Umeå University's logo

This study introduces two new fluorine-free ionic liquids (ILs) produced by coupling biomass-derived heterocyclic anions, i.e., tetrahydro-2H-pyran-4-carboxylate (THP) and furan-3-carboxylate (3-FuA), and tetrahydroxyphosphonium cation (P4444). The (P4444)(3-FuA) IL exhibits slightly higher thermal stability, displays a lower glass-transition temperature and significantly higher ionic conductivity than (P4444)(THP). This improvement arises from π-electron delocalization in the (3-FuA) anion, by dispersing the negative charge over the ring, weakening the cation–anion attractions, and thus enhancing the ion mobility. Owing to the favorable ion transport characteristics, (P4444)(3-FuA) performs exceptionally well as a supercapacitor electrolyte. When paired with multiwalled carbon nanotubes (MWCNT)-based electrodes, (P4444)(3-FuA) delivers an areal capacitance of 430 mF cm−2 at 2 mV s−1, an energy density of 86 µWh cm−2 at 0.298 mA cm−2, and a power density of 1492 µW cm−2 at 0.995 mA cm−2, while maintaining 97% Coulombic efficiency after 6 000 cycles at 60°C. In comparison, the (P4444)(THP) IL demonstrate a lower capacitance performance, albeit with robust long-term stability. Overall, both the ILs display enhanced capacitance with increasing temperature, underscoring their potential as fluorine-free electrolytes for supercapacitors operating under elevated thermal conditions.