Enhancing the Capacitive Performance of Electric Double-Layer Capacitors with Ionic Liquid Mixtures

Formulating room-temperature ionic liquid (RTIL) mixed electrolytes was recently proposed as an effective and convenient strategy to increase the capacitive performance of electrochemical capacitors. Here we investigate the electrical double-layer (EDL) structure and the capacitance of two RTILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), and their mixtures with onion-like carbon electrodes using experiment and classical density functional theory. The principal difference between these ionic liquids is the smaller diameter of the BF4– anion relative to the TFSI– anion and the EMI+ cation. A volcano-shaped trend is identified for the capacitance versus the composition of the RTIL mixture. The mixture effect, which makes more counterions pack on and more co-ions leave from the electrode surface, leads to an increase of the counterion density within the EDL and thus a larger capacitance. These theoretical predictions are in good agreement with our experimental observations and offer guidance for designing RTIL mixtures for EDL supercapacitors.