ORIGINAL PAPER Temperature stable supercapacitors based on ionic liquid and mixed functionalized carbon nanomaterials R. S. Borges & H. Ribeiro & R. L. Lavall & G. G. Silva Received: 23 February 2012 / Revised: 20 May 2012 / Accepted: 23 May 2012 # Springer-Verlag 2012 Abstract The ionic liquid 1-butyl-2,3-dimethylimidazo- lium bis(trifluoromethylsulfonyl)imide (BDMIM-TFSI) showed a conductivity of 1.65 mS cm -1 and an electrochem- ical stability window of 4.4 V at room temperature. Two types of electrodes based on carbon nanomaterials were prepared: (1) with alternating layers of two oppositely charged functionalized double-walled carbon nanotubes (DWCNTs) and (2) with the functionalized DWCNTs and graphene oxide nanoplatelets. The electrodes presented a porous morphology and a connected pathway between the carbon nanotubes and graphene oxide platelets. Electro- chemical capacitors based on the carbon nanomaterials and BDMIM-TFSI were produced in a stacking configuration and were characterized at 25 °C, 60 °C, and 100 °C. The supercapacitors with electrodes based on the three alternating layers of two oppositely charged DWCNTs and graphene oxide presented higher values of capacitance, which were attributed to a morphology favorable to providing ionic access to the carbonaceous surface. Box-like voltammetric curves were used to calculate the capacitance in a 4-V potential window at 100 °C. Keywords 1-Butyl-2,3-dimethylimidazolium bis (trifluoromethylsulfonyl)imide . Double-walled carbon nanotubes . Graphene oxide nanoplatelets . Electrochemical capacitors Introduction The current electric double layer capacitors or supercapaci- tors can operate at very high charge–discharge rates and can have lifetimes of over a million cycles [1, 2]. However, some specific problems must be overcome to ensure broader applications of these devices. Some of the limitations are as follows: (1) lower energy storage than batteries [1–3], (2) failure to filter voltage ripple [4], and (3) instability in applications at temperatures of 60 °C and higher [5, 6]. The use of ionic liquids as an electrolyte can contribute to the safer use of supercapacitors for harvesting energy; for instance, when vehicles must be stopped, their operation involves temperatures of at least 60 °C. Ionic liquids of cyclic cations exhibit unique properties including inflammability, negligible vapor pressure, high thermal and chemical stability, and conductivity values higher than 1 mS cm -1 [7–11]. These ionic liquids have been used in many electrochemical devices such as lithium batteries [7, 12, 13], capacitors [3, 5, 7, 14], solar cells, fuel cells, and others [7]. The ionic liquid 1-butyl-2,3-dimethy- limidazolium bis(trifluoromethylsulfonyl)imide (BDMIM- TFSI) and other similar ionic liquids have been studied by groups interested in their basic physicochemical properties [8, 15, 16] and their applications in batteries [13] and super- capacitors [5]. Carbonaceous materials, and high surface area amor- phous carbon in particular, are widely used as electrodes in supercapacitors [6, 17, 18]. Ionic liquids have larger anions and cations than aqueous electrolytes, and the size of the ionic species should be paired with the porosity range of the carbonaceous at the electrodes to be able to take advantage of the double layer capacitance [19–21]. Carbon nanotubes (CNT) have been extensively investi- gated for applications in electrochemical capacitors [22–26]. R. S. Borges : H. Ribeiro : R. L. Lavall : G. G. Silva (*) Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, 31270-901, Belo Horizonte, Brazil e-mail: glaura@qui.ufmg.br J Solid State Electrochem DOI 10.1007/s10008-012-1785-5