High performance supercapacitor under extremely low environmental temperature

Supercapacitors with a broad operational temperature range, especially with the capability of operating at low temperature (below -50°C), are required for applications in extreme environments. This paper presents a high performance supercapacitor, consisting of highly aligned nano-porous graphene (A-NPG) with an eutectic mixture of ionic liquids (ILs), 1-butyl-4-methylpyridinium tetrafluoroborate (BMPBF₄) in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF₄), which exhibits a high capacitance performance over a temperature range from -50°C to 80°C. The experimental results reveal a reducing of melting temperature T_m of the eutectic mixture to -74°C, compared with -39°C of BMIBF₄. The increased ionic conductivity and reduced T_m of the eutectic mixture, combined with ultrahigh density A-NPG electrodes, enable the supercapacitor to show a high capacitance of 125 F cm^-3 at room temperature and 106 F cm^-3 at -50°C. In contrast, the supercapacitor with pure BMIBF₄ shows a capacitance of 119 F cm^-3 and 16.5 F cm^-3 at room temperature and -50°C, respectively.