Best Creative Work

Emmanuel Pameté Yambou received an award from the Polish Academy of Sciences (PAN) for the "best creative work" published in 2020 in the field of Exact Science and Earth Sciences.

Best Poster Prize

Emmanuel Pameté Yambou has been awarded the Best Poster Prize at the 71st Annual Meeting of the International Society of Electrochemistry 30 August - 4 September, 2020, Belgrade online, Serbia.

Abstract:

Reduction of faradaic contributions in carbon/carbon cells using a “water-in-salt” electrolyte

Emmanuel Pameté Yambou* and François Béguin

Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology,

Berdychowo4, 60-965 Poznan, Poland

*emmanuel.yambou@doctorate.put.poznan.pl

Environmentally friendly aqueous alkali sulfate electrolytes with pH close to neutrality have been recently proposed for carbon/carbon supercapacitors (SCs) demonstrating enhanced voltages up to U = 1.6 V [1]. The enhanced voltages in SCs using neutral aqueous electrolytes exceeding the electrochemical stability window of water (1.23 V) is due to the increased local pH inside the porosity of negative carbon electrode due to the electrochemical reduction water producing OH- anions and causing important di-hydrogen evolution over-potential [2]. However, it has now been established that carbon/carbon cell in 1 mol L-1 Li2SO4 keeps good state-of-health under potentiostatic floating only up to 1.5 V, beyond this voltage oxidation of positive carbon electrode occurs owing to the generation of oxygenated functionalities demonstrated by the CO and CO2 evolution as side products [3]. Besides, reduction of water at the negative electrode generating hydrogen gas presents another challenge at high voltages contributing to pressure increase inside the cell and consequently resulting deterioration of SC performance during long-term aging/cycling [4].

In this work, we implement LiTFSI in water at two different concentrations (1 mol kg-1 and 20 mol kg-1, pH = 6.1) in carbon/carbon supercapacitors to demonstrate the influence of electrolyte concentration on the electrochemical performance of the cell. Three-electrode cell investigations show that the hydrogen adsorption under negative polarization of carbon electrode in 20 mol kg-1 aqueous LiTFSI is dramatically reduced in comparison to 1 mol kg-1 LiTFSI. Two electrode cell data demonstrates nearly rectangular CV characteristics similar to an electrical double-layer capacitor for carbon/carbon cell in 20 mol kg-1 LiTFSI. Thanks to the reduced faradaic contribution at the concentration of 20 mol kg-1 LiTFSI owing to the negligible hydrogen adsorption and desorption at the negative carbon electrode, the cell demonstrates low self-discharge of ΔU = 0.178 V. Further analyses of self-discharge data suggest the shift of activation controlled mechanisms to diffusion controlled mechanisms with the increase of concentration from 1 mol kg-1 LiTFSI to 20 mol kg-1 LiTFSI. Thermo-programmed desorption performed on the carbon electrodes in 20 mol kg-1 LiTFSI suggest a sudden mass loss at around 450 °C owing to the decomposition of LiTFSI trapped inside the electrode porosity.

References:

[1] Demarconnay L., Raymundo-Piñero E., Béguin F., Electrochem. Comm., 12 (2010) 1275.

[2] Gao Q., Demarconnay L., Raymundo-Piñero E., Béguin F., Energy & Environ. Sci., 5 (2012) 9611.

[3] Ratajczak P., Jurewicz K., Skowron P., Abbas Q., Béguin F., Electrochimica Acta, 130 (2014) 344.

[4] He M., Fic K., Frackowiak E., Novak P., Berg E. J., Energy Environ. Sci., 9 (2016) 623.

[5] Suo L., Borodin O., Gao T., Olguin M., Ho J., Fan X., Luo C., Wang C., Xu K., Science, 350 (2015) 938.

Acknowledgements:

The Polish National Science Center (NCN) is acknowledged for supporting the MAESTRO project UMO-2016/22/A/ST4/00092

Best Oral presentation Award

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.