Carbon Capsules of Ionic Liquid for Enhanced Performance of Electrochemical Double-Layer Capacitors

Ion accessibility, large surface area, and complete wetting of a carbonaceous electrode by the electrolyte are crucial for high-performance electrochemical double-layer capacitors. Herein, we report a facile and scalable method to prepare electrode-electrolyte hybrid materials, where an ionic liquid (IL) electrolyte is encapsulated within a shell of reduced graphene oxide (rGO) nanosheets as the active electrode material (called rGO-IL capsules). These structures were templated using a Pickering emulsion consisting of a dispersed phase of 1-methyl-3-butylimidazolium hexafluorophosphate ([bmim][PF₆]) and a continuous water phase; graphene oxide nanosheets were used as the surfactant, and interfacial polymerization yielded polyurea that bound the nanosheets together to form the capsule shell. This method prevents the aggregation and restacking of GO nanosheets and allows wetting of the materials by IL. The chemical composition, thermal properties, morphology, and electrochemical behavior of these new hybrid architectures are fully characterized. Specific capacitances of 80 F g^-1 at 18 °C and 127 F g^-1 at 60 °C were achieved at a scan rate of 10 mV s^-1 for symmetric coin cells of rGO-IL capsules. These architected materials have higher capacitance at low temperature (18 °C) across many scan rates (10-500 mV s^-1) compared with analogous cells with the porous carbon YP-50. These results demonstrate a distinct and important methodology to enhance the performance of electrochemical double-layer capacitors by incorporating electrolyte and carbon material together during synthesis.