This work provides a study of mixtures of the azepanium-based ionic liquid (IL) N-methyl, N-butyl-azepanium bis[(trifluoromethane) sulfonyl]imide (Azp14TFSI) and propylene carbonate (PC) as electrolyte components in electrochemical double layer capacitors (EDLCs). The considered mixtures’ properties were then compared to the properties of mixtures of N-butyl, N-methylpyrrolidinium bis[(trifluoromethane) sulfonyl]imide (Pyr14TFSI) and PC in terms of viscosity, conductivity and electrochemical behavior. The mixtures’ operative potentials were found to be comparable to each other, leading to operative voltages as high as 3.5V, while retaining the low viscosities and high conductivities of PC based EDLC electrolytes.