Nanoporous carbon-based supercapacitors store electricity through adsorptionof ions from the electrolyte at the surface of the electrodes. Room temperatureionic liquids, which show the largest ion concentrations among organic liquidelectrolytes, should in principle yield larger capacitances. Here we show byusing electrochemical measurements that the capacitance is not significantlyaffected when switching from a pure ionic liquid to a conventional organicelectrolyte using the same ionic species. By performing additional moleculardynamics simulations, we interpret this result as an increasing difficulty ofseparating ions of opposite charges when they are more concentrated, i.e. inthe absence of a solvent which screens the Coulombic interactions. The chargingmechanism consistently changes with ion concentration, switching fromcounter-ion adsorption in the diluted organic electrolyte to ion exchange inthe pure ionic liquid. Contrarily to the capacitance, in-pore diffusioncoefficients largely depend on the composition, with a noticeable slowing ofthe dynamics in the pure ionic liquid.
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