Fluorine‐Free Ionic Liquid Electrolytes Featuring Biomass‐Derived Heterocyclic Anions for Variable Temperature Supercapacitors

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 (P 4444 ). The (P 4444 )(3‐FuA) IL exhibits slightly higher thermal stability, displays a lower glass‐transition temperature and significantly higher ionic conductivity than (P 4444 )(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, (P 4444 )(3‐FuA) performs exceptionally well as a supercapacitor electrolyte. When paired with multiwalled carbon nanotubes (MWCNT)‐based electrodes, (P 4444 )(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 (P 4444 )(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.