Chemically Cross-Linked Poly(2-hydroxyethyl methacrylate)-Supported Deep Eutectic Solvent Gel Electrolytes for Eco-Friendly Supercapacitors

Nonvolatile, ion-dense electrolytes, such as ionic liquids and deep eutectic solvents (DESs) are attractive candidates for safe, high-performance electrochemical/electrostatic energy storage devices. In this report, an inexpensive and eco-friendly DES (1:2 mixture of choline chloride/ethylene glycol) has been incorporated into a solid-state gel electrolyte using a chemically cross-linked polymer scaffold formed through insitu UV copolymerization of 2-hydroxyethyl methacrylate (HEMA) and poly(ethylene glycol) diacrylate. A high room-temperature ionic conductivity of 5.7 mScm(-1) has been obtained for a DES gel containing 13.2 vol% polymer scaffold. Tunable DES gel compressive elastic modulus values, between 14 kPa and 1.0 MPa, could be obtained by varying the 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 22.9vol% polymer-supported DES gel as electrolyte/separator in a supercapacitor prototype with activated carbon fabric electrodes, a specific capacitance of 33.3 Fg(-1) and an energy density of 15.8 Whkg(-1) (based on the mass of two electrodes) were obtained during discharge at a current density of 0.01 Ag-1. This work supports the notion that chemically cross-linked polymer-supported DES gel electrolytes are suitable for solid-state, flexible supercapacitor applications.