Fluorine-Free Cyclic Carboxylate Ionic Liquids as Supercapacitor Electrolytes

Two fluorine-free phosphonium-based ionic liquids (ILs), (P4444)(CH) and (P4444)(CP), incorporating cyclohexane- and cyclopentane-carboxylate anions, are synthesized to elucidate the role of ring geometry on their properties. The (P4444)(CH) forms a deeply amorphous system with a low glass-transition temperature (Tg) at −55 °C) and a single-step thermal degradation onset at 244 °C, indicating a disordered yet thermally stable liquid phase. In contrast, the (P4444)(CP) exhibits a higher Tg at −49 °C with a higher decomposition onset of 286 °C. The (P4444)(CH) displays an increase in ionic conductivities from 0.058 30 °C to 1.367 mS cm–1 100 °C, whereas (P4444)(CP) consistently exhibits higher values, rising from 0.078 to 2.618 mS cm–1 at the same temperatures. When employed as electrolytes in activated-carbon supercapacitors (SCs) tested over 30–90 °C and 0.5–4.0 V, the (P4444)(CP) enables markedly enhanced capacitive performance. The (P4444)(CP)-based device delivers 140.75 mF cm–2 (at 1 mV s–1) and 117.51 mF cm–2 (at 0.195 mA cm–2), along with an energy density of 65.29 μWh cm–2, substantially exceeding the (P4444)(CH)-based system (85.36 mF cm–2, 47.42 μWh cm–2). The devices with both ILs reveal low interfacial resistance and excellent cycling stability, maintaining Coulombic efficiencies exceeding 99% over 10,000 charge–discharge cycles.