Emmanuel Pameté Yambou won the prize for the ‘Best Oral presentation’ during the 8th International Conference on Carbon for Energy Storage and Environmental Protection (CESEP’19), 21-24th October 2019, Alicante, Spain.
Title: Low temperature performance of carbon/carbon EDLCs down to -50°C in ionic liquid binary mixture
Authors: Emmanuel Pameté Yambou, Barbara Górska and François Béguin
ABSTRACT Ionic liquids (ILs) are a solvent-free perspective class of electrolytes for high voltage electrical double-layer capacitors (EDLCs). They are characterized by high electrochemical stability, as well as they are neither volatile nor flammable. Therefore, ILs are superior to conventional organic electrolytes, e.g., 1 mol L‑1 TEABF4 in acetonitrile, which pose safety and toxicity concerns. However, the relatively high melting point of ILs restricts the scope of their applicability at low temperature compared to traditional EDLCs performing from -40 °C up to +70 °C. To overcome these limitations and to extend the operating range of EDLCs with ILs to sub-ambient temperature, their binary mixtures have been proposed as electrolytes [1]. Recently, a graphite oxide-based EDLC incorporating a binary mixture (1:1 by molar ratio) of 1-butyl-4-methyl-pyridinium tetrafluoroborate (BMPBF4) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) could perform even down to -50 °C [2]. , as graphite oxide has a relatively low density, the resulting volumetric capacitance of such EDLC is very low. In this work, binary mixtures with various molar ratios were formulated using two ILs with a common 1‑ethyl-3-methylimidazolium cation, [EMIm]+, coupled with bis(trifluosulfonyl)imide [FSI]- or tetrafluoroborate [BF4]- anions. Their thermal properties, viscosity and conductivity were determined. Accordingly, the (EMImFSI)0.5(EMImBF4 )0.5 mixture, which was liquid down to -97 °C as well as displayed relatively low viscosity of 33.0 mP s and relatively good conductivity of 12.1 mS cm-1 (at 20 °C), was selected as electrolyte for carbon-based EDLCs. To uphold the low-temperature performance of capacitors, we applied electrodes made of mesoporous materials: carbon black SC2A (by Cabot) and a home-made templated carbon MP98B with mesopores (average size of 3.5 nm) well-fitted for the electrolyte ions. Both EDLCs operated down to -50 °C, while the cell based on MP98B exhibited better charge propagation and greater capacitance (140 F g-1† vs 115 F g-1† @ 20 °C). During the presentation, the electrochemical properties of EDLCs at low temperature will be discussed in light of the porous texture of carbons and thermal properties of IL mixtures. †capacitance values are expressed per average active mass in one electrode.
Acknowledgements The Polish National Science Center (NCN) is acknowledged for supporting the MAESTRO project UMO-2016/22/A/ST4/00092.
References [1] M. Kunze, S. Jeong, E. Paillard, M. Winter, S. Passerini, Melting behavior of pyrrolidinium-based ionic liquids and their binary mixtures, The Journal of Physical Chemistry C, 114 (2010) 12364-12369. [2] Y. Zhou, M. Ghaffari, M. Lin, H. Xu, H. Xie, C.M. Koo, Q. Zhang, High performance supercapacitor under extremely low environmental temperature, RSC Advances, 5 (2015) 71699-71703.
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