Nanoconfinement of Ionic Liquid into Porous Carbon Electrodes

Planar electrochemical double-layer capacitors (supercapacitors) are strategic elements for the realization of miniaturized autonomous devices requiring energy storage and conversion capabilities. In particular, supercapacitors fabricated with nanoporous carbon electrodes and ionic liquids as electrolytes are very promising for a wide range of applications. The understanding and control of the interactions of the ionic liquid with the porous carbon interface is both practically and fundamentally interesting, because of the effects of surface confinement on the structural and functional properties of the ionic liquid. In particular, the role of the morphology in the ionic liquid confinement has attracted huge interest in many disciplines. Here we report direct experimental evidence of the solid-like structuring of confined 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([Bmim][NTf2]), which extends several tens of nanometers from the interface of nanoporous carbon thin films. These solid-like structures coexist with a huge amount of ionic liquid in its bulk phase. The presence of a solid-like phase occurring at the interface affects the double-layer organization of the ionic liquid at the electrified interface of nanoporous carbon based planar supercapacitors. Our results suggest the presence of the solid-like structured ionic liquid domains not only on the upper carbon thin film interface but also into the bulk of the nanoporous matrix.