Eutectic Mixtures of Ionic Liquids Electrolytes for Electric Double Layer Capacitors

Eutectic mixtures of oxygen-containing spirobipyrrolidinium-based room temperature ionic liquids (RTIL) were applied as electrolytes for electric double layer capacitors (EDLC). The mixtures of RTILs exhibit lower melting points with conductivity, and viscosity comparable with 2MOP BF4. The potential window of mixed RTIL was about 5.5 V (at 50 μA/cm2 threshold). The RTIL mixtures exhibit higher breakdown potential (> 4.6 V) as compared to conventional molecular solvent (acetonitrile)-based cells (~4.0 V) in EDLC tests. However, these RTIL mixtures lead to higher equivalent series resistance (ESR) initiating around 3.9 V and lower capacitance. Cycling of the negative electrode at 4.6 V led to degradation of the active carbon material, while the positive electrode maintained its integrity.