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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.

This study explores the synthesis and electrochemical performance of graphene oxide co-doped with sodium and potassium (Na–K–GO) as electrode materials for supercapacitors (SCs) designed to operate at 60°C over an extended voltage window. The Na–K–GO is employed as the electrode material, while a fluorine-free ionic liquid (IL), [P4444][MEEA]—comprising a tetrabutylphosphonium cation and a 2-2-(2-methoxyethoxy)ethoxy anion—served as the electrolyte, enabling stable operation over a wide voltage window at elevated temperatures. Using this combination, three coin-cell SCs are fabricated: two symmetric devices (SC-1 and SC-2) and one asymmetric device (SC-3). All the three exhibited remarkable charge storage abilities, a retaining performance over 10 000 charge–discharge cycles at 60°C. Among the three devices, SC-3 exhibited the best overall electrochemical performance, delivering a high specific capacitance of 47.01 F g−1 and an energy density of 27.77 Wh kg−1 at 0.5 A g−1. Even at a higher current density of 1 A g−1, SC-3 maintained a maximum power density of 1000 W kg−1 while sustaining an energy density of 14.21 Wh kg−1, reflecting its strong rate capability. Moreover, the long-term cycling tests at 2 A g−1 demonstrated an outstanding durability of SC-3, which retained 99% coulombic efficiency after 10 000 cycles, significantly outperforming the SC-2 (90%) and SC-1 (79%).