Single-Layered Mesoporous Carbon Sandwiched Graphene\nNanosheets for High Performance Ionic Liquid Supercapacitors

Abstract

Ionic\nliquid based supercapacitors generally using nanoporous carbon\nas electrode materials hold promise for future energy storage devices\nwith improved energy density, but their power performances are limited\nby the high viscosity and relatively large size of ionic liquid electrolytes.\nUnderstanding the relationship between the pore size of nanoporous\ncarbon, the ionic liquid electrolyte diffusivity, and the energy/power\ndensity is critical for the development of ionic liquid based supercapacitors\nwith high performance. Herein, we report the synthesis of single-layered\nmesoporous carbon sandwiched graphene nanosheets (sMC@G) with mesopore-dominant\n(82% ∼89%) high surface area and tunable mesopore sizes (4.7,\n6.8, 9.4, 10.6, and 13.9 nm). When using 1-ethyl-3-methylimidazolium\ntetrafluoroborate (EMImBF₄) as the electrolyte with a cation\nsize of 0.76 nm, it is demonstrated that the ion diffusion coefficient\nincreases a little when the mesopore size is not larger than 6.8 nm,\nand then jumps dramatically in the range 6.8–10.6 nm. When\nthe pore size is enlarged to 13.9 nm, the ion diffusion coefficient\nincreases slightly, approaching the bulk diffusion coefficient of\nthe electrolyte. A size ratio of mesopore over electrolyte ion of\n14 is recommended for fast ion/electrolyte transport and therefore\nimproves the power density (14.7 kW kg^–1 at 20 A\ng^–1) without compromising the energy density (130\nWh kg^–1 at 1 A g^–1). The performance\nof sMC@G is superior to other porous carbon materials used in ionic\nliquid electrolyte supercapacitors.