In this article, we explore electrical double-layer capacitors (EDLCs) based on ionic liquids (IL), focusing on their stability and efficiencies (Coulombic and energetic). We analyze the conventional voltammetric methods to determine the electrochemical stability window, pointing out that a simple analysis of Coulombic efficiency, under different current and voltage conditions in galvanostatic experiments, has the advantage of being more representative for real cells, especially when studied together with energy efficiency. In addition, we study how IL-based EDLCs differ from aqueous and organic ones in terms of energy efficiency correlating with the physicochemical properties of electrolytes. Finally, we discuss the high resistance of this type of EDLC that has a detrimental effect on energy efficiency and cycling performance, even when the Coulombic efficiency is close to 100%.

The authors would like to thank the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq)/Brazil for ́ the financial support (Process No. 458487/2014-7). The authors would like to acknowledge the Instituto Nacional de Ciencia e Tecnologia em Nanomateriais de Carbono (INCT ̂ Nanocarbono)/CNPq. R.L.L. is recipient of fellowship from CNPq (Grant No. 313304/2017 - 3). M.C.G.S. thankfully acknowledge the scholarship received from the Coordenaca̧ õ de Aperfeicoamento de Pessoal de Nivel Superior (CAPES) - Brazil.