(642e) Electrochemical Performance of Binary Mixture of Ionic Liquids Electrolytes in Supercapacitors

Ionic liquids (ILs), as promising electrolyte candidates for batteries and supercapacitors have been attracting increasing research interest. Recently, it was reported that a binary mixture of ionic liquids extends the operation temperature range for supercapacitors to -50 - 100 °C [1], which makes the mixture of ionic liquids potential electrolytes for applications in severe weather conditions. In this work, a equimolar mixture of 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C₃mpy][Tf₂N]), 1-methyl-1-butylpiperidinium bis(trifluoromethylsulfonyl)imide ([C₄mpip][Tf₂N]) was investigated by classic molecular dynamics (MD) simulation. Differential scanning calorimetry (DSC) measurement verified that the binary mixture exhibited lower glass transition temperature than either of the pure ILs. Moreover, the binary mixture gave rise to the higher conductivity than the neat ILs at lower temperature range.  In order to study its capacitive performance in supercapacitors, the mixture and the neat ILs were used as electrolytes near onion-like carbon (OLC) electrode at varying temperatures.  Both the differential capacitance and integral capacitance exhibited independence of the electrical potential applied for three electrolytes, which is in agreement with previous report on a different IL [2]. Anomalous temperature dependence of the capacitance was observed and it was dominated by the electrical double layer (EDL) thickness, which is for the first time substantiated in MD simulation. In summary, this study demonstrated that the binary mixture of ILs outperformed neat ILs as supercapacitor electrolyte with widened operation temperature, higher conductivity and a near-flat capacitance as a function of electric potential. The anomalous temperature dependence of the capacitance dominated by the EDL thickness provides a theoretical model for interpreting the anomalous temperature dependence observed in experiments.

References

[1] R. Y. Lin, P. L. Taberna, S. Fantini, V. Presser, C. R. Perez, F. Malbosc, N. L. Rupesinghe, K. B. K. Teo, Y. Gogotsi, P. Simon, Capacitive Energy Storage from -50 to 100  °C Using an Ionic Liquid Electrolyte,Journal of Physical Chemistry Letters,2011, 2(19):2396-2401

[2] G. Feng, D. Jiang, and P. T. Cummings, Curvature Effect on the Capacitance of Electric Double Layers at Ionic Liquid/Onion-like Carbon Interfaces,Journal of Chemical Theory and Computation, 2012, 8(3):1058–1063