Eutectic mixtures of ionic liquids electrolytes for electric double layer capacitors

Eutectic mixtures of oxygen-containing spirobipyrrolidinium-based ionic liquids were investigated as prospective room temperature ionic liquid electrolytes (RTILs) for supercapacitor application. These in-house developed novel ionic liquids (IL) possess wide voltage window (6-7 V) and solubility (3-5.5 M) in conventional solvents, such as acetonitrile and propylene carbonate. The potential window of mixed RTIL was about 5.5 V (at 50 mu A/cm(2) threshold). The RTIL mixtures exhibit higher breakdown potential (> 4.6 V) as compared to conventional molecular solvent (acetonitrile)-based cells (similar to 4.0 V) in supercapacitor tests. However, these RTIL mixtures (neat, i.e. w/o any solvent addition) lead to high 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.