Development of Ionic Liquids Applicable as Draw Solutes for Forward Osmosis and Electrolyte Additives for Aqueous Zinc Batteries = 유도용질과 수계 아연전지 첨가제로의 적용이 가능한 이온성 액체의 개발

다국어 초록 (Multilingual Abstract)

This study focuses on the development of functional ionic liquids （ILs） applicable as draw solutes for forward osmosis （FO） and electrolyte additives for aqueous zinc ion batteries （AZIBs）. Water scarcity caused by climate change, industrialization, and population growth has intensified the demand for efficient water treatment technologies, while the rapid expansion of renewable energy systems has increased the need for advanced energy storage technologies. Although forward osmosis has attracted considerable attention as an energy efficient desalination process, the recovery of draw solutes remains a major challenge. Likewise, aqueous zinc ion batteries offer high safety and low cost but suffer from limited cycling stability due to dendrite formation and parasitic reactions at the Zn anode. For the FO application, a series of dodecyltrimethylammonium and octyltrimethylammonium based ionic liquids were synthesized, and the effects of anion structure on their physicochemical properties were systematically evaluated. Among the synthesized ILs, [N11112][OTf], [N11112][BF4], and [N1118][BF4] exhibited upper critical solution temperature （UCST） behavior at approximately 35, 45, and 39 oC, respectively, demonstrating reversible thermoresponsive phase transitions. These ILs achieved high recovery efficiencies ranging from 96.6% to 99.5% through temperature induced phase separation, enabling effective draw solute regeneration without additional separation processes. Thermal analyses further confirmed sufficient thermal stability of the selected ionic liquids under FO operating conditions. FO performance evaluation revealed that [N1118][BF4] exhibited the highest water flux, reaching 37.53 LMH at a concentration of 20 wt%. However, this IL also showed a relatively high reverse solute flux. In contrast, [N11112][OTf] exhibited a lower water flux but demonstrated an exceptionally low reverse solute flux of 6.43 g m-2 h-1 under AL-FS conditions at 20 wt%. Furthermore, [N11112][OTf] exhibited the lowest specific reverse solute flux （Js/Jw）, indicating minimal draw solute loss per unit volume of water produced. These characteristics provide significant advantages in terms of process economy and operational stability for practical FO applications. These results demonstrate that water flux alone is insufficient for evaluating draw solute performance and highlight the importance of specific reverse solute flux as a practical indicator of draw solute efficiency. In addition, stable water flux and UCST behavior were maintained throughout four consecutive recycling cycles, confirming excellent reusability. Antibacterial activity tests demonstrated that all synthesized ILs exhibited strong antibacterial performance against both Escherichia coli and Staphylococcus aureus. In particular, [N1118][BF4] achieved inhibition rates of 99.74% against E. coli and 100% against S. aureus. These results suggest that the quaternary ammonium framework can contribute to mitigating biofouling in FO systems. Furthermore, quaternary ammonium based ionic liquids were evaluated as electrolyte additives for aqueous zinc ion batteries. 1H NMR and Raman spectroscopy analyses revealed that the ionic liquid additives did not significantly alter the bulk structure of the ZnSO4 electrolyte but influenced the local electrolyte environment and hydrogen bonding distribution. Electrochemical evaluation using Zn||Zn symmetric cells demonstrated improved cycling stability and reduced voltage polarization in the presence of ionic liquid additives. Among the investigated additives, 0.1 mM [N1114][OTf] exhibited the most favorable electrochemical performance. Electrochemical impedance spectroscopy （EIS） and distribution of relaxation times （DRT） analysis revealed reduced charge transfer resistance and facilitated Zn2+ transport at the Zn/electrolyte interface, resulting in improved cycling stability and reduced voltage polarization. Post cycling characterization showed that the ionic liquid containing electrolyte produced a more compact and uniform Zn deposition morphology, as confirmed by scanning electron microscopy （SEM） analysis. In addition, X-ray diffraction （XRD） analysis revealed changes in the crystallographic growth behavior of Zn deposits without altering the fundamental crystal structure of metallic Zn. These findings suggest that quaternary ammonium based ionic liquids can effectively regulate the electrolyte environment and Zn deposition behavior, thereby enhancing the electrochemical stability of aqueous zinc ion batteries. Overall, this study demonstrates that rational molecular design of ionic liquids can simultaneously provide thermoresponsive recovery capability, antibacterial functionality, and enhanced electrochemical stability. In particular, [N11112][OTf] exhibited excellent performance as a draw solute owing to its low specific reverse solute flux and high recovery efficiency, while quaternary ammonium based ionic liquids showed considerable potential as electrolyte additives for improving the cycling performance and stability of aqueous zinc ion batteries. Therefore, this work presents a versatile design strategy for functional ionic liquids applicable to both water treatment and energy storage technologies, highlighting their potential as next-generation draw solutes for forward osmosis and electrolyte additives for aqueous zinc ion batteries.

목차 (Table of Contents)

• Part. 1 — Dual-functional ionic liquids exhibiting upper critical solution temperature behavior and antibacterial activity as draw solutes for forward osmosis
• Ⅰ. Introduction 2
• 1. Research background 2
• 1.1 Water scarcity and treatment challenges 2
• 1.2 Forward osmosis and draw solutes 3
• 1.3 Thermoresponsive ionic liquids 4
• 1.4 Biofouling in forward osmosis (FO) systems 5
• 2. Research objective 6
• Ⅱ. Experimental 9
• 1. Materials and characterization 9
• 2. Synthesis of ionic liquids 10
• 3. Forward osmosis setup and performance evaluation 13
• 4. Recycling test 14
• 5. Antibacterial activity 15
• Ⅲ. Results and discussion 17
• 1. Synthesis and structural characterization of ionic liquids 17
• 1.1 Synthesis of ionic liquids 17
• 1.2 Structural analysis by proton nuclear magnetic resonance spectroscopy (1H NMR) 18
• 1.3 Structural analysis by fluorine nuclear magnetic resonance spectroscopy (19F NMR) 27
• 1.4 Structural analysis by Fourier transform infrared spectroscopy (FT-IR) 29
• 2. Performance evaluation of ionic liquids 32
• 2.1 Thermal responsiveness and upper critical solution temperature (UCST) behavior 32
• 2.2 Recovery efficiency 39
• 3. Thermal stability and intermolecular interactions 42
• 3.1 Thermogravimetric analysis 42
• 3.2 Conductivity and ion dissociation behavior 48
• 4. Forward osmosis (FO) performance 56
• 4.1 Water flux performance 56
• 4.2 Reverse solute flux and specific solute flux 63
• 4.3 Recycling stability 68
• 5. Practical applicability of the synthesized ionic liquids 71
• 5.1 Antibacterial activity 71
• 5.2 Membrane surface analysis 75
• 5.3 Corrosion and compatibility evaluation 84
• Ⅳ. Conclusions 87
• References 90
• Part. 2 — Design and application of quaternary ammonium ionic liquids additives for aqueous zinc battery
• Ⅰ. Introduction 109
• 1. Research background 109
• 1.1 Energy storage systems and limitations of lithium ion batteries 109
• 1.2 Advantages of aqueous zinc ion battery 110
• 1.3 Challenges and interfacial instability of Zn anodes 111
• 1.4 Electrolyte engineering for stable Zn anodes 112
• 1.5 Ionic liquid-based electrolyte additives 113
• 2. Research objective 114
• Ⅱ. Experimental 118
• 1. Synthesis and characterization 118
• 1.1 Materials 118
• 1.2 Synthesis of ionic liquid additives 118
• 1.3 Structural characterization of ionic liquid additives 121
• 2. Electrolyte characterizations 121
• 2.1 Raman spectroscopy analysis 121
• 2.2 Proton nuclear resonance (1H NMR) spectroscopy 122
• 2.3 Contact angle of Zn electrolytes for Zn plates 122
• 3. Electrolyte measurements and post cycling characterizations 124
• 3.1 Assembly of ZnǀǀZn symmetric cells 124
• 3.2 Galvanostatic cycling test 124
• 3.3 Electrochemical impedance spectroscopy test 124
• 3.4 Scanning electron microscopy (SEM) 125
• 3.5 X-ray diffraction (XRD) 125
• Ⅲ. Results and discussion 126
• 1. Molecular design and structural characterization 126
• 1.1 Synthesis of ionic liquids 126
• 1.2 Structural analysis by proton nuclear magnetic resonance spectroscopy (1H NMR) 127
• 1.3 Structural analysis by Fourier transform infrared spectroscopy (FT-IR) 132
• 2. Influence of ionic liquid additives on electrolyte structure 135
• 2.1 Raman analysis of electrolyte environment 135
• 2.2 Chemical shift analysis by proton nuclear resonance (1H NMR) spectroscopy 138
• 2.3 Contact angle of Zn electrolytes for Zn plates 141
• 3. Electrochemical performance of ionic liquids additives 144
• 3.1 Cycling stability of ZnǀǀZn symmetric cells 144
• 3.2 Electrochemical impedance spectroscopy (EIS) ZnǀǀZn symmetric cell 148
• 4. Structural analysis of cycled Zn electrodes 152
• 4.1 X-ray diffraction (XRD) analysis of Zn electrodes 152
• 4.2 SEM analysis of cycled Zn electrodes 155
• Ⅳ. Conclusions 158
• Reference 161
• 국문초록 169
• 감사의 글 172