(576g) Chemically Cross-Linked Poly(2-Hydroxyethyl Methacrylate)-Supported Deep Eutectic Solvent Gel Electrolytes

Nonvolatile, ion-dense electrolytes, such as ionic liquids (ILs) and deep eutectic solvents (DESs), are attractive candidates for safe, high performance electrochemical/electrostatic energy storage devices. Creating solid-state, low volatility gel polymer electrolytes offers an appealing way to further leverage ILs/DESs for future electrical energy storage device applications due to their leak-proof nature and robust flexibility. In this work, we describe the first-time incorporation of an inexpensive and eco-friendly DES (a 1:2 molar ratio mixture of choline chloride:ethylene glycol) into a solid-state gel electrolyte by employing UV-initiated free radical copolymerization of 2-hydroxyethyl methacrylate (HEMA) and poly(ethylene glycol) diacrylate (PEGDA) in situ within the DES to create a chemically cross-linked polymer scaffold. A relatively small amount of PEGDA (1:30 molar ratio with HEMA) is included to serve as a chemical cross-linker in order to create a covalently-bonded polymer network within the DES. A high room temperature ionic conductivity of 5.7 mS/cm has been obtained for a DES gel containing a 13.2 vol.% copolymer scaffold. Tunable DES gel compressive elastic modulus values, which could be varied between 14 kPa and 1.0 MPa, were obtained by varying the total polymer content. Clear evidence of hydrogen bonding interactions between the poly(HEMA) scaffold and the DES anion was revealed through species self-diffusivity measurements, as well as by FTIR spectroscopy. Using a 22.9 vol.% polymer-supported DES gel as electrolyte/separator in a supercapacitor prototype with activated carbon fabric electrodes, a specific capacitance of 33.3 F/g and an energy density of 15.8 Wh/kg (based on the mass of two electrodes) were obtained during discharge at a current density of 10 mA/g. This study supports the notion that chemically cross-linked polymer-supported DES gel electrolytes are suitable for energy storage applications, and opens the door to future investigations of how polymer-DES interactions can be exploited in order to maximize the performance of polymer-supported DES gel composites.