Study on Interactions between Ions and Molecules in Water-in-salt Electrolytes and Development of Moisture Adsorption Energy Storage Devices

Abstract

Desalination technology is essential for securing future water resources. However, the process generates superconcentrated aqueous solutions as byproducts. The byproducts with very low dissolved oxygen levels are often discarded into the ocean, leading to dead ocean for marine life. Therefore, effort to utilizing the byproducts into high-value materials should be necessary for the future. In this sense, the concept of Water-in-Salt Electrolyte (WiSE) was introduced in 2015 as a superconcentrated aqueous solution, and significant efforts have been made to use it as an electrolyte in aqueous batteries. WiSE offers several advantages: it is safer from explosion, cost-effective, and has high ionic conductivity than organic solvents. Additionally, it provides a wider electrical stability window range and contains a large amount of ions, making it an advantageous electrolyte. However, the water in WiSE undergoes a phase change into ice crystals at sub-zero temperatures, losing its energy storage capabilities. Researcher also have been tried to utilize WiSE in Electrical Double Layer Capacitors (EDLCs), but there have been challenges in optimizing the energy storage performances due to lack of fundamental material chemistry for EDLCs. Therefore, in this thesis, we focused on the fundamental material chemistry required to use WiSE as an electrolyte of EDLCs. We identified the optimal operating voltage and concentration, demonstrating stable operation at low temperatures, and explored the sustainable energy potential by utilizing WiSE's ability to absorb moisture from the air. In the Part 2, potassium acetate (KOAc)-based WiSE was prepared in the concentration range from 1 m to 27 m. The superconcentrated aqueous electrolyte is still transparent and clear liquid but more viscous than dilute electrolytes. Concentration-dependent ion-molecular interaction in a bulk WiSE was systemically investigated using spectroscopic and electrochemical analysis. We found that most of water molecules are strongly confined to ions, leading to reduced activity. Furthermore, we found that KOAc-based WiSEs have concentration- dependent anti-freezing properties. Phase transition behavior was analyzed in the range of -120 ℃ to 80 ℃ using differential scanning calorimetry (DSC) and X-ray diffractometer (XRD). Interpretation was conducted based on competition between thermal energy of ions and molecules and bond energy of them. Finally, based on our analysis and understaning, phase diagram of KOAc WiSE was constructed in the temperature range from -120 ℃ and - In the Part 3, we applied WiSE as an electrolyte of EDLCs. First of all, electrolytes are soaked into a pores of activated carbon electrode. We found that phase transition behavior of WiSE is not significantly different between at bulk state and in pores of electrode, which guarantee anti-freezing properties of WiSE. And We fabricated electrical double layer capacitors (EDLCs) using various concentration of KOAc WiSEs. Ion adsorption and desorption behavior of ions at the surface of porous carbon electrodes is also interpreted through self-discharge profiles. Optimal cutoff voltage of KOAc WiSE-based EDLCs was determined using a various criterions including coulombic efficiency, energy density, CPE ideality factor, energy efficiency, and power density. Finally, energy storage property of WiSE-based EDLCs was evaluated in the temperature range from -20 ℃ to 80 ℃. And temperature cycling during a galvanostatic charge and discharge was also demonstrated. In the Part 4, poly acrylamide (PAAm)-based quasi-solid electrolytes cooperated with KOAc WiSE was synthesized and solid state EDLCs was fabricated. Solid state energy storage devices have attracted many interest due to their high volumetric energy density. However, low ionic conductivity and interface adhesion, which is an origin of high IR drop, limits their universal use. In this part, quasi-solid electrolytes are infiltrated into a pores of carbon electrodes to enhancing interface properties of EDLCs. Mechanically robustness of the fabricated EDLCs were analyzed against hammering. Stacked cell was fabricated by embedding three of single cell into a one hydrogel to enhancing volumetric energy density. Solid state EDLCs have also anti-freezing property, due to anti-freezing property of WiSE. Therefore, sub-zero temperature operation was also demonstrated upto -20 ℃. Furthermore, self-recovery property of the quasi-solid electrolyte are demonstrated. When EDLCs lose their energy storage properties by being dried in a higher temperature, hygroscopic KOAc adsorb water from the air and recover energy storage property their self. In our knowledge, demonstrating self-recovery properties of the EDLCs by adsorbing moisture from the air is the first time ever. In the Part 5, as a last part, we demonstrated moisture adsorbing alkaline Al-air battery. Lots of energy typically loses in an alkaline electrolyte system because of severe self-corrosion of Al anode. However, we found that reduced activity of water molecules in WiSE inhibited self-corrosion of aluminum in an alkaline electrolyte. Therefore, Al-air battery in an alkaline WiSE is stably operated. Futhermore, energy sources of metal-air battery are metal anode, oxygen and H2O. Among them, H2O is typically supplied from aqueous electrolytes. However, in this research, we demonstrated Al-air battery which collecting H2O from the air by adapting a hygroscopic property of dried KOAc WiSE. The hygroscopic Al-air battery with minimum amount of water is stably operated near 1.0 V for a long time. This research demonstrated a one possibility for utilizing the future sustainable energy sources from the air.