Solvation Dynamics of Choline Halide-Based Deep Eutectic Solvents

Abstract

The intermittent nature of renewable energy sources requires reliable and cheap long-term energy storage technologies. Redox flow batteries (RFBs) are reliable, scalable, and cost-efficient. Electrolytes are crucial for RFB's performance. Deep eutectic solvents (DESs) are a new class of ionic liquids with relatively cheap and easy synthesis. In addition, their properties are tunable, making DES an excellent candidate for many energy and industry applications. A DES mixture is created by combining hydrogen-bond acceptor (HBA) and donor (HBD) in a specified ratio, producing a mixture with a lower melting point than its constituent components. Varying the type and ratio of HBA results in tunning DES properties. Also, adding co-solvents, such as water, can provide further tunning to DES mixtures. This dissertation aims to study the potential of water addition, HBA choice, and the compositional variation to optimize the solvation dynamics and physicochemical properties of choline halide-based DESs. Besides the most famous Ethaline (choline chloride ChCl): ethylene glycol (EG), 1:2), we reported the first choline fluoride (ChF) and EG-based DES mixture preparation with 1:2 molar ratio. We studied physical properties upon water addition by conductivity, density, viscosity, and ET(30) polarity. NMR, FT-IR, and Raman spectroscopies were used to characterize their structures. Femtosecond transient-absorption spectroscopy (fs-TSA) and NMR-diffusometry were used to study solvation dynamics, providing a powerful tool for studying charge transport properties. Volumetric properties were used to study the effect of water in EthalineF/water and Ethaline/water mixtures. This work also investigated the eutectic enhancement of dynamics and charge transport in Ethaline. We provided excellent tools, such as using B30 as a probe molecule in time-resolved spectroscopy to study solvation dynamics and correlating DES-DES self-interactions to experimental conductivities. Also, proved that water (>1 wt.%) can accelerate charge transport and enhance physical properties. The vibrational spectroscopies indicate that EthalineF has stronger hydrogen bond interactions than Ethaline. In addition, we reported the effect of the solvodynamic radius on the observed lower conductivity in EthalineF compared to Ethaline. Furthermore, based on the NMR diffusion measurements, we assigned the two F-NMR peaks to be EG and choline associated fluorides.