Burt, Ryan and Breitsprecher, Konrad and Daffos, Barbara and Taberna, Pierre-Louis and Simon, Patrice and Birkett, Greg and Zhao, X. S. and Holm, Christian and Salanne, Mathieu Capacitance of Nanoporous Carbon-Based Supercapacitors Is a Trade-Off between the Concentration and the Separability of the Ions. (2016) The Journal of Physical Chemistry Letters, 7 (19). 4015-4021. ISSN 1948-7185
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Official URL: http://dx.doi.org/10.1021/acs.jpclett.6b01787
Abstract
Nanoporous carbon-based supercapacitors store electricity through adsorption of ions from the electrolyte at the surface of the electrodes. Room temperature ionic liquids, which show the largest ion concentrations among organic liquid electrolytes, should in principle yield larger capacitances. Here, we show by using electrochemical measurements that the capacitance is not significantly affected when switching from a pure ionic liquid to a conventional organic electrolyte using the same ionic species. By performing additional molecular dynamics simulations, we interpret this result as an increasing difficulty of separating ions of opposite charges when they are more concentrated, that is, in the absence of a solvent that screens the Coulombic interactions. The charging mechanism consistently changes with ion concentration, switching from counterion adsorption in the diluted organic electrolyte to ion exchange in the pure ionic liquid. Contrarily to the capacitance, in-pore diffusion coefficients largely depend on the composition, with a noticeable slowing of the dynamics in the pure ionic liquid.
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