新穎液流式電池去離子技術於高效能脫鹽之研究：從電雙層電荷儲存到法拉第反應分離

Abstract

因應台灣水資源發展的挑戰與困境，具低能耗、高效率的新型電化學水處理技術成為開發多元化水源的途徑之一，可應用於廢水再生與海水淡化等領域。本研究首先選定三通道電化學系統，藉由多孔性碳電極與離子交換薄膜之排列，控制通道之間的離子移動，利用充電階段碳電極的電雙層電荷儲存與放電階段的透析分離機制，達到連續脫鹽與濃縮之成效。相比典型電容去離子系統，此三通道的結構不僅克服的半連續產水限制，其脫鹽量亦於選定之操作電壓範圍(1.0~2.5 V)中顯著提高至少3倍。然而，採用多孔碳電極的電容透析系統仍舊受限於充電與放電的兩段式操作。 為實現多通道電化學技術的連續操作，本研究建構具四通道的新穎液流式電池去離子系統，引入具法拉第反應特性的液流式電池材料電解液：鐵氰化鉀/亞鐵氰化鉀。在外部電場(0.2~0.8 V)的作用下，亞鐵氰化鉀於陽極流道氧化成鐵氰化鉀，並循環至陰極流道後再被還原成亞鐵氰化鉀，利用其連續的法拉第反應及多通道的系統結構實現液流式電池去離子系統的同步脫鹽與濃縮。於此基礎上，結合靜電紡絲碳纖維材料，並藉由調控氮氣環境下的碳化溫度(700~900℃)控制其材料特性。經900℃的碳化處理後，電紡碳纖維展示高石墨化(Id/Ig=0.92)、高石墨氮比(72%)以及低材料電阻(37 Ω)。證實高碳化溫度直接提升電紡碳纖維的材料導電性。並且在循環伏安曲線中呈現明顯的氧化還原峰與高反應電流，佐證電紡碳電極維持了鐵氰化鉀/亞鐵氰化鉀之間良好的電化學可逆性以及顯著提高其法拉第反應活性。因此，將碳化控制後的靜電紡絲碳纖維作為液流式電池去離子系統之電極材料，其高導電特性在提升法拉第反應活性以促進其電荷傳輸速率的同時有效降低系統整體電阻，大幅提升系統脫鹽效能與能源表現。在0.4 V的操作電壓下處理100 mM的氯化鈉溶液，碳纖維電極將液流式電池去離子的脫鹽速率提升1.4倍、達到97.5 μg/min/cm2，並在維持96%高電荷利用效率的前提下降低脫鹽能耗(0.012 kWh/mol)。進階選擇廢水再生與海水淡化等兩大標的進行實驗室規模之實際水樣脫鹽測試，不僅其脫鹽速率分別達148.3 μg/min/cm2與 81.6 μg/min/cm2，更只需以0.013與0.016 kWh/mol的能耗成功去除98%以上的水中離子，達到低能耗高效脫鹽的目標。 進一步透過生命週期評估法檢視液流式電池去離子技術的環境影響，結果顯示其標準化衝擊(1.61×102)遠低於典型薄膜電容去離子技術(4.97×103)，凸顯液流式電池去離子技術的發展前景。然而，該技術之衝擊則略高於液流式電容去離子技術(1.61×101)，主要可歸因於系統材料與電池材料對的選擇。因此，未來可選用低環境衝擊之材料以提升本技術之環境友善性，期可藉由本研究成果推動液流式電池去離子技術之發展，滿足零排放水處理技術之願景。

Water scarcity and energy crisis are expanding human-related conditions that should be included in risk assessment. To date, electro-driven ion separation technology is one of the key research topics for high-efficient desalination. This study aims at proposing electrochemical ion separation technology with use of advanced electrode materials in multi-chamber system for high-efficient desalination. A three-chamber membrane stack configuration based on an integrated capacitive-electrodialysis process (CapED) was firstly proposed for continuous and energy-efficient desalination. The three chambers between two capacitive electrodes were separated by a CEM and an AEM, which enabled simultaneous and continuous generation of desalinated and concentrated solutions. During charging, capacitive electrosorption and electrodialytic separation accounted for desalination. During discharging, the energy stored by capacitive electrosorption was utilized for desalination through dialytic separation. Therefore, continuous desalination could be achieved by a cyclic charging/discharging process. In addition, the salt removal amount was enhanced by increasing the voltage with respect to the different capacitive and electrodialytic contributions. However, the CapED system using capacitive electrodes was still limited by the two-stage operation of charging and discharging. To achieve one-stage continuous ion separation, a redox-flow battery desalination (FBD) was developed, utilizing the reversible redox reactions of the Fe(CN)63−/4− electrolyte. The electrospun carbon fiber (CF) with tailor-made properties was developed with a carbonization temperature of 900℃ for enhancing FBD system performance. With a carbonization temperature of 900℃, the CF (CF-900) showed a high degree of graphitization (Id/Ig = 0.92), highest graphitic-N content (72%) and low electrode resistance. The cyclic voltammetry curve of CF-900 was composed of evident redox peaks with a high current level, revealing good electrochemical reversibility and high electrochemical activity of the redox reactions. At 0.4 V, high average salt removal rate (ASRR) of 97.5 μg/min/cm2 was achieved while desalinating 10 mM NaCl with addition of CF-900 electrode, which was 1.4-fold higher than in the absence of the CF electrode. The addition of CF-900 greatly improved the charge efficiency (96%) with a relatively lower energy consumption of 0.0112 kWh/mol. Moreover, the application of high-conductivity CF-900 electrode demonstrated high ASRR (148.3 μg/min/cm2 for wastewater reclamation and 81.6 μg/min/cm2 for seawater desalination) with low energy consumption, making the strategy promising for achieving an efficient FBD process. In addition, a comprehensive life cycle assessment was conducted to evaluate the environmental impact of the FBD process. The assessment considered factors such as applied voltage and influent concentration, which were found to significantly influence the deionization performance. The findings highlighted the wide salinity working range and superior performance of FBD in comparison to membrane capacitive deionization (MCDI) and flow-electrode capacitive deionization (FCDI). Notably, the normalized impact assessment revealed that FBD (1.61×102) exhibited significantly lower environmental impact compared to MCDI (4.97×103), indicating promising prospects for the development of FBD. However, it should be noted that FBD's impact results were slightly higher than those of FCDI (1.61×101), primarily attributed to the choice of components and redox materials. Therefore, the materials with low environmental impact can be chosen for FBD to enhance the environmental friendliness of this technology. Overall, this research contributes significant insights into the technological and environmental aspects of FBD, promoting the development of sustainable and highly efficient desalination technology for environmental sustainability.