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
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
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