ORIGINAL PAPER On the interaction of carbon electrodes and non conventional electrolytes in high-voltage electrochemical capacitors Gelines Moreno-Fernández 1 & Christoph Schütter 2,3,4 & José M. Rojo 1 & Stefano Passerini 4 & Andrea Balducci 2,3,4 & Teresa A. Centeno 5 Received: 17 August 2017 /Revised: 12 October 2017 /Accepted: 13 October 2017 # Springer-Verlag GmbH Germany 2017 Abstract This study is essentially based on innovative electro- lytes such as the organic salt N-methyl-N-butylpyrrolidinium tetrafluoroborate (Pyr 14 BF 4 ) dissolved in propylene carbonate (PC) and the pure ionic liquid (N-butyl-N-methyl- pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr 14 TFSI) and its solution in PC. Activated carbon cloths were used as self-standing binder-free electrodes. It is found that the presence of impurities in carbon electrodes may lead to electrolyte de- composition and electrode degradation which notably affect the electrochemical double-layer capacitor (EDLC) performance. Such processes greatly depend on the composition of both the electrode and the electrolyte, being much less significant with solvent-containing electrolytes. By raising the operation tem- perature to 60 °C, the EDLC performance in the ionic liquid Pyr 14 TFSI is notably improved due to a relevant decrease in the viscosity and increase in ionic conductivity. By contrast, the presence of impurities, e.g., Zn and Al, in the electrodes re- markably reduces the electrolyte stability and a thick layer of decomposition products completely covers the carbon fibers after cycling at high temperature. The ionic liquid in solution maintains the high maximum operative voltage of the net ionic liquid whereas its viscosity and ionic conductivity are close to those of the conventional Et 4 NBF 4 /PC. Furthermore, the pres- ence of propylene carbonate as solvent prevents to some extent the ionic liquid degradation. Keywords Electrochemical capacitor . Carbon . Ionic liquids . Electrolyte decomposition . Electrode damage Introduction Electrochemical double-layer capacitors (EDLCs), also called supercapacitors, are nowadays very reliable systems for electri- cal energy storage [1, 2]. The energy density of commercial EDLCs, of ca. 5 Wh kg -1 , is sufficient for many current appli- cations, but a substantial increment is highly required by the energy needs in a short-medium-term future. Therefore, the increase in the energy storage capability of these devices is the main target of the intensive research that is being accom- plished currently. The strategy mainly relies on two approaches: 1. The enhancement of the electrode capacitance. The ma- jority of the supercapacitors in the market utilize carbon materials as electrodes and, then, tremendous efforts have been made for the development of novel carbon materials with a much higher specific surface area and optimized physicochemical properties [2–4]. 2. The increase in the operating voltage. In the last years, a large variety of new electrolytes such as ionic liquids have been proposed for 3.0–3.5 V-EDLCs [2, 4, 5] in order to surpass the limits in the operating voltage of aqueous (1 V) and organic (2.5–3 V) electrolytes. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10008-017-3809-7) contains supplementary material, which is available to authorized users. * Teresa A. Centeno teresa@incar.csic.es 1 Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), C/ Sor Juana Inés de la Cruz, 3,Cantoblanco, 28049 Madrid, Spain 2 Institute for Technical Chemistry and Environmental Chemistry, Friedrich-Schiller University Jena, Jena, Germany 3 Center for Energy and Environmental Chemistry Jena (CEEC Jena), Philosophenweg 7a, 07743 Jena, Germany 4 Helmholtz Institute Ulm-Karlsruhe Institute of Technology, Helmholtzstrasse 11, 89081 Ulm, Germany 5 Instituto Nacional del Carbón (INCAR-CSIC), Francisco Pintado Fe 26, 33011 Oviedo, Spain J Solid State Electrochem https://doi.org/10.1007/s10008-017-3809-7