以石墨氈電極提高氧化還原液流式電池於高效率鹵水濃縮之研析

Abstract

逆滲透濃縮液(Reverse osmosis rejects, ROR)為高鹽度的鹵水，若未經妥善處理而直接排放到環境中，將造成環境危害。為降低其對環境的衝擊，可透過高水回收率的濃縮程序大幅減少鹵水體積，然而，鹵水濃縮往往需要耗費大量能源，因此以低能源消耗進行濃縮程序為濃縮技術目標。氧化還原液流式電池脫鹽技術(redox-flow battery desalination, RFB)為近年新興的電化學離子分離技術，利用電池材料對：鐵氰化鉀與亞鐵氰化鉀作為電解液，通過施加低電壓促使電池材料對之間的氧化還原反應，進一步驅使淡室中離子遷移並集中至濃室，得以實現以低能耗濃縮鹵水的目標。本研究目的為探討RFB技術於高水回收率鹵水濃縮之可行性，並通過使用經熱處理之石墨氈(graphite felt, GF)電極提升RFB濃縮表現。研究結果顯示，於RFB系統中使用經400℃熱處理2小時的GF電極可顯著提高離子分離速率，相比於未使用GF電極時之RFB系統，其離子分離速率可顯著提升至1.7倍。進一步測試系統水回收率(50%~90%)之影響，發現離子分離速率並未因水回收率提高而減少，且能源消耗量在不同的水回收率條件下始終維持相同水平，顯示RFB在高水回收率的鹵水濃縮應用上具有潛在的優勢。為驗證RFB在實際工業ROR濃縮的可行性，以桃園市某工業廢水處理廠之ROR為例，以90%水回收率進行RFB操作，結果顯示在5小時的濃縮過程中，鹵水導電度從8.5 mS/cm增加至約58.7 mS/cm，僅需39.8 kJ/mol的低能耗便可達到約6.9倍的濃縮。結論而言，以GF電極提升濃縮性能的RFB系統，於低能耗鹵水濃縮技術中具有高發展潛力，期望此研究結果能為RFB技術應用提供新的應用方向，並推動低能耗鹵水濃縮技術的進一步發展。

In the present work, a redox-flow battery (RFB) with a four-chamber architecture was used as a promising electrochemical ion separation process with high water recovery for brine concentration. Herein, the thermally treated graphite felt (GF) electrodes was used to promote the redox reactions of the ferri/ferrocyanide electrolyte in the energy-efficient RFB system. The GF electrodes were pretreated at 400℃ in air for 2 hours (GF400-2), leading to notable improvements in electrochemical performance. Specifically, GF400-2 exhibited a low charge-transfer resistance of 1.37 Ω in electrochemical impedance spectroscopy analysis, and a 4.9-fold increase in peak current observed in cyclic voltammetry analysis. When treating a 4.5 g/L NaCl solution at 0.4 V, the RFB with GF400-2 demonstrated 1.7-fold higher average salt concentration rate (ASCR) than that of the RFB without GF electrodes. Moreover, increasing the water recovery from 50% to 90% had minimal effects on the energy consumption for brine concentrations in the RFB system. At 90% water recovery, when treating NaCl solutions with salinities ranging from 1.5 to 14.0 g/L at 0.4 V, a relatively higher ASCR and charge efficiency were observed at 4.5 g/L initial salt concentration. These features highlight the potential application for high water recovery in concentrating the real industrial reverse osmosis rejects (ROR) with an ionic conductivity of 8.5 mS/cm. As demonstrated, the RFB with GF400-2 efficiently concentrated the ROR by a factor of 6.9, achieving a high ASCR of 138.2 µg/min/cm2, while exhibiting a low energy consumption of 0.71 kWh/m3. These findings offer valuable perspectives on brine management with an energy-efficient, high-water recovery RFB, establishing its significance in this domain.