Eutectic mixture of Protic Ionic Liquids as an Electrolyte for Activated Carbon-Based Supercapacitors

One of the drawbacks of ionic liquids is that some of them are solid or very viscous at room temperature; this prevents their use as electrolytes in energy storage systems. One solution consists of a binary mixture of pure, solid salts with the formation of a eutectic, allowing it to be used at room temperature. This work describes, for the first time, the formulation and use of a binary mixture of protic ionic liquids (PILs) based on the same H-bond donor, pyrrolidinium cation, with nitrate ([Pyrr][NO3]) and bis(trifluoromethanesulfonyl) imide ([Pyrr][TFSI]) anions as an electrolyte for carbon-based supercapacitors. The physico-chemical and thermal properties of mixtures as a function of composition and temperature were fully investigated and related to their electrochemical behavior as an electrolyte. The electrochemical characterization of some selected optimal compositions (x[Pyrr][NO3] = 0.64, 0.72, 0.80) shows, at first, good capacitive behavior (up to 148 F g−1) at a potential voltage of 2.0 V. Moreover, above 2.0 V, anion intercalation was observed, increasing the specific capacitance up to 209 F g−1. At 2.5 V during cycling, the characteristic peak of anion intercalation/deintercalation decreases, caused by saturation of the positive electrode by anions. Finally, accelerated aging tests showed good capacity retention (> 80%) at 2.0 V during 110 hours for x[Pyrr][NO3] = 0.72, at 25 °C. According to these results, the use of eutectic PIL binary mixtures is a promising way to design adapted electrolytes according to the material electrode for energy devices at room temperature.

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