Multi-oxirane additives as property modifiers for DGEBA-based structural electrolytes

Structural supercapacitors require electrolytes that can efficiently transport ions while bearing mechanical loads, known as structural electrolytes (SEs). Here, we report the addition of the multi-oxirane epoxies, reactive epoxy additives (REAs), as a strategy to tune the microstructure of an established bicontinuous SE, formed by reaction induced phase separation (RIPS). Specifically, trimethylolpropane triglycidyl ether (TGETMP), 4-(diglycidylamino)phenyl glycidyl ether (GEDGP), tris(4-hydroxyphenyl)methane triglycidyl ether (TGETHP), and tetraglycidyl-4,4'-methylenedianiline (TGEMDA) were introduced into an SE based on a blend of bisphenol A diglycidyl ether (DGEBA) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMIM-TFSI), (DGEBA_IL40). Additive concentrations were varied between 1 and 50 wt% and the SEs were characterized using scanning electron microscopy, impedance spectroscopy, three-point bending tests and dynamic mechanical thermal analysis. At 2.5 wt% addition, all REAs enhanced ionic conductivity, but reduced Young's modulus, relative to the unmodified SE based on DGEBA_IL40. TGETMP showed the most promising performance by increasing ionic conductivity by 90% and reducing Young's modulus only by 20%. Small-angle neutron scattering of the curing process of DGEBA_IL40 with the addition of 2.5 wt% TGETMP showed a delay in phase separation kinetics and an increase in the cluster correlation length from about 30 Å to 63 Å compared to the unmodified system. Carbon aerogel-modified carbon fibre (CF-CAG) structural electrodes were introduced into the SEs, to explore whether the bicontinuous microstructure would be retained in a multifunctional device. The unmodified SE (DGEBA_IL40) showed epoxy sheathing on CF-CAG with bead-like morphology around fibres and strut-like morphology in the bulk, showing that the presence of CF-CAG influenced local curing of the SE at the electrode/electrolyte interface. Encouragingly, the addition of 2.5 wt% TGETMP to DGEBA_IL40 reduced both epoxy sheathing and bead formation around CF-CAG.