Emmanuel Pameté Yambou presented a talk entitled "Extending the operating temperature range of ionic liquid-based EDLCs down to -50 °C by implementing binary mixtures and carbons with optimised porous texture" at the 70th Annual Meeting of the International Society of Electrochemistry (Electrochemistry: Linking Resources to Sustainable Development, 4 to 9 August 2019, Durban, South Africa).
Abstract:
Extending the operating temperature range of ionic liquid-based EDLCs down to -50 °C by implementing binary mixtures and carbons with optimised porous texture
Emmanuel Pameté Yambou, Barbara Górska, Vladimir Pavlenko and François Béguin
Poznan University of Technology, ul. Berdychowo 4, 60-965, Poznan, Poland
emmanuel.yambou@doctorate.put.poznan.pl
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]. Nevertheless, as graphite oxide has a relatively low density, the resulting volumetric capacitance of such EDLC is very low.
The present study is focused on the use of imidazolium-based ILs, due to their relatively low viscosity and relatively high conductivity in contrast to their piperidinium or pyrrolidinium counterparts. Binary mixtures with various molar ratios were formulated using two ILs with 1-ethyl-3-methylimidazolium cation, [EMIm]+, coupled with bis(trifluosulfonyl)imide [FSI]- or tetrafluoroborate [BF4]- anions. Some mixtures did not show any first order transition unlike the parent ILs, and remained liquid below -90 ºC, where they underwent devitrification. The temperature dependence of the dynamic viscosity and ionic conductivity of the pure ILs and their mixtures followed the Vogel-Tamman-Fulcher equation, while the corresponding Walden plot revealed a high degree of ionicity classifying them as “good ILs”. Accordingly, we selected the EMImFSI/EMImBF4 (1:1 molar ratio) mixture, as it exhibited only a glass transition at -97°C, relatively low viscosity of 33.0 mP s and relatively good conductivity of 12.1 mS cm-1 (at 20 °C).
To uphold the low-temperature performance of carbon-based EDLCs in the EMImFSI/EMIMBF4 (1:1 molar ratio) electrolyte, we applied electrodes made of mesoporous materials: SC2A carbon black (by Cabot, SDFT = 1502 m2 g-1; Vmeso = 1.05 cm3 g-1) and an home-made carbon [3] MP98B (SDFT = 1556 m2 g-1;Vmeso = 1.30 cm3 g-1) having well-defined mesopores with average size of 3.5 nm. Both EDLCs operated down to -50 ºC, while at this temperature the cell based on MP98B exhibited lower resistance and higher gravimetric capacitance (106 F g-1* vs 73 F g-1*).
* the capacitance values are expressed per average active mass in one electrode.
References
[1] M. Kunze, S. Jeong, E. Paillard, M. Winter, S. Passerini,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, RSC Advances, 5 (2015) 71699-71703.
[3] T. Morishita, T. Tsumura, M. Toyoda, J. Przepiórski, A. Morawski, H. Konno, M. Inagaki, Carbon, 48 (2010) 2690-2707.
Acknowledgements
The Polish National Science Center (NCN) is acknowledged for supporting the MAESTRO project UMO-2016/22/A/ST4/00092.
